Serotonin deficiency shortens the duration of forward movement in Caenorhabditis elegans

Phylogenetic analyses of 5-hydroxytryptamine 3 (5-HT3) receptors in Metazoa

The 5-hydroxytrptamine 3 (5-HT3) receptor is a member of the 'Cys-loop' family and the only pentameric ligand gated ion channel among the serotonin receptors. 5-HT3 receptors play an important role in controlling growth, development, and behaviour in animals. Several 5-HT3 receptor antagonists are used to treat diseases (e.g., irritable bowel syndrome, nausea and emesis). Humans express five different subunits (A-E) enabling a variety of heteromeric receptors to form but all contain 5HT3A subunits. However, the information available about the 5-HT3 receptor subunit occurrence among the metazoan lineages is minimal. In the present article we searched for 5-HT3 receptor subunit homologs from different phyla in Metazoa. We identified more than 1000 5-HT3 receptor subunits in Metazoa in different phyla and undertook simultaneous phylogenetic analysis of 526 5HT3A, 358 5HT3B, 239 5HT3C, 70 5HT3D, and 173 5HT3E sequences. 5-HT3 receptor subunits were present in species belonging to 11 phyla: Annelida, Arthropoda, Chordata, Cnidaria, Echinodermata, Mollusca, Nematoda, Orthonectida, Platyhelminthes, Rotifera and Tardigrada. All subunits were most often identified in Chordata phylum which was strongly represented in searches. Using multiple sequence alignment, we investigated variations in the ligand binding region of the 5HT3A subunit protein sequences in the metazoan lineage. Several critical amino acid residues important for ligand binding (common structural features) are commonly present in species from Nematoda and Platyhelminth gut parasites through to Chordata. Collectively, this better understanding of the 5-HT3 receptor evolutionary patterns raises possibilities of future pharmacological challenges facing Metazoa including effects on parasitic and other species in ecosystems that contain 5-HT3 receptor ligands.

d-Serine and d-Alanine Regulate Adaptive Foraging Behavior in Caenorhabditis elegans via the NMDA Receptor

d-Serine (d-Ser) is a coagonist for NMDA-type glutamate receptors and is thus important for higher brain function. d-Ser is synthesized by serine racemase and degraded by d-amino acid oxidase. However, the significance of these enzymes and the relevant functions of d-amino acids remain unclear. Here, we show that in the nematode Caenorhabditis elegans, the serine racemase homolog SERR-1 and d-amino acid oxidase DAAO-1 control an adaptive foraging behavior. Similar to many organisms, C. elegans immediately initiates local search for food when transferred to a new environment. With prolonged food deprivation, the worms exhibit a long-range dispersal behavior as the adaptive foraging strategy. We found that serr-1 deletion mutants did not display this behavior, whereas daao-1 deletion mutants immediately engaged in long-range dispersal after food removal. A quantitative analysis of d-amino acids indicated that d-Ser and d-alanine (d-Ala) are both synthesized and suppressed during food deprivation. A behavioral pharmacological analysis showed that the long-range dispersal behavior requires NMDA receptor desensitization. Long-term pretreatment with d-Ala, as well as with an NMDA receptor agonist, expanded the area searched by wild-type worms immediately after food removal, whereas pretreatment with d-Ser did not. We propose that d-Ser and d-Ala are endogenous regulators that cooperatively induce the long-range dispersal behavior in C. elegans through actions on the NMDA receptor.SIGNIFICANCE STATEMENT In mammals, d-serine (d-Ser) functions as an important neuromodulator of the NMDA-type glutamate receptor, which regulates higher brain functions. In Caenorhabditis elegans, previous studies failed to clearly define the physiological significance of d-Ser, d-alanine (d-Ala), and their metabolic enzymes. In this study, we found that these d-amino acids and their associated enzymes are active during food deprivation, leading to an adaptive foraging behavior. We also found that this behavior involved NMDA receptor desensitization.

Serotonin in Animal Cognition and Behavior

Serotonin (5-hydroxytryptamine, 5-HT) is acknowledged as a major neuromodulator of nervous systems in both invertebrates and vertebrates. It has been proposed for several decades that it impacts animal cognition and behavior. In spite of a completely distinct organization of the 5-HT systems across the animal kingdom, several lines of evidence suggest that the influences of 5-HT on behavior and cognition are evolutionary conserved. In this review, we have selected some behaviors classically evoked when addressing the roles of 5-HT on nervous system functions. In particular, we focus on the motor activity, arousal, sleep and circadian rhythm, feeding, social interactions and aggressiveness, anxiety, mood, learning and memory, or impulsive/compulsive dimension and behavioral flexibility. The roles of 5-HT, illustrated in both invertebrates and vertebrates, show that it is more able to potentiate or mitigate the neuronal responses necessary for the fine-tuning of most behaviors, rather than to trigger or halt a specific behavior. 5-HT is, therefore, the prototypical neuromodulator fundamentally involved in the adaptation of all organisms across the animal kingdom.

Neurotoxicity of nonylphenol exposure on Caenorhabditis elegans induced by reactive oxidative species and disturbance synthesis of serotonin

The present study was performed to evaluate the neurobehavioural deficit induced by nonylphenol (NP), a well-known xenobiotic chemical. The neurotoxic mechanism from oxidative stress and serotonin-related progress was also investigated. Caenorhabditis elegans was exposed at different levels of NP ranging from 0 to 200 μg L^-1 for 10 days. The results revealed that from a relatively low concentration (i.e., 10 μg L^-1), significant effects including decreased head thrashes, body bends and forging behaviour could be observed, along with impaired learning and memory behaviour plasticity. The level of reactive oxygen species (ROS) in head was significantly elevated with the increase of NP concentrations from 10 to 200 μg L^-1. Through antioxidant experiment, the oxidative damage caused by NP restored to some extent. At a NP concentration of 200 μg L^-1, the significant increased expression of stress-related genes, including sod-1, sod-3, ctl-2, ctl-3 and cyp-35A2 gene, was observed from integrated gene expression profiles. In addition, in comparison with wild-type N2 worms, the ROS accumulation was increased significantly with the mutation of sod-3. Tryptophan hydroxylase (TPH) in ADF and NSM neurons sharply decreased at the concentrations of 10-200 μg L^-1. The transcription of TPH synthesis-related genes and serotonin-related genes were both suppressed, including tph-1, cat-1, cat-4, ser-1, and mod-5. Overall, these results indicated that NP could induce neurotoxicity on Caenorhabditis elegans through excessive induction of ROS and disturbance synthesis of serotonin. The conducted research opened up new avenues for more effective exploration of neurotoxicity caused by NP.

Insulin Signaling Deficiency Produces Immobility in Caenorhabditis elegans That Models Diminished Motivation States in Man and Responds to Antidepressants

Defects in insulin signaling have been reported in schizophrenia and major depressive disorder, which also share certain negative symptoms such as avolition, anhedonia, and apathy. These symptoms reflect diminished motivational states, which have been modeled in rodents as increased immobility in the forced swimming test. We have discovered that loss-of-function mutations in the insulin receptor (daf-2) and syntaxin (unc-64) genes in Caenorhabditis elegans, brief food deprivation, and exposure to DMSO produce immobility and avolition in non-dauer adults. The animals remain responsive to external stimuli; however, they fail to forage and will remain in place for >12 days or until they die. Their immobility can be prevented with drugs used to treat depression and schizophrenia and that reduce immobility in the forced swimming test. This includes amitriptyline, amoxapine, clozapine, and olanzapine, but not benzodiazepines and haloperidol. Recovery experiments confirm that immobility is induced and maintained by excessive signaling via serotonergic and muscarinic cholinergic pathways. The immobility response described here represents a potential protophenotype for avolition/anhedonia in man. This work may provide clues about why there is a significant increase in depression in patients with diabetes and suggest new therapeutic pathways for disorders featuring diminished motivation as a prominent symptom.

Food Search Strategy Changes in Caenorhabditis elegans under Chronic Starvation Conditions

Starvation is a primary threat to survival in nature. This study investigated the effects of starvation on animal behavior and neural function using a nematode model. Nematodes exhibit chemotactic responses to various compounds, including diacetyl produced by food bacteria. Locomotion, chemotactic behavior, and olfactory adaptation were measured following chronic starvation. Our results revealed a starvation-dependent reduction in locomotor activity. Chemotaxis response to the odorant diacetyl was attenuated after 2-38 hr of starvation. However, chemotactic behavior increased significantly after 48 hr of starvation compared with that after 38 hr of starvation, suggesting that food search behavior was enhanced after 48 hr of starvation. Inhibition of diacetyl adaptation was observed in the nematodes after 48 hr of starvation. However, exogenous exposure to serotonin during 48 hr of starvation caused the inhibition of diacetyl adaptation to be attenuated in following 24 hr period of normal feeding.Therefore, the inhibitory effects of starvation on olfactory adaptation may reduce chemotaxis response to the odorant diacetyl in a manner mediated by serotonin.

NPR-9, a Galanin-Like G-Protein Coupled Receptor, and GLR-1 Regulate Interneuronal Circuitry Underlying Multisensory Integration of Environmental Cues in Caenorhabditis elegans

C. elegans inhabit environments that require detection of diverse stimuli to modulate locomotion in order to avoid unfavourable conditions. In a mammalian context, a failure to appropriately integrate environmental signals can lead to Parkinson’s, Alzheimer’s, and epilepsy. Provided that the circuitry underlying mammalian sensory integration can be prohibitively complex, we analyzed nematode behavioral responses in differing environmental contexts to evaluate the regulation of context dependent circuit reconfiguration and sensorimotor control. Our work has added to the complexity of a known parallel circuit, mediated by interneurons AVA and AIB, that integrates sensory cues and is responsible for the initiation of backwards locomotion. Our analysis of the galanin-like G-protein coupled receptor NPR-9 in C. elegans revealed that upregulation of galanin signaling impedes the integration of sensory evoked neuronal signals. Although the expression pattern of npr-9 is limited to AIB, upregulation of the receptor appears to impede AIB and AVA circuits to broadly prevent backwards locomotion, i.e. reversals, suggesting that these two pathways functionally interact. Galanin signaling similarly plays a broadly inhibitory role in mammalian models. Moreover, our identification of a mutant, which rarely initiates backwards movement, allowed us to interrogate locomotory mechanisms underlying chemotaxis. In support of the pirouette model of chemotaxis, organisms that did not exhibit reversal behavior were unable to navigate towards an attractant peak. We also assessed ionotropic glutamate receptor GLR-1 cell-specifically within AIB and determined that GLR-1 fine-tunes AIB activity to modify locomotion following reversal events. Our research highlights that signal integration underlying the initiation and fine-tuning of backwards locomotion is AIB and NPR-9 dependent, and has demonstrated the suitability of C. elegans for analysis of multisensory integration and sensorimotor control.

Multiple environmental cues are sensed by an organism in order to coordinate behavioral responses. Consequently, organisms must be able to simultaneously detect and integrate multiple external stimuli in order to appropriately modify their behavior. Identifying the unique circuits mediating the response to individual stimuli and points of overlap is essential to understanding how multiple signals can be integrated for a coordinated behavioral response. In order to analyze individual circuits, we have used the model organism C. elegans. We have identified that a C. elegans neuropeptide receptor (NPR-9) and a glutamate receptor (GLR-1) function in a single interneuron to play a broad regulatory role in multiple neural circuits. Our research has identified that interneuron AIB is involved in the integration of signals from numerous sensory neurons. Moreover, regulation of AIB via a neuropeptide receptor (NPR-9) and a glutamate receptor (GLR-1) coordinates AIB activity in the context of multisensory integration. Long-range chemotaxis behavior, in which an organism alters locomotory patterns based on odorant sensation, is also regulated by NPR-9. Our analysis indicates that reversals, and thus the pirouette model, are sufficient for chemotaxis.

Drug discovery based on genetic and metabolic findings in schizophrenia

Recent progress in the genetics of schizophrenia provides the rationale for re-evaluating causative factors and therapeutic strategies for this disease. Here, we review the major candidate susceptibility genes and relate the aberrant function of these genes to defective regulation of energy metabolism in the schizophrenic brain. Disturbances in energy metabolism potentially lead to neurodevelopmental deficits, impaired function of the mature nervous system and failure to maintain neurites/dendrites and synaptic connections. Current antipsychotic drugs do not specifically address these underlying deficits; therefore, a new generation of more effective medications is urgently needed. Novel targets for future drug discovery are identified in this review. The coordinated application of structure-based drug design, systems biology and research on model organisms may greatly facilitate the search for next-generation antipsychotic drugs.

Evaluation of Head Movement Periodicity and Irregularity during Locomotion of Caenorhabditis elegans

Caenorhabditis elegans is suitable for studying the nervous system, which controls behavior. C. elegans shows sinusoidal locomotion on an agar plate. The head moves not only sinusoidally but also more complexly, which reflects regulation of the head muscles by the nervous system. The head movement becomes more irregular with senescence. To date, the head movement complexity has not been quantitatively analyzed. We propose two simple methods for evaluation of the head movement regularity on an agar plate using image analysis. The methods calculate metrics that are a measure of how the head end movement is correlated with body movement. In the first method, the length along the trace of the head end on the agar plate between adjacent intersecting points of the head trace and the quasi-midline of the head trace, which was made by sliding an averaging window of 1/2 the body wavelength, was obtained. Histograms of the lengths showed periodic movement of the head and deviation from it. In the second method, the intersections between the trace of the head end and the trace of the 5 (near the pharynx) or 50% (the mid-body) point from the head end in the centerline length of the worm image were marked. The length of the head trace between adjacent intersections was measured, and a histogram of the lengths was produced. The histogram for the 5% point showed deviation of the head end movement from the movement near the pharynx. The histogram for the 50% point showed deviation of the head movement from the sinusoidal movement of the body center. Application of these methods to wild type and several mutant strains enabled evaluation of their head movement periodicity and irregularity, and revealed a difference in the age-dependence of head movement irregularity between the strains. A set of five parameters obtained from the histograms reliably identifies differences in head movement between strains.

An optogenetic application of proton pump ArchT to C. elegans cells

Application of novel light-driven ion channel/pumps would benefit optogenetic studies of Caenorhabditis elegans. A recent study showed that ArchT, a novel light-driven outward proton pump, is >3 times more light-sensitive than the Arch proton pump. Here we report the silencing effect of ArchT in C. elegans cells. ArchT expressed by using a body-wall muscle or pan-neuronal-promoters caused a quick and reliable locomotion paralysis when worms were illuminated by green light. Unlike the report on mouse neurons, however, light sensitivity of ArchT is similar to that of Arch in C. elegans. ArchT-mediated acute silencing of serotonergic neurons quickly triggered backward locomotion. This response was abolished in the presence of exogenously added serotonin, suggesting that, in a normal situation, serotonin is secreted in a constitutive fashion to repress backward movement.

Sex differences in behavioral decision-making and the modulation of shared neural circuits

Animals prioritize behaviors according to their physiological needs and reproductive goals, selecting a single behavioral strategy from a repertoire of possible responses to any given stimulus. Biological sex influences this decision-making process in significant ways, differentiating the responses animals choose when faced with stimuli ranging from food to conspecifics. We review here recent work in invertebrate models, including C. elegans, Drosophila, and a variety of insects, mollusks and crustaceans, that has begun to offer intriguing insights into the neural mechanisms underlying the sexual modulation of behavioral decision-making. These findings show that an animal's sex can modulate neural function in surprisingly diverse ways, much like internal physiological variables such as hunger or thirst. In the context of homeostatic behaviors such as feeding, an animal's sex and nutritional status may converge on a common physiological mechanism, the functional modulation of shared sensory circuitry, to influence decision-making. Similarly, considerable evidence suggests that decisions on whether to mate or fight with conspecifics are also mediated through sex-specific neuromodulatory control of nominally shared neural circuits. This work offers a new perspective on how sex differences in behavior emerge, in which the regulated function of shared neural circuitry plays a crucial role. Emerging evidence from vertebrates indicates that this paradigm is likely to extend to more complex nervous systems as well. As men and women differ in their susceptibility to a variety of neuropsychiatric disorders affecting shared behaviors, these findings may ultimately have important implications for human health.

Role of the evolutionarily conserved starvation response in anorexia nervosa

This review will summarize recent findings concerning the biological regulation of starvation as it relates to anorexia nervosa (AN), a serious eating disorder that mainly affects female adolescents and young adults. AN is generally viewed as a psychosomatic disorder mediated by obsessive concerns about weight, perfectionism and an overwhelming desire to be thin. By contrast, the thesis that will be developed here is that, AN is primarily a metabolic disorder caused by defective regulation of the starvation response, which leads to ambivalence towards food, decreased food consumption and characteristic psychopathology. We will trace the starvation response from yeast to man and describe the central role of insulin (and insulin-like growth factor-1 (IGF-1))/Akt/ F-box transcription factor (FOXO) signaling in this response. Akt is a serine/threonine kinase downstream of the insulin and IGF-1 receptors, whereas FOXO refers to the subfamily of Forkhead box O transcription factors, which are regulated by Akt. We will also discuss how initial bouts of caloric restriction may alter the production of neurotransmitters that regulate appetite and food-seeking behavior and thus, set in motion a vicious cycle. Finally, an integrated approach to treatment will be outlined that addresses the biological aspects of AN.

Age- and subcaste-related patterns of serotonergic immunoreactivity in the optic lobes of the ant Pheidole dentata

Serotonin, a biogenic amine known to be a neuromodulator of insect behavior, has recently been associated with age-related patterns of task performance in the ant Pheidole dentata. We identified worker age- and subcaste-related patterns of serotonergic activity within the optic lobes of the P. dentata brain to further examine its relationship to polyethism. We found strong immunoreactivity in the optic lobes of the brains of both minor and major workers. Serotonergic cell bodies in the optic lobes increased significantly in number as major and minor workers matured. Old major workers had greater numbers of serotonergic cell bodies than minors of a similar age. This age-related increase in serotonergic immunoreactivity, as well as the presence of diffuse serotonin networks in the mushroom bodies, antennal lobes, and central complex, occurs concomitantly with an increase in the size of worker task repertoires. Our results suggest that serotonin is associated with the development of the visual system, enabling the detection of task-related stimuli outside the nest, thus playing a significant role in worker behavioral development and colony-wide division of labor.

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.

In vitro and in vivo characterization of p-amino-phenethyl-m-trifluoromethylphenyl piperazine (PAPP), a novel serotonergic agonist with anthelmintic activity against Haemonchus contortus, Teladorsagia circumcincta and Trichostrongylus colubriformis

The neurotransmitter serotonin (5-hydroxy tryptamine or 5HT) regulates key physiological processes in nematodes such as locomotion and feeding. PAPP (p-amino-phenethyl-m-trifluoromethylphenyl piperazine) is a known agonist of the 5-HT(1Hc) receptor of the barber pole worm, Haemonchus contortus. In this study, PAPP was highly active against L3-stage larvae of H. contortus and Trichostrongylus colubriformis in an in vitro larval migration assay, with EC50 values of 9.36 and 11.8 microM, respectively, that were comparable to levamisole (10.2 microM) and superior to pyrantel (55.39 microM). When administered orally or subcutaneously to nematode infected gerbils, PAPP provided >99% efficacy against H. contortus and >98% efficacy against Teladorsagia circumcincta at 100 mg/kg, comparable to levamisole at 10 mg/kg. Drug titration revealed significant activity down to 50 mg/kg against these two species. Spectrum was limited, however, with somewhat lower efficacy (83%) in T. colubriformis infected gerbils at 100 mg/kg. Oral delivery of hydrochloride, acetate and phosphate salts of PAPP to nematode infected gerbils did not result in an increase in either potency or spectrum. The finding that PAPP exhibits significant anthelmintic activity suggests that the nematode-specific serotonergic system is a viable target for future anthelmintic discovery.

Embryogenesis of the serotonergic system in the earthwormEisenia fetida (Annelida, Oligochaeta): Immunohistochemical and biochemical studies

Organization of the serotonergic system and changes of the serotonin (5-HT) content were studied during the embryogenesis of the earthworm Eisenia fetida, using immunocytochemistry and HPLC. A gradual emergence of 5-HT immunoreactive (IR) cells and their axon projections in the several ganglia of the central (CNS) and peripheral nervous system are described in the context of a staged time-scale of development. The first 5-HT-IR neurons appear in the subesophageal ganglion at an early embryonic stage (E2), followed by neurons in some rostrally located ventral ganglia. In the cerebral ganglion, 5-HT-IR cells can be detected only from stage E5. The number of labeled cells in each ganglion of the embryo increases until hatching, when it is still considerably lower than that observed in adults. This shows that the development of the 5-HTergic system is far from complete by the end of embryogenesis. Organization of 5-HT-IR innervation of the body wall starts by stages E3 to E4. In the stomatogastric nervous system the first 5-HT-IR fibers can be detected by stage E5. By stage E9 5-HT immunopositive neurons can be observed in both the stomatogastric ganglia and the enteric plexus. Both 5-HT levels and the numbers of the labeled cells show a significant increase before hatching, which indicate a functional maturation of the 5-HTergic system. Based on the early appearance of 5-HT, we suppose that it may play a regulatory role in both the gangliogenesis and the maturation of peripheral functions necessary during postembryonic life.