Evidence for local synthesis of melatonin in golden hamster retina

Daily variations in melatonin content of retinas of pinealectomized and 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-operated animals). Moreover, mean retinal melatonin levels augmented significantly after pinealectomy. In vitro the augmented melatonin levels found in retinas incubated in darkness for 8 h was suppressed by exposure to light, indicating the ability of hamster retina to regulate melatonin synthesis in isolated conditions. Taken together, the in vivo and in vitro results support daily variations of melatonin content of exclusive retinal origin.