CALCIUM CURRENTS AND INTERNAL CALCIUM IN CAUDO-DORSAL NEUROENDOCRINE CELLS IN THE SNAIL LYMNAEA-STAGNALIS

Voltage-activated calcium currents and changes in internal calcium in relation to electrical activity were studied in the neuroendocrine Cau do-Dorsal Cells (CDCs) of the pond snail Lymnaea stagnalis. The CDCs c ontrol egg-laying via the release of a number of peptides during a pro longed period of spiking activity called afterdischarge. Under whole-c ell voltage-clamp conditions two distinct dihydropyridine-sensitive e high-voltage-activated (HVA) calcium currents were demonstrated in iso lated CDCs, which differed in voltage-dependence of activation and in kinetics and voltage-dependence of inactivation. Stimulation of severa l second messenger routes (cAMP, cGMP and PKC) increased the amplitude s of both HVA calcium currents. Phosphorylation by protein kinases app eared to be a critical step in the modulation of the HVA calcium chann els. Fluctuations in the intracellular calcium concentration ([Ca2+](i )) during an afterdischarge were measured in CDCs in the intact centra l nervous system, using the fluorescent dye fura-a. During an electric ally-induced discharge, the intracellular calcium level increased. How ever, maximal calcium levels were only reached at the final phase of t he discharge or several minutes after the cessation of firing. This su ggests that calcium rises during the discharge require action potentia l driven influx of extracellular calcium through calcium channels, whe reas the prolonged high level of calcium following the discharge is no t directly dependent on action potentials.