Pharmacological characterisation of multiple components in the enhancement by pregnanolone and propofol of [3H]flunitrazepam binding to GABAA receptors.

The enhancement by pregnanolone (5 beta-pregnan-3 alpha-ol-20-one) and propofol (2,6-diisopropylphenol) of [3H]flunitrazepam (FNZ) binding to GABAA receptors in rat whole brain homogenate has been investigated. Two components in the concentration-effect relationship for pregnanolone were distinguished by the sensitivity of one component to antagonism by bicuculline and enhancement by muscimol, and the selective but weak antagonism of the bicuculline-insensitive component by 11-ketoprogesterone (4-pregnen-3,11,20-trione). Unlike pregnanolone, the enhancement by propofol of [3H]FNZ binding appeared to comprise a single component which was insensitive to 11-ketoprogesterone and was only slightly antagonised by bicuculline and slightly enhanced by muscimol. These results provide evidence for distinct GABA-dependent and GABA-independent components of the action of pregnanolone in the enhancement of [3H]FNZ binding, with the GABA-independent component being sensitive to 11-ketoprogesterone. The data also support the suggestion of different binding sites for pregnanolone and propofol.