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

Jun Wang and Robert S. Zucker

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.

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