Jean-Pierre Chambon

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.

Behavioural evidence for a selective GABA-A antagonistic activity of SR 95103 and SR 42641 after intrastriatal injection in mice

Two pyridazinyl GABA derivatives, SR 95103 and SR 42641 have recently been described as selective GABAA receptor antagonists. We have now investigated the behavioural effects of SR 95103 and SR 42641 after intrastriatal injection in mice. When injected into the right striatum, SR 95103 (0.01-0.5 microgram), SR 42641 (0.0001-0.01 microgram) and bicuculline methiodide (0.005-0.05 microgram) induced contralateral rotations which were antagonized by intraperitoneal injection of muscimol. In contrast, the intrastriatal injection of the GABAA receptor agonist muscimol induced ipsilateral rotations. Muscimol-induced turning was antagonized by SR 95103 (10-30 mg/kg), SR 42641 (1-10 mg/kg) and (+)-bicuculline (0.125-0.5 mg/kg) injected intraperitoneally, but not by strychnine. Intrastriatal glycine also induced ipsilateral rotations which were antagonized by strychnine (0.01-0.3 mg/kg i.p.) but not by (+)-bicuculline, SR 95103 or SR 42641. These results suggest that SR 95103 and SR 42641 induce turning through a selective blockade of GABAA receptors within the striatum.

The sensitivity of γ-aminobutyric acid antagonists to thiocyanate is related to the absence of a functional anionic group in their structure

Pyridazinyl derivatives of gamma-aminobutyric acid (GABA) have recently been shown to be selective, reversible and competitive GABAA antagonists. Unlike what is observed with all other GABAA antagonists, the affinity of these compounds for the GABAA receptor is not modified by thiocyanate. The chemical structure of these pyridazinyl-GABA derivatives differs from that of other GABAA antagonists by the presence of a free carboxylic group in their structure. We speculated that this could explain their lack of sensitivity to thiocyanate. Consequently, we synthesized three structural analogues of these pyridazinyl-GABA derivatives in which we replaced the free carboxyl group by a cyano group. These compounds displaced [3H]GABA from rat brain membranes and reversed the GABA-induced enhancement of [3H]diazepam binding. However their affinity for the GABAA receptor increased 10- to 20-fold in the presence of thiocyanate. Thus, sensitivity to thiocyanate appears to be related more to the absence of an anionic functional group than to the agonist or antagonist nature of the GABAA ligand.

Electroencephalographic study of SR 95103, a GABAA antagonist: Interaction with inhibitory amino acids and muscimol

SR 95103 has recently been described as a selective GABAA antagonist. In this study, the electroencephalographic (EEG) effects of SR 95103 were investigated as well as its central interaction with inhibitory amino acids and muscimol. Slow intravenous infusions of SR 95103 in rats induced epileptiform EEG activities which were antagonized by intracerebroventricularly injected muscimol, GABA and taurine whereas glycine did not modify and even facilitated the effects of SR 95103. These results suggest that the EEG effects of SR 95103 are due to the specific GABAA antagonistic properties of this compound.