Short‐Photoperiod Exposure Reduces Vasopressin (V 1a ) Receptor Binding But Not Arginine‐Vasopressin‐Induced Flank Marking in Male Syrian Hamsters

In Syrian hamsters, socially relevant information is communicated with a form of scent marking known as flank marking. There is substantial evidence that arginine‐vasopressin acting on V 1a vasopressin receptors (V 1a R) in the medial preoptic‐anterior hypothalamic continuum (MPOA‐AH) regulates the expression of flank marking. Previous studies have shown that the expression of flank marking is also influenced by the circulating concentrations of gonadal hormones. In hamsters housed in long ‘summer‐like’ photoperiods (i.e. >12.5 h of light/day), castration reduces flank marking and administration of testosterone restores precastration levels of flank marking. When exposed to short ‘winter‐like’ photoperiods (i.e. <12.5 h of light/day), hamsters undergo gonadal regression and the circulating levels of testosterone decline. Surprisingly, flank marking induced during social encounters is not reduced in hamsters exposed to short photoperiods despite the low circulating concentrations of testosterone. In the present study, it was hypothesized that reductions in testosterone, caused by exposure to short photoperiod, would not reduce the ability of vasopressin to stimulate flank marking by its actions in the MPOA‐AH. The amount of flank marking induced by vasopressin injected into the MPOA‐AH did not significantly differ between hamsters housed in long and short photoperiods; however, short photoperiod‐exposed males had significantly less V 1a R binding in the MPOA than long photoperiod‐exposed males. These results support the hypothesis that the sensitivity of the MPOA‐AH to vasopressin is not reduced in short photoperiod‐exposed males, despite decreases in serum testosterone. However, by contrast to our predictions, short photoperiod‐exposed males have significantly reduced V 1a R binding in the MPOA‐AH compared to long photoperiod‐exposed males.