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Photolysis‐induced suppression of inhibition in rat hippocampal CA1 pyramidal neurons

1 Whole cell patch clamp recording, Ca2+ measurement with ratiometric fluorescent dyes and photolysis of caged Ca2+ were combined to investigate the depolarization‐ and photolysis‐induced suppression of inhibition (DSI and PSI) in rat hippocampal CA1 pyramidal cells. 2 A 5‐s depolarization from −70 mV to 0 mV or a 6‐s photolysis of nitrophenyl‐EGTA (NPE) in cell bodies could each depress the frequency of spontaneous inhibitory postsynaptic currents (IPSCs) and the amplitude of evoked IPSCs while elevating intracellular Ca2+ concentration ([Ca2+]i). 3 Within a cell the elevation of [Ca2+]i induced by depolarization was inversely related to that induced by photolysis, suggesting that higher [NPE] is more effective in releasing caged Ca2+ but also increases buffer capacity to reduce [Ca2+]i rises caused by Ca2+ influx through voltage‐dependent Ca2+ channels. 4 Both DSI and PSI were linearly related to [Ca2+]i, with a 50 % reduction in transmission occurring at about 3.6‐3.9 μM. 5 [Ca2+]i recovered more quickly than DSI, indicating that the duration of DSI is not set simply by the duration of [Ca2+]i elevation, but rather entails other rate‐limiting processes. 6 We conclude that DSI is activated by micromolar [Ca2+]i acting far from sites of Ca2+ entry through channels in the plasma membrane.