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Ryanodine receptor‐mediated intracellular calcium release in rat cerebellar Purkinje neurones.

1. Ryanodine receptor‐mediated Ca2+ release was investigated in Purkinje neurones of rat cerebellar slices by using whole‐cell patch‐clamp recordings combined with fluorometric digital imaging of cytoplasmic Ca2+ concentration ([Ca2+]i). 2. Caffeine caused a transient increase in [Ca2+]i in the somata and dendrites of Purkinje neurones. Caffeine‐induced Ca2+ transients were not associated with a membrane inward current and persisted in Ca(2+)‐free external solutions, indicating that they are caused by Ca2+ released from intracellular stores. The amplitudes of the caffeine‐mediated elevations in [Ca2+]i were strongly dependent on the baseline level of [Ca2+]i. 3. Intracellular application of Ruthenium Red through the patch pipette blocked caffeine‐induced Ca2+ transients in Purkinje neurones. Ryanodine when applied either intra‐ or extracellularly caused a use‐dependent block of caffeine‐induced Ca2+ release. 4. Depolarization‐induced Ca2+ transients were strongly prolonged by caffeine. Several lines of evidence suggest that these prolongations reflect Ca(2+)‐induced Ca2+ release. 5. Despite the presence of skeletal muscle type ryanodine receptors in Purkinje neurones, depolarizing pulses failed to induce any changes in [Ca2+]i when the influx of Ca2+ through voltage‐gated channels was prevented by using Ca(2+)‐free solution, or when applying blockers of voltage‐gated Ca2+ channels. 6. Dendritic Ca2+ transients produced by stimulation of the excitatory climbing fibre synaptic input were also prolonged by caffeine, indicating that ryanodine receptor‐mediated release of Ca2+ may be involved in synaptic signalling in cerebellar Purkinje neurones. 7. Ryanodine receptor‐mediated release of Ca2+ in cerebellar Purkinje neurones can be explained by a model in which release of Ca2+ is strongly facilitated by the co‐operative action of Ca2+, caffeine and/or ryanodine. Our results suggest that Ca2+ release in these central neurones becomes prominent only during episodes of intensive electrical activity associated with increased Ca2+ entry.