Actions of Zn2+ on spontaneous, stimulus and transmitter evoked events in Helix neurons

Modulatory actions of Zn2+ (0.05-2 mM) on spontaneous, stimulus evoked excitatory postsynaptic potentials (EPSPs) or currents (EPSCs) and GABA, L-glutamate induced depolarizations were examined in current- and voltage-clamp conditions. Identified and unidentified Helix pomatia L. neurons were used in the experiments. Zn2+ increased the frequency of the spontaneous EPSPs or EPSCs in a dose-dependent manner but the excitation slowly declined with time. Two phasic effects of Zn2+ on stimulus evoked EPSPs or EPSCs were characteristic in both normal and low Ca(2+)-high Mg2+ salines. Zn2+ in a low dose (0.05-0.4 mM) increased, but in a higher dose decreased the amplitude of the evoked EPSP or EPSCs. Higher than 1 mM Zn2+ sustained the voltage dependence of the stimulus evoked EPSPs or EPSCs. Sodium-potassium dependent depolarizations of GABA and glutamate responses were attenuated by zinc ions. The zinc ion sensitivity of neurons derived from active animals was higher then of hibernating species (IC50 = 0.73 +/- 0.006; 1.33 +/- 0.12, respectively, mean +/- SEM, n = 12). We conclude that zinc ions have both pre- and post-synaptic effects. Zinc induced actions on Helix neuronal synapses based on the modulation of a complex phenomenon appear to be more than one mechanism.