Corticotropin-releasing factor receptor densities vary with photoperiod and sociality.

Life in social groups relies on prosocial behaviors as well as on reduction of antisocial behaviors such as aggression and territoriality. The mechanisms supporting variation in behaviors that give rise to group living (sociality) are largely unknown. Female meadow voles exhibit natural seasonal variation in sociality: females are aggressive and territorial in summer, while in winter they share burrows and nest in mixed-sex groups. This behavioral shift is paralleled in the lab by day length-dependent variation in partner preference formation and social huddling. We exploit natural variation in meadow vole sociality in order to examine changes in neural pathways that coincide with environmental and behavioral variations. Mounting evidence suggests that the corticotropin-releasing factor system, encompassing multiple peptides and two receptor subtypes (CRF1 and CRF2), may play an important role in regulating social behaviors. We report day-length dependent variation in CRF1 and CRF2 receptor binding in female meadow voles, and relate these findings to previously collected oxytocin receptor (OTR) binding data and behavioral data for the same individuals. CRF1 receptor binding was greater in summer-like long day lengths (LD), particularly in the hippocampus, while CRF2 receptor binding was greater in winter-like short day lengths (SD) in the cingulate cortex and hippocampus. OTR varied with day length in the bed nucleus of the stria terminalis, nucleus accumbens, and hippocampus. SD voles huddled more extensively than LD voles, and greater huddling time was associated with more CRF1 receptor binding and less CRF2 receptor binding in subregions of the lateral septum. CRF2 receptor associations with behavior mirrored those of OTR in the lateral septum. Finally, estradiol treatment affected density of CRF receptors in multiple brain regions. CRF receptors and their ligands are promising candidates for enhancing understanding of the regulation of non-sexual social behavior between group living peers.