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 regulation by adrenergic pathways has been well documented. Recent evidence points to adrenergically stimulated nitric oxide generation as a mechanism for coupling this pathway. To what extent nitric oxide (NO) signalling can influence adrenergically stimulated melatonin synthesis has not been investigated. Cyclic guanosine 3′,5′‐monophospate (cGMP) signal transduction in the bovine pineal has also received little attention. We describe in the present report: 1) a dose‐dependent elevation of cGMP in response to the nitrovasodilators, sodium nitroprusside (SNP) and 3‐morpholino‐sydnonimine (SIN‐l), 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 evidence for the presence of nitric oxide synthase (NOS) (EC 1.14.23.‐) in the intact bovine pineal gland and in cultured bovine pinealocytes. These data support the view that NOS is a component of the cGMP‐generating system in mammalian pinealocytes. Although NO‐donor molecules are also potent activators of cGMP accumulation, they may have other important actions in the pineal, namely the inhibition of adrenergic‐stimulated melatonin synthesis. As SNP and SIN‐1 exerted this inhibitory effect on cells regardless of whether they were stimulated by isoproterenol, forskolin or 8‐Br‐CAMP it would appear that NO‐donors can act ‘downstream’ from the receptor/adenylate cyclase level.