Modulation of dopamine-mediated changes in horizontal cell receptive field organization in the mudpuppy retina.

Abstract

Light adaptation allows the retina to change its functional range so that it is appropriate for the ambient light conditions. There are many mechanisms of light adaptation in the retina, one of which is the uncoupling of horizontal cells. This dissertation investigates which type of photoreceptor mediates light-evoked uncoupling and the role of acetylcholine in modulating horizontal cell uncoupling in mudpuppy (Necturus maculosus). In order to determine whether rods or cones mediate light-evoked uncoupling of horizontal cells, the relative effectiveness of adapting lights of different wavelengths was measured in dark-adapted retinas. Diffuse blue or red adapting lights were adjusted in intensity to be equally effective for rods or cones. When adapting lights were rod-matched the uncoupling effect of the red light was significantly greater than that of the blue light, but when adapting lights were cone-matched the effect of blue light was not significantly greater than that of red light. However, the adapting lights elicited both rod- and cone-driven responses in horizontal cells. The results suggest that the light-evoked uncoupling of horizontal cells in mudpuppy is mainly due to activation of cones, although a small contribution from rods could not be ruled out. Since it was previously shown that light-evoked uncoupling of horizontal cells is mediated by an increase in dopamine release, these results suggest that light evoked dopamine release is due mainly to activation of cones. Cholinergic modulation of horizontal cell coupling was also studied. The cholinergic agonist carbachol and the selective nicotinic agonist, 1,1-dimethyl-4-phenylpiperazinium (DMPP), uncoupled the horizontal cells, but the muscarinic agonist, oxotremorine, did not. The uncoupling effects of carbachol and DMPP were blocked by the dopamine receptor antagonist fluphenazine, and carbachol also caused an increase in the release of $\sp3$H-dopamine from retinas. These results indicate that carbachol uncoupled horizontal cells by stimulating dopamine release via nicotinic receptors. d-Tubocurarine increased horizontal cell coupling, indicating that tonic cholinergic input was present in dark-adapted retinas. However, d-tubocurarine did not reduce light-evoked uncoupling of horizontal cells, suggesting that cholinergic neurons are not an essential part of the direct pathway by which light causes an immediate increase in dopamine release.