Ultrastructural aspects of peptidergic modulation in the peripheral nervous system of Helix pomatia

Chemical-neuroanatomical organization of peripheral sensory-efferent systems in the pond snail (Lymnaea stagnalis)

Perception and processing of chemical cues are crucial for aquatic gastropods, for proper elaboration of adaptive behavior. The pond snail, Lymnaea stagnalis, is a model species of invertebrate neurobiology, in which peripheral sensory neurons with different morphology and transmitter content have partly been described, but we have little knowledge regarding their functional morphological organization, including their possible peripheral intercellular connections and networks. Therefore the aim of our study was to characterize the sensory system of the tentacles and the lip, as primary sensory regions, and the anterior foot of Lymnaea with special attention to the transmitter content of the sensory neurons, and their relationship to extrinsic elements of the central nervous system. Numerous bipolar sensory cells were demonstrated in the epithelial layer of the peripheral organs, displaying immunoreactivity to antibodies raised against tyrosine hydroxylase, histamine, glutamate and two molluscan type oligopeptides, FMRFamide and Mytilus inhibitory peptide. A subepithelial plexus was formed by extrinsic serotonin and FMRFamide immunoreactive fibers, whereas in deeper regions axon processess of different origin with various immunoreactivities formed networks, too. HPLC-MS assay confirmed the presence of the low molecular weight signal molecules in the three examined areas. Following double-labeling immunohistochemistry, close arrangements were observed, formed by sensory neurons and extrinsic serotonergic (and FMRFamidergic) fibers at axo-dendritic, axo-somatic and axo-axonic levels. Our results suggest the involvement of a much wider repertoire of signal molecules in peripheral sensory processes of Lymnaea, which can locally be modified by central input, hence influencing directly the responses to environmental cues.

Nitric oxide-coupled signaling in odor elicited molecular events in the olfactory center of the terrestrial snail, Helix pomatia

Olfaction, a chemosensory modality, plays a pivotal role in the orientation and behavior of invertebrates. The central olfactory processing unit in terrestrial stylomatophoran snails is the procerebrum, which contains NO synthesizing interneurons, whose oscillatory currents are believed to be the base of odor evoked memory formation. Nevertheless, in this model the up- and downstream events of molecular cascades that trigger and follow NO release, respectively, have not been studied. Immunocytochemistry and flow cytometry studies performed on procerebral neural perikarya isolated from the snail Helix pomatia revealed cell populations with discrete DAF-2 fluorescence, indicating the release of different amounts of NO. Glutamate increased the intensity of DAF-2 fluorescence, and the number of DAF-2 positive non-bursting interneurons, through a mechanism likely to involve an NMDA-like receptor. Similarly to glutamate, NO activation induced an increase in intracellular cGMP levels through activation of soluble guanylyl cyclase. Immunohistochemical localization of proteins possessing the phosphorylated target sequence of AGC family kinases (RXXS/T-P), among them protein kinase A (RRXS/T-P), showed striking similarities to the distribution of NOS/cGMP. Activators of cyclic nucleotide synthesis increased the AGC-kinase-dependent phosphorylation of discrete proteins with 28, 45, and 55kDamw. Importantly, exposure of snails to an attractive odorant induced hyperphosphorylation of the 28kDa protein, and increased levels of cGMP synthesis. Protein S-nitrosylation and intercellular activation of protein kinase G were also suggested as alternative components of NO signaling in the snail procerebrum. The present results from Helix pomatia indicate an important role for procerebrum NO/cGMP/PKA signaling pathways in the regulation of olfactory (food-finding) behavior.

Nerve-granular cell communication in the atrium of the snail Achatina achatina occurs via the cardioexcitatory transmitters serotonin and FMRFamide

In the present study, the anatomical association and functional interaction between nerve fibres and granular cells in the atrium of the snail Achatina achatina are investigated using a combination of scanning electron microscopy (SEM), pharmacological and immunofluorescence techniques. The SEM studies support a close anatomical association of axons with granular cells and new features of surface morphology are revealed. Pharmacological experiments showed that both serotonin and FMRFamide were able to induce degranulation of granular cells and the release of cysteine-rich atrial secretory protein. Serotonin- and FMRFamide-immunoreactive nerve fibres were observed at variable distances from granular cells, ranging from close contact to distances as far as the diameter of a muscle bundle. These results suggest that serotonin and FMRFamide play a role as paracrine excitatory transmitters in nerve-to-granular cell communication.

Excitatory neurotransmitters in the tentacle flexor muscles responsible for space positioning of the snail olfactory organ

Recently, three novel flexor muscles (M1, M2 and M3) in the posterior tentacles of the snail have been described, which are responsible for the patterned movements of the tentacles of the snail, Helix pomatia. In this study, we have demonstrated that the muscles received a complex innervation pattern via the peritentacular and olfactory nerves originating from different clusters of motoneurons of the cerebral ganglia. The innervating axons displayed a number of varicosities and established neuromuscular contacts of different ultrastructural forms. Contractions evoked by nerve stimulation could be mimicked by external acetylcholine (ACh) and glutamate (Glu), suggesting that ACh and Glu are excitatory transmitters at the neuromuscular contacts. Choline acetyltransferase and vesicular glutamate transporter immunolabeled axons innervating flexor muscles were demonstrated by immunohistochemistry and in Western blot experiments. Nerve- and transmitter-evoked contractions were similarly attenuated by cholinergic and glutamatergic antagonists supporting the dual excitatory innervation. Dopamine (DA, 10⁻⁵ M) oppositely modulated thin (M1/M2) and thick (M3) muscle responses evoked by stimulation of the olfactory nerve, decreasing the contractions of the M1/M2 and increasing those of M3. In both cases, the modulation site was presynaptic. Serotonin (5-HT) at high concentration (10⁻⁵ M) increased the amplitude of both the nerve- and the ACh-evoked contractions in all muscles. The relaxation rate was facilitated suggesting pre- and postsynaptic site of action. Our data provided evidence for a DAergic and 5-HTergic modulation of cholinergic nerves innervating flexor muscles of the tentacles as well as the muscles itself. These effects of DA and 5-HT may contribute to the regulation of sophisticated movements of tentacle muscles lacking inhibitory innervation.

The nociceptin/orphanin FQ-like opioid peptide in nervous periesophageal ganglia of land snail Helix aspersa

The neuropeptide nociceptin/orphanin FQ (N/OFQ) and its receptor are members of the endogenous opioid peptide family. In mammals N/OFQ modulates a variety of biological functions such as nociception, food intake, endocrine, control of neurotransmitter release, among others. In the molluscs Cepea nemoralis and Helix aspersa the administration of N/OFQ produces a thermopronociceptive effect. However, little is known about its existence and anatomic distribution in invertebrates. The aim of this study was to provide a detailed anatomical distribution of N/OFQ like peptide immunoreactivity (N/OFQ-IL), to quantify the tissue content of this peptide, as well as to demostrate molecular evidence of N/OFQ mRNA in the nervous tissue of periesophageal ganglia of the land snail H. aspersa. Immunohistochemical, immunocytochemical, radioimmunoanalysis (RIA) and reverse transcription-polymerase chain reaction (RT-PCR) techniques were used. With regard to RT-PCR, the primers to detect expression of mRNA transcripts from H. aspersa were derived from the rat N/OFQ opioid peptide. We show a wide distribution of N/OFQ-IL in neurons and fibers in all perioesophageal ganglia, fibers of the neuropile, nerves, periganglionar connective tissue, aortic wall and neurohemal sinuses. The total amount of N/OFQ-IL in the perioesophageal ganglia (7.75 ± 1.75 pmol/g of tissue) quantified by RIA was similar to that found in mouse hypothalamus (10.1 ± 1.6 pmol/g of tissue). In this study, we present molecular evidence of N/OFQ mRNA expression. Some N/OFQ-IL neurons have been identified as neuroendocrine or involved in olfaction, hydro-electrolyte regulation, feeding, and thermonociception. Therefore, we suggest that N/OFQ may participate in these snail functions.

Organization of the serotonergic innervation of the feeding (buccal) musculature during the maturation of the pond snail Lymnaea stagnalis: a morphological and biochemical study

The serotonergic innervation of the buccal musculature responsible for feeding (radula protraction) was investigated during the maturation of the pond snail, Lymnaea stagnalis L., applying light and electron microscopic immunohistochemistry and biochemical approaches. According to epifluorescence and laser confocal microscopy, the first 5-HT-like-immunoreactive (5-HTLIR) processes appeared on the surface of the musculature at the postmetamorphic E80% embryonic stage. Until hatching, the innervation continued to increase in density, showing axon arborizations with projections into the deeper muscle levels. An adult-like pattern of 5-HTLIR innervation appeared at P2-P3 juvenile stages. At the ultrastructural level, close (16-20 nm) but mostly unspecialized neuromuscular contacts were formed by both unlabeled and 5-HTLIR axon profiles from the E80% embryonic stage. Labeled processes were also found located relatively far from the muscle cells. An HPLC assay showed a gradual increase of the 5-HT level in the buccal mass during development. The buccal mass was characterized by a single-component high-affinity 5-HT uptake system, and 5-HT release could be evoked by 100 mM K(+) and blocked in Ca(2+) -free medium. It is suggested that 5-HT plays a wide modulatory role in the peripheral feeding system and is also involved in the functional maturation of the muscle system.

Peptidergic modulation of serotonin and nerve elicited responses of the salivary duct muscle in the snail, Helix pomatia

In the present study, the ability of a range of endogenous neuropeptides to modulate neuromuscular transmission was examined in the salivary duct neuromuscular preparation of the terrestrial snail, Helix pomatia. Immunohistochemical and physiological techniques were used to localize the neuropeptides (GSPYFVamide, CARP, FMRFamide and APGWamide) and to investigate whether contractions elicited by the stimulation of the salivary nerve or by exogenously applied 5-HT are subject to peptidergic modulation. All of the neuropeptides studied decreased the tonus by a direct action on the muscle fibers in a concentration dependent manner in a range of 10(-9) to 10(-6)M. Neuropeptides distinctly affected the 5-HT evoked contraction or relaxation and GSPYFVa and APGWa decreased also the amplitude of contractions elicited by the stimulation of the salivary nerve. All four neuropeptides facilitated the relaxation phase providing further evidence for the postsynaptic action of neuropeptides. Low Ca(2+)/high Mg(2+) saline abolished the nerve-elicited contractions, however the denervated muscle retained the ability to contract due to the mobilization of the Ca(2+) from intracellular stores. It was concluded, that peptides belonging to different peptide families exerted their effects through pre- and postsynaptic mechanisms. The modulatory effect of neuropeptides can be assigned to the partial co-localization of 5-HT and neuropeptides in the nerves innervating muscles of the salivary duct, as it was demonstrated by double-labeling immunohistochemistry. A double origin of the 5-HTergic innervation was demonstrated, including efferents originating from both the cerebral and visceral ganglia.

Atrial granular cells of the snailAchatina fulicarelease proteins into hemolymph after stimulation of the heart nerve

The atrium of the gastropod mollusc Achatina fulica receives rich innervation and contains numerous granular cells (GCs). We studied the atrial innervation and discovered that axon profiles typical in appearance of peptidergic neurons form close unspecialized membrane contacts with GCs. Then,we investigated, at both morphological and biochemical levels, the effect of electrical stimulation of the heart nerve on GCs of Achatina heart perfused in situ. The ultrastructural study demonstrated changes in granule morphology consistent with secretion. These events included alteration of granule content, intracellular granule fusion and formation of complex degranulation channels, within which the granule matrix solubilized. It was shown that electrical stimulation resulted in a significant increase of the total protein concentration in the perfusate. Furthermore, SDS-PAGE analysis of the perfusate revealed three new proteins with molecular masses of 16, 22,and 57 kDa. Affinity-purified polyclonal antibodies against the 16 kDa protein were obtained; the whole-mount immunofluorescence technique revealed the presence of this protein in the granules of atrial GCs. In GCs of the stimulated atrium, a progressive loss of their granular content was observed. The results suggest that the central nervous system can modulate the secretory activity of the atrial GCs through non-synaptic pathways.

Electrical properties and cell-to-cell communication of the salivary gland cells of the snail, Helix pomatia

The aim of the present study was to assess the cellular mechanism of secretion in the salivary gland of the snail, Helix pomatia, using electrophysiological, electron microscopic and immunohistochemical techniques. A homogeneously distributed membrane potential (-56.6 +/- 9.8 mV) was determined mainly by a K+ -electrochemical gradient and partly by the contribution of the electrogenic Na+ -pump and Cl- conductance. Low resistance electrical coupling sites were identified physiologically. Transmission electron microscopy and innexin 2 antibody revealed the presence of gap-junction-like membrane structures between gland cells. It is suggested that gap-junctions are sites of electrotonic intercellular communication, which integrate the gland cells into a synchronized functional unit in the acinus. Stimulation of the salivary nerve elicited secretory potentials (depolarization) which could be mimicked by local application of acetylcholine, dopamine or serotonin. In voltage-clamp experiments four major conductances were identified: a delayed rectifier (IK), a transient (IA) and a Ca2+ -activated outward K+ current (IK(Ca)) and Ca2+ -inward currents (ICa). It is suggested that one or more of these conductances may give rise to a stimulus activated secretory potential leading to excitation-secretion coupling and subsequent the release of the mucus from the gland cells.

G-protein coupled activation of potassium channels by endogenous neuropeptides in snail neurons

Members of the mytilus inhibitory peptide (MIP) family play an important role in the modulation of many physiological processes in molluscs. The signal transduction pathways affected by the MIP effect have not, however, been elucidated. Application of guanosine 5'-[gamma-thio]triphosphate tetralithium salt (GTPgammaS), guanosine 5'-[beta-thio]diphosphate trilithium salt (GDPbetaS), the G-protein inhibitor suramin and pertussis toxin (PTX) demonstrated the involvement of the PTX-insensitive G-protein in the signal transduction pathway mediating MIP effects. Both G-protein alpha(i) and betagamma subunits were identified in D-neurons of Helix pomatia by immunoblotting. Their role in signal transduction was shown in electrophysiological experiments, which supported the notion that, in addition to the Galpha subunit, the betagamma dimer also participates in the neuropeptide-induced activation of K-channels in snail neurons. Finally, neuropeptide-activated responses were inhibited by the activation of adenylyl cyclase and by blockers of the phospholipase pathway. We suggest that bifurcation of the signal transduction takes place at the level of G-protein subunits. The alpha subunit may have a direct effect on adenylyl cyclase, while the betagamma subunit may have a direct effect on phospholipase enzymes.

Functional morphology of the salivary gland of the snail, Helix pomatia: a histochemical and immunocytochemical study

Functional morphology of Helix pomatia salivary gland cells was studied at light microscopic level by using different histochemical methods. Three cell types could be demonstrated in the salivary gland: mucocytes, granular and vacuolated cells. The distribution and the number of the different cell types were different in active and inactive snails. In active feeding animals, dilatated interlobular salivary ducts were observed, which were never present in inactive ones. In active animals an additional cell type, the cystic cell could also be observed. Periodic acid Schiff staining revealed both mucuos and serous elements in the salivary gland. Furthermore, hematoxyline-eosin staining indicated the occurrence of a cell layer with high mitotic activity in the acini. Applying immunohistochemical methods with monoclonal mouse anti-human Ki-67 clone, B56 and polyclonal rabbit anti-human Ki-67 antibodies, we also were able to demonstrate the occurrence of dividing cells in the salivary gland. Analysis of 1-2 microm semi-thin Araldite sections stained with toluidine-blue showed that the saliva can be released, in addition to possible exocytosis, by the lysis of cystic cells. Using an apoptosis kit, we could also establish that this process was due to rather an apoptotic than a necrotic mechanism. In the salivary gland of active snails, where an intensive salivation takes place, significantly more apoptotic cells occurred, if compared to that of inactive animals. It is suggested that programmed cell death may also be involved in the saliva release.

A Possible Stimulatory Effect of FMRFamide on Neural Nitric Oxide Production in the Central Nervous System of Helix lucorum L

The anatomical and functional relationship between neurons expressing nitric oxide (NO) synthase and molluscan cardioexcitatory (FMRFamide)-like neuropeptides was studied in the central ganglia of Helix lucorum (Pulmonata, Gastropoda), applying NADPHdiaphorase (NADPHd) histochemistry to visualize NO synthase and immunocytochemistry to demonstrate FMRFamide (FMRFa) at the light microscopic level. The NO production of the ganglia was detected by the colorimetric Griess determination of nitrite, a breakdown product of NO. Effects of the NO synthase substrate amino acid L-arginine, the NO synthase inhibitor Nomega-nitro-L-arginine (NOARG), synthetic FMRFa and the FMRFa sensitive ion channel blocker amiloride hydrochloride on nitrite production were also tested. NADPHd reaction labeled nerve cells and fibers in the procerebra, mesocerebra and metacerebra within the cerebral ganglia, and cell clusters in the postcerebral ganglia. FMRFa immunolabeling could be observed within subpopulations of NADPHd positive cells and in pericellular varicose fibers surrounding NADPHd stained neurons. Nitrite production of the ganglia was stimulated by L-arginine (10- 20 mM) but was decreased by NOARG (1-2 mM). Synthetic FMRFa (0.830-3.340 mM) increased the nitrite production in a dose dependent manner, but was ineffective in the presence of NOARG. Amiloride hydrochloride (7.890 mM) reduced the FMRFa evoked nitrite production in all ganglia. This is the first description of an anatomical relationship between putative NO producing and FMRFa containing cells, suggesting a possible regulatory role of FMRFa in the NO mediated signaling in an invertebrate nervous system.

Dopamine and serotonin receptors mediating contractions of the snail, Helix pomatia, salivary duct

The combination of high performance liquid chromatography, bioassay and immunocytochemistry was applied to study the regulation of the salivary duct muscle of the snail, Helix pomatia. The major function of the duct appears to be to propel the saliva toward the buccal cavity during feeding. It has been established that serotonin and dopamine applied exogenously mimic the effect on the duct exerted by the stimulation of the salivary nerve. Immunohistochemistry revealed the presence of serotonin, but not dopaminergic nerve elements in the nerve and along the duct surface. However, both serotonin (14.9-15.5 pmol/mg) and dopamine (0.38-0.58 pmol/mg), as well as the synthesizing enzymes (tyrozine hydroxylase 0.28 pmol/mg tissue/h and DOPA 0.32 nmol synthesized DA/mg tissue/h) could regularly be assayed in the salivary duct by high performance liquid chromatography. When released following the stimulation of the salivary nerve, both monoamines were shown to interact with distinct membrane receptors. Dopamine elicited a sustained increase of the muscle tone in concentration-dependent manner (K(d)=1.5 microM). Mammalian D(1) receptor antagonist flupenthixol and fluphenazine attenuated, whereas the D(1) receptor agonist SKF-38393 mimicked the effect elicited by exogenous dopamine. Serotonin had a double effect on the salivary duct: a relaxing and a contracting one with different K(d) values 76 nM and 2.4 microM, respectively. 5-HT(2) receptor antagonist ritanserin and ketanserin attenuated the serotonin-induced relaxation. In contrast 5-HT(3) antagonist metoclopramide and MDL2222 decreased and 5-HT(3) receptor agonist 1-(m-chlorophenyl)-biguanide mimicked the serotonin-induced contraction, suggesting that serotonin exerted its action on two different receptor subtypes. The release of radiolabeled serotonin and dopamine upon nerve stimulation was found to be Ca-dependent. Furthermore, the increase in serotonin concentration induced a decrease of the potency of dopamine to elicit sustained contraction. These results provide evidence for the transmitter role of serotonin and dopamine in salivary duct. It is concluded that receptors reveal a pharmacological profile related to vertebrate D(1), 5-HT(2) and 5-HT(3) receptor subtypes. Moreover, it was found that the process of conveying the saliva is modulated by an interaction of dopamine and serotonin.

Ultrastructure of neuromuscular contacts in the embryonic pond snail Lymnaea stagnalis L

Ultrastructural characteristics of muscle fibers and neuromuscular contacts were investigated during two stages of embryogenesis of the pulmonate snail Lymnaea stagnalis. The first muscle cells appear as early as during metamorphosis (50-55% of embryonic development), whereas previously, in the trochophore/veliger stages (25-45%), muscular elements cannot be detected at all. The first muscle fibers contain large amounts of free numbers, a well-developed rER system and only a few irregularly arranged contractile elements. The nucleus is densely packed with heterochromatine material. At 75% adult-like postmetamorphic stage, the frequency of muscle fibers increases significantly, but, bundles of muscle fibers cannot yet be observed. Furthermore the muscle cells are characterized by large numbers of free ribosomes and numerous rER elements. Fine axon bundles and single axon processes, both accompanied by glial elements, can already be found at this time. Axon varicosities with different vesicle and/or granule contents form membrane contacts with muscle fibers, but without revealing membrane specialization on the pre- or postsynaptic side. The late development of the muscle system and neuromuscular contacts during Lymnaea embryogenesis correlates well with the maturation of different forms of behavior of adult, free-living life, and also with the peripheral appearance of chemically identified components of the embryonic nervous system of central origin.

Development and distribution of the peptidergic system in larval and adult Patiriella: comparison of sea star bilateral and radial nervous systems

Development of the larval peptidergic system in the sea star Patiriella regularis and structure of the adult nervous system in Patiriella species were documented in an immunofluorescence investigation using antisera to the sea star neuropeptide GFNSALMFamide 1 (S1) and confocal microscopy. P. regularis has planktotrophic development through bipinnaria and brachiolaria larvae. In early bipinnaria, two groups of immunoreactive cells appeared on either side of the anterior region and proliferated to form a pair of dorsolateral ganglia. The ganglia gave rise to fine varicose fibres that innervated the preoral and adoral ciliated bands. Peptidergic cells also innervated the postoral ciliated band, and a nerve tract connected the pre- and postoral bands. Fully developed bipinnaria had a well-developed peptidergic system, the organisation of which reflected the bilateral larval body plan. As the brachiolar attachment complex differentiated at the anterior end, the ganglia became positioned on either side of the anterior projection, from which they innervated the complex. It is suggested, based on the distribution of S1-like immunoreactivity in association with ciliary and attachment structures, that the peptidergic system functions in modulation of feeding, swimming, and settlement. The larval peptidergic system degenerates as the larval body is resorbed during metamorphosis. In adults, S1-like immunoreactivity was intense in the axonal region of the ectoneural nervous system and in hyponeural perikarya. Immunoreactive cells in the neuroepithelium connected with the surface and may be sensory. Examination of immunoreactivity in several Patiriella species attests to the highly conserved organisation of the peptidergic system in adult asteroids.