Differential circadian rhythms in pineal and hypothalamic 5-HT induced by artificial photoperiods or melatonin

Abstract

CHANGES in daylength are principally responsible for synchronising breeding with time of year in mammals and birds¹: for example, ferrets begin to breed in spring and oestrus ends in autumn. Oestrus can be provoked by exposure to long photo-periods (such as 14 h light: 10 h dark, LD 14 : 10) during winter² and can be terminated prematurely if short photo-periods (LD 8 : 16) are given during summer³. The pineal is essential for both effects, which are prevented by removal of the gland^3,4 or its autonomic nerve supply⁵ from the superior cervical ganglia (C.A.Y. unpublished results). The pineal contains large concentrations of the transmitter 5-hydroxytryptamine (5-HT), which has been implicated in the neural control of circadian rhythms, which themselves may be related to the genesis of annual rhythms¹. In the pineal of the rat concentrations of 5-HT vary in phase with photoperiod, and this rhythm is prevented by ganglionectomy⁶. Inhibition of cerebral 5-HT synthesis prevents circadian rhythms in concentrations of both plasma adrenal corticosteroids⁷ and luteinising hormone⁸. Lesions of the suprachiasmatic nuclei, which contain large concentrations of 5-HT^9,10, abolish circadian rhythms in eating and drinking¹¹, in running¹² and in concentrations of pineal n-acetyltransferase¹³, which is the rate limiting enzyme in the synthesis of melatonin, a putative pineal hormone. The pineal must transmit information about photoperiod to neural structures regulating the pituitary, but the nature of the link is unknown. We report here that exposure of ferrets to either long or short photoperiods induces differential 5-HT rhythms in the pineal and hypothalamus, which are abolished by autonomic denervation, and that melatonin induces 5-HT rhythms characteristic of animals kept in short photoperiods.