Segregation of leech neurones by the effect of sparteine on action potential duration.

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Abstract
Sparteine (SPT) and 3- or 4-aminopyridine were applied to leech segmental ganglia and the electrophysiological responses of the Retzius (R) and sensory neurons responding to pressure (P), touch (T) and noxious (N) stimuli analyzed. SPT 0.05-0.5 mM when presented via the bath to the whole ganglion prolonged the action potentials of these neurons to characteristically different degrees; the cells were clearly segregated in the order R > N > P > T at 0.5 mM, regardless of exposure time. The plateau of the prolonged action potentials in the R and N cells was sustained by either Ca or Sr and was blocked by Mn, in normal or Na-free Ringer. These responses were similar to those seen in the same cell types with TEA [tetraethylammonium chloride] under the same condition. The SPT prolongation of action potentials was favored by alkalinization of the Ringer solution. This implies that the drug acted in its uncharged form. SPT was ineffective when applied by pressure into the somata of these 4 neurons. This may be because SPT was charged at the intracellular pH or because it acted at some external membrane site. The aminopyridines when applied in the bath had no effect on the repolarization of these 4 neurons. TEA and SPT probably act on repolarization by similar mechanisms. The parameter of membrane function principally affected is probably a K current which contributes to repolarization to different degrees in the 4 cells and which may be activated by Ca.