Ultrastructural evidence for synthesis, storage and release of insulin-related peptides in the central nervous system of Lymnaea stagnalis

Abstract

The cerebral neuroendocrine Light Green Cells of the pulmonate snail Lymnaea stagnate, which control body growth and associated processes, stain positively with an affinity-purified antiserum raised to a large part of the C-chain of pro-molluscan insulin-related peptides. At the ultrastructural level, the rough endoplasmic reticulum is immunonegative, the Golgi apparatus is slightly positive and secretory granules in the process of budding from the Golgi apparatus are strongly positive. These observations indicate that the Light Green Cells synthesize molluscan insulin-related peptides, which are processed before packing by the Golgi apparatus into secretory granules. The two morphologically distinct secretory granule types, i.e. with pale and dark contents, respectively, are equally immunoreactive with antiserum raised to the C-chain of molluscan insulin-related peptides. Secretory granules within lysosomal structures reveal various degrees of immunoreactivity, indicating their graded breakdown. The Light Green Cells release secretory material by the process of exocytosis into the haemolymph from neurohaemal axon terminals located in the periphery of the median lip nerve. The electron-dense (tannic acid method) released contents are clearly immunopositive. The same holds for secretory granule contents released from Light Green Cells axon profiles in the centre of the lip nerve. Some immunoreactivity is also present in the intercellular space between these axon profiles. It is concluded that molluscan insulin-related peptides may act in two ways, namely (1) as neurohormones via the haemolymph at peripheral targets (2) in a non-synaptic (paracrine) fashion at targets within the central nervous system.