SENSITIZATION OF G-PROTEIN-COUPLED BENZODIAZEPINE RECEPTORS IN THE STRIATUM OF 6-HYDROXYDOPAMINE-LESIONED RATS

The nonselective benzodiazepine (BZ) agonist diazepam is a potent inhi bitor of adenylyl cyclase (AC) activity in the rat striatum. To examin e this inhibitory action of diazepam further, its effects were examine d in 6-hydroxydopamine-lesioned animals, which reportedly exhibit sens itization of the striatal AC pathway. As previously observed, inhibiti on 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 sensi tization 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 [H-3]diazepam bindin g revealed a significant increase in the density of binding sites in d enervated striata, with no change in affinity. A time-dependent increa se in [alpha-P-32] GTP labeling of two distinct striatal proteins with apparent molecular masses of 40 and 45 kDa, suggestive of the alpha s ubunits of G(i) and G(s), respectively, was observed. There was a sign ificant increase in basal [alpha-P-32] GTP binding to both proteins in lesioned striata. In addition, diazepam stimulated [alpha-P-32] GTP b inding to the 40-kDa protein, especially in lesioned striata. These da ta indicate that the sensitization of the receptor-mediated inhibitory effect of diazepam on AC activity in denervated stria ia may involve up-regulation of BZ receptors as well as enhanced functional coupling of these receptors to inhibitory G proteins.