Binding sites for [3H]SCH 23390 in retina: properties and possible relationship to dopamine D1-receptors mediating stimulation of adenylate cyclase.

Bovine, rat and chick embryo retinal membranes contain high affinity, saturable and stereospecific binding sites for the selective dopamine D1-receptor antagonist, [3H]SCH 23390 (R-(+)-8-chloro-2,3,4,5-tetrahydro-3- methyl-5-phenyl-lH-3-benzazepin-7-ol). Saturation studies and Scatchard analyses showed a single class of [3H]SCH 23390 binding sites with Kd (apparent dissociation constant) values of 0.5-1.4 nM for the different species studied. A high ratio of specific to non-specific binding was found over a wide range of radioligand concentrations. The Bmax (binding site number) for [3H]SCH 23390 in calf retina was 307 +/- 38 fmol/mg protein, significantly greater than Bmax values previously obtained for binding of [3H]spiroperidol and [3H]ADTN (2-amino-6,7-dihydroxy-1,2,3,4-tetrahydronaphthalene) to dopamine D2-receptors in calf retina. Relative affinities (Ki values) of dopamine antagonists for calf retinal [3H]SCH 23390 binding sites were similar to those reported for [3H]SCH 23390 binding in rat striatum and also were in general agreement with potencies for antagonism of retinal dopamine-stimulated adenylate cyclase. However, they differed markedly from the relative affinities for retinal D2-receptor sites. Additional data indicated that SCH 23390 did not bind significantly to retinal D2- or serotonergic receptors and had 30- to 80-fold less affinity for alpha 2-noradrenergic than for the [3H]SCH 23390 sites. Competition studies indicated a high degree of selectivity for dopamine agonist over other agonists for [3H]SCH 23390 binding sites with Ki values in the range expected for interaction with dopamine receptors mediating stimulation of adenylate cyclase. Affinity for dopamine was decreased in the presence of the GTP analogue, Gpp(NH)p. In the presence of sodium ions the affinities of dopamine agonists for [3H]SCH 23390 binding sites were markedly and selectively decreased; the sensitivity to dopamine for stimulation of adenylate cyclase activity was also decreased in the presence of sodium ions. Modulation by sodium ions was previously observed for D2- but not for a D1-receptor interaction. It is proposed that [3H]SCH 23390 binds to a unique class of receptors, most likely D1-receptors coupled to adenylate cyclase in retina. [3H]SCH 23390 provides a potent new tool for study of these receptors. In retina D1-receptors positively coupled to cyclase as well as D2- and other receptors that may be negatively coupled to cyclase, appear to be regulated by sodium ions as well as by guanine nucleotides.