Blocking L-type calcium channels enhances long-term depression induced by low-frequency stimulation at hippocampal CA1 synapses

Specific contributions of voltage-dependent calcium channels (VDCCs) to induction of long-term depression (LTD) have not been thoroughly elucidated. The present study examined roles of T- and L-type VDCCs in N-methyl-d-aspartate (NMDA) receptor-dependent LTD induced at several different levels of synaptic activation (0.5- to 10-Hz presynaptic stimulations) at Schaffer collateral-CA1 synapses in rat hippocampal slices. Blockade of T-type VDCCs with nickel ions failed to change LTD magnitude at all levels of stimulation. However, blockade of L-type VDCCs reduced LTD in response to stimulation at 1 and 2 Hz and, conversely, enhanced LTD at a lower frequency (0.5 Hz). The enhancement of 0.5-Hz LTD under L-type VDCC blockade was shown pharmacologically to depend on NMDA receptors (NMDARs) and intracellular Ca2+ release. Calcium imaging revealed that contribution of L-type VDCC-mediated calcium influx to the total calcium increase was greater during 0.5-Hz stimulation than during 1.0-Hz stimulation. This finding, combined with the reported suppression of NMDARs mediated by L-type VDCCs, may be relevant to the present enhancement of 0.5-Hz LTD due to L-type VDCC blockade.