Contribution of T-type VDCC to TEA-induced long-term synaptic modification in hippocampal CA1 and dentate gyrus.

We have previously reported that exposure to the K+ channel blocker tetraethylammonium (TEA), 25 mM, induces long-term potentiation (LTP) in CA1, but not in the dentate gyrus (DG), of the rat hippocampal slice. During TEA application, stimulation of excitatory afferents results in a strong depolarizing potential after the fast excitatory postsynaptic potential (EPSP) in CA1, but not in DG. We hypothesized that the differential effect of TEA on long-term synaptic modification in CA1 and DG results from different levels of TEA-elicited depolarization in the two cell types. Additional pharmacological studies showed that blockade of T-type voltage-dependent calcium channels (VDCCs) decreased both the magnitude of LTP and the late, depolarizing potential in CA1. Blockade of L-type VDCCs had no such effect. Using computer models of morphologically reconstructed CA1 pyramidal cells and DG granule cells, we tested our hypothesis by simulating the relative intracellular Ca2+ accumulation and membrane potential changes mediated by T-type and L-type VDCCs. Simulation results using pyramidal cell models showed that, with decreased maximum conductance of TEA-sensitive potassium channels, synaptic inputs elicited strong depolarizing potentials similar to those observed with intracellular recording. During this depolarization, VDCCs were opened and resulted in a large intracellular Ca2+ accumulation that presumably caused LTP. When T-type VDCCs were blocked, the magnitudes of both the Ca2+ accumulation and the late depolarizing potential were decreased substantially. Simulated blockade of L-type VDCCs had only a minor effect. Together, our modeling and experimental studies indicate that T-type VDCCs, rather than L-type VDCCs, are primarily responsible for facilitating the depolarizing potential caused by TEA and for the consequent Ca2+ influx. Thus, our findings strongly suggest that the induction of TEA-LTP in CA1 depends primarily on T-type, rather than L-type, VDCCs. Simulation results using modeled granule cells suggests that the failure of TEA to induce LTP in DG is partly due to a low density of T-type VDCCs in granule cell membranes.