THE ACTIVATION OF NICOTINIC ACETYLCHOLINE-RECEPTORS IN THE NEMATODE PARASITE ASCARIS-SUUM BY THE APPLICATION OF LEVAMISOLE TO THE CYTOPLASMIC SURFACE OF MUSCLE MEMBRANE

The patch-clamp technique was used to examine activation by levamisole of nicotinic acetylcholine receptors in muscle vesicle preparations f rom Ascaris suum. Previous experiments have shown that levamisole, whe n applied at concentrations greater than 30 muM to the extracellular s urface of the membrane, activated channels which showed characteristic s of a voltage-sensitive open channel-block and desensitisation. In th is study initial experiments were performed where channels were activa ted with levamisole (2 muM) applied in the patch-pipette. Isolated ins ide-out patches were used. In addition, levamisole (30-926 muM) was ad ded to the cytoplasmic membrane surface via the bath solution. In eigh t out of nine experiments, addition of levamisole to the bath solution resulted in an increase in channel activity, a voltage-sensitive open channel-block and desensitisation. The open channel-block occurred at hyperpolarised potentials, an observation consistent with levamisole (a cationic substance) blocking the channel from the extracellular sur face. Thus it was concluded that levamisole crossed from the cytoplasm ic side of the membrane, via the lipid phase, to the extracellular sur face of the patch. In the presence of high cytoplasmic concentrations of levamisole, open channel-block was not observed at depolarised pote ntials, suggesting channel asymmetry.