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 from Ascaris suum. Previous experiments have shown that levamisole, when applied at concentrations greater than 30 muM to the extracellular surface of the membrane, activated channels which showed characteristics of a voltage-sensitive open channel-block and desensitisation. In this study initial experiments were performed where channels were activated with levamisole (2 muM) applied in the patch-pipette. Isolated inside-out patches were used. In addition, levamisole (30-926 muM) was added to the cytoplasmic membrane surface via the bath solution. In eight 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 surface. Thus it was concluded that levamisole crossed from the cytoplasmic side of the membrane, via the lipid phase, to the extracellular surface of the patch. In the presence of high cytoplasmic concentrations of levamisole, open channel-block was not observed at depolarised potentials, suggesting channel asymmetry.