LONG-TERM DEPRESSION IN RAT VISUAL-CORTEX IS ASSOCIATED WITH A LOWER RISE OF POSTSYNAPTIC CALCIUM THAN LONG-TERM POTENTIATION

To test the hypothesis that an input-associated rise of Ca2+ at postsy naptic sites beyond a certain threshold leads to the induction of long -term potentiation (LTP) while a lower rise below the threshold leads to long-term depression (LTD), the method of microscopic Ca2+ fluorome try was employed simultaneously with recordings of synaptic activity f rom layer II/III of visual cortical slices prepared from young rats. T he conventional Ca2+ indicators, such as fura-2 or fluo-3, may interfe re with intracellular processes for the induction of LTP/LTD because o f their strong Ca2+-chelating action. To minimize such a problem, anot her Ca2+ indicator, rhod-2, was used since it has a much weaker Ca2+-c helating action than those indicators. In 16 slices loaded with rhod-2 through the perfusion medium, tetanic stimulation of theta-burst type was applied to layer IV of the cortex and changes in Ca2+ concentrati on were analyzed in layer II/III from which field potentials to test s timulation of layer IV were recorded simultaneously. In 7 slices in wh ich weak tetanic stimulation consisting of 0.1 ms duration pulses was applied to layer IV, LTD of field responses was induced, while LTP was induced in 6 of the 9 slices in which strong tetanus consisting of 0. 2 ms pulses was applied. In the 6 slices in which LTP was induced, the peak rise of fluorescence intensity during tetanus was 13.9 +/- 0.2 ( S.E.M.) %, which was significantly (t-test, P < 0.01) higher than that (10.4 +/- 0.3%) in the 9 slices in which LTD was induced. In another series of experiments, rhod-2 was injected directly into 12 pyramidal cell-like neurons in layer II/III through patch pipettes, and changes in Ca2+ concentration in apical dendritic areas during tetanus were me asured simultaneously with recordings of excitatory postsynaptic poten tials (EPSPs) evoked by test stimulation of layer IV. It was found tha t LTP of EPSPs was induced in 4 cells which exhibited a strong rise of dendritic Ca2+ signal (197.1 +/- 18.5%) while LTD was induced in othe r 5 cells which showed a weak rise of the signal (31.0 +/- 4.1%). Thes e results seem consistent with the above-mentioned, Ca2+-switching hyp othesis for the induction of LTP and LTD in visual cortex.