Immunocytochemical and radioimmunometrical demonstration of serotonin- and neuropeptide-immunoreactivities in the adult rat tapeworm, Hymenolepis diminuta (Cestoda, Cyclophyllidea)

The interplay of helminthic neuropeptides and proteases in parasite survival and host immunomodulation

Neuropeptides comprise a diverse and broad group of neurotransmitters in vertebrates and invertebrates, with critical roles in neuronal signal transduction. While their role in controlling learning and memory in the brains of mammals is known, their extra-synaptic function in infection and inflammation with effects on distinct tissues and immune cells is increasingly recognized. Helminth infections especially of the central nervous system (CNS), such as neurocysticercosis, induce neuropeptide production by both host and helminth, but their role in host–parasite interplay or host inflammatory response is unclear. Here, we review the neurobiology of helminths, and discuss recent studies on neuropeptide synthesis and function in the helminth as well as the host CNS and immune system. Neuropeptides are summarized according to structure and function, and we discuss the complex enzyme processing for mature neuropeptides, focusing on helminth enzymes as potential targets for novel anthelminthics. We next describe known immunomodulatory effects of mammalian neuropeptides discovered from mouse infection models and draw functional parallels with helminth neuropeptides. Last, we discuss the anti-microbial properties of neuropeptides, and how they may be involved in host–microbiota changes in helminth infection. Overall, a better understanding of the biology of helminth neuropeptides, and whether they affect infection outcomes could provide diagnostic and therapeutic opportunities for helminth infections.

The Role of the Intestinal Epithelium in the "Weep and Sweep" Response during Gastro-Intestinal Helminth Infections

Simple Summary

The immune system actively combats intruders such as bacteria, viruses, fungi, and protozoan and metazoan parasites using leukocytes. During an infection white blood cells are activated to internalize bacteria or viruses and release a number of molecules to kill pathogens. Unfortunately, those mechanisms are ineffective against larger intruders like helminths, which are too large to be killed by a single immune cell. To eliminate gastro-intestinal helminths an integrated response involving the nervous, endocrine, and immune systems are used to expel the parasites. This is achieved through increased gut hydration and muscle contractions which detach worms from the gut and lead to release outside the body in a “weep and sweep” response. Epithelial cells of the intestine are significant players in this process, being responsible for detecting the presence of helminths in the gut and participating in the regulation of parasite expulsion. This paper describes the role of the gut epithelium in detecting and eliminating helminths from the intestine.

Abstract

Helminths are metazoan parasites infecting around 1.5 billion people all over the world. During coevolution with hosts, worms have developed numerous ways to trick and evade the host immune response, and because of their size, they cannot be internalized and killed by immune cells in the same way as bacteria or viruses. During infection, a substantial Th₂ component to the immune response is evoked which helps restrain Th₁-mediated tissue damage. Although an enhanced Th₂ response is often not enough to kill the parasite and terminate an infection in itself, when tightly coordinated with the nervous, endocrine, and motor systems it can dislodge parasites from tissues and expel them from the gut. A significant role in this “weep and seep” response is attributed to intestinal epithelial cells (IEC). This review highlights the role of various IEC lineages (enterocytes, tuft cells, Paneth cells, microfold cells, goblet cells, and intestine stem cells) during the course of helminth infections and summarizes their roles in regulating gut architecture and permeability, and muscle contractions and interactions with the immune and nervous system.

Serotonin stimulates Echinococcus multilocularis larval development

Background

Serotonin is a phylogenetically ancient molecule that is widely distributed in most metazoans, including flatworms. In addition to its role as a neurotransmitter, serotonin acts as a morphogen and regulates developmental processes. Although several studies have focused on the serotonergic nervous system in parasitic flatworms, little is known on the role of serotonin in flatworm development.

Methods

To study the effects of serotonin on proliferation and development of the cestode Echinococcus multilocularis, we cloned the genes encoding the E. multilocularis serotonin transporter (SERT) and tryptophan hydroxylase (TPH), analyzed gene expression by transcriptome analysis and whole mount in situ hybridization (WMISH) and performed cell culture experiments.

Results

We first characterized orthologues encoding the SERT and TPH, the rate-limiting enzyme in serotonin biosynthesis. WMISH and transcriptomic analyses indicated that the genes for both SERT and TPH are expressed in the parasite nervous system. Long-term treatment of parasite stem cell cultures with serotonin stimulated development towards the parasite metacestode stage. Mature metacestode vesicles treated with serotonin showed increased rates of incorporation of the thymidine analogue 5-ethynyl-2′-deoxyuridine (EdU), indicating stimulated cell proliferation. In contrast, treatment with the selective serotonin reuptake inhibitor paroxetine strongly affected the viability of parasite cells. Paroxetine also caused structural damage in metacestode vesicles, suggesting that serotonin transport is crucial for the integrity of parasite vesicles.

Conclusions

Our results indicate that serotonin plays an important role in E. multilocularis development and proliferation, providing evidence that the E. multilocularis SERT and TPH are expressed in the nervous system of the protoscolex. Our results further suggest that the E. multilocularis SERT has a secondary role outside the nervous system that is essential for parasite integrity and survival. Since serotonin stimulated E. multilocularis metacestode development and proliferation, serotonin might also contribute to the formation and growth of the parasite in the liver.

Modulation of the immune response by helminths: a role for serotonin?

The mammalian gut is a remarkable organ: with a nervous system that rivals the spinal cord, it is the body’s largest repository of immune and endocrine cells and houses an immense and complex microbiota. Infection with helminth parasites elicits a conserved program of effector and regulatory immune responses to eradicate the worm, limit tissue damage, and return the gut to homeostasis. Discrete changes in the nervous system, and to a lesser extent the enteroendocrine system, occur following helminth infection but the importance of these adaptations in expelling the worm is poorly understood. Approximately 90% of the body’s serotonin (5-hydroxytryptamine (5-HT)) is made in enterochromaffin (EC) cells in the gut, indicative of the importance of this amine in intestinal function. Signaling via a plethora of receptor subtypes, substantial evidence illustrates that 5-HT affects immunity. A small number of studies document changes in 5-HT levels following infection with helminth parasites, but these have not been complemented by an understanding of the role of 5-HT in the host–parasite interaction. In reviewing this area, the gap in knowledge of how changes in the enteric serotonergic system affects the outcome of infection with intestinal helminths is apparent. We present this as a call-to-action by investigators in the field. We contend that neuronal EC cell–immune interactions in the gut are essential in maintaining homeostasis and, when perturbed, contribute to pathophysiology. The full affect of infection with helminth parasites needs to define, and then mechanistically dissect the role of the enteric nervous and enteroendocrine systems of the gut.

Murine autoimmune arthritis is exaggerated by infection with the rat tapeworm, Hymenolepis diminuta

Infection with helminth parasites triggers strong and stereotypic immune responses in humans and mice, which can protect against specific experimentally-induced autoimmune diseases. We have shown that infection with the rat tapeworm, Hymenolepis diminuta, confers a protective effect on FCA-induced joint inflammation. Here, we investigated the effect of a prophylactic infection with H. diminuta on the K/BxN-serum model of polyarthritis in BALB/c mice. Mice were infected with 10 cysticercoids of H. diminuta by oral gavage and 8 days later arthritis was induced by i.p. injection of K/BxN arthritogenic serum. Joint swelling and pain measurements were recorded throughout a 13 day time course. At necropsy, joints and blood serum were collected. K/BxN-treated mice developed joint inflammation in the front paws, hind paws and knees as shown by increased swelling, mechanical allodynia and myeloperoxidase activity. Mice infected with H. diminuta had more severe disease, with increased eosinophil peroxidase activity in their paws and greater inflammatory infiltrate and synovitis in the knee joints. Hymenolepis diminuta-infected mice displayed significant increases in serum levels of C5a and mast cell protease-1 compared with K/BxN-serum only treatment, the latter being indicative of mast cell activation. In contrast to the protective effect of infection with H. diminuta in FCA-induced monoarthritis, infection with this helminth exacerbated K/BxN serum-induced polyarthritis in BALB/c mice. This correlated with increases in C5a and mast cell activation: factors critical in the development of K/BxN-induced arthritis. Thus, while data accumulate from animal models showing that infection with helminth parasites may be beneficial for a variety of auto-inflammatory diseases, our findings demonstrate the potential for helminths to exacerbate disease. Hence care is needed when helminth therapy is translated into a clinical setting.

Cytochemical observations on cholinergic, serotoninergic and peptidergic neuronal pathways in Cephalochlamys namaquensis

The localization and distribution of cholinergic, serotoninergic (5-HT, serotonin) and peptidergic components of the nervous system of adult Cephalochlamys namaquensis (Cestoda: Pseudophyllidea) have been determined using enzyme histochemical and immunocytochemical techniques interfaced with light and confocal scanning laser microscopy. All three classes of neuroactive substance showed a similar pattern of staining, occurring extensively throughout the central and peripheral nervous systems of the parasite. There were some minor regional differences in staining, suggesting specific roles for certain classes of neurone, and nerve cell bodies were most evident following immunostaining for serotonin. The general overlap in the distribution of staining may be indicative of some co-localization of neurotransmitter and/or neuromodulatory substances.

Immunoelectron microscopical studies of regulatory peptides in the nervous system of the monogenean parasite, Diclidophora merlangi

Specific antisera, directed against the highly conserved C-terminal hexapeptide amide of mammalian pancreatic poly-peptide (PP) and the invertebrate peptide FMRFamide, have been used in conjunction with post-embedding, IgG-conjugated colloidal gold immunostaining to demonstrate peptide immunoreactivity at subcellular level in the nervous system of adult Diclidophora merlangi. Gold labelling revealed that immunoreactivity for PP and FMRFamide was localized exclusively in dense-cored vesicles occupying the majority of axons in the central nervous system. Double-labelling demonstrated an apparent co-localization of PP and FMRFamide in the same dense-cored vesicles. Antigen pre-absorption experiments indicated cross-reactivity of the two antisera as unlikely, and that some if not all of the PP/FMRFamide immunostaining in the parasite was due to a neuropeptide F-like peptide.

Functional morphology of the platyhelminth nervous system

As the most primitive metazoan phylum, the Platyhelminthes occupies a unique position in nervous system evolution. Centrally, their nervous system consists of an archaic brain from which emanate one or more pairs of longitudinal nerve cords connected by commissures; peripherally, a diverse arrangement of nerve plexuses of varying complexity innervate the subsurface epithelial and muscle layers, and in the parasitic taxa they are most prominent in the musculature of the attachment organs and egg-forming apparatus. There is a range of neuronal-cell types, the majority being multi- and bipolar. The flatworm neuron is highly secretory and contains a heterogeneity of vesicular inclusions, dominated by densecored vesicles, whose contents may be released synaptically or by paracrine secretion for presumed delivery to target cells via the extracellular matrix. A wide range of sense organ types is present in flatworms, irrespective of life-styles. The repertoire of neuronal substances identified cytochemically includes all of the major candidate transmitters known in vertebrates. Two groups of native flatworm neuropeptides have been sequenced, neuropeptide F and FMRFamide-related peptides (FaRPs), and immunoreactivities for these have been localised in dense-cored neuronal vesicles in representatives of all major fiatworm groups. There is evidence of co-localisation of peptidergic and cholinergic elements; serotoninergic components generally occupy a separate set of neurons. The actions of neuronal substances in flatworms are largely undetermined, but FaRPs and 5-HT are known to be myoactive in all of the major groups, and there is immuno-cytochemical evidence that they have a role in the mechanism of egg assembly.

The gull-tapeworm, Diphyllobothrium dendriticum and neuropeptide F: an immunocytochemical study

Neuropeptide F (Moniezia expansa) immunoreactivity (NPF-IR) has been detected in the nervous system of plerocercoid and adult stages of the gull-tapeworm Diphyllobothrium dendriticum, using immunocytochemical methodology. The application of the antiserum for this authentic flatworm neuropeptide to whole-mounts and frozen sections of the worm has resulted in new information about its neuroanatomy. Thus, at regular intervals, transverse nerves extend from the main nerve cords laterally, joining the longitudinal lateral minor cords in the cortical parenchyma. In the adult worm, the transverse nerves are located at the posterior border of each proglottis. The medullary parenchyma lacks NPF-IR. The NPF-immunoreactive cell bodies are bi- to multipolar and preferentially located in the peripheral nervous system, in close association with the holdfast musculature of the scolex and the extensive body musculature. NPF-IR was observed in the innervation to the muscular ducts of the reproductive system. The pattern of NPF-IR was compared with that recorded for RFamide- and 5-HT-IR and double-immunostaining has revealed separate populations of serotoninergic and peptidergic neurones.

Flatworm nerve–muscle: structural and functional analysis

Platyhelminthes occupy a unique position in nerve–muscle evolution, being the most primitive of metazoan phyla. Essentially, their nervous system consists of an archaic brain and associated pairs of longitudinal nerve cords cross-linked as an orthogon by transverse commissures. Confocal imaging reveals that these central nervous system elements are in continuity with an array of peripheral nerve plexuses which innervate a well-differentiated grid work of somatic muscle as well as a complexity of myofibres associated with organs of attachment, feeding, and reproduction. Electrophysiological studies of flatworm muscles have exposed a diversity of voltage-activated ion channels that influence muscle contractile events. Neuronal cell types are mainly multi- and bi-polar and highly secretory in nature, producing a heterogeneity of vesicular inclusions whose contents have been identified cytochemically to include all three major types of cholinergic, aminergic, and peptidergic messenger molecules. A landmark discovery in flatworm neuro biology was the biochemical isolation and amino acid sequencing of two groups of native neuropeptides: neuro peptide F and FMRFamide-related peptides (FaRPs). Both families of neuropeptide are abundant and broadly distributed in platyhelminths, occurring in neuronal vesicles in representatives of all major flatworm taxa. Dual localization studies have revealed that peptidergic and cholinergic substances occupy neuronal sets separate from those of serotoninergic components. The physiological actions of neuronal messengers in flatworms are beginning to be established, and where examined, FaRPs and 5-HT are myoexcitatory, while cholinomimetic substances are generally inhibitory. There is immunocytochemical evidence that FaRPs and 5-HT have a regulatory role in the mechanism of egg assembly. Use of muscle strips and (or) muscle fibres from free-living and parasitic flatworms has provided baseline information to indicate that muscle responses to FaRPs are mediated by a G-protein-coupled receptor, and that the signal transduction pathway for contraction involves the second messengers cAMP and protein kinase C.

NO nerves and their targets in a tapeworm: An immunocytochemical study of cGMP in Hymenolepis diminuta

We studied the pattern of cGMP immunostaining (IS) after stimulation with a nitric oxide donor in the presence of an inhibitor of phosphodiesterase in adult Hymenolepis diminuta. cGMP-IS was detected in the peripheral nervous system, especially in nerve fibres close to the body muscle fibres. cGMP-IS also occurred in terminals beneath the basal lamina of the tegument and between the muscle fibres of the suckers. The pattern of cGMP-IS was compared to that of 5-HT-IS and GYIRFamide-IS. TRITC-conjugated phalloidin was used to stain the musculature.

Localization of a sodium-dependent high-affinity serotonin transporter and recruitment of exogenous serotonin by the cestode Hymenolepis diminuta: an autoradiographic and immunohistochemical study

Uptake of serotonin by tissues of intact and hemitransected Hymenolepis diminuta was studied by autoradiography and immunohistochemistry. Hemitransected worms were incubated in balanced saline containing 10 µM [3H]serotonin and washed extensively. The density of silver grains over the serotonin-immunoreactive longitudinal nerve cords and commissural rings, male reproductive system, tissues surrounding the genital pouch, and deep longitudinal muscles was significantly greater than that over the parenchyma. The presence of serotonin in spermatozoa suggested a role for this amine in spermatozoon activity. In contrast, uptake of 10 µM [3H]serotonin in sodium-free saline was significantly reduced compared with that in balanced saline in all tissues examined except the parenchyma. Analysis of the data revealed that the sodium-dependent high-affinity serotonin transport system is localized primarily in the serotonergic-like neurons of H. diminuta, which suggests possible recycling of neuronally released serotonin. Following incubation of intact worms in vitro for 12 h in 5 µM [3H]serotonin, the density of silver grains was significantly higher over the serotonin-immunoreactive nerves, elements of the male reproductive system, tissues surrounding the genital pouch, and deep longitudinal muscles than over the parenchyma. These results demonstrate recruitment of exogenous serotonin by intact H. diminuta and suggest sequestration and concentration by the serotonin-immunoreactive neurons via the sodium-dependent high-affinity transporter. These data further suggest that although H. diminuta can synthesize serotonin, it may obtain serotonin from the host. Nonetheless, the amount of serotonin recruited by H. diminuta from the host in vivo compared with that which they synthesize is not known.

Immunology and biochemistry of Hymenolepis diminuta

This review is an account of modern research into the immunology and biochemistry of the rat tapeworm, Hymenolepis diminuta. The first half of the review is devoted to the immunological responses of the host to the parasite. It describes the specific responses that occur when the host is exposed to a primary infection, and the changes that occur when further infections are superimposed on the primary one. The aquisition of immunity to the tapeworm and its persistence in the absence of the infection are also discussed, as well as the non-specific responses of the host to the parasite. The second half of the review is concerned with biochemistry, summarizing the early biochemical work that has been carried out on the tapeworm and describing the metabolic pathways now thought to be characteristic of the parasite. What little information that exists on intermediary metabolism in eggs and larvae is summarized here. Much of this section is concerned with the role of mitochondria in H. diminuta, especially the control of the critical branchpoint (PK/PEPCK), which partitions carbon into either the cytosol or the mitochondrion. The role of 5-hydroxytryptamine in controlling both worm behaviour and metabolism is discussed, followed by a brief look at some other effectors that may prove in the future to have great significance in regulating the parasite. Finally, there is a detailed consideration of strain variation within H. diminuta and of the impact on the tapeworm of components of the immune system, formerly described as the 'crowding effect'. The review concludes with a brief discussion of evolutionary aspects of the rat-tapeworm relationship and a comprehensive bibliography.

Transport of exogenous 5-hydroxytryptamine across the outer plasma membrane of the syncytial tegument ofHymenolepis diminutais by simple diffusion

The uptake of 5-hydroxytryptamine (5HT) from a 10 μM solution of exogenous [3H]5HT into the tegument of Hymenolepis diminuta was linear for the first 20 min of incubation. The rate of transport was 0.04 ± 0.01 pmol∙mg wet mass−1∙min−1, and there were no significant differences in the rate of uptake by the anterior, middle, and posterior regions of the body. The initial uptake was not Na+-dependent, was not saturable at up to 100 μM, was not highly temperature-dependent (Q10~ 1.2), and displayed activation energy of 11.8 kJ∙mol−1. Furthermore, uptake was not inhibited by p-chloromercuriphenyl sulphonic acid, imipramine, amiloride, or 5HT analogues, which collectively support a non-carrier-mediated uptake mechanism. Washing of the tissues with 10 mM 5HT after incubation in 10 μM [3H]5HT displaced less than 10% of the remaining [3H]5HT associated with the tissues, and little radioactivity was extracted by washing in acetone or chloroform. The uptake of [3H]5HT, however, was pH-dependent, the rate of uptake being closely correlated with the proportion of unprotonated 5HT. Only a small portion of the transported [3H]5HT was metabolized to a product associated with 5-hydroxyindoleacetic acid, and metabolism was significantly inhibited by the monoamine oxidase inhibitors iproniazid phosphate, deprenyl, and clorgyline. The present study showed that small amounts of [3H]5HT were taken up by H. diminuta by simple diffusion, little of the [3H]5HT was adsorbed to the surface of the worms or dissolved in the lipid phase of the plasma membrane, and some of the [3H]5HT taken up was metabolized by a monoamine oxidase-like enzyme.

NO nerves in a tapeworm. NADPH-diaphorase histochemistry in adult Hymenolepis diminuta

The free radical nitric oxide (NO), which is synthesized by nitric oxide synthase (NOS), has recently been discovered to function as a neuronal messenger. The presence of NOS was detected in the nervous system of adult Hymenolepis diminuta with NADPH-diaphorase (NADPH-d) histochemistry. The NADPH-d histochemical reaction is regarded as a selective marker for NOS in neuronal tissue. NADPH-d staining was observed in nerve fibres in the main and minor nerve cords and the transverse ring commissures, and in cell bodies in the brain commissure, along the main nerve cords, in the suckers and the rostellar sac. NADPH-d staining was also observed in the wall of the internal seminal vesicle and the genital atrium. The pattern of NADPH-d staining was compared with that of the 5-HT immunoreactive nervous elements. The NADPH-d staining reaction and the 5-HT immunoreactivity occur in separate sets of neurons. This is the first time the NADPH-d reaction has been demonstrated in the nervous system of a flatworm, indicating that NOS is present and that NO can be produced at this level of evolution.

Immunocytochemical localization of serotonin (5-HT) in the nervous system of the hydatid organism, Echinococcus granulosus (Cestoda, Cyclophyllidea)

The localization and distribution of the serotoninergic components of the nervous system in the hydatid organism, Echinococcus granulosus, were determined by immunocytochemical techniques in conjunction with confocal scanning laser microscopy (CSLM). The distribution of serotonin immunoreactivity (IR) paralleled that previously described for cholinesterase activity, although it was more widespread. Nerve cell bodies and nerve fibres immunoreactive for 5-HT were present throughout the central nervous system (CNS), occurring in the paired lateral, posterior lateral and rostellar ganglia, their connecting commissures and nerve rings in the scolex and in the ten longitudinal nerve cords that run posteriorly throughout the body of the worm. A large population of nerve cell bodies was associated with the lateral nerve cords. In the peripheral nervous system (PNS), immunoreactive nerve fibres occurred in well-developed nerve plexuses innervating the somatic musculature and the musculature of the rostellum and suckers. The genital atrium and associated reproductive ducts were richly innervated with serotoninergic nerve cell bodies and nerve fibres.

FMRFamide-related peptides in Hymenolepis diminuta: immunohistochemistry and radioimmunoassay

The localization of FMRFamide-related peptide (FaRP) immunoreactivity was determined during different stages of development of the rat tapeworm Hymenolepis diminuta. In the adult worm (14 days old), FaRP immunostaining was most intense in the scolex and concentrated in the central nervous system (cerebral ganglia and transverse commissures) and around the lips of the suckers. In the strobila, medial and lateral longitudinal nerve cords (LNCs) and ladder-like connecting commissures were the only tissue stained. Immunoreactivity in the medial LNCs of the adult tapeworms extended only to and included proglottides containing developing testis and seminal receptacle but disappeared in proglottides in which primordial ovaries were first detected. Radioimmunoassay confirmed that FaRPs were concentrated in the scolex/neck region of the adult worm (3.9 +/- 1.5 pmol mg protein-1), whereas the lowest concentrations (0.2 +/- 0.19 pmol mg protein-1) were recovered from the regions of the strobila containing shelled eggs. The pattern of FaRP immunoreactivity observed in 5- and 7-day-old worms was similar to that seen in adult worms, but in 2- and 3-day-old worms the pattern of immunoreactivity observed in the cerebral ganglia, transverse commissures, and LNCs differed significantly as compared with that seen in older worms. These results indicate differential utilization and/or roles for FaRPs during development and suggest both central and sensory roles in this tapeworm.

Serotoninergic and peptidergic nerve elements in the protoscolex of Echinococcus granulosus (Cestoda, Cyclophyllidea)

The localisation and distribution of 5-hydroxytryptamine (5-HT, or serotonin) and neuropeptides in the nervous system of the protoscolex of the hydatid organism Echinococcus granulosus were determined by an indirect immunofluorescence technique. Nerve-cell bodies immunoreactive for 5-HT occurred in the lateral ganglia and in association with the lateral longitudinal nerve cords. 5-HT immunostaining was also evident in the central nerve ring, in the rostellar nerves and in the nerve plexus innervating the suckers. Of the antisera used to screen the protoscolex for neuropeptide immunoreactivity (IR), immunostaining was obtained with those raised against pancreatic polypeptide (PP), peptide YY (PYY), substance P (SP), peptide histidine isoleucine (PHI) and vasoactive intestinal peptide (VIP). The most extensive pattern of IR occurred with antisera to PP and PYY. Immunoreactive nerve elements were evident in the lateral ganglia, central nerve ring, rostellar nerves, rostellar ganglia, sucker plexus and longitudinal nerve cords. The distribution of SP-, PHI- and VIP-IRs was more restricted: SP-IR occurred in the lateral ganglia and sucker nerves, whilst PHI- and VIP-immunoreactive nerve elements were associated with the lateral longitudinal nerve cords. Protoscoleces cultured in vitro for 29 days were also examined and neuroanatomical changes noted. A greater development of the longitudinal nerve cords and their cross-connectives in the body of the worm was evident, and a group of nerve cells were seen to develop at the posterior end of the main lateral nerve cords.

[The nervous system of Fibricola seoulensis by acetylcholinesterase histochemistry]

F. seoulensis were obtained from artificially infected albino rats at 3, 4, 5, 6, 7 days after infection. The worms and metacercariae were washed in physiological saline solution, and fixed with 10% neutral formalin. The acetylcholinesterase (AchE) stained by enzyme histochemistry using acetylthiocholine iodide as substrate. Eserine, iso-OMPA and BW284C51 were used as inhibitors of AchE. The nervous system consists of three pairs longitudinal nerve trunks interconnected with excretory plexus in posterior half, and pharynx and oral sucker in anterior half of metacercariae and adults. The longitudinal nerve trunks are interconnected with transverse commissures and numerous circular commissures. Considerable numbers of circular commissures are interconnected with longitudinal nerve trunks lying on the surface of the worms. At each stage of juvenile worms, AchE and nonspecific cholinesterase activities were observed in the oral sucker, ventral sucker, pharynx and nerve system. Isozymes of AchE in F. seoulensis were separated into the two bands, 69 kDa and 132 kDa. The major band was 69 kDa.

Cytochemical observations on the nervous system of adult Corrigia vitta

Adult Corrigia vitta (Trematoda: Dicrocoelidea) inhabit the pancreatic duct of the fieldmouse, Apodemus sylvaticus, where, in numbers, they may occlude the duct lumen and prevent the flow of pancreatic secretions. Enzyme histochemical and immunocytochemical techniques, in conjunction with confocal scanning laser microscopy, have been used to examine the localization and distribution of cholinergic, serotoninergic (5-HT, serotonin) and peptidergic components of the nervous system of the adult worm. All three classes of neuronal mediator showed a common pattern of staining, occurring throughout the central and peripheral nervous systems. Of the four peptide immunoreactivities (IR) demonstrated (pancreatic polypeptide (PP), peptide YY (PYY), substance P (SP), FMRFamide), PP-IR was the most predominant, occurring not only within the central ganglia and longitudinal nerve cords, but also in subtegumental plexuses and in fibres associated with the egg-forming apparatus. PYY and FMRFamide IRs were evident throughout the central and peripheral nervous systems; FMRFamide immunostaining, in particular, highlighted innervation of the ootype and immunoreactive cell bodies around the Mehlis' gland. Both SP- and 5-HT-IRs were restricted to the cerebral ganglia, ventral nerve cords and associated cell bodies. The distribution patterns of these peptides and 5-HT within the nervous system of C. vitta suggest they are likely to function as neuronal mediators. PP, PYY and FMRFamide may also serve in regulating egg production.

The cholinergic, serotoninergic and peptidergic components of the nervous system of Moniezia expansa (Cestoda, Cyclophyllidea)

The central (CNS) and peripheral (PNS) nervous systems of the cyclophyllidean tapeworm, Moniezia expansa, were examined for the presence of cholinergic, serotoninergic and peptidergic elements using enzyme cytochemical and immunocytochemical techniques in conjunction with light and confocal scanning laser microscopy. Cholinesterase activity and 5-hydroxytryptamine- and regulatory peptide-immunoreactivities (IRs) were localized to the nerve fibres and cell bodies of all of the major neuronal components in the CNS of the worm, including the cerebral ganglia and connecting commissure, the 10 longitudinal nerve cords and associated transverse ring commissures. Although each of the 3 systems appeared well developed and comprised a significant portion of the nervous system, the serotoninergic constituent was the most highly developed, consisting of a vast array of nerve fibres and cell bodies distributed throughout the strobila of the worm. A close association of cholinesterase reactivity and peptide-IRs was evident throughout the CNS, indicating the possible co-localization of acetylcholine and neuropeptides. Within the PNS, cholinergic activity and serotoninergic- and peptidergic-IRs occurred in the subtegumental network of nerve fibres and somatic musculature. Although all 3 neurochemical elements were present in the acetabula, they were found in different nerve fibres; only cholinergic and peptidergic cell bodies were found. The common genital opening, vagina and ootype regions of the reproductive system displayed a rich innervation of all 3 types of neuronal populations. Within the peptidergic system, immunostaining with antisera raised to the C-terminus of the neuropeptide Y superfamily of peptides and the invertebrate peptides, neuropeptide F (M. expansa) and FMRFamide was the most prevalent. Limited positive-IR for substance P and neurokinin A were also recorded in the CNS of the worm.

Immunocytochemical evidence for the presence of substance P-like peptide in Diphyllobothrium dendriticum

Substance P immunoreactivity (SP-IR) was detected in the nervous system of the gull-tapeworm Diphyllobothrium dendriticum. The distribution of the SP-IR neurons in the plerocercoid differs from that of other peptidergic and aminergic neurons in the worm. As well as occurring in the ganglionic commissure and along the two main nerve cords, SP-IR neurons are located laterally to the main nerve cords but not dorsally or ventrally. The SP-IR neurons have projections extending to the surface. Bipolar SP-IR neurons with processes to the surface also occur in the tip of the scolex. A sensory function for the SP-IR neurons is suggested.

Electron immunogold labeling of regulatory peptide immunoreactivity in the nervous system of Moniezia expansa (Cestoda: Cyclophyllidea)

An electron immunogold-labeling technique was used in conjunction with a post-embedding procedure to demonstrate for the first time the ultrastructural distribution of the parasitic platyhelminth neuropeptide, neuropeptide F (NPF), in the nervous system of the cestode Moniezia expansa. Two axon types, distinguished by their populations of different-sized electron-dense vesicles, were identified. Immunogold labeling demonstrated an apparent homogeneity of PP, FMRFamide and NPF (M. expansa) antigenic sites throughout the larger dense-cored vesicles within the central nervous system. Triple labeling clearly demonstrated the co-localisation of immunoreactivities (IR) for NPF, PP and FMRFamide within the same dense-cored vesicles. The presence of NPF-IR within the vesicles occupying the perikaryon of the neuronal cell body indicated that the peptides had undergone post-translational C-terminal amidation prior to entering the axon. Antigen pre-absorption experiments using NPF prevented labeling with either PP or FMRFamide antisera, and the failure of these antisera to block NPF-IR supports the view that some, if not all, of the PP/FMRFamide-IR is due to NPF-like peptides.