Suprachiasmatic Lesions Prevent an Antigonadal Effect of Melatonin

Abstract

The suprachiasmatic region of the hypothalamus has been implicated in the control of seasonal reproduction in male hamsters. Lesions of the suprachiasmatic nuclei (SCN) prevent gonadal regression in hamsters exposed to short daily photoperiods and accelerate recrudescence in hamsters with previously regressed testes. Removal or denervation of the pineal gland has similar effects, perhaps because of the loss of pineal melatonin; melatonin has been shown to have antigonadal effects in both intact and pinealectomized hamsters. If the influence of SCN lesions on gonadal condition is mediated by effects on pineal function, then SCN lesioned and pinealectomized hamsters should be functionally similar. In particular, antigonadal effects of exogenous melatonin that occur in pinealectomized hamsters should also occur in SCN lesioned hamsters. One such antigonadal effect is the inhibition by melatonin of testicular recrudescence induced by long photoperiods. In these studies, capsules filled with melatonin slowed testicular recrudescence in both intact and pinealectomized hamsters but did not affect recrudescence in SCN lesioned hamsters. Among hamsters not treated with melatonin, the testes of SCN lesioned animals recrudesced more rapidly than those of intact or pinealectomized hamsters exposed to the same stimulatory photoperiod. Suprachiasmatic lesions and pinealectomy are therefore not functionally equivalent treatments under these experimental conditions. In addition to their known effects on pineal function, suprachiasmatic lesions may interrupt neural systems adjacent to the SCN that are involved in the regulation of gonadotropin and prolactin secretion. Damage to these systems in animals with lesions aimed at the SCN may account for some of the differences between SCN lesioned and pinealectomized hamsters.