Calcium Currents and Internal Calcium in Caudo-Dorsal Neuroendocrine Cells in the Snail Lymnaea Stagnalis

Abstract

Voltage-activated calcium currents and changes in internal calcium in relation to electrical activity were studied in the neuroendocrine Caudo-Dorsal Cells (CDCs) of the pond snail Lymnaea stagnalis. The CDCs control egg-laying via the release of a number of peptides during a prolonged period of spiking activity called afterdischarge. Under whole-cell voltage-clamp conditions two distinct dihydropyridine-sensitive high-voltage-activated (HVA) calcium currents were demonstrated in isolated CDCs, which differed in voltage-dependence of activation and in kinetics and voltage-dependence of inactivation. Stimulation of several second messenger routes (cAMP, cGMP and PKC) increased the amplitudes of both HVA calcium currents. Phosphorylation by protein kinases appeared to be a critical step in the modulation of the HVA calcium channels. Fluctuations in the intracellular calcium concentration ([Ca2+]i) during an afterdischarge were measured in CDCs in the intact central nervous system, using the fluorescent dye fura-2. During an electrically-induced discharge, the intracellular calcium level increased. However, maximal calcium levels were only reached at the final phase of the discharge or several minutes after the cessation of firing. This suggests that calcium rises during the discharge require action potential driven influx of extracellular calcium through calcium channels, whereas the prolonged high level of calcium following the discharge is not directly dependent on action potentials.