Perturbation of benzodiazepine receptor binding by pyrazolopyridines involves picrotoxinin/barbiturate receptor sites

The two pyrazolopyridines, etazolate (SQ20009) and cartazolate (SQ65396), enhance the binding of [³H]diazepam to benzodiazepine receptor sites in rat brain. This enhancement is due to a change in affinity without a change in maximal binding. Pentobarbital also enhances [³H]diazepam binding by lowering the K(D). Pentobarbital gives a maximal enhancement of benzodiazepine binding slightly greater than that induced by etazolate, but maximal concentrations of etazolate cannot increase further the maximal enhancement by pentobarbital. By contrast, the enhancement of benzodiazepine binding by γ-aminobutyric acid (GABA) is fully additive with the effect of either etazolate or pentobarbital. The enhancement by γ-aminobutyric acid is blocked specifically by the two γ-aminobutyric acid mimetic compounds, imidazole acetic acid and 4,5,6,7-tetrahydroisoxazolo [5,4-c]pyridine-3-ol (THIP), compounds which do not inhibit the enhancement of diazepam binding by pentobarbital or etazolate. The effects of etazolate and pentobarbital, but not those of GABA, can be blocked competitively by 1 to 10 μM picrotoxinin. Etazolate and cartazolate, like pentobarbital competitively inhibit α-[³H]dihydropicrotoxinin binding with potencies correlating well with their stimulatory properties on benzodiazepine binding. These results suggest that pyrazolopyridines, like pentobarbital, stimulate [³H]diazepam binding through a picrotoxinin-sensitive site which is distinct from the γ-aminobutyric acid receptor site. These in vitro interactions between the etazolate and benzodiazepine receptor sites can be modulated also by the GABA antagonist, bicuculline, consistent with the existence of a complex containing three receptors (GABA, benzodiazepines, and etazolate/barbiturates/picrotoxinin). Bicuculline reverses completely the effects of etazolate and GABA on benzodiazepine binding but reverses only partially the enhancement by pentobarbital. This difference between the effects of pentobarbital and etazolate, as well as differences in their maximal enhancing effects mentioned above, suggests that pentobarbital may have an action similar to etazolate on some bicuculline-sensitive benzodiazepine binding sites, but, in addition, pentobarbital can enhance some other benzodiazepine sites in a manner which is neither mimicked by etazolate nor blocked by bicuculline.