Adaptive changes in the number of G s- and G i-proteins underlie adenylyl cyclase sensitization in morphine-treated rat striatal neurons

Abstract

In the present study we investigated the possible role of changes in the number of membrane-bound G-proteins in the sensitization of dopamine D 1 receptor-stimulated adenylyl cyclase, observed in primary cultures of rat striatal neurons chronically exposed to morphine. Whereas exposure of these neurons to 10 μM morphine for 7 days caused a profound increase in cyclic AMP production, induced by the dopamine D 1 receptor agonist SKF 38393 (1 μM), Scatchard analysis of [ 125I]SCH 23982 binding to membrane preparations revealed that neither the B max nor the K d values of dopamine D 1 receptor binding sites were affected. Interestingly, immunoblotting experiments revealed an increase (of more than 50%) in the number of stimulatory G-proteins (G αs) in neurons displaying an enhanced adenylyl cyclase activity. In morphine-treated neurons, the number of inhibitory G-proteins (G αi) appeared to be slightly reduced (by about 16%). Moreover, the observation that cholera toxin (0.1 nM)-stimulated cyclic AMP production, unlike that induced by forskolin (1 μM), was enhanced in morphine-treated neurons, indicates a causal relationship between the reciprocal changes in G-protein number and the increase of dopamine D 1 receptor-stimulated adenylyl cyclase activity. The possible role of these changes in G-protein number in the development of morphine tolerance and dependence is discussed.