Ionic mechanisms of the fast (nicotinic) phase of the acetylcholine response of identified Planorbarius corneus neurons

Abstract

Responses to electrophoretic application of acetylcholine and suberyldicholine were investigated in identified neurons (LPed-2 and LPed-3) isolated from the left pedal ganglion ofPlanorbarius corneus. When microelectrodes filled with potassium chloride were used the reversal potentials of responses to acetylcholine and suberyldicholine were less negative than when microelectrodes filled with potassium sulfate were used; these reversal potentials were shifted toward depolarization if chloride ions in the medium were replaced by sulfate. These facts indicate that the responses in both LPed-2 and LPed-3 depend on chloride ions. Reversal potentials for acetylcholine and suberyldicholine in LPed-3 were virtually identical (−51 and −50 mV respectively), but in LPed-2 they differed significantly (−46 and −62 mV respectively). Replacement of sodium ions by Tris ions shifted the reversal potential for acetylcholine in LPed-2 toward hyperpolarization but did not change the reversal potential for suberyldicholine. Benzohexonium had the same action. The reversal potential for acetylcholine in medium with a reduced sodium concentration or in the presence of benzohexonium was the same as for suberyldicholine. It is concluded that on neuron LPed-2 acetylcholine activates both acetylcholine receptors which control conductance for chloride ions and acetylcholine receptors which change conductance for sodium ions, whereas suberyldicholine acts only on acetylcholine receptors responsible for the chloride conductance of the membrane.