EFFECTS OF 5-HT3 RECEPTOR ANTAGONISTS ON BINDING AND FUNCTION OF MOUSE AND HUMAN GABA(A) RECEPTORS

Both 5-HT3 receptor antagonists and benzodiazepine receptor ligands have effects on anxiety, and alter the behavioral action of ethanol. For these reasons, we tested the ability of several 5-HT3 receptor antagonists to inhibit the ligand binding and function of the gamma-aminobutyric acid(A)/benzodiazepine receptor Cl- channel complex of mouse brain membranes. MDL 72222 (1-a-H-3-a-5-aH-optropan-3yl-3,5-dichlorobenzoate) and LY 278584 (1-methyl-N-(8-methyl-8-azabicyclo[3.2.1.]oct-3-yl)-1H-indazole-3-carboxamide) inhibited [H-3]flunitrazepam binding with K-i values of approximately 20 mu M; ICS 205-930 (3 alpha-tropanyl-1H-indole-3-carboxylic acid ester) was more potent with a K-i of 0.8 mu M. ICS 205-930 (50 mu M) had no effect on [H-3]muscimol binding. ICS 205-930, MDL 72222, and LY 278584 all inhibited the binding of [S-35]TBPS (tert-butylbicyclophosphorothionate) with K-i values of approximately 10 mu M and reduced muscimol-dependent Cl-36(-) flux into mouse cortical microsacs by 30-45% at a concentration of 10 mu M. ICS 205-930, MDL 72222, and LY 278584 (at micromolar concentrations) reduced GABA-gated chloride currents studied in Xenopus oocytes expressing human alpha(1) beta(1) gamma(2S) GABA(A) receptor subunits. ICS 205-930 differed from the other two 5-HT3 receptor antagonists in that it induced a biphasic effect on GABA-gated currents: at concentrations from 0.1 to 5 mu M it potentiated GABA responses, whereas at higher concentrations (50-100 mu M) it produced inhibition. The stimulatory action induced by ICS 205-930 was due to interaction at the benzodiazepine recognition site because expression of the gamma(2) subunit was required and Ro 15-1788 (1 mu M) completely prevented the potentiation caused by ICS 205-930. Thus, several 5-HT3 receptor antagonists inhibit benzodiazepine binding and affect GABA(A) receptor function. These actions are most pronounced for ICS 205-930 and likely involve direct affects on the GABA/benzodiazepine complex rather than interactions with 5-HT3 receptors.