Calcium influx and poststimulus current during early adaptation in Aplysia giant neurons.

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Abstract
During action-potential trains elicited by depolarizing current, the giant neurons R2 and LP1 of Aplysia exhibit an early phase of rapid spike frequency adaptation. Measurements of absorbance changes of intracellular Arsenazo III indicate a rapidly rising free intracellular Ca concentration ([Ca]i) during early adaptation. Voltage-clamp analysis of slow membrane currents reveals an outward, hyperpolarizing current activated during these spike trains. This poststimulus current (PSC) is carried largely by K+; after a spike train it shows a prolonged decay with fast and slow exponential phases. The return of the Arsenazo III absorbance to base line after a spike train seems to have 1 time constant close to that of the fast phase of the PSC. Using action-potential trains of varied duration of frequency, the peak absorbance change, indicative of the peak [Ca]i level and the peak PSC magnitude vary proportionately. Detectable increases in [Ca]i during spike trains were prevented by bathing the cells in low-Ca seawater, application of La or Co, or injection of intracellular ethylene glycol-bis(.beta.-aminoethyl ether)-N,N''-tetraacetic acid (EGTA). This resulted in profound reductions of the PSC and attenuation of early adaptation. Measurement of the magnitude of the PSC during voltage-clamp commands to depolarized levels in the range of the interspike interval voltage during adaptation indicated that the PSC could be a dominant force in bringing about early adaptation.