Reduced light responsiveness of the cone pathway during prolonged darkness does not result from an increase of dopaminergic activity in the fish retina.

Dopaminergic activity in the fish retina during prolonged darkness was analyzed by monitoring total dopamine (DA) content, dihydroxyphenylacetic acid (DOPAC) and endogenous release of DA. Whereas DOPAC values drop to a third within the first 90 min of darkness, the releasable pool of DA increases by a third during this period. Endogenous release of DA drops to about 25% within the first 30 min and remains at this low level during continuous darkness. These data demonstrate that the electrophysiologically observed sensitivity reduction of horizontal cells during prolonged darkness is not due to an increase of dopaminergic activity during this period.