Photoperiod and melatonin influence seasonal gonadal cycles in the grasshopper mouse (Onychomys leucogaster)

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Abstract
Development of the reproductive apparatus was delayed in grasshopper mice maintained from birth in short photoperiods (10 h light/day). The inhibitory effects of short photoperiods on sexual maturation eventually waned and mice in 10L:14D [light/dark cycle] became reproductively active. Adult mice transferred from long (14 h light/day) to short photoperiods underwent testicular regression after 10 wk and complete gonadal redevelopment after 30 wk. A similar phenomenon was observed in adult female mice; estrous cycles ceased within 3 wk and resumed after 13 wk in the short photoperiod. The regressive effects of short photoperiods on the male reproductive system were mimicked by daily injections of melatonin administered to mice housed in 14L:10D. Responsiveness of the female reproductive system to melatonin was reduced among photorefractory as compared to photosensitive mice. Apparently the initial rate of sexual maturation and the timing of seasonal breeding in adult mice are regulated by photoperiod; effects of short daylengths on the neuroendocrine-reproductive axis appear to be mediated by the pineal gland.