VOLTAGE SENSITIVITY OF SMALL, FOCAL TRANSIENT POTASSIUM DEPOLARIZATIONS IN SNAIL NEURONS - RELEVANCE FOR DIAGNOSIS OF CHEMICAL SYNAPTIC ACTIVITY

The depolarization produced by the focal iontophoresis of potassium ions onto the somata of neurons of the central nervous system of the Mexican snail, Helix aspersa, was investigated. Reproducible small depolarizations can be evoked by constant iontophoretic current pulses. The relationship between the size of the depolarization and the distance of the iontophoretic electrode from the cell body was studied. Reliable depolarizations with minimal artifact and little undesirable effects of current were obtained at a distance from the cell soma of about 2 μm. The depolarization produced by potassium was strongly voltage dependent and grew almost linearly with hyperpolarization of the cell. The apparent reversal potential obtained by extrapolation was somewhat negative (range, O to - 20 mV). Depolarization of the cell reduced the size of the potassium response. The relationship between the size of the evoked response and the membrane potential, however, deviated from linearity, and the evoked response vanished with strong depolarizations. No reversal of the potassium depolarization was seen. The depolarization produced by potassium was accompanied by an increase in membrane conductance. Membrane rectifications found in the examined neurons do not account for the voltage-dependent behavior or the increased conductance found during the potassium-evoked depolarization. Indeed, voltage dependence and increased conductance were found in the absence of membrane rectification. Cobalt chloride blocked almost all spontaneous and evoked synaptic activity. The size of the potassium depolarization, the accompanying conductance increase, and the voltage-sensitive behavior, however, were not affected. The Goldman equation predicts the behavior of the potassium depolarization. Hyperpolarization should produce a linear growth in the evoked response, which extrapolates to zero membrane potential. Depolarization should reduce the evoked response asymptotically to zero. According to predictions from the Goldman equation, a chemical synaptic process associated with an increase in extracellular potassium should increase the voltage-dependent slope of the synaptic process and shift the observed reversal potential to more depolarizing values. The relevance of these results is discussed in relation to potasium depolarizations found in invertebrates and to the mechanism of primary afferent depolarization. In general, only the demonstration of an iondependent reversal in sign of the response with membrane polarization guarantees the presence of a chemical synaptic process.