Dopamine turnover in rat retina: a 24-hour light-dependent rhythm.

To examine the pattern of retinal dopamine (DA) turnover during 24 h, DA and 3,4-dihydroxyphenylacetic acid (DOPAC) were measured at 2-h intervals in rats kept in a regular 12-h light-dark cycle. DA and DOPAC were higher during the light and lower during the dark hours. These rhythmic variations were abolished and DA and DOPAC remained low during 24 h of light deprivation. Findings indicate that DA turnover in the retina is accelerated during the light and suppressed during the dark phases of the diurnal light-dark cycle. These fluctuations in retinal DA neuronal activity are not truly circadian but are dependent on environmental lighting as an external stimulator.