Social animals display a diverse set of behaviors when they encounter other animals depending on whether they identify as a competitor, mating partner, predator, or prey. Fighting and mating, especially, are two main social behaviors in adult males. While males attack conspecific male invaders to protect their territory, they mate with female partners to reproduce. The neural circuits underlying the generation of those innate social behaviors have fascinated neuroscientists for nearly a century.

Decades of research, including recent advances using precise manipulation tools such as optogenetics, have firmly established the medial preoptic nucleus (MPN) and ventrolateral part of ventromedial hypothalamus (VMHvl) as critical sites for eliciting male sexual and aggressive behaviors, respectively. However, these hypothalamic regions do not work in isolation. Instead, their activation depends on inputs from various upstream regions that convey information regarding the sensory cues of the conspecifics, the context, and the internal state and past experience of the subject animal. Some inputs that carry the olfactory information (a key sensory modality for rodents) have been identified, but much remains unknown regarding the source of inputs that carry more complex and synthesized information essential for the voluntary initiation of the behaviors.

In our recent study, we sought to address whether the posterior amygdala (PA) is a key region for generating the initiation signals of social behaviors (Yamaguchi et al., 2020). PA, also known as the amygdalohippocampal area, is a cortical-like amygdala region that consists primarily of excitatory glutamatergic neurons and sends dense projections into the hypothalamic areas, including MPN and VMHvl. PA contains abundant estrogen receptor alpha (Esr1) like MPN and VMHvl. Thus, it is under the strong influence of estrogen that is known to regulate social behaviors profoundly. Despite the strong connection of PA with hypothalamus and its rich expression of Esr1, PA has been largely ignored in neuroscientific studies. Little is known regarding how PA cells respond and what the functions of PA is in social behaviors.

Given that MPN and VMHvl play fundamentally different roles in social behaviors, we hypothesized that PA neurons projecting into the MPN and VMHvl are distinct and play differential roles in male sexual and aggressive behaviors. To test this hypothesis, we injected two variants of the retrograde neural tracers each into the MPN and VMHvl of the same animal and found minimal overlap between the tracers of different colors, supporting the existence of two anatomically distinct PA populations.

To monitor neural activities of male PAEsr1+ cells projecting into the MPN and VMHvl during social encounters, we performed fiber photometric recordings, a technique that monitors the bulk neural activities through an optic fiber in free-moving animals. We found that PAEsr1+ neurons projecting into the MPN are more active during female investigation and sexual behaviors, while PAEsr1+ neurons projecting into the VMHvl show stronger responses during male investigation and inter-male aggression. Importantly, PAEsr1+ cells projecting into the MPN start to rise in activity prior to the mounting onset, consistent with a potential role in driving sexual behaviors.

To understand how the PA functions in male mating and fighting, we artificially controlled the neural activity of male PA cells using pharmacogenetics (inhibitory and excitatory Designer Receptors Exclusively Activated by Designer Drugs) and observed changes in social behaviors. Most striking is that after inactivation of PAEsr1+ neurons projecting into the MPN, male animals could barely initiate sexual behaviors. None of the males achieved intromission even though they were highly sexually experienced and encountered sexually-receptive females. In contrast, the inter-male aggression was not affected by inhibiting the PAEsr1+ MPN projecting cells, but by inactivation of PAEsr1+ VMHvl projecting cells.

Conversely, pharmacogenetic activation of PAEsr1+ neurons projecting into the MPN and VMHvl neurons promoted sexual and aggressive behaviors, respectively. While activating PAEsr1+ MPN projecting cells elicited repeated mounting attempts towards non-receptive females despite the females’ resistance, activating PAEsr1+ VMHvl projecting cells promoted aggression towards both male and female intruders. These data supported indispensable roles of the PA in driving aggression and sexual behaviors in male mice.

In summary, we identified two largely distinct subpopulations in the PA, which directly control hypothalamic activity and regulate male sexual and aggressive behaviors. Our study provides a new circuit level understanding regarding how innate social behaviors are initiated.

—Takashi Yamaguchi, PhD

Read the paper “Posterior amygdala regulates sexual and aggressive behaviors in male mice” in Nature Neuroscience, published July 27, 2020.