SIMILAR EFFECTS OF CARBACHOL AND DOPAMINE ON NEURONS IN THE DISTAL RETINA OF THE TIGER SALAMANDER

Though there is considerable evidence that dopamine is an important re tinal neuromodulator that mediates many of the changes in the properti es of retinal neurons that are normally seen during light adaptation, the mechanism by which dopamine release is controlled remains poorly u nderstood. In this paper, we present evidence which indicates that dop amine release in the retina of the tiger salamander, Ambystoma tigrinu m, is driven excitatorily by a cholinergic input. We compared the effe cts of applying carbachol to those of dopamine application on the resp onses of rods, horizontal cells, and bipolar cells recorded intracellu larly from the isolated, perfused retina of the tiger salamander. Micr omolar concentrations of dopamine reduced the amplitudes of rod respon ses throughout the rods' operating range. The ratio of amplitudes of t he cone-driven to rod-driven components of the responses of both horiz ontal and bipolar cells was increased by activation of both D1 and D2 dopamine receptors. Dopamine acted to uncouple horizontal cells and al so off-center bipolar cells, the mechanism in the case of horizontal c ells depending only upon activation of D1 receptors. Carbachol, a spec ific cholinomimetic, applied in five- to ten-fold higher concentration s, produced effects that were essentially identical to those of dopami ne. These effects of carbachol were blocked by application of specific dopamine blockers, however, indicating that they are mediated seconda rily by dopamine. We propose that the dopamine-releasing amacrine cell s in the salamander are under the control of cells, probably amacrine cells, which secrete acetylcholine as their transmitter.