Experimental psychiatric illness and drug abuse models: from human to animal, an overview

"Genetic influences impacting nicotine use and abuse during adolescence: insights from human and rodent studies"

Nicotine use continues to be a major public health concern, with an alarming recent rise in electronic cigarette consumption. Heritability estimates of nicotine use and abuse range from 40-80%, providing strong evidence that genetic factors impact nicotine addiction-relevant phenotypes. Although nicotine use during adolescence is a key factor in the development of addiction, it remains unclear how genetic factors impact adolescent nicotine use and abuse. This review will discuss studies investigating genetic factors impacting nicotine use during adolescence. Evidence from both rodent and human studies will be summarized and integrated when possible. Human adolescent studies have largely included candidate gene studies for genes identified in adult populations, such as genes involved in nicotine metabolism, nicotinic acetylcholine receptor signaling, dopaminergic signaling, and other neurotransmitter signaling systems. Alternatively, rodent studies have largely taken a discovery-based approach identifying strain differences in adolescent nicotine addiction-relevant behaviors. Here, we aim to answer the following three questions by integrating human and rodent findings: 1) Are there genetic variants that uniquely impact nicotine use during adolescence? 2) Are there genetic variants that impact both adolescent and adult nicotine use? and 3) Do genetic factors in adolescence significantly impact long-term consequences of adolescent nicotine use? Determining answers for these three questions will be critical for the development of preventative measures and treatments for adolescent nicotine use and addiction.

Acute MDPV Binge Paradigm on Mice Emotional Behavior and Glial Signature

3,4-Methylenedioxypyrovalerone (MDPV), a widely available synthetic cathinone, is a popular substitute for classical controlled drugs of abuse, such as methamphetamine (METH). Although MDPV poses public health risks, its neuropharmacological profile remains poorly explored. This study aimed to provide evidence on that direction. Accordingly, C57BL/6J mice were exposed to a binge MDPV or METH regimen (four intraperitoneal injections every 2 h, 10 mg/kg). Locomotor, exploratory, and emotional behavior, in addition to striatal neurotoxicity and glial signature, were assessed within 18–24 h, a known time-window encompassing classical amphetamine dopaminergic neurotoxicity. MDPV resulted in unchanged locomotor activity (open field test) and emotional behavior (elevated plus maze, splash test, tail suspension test). Additionally, striatal TH (METH neurotoxicity hallmark), Iba-1 (microglia), GFAP (astrocyte), RAGE, and TLR2/4/7 (immune modulators) protein densities remained unchanged after MDPV-exposure. Expectedly, and in sheer contrast with MDPV, METH resulted in decrease general locomotor activity paralleled by a significant striatal TH depletion, astrogliosis, and microglia arborization alterations (Sholl analysis). This comparative study newly highlights that binge MDPV-exposure comes without evident behavioral, neurochemical, and glial changes at a time-point where METH-induced striatal neurotoxicity is clearly evident. Nevertheless, neuropharmacological MDPV signature needs further profiling at different time-points, regimens, and brain regions.

CXCR4 inhibition with AMD3100 attenuates amphetamine induced locomotor activity in adolescent Long Evans male rats

Adolescent psychostimulant abuse has been on the rise over the past decade. This trend has demonstrable ramifications on adolescent behavior and brain morphology, increasing risk for development of addiction during adolescence and in later adulthood. Neuroimmune substrates are implicated in the etiology of substance use disorders. To add to this body of work, the current study was developed to explore the role of a chemokine receptor, CXC Chemokine Receptor 4 (CXCR4), in the development of amphetamine (AMPH) sensitization. We targeted CXCR4 as it is implicated in developmental processes, dopaminergic transmission, neuroimmune responses, and the potentiation of psychostimulant abuse pathology. To evaluate the role of CXCR4 activity on the development of AMPH sensitization, a CXCR4 antagonist (Plerixafor; AMD3100) was administered to rats as a pretreatment variable. Specifically, adolescent Long Evans male rats (N = 37) were divided into four groups: (1) AMD3100 (IP, 4.0 mg/kg) + AMPH (IP, 4.0 mg/kg), (2) saline (SAL; 0.9% NaCl) + AMPH, (3) AMD3100 + SAL, and (4) SAL + SAL. Animals were first habituated to locomotor activity (LMA) chambers, then injected with a pretreatment drug (AMD3100 or SAL) followed by AMPH or SAL every other for four days. After a one-week withdrawal period, all animals were administered a low challenge dose of AMPH (IP, 1.0 mg/kg). AMPH-injected rats displayed significantly more locomotor activity compared to controls across all testing days. CXCR4 antagonism significantly attenuated AMPH-induced locomotor activity. On challenge day, AMD3100 pre-treated animals exhibited diminutive AMPH-induced locomotor activity compared to SAL pre-treated animals. Postmortem analyses of brain tissue revealed elevated CXCR4 protein levels in the striatum of all experimental groups. Our results implicate CXCR4 signaling in the development of AMPH sensitization and may represent an important therapeutic target for future research in psychostimulant abuse.

A Systematic Review of the MDMA Model to Address Social Impairment in Autism

BACKGROUND

Autism Spectrum Disorder (ASD) is a neurodevelopmental disorder characterised by repetitive behaviours, cognitive rigidity/inflexibility, and social-affective impairment. Unfortunately, no gold-standard treatments exist to alleviate the core socio-behavioural impairments of ASD. Meanwhile, the prosocial empathogen/entactogen 3,4-methylene-dioxy-methamphetamine (MDMA) is known to enhance sociability and empathy in both humans and animal models of psychological disorders.

OBJECTIVE

We review the evidence obtained from behavioural tests across the current literature, showing how MDMA can induce prosocial effects in animals and humans, where controlled experiments were able to be performed.

METHODS

Six electronic databases were consulted. The search strategy was tailored to each database. Only English-language papers were reviewed. Behaviours not screened in this review may have affected the core ASD behaviours studied. Molecular analogues of MDMA have not been investigated.

RESULTS

We find that the social impairments may potentially be alleviated by postnatal administration of MDMA producing prosocial behaviours in mostly the animal model.

CONCLUSION

MDMA and/or MDMA-like molecules appear to be an effective pharmacological treatment for the social impairments of autism, at least in animal models. Notably, clinical trials based on MDMA use are now in progress. Nevertheless, larger and more extended clinical studies are warranted to prove the assumption that MDMA and MDMA-like molecules have a role in the management of the social impairments of autism.

A continuum hypothesis of psychotomimetic rapid antidepressants

Ketamine, classical psychedelics and sleep deprivation are associated with rapid effects on depression. Interestingly, these interventions also have common psychotomimetic actions, mirroring aspects of psychosis such as an altered sense of self, perceptual distortions and distorted thinking. This raises the question whether these interventions might be acute antidepressants through the same mechanisms that underlie some of their psychotomimetic effects. That is, perhaps some symptoms of depression can be understood as occupying the opposite end of a spectrum where elements of psychosis can be found on the other side. This review aims at reviewing the evidence underlying a proposed continuum hypothesis of psychotomimetic rapid antidepressants, suggesting that a range of psychotomimetic interventions are also acute antidepressants as well as trying to explain these common features in a hierarchical predictive coding framework, where we hypothesise that these interventions share a common mechanism by increasing the flexibility of prior expectations. Neurobiological mechanisms at play and the role of different neuromodulatory systems affected by these interventions and their role in controlling the precision of prior expectations and new sensory evidence will be reviewed. The proposed hypothesis will also be discussed in relation to other existing theories of antidepressants. We also suggest a number of novel experiments to test the hypothesis and highlight research areas that could provide further insights, in the hope to better understand the acute antidepressant properties of these interventions.

Neuropeptide and cytokine regulation of pain in the context of substance use disorders

Substance use disorders (SUDs) are frequently accompanied by affective symptoms that promote negative reinforcement mechanisms contributing to SUD maintenance or progression. Despite their widespread use as analgesics, chronic or excessive exposure to alcohol, opioids, and nicotine produces heightened nociceptive sensitivity, termed hyperalgesia. This review focuses on the contributions of neuropeptide (CRF, melanocortin, opioid peptide) and cytokine (IL-1β, TNF-α, chemokine) systems in the development and maintenance of substance-induced hyperalgesia. Few effective therapies exist for either chronic pain or SUD, and the common interaction of these disease states likely complicates their effective treatment. Here we highlight promising new discoveries as well as identify gaps in research that could lead to more effective and even simultaneous treatment of SUDs and co-morbid hyperalgesia symptoms.

Caenorhabditis elegans as a model system to identify therapeutics for alcohol use disorders

Alcohol use disorders (AUDs) cause serious problems in society and few effective treatments are available. Caenorhabditis elegans (C. elegans) is an excellent invertebrate model to study the neurobiological basis of human behavior with a conserved, fully tractable genome, and a short generation time for fast generation of data at a fraction of the cost of other organisms. C. elegans demonstrate movement toward, and concentration-dependent self-exposure to various psychoactive drugs. The discovery of opioid receptors in C. elegans provided the impetus to test the hypothesis that C. elegans may be used as a medications screen to identify new AUD treatments. We tested the effects of naltrexone, an opioid antagonist and effective treatment for AUDs, on EtOH preference in C. elegans. Six-well agar test plates were prepared with EtOH placed in a target zone on one side and water in the opposite target zone of each well. Worms were treated with naltrexone before EtOH preference testing and then placed in the center of each well. Wild-type worms exhibited a concentration-dependent preference for 50, 70 and 95% EtOH. Naltrexone blocked acute EtOH preference, but had no effect on attraction to food or benzaldehyde in wild-type worms. Npr-17 opioid receptor knockout mutants did not display a preference for EtOH. In contrast, npr-17 opioid receptor rescue mutants exhibited significant EtOH preference behavior, which was attenuated by naltrexone. Chronic EtOH exposure induced treatment resistance and compulsive-like behavior. These data indicate that C. elegans can serve as a model system to identify compounds to treat AUDs.

Caenorhabditis elegans Show Preference for Stimulants and Potential as a Model Organism for Medications Screening

The nematode Caenorhabditis elegans (C. elegans) is a popular invertebrate model organism to study neurobiological disease states. This is due in part to the intricate mapping of all neurons and synapses of the entire animal, the wide availability of mutant strains, and the genetic and molecular tools that can be used to manipulate the genome and gene expression. We have shown that, C. elegans develops a conditioned preference for cues that had previously been paired with either cocaine or methamphetamine exposure that is dependent on dopamine neurotransmission, similar to findings using place conditioning with rats and mice. In the current study, we show C. elegans also display a preference for, and self-exposure to, cocaine and nicotine. This substance of abuse (SOA) preference response can be selectively blocked by pretreatment with naltrexone and is consistent with the recent discovery of an opioid receptor system in C. elegans. In addition, pre-exposure to the smoking cessation treatment varenicline also inhibits self-exposure to nicotine. Exposure to concentrations of treatments that inhibit SOA preference/self-exposure did not induce any significant inhibition of locomotor activity or affect food or benzaldehyde chemotaxis. These data provide predictive validity for the development of high-throughput C. elegans behavioral medication screens. These screens could enable fast and accurate generation of data to identify compounds that may be effective in treating human addiction. The successful development and validation of such models would introduce powerful and novel tools in the search for new pharmacological treatments for substance use disorders, and provide a platform to study the mechanisms that underlie addictions.

A Possible Role of Anhedonia as Common Substrate for Depression and Anxiety

Depression and anxiety are often comorbid, in up to 70% of cases, and the level of one or the other may fluctuate, leading now to a diagnosis of depression, now to a diagnosis of anxiety. For these reasons, and for the presence of many other common factors, it has been suggested that both are part of the same continuum of problems and that they have a common substrate. This paper proposes the possibility that anhedonia may be an important component of this possible common substrate, and it tries to identify the mechanism with which anhedonia could contribute to causing both depression and anxiety. It also proposes an explanation why an intense pleasure could improve both depression and anxiety.

Caenorhabditis elegans as a Model to Study the Molecular and Genetic Mechanisms of Drug Addiction

Drug addiction takes a massive toll on society. Novel animal models are needed to test new treatments and understand the basic mechanisms underlying addiction. Rodent models have identified the neurocircuitry involved in addictive behavior and indicate that rodents possess some of the same neurobiologic mechanisms that mediate addiction in humans. Recent studies indicate that addiction is mechanistically and phylogenetically ancient and many mechanisms that underlie human addiction are also present in invertebrates. The nematode Caenorhabditis elegans has conserved neurobiologic systems with powerful molecular and genetic tools and a rapid rate of development that enables cost-effective translational discovery. Emerging evidence suggests that C. elegans is an excellent model to identify molecular mechanisms that mediate drug-induced behavior and potential targets for medications development for various addictive compounds. C. elegans emit many behaviors that can be easily quantitated including some that involve interactions with the environment. Ethanol (EtOH) is the best-studied drug-of-abuse in C. elegans and at least 50 different genes/targets have been identified as mediating EtOH's effects and polymorphisms in some orthologs in humans are associated with alcohol use disorders. C. elegans has also been shown to display dopamine and cholinergic system-dependent attraction to nicotine and demonstrate preference for cues previously associated with nicotine. Cocaine and methamphetamine have been found to produce dopamine-dependent reward-like behaviors in C. elegans. These behavioral tests in combination with genetic/molecular manipulations have led to the identification of dozens of target genes/systems in C. elegans that mediate drug effects. The one target/gene identified as essential for drug-induced behavioral responses across all drugs of abuse was the cat-2 gene coding for tyrosine hydroxylase, which is consistent with the role of dopamine neurotransmission in human addiction. Overall, C. elegans can be used to model aspects of drug addiction and identify systems and molecular mechanisms that mediate drug effects. The findings are surprisingly consistent with analogous findings in higher-level organisms. Further, model refinement is warranted to improve model validity and increase utility for medications development.

Reinforcement principles for addiction medicine; from recreational drug use to psychiatric disorder

The transition from recreational drug use to addiction can be conceptualized as a pathological timeline whereby the psychological mechanisms responsible for disordered drug use evolve from positive reinforcement to favor elements of negative reinforcement. Abused substances (ranging from alcohol to psychostimulants) are initially ingested at regular occasions according to their positive reinforcing properties. Importantly, repeated exposure to rewarding substances sets off a chain of secondary reinforcing events, whereby cues and contexts associated with drug use may themselves become reinforcing and thereby contribute to the continued use and possible abuse of the substance(s) of choice. Indeed, the powerful reinforcing efficacy of certain drugs may eclipse that of competing social rewards (such as career and family) and lead to an aberrant narrowing of behavioral repertoire. In certain vulnerable individuals, escalation of drug use over time is thought to drive specific molecular neuroadaptations that foster the development of addiction. Research has identified neurobiological elements of altered reinforcement following excessive drug use that comprise within-circuit and between-circuit neuroadaptations, both of which contribute to addiction. Central to this process is the eventual potentiation of negative reinforcement mechanisms that may represent the final definitive criterion locking vulnerable individuals into a persistent state of addiction. Targeting the neural substrates of reinforcement likely represents our best chances for therapeutic intervention for this devastating disease.

A critical assessment of the scientific basis, and implementation, of regulations for the safety assessment and marketing of innovative tobacco-related products

Our scientific, logistical, ethical and animal welfare-related concerns about the latest US Food and Drug Administration (FDA) regulations for existing and so-called 'new' tobacco products, aimed at reducing harmful exposures, are explained. Such claims for sales in the USA now have to be based on a wide range of information, a key part of which will increasingly be data on safety and risk. One of the pathways to achieve marketing authorisation is to demonstrate substantial equivalence (SE) with benchmark products, called predicates. However, the regulations are insufficiently transparent with regard to: a) a rationale for the cut-off date for 'old' and 'new' products, and for exempting the former from regulation; b) the scientific validity and operation of SE; c) options for product labelling to circumvent SE; d) the experimental data required to support, and criteria to judge, a claim; and e) a strategy for risk assessment/management. Scientific problems related to the traditional animal methods used in respiratory disease and inhalation toxicology, and the use of quantitative comparators of toxicity, such as the No Observed Adverse Effect Level, are discussed. We review the advantages of relevant in vitro, mechanism-based, target tissue-oriented technologies, which an advisory report of the Institute of Medicine of the US National Academy of Sciences largely overlooked. These benefits include: a) the availability, for every major site in the respiratory tract, of organotypic human cell-based tissue culture systems, many of which are already being used by the industry; b) the accurate determination of concentrations of test materials received by target cells; c) methods for exposure to particulate and vapour phases of smoke, separately or combined; d) the ability to study tissue-specific biotransformation; and e) the use of modern, human-focused methodologies, unaffected by species differences. How data extrapolation, for risk assessment, from tissue culture to the whole animal, could be addressed, is also discussed. A cost (to animal welfare)-benefit (to society, including industry and consumers) analysis was conducted, taking into account the above information; the potential for animal suffering; the extensive data already available; the existence of other, less hazardous forms of nicotine delivery; the fact that much data will be generated solely for benchmarking; and that many smokers (especially nicotine-dependents) ignore health warnings. It is concluded that, in common with policies of several tobacco companies and countries, the use of laboratory animals for tobacco testing is very difficult, if not impossible, to justify. Instead, we propose and argue for an integrated testing scheme, starting with extensive chemical analysis of the ingredients and by-products associated with the use of tobacco products and their toxicity, followed by use of in vitro systems and early clinical studies (involving specific biomarkers) with weight-of-evidence assessments at each stage. Appropriate adjustment factors could be developed to enable concentration-response data obtained in vitro, with the other information generated by the strategy, to enable the FDA to meet its objectives. It is hoped that our intentionally provocative ideas will stimulate further debate on this contentious area of regulatory testing and public safety.

Experimental Models on Effects of Psychostimulants

Psychostimulants are a diverse group of substances that cause an increase in psychomotor activity at least in part through their actions on catecholaminergic systems including the dopaminergic mesolimbic pathways. Animal models used to study addiction are based on the psychomotor stimulant theory of addiction. The basics of this theory are that the reinforcing effects and the addition liabilities of the drugs can be predicted from their ability to induce psychomotor activation. This approach focuses on the ability of the drugs to directly control the animal's behavior and to induce psychomotor stimulation, and is consistent with the behavioral definition of addiction and behavioral sensitization. Animal experiments have the advantage over clinical studies of lower variation and fewer confounding effects.

Compulsive-like responding for opioid analgesics in rats with extended access

The abuse of prescription opioids that are used for the treatment of chronic pain is a major public health concern, costing ∼$53.4 billion annually in lost wages, health-care costs, and criminal costs. Although opioids remain a first-line therapy for the treatment of severe chronic pain, practitioners remain cautious because of the potential for abuse and addiction. Opioids such as heroin are considered very rewarding and reinforcing, but direct and systematic comparisons of compulsive intake between commonly prescribed opioids and heroin in animal models have not yet been performed. In the present study, we evaluated the potential for compulsive-like drug seeking and taking, using intravenous self-administration of oxycodone, fentanyl, and buprenorphine in rats allowed long access sessions (12 h). We measured compulsive-like intake using an established escalation model and responding on a progressive ratio schedule of reinforcement. We compared the potential for compulsive-like self-administration of these prescription opioids and heroin, which has been previously established to induce increasing intake that models the transition to addiction in humans. We found that animals that self-administered oxycodone, fentanyl, or heroin, but not buprenorphine had similar profiles of escalation and increases in breakpoints. The use of extended access models of prescription opioid intake will help better understand the biological factors that underlie opioid dependence.

Nicotinic, glutamatergic and dopaminergic synaptic transmission and plasticity in the mesocorticolimbic system: focus on nicotine effects

Cigarette smoking is currently the leading cause of preventable deaths and disability throughout the world, being responsible for about five million premature deaths/year. Unfortunately, fewer than 10% of tobacco users who try to stop smoking actually manage to do so. The main addictive agent delivered by cigarette smoke is nicotine, which induces psychostimulation and reward, and reduces stress and anxiety. The use of new technologies (including optogenetics) and the development of mouse models characterised by cell-specific deletions of receptor subtype genes or the expression of gain-of-function nAChR subunits has greatly increased our understanding of the molecular mechanisms and neural substrates of nicotine addiction first revealed by classic electrophysiological, neurochemical and behavioural approaches. It is now becoming clear that various aspects of nicotine dependence are mediated by close interactions of the glutamatergic, dopaminergic and γ-aminobutyric acidergic systems in the mesocorticolimbic system. This review is divided into two parts. The first provides an updated overview of the circuitry of the ventral tegmental area, ventral striatum and prefrontal cortex, the neurotransmitter receptor subtypes expressed in these areas, and their physiological role in the mesocorticolimbic system. The second will focus on the molecular, functional and behavioural mechanisms involved in the acute and chronic effects of nicotine on the mesocorticolimbic system.

Medications development for substance-use disorders: contextual influences (dis)incentivizing pharmaceutical-industry positioning

INTRODUCTION

The significant contribution of substance-use disorders (SUDs) to the global-disease burden and associated unmet medical needs has not engendered a commensurate level of pharma-industry research and development (R&D) for novel SUD therapeutics invention. Analysis of contextual factors shaping this position suggests potential routes toward incentivizing R&D commitment for that purpose.

AREAS COVERED

This article considers multiple primary factors that have consorted to disincentivize pharma industry's operating in the SUD space: ill-understood pathology; variegated treatments and patient profiles; involved clinical trials; and - with particular reference to SUDs-negative cultural/business stigmas and shallow commercial precedent. Industry incentivization for SUD drug innovation requires progress on several fronts, including: translational experimental data and systems; personalized, holistic SUD treatment approaches; interactions among pharma, nonindustry constituencies, and the medical profession with vested interests in countering negative stereotypes and expanding SUD treatment options; and public-private alliances focused on improving SUD pharmacotherapy.

EXPERT OPINION

Given the well-entrenched business stance whereby the prospect of future profits in major markets largely determines drug-company R&D investment trajectory, strategic initiatives offering substantial reductions in the risks and opportunity (i.e., time and money) costs associated with SUD drug discovery are likely to be the most potent drivers for encouraging mainstream industry positioning in this therapeutic area. Such initiatives could originate from front-loaded R&D operational and back-loaded patent, regulatory, marketing and health-care policy reforms. These may be too involved and protracted for the turbulent pharmaceutical industry to entertain amid its recent retrenchment from psychiatric/CNS diseases and intense pressures to increase productivity and shareholder value.

Operant alcohol self-administration in dependent rats: focus on the vapor model

Alcoholism (alcohol dependence) is characterized by a compulsion to seek and ingest alcohol (ethanol), loss of control over intake, and the emergence of a negative emotional state during withdrawal. Animal models are critical in promoting our knowledge of the neurobiological mechanisms underlying alcohol dependence. Here, we review the studies involving operant alcohol self-administration in rat models of alcohol dependence and withdrawal with the focus on the alcohol vapor model. In 1996, the first articles were published reporting that rats made dependent on alcohol by exposure to alcohol vapors displayed increased operant alcohol self-administration during acute withdrawal compared with nondependent rats (i.e., not exposed to alcohol vapors). Since then, it has been repeatedly demonstrated that this model reliably produces physical and motivational symptoms of alcohol dependence. The functional roles of various systems implicated in stress and reward, including opioids, dopamine, corticotropin-releasing factor (CRF), glucocorticoids, neuropeptide Y (NPY), γ-aminobutyric acid (GABA), norepinephrine, and cannabinoids, have been investigated in the context of alcohol dependence. The combination of models of alcohol withdrawal and dependence with operant self-administration constitutes an excellent tool to investigate the neurobiology of alcoholism. In fact, this work has helped lay the groundwork for several ongoing clinical trials for alcohol dependence. Advantages and limitations of this model are discussed, with an emphasis on what future directions of great importance could be.

Neuronal extracellular signal-regulated kinase (ERK) activity as marker and mediator of alcohol and opioid dependence

Early pioneering work in the field of biochemistry identified phosphorylation as a crucial post-translational modification of proteins with the ability to both indicate and arbitrate complex physiological processes. More recent investigations have functionally linked phosphorylation of extracellular signal-regulated kinase (ERK) to a variety of neurophysiological mechanisms ranging from acute neurotransmitter action to long-term gene expression. ERK phosphorylation serves as an intracellular bridging mechanism that facilitates neuronal communication and plasticity. Drugs of abuse, including alcohol and opioids, act as artificial yet powerful rewards that impinge upon natural reinforcement processes critical for survival. The graded progression from initial exposure to addiction (or substance dependence) is believed to result from drug- and drug context-induced adaptations in neuronal signaling processes across brain reward and stress circuits following excessive drug use. In this regard, commonly abused drugs as well as drug-associated experiences are capable of modifying the phosphorylation of ERK within central reinforcement systems. In addition, chronic drug and alcohol exposure may drive ERK-regulated epigenetic and structural alterations that underlie a long-term propensity for escalating drug use. Under the influence of such a neurobiological vulnerability, encountering drug-associated cues and contexts can produce subsequent alterations in ERK signaling that drive relapse to drug and alcohol seeking. Current studies are determining precisely which molecular and regional ERK phosphorylation-associated events contribute to the addiction process, as well as which neuroadaptations need to be targeted in order to return dependent individuals to a healthy state.

Biological substrates of addiction

This review is an introduction to addiction, the reward circuitry, and laboratory addiction models. Addiction is a chronic disease hallmarked by a state of compulsive drug seeking that persists despite negative consequences. Most of the advances in addiction research have centered on the canonical and contemporary drugs of abuse; however, addictions to other activities and stimuli also exist. Substances of abuse have the potential to induce long-lasting changes in the brain at the behavioral, circuit, and synaptic levels. Addiction-related behavioral changes involve initiation, escalation, and obsession to drug seeking and much of the current research is focused on mapping these manifestations to specific neural pathways. Drug abuse is well known to recruit components of the mesolimbic dopamine system, including the nucleus accumbens and ventral tegmental area. In addition, altered function of a wide variety of brain regions is tightly associated with specific manifestations of drug abuse. These regions peripheral to the mesolimbic pathway likely play a role in specific observed comorbidities and endophenotypes that can facilitate, or be caused by, substance abuse. Alterations in synaptic structure, function, and connectivity, as well as epigenetic and genetic mechanisms are thought to underlie the pathologies of addiction. In preclinical models, these persistent changes are studied at the levels of molecular pharmacology and biochemistry, ex vivo and in vivo electrophysiology, radiography, and behavior. Coordinating research efforts across these disciplines and examining cell type- and circuit-specific phenomena are crucial components for translating preclinical findings to viable medical interventions that effectively treat addiction and related disorders. WIREs Cogn Sci 2014, 5:151-171. doi: 10.1002/wcs.1273 Conflict of interest: The authors have declared no conflicts of interest for this article. For further resources related to this article, please visit the WIREs website.

Escalation of drug self-administration as a hallmark of persistent addiction liability

Drug addiction is a progressive, relapsing disease comprised of interlocking stages of disordered motivation. Numerous animal models describing various stages of the addiction process have been developed over the past few decades, providing considerable advantages for the modeling of drug addiction compared with other complex psychiatric disease states. Escalation of drug self-administration has emerged as a widely accepted operant conditioning model of excessive drug intake. We further argue here that drug-escalated animals represent a comprehensive model of addiction according to the manifestations of behavioral neuroadaptations resulting directly or indirectly from excessive drug consumption. In particular, drug-escalated animals exhibit a host of symptoms in line with multiple Diagnostic and Statistical Manual of Mental Disorders criteria for substance dependence, which can be summarized as an emergence of uncontrollable drug-taking and drug-seeking behaviors as a consequence of within-circuit and between-circuit neuroadaptations. Such a transition from impulsive drug sampling to compulsive intake represents a highly valid conceptualization of the addiction timeline in humans, and further investigation of persistent or near-permanent (e.g. epigenetic) neuroadaptations generated by operant drug intake escalation models will continue to provide mechanisms and therapeutic interventions for reversing the aberrant neuroplasticity underlying addiction.

Opportunities for bioinformatics in the classification of behavior and psychiatric disorders

A bioinformatics approach to behavioral neuroscience provides both unique opportunities and challenges for research on behavior. A major challenge has been to describe, define, and discriminate among abstract behavioral processes, in large part by distinguishing among the biological mechanisms of unique but not entirely discrete, entities of behavior. Understanding the complexity of neurobiology and behavior requires integration of data across diverse biological systems, types of data, and levels of scale. With the perspective and application of bioinformatics, we can uncover the relationships among these systems and take steps forward in realizing the common and distinct bases of psychiatric disease.