Effects of phenothiazine-class antipsychotics on the function of α7-nicotinic acetylcholine receptors

The effects of phenothiazine-class antipsychotics (chlorpromazine, fluphenazine, phenothiazine, promazine, thioridazine, and triflupromazine) upon the function of the cloned α₇ subunit of the human nicotinic acetylcholine receptor expressed in Xenopus oocytes were tested using the two-electrode voltage-clamp technique. Fluphenazine, thioridazine, triflupromazine, chlorpromazine, and promazine reversibly inhibited acetylcholine (100 μM)-induced currents with IC₅₀ values of 3.8; 5.8; 6.1; 10.6 and 18.3 μM, respectively. Unsubstituted phenothiazine did not have a significant effect up to a concentration of 30 μM. Inhibition was further characterized using fluphenazine, the strongest inhibitor. The effect of fluphenazine was not dependent on the membrane potential. Fluphenazine (10 μM) did not affect the activity of endogenous Ca²⁺-dependent Cl⁻ channels, since the extent of inhibition by fluphenazine was unaltered by intracellular injection of the Ca²⁺ chelator BAPTA and perfusion with Ca²⁺-free bathing solution containing 2 mM Ba²⁺. Inhibition by fluphenazine, but not by chlorpromazine was reversed by increasing acetylcholine concentrations. Furthermore, specific binding of [¹²⁵I] α-bungarotoxin, a radioligand selective for α₇-nicotinic acetylcholine receptor, was inhibited by fluphenazine (10 μM), but not by chlorpromazine in oocyte membranes. In hippocampal slices, epibatidine-evoked [³H] norepinephrine release was also inhibited by fluphenazine (10 μM) and chlorpromazine (10 μM). Our results indicate that phenothiazine-class typical antipsychotics inhibit, with varying potencies, the function of α₇-nicotinic acetylcholine receptor.