TRANSIENT CHANGES IN INTRACELLULAR CALCIUM ASSOCIATED WITH A PROLONGED INCREASE IN EXCITABILITY IN NEURONS OF APLYSIA-CALIFORNICA

1. Transient stimulation of an afferent input to the bag cell neurons of Aplysia californica triggers a 30-min period of spontaneous firing termed the afterdischarge. Measurement of free calcium ion concentrati ons using calcium-sensitive electrodes revealed a biphasic pattern of elevation of intracellular calcium levels during the afterdischarge. B asal calcium levels at the soma were found to rise rapidly during affe rent stimulation and then to decline before the onset of spontaneous f iring. This early peak in intracellular calcium was followed by a slow er, transient elevation of calcium levels during the period of rapid f iring that occurs in the first few minutes of afterdischarge. Stimulat ion of clusters of bag cell neurons in a calcium-free external medium failed to trigger afterdischarge and produced no changes in basal intr acellular calcium levels. 2. When calcium ions in the external medium were replaced by barium ions, stimulation of clusters of bag cell neur ons triggered afterdischarges that were characterized by long-duration action potentials. Intracellular calcium levels during these afterdis charges rose slowly over the first few minutes of spontaneous firing. Because calcium-sensitive microelectrodes do not respond to barium ion s, these data suggest that stimulation of afterdischarge triggers calc ium release from an intracellular compartment. 3. During afterdischarg es in barium-containing external media, each broadened action potentia l produced a discrete transient elevation of intracellular calcium lev els. A similar effect was observed in isolated bag cell neurons in pri mary culture when action potentials were stimulated by depolarizing cu rrent pulses in a barium-containing medium. These data suggest that, u nder these conditions, individual action potentials trigger the releas e of intracellular calcium from some intracellular pool.