Pharmacological and biochemical characterization of the D-1 dopamine receptor mediating acetylcholine release in rabbit retina

Superfusion with dopamine (0.1 μM-10 mM) evokes calcium-dependent [³H]acetylcholine release from rabbit retina labeled in vitro with [³H]choline. This effect is antagonized by the D-1 dopamine receptor antagonist SCH 23390. Activation or blockade of D-2 dopamine, alpha-2 or beta receptors did not stimulate or attenuate the release of [³H]acetylcholine from rabbit retina. Dopamine receptor agonists evoke the release of [³H]acetylcholine with the following order of potency: apomorphine ≥ SKF(R)82526 > SKF 85174 > SKF(R)38393 ≥ pergolide ≥ dopamine (EC₅₀ = 4.5 μM) > SKF(S)82526 ≥ SKF(S)38393. Dopamine receptor antagonists inhibited the dopamine-evoked release of [³H]acetylcholine: SCH 23390 (IC⁵⁰ = 1 nM) > (+)-butaclamol ≥ cis-fluphenthixol >fluphenazine > perphenazine > trans-flupenthixol > R-sulpiride. The potencies of dopamine receptors agonists and antagonists at the dopamine receptor mediating [³H]acetylcholine release is characteristic of the D-1 dopamine receptor. These potencies were correlated with the potencies of dopamine receptor agonists and antagonists at the D-1 dopamine receptor in rabbit retina as labeled by [³H]SCH 23390, or as determined by adenylate cyclase activity. [³H]SCH 23390 binding in rabbit retinal membranes was stable, saturable and reversible. Scatchard analysis of [³H]SCH 23390 saturation data revealed a single high affinity binding site (K(d) = 0.175 ± 0.002 nM) with a maximum binding of 482 ± 12 fmol/mg of protein. The potencies of dopamine receptor agonists to stimulate [³H]acetylcholine release were correlated with their potencies to stimulate adenylate cyclase (r = 0.784, P < .05, n = 7) and with their affinities at [³H]SCH 23390 binding sites (r = 0.755, P > .05, n = 8). The potencies of antagonists to inhibit dopamine-evoked [³H]acetylcholine release were correlated with their potencies to inhibit the dopamine-stimulated adenylate cyclase (r = 0.759, P < .05, n = 5) and with their affinities at [³H]SCH 23390 binding sites (r = 0.998, P < .01, n = 7). We conclude that in rabbit retina dopamine evokes calcium-dependent [³H]acetylcholine release through activation of a site with the pharmacological characteristics of a D-1 dopamine receptor.