Action of pyrazolopyridines as modulators of [ 3H]flunitrazepam binding to the GABA/benzodiazepine receptor complex of the cerebellum

Abstract

The pyrazolopyridines etazolate (SQ 20009) and cartazolate (SQ 65396) have strong modulatory effects on the GABA/benzodiazepine receptor complex of rat cerebellum. Thus, etazolate and cartazolate directly stimulate [ 3H]flunitrazepam binding (with EC 50 values of 1.2 μM and 0.3 μM respectively) by increasing the apparent affinity of [ 3H]flunitrazepam for its binding sites. Stimulation of [ 3H]flunitrazepam binding by pyrazolopyridines is dependent on the presence of certain anions like chloride, bromide, iodide, nitrite, nitrate but not fluoride, acetate, formate or sulfate. It is inhibited by bicuculline-methiodide, and by the ‘chloride channel drugs’ picrotoxinin and IPTBO. isoTHAZ, a GABA analogue with GABA antagonist properties in vivo, fails to inhibit binding stimulated by etazolate but antagonizes [ 3H]flunitrazepam binding stimulated by GABA. The pyrazolopyridines have also indirect effects on benzodiazepine receptor binding since they enhance the apparent sensitivity of those GABA recognition sites which are coupled to benzodiazepine binding sites. Thus, in the presence of 10 μM etazolate, GABA and muscimol enhance [ 3H]flunitrazepam binding, with EC 50 values of 109 nM and 12 nM respectively. This sensitization effect is partially dependent on the presence of chloride ions. The pyrazolopyridines facilitate also the stimulation of benzodiazepine receptor binding by β-alanine and taurine and by the rigid and flattened GABA analogues THIP and piperidine-4-sulfonic acid. Taken together, these results suggest that the pyrazolopyridines modulate [ 3H]flunitrazepam binding by acting at a site closely related to GABA receptor-regulated chloride ion channels.