EVIDENCE FOR LOCAL SYNTHESIS OF MELATONIN IN GOLDEN-HAMSTER RETINA

DAILY variations in melatonin content of retinas of pinealectomized an d sham-operated golden hamsters were studied. Melatonin content showed significant daily variations with maximal values at night (i.e. early in the night in pinealectomized hamsters and late at night in sham-op erated animals). Moreover, mean retinal melatonin levels augmented sig nificantly after pinealectomy. In vitro the augmented melatonin levels found in retinas incubated in darkness for 8 h was suppressed by expo sure to light, indicating the ability of hamster retina to regulate me latonin synthesis in isolated conditions. Taken together, the in vivo and in vitro results support daily variations of melatonin content of exclusive retinal origin.