Distinct Dynamics of Theta and Gamma Rhythmicity during Social Interaction Suggest Differential Mode of Action in the Medial Amygdala of Sprague Dawley Rats and C57BL/6J Mice.

The medial nucleus of the amygdala (MeA) is known to regulate social behavior. This brain area is functionally positioned in a crossroads between sensory information processing and behavioral modulation. On the one hand, it receives direct chemosensory input from the accessory olfactory bulb. On the other hand, it orchestrates various behavioral outputs via brain-wide projections under the regulation of multiple neuromodulatory systems. Previously, we showed that adult male Sprague Dawley (SD) rats and C57BL/6J mice, the most widely used rodent models in neuroscience research, differ in their dynamics of motivation to interact with a novel same-sex conspecific and that this difference correlates with the level of c-Fos expression in the MeA. Here we used chronically implanted electrodes to compare rhythmic local field potential signals recorded from these animals during free and restricted social interactions. We found a significant induction of rhythmicity in the theta (4-12 Hz) and gamma (30-80 Hz) bands during both free and restricted social interaction in both rats and mice. However, the induction of gamma rhythmicity, thought to reflect activity of local neuronal networks, was significantly higher in rats than mice. Nevertheless, in contrast to rats, mice exhibited induction of rhythmicity, in both the theta and gamma bands, in synchrony with investigation of social, but not object stimuli. These results suggest that during interaction with a novel same-sex conspecific, the MeA of C57BL/6J mice is mostly involved in sensory information processing while in SD rats it is mainly active in modulating the social motivation state of the animal.