States of excitability in ovulation hormone producing neuroendocrine cells of Lymnaea stagnalis (gastropoda) and their relation to the egg-laying cycle.

The electrotonically coupled network of about 100 neuroendocrine caudodorsal cells (CDC) of the freshwater snail Lymnaea stagnalis exhibits three states of excitability with distinct electrophysiological characteristics. Transitions between these states occur spontaneously or can be induced experimentally. The CDC produce an ovulation hormone, and the excitability states are clearly related to the egg-laying cycle of the snail. Two hours before egg laying, the cells enter an active state, which lasts one hour. This phase is characterized by a spontaneous firing pattern, which in preparations can be evoked as an afterdischarge, and during which the hormone is thought to be released. After this, the cells enter an inhibited state in which no other activity than directly stimulus-dependent ortho- and antidromic action potentials can be evoked. This phase lasts till about four hours after egg laying. The subsequent resting state is characterized by facilitation of the responses upon repetitive stimulation of the cells, leading to depolarization of the network and additional action potentials. In this phase, an afterdischarge can be evoked, which brings the cells into the active state again.