Control of Hydroxyindole O-Methyltransferase Activity in the Rat Pineal Gland by Environmental Lighting

Abstract

The mammalian plineal rland has been shown to contain several enzymes involved in the formation and metabolism of physiologically active indoles and amines (1-3). The rat plineal (lan(l contains the highest rel)orted tissue levels of serotonin and 5-hvdroxytryptophan decarboxylase (4, 5). In addition, the plineal gland is unique in that it is the only organ that contai ns hydloxvindole O-methvltransferase, an enzyme that converts -acetvlserotonin to melatonin (6). Since N-acetylation of serotonin has been shown to take place in several tissues (7) and since the activity of hdroxvindole O-methvltransferase in the rat l)ineal is relatively low (8), it appears that meth!lation is the rate-limiting step in the formation of melatonin and perhaps other methoxyindoles in this s)ecies. The wveight, morplholog-, and chemical composition of tihe rat p)ineal can be altered by exlosing rats to continuous light or dlarkness; rats kept in light have small plineal glands (9), with decreasedl cytol)lasmic basolphilia, nucleolar size (10), and serotonin content (4). In l)reliminary relorlts it was shown that light also affected the hydroxyin(olle O-methltlransferase activity in the pineal -land; rats kept in constant dark had 3 to 10 times more melatonin-forminl-l, enznyme activitv than rats kept in the light (8). In the hen, explosure to light increased the activity of hy(rloxyindole ()-ncthyltransferase (11). This rel)ort will describe the tinme course of the effect of light an(d darkness on hldroxvindlole O-lmethltransferase activity in the rat l)ineal. It will be shovn that there is a ditirnal cycle in the enzymatic synthesis of melatonin and that this cycle is re,ulated by environmental lihltin. 'lhe information about lighting. reaches the l)ineal land( 1by a neural route ilnv0olving the eves all(l the symlpathetic nervous system.