Calcium oscillations in rhythmically active respiratory neurones in the brainstem of the mouse

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Abstract
1. The rhythmically active respiratory network in the brainstem slice of the mouse was investigated under in vitro conditions using patch clamp and microfluorometric techniques. Rhythmic respiratory activity persisted over the whole course of an experiment. 2. Electrophysiologically recorded rhythmic activity in respiratory neurones was accompanied by oscillations in intracellular calcium, which displayed a maximal concentration of 300 nM and decayed to basal levels with a mean time constant of 1.6 +/- 0.9 s. 3. Elevations of calcium concentrations were highly correlated with the amplitude of rhythmic membrane depolarization of neurones, indicating that they were initiated by a calcium influx across the plasma membrane through voltage-gated calcium channels. 4. Voltage clamp protocols activating either high voltage-activated (HVA) or both HVA and low voltage-activated (LVA) calcium channels showed that intracellular calcium responses were mainly evoked by calcium currents through HVA channels. 5. Somatic calcium signals depended linearly on transmembrane calcium fluxes, suggesting that calcium-induced calcium release did not substantially contribute to the response. 6. For calcium elevations below 1 microM, decay time constants were essentially independent of the amplitude of calcium rises, indicating that calcium extrusion was adequately approximated by a linear extrusion mechanism. 7. Cytosolic calcium oscillations observed in neurones of the ventral respiratory group provide further evidence for rhythmic activation of calcium-dependent conductances or second messenger systems participating in the generation and modulation of rhythmic activity in the central nervous system.