Biochemical characterization of the interaction of three pyridazinyl-GABA derivatives with the GABAA receptor site.

An arylaminopyridazine derivative of gamma-aminobutyric acid (GABA), SR 95103, has been shown to be a selective antagonist of GABA at the GABAA receptor site. Subsequent structure-activity studies showed that suppressing the methyl in the 4-position of the pyridazine ring, and substituting the phenyl ring at the para position with a chlorine (SR 42641) or a methoxy group (SR 95531) led to compounds which exhibited the highest affinities for the GABA receptor site in this series. In the present study we examined the biochemical interaction of these compounds with the GABA receptor as well as their biochemical selectivity for this receptor. SR 95531 and SR 42641 displaced [3H]GABA from rat brain membranes with apparent Ki values of 0.15 microM and 0.28 microM respectively and Hill numbers near 1.0. The two compounds antagonized the GABA-elicited enhancement of [3H]diazepam-binding in a concentration-dependent manner without affecting [3H]diazepam-binding per se. Scatchard and Lineweaver-Burk analysis of the interaction of the two compounds with the GABAA receptor sites, revealed that the compounds were competitive at the high affinity site, but non-competitive at the low affinity site. Neither compound interacted with other GABAergic processes or with a variety of central receptor sites. When administered intravenously, SR 95531 and SR 42641 elicited tonic-clonic seizures in mice. Based on these results, it is postulated that SR 95531 and SR 42641 are specific, potent and competitive GABAA antagonists.