Inhibition of skeletal muscle nicotinic receptors by the atypical antipsychotic clozapine

We have previously observed that certain atypical antipsychotic drugs reduce the amplitude and duration of miniature end-plate currents (EPCs) at the frog neuromuscular junction (Effects of atypical antipsychotics on vertebrate neuromuscular transmission. Nguyen, Q.-T., Yang, J., Miledi, R. Neuropharmacology 42, 2002, 670-676), therefore suggesting that these drugs act on nicotinic acetylcholine receptors. In this study we examined the effects of the atypical antipsychotic clozapine on nicotinic receptors of frog neuromuscular end-plates or in Xenopus oocytes expressing the alpha(1)beta(1)gammadelta mouse skeletal muscle nicotinic receptor. At neuromuscular junctions. postsynaptic currents were reduced by micromolar concentrations of clozapine. This compound also acted presynaptically by increasing the quantal content of EPCs of muscles without noticeably affecting paired-pulse facilitation. In oocytes, clozapine inhibited alpha(1)beta(1)gammadelta receptors with an IC50 of 10 muM and a Hill coefficient of 1. Blockage of alpha(1)beta(1)gammadelta receptors by clozapine bears several hallmarks of open-channel blockers. including faster response decays, strong voltage dependence of the block, large rebound currents upon wash, and reduction of peak responses even at saturating concentrations of acetylcholine. However, clozapine increased the EC50 for acetylcholine and its blocking effect was enhanced by preincubation. These results suggest that clozapine antagonizes muscle nicotinic receptors by blocking open channels, and possibly also by another mechanism which still remains to be investigated. (C) 2002 Elsevier Science Ltd. All rights reserved.