Abstract
We investigated the effects of dopamine and its agonists and antagonists on the receptive field properties of ganglion cells in the isolated eyecup preparation of the rabbit. In general, dopamine (20–250 μM) reduced the overall sensitivity of ganglion cells to light stimuli while increasing the spontaneous activity of off-center cells and decreasing the spontaneous activity of on-center cells and on-off directionally selective cells. Neither ( − )-apomorphine (8–82 μM) nor the selective D-2 agonist LY 141865 (7–85 μM) mimicked the effects of exogenous dopamine. Instead, both drugs altered the responses of ganglion cells in a manner similar to that of the selective D-1 antagonist SCH 23390. The latter at 4–41 μM: (1) selectively reduced the antagonistic surround responses of off-center cells; (2) changed the sustained excitatory responses of on-center sustained cells to spots of light into sustained inhibitory responses; (3) selectively reduced the leading edge responses of on-off directionally selective cells to moving light stimuli, and (4) decreased the spontaneous activity of off-center cells while increasing the spontaneous activity of on-center cells. The effects of the selective D-2 antagonist S-sulpiride (37–116 μM) on the responses of on-center cells resembled those of exogenous dopamine, while for off-center cells the effects of S-sulpiride were similar to those of ( − )-apomorphine and LY 141865. Results were compared with those obtained previously with dopamine antagonists haloperidol, fluphenazine and cis-flupenthixol 14 on ganglion cell responses in the intact rabbit eye. These three drugs were clearly acting at D-1 receptors. The present findings support a physiological role for D-2 receptors in visual processing in the rabbit retina, in particular the hypothesis that endogenous dopamine release is modulated by inhibitory D-2 autoreceptors. 4,5 They also suggest that one function of dopaminergic neurons may be to modulate the sensitivity of ganglion cells to light stimuli.
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