The action of cobalt, cadmium and thallium on presynaptic currents in mouse motor nerve endings

The action of cobalt, cadmium and thallium on presynaptic currents was investigated by recording extracellular potentials from mouse motor nerve terminals. Recorded waveforms consisted of two negative deflections preceded by a small positivity. The second negative deflection could be blocked by the potassium channel blockers tetraethylammonium (TEA) and 3,4-diaminopyridine (3,4-DAP). Application of either divalent cations (cobalt, cadmium) or monovalent thallous ions to the bath, even in mM concentrations, did not change these waveforms significantly. After application of high TEA and 3,4-DAP concentrations (10 mM and 250 microM, respectively), a prolonged positive-going wave arose, which could be blocked reversibly by bath application of cobalt and cadmium, but not thallium. The concentration-inhibition curve for cadmium suggested two apparent dissociation constants, whereas cobalt seemed to have only one apparent dissociation constant. It was concluded that the long-lasting positive wave is driven by calcium influx, since it was competitively antagonized by the application of cobalt and cadmium. A different, short-lasting positive wave arose using lower concentrations of potassium channel blockers, and this wave could not be blocked by cobalt, cadmium, or thallium.