Modification of Hypothalamic Electrical Activity by Pineal Indoles

Abstract

In a number of mammalian species, the pineal gland has been shown to mediate the inhibitory effects of short photoperiods on the secretion of several pituitary hormones. Although most studies have centred on gonadotropin secretion, in the last twenty five years there have been reports which have provided evidence for pineal-mediated inhibition of the production and release of each of the adenohypophyseal hormones (see review Mess et al., 1979). The Syrian hamster (Mesocricetus auratus) is a highly photosensitive species, in which the pineal can readily be shown to exert a dramatic and potent influence on the release of luteinising hormone (LH), follicle stimulating hormone (FSH) and prolactin (Reiter, 1980). Light restriction, obtained by blinding hamsters or placing them in continuous darkness or short photoperiod, results in reproductive involution. In males, after 50 days of light restriction, the testes are reduced to less than 20% of their original weight and do not produce spermatozoa (Hoffman and Reiter, 1965). These changes are associated with a reduction in the pituitary content of both LH and FSH (Reiter and Johnson, 1974a), and of prolactin (Reiter et al., 1975). Turek et al. (1976) found that serum LH and FSH levels in animals exposed to short photoperiods were lower than in hamsters maintained in a L:D, 14:10 lighting cycle. In light restricted females, the oestrous cycles are interrupted and the uteri become infantile (Johnson and Reiter, 1978). Although the ovaries tend to increase in weight under these conditions, there are pronounced changes in ovarian morphology. There is extensive proliferation of interstitial tissue, a small number of antral follicles and a distinct lack of corpora lutea (Vaughan et al., 1981). The relationship between reproductive collapse and pituitary gonadotropin secretion is more complicated in female hamsters. In fact, LH levels actually appear to rise when animals are blinded (Reiter and Johnson, 1974b). Presumably the occurrence of daily surges in both LH and FSH in female hamsters exposed to short days (Seegal and Goldman, 1975) underlies the disruption of normal reproductive function.