Siberian hamsters have decreased gonadotropin levels and testis size after short-day (SD) exposure. Upon transfer from short to long days, FSH and testis weight increase rapidly, whereas LH and T remain low for much longer. We investigated whether an additional environmental stimulus, specifically a female, could trigger an earlier release of LH and whether the response to the female was dependent on photoperiod. An increase in serum LH was induced in long day (LD), but not SD, males within minutes of female exposure. The ability of SD males to secrete LH upon female exposure was regained within 4 d of photostimulation. FSH was not secreted after female exposure, but varied with photoperiod. Thus, FSH and LH are differentially regulated by photoperiod and female exposure. In subsequent studies melatonin injections and a GnRH antagonist were used to show that photoperiod modulates the endocrine responsiveness of a male to a female via melatonin and that female-induced LH release is GnRH dependent. Collectively, these results suggest separation of gonadotropin signaling pathways by environmental stimuli and provide an excellent model to elucidate the effects of photoperiod on the processing of social and chemosensory inputs to the GnRH neurons of the hypothalamus.
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