Spike aftercurrents in R15 of Aplysia: their relationship to slow inward current and calcium influx

Abstract

Spikes in the bursting neuron, R15 of A. californica, are followed by depolarizing afterpotentials and often by delayed hyperpolarizing afterpotentials as well. Placing the cell in a voltage clamp after a spike allows measurement of the depolarizing aftercurrent (DAC) and hyperpolarizing aftercurrent (HAC) that underlie the afterpotentials. Subthreshold depolarizations give rise to small DAC and HAC. The DAC and the slow inward current (SIC) of R15 are reduced or blocked in a similar manner by many experimental manipulations, e.g., application of dopamine, zero-Ca seawater, zero-Na seawater or Ca-channel blockers (Mn2+ and La3+), or cooling the cell from 21.degree.-22.degree. C to 10.degree. C. Neither the DAC nor the SIC were blocked by tetrodotoxin (100 .mu.M) and neither was sensitive to altered extracellular K. Both the DAC and SIC become larger as the holding potential of the cell is progressively depolarized from -70 to -40 mV. DAC are sensitive to the injection of intracelluar Ca chelators (EGTA (ethylene glycol-bis(.beta.-aminoethyl ether)-N,N1-tetraacetic acid) or EDTA). DAC amplitude is .apprx. 90% reduced by intracellular EGTA concentration near 1 mM. The SIC is unchanged or much less affected the Ca buffers. DAC are also more sensitive to low (1 mM) extracellular Ca than is the SIC. The HAC is also a Ca-dependent current. It is blocked by an experimental manipulation reducing Ca influx or intracellular Ca accumulation, i.e., reduced extracellular Ca, Ca-channel blockers or intracellular EGTA. The DAC and the SIC are carried by the same conductance mechanism. In the case of the DAC, the conductance might be activated by a rise in intracellular Ca activity accompanying the spike and, in the case of the SIC, depolarization per se may be the most important activating condition.