Evidence for the absorption and synthesis of 5-hydroxytryptamine in perfused muscle and intestinal tissue and whole worms of adult Ascaris suum

Effects of 5-HT (serotonin) on reproductive behaviour in Heterodera schachtii (Nematoda)

The effects of serotonin (5-hydroxytryptamine or 5-HT) on the reproductive behaviour of Heterodera schachtii was examined. Male and juvenile H. schachtii were incubated in 5-HT for 15 min. Stylet thrusting was stimulated by 10 mM 5-HT, whereas 0.1 mM 5-HT was without effect. Prolonged treatment of males with 10 mM 5-HT induced eversion of the spicules and extrusion of materials from the reproductive tract. Treatment with 60 mM triethanolamine and Wright's stain indicated that extrusions did not contain spermatids or spermatozoa. We confirm the effects of 5-HT on reproductive behaviour and suggest that the stylet and pharynx may subserve a reproductive function during later life stages in H. schachtii.

The range and biological activity of FMRFamide-related peptides and classical neurotransmitters in nematodes

Nematodes include both major parasites of humans, livestock and plants in addition to free-living species such as Caenorhabditis elegans. The nematode nervous system (especially in C. elegans) is exceptionally well defined in terms of the number, location and projections of the small number of neurons in the nervous system and their integration into circuits involved in regulatory behaviours vital to their survival. This review will summarize what is known about the biological activity of neurotransmitters in nematodes: the biosynthetic pathways and genes involved, their receptors, inactivation mechanisms and secondary messenger signalling systems. It will cover the 'classical' transmitters, such as acetylcholine (ACh), GABA, glutamate, serotonin, dopamine, octopamine, noradrenaline and nitric oxide. The localization of peptides throughout the nematode nervous system is summarized, in addition to the isolation of nematode neuropeptides by both traditional biochemical techniques and more modern genetic means. The major contribution of the completion of the C. elegans genome-sequencing program is highlighted throughout. Efforts to unravel neurotransmitter action in various physiological actions such as locomotion, feeding and reproduction are detailed as well as the various inactivation mechanisms for the current complement of nematode transmitters.

Exploring the neurotransmitter labyrinth in nematodes

Nematodes include both free-living species such as Caenorhabditis elegans and major parasites of humans, livestock and plants. The apparent simplicity and uniformity of their nervous system belies a rich diversity of putative signalling molecules, particularly neuropeptides. This new appreciation stems largely from the genome-sequencing project with C. elegans, which is due to be completed by the end of 1998. The project has provided additional insights into other aspects of nematode neurobiology, as have studies on the mechanism of action of anthelmintics. Here, progress on the identification, localization, synthesis and physiological actions of transmitters identified in nematodes is explored.

Levels of biogenic amines in larvae and adults of the rat hookworm, Nippostrongylus brasiliensis (Nematoda)

Norepinephrine, 5-hydroxytryptophan, octopamine, dihydroxyphenylacetic acid, N-acetyldopamine, dopamine, 5-hydroxyindoleacetic acid, N-acetylserotonin, tyramine, tryptophan and serotonin in larvae (third free stage and parasitic stages) and adult males and females (at defined ages during the intestinal phase) of the parasitic nematode Nippostrongylus brasiliensis were quantified simultaneously by high-performance liquid chromatography with electrochemical detection. Biogenic amine levels depended on the stage, the age and the sex of parasites and on environmental conditions. Their physiological roles in reproductively competent adults of this nematode are discussed in relation to exuviation and egg laying. Parallel fluctuations in free ecdysteroids and norepinephrine were observed in females from the same worm populations.

Neuronal localization of serotonin in the nematode Ascaris suum

We have used immunocytochemical techniques to investigate the distribution of serotonin-like immunoreactivity in the nematode Ascaris suum. Antisera raised against serotonin (5-hydroxytryptamine, 5-HT) conjugated to bovine serum albumin (BSA) labelled a pair of neurons in the pharynx of both sexes and five cells in the ventral cord of the male tail. The labelling was blocked by 5-HT or by 5-HT conjugated to BSA. The 5-HT-immunoreactive cells in the pharynx resemble neurosecretory cells and are probably homologous to the neurosecretory motor neurons (NSM) in Caenorhabditis elegans; the cells in the male tail appear to be motor neurons that are homologous to CP neurons in C. elegans. Other cells that stain with 5-HT antisera have been observed in C. elegans but are not seen in Ascaris.

Metabolism and inactivation of neurotransmitters in nematodes

The nematode nervous system employs many of the same neurotransmitters as are found in higher animals. The inactivation of neurotransmitters is absolutely essential for the correct functioning of the nervous system. In this article we discuss the various mechanisms used generally in animal nervous systems for synaptic inactivation of neurotransmitters and review the evidence for similar mechanisms operating in parasitic and free-living nematodes. The sequencing of the entire Caenorhabditis elegans genome means that the sequence of nematode genes can be accessed from the C. elegans database (ACeDB) and this wealth of information together with the increasing knowledge of the genetics of this free-living nematode will have great impact on all aspects of nematode neurobiology. The review will provide an insight into how this information may be exploited to identify and characterize target proteins for the development of novel anti-nematode drugs.

The action of serotonin and the nematode neuropeptide KSAYMRFamide on the pharyngeal muscle of the parasitic nematode, Ascaris suum

The pharyngeal component of the enteric nervous system of the parasitic nematode, Ascaris suum exhibits immunoreactivity for serotonin (5-hydroxytryptamine or 5-HT) and for FMRFamide-like peptides. This paper describes the application of an in vitro pharmacological approach to investigate the functional role of 5-HT and FMRFamide-like peptides. The pharyngeal pumping behaviour of Ascaris suum was monitored using a modified pressure transducer system which measures pharyngeal pressure changes and therefore pumping. The pharynx did not contract spontaneously; however, 5-HT (10-1000 microM) stimulated pumping at a frequency of 0.5 Hz. FMRFamide had no apparent effect on pharyngeal pumping. The native nematode FMRFamide-related peptide (FaRP), KSAYMRFamide inhibited the pumping elicited by 5-HT. The duration of inhibition was dose-dependent (0.1-1000 nM) with a threshold of 0.1 nM. In 4 preparations, the inhibition of the pharyngeal muscle was preceded by an initial excitation and increase in the amplitude of pharyngeal pressure changes. The pharynx is involved in various nematode processes, including feeding, regulation of hydrostatic pressure and excretion. The role of 5-HT and KSAYMRFamide in the pharyngeal function of nematodes is discussed.

Immunocytochemical demonstration of peptidergic and serotoninergic components in the enteric nervous system of the roundworm, Ascaris suum (Nematoda, Ascaroidea)

The localization and distribution of neuropeptides and an indoleamine (serotonin or 5-hydroxytryptamine) in the enteric nervous system (ENS) of the pig roundworm, Ascaris suum, have been determined by the application of an indirect immunofluorescence technique in conjunction with confocal scanning laser microscopy. Whole-mount preparations of pharyngeal, intestinal and rectal regions were screened with antisera to 23 vertebrate peptides, 2 invertebrate peptides and serotonin (= 5-HT). Positive immunoreactivity (IR) was obtained with antisera to pancreatic polypeptide (PP), peptide YY (PYY), FMRFamide, gastrin and serotonin. The only IR observed in the ENS was that evident in the nerve supply to the pharynx and rectal region; no IR was associated with any region of the intestine. The most extensive patterns of IR occurred with antisera to PYY, FMRFamide and serotonin. In the pharyngeal component of the ENS, IR was evident in the lateral and dorsal longitudinal pharyngeal nerves, pharyngeal commissures, nerve plexus, and associated nerve cells and fibres. In contrast, the distribution of IR to the PP and gastrin antisera was more restricted and displayed a lower intensity of immunostaining. The other component of the ENS, the rectal enteric system, only yielded immunostaining to FMRFamide. The possible role of neuropeptides and serotonin in the nutritional biology of nematodes is discussed.

Neuromuscular transmission in nematode parasites and antinematodal drug action

Some anthelmintic drugs interfere selectively with nematode neuromuscular transmission. These drugs include: the nicotinic agonists, e.g. levamisole, the gamma-amino butyric acid agonist piperazine, and the avermectins which open Cl- channels. The physiology and pharmacology of neuromuscular transmission in nematodes is reviewed and the actions of antinematodal drugs which interfere with the transmission described. The results of experiments on the large porcine-intestinal nematode parasite, Ascaris suum, form the basis of the account presented but experiments on other nematodes suggest that these observations may be generalized. Results of some experiments on the small free living nematode Caenorhabditis elegans are also included.

Pyridoxal 5'-phosphate dependent enzymes in the nematode Nippostrongylus brasiliensis

Eight classes of pyridoxal 5'-phosphate dependent enzymes have been investigated in Nippostrongylus brasiliensis in parallel with rat tissues. The range of decarboxylases detected in N. brasiliensis was limited in comparison with rat tissues. N. brasiliensis possessed a highly active L-serine hydroxymethyltransferase, but in contrast with rat liver, 5-aminolevulinic acid synthetase was absent. Similar levels of L-serine and L-threonine dehydratase activities were detected in N. brasiliensis and rat liver, and both organisms lacked L-alanine racemase, L-tryptophan synthetase and L-methionine gamma-lyase. The demonstration of cystathionine beta-synthase and gamma-cystathionase in N. brasiliensis suggests the presence of a functional trans-sulphuration sequence. The substrate specificities of the nematode cystathionine beta-synthase and gamma-cystathionase varied significantly from those of the corresponding mammalian enzymes. Particularly striking was the ability of N. brasiliensis cystathionine beta-synthase to catalyse the non-mammalian 'activated L-serine sulphydrase' reaction (L-cysteine + R-SH----cysteine thioether + H2S). N. brasiliensis and rat liver exhibited comparable abilities to transaminate amino acids via the 2-oxoglutarate: glutamate system.

N-acetylation of biogenic amines in Ascaridia galli

The metabolism of 5-hydroxytryptamine (5-HT), octopamine (OA) and dopamine (DA) by adult male Ascaridia galli was investigated by incubating worms cut into 5 mm lengths with radio-isotope labelled amine. [14C]5-hydroxytryptamine and [14C]octopamine were taken up by the tissues and were metabolized to apolar products that co-chromatographed using both high-performance liquid chromatography and thin-layer chromatography with N-acetyl 5-hydroxytryptamine (NA5-HT) and N-acetyloctopamine (NAOA), respectively. N-acetylation was by far the most important reaction detected under these experimental conditions. A brief incubation of cut worms with [3H]dopamine resulted in the formation of a radio-isotope labelled metabolite that co-chromatographed with N-acetyldopamine (NADA) on reversed-phase high-performance liquid chromatography. The N-acetyltransferase activity towards 5-hydroxytryptamine and octopamine was detected in crude homogenates of male worms only when the co-substrate, acetyl CoA, was added to the reaction mixture. This enzyme activity appeared to be mainly localized in a 40 000 g supernatant fraction. The failure of previous studies to detect N-acetylation of biogenic amines in tissue homogenates of nematodes may have been due to the low levels of acetyl CoA present in these enzyme preparations.

Hallucinogenic and neuroleptic drug interactions with potential neurotransmitter receptors in parasitic nematodes

Receptors potentially involved in neurotransmitting have been characterised in the muscle tissue and in whole worms of the nematodes Ascaris suum and Onchocerca volvulus, respectively. Binding studies revealed a high affinity for LSD with apparent KD values of 94 nM for A. suum and 120 nM for O. volvulus, whereas those of the neuroleptics haloperidol, spiperone and mianserin were found to be in the micromolar range. A variety of neurotransmitter antagonists, known to bind with high affinities either to mammalian D1/2 or to 5-HT1/2 receptors, were tested for their ability to bind to the nematode receptor. Results from these displacement experiments using tritiated LSD, mianserin, spiperone and haloperidol show distinct specificities of the nematode receptors compared to known receptor classes of mammals. With respect to this novel specificity, the nematode receptors seem to be unique and clearly distinct from those of the hosts.

Biogenic amines and GABA in the larval and adult forms of the nematode Nippostrongylus brasiliensis

1. Simultaneous detection (HPLC and electrochemical detection) of biological extracts of larval and adult stages of Nippostrongylus brasiliensis was performed in order to assay biogenic amines. 2. Gamma-amino-butyric acid was assayed in the same samples. 3. Tryptophan, 5-hydroxyindoleacetic acid were at the same level in adults and larvae. 4. 5-Hydroxytryptophan, serotonin, dihydroxyphenylalanine and dopamine were significantly higher in larvae in which gamma-amino-butyric acid was not detected.

N-acetylation of serotonin, octopamine and dopamine by adult Brugia pahangi

The metabolism of biogenic amines by the filarial worm, Brugia pahangi, was investigated by incubating cut worms with radio-labelled amine substrates. Two-dimensional thin-layer chromatography and analysis on two high-performance liquid chromatography systems showed that [14C]5-hydroxytryptamine was metabolised to a less polar compound that was identified as N-acetyl 5-hydroxytryptamine. N-Acetyloctopamine and N-acetyldopamine were also formed when cut B. pahangi were incubated with [14C]octopamine and [3H]dopamine, respectively. N-Acetyltransferase activity towards 5-hydroxytryptamine was readily detected in nematode homogenates. This enzyme was localised in a 50,000 x g supernatant and required the addition of the co-substrate, acetyl CoA, for activity. No evidence was obtained for the involvement of monoamine oxidases in the metabolism of 5-HT in these filarial worms.

Tissue and ultrastructural localization of 5-hydroxytryptamine (serotonin) in the tissues of Ascaris suum with energy dispersive X-ray spectrometry of immunoreactive structures

Muscle, hypodermis and gastrointestinal epithelial cells from adult female Ascaris lumbricoides var. suum were found to contain serotonin based upon glyoxylic acid induced histofluorescence and indirect immunolabeling with an antiserotonin monoclonal antibody conjugated to protein A-colloidal gold. Histofluorescence indicated that muscle-hypodermis and intestinal epithelial cells contained significant concentrations of 5-hydroxytryptamine while fluorescence was absent in the nerve cord and cuticle. Immunolabeling at the ultrastructural level indicated that serotonin was sequestered in electron-opaque patches, dense vesicles and mitochondria of the muscle-hypodermis and intestinal tissue. Perfusion of whole worms and isolated tissues with 10(4) M-serotonin further indicated: (1) immunolabeled patches and dense vesicles were often associated with cytoskeletal elements, (2) serotonin did not appear to enter the intestinal or muscle cells by endocytosis, (3) immunolabeled patches examined with energy dispersive X-ray spectrometry (X-ray microanalysis) were found to contain iron at concentrations approximately double that of the surrounding cytoplasm.

Serotonin receptors in the tissues of adult Ascaris suum

Serotonin (5-hydroxytryptamine, 5-HT) receptors in the muscle and intestinal tissues of adult Ascaris suum have been investigated. [3H] lysergic acid diethylamide (LSD) exhibited specific and saturable binding to membranes prepared from both intestine and muscle. The intestinal tissue membranes had an equilibrium dissociation constant (Kd) of 2.70 nM for LSD and a Kd of 2.50 microM for 5-HT. As compared to the intestine, the muscle membranes had comparatively higher affinity for both LSD (Kd = 1.80 nM) and 5-HT (Kd = 0.68 microM). The muscle membranes also had a high binding affinity for ketanserin, a 5-HT2 antagonist, (Kd = 16.7 nM) whereas intestinal membranes exhibited no specific binding of ketanserin. Serotonin significantly inhibited the binding of LSD to the intestinal and muscle tissue membranes while adrenergic and cholinergic drugs and histamine did not. This suggested that the binding of LSD, 5-HT and ketanserin to the parasite membranes was specific. Collectively, the data demonstrated the presence of a serotonin receptor in the muscle and intestinal tissues of the adult A. suum. The receptor in the muscle was pharmacologically similar to the mammalian serotonin type 2 receptor.