High intracellular calcium levels during and after electrical discharges in molluscan peptidergic neurons.

Intracellular calcium levels ([Ca2+]i) during and following electrical activity of the neuroendocrine caudodorsal cells of the pond snail (Lymnaea stagnalis) were measured in situ and is dissociated cells by combining electrical recordings and Fura-2 measurements. Caudodorsal cells are typical neuroendocrine cells that control egg laying via the release of a set of peptides during a stereotyped discharge of action potentials. Single action potentials or short trains of spikes in dissociated caudodorsal cells induced only small but consistent increases in [Ca2+]i. With longer or repeated spike trains, larger [Ca2+]i transients were measured, indicating accumulation of calcium. The calcium channel blocker Ni2+ suppressed the calcium elevation, suggesting that calcium influx occurred through voltage-activated calcium channels. Calcium levels in caudodorsal cells in situ were measured before, during and after the stereotyped firing pattern, a approximately 35-min discharge of regular spiking. Basal calcium levels in caudodorsal cells in situ were about 125 nM. During the initial phase of the discharge, the intracellular calcium level increased to approximately 250 nM. Maximal calcium levels (300-600 nM) were only reached at the final phase of the discharge or several minutes after the cessation of firing. Calcium levels remained elevated for up to 1 h after the end of the discharge. During this time, caudodorsal cells were characterized by very low excitability. We suggest that the prolonged, elevated level of calcium following the discharge need not be directly dependent on action potentials. The long-lasting [Ca2+]i elevation may cause the release of neuropeptides to outlast the duration of electrical activity, thus uncoupling release from spiking. In addition, it may cause reduced excitability of neuroendocrine cells following a period of spiking, thereby inducing a refractory period.