Pharmacological actions of essential (zinc, copper) and toxic metal ions (lead, mercury) on neurons and neuronal synapses of the snail, Helix pomatia L.

Actions of zinc, copper, lead and mercury ions on identified neurons and neuronal synapses of the snail, Helix pomatia L. were examined by use of electrophysiological methods. The studied metal ions depolarized the membrane and decreased the resistance but the neuronal excitability changed differentially. Copper and zinc transiently increased the amplitude and/or frequency of the spontaneous EPSPs or EPSCs in time- and dose-dependent ways. Lead attenuated the evoked EPSPs or EPSCs but copper and in some neurons zinc and mercury induced dose-dependent biphasic effects. Metal ions influenced the potential dependence, reversal potential, rate of rise and rate of fall of evoked responses differentially. The actions of zinc and lead on evoked EPSPs were reversible but those of copper and mercury were not and DTT treatment was ineffective. The affinity of the metal ions to the binding site(s) was low and the binding kinetics slow. The sensitivity of the binding site decreased in the order of mercury>copper>lead>zinc according to the IC50 values. The binding stoichiometry (nH value) changed from 1.4 to 2.6 for the various metal ions.