The anaesthetic 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 20599 [(2β,3α,5α)-21-chloro-3-hydroxy-2-(4-morpholinyl)pregnan-20-one methanesulphonate], and the ability of the compound to allosterically regulate the activity of the GABA(A) receptor, have been studied in comparison to the properties of established intravenous general-anaesthetic agents. Intravenously administered Org 20599 produced a rapid onset, short duration loss of the righting reflex in mice. The anaesthetic potency of Org 20599 was comparable to that of the steroids 5α-pregnan-3α-ol-20-one or alphaxalone, and exceeded that of propofol, thiopentone or pentobarbitone. Org 20599 and the reference anaesthetic agents allosterically displaced the binding of [³⁵S]-t-butylbicyclophosphorothionate (TBPS) from GABA(A) receptors of rat-brain membranes with the order of potency: 5α-pregnan-3α-ol-20-one > Org 20599 > alphaxalone > propofol > thiopentone > pentobarbitone. At human recombinant α₁, β₂, γ(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 order of potency in the aforementioned assays was identical to that determined from the displacement of TBPS binding. At concentrations greater than those required for potentiation of GABA, the anaesthetics exhibited GABA-mimetic effects with a rank order of potency that paralleled their modulatory activity. Such direct agonism varied greatly in maximal effect between compounds. The modulatory and direct agonist actions of Org 20599 were additionally confirmed utilizing rat hippocampal neurones in culture. The results indicate Org 20599 to be a potent and short-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.