Place preference induced by nucleus accumbens amphetamine is impaired by local blockade of Group II metabotropic glutamate receptors in rats

Acute Administration of Ojeok-san Ameliorates Pain-like Behaviors in Pre-Clinical Models of Inflammatory Bowel Diseases

(1) Background: Gastrointestinal pain and fatigue are the most reported concerns of patients with inflammatory bowel disease (IBD). Commonly prescribed drugs focus on decreasing excessive inflammation. However, up to 20% of IBD patients in an "inactive" state experience abdominal pain. The medicinal herb Ojeok-san (OJS) has shown promise in the amelioration of visceral pain. However, no research on OJS has been conducted in preclinical models of IBD. The mechanism by which OJS promotes analgesia is still elusive, and it is unclear if OJS possesses addictive properties. (2) Aims: In this study, we examined the potential of OJS to promote analgesic effects and rewarding behavior. Additionally, we investigated if tumor necrosis factor alpha (TNFα) from macrophages is a primary culprit of IBD-induced nociception. (3) Methods: Multiple animal models of IBD were used to determine if OJS can reduce visceral nociception. TNFα-macrophage deficient mice were used to investigate the mechanism of action by which OJS reduces nociceptive behavior. Mechanical sensitivity and operant conditioning tests were used to determine the analgesic and rewarding effects of OJS. Body weight, colon length/weight, blood in stool, colonic inflammation, and complete blood count were assessed to determine disease progression. (4) Results: OJS reduced the evoked mechanical nociception in the dextran sulphate sodium model of colitis and IL-10 knockout (KO) mice and delayed aversion to colorectal distension in C57BL/6 mice. No rewarding behavior was observed in OJS-treated IL-10 KO and mdr1a KO mice. The analgesic effects of OJS are independent of macrophage TNFα levels and IBD progression. (5) Conclusions: OJS ameliorated elicited mechanical and visceral nociception without producing rewarding effects. The analgesic effects of OJS are not mediated by macrophage TNFα.

Amphetamine acts within the lateral hypothalamic area to elicit affectively neutral arousal and reinstate drug-seeking

Psychostimulants, including amphetamine (AMPH), exert robust arousal-enhancing, reinforcing and locomotor-activating effects. These behavioural actions involve drug-induced elevations in extracellular norepinephrine (NE) and dopamine (DA) within a variety of cortical and subcortical regions. The lateral hypothalamic area (LHA), including the lateral hypothalamus proper, perifornical area and adjacent dorsomedial hypothalamus, is implicated in appetitive- and arousal-related processes. The LHA is innervated by both NE and DA projections and systemically administered AMPH has been demonstrated to activate LHA neurons. Combined, these and other observations suggest the LHA may be a site of action in the behavioural effects of psychostimulants. To test this hypothesis, we examined the degree to which AMPH (10 nmol, 25 nmol) acts within the LHA to exert arousing, locomotor-activating and reinforcing actions in quietly resting/sleeping rats. Although intra-LHA AMPH robustly increased time spent awake, this occurred in the absence of pronounced locomotor activation or reinforcing actions, as measured in a conditioned place preference (CPP) paradigm. Arousing and stressful conditions or drug re-exposure can elicit relapse in humans and reinstate drug-seeking in animals. Given the LHA is also implicated in the reinstatement of drug-seeking behaviour, additional studies examined whether AMPH acts within the LHA to reinstate an extinguished CPP produced with systemic AMPH administration. Our results demonstrate that AMPH action within the LHA is sufficient to reinstate drug-seeking behaviour, as measured in this paradigm. Collectively, these observations demonstrate that psychostimulants act within the LHA to elicit affectively neutral arousal and reinstate drug-seeking behaviour.

Metabotropic glutamate receptor blockade in nucleus accumbens shell shifts affective valence towards fear and disgust

Glutamatergic inputs to the nucleus accumbens (NAc) modulate both appetitive and fearful motivation. It has been suggested that pathological disturbances of glutamate signaling in NAc contribute to motivation disorders, ranging from excessive desire in drug addiction to paranoia in schizophrenia. Metabotropic glutamate receptors are of special interest, as metabotropic Group II receptor (mglu2/3) agonists have been proposed as potential treatments for both addiction and schizophrenia. Here we tested whether local mglu2/3 receptor blockade in the medial shell of the rat NAc can generate intense distortions of motivation or affect, which might model clinical dysfunction. We found that microinjection of the mglu2/3 antagonist LY341495 at sites throughout medial shell suppressed appetitive motivation to eat and drink. Simultaneously, LY341495 microinjections generated fearful motivation in the form of defensive treading or burying. To assess whether the valence shift extended into a parallel hedonic shift from affective 'liking' to 'disliking' we employed the taste reactivity test, which measures affective orofacial reactions to the sensory pleasure or displeasure of tastes. We found that LY341495 microinjections reduced positive 'liking' reactions to sucrose and enhanced 'disliking' reactions. Overall, mglu2/3 antagonism at most shell sites produced a similar valence shift from positive to negative. This pattern comprised (i) generation of fearful behaviors, and (ii) induction of aversive affective reactions, together with (iii) loss of appetitive ingestion and (iv) loss of 'liking' for rewards. These results are discussed in terms of implications for clinical disorders and the influence of corticolimbic glutamate inputs to NAc in the generation of motivation and affect.

Hypothesis-driven medication discovery for the treatment of psychostimulant addiction

Psychostimulant abuse is a serious social and health problem, for which no effective treatments currently exist. A number of review articles have described predominantly 'clinic'-based pharmacotherapies for the treatment of psychostimulant addiction, but none have yet been shown to be definitively effective for use in humans. In the present article, we review various 'hypothesis'- or 'mechanism'-based pharmacological agents that have been studied at the preclinical level and evaluate their potential use in the treatment of psychostimulant addiction in humans. These compounds target brain neurotransmitter or neuromodulator systems, including dopamine (DA), gamma-aminobutyric acid (GABA), endocannabinoid, glutamate, opioid and serotonin, which have been shown to be critically involved in drug reward and addiction. For drugs in each category, we first briefly review the role of each neurotransmitter system in psychostimulant actions, and then discuss the mechanistic rationale for each drug's potential anti-addiction efficacy, major findings with each drug in animal models of psychostimulant addiction, abuse liability and potential problems, and future research directions. We conclude that hypothesis-based medication development strategies could significantly promote medication discovery for the effective treatment of psychostimulant addiction.

Glutamatergic substrates of drug addiction and alcoholism

The past two decades have witnessed a dramatic accumulation of evidence indicating that the excitatory amino acid glutamate plays an important role in drug addiction and alcoholism. The purpose of this review is to summarize findings on glutamatergic substrates of addiction, surveying data from both human and animal studies. The effects of various drugs of abuse on glutamatergic neurotransmission are discussed, as are the effects of pharmacological or genetic manipulation of various components of glutamate transmission on drug reinforcement, conditioned reward, extinction, and relapse-like behavior. In addition, glutamatergic agents that are currently in use or are undergoing testing in clinical trials for the treatment of addiction are discussed, including acamprosate, N-acetylcysteine, modafinil, topiramate, lamotrigine, gabapentin and memantine. All drugs of abuse appear to modulate glutamatergic transmission, albeit by different mechanisms, and this modulation of glutamate transmission is believed to result in long-lasting neuroplastic changes in the brain that may contribute to the perseveration of drug-seeking behavior and drug-associated memories. In general, attenuation of glutamatergic transmission reduces drug reward, reinforcement, and relapse-like behavior. On the other hand, potentiation of glutamatergic transmission appears to facilitate the extinction of drug-seeking behavior. However, attempts at identifying genetic polymorphisms in components of glutamate transmission in humans have yielded only a limited number of candidate genes that may serve as risk factors for the development of addiction. Nonetheless, manipulation of glutamatergic neurotransmission appears to be a promising avenue of research in developing improved therapeutic agents for the treatment of drug addiction and alcoholism.

Measuring reward with the conditioned place preference (CPP) paradigm: update of the last decade

Conditioned place preference (CPP) continues to be one of the most popular models to study the motivational effects of drugs and non-drug treatments in experimental animals. This is obvious from a steady year-to-year increase in the number of publications reporting the use this model. Since the compilation of the preceding review in 1998, more than 1000 new studies using place conditioning have been published, and the aim of the present review is to provide an overview of these recent publications. There are a number of trends and developments that are obvious in the literature of the last decade. First, as more and more knockout and transgenic animals become available, place conditioning is increasingly used to assess the motivational effects of drugs or non-drug rewards in genetically modified animals. Second, there is a still small but growing literature on the use of place conditioning to study the motivational aspects of pain, a field of pre-clinical research that has so far received little attention, because of the lack of appropriate animal models. Third, place conditioning continues to be widely used to study tolerance and sensitization to the rewarding effects of drugs induced by pre-treatment regimens. Fourth, extinction/reinstatement procedures in place conditioning are becoming increasingly popular. This interesting approach is thought to model certain aspects of relapse to addictive behavior and has previously almost exclusively been studied in drug self-administration paradigms. It has now also become established in the place conditioning literature and provides an additional and technically easy approach to this important phenomenon. The enormous number of studies to be covered in this review prevented in-depth discussion of many methodological, pharmacological or neurobiological aspects; to a large extent, the presentation of data had to be limited to a short and condensed summary of the most relevant findings.