Effects of submicromolar concentrations of dopamine on photoreceptor to horizontal cell communication

Dopamine has been postulated to act as an intraretinal messenger for light adaptation by biasing retinal circuits to favor cone over rod inputs to second- and third-order neurons. As an experimental test, we studied the effects of dopamine and related ligands on rod to horizontal cell synaptic transfer. Rods and horizontal cells (HC) were recorded from simultaneously. Red and green light-emitting diodes were modulated sinusoidally in counterphase at 1 Hz and their relative intensities adjusted to elicit a rod null. Dark-adapted HC's also showed a null, but exposure to 0.5-1.0 microM dopamine, which corresponds to the endogenous levels, elicited a large imbalance in the HC response while the rod null was maintained. Similar effects were achieved with either a D1 dopamine agonist, SKF 38393 (20 microM) or a D2 dopamine agonist, quinpirole HCl (1 microM). Correspondingly, the frequency range over which the HC responded to red flickering lights increased very substantially. Exposure to a D2, but not a D1 dopamine agonist increased rod flicker, but not as dramatically as in the HC. These data indicate that the synaptic gains of rod and cone transmission to a second order retinal neuron are modifiable by endogenous levels of dopamine. Secondly, the bandpass of rod flicker is altered by dopamine, acting through a D2 dopamine receptor.