D2-dopamine receptor blockade modulates temporal resolution in goldfish

A possible effect of dopamine on the temporal resolution of goldfish was investigated in a behavioral, two-alternative, forced-choice procedure. Flicker fusion frequency (FFF) was measured before and after bilateral intravitreal injections of D1- or D2-dopamine receptor (D1-/D2-R) antagonists, or after depletion of retinal dopamine by bilateral intravitreal injections of the dopaminergic neurotoxin 6-hydroxydopamine (6-OHDA). Prior to drug injections, fish achieved FFFs of 33-39 Hz. A D1-R antagonist, SCH 23390, reduced FFF by about 12% (P > 0.1). whereas a D2 antagonist, sulpiride, reduced the relative FFF by 25% (P < 0.03). Depletion of retinal dopamine with 6-OHDA induced a gradual reduction in the FFF to a maximal reduction of 50% (P < 0.001) at 2 weeks postinjection. There was recovery to control levels after 3-4 weeks postinjection. The recovery of FFF, at least in one animal, was due to the return of retinal dopamine because FFF could be reduced by intravitreal injections of sulpiride during the recovery phase. These experiments demonstrate that retinal dopamine, particularly acting on D2-R, is important for photopic temporal resolution.