Seasonality and melatonin receptors in the pars tuberalis in some long day breeders.

The density of high-affinity melatonin receptors in the pars tuberalis of different long day breeders exhibits a seasonal variation, with high values in spring-summer and low values in autumn-winter. In all the studies, this decrease in receptor density was always associated with both a short photoperiod and sexual inactivity. Moreover, during the hibernation period, the reduction in receptor number was more marked in hypothermic than in normothermic animals. These seasonal changes in melatonin receptors were restricted to the pars tuberalis, which suggests that this structure represents an important target site for mediation of the photoperiodic effect of melatonin on seasonal functions. The mechanism of melatonin action at this level is discussed.