Neuroendocrine integrative mechanisms in mammalian pineal gland: Effects of steroid and adenohypophysial hormones on melatonin synthesis in vitro

Abstract

The time course for the decrease in norepinephrine concentration of rat pineal explants in culture indicated a significant fall starting at the 4th hour and completed after 16–24 h of incubation. Significant decreases of serotonin and 5-hydroxyindoleacetic acid (HIAA) levels in tissue, an increase of HIAA/serotonin ratio, and an increase of melatonin production rate in vitro were also observed as a function of the incubation time. Estradiol (10 −7−10 −5 M) increased rat pineal melatonin content, testosterone (10 −5 M) decreased it and progesterone was devoid of activity when incubated with explants for up to 6 h. The in vitro stimulatory effect of estradiol on rat pineal methoxyindole synthesis was blocked by propranolol but not by phentolamine; propranolol also blocked the increase of nuclear estradiol-receptor complex produced by estrogen exposure of pineal explants. TSH (1–100 ng/ml), growth hormone (10–100 ng/ml) and LH (10 ng/ml) augmented rat pineal melatonin content while 100 ng/ml of FSH decreased it significantly. Prolactin exerted a biphasic effect on rat pineal explants, the lowest concentration augmenting melatonin content while the high concentration depressed it. Deep, intermediate and superficial segments of guinea-pig pineal glands showed an increase in melatonin concentration after a 6-h incubation in the presence of 10 −7−10 −5 M estradiol.