An early sodium and a late calcium phase in the afterdischarge of peptide-secreting neurons of Aplysia.

Following brief electrical stimulation of a pleuroabdominal connective nerve, the clusters of peptidergic bag cell neurons of Aplysia generate a long-lasting (or approximately 30 min) synchronous afterdischarge. In a normal seawater medium, the afterdischarge comprises an early phase of rapid firing (2-6 Hz) lasting for less than one minute, followed by a second, prolonged phase of lowering firing rate (less than 0.5 Hz). We have found that the transition from the first, rapid-firing phase to the prolonged second phase of afterdischarge is associated with an increase both in the width and height of intracellularly recorded action potentials. In addition, we find that in the presence of the potassium channel-blocker, TEA, such biphasic afterdischarges may be triggered by depolarizing current that evokes action potentials in a single bag cell neuron in a cluster. Afterdischarges resembling either the first or the second phase of a normal afterdischarge may be produced by using media deficient in calcium or sodium, respectively, in combination with potassium channel-blockers. Brief stimulation in media deficient in calcium give rise to afterdischarges of high firing frequency (2-6 Hz) lasting for less than one minute (mean duration = 41 s) while, in a sodium-free medium, longer afterdischarges (mean duration = 18 min) of lower firing rate (less than 0.3 Hz) were generated. At the end of afterdischarge in a normal seawater medium, containing both sodium and calcium ions, the bag cell neurons become refractory to stimulation. Thus, further stimulation either fails to induce afterdischarge or results in afterdischarges that are attenuated both in frequency of firing and in duration compared with the first afterdischarge. In these experiments we found that the brief afterdischarges, observed in low-calcium media, which resemble the first phase of a normal afterdischarge, did not induce subsequent refractoriness while afterdischarges evoked in the normal calcium, sodium-free medium did result in refractoriness to further afterdischarge. Our data indicate that both phases of a bag cell afterdischarge are due to intrinsic bag cell mechanisms and suggest that the firing pattern in the first phase is largely sodium dependent and that of the second, slow phase is calcium dependent. Moreover, our data indicate that it is the second, calcium-dependent phase that induces the refractoriness that follows bag cell afterdischarge.