THE HUMAN NEUROBLASTOMA CELL-LINE, IMR-32 POSSESSES A GABA(A) RECEPTOR LACKING THE BENZODIAZEPINE MODULATORY SITE

GABA(A) receptors were identified in IMR-32 cell membranes by the binding of [S-35]t-butylbicyclophosphorothionate ([S-35]TBPS) to the chloride channel. GABA (IC50 2.2 muM), muscimol (IC50 0.8 muM), picrotoxin (IC50 1.7 muM), pentobarbitone (IC50 108 muM), etomidate (IC50 53 muM), chlormethiazole (IC50 98 muM) and Ro 5-3663 (IC, 280 muM) all inhibited [S-35]TBPS binding. The potency of these drugs at the [S-35]TBPS binding site in IMR-32 cell membranes did not correlate with their potency on [S-35]TBPS binding to rat cortical membranes (linear correlation of pIC50 values, r = 0.75, NS). No specific binding of the benzodiazepine ligands [H-3]flunitrazepam or [H-3]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 muM) but not by the GABA(B) agonist baclofen (100 muM). In the absence of exogenous GABA chloride efflux was stimulated by chlormethiazole (1-100 muM) in a picrotoxin-sensitive manner. Flurazepam (1-100 muM) 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.