Multiple actions of fenamates and other nonsteroidal anti-inflammatory drugs on GABAA receptors.

The nonsteroidal anti-inflammatory drug (NSAID) niflumic acid, a fenamate in structure, has many molecular targets, one of them being specific subtypes of the main inhibitory ligand-gated anion channel, the GABAA receptor. Here, we report on the effects of other fenamates and other classes of NSAIDs on brain picrotoxinin-sensitive GABAA receptors, using an autoradiographic assay with [35S]TBPS as a ligand on mouse brain sections. We found that the other fenamates studied (flufenamic acid, meclofenamic acid, mefenamic acid and tolfenamic acid) affected the autoradiographic signal at low micromolar concentrations in a facilitatory-like allosteric fashion, i.e., without having affinity to the [35S]TBPS binding site. Unlike niflumic acid that shows clear preference for inhibiting cerebellar granule cell layer GABAA receptors, the other fenamates showed little brain regional selectivity, indicating that their actions are not receptor-subtype selective. Of the non-fenamate NSAIDs studied at 100 μM concentration, diclofenac induced the greatest inhibition of the binding, which is not surprising as it has close structural similarity with the potent fenamate meclofenamic acid. Using two-electrode voltage-clamp assays on Xenopus oocytes, the effect of niflumic acid was found to be dependent on the β subunit variant and the presence of γ2 subunit in rat recombinant α1β and α1βγ2 GABAA receptors, with the β1 allowing the niflumic acid inhibition and β3 the stimulation of the receptor-mediated currents. In summary, the fenamate NSAIDs constitute an interesting class of compounds that could be used for development of potent GABAA receptor allosteric agonists with other targets to moderate inflammation, pain and associated anxiety/depression.