Role of the pineal gland and melatonin in the photoperiodic control of hypothalamic gonadotropin-releasing hormone in the male jerboa (Jaculus orientalis), a desert rodent

The neuroendocrine mechanism underlying seasonal changes in gonadal activity of the jerboa, a desert hibernating rodent adapted to harsh climatic conditions, are poorly understood. We investigated the role of the pineal gland and melatonin in the photoperiodic control of hypothalamic gonadotropin-releasing hormone (GnRH). Intact and pinealectomized male jerboas were subjected to short photoperiod, while others were kept under long photoperiod and injected daily with melatonin or vehicle. Testes activity was monitored by evaluating the testes volume during 10 weeks. GnRH immunoreactivity was investigated quantitatively with image analysis. Following melatonin administration, the hormone peaked in plasma after 30 min, with return to control levels 2.5 h later. Exposure to short photoperiod and melatonin resulted in marked increase in the number of GnRH-containing cells in the preoptic area and mediobasal hypothalamus, whereas GnRH immunoreactivity of fibers and terminals in the median eminence decreased under these conditions. The findings indicate that in the jerboa short photoperiod induces testicular regression by prolonging the duration of melatonin as an endocrine signal. This mechanism probably involves inhibition of GnRH release in the median eminence, with consequent accumulation of GnRH in perikarya of the preoptic area and mediobasal hypothalamus. Interestingly, GnRH cells of the median eminence did not appear to be influenced by the photoperiod and pineal melatonin, whereas their number was increased by exogenous melatonin. The latter data suggest for the first time the involvement of an extrapineal melatonin, whose origin remains to be identified, in the modulation of the GnRH regulatory system in rodents.