Endocrinology, Vol 120, 1682-1684, Copyright © 1987 by Endocrine Society

ARTICLES

The Syrian hamster pineal gland responds to isoproterenol in vivo at night

GM Vaughan and RJ Reiter

Failure of isoproterenol (ISO) injections to raise pineal melatonin content has generated doubt about beta-adrenergic control of the melatonin rhythm in Syrian hamsters. However, the effect of ISO injected at night after light-induced reduction of pineal melatonin has not been reported. In this study, light exposure began at 6 1/4 h into one (normally 10-h) dark phase. The hamsters were injected with either ISO (1 mg/kg) or vehicle 15 min later when pineal melatonin content was low. Light exposure continued. Two h after ISO but not vehicle injection, pineal melatonin content rose more than six-fold. In other animals injected at the end of the usual light phase then kept in light for 2 h, pineal melatonin was equally low after ISO or vehicle injection. The Syrian hamster pineal gland can respond in vivo to a beta-adrenergic agonist injected at the physiologically relevant time of the normal nocturnal melatonin surge. This finding, taken together with the previously reported inhibition of the endogenous nocturnal melatonin surge with a beta-blocking drug, suggests that a beta- adrenergic mechanism controls the hamster pineal melatonin rhythm.