Stewart A Bloomfield, Ph.D.

Masked excitatory crosstalk between the ON and OFF visual pathways in the mammalian retina
Farajian, Reza; Pan, Feng; Akopian, Abram; Volgyi, Bela; Bloomfield, Stewart A
2011 Sep 15;589(Pt 18):4473-4489, Journal of physiology
Abstract A fundamental organizing feature of the visual system is the segregation of ON and OFF responses into parallel streams to signal light increment and decrement. However, we found that blockade of GABAergic inhibition unmasks robust ON responses in OFF alpha-ganglion cells (alpha-GCs). These ON responses had the same centre-mediated structure as the classic OFF responses of OFF alpha-GCs, but were abolished following disruption of the ON pathway with l-AP4. Experiments showed that both GABA(A) and GABA(C) receptors are involved in the masking inhibition of this ON response, located at presynaptic inhibitory synapses on bipolar cell axon terminals and possibly amacrine cell dendrites. Since the dendrites of OFF alpha-GCs are not positioned to receive excitatory inputs from ON bipolar cell axon terminals in sublamina-b of the inner plexiform layer (IPL), we investigated the possibility that gap junction-mediated electrical synapses made with neighbouring amacrine cells form the avenue for reception of ON signals. We found that the application of gap junction blockers eliminated the unmasked ON responses in OFF alpha-GCs, while the classic OFF responses remained. Furthermore, we found that amacrine cells coupled to OFF alpha-GCs display processes in both sublaminae of the IPL, thus forming a plausible substrate for the reception and delivery of ON signals to OFF alpha-GCs. Finally, using a multielectrode array, we found that masked ON and OFF signals are displayed by over one-third of ganglion cells in the rabbit and mouse retinas, suggesting that masked crossover excitation is a widespread phenomenon in the inner mammalian retina
— id: 137837, year: 2011, vol: 589, page: 4473, stat: Journal Article,

Cadherin-6 mediates axon-target matching in a non-image-forming visual circuit
Osterhout, Jessica A; Josten, Nicko; Yamada, Jena; Pan, Feng; Wu, Shaw-Wen; Nguyen, Phong L; Panagiotakos, Georgia; Inoue, Yukiko U; Egusa, Saki F; Volgyi, Bela; Inoue, Takayoshi; Bloomfield, Stewart A; Barres, Ben A; Berson, David M; Feldheim, David A; Huberman, Andrew D
2011 Aug 25;71(4):632-639, Neuron
Neural circuits consist of highly precise connections among specific types of neurons that serve a common functional goal. How neurons distinguish among different synaptic targets to form functionally precise circuits remains largely unknown. Here, we show that during development, the adhesion molecule cadherin-6 (Cdh6) is expressed by a subset of retinal ganglion cells (RGCs) and also by their targets in the brain. All of the Cdh6-expressing retinorecipient nuclei mediate non-image-forming visual functions. A screen of mice expressing GFP in specific subsets of RGCs revealed that Cdh3-RGCs which also express Cdh6 selectively innervate Cdh6-expressing retinorecipient targets. Moreover, in Cdh6-deficient mice, the axons of Cdh3-RGCs fail to properly innervate their targets and instead project to other visual nuclei. These findings provide functional evidence that classical cadherins promote mammalian CNS circuit development by ensuring that axons of specific cell types connect to their appropriate synaptic targets
— id: 138009, year: 2011, vol: 71, page: 632, stat: Journal Article,

Light increases the gap junctional coupling of retinal ganglion cells
Hu, Edward H; Pan, Feng; Volgyi, Bela; Bloomfield, Stewart A
2010 Nov 1;588(Pt 21):4145-4163, Journal of physiology
We examined the effect of light adaptation on the gap junctional coupling of alpha-ganglion cells (alpha-GCs) in rabbit and mouse retinas. We assayed changes in coupling by measuring parameters of tracer coupling following injection of alpha-GCs with Neurobiotin and the concerted spike activity of alpha-GC neighbours under dark- and light-adapted conditions. We found that light adaptation using mesopic or photopic background lights resulted in a dramatic increase in the labelling intensity, number, and spatial extent of ganglion and amacrine cells coupled to OFF alpha-GCs when compared to levels seen under dark adaptation. While this augmentation of coupling by light did not produce an increase in the concerted spontaneous activity of OFF alpha-GC neighbours, it did significantly increase correlated light-evoked spiking. This was seen as an increase in the number of correlated spikes for alpha-GC neighbours and an extension of correlations to second-tier neighbours that was not seen under dark-adapted conditions. Pharmacological studies in the rabbit retina indicated that dopamine mediates the observed changes in coupling by differentially activating D1 and D2 receptors under different adaptation states. In this scheme, activation of dopamine D1 receptors following light exposure triggers cAMP-mediated intracellular pathways resulting in an increase in gap junctional conductance. Overall, our results indicate that as we move from night to day there is an enhanced electrical coupling between alpha-GCs, thereby increasing the concerted activity believed to strengthen the capacity and efficiency of information flow across the optic nerve
— id: 114174, year: 2010, vol: 588, page: 4145, stat: Journal Article,

Connexin36 is required for gap junctional coupling of most ganglion cell subtypes in the mouse retina
Pan, Feng; Paul, David L; Bloomfield, Stewart A; Volgyi, Bela
2010 Mar 15;518(6):911-927, Journal of comparative neurology
Converging evidence indicates that electrical synaptic transmission via gap junctions plays a crucial role in signal processing in the retina. In particular, amacrine and ganglion cells express numerous gap junctions, resulting in extensive electrical networks in the proximal retina. Both connexin36 (Cx36) and connexin45 (Cx45) subunits are widely distributed in the inner plexiform layer (IPL) and therefore are likely contribute to gap junctions formed by a number of ganglion cell subtypes. In the present study, we used the gap junction-permeant tracer Neurobiotin to compare the coupling pattern of different ganglion cell subtypes in wild-type (WT) and Cx36 knockout (KO) mouse retinas. We found that homologous ganglion-to-ganglion cell coupling was lost for two subtypes after deletion of Cx36, whereas two other ganglion cell subtypes retained homologous coupling in the KO mouse. In contrast, deletion of Cx36 resulted in a partial or complete loss of ganglion-to-amacrine cell heterologous coupling in 9 of 10 ganglion cell populations studied. Overall, our results indicate that Cx36 is the predominant subunit of gap junctions in the proximal mouse retina, expressed by most ganglion cell subtypes, and thereby likely plays a major role in the concerted activity generated by electrical synapses
— id: 106370, year: 2010, vol: 518, page: 911, stat: Journal Article,

GABA blockade unmasks an OFF response in ON direction selective ganglion cells in the mammalian retina
Ackert, Jessica M; Farajian, Reza; Volgyi, Bela; Bloomfield, Stewart A
2009 Sep 15;587(Pt 18):4481-4495, Journal of physiology
One unique subtype of retinal ganglion cell is the direction selective (DS) cell, which responds vigorously to stimulus movement in a preferred direction, but weakly to movement in the opposite or null direction. Here we show that the application of the GABA receptor blocker picrotoxin unmasks a robust excitatory OFF response in ON DS ganglion cells. Similar to the characteristic ON response of ON DS cells, the masked OFF response is also direction selective, but its preferred direction is opposite to that of the ON component. Given that the OFF response is unmasked with picrotoxin, its direction selectivity cannot be generated by a GABAergic mechanism. Alternatively, we find that the direction selectivity of the OFF response is blocked by cholinergic drugs, suggesting that acetylcholine release from presynaptic starburst amacrine cells is crucial for its generation. Finally, we find that the OFF response is abolished by application of a gap junction blocker, suggesting that it arises from electrical synapses between ON DS and polyaxonal amacrine cells. Our results suggest a novel role for gap junctions in mixing excitatory ON and OFF signals at the ganglion cell level. We propose that OFF inputs to ON DS cells are normally masked by a GABAergic inhibition, but are unmasked under certain stimulus conditions to mediate optokinetic signals in the brain
— id: 102401, year: 2009, vol: 587, page: 4481, stat: Journal Article,

The diverse functional roles and regulation of neuronal gap junctions in the retina
Bloomfield, Stewart A; Volgyi, Bela
2009 Jul;10(7):495-506, Nature reviews. Neuroscience
Electrical synaptic transmission through gap junctions underlies direct and rapid neuronal communication in the CNS. The diversity of functional roles that electrical synapses have is perhaps best exemplified in the vertebrate retina, in which gap junctions are formed by each of the five major neuron types. These junctions are dynamically regulated by ambient illumination and by circadian rhythms acting through light-activated neuromodulators such as dopamine and nitric oxide, which in turn activate intracellular signalling pathways in the retina.The networks formed by electrically coupled neurons are plastic and reconfigurable, and those in the retina are positioned to play key and diverse parts in the transmission and processing of visual information at every retinal level
— id: 100482, year: 2009, vol: 10, page: 495, stat: Journal Article,

Tracer coupling patterns of the ganglion cell subtypes in the mouse retina
Volgyi, Bela; Chheda, Samir; Bloomfield, Stewart A
2009 Feb 10;512(5):664-687, Journal of comparative neurology
It is now clear that electrical coupling via gap junctions is prevalent across the retina, expressed by each of the five main neuronal types. With the introduction of mutants in which selective gap junction connexins are deleted, the mouse has recently become an important model for studying the function of coupling between retinal neurons. In this study we examined the tracer-coupling pattern of ganglion cells by injecting them with the gap junction-permanent tracer Neurobiotin to provide, for the first time, a comprehensive survey of ganglion cell coupling in the wildtype mouse retina. Murine ganglion cells were differentiated into 22 morphologically distinct subtypes based on soma-dendritic parameters. Most (16/22) ganglion cell subtypes were tracer-coupled to neighboring ganglion and/or amacrine cells. The amacrine cells coupled to ganglion cells displayed either polyaxonal or wide-field morphologies with extensive arbors. We found that different subtypes of ganglion cells were never coupled to one another, indicating that they subserved independent electrical networks. Finally, we found that the tracer-coupling patterns of the 22 ganglion cell populations were largely stereotypic across the 71 retinas studied. Our results indicate that electrical coupling is extensive in the inner retina of the mouse, suggesting 0
— id: 92176, year: 2009, vol: 512, page: 664, stat: Journal Article,

Distribution and functional roles of neuronal gap junctions in the mouse retina
Bloomfield, Stewart A; Volgyi, Bela
Eye, retina, and visual system of the mouse Cambridge, MA : MIT Press, 2008,
(from the chapter) In this chapter we review the distribution of gap junctions in the mouse retina and recent work detailing their significant and varied functional roles in visual processing.
— id: 5248, year: 2008, vol: , page: 120, stat: Chapter,

Synaptic regulation of the light-dependent oscillatory currents in starburst amacrine cells of the mouse retina
Petit-Jacques, Jerome; Bloomfield, Stewart A
2008 Aug;100(2):993-1006, Journal of neurophysiology
Responses of on-center starburst amacrine cells to steady light stimuli were recorded in the dark-adapted mouse retina. The response to spots of dim white light appear to show two components, an initial peak that correspond to the onset of the light stimulus and a series of oscillations that ride on top of the initial peak relaxation. The frequency of oscillations during light stimulation was three time higher than the frequency of spontaneous oscillations recorded in the dark. The light-evoked responses in starburst cells were exclusively dependent on the release of glutamate likely from presynaptic bipolar axon terminals and the binding of glutamate to AMPA/kainate receptors because they were blocked by 6-cyano-7-nitroquinoxalene-2,3-dione. The synaptic pathway responsible for the light responses was blocked by AP4, an agonist of metabotropic glutamate receptors that hyperpolarize on-center bipolar cells on activation. Light responses were inhibited by the calcium channel blockers cadmium ions and nifedipine, suggesting that the release of glutamate was calcium dependent. The oscillatory component of the response was specifically inhibited by blocking the glutamate transporter with d-threo-beta-benzyloxyaspartic acid, suggesting that glutamate reuptake is necessary for the oscillatory release. GABAergic antagonists bicuculline, SR 95531, and picrotoxin increased the amplitude of the initial peak while they inhibit the frequency of oscillations. TTX had a similar effect. Strychnine, the blocker of glycine receptors did not affect the initial peak but strongly decreased the oscillations frequency. These inhibitory inputs onto the bipolar axon terminals shape and synchronize the oscillatory component
— id: 93305, year: 2008, vol: 100, page: 993, stat: Journal Article,

Response properties of a unique subtype of wide-field amacrine cell in the rabbit retina
Bloomfield, Stewart A; Volgyi, Bela
2007 Jul-Aug;24(4):459-469, Visual neuroscience
We studied the morphology and physiology of a unique wide-field amacrine cell in the rabbit retina. These cells displayed a stereotypic dendritic morphology consisting of a large, circular and monostratified arbor that often extended over 2 mm. Their responses contained both somatic and dendritic sodium spikes suggesting active propagation of synaptic signals within the dendritic arbor. This idea is supported by the enormous size of their ON-OFF receptive fields. Interestingly, these cells exhibited separate ON and OFF receptive fields that, while concentric, were vastly different in size. Whereas the ON receptive field of these cells extended nearly 2 mm, the OFF receptive field was typically 75% smaller. Blockade of voltage-gated sodium channels with QX-314 dramatically reduced the large ON receptive field, but had little effect on the smaller OFF receptive field. These results indicate a spatial disparity in the location of on- and off-center bipolar cell inputs to the dendritic arbor of wide-field amacrine cells. In addition, the active propagation of signals suggests that synaptic inputs are integrated both locally and globally within the dendritic arbor
— id: 75383, year: 2007, vol: 24, page: 459, stat: Journal Article,

Light-induced changes in spike synchronization between coupled ON direction selective ganglion cells in the mammalian retina
Ackert, Jessica M; Wu, Synphen H; Lee, Jacob C; Abrams, Joseph; Hu, Edward H; Perlman, Ido; Bloomfield, Stewart A
2006 Apr 19;26(16):4206-4215, Journal of neuroscience
Although electrical coupling via gap junctions is prevalent among ganglion cells in the vertebrate retina, there have been few direct studies of their influence on the light-evoked signaling of these cells. Here, we describe the pattern and function of coupling between the ON direction selective (DS) ganglion cells, a unique subtype whose signals are transmitted to the accessory optic system (AOS) where they initiate the optokinetic response. ON DS cells are coupled indirectly via gap junctions made with a subtype of polyaxonal amacrine cell. This coupling underlies synchronization of the spontaneous and light-evoked spike activity of neighboring ON DS cells. However, we find that ON DS cell pairs show robust synchrony for all directions of stimulus movement, except for the null direction. Null stimulus movement evokes a GABAergic inhibition that temporally shifts firing of ON DS cell neighbors, resulting in a desynchronization of spike activity. Thus, detection of null stimulus movement appears key to the direction selectivity of ON DS cells, evoking both an attenuation of spike frequency and a desynchronization of neighbors. We posit that active desynchronization reduces summation of synaptic potentials at target AOS cells and thus provides a secondary mechanism by which ON DS cell ensembles can signal direction of stimulus motion to the brain
— id: 64173, year: 2006, vol: 26, page: 4206, stat: Journal Article,

Morphology and tracer coupling pattern of alpha ganglion cells in the mouse retina
Volgyi, Bela; Abrams, Joseph; Paul, David L; Bloomfield, Stewart A
2005 Nov 7;492(1):66-77, Journal of comparative neurology
Alpha cells are a type of ganglion cell whose morphology appears to be conserved across a number of mammalian retinas. In particular, alpha cells display the largest somata and dendritic arbors at a given eccentricity and tile the retina as independent on- (ON) and off-center (OFF) subtypes. Mammalian alpha cells also express a variable tracer coupling pattern, which often includes homologous (same cell type) coupling to a few neighboring alpha cells and extensive heterologous (different cell type) coupling to two to three amacrine cell types. Here, we use the gap junction-permeant tracer Neurobiotin to determine the architecture and coupling pattern of alpha cells in the mouse retina. We find that alpha cells show the same somatic and dendritic architecture described previously in the mammal. However, alpha cells show varied tracer coupling patterns related to their ON and OFF physiologies. ON alpha cells show no evidence of homologous tracer coupling but are coupled heterologously to at least two types of amacrine cell whose somata lie within the ganglion cell layer. In contrast, OFF alpha cells are coupled to one another in circumscribed arrays as well as to two to three types of amacrine cell with somata occupying the inner nuclear layer. We find that homologous coupling between OFF alpha cells is unaltered in the connexin36 (Cx36) knockout (KO) mouse retina, indicating that it is not dependent on Cx36. However, a subset of the heterologous coupling of ON alpha cells and all the heterologous coupling of OFF alpha cells are eliminated in the KO retina, suggesting that Cx36 comprises most of the junctions made with amacrine cells
— id: 61844, year: 2005, vol: 492, page: 66, stat: Journal Article,

Control of late off-center cone bipolar cell differentiation and visual signaling by the homeobox gene Vsx1
Chow, Robert L; Volgyi, Bela; Szilard, Rachel K; Ng, David; McKerlie, Colin; Bloomfield, Stewart A; Birch, David G; McInnes, Roderick R
2004 Feb 10;101(6):1754-1759, Proceedings of the National Academy of Sciences of the United States of America
Retinal bipolar cells are interneurons that transmit visual signals from photoreceptors to ganglion cells. Although the visual pathways mediated by bipolar cells have been well characterized, the genes that regulate their development and function are largely unknown. To determine the role in bipolar cell development of the homeobox gene Vsx1, whose retinal expression is restricted to a major subset of differentiating and mature cone bipolar (CB) cells, we targeted the gene in mice. Bipolar cell fate was not altered in the absence of Vsx1 function, because the pan-bipolar markers Chx10 and Ret-B1 continued to be expressed in inner nuclear layer neurons labeled by the Vsx1-targeting reporter gene, tauLacZ. The specification, number, and gross morphology of the subset of on-center and off-center (OFF)-CB cells defined by tauLacZ expression from the Vsx1 locus were also normal in Vsx1(tauLacZ)/Vsx1(tauLacZ) mice. However, the terminal differentiation of OFF-CB cells in the retina of Vsx1(tauLacZ)/Vsx1(tauLacZ) mice was incomplete, as demonstrated by a substantial reduction in the expression of at least four markers (recoverin, NK3R, Neto1, and CaB5) for these interneurons. These molecular abnormalities were associated with defects in retinal function and documented by electroretinography and in vitro ganglion cell recordings specific to cone visual signaling. In particular, there was a general reduction in the light-mediated activity of OFF, but not on-center, ganglion cells. Thus, Vsx1 is required for the late differentiation and function of OFF-CB cells and is associated with a heritable OFF visual pathway-specific retinal defect
— id: 94044, year: 2004, vol: 101, page: 1754, stat: Journal Article,

Connexin36 is essential for transmission of rod-mediated visual signals in the mammalian retina
Deans, Michael R; Volgyi, Bela; Goodenough, Daniel A; Bloomfield, Stewart A; Paul, David L
2002 Nov 14;36(4):703-712, Neuron
To examine the functions of electrical synapses in the transmission of signals from rod photoreceptors to ganglion cells, we generated connexin36 knockout mice. Reporter expression indicated that connexin36 was present in multiple retinal neurons including rod photoreceptors, cone bipolar cells, and AII amacrine cells. Disruption of electrical synapses between adjacent AIIs and between AIIs and ON cone bipolars was demonstrated by intracellular injection of Neurobiotin. In addition, extracellular recording in the knockout revealed the complete elimination of rod-mediated, on-center responses at the ganglion cell level. These data represent direct proof that electrical synapses are critical for the propagation of rod signals across the mammalian retina, and they demonstrate the existence of multiple rod pathways, each of which is dependent on electrical synapses
— id: 94045, year: 2002, vol: 36, page: 703, stat: Journal Article,

Feedback inhibition in the inner plexiform layer underlies the surround-mediated responses of AII amacrine cells in the mammalian retina
Volgyi, Bela; Xin, Daiyan; Bloomfield, Stewart A
2002 Mar 1;539(Pt 2):603-614, Journal of physiology
Intracellular recordings were made from narrow-field, bistratified AII amacrine cells in the isolated, superfused retina-eyecup of the rabbit. Pharmacological agents were applied to neurons to dissect the synaptic pathways subserving AII cells so as to determine the circuitry generating their off-surround responses. Application of the GABA antagonists, picrotoxin, bicuculline and 1,2,5,6-tetrahydropyridine-4-yl methylphosphinic acid (TPMPA) all increased the on-centre responses of AII amacrine cells, but attenuated the off-surround activity. At equal concentrations, picrotoxin was approximately twice as effective as bicuculline or TPMPA in modifying the response activity of AII amacrine cells. These results indicate that the mechanism underlying surround inhibition of AII amacrine cells includes activation of both GABA(A) and GABA(C) receptors in an approximately equal ratio. Application of the GABA antagonists also increased the size of on-centre receptive fields of AII amacrine cells. Again, picrotoxin was most effective, producing, on average, a 54 % increase in the size of the receptive field, whereas bicuculline and TPMPA produced comparable 34 and 33 % increases, respectfully. Application of the voltage-gated sodium channel blocker TTX produced effects on AII amacrine cells qualitatively similar to those of the GABA blockers. Intracellular application of the chloride channel blocker 4,4'-dinitro-stilbene-2,2'-disulphonic acid (DNDS) abolished the direct effects of GABA on AII amacrine cells. Moreover, DNDS increased the amplitude of both the on-centre and off-surround responses. The failure of DNDS to block the off-surround activity indicates that it is not mediated by direct GABAergic inhibition. Taken together, our results suggest that surround receptive fields of AII amacrine cells are generated indirectly by the GABAergic, reciprocal feedback synapses from S1/S2 amacrine cells to the axon terminals of rod bipolar cells
— id: 39699, year: 2002, vol: 539, page: 603, stat: Journal Article,

Plasticity of AII amacrine cell circuitry in the mammalian retina
Bloomfield SA
2001 ;131(2):185-200, Progress in brain research
— id: 21169, year: 2001, vol: 131, page: 185, stat: Journal Article,

Rod vision: pathways and processing in the mammalian retina
Bloomfield SA; Dacheux RF
2001 May;20(3):351-384, Progress in retinal & eye research
Bipolar cells in the mammalian retina are postsynaptic to either rod or cone photoreceptors, thereby segregating their respective signals into parallel vertical streams. In contrast to the cone pathways, only one type of rod bipolar cell exists, apparently limiting the routes available for the propagation of rod signals. However, due to numerous interactions between the rod and cone circuitry, there is now strong evidence for the existence of up to three different pathways for the transmission of scotopic visual information. Here we survey work over the last decade or so that have defined the structure and function of the interneurons subserving the rod pathways in the mammalian retina. We have focused on: (1) the synaptic ultrastructure of the interneurons; (2) their light-evoked physiologies; (3) localization of specific transmitter receptor subtypes; (4) plasticity of gap junctions related to changes in adaptational state; and (5) the functional implications of the existence of multiple rod pathways. Special emphasis has been placed on defining the circuits underlying the different response components of the AII amacrine cell, a central element in the transmission of scotopic signals
— id: 21215, year: 2001, vol: 20, page: 351, stat: Journal Article,

Localization of Kv3 potassium channel subunits in the mouse retina
Ozaita, A.; Volgyi, B.; Bloomfield, S. A.; Rudy, B.
2001 ;27(2):2148-2148, Abstracts (Society for Neuroscience)
Kv3 K+ channels have been shown to play critical roles in fast spike repolarization and in enabling high frequency firing in cortical interneurons and in brainstem auditory neurons. We examined the cellular and subcellular distribution of Kv3.1a, Kv3.1b and Kv3.2 subunits in mouse retina. Expression of Kv3.1b was detected in few ganglion cells and two types of amacrine cells. One type had small somata and were distributed in mirror symmetry in the inner nuclear layer (INL) and the ganglion cell layer (GCL), in neurons projecting to strata 2 and 4 of the inner plexiform layer (IPL). These cells coexpressed Kv3.1b, calretinin and choline acetyltransferase. This group of Kv3.1b expressing neurons is both morphologically and neurochemically identical to the starburst amacrine cells. Interestingly, Kv3.1a immunoreactivity was restricted to the dendritic processes of the starburst cells in strata 2 and 4. Kv3.2 subunits were detected in the somata of a few ganglion cells, in scattered amacrine cell bodies and proximal dendrites located in the INL and in diffuse fiber-like structures throughout the IPL. Thus, Kv3.1b and Ky3.2 showed both somatic and dendritic localization in retinal amacrine and ganglion cells whereas Kv3.1a subunits were restricted to the processes in the IPL. These results indicate that Kv3.1 and Kv3.2 subunits are located in specific cell types in the retina. The subcellular segregation of the three subunits may underlie a physiological disparity in the spiking activity or modulation of different parts of the neuron. Future studies will investigate the functional roles of Kv3 channels in the retina
— id: 92528, year: 2001, vol: 27, page: 2148, stat: Journal Article,

Morphology and physiology of the polyaxonal amacrine cells in the rabbit retina
Volgyi B; Xin D; Amarillo Y; Bloomfield SA
2001 Nov 5;440(1):109-125, Journal of comparative neurology
We examined the morphology and physiological response properties of the axon-bearing, long-range amacrine cells in the rabbit retina. These so-called polyaxonal amacrine cells all displayed two distinct systems of processes: (1) a dendritic field composed of highly branched and relatively thick processes and (2) a more extended, often sparsely branched axonal arbor derived from multiple thin axons emitted from the soma or dendritic branches. However, we distinguished six morphological types of polyaxonal cells based on differences in the fine details of their soma/dendritic/axonal architecture, level of stratification within the inner plexiform layer (IPL), and tracer coupling patterns. These morphological types also showed clear differences in their light-evoked response activity. Three of the polyaxonal amacrine cell types showed on-off responses, whereas the remaining cells showed on-center responses; we did not encounter polyaxonal cells with off-center physiology. Polyaxonal cells respected the on/off sublamination scheme in that on-off cells maintained dendritic/axonal processes in both sublamina a and b of the IPL, whereas processes of on-center cells were restricted to sublamina b. All polyaxonal amacrine cell types displayed large somatic action potentials, but we found no evidence for low-amplitude dendritic spikes that have been reported for other classes of amacrine cell. The center-receptive fields of the polyaxonal cells were comparable to the diameter of their respective dendritic arbors and, thus, were significantly smaller than their extensive axonal fields. This correspondence between receptive and dendritic field size was seen even for cells showing extensive homotypic and/or heterotypic tracer coupling to neighboring neurons. These data suggest that all polyaxonal amacrine cells are polarized functionally into receptive dendritic and transmitting axonal zones
— id: 26530, year: 2001, vol: 440, page: 109, stat: Journal Article,

Surround inhibition of mammalian AII amacrine cells is generated in the proximal retina
Bloomfield SA; Xin D
2000 Mar 15;523 Pt 3:771-783, Journal of physiology
1. Intracellular recordings were obtained from neurons in the superfused retina-eyecup preparation of the rabbit under dark-adapted conditions. Neurotransmitter agonists and antagonists were applied exogenously via the superfusate to dissect the synaptic pathways pharmacologically and thereby determine those pathways responsible for the generation of the on-centre/off-surround receptive fields of AII amacrine cells. 2. Application of the metabotropic glutamate receptor agonist, APB, reversibly blocked both the on-centre and off-surround responses of AII cells. These data were consistent with the idea that both the centre- and surround-mediated responses are derived from inputs from the presynaptic rod bipolar cells. 3. Whereas rod bipolar cells showed on-receptive fields approximately 100 microm across, we found no evidence for an antagonistic off-surround response using light stimuli which effectively elicited the off-surrounds of AII amacrine cells. These results indicated that the surrounds of AII cells are not derived from rod bipolar cell inputs. 4. Application of the ionotropic glutamate receptor antagonists CNQX or DNQX enhanced the on-centre responses of AII cells but attenuated the off-surround responses. These data indicated that the centre- and surround-mediated responses could not both be derived from signals crossing the rod bipolar-to-AII cell synapse. 5. Application of the glycine antagonist, strychnine, had only minor and variable effects on AII cell responses. However, the GABA antagonists picrotoxin and bicuculline enhanced the on-centre response but attenuated or completely blocked the off-surround response of AII cells. The GABA antagonists had no effect on the responses of horizontal cells indicating that their effects on AII cell responses reflected actions on inner retinal circuitry rather than feedback circuitry in the outer plexiform layer. 6. Application of the voltage-gated sodium channel blocker TTX enhanced the on-centre responses of AII cells but attenuated or abolished their off-surround responses. 7. Taken together, our results suggest that the on-centre responses of AII cells result from the major excitatory drive from rod bipolar cells. However, the surround receptive fields of AII cells appear to be generated by lateral, inhibitory signals derived from neighbouring GABAergic, on-centre amacrine cells. A model is presented whereby the S1 amacrine cells produce the surround receptive fields of AII amacrine cells via inhibitory, feedback circuitry to the axon terminals of rod bipolar cells
— id: 11802, year: 2000, vol: 523 Pt 3, page: 771, stat: Journal Article,

Modulation of the tracer coupling pattern of alpha ganglion cells in the rabbit retina
Hu, EH; Bloomfield, SA
2000 MAR 15 ;41(4):S936-S936, Investigative ophthalmology & visual science. IOVS
— id: 54626, year: 2000, vol: 41, page: S936, stat: Journal Article,

Novel interactions with AMPA receptor binding protein (ABP)
Silverman, J. B.; Bloomfield, S.; Ziff, E. B.
2000 ;26(1-2):?-?, Abstracts (Society for Neuroscience)
AMPA receptors mediate the majority of fast synaptic neurotransmission at excitatory synapses within the CNS. One approach to studying these receptors is to construct a blueprint of the proteins located within the PSD which interact with AMPA receptor subunits. Towards this goal, a number of PDZ-containing proteins have been shown to interact with the cytoplasmic C-terminus of GluR2, the dominant AMPA receptor subunit. ABP is one such protein that contains six or seven PDZ domains, depending on its form. PDZ domain 5 of ABP shows the highest binding affinity for GluR2, and therefore it is likely that this domain mediates the interaction with AMPA receptors in vivo. Presumably, the rest of the PDZ domains of ABP are free to bind other proteins. These interactions may be responsible for anchoring AMPA receptors at the synapse, or alternatively, may be involved with the transport of AMPA receptors into and out of the postsynaptic membrane. Therefore, in order to understand the processing of AMPA receptors it is necessary to decipher the identity of additional proteins that interact with ABP. Using the Yeast Two-Hybrid system, a rat brain cDNA library was screened for potential ABP binding partners. Specifically, the first three PDZ domains of ABP were used as bait to discover novel interactions. Additional biochemical methods have been used to prove the specificity of binding and the function of these novel interacting proteins
— id: 92637, year: 2000, vol: 26, page: ?, stat: Journal Article,

Effects of GABA blockers on the response properties of amacrine cells in the rabbit retina
Volgyi, B; Bloomfield, SA
2000 MAR 15 ;41(4):S619-S619, Investigative ophthalmology & visual science. IOVS
— id: 54621, year: 2000, vol: 41, page: S619, stat: Journal Article,

Effects of TTX and GABA blockers on the response properties of an amacrine cells in the dark adapted rabbit retina
Xin, D; Volgyi, B; Bloomfield, SA
2000 MAR 15 ;41(4):S619-S619, Investigative ophthalmology & visual science. IOVS
— id: 54620, year: 2000, vol: 41, page: S619, stat: Journal Article,

Bi-directional movement of neurobiotin across gap junctions connecting alpha-ganglion cells to wide-field amacrine cells
Hu, E H; Xin, D; Bloomfield, S A
1999 May 9-14;40(4):S813-S813, Investigative ophthalmology & visual science. IOVS
— id: 15915, year: 1999, vol: 40, page: S813, stat: Journal Article,

Comparison of the responses of AII amacrine cells in the dark- and light-adapted rabbit retina
Xin D; Bloomfield SA
1999 Jul-Aug;16(4):653-665, Visual neuroscience
We studied the light-evoked responses of AII amacrine cells in the rabbit retina under dark- and light-adapted conditions. In contrast to the results of previous studies, we found that AII cells display robust responses to light over a 6-7 log unit intensity range, well beyond the operating range of rod photoreceptors. Under dark adaptation, AII cells showed an ON-center/OFF-surround receptive-field organization. The intensity-response profile of the center-mediated response component followed a dual-limbed sigmoidal function indicating a transition from rod to cone mediation as stimulus intensities were increased. Following light adaptation, the receptive-field organization of AII cells changed dramatically. Light-adapted AII cells showed both ON- and OFF-responses to stimulation of the center receptive field, but we found no evidence for an antagonistic surround. Interestingly, the OFF-center response appeared first following rapid light adaptation and was then replaced gradually over a 1-4 min period by the emerging ON-center response component. Application of the metabotropic glutamate receptor agonist APB, the ionotropic glutamate blocker CNQX, 8-bromo-cGMP, and the nitric oxide donor SNAP all showed differential effects on the various center-mediated responses displayed by dark- and light-adapted AII cells. Taken together, these pharmacological results indicated that different synaptic circuits are responsible for the generation of the different AII cell responses. Specifically, the rod-driven ON-center responses are apparently derived from rod bipolar cell synaptic inputs, whereas the cone-driven ON-center responses arise from signals crossing the gap junctions between AII cells and ON-center cone bipolar cells. Additionally, the OFF-center response of light-adapted AII cells reflects direct synaptic inputs from OFF-center cone bipolar cells to AII dendritic processes in the distal inner plexiform layer
— id: 8495, year: 1999, vol: 16, page: 653, stat: Journal Article,

Dark- and light-induced changes in coupling between horizontal cells in mammalian retina
Xin D; Bloomfield SA
1999 Mar 1;405(1):75-87, Journal of comparative neurology
Retinal horizontal cells exhibit large receptive fields derived from their extensive electrical coupling by means of gap junctions. The conductance of these gap junctions seems to be regulated by dopamine acting through a cAMP-mediated cascade. There is now abundant evidence that extracellular dopamine levels vary with changes in ambient light intensity, suggesting that changes in the dark/light adaptational state of the retina can modulate coupling between horizontal cells. We studied this question in the mammalian retina by determining the effects of ambient light levels, in the form of changing background light intensity, on the coupling profiles of A- and B-type horizontal cells in the rabbit. Changes in coupling were assessed by measurements of the space constants of the syncytium formed by horizontal cells and the intercellular spread of the biotinylated tracer Neurobiotin. Our results indicate that dark-adapted horizontal cells show relatively weak coupling. However, presentation of background lights as dim as one-quarter log unit above rod threshold resulted in increases in both the averaged extent of tracer coupling and space constants of A- and B-type horizontal cells. Coupling expanded further as background light intensities were increased by 1-1.5 log units, after which additional light adaptation brought about an uncoupling of cells. Coupling reached its minimum at light intensities about 3 log units above rod threshold, after which, with further light adaptation, it stabilized at levels close to those seen in dark-adapted retinas. Our results indicate that electrical coupling between mammalian horizontal cells is modulated dramatically by changes in the adaptational state of the retina: coupling is maximized under dim ambient light conditions and diminishes as the retina is dark or light adapted from this level
— id: 7939, year: 1999, vol: 405, page: 75, stat: Journal Article,

A comparison of receptive-field and tracer-coupling size of amacrine and ganglion cells in the rabbit retina
Bloomfield SA; Xin D
1997 Nov-Dec;14(6):1153-1165, Visual neuroscience
Recent studies have shown that amacrine and ganglion cells in the mammalian retina are extensively coupled as revealed by the intercellular movement of the biotinylated tracers biocytin and Neurobiotin. These demonstrations of tracer coupling suggest that electrical networks formed by proximal neurons (i.e. amacrine and ganglion cells) may underlie the lateral propagation of signals across the inner retina. We studied this question by comparing the receptive-field size, dendritic-field size, and extent of tracer coupling of amacrine and ganglion cells in the dark-adapted, superfused, isolated retina eyecup of the rabbit. Our results indicate that while the center-receptive fields of proximal neurons are approximately 15% larger than their corresponding dendritic diameters, this slight difference can be explained by factors other than electrical coupling such as tissue shrinkage associated with histological processing. However, the extent of tracer coupling of amacrine and ganglion cells was, on average, about twice the size of the corresponding receptive fields. Thus, the receptive field of an individual proximal neuron matched far more closely to its dendritic diameter than to the size of the tracer-coupled network of cells to which it belonged. The exception to this rule was the AII amacrine cells for which center-receptive fields were 2-3 times the size of their dendritic diameters but matched closely to the size of the tracer-coupled arrays. Thus, with the exception of AII cells, our data indicate that tracer coupling between proximal neurons is not associated with an enlargement of their receptive fields. Our results, then, provide no evidence for electrical coupling or, at least, indicate that extensive lateral spread of visual signals does not occur in the proximal mammalian retina
— id: 12171, year: 1997, vol: 14, page: 1153, stat: Journal Article,

Light-induced modulation of coupling between AII amacrine cells in the rabbit retina
Bloomfield SA; Xin D; Osborne T
1997 May-Jun;14(3):565-576, Visual neuroscience
The rod-driven, AII amacrine cells in the mammalian retina maintain homologous gap junctions with one another as well as heterologous gap junctions with on-cone bipolar cells. We used background illumination to study whether changes in the adaptational state of the retina affected the permeabilities of these two sets of gap junctions. To access changes in permeability, we injected single AII amacrine cells with the biotinylated tracer, Neurobiotin, and measured the extent of tracer coupling to neighboring AII cells and neighboring cone bipolar cells. We also measured the center-receptive field size of AII cells to assess concomitant changes in electrical coupling. Our results indicate that in well dark-adapted retinas, AII cells form relatively small networks averaging 20 amacrine cells and covering about 75 microns. The size of these networks matched closely to the size of AII cell on-center receptive fields. However, over most of their operating range, AII cells formed dramatically larger networks, averaging 326 amacrine cells, which corresponded to an increased receptive-field size. As the retina was light adapted beyond the operating range of the AII cells, they uncoupled to form networks comparable in size to those seem in well dark-adapted retinas. Our results, then, indicate that the adaptational state of the retina has a profound effect on the extent of electrical coupling between AII amacrine cells. Although we observed light-induced changes in the number of tracer-coupled cone bipolar cells, these appeared to be an epiphenomenon of changes in homologous coupling between AII amacrine cells. Therefore, in contrast to the robust changes in AII-AII coupling produced by background illumination, our data provided no evidence of a light-induced modulation of coupling between AII cells and on-cone bipolar cells
— id: 7117, year: 1997, vol: 14, page: 565, stat: Journal Article,

Tracer coupling pattern of amacrine and ganglion cells in the rabbit retina
Xin D; Bloomfield SA
1997 Jul 14;383(4):512-528, Journal of comparative neurology
We examined the tracer coupling pattern of more than 15 morphological types of amacrine and ganglion cells in the rabbit retina. Individual cells were injected intracellularly with the biotinylated tracer Neurobiotin, which was then allowed to diffuse across gap junctions to label neighboring neurons. We found that homologous and/or heterologous tracer coupling was common for most proximal neurons. In fact, the starburst amacrine cell was the only amacrine cell type that showed no evidence of coupling. The remaining types of amacrine cell were coupled exclusively to other amacrines, either homologously or, more often, through a combination of homologous and heterologous junctions. In only one case did we visualize labeled ganglion cells following injection of Neurobiotin into an amacrine cell. In contrast, injection of Neurobiotin into ganglion cells almost always resulted in the labeling of amacrine cells. Taken together, these results suggest a directionality to the movement of tracer across gap junctions connecting amacrine and ganglion cells. We found that the coupling pattern for a given morphological type of cell was generally stereotypic and consistent across retinas. The notable exceptions to this finding were alpha ganglion cells and cells with morphology corresponding to that of on-off direction selective ganglion cells. In both cases, individual cells showed either extensive coupling to both amacrine and ganglion cells or no coupling at all. A notable finding was that, in every case, the neighboring cells within a tracer-coupled array were always within one gap junction of the injected neuron. Furthermore, in many cases, the array formed by the somata of tracer-coupled cells was almost perfectly coincident with the dendritic arbor of the injected cell. Thus, our results indicate that whereas coupling is extensive within the proximal retina, individual cells partake in coupled networks that are stereotypic and highly circumscribed
— id: 7284, year: 1997, vol: 383, page: 512, stat: Journal Article,

Effect of spike blockade on the receptive-field size of amacrine and ganglion cells in the rabbit retina
Bloomfield SA
1996 May;75(5):1878-1893, Journal of neurophysiology
1. Intracellular recordings were obtained from 21 amacrine cells and 12 ganglion cells in the isolated, superfused retina-eyecup of the rabbit. Cells were subsequently labeled with horseradish peroxidase (HRP) or N-(2-aminoethyl)-biotinamide hydrochloride (Neurobiotin) for morphologic identification. 2. Initial experiments performed on three amacrine cells and three ganglion cells showed that 1 microM tetrodotoxin (TTX) abolished all spiking. This included both large-amplitude and small-amplitude spikes recorded in many amacrine cells, indicating that they are mediated by voltage-gated sodium channels. 3. The center-receptive-field size of 18 amacrine cells and 9 ganglion cells was measured with the use of a 50-microns-wide/6.0-mm-long rectangular slit of light that was displaced along its minor axis (parallel to the visual streak) in steps as small as 3 microns. The retina was then bathed in 1 microM TTX, or individual cells were injected with 50 mM QX-314, a quatemary lidocaine derivative, to abolish all spiking, and the center-receptive field of each cell was then remeasured. 4. Although TTX blocked spiking in all ganglion cells (dendritic diameters ranging from 302 to 969 microns), it produced no significant change in the size of their center-receptive fields. This finding argues that passive, electrotonic spread of synaptic inputs to ganglion cell dendritic arbors is adequate for efficient propagation from terminal branches to the soma; active propagation via voltage-gated sodium channels plays no apparent role. 5. In contrast, TTX and QX-314 had variable effect on the receptive fields of amacrine cells, which was related to the size of their dendritic arbors. Whereas TTX had no significant effect on the receptive-field size of amacrine cells whose dendritic arbors were < 525 microns across, the center-receptive fields of larger amacrine cells were reduced, on average, by 40%; QX-314 produced a very similar average reduction of 39%. Moreover, for these larger cells, there was a direct relationship between the magnitude of the reduction in receptive-field size produced by TTX or QX-314 and the size of a cell's dendritic arbor. This relationship was true whether the change in receptive-field size was measured in absolute terms or as percent reduction from control values. 6. Interestingly, TTX and QX-314 also significantly reduced the amplitude of slow potentials recorded in amacrine cells by an average of 22 and 24%, respectively. However, the amplitude of slow potentials recorded in ganglion cells were relatively uneffected by TTX. 7. These findings are consistent with the idea that, for amacrine cells with dendritic arbors spanning > 525 microns, active propagation of synaptic signals mediated by voltage-gated sodium channels is necessary for efficient movement of information across a cell's dendritic arbor and thus plays a major role in shaping their receptive fields. Although the TTX effects may also reflect an indirect contribution from altered synaptic input derived from presynaptic spiking neurons, the strong similarity between the effects of TTX and QX-314 argues that any such contribution was minor. For smaller amacrine cells, passive, electrotonic spread of signals appears adequate for efficient propagation within their limited dendritic arbors
— id: 8445, year: 1996, vol: 75, page: 1878, stat: Journal Article,

A comparison of receptive field and tracer coupling size of horizontal cells in the rabbit retina
Bloomfield SA; Xin D; Persky SE
1995 Sep-Oct;12(5):985-999, Visual neuroscience
The large receptive fields of retinal horizontal cells are thought to reflect extensive electrical coupling via gap junctions. It was shown recently that the biotinylated tracers, biocytin and Neurobiotin, provide remarkable images of coupling between many types of retinal neuron, including horizontal cells. Further, these demonstrations of tracer coupling between horizontal cells rivaled the size of their receptive fields, suggesting that the pattern of tracer coupling may provide some index of the extent of electrical coupling. We studied this question by comparing the receptive field and tracer coupling size of dark-adapted horizontal cells recorded in the superfused, isolated retina-eyecup of the rabbit. Both the edge-to-edge receptive field and space constants (lambda) were computed for each cell using a long, narrow slit of light displaced across the retinal surface. Cells were subsequently labeled by iontophoretic injection of Neurobiotin. The axonless A-type horizontal cells showed extensive, homologous tracer coupling in groups greater than 1000 covering distances averaging about 2 mm. The axon-bearing B-type horizontal cells were less extensively tracer coupled, showing homologous coupling of the somatic endings in groups of about 100 cells spanning approximately 400 microns and a separate homologous coupling of the axon terminal endings covering only about 275 microns. Moreover, we observed a remarkable, linear relationship between the size of the receptive fields of each of the three horizontal cell endings and the magnitude of their tracer coupling. Our findings suggest that the extent of tracer coupling provides a strong, linear index of the magnitude of electrical current flow, as derived from receptive-field measures, across groups of coupled horizontal cells. These data thus provide the first direct evidence that the receptive-field size of horizontal cells is related to the extent of their coupling via gap junctions
— id: 7065, year: 1995, vol: 12, page: 985, stat: Journal Article,

RESPONSE PROPERTIES OF LONG-RANGE AMACRINE CELLS IN THE RABBIT RETINA
BLOOMFIELD, SA; XIN, D
1995 MAR 15 ;36(4):S623-S623, Investigative ophthalmology & visual science. IOVS
— id: 87341, year: 1995, vol: 36, page: S623, stat: Journal Article,

THE TRACER COUPLING PATTERN OF AMACRINE AND GANGLION-CELLS IN THE RABBIT RETINA
XIN, D; BLOOMFIELD, SA; ANDREWS, AL
1995 MAR 15 ;36(4):S602-S602, Investigative ophthalmology & visual science. IOVS
— id: 87339, year: 1995, vol: 36, page: S602, stat: Journal Article,

Orientation-sensitive amacrine and ganglion cells in the rabbit retina
Bloomfield SA
1994 May;71(5):1672-1691, Journal of neurophysiology
1. Intracellular recordings were obtained from amacrine and ganglion cells in the isolated, superfused retina-eyecup preparation of the rabbit to test the orientation sensitivity of their responses. Cell identification was based on morphological criteria following injection of horseradish peroxidase (HRP) or N-(2-aminoethyl)-biotinamide hydrochloride (Neurobiotin) to visualize soma-dendritic architectures. 2. In terms of the physiological mechanisms generating their sensitivity, two types of orientation-sensitive amacrine cell and a single type of orientation-sensitive ganglion cell were found. These cell types were termed orientation selective and orientation biased. Cells were subtypes further into on- or off-center receptive-field categories. 3. The receptive fields of orientation-selective amacrine and ganglion cells were composed of two inhibitory fields that flanked the excitatory center receptive field along the preferred orientation. These inhibitory flanks produced a center receptive-field anisotropy with its major axis corresponding to the preferred orientation: either parallel or orthogonal to the visual streak. When a stimulus was oriented orthogonal to the preferred orientation (i.e., at the null orientation), the inhibitory fields were stimulated, resulting in a null inhibition that blocked the center-mediated excitation. Stimulation of these inhibitory flanks was absolutely essential to evoke the orientation selectivity of these cells. The null response reflected inhibition associated with a conductance increase and not disfacilitation. 4. Orientation-biased amacrine cells displayed a center receptive-field anisotropy with its major axis oriented either parallel or orthogonal to the visual streak. These cells preferred light stimuli oriented along the major axis of the center receptive field. However, whereas the excitatory response of these cells was reduced when a stimulus was rotated from the preferred orientation, there was no corresponding hyperpolarization. No null inhibition was detected even after modulation of the membrane potential with extrinsic current. 5. Although orientation-biased amacrine cells were morphologically heterogeneous, they all displayed dendritic arbors that were markedly elongated along an axis corresponding to their physiological preferred orientation. Thus it appears that the elongated dendritic fields of these cells may provide for the anisotropy of their center receptive fields and, in turn, their orientation sensitivity. 6. Orientation-selective amacrine cells formed a rather homogeneous morphological group of cells. These neurons displayed large, radially symmetric dendritic arbors with diameters averaging 1,100 microns. There were no asymmetries in their dendritic fields and thus no clear structural basis for their orientation selectivity. 7. In contrast, orientation-selective ganglion cells displayed diverse soma-dendritic architecture and thus could not be placed into a single morphological class.(ABSTRACT TRUNCATED AT 400 WORDS)
— id: 12972, year: 1994, vol: 71, page: 1672, stat: Journal Article,

A unique morphological subtype of horizontal cell in the rabbit retina with orientation-sensitive response properties
Bloomfield SA
1992 Jun 1;320(1):69-85, Journal of comparative neurology
Intracellular recordings were obtained from horizontal cells in the rabbit retina to assess the orientation sensitivity of their visual responses to moving and stationary rectangular slits of light. Cells were subsequently labeled with horseradish peroxidase (HRP) for morphological identification. The responses of A-type horizontal cells and those of the somatic and axon terminal endings of B-type horizontal cells (with the exception of one cell) were found to be insensitive to the orientation of light stimuli. However, 20 horizontal cells encountered within or just superior to the visual streak displayed clear orientation-sensitive response properties. These cells were divided into two groups: the majority (70%) showed preference for light stimuli oriented parallel to the visual streak, whereas the remainder preferred stimuli oriented orthogonal to the visual streak. Analysis of the shape of the receptive fields of these cells by means of a narrow, displaced slit of light revealed an anisotropy with the major or elongated axis of the receptive field of each cell aligned along the same angle as its physiological preferred orientation. Morphologically, the orientation-sensitive horizontal cells formed a homogeneous group with an architecture corresponding to that of elongated A-type or Ae-type horizontal cells reported previously in the rabbit retina. These cells showed a marked elongation of their dendritic arbors with the major axes oriented either parallel or orthogonal to the visual streak. Furthermore, the orientation of the dendritic arbor of each cell matched that of its physiological preferred orientation. The present results, then, suggest strongly that the orientation sensitivity of Ae-type horizontal cells results directly from the asymmetry in their dendritic arbors. The spatial location and specialized physiology of Ae-type horizontal cells suggest that they play a role in the formation of orientation-sensitive properties exhibited by more proximal neurons in the rabbit retina
— id: 13586, year: 1992, vol: 320, page: 69, stat: Journal Article,

Relationship between receptive and dendritic field size of amacrine cells in the rabbit retina
Bloomfield SA
1992 Sep;68(3):711-725, Journal of neurophysiology
1. Intracellular recordings were obtained from 40 amacrine cells in the isolated, superfused retina eyecup of the rabbit. Cells were subsequently labeled with horseradish peroxidase for morphological identification. Many of these cells displayed dendritic morphology consistent with that of amacrine cells described in prior anatomic studies, including starburst, A17, AII, and DAPI-3 cells. 2. The center receptive field of amacrine cells was measured with a 50- or 95-microns-wide, 6.0-mm-long rectangular slit of light that was displaced along its minor axis (parallel to the visual streak) in increments as small as 3 microns. The extent of the receptive field was calculated as the total distance over which the displaced slit could evoke a center response. Area summation of amacrine cells was measured with concentric spots of light with increasing diameters centered over the cell. 3. For a single amacrine cell, the receptive field size was comparable to the extent of its dendritic arbor. For the total population of amacrine cells, there was a strong, linear relationship between receptive field and dendritic field size. The receptive fields were, on average, 27% larger than the corresponding dendritic arbors, but this discrepancy can be accounted for entirely by tissue shrinkage associated with histological processing and a small imprecision of the light stimuli. Area summation measurements were consistent with those of receptive fields and were also related linearly to the dendritic field size of cells. 4. These findings indicate that even when the slit of light was placed at the distal edges of the dendritic arbor, synaptic inputs activated there were propagated effectively to the soma and recorded by microelectrodes placed there. In addition, amacrine cells were capable of summating synaptic inputs distributed throughout the entire arbor. 5. These results are inconsistent with the findings of prior computational modeling studies of passive, dendritic current flow in A17 and starburst amacrine cells that synaptic inputs on distal dendritic branches are isolated electrically from the soma and that these branches form autonomous, functional subunits. 6. The majority of amacrine cells encountered displayed light-evoked and/or spontaneous action potentials. These action potentials often took the form of high-amplitude somatic and low-amplitude dendritic spikes. On average, spiking amacrine cells showed considerably larger dendritic fields than nonspiking amacrine cells. In fact, all amacrine cells with arbors greater than 436 microns, which formed 45% of the total population, displayed spike activity.(ABSTRACT TRUNCATED AT 400 WORDS)
— id: 13456, year: 1992, vol: 68, page: 711, stat: Journal Article,

Two types of orientation-sensitive responses of amacrine cells in the mammalian retina
Bloomfield SA
1991 Mar 28;350(6316):347-350, Nature
Neurons sensitive to the orientation of light stimuli exist throughout the mammalian visual system, suggesting that this spatial feature is a fundamental cue used by the brain to decipher visual information. The most peripheral neurons known to show orientation sensitivity are the retinal ganglion cells. Considerable morphological and pharmacological data suggest that the orientation sensitivity of ganglion cells is formed, at least partly, by the amacrine cells, which are laterally oriented interneurons presynaptic to the ganglion cells in the inner plexiform layer. So far there have been few studies of the responses of amacrine cells to oriented visual stimuli and their role in forming orientation-sensitive responses in the retina remains unclear. Here I report the novel finding of a population of amacrine cells in the rabbit retina which are orientation-sensitive. These amacrine cells can be divided into two subtypes, whose orientation sensitivity is manufactured by two distinct mechanisms. The orientation sensitivity of the first subtype of amacrine cell is formed from the interactions of excitatory, centre-receptive field synaptic inputs and inhibitory inputs of opposite polarity, whereas that for cells of the second subtype seems to be the product of a marked asymmetry in their dendritic arbors
— id: 14095, year: 1991, vol: 350, page: 347, stat: Journal Article,

Dendritic arbors of large-field ganglion cells show scaled growth during expansion of the goldfish retina: a study of morphometric and electrotonic properties
Bloomfield SA; Hitchcock PF
1991 Apr;11(4):910-917, Journal of neuroscience
The retina of the goldfish grows by a balloon-like expansion and by the addition of new neurons at the margin. It has been proposed that as a consequence of this expansion the dendritic arbors of ganglion cells in central retina grow in a uniform manner without the addition of new branches. In the present study, we have examined this proposal by comparing the geometries of individual dendritic arbors of large-field ganglion cells from the retinas of small/young and large/old fish. These comparisons were based on measurements of several parameters of dendritic morphology, including number of segments and branches, branch angles, changes in diameter at branch points, and proximal versus distal distribution of arbor length. In addition, we used passive, steady-state cable modeling as an independent method of estimating the functional architectures of small and large dendritic arbors. Our morphometric data indicate that, though they are very different in absolute size, dendritic arbors of small and large ganglion cells have remarkably similar architectures. Analysis with steady-state cable equations indicates that the arbors from small and large cells have equivalent electrotonic lengths and show comparable propagation of synaptic currents. These data are consistent with the hypothesis that dendritic arbors of small and large ganglion cells are scaled versions of one another. We conclude that the growth of these cells during the expansion of the retina is the result of the addition of dendritic mass to an arbor whose basic geometry remains unchanged
— id: 14078, year: 1991, vol: 11, page: 910, stat: Journal Article,

Dendritic current flow in relay cells and interneurons of the cat's lateral geniculate nucleus
Bloomfield, S A; Sherman, S M
1989 May;86(10):3911-3914, Proceedings of the National Academy of Sciences of the United States of America
We used a passive, steady-state cable model to simulate current flow within the dendritic arbors of relay cells and interneurons in the cat's lateral geniculate nucleus. In confirmation of our previous work on relay cells, we found them to be electronically compact; thus a postsynaptic potential generated anywhere in a relay cell's dendritic arbor spreads with relatively little attenuation throughout the arbor and to its soma. An interneuron is very different. Its arbor is much more extensive electronically with the result that a postsynaptic potential significantly affects only local areas of the dendritic arbor, and only inputs to proximal dendrites or to the soma will much affect the soma. Since much of the interneuron's synaptic output derives from dendritic terminals that are both presynaptic and postsynaptic, its dendritic arbor may contain many local circuits that perform neuronal computations independently of each other, and this processing might be invisible to the soma. Furthermore, these interneurons possess conventional axonal outputs, and these contact postsynaptic profiles that are quite different from the postsynaptic targets of the dendritic terminals. Presumably, the axonal output reflects the integrated computations performed on proximal synaptic inputs, and it uses conventional action potentials to convey this output. We suggest that the interneuron does double duty: its dendritic arbor is used for many independent local circuits that perform one set of neuronal computations, and its axonal output represents conventional neuronal integration of proximal synaptic inputs
— id: 106578, year: 1989, vol: 86, page: 3911, stat: Journal Article,

Postsynaptic potentials recorded in neurons of the cat's lateral geniculate nucleus following electrical stimulation of the optic chiasm
Bloomfield, S A; Sherman, S M
1988 Dec;60(6):1924-1945, Journal of neurophysiology
1. We recorded intracellularly from X and Y cells of the cat's lateral geniculate nucleus and measured the postsynaptic potentials (PSPs) evoked from electrical stimulation of the optic chiasm. We used an in vivo preparation and computer averaged the PSPs to enhance their signal-to-noise ratio. 2. The vast majority (46 of 50) of our sample of X and Y cells responded to stimulation of the optic chiasm with an excitatory postsynaptic potential (EPSP) followed by an inhibitory postsynaptic potential (IPSP); these were tentatively identified as relay cells. We quantified several parameters of these PSPs, including amplitude, latency, time to peak (i.e., rise time), and duration. 3. Among the relay cells, the latencies of both the EPSP and action potential evoked by optic chiasm stimulation were shorter in Y cells than in X cells. Furthermore, the difference between the latencies of the EPSP and action potential was shorter for Y cells than for X cells. This means that the EPSPs generated in Y cells reached threshold for generation of action potentials faster than did those in X cells. The EPSPs of Y cells also displayed larger amplitudes and faster rise times than did those in X cells, but neither of these distinctions was sufficient to explain the shorter latency difference between the EPSP and action potential for Y cells. 4. The EPSPs recorded in relay Y cells had longer durations than did those in relay X cells. Our data suggest that the subsequent IPSP actively terminates the EPSP, which, in turn, suggests that the time interval between EPSP and IPSP onsets is longer in Y cells than in X cells. Furthermore, we found that, for individual Y cells, the latency and duration of the evoked EPSP were inversely related. These observations lead to the conclusion that the latency of IPSPs activated from the optic chiasm is relatively constant among Y cells and thus independent of the EPSP latencies. Thus the excitation and inhibition produced in individual geniculate Y cells may originate from different populations of retinogeniculate axons. 5. The IPSPs recorded in geniculate relay cells following optic chiasm stimulation could be divided into three groups based on their durations. The majority of both X and Y cells showed short-duration IPSPs, whereas the remainder of Y cells displayed medium-duration IPSPs, and the remaining X cells displayed long-duration IPSPs. A positive correlation was seen between the time to peak and duration of these IPSPs.(ABSTRACT TRUNCATED AT 400 WORDS)
— id: 106579, year: 1988, vol: 60, page: 1924, stat: Journal Article,

Passive cable properties and morphological correlates of neurones in the lateral geniculate nucleus of the cat
Bloomfield, S A; Hamos, J E; Sherman, S M
1987 Feb;383:653-692, Journal of physiology
1. We used an in vivo preparation of the cat to study the passive cable properties of sixteen X and twelve Y cells in the lateral geniculate nucleus. Cells were modelled as equivalent cylinders according to Rall's formulations (Rall, 1959a, 1969, 1977). We injected intracellular current pulses into these geniculate neurones, and we analysed the resulting voltage transients to obtain the cable parameters of these cells. In addition, fifty-four physiologically characterized neurones were labelled with horseradish peroxidase (HRP) and analysed morphologically. 2. Analysis of HRP-labelled geniculate neurones showed that the dendritic branching pattern of these cells adheres closely to the 3/2 power rule. That is, at each branch point, the diameter of the parent branch raised to the 3/2 power equals the sum of the diameters of the daughter dendrites after each is raised to the 3/2 power. Furthermore, preliminary data indicate that the dendritic terminations emanating from each primary dendrite occur at the same electrotonic distance from the soma. These observations suggest that both X and Y cells meet the geometric constraints necessary for reduction of their dendritic arbors into equivalent cylinders. 3. We found a strong linear relationship between the diameter of each primary dendrite and the membrane surface area of the arbor emanating from it. We used this relationship to derive an algorithm for determining the total somatic and dendritic membrane surface area of an X and Y cell simply from knowledge of the diameters of its soma and primary dendrites. 4. Both geniculate X and Y cells display current-voltage relationships that were linear within +/- 20 mV of the resting membrane potential. This meant that we could easily remain within the linear voltage range during the voltage transient analyses. 5. X and Y cells clearly differ in terms of many of their electrical properties, including input resistance, membrane time constant and electrotonic length. The difference in input resistance between X and Y cells cannot be attributed solely to the smaller average size of X cells, but it also reflects a higher specific membrane resistance (Rm) of the X cells. Furthermore, X cells exhibit electrotonic lengths slightly larger than those of Y cells, but both neuronal types display electrotonic lengths of roughly 1. This indicates that even the most distally located innervation to these cells should have considerable influence on their somatic and axonal responses.(ABSTRACT TRUNCATED AT 400 WORDS)
— id: 106580, year: 1987, vol: 383, page: 653, stat: Journal Article,

A functional organization of ON and OFF pathways in the rabbit retina
Bloomfield, S A; Miller, R F
1986 Jan;6(1):1-13, Journal of neuroscience
Intracellular electrophysiological recordings were obtained from amacrine and ganglion cells in an isolated, superfused retina-eyecup preparation of the rabbit. Cells were characterized physiologically, after which cell-staining was accomplished by intracellular iontophoresis of HRP. A computer-assisted image-processing system was used to study the dendritic stratification pattern of HRP-labeled neurons within the inner plexiform layer (IPL). Our results support the concept that the IPL is functionally divided into a distal OFF region and proximal ON layer. ON and OFF ganglion and amacrine cells show dendritic arborizations consistent with this division and ON-OFF ganglion cells have processes in both portions of the IPL. It appears that these functional subdivisions of the IPL reflect excitatory, but not necessarily inhibitory, inputs. Thus, the pattern of dendritic arborization of a cell appears to predict its physiological response polarity, regardless of the type of inhibition it receives
— id: 106581, year: 1986, vol: 6, page: 1, stat: Journal Article,

Roles of aspartate and glutamate in synaptic transmission in rabbit retina. I. Outer plexiform layer
Bloomfield, S A; Dowling, J E
1985 Mar;53(3):699-713, Journal of neurophysiology
Intracellular recordings were obtained from horizontal and bipolar cells of the superfused, isolated retina-eyecup of the rabbit. The putative neurotransmitters aspartate, glutamate, and several analogues were added to the superfusate while the membrane potential and light-responsiveness of the retinal neurons were monitored. Both L-aspartate and L-glutamate mimicked the actions of the endogenous photoreceptor transmitter on horizontal cells, on-bipolar cells, and off-bipolar cells. At applied concentrations of 2.5-20 mM, the actions of L-aspartate and L-glutamate were indistinguishable. D-aspartate potentiated the effects of both L-aspartate and L-glutamate on horizontal cells. This suggests that active uptake systems for these amino acids exist in the outer plexiform layer (OPL) of the rabbit retina. The glutamate analogue kainate produced effects similar to those of aspartate and glutamate on second-order neurons, but at concentrations lower by over two orders of magnitude. The glutamate analogue quisqualate had effects similar to kainate but with much less potency. The aspartate analogue n-methyl DL-aspartate (NMDLA) antagonized the effects of the photoreceptor transmitter on horizontal and off-bipolar cells. This action of NMDLA was only observed at low concentrations (50 microM). In addition, NMDLA could block the effects of exogenously applied kainate. The NMDLA had no clear effects on on-bipolar cells. The glutamate analogue 2-amino-4-phosphonobutyrate reversibly blocked the responses of on-bipolar cells but had no effect on either horizontal or off-bipolar cell responses. This suggests that on-bipolar cells possess a unique synaptic receptor. The aspartate analogue 2-amino-3-phosphonoproprionate did not show this selectivity, suggesting that this unique receptor is a glutamate-preferring receptor. The antagonists alpha-methyl glutamate, alpha-amino adipate, and glutamate diethyl ester all showed only a weak ability to antagonize the actions of the photoreceptor transmitter on second-order neurons. The results of this study indicate that glutamate or a glutamate-like substance is the likely transmitter of rods and cones in the rabbit retina. A comparison of the present findings with those previously obtained in lower vertebrate retinas suggests that the basic pharmacological design of the OPL of all vertebrate retinas is very similar
— id: 106583, year: 1985, vol: 53, page: 699, stat: Journal Article,

Roles of aspartate and glutamate in synaptic transmission in rabbit retina. II. Inner plexiform layer
Bloomfield, S A; Dowling, J E
1985 Mar;53(3):714-725, Journal of neurophysiology
Intracellular recordings were obtained from amacrine and ganglion cells in the superfused, isolated retina-eyecup of the rabbit. The putative neurotransmitters aspartate, glutamate, and several of their analogues were added to the superfusate while the membrane potential and light-responsiveness of the retinal neurons were monitored. Both L-aspartate and L-glutamate displayed excitatory actions on the activity of the vast majority of amacrine and ganglion cells studied. However, these agents occasionally appeared to inhibit the responses of the inner retinal neurons by producing hyperpolarization of the membrane potential and blockage of the light-evoked responses. In either case, the effects of aspartate and glutamate were indistinguishable. The glutamate analogues kainate and quisqualate produced strong excitatory effects on the responses of amacrine and ganglion cells at concentrations some 200-fold less than those needed to obtain similar effects with aspartate or glutamate. The aspartate analogue, n-methyl DL-aspartate (NMDLA), also produced strong excitatory effects but was approximately three times less potent than kainate or quisqualate. On one occasion, we encountered a ganglion cell that was depolarized by kainate, but hyperpolarized by NMDLA. The glutamate antagonist alpha-methyl glutamate and the aspartate antagonist alpha-amino adipate effectively blocked the responses of amacrine and ganglion cells. However, on any one cell, one antagonist was always clearly more potent than the other. We examined the actions of the glutamate analogue 2-amino-4-phosphonobutyrate (APB) on the responses of inner retinal neurons and found that it selectively abolished all 'on' activity in the inner retina. Together with our finding that APB selectively abolishes on-bipolar cell responses (see Ref. 6), these data support the hypothesis that on-bipolar cells subserve the 'on' activity of amacrine and ganglion cells. Our data suggest that aspartate and glutamate are excitatory transmitters in the inner retina, possibly being released from bipolar cell axon terminals in the inner plexiform layer
— id: 106582, year: 1985, vol: 53, page: 714, stat: Journal Article,

Electroanatomy of a unique amacrine cell in the rabbit retina
Miller, R F; Bloomfield, S A
1983 May;80(10):3069-3073, Proceedings of the National Academy of Sciences of the United States of America
Intracellular electrophysiological recordings were obtained from a specialized class of 'starburst' amacrine cells by using an isolated superperfused retina-eyecup preparation of the rabbit. These cells were injected intracellularly with horseradish peroxidase and identified with light microscopy. A computer-controlled image-processing system was used to map and display the three-dimensional dendritic organization and provide information on length and sublaminar distribution of dendritic processes. Starburst amacrines show an unusual dendritic architecture that includes thin intermediate dendritic segments. Analysis with steady-state cable equations suggests that these thin segments may provide electrical isolation of distal processes, raising the possibility that a single dendrite, which lies beyond the thin segment, may constitute a functional subunit of the cell
— id: 106584, year: 1983, vol: 80, page: 3069, stat: Journal Article,

A physiological and morphological study of the horizontal cell types of the rabbit retina
Bloomfield, S A; Miller, R F
1982 Jul 1;208(3):288-303, Journal of comparative neurology
A perfused, isolated retina-eyecup preparation of the rabbit was utilized to correlate the physiology and morphology of horizontal cells. Neurons were physiologically characterized by intracellular recording techniques and subsequently stained with intracellular iontophoretically injected horseradish peroxidase for morphological identification. Three types of rabbit horizontal cell recordings have been differentiated, based on variations in response waveform, amplitude-intensity properties, and area summation characteristics. These three types have been unequivocally associated with the axonless A-type horizontal cells and the somatic and axon terminal endings (each displaying its own distinct physiology) of B-type horizontal cells first described in studies using Golgi-impregnation techniques (Fisher and Boycott, '74). In addition, the sizes of A-type horizontal cells were found to be directly related to their retinal eccentricities from the optic desk. However a unique subclass of A-type cells has been discovered (elongated or Ae type) which displayed the largest dendritic field of any cells studied here, yet had the smallest eccentricities--within 1.4 mm of the optic disk. Moreover, elongated A-type cells exhibited long asymmetrical dendritic fields which were oriented parallel with the visual streak. The unique asymmetry and orientation of these cells suggests that they may have orientation-biased receptive field properties. Physiological evidence for an orientation-biased horizontal cell is presented in support of this notion
— id: 106585, year: 1982, vol: 208, page: 288, stat: Journal Article,

A physiological-morphological study of neuronal pathways in the rabbit retina
Bloomfield, Stewart Allen
[S.l. : s.n.], 1981,
Thesis (Ph.D.) - Washington Univ, 1981
— id: 2104, year: 1981, vol: , page: , stat: ,