Dynamics of postsynaptic calcium involved in plasticity of visual cortex

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In the developing visual cortex, repetitive stimulation of afferent pathways was reported to induce long-term potentiation (LTP) or long-term depression (LTD) of synaptic transmission in layr II/III of the cortex. It was not clearly known, however, what factors determine the direction of long-term changes in synaptic efficacy, i.e., LT or LTD.The present results were designed to address this question.Thin slices of the visual cortex were prepared from young rats (postnatal days 20-26). Under visual control, neurons in layr II/III were loaded with a fluorescent Ca^<2+> indicator, rhod 2, through glass micropipettes for patch-clamp recording. Changes in fluorescence intensity were recorded simultaneously with excitaory postsynaptic potentials (EPSPs) evoked by test stimulation of layr IV.Tetanic stimulation of layr IV which induced LTP of EPSPs led to a massive increase in fluorescence intensity at dendritic hot spots. On the other hand, tetanic stimulation which induced a moderate rise of fluorescence led to LTD of EPSPs. In sum, it is suggested that a rise of postsynaptic Ca^<2+> during synaptic inputs beyond a certain threshold induces LTP while a weak or moderate rise below the threshold induces LTD.