Addiction research in a simple animal model: the nematode Caenorhabditis elegans

Noninvasive Electrophysiology: Emerging Prospects in Aquatic Neurotoxicity Testing

The significance of neurotoxicological risks associated with anthropogenic pollution is gaining increasing recognition worldwide. In this regard, perturbations in behavioral traits upon exposure to environmentally relevant concentrations of neurotoxic and neuro-modulating contaminants have been linked to diminished ecological fitness of many aquatic species. Despite an increasing interest in behavioral testing in aquatic ecotoxicology there is, however, a notable gap in understanding of the neurophysiological foundations responsible for the altered behavioral phenotypes. One of the canonical approaches to explain the mechanisms of neuro-behavioral changes is functional analysis of neuronal transmission. In aquatic animals it requires, however, invasive, complex, and time-consuming electrophysiology techniques. In this perspective, we highlight emerging prospects of noninvasive, in situ electrophysiology based on multielectrode arrays (MEAs). This technology has only recently been pioneered for the detection and analysis of transient electrical signals in the central nervous system of small model organisms such as zebrafish. The analysis resembles electroencephalography (EEG) applications and provides an appealing strategy for mechanistic explorative studies as well as routine neurotoxicity risk assessment. We outline the prospective future applications and existing challenges of this emerging analytical strategy that is poised to bring new vistas for aquatic ecotoxicology such as greater mechanistic understanding of eco-neurotoxicity and thus more robust risk assessment protocols.

Increased dopaminergic neurotransmission results in ethanol dependent sedative behaviors in Caenorhabditis elegans

Ethanol is a widely used drug, excessive consumption of which could lead to medical conditions with diverse symptoms. Ethanol abuse causes dysfunction of memory, attention, speech and locomotion across species. Dopamine signaling plays an essential role in ethanol dependent behaviors in animals ranging from C. elegans to humans. We devised an ethanol dependent assay in which mutants in the dopamine autoreceptor, dop-2, displayed a unique sedative locomotory behavior causing the animals to move in circles while dragging the posterior half of their body. Here, we identify the posterior dopaminergic sensory neuron as being essential to modulate this behavior. We further demonstrate that in dop-2 mutants, ethanol exposure increases dopamine secretion and functions in a DVA interneuron dependent manner. DVA releases the neuropeptide NLP-12 that is known to function through cholinergic motor neurons and affect movement. Thus, DOP-2 modulates dopamine levels at the synapse and regulates alcohol induced movement through NLP-12.

Effects of Ethanol on Sensory Inputs to the Medial Giant Interneurons of Crayfish

Crayfish are capable of two rapid, escape reflexes that are mediated by two pairs of giant interneurons, the lateral giants (LG) and the medial giants (MG), which respond to threats presented to the abdomen or head and thorax, respectively. The LG has been the focus of study for many decades and the role of GABAergic inhibition on the escape circuit is well-described. More recently, we demonstrated that the LG circuit is sensitive to the acute effects of ethanol and this sensitivity is likely mediated by interactions between ethanol and the GABAergic system. The MG neurons, however, which receive multi-modal sensory inputs and are located in the brain, have been less studied despite their established importance during many naturally occurring behaviors. Using a combination of electrophysiological and neuropharmacological techniques, we report here that the MG neurons are sensitive to ethanol and experience an increase in amplitudes of post-synaptic potentials following ethanol exposure. Moreover, they are affected by GABAergic mechanisms: the facilitatory effect of acute EtOH can be suppressed by pretreatment with a GABA receptor agonist whereas the inhibitory effects resulting from a GABA agonist can be occluded by ethanol exposure. Together, our findings suggest intriguing neurocellular interactions between alcohol and the crayfish GABAergic system. These results enable further exploration of potentially conserved neurochemical mechanisms underlying the interactions between alcohol and neural circuitry that controls complex behaviors.

Nicotine affects protein complex rearrangement in Caenorhabditis elegans cells

Nicotine may affect cell function by rearranging protein complexes. We aimed to determine nicotine-induced alterations of protein complexes in Caenorhabditis elegans (C. elegans) cells, thereby revealing links between nicotine exposure and protein complex modulation. We compared the proteomic alterations induced by low and high nicotine concentrations (0.01 mM and 1 mM) with the control (no nicotine) in vivo by using mass spectrometry (MS)-based techniques, specifically the cetyltrimethylammonium bromide (CTAB) discontinuous gel electrophoresis coupled with liquid chromatography (LC)-MS/MS and spectral counting. As a result, we identified dozens of C. elegans proteins that are present exclusively or in higher abundance in either nicotine-treated or untreated worms. Based on these results, we report a possible network that captures the key protein components of nicotine-induced protein complexes and speculate how the different protein modules relate to their distinct physiological roles. Using functional annotation of detected proteins, we hypothesize that the identified complexes can modulate the energy metabolism and level of oxidative stress. These proteins can also be involved in modulation of gene expression and may be crucial in Alzheimer's disease. The findings reported in our study reveal putative intracellular interactions of many proteins with the cytoskeleton and may contribute to the understanding of the mechanisms of nicotinic acetylcholine receptor (nAChR) signaling and trafficking in cells.

The protein arginine methyltransferase PRMT5 promotes D2-like dopamine receptor signaling

Protein arginine methylation regulates diverse functions of eukaryotic cells, including gene expression, the DNA damage response, and circadian rhythms. We showed that arginine residues within the third intracellular loop of the human D2 dopamine receptor, which are conserved in the DOP-3 receptor in the nematode Caenorhabditis elegans, were methylated by protein arginine methyltransferase 5 (PRMT5). By mutating these arginine residues, we further showed that their methylation enhanced the D2 receptor-mediated inhibition of cyclic adenosine monophosphate (cAMP) signaling in cultured human embryonic kidney (HEK) 293T cells. Analysis of prmt-5-deficient worms indicated that methylation promoted the dopamine-mediated modulation of chemosensory and locomotory behaviors in C. elegans through the DOP-3 receptor. In addition to delineating a previously uncharacterized means of regulating GPCR (heterotrimeric guanine nucleotide-binding protein-coupled receptor) signaling, these findings may lead to the development of a new class of pharmacological therapies that modulate GPCR signaling by changing the methylation status of these key proteins.

Drug-dependent behaviors and nicotinic acetylcholine receptor expressions in Caenorhabditis elegans following chronic nicotine exposure

Nicotine, the major psychoactive compound in tobacco, targets nicotinic acetylcholine receptors (nAChRs) and results in drug dependence. The nematode Caenorhabditis elegans' (C. elegans) genome encodes conserved and extensive nicotinic receptor subunits, representing a useful system to investigate nicotine-induced nAChR expressions in the context of drug dependence. However, the in vivo expression pattern of nAChR genes under chronic nicotine exposure has not been fully investigated. To define the role of nAChR genes involved in nicotine-induced locomotion changes and the development of tolerance to these effects, we characterized the locomotion behavior combining the use of two systems: the Worm Tracker hardware and the WormLab software. Our results indicate that the combined system is an advantageous alternative to define drug-dependent locomotion behavior in C. elegans. Chronic (24-h dosing) nicotine exposure at 6.17 and 61.7μM induced nicotine-dependent behaviors, including drug stimulation, tolerance/adaption, and withdrawal responses. Specifically, the movement speed of naïve worms on nicotine-containing environments was significantly higher than on nicotine-free environments, suggesting locomotion stimulation by nicotine. In contrast, the 24-h 6.17μM nicotine-treated worms exhibited significantly higher speeds on nicotine-free plates than on nicotine-containing plates. Furthermore significantly increased locomotion behavior during nicotine cessation was observed in worms treated with a higher nicotine concentration of 61.7μM. The relatively low locomotion speed of nicotine-treated worms on nicotine-containing environments also indicates adaption/tolerance of worms to nicotine following chronic nicotine exposure. In addition, this study provides useful information regarding the comprehensive in vivo expression profile of the 28 "core" nAChRs following different dosages of chronic nicotine treatments. Eleven genes (lev-1, acr-6, acr-7, acr-11, lev-8, acr-14, acr-16, acr-20, acr-21, ric-3, and unc-29) were significantly up-regulated following 61.7μM nicotine treatment, in which worms showed significantly increased locomotion behavior. This study provides insights into the linkage between nicotine-induced locomotion behavior and the regulation of nicotinic acetylcholine receptors.

The role of the BK channel in ethanol response behaviors: evidence from model organism and human studies

Alcohol abuse is a significant public health problem. Understanding the molecular effects of ethanol is important for the identification of at risk individuals, as well as the development of novel pharmacotherapies. The large conductance calcium sensitive potassium (BK) channel has emerged as an important player in the behavioral response to ethanol in genetic studies in several model organisms and in humans. The BK channel, slo-1, was identified in a forward genetics screen as a major ethanol target in C. elegans for the effects of ethanol on locomotion and egg-laying behaviors. Regulation of the expression of the BK channel, slo, in Drosophila underlies the development of rapid tolerance to ethanol and benzyl alcohol sedation. Rodent expression studies of the BK-encoding KCNMA1 gene have identified regulation of mRNA levels in response to ethanol exposure, and knock out studies in mice have demonstrated that the β subunits of the BK channel, β1 and β4, can modulate ethanol sensitivity of the channel in electrophysiological preparations, and can influence drinking behavior. In human genetics studies, both KCNMA1 and the genes encoding β subunits of the BK channel have been associated with alcohol dependence. This review describes the genetic data for a role for BK channels in mediating behavioral responses to ethanol across these species.

Effects of midazolam, pentobarbital and ketamine on the mRNA expression of ion channels in a model organism Daphnia pulex

Background

Over the last few decades intensive studies have been carried out on the molecular targets mediating general anesthesia as well as the effects of general anesthetics. The γ-aminobutyric acid type A receptor (GABA_AR) has been indicated as the primary target of general anaesthetics such as propofol, etomidate and isoflurane, and sedating drugs including benzodiazepines and barbiturates. The GABA_AR is also involved in drug tolerance and dependence. However, the involvement of other ion channels is possible.

Methods

Using reverse transcription and quantitative PCR techniques, we systematically investigated changes in the mRNA levels of ion channel genes in response to exposure to midazolam, pentobarbital and ketamine in a freshwater model animal, Daphnia pulex. To retrieve the sequences of Daphnia ion channel genes, Blast searches were performed based on known human or Drosophila ion channel genes. Retrieved sequences were clustered with the maximum-likelihood method. To quantify changes in gene expression after the drug treatments for 4 hours, total RNA was extracted and reverse transcribed into cDNA and then amplified using quantitative PCR.

Results

A total of 108 ion channel transcripts were examined, and 19, 11 and 11 of them are affected by midazolam (100 μM), pentobarbital (200 μM) and ketamine (100 μM), respectively, covering a wide variety of ion channel types. There is some degree of overlap with midazolam- and pentobarbital-induced changes in the mRNA expression profiles, but ketamine causes distinct changes in gene expression pattern.

In addition, flumazenil (10 μM) eliminates the effect of midazolam on the mRNA expression of the GABA_A receptor subunit Rdl, suggesting a direct interaction between midazolam and GABA_A receptors.

Conclusions

Recent research using high throughput technology suggests that changes in mRNA expression correlate with delayed protein expression. Therefore, the mRNA profile changes in our study may reflect the molecular targets not only in drug actions, but also in chronic drug addiction. Our data also suggest the possibility that hypnotic/anesthetic drugs are capable of altering the functions of the nervous system, as well as those non-nerve tissues with abundant ion channel expressions.

Amphetamine activates an amine-gated chloride channel to generate behavioral effects in Caenorhabditis elegans

Amphetamine is a highly addictive psychostimulant, which is thought to generate its effects by promoting release of dopamine through reverse activation of dopamine transporters. However, some amphetamine-mediated behaviors persist in dopamine transporter knock-out animals, suggesting the existence of alternative amphetamine targets. Here we demonstrate the identification of a novel amphetamine target by showing that in Caenorhabditis elegans, a large fraction of the behavioral effects of amphetamine is mediated through activation of the amine-gated chloride channel, LGC-55. These findings bring to light alternative pathways engaged by amphetamine, and urge rethinking of the molecular mechanisms underlying the effects of this highly-addictive psychostimulant.

Cocaine tolerance in honey bees

Increasingly invertebrates are being used to investigate the molecular and cellular effects of drugs of abuse to explore basic mechanisms of addiction. However, in mammals the principle factors contributing to addiction are long-term adaptive responses to repeated drug use. Here we examined whether adaptive responses to cocaine are also seen in invertebrates using the honey bee model system. Repeated topical treatment with a low dose of cocaine rendered bees resistant to the deleterious motor effects of a higher cocaine dose, indicating the development of physiological tolerance to cocaine in bees. Cocaine inhibits biogenic amine reuptake transporters, but neither acute nor repeated cocaine treatments caused measurable changes in levels of biogenic amines measured in whole bee brains. Our data show clear short and long-term behavioural responses of bees to cocaine administration, but caution that, despite the small size of the bee brain, measures of biogenic amines conducted at the whole-brain level may not reveal neurochemical effects of the drug.

Genes, hormones, and circuits: an integrative approach to study the evolution of social behavior

Tremendous progress has been made in our understanding of the ultimate and proximate mechanisms underlying social behavior, yet an integrative evolutionary analysis of its underpinnings has been difficult. In this review, we propose that modern genomic approaches can facilitate such studies by integrating four approaches to brain and behavior studies: (1) animals face many challenges and opportunities that are ecologically and socially equivalent across species; (2) they respond with species-specific, yet quantifiable and comparable approach and avoidance behaviors; (3) these behaviors in turn are regulated by gene modules and neurochemical codes; and (4) these behaviors are governed by brain circuits such as the mesolimbic reward system and the social behavior network. For each approach, we discuss genomic and other studies that have shed light on various aspects of social behavior and its underpinnings and suggest promising avenues for future research into the evolution of neuroethological systems.

d-Amphetamine stimulates unconditioned exploration/approach behaviors in crayfish: towards a conserved evolutionary function of ancestral drug reward

In mammals, rewarding properties of drugs depend on their capacity to activate a dopamine-mediated appetitive motivational seeking state--a system that allows animals to pursue and find all kinds of objects and events needed for survival. With such states strongly conserved in evolution, invertebrates have recently been developed into a powerful model in addiction research, where a shared ancestral brain system for the acquisition of reward can mediate drug addiction in many species. A conditioned place preference paradigm has illustrated that crayfish seek out environments that had previously been paired with psychostimulant and opioid administration. The present work demonstrates that the administration of D-amphetamine stimulates active explorative behaviors in crayfish through the action of the drug within their head ganglion. Crayfish, with a modularly organized and experimentally accessible, ganglionic nervous system offers a unique model to investigate (1) the fundamental, biological mechanisms of addictive drug reward; (2) how an appetitive/seeking disposition is implemented in a simple neural system, and (3) how it mediates the rewarding actions of major drugs of abuse.

Advances in animal models of drug addiction

Drug addiction is a syndrome of impaired response inhibition and salience attribution, which involves a complex neurocircuitry underlying drug reinforcement, drug craving, and compulsive drug-seeking and drug-taking behaviors despite adverse consequences. The concept of disease stages with transitions from acute rewarding effects to early- and end-stage addiction has had an important impact on the design of nonclinical animal models. This chapter reviews the main advances in nonclinical paradigms that aim to at model (1) positive and negative reinforcing effects of addictive drugs; (2) relapse to drug-seeking behavior; (3) reconsolidation of drug cue memories, and (4) compulsive/impulsive drug intake. In addition, recent small animal neuroimaging studies and invertebrate models will be briefly discussed (see also Bifone and Gozzi, Animal models of ADHD, 2011). Continuous improvement in modeling drug intake, craving, withdrawal symptoms, relapse, and comorbid psychiatric associations is a necessary step to better understand the etiology of the disease and to ultimately foster the discovery, validation and optimization of new efficacious pharmacotherapeutic approaches. The modeling of specific subprocesses or constructs that address clinically defined criteria will ultimately increase our understanding of the disease as a whole. Future research will have to address the questions of whether some of these constructs can be reliably used as outcome measures to assess the effects of a treatment in clinical settings, whether changes in those measures can be a target of therapeutic efforts, and whether they relate to biological markers of traits such as impulsivity, which contribute to increased drug-seeking and may predict binge-like patterns of drug intake.

Drug-sensitive reward in crayfish: an invertebrate model system for the study of SEEKING, reward, addiction, and withdrawal

In mammals, rewarding properties of drugs depend on their capacity to activate appetitive motivational states. With the underlying mechanisms strongly conserved in evolution, invertebrates have recently emerged as a powerful new model in addiction research. In crayfish natural reward has proven surprisingly sensitive to human drugs of abuse, opening an unlikely avenue of research into the basic biological mechanisms of drug addiction. In a series of studies we first examined the presence of natural reward systems in crayfish, then characterized its sensitivity to a wide range of human drugs of abuse. A conditioned place preference (CPP) paradigm was used to demonstrate that crayfish seek out those environments that had previously been paired with the psychostimulants cocaine and amphetamine, and the opioid morphine. The administration of amphetamine exerted its effects at a number of sites, including the stimulation of circuits for active exploratory behaviors (i.e., SEEKING). A further study examined morphine-induced reward, extinction and reinstatement in crayfish. Repeated intra-circulatory infusions of morphine served as a reward when paired with distinct visual or tactile cues. Morphine-induced CPP was extinguished after repeated saline injections. Following this extinction phase, morphine-experienced crayfish were once again challenged with the drug. The priming injections of morphine reinstated CPP at all tested doses, suggesting that morphine-induced CPP is unrelenting. In an exploration of drug-associated behavioral sensitization in crayfish we concurrently mapped measures of locomotion and rewarding properties of morphine. Single and repeated intra-circulatory infusions of morphine resulted in persistent locomotory sensitization, even 5 days following the infusion. Moreover, a single dose of morphine was sufficient to induce long-term behavioral sensitization. CPP for morphine and context-dependent cues could not be disrupted over a drug free period of 5 days. This work demonstrates that crayfish offer a comparative and complementary approach in addiction research. Serving as an invertebrate animal model for the exposure to mammalian drugs of abuse, modularly organized and experimentally accessible nervous systems render crayfish uniquely suited for studying (1) the basic biological mechanisms of drug effects, (2) to explore how the appetitive/seeking disposition is implemented in a simple neural system, and (3) how such a disposition is related to the rewarding action of drugs of abuse. This work aimed to contribute an evolutionary, comparative context to our understanding of a key component in learning, and of natural reward as an important life-sustaining process.

Translational regulation of p53 as a potential tumor therapy target

The tumor suppressor p53 is a central player in apoptosis induction in response to oncogenic stimuli and DNA damage. As activation of p53 has been suggested as a prime strategy for future tumor therapy, inhibition of negative regulators of p53 activity would be a similarly desirable strategy. The small worm Caenorhabditis elegans is a model organism in which many conserved biological pathways, including the core apoptotic machinery, were elucidated. The discovery of a worm p53 homolog cep-1/p53 (which stands for C. elegans p53) that specifically induces apoptosis upon DNA damage through a pathway that is conserved from worm to man opened the way for the use of C. elegans genetics to uncover regulatory mechanisms – and hence novel therapeutic targets – of p53-mediated apoptosis. The authors have recently reported a novel mechanism of C. elegans cep-1/p53 regulation through germ line defective-1-mediated translational repression. This review discusses the potential of the worm system to screen for apoptosis-inducing cancer drugs and to identify novel p53 regulators whose human counterparts might become potential tumor therapy targets.

Feeding Responses of Free-flying Honeybees to Secondary Compounds Mimicking Floral Nectars

The role of secondary compounds (SC) in deterring herbivores and pathogens from vegetative parts of plants is well established, whereas their role in plant reproductive organs such as floral nectar is unclear. The present study aimed to reveal the response of free-flying honeybees to naturally occurring concentrations of four SC in floral nectar. We selected nicotine, anabasine, caffeine, and amygdalin, all of which are found in nectar of various plants. In repeated paired-choice experiments, we offered 20% sucrose solution as control along with test solutions of 20% sucrose with various concentrations of the above SC. Except for anabasine, naturally occurring concentrations of SC did not have a deterring effect. Furthermore, low concentrations of nicotine and caffeine elicited a significant feeding preference. SC can, therefore, be regarded as postingestive stimulants to pollinators, indicating that the psychoactive alkaloids in nectar may be a part of their mutualistic reward. Further studies are needed to test our hypothesis that psychoactive alkaloids in nectar impose dependence or addiction effects on pollinators.