N-acetyltransferase activity responds to environmental lighting in the eye as well as in the pineal gland

Abstract

N-acetyltransferase (NAT), which converts serotonin to N-acetylserotonin, is widely distributed in the vertebrate body^1,2. In the pineal gland, however, NAT activity shows circadian changes which respond to environmental lighting both in vivo and in vitro^3,4. NAT is responsible for rhythmic production of the pineal hormone melatonin, which functions in body colour changes⁶, photoperiodic control of reproduction^7,8, and behavioural circadian rhythms^9,10. The rhythmic changes in NAT have so far been thought unique to the pineal gland. Here we report data for ocular NAT showing a change in a daily light–dark cycle and modification by environmental lighting. The finding of light modified NAT in the eye is significant because it may explain the failure of some investigators¹¹ to find effects of pinealectomy and it may also explain the occurrence of rhythmic melatonin in the blood of some species after pinealectomy^12–14. The prerequisites for the production of melatonin exist in the eye as well as in the pineal gland: serotonin¹⁵, hydroxyindole-O-methyltransferase activity¹⁶ and N-acetylindole¹⁷ are present. Furthermore, labelled serotonin is converted to melatonin by the retina in vitro¹⁸. Our finding of NAT and its modification by light and dark completes the argument for ocular capacity for melatonin production in a cyclic manner modified by environmental lighting.