Coordinated modulation of cellular signaling through ligand-gated ion channels in Hydra vulgaris (Cnidaria, Hydrozoa)

Cnidarians lack well developed organs, but they have evolved the molecular and cellular components needed to assemble a nervous system. The apparent 'simplicity' of the cnidarian nervous net does not occur at the cellular level, but rather in the organisation of conducting systems. Cnidarian neurons are in fact electrically excitable, show the typical extended morphology and are connected by chemical synapses or gap junctions. They have been regarded as peptidergic, given the wealth of neuropeptides generally distributed along neurites and in cell bodies, supporting the hypothesis of a modulatory role in neurotransmission. However, the presence of clear-cored, as well as dense-cored synaptic vesicles in cnidarian neurons suggests both fast and slow synaptic transmission mechanisms. In fact, biochemical and functional evidence indicates that classical neurotransmitters and their metabolic partners are present in cnidarian tissues, where they are involved in coordinating motility and behavior. We have identified and characterized in Hydra tissues receptors to the inhibitory and excitatory amino acid neurotransmitters, GABA, glycine and NMDA, that are similar to mammalian ionotropic receptors in terms of their biochemical and pharmacological properties. These receptors appear to regulate pacemaker activities and their physiological correlates; in the live animal, they also affect feeding behavior, namely the duration and termination of the response elicited by reduced glutathione, with opposite actions of GABA and glycine or NMDA, respectively. These results suggest that modulation of cellular signaling through ligand-gated-ion channels is an ancient characteristic in the animal kingdom, and that the pharmacological properties of these receptors have been highly conserved during evolution.     

Fast Neurotransmission Related Genes Are Expressed in Non Nervous Endoderm in the Sea Anemone Nematostella vectensis

Cnidarian nervous systems utilize chemical transmission to transfer signals through synapses and neurons. To date, ample evidence has been accumulated for the participation of neuropeptides, primarily RFamides, in neurotransmission. Yet, it is still not clear if this is the case for the classical fast neurotransmitters such as GABA, Glutamate, Acetylcholine and Monoamines. A large repertoire of cnidarian Fast Neurotransmitter related Genes (FNGs) has been recently identified in the genome of the sea anemone, Nematostella vectensis. In order to test whether FNGs are localized in cnidarian neurons, we characterized the expression patterns of eight Nematostella genes that are closely or distantly related to human central and peripheral nervous systems genes, in adult Nematostella and compared them to the RFamide localization. Our results show common expression patterns for all tested genes, in a single endodermal cell layer. These expressions did not correspond with the RFamide expressing nerve cell network. Following these results we suggest that the tested Nematostella genes may not be directly involved in vertebrate-like fast neurotransmission.     

Monoamines and neuropeptides as transmitters in the sedentary polychaete Sabellastarte magnifica: actions on the longitudinal muscle of the body wall.

Pharmacological studies on the body wall musculature of the sedentary polychaete Sabellastarte magnifica show a potential neurotransmitter role for monoamines and neuropeptides in this organism. All catecholamines induced contraction of longitudinal muscle strips, while serotonin and the neuropeptides FMRFamide and substance P caused a relaxation of both resting and active muscle. In addition, we demonstrate catecholaminergic and serotonergic pathways in the nervous system of this sabellid, using immunohistochemistry and catecholamine-induced fluorescence. The presence of neuropeptide-containing fibers in the nervous system of this polychaete has been previously reported. Together these results suggest that catecholamines act as excitatory transmitters on the longitudinal muscle cells of the body wall of S. magnifica, while serotonin and FMRFamide, and possible substance P, are inhibitory transmitters. The possibility of coexistence of serotonin and FMRFamide within the same neuronal cell bodies and fibers of this polychaete is also explored.     

The Ferrier Lecture, 1983. Amino acids and peptides: fast and slow chemical signals in the nervous system?

The classical monoamine neurotransmitters, acetylcholine and the catecholamines, are used by only a small proportion of synapses in mammalian c.n.s. The amino acids GABA (gamma-aminobutyric acid) and L-glutamate may be the principal inhibitory and excitatory neurotransmitters used for fast point-to-point transmission in the c.n.s. The monoamines and the large number of neuropeptides (over 30) now known to exist in c.n.s. may be chemical signals used for a different type of chemically addressed form of information transmission between neurons in c.n.s. characterized by less precise spatial connections, a slower time course and a far richer diversity of chemical signals than used in classical synaptic neurotransmission. In this context the brain can be viewed as a neuroendocrine secretory organ of great complexity.     

Weighing the role of hypothalamic feeding neurotransmitters

Growing health problems related to obesity have focused considerable attention on a number of neurotransmitters, particularly hypothalamic neuropeptides, involved in regulating energy homeostasis and food intake. As the fast-acting transmitters GABA and glutamate underlie the majority of fast synaptic activity in the hypothalamus, understanding neuropeptide modulation of amino acid transmitter actions may be key to a full appreciation of how the brain controls caloric balances.     

Opioid receptor dependent long-term potentiation: Peptidergic regulation of synaptic plasticity in the hippocampus

Rapid progress has been made towards understanding the synaptic physiology of excitatory amino acid transmission in the hippocampus. By comparison, the function of opioid peptides localized to some of the same pathways which use glutamate for fast excitation is poorly understood. Here I consider new evidence specifically implicating opioid peptides in long-term potentiation (LTP) induced by high-frequency stimulation of pathways which combine glutamate and opioid neurotransmission. This form of LTP is unique in that it depends on activation of opioid receptors, and unlike many excitatory systems in brain, it does not require activation of the (NMDA) type of glutamate receptor. Thus one of the main functions of opioids in the hippocampus may be to regulate activity-dependent changes in synaptic strength and neuronal excitability. At another level, “opioid” LTP may provide basic insights into peptidergic transmission and its functional interactions with classical neurotransmitters in the brain.     

Neuronal messengers in the human cerebral circulation

In recent years our knowledge of the nervous control of the cerebral circulation has increased. The use of denervations and retrograde tracing in combination with immunohistochemical techniques has demonstrated that cerebral vessels are supplied with sympathetic, parasympathetic, and sensory nerve fibers and possibly central pathways containing a multiplicity of new transmitter substances in addition to the classical transmitters. The majority of these transmitters are neuropeptides. More recently it has been suggested that a gaseous transmitter, nitric oxide (NO) also could participate in the neuronal regulation of cerebral blood flow. Although little is known about the physiological actions and inter-relationships among all these putative neurotransmitters, their presence within cerebrovascular nerve fibers will make it necessary to revise our view on the mechanisms of cerebrovascular neurotransmission.     

Immunohistochemical Localization of Neuropeptides and Neurotransmitters in the Nucleus Solitarius

The nucleus tractus solitarii (NTS), which receives visceralafferent information from the cardiovascular, respiratory, gastrointestinaland taste systems, contains multiple neurotrasmitters and neuropeptidesthroughout its rostral to caudal extent. The neurotransmittersand neuropeptides immunoreactivity is located predominatelyin varicose fibers and small puncta throughout the neuropil.In addition, immunoreactive NTS neurons for a variety of neurotransmittersand neuropeptides are present in subnuclear regions. The neuroactive substances localized immunohistochemically inthe NTS include acetylcholine, the neuropeptides, substanceP, methionine- and leucine-enkephalin, ß-endorphin,cholecystokinin, neurotensin, galanin, calcitonin gene-relatedpeptide, somatostatin, FMRMamide, neuropeptide Y, angiotensinII, vasoactive intestinal polypeptide, vasopressin, oxytocin,thyrotropin-releasing hormone, luteinizing hormone-releasinghormone, atrial natriuretic peptide, the catecholamines, dopamine,norepinephrine, epinephrine, serotonin, histamine and the aminoacids, GABA and glutamate. The pattern of innervation for eachneurotransmitter and neuropeptide is not homogeneously distributedthroughout the NTS. Each substance has a unique pattern withinthe NTS as each subnuclear region contains different immunohistochemicalstaining patterns and densities of fibers. At the ultrastructural level both neurotransmitters and neuropeptidesare present in synaptic terminals that are in contact with differentparts of the neuronal membranes. Typically, the labeled terminalscontain both small, clear vesicles and large, dense core vesicleswith the exception of synaptic terminals containing acetylcholine,GABA and glutamate which do not typically have the large, densecore vesicles. The most frequent post-synaptic target are dendritesand spinous processes. Less frequently, synaptic contacts arepresent on the cell soma. Chem. Senses 21: 367–376, 1996.     

Roles of glutamine in neurotransmission

Glutamine (Gln) is found abundantly in the central nervous system (CNS) where it participates in a variety of metabolic pathways. Its major role in the brain is that of a precursor of the neurotransmitter amino acids: the excitatory amino acids, glutamate (Glu) and aspartate (Asp), and the inhibitory amino acid, γ-amino butyric acid (GABA). The precursor-product relationship between Gln and Glu/GABA in the brain relates to the intercellular compartmentalization of the Gln/Glu(GABA) cycle (GGC). Gln is synthesized from Glu and ammonia in astrocytes, in a reaction catalyzed by Gln synthetase (GS), which, in the CNS, is almost exclusively located in astrocytes (Martinez-Hernandez et al., 1977). Newly synthesized Gln is transferred to neurons and hydrolyzed by phosphate-activated glutaminase (PAG) to give rise to Glu, a portion of which may be decarboxylated to GABA or transaminated to Asp. There is a rich body of evidence which indicates that a significant proportion of the Glu, Asp and GABA derived from Gln feed the synaptic, neurotransmitter pools of the amino acids. Depolarization-induced-, calcium- and PAG activity-dependent releases of Gln-derived Glu, GABA and Asp have been observed in CNS preparations in vitro and in the brain in situ. Immunocytochemical studies in brain slices have documented Gln transfer from astrocytes to neurons as well as the location of Gln-derived Glu, GABA and Asp in the synaptic terminals. Patch-clamp studies in brain slices and astrocyte/neuron co-cultures have provided functional evidence that uninterrupted Gln synthesis in astrocytes and its transport to neurons, as mediated by specific carriers, promotes glutamatergic and GABA-ergic transmission. Gln entry into the neuronal compartment is facilitated by its abundance in the extracellular spaces relative to other amino acids. Gln also appears to affect neurotransmission directly by interacting with the NMDA class of Glu receptors. Transmission may also be modulated by alterations in cell membrane polarity related to the electrogenic nature of Gln transport or to uncoupled ion conductances in the neuronal or glial cell membranes elicited by Gln transporters. In addition, Gln appears to modulate the synthesis of the gaseous messenger, nitric oxide (NO), by controlling the supply to the cells of its precursor, arginine. Disturbances of Gln metabolism and/or transport contribute to changes in Glu-ergic or GABA-ergic transmission associated with different pathological conditions of the brain, which are best recognized in epilepsy, hepatic encephalopathy and manganese encephalopathy.     

Neurotransmitter-induced changes in the intracellular calcium concentration suggest a differential central modulation of CCAP neuron subsets in Drosophila

The complete sequencing of the Drosophila melanogaster genome allowed major progress in the research on invertebrate neuropeptide signaling. However, it is still largely unknown how the insect CNS exerts synaptic control over the secretory activity of peptidergic neurons; afferent pathways and employed chemical transmitters remain largely unidentified. In the present study, we set out to identify neurotransmitters mediating synaptic input onto CCAP-expressing neurons (N(CCAP)), which play an important role in the regulation of ecdysis-related events. By in vitro and in situ calcium imaging with synthetic and genetically encoded calcium indicators, we provide evidence that differential neurotransmitter inputs control the activity of N(CCAP) subsets. In short-term culture, almost all N(CCAP) showed increases of the free intracellular calcium concentration after application of acetylcholine (ACh) and nicotine, whereas only some N(CCAP) responded to glutamate and GABA. In the intact ventral ganglia of both larvae and adults, only few N(CCAP) showed intracellular calcium-rises or calcium-oscillations after application of cholinergic agonists indicating a prevailing central inhibition of most N(CCAP) during these developmental stages. In larvae, responding N(CCAP) were primarily located in the third thoracic neuromere. At least one N(CCAP) pair in this neuromere belonged to a morphologically distinct subset with neurohemal endings on the body wall muscles. Our results suggest that N(CCAP) express functional receptors for ACh, glutamate, and GABA, and indicate that these transmitters are involved in a context-dependent regulation of functionally distinct N(CCAP) subsets.     

IPF, a vesicular uptake inhibitory protein factor, can reduce the Ca(2+)-dependent, evoked release of glutamate, GABA and serotonin

Synaptic vesicles in the nerve terminal play a pivotal role in neurotransmission. Neurotransmitter accumulation into synaptic vesicles is catalyzed by distinct vesicular transporters, harnessing an electrochemical proton gradient generated by V-type proton-pump ATPase. However, little is known about regulation of the transmitter pool size, particularly in regard to amino acid neurotransmitters. We previously provided evidence for the existence of a potent endogenous inhibitory protein factor (IPF), which causes reduction of glutamate and GABA accumulation into isolated, purified synaptic vesicles. In this study we demonstrate that IPF is concentrated most in the synaptosomal cytosol fraction and that, when introduced into the synaptosome, it leads to a decrease in calcium-dependent exocytotic (but not calcium-independent) release of glutamate in a concentration-dependent manner. In contrast, alpha-fodrin (non-erythroid spectrin), which is structurally related to IPF and thought to serve as the precursor for IPF, is devoid of such inhibitory activity. Intrasynaptosomal IPF also caused reduction in exocytotic release of GABA and the monoamine neurotransmitter serotonin. Whether IPF affects vesicular storage of multiple neurotransmitters in vivo would depend upon the localization of IPF. These results raise the possibility that IPF may modulate synaptic transmission by acting as a quantal size regulator of one or more neurotransmitters.     

gamma-Aminobutyric acid(A) neurotransmission and cerebral ischemia

In this review, we present evidence for the role of gamma-aminobutyric acid (GABA) neurotransmission in cerebral ischemia-induced neuronal death. While glutamate neurotransmission has received widespread attention in this area of study, relatively few investigators have focused on the ischemia-induced alterations in inhibitory neurotransmission. We present a review of the effects of cerebral ischemia on pre and postsynaptic targets within the GABAergic synapse. Both in vitro and in vivo models of ischemia have been used to measure changes in GABA synthesis, release, reuptake, GABA(A) receptor expression and activity. Cellular events generated by ischemia that have been shown to alter GABA neurotransmission include changes in the Cl(-) gradient, reduction in ATP, increase in intracellular Ca(2+), generation of reactive oxygen species, and accumulation of arachidonic acid and eicosanoids. Neuroprotective strategies to increase GABA neurotransmission target both sides of the synapse as well, by preventing GABA reuptake and metabolism and increasing GABA(A) receptor activity with agonists and allosteric modulators. Some of these strategies are quite efficacious in animal models of cerebral ischemia, with sedation as the only unwanted side-effect. Based on promising animal data, clinical trials with GABAergic drugs are in progress for specific types of stroke. This review attempts to provide an understanding of the mechanisms by which GABA neurotransmission is sensitive to cerebral ischemia. Furthermore, we discuss how dysfunction of GABA neurotransmission may contribute to neuronal death and how neuronal death can be prevented by GABAergic drugs.     

A role for Melanin-Concentrating Hormone in learning and memory

The neurobiological substrate of learning process and persistent memory storage involves multiple brain areas. The neocortex and hippocampal formation are known as processing and storage sites for explicit memory, whereas the striatum, amygdala, neocortex and cerebellum support implicit memory. Synaptic plasticity, long-term changes in synaptic transmission efficacy and transient recruitment of intracellular signaling pathways in these brain areas have been proposed as possible mechanisms underlying short- and long-term memory retention. In addition to the classical neurotransmitters (glutamate, GABA), experimental evidence supports a role for neuropeptides in modulating memory processes. This review focuses on the role of the Melanin-Concentrating Hormone (MCH) and receptors on memory formation in animal studies. Possible mechanisms may involve direct MCH modulation of neural circuit activity that support memory storage and cognitive functions, as well as indirect effect on arousal.     

Independent origins of neurons and synapses: insights from ctenophores

There is more than one way to develop neuronal complexity, and animals frequently use different molecular toolkits to achieve similar functional outcomes. Genomics and metabolomics data from basal metazoans suggest that neural signalling evolved independently in ctenophores and cnidarians/bilaterians. This polygenesis hypothesis explains the lack of pan-neuronal and pan-synaptic genes across metazoans, including remarkable examples of lineage-specific evolution of neurogenic and signalling molecules as well as synaptic components. Sponges and placozoans are two lineages without neural and muscular systems. The possibility of secondary loss of neurons and synapses in the Porifera/Placozoa clades is a highly unlikely and less parsimonious scenario. We conclude that acetylcholine, serotonin, histamine, dopamine, octopamine and gamma-aminobutyric acid (GABA) were recruited as transmitters in the neural systems in cnidarian and bilaterian lineages. By contrast, ctenophores independently evolved numerous secretory peptides, indicating extensive adaptations within the clade and suggesting that early neural systems might be peptidergic. Comparative analysis of glutamate signalling also shows numerous lineage-specific innovations, implying the extensive use of this ubiquitous metabolite and intercellular messenger over the course of convergent and parallel evolution of mechanisms of intercellular communication. Therefore: (i) we view a neuron as a functional character but not a genetic character, and (ii) any given neural system cannot be considered as a single character because it is composed of different cell lineages with distinct genealogies, origins and evolutionary histories. Thus, when reconstructing the evolution of nervous systems, we ought to start with the identification of particular cell lineages by establishing distant neural homologies or examples of convergent evolution. In a corollary of the hypothesis of the independent origins of neurons, our analyses suggest that both electrical and chemical synapses evolved more than once.     

GABA in the nervous system of the planarian Polycelis nigra

Gamma-aminobutyric acid (GABA) is an important inhibitory neurotransmitter in vertebrates and it has a similar inhibitory role in several invertebrate taxa. The transmitters serotonin, octopamine, catecholamines and histamine are present in flatworms while evidence for GABA is still lacking. Therefore, we have studied the occurrence of GABA-like immunoreactivity (IR) in the planarian nervous system. Specimens of Polycelis nigra were fixed in 4% 1-ethyl-3-(3-dimethyl-aminopropyl) carbodiimide with 2% paraformaldehyde. The GABA-antiserum was raised in rabbits against GABA conjugated to keyhole limpet hemocyanin. Preabsorption with GABA-ovalbumin conjugate abolished all IR. The results were further confirmed with an monoclonal antibody and high pressure liquid chromatography (HPLC). In P. nigra GABA-like IR was seen as long, often varicose, sparsely distributed fibers in the ventral longitudinal nerve cords. IR was also located in a few cell somata in the brain and in the neuropil of the brain. The IR was restricted to the central nervous system and was absent in peripheral nerves and plexuses. The HPLC analysis supported the presence of GABA.Our results suggest that GABA is an interneuronal transmitter in P. nigra. The results also suggest a phylogenetically old origin of GABAergic neurotransmission.     

Coexistence and corelease of cholinergic and peptidergic transmitters in frog sympathetic ganglia

Both acetylcholine (ACh) and a peptide that resembles luteinizing hormone-releasing hormone (LHRH) serve as transmitters in sympathetic ganglia of the bullfrog. Although ACh is contained and released from both preganglionic B fibers, which form synaptic contacts with only B cells in the ganglia, and preganglionic C fibers, which are in synaptic contact with C cells only, the LHRH-like peptide is contained and released exclusively from preganglionic C fibers. The same preganglionic C fibers appear to supply both ACh and the LHRH-like peptide because the thresholds for the cholinergic fast excitatory postsynaptic potential (EPSP) correlate well with the thresholds for the peptidergic late slow EPSP recorded in the same C cell. Further, anatomical studies showed that almost all nerve terminals on C cells contained the LHRH-like peptide. Some of these same terminals must also contain and release. ACh, mediating the cholinergic fast EPSPs with millisecond synaptic delays. Therefore at least some, if not all, terminals of preganglionic C fibers contain and release both cholinergic and peptidergic transmitters.     

Regulation of neurotransmitter release by metabotropic glutamate receptors

The G protein-coupled metabotropic glutamate (mGlu) receptors are differentially localized at various synapses throughout the brain. Depending on the receptor subtype, they appear to be localized at presynaptic and/or postsynaptic sites, including glial as well as neuronal elements. The heterogeneous distribution of these receptors on glutamate and nonglutamate neurons/cells thus allows modulation of synaptic transmission by a number of different mechanisms. Electrophysiological studies have demonstrated that the activation of mGlu receptors can modulate the activity of Ca(2+) or K(+) channels, or interfere with release processes downstream of Ca(2+) entry, and consequently regulate neuronal synaptic activity. Such changes evoked by mGlu receptors can ultimately regulate transmitter release at both glutamatergic and nonglutamatergic synapses. Increasing neurochemical evidence has emerged, obtained from in vitro and in vivo studies, showing modulation of the release of a variety of transmitters by mGlu receptors. This review addresses the neurochemical evidence for mGlu receptor-mediated regulation of neurotransmitters, such as excitatory and inhibitory amino acids, monoamines, and neuropeptides.     

Peptides and neurotransmission in the central nervous system

Radioimmunoassays of brain extracts have shown that several peptides occur in high concentrations in the CNS. The releasing-factor peptides TRF, LRF, somatostatin, CRF and GRF have the highest concentration in the hypothalamic extracts. High levels of somatostatin, CCK octapeptide, neuropeptide Y (NPY) and vasoactive intestinal peptide (VIP) are found in cortical extracts. Substance P, CCK, NPY, and enkephalins are present in high concentrations in basal ganglia and mesolimbic areas. Pharmacological doses of these peptides result in several behavioural and vegetative effects. Immunocytochemical studies show that the CNS peptides are localised in neurones and in synaptic vesicles. In vitro studies with brain tissues show that peptides are capable of modifying the ongoing classical neurotransmission. In depressive patients several neuropeptides (CCK, CRF and NPY) have been shown to have low CSF levels. Patients dying of senile dementia have low cortical levels of somatostatin, CRF and substance P. In schizophrenic patients CCK peptides have shown to improve some symptoms. At present the therapeutic potentials of peptides are poorly known. More studies are required to understand their role in neurotransmission and related pathological states.     

Effect of Motor and Premotor Cortex Ablation on Concentrations of Amino Acids, Monoamines, and Acetylcholine and on the Ultrastructure in Rat Striatum. A Confirmation of Glutamate as the Specific Cortico-Striatal Transmitter

At 1, 2, and 4 weeks after unilateral premotor and motor cortex ablation in rats, a significant and lasting decrease in glutamate levels in the ipsilateral versus contralateral striatum was observed. A significant corresponding fall in aspartate was seen only after 1 week. In contrast, there was a large increase in the striatal concentrations of lysine, threonine, alanine, and glutamine 1 week after the cortical ablation. This correlates with the extensive glial proliferation in the deafferented ipsilateral striatum. Four weeks after cortical ablation the GABA concentration was significantly increased. There was no decrease in other putative transmitters (dopamine, serotonin, acetylcholine, glycine and taurine), nor was a glutamate decrease observed in the hippocampus or in the hypothalamus, which do not receive direct premotor and motor cortical inputs. Both biochemical and morphological evidence for a minor contralateral cortico-striatal projection was obtained. Correlating with the fall in glutamate, ultrastructural observations indicated the degeneration of two types of striatal synapses, i.e., those of the axo-spinous type III and of the axo-dendritic type VII. Frontal cortex ablation clearly affects, in opposite directions, the metabolism of various striatal amino acids but not that of acetylcholine and the monoamine transmitters. The results strongly support the view that glutamate is the transmitter of the cortico-striatal fibers.     

Analysis of classical neurotransmitter markers in tapeworms: Evidence for extensive loss of neurotransmitter pathways

Parasitic flatworms have complex neuromuscular systems that serve important functions in their life cycles. However, our understanding of neurotransmission in parasitic flatworms is limited. Pioneering studies have suggested the presence of several classical neurotransmitter systems, but their molecular components have not been characterized in most cases. Because these components are conserved in bilaterian animals, we searched the genomes of parasitic flatworms for orthologs of genes required for neurotransmitter synthesis, vesicular transport, reuptake, and reception. Our results indicate that tapeworms have lost the genes that are specifically required in other animals for synaptic signaling using the classical neurotransmitters dopamine, tyramine, octopamine, histamine and gamma-aminobutyric acid (GABA). These results imply that these signaling pathways are either absent in these parasites, or that they require completely different molecular components in comparison with other animals. The orthologs of genes related to histaminergic and GABA signaling are also missing in trematodes (although Schistosoma-specific histaminergic receptors have been previously described). In contrast, conserved genes required for glutamatergic, serotonergic and cholinergic signaling could be found in all analyzed flatworms. We analyzed the expression of selected markers of each pathway in the tapeworm Hymenolepis microstoma by whole-mount in situ hybridization. Each marker was specifically expressed in the nervous system, although with different patterns. In addition, we analyzed the expression of proprotein convertase 2 as a marker of peptidergic cells. This gene showed the widest expression in the nervous system, but was also expressed in other tissues, suggesting additional roles of peptidergic signaling in tapeworm development and reproduction.