Serotonin regulates repolarization of the C. elegans pharyngeal muscle

Antipsychotic drugs up-regulate tryptophan hydroxylase in ADF neurons of Caenorhabditis elegans: role of calcium-calmodulin-dependent protein kinase II and transient receptor potential vanilloid channel

Antipsychotic drugs produce acute behavioral effects through antagonism of dopamine and serotonin receptors, and long-term adaptive responses that are not well understood. The goal of the study presented here was to use Caenorhabditis elegans to investigate the molecular mechanism or mechanisms that contribute to adaptive responses produced by antipsychotic drugs. First-generation antipsychotics, trifluoperazine and fluphenazine, and second-generation drugs, clozapine and olanzapine, increased the expression of tryptophan hydroxylase-1::green fluorescent protein (TPH-1::GFP) and serotonin in the ADF neurons of C. elegans. This response was absent or diminished in mutant strains lacking the transient receptor potential vanilloid channel (TRPV; osm-9) or calcium/calmodulin-dependent protein kinase II (CaMKII; unc-43). The role of calcium signaling was further implicated by the finding that a selective antagonist of calmodulin and a calcineurin inhibitor also enhanced TPH-1::GFP expression. The ADF neurons modulate foraging behavior (turns/reversals off food) through serotonin production. We found that short-term exposure to the antipsychotic drugs altered the frequency of turns/reversals off food. This response was mediated through dopamine and serotonin receptors and was abolished in serotonin-deficient mutants (tph-1) and strains lacking the SER-1 and MOD-1 serotonin receptors. Consistent with the increase in serotonin in the ADF neurons induced by the drugs, drug withdrawal after 24-hr treatment was accompanied by a rebound in the number of turns/reversals, which demonstrates behavioral adaptation in serotonergic systems. Characterization of the cellular, molecular, and behavioral adaptations to continuous exposure to antipsychotic drugs may provide insight into the long-term clinical effects of these medications.

Neural and molecular dissection of a C. elegans sensory circuit that regulates fat and feeding

A major challenge in understanding energy balance is deciphering the neural and molecular circuits that govern behavioral, physiological, and metabolic responses of animals to fluctuating environmental conditions. The neurally expressed TGF-beta ligand DAF-7 functions as a gauge of environmental conditions to modulate energy balance in C. elegans. We show that daf-7 signaling regulates fat metabolism and feeding behavior through a compact neural circuit that allows for integration of multiple inputs and the flexibility for differential regulation of outputs. In daf-7 mutants, perception of depleting food resources causes fat accumulation despite reduced feeding rate. This fat accumulation is mediated, in part, through neural metabotropic glutamate signaling and upregulation of peripheral endogenous biosynthetic pathways that direct energetic resources into fat reservoirs. Thus, neural perception of adverse environmental conditions can promote fat accumulation without a concomitant increase in feeding rate.

Manipulation of serotonin signal suppresses early phase of behavioral aging in Caenorhabditis elegans

Aging is associated with progressive changes in behavioral functions, in part caused by muscle frailty, called sarcopenia. However, it was not clear whether certain neurotransmitters are directly involved in behavioral aging. Here we investigated aging of locomotion behaviors with an associative learning property, called basal and enhanced slowing response in Caenorhabditis elegans. Basal slowing response is a modest slowdown in response to food, while enhanced slowing response is a greater slowdown response when animals experience starvation. The behaviors are mediated by dopamine and serotonin, respectively. During aging, basal slowing response was increased, resulting in a diminished difference between the two slowing responses. The behavioral change occurred during early phase of aging prior to the timing when sarcopenia was observed in previous studies. Interestingly, expression of a serotonin biosynthesis marker, tph-1Colon, two colonsGFP, was increased in old animals. Serotonin receptor antagonists and deletion mutants of their target receptor genes (ser-1 and ser-4) partially suppressed age-related changes in locomotion behaviors. Thus, manipulating serotonin signal at receptor levels suppresses early phase of locomotion aging.

Serotonin regulates C. elegans fat and feeding through independent molecular mechanisms

We investigated serotonin signaling in C. elegans as a paradigm for neural regulation of energy balance and found that serotonergic regulation of fat is molecularly distinct from feeding regulation. Serotonergic feeding regulation is mediated by receptors whose functions are not required for fat regulation. Serotonergic fat regulation is dependent on a neurally expressed channel and a G protein-coupled receptor that initiate signaling cascades that ultimately promote lipid breakdown at peripheral sites of fat storage. In turn, intermediates of lipid metabolism generated in the periphery modulate feeding behavior. These findings suggest that, as in mammals, C. elegans feeding behavior is regulated by extrinsic and intrinsic cues. Moreover, obesity and thinness are not solely determined by feeding behavior. Rather, feeding behavior and fat metabolism are coordinated but independent responses of the nervous system to the perception of nutrient availability.

Evidence for a role for cyclic AMP in modulating the action of 5-HT and an excitatory neuropeptide, FLP17A, in the pharyngeal muscle of Caenorhabditis elegans

The feeding activity of the nematode Caenorhabditis elegans is regulated by an anatomically well-defined network of 20 enteric neurones that employs small molecule and neuropeptidergic signalling. Two of the most potent excitatory agents are 5-HT and the neuropeptide FLP17A. Here we have examined the role of cAMP in modulating their excitatory actions by pharmacological manipulation of the level of cAMP. Application of the membrane permeable cAMP analogue, dibutyryl-cAMP (1 microM), enhanced the excitatory response to both FLP17A and 5-HT. Furthermore, the adenylyl cyclase activator, forskolin (50 nM), significantly enhanced the excitatory response to both FLP17A and 5-HT. The phosphodiesterase inhibitor, ibudilast (10 microM), enhanced the excitatory response to FLP17A. The protein kinase inhibitor, H-9 dihydrochloride (10 microM) significantly reduced the excitatory response to 5-HT. H-9 dihydrochloride also had a direct effect on pharyngeal activity. The effect of FLP17A and 5-HT on two mutants, egl-8 (loss-of-function phospholipase-Cbeta) and egl-30 (loss-of-function Galphaq) was also investigated. Both these mutants have a lower pharyngeal pumping rate than wild-type which has to be considered when interpreting the effects of these mutations on the excitatory responses to FLP17A and 5HT. However, even taking into consideration the lower basal activity of these mutants, it is clear that the percentage increase in pharyngeal pumping rate induced by FLP17A is greatly reduced in both mutants compared to wild-type. In the case of 5-HT, the effect of the mutant backgrounds on the response was less pronounced. Overall, the data support a role for cAMP in modulating the excitatory action of both FLP17A and 5-HT on C. elegans pharyngeal pumping and furthermore implicate an EGL-30 dependent pathway in the regulation of the response to FLP17A.

An antidepressant that extends lifespan in adult Caenorhabditis elegans

The mechanisms that determine the lifespan of an organism are still largely a mystery. One goal of ageing research is to find drugs that would increase lifespan and vitality when given to an adult animal. To this end, we tested 88,000 chemicals for the ability to extend the lifespan of adult Caenorhabditis elegans nematodes. Here we report that a drug used as an antidepressant in humans increases C. elegans lifespan. In humans, this drug blocks neural signalling by the neurotransmitter serotonin. In C. elegans, the effect of the drug on lifespan is reduced or eradicated by mutations that affect serotonin synthesis, serotonin re-uptake at synapses, or either of two G-protein-coupled receptors: one that recognizes serotonin and the other that detects another neurotransmitter, octopamine. In vitro studies show that the drug acts as an antagonist at both receptors. Testing of the drug on dietary-restricted animals or animals with mutations that affect lifespan indicates that its effect on lifespan involves mechanisms associated with lifespan extension by dietary restriction. These studies indicate that lifespan can be extended by blocking certain types of neurotransmission implicated in food sensing in the adult animal, possibly leading to a state of perceived, although not real, starvation.

Tyramine and octopamine independently inhibit serotonin-stimulated aversive behaviors in Caenorhabditis elegans through two novel amine receptors

Biogenic amines modulate key behaviors in both vertebrates and invertebrates. In Caenorhabditis elegans, tyramine (TA) and octopamine (OA) inhibit aversive responses to 100%, but not dilute (30%) octanol. TA and OA also abolish food- and serotonin-dependent increases in responses to dilute octanol in wild-type but not tyra-3(ok325) and f14d12.6(ok371) null animals, respectively, suggesting that TA and OA modulated responses to dilute octanol are mediated by separate, previously uncharacterized, G-protein-coupled receptors. TA and OA are high-affinity ligands for TYRA-3 and F14D12.6, respectively, based on their pharmacological characterization after heterologous expression. f14d12.6::gfp is expressed in the ASHs, the neurons responsible for sensitivity to dilute octanol, and the sra-6-dependent expression of F14D12.6 in the ASHs is sufficient to rescue OA sensitivity in f14d12.6(ok371) null animals. In contrast, tyra-3::gfp appears not to be expressed in the ASHs, but instead in other neurons, including the dopaminergic CEP/ADEs. However, although dopamine (DA) also inhibits 5-HT-dependent responses to dilute octanol, TA still inhibits in dop-2; dop-1; dop-3 animals that do not respond to DA and cat-2(tm346) and Pdat-1::ICE animals that lack significant dopaminergic signaling, suggesting that DA is not an intermediate in TA inhibition. Finally, responses to TA and OA selectively desensitize after preexposure to the amines. Our data suggest that although tyraminergic and octopaminergic signaling yield identical phenotypes in these olfactory assays, they act independently through distinct receptors to modulate the ASH-mediated locomotory circuit and that C. elegans is a useful model to study the aminergic modulation of sensory-mediated locomotory behaviors.

Effects of genetic mutations and chemical exposures on Caenorhabditis elegans feeding: evaluation of a novel, high-throughput screening assay

Background

Government agencies have defined a need to reduce, refine or replace current mammalian-based bioassays with testing methods that use alternative species. Invertebrate species, such as Caenorhabditis elegans, provide an attractive option because of their short life cycles, inexpensive maintenance, and high degree of evolutionary conservation with higher eukaryotes. The C. elegans pharynx is a favorable model for studying neuromuscular function, and the effects of chemicals on neuromuscular activity, i.e., feeding. Current feeding methodologies, however, are labor intensive and only semi-quantitative.

Methodology/Principal Findings

Here a high-throughput assay is described that uses flow cytometry to measure C. elegans feeding by determining the size and intestinal fluorescence of hundreds of nematodes after exposure to fluorescent-labeled microspheres. This assay was validated by quantifying fluorescence in feeding-defective C. elegans (eat mutants), and by exposing wild-type nematodes to the neuroactive compounds, serotonin and arecoline. The eat mutations previously determined to cause slow pumping rates exhibited the lowest feeding levels with our assay. Concentration-dependent increases in feeding levels after serotonin exposures were dependent on food availability, while feeding levels decreased in arecoline-exposed nematodes regardless of the presence of food. The effects of the environmental contaminants, cadmium chloride and chlorpyrifos, on wild-type C. elegans feeding were then used to demonstrate an application of the feeding assay. Cadmium exposures above 200 µM led to a sharp drop in feeding levels. Feeding of chlorpyrifos-exposed nematodes decreased in a concentration-dependent fashion with an EC₅₀ of 2 µM.

Conclusions/Significance

The C. elegans fluorescence microsphere feeding assay is a rapid, reliable method for the assessment of neurotoxic effects of pharmaceutical drugs, industrial chemicals or environmental agents. This assay may also be applicable to large scale genetic or RNAi screens used to identify genes that are necessary for the development or function of the pharynx or other neuromuscular systems.

Responses of Heterodera glycines and Meloidogyne incognita to exogenously applied neuromodulators

Biogenic amines regulate important behaviours in nematodes and are associated with pharyngeal activity in plant-parasitic nematodes. A robust behavioural assay based upon nematode body movements was developed to expand the study of these and other neuroregulators in plant-parasitic nematodes. Dopamine, octopamine and serotonin each had significant but differing effects on the behaviour of soybean cyst nematode Heterodera glycines and root-knot nematode Meloidogyne incognita juveniles. Body movement frequency was increased twofold in H. glycines by 5 mM dopamine (P = 0.0001), but decreased by 50 mM dopamine in H. glycines (88%) and M. incognita (53%) (P < 0.0001). Movement frequency in both species was increased by 50-70% (P < 0.0001) by 50 mM octopamine, and 5 mM octopamine increased M. incognita movement frequency more than twofold (P < 0.0001). Movement frequency in each species was reduced by more than 90% by 5 mM serotonin (P < 0.0001). While amplitude of body movement in H. glycines was unaffected by any amine, it was significantly reduced in M. incognita by all amines (P < 0.0006). Stylet pulsing frequencies in either species were unaffected by dopamine or octopamine, but 5 mM serotonin stimulated pulsing in H. glycines by nearly 13-fold (P < 0.0001) and in M. incognita by more than 14-fold (P < 0.0001). The invertebrate neuropeptide FLRFamide (N-Phe-Leu-Arg-Phe) increased M. incognita body movement frequency 45% (P = 0.02) at 1 mM but did not affect stylet activity. Finally, H. glycines egg hatch was completely suppressed by 50 mM serotonin, and partially suppressed by 50 mM dopamine (75%; P < 0.0001) and 50 mM octopamine (55%; P < 0.0001).

Genes encoding putative biogenic amine receptors in the parasitic nematode Brugia malayi

Filarial nematodes, such as Brugia malayi, cause major health problems worldwide. The lack of a vaccine against B. malayi, combined with ineffective chemotherapy against the adult has prompted the examination of biogenic amine receptors (BARs) as possible targets for drug discovery. We employed bioinformatics to identify genes encoding putative B. malayi BARs. Surprisingly, the B. malayi genome contains half of the genes predicted to encode BARs in the genomes of free-living nematodes such as Caenorhabditis elegans or C. briggsae; however, all of the predicted B. malayi receptors have clear orthologues in C. elegans. The B. malayi genes encode each of the major BAR subclasses, including three serotonin, two dopamine and two tyramine/octopamine receptors and the structure of orthologous BAR genes is conserved. We find that potential G-protein coupling and ligand-specificity of individual BARs may be predicted by phylogenetic comparisons. Our results provide a framework for how G-protein coupled receptors may be targeted for drug development in medically important parasitic nematodes.

The genetics of pathogen avoidance in Caenorhabditis elegans

Much attention is rightly focused on how microbes cause disease, but they can also affect other aspects of host physiology, including behaviour. Indeed, pathogen avoidance behaviours are seen across animal taxa and are probably of major importance in nature. Here, we review what is known about the molecular genetics underlying pathogen avoidance in the nematode Caenorhabditis elegans. In its natural environment, the soil, this animal feeds on microbes and is continuously exposed to a diverse mix of microorganisms. Nematodes that develop efficient behavioural responses that enhance their attraction to sources of food and avoidance of pathogens will have an evolutionary advantage. C. elegans can specifically detect natural products of bacteria, including surfactants (such as serrawettin) and acylated homoserine lactone autoinducers, and it can learn to avoid pathogenic species. To date, several distinct mechanisms have been shown to be involved in pathogen avoidance. They are based on G protein-like, insulin-like and neuronal serotonin signalling. We discuss recent findings on the mechanisms of pathogen recognition in C. elegans, the relationship between alternative behavioural defences and also between these and other life-history traits. We propose that the selective pressure associated with avoidance behaviours influence both pathogen and host evolution.

UNC-31 (CAPS) is required for dense-core vesicle but not synaptic vesicle exocytosis in Caenorhabditis elegans

Previous studies indicated that CAPS (calcium-dependent activator protein for secretion) functions as an essential component for the Ca2+-dependent exocytosis of dense-core vesicles in neuroendocrine cells. However, recent mouse knock-out studies suggested an alternative role in the vesicular uptake or storage of catecholamines. To genetically assess the functional role of CAPS, we characterized the sole Caenorhabditis elegans CAPS ortholog UNC-31 (uncoordinated family member) and determined its role in dense-core vesicle-mediated peptide secretion and in synaptic vesicle recycling. Novel assays for dense-core vesicle exocytosis were developed by expressing a prepro-atrial natriuretic factor-green fluorescent protein fusion protein in C. elegans. unc-31 mutants exhibited reduced peptide release in vivo and lacked evoked peptide release in cultured neurons. In contrast, cultured neurons from unc-31 mutants exhibited normal stimulated synaptic vesicle recycling measured by FM4-64 [N-(3-triethylammoniumpropyl)-4-(6-(4-diethylamino)phenyl)hexatrienyl)pyridinium dibromide] dye uptake. Conversely, UNC-13, which exhibits sequence homology to CAPS/UNC-31, was found to be essential for synaptic vesicle but not dense-core vesicle exocytosis. These findings indicate that CAPS/UNC-31 function is not restricted to catecholaminergic vesicles but is generally required for and specific to dense-core vesicle exocytosis. Our results suggest that CAPS/UNC-31 and UNC-13 serve parallel and dedicated roles in dense-core vesicle and synaptic vesicle exocytosis, respectively, in the C. elegans nervous system.

Are Caenorhabditis elegans receptors useful targets for drug discovery: pharmacological comparison of tyramine receptors with high identity from C. elegans (TYRA-2) and Brugia malayi (Bm4)

The biogenic amine, tyramine (TA), modulates a number of key processes in nematodes and a number of TA-specific receptors have been identified. In the present study, we have identified a putative TA receptor (Bm4) in the recently completed Brugia malayi genome and compared its pharmacology to its putative Caenorhabditis elegans orthologue, TYRA-2, under identical expression and assay conditions. TYRA-2 and Bm4 are the most closely related C. elegans and B. malayi BA receptors and differ by only 14aa in the TM regions directly involved in ligand binding. Membranes from HEK-293 cells stably expressing Bm4 exhibited specific, saturable, high affinity, [(3)H]LSD and [(3)H]TA binding with K(d)s of 18.1+/-0.93 and 15.1+/-0.2 nM, respectively. More importantly, both TYRA-2 and Bm4 TA exhibited similar rank orders of potencies for a number of potential tyraminergic ligands. However, some significant differences were noted. For example, chloropromazine exhibited an order of magnitude higher affinity for Bm4 than TYRA-2 (pK(i)s of 7.6+/-0.2 and 6.49+/-0.1, respectively). In contrast, TYRA-2 had significantly higher affinity for phentolamine than Bm4. These results highlight the utility of the nearly completed B. malayi genome and the importance of using receptors from individual parasitic nematodes for drug discovery.

Molecular signaling involved in regulating feeding and other mitivated behaviors

The metabolic and nutritional status of an organism influences multiple behaviors in addition to food intake. When an organism is hungry, it employs behaviors that help it locate and ingest food while suppressing behaviors that are not associated with this goal. Alternatively, when an organism is satiated, food-seeking behaviors are repressed so that the animal can direct itself to other goal-oriented tasks such as reproductive behaviors. Studies in both vertebrate and invertebrate model systems have revealed that food-deprived and -satiated behaviors are differentially executed and integrated via common molecular signaling mechanisms. This article discusses cellular and molecular mechanisms for how insulin, neuropeptide Y (NPY), and serotonin utilize common signaling pathways to integrate feeding and metabolic state with other motivated behaviors. Insulin, NPY, and serotonin are three of the most well-studied molecules implicated in regulating such behaviors. Overall, insulin signaling allows an organism to coordinate proper behavioral output with changes in metabolism, NPY activates behaviors required for locating and ingesting food, and serotonin modulates behaviors performed when an organism is satiated. These three molecules work to ensure that the proper behaviors are executed in response to the feeding state of an organism.

The serotonin receptor SER-1 (5HT2ce) contributes to the regulation of locomotion inCaenorhabditis elegans

Serotonin (5-hydroxytryptamine: 5HT) is an important neuroactive substance in the model roundworm, Caenorhabditis elegans. Aside from having effects in feeding and egg-laying, 5HT inhibits motility and also modulates several locomotory behaviors, notably food-induced slowing and foraging. Recent evidence showed that a serotonergic 5HT2-like receptor named SER-1 (also known as 5HT2ce) was responsible for the effect of 5HT on egg-laying. Here we confirm this observation and show that SER-1 also plays an important role in locomotion. A mutant lacking SER-1 was found to be highly resistant to exogenous 5HT in the absence of food and this resistant phenotype was rescued by reintroducing the SER-1 gene in a mutant background. Pharmacological studies showed that the same antagonists that blocked the activity of recombinant SER-1 in vitro also inhibited the effect of 5HT on motility, suggesting the same receptor was responsible for both effects. When tested for locomotory behaviors, the SER-1 mutant was found to be moderately defective in food-induced slowing. In addition, the mutant changed direction more frequently than the wildtype when searching for food, suggesting that SER-1 may play a role in navigational control during foraging. Both these effects required the presence of MOD-1, a 5HT gated chloride channel, and the results indicate that SER-1 and MOD-1 modulate these behaviors through a common pathway. On the basis of expression analysis of a ser-1::GFP translational fusion, SER-1 is prominently located in central, integrating neurons of the head ganglia (RIA and RIC) but not the body wall musculature. The evidence suggests that SER-1 controls locomotion through indirect modulation of neuromuscular circuits and has effects both on speed and direction of movement.

Electrical signals during nectar sucking in the carpenter ant Camponotus mus

Ants of the same size can vary their intake rate of a given sucrose solution depending on the colony's needs for carbohydrates. As this capacity has not yet been described for another insect, the question of how they can do that was the focus of our work. When viscosity and ant-morphometry remain constant, changes in intake rate can only be attributed to the sucking forces. The aim of this study was to analyze the nectar sucking activity in the ant Camponotus mus. Feeding behavior seems to be under motivational control; therefore, we developed a non-invasive experimental device. We recorded the electrical signal generated during nectar feeding by offering ants sucrose solutions of different concentrations (from 10%w/w to 70%w/w). The signal frequency was between 2 and 12 peaks/s. We could distinguish two different patterns of electrical signal during feeding depending on the solution concentration. Only the more concentrated solutions reached frequencies higher than 7 peaks/s and the signal performance was quite irregular. For the other concentrations (10%, 30% and 50%), signal frequencies were lower than 6 peaks/s and the signal pattern was sinusoidal, regular and decreased with intake in all cases. We discuss the possible implications of these two signal patterns.

Anatomy, physiology and pharmacology of Caenorhabditis elegans pharynx: a model to define gene function in a simple neural system

Invertebrate neuroscience has provided a number of very informative model systems that have been extensively utilized in order to define the neurobiological bases of animal behaviours (Sattelle and Buckingham in Invert Neurosci 6:1-3, 2006). Most eminent among these are a number of molluscs, including Aplysia californica, Lymnaea stagnalis and Helix aspersa, crustacean systems such as the crab stomatogastric ganglion and a wide-range of other arthropods. All of these have been elegantly exploited to shed light on the very important phenomenon of the molecular and cellular basis for synaptic regulation that underpins behavioural plasticity. Key to the successful use of these systems has been the ability to study well-defined, relatively simple neuronal circuits that direct and regulate a quantifiable animal behaviour. Here we describe the pharyngeal system of the nematode C. elegans and its utility as a model for defining the genetic basis of behaviour. The circuitry of the nervous system in this animal is uniquely well-defined. Furthermore, the feeding behaviour of the worm is controlled by the activity of the pharynx and this in turn is regulated in a context-dependent manner by a simple nervous system that integrates external signals, e.g. presence or absence of food, and internal signals, e.g. the nutritional status of the animal to direct an appropriate response. The genetics of C. elegans is being effectively exploited to provide novel insight into genes that function to regulate the neuronal network that controls the pharynx. Here we summarise the progress to date and highlight topics for future research. Two main themes emerge. First, although the anatomy of the pharyngeal system is very well-defined, there is a much poorer understanding of its neurochemistry. Second, it is evident that the neurochemistry is remarkably complex for such a simple circuit/behaviour. This suggests that the pharyngeal activity may be subject to exquisitely precise regulation depending on the animal's environment and status. This therefore provides a very tractable genetic model to investigate neural mechanisms for signal integration and synaptic plasticity in a well-defined neuronal network that directs a quantifiable behaviour, feeding.

Both insulin and calcium channel signaling are required for developmental regulation of serotonin synthesis in the chemosensory ADF neurons of Caenorhabditis elegans

Proper calcium channel and insulin signaling are essential for normal brain development. Leaner mice with a mutation in the P/Q-type voltage-gated calcium channel, Cacna1a, develop cerebellar atrophy and mutations in the homologous human gene are associated with increased migraine and seizure tendency. Similarly, abnormalities in insulin signaling are associated with abnormal brain growth and migraine tendency. Previously, we have shown that in the ADF chemosensory neurons of Caenorhabditis elegans UNC-2/Ca(2+) channel function affects TGF-beta-dependent developmental regulation of tryptophan hydroxylase, the rate-limiting enzyme in serotonin synthesis. Here we show that developmental expression of a tryptophan hydroxylase: :GFP reporter construct is similarly decreased by reduction-of-function mutations in the daf-2/insulin receptor. This decreased expression of tryptophan hydroxylase observed in both the daf-2 and unc-2 mutant backgrounds is suppressible either genetically by reduction-of-function mutations in the daf-16/forkhead transcription factor, an effector of the DAF-2/insulin receptor, or pharmacologically by the serotonin receptor antagonist cyproheptadine. Overall, these data suggest that both UNC-2 and DAF-2 function are required in the developmental regulation of DAF-16 and serotonin-dependent inhibition of tryptophan hydroxylase expression.

Role of a FMRFamide-like family of neuropeptides in the pharyngeal nervous system of Caenorhabditis elegans

The nervous system of C. elegans has a remarkable abundance of flp genes encoding FMRFamide-like (FLP) neuropeptides. To provide insight into the physiological relevance of this neuropeptide diversity, we have tested more than 30 FLPs (encoded by 23 flps) for bioactivity on C. elegans pharynx. Eleven flp genes encode peptides that inhibit pharyngeal activity, while eight flp genes encode peptides that are excitatory. Three potent peptides (inhibitory, FLP-13A, APEASPFIRFamide; excitatory, FLP-17A, KSAFVRFamide; excitatory, FLP-17B, KSQYIRFamide) are encoded by flp genes, which, according to reporter gene constructs, are expressed in pharyngeal motoneurons. Thus, they may act through receptors localized on the pharyngeal muscle. The two other potent peptides, FLP-8 (excitatory AF1, KNEFIRFamide,) and FLP-11A (inhibitory, AMRNALVRFamide), appear to be expressed in extrapharyngeal neurons and are therefore likely to act either indirectly or as neurohormones. Intriguingly, a single neuron can express peptides that have potent but opposing biological activity in the pharynx. Only five flp genes encode neuropeptides that have no observable effect on the pharynx, but none of these have shown reporter gene expression in the pharyngeal nervous system. To examine the roles of multiple peptides produced from single precursors, a comparison was made between the bioactivity of different neuropeptides for five flp genes (flp-3, flp-13, flp-14, flp-17, and flp-18). For all but one gene (flp-14), the effects of peptides encoded by the same gene were similar. Overall, this study demonstrates the impressive neurochemical complexity of the simple circuit that regulates feeding in the nematode, C. elegans.

Actions of glutamate and ivermectin on the pharyngeal muscle of Ascaridia galli: a comparative study with Caenorhabditis elegans

The actions of glutamate and ivermectin were examined in the pharynx of Ascaridia galli and the results compared with those on the pharynx of Caenorhabditis elegans. In both preparations glutamate elicits a depolarization and inhibition of pharyngeal pumping, but the response of the pharynx of A. galli was much less than for C. elegans. This may be either because the pharyngeal membrane potential of the former is closely linked to the equilibrium potential for chloride ions (E(Cl)) while that of C. elegans is independent of E(Cl), or that there is a lower density of glutamate receptors on the pharyngeal muscle of A. galli compared with C. elegans. The maximum depolarization to glutamate of the pharyngeal muscle was 4.5+/-0.8 mV in A. galli while it was >25 mV in C. elegans. Picrotoxin was a weak antagonist of the glutamate response in both species. Flufenamic acid, pentobarbitone and flurazepam had no significant effect on either preparation at concentrations up to 100 microM. Three glutamate receptor agonists, ibotenate, kainate and quisqualate were all more potent than glutamate on the A. galli pharyngeal muscle. In contrast, only ibotenate was more potent than glutamate in C. elegans pharynx, the other two agonists being approximately 20 times less potent. The potency of ivermectin differed markedly between the two species, being approximately three orders of magnitude less potent on the pharynx of A. galli compared with C. elegans. This study demonstrates clear differences between the properties of the pharyngeal muscle of the two species and shows that care must be taken when extrapolating data from free-living to parasitic species of nematode.

SER-7, a Caenorhabditis elegans 5-HT7-like receptor, is essential for the 5-HT stimulation of pharyngeal pumping and egg laying

Serotonin (5-HT) stimulates both pharyngeal pumping and egg laying in Caenorhabditis elegans. Four distinct 5-HT receptors have been partially characterized, but little is known about their function in vivo. SER-7 exhibits most sequence identity to the mammalian 5-HT7 receptors and couples to a stimulation of adenyl cyclase when expressed in COS-7 cells. However, many 5-HT7-specific agonists have low affinity for SER-7. 5-HT fails to stimulate pharyngeal pumping and the firing of the MC motorneurons in animals containing the putative ser-7(tm1325) and ser-7(tm1728) null alleles. In addition, although pumping on bacteria is upregulated in ser-7(tm1325) animals, pumping is more irregular. A similar failure to maintain "fast pumping" on bacteria also was observed in ser-1(ok345) and tph-1(mg280) animals that contain putative null alleles of a 5-HT2-like receptor and tryptophan hydroxylase, respectively, suggesting that serotonergic signaling, although not essential for the upregulation of pumping on bacteria, "fine tunes" the process. 5-HT also fails to stimulate egg laying in ser-7(tm1325), ser-1(ok345), and ser-7(tm1325) ser-1(ok345) animals, but only the ser-7 ser-1 double mutants exhibit an Egl phenotype. All of the SER-7 mutant phenotypes are rescued by the expression of full-length ser-7gfp translational fusions. ser-7gfp is expressed in several pharyngeal neurons, including the MC, M2, M3, M4, and M5, and in vulval muscle. Interestingly, 5-HT inhibits egg laying and pharyngeal pumping in ser-7 null mutants and the 5-HT inhibition of egg laying, but not pumping, is abolished in ser-7(tm1325);ser-4(ok512) double mutants. Taken together, these results suggest that SER-7 is essential for the 5-HT stimulation of both egg laying and pharyngeal pumping, but that other signaling pathways can probably fulfill similar roles in vivo.

The Caenorhabditis elegans lev-8 gene encodes a novel type of nicotinic acetylcholine receptor alpha subunit

We have cloned Caenorhabditis elegans lev-8 and demonstrated that it encodes a novel nicotinic acetylcholine receptor (nAChR) subunit (previously designated ACR-13), which has functional roles in body wall and uterine muscles as part of a levamisole-sensitive receptor. LEV-8 is an alpha subunit and is the first to be described from the ACR-8-like group, a new class of nAChR with atypical acetylcholine-binding site (loop C) and channel-lining motifs. A single base pair change in the first intron of lev-8 in lev-8(x15) mutants leads to alternative splicing and the introduction of a premature stop codon. lev-8(x15) worms are partially resistant to levamisole-induced egg laying and paralysis, phenotypes rescued by expression of the wild-type gene. lev-8(x15) worms also show reduced rates of pharyngeal pumping. Electrophysiological recordings from body wall muscle show that currents recorded in response to levamisole have reduced amplitude in lev-8(x15) compared with wild-type animals. Consistent with these phenotypic observations, green fluorescent protein fused to LEV-8 is expressed in body wall and uterine muscle, motor neurons and epithelial-derived socket cells. Thus, LEV-8 is a levamisole receptor subunit and exhibits the most diverse expression pattern of any invertebrate nAChR subunit studied to date.

The GAR-3 muscarinic receptor cooperates with calcium signals to regulate muscle contraction in the Caenorhabditis elegans pharynx

Muscarinic acetylcholine receptors regulate the activity of neurons and muscle cells through G-protein-coupled cascades. Here, we identify a pathway through which the GAR-3 muscarinic receptor regulates both membrane potential and excitation-contraction coupling in the Caenorhabditis elegans pharyngeal muscle. GAR-3 signaling is enhanced in worms overexpressing gar-3 or lacking GPB-2, a G-protein beta-subunit involved in RGS-mediated inhibition of G(o)alpha- and G(q)alpha-linked pathways. High levels of signaling through GAR-3 inhibit pharyngeal muscle relaxation and impair feeding--but do not block muscle repolarization--when worms are exposed to arecoline, a muscarinic agonist. Loss of gar-3 function results in shortened action potentials and brief muscle contractions in the pharyngeal terminal bulb. High levels of calcium entry through voltage-gated channels also impair terminal bulb relaxation and sensitize worms to the toxic effects of arecoline. Mutation of gar-3 reverses this sensitivity, suggesting that GAR-3 regulates calcium influx or calcium-dependent processes. Because the effects of GAR-3 signaling on membrane depolarization and muscle contraction can be separated, we conclude that GAR-3 regulates multiple calcium-dependent processes in the C. elegans pharyngeal muscle.

Tyramine Functions Independently of Octopamine in the Caenorhabditis elegans Nervous System

Octopamine biosynthesis requires tyrosine decarboxylase to convert tyrosine into tyramine and tyramine beta-hydroxylase to convert tyramine into octopamine. We identified and characterized a Caenorhabditis elegans tyrosine decarboxylase gene, tdc-1, and a tyramine beta-hydroxylase gene, tbh-1. The TBH-1 protein is expressed in a subset of TDC-1-expressing cells, indicating that C. elegans has tyraminergic cells that are distinct from its octopaminergic cells. tdc-1 mutants have behavioral defects not shared by tbh-1 mutants. We show that tyramine plays a specific role in the inhibition of egg laying, the modulation of reversal behavior, and the suppression of head oscillations in response to anterior touch. We propose a model for the neural circuit that coordinates locomotion and head oscillations in response to anterior touch. Our findings establish tyramine as a neurotransmitter in C. elegans, and we suggest that tyramine is a genuine neurotransmitter in other invertebrates and possibly in vertebrates as well.

Biogenic amine receptors in parasitic nematodes: what can be learned from Caenorhabditis elegans?

The biogenic amines, serotonin, octopamine, tyramine and dopamine regulate many essential processes in parasitic nematodes, such as pharyngeal pumping, muscle contraction, and egg-laying, as well as more complex behaviors, such as mechanosensation and foraging, making biogenic amine receptors excellent targets for drug discovery. This review is designed to summarize our knowledge of nematode biogenic amine signaling and preliminarily identify some of the key receptors involved in the regulation of biogenic amine-dependent behaviors through an analysis of the free-living nematode, Caenorhabditis elegans.

Acanthocheilonema viteae: octopamine and its physiological role

Octopamine acts as an important neurotransmitter and neuromodulator in arthropods, mollusks, and nematodes. In mammals, however, no definite function for this amine has yet been described. By virtue of this difference in the neurophysiological requirement of the mammalian host and nematodes, octopamine offers good opportunity for exploring this area deeply with a view to identify a unique target for filarial chemotherapy. Results of the present study indicated that Acanthocheilonema viteae, the rodent filarial parasite, utilized tyrosine as a precursor for producing octopamine and some other biogenic amines. Octopamine exhibited specific saturable binding with the membrane prepared from the anterior portion of the filariid. This amine induced concentration dependent increase in the membrane potential which possibly caused tonic paralysis of the filariid. The rate of micro filarial release by the female worms also declined in the presence of this amine. The study thus provided preliminary evidences for the presence of an octopamine neurotransmitter system and also about some of the roles it plays in A. viteae.

Function and evolution of the serotonin-synthetic bas-1 gene and other aromatic amino acid decarboxylase genes in Caenorhabditis

Background

Aromatic L-amino acid decarboxylase (AADC) enzymes catalyze the synthesis of biogenic amines, including the neurotransmitters serotonin and dopamine, throughout the animal kingdom. These neurotransmitters typically perform important functions in both the nervous system and other tissues, as illustrated by the debilitating conditions that arise from their deficiency. Studying the regulation and evolution of AADC genes is therefore desirable to further our understanding of how nervous systems function and evolve.

Results

In the nematode C. elegans, the bas-1 gene is required for both serotonin and dopamine synthesis, and maps genetically near two AADC-homologous sequences. We show by transformation rescue and sequencing of mutant alleles that bas-1 encodes an AADC enzyme. Expression of a reporter construct in transgenics suggests that the bas-1 gene is expressed, as expected, in identified serotonergic and dopaminergic neurons. The bas-1 gene is one of six AADC-like sequences in the C. elegans genome, including a duplicate that is immediately downstream of the bas-1 gene. Some of the six AADC genes are quite similar to known serotonin- and dopamine-synthetic AADC's from other organisms whereas others are divergent, suggesting previously unidentified functions. In comparing the AADC genes of C. elegans with those of the congeneric C. briggsae, we find only four orthologous AADC genes in C. briggsae. Two C. elegans AADC genes – those most similar to bas-1 – are missing from C. briggsae. Phylogenetic analysis indicates that one or both of these bas-1-like genes were present in the common ancestor of C. elegans and C. briggsae, and were retained in the C. elegans line, but lost in the C. briggsae line. Further analysis of the two bas-1-like genes in C. elegans suggests that they are unlikely to encode functional enzymes, and may be expressed pseudogenes.

Conclusions

The bas-1 gene of C. elegans encodes a serotonin- and dopamine-synthetic AADC enzyme. Two C. elegans AADC-homologous genes that are closely related to bas-1 are missing from the congeneric C. briggsae; one or more these genes was present in the common ancestor of C. elegans and C. briggsae. Despite their persistence in C. elegans, evidence suggests the bas-1-like genes do not encode functional AADC proteins. The presence of the genes in C. elegans raises questions about how many 'predicted genes' in sequenced genomes are functional, and how duplicate genes are retained or lost during evolution. This is another example of unexpected retention of duplicate genes in eukaryotic genomes.

IRI-1, a LIN-15B homologue, interacts with inositol-1,4,5-triphosphate receptors and regulates gonadogenesis, defecation, and pharyngeal pumping in Caenorhabditis elegans

Inositol-1,4,5-triphosphate receptors (IP(3)Rs) are ligand-gated Ca(2+) channels that control Ca(2+) release from intracellular stores. They are central to a wide range of cellular responses. IP(3)Rs in Caenorhabditis elegans are encoded by a single gene, itr-1, and are widely expressed. Signaling through IP(3) and IP(3)Rs is important in ovulation, control of the defecation cycle, modulation of pharyngeal pumping rate, and embryogenesis. To further elucidate the molecular basis of the diversity of IP(3)R function, we used a yeast two-hybrid screen to search for proteins that interact with ITR-1. We identified an interaction between ITR-1 and IRI-1, a previously uncharacterized protein with homology to LIN-15B. Iri-1 is widely expressed, and its expression overlaps significantly with that of itr-1. In agreement with this observation, iri-1 functions in known itr-1-mediated processes, namely, upregulation of pharyngeal pumping in response to food and control of the defecation cycle. Knockdown of iri-1 in an itr-1 loss-of-function mutant potentiates some of these effects and sheds light on the signaling pathways that control pharyngeal pumping rate. Knockdown of iri-1 expression also results in a sterile, evl phenotype, as a consequence of failures in early Z1/Z4 lineage divisions, such that gonadogenesis is severely disrupted.

Slow Ca2+ dynamics in pharyngeal muscles in Caenorhabditis elegans during fast pumping

The pharyngeal muscles of Caenorhabditis elegans are composed of the corpus, isthmus and terminal bulb from anterior to posterior. These components are excited in a coordinated fashion to facilitate proper feeding through pumping and peristalsis. We analysed the spatiotemporal pattern of intracellular calcium dynamics in the pharyngeal muscles during feeding. We used a new ratiometric fluorescent calcium indicator and a new optical system that allows simultaneous illumination and detection at any two wavelengths. Pumping was observed with fast, repetitive and synchronous spikes in calcium concentrations in the corpus and terminal bulb, indicative of electrical coupling throughout the muscles. The posterior isthmus, however, responded to only one out of several pumping spikes to produce broad calcium transients, leading to peristalsis, the slow and gradual motion needed for efficient swallows. The excitation-calcium coupling may be uniquely modulated in this region at the level of calcium channels on the plasma membrane.

The voltage-gated calcium channel UNC-2 is involved in stress-mediated regulation of tryptophan hydroxylase

Migraine is an episodic pain disorder whose pathophysiology is related to deficiency of serotonin signaling and abnormal function of the P/Q-type calcium channel, CACNA1A. Because the relationship of the CACNA1A channel to serotonin signaling is unknown and potentially of therapeutic interest we have used genetic analysis of the Caenorhabditis elegans ortholog of this calcium channel, UNC-2, to help identify candidate downstream effectors of the human channel. By genetic dissection of the lethargic mutant phenotype of unc-2, we have established an epistasis pathway showing that UNC-2 function antagonizes a transforming growth factor (TGF)-beta pathway influencing movement rate. This same UNC-2/TGF-beta pathway is required for accumulation of normal serotonin levels and stress-induced modulation of tryptophan hydroxylase (tph) expression in the serotonergic chemosensory ADF neurons, but not the NSM neurons. We also show that transgenic expression of the migraine-associated Ca2+ channel, CACNA1A, in unc-2 animals can functionally substitute for UNC-2 in stress-activated regulation of tph expression. The demonstration that these evolutionarily related channels share a conserved ability to modulate tph expression through their effects on TGF-beta signaling provides the first specific example of how CACNA1A function may influence levels of the critical migraine neurotransmitter serotonin.

SER-7b, a constitutively active Galphas coupled 5-HT7-like receptor expressed in the Caenorhabditis elegans M4 pharyngeal motorneuron

Serotonin plays a key role in the regulation of pharyngeal pumping in nematodes. We have isolated a Caenorhabditis elegans cDNA (C09B7.1b, ser-7b) with greatest identity to the 5-HT7 receptor family. Membranes from COS-7 cells expressing SER-7b exhibit saturable [3H]-LSD binding (Kd = 45 nm) that is inhibited by serotonin (5-HT) and tryptamine, but not by other physiological biogenic amines. Expression of SER-7b in COS-7 cells results in dramatic increase in basal cAMP levels over untransfected cells that is dependent on expression level. 5-HT further elevates cAMP levels in a dose-dependent manner (pEC50 = 7.5 +/- 0.5). Mammalian 5-HT7 receptor inverse agonists reduce constitutive activity, with methiothepin the most potent (pIC50 = 7.8 +/- 0.1). Ser-7::GFP transcriptional fusions reveal that SER-7b appears to be expressed solely in the M4 pharyngeal motorneuron after hatching. This is the first report of a Galphas coupled biogenic amine receptor in nematodes and the localization of SER-7b in the M4 pharyngeal motorneuron suggests that SER-7b may play a role in the regulation of pharyngeal pumping.

Food transport in the C. elegans pharynx

Pumping of the C. elegans pharynx transports food particles (bacteria) posteriorly. We examined muscle motions to determine how this posterior transport is effected. We find that the motions of the middle section of the pharynx, the anterior isthmus, are delayed relative to the anterior section, the corpus. Simulations in which particles are assumed to move at mean fluid velocity when not captured by the walls of the pharyngeal lumen show that delayed isthmus motions do indeed cause net particle transport; however, the amount is much less than in the real pharynx. We propose that the geometry of the pharyngeal lumen forces particles to the center, where they move faster than mean fluid velocity. When this acceleration is incorporated into the simulation, particles are transported efficiently. The transport mechanism we propose explains past observations that the timing of muscle relaxation is important for effective transport. Our model also makes a prediction, which we confirm, that smaller bacteria are better food sources for C. elegans than large ones.