Regulation of neurosecretory activity in the freshwater pulmonate Lymnaea stagnalis (L.).

The neurosecretory Dark Green Cells (DGC) in the pleural and parietal ganglia of the freshwater pulmonate Lymnaea stagnalis are involved in osmoregulation, producing a diuretic hormone. The secretory activity of these cells is dependent on the osmolarity of the environment. In order to investigate whether the regulation of this activity is under nervous control, two experiments were carried out in which the right pleural and parietal ganglion complex (PPC) was transplanted into acceptor snails. The light and electron microscopic observations in the first experiment-transplantation of the PPC into control snails, kept under normal conditions-indicate that degeneration in transplants occurs only during the first week of transplantation and is mainly restricted to neuronal processes of non-neurosecretory cells (non-NSC) of which the cell bodies are located outside the PPC. Degenerated nervous elements are eliminated by phagocytozing amoebocytes and glial cells. Degeneration of neurosecretory elements was only very seldom observed. Like most non-NSC, the DGC and the three other neurosecretory cell (NSC) types in the right parietal ganglion show a normal morphology. Moreover, up to at least 6 weeks after transplantation the NSC remain capable of synthesizing, transporting and releasing their neurosecretory material (NSM). Apparently these processes can proceed in the absence of nervous input from outside the PPC. After 3 and 6 weeks of transplantation outgrowth was noted of both non-NSC and NSC axons. In the second experiment groups of acceptor snails were exposed during one week to different environmental osmolarities. Thereafter the NSC activities of the transplanted DGC and of the DGC in the acceptor's own nervous system were determined by quantitative light and electron microscopy. The following observations apply to both transplanted and acceptor DGC. Compared with the control condition (tap water) exposure of snails to demineralized water leads to a significant increase in nuclear and cytoplasmic volumes and in the extent of the granular endoplasmic reticulum (GER) and the Golgi apparatus. Moreover, the activity of the Golgi apparatus is strongly enhanced. In the DGC axon endings located in the pleuroparietal connective the release of NSM is raised markedly above control level. Conversely, exposure to a 0.08 M NaCl solution (hypertonic to the blood) reduces the secretory activity of the DGC, as is obvious from the decrease of nuclear and cytoplasmic volumes, the smaller extent of the GER and of the Golgi apparatus and the lower Golgi activity. The number of cytosomes is increased dramatically, which accounts for the breakdown of elementary granules and of other cytoplasmic constituents. The NSM of the DGC is partly broken down in the axon endings by lysosomal action. Its release takes place at a low rate. Since these changes in neurosecretory activity occur in a similar fashion in transplanted and in acceptor DGC it is concluded that the response of the DGC to environmental osmolarity is not regulated via nervous pathways to the PPC. The only significant difference between transplanted and acceptor DGC concerns the higher number of clear vesicles in the DGC axon endings of transplants. Some explanations for this phenomenon are discussed. Possible mechanisms of regulation of DGC neurosecretory activity are considered. [ABSTRACT FROM AUTHOR]