Selective reduction of alcohol drinking in Sardinian alcohol-preferring rats by a sigma-1 receptor antagonist

Ethanol drinking at adulthood is sensitive to S1-R antagonism and is promoted by binge ethanol self-administration at adolescence

BACKGROUND

Binge drinking at adolescence is a risk factor for problematic alcohol (ethanol) consumption later in life, yet the murine studies that modelled this phenomenon via ethanol self-administration have provided mixed findings. Antagonism of the sigma-1 receptor (S1-R) system at adolescence modulates ethanol's motivational effects and intake. It is still unknown, however, whether this antagonism would protect against enhanced ethanol intake at adulthood after adolescent binge ethanol exposure.

METHODS

Exp. 1 and 2 tested adults male or female Wistar rats -exposed or not to ethanol self-administration at adolescence (postnatal days 31-49; nine 2-hour sessions of access to 8-10% ethanol)- for ethanol intake using 24-h two-bottle choice test (Exp. 1) or time restricted, single-bottle, tests (Exp. 2). Experiments 2-5 evaluated, in adolescent or adult rats, the effects of the S1-R antagonist S1RA on ethanol intake and on ethanol-induced conditioned taste or place aversion. Ancillary tests (e.g., novel object recognition, ethanol-induced locomotor activity) were also conducted.

RESULTS

Adolescent ethanol exposure promoted ethanol consumption at both the restricted, single-bottle, and at the two-bottle choice tests conducted at adulthood. S1RA administration reduced ethanol intake at adulthood and facilitated the development of ethanol-induced taste (but not place) aversion.

CONCLUSIONS

S1RA holds promise for lessening ethanol intake after chronic and substantial ethanol exposure in adolescence that results in heightened ethanol exposure at adulthood. This putative protective effect of S1-R antagonism may relate to S1RA exacerbating the aversive effects of this drug.

Knocking Out Sigma-1 Receptors Reveals Diverse Health Problems

Sigma-1 receptor (Sig-1R) is a protein present in several organs such as brain, lung, and heart. In a cell, Sig-1R is mainly located across the membranes of the endoplasmic reticulum and more specifically at the mitochondria-associated membranes. Despite numerous studies showing that Sig-1R could be targeted to rescue several cellular mechanisms in different pathological conditions, less is known about its fundamental relevance. In this review, we report results from various studies and focus on the importance of Sig-1R in physiological conditions by comparing Sig-1R KO mice to wild-type mice in order to investigate the fundamental functions of Sig-1R. We note that the Sig-1R deletion induces cognitive, psychiatric, and motor dysfunctions, but also alters metabolism of heart. Finally, taken together, observations from different experiments demonstrate that those dysfunctions are correlated to poor regulation of ER and mitochondria metabolism altered by stress, which could occur with aging.

Characterization of a differential reinforcement of low rates of responding task in non-deprived male and female rats: Role of Sigma-1 receptors

Impulsive action can be defined as the inability to withhold a response and represents one of the dimensions of the broad construct impulsivity. Here, we characterized a modified differential reinforcement of low rates of responding (DRL) task developed in our laboratory, in which impulsive action is measured in ad libitum fed/watered subjects. Specifically, we first determined the effects of both sex and estrous cycle on impulsive action by systematically comparing male and estrous-synchronized female subjects. In addition, we evaluated the convergent validity of this modified DRL task by testing the effects of the D2R/5HT2AR antagonist, aripiprazole, and the noncompetitive NMDAR antagonist, MK-801. Finally, we tested the effects of the selective antagonist BD-1063 and agonist PRE-084 of Sigma-1 receptor (Sig-1R) on impulsive action using this modified DRL task. We found that female rats showed and increased inability to withhold a response when compared to males, and this effect was driven by the metestrus/diestrus phase of the estrous cycle. In addition, aripiprazole and MK-801 fully retained their capability to reduce and increase impulsive action, respectively. Finally, the selective Sig-1R antagonist, BD-1063 dose-dependently reduced the inability to withhold a response in both sexes, though more potently in female rats. In summary, we show that impulsive action, as measured in a modified DRL task which minimizes energy-homeostatic influences, is a function of both sex and estrous cycle. Furthermore, we validate the convergent validity of the task and provide evidence that Sig-1R antagonism may represent a novel pharmacological strategy to reduce impulsive action.

Antagonism of Sigma-1 receptor blocks heavy alcohol drinking and associated hyperalgesia in male mice

BACKGROUND

Alcohol use disorder (AUD) is a complex psychiatric disease characterized by high alcohol intake as well as hyperkatifeia and hyperalgesia during withdrawal. A role for Sigma-1 receptors (Sig-1Rs) in the rewarding and reinforcing effects of alcohol has started to emerge in recent years, as rat studies have indicated that Sig-1R hyperactivity may result in excessive alcohol drinking. Sig-1R studies in mice are very scarce, and its potential role in alcohol-induced hyperalgesia is also unknown.

METHODS

In this study, we investigated the role of Sig-1R in alcohol drinking and associated hyperalgesia in male mice, using an intermittent access 2-bottle choice model of heavy drinking.

RESULTS

The Sig-1R antagonist BD-1063 was found dose dependently to reduce both alcohol intake and preference, without affecting either water or sucrose intake, suggesting that the effects are specific for alcohol. Notably, the ability of BD-1063 to suppress ethanol intake correlated with the individual baseline levels of alcohol drinking, suggesting that the treatment was more efficacious in heavy drinking animals. In addition, BD-1063 reversed alcohol-induced hyperalgesia during withdrawal, assessed using an automatic Hargreaves test, without affecting thermal sensitivity in alcohol-naïve animals or locomotor activity in either group.

CONCLUSIONS

These data show that Sig-1R antagonism dose-dependently reduced ethanol consumption in heavy drinking mice as well as its efficacy in reducing alcohol-induced hyperalgesia. These findings provide a foundation for the development of novel treatments for AUD and associated pain states.

The Sigma-2 receptor / transmembrane protein 97 (σ2R/TMEM97) modulator JVW-1034 reduces heavy alcohol drinking and associated pain states in male mice

Alcohol Use Disorder (AUD) is a chronic relapsing disorder characterized by compulsive alcohol intake, loss of control over alcohol intake, and a negative emotional state when access to alcohol is prevented. AUD is also closely tied to pain, as repeated alcohol drinking leads to increased pain sensitivity during withdrawal. The sigma-2 receptor, recently identified as transmembrane protein 97 (σ2R/TMEM97), is an integral membrane protein involved in cholesterol homeostasis and lipid metabolism. Selective σ2R/Tmem97 modulators have been recently shown to relieve mechanical hypersensitivity in animal models of neuropathic pain as well as to attenuate alcohol withdrawal signs in C. elegans and to reduce alcohol drinking in rats, suggesting a potential key role for this protein in alcohol-related behaviors. In this study, we tested the effects of a potent and selective σ2R/TMEM97 ligand, JVW-1034, on heavy alcohol drinking and alcohol-induced heightened pain states in mice using an intermittent access model. Administration of JVW-1034 decreased both ethanol intake and preference for ethanol, without affecting water intake, total fluid intake, or food intake. Notably, this effect was specific for alcohol, as JVW-1034 had no effect on sucrose intake. Furthermore, JVW-1034 reduced both thermal hyperalgesia and mechanical hypersensitivity in ethanol withdrawn mice. Our data provide important evidence that modulation of σ2R/TMEM97 with small molecules can mediate heavy alcohol drinking as well as chronic alcohol-induced heightened pain sensitivity, thereby identifying a promising novel pharmacological target for AUD and associated pain states.

Sigma-1 antagonism inhibits binge ethanol drinking at adolescence

BACKGROUND

Ethanol use during adolescence is a significant health problem, yet the pharmacological treatments to reduce adolescent binge drinking are scarce. The present study assessed, in male and female adolescent Wistar rats, if the sigma-1 receptor (S1-R) antagonists S1RA or BD-1063 disrupted ethanol drinking.

METHODS

Three times a week, for two weeks, the rats received the S1-R antagonists. Thirty min later they were exposed, for 2 h, to a bottle of 8% or 10 % v/v ethanol. A 24 h, two-bottle, ethanol intake test was conducted after termination of these procedures. A subset of these rats was tested for recognition memory via the novel object recognition test.

RESULTS

The rats given 64 mg/kg S1RA drank, in each binge session, significantly less than vehicle counterparts. Male rats given 4 or 16 mg/kg S1RA drank significantly less than those given 0 mg/kg in session 3 or in session 1 and 2, respectively; whereas female rats given 4 or 16 mg/kg drank significantly less than females given 0 mg/kg in session 2-5 or in sessions 2-6, respectively. Administration of 32 mg/kg, but not of 2 or 8 mg/kg, BD-1063 suppressed, across sessions, ethanol drinking. S1-R antagonism reduced absolute ethanol drinking at the two-bottle choice post-test. Recognition memory was not affected by the ethanol exposure.

CONCLUSIONS

The results indicate that S1-R antagonists may be promising targets to prevent increases in ethanol intake at adolescence. The persistent effect of S1-R antagonism in free-choice drinking suggests that modulation of the S1-R is altering plastic effects associated with ethanol exposure.

Prediction of ethanol self-administration in pre-weanling, adolescent, and young adult rats

Alcohol (ethanol) use is almost normative by late adolescence, in most western countries. It is important to identify factors that distinguish those who progress from alcohol initiation to sustained use of the drug, from those that keep a controlled pattern of drinking. The factors precipitating this transition may change across development. This study analyzed associations between behavioral endophenotypes and ethanol intake at three developmental periods. Exp. 1 measured ethanol drinking at postnatal day 18, via an intraoral infusion procedure, in male or female pre-weanling rats screened for anxiety response in the light-dark box test and for distance traveled in a novel open field. Exp. 2 measured, in juvenile/adolescent or young adult rats, the association between shelter seeking, exploratory/risk-taking behaviors, anxiety or hedonic responses, and ethanol intake. Ethanol intake in pre-weanlings was explained by distance traveled in a novel environment, whereas anxiety responses, measured in the multivariate concentric square field apparatus (MSCF), selectively predicted ethanol intake at adolescence, but not at adulthood. Those juvenile/adolescents with lower mean duration of visit to areas of the MSCF that evoke anxiogenic responses exhibited heightened ethanol intake. These findings suggest that the association between anxiety and ethanol intake may be specifically relevant during adolescence.

Sigma receptor-induced heavy drinking in rats: Modulation by the opioid receptor system

Alcohol use disorder (AUD) is a major cause of morbidity and mortality worldwide, for which new efficacious treatments are necessary. The opioid receptor system is a mediator of the rewarding effects of alcohol; in particular, while activation of μ opioid receptors enhances ethanol intake in rodents, opioid-receptor antagonists, such as naloxone and naltrexone, reduce its pleasurable and reinforcing effects, thereby decreasing alcohol. Sigma receptors (Sig-Rs) have been proposed as modulators of the effects of alcohol and, therefore, as a potential new pharmacological target for AUD. Somewhat analogously to μ opioid ligands, SigR agonists increase, while SigR antagonists decrease alcohol intake in animal models of excessive alcohol drinking. However, a potential cross-talk between these two receptor systems in relation to alcohol consumption has so far not been investigated. Here, we addressed this question pharmacologically, by testing the effects of either activating or inhibiting opioid receptors on the heavy alcohol drinking induced by chronic stimulation of SigR in alcohol-preferring rats. We found that the opioid receptor agonist morphine, which per se increases ethanol intake, at a sub-threshold dose reduces the binge-like drinking induced by the repeated treatment with the SigR agonist 1,3-di-o-tolylguanidine (DTG); conversely, the opioid receptor antagonist naltrexone, which per se reduces ethanol intake, at a sub-threshold dose potentiates the DTG-induced binge-like drinking. Our data show a cross-talk between the opioid and SigR systems relevant to the modulation of alcohol drinking, which provides important insights into the neurobiology of AUD and may lead to the development of novel therapies, either standalone or in combination.

Sigma-1 receptor antagonist, PD144418, selectively reduces female motivation for food during negative energy balance

Sigma-1 (σ1) receptors have been investigated for their involvement in learning, rewarding and motivational processes. PD144418, a σ1 receptor antagonist, has been found to produce a dose-dependent attenuation of locomotor activity induced by cocaine, and by itself, does not suppress basal locomotor activity in mice. Moreover, PD144418 decreases the motivational effort of a food-reinforced behavior in male rats, without altering appetite or food palatability. It remains unknown whether the PD144418 can alter the motivational effort of a food-reinforced behavior in response to altered energy homeostasis, as is the case under 24 -h food deprivation. Additionally, while the previous experiments indicate effects in male rats, there has been no research examining the effects of PD144418, or any other σ1 receptor antagonist, on motivational aspects of feeding in females. The present study examined the effects of PD144418 on motivational aspects of feeding in male and female rats using an operant task under sated or food deprived conditions. Results indicated that when animals are sated, at the highest dose (10 μmol/kg), under a progressive ratio (PR) reinforcement schedule, PD144418 significantly attenuated the breakpoint and the number of active lever responses for sucrose pellets in both males and females. When animals are in a state of energy deficit, as is the case following 24-hr food deprivation, PD144418 does not alter motivationally driven operant responding as measured by the breakpoint in either sex but does alter the number of earned reinforcers responses in females.

Role of Sigma Receptors in Alcohol Addiction

Pharmacological treatments for alcohol use disorder (AUD) are few in number and often ineffective, despite the significant research carried out so far to better comprehend the neurochemical underpinnings of the disease. Hence, research has been directed towards the discovery of novel therapeutic targets for the treatment of AUD. In the last decade, the sigma receptor system has been proposed as a potential mediator of alcohol reward and reinforcement. Preclinical studies have shown that the motivational effects of alcohol and excessive ethanol consumption involve the recruitment of the sigma receptor system. Furthermore, sigma receptor antagonism has been shown to be sufficient to inhibit many behaviors related to AUDs. This paper will review the most current evidence in support of this receptor system as a potential target for the development of pharmacological agents for the treatment of alcohol addiction.

Neuroprotection in non-transgenic and transgenic mouse models of Alzheimer's disease by positive modulation of σ1 receptors

The sigma-1 (σ1) receptor is an endoplasmic reticulum (ER) chaperone protein, enriched in mitochondria-associated membranes. Its activation triggers physiological responses to ER stress and modulate Ca2+ mobilization in mitochondria. Small σ1 agonist molecules activate the protein and act behaviorally as antidepressant, anti-amnesic and neuroprotective agents. Recently, several chemically unrelated molecules were shown to be σ1 receptor positive modulators (PMs), with some of them a clear demonstration of their allostericity. We here examined whether a σ1 PM also shows neuroprotective potentials in pharmacological and genetic models of Alzheimer's disease (AD). For this aim, we describe (±)-2-(3-chlorophenyl)-3,3,5,5-tetramethyl-2-oxo-[1,4,2]-oxazaphosphinane (OZP002) as a novel σ1 PM. OZP002 does not bind σ1 sites but induces σ1 effects in vivo and boosts σ1 agonist activity. OZP002 was antidepressant in the forced swim test and its effect was blocked by the σ1 antagonist NE-100 or in σ1 receptor knockout mice. It potentiated the antidepressant effect of the σ1 agonist igmesine. In mice tested for Y-maze alternation or passive avoidance, OZP002 prevented scopolamine-induced learning deficits, in a NE-100 sensitive manner. Pre-administered IP before an ICV injection of amyloid Aβ25-35 peptide, a pharmacological model of Alzheimer's disease, OZP002 prevented the learning deficits induced by the peptide after one week in the Y-maze, passive avoidance and novel object tests. Biochemical analyses of the mouse hippocampi showed that OZP002 significantly decreased Aβ25-35-induced increases in reactive oxygen species, lipid peroxidation, and increases in Bax, TNFα and IL-6 levels. Immunohistochemically, OZP002 prevented Aβ25-35-induced reactive astrogliosis and microgliosis in the hippocampus. It also alleviated Aβ25-35-induced decreases in synaptophysin level and choline acetyltransferase activity. Moreover, chronically administered in APPswe mice during 2 months, OZP002 prevented learning deficits (in all tests plus place learning in the water-maze) and increased biochemical markers. This study shows that σ1 PM with high neuropotective potential can be identified, combining pharmacological efficacy, selectivity and therapeutic safety, and identifies a novel promising compound, OZP002.

Sigma-1 receptor antagonist PD144418 suppresses food reinforced operant responding in rats

Sigma-1 (σ1) receptors have been investigated for their involvement in learning, rewarding and motivational processes, particularly as it relates to substances of abuse. Few studies have examined the effects of σ1 receptor agonists and antagonists on the rewarding and motivational properties of natural reinforcers, such as food. Studies that have investigated σ1 receptor agonists and antagonists has produced conflicting results. σ1 receptor antagonist PD144418 has been found to produce a dose-dependent attenuation of locomotor activity induced by cocaine, and by itself, does not suppress basal locomotor activity in mice. However, its effects on reward and motivation as it relates to food are unknown. The present study examined the involvement of σ1 receptors in mediating the rewarding and motivational properties of food using an operant task. The results indicated that at the highest dose (10 μmol/kg), PD144418 significantly attenuated the number of active lever responses for chow pellets but did not decrease the number of active lever responses for sucrose pellets under a fixed ratio (FR2) schedule of reinforcement. However, under a progressive ratio (PR) reinforcement schedule, 10 μmol/kg of PD14418 significantly reduced the breakpoint, a measure indicative of effort or motivation, for both chow and sucrose pellets. When ad libitum chow or sucrose pellets were made freely available (i.e. no lever press required) inside the operant chamber, 10 μmol/kg, PD144418 did not have an effect on number of pellets consumed. These findings indicate that PD144418 reduces the motivational effort of a food reinforced behavior.

A behavioral and pharmacological characterization of palatable diet alternation in mice

Obesity and eating disorders are widespread in Western societies. Both the increased availability of highly palatable foods and dieting are major risk factors contributing to the epidemic of disorders of feeding. The purpose of this study was to characterize an animal model of maladaptive feeding induced by intermittent access to a palatable diet alternation in mice. In this study, mice were either continuously provided with standard chow food (Chow/Chow), or provided with standard chow for 2days and a high-sucrose, palatable food for 1day (Chow/Palatable). Following stability of intake within the cycling paradigm, we then investigated the effects of several pharmacological treatments on excessive eating of palatable food: naltrexone, an opioid receptor antagonist, SR141716A, a cannabinoid-1 receptor antagonist/inverse agonist, and BD-1063, a sigma-1 receptor antagonist. Over successive cycles, Chow/Palatable mice showed an escalation of palatable food intake within the first hour of renewed access to palatable diet and displayed hypophagia upon its removal. Naltrexone, SR141716A, and BD-1063 all reduced overconsumption of palatable food during this first hour. Here we provide evidence of strong face and convergent validity in a palatable diet alternation model in mice, confirming multiple shared underlying mechanisms of pathological eating across species, and thus making it a useful therapeutic development tool.

Neuroscience of Compulsive Eating Behavior

A systematic characterization of compulsivity in pathological forms of eating has been proposed in the context of three functional domains: (1) habitual overeating; (2) overeating to relieve a negative emotional state; and (3) overeating despite aversive consequences. In this review, we provide evidence supporting this hypothesis and we differentiate the nascent field of neurocircuits and neurochemical mediators of compulsive eating through their underlying neuropsychobiological processes. A better understanding of the neurobiological mechanisms that lead to compulsive eating behavior can improve behavioral and pharmacological intervention for disorders of pathological eating.

Rat animal models for screening medications to treat alcohol use disorders

The purpose of this review is to present animal research models that can be used to screen and/or repurpose medications for the treatment of alcohol abuse and dependence. The focus will be on rats and in particular selectively bred rats. Brief introductions discuss various aspects of the clinical picture, which provide characteristics of individuals with alcohol use disorders (AUDs) to model in animals. Following this, multiple selectively bred rat lines will be described and evaluated in the context of animal models used to screen medications to treat AUDs. Next, common behavioral tests for drug efficacy will be discussed particularly as they relate to stages in the addiction cycle. Tables highlighting studies that have tested the effects of compounds using the respective techniques are included. Wherever possible the Tables are organized chronologically in ascending order to describe changes in the focus of research on AUDs over time. In general, high ethanol-consuming selectively bred rats have been used to test a wide range of compounds. Older studies usually followed neurobiological findings in the selected lines that supported an association with a propensity for high ethanol intake. Most of these tests evaluated the compound's effects on the maintenance of ethanol drinking. Very few compounds have been tested during ethanol-seeking and/or relapse and fewer still have assessed their effects during the acquisition of AUDs. Overall, while a substantial number of neurotransmitter and neuromodulatory system targets have been assessed; the roles of sex- and age-of-animal, as well as the acquisition of AUDs, ethanol-seeking and relapse continue to be factors and behaviors needing further study. This article is part of the Special Issue entitled "Alcoholism".

Pathological Overeating: Emerging Evidence for a Compulsivity Construct

Compulsive eating behavior is a transdiagnostic construct that is characteristic of medical and psychiatric conditions such as forms of obesity and eating disorders. Although feeding research is moving toward a better understanding of the proposed addictive properties of food, the components and the mechanisms contributing to compulsive eating are not yet clearly defined or understood. Current understanding highlights three elements of compulsive behavior as it applies to pathological overeating: (1) habitual overeating; (2) overeating to relieve a negative emotional state; and (3) overeating despite aversive consequences. These elements emerge through mechanisms involving pathological habit formation through an aberrant learning process, the emergence of a negative emotional state, and dysfunctions in behavioral control. Dysfunctions in systems within neurocircuitries that comprise the basal ganglia, the extended amygdala, and the prefrontal cortex result in compulsive eating behaviors. Here, we present evidence to relate compulsive eating behavior and addiction and to characterize their underlying neurobiological mechanisms. A major need to improve understanding of compulsive eating through the integration of complex motivational, emotional, and cognitive constructs is warranted.

Genetic studies of alcohol dependence in the context of the addiction cycle

Family, twin and adoption studies demonstrate clearly that alcohol dependence and alcohol use disorders are phenotypically complex and heritable. The heritability of alcohol use disorders is estimated at approximately 50-60% of the total phenotypic variability. Vulnerability to alcohol use disorders can be due to multiple genetic or environmental factors or their interaction which gives rise to extensive and daunting heterogeneity. This heterogeneity makes it a significant challenge in mapping and identifying the specific genes that influence alcohol use disorders. Genetic linkage and (candidate gene) association studies have been used now for decades to map and characterize genomic loci and genes that underlie the genetic vulnerability to alcohol use disorders. These approaches have been moderately successful in identifying several genes that contribute to the complexity of alcohol use disorders. Recently, genome-wide association studies have become one of the major tools for identifying genes for alcohol use disorders by examining correlations between millions of common single-nucleotide polymorphisms with diagnosis status. Genome-wide association studies are just beginning to uncover novel biology; however, the functional significance of results remains a matter of extensive debate and uncertainty. In this review, we present a select group of genome-wide association studies of alcohol dependence, as one example of a way to generate functional hypotheses, within the addiction cycle framework. This analysis may provide novel directions for validating the functional significance of alcohol dependence candidate genes. This article is part of the Special Issue entitled "Alcoholism".

Sigma Receptors and Substance Use Disorders

Thanks to advances in neuroscience, addiction is now recognized as a chronic brain disease with genetic, developmental, and cultural components. Drugs of abuse, including alcohol, are able to produce significant neuroplastic changes responsible for the profound disturbances shown by drug addicted individuals. The current lack of efficacious pharmacological treatments for substance use disorders has encouraged the search for novel and more effective pharmacotherapies. Growing evidence strongly suggests that Sigma Receptors are involved in the addictive and neurotoxic properties of abused drugs, including cocaine , methamphetamine , and alcohol. The present chapter will review the current scientific knowledge on the role of the Sigma Receptor system in the effects of drugs and alcohol, and proposes that this receptor system may represent a novel therapeutic target for the treatment of substance use disorders and associated neurotoxicity.

Sigma Receptors and Alcohol Use Disorders

Although extensive research has focused on understanding the neurobiological mechanisms underlying alcohol addiction, pharmacological treatments for alcohol use disorders are very limited and not always effective. This constraint has encouraged the search for novel pharmacological targets for alcoholism therapy. Sigma receptors were shown to mediate some of the properties of cocaine and amphetamine, which was attributed to the direct binding of psychostimulants to these receptors. More recently, the role of sigma receptors in the rewarding and reinforcing effects of alcohol was also proposed, and it was suggested that their hyperactivity may result in excessive alcohol drinking. This chapter reviews current knowledge on the topic, and suggests that the sigma receptor system may represent a new therapeutic target for the treatment of alcohol use disorders.

Ethanol Stimulates Endoplasmic Reticulum Inositol Triphosphate and Sigma Receptors to Promote Withdrawal-Associated Loss of Neuron-Specific Nuclear Protein/Fox-3

BACKGROUND

Prior studies demonstrate that ethanol (EtOH) exposure induces the release of intracellular calcium (CA(2+) ) in modulation of γ-aminobutyric acid-ergic tone and produces concomitant alterations in sigma (σ)-1 protein expression that may contribute to the development EtOH dependence. However, the influence of CA(2+) released from endoplasmic reticulum (ER)-bound inositol triphosphate (IP3) and σ-1 receptors in regulating hippocampal function has yet to be delineated.

METHODS

Rat hippocampal explants were subjected to chronic intermittent EtOH (CIE) exposure with or without the addition of IP3 inhibitor xestospongin C (0 to 0.5 μM) or σ-1 receptor antagonist BD-1047 (0 to 80 μM). Hippocampal viability was assessed via immunohistochemical labeling of neuron-specific nuclear protein (NeuN)/Fox-3 in CA1, CA3, and dentate gyrus (DG) subregions.

RESULTS

Exposure to CIE produced consistent and significant decreases of NeuN/Fox-3 in each primary cell layer of the hippocampal formation. Co-exposure to xestospongin reversed these effects in the CA1 subregion and significantly attenuated these effects in the CA3 and DG regions. Xestospongin application also significantly increased NeuN/Fox-3 immunofluorescence in EtOH-naïve hippocampi. Co-exposure to 20 μM BD-1047 also reversed the loss of NeuN/Fox-3 during CIE exposure in each hippocampal cell layer, whereas exposure to 80 μM BD-1047 did not alter NeuN/Fox-3 in EtOH-treated hippocampi. By contrast, 80 μM BD-1047 application significantly increased NeuN/Fox-3 immunofluorescence in EtOH-naïve hippocampi in each subregion.

CONCLUSIONS

These data suggest that EtOH stimulates ER IP3 and σ-1 receptors to promote hippocampal loss of NeuN/Fox-3 during CIE.

Ethanol-related behaviors in mice lacking the sigma-1 receptor

RATIONALE

The Sigma-1 receptor (Sig-1R) is a chaperone protein that has been implicated in drug abuse and addiction. Multiple studies have characterized the role the Sig-1R plays in psychostimulant addiction; however, fewer studies have specifically investigated its role in alcohol addiction. We have previously shown that antagonism of the Sig-1R reduces excessive drinking and motivation to drink, whereas agonism induces binge-like drinking in rodents.

OBJECTIVES

The objectives of these studies were to investigate the impact of Sig-1R gene deletion in C57Bl/6J mice on ethanol drinking and other ethanol-related behaviors.

METHODS

We used an extensive panel of behavioral tests to examine ethanol actions in male, adult mice lacking Oprs1, the gene encoding the Sig-1R. To compare ethanol drinking behavior, Sig-1 knockout (KO) and wild type (WT) mice were subject to a two-bottle choice, continuous access paradigm with different concentrations of ethanol (3-20% v/v) vs. water. Consumption of sweet and bitter solutions was also assessed in Sig-1R KO and WT mice. Finally, motor stimulant sensitivity, taste aversion and ataxic effects of ethanol were assessed.

RESULTS

Sig-1R KO mice displayed higher ethanol intake compared to WT mice; the two genotypes did not differ in their sweet or bitter taste perception. Sig-1R KO mice showed lower sensitivity to ethanol stimulant effects, but greater sensitivity to its taste aversive effects. Ethanol-induced sedation was instead unaltered in the mutants.

CONCLUSIONS

Our results prove that the deletion of the Sig-1R increases ethanol consumption, likely by decreasing its rewarding effects, and therefore indicating that the Sig-1R is involved in modulation of the reinforcing effects of alcohol.

Diet-induced obesity and diet-resistant rats: differences in the rewarding and anorectic effects of D-amphetamine

RATIONALE

Obesity is a leading public health problem worldwide. Multiple lines of evidence associate deficits in the brain reward circuit with obesity.

OBJECTIVE

Whether alterations in brain reward sensitivity precede or are a consequence of obesity is unknown. This study aimed to investigate both innate and obesity-induced differences in the sensitivity to the effects of an indirect dopaminergic agonist.

METHODS

Rats genetically prone to diet-induced obesity (DIO) and their counterpart diet-resistant (DR) were fed a chow diet, and their response to D-amphetamine on intracranial self-stimulation and food intake were assessed. The same variables were then evaluated after exposing the rats to a high-fat diet, after DIO rats selectively developed obesity. Finally, gene expression levels of dopamine receptors 1 and 2 as well as tyrosine hydroxylase were measured in reward-related brain regions.

RESULTS

In a pre-obesity state, DIO rats showed innate decreased sensitivity to the reward-enhancing and anorectic effects of D-amphetamine, as compared to DR rats. In a diet-induced obese state, the insensitivity to the potentiating effects of D-amphetamine on intracranial self-stimulation (ICSS) threshold persisted and became more marked in DIO rats, while the anorectic effects were comparable between genotypes. Finally, innate and obesity-induced differences in the gene expression of dopamine receptors were observed.

CONCLUSIONS

Our results demonstrate that brain reward deficits antedate the development of obesity and worsen after obesity is fully developed, suggesting that these alterations represent vulnerability factors for its development. Moreover, our data suggests that the reward-enhancing and anorectic effects of D-amphetamine are dissociable in the context of obesity.

Sigma-1 receptor mediates acquisition of alcohol drinking and seeking behavior in alcohol-preferring rats

Sigma-1 receptor (Sig-1R) has been proposed as a novel therapeutic target for drug and alcohol addiction. We have shown previously that Sig-1R agonists facilitate the reinforcing effects of ethanol and induce binge-like drinking, while Sig-1R antagonists on the other hand block excessive drinking in genetic and environmental models of alcoholism, without affecting intake in outbred non-dependent rats. Even though significant progress has been made in understanding the function of Sig-1R in alcohol reinforcement, its role in the early and late stage of alcohol addiction remains unclear. Administration of the selective Sig-1R antagonist BD-1063 dramatically reduced the acquisition of alcohol drinking behavior as well as the preference for alcohol in genetically selected TSRI Sardinian alcohol preferring (Scr:sP) rats; the treatment had instead no effect on total fluid intake, food intake or body weight gain, proving selectivity of action. Furthermore, BD-1063 dose-dependently decreased alcohol-seeking behavior in rats trained under a second-order schedule of reinforcement, in which responding is maintained by contingent presentation of a conditioned reinforcer. Finally, an innate elevation in Sig-1R protein levels was found in the nucleus accumbens of alcohol-preferring Scr:sP rats, compared to outbred Wistar rats, alteration which was normalized by chronic, voluntary alcohol drinking. Taken together these findings demonstrate that Sig-1R blockade reduces the propensity to both acquire alcohol drinking and to seek alcohol, and point to the nucleus accumbens as a potential key region for the effects observed. Our data suggest that Sig-1R antagonists may have therapeutic potential in multiple stages of alcohol addiction.

Addiction and the adrenal cortex

Substantial evidence shows that the hypophyseal–pituitary–adrenal (HPA) axis and corticosteroids are involved in the process of addiction to a variety of agents, and the adrenal cortex has a key role. In general, plasma concentrations of cortisol (or corticosterone in rats or mice) increase on drug withdrawal in a manner that suggests correlation with the behavioural and symptomatic sequelae both in man and in experimental animals. Corticosteroid levels fall back to normal values in resumption of drug intake. The possible interactions between brain corticotrophin releasing hormone (CRH) and proopiomelanocortin (POMC) products and the systemic HPA, and additionally with the local CRH–POMC system in the adrenal gland itself, are complex. Nevertheless, the evidence increasingly suggests that all may be interlinked and that CRH in the brain and brain POMC products interact with the blood-borne HPA directly or indirectly. Corticosteroids themselves are known to affect mood profoundly and may themselves be addictive. Additionally, there is a heightened susceptibility for addicted subjects to relapse in conditions that are associated with change in HPA activity, such as in stress, or at different times of the day. Recent studies give compelling evidence that a significant part of the array of addictive symptoms is directly attributable to the secretory activity of the adrenal cortex and the actions of corticosteroids. Additionally, sex differences in addiction may also be attributable to adrenocortical function: in humans, males may be protected through higher secretion of DHEA (and DHEAS), and in rats, females may be more susceptible because of higher corticosterone secretion.

Animal models for medications development targeting alcohol abuse using selectively bred rat lines: neurobiological and pharmacological validity

The purpose of this review paper is to present evidence that rat animal models of alcoholism provide an ideal platform for developing and screening medications that target alcohol abuse and dependence. The focus is on the 5 oldest international rat lines that have been selectively bred for a high alcohol-consumption phenotype. The behavioral and neurochemical phenotypes of these rat lines are reviewed and placed in the context of the clinical literature. The paper presents behavioral models for assessing the efficacy of pharmaceuticals for the treatment of alcohol abuse and dependence in rodents, with particular emphasis on rats. Drugs that have been tested for their effectiveness in reducing alcohol/ethanol consumption and/or self-administration by these rat lines and their putative site of action are summarized. The paper also presents some current and future directions for developing pharmacological treatments targeting alcohol abuse and dependence.

Antagonism of sigma-1 receptors blocks compulsive-like eating

Binge eating disorder is an addiction-like disorder characterized by episodes of rapid and excessive food consumption within discrete periods of time which occur compulsively despite negative consequences. This study was aimed at determining whether antagonism of Sigma-1 receptors (Sig-1Rs) blocked compulsive-like binge eating. We trained male wistar rats to obtain a sugary, highly palatable diet (Palatable group) or a regular chow diet (Chow control group), for 1 h a day under fixed ratio 1 operant conditioning. Following intake stabilization, we evaluated the effects of the selective Sig-1R antagonist BD-1063 on food responding. Using a light/dark conflict test, we also tested whether BD-1063 could block the time spent and the food eaten in an aversive, open compartment, where the palatable diet was offered. Furthermore, we measured Sig-1R mRNA and protein expression in several brain areas of the two groups, 24 h after the last binge session. Palatable rats rapidly developed binge-like eating, escalating the 1 h intake by four times, and doubling the eating rate and the regularity of food responding, compared to Chow rats. BD-1063 dose-dependently reduced binge-like eating and the regularity of food responding, and blocked the increased eating rate in Palatable rats. In the light/dark conflict test, BD-1063 antagonized the increased time spent in the aversive compartment and the increased intake of the palatable diet, without affecting motor activity. Finally, Palatable rats showed reduced Sig-1R mRNA expression in prefrontal and anterior cingulate cortices, and a two-fold increase in Sig-1R protein expression in anterior cingulate cortex compared to control Chow rats. These findings suggest that the Sig-1R system may contribute to the neurobiological adaptations driving compulsive-like eating, opening new avenues of investigation towards pharmacologically treating binge eating disorder.

Ethanol exposure in early adolescence inhibits intrinsic neuronal plasticity via sigma-1 receptor activation in hippocampal CA1 neurons

BACKGROUND

We demonstrated previously that rats exposed to chronic intermittent ethanol (CIE) vapors in early adolescence show increased magnitudes of long-term potentiation (LTP) of excitatory transmission when recorded at dendritic synapses in hippocampus. Large amplitude LTP following CIE exposure is mediated by sigma-1 receptors; however, not yet addressed is the role of sigma-1 receptors in modulating the intrinsic properties of neurons to alter their action potential firing during LTP.

METHODS

Activity-induced plasticity of spike firing was investigated using rat hippocampal slice recordings to measure changes in both field excitatory postsynaptic potentials (fEPSPs) and population spikes (pop. spikes) concomitantly at dendritic inputs and soma of CA1 pyramidal neurons, respectively.

RESULTS

We observed unique modifications in plasticity of action potential firing in hippocampal slices from CIE exposed adolescent rats, where the induction of large amplitude LTP by 100 Hz stimulations was accompanied by reduced CA1 neuronal excitability--reflected as decreased pop. spike efficacy and impaired activity-induced fEPSP-to-spike (E-S) potentiation. In contrast, LTP induction in ethanol-naïve control slices resulted in increased spike efficacy and robust E-S potentiation. E-S potentiation impairments emerged at 24 hours after CIE treatment cessation, but not before the alcohol withdrawal period, and were restored with bath-application of the sigma-1 receptor selective antagonist BD1047, but not the NMDA receptor antagonist d-AP5. Further evidence revealed a significantly shortened somatic fEPSP time course in adolescent CIE-withdrawn hippocampal slices during LTP; however, paired-pulse data show no apparent correspondence between E-S dissociation and altered recurrent feedback inhibition.

CONCLUSIONS

Results here suggest that acute withdrawal from adolescent CIE exposure triggers sigma-1 receptors that act to depress the efficacy of excitatory inputs in triggering action potentials during LTP. Such withdrawal-induced depression of E-S plasticity in hippocampus probably entails sigma-1 receptor modulation of 1 or several voltage-gated ion channels controlling the neuronal input-output dynamics.

Activation of σ-receptors induces binge-like drinking in Sardinian alcohol-preferring rats

Sigma (σ) receptors have been implicated in the behavioral and motivational effects of alcohol and psychostimulants. Sigma receptor antagonists reduce the reinforcing effects of alcohol and excessive alcohol intake in both genetic (alcohol-preferring rats) and environmental (chronic alcohol-induced) models of alcoholism. The present study tested the hypothesis that pharmacological activation of σ-receptors facilitates ethanol reinforcement and induces excessive, binge-like ethanol intake. The effects of repeated subcutaneous treatment with the selective σ-receptor agonist 1,3-di-(2-tolyl)guanidine (DTG; 15 mg/kg, twice a day for 7 days) on operant ethanol (10%) self-administration were studied in Sardinian alcohol-preferring (sP) rats. To confirm that the effect of DTG was mediated by σ-receptors, the effects of pretreatment with the selective σ-receptor antagonist BD-1063 (7 mg/kg, subcutaneously) were determined. To assess the specificity of action, the effects of DTG on the self-administration of equally reinforcing solutions of saccharin or sucrose were also determined. Finally, gene expression of opioid receptors in brain areas implicated in ethanol reinforcement was analyzed in ethanol-naive sP rats treated acutely or repeatedly with DTG, because of the well-established role of the opioid system in alcohol reinforcement and addiction. Repeatedly administered DTG progressively and dramatically increased ethanol self-administration in sP rats and increased blood alcohol levels, which reached mean values close to 100 mg% in 1 h drinking sessions. Repeated DTG treatment also increased the rats' motivation to work for alcohol under a progressive-ratio schedule of reinforcement. BD-1063 prevented the effects of DTG, confirming that σ-receptors mediate the effects of DTG. Repeated DTG treatment also increased the self-administration of the non-drug reinforcers saccharin and sucrose. Naive sP rats repeatedly treated with DTG showed increased mRNA expression of μ- and δ-opioid receptors in the ventral tegmental area. These results suggest a key facilitatory role for σ-receptors in the reinforcing effects of alcohol and identify a potential mechanism that contributes to binge-like and excessive drinking.