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

Recent advances in drug discovery efforts targeting the sigma 1 receptor system: Implications for novel medications designed to reduce excessive drug and food seeking

Psychiatric disorders characterized by uncontrolled reward seeking, such as substance use disorders (SUDs), alcohol use disorder (AUD) and some eating disorders, impose a significant burden on individuals and society. Despite their high prevalence and substantial morbidity and mortality rates, treatment options for these disorders remain limited. Over the past two decades, there has been a gradual accumulation of evidence pointing to the sigma-1 receptor (S1R) system as a promising target for therapeutic interventions designed to treat these disorders. S1R is a chaperone protein that resides in the endoplasmic reticulum, but under certain conditions translocates to the plasma membrane. In the brain, S1Rs are expressed in several regions important for reward, and following translocation, they physically associate with several reward-related GPCRs, including dopamine receptors 1 and 2 (D1R and D2R). Psychostimulants, alcohol, as well as palatable foods, all alter expression of S1R in regions important for motivated behavior, and S1R antagonists generally decrease behavioral responses to these rewards. Recent advances in structural modeling have permitted the development of highly-selective S1R antagonists with favorable pharmacokinetic profiles, thus providing a therapeutic avenue for S1R-based medications. Here, we provide an up-to-date overview of work linking S1R with motivated behavior for drugs of abuse and food, as well as evidence supporting the clinical utility of S1R antagonists to reduce their excessive consumption. We also highlight potential challenges associated with targeting the S1R system, including the need for a more comprehensive understanding of the underlying neurobiology and careful consideration of the pharmacological properties of S1R-based drugs.

Long-term Administration of 3-Di-O-Tolylguanidine Modulates Spatial Learning and Memory in Rats and Causes Transition in the Concentration of Neurotransmitters in the Hippocampus, Prefrontal Cortex and Striatum

The sigma-1 and sigma-2 (σ₁ and σ₂) receptors are found in high concentrations in the brain, and their altered expression leads to a variety of neuropsychiatric disorders. 3-di-tolylguanidine (DTG) stimulates the activity of both of these receptors. We assessed the effects of administering DTG to adult male Sprague Dawley rats on learning and memory consolidation processes and on the levels of neurotransmitters in selected brain structures. Spatial learning and memory were evaluated in the water maze test. The DTG was administered orally at daily doses of 3 mg/kg (DTG3), 10 mg/kg (DTG10) or 30 mg/kg (DTG30) for 10 weeks before and during the water-maze test. After completion of the experiment, the concentration of monoamines and their metabolites as well as amino acids in structures involved in cognitive performance - the hippocampus, prefrontal cortex, and striatum - were determined using high performance liquid chromatography (HPLC). The DTG10 group showed an improvement in memory processes related to the "new" platform location, whereas the DTG30 group was worse at finding the "old" platform location. Since the administration of DTG led to differences in dopaminergic transmission, it was assumed to influence memory processes in this way. Changes in histidine, serine, alanine, taurine, and glutamic acid levels in selected structures of the brains of rats with memory impairment were also observed. We conclude that long-term administration of DTG modulates spatial learning and memory in rats and changes the concentrations of neurotransmitters in the hippocampus, prefrontal cortex, and striatum..

Administration of the sigma-1 receptor agonist PRE-084 at emerging adulthood, but not at early adolescence, attenuated ethanol-induced conditioned taste aversion in female rats

Ethanol-induced conditioned taste aversion (CTA) is greater in late adolescence or young adulthood than in early adolescence. The role of the sigma receptor system in this age-related difference has not been extensively explored, particularly in female rats. This study assessed the effects of the activation of sigma-1 receptors (S1-R), via the selective S1-R agonist PRE-084, on ethanol-induced CTA at early or at terminal adolescence/emerging adulthood (28 or 56 days-old at the beginning of the procedures, respectively) in female Wistar rats. The modulation of binge-like ethanol intake by PRE-084 was assessed at terminal adolescence. S1-R activation at the acquisition of ethanol-induced CTA attenuated such learning at terminal but not at early adolescence. PRE-084 did not significantly affect ethanol binge drinking in the terminal adolescents. These results highlight the role of S1-R in ethanol-induced CTA and suggest that differential functionality of this transmitter system may underlie age-specific sensitivities to the aversive effects of ethanol.

Pituitary adenylate cyclase-activating polypeptide type 1 receptor within the nucleus accumbens core mediates excessive alcohol drinking in alcohol-preferring rats

Alcohol use disorders (AUD) have a strong component of heritability; however, the neurobiological mechanisms mediating the propensity to consume excessive amounts of alcohol are still not well understood. Pituitary adenylate cyclase-activating polypeptide (PACAP), a highly conserved neuropeptide which exerts its effects mainly through the PAC1 receptor (PAC1R), has been suggested to be one of the mediators of the effects of drugs of abuse and alcohol. Here, we investigated the role of the PACAP/PAC1R system in excessive alcohol drinking in alcohol-preferring rats, an established animal model of AUD. Intracerebroventricular (i.c.v.) administration of the PAC1R antagonist PACAP(6-38) blocked excessive alcohol drinking and motivation to drink in Sardinian alcohol-preferring (Scr:sP) rats, without affecting water, saccharin, or sucrose intake. Notably, PACAP(6-38) did not affect ethanol responding in outbred Wistar rats. PACAP(6-38) also significantly reduced alcohol-seeking behavior under a second-order schedule of reinforcement. Using immunohistochemistry, a significant increase in the number of PAC1R positive cells was observed selectively in the nucleus accumbens (NAcc) Core of Scr:sP rats, compared to Wistar rats following alcohol drinking. Finally, excessive drinking in Scr:sP rats was suppressed by intra-NAcc Core, but not intra-NAcc Shell, PACAP(6-38), as well as by virally-mediated PAC1R knockdown in the NAcc Core. The present study shows that hyperactivity of the PACAP/PAC1R system specifically in the NAcc Core mediates excessive drinking of alcohol-preferring rats, and indicates that this system may represent a novel target for the treatment of AUD.

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.

An operant ethanol self-administration paradigm that discriminates between appetitive and consummatory behaviors reveals distinct behavioral phenotypes in commonly used rat strains

Alcohol use disorder (AUD) constitutes a major burden to global health. Recently, the translational success of animal models of AUD has come under increased scrutiny. Efforts to refine models to gain a more precise understanding of the neurobiology of addiction are warranted. Appetitive responding for ethanol (seeking) and its consumption (taking) are governed by distinct neurobiological mechanisms. However, consumption is often inferred from appetitive responding in operant ethanol self-administration paradigms, preventing identification of distinct experimental effects on seeking and taking. In the present study, male Long-Evans, Wistar, and Sprague-Dawley rats were trained to lever press for ethanol using a lickometer-equipped system that precisely measures both appetitive and consummatory behavior. Three distinct operant phenotypes emerged during training: 1) Drinkers, who lever press and consume ethanol; 2) Responders, who lever press but consume little to no ethanol; and 3) Non-responders, who do not lever press. While the prevalence of each phenotype differed across strains, appetitive and consummatory behavior was similar across strains within each phenotype. Appetitive and consummatory behaviors were significantly correlated in Drinkers, but not Responders. Analysis of drinking microstructure showed that greater consumption in Drinkers relative to Responders is due to increased incentive for ethanol rather than increased palatability. Importantly, withdrawal from chronic ethanol exposure resulted in a significant increase in appetitive responding in both Drinkers and Responders, but only Drinkers exhibited a concomitant increase in ethanol consumption. Together, these data reveal important strain differences in appetitive and consummatory responding for ethanol and uncover the presence of distinct operant phenotypes.

Viral-Mediated Knockdown of Nucleus Accumbens Shell PAC1 Receptor Promotes Excessive Alcohol Drinking in Alcohol-Preferring Rats

Alcohol use disorder (AUD) is a chronic, relapsing disorder whose genetic and environmental susceptibility components are not fully understood. Neuropeptidergic signaling has been repeatedly implicated in modulating excessive alcohol drinking, especially within sub-regions of the striatum. Here, we investigated the potential involvement of the selective receptor for pituitary adenylate cyclase-activating polypeptide (PACAP), PAC1R, in the nucleus accumbens shell (NAcc Shell) in excessive alcohol drinking in alcohol-preferring rats, an established animal model of the genetic propensity for alcoholism. Scr:sP alcohol-preferring rats were trained to operantly self-administer alcohol and then either an AAV virus short-hairpin RNA (shRNA) targeted to knockdown PAC1R, or an AAV control virus were microinfused into the NAcc Shell. NAcc Shell PAC1R shRNA knockdown virus was confirmed to significantly decrease PAC1R levels in the NAcc Shell. The effects of NAcc Shell PAC1R shRNA knockdown on ethanol self-administration were investigated using a Fixed Ratio (FR) 1 and a Progressive Ratio (PR) schedule of reinforcement. The effect of PAC1R knockdown on self-administration of an alternative reinforcer, saccharin, was also assessed. The results showed that the reduction in PAC1R in the NAcc Shell led to excessive ethanol drinking, increased preference for ethanol, and higher motivation to drink. NAcc Shell PAC1R shRNA knockdown did not comparably increase saccharin self-administration, suggesting selectivity of action. These data suggest that NAcc Shell PAC1R may serves as a "brake" on alcohol drinking, and thereby the loss of function of PAC1R leads to excessive alcohol consumption. Therefore, the PACAP/PAC1R system may represent a novel target for the treatment of AUD.

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.

Sigmar1's Molecular, Cellular, and Biological Functions in Regulating Cellular Pathophysiology

The Sigma 1 receptor (Sigmar1) is a ubiquitously expressed multifunctional inter-organelle signaling chaperone protein playing a diverse role in cellular survival. Recessive mutation in Sigmar1 have been identified as a causative gene for neuronal and neuromuscular disorder. Since the discovery over 40 years ago, Sigmar1 has been shown to contribute to numerous cellular functions, including ion channel regulation, protein quality control, endoplasmic reticulum-mitochondrial communication, lipid metabolism, mitochondrial function, autophagy activation, and involved in cellular survival. Alterations in Sigmar1’s subcellular localization, expression, and signaling has been implicated in the progression of a wide range of diseases, such as neurodegenerative diseases, ischemic brain injury, cardiovascular diseases, diabetic retinopathy, cancer, and drug addiction. The goal of this review is to summarize the current knowledge of Sigmar1 biology focusing the recent discoveries on Sigmar1’s molecular, cellular, pathophysiological, and biological functions.

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.

Novel Highly Potent and Selective Sigma1 Receptor Antagonists Effectively Block the Binge Eating Episode in Female Rats

In this paper, the benzo-cracking approach was applied to the potent sigma1 (σ₁) receptor antagonist 1 to afford the less conformationally constrained 1,3-dioxane derivatives 2 and 3. To evaluate the effect of the increase in the distance between the two hydrophobic structural elements that flank the basic function, the cis and trans diastereomers of 4 and 5 were also prepared and studied. Compounds 2 and 3 showed affinity values at the σ₁ receptor significantly higher than that of the lead compound 1. In particular, 3 displayed unprecedented selectivity over the σ₂ receptor, the phencyclidine site of the NMDA receptor, and opioid receptor subtypes, as well as over the dopamine transporter. Docking results supported the structure–activity relationship studies. Due to its interesting biological profile, derivative 3, selected for an in vivo study in a validated preclinical model of binge eating, was able to counteract the overeating of palatable food only in binging rats, without affecting palatable food intake in the control group and anxiety-like and depression-related behaviors in female rats. This result strengthened the involvement of the σ₁ receptor in the compulsive-like eating behavior and supported the σ₁ receptor as a promising target for the management of eating disorders.

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.

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.

Quetiapine Abuse Fourteen Years Later: Where Are We Now? A Systematic Review

Background: Quetiapine, an atypical antipsychotic endowed with weak dopamine antagonist, potent 5-HT_2A-blocking, partial 5-HT_1A-agonist, anti-H₁ histamine, adrenolytic, and sigma₁ receptor agonist activities, since an original 2004 report is increasingly misused. Although some of its pharmacodynamics might explain some motives for voluptuary use, most of its actions are directed at setting-off those motives. Hence, it is possible that its popularity in special populations is due to the fact that the unpleasant or unwanted effects of addiction substances are somehow soothed by quetiapine. Currently, quetiapine is tested in substance use disorders, showing some promise, but it is likely to be misused in certain contexts. Objectives: To review the evidence for the use of quetiapine as addiction substance and investigate the characteristics of populations involved in such addiction. Methods: A systematic review of literature on various databases retrieved on September 7, 2018 87 records to comment. Results. We reviewed the evidence for quetiapine's addictive potential in the light of its pharmacodynamics properties and presented two cases of recreational quetiapine use, by a 35-year old male patient with past addictive behavior and by a 50-year-old woman with major depressive disorder and conversion disorder. We found quetiapine to be abused mainly by addict populations and people with law involvement. Conclusions/Importance: There is no reason to include quetiapine among regulated substances, but monitoring of its use in selected populations is warranted. Psychiatrists and physicians working in the penitentiary system should be aware of the addictive potential of quetiapine and adopt measures restricting its use.

Sigma-1 receptor ligand PD144418 and sigma-2 receptor ligand YUN-252 attenuate the stimulant effects of methamphetamine in mice

RATIONALE

Previous research indicates that the selective sigma-1 receptor ligand PD144418 and the selective sigma-2 ligands YUN-252 can inhibit cocaine-induced hyperactivity. The effects of these ligands on other stimulants, such as methamphetamine, have not been reported.

OBJECTIVES

The present study examined the effects of PD144418 and YUN-252 pretreatment on methamphetamine-induced hyperactivity after acute treatment.

METHODS

Mice (n = 8-14/group) were injected with PD144418 (3.16, 10, or 31.6 μmol/kg), YUN-252 (0.316, 3.16, 31.6 μmol/kg), or saline. After 15 min, mice injected with 2.69 μmol/kg methamphetamine or saline vehicle, where distance traveled during a 60-min period was recorded. Additionally, the effect of PD144418 on the initiation and expression of methamphetamine sensitization was determined by treating mice (n = 8-14/group) with PD144418, methamphetamine or saline repeatedly over a 5-day period, and testing said mice with a challenge dose after a 7-day withdrawal period.

RESULTS

Results indicate that both PD144418 and YUN-252, in a dose-dependent manner, attenuated hyperactivity induced by an acute methamphetamine injection. Specifically, 10 μmol/kg or 31.6 μmol/kg of PD144418 and 31 μmol/kg of YUN-252 suppressed methamphetamine-induced hyperactivity. In regard to methamphetamine sensitization, while 10 μmol/kg PD144418 prevented the initiation of methamphetamine sensitization, it did not have an effect on the expression.

CONCLUSIONS

Overall, the current results suggest an intriguing potential for this novel sigma receptor ligand as a treatment for the addictive properties of methamphetamine. Future analysis of this novel sigma receptor ligand in assays directly measuring reinforcement properties will be critical.

Investigation of the Role of Chirality in the Interaction with σ Receptors and Effect on Binge Eating Episode of a Potent σ1 Antagonist Analogue of Spipethiane

The enantiomers of the potent σ1 receptor antagonist (±)-1 were synthesized and evaluated for their affinity at σ1, σ2 receptors and dopamine transporter (DAT). Analogously to (±)-1, both of the enantiomers showed very high affinity for the σ1 receptor and unprecedented selectivity over both the σ2 receptor and DAT. The lack of enantioselectivity between (+)-1 and (-)-1 indicated that the center of chirality in the 2-position of the benzothiochromane nucleus does not play a crucial role in the interaction with any of the studied targets. Docking studies confirmed that the configuration of the enantiomers has only marginal effects on the molecular interactions with the σ1 receptor. In in vivo studies in a female rat model of binge eating, (±)-1 dose-dependently decreased the binge eating episode elicited by a history of intermittent food restriction and stress, confirming and strengthening the important role played by the σ1 receptor in bingeing-related eating disorders.

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.

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.

Neuropharmacology of compulsive eating

Compulsive eating behaviour is a transdiagnostic construct observed in certain forms of obesity and eating disorders, as well as in the proposed construct of 'food addiction'. Compulsive eating can be conceptualized as comprising three elements: (i) habitual overeating, (ii) overeating to relieve a negative emotional state, and (iii) overeating despite adverse consequences. Neurobiological processes that include maladaptive habit formation, the emergence of a negative affect, and dysfunctions in inhibitory control are thought to drive the development and persistence of compulsive eating behaviour. These complex psychobehavioural processes are under the control of various neuropharmacological systems. Here, we describe the current evidence implicating these systems in compulsive eating behaviour, and contextualize them within the three elements. A better understanding of the neuropharmacological substrates of compulsive eating behaviour has the potential to significantly advance the pharmacotherapy for feeding-related pathologies.This article is part of a discussion meeting issue 'Of mice and mental health: facilitating dialogue between basic and clinical neuroscientists'.

Effects of Long-Term Alcohol Drinking on the Dopamine D2 Receptor: Gene Expression and Heteroreceptor Complexes in the Striatum in Rats

BACKGROUND

Reduced dopamine D2 receptor (D2R) ligand binding has repeatedly been demonstrated in the striatum of humans with alcohol use disorder (AUD). The attenuated D2R binding has been suggested to reflect a reduced D2R density, which in turn has been proposed to drive craving and relapse. However, results from rodent studies addressing the effects of alcohol drinking on D2R density have been inconsistent.

METHODS

A validated alcohol drinking model (intermittent access to 20% alcohol) in Wistar rats was used to study the effects of voluntary alcohol drinking (at least 12 weeks) on the D2R in the striatum compared to age-matched alcohol-naïve control rats. Reverse transcriptase quantitative PCR was used to quantify isoform-specific Drd2 gene expression levels. Using bisulfite pyrosequencing, DNA methylation levels of a regulatory region of the Drd2 gene were determined. In situ proximity ligation assay was used to measure densities of D2R receptor complexes: D2R-D2R, adenosine A2A receptor (A2AR)-D2R, and sigma1 receptor (sigma1R)-D2R.

RESULTS

Long-term voluntary alcohol drinking significantly reduced mRNA levels of the long D2R isoform in the nucleus accumbens (NAc) but did not alter CpG methylation levels in the analyzed sequence of the Drd2 gene. Alcohol drinking also reduced the striatal density of D2R-D2R homoreceptor complexes, increased the density of A2AR-D2R heteroreceptor complexes in the NAc shell and the dorsal striatum, and decreased the density of sigma1R-D2R heteroreceptor complexes in the dorsal striatum.

CONCLUSIONS

The present results on long-term alcohol drinking might reflect reduced D2R levels through reductions in D2R-D2R homoreceptor complexes and gene expression. Furthermore, based on antagonistic interactions between A2AR and D2R, an increased density of A2AR-D2R heteroreceptor complexes might indicate a reduced affinity and signaling of the D2R population within the complex. Hence, both reduced striatal D2R levels and reduced D2R protomer affinity within the striatal A2AR-D2R complex might underlie reduced D2R radioligand binding in humans with AUD. This supports the hypothesis of a hypodopaminergic system in AUD and suggests the A2AR-D2R heteroreceptor complex as a potential novel treatment target.

(+)-Methyl (1R,2S)-2-{[4-(4-Chlorophenyl)-4-hydroxypiperidin-1-yl]methyl}-1-phenylcyclopropanecarboxylate [(+)-MR200] Derivatives as Potent and Selective Sigma Receptor Ligands: Stereochemistry and Pharmacological Properties

Methoxycarbonyl-1-phenyl-2-cyclopropylmethyl based derivatives cis-(+)-1a [cis-(+)-MR200], cis-(-)-1a [cis-(-)-MR201], and trans-(±)-1a [trans-(±)-MR204], have been identified as new potent sigma (σ) receptor ligands. In the present paper, novel enantiomerically pure analogues were synthesized and optimized for their σ receptor affinity and selectivity. Docking studies rationalized the results obtained in the radioligand binding assay. Absolute stereochemistry was unequivocally established by X-ray analysis of precursor trans-(+)-5a as camphorsulfonyl derivative 9. The most promising compound, trans-(+)-1d, showed remarkable selectivity over a panel of more than 15 receptors as well as good chemical and enzymatic stability in human plasma. An in vivo evaluation evidenced that trans-(+)-1d, in contrast to trans-(-)-1d, cis-(+)-1d, or cis-(-)-1d, which behave as σ₁ antagonists, exhibited a σ₁ agonist profile. These data clearly demonstrated that compound trans-(+)-1d, due to its σ₁ agonist activity and favorable receptor selectivity and stability, provided an useful tool for the study of σ₁ receptors.

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.

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.

Sigma1 Pharmacology in the Context of Cancer

Sigma1 (also known as sigma-1 receptor, Sig1R, σ1 receptor) is a unique pharmacologically regulated integral membrane chaperone or scaffolding protein. The majority of publications on the subject have focused on the neuropharmacology of Sigma1. However, a number of publications have also suggested a role for Sigma1 in cancer. Although there is currently no clinically used anti-cancer drug that targets Sigma1, a growing body of evidence supports the potential of Sigma1 ligands as therapeutic agents to treat cancer. In preclinical models, compounds with affinity for Sigma1 have been reported to inhibit cancer cell proliferation and survival, cell adhesion and migration, tumor growth, to alleviate cancer-associated pain, and to have immunomodulatory properties. This review will highlight that although the literature supports a role for Sigma1 in cancer, several fundamental questions regarding drug mechanism of action and the physiological relevance of aberrant SIGMAR1 transcript and Sigma1 protein expression in certain cancers remain unanswered or only partially answered. However, emerging lines of evidence suggest that Sigma1 is a component of the cancer cell support machinery, that it facilitates protein interaction networks, that it allosterically modulates the activity of its associated proteins, and that Sigma1 is a selectively multifunctional drug target.

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-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.