Structural aspects, potassium stimulation and calcium dependence of nonsynaptic neuropeptide release by the egg laying controlling caudodorsal cells of Lymnaea stagnalis

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.

Exocytotic release of vasoative intestinal polypeptide and serotonin from mucosal nerve fibres and endocrine cells of the intestine of the goldfish (Carassius auratus) and the tilapia (Oreochromis mossambicus): an ultrastructural study

In earlier studies were determined the effect, presence and ultrastructure of vasoactive intestinal polypeptide (VIP) and 5-hydroxytryptamine (5-HT)-containing nerve fibres in the tilapia and goldfish intestinal mucosa. 5-HT-labelled varicosities were found close to the epithelial cells; however, synaptic membrane specializations have never been observed. VIP-like immunoreactive nerve fibres appear to be located less frequently close to the goldfish epithelium, as in the tilapia intestine, in which the distance between the VIP- or 5-HT-labelled varicosities and the epithelial cells was also rather large (more than 2 micros). To establish a possible role of VIP and 5-HT as neurotransmitters involved in the regulation of fish intestinal epithelium both electron microscopical and immunoelectron microscopical methods were used to visualize the release of 5-HT and VIP from nerve fibres. We found exocytoses from VIP-ergic and serotonergic varicosities in the muscle layers of both fish. Directly underneath the intestinal epithelium of the goldfish, it was demonstrated that 5-HT could be released from scarce varicosities. The release of 5-HT in the tilapia intestinal mucosa could only be observed from endocrine cells.

A pattern confirmed and refined--synaptic, nonsynaptic and parasynaptic exocytosis

Neurons are now known to produce a variety of types of chemical transmitters. Classical transmitters are stored within synaptic vesicles which undergo synaptic exocytosis in association with presynaptic thickenings. The larger, dense-cored secretory granules present in most neurons contain neuropeptides and mainly discharge their contents at morphologically undifferentiated (i.e. nonsynaptic) sites. The synaptic character of vesicle discharge enables transmitters to exercise a highly focal action, whereas nonsynaptic release probably relates to the slow rate of degradation of many neuropeptides and their consequent widespread diffusion and sphere of action. However, one variant of the basic pattern, involving the restriction of granule discharge to areas of the terminal plasmalemma situated adjacent to the postsynaptic cells (i.e. a parasynaptic configuration), enables a degree of targeted peptide discharge to be achieved. The diversity of patterns of neural exocytosis adds a further dimension to the complexity of nervous function.

Light and electron microscopic immunocytochemical demonstration of synthesis, storage, and release sites of the neuropeptide calfluxin in Lymnaea stagnalis

The cerebral caudodorsal cells (CDC) of the pulmonate snail Lymnaea stagnalis control egg-laying and associated behaviors. They produce various peptides derived from two precursor molecules, proCDCH-I and II, one of which is calfuxin (CaFl). CaFL is involved in the control of the activity of a female accessory sex gland, the albumen gland. At the light microscope level, using an antibody raised against synthetic CaFl, immunoreactivity was demonstrated in all CDC somata as well as in the neurohemal CDC terminals in the periphery of the cerebral commissure and in the CDC axon collaterals in the inner region of the commissure. A group of small neurons in each cerebral ganglion was also immunopositive. At the ultrastructural level, secretory granules (SG) and large electron-dense granules (LG), formed by the Golgi apparatus and thought to be involved in intracellular degradation of secretory material, were clearly immunolabeled. The density of immunolabeling of LG was 3.3 times greater than that of SG, indicating that CaFl is preferentially packed into LG. In the LG, the density of immunolabeling with anti-alpha CDCP (alpha CDCP is also a peptide derived from proCDCH-I and II) was 10 times greater than in SG, suggesting that CaFl and alpha CDCP are processed and sorted in (quantitatively) different ways. In the neurohemal terminals SG release their CaFl-immunopositive contents into the hemolymph by the process of exocytosis, whereas collaterals release such contents into the intracellular space of the intercerebral commissure. It is proposed that neurohemally released CaFl acts upon the albumen gland, whereas CaFl released from the collaterals may influence the activity of central neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Three-dimensional architecture of identified cerebral neurosecretory cells in an insect

The organization of identified neurosecretory cell groups in the larval brain of the tobacco hornworm, Manduca sexta, was investigated immunocytologically. Computer-assisted three-dimensional reconstruction was used to examine the architecture of the neurosecretory cell groups. The group III lateral neurosecretory cells (L-NSC-III) which produce the prothoracicotropic hormone are located dorsolaterally in the protocerebrum and extend axons medially that decussate to the contralateral lobe prior to exiting the brain through the nervi corporis cardiaci I + II. The group IIa2 medial neurosecretory cells (M-NSC IIa2) are located anteriorly in the medial dorsal protocerebrum. The axons of these cells also exit the brain via the contralateral nervi corporis cardiaci I + II. However, their axons traverse a different pathway through the brain from that of the L-NSC III axons. Each of the cell groups possesses elaborate dendrites with terminal varicosities. The dendrites can be classified into specific fields based upon their location and projection pattern within the brain. The dendrites for these two neurosecretory cell groups overlap in specific regions of the protocerebral neuropil. After the axons of these neurosecretory cells exit the brain through the retrocerebral nerve, they innervate the corpus allatum where they arborize to form neurohemal terminals in strikingly different patterns. The L-NSC III penetrate throughout the glandular structure and the M-NSC IIa2 terminals are restricted to the external sheath. A third group of cerebral neurosecretory cells, the ventromedial neurons (VM) which stain with the monoclonal antibody to prothoracicotropic hormone in Manduca, are located anteriorly in the medial region of the brain. The axons of these cells do not exit the brain to the retrocerebral complex, but rather pass through the circumesophageal connectives and ventral nerve cord. These neurons appear to be the same VM neurons that produce eclosion hormone. One dendritic field of the L-NSC III terminates in close apposition to the VM neurons. The distinct morphologies of these neurosecretory cell groups in relation to other cell groups and the distribution of neuropeptides within the neurons suggest that insect neurosecretory cells, like their vertebrate counterparts, may have multiple regulatory roles.

Immuno-electron microscopy of sorting and release of neuropeptides in Lymnaea stagnalis

The cerebral caudodorsal cells of the pulmonate snail Lymnaea stagnalis control egg laying and egg laying behavior by releasing various peptides derived from two precursors. The biosynthesis, storage, intracellular breakdown and release of three caudodorsal cell peptides were studied by means of immuno-electron microscopy using antisera raised to fragments of these peptides: (1) Caudodorsal Cell Hormone-I (CDCH-I; derived from precursor I), (2) Caudodorsal Cell Hormone-II (CDCH-II; from precursor II), and (3) alpha-Caudodorsal Cell Peptide (alpha CDCP; from both precursors). After affinity purification of the antisera, the specificity of the sera was confirmed with dotting immunobinding assays. From the ultrastructural immunocytochemical data it has been concluded that the precursor molecules are cleaved at the level of the Golgi apparatus after which the C-terminal parts (containing alpha CDCP) and N-terminal parts (containing DCDH-I or CDCH-II) are sorted and preferentially packaged into large electron-dense granules (MD 150 nm), respectively. Very probably, the content of the large electron-dense granules is degraded within the cell body. The immunoreactivity of the secretory granules increases during discharge from the Golgi apparatus, indicating further processing. At least a portion of the secretory granules contains all three peptides, as shown by double and triple immunopositive stainings whereas other granules appear to contain only one or two of these peptides. The caudodorsal cells release multiple peptides via exocytosis from neurohemal axon terminals into the hemolymph and from blindly ending axon collaterals into the intercellular space of the cerebral commissure (nonsynaptic release).

Discharge induction in molluscan peptidergic cells requires a specific set of autoexcitatory neuropeptides

The peptidergic caudodorsal cells of the pond snail Lymnaea stagnalis generate long lasting discharges of synchronous spiking activity to release their products. During caudodorsal cell discharges a peptide factor is released which induces similar discharges in silent caudodorsal cells [Ter Maat A. et al. (1988) Brain Res. 438, 77-82]. To identify this factor, the electrophysiological effects of putative caudodorsal cell gene products, calfluxin, caudodorsal cell hormone, four alpha caudodorsal cell peptides and three beta caudodorsal cell peptides, were tested individually and in various combinations. Calfluxin, alpha caudodorsal cell peptide and beta 1 caudodorsal cell peptide each had no effect on membrane potential or excitability of the caudodorsal cells. All other caudodorsal cell peptides caused excitatory responses, but did not induce discharges. Instead, only a specific combination of four caudodorsal cell peptides, caudodorsal cell hormone and alpha caudodorsal cell peptide (1-11, 3-11 and 3-10), evoked caudodorsal cell discharges with similar characteristics to electrically evoked discharges. Incomplete versions of this combination failed to cause a discharge. In addition, antibodies to caudodorsal cell hormone or alpha caudodorsal cell peptide reduced caudodorsal cell excitability and prevented the generation of discharges by electrical stimulation. These results suggest that excitatory autotransmission caused by four caudodorsal cell peptides provides a means to amplify excitatory inputs, thus leading to the generation of the all-or-nothing caudodorsal cell discharge.

Secretory activity and postembryonic development of the tentacle sensory system controlling growth hormone-producing neurons in Lymnaea stagnalis

The cerebral neuroendocrine peptidergic light green cells (LGC) of the freshwater snail Lymnaea stagnalis regulate body growth. The LGC are controlled by a tentacle sensory system that consists of two types (S1 and S2) of primary sensory neuron located at the base of each tentacle. Sensory (S2) axons make synaptic contacts (type A synapse-like structures) with the somata and axons of the LGC, where they release the contents of secretory granules, by exocytosis (demonstrated with the ultrastructural tannic acid-Ringer incubation method). Ultracytochemistry indicates that the granule contents are glycoproteinaceous. Furthermore, the S2 axons release secretory material in a nonsynaptic fashion into the interneuronal space of the central nervous system (CNS), at the level of the neuropiles of the cerebral ganglia and of the cerebral commissure. This release occurs by exocytosis from nonsynaptic release sites. It is proposed that the tentacle sensory system not only (synaptically) controls LGC activity but also influences other, remote neuronal targets in the CNS in a nonsynaptic ("at long distance," "paracrine," "hormone-like") fashion. Already in newly hatched snails (with a shell height of 1 mm) S2 axons show a fair rate of exocytotic activity, in both synaptic and nonsynaptic respects. During postembryonic development the secretory capacity of the S2 sensory neurons increases markedly, by increases in (1) the number of axons, (2) the size of the secretory granules, and (3) exocytosis activity. This increased capacity may meet a growing demand of the developing CNS, including the LGC, for neurochemical input from the tentacle sensory system.

Developmental and comparative aspects of nonsynaptic release by the egg-laying controlling caudodorsal cells of basommatophoran snails

In an immunoelectron microscope study the postembryonic development of the cerebral caudodorsal cells (CDC) in the freshwater snail Lymnaea stagnalis was studied as well as the development of similar neurons in other basommatophoran families. The CDC of adult L. stagnalis control egg-laying and associated behaviors by releasing various peptides, including the ovulation hormone CDCH. The CDC release peptides from neurohemal axon terminals and from nonsynaptic release sites of axon collaterals. During postembryonic development the collateral system develops synchronously with the neurohemal area. The first collaterals appear in the cerebral commissure of juvenile snails (10 mm shell height; S = 10). Up to S = 30 they gradually increase in size and length and eventually run through the entire inner compartment. Secretory granules in both collaterals and neurohemal axon terminals increase in size as well. Immunoelectron microscopy combined with the TARI-method for the demonstration of exocytosis indicates that CDCH-release from collaterals and neurohemal terminals occurs already in S = 10; exocytosis of immunoreactive granule contents takes place from nonsynaptic release sites, unspecialized areas of the axolemma of the collaterals. Release activity in the collaterals gradually increases up to S greater than or equal to 20. Neurohemal release activity shows a similar picture except for a steep increase in adult snails. A distinct glial sheath, separating the neurohemal area from the collateral system, appears around S = 15. Representatives of three families of Basommatophora, viz. the lymnaeid L. ovata, the planorbid Planorbis planorbis, and the bulinid Bulinus truncatus possess a well-developed collateral system showing many signs of exocytosis. A glial sheath separates the collaterals from the neurohemal area. Secretory granules of the CDC in L. ovata stain weakly positive with the anti-CDCH antiserum. Since the other Basommatophora did not show immunoreactivity, the chemical structure of egg laying peptides in Basommatophora seems to be genus specific. Apparently the secretory activity of both the neurohemal area and the collateral system is not only important in the sexually mature animal, being involved in the control of egg laying and egg-laying behavior, but also in the juvenile snail. The finding of a collateral system in representatives of three basommatophoran families strongly indicates the importance of the system for the control of reproduction in basommatophoran snails in general.

Quantitative immunoelectron microscopy and tannic acid study of dynamics of neurohaemal and non-synaptic peptide release by the caudodorsal cells of Lymnaea stagnalis

The caudodorsal cells (CDCs) of the freshwater snail Lymnaea stagnalis control egg laying and egg-laying associated behaviours by releasing various peptides including the ovulation hormone CDCH. Previously it has been shown that release occurs (1) into the haemolymph from neurohaemal axon terminals in the outer compartment of the cerebral commissure, and (2) into the intercellular space of the central nervous system from non-synaptic release sites of axon collaterals in the inner compartment of the commissure. Outer and inner compartments are separated by a sheath of glial cells. In the present study the secretory dynamics of neurohaemal and collateral release have been studied. Immunoelectron microscopy with an antibody against a synthesized fragment of the egg-laying hormone [CDCH] indicates that CDCH is released by exocytosis from both sites: positive immunoreaction was found for the contents of secretory granules and contents that underwent exocytosis, and furthermore in the intercellular spaces of the inner and outer compartments. Quantitative (immunogold) electron microscopy combined with either the tannic acid-glutaraldehyde-osmium tetroxide (TAGO) method or the tannic acid-Ringer incubation (TARI) method for the visualization and quantification of exocytosis of CDCH, shows different dynamics of neurohaemal and collateral CDCH release. Neurohaemal release is strongly increased during electrical activity of the CDCs (active state). This increase does not only appear from an increased number of (immunopositive) exocytoses (3X) but also from increases in (1) the percentages of all stationary and all exocytosing granule contents that are immunopositive (both increase from 70% to 85%), (2) the degree of immunopositivity per exteriorized granule content (2X) and (3) the degree of immunopositivity in the intercellular space of the neurohaemal area (5X). Collaterals show a different picture: CDCH release particularly occurs during electrical silence (resting and inhibited states). No effect was noted of the electrical state of the CDCs on the percentages of CDCH-immunoreactive stationary or exteriorized granule contents, nor on the degree of immunopositivity of the exteriorized contents. Furthermore, the degree of immunopositivity in the intercellular space of the inner compartment is drastically decreased (8X). Finally, both in the resting and the active state, the percentage of CDCH-positive exocytosing contents in the collaterals is smaller than that of CDCH-positive stationary contents whereas in the neurohaemal area these percentages do not differ.(ABSTRACT TRUNCATED AT 400 WORDS)