Daily and circadian expression patterns of mt1 melatonin receptor mRNA in the rat pars tuberalis.

Abstract

The mammalian pineal gland transduces lighting conditions into an endocrine message, the nocturnal synthesis and secretion of melatonin (17,20). Photoperiod is then coded hormonally by the duration of the nocturnally elevated plasma melatonin levels (7,14,23). Herewith, melatonin is involved in the regulation of seasonal biological rhythms, like reproduction (14,15,23). Among the large number of structures containing melatonin receptors, the pars tuberalis (PT) of the pituitary is the only one presenting a high number of 2-I-melatonin binding sites in all the mammalian species studied so far (14). To date, in mammals, two different melatonin receptor cDNAs have been cloned, called Mel1a and Mel1b (mt1 and MT2, respectively) (18,19). These authors showed also that the distribution of mt1 mRNA is coincident with that of 2-125Imelatonin binding. In response to the photoperiodic changes, seasonal variations of 2-125I-melatonin binding in the PT of seasonal breeders have been described (2,4,13,16,21,22,23). At present time, these receptors are thought to be involved in mediating the photoperiodic effects of melatonin in the seasonal control of prolactin secretion (8,9,10,12).