BRAIN AND PITUITARY MELATONIN RECEPTORS IN MALE-RAT DURING POSTNATAL AND PUBERTAL DEVELOPMENT AND THE EFFECT OF PINEALECTOMY AND TESTOSTERONE MANIPULATION

Using quantitative autoradiography, melatonin receptors have been stud ied during post-natal and pubertal development of the rat in 2 brain a nd 2 pituitary structures. In the pars distalis of anterior pituitary, melatonin receptors decrease gradually in density after birth and dis appear in 30 day-old animals. In contrast melatonin binding is only ex pressed in the paraventricular nuclei of the thalamus at the age of 21 -23 days and is always present in adult animals. In the suprachiasmati c nuclei and in the pars tuberalis of the pituitary, melatonin recepto r density decreases after birth, remains stable for approximately 1 mo nth and increases again at puberty to reach the birth values in the ad ult. This increase was absent in pinealectomized and in castrated anim als but present in castrated animals receiving testosterone suggesting that it depends upon circulating testosterone and melatonin levels. T hese results show that melatonin receptors are differentially regulate d during post-natal development in each of the 4 structures studied, a nd that melatonin and testosterone are 2 factors which could be involv ed in the regulation of melatonin receptor density in the suprachiasma tic nuclei and pars tuberalis.