Sensitization of G Protein-Coupled Benzodiazepine Receptors in the Striatum of 6-Hydroxydopamine-Lesioned Rats
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The nonselective benzodiazepine (BZ) agonist diazepam is a potent inhibitor of adenylyl cyclase (AC) activity in the rat striatum. To examine this inhibitory action of diazepam further, its effects were examined in 6-hydroxydopamine-lesioned animals, which reportedly exhibit sensitization of the striatal AC pathway. As previously observed, inhibition of AC activity by diazepam was biphasic, with the first phase being receptor-mediated, whereas the second phase involves a direct action on the enzyme itself. In the presence of NaCl (120 mM), a marked sensitization to the receptor-mediated inhibitory effect of diazepam on AC activity was observed in striatal membranes of lesioned animals. EC50 values were 10.4 +/- 1.1 and 4.8 +/- 0.9 nM (p < 0.05) for intact and lesioned striata, respectively. An examination of [3H]diazepam binding revealed a significant increase in the density of binding sites in denervated striata, with no change in affinity. A time-dependent increase in [alpha-32P]GTP labeling of two distinct striatal proteins with apparent molecular masses of 40 and 45 kDa, suggestive of the alpha subunits of Gi and Gs, respectively, was observed. There was a significant increase in basal [alpha-32P]GTP binding to both proteins in lesioned striata. In addition, diazepam stimulated [alpha-32P]GTP binding to the 40-kDa protein, especially in lesioned striata. These data indicate that the sensitization of the receptor-mediated inhibitory effect of diazepam on AC activity in denervated striata may involve up-regulation of BZ receptors as well as enhanced functional coupling of these receptors to inhibitory G proteins.
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