MELATONIN REDUCES NITRIC-OXIDE SYNTHASE ACTIVITY IN RAT HYPOTHALAMUS

In this report, rat hypothalamic nitric oxide synthase (NOS) activity is shown to be partially inhibited by physiological concentrations of the pineal hormone melatonin. In vitro studies demonstrate that 1 nM m elatonin, which approximates the physiological concentration of the ho rmone at night, significantly inhibited NOS activity. In vivo studies show that administering melatonin or collecting the hypothalamus from animals at night, when endogenous melatonin levels are elevated, resul ts in a significant decrease of NOS activity. Results also show that c almodulin may be involved in this process since its presence in the in cubation medium prevents the inhibitory effect of melatonin on NOS act ivity.