Pacemaking mechanism of the afterdischarge of the ovulation hormone-producing caudo-dorsal cells in the gastropod mollusc Lymnaea stagnalis

Abstract

The ovulation hormone-producing caudo-dorsal cells (CDC) of Lymnaea stagnalis have three states of excitability (active, inhibited, and resting), which are related to the egg-laying cycle. Active state CDC produce a firing pattern of prolonged spiking activity (1 spike/2 s), which in the animal occurs shortly before egg laying. In preparations it is evoked as an afterdischarge upon repetitive stimulation of CDC. The afterdischarge is not synaptically driven, but rests on a pacemaking mechanism. CDC are silent in the inhibited and resting states, which follow egg laying. In these states the membrane potential is mainly dependent on [K+]0. In the active state the ratio of the K+, Na+, and Ca2+ permeabilities has changed considerably, probably resulting from an increased permeability to Na+ and Ca2+. The firing rate in the afterdischarge is dependent on the membrane potential, which is confirmed experimentally by varying [K+]0. [Na+]0 and [Ca2+]0 directly influence the firing rate. Firing stops in Na+-free saline, but is enhanced by Ca2+-free or high-Mg2+ saline. TTX does not affect firing. Relatively high concentrations of Co2+ and La3+ (2 x 10(-3) M) strongly inhibit CDC. Regular firing can be changed into bursting by various means, such as high K+ or addition of 1 mM Ba2+. Bursting normally occurs at the beginning of the afterdischarge. Postburst hyperpolarizations are reduced in Ca2+-free saline and by low Co2+ (10(-4)--5 X 10(-4) M). Active CDC are driven by a pacemaking mechanism constituted by a voltage-dependent Na+/Ca2+ channel and a Ca2+-dependent K+ channel, thus resembling that of bursting pacemakers. The pacemaking mechanism is inactive in the resting and inhibited state.