THE ANESTHETIC ACTION AND MODULATION OF GABA(A) RECEPTOR ACTIVITY BY THE NOVEL WATER-SOLUBLE AMINOSTEROID ORG-20599

The anaesthetic profile of a novel water-soluble aminosteroid, Org 205 99 [(2 beta,3 alpha,5 1-chloro-3-hydroxy-2-(4-morpholinyl)pregnan-20-o ne methanesulphonate], and the ability of the compound to allosterical ly regulate the activity of the GABA(A) receptor, have been studied in comparison to the properties of established intravenous general-anaes thetic agents. Intravenously administered Org 20599 produced a rapid o nset, short duration loss of the righting reflex in mice. The anaesthe tic potency of Org 20599 was comparable to that of the steroids 5 alph a-pregnan-3 alpha-ol-20-one or alphaxalone, and exceeded that of propo fol, thiopentone or pentobarbitone. Org 20599 and the reference anaest hetic agents allosterically displaced the binding of [S-35]-t-butylbic yclophosphorothionate (TBPS) from GABA(A) receptors of rat-brain membr anes with the order of potency: 5 alpha-pregan-3 alpha-ol-20-one > Org 20599 > alphaxalone > propofol > thiopentone > pentobarbitone. At hum an recombinant alpha(1), beta(2), gamma(2L) subunit-containing GABA(A) receptors expressed in Xenopus laevis oocytes, the anaesthetic agents produced a concentration-dependent and reversible potentiation of the peak amplitude of GABA-evoked currents. A similar positive allosteric action of Org 20599 was observed for the GABA(A) receptors expressed by bovine adrenal chromaffin cells maintained in culture. The rank ord er of potency in the aforementioned assays was identical to that deter mined from the displacement of TBPS binding. At concentrations greater than those required for potentiation of GABA, the anaesthetics exhibi ted GABA-mimetic effects with a rank order of potency that paralleled their modulatory activity. Such direct agonism varied greatly in maxim al effect between compounds. The modulatory and direct agonist actions of Org 20599 were additionally confirmed utilizing rat hippocampal ne urones in culture. The results indicate Org 20599 to be a potent and s hort-acting intravenous anaesthetic agent in mice and suggest positive allosteric regulation of GABA(A) receptor function to be a plausible molecular mechanism of action for the drug. Copyright (C) 1996 Elsevie r Science Ltd.