Patterns of neural activity are critical for sculpting the immature brain, and disrupting this activity is thought to underlie many neurodevelopmental disorders. In fact, specific developmental processes are highly dependent on sensory experience and levels of circuit activity. For example, high-frequency bursts of activity are conducive to evoking synaptic plasticity, while periods of sustained slow-wave activity are conducive to synaptic homeostasis and memory consolidation. This neural activity can be generated by the spontaneous activity within the brain or through sensory-evoked stimulation by the environment.

For the mammalian infant, the primary source of sensory-evoked environmental stimulation is the maternal caregiver and it has long been shown to impact neurobehavioral development, where poor maternal care leads to long-term neurobehavioral deficits. One hypothesis is that the maternal caregiver imposes a direct impact on the development of the infant brain, resulting in lasting changes to the neural circuitry. While previous reports demonstrate the slow emergence of mature-like cortical activity over the early postnatal period in anesthetized and/or socially isolated infant rats, the immediate impact of natural maternal stimulation on infant cortical activity has not been previously described, in part due to the difficulties of obtaining recordings within the nest.

A recent paper published in Current Biology by Drs. Emma C. Sarro, Donald A. Wilson, and Regina M. Sullivan explored for the first time rat pups’ brain electrical activity within the natural nest/litter environment specifically related to maternal interactions. Sarro et al. implanted infant rats with wireless electrodes targeting either the neocortex or amygdala regions. This provided an opportunity to explore the immediate effects of natural maternal and infant interactions (i.e., nursing, grooming) within the nest on the infant brain activity.

Local field potentials (LFPs) were recorded in these freely behaving infant rats from postnatal days ~12 to 19 during natural interactions with their mother. Sarro et al. show that the mere absence of the mother from the nest increased the magnitude of high-frequency oscillatory activity, conducive to synaptic plasticity, where even further isolation of the pup from the other littermates induced greater increase of high-frequency oscillations. The mother’s return to the nest dampened this activity, and nipple attachment induced an increase in slow-wave activity, conducive to synaptic homeostasis and memory consolidation. Additionally, maternal stimulation of pups (grooming, milk ejection) consistently produced rapid, transient high-frequency oscillations. Interestingly, as pups approached independence from the mother, or weaning, the magnitude of these maternal effects decreased.

This study demonstrates a powerful influence of the maternal presence and mother–infant interactions on brain development. The authors suggest that this mechanism may allow variation in maternal care to create variability in brain development and behavioral outcome. Thus, the nature and duration of specific maternal behaviors as well as the temporal patterning of interaction and absence could induce robust individual differences in arousal memory formation and general brain maturation. Alongside previous work by the Sullivan lab demonstrating the long-lasting neurobehavioral consequences of poor maternal care, the maternal modulation of infant brain state described here may contribute to these effects.

—Emma C. Sarro, PhD

Read the paper “Maternal regulation of infant brain state” in Current Biology, published July 21, 2014.