Peptide processing and release by the neuroendocrine caudodorsal cells of Lymnaea stagnalis during an egg-laying cycle

Abstract

The freshwater mollusc Lymnaea stagnalis exhibits an egg-laying cycle that takes about 24 h. The cycle is under control of the cerebral neuroendocrine Caudodorsal Cells (CDC), which release various peptides, one of which is the ovulation-inducing hormone, CDCH. During the egg-laying cycle the CDC reveal three consecutive states of electrical activity, the resting, the active and the inhibited state. Processing and release of CDCH have been studied in relation to the states of electrical (and secretory) activity of the CDC, with quantitative immuno-electron microscopy and a newly raised CDCH antiserum. In somata of CDC in the resting state 1 (just before activation of the CDC), in the active state (30 min after activation) and in the inhibited state (3 h after activation) more secretory granules are immunolabelled, and to a higher degree, than in somata of CDC in the resting state 2 (10 h after activation). Axon terminals of CDC in resting states 1 and 2 are equally immunoreactive, whereas in terminals in the active state more granules are labelled, and to a higher degree, than in the resting states. Secretory granules in terminals in the inhibited state are intermediate in both respects. Immuno-electron microscopy combined with the tannic acid method for the demonstration of exocytosis showed that in terminals in the active state, the percentage of immunolabelled exocytosed granule contents is much higher than in the other states. The same holds for the degree of immunopositivity of these contents. It is proposed that the activity of the CDC depends on their state of electrical and secretory activity, not only with respect to the rate of CDCH release (maximal in active state) but also with regard to the rate of precursor processing (maximal in resting state 2) and posttranslational modification of CDCH within secretory granules in the neurohemal axon terminals (maximal in active state).