Short-Photoperiod Exposure Reduces Vasopressin (V1a) Receptor Binding But Not Arginine-Vasopressin-Induced Flank Marking in Male Syrian Hamsters

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Abstract
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 V1a vasopressin receptors (V1aR) 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. 1aR 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 V1aR binding in the MPOA‐AH compared to long photoperiod‐exposed males.