Characterization of the interaction between a novel convulsant agent, norbiphen, and GABA\u3csub\u3eA\u3c/sub\u3e and other ligand-gated ion channels

A hybrid molecule composed of the antimicrobial, norfloxacin, linked to the non-steroidal anti-inflammatory drug (NSAID), biphenylacetic acid, which we have termed norbiphen, is a lethal convulsant in vivo and an antagonist of rodent GABA receptors in vitro. In the present study, the selectivity, molecular site(s) and mechanism of action of this novel convulsant were investigated using electrophysiological techniques. Sub-maximal GABA-evoked currents recorded from rodent hippocampal neurons were reversibly inhibited by norbiphen (1 μM) to 5±2% of control whereas glutamate, NMDA and glycine activated responses were little or unaffected. Sub-maximal GABA-evoked currents recorded from oocytes expressing recombinant human α1β2γ2s or α1β2 GABA receptors were also reversibly inhibited by norbiphen (1-1000 nM) with an IC (±s.e.m.) of 5.7±1 and 8.8±1 nM, respectively. Similarly, GABA currents recorded from α1β1γ2s, α1β1 and β2γ2s receptors were inhibited with IC s of 16.1±1, 18.8±1 and 4.2±1 nM, respectively. In contrast, norbiphen (100 nM) had little or no effect at ρ1 GABA homomers. At α1β2γ2s receptors, norbiphen had no affect on the GABA reversal potential, and inhibition was not voltage-dependent, suggesting that this compound does not act at the ion channel. The GABA concentration response curve was shifted in a competitive-like fashion by norbiphen (10-300 nM) and a Schild analysis of these data yielded a slope of 0.94±0.1 and a pA of 7.77. Our data reveal a novel, selective and highly potent antagonist of GABA receptors. Norbiphen should be a valuable agent in future studies of this receptor complex. © 2002 Elsevier Science Ltd. All rights reserved. A A 50 50 C 2