Regulation of neurosecretory activity in the freshwater pulmonate Lymnaea stagnalis (L.). A quantitative electron microscopical study

About a snail, a toad, and rodents: animal models for adaptation research

Neural adaptation mechanisms have many similarities throughout the animal kingdom, enabling to study fundamentals of human adaptation in selected animal models with experimental approaches that are impossible to apply in man. This will be illustrated by reviewing research on three of such animal models, viz. (1) the egg-laying behavior of a snail, Lymnaea stagnalis: how one neuron type controls behavior, (2) adaptation to the ambient light condition by a toad, Xenopus laevis: how a neuroendocrine cell integrates complex external and neural inputs, and (3) stress, feeding, and depression in rodents: how a neuronal network co-ordinates different but related complex behaviors. Special attention is being paid to the actions of neurochemical messengers, such as neuropeptide Y, urocortin 1, and brain-derived neurotrophic factor. While awaiting new technological developments to study the living human brain at the cellular and molecular levels, continuing progress in the insight in the functioning of human adaptation mechanisms may be expected from neuroendocrine research using invertebrate and vertebrate animal models.

The sodium influx stimulating peptide of the pulmonate freshwater snail Lymnaea stagnalis

In Lymnaea stagnalis integumental Na+ uptake is stimulated by the sodium influx stimulating (SIS)-peptide. Its primary structure was determined as: SRTQSRFAS- YELMGTEGTECVTTKTISQICYQCATRHEDSFVQVYQECCKKEMGLREYCEEIYTELPIRSGLWQPN++ +. Antisera raised against parts of SIS-peptide stained neurons in the visceral, parietal, and pleural ganglia, and in the proximal parts of the intestinal, anal, and right internal pallial nerves. Locations and axon projection patterns of these neurons suggest that they represent the previously described neurosecretory yellow cells.

Morphological basis for nonsynaptic communication within the central nervous system by exocytotic release of secretory material from the egg-laying stimulating neuroendocrine caudodorsal cells of Lymnaea stagnalis

The fine structure of the axons of the cerebral, egg-laying stimulating caudodorsal cells of the snail Lymnaea stagnalis has been studied with various light and electron microscope techniques. Special attention was paid to exocytotic release of secretory material (demonstrated with the tanic acid method) from nonsynaptic release sites in the cerebral commissure. This phenomenon has been compared with neurohaemal release. The commissure consists of two morphological compartments, separated by a sheath of glial cells. The outer compartment is formed by the neurohaemal area of the caudodorsal cells, the inner consists of thousands of, mainly unidentified, axons. Furthermore, ventral caudodorsal cells send axons through the inner compartment. These give rise to collaterals, which divide into smaller collaterals, forming an extensive network ("collateral system") throughout the inner compartment. Eventually, collaterals end blindly within the inner compartment. They contain the same three morphological types of secretory granule as the neurohaemal axon terminals. The collaterals never form synaptic contacts; exocytotic release of the contents of secretory granules takes place at nonsynaptic release sites. These sites occur rather dispersed and do not face one particular type of neighbouring neural element. As in the neurohaemal area, both single and multiple exocytoses occur. Widened intercellular spaces, filled with flocculent, electron-dense material, occur near highly active nonsynaptic release sites. The spaces are often bordered by glial cells and may facilitate diffusion of released secretory material through the inner compartment. Apparently, a ventral caudodorsal cell releases secretory material in two fashions: from neurohaemal axon terminals into the haemolymph, and nonsynaptically, from the collaterals into the intercellular space of the central nervous system. Possible functions of the glial sheath between the neurohaemal area and the inner compartment are proposed. Most likely, the collateral system enables the caudodorsal cells to communicate with targets within the central nervous system in a nonsynaptic fashion. A possible target is the cerebral Ring Neuron, which sends an axon branch through the inner compartment and, as was previously shown neurophysiologically, is controlled by the caudodorsal cells in a nonsynaptic fashion.

Morphological and biochemical changes in mucous cells of the murine sublingual salivary gland during the carbamylcholine-induced secretory cycle

Changes in these cells have been evaluated over 6 h following cholinergic stimulation. Carbamylcholine administration resulted in the release of almost 50 per cent of secretory material within 15 min, which caused a reduction of 33 per cent in cell size. After 2 h the cells were depleted of secretory material. However, in the second hour the release of secretory material was accompanied by an enlargement of the nucleus, Golgi complexes and rough endoplasmic reticulum (RER), which suggests an elevation of biosynthetic activity. The enlargement of the RER was not the result of an increase in RNA, i.e. in the number of ribosomes, but of dilatation of its cisternal spaces. Before release took place, there was a continuous coalescence of secretory granules. After this extensive fusion, which is probably the result of an altered physiological state of the granule membrane and subsequent water uptake caused by cholinergic stimulation, the viscous mucins could be squeezed out, water transport is likely to assist in this ejection. Refilling of the mucous cells was almost complete within 6 h after stimulation.

Neuritic growth and neuroma formation by an isolated molluscan ganglion

The isolated, paired buccal ganglia of the snail Helisoma trivolvis were cultured in vivo (i.e., in host snails) for periods up to 377 days. Such ganglia typically become enveloped in a white mass of fibrous tissue which reaches a maximum plateau of size within four weeks. The scanning electron microscope revealed the newly formed tissue to be composed of fine fibers which also pervaded the neuropil. Injection of individual neurons with Lucifer Yellow CH filled many new neurites within the tissue mass. It is concluded that these masses are largely of neuronal origin and they are therefore designated as neuromas. The maximum size of neuromas may be limited by intrinsic mechanisms which limit neuronal volume.

Ultrastructural dynamics of exocytosis in the ovulation-neurohormone producing caudo-dorsal cells of the freshwater snail Lymnaea stagnalis (L.)

The ultrastructural dynamics of exocytosis in the ovulation-stimulating neurosecretory Caudo-Dorsal Cells (CDC) of the freshwater snail L. stagnalis were studied after incubation of cerebral ganglia in Ringer's solutions with different concentrations of K+ and Ca2+. Detection of exocytosis was facilitated by the use of the tannic acid-glutaraldehyde fixation method (TAGO-method). In control Ringer (low K+) the frequency of exocytosis was rather low. Exocytosis mainly occurred as "terminal" exocytosis (TE); "intracellular" (ICE) and, particularly, "multiple" exocytosis (ME) took place infrequently. Incubation in high K+-containing Ringer strongly increased exocytotic activity. Compared to the controls the total frequency of exocytosis was 50 X as high, whereas TE, ICE and ME occurred 6 X, 47 X, and more than 300 X as frequently, respectively. In high K+/Ca2+-free Ringer the total frequency of exocytosis was only 2 X as high as in control Ringer. It is concluded that TE, ICE, and ME are normal, Ca2+-dependent exocytotic phenomena. The significance of their dynamics in response to K+-stimulation is discussed. The extremely high frequency of exocytosis, as well as the presence of "unaltered granule contents in transit", is explained by assuming that an exocytotic event in the CDC lasts rather long, viz. some minutes. The results may reflect the physiological mechanism by which the CDC release their ovulation hormone. The possible involvement of "clear" and "large" electron lucent vesicles in membrane reuptake after exocytosis is considered.

The morphology of neurosecretory neurones in the pond snail, Lymnaea stagnalis, by the injection of Procion Yellow and horseradish peroxidase

The morphology of neurosecretory neurones, the Dark Green Cells, Yellow Cells, Yellow-green Cells, Light Green Cells, Caudodorsal Cells and Canopy Cells, in the central nervous system of the snail,Lymnaea stagnalis, was investigated by the intracellular injection of Procion Yellow and, for the Yellow Cells only, of horseradish peroxidase. The cerebral ganglia neurosecretory cells (Light Green Cells, Caudodorsal Cells and Canopy Cells) had discrete neurohaemal organs and their axons projected exclusively to nerves and connectives close to the central nervous system. The Light Green Cells had single, undividing axons, which projected exclusively to the ipsilateral median lip nerve. Hormone release is thought to take place principally from the lateral edges of axons, at various points along their lengths, within the median lip nerve. The Caudodorsal Cells projected to the cerebral commissure, where their axons often branched before terminating at the edge of the neuropil. The degree of axonal branching and the location of the Caudodorsal Cell terminals varied widely in different cells. Axon terminals penetrated the perineurium and travelled for several hundred micrometres within the connective tissue sheath of the cerebral commissure. Again, release of neurosecretory material at various points along their lengths seems likely. The Canopy Cells (a pair of individually identifiable giant cells) had a single axon, which projected to the contralateral cerebral ganglion via the cerebral commissure. Axons of left and right Canopy Cells were closely apposed in the cerebral commissure and this is the likely site of the electrotonic junction known to connect them. Neurohaemal organs for the Caudodorsal Cells are the ipsilateral lateral lobe, cerebral commissure and contralateral median lip nerve. Neurosecretory neurones whose cell bodies were located in the pleural, parietal and visceral ganglia (Yellow Cells, Yellow-green Cells and Dark Green Cells) had extensive non-localized neurohaemal areas in the connective tissue sheath surrounding the central ganglia as well as peripheral nerve projections. The Yellow Cells had one or two axons, which, in neurones located in the visceral and right parietal ganglia, projected extraganglionically to the central sheath or to the intestinal and internal right parietal nerves. These nerve projections are appropriate for the innervation of the kidney, the peripheral target organ of the Yellow Cells. Yellow Cells, located in the pleural ganglia, only had axonal projections to the central sheath. Yellow Cells and Yellow-green Cells had well developed dendritic branching terminating in the central neuropil. Yellow-green Cells project mainly to the anal and external right parietal nerves. Pleural ganglia Dark Green Cells had a few terminals located beneath the perineurium of the pleural ganglia but most of their axonal projections were to peripheral nerves. All Dark Green Cells projected to the ipsilateral pedal ganglion and then to pedal nerves. In addition, some pleural Dark Green Cells had further projections to the internal and external right parietal nerves and median lip nerve of the cerebral ganglion. The widespread distribution of Dark Green Cell axons was consistent with their supposed role in regulating ion and water transport across the skin of the foot and mantle. The electrotonic junctions known to connect Dark Green Cells whose cell bodies are close together on the pleural ganglion surface are located in the pleural ganglion, pleuro-pedal connective and pedal ganglion.

Ultrastructural analysis of peptide-hormone release by exocytosis

The release of neuropeptides and other proteinaceous hormones by exocytosis was studied in some detail by means of tannic acid in combination with glutaraldehyde (TAGO-method). This method strongly enhances the electron density of extracellular proteins including exocytosed secretory products, and therefore facilitates visualization of the release process in qualitative and quantitative respects. This study included a variety of neurosecretory cell types in the CNS of the freshwater snail L. stagnalis, the storage and glandular lobes of the corpora cardiaca of the locust L. migratoria, and the posterior pituitary of the rat. In addition, peptide secreting cells in the anterior pituitary were examined. The cytochemical specificity of the TAGO-method as well as the significance of "frozen" contents of exocytosing secretory granules in mammotrophs and somatotrophs of the rat is discussed.

Sprouting and functional regeneration of an identified neuron in Helisoma

A pair of identified neurons in the snail, Helisoma, rapidly regenerate after severance of their axons and functionally reinnervate their former target organs, the salivary glands. Functional reinnervation occurred in 40% of the preparations within 6 days and in 93% of those examined on or after day 7 following axon severance. Neuronal regeneration is highly specific. For instance, in each case in which the salivary glands were found to be reinnervated and simultaneous records were taken from gland cells and their normal effector neuron, this neuron (neuron no. 4) was found to have reinnervated the glands. In addition, regeneration following total denervation of the feeding musculature can result in feeding movements which appear normal on visual inspection and by myographic recordins. Neural regeneration is manifested by extensive axonal sprouting. The site of initiation of sprouts is a function of the crush site, but sprouting is not totally restricted to the vicinity of the crush. In addition, the initial regeneration and axonal sprouting following severance of these axons is not dependent upon the presence of their original target organs.

Anatomy and ultrastructure of the axons and terminals of neurons R3-R14 in Aplysia

Using light and electron microscopy and autoradiography, we have traced the axons of neurons R3-R14 in the parietovisceral ganglion (PVG) of Aplysia to terminal fields associated with vascular tissue. The axons are identified by their large size (15-30 micrometer diameter), extensive glial infolding, characteristic dense core vesicles (DCV; approximately 180 nm diameter), and specific, rapid uptake of 3H-glycine. Each neuron in this homogeneous group sends an axon via the branchial nerve to the pericardial region surrounding the junction of the efferent gill vein and the heart. R14 also sends axons to major arteries near the PVG. The R3-R14 axons branch extensively; we estimate that there are at least several hundred per cell. Branches along axons in the branchial nerve exit the nerve, subdivide, and end blindly in the sheath which is bathed by hemolymph. Similar blind endings from R3R14 occur in the sheath of the PVG (Coggeshall, '67). Axonal branches in the pericardial region and the special R14 axons in the arterial walls form both varicose endings near and terminals in contact with vasvular smooth muscle. All R3-R14 endings are free of glia, packed with DCV, show occasional omega-shaped profiles and rapidly take up 3H-glycine. R3-R14 manufacture specific low molecular weight peptides (Gainer and Wollberg, '74), and both the cell bodies (Iliffe et al., '77) and the germinals contain unusually high concentrations of glycine. The presence of peptides as putative neurohormones and sheath endings (neurohormonal release areas) are consistent with R3-R14 being neurosecretory (Coggeshall et al., '66). While glycine could not be a circulating hormone due to its high circulating levels (Iliffe et al., '77), glycine could act as a local chemical messenger between R3-R14 and smooth muscle. The terminal morphology of R3-R14 is consistent with these neurons having both synaptic-type and neurosecretory-type functions.

The anatomy of neurosecretory neurones in the pond snail Lymnaea stagnalis (L.)

The anatomy of three neurosecretory cell types in the central nervous system (c.n.s.) of the gastropod mollusc Lymnaea stagnalis (L.)- the Dark Green Cells, Yellow Cells and Yellow-green Cells-has been studied by using bright and dark field illumination of material stained for neurosecretion by the Alcian Blue-Alcian Yellow technique. The neuronal geometry of single and groups of neurosecretory cells of the various types has been reconstructed from serial sections, and the likely destination of most of their processes has been determined. Dark Green Cells are monopolar, occur exclusively within the central nervous system (c.n.s.), have few or no branches terminating in neuropile, and send axons to the surface of the pleuro-parietal and pleuro-cerebral connectives. The majority of Dark Green Cell axons however (80-85%), project down nerves which innervate ventral and anterior parts of the head-foot, the neck and the mantle. Dark Green Cell axons can be found in small nerves throughout these areas, and may terminate in a find plexus of axons on the surfaces of the nerves. Since previous experimental work has shown that the Dark Green Cells are involved in osmotic or ionic regulation, these results suggest that the target organ of the Dark Green Cells may be the skin. Yellow Cells occur both within and outside the c.n.s. They are usually monopolar, but can be bipolar. They have several axons which normally arise separately from a single pole of the cell body, or close to it. One or more processes leave the cell proximal to the point where separate axons arise, and may run unbranched for some distance through neuropile before terminating in fine brances and blobs of various sizes. These branches may release hormone inside the c.n.s. Yellow-green Cells are mono-, bi- or multi-polar, and like the Yellow Cells are found both within and outside the c.n.s. Some Yellow-green Cells, though not all, have projections which terminate in neuropile in fine branches and blobs. Yellow-green Cell bodies which occur in nerves can project back along the nerve into the c.n.s. The axons of Yellow Cells and Yellow-green Cells project to release sites in various ways. Some project into the connective tissue shealth of the c.n.s., which serves as a neurohaemal organ, either directly through the surface of a ganglion, or from the pleuro-cerebral or pleuro-parietal connectives. Other axons leave the c.n.s. via nerves leaving the left and right parietal and visceral ganglia; projections into the intestinal, anal, and internal right parietal nerves being most numerous. Axons which may be from either, or both Yellow Cells and Yellow-green Cells, can be found along the entire unbranched lengths of these nerves, and in subsequent branches which innervate organs lying in the anterior turn of the shell. All of these orgnas are closely associated with the lung cavity...

Neuronal and non-neuronal control of the neurosecretory caudo-dorsal cells of the freshwater snail Lymnaea stagnalis (L.)

The cerebral ganglia of the freshwater snail Lymnaea stagnalis contain two clusters of neurosecretory Caudo-Dorsal Cells (CDC). These cells produce a neurohormone which stimulates ovulation. Ganglion transplantation and quantitative electron microscopy show that neuronal isolation of the cerebral ganglia complex (CCC) results in an activation of the CDC. It was, therefore, concluded that the CDC are controlled by an inhibitory neuronal input originating outside the cerebral ganglia. Ultrastructural studies on synaptic degeneration in the CCC suggest that this input reaches the CDC via a special type of synapse-like structure, the type C-SLS. Furthermore, transplantation of CCC into acceptor snails leads to a reduced release and an increased intracellular brekdown of neurohormone in the CDC of the nervous system of the acceptors. It is supposed that these phenomena are caused by the release of an (unknown) factor from the transplanted CCC. Special attention was given to the formation and degradation of a peculiar type of neurohormone granule, the large electron dense granule. The physiological significance of the neuronal and non-neuronal control mechanisms which regulate CDC activity is discussed.