Down-regulation of the nocturnally elevated guanylyl cyclase activity in the rat pineal gland.

Previous studies have shown that in the rat pineal, the cytosolic and the particulate forms of guanylyl cyclase (GC) activity undergo a biphasic 24-h rhythm with two prominent peaks, one in the middle of the light phase and the other in the middle of the dark phase. In this study we investigated whether the well established photo-neural adrenergic regulatory processes identified for pineal melatonin synthesis also apply to the nocturnal elevation of GC activity. A 10-min light pulse given in the middle of the dark phase decreases the cytosolic and the particulate forms of GC. Administration of the beta-receptor blocker propranolol did not depress the nocturnally elevated GC activity. Sympathetic denervation of the pineal gland by means of superior cervical ganglionectomy did not noticeably affect nocturnal GC activity studied 6 days and 2 months after surgery. In vitro, administration of the nitric oxide synthase blocker NG-mono-methyl-L-arginine (NMMA) for 10 min did not change the cytosolic form of GC activity. The results obtained reveal that in the rat pineal, the down-regulation of the nocturnally elevated GC activity does not appear to be adrenergically mediated.