Differential modulation of α3β2 and α3β4 neuronal nicotinic receptors expressed in Xenopus oocytes by flufenamic acid and niflumic acid

Abstract

Effects of flufenamic acid (FFA) and niflumic acid (NFA), which are often used to block Ca2+-activated CI current, have been investigated in voltage- clamped Xenopus oocytes expressing α3β2 and α3β4 nicotinic ACh receptors (nAChRs). NFA and FFA inhibit α3β2 nAChR-mediated inward currents and potentiate α3β4 nAChR-mediated inward currents in normal, Cl -free and Ca2+-free solutions to a similar extent. The concentration-dependence of the inhibition of α3β2 nAChR-mediated ion current yields IC50 values of 90 μM for FFA and of 260 μM for NFA. The potentiation of α3β4 nAChR- mediated ion current by NFA yields an EC50 value of 30 μM, whereas the effect of FFA does not saturate for concentrations of up to 1 mM. At 100 μM, FFA reduces the maximum of the concentration-effect curve of ACh for α3β2 nAChRs, but leaves the EC50 of ACh unaffected. The same concentration of FFA potentiates α3β4 nAChR-mediated ion currents for all ACh concentrations and causes a small shift of the concentration-effect curve of ACh to lower agonist concentrations. The potentiation, like the inhibition, is most likely due to a noncompetitive effect of FFA. Increasing ACh-induced inward current either by raising the agonist concentration from 10 μM to 200 μM or by coapplication of 10 μM ACh and 200 μM FFA causes a similar enhancement of block of the α3β4 nAChR-mediated ion current by Mg2+. This suggests that the effects of FFA and of an increased agonist concentration result in a similar functional modification of the α3β4 nAChR-operated ion channel. It is concluded that α3β4 and α3β2 nAChRs are oppositely modulated by FFA and NFA through a direct β-subunit-dependent effect.