COMPARISON OF THE EFFECTS OF FLICKERING AND STEADY LIGHT ON DOPAMINE RELEASE AND HORIZONTAL CELL COUPLING IN THE MUDPUPPY RETINA

1. The effects of flickering adapting illumination (repetitive flashes) on horizontal cell responses to illumination of the center and surround portions of the receptive field were compared with those of steady adapting illumination in dark-adapted mudpuppy retinas. 2. Exposure to flickering adapting light caused an increase in amplitude of responses to small spots in the receptive-field center and a decrease in the response to a concentric annulus. This is interpreted as due to an increase in coupling resistance between horizontal cells. 3. The uncoupling effect of flickering adapting light was no greater than that of the same quantity of steady adapting light at the same intensity, even when the rate of flickering was varied by a factor of 10. 4. The uncoupling effect of flickering light was blocked by the dopamine antagonists fluphenazine and SCH23390, indicating that it is mediated by dopamine release. 5. The uncoupling effect of flickering light was also blocked in the presence of 2-amino-4-phosphonobutyrate (APB), which prevents light responses of on-center but not off-center bipolar cells, suggesting that flickering light increases dopamine release via the on-pathway. 6. The gamma-aminobutyric acid (GABA) antagonist bicuculline had an uncoupling effect similar to that of adapting illumination. This effect was blocked by dopamine antagonists, indicating that there is tonic GABA-mediated inhibition of dopamine release in mudpuppy retina similar to that previously reported by others in fish and turtle retinas. 7. The uncoupling effect of bicuculline was not reversed by APB. However, APB alone caused an increase in coupling that was rapidly reversed by bicuculline. This indicates that the effect of APB on horizontal cell coupling was not due to a direct action of APB on dopamine-releasing cells. 8. The results indicate that flickering light affects horizontal cell coupling via the same mechanism (increased release of dopamine) and pathway (via on-center bipolar cells) as shown previously for steady light. Although the magnitude of this effect of flickering light may depend on the parameters of the flickering light stimulus, there is no evidence that its uncoupling action on horizontal cells involves a separate pathway or transmitter mechanism.