Effects of transition metal ions on spontaneous electrical activity and chemical synaptic transmission of neurons in the medicinal leech

Leech neurons exposed to salines containing inorganic Ca2+-channel blockers generate rhythmic bursts of impulses. According to an earlier model, these blockers unmask persistent Nat currents that generate plateau-like depolarizations, each triggering a burst of impulses. The resulting increase in intracellular Nat activates an outward Na+/K+ pump current that contributes to burst termination. We tested this model by examining systematically the effects of six transition metal ions (Co2+, Ni2+, Mn2+, Cd2+, La3+, and Zn2+) on the electrical activity of neurons in isolated leech ganglia. Each ion induced bursting activity, but the amplitude, form, and persistence of bursting differed with the ion used and its concentration relative to Ca2+ All ions tested suppressed chemical synaptic transmission between identified motor neurons, consistent with block of voltage-dependent Ca2+ currents in these cells. In addition, a strong correlation between suppression of synaptic transmission and burst amplitudes was obtained. Finally, burst duration was increased and the rate of repolarization decreased in reduced Kt saline, as expected for pump-dependent repolarization. These results provide further support for the hypothesis that a novel form of oscillatory electrical activity driven by persistent Naf currents and the Na+/K+ pump occurs in leech ganglia exposed to Ca2+-channel blockers.