Photoperiodic mechanisms controlling scent marking: interactions of vasopressin and gonadal steroids.

Microinjection of arginine vasopressin (Avp) into the rostral hypothalamus of Syrian hamsters induces a form of scent marking known as flank marking. The ability of Avp to stimulate flank marking is mediated by the vasopressin 1a receptor (Avpr1a). In hamsters housed in long 'summer-like' photoperiods, the amount of flank marking and the number of Avpr1a receptors in the rostral hypothalamus are regulated by testosterone. However, hamsters housed in short 'winter-like' photoperiods for 6-8 weeks continue to flank mark at high levels despite significant reductions in the circulating levels of testosterone. In the present study, we compared the effects of gonadal steroids on Avp-induced flank marking and Avpr1a binding and affinity in hamsters housed in short photoperiods and those housed in long photoperiods. In long-photoperiod-housed hamsters, castration significantly reduced the amount of Avp-induced flank marking; however, in short-photoperiod-housed hamsters there were no significant differences between gonadally regressed and castrated hamsters. Surprisingly, Avpr1a receptor binding, but not affinity, in the medial preoptic area and the medial preoptic nucleus was significantly reduced in long-photoperiod-housed castrates as well as short-photoperiod-housed gonadally regressed and castrated hamsters, compared with long-photoperiod-housed gonadally intact hamsters. These data demonstrate that in short photoperiods Avp-induced flank marking is independent of gonadal hormones, despite gonadal steroid-dependent reductions in Avpr1a binding.