Abstract

We have previously reported that, 30 min after a single injection of 7.5 mg/kg d-amphetamine sulfate, there was a significant 25% decrease in the apparent Vmax for stimulation of adenylate cyclase activity by the D1 receptor-selective partial agonist SKF 38393 in rat striatal membranes, as compared with saline-injected controls. This desensitization was seen in the striatal but not the mesolimbic forebrain. In the present study this desensitization was further characterized by using various ligands that interact with the three components of the D1 receptor-coupled adenylate cyclase complex to determine the site of modification that resulted in the desensitization. The desensitization was not associated with a change in the stimulation of adenylate cyclase at the level of the catalytic subunit or the guanyl nucleotide-regulatory protein Ns. Receptor number, as assessed by the binding of the D1 selective antagonist [3H]SCH 23390, was unaltered in the desensitized state. In contrast, the number of high affinity binding sites, as measured with the agonist [3H]dopamine was decreased 30% by acute amphetamine exposure. This suggests that the amphetamine-induced desensitization may be the result of an uncoupling of the receptor from Ns. In order to further assess the effects of amphetamine on receptor/Ns coupling, we measured the ability of the guanyl nucleotide guanosine-5'-(beta,gamma-imido)triphosphate [Gpp(NH)p] to decrease high affinity [3H]dopamine binding to striatal membranes. The inclusion of 100 microM Gpp(NH)p in the assay decreased the number of receptors in the high affinity state by 40% and 52% in membranes from saline- and amphetamine-pretreated rats, respectively. These results imply that amphetamine treatment does not modify the ability of Gpp(NH)p to decrease high affinity agonist binding. It is possible that amphetamine treatment decreases the number of receptors that can couple to Ns but the remaining receptors can still form a high affinity complex and are sensitive to the effects of Gpp(NH)p. We also report that maximal D2 dopamine receptor-mediated inhibition of forskolin-stimulated adenylate cyclase activity was decreased in striatal membranes prepared from amphetamine-treated rats as compared with saline-injected controls, implying that the D2 pathway was desensitized by amphetamine treatment. Conversely, acute amphetamine injection did not alter the ability of either the adenosine A2 receptor to stimulate or the muscarinic cholinergic receptor to inhibit adenylate cyclase activity in the rat striatum. These results suggest that acute amphetamine treatment produces a dopamine receptor-specific or homologous desensitization.