Abstract

The binding of the rigid dopamine analogue [³H]amino-6,7-dihydroxy-1,2,3,4-tetrahydronapthalene ([³H]ADTN) to rat striatal membranes was characterized in the presence and absence of purine nucleotides. At 37°, when assays were carried out in the presence of ATP or GTP, the binding of [³H]ADTN increased for approximately 5 min and then progressively declined. However, when assays were carried out at 20°, [³H]ADTN binding reached equilibrium and was stable in both the absence and presence of purine nucleotides. Scatchard analysis of binding isotherms showed that the affinity of the binding sites for [³H]ADTN decreased by approximately 3-fold in the presence of either 0.3 mM ATP or GTP. An unexpected finding was that the density of binding sites increased by approximately 5-fold. In contrast, the nonhydrolyzable purine nucleotide analogues guanylylimidodiphosphate and adenylylimidodiphosphate did not affect the binding of [³H]ADTN. The pharmacological specificity of [³H]ADTN binding, determined either in the absence or presence of ATP or GTP, was characteristic of binding to dopamine receptors in that dopamine, epinine, and (±)-ADTN were more potent than (-)-norepinephrine or (-)-epinephrine in inhibiting [³H]ADTN binding, and binding was inhibited by a variety of neuroleptics including (+)-butaclamol and (α)-flupenthixol. However, there were effects of purine nucleotides on the affinities of the receptor for the partial agonist apomorphine and for a variety of antagonists. In the presence of nucleotides, antagonists were 2 to 10 times less potent in inhibiting [³H]ADTN binding, whereas apomorphine was 300-fold less potent. Exposure of striatal membranes to UV irradiation (λ_max = 310 nm) for 30 min reduced [³H]ADTN binding observed in the absence of nucleotide by 60% and eliminated the increase in binding observed in the presence of nucleotide. Neither [³H]spiroperidol binding nor dopamine- or ADTN-stimulated adenylate cyclase activity was affected by this treatment. The results suggest that, in the presence of ATP or GTP, [³H]ADTN binds to a second class of binding sites in the striatum. These additional sites (B_max ∼ 20 pmoles/mg of protein) are not associated with either [³H]spiroperidol binding or dopamine-stimulated adenylate cyclase activity.