Activity induced elevations of intracellular calcium concentration in neurons of the deep cerebellar nuclei

1. Depolarization-induced changes in the cytosolic free calcium concentration ([Ca²⁺](i)) were examined in slice-cultured neurons of the deep cerebellar nuclei by combined intracellular and multisite fura-2 recording techniques. 2. Firing of tetrodotoxin (TTX)-sensitive action potentials induced by depolarizing current pulses caused large elevations in somatic as well as proximal dendritic [Ca²⁺](i). In the dendrites, rise and decay times of [Ca²⁺](i) were faster than in the soma. [Ca²⁺](i) changes associated with depolarizations to ≤-40 mV in the presence of TTX were small compared with changes induced by Na⁺ spike firing, suggesting that Ca²⁺ influx through high voltage-activated Ca²⁺ channels is a major cause for Na⁺ spike-associated [Ca²⁺](i) increases. 3. During sustained Na⁺ spike firing at a constant frequency (>20 Hz), [Ca²⁺](i) approached a constant level, after ~1 s in the dendrites and 2 s in the soma, respectively. The amplitude of the attained level was positively correlated with the firing frequency. We suggest that during tonic activity [Ca²⁺](i) reaches a steady state determined by Ca²⁺ influx and extrusion. 4. TTX-resistant plateau potentials caused substantially greater [Ca²⁺](i) increases in the dendrites than in the soma. In the dendrites, plateau-associated Ca²⁺ transients were comparable in amplitude to Ca²⁺ transients triggered by short (50 ms) Na⁺ spike trains, in the soma, they were considerably smaller. 5. Low-threshold spikes (LTSs) in association with a burst of Na⁺ spikes induced a sharp increase in [Ca²⁺](i) both in the soma and in dendrites. A major fraction of this increase appeared to be mediated by Na⁺ spikes, which occurred superimposed on the LTS, and consequently, only a minor fraction directly by the LTS. 6. We conclude that voltage-gated Ca²⁺ channels are localized both at somatic as well as dendritic sites in neurons of the DCN and mediate Ca²⁺ influx during Na⁺ spikes. TTX-resistant plateau potentials appear to be generated in the dendrites and to spread only passively into the soma.