PK 11195 blockade of benzodiazepine‐induced inhibition of forskolin‐stimulated adenylate cyclase activity in the striatum

Abstract

1 The effects of benzodiazepine receptor antagonists on the inhibition of forskolin-stimulated adenylate cyclase (AC) activity by various benzodiazepine (BZ) and indoleamine agonists in the rat striatum were investigated. 2 A biphasic inhibition of forskolin-stimulated AC activity by the peripheral-type agonist, Ro5-4864, and a multiphasic inhibition by the non-selective BZ, diazepam, was observed. One phase of AC inhibition is consistent with a Gi-coupled receptor-mediated action, whereas the other phases appear to involve a direct effect on the enzyme itself. 3 While the central-type antagonist, flumazenil, had no effect on the ability of Ro5-4864 to inhibit AC activity, the peripheral-type receptor ligand, PK 11195, abolished the first phase of inhibition. 4 PK 11195 and pertussis toxin were found to block the inhibitory effect of various BZs and the indoleamines, melatonin and 2-iodomelatonin, on induced AC activity. 5 Saturation binding studies, conducted at 30'C with [3H]-diazepam revealed a single binding site in the rat striatum (KD = 19.3 + 0.80 nM) which significantly decreased in affinity in the presence of GTP (KD = 30.5+2.6 nM; P<0.05). No significant change in B,,,, was observed. 6 These findings indicate the presence of G,-coupled BZ receptors in the rat striatum. Thus, suppression of cyclic AMP production may contribute to the diverse neuropharmacological effects of BZs, melatonin and related drugs.

Sensitization of G Protein‐Coupled Benzodiazepine Receptors in the Striatum of 6‐Hydroxydopamine‐Lesioned Rats

“…In the diazepamstimulated GTP binding experiments, only the 40-kDa band, which presumably represents Ga, was detected, and the intensity of this band increased in lesioned tissue. This is consistent with evidence that G~-coupled BZ receptors are present in the striatum (Tenn et al, 1996) and with the present functional data that indicate a potentiation of BZ receptor-mediated inhibition of AC activity. Because G is in greater abundance than G. in the brain (Goldsmith et al, 1987), this could account for only the G 1a subunit being labeled in the diazepam-stimulated GTP binding experiments, which were conducted for shorter incubation times as compared with basal studies.…”

“…In conclusion, this report augments earlier evidence that novel BZ receptors, coupled to a pertussis toxinsensitive inhibitory G protein (G1), mediate suppression of the AC-cAMP pathway in the brain (Dan'ura et al, 1988;Niles and Hashemi, 1990;Tenn et al, 1996). Earlier studies have shown that the central-type BZ antagonist flumazenil does not block the inhibitory effects of either the peripheral-type agonist Ro 5-4864 or diazepam on AC activity in the brain (Dan' ura et al, 1988;Niles and Hashemi, 1990).…”

“…Moreover, central-type BZ agonists, acting via BZ-GABAA receptors, can suppress dopaminergic activity in the striatum of 6-OHDA-lesioned animals (Tenn and Niles, 1995). However, it is unlikely that this interaction is involved in the functional sensitization presently described, because the central-type BZ receptors on the BZ-GABAA receptor complex are quite distinct from the G protein-coupled BZ receptors, which mediate inhibition of AC activity (Tenn et al, 1996).…”

“…There is increasing evidence that another BZ receptor mediates inhibition of adenylyl cyclase ATP pyrophosphate-lyase (cyclizing); EC 4.6.1.1] (AC) activity in the CNS (Dan'ura et al, 1988;Niles and Hashemi, 1990). This BZ receptor is coupled to a pertussis toxin-sensitive G protein, presumably G, (Dan'ura et al, 1988;Tenn et al, 1996). It is interesting that the PBR agonist Ro5-4864 inhibits AC via this receptor, which is not affected by the central-type BZ antagonist fiumazenil (Dan'ura et al, 1988;Niles and Hashemi, 1990) but is blocked by the PBR ligand PK1 1195 (Tenn et al, 1996).…”

“…This BZ receptor is coupled to a pertussis toxin-sensitive G protein, presumably G, (Dan'ura et al, 1988;Tenn et al, 1996). It is interesting that the PBR agonist Ro5-4864 inhibits AC via this receptor, which is not affected by the central-type BZ antagonist fiumazenil (Dan'ura et al, 1988;Niles and Hashemi, 1990) but is blocked by the PBR ligand PK1 1195 (Tenn et al, 1996). These findings, together with evidence that PBRs may be present on nonmitochondrial membranes (Olson et al, 1988;Mukherjee and Das, 1989), suggest that G protein-coupled BZ receptors are a PBR subtype localized in the plasma membrane.…”