THE EFFECT OF NO-DONORS IN BOVINE AND RAT PINEAL CELLS - STIMULATION OF CGMP AND CGMP-INDEPENDENT INHIBITION OF MELATONIN SYNTHESIS

The presence of soluble guanylate cyclase in the pineal and its regula tion by adrenergic pathways has been well documented. Recent evidence points to adrenergically stimulated nitric oxide generation as a mecha nism for coupling this pathway. To what extent nitric oxide (NO) signa lling can influence adrenergically stimulated melatonin synthesis has not been investigated. Cyclic guanosine 3',5'-monophosphate (cGMP) sig nal transduction in the bovine pineal has also received little attenti on. We describe in the present report: 1) a dose-dependent elevation o f cGMP in response to the nitrovasodilators, sodium nitroprusside (SNP ) and 3-morpholino-sydnonimine (SIN-1), 2) a dose-dependent inhibition of melatonin synthesis by SNP and SIN-1, but not by 8-Br-cGMP in both bovine and rat pineal cell cultures, which is not due to cytotoxicity as judged by two different approaches, and 3) immunohistochemical evi dence for the presence of nitric oxide synthase (NOS) (EC 1.14.23.-) i n the intact bovine pineal gland and in cultured bovine pinealocytes. These data support the view that NOS is a component of the cGMP-genera ting system in mammalian pinealocytes. Although NO-donor molecules are also potent activators of cGMP accumulation, they may have other impo rtant actions in the pineal, namely the inhibition of adrenergic-stimu lated melatonin synthesis, As SNP and SIN-1 exerted this inhibitory ef fect on cells regardless of whether they were stimulated by isoprotere nol, forskolin or 8-Br-cAMP it would appear that NO-donors can act 'do wnstream' from the receptor/adenylate cyclase level.