Oxytocin, a hormone produced in the paraventricular and supraoptic nuclei in the brain, influences maternal functions and behaviors, including parturition, lactation, and bonding. In this study, New York University (NYU) researchers sought to understand precisely how and where the maternal mouse brain responds to pup distress cries in relation to oxytocin signals (Marlin et al. 2015).
The NYU researchers tested the responsiveness of virgin females and experienced mothers (dams) to retrieve pups that had been experimentally removed from a nest. As expected, wild-type C57BL/6 virgin females did not retrieve pups unless co-housed with dams for more than 2 days. In contrast, virgin females carrying an oxytocin-IRES-cre transgene (024234) and infected with a channelrhodopsin adenovirus (a vector for activating oxytoxin-expressing neurons with light) rapidly retrieved pups similar to wild-type dams when those oxytocin neurons were stimulated. Likewise, wild-type virgin females that received stereotactic injections of oxytocin also responded to pup distress calls.
Examination of the primary auditory cortex (AI) in oxytocyin receptor green fluorescent protein-expressing reporter mice (OTXR-eGFP) revealed significantly more oxytocin-positive cells in the left AI than in the right AI. This data suggested that pup retrieval behavior is functionally lateralized, with oxytocin-sensitive neural circuits in the left AI playing a greater role in regulating this behavior. Consistent with this hypothesis, pup retrieval behavior was blocked by pre-treating the left AI with muscimol, a GABAergic neuron inhibitor and neural sedative. Infusing muscimol into the right AI had no effect on pup retrieval.
Examination of excitatory and inhibitory post-synaptic currents in both the left and right AIs and in the left AIs of experience dams versus naïve virgins revealed:
These data suggest that the heightened pup retrieval behavior in experienced dams is not simply due to stronger excitatory synaptic inputs to their left AI. Instead, signals in the left AI are more responsive to smaller oxytocin-induced changes in the balance between excitatory and inhibitory synaptic signals, resulting in a greater sensitivity to pup distress.
Together, these findings lend further support to previous research describing dominance in the right ear/left hemisphere circuitry in stimulating maternal responses to auditory cues from offspring. Further, this research suggests that behavior and neural circuitry can be modified through hormonal influence.
