Interactions between three pyridazinyl-GABA derivatives and central GABA and glycine receptors in the rat, an in vivo microiontophoretic study

Three pyridazinyl-γ-aminobutyric acid (GABA) derivatives, SR 95103, SR 42641 and SR 95531, have previously been shown to be specific, competitive and reversible GABA A antagonists. For all three compounds selectivity was claimed mainly on the basis of biochemical results. However the absence of an interaction with the binding site for strychnine does not preclude an interaction with the glycine receptor. Therefore the interaction between these compounds and GABA- and glycine-elicited responses in the rat cuneate nucleus was examined in vivo by microiontophoretic techniques using extracellular recordings. Preliminary experiments in the somesthetic cortex of the rat (18 cells) confirmed that SR 95103 (100 mM; 0–100 nA) blocked GABA-evoked responses. In the cuneate nucleus SR 95103 (100 mM), SR 42641 (5 mM), SR 95531 (5 mM) and bicuculline methochloride (BMC; 5mM) blocked GABA-elicited responses in a dose-dependent, competitive and reversible fashion for ejection currents up to 100 nA. The compound SR 95103 appeared to be less potent than bicuculline and also antagonized glycine-induced responses. Both SR 42641 and SR 95531 appeared to be equipotent to bicuculline and selective for the GABA receptor. Based on these results, it is postulated that SR 42641 and SR 95531 are potent and reversible GABA A antagonists and could be useful tools to further characterize the GABA receptor.