Nocturnal accumulation of cyclic 3′,5′-guanosine monophosphate (cGMP) in the chick pineal organ is dependent on activation of guanylyl cyclase-B

The role of cGMP in the avian pineal is not well understood. Although the light-sensitive secretion of melatonin is a well-known output of the circadian oscillator, pharmacologically elevated cGMP levels do not result in altered melatonin secretory amplitude or phase. This suggests that pineal cGMP signalling does not couple the endogenous circadian oscillator to the expression of melatonin rhythms. Nonetheless, the free-running rhythm of cGMP signalling implies a link to the circadian oscillator in chick pinealocytes. As the circadian rhythm of cGMP levels in vitro is not altered by pharmacological inhibition of phosphodiesterase activity, we infer that the synthesis, rather than the degradation of cGMP, is under circadian control. In vitro experiments with the nitric oxide synthase (NOS) inhibitor N-G-nitro-L-arginine as well as with an inhibitor of the NO-sensitive soluble guanylyl cyclase (sGC), showed that the NOS-sGC pathway does not play a major role in the circadian control of cGMP generation. In organ culture experiments, we demonstrated that C-type natriuretic peptide (CNP), but not atrial natriuretic peptide (ANP), elevated daytime levels of cGMP. As CNP acts on the membrane guanylyl cyclase isoform B (GC-B), which is expressed at very high levels in mammalian pineals, we investigated the effect of the membrane GC-specific inhibitor HS-142 on chick pineal cGMP levels. CNP-induced daytime cGMP levels were reduced by HS-142. More importantly, 'spontaneously' high nocturnal levels of cGMP in vitro were reduced to daytime levels by acute addition of HS-142. These data implicate endogenous nocturnal CNP release and subsequent activation of GC-B as the major input driving cGMP synthesis in the chick pineal organ.