Differential neurotoxicological effects of lead on voltage-dependent and receptor-operated ion channels

Mouse neuroblastoma cells of the clone N1E-115 express a variety of ion channels and receptors, including a number that is also involved in neurotransmission. Effects of Pb2+ on several of these ion channels have been investigated under experimental conditions that allow electrophysiological recording of membrane current carried by distinct types of ion channels. In whole-cell voltage clamp experiments voltage-dependent calcium channels are blocked by Pb2+ at micromolar concentrations, while voltage-dependent sodium channels are not affected by Pb2+. The neuronal type nicotinic acetylcholine (ACh) receptor-ion channel complex is sensitive to low concentrations of Pb2+. At 1 nM-3 microM, Pb2+ reduces the peak amplitude of the ACh-induced inward current to 74%-10% of the control value in a concentration-dependent manner. However, at Pb2+ concentration between 10 and 100 microM this blocking effect is reduced and kinetics of decay of the ACh-induced inward current are slowed. The effects of Pb2+ on the nicotinic receptor-mediated inward current amplitude can be described by the sum of two sigmoidal concentration-effect curves with an IC50 value of 19 nM and an EC50 of 21 microM. The serotonin 5-HT3 receptor-ion channel complex is less sensitive to Pb2+. The serotonin-induced inward current is blocked by Pb2+ with an IC50 value of 49 microM. In single channel patch clamp experiments internal Pb2+ causes activation of calcium-activated potassium channels in N1E-115 cells. The two types of calcium-activated potassium channels show differential sensitivity: the low conductance (SK) channel is more sensitive to Pb2+ than the high conductance (BK) channel. At micromolar concentrations Pb2+ also induces an ion current mediated by metal ion-activated ion channels. Opening of these channels, which have a single channel conductance of 24 pS and a reversal potential of 0 mV, depends on Pb2+ concentration. These effects of Pb2+ support the hypothesis that Pb2+ affects synaptic transmission by blocking presynaptic voltage-dependent calcium channels. On the other hand, effects on other sensitive target sites, the neuronal nicotinic ACh receptor in particular, clearly indicate that other targets may be involved in the toxic effects of Pb2+ on the nervous system.