Photoperiodic regulation of vasopressin receptor binding in female Syrian hamsters.

During long "summer-like" photoperiods, female Syrian hamsters display a regular 4-day estrous cycle. However, during short "winter-like" photoperiods (<12.5 h of light/day) hamsters become anestrus. Short photoperiod exposure eliminates reproductive behavior but social behaviors such as aggression and scent marking continue to be displayed. In long photoperiods, the types and intensity of social behaviors change as a function of the estrous cycle. For example, aggression and scent marking tend to occur at higher levels on diestrus 1 and diestrus 2 than on proestrus or estrus. Aggression and scent marking may be regulated, at least in part, by changes in the density of arginine vasopressin-V(1a) receptors (V(1a)R). In Experiment 1, it was hypothesized that the density of V(1a)R would change across the estrous cycle in several subcortical regions implicated in the regulation of aggression and scent marking. In Experiment 2, it was hypothesized that exposure to short photoperiod would alter the density of V(1a)R in several regions involved in the regulation of social behavior. Interestingly, there were no dramatic changes in V(1a)R binding across the estrous cycle within any of the neuroanatomical areas measured. However, in hamsters housed in short photoperiod, there were lower levels of V(1a)R binding within the medial preoptic nucleus (MPN), medial preoptic area (MPO), lateral hypothalamus (LH), central nucleus of the amygdala (Ce) and bed nucleus of the stria terminalis (BST) than in hamsters housed in long photoperiod. These data suggest that photoperiodic mechanisms can alter the density of V(1a)R in subset of V(1a) binding sites thought to be involved in the regulation of social behaviors in female hamsters.