Sensitization of G Protein‐Coupled Benzodiazepine Receptors in the Striatum of 6‐Hydroxydopamine‐Lesioned Rats
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Abstract: 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 [α‐32P]GTP labeling of two distinct striatal proteins with apparent molecular masses of 40 and 45 kDa, suggestive of the α subunits of Gi and Gs, respectively, was observed. There was a significant increase in basal [α‐32P]GTP binding to both proteins in lesioned striata. In addition, diazepam stimulated [α‐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.
