The role of NMDA receptors in long-term potentiation (LTP) and depression (LTD) in rat visual cortex

The purpose of the present study was to improve our understanding of the role of NMDA receptors in neocortical synaptic plasticity. In slices of rat visual cortex the field potential elicited in layer III in response to white matter stimulation consisted of two components with peak latencies at 5-8 ms (EPSP1) and 12-19 ms (EPSP2). EPSP2 appeared to be polysynaptic since it did not follow stimulation at 0.5 Hz. EPSP1 consisted of both kainate/AMPA and NMDA receptor activity, as revealed by bath application of DNQX and APV. EPSP2 displayed a variable sensitivity to bath-applied APV. Tetanic stimulation of the white matter in normal medium consistently induced long-term potentiation of EPSP1. In the presence of APV, LTP of EPSP1 was induced only when EPSP2 was still present, while there was no change, or LTD was induced, when EPSP2 was completely blocked by APV. In rat visual cortex, blockade of NMDA receptor participation in the postsynaptic response to tetanic stimulation reduces the probability for LTP induction but does not necessarily prevent LTP; synaptic strength may still change in either direction depending, in part, on factors affecting the magnitude of postsynaptic depolarization during tetanus.