Different threshold levels of postsynaptic [Ca2+]i have to be reached to induce LTP and LTD in neocortical pyramidal cells.

Changes in [Ca2+]i were measured in layer II-III pyramid cells of the rat visual cortex slices during application of either LTP or LTD inducing stimulation protocols. At dendritic sites activated by the stimulated afferents [Ca2+]i reached higher amplitudes and decayed more slowly with LTP than with LTD inducing stimuli. In the presence of Ca2+ chelators, the stimulation protocol that would normally produce LTP induced either LTD or failed to induce a synaptic modifications altogether. These results support the hypothesis that the polarity of synaptic gain changes depends on the magnitude of postsynaptic [Ca2+]i responses, the induction of LTP requiring a more pronounced surge of [Ca2+]i than the induction of LTD.