Diurnal rhythm in rat pineal cyclic nucleotide phosphodiesterase activity

Abstract

MANY substances in the rat pineal organ exhibit marked diurnal variations; some of these fluctuations are circadian, that is, cued but not driven by environmental lighting. The content of the pineal hormone melatonin¹ for example, is seven- to tenfold higher during the dark phase of the diurnal cycle than during the light phase². Similarly, the activities of the melatonin-synthesising enzymes 5-hydroxytryptarnine-N-acetyl transferase (NAT) and hydroxyindole-O-methyl transferase (HIOMT) are also higher in the dark (by factors of 15–70-fold and threefold, respectively^3,4). The immediate precursor of melatonin, N-acetyl 5-HT is 10–30-fold higher in the dark than in the light⁵. In contrast the pineal content of the precursor of N-acetyl 5-HT, 5-HT is twofold higher in the light than in the dark phase of the diurnal cycle⁶, suggesting that changes in NAT activity regulate changes in melatonin biosynthesis. These changes have been shown to be controlled by environmental light acting through the retina by way of central nervous pathways to the superior cervical ganglion, and thence through postganglionic sympathetic fibres to the pineal body⁷. Nor-adrenaline released from these fibres acts on β adrenoceptors on the parenchymal pineal cells to stimulate adenylate cyclase, causing an increase in intracellular cyclic AMP which is followed by increased NAT activity and melatonin biosynthesis^8,9. Decentralisation, superior cervical ganglionectomy, administration of β-adrenoceptor blocking drugs or protein synthesis inhibitors can all block the light–dark changes observed in NAT activity in this organ^10,11.