Abstract

In addition to blocking cyclooxygenases, members of the fenamate group of nonsteroidal anti-inflammatory drugs have been proposed to affect brain GABA_A receptors. Using quantitative autoradiography with GABA_A receptor-associated ionophore ligand [³⁵S]t-butylbicyclophosphorothionate (TBPS) on rat brain sections, one of the fenamates, niflumate, at micromolar concentration was found to potentiate GABA actions in most brain areas, whereas being in the cerebellar granule cell layer an efficient antagonist similar to furosemide. With recombinant GABA_A receptors expressed in Xenopus laevis oocytes, we found that niflumate potentiated 3 μM GABA responses up to 160% and shifted the GABA concentration-response curve to the left in α1β2γ2 receptors, the predominant GABA_A receptor subtype in the brain. This effect needed the γ2 subunit, because on α1β2 receptors, niflumate exhibited solely an antagonistic effect at high concentrations. The potentiation was not abolished by the specific benzodiazepine site antagonist flumazenil. Niflumate acted as a potent antagonist of α6β2 receptors (with or without γ2 subunit) and of αXβ2γ2 receptors containing a chimeric α1 to α6 subunit, which suggests that niflumate antagonism is dependent on the same transmembrane domain 1- and 2-including fragment of the α6 subunit as furosemide antagonism. This antagonism was noncompetitive because the maximal GABA response, but not the potency, was reduced by niflumate. These data show receptor subtype-dependent positive and negative modulatory actions of niflumate on GABA_A receptors at clinically relevant concentrations, and they suggest the existence of a novel positive modulatory site on α1β2γ2 receptors that is dependent on the γ2 subunit but not associated with the benzodiazepine binding site.