title: Modulation of the chloride ionophore by benzodiazepine receptor ligands: influence of gamma-aminobutyric acid and ligand efficacy.
abstract: t-Butylbicyclophosphorothionate (TBPS) produces dose-dependent enhancement of [3H]propyl beta-carboline-3-carboxylate ([3H]PCC, 40 pM) binding to the benzodiazepine1 (BZ1) receptor subtype in hippocampus. Furthermore, TBPS enhancement of [3H]PCC binding was antagonized by micromolar concentrations of gamma-aminobutyric acid (GABA) in a way reversible by bicuculline. BZ receptor ligands that are "GABA positive" (i.e., enhance GABA neurotransmission) allosterically inhibited [35S]TBPS binding, whereas "GABA-negative" ligands (i.e., inhibit GABA neurotransmission) produced the opposite effect. The efficacy of the ligands as modulators of [35S]TBPS binding was consistent with their reported in vivo pharmacology. The effects of positive and negative ligands on [35S]TBPS binding were modulated by micromolar concentrations of GABA. Examination of the kinetics of [35S]TBPS binding suggested the presence of slowly and rapidly dissociating components. The GABA-positive clonazepam stabilized the rapidly dissociating component of [35S]TBPS binding, whereas methyl beta-carboline-3-carboxylate had a similar effect on the slowly dissociating component. It is speculated that the slowly dissociating component of [35S]TBPS binding is associated with a closed chloride channel, whereas the opposite is proposed for the rapidly dissociating component. The differential effects of GABA-positive versus GABA-negative ligands on [35S]TBPS binding and the modulatory effect of GABA provide further evidence to suggest that [35S]TBPS labels a site near the chloride ionophore linked to the GABA-BZ receptor complex.
mesh_terms: D000818:Animals; D001643:Bridged Bicyclo Compounds / Q000378:metabolism* / Q000494:pharmacology; D019086:Bridged Bicyclo Compounds, Heterocyclic; D001952:Bridged-Ring Compounds / Q000378:metabolism*; D002243:Carbolines / Q000378:metabolism / Q000494:pharmacology; D002540:Cerebral Cortex / Q000378:metabolism; D002712:Chlorides / Q000378:metabolism*; D002998:Clonazepam / Q000494:pharmacology; D066298:In Vitro Techniques; D007476:Ionophores / Q000378:metabolism*; D007700:Kinetics; D008024:Ligands; D008297:Male; D051381:Rats; D011919:Rats, Inbred Strains; D011963:Receptors, GABA-A / Q000032:analysis / Q000187:drug effects / Q000378:metabolism*; D013462:Sulfur Radioisotopes; D014316:Tritium; D005680:gamma-Aminobutyric Acid / Q000494:pharmacology*
chemical_list: D001643:Bridged Bicyclo Compounds; D019086:Bridged Bicyclo Compounds, Heterocyclic; D001952:Bridged-Ring Compounds; D002243:Carbolines; D002712:Chlorides; D007476:Ionophores; D008024:Ligands; D011963:Receptors, GABA-A; D013462:Sulfur Radioisotopes; D014316:Tritium; C026072:1-methyl-beta-carboline-3-carboxylic acid; D005680:gamma-Aminobutyric Acid; D002998:Clonazepam; C037476:tert-butylbicyclophosphorothionate; C028585:propyl beta-carboline-3-carboxylate