Two different presynaptic calcium currents in mouse motor nerve terminals

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Abstract
Extracellular recordings of potential changes under the perineural sheath of nerve bundles close to some of the nerve terminals were performed using the M. triangularis sterni of the mouse. The nerve signals consisted of a predominant doublepeaked negativity which was often preceded by a small positive deflection. While the first negative peak is related to the propagating nerve action potential, the second negative deflection can be attributed to a potassium conductance since it was selectively blocked by tetraethylammonium (TEA) or 3,4-diaminopyridine (3,4-DAP). Combined application of TEA and 3,4-DAP gave rise to a prolonged positive-going wave which was blocked by Cd2+, thus, indicating its underlying cause to be a Ca current. Ionophoretic application of TEA and Cd2+ to the endplates affected potassium and calcium components of the subendothelial signals, respectively, thus indicating their presynaptic origin. This finding is supported by the decrease of the amplitude of these components with increasing distance from the endplate region. Maximal effects on K conductance attainable with 3,4-DAP could still be potentiated by TEA, indicating the presence of at least two distinct sets of K channels. The prolonged positive potential induced by TEA and 3,4-DAP consisted of a fast and slow component, both of which can be attributed to Ca conductances with different characteristics. The fast positive signal component is attributed to the voltage-dependent Ca channel, responsible for the initiation of transmitter release. Its amplitude and duration depend on extracellular Ca2+-concentration. The fast component is still present when Ca2+ is substituted by Sr2+ or Ba2+. It is blocked by Cd2+ and Mn2+ in the millimolar range, but remains unaffected by organic Ca-antagonists. The slow positive signal component, whose physiological role remains to be elucidated, also depends on extracellular Ca2+-concentration. It is reduced by frequent nerve stimulation. The slow component can be carried by Sr2+ or Ba2+ and it is blocked by Cd2+ and Mn2+ in the micromolar range. In contrast to the fast ‘Ca potential’ the slow one is reduced by verapamil and diltiazem but not by the 1,4-dihydropyridines nitrendipine and nisoldipine. Concentration-response curves can best be described assuming two dissociation constants.

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