On the Role of Glial Cells in Dual Secretory Dynamics of the Caudodorsal Cells of l yMnaea Stagnalis

Abstract

The Caudodorsal cells (CDC) of Lymnaea stagnalis are neurons that control egg laying and egg-laying behaviour by releasing various peptides, including the ovulation hormone CDCH. Release occurs in a dual fashion, viz. 1. into the haemolymph, from neurohacmal axon terminals in the outer compartment of the cerebral commissure and 2. into the intercellular space of the central nervous system, from nonsynaptic release sites of axon collaterals in the inner compartment of the commissure. Previous studies have shown that neurohaemal CDCH-release is maximal during electrical activity of the CDC (active state) whereas CDCH-release from the collaterals particularly occurs during electrical silence (resting and inhibition states). Inner and outer compartment are separated by a continuous sheath of glial cells. Immunoelectron microscopy with an antibody against a synthesized fragment of CDCH (CDCH20-36) including the use of the TAGO-method for the visualization of CDCH-release, shows that in the active state CDCH-immunoreactivity of the intercellular space is much higher (x5) in the outer than in the inner compartment. Apparently, the glial sheath prevents displacement of CDCH from one compartment to the other. In the cells of the sheath many of the immunogold particles are located over small, electron-lucent vesicles, suggesting endocytotic uptake of CDCH. Furthermore, the sheath cells appear able to block and to take up protein A and trypan blue; other substances like tannic acid are not prevented from crossing the sheath. It is concluded that the glial sheath acts as a selective barrier, preventing only particular substances like CDCH from passing it. By blocking and, possibly, by ingesting CDCH (and other CDC-peptides), the sheath may contribute to the control of peripheral and central targets, exerted by CDCH secreted from the neurohaemal area and from the collateral system, respectively.