The human neuroblastoma cell line, IMR-32 possesses a GABA A receptor lacking the benzodiazepine modulatory site

Abstract

GABA A receptors were identified in IMR-32 cell membranes by the binding of [ 35 S]t- butyl-bicyclophosphorothionate ([ 35S]TBPS) to the chloride channel. GABA (IC 50 2.2μM), muscimol (IC 50 0.8 μM), picrotoxin (IC 50 1.7 μM), pentobarbitone (IC 50 108 μM), etomidate (IC 50 53 μM), chlormethiazole (IC 50 98 μM) and Ro 5-3663 (IC 50 280 μM) all inhibited [ 35S]TBPS binding. The potency of these drugs at the [ 35S]TBPS binding site in IMR-32 cell membranes did not correlate with their potency on [ 35S]TBPS binding to rat cortical membranes (linear correlation of pIC 50 values, r = 0.75, NS). No specific binding of the benzodiazepine ligands [ 3H]flunitrazepam or [ 3H]Ro 15-4513 to IMR-32 cell membranes was observed. Chloride efflux from IMR-32 cells was studied using the fluorescent dye 6-methoxy- N-(3-sulphopropyl) quinolinium. Chloride efflux was stimulated by GABA and muscimol (0.1–100 μM) but not by the GABA B agonist baclofen (100 μM). In the absence of exogenous GABA chloride efflux was stimulated by chlormethiazole (1–100 μM) in a picrotoxin-sensitive manner. Flurazepam (1–100 μM) both alone and in the presence of GABA had no effect on chloride efflux. It is concluded that IMR-32 cells contain a functional GABA A receptor which differs from that in rat cortex both in its general pharmacology and specifically in the absence of the allosteric modulatory site sensitive to benzodiazepines.