MELATONIN EFFECT ON [H-3] GLUTAMATE UPTAKE AND RELEASE IN THE GOLDEN-HAMSTER RETINA

The effect of melatonin on [H-3]glutamate uptake and release in the go lden hamster retina was studied. In retinas excised in the middle of t he dark phase, i.e., at 2400 h, melatonin (0.1 and 10 nM) significantl y increased [H-3]glutamate uptake, and this effect persisted in a Ca2-free medium. On the other hand, melatonin significantly increased [H- 3]glutamate release in retinas excised at 2400 h, but this effect was Ca2+ sensitive. Melatonin significantly increased Ca-45(2+) uptake by a crude synaptosomal fraction from retinas of hamsters killed at 2400 h. In retinas excised at 1200 h, melatonin had no effect on [H-3]gluta mate uptake, [H-3]glutamate release, or Ca-45(2+) uptake at any concen tration tested. Cyclic GMP analogues, i.e., 8-bromoguanosine 3',5'-cyc lic monophosphate and 2'-O-dibutyrylguanosine 3',5'-cyclic monophospha te, significantly increased [H-3]glutamate uptake, [H-3]glutamate rele ase, and Ca-45(2+) uptake by tissue removed at 1200 and 2400 h, sugges ting that the effects of melatonin could correlate with a previously d escribed effect of melatonin on cyclic GMP levels in the golden hamste r retina. Taking into account the key role of glutamate in visual mech anisms, the results suggest the participation of melatonin in retinal physiology.