Examination of oxytocin receptor distribution and social behavior across a South American rodent clade

Oxytocin signaling has been implicated in many aspects of social behavior, including social attachment, anxiety-reduction, and trust. Differential expression of the oxytocin receptor in mammalian brains has also been linked to variation in mating system (monogamy vs. promiscuity) in vole species. The oxytocin neuropeptide is well suited to modulate behavior because of its localization in specific neural pathways, its slow and enduring effects, and its durable response to physiological changes (Insel, 1992). Although most studies linking behavior with neural pathways focus on reproductive and parental care, a complete understanding of social behavior necessitates understanding the neural circuitry linked with non-reproductive social behaviors. Non-reproductive social behavior, i.e. social relationships between peers, forms the basis of many mammalian societies (Anacker and Beery, 2013). The distribution of the oxytocin receptor and its peptide may play an important role in regulating social behavior between peers in a species-specific manner. In the present study, I examined oxytocin receptor distribution in eight species of South American rodents in the Ctenomys genus and its nearest relatives. These species display natural variations in social behavior and can provide a link between non-reproductive social behaviors and the neural circuitry underlying that behavior. Using oxytocin receptor autoradiography assays, I examined key brain regions expressing the oxytocin receptor in five solitary and three social Ctenomyid rodents. Oxytocin receptor distribution in all brain regions showed significant between-species variations. Variations in oxytocin receptors between social and solitary species were present in the central amygdala (CeA), basolateral amygdala (BLA), and nucleus accumbens core (NAcc-Core). Sex differences in receptor expression were also noted in 6 species. Phylogenetic analysis was conducted to determine in which regions oxytocin receptor distribution was constrained by phylogeny. Evidence for significant association of OTR density with the phylogenetic tree was observed for the BLA, CeA, NAcc-Core, Ventral Medial Hypothalamus, and Bed Nucleus Stria Terminalis. Given the present clustering of social species in one region of the phylogeny, it is unclear whether OTR expression in the CeA, BLA, and NAcc-Core is indicative solely of shared evolutionary history or also of shared social behavior. The planned addition of a social species that is not part of this cluster should resolve this question. Groundwork was also laid for future examination of oxytocin peptide expression in these samples