The effects of dopamine on isolated retinal cell responses to glutamate

Dopamine is known to enhance glutamate-induced currents in retinal horizontal and OFF bipolar cells. The impact of this dopaminergic modulation of glutamate responses on membrane potential is less well understood. Glutamate responses of dissociated retinal cells were studied with the voltage probe oxonol, DiBaC4(5). In general, glutamate voltage responses were found to increase in the presence of dopamine or selective agonists. Effects of dopamine antagonists were both consistent and prominent, but presented a more complex picture. Dopamine and the D1 or D2 antagonists further increased glutamate voltage responses. These results imply at least two dopamine receptor mechanisms with different D1/D2 affinities. One possibility for a second site of action is dopamine modulated potassium currents. Such currents tend to reduce the voltage amplitude of glutamate responses. Inhibition of these currents by dopamine antagonists might further augment glutamate voltage responses.