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

1. Whole cell patch clamp recording, Ca(2+) measurement with ratiometric fluorescent dyes and photolysis of caged Ca(2+) 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 Ca(2+) concentration ([Ca(2+)](i)). 3. Within a cell the elevation of [Ca(2+)](i) induced by depolarization was inversely related to that induced by photolysis, suggesting that higher [NPE] is more effective in releasing caged Ca(2+) but also increases buffer capacity to reduce [Ca(2+)](i) rises caused by Ca(2+) influx through voltage-dependent Ca(2+) channels. 4. Both DSI and PSI were linearly related to [Ca(2+)](i), with a 50 % reduction in transmission occurring at about 3.6--3.9 microM. 5. [Ca(2+)](i) recovered more quickly than DSI, indicating that the duration of DSI is not set simply by the duration of [Ca(2+)](i) elevation, but rather entails other rate-limiting processes. 6. We conclude that DSI is activated by micromolar [Ca(2+)](i) acting far from sites of Ca(2+) entry through channels in the plasma membrane.