Light-induced stimulation of retinal dopamine: a dose-response relationship.

Light stimulates dopamine (DA) release in the retina. The purpose of this study was to determine the threshold and dose-response relationship between ocular light exposure and retinal DA synthesis in vivo. Groups of dark-adapted rats were exposed to 0, 1, 3, 5, 10, 25, 50, 100 or 1000 microwatts per square centimeter (microW/cm2) of white light for 15 min. Retinal DA and dihydroxyphenylalanine (DOPA) were subsequently quantified by liquid chromatography with electrochemical detection. Both the DA and DOPA data fit hyperbolic curves significantly (P less than 0.01). Exposure to white light at 25 microW/cm2 or greater appears to elicit the maximum response of these neurons. Threshold irradiation is calculated to be 3-5 microW/cm2. These results indicate that retinal DA synthesis and presumably DA neuron activity have a graded response to increasing irradiances of white light.