Photoperiodic modulation of GnRH mRNA in the male Syrian hamster.
Male Syrian hamsters (Mesocricetus auratus) are seasonal breeders. They show marked testicular regression when exposed to short autumnal photoperiods, and then remain sexually quiescent for several months. By mid-winter, however, they show a loss in responsiveness to the inhibitory influence of short photoperiods and their testes begin to recrudesce. To shed light on the neuroendocrine mechanism responsible for mediating these reproductive changes, we examined the influence of photoperiod on the expression of GnRH mRNA in the hamster forebrain. Adult males were either exposed to short photoperiods (6L:18D) for 16 weeks or were maintained under long photoperiods (14L:10D); additional animals were exposed to short or long photoperiods for 22 weeks. As expected, exposure to short photoperiods for 12 weeks resulted in a marked decrease (P<0.01) in testicular mass and serum testosterone levels, but after 22 weeks these reproductive parameters were once again significantly elevated (P<0.01). In contrast, quantitative in situ hybridization histochemistry revealed no difference (P>0.05) between the GnRH mRNA levels of the short-photoperiod hamsters and their aged-matched long-photoperiod controls, although an age-related decrease (P<0.05) was evident in both photoperiod-treatment groups. These data emphasize that GnRH mRNA is highly expressed in hamsters even when their reproductive axis has been rendered sexually quiescent by exposure to short photoperiods, and that photoperiod-induced changes in GnRH secretion, rather than synthesis, are more likely to regulate the timing of the breeding season. On the other hand, the data indicate that GnRH mRNA levels show an aging-related decrease, regardless of photoperiod, suggesting that in the long term a decrease in GnRH gene expression may contribute to the reduced fertility of old hamsters.