Physiological concentrations of melatonin inhibit the norepinephrine-induced activation of prostaglandin E₂ and cyclic AMP production in rat hypothalamus: A mechanism involving inhibition of nitric oxide synthase

In this paper, we summarize the results of in vitro studies showing that physiological concentrations of melatonin inhibit the norepinephrine-induced activation of prostaglandin E₂ (PGE₂) and cyclic AMP production in rat medial basal hypothalamus (MBH). Interestingly, a concentration of melatonin as low as 1 nM, which is roughly equivalent to the nocturnal serum physiological concentration of the hormone in the rat, significantly inhibit PGE₂ and cyclic AMP production in the MBH. The suppressive effect of melatonin may be mediated by an inhibition of nitric oxide synthase (NOS) activity, since the stimulatory effect of sodium nitroprusside (SNP), a spontaneous generator of NO, was not prevented by melatonin. Melatonin also inhibited NOS activity in rat MBH in a dose-dependent manner. The results suggest the existence of a new or an ancillary means by which melatonin may regulate the physiology of the hypothalamus-pituitary unit.