Tritiated dopamine was used to label the dopamine receptor in membranes isolated from the rat corpus striatum. Scatchard analysis of displacement of [3Hldopamine by nonradioactive dopamine indicated the presence of two binding sites. The similarities in affinity, capacity, and drug specificity of the high-affinity site in the striatal membranes from rat and the binding site in the membranes from the calf caudate nucleus suggest that [3Hjdopamine labels the same site in both species. In order to determine what conformation of dopamine is preferred at the dopamine receptor site, conformationally restricted analogs of dopamine-namely, the cis and trans 2-amino-1(3,4-dihydroxyphenyl)cyclobutane hydrochlorides-were tested for their affinity to the receptor. Compared to the cis conformation, the trans-restricted analogs had more affinity for the receptor site, indicating that dopamine probably interacts with the receptor in the trans conformation.There is considerable interest in the relationship between conformation of neurotransmitters at their receptors and biological activity (1, 2). The putative neurotransmitter dopamine is probably involved in parkinsonism (3), Huntington chorea (4), drug-induced dyskinesia (5, 6), schizophrenia (7), and manic depressive psychosis (8,9). The preferred conformation of dopamine in the solid state is the trans extended form, and in solution it exists primarily in the trans and gauche conformations with the trans form being slightly predominant (10, 11).However, it is realized that the preferred conformation of dopamine in the dopamine-receptor complex need not be the same as that in solution or in the solid state (11, 2). Nevertheless, investigations with dopamine-sensitive adenylate cyclase support the idea that the trans extended form of dopamine interacts with the dopamine receptor. Among the semirigid analogs of dopamine, the compounds with the greatest potency are those with the ethylamine side chain of dopamine in the fully extended form, as in apomorphine and 2-amino-6,7-dihydroxytetralin (12)(13)(14)(15)(16). On the other hand, there are significant differences indrugaffinity,subcellular localization, and ontogenetic developmental pattern between the dopamine receptor recognition site and adenylate cyclase (17)(18)(19)(20). Thus, the significance of the dopamine-sensitive adenylate cyclase data in relation to dopamine-mediated behaviors is not known.Apomorphine and 2-amino-6,7-dihydroxytetralin have an affinity for the dopamine receptor binding site as great or greater than that of dopamine (21). This affinity, however, may not be due to these compounds having the dopamine moiety in a fully extended form, because no binding study has been carried out comparing the affinity of isomeric compounds that possess a cis and trans relationship between the catechol and amino groups found in dopamine. Therefore, to determine the preferred conformation of dopamine at its receptor site, we Ci/mmol, New England Nuclear) which was added in 100 gl of 0.1% ascorbic acid, 1.8 ml of...
