Effects of muscarinic M(1) and M(4) acetylcholine receptor stimulation on extinction and reinstatement of cocaine seeking in male mice, independent of extinction learning

Targeting muscarinic receptors for the treatment of alcohol use disorders: Opportunities and hurdles for clinical development

Emerging evidence suggests muscarinic acetylcholine receptors represent novel targets to treat alcohol use disorder. In this review, we draw from literature across medicinal chemistry, molecular biology, addiction and learning/cognition fields to interrogate the proposition for muscarinic receptor ligands in treating various aspects of alcohol use disorder, including cognitive dysfunction, motivation to consume alcohol and relapse. In support of this proposition, we describe cholinergic dysfunction in the pathophysiology of alcohol use disorder at a network level, including alcohol-induced adaptations present in both human post-mortem brains and reverse-translated rodent models. Preclinical behavioural pharmacology implicates specific muscarinic receptors, in particular, M4 and M5 receptors, as potential therapeutic targets worthy of further interrogation. We detail how these receptors can be selectively targeted in vivo by the use of subtype-selective allosteric modulators, a strategy that overcomes the issue of targeting a highly conserved orthosteric site bound by acetylcholine. Finally, we highlight the intense pharma interest in allosteric modulators of muscarinic receptors for other indications that provide an opportunity for repurposing into the alcohol use disorder space and provide some currently unanswered questions as a roadmap for future investigation.

Co-stimulation of muscarinic M1 and M4 acetylcholine receptors prevents later cocaine reinforcement in male and female mice, but not place-conditioning

Acute stimulation of M1 or M4 muscarinic cholinergic receptors reduces cocaine abuse-related effects in mice and rats. The combined activation of these receptor subtypes produces synergistic effects on some behavioural endpoints in mice. M1 and M1 + M4 receptor stimulation in a cocaine vs. food choice assay in rats and microdialysis in rats showed delayed and lasting "anticocaine effects". Here, we tested whether these putative lasting neuroplastic changes are sufficient to occlude the reinforcing effects of cocaine at the behavioural level in mice. Mice were pre-treated with the M1 receptor partial agonist VU0364572, M4 receptor positive allosteric modulator VU0152100, or VU0364572 + VU0152100 two weeks prior to acquisition of cocaine intravenous self-administration (IVSA). Male C57BL/6JRj mice received vehicle, VU0364572, VU0152100, or VU0364572 + VU0152100. Female mice were tested with two VU0364572 + VU0152100 dose combinations or vehicle. To attribute potential effects to either reduced rewarding effects or increased aversion to cocaine, we tested VU0364572 alone and VU0364572 + VU0152100 in acquisition of cocaine-conditioned place preference (CPP) in male mice using an unbiased design. The acquisition of cocaine IVSA was drastically reduced and/or slowed in male and female mice receiving VU0364572 + VU0152100, but not either drug alone. Food-maintained operant behaviour was unaffected, indicating that the treatment effects were cocaine-specific. No treatment altered the acquisition of cocaine-CPP, neither in the post-test, nor in a challenge 14 days later. The cocaine IVSA findings confirm unusual long-lasting "anticocaine" effects of muscarinic M1 + M4 receptor stimulation. Thus, in mice, simultaneous stimulation of both receptor subtypes seems to produce potential neuroplastic changes that yield lasting effects.

Emerging GPCR targets for AUD: Insights from preclinical studies

G protein-coupled receptors (GPCRs) are the largest group of membrane receptors in the central nervous system and one of the key proteins for signal transduction between cells. Currently, many drugs available on the market act via GPCRs and these receptors remain attractive targets for the treatment of brain disorders, including alcohol use disorder (AUD). Here, we describe the most recent literature, with a primary focus on the past 5 years, on GPCR targets with the potential for reducing behaviours associated with excessive alcohol intake. Specifically, we focus on preclinical evidence of compounds with attractive pharmacological profiles and potential for future clinical investigation for the treatment of AUD.

Prevention of MK-801-induced amnestic effect with combined activation of 5-HT1A and muscarinic receptors in mice

BACKGROUND

Muscarinic or 5-HT1A receptors are crucial in learning and memory processes, and their expression is evident in the brain areas involved in cognition. The administration of the activators of these receptors prevents the development of cognitive dysfunctions in animal models of schizophrenia induced by MK-801 (N-methyl-d-aspartate receptor antagonist) administration. GABAergic dysfunction is considered as one of the most important causes of MK-801-induced spatial learning deficits.

METHODS

Novel object recognition (NOR) and Morris water maze (MWM) tests were used to study the anti-amnestic effect of the biased 5-HT1A receptor agonist (F15599) alone or in combinations with VU0357017 (M1 receptor allosteric agonist), VU0152100 (M4 receptor positive allosteric modulator), and VU0238429 (M5 receptor positive allosteric modulator) on MK-801-induced dysfunctions. The compounds were administered for 5 consecutive days. Animals tested with the MWM underwent 5-day training. Western blotting was used to study the expressions of 5-HT1A receptors and the level of GAD65 in the frontal cortices (FCs) and hippocampi of the animals.

RESULTS

F15599 prevented the amnestic effect induced by MK-801 in the MWM at a dose of 0.1 mg/kg. The co-administration of the compound with muscarinic receptors activators had no synergistic effect. The additive effect of the combinations was evident in the prevention of declarative memory dysfunctions investigated in NOR. The administration of MK-801 impaired 5-HT1A expression in the hippocampi and decreased GAD65 levels in both the FCs and hippocampi. The administration of muscarinic ligands prevented these MK-801-induced deficits only in the hippocampi of MWM-trained animals. No effects of the compounds were observed in untrained mice.

CONCLUSION

Our results indicate that F15599 prevents schizophrenia-related spatial learning deficits in the MWM; however, the activity of the compound is not intensified with muscarinic receptors activators. In contrast, the combined administration of the ligands is effective in the NOR model of declarative memory. The muscarinic receptors activators reversed MK-801-induced 5-HT1A and GAD65 dysfunctions in the hippocampi of MWM-trained mice, but not in untrained mice.

Muscarinic M1 and M4 receptor agonists for schizophrenia: promising candidates for the therapeutic arsenal

INTRODUCTION

Successful phase 3 trials of KarXT in people with schizophrenia herald a new era of treating the disorder with drugs that do not target the dopamine D2 receptor. The active component of KarXT is xanomeline, a muscarinic (CHRM) M1 and M4 agonist, making muscarinic receptors a viable target for treating schizophrenia.

AREAS COVERED

This review covers the process of taking drugs that activate the muscarinic M1 and M4 receptors from conceptualization to the clinic and details the mechanisms by which activating the CHRM1 and 4 can affect the broad spectrum of symptoms experienced by people with schizophrenia.

EXPERT OPINION

Schizophrenia is a syndrome which means drugs that activate muscarinic M1 and M4 receptors, as was the case for antipsychotic drugs acting on the dopamine D2 receptor, will not give optimal outcomes in everyone within the syndrome. Thus, it would be ideal to identify people who are responsive to drugs activating the CHRM1 and 4. Given knowledge of the actions of these receptors, it is possible treatment non-response could be restricted to sub-groups within the syndrome who have deficits in cortical CHRM1 or those with one of the cognitive endophenotypes that may be identifiable by changes in the blood transcriptome.

Establishment of multi-stage intravenous self-administration paradigms in mice

Genetically tractable animal models provide needed strategies to resolve the biological basis of drug addiction. Intravenous self-administration (IVSA) is the gold standard for modeling psychostimulant and opioid addiction in animals, but technical limitations have precluded the widespread use of IVSA in mice. Here, we describe IVSA paradigms for mice that capture the multi-stage nature of the disorder and permit predictive modeling. In these paradigms, C57BL/6J mice with long-standing indwelling jugular catheters engaged in cocaine- or remifentanil-associated lever responding that was fixed ratio-dependent, dose-dependent, extinguished by withholding the drug, and reinstated by the presentation of drug-paired cues. The application of multivariate analysis suggested that drug taking in both paradigms was a function of two latent variables we termed incentive motivation and discriminative control. Machine learning revealed that vulnerability to drug seeking and relapse were predicted by a mouse's a priori response to novelty, sensitivity to drug-induced locomotion, and drug-taking behavior. The application of these behavioral and statistical-analysis approaches to genetically-engineered mice will facilitate the identification of neural circuits driving addiction susceptibility and relapse and focused therapeutic development.

Jugular Vein Catheter Design and Cocaine Self-Administration Using Mice: A Comprehensive Method

Intravenous self-administration (IVSA) is a behavioral method of voluntary drug intake in animal models which is used to study the reinforcing effects of drugs of abuse. It is considered to have greater face validity in the study of substance use and abuse than other assays, and thus, allows for valuable insight into the neurobiological basis of addiction, and the development of substance abuse disorders. The technique typically involves surgically inserting a catheter into the jugular vein, which enables the infusion of drug solution after the performance of a desired operant behavior. Two nose- poke ports or levers are offered as manipulanda and are randomly assigned as active (reinforced) or inactive (non-reinforced) to allow for the examination of discrimination in the assessment of learning. Here, we describe our methodological approach to this assay in a mouse model, including construction and surgical implantation of a jugular vein catheter, set up of operant chambers, and considerations during each phase of the operant task.

Chronic escalating-dose and acute binge cocaine treatments change the hippocampal cholinergic muscarinic system on drug presence and after withdrawal

Cocaine addiction is a relapsing disorder with loss of control in limiting drug intake. Considering the involvement of acetylcholine in the neurobiology of the disease, our aim was to evaluate whether cocaine induces plastic changes in the hippocampal cholinergic muscarinic system. Male Swiss-Webster mice received saline or cocaine (ip) three times daily (60-min intervals) either acutely or in an escalating-dose binge paradigm for 14 days. Locomotor activity was measured in all treatment days. Dopaminergic and cholinergic muscarinic receptors (D₁R, D₂R, M₁-M_5, mAChRs), choline acetyltransferase (ChAT), vesicular acetylcholine transporter (VAChT) and acetylcholinesterase (AChE) were quantified in the hippocampus by immunoblotting one hour after the last injection (on drug) or after 14 days of abstinence (withdrawal). Escalating-dose group showed cocaine-induced locomotor sensitization from day 2. M₃ mAChR and ChAT significantly increased after the on-drug acute binge treatment. Escalating-dose on-drug group showed increased ChAT, M₁, M₅ mAChR and D₂R; and decreased D₁R. Acute-binge withdrawal group showed increased VAChT, M₂ mAChR, D₁R, and D₂R; and decreased M₁ mAChR. Escalating-dose withdrawal group presented increased D₁R and VAChT and decreased M₁ mAChR and D₂R. Locomotor activity was negatively correlated with M₁ mAChR and AChE in on-drug group and positively correlated with VAChT in withdrawal group. M₁ mAChR was positively correlated with M₂ mAChR and ChAT in on-drug group, whereas ChAT was positively correlated with M₅ mAChR in withdrawal group. The results indicate that cocaine induced an increase in the hippocampal cholinergic tone in the presence of the drug, whereas withdrawal causes a resetting in the system.

Effects of acute and repeated administration of the selective M4 PAM VU0152099 on cocaine versus food choice in male rats

Ligands that stimulate muscarinic acetylcholine receptors 1 and 4 (M1 , M4 ) have shown promising effects as putative pharmacotherapy for cocaine use disorder in rodent assays. We have previously shown reductions in cocaine effects with acute M4 stimulation, as well as long-lasting, delayed reductions in cocaine taking and cocaine seeking with combined M1 /M4 receptor stimulation or with M1 stimulation alone. M4 stimulation opposes dopaminergic signalling acutely, but direct dopamine receptor antagonists have proved unhelpful in managing cocaine use disorder because they lose efficacy with long-term administration. It is therefore critical to determine whether M4 approaches themselves can remain effective with repeated or chronic dosing. We assessed the effects of repeated administration of the M4 positive allosteric modulator (PAM) VU0152099 in rats trained to choose between intravenous cocaine and a liquid food reinforcer to obtain quantitative measurement of whether M4 stimulation could produce delayed and lasting reduction in cocaine taking. VU0152099 produced progressively augmenting suppression of cocaine choice and cocaine intake, but produced neither rebound nor lasting effects after treatment ended. To compare and contrast effects of M1 versus M4 stimulation, we tested whether the M4 PAM VU0152100 suppressed cocaine self-administration in mice lacking CalDAG-GEFI signalling factor, required for M1 -mediated suppression of cocaine self-administration. CalDAG-GEFI ablation had no effect on M4 -mediated suppression of cocaine self-administration. These findings support the potential usefulness of M4 PAMs as pharmacotherapy to manage cocaine use disorder, alone or in combination with M1 -selective ligands, and show that M1 and M4 stimulation modulate cocaine-taking behaviour by distinct mechanisms.

CalDAG-GEFI mediates striatal cholinergic modulation of dendritic excitability, synaptic plasticity and psychomotor behaviors

CalDAG-GEFI (CDGI) is a protein highly enriched in the striatum, particularly in the principal spiny projection neurons (SPNs). CDGI is strongly down-regulated in two hyperkinetic conditions related to striatal dysfunction: Huntington’s disease and levodopa-induced dyskinesia in Parkinson’s disease. We demonstrate that genetic deletion of CDGI in mice disrupts dendritic, but not somatic, M1 muscarinic receptors (M1Rs) signaling in indirect pathway SPNs. Loss of CDGI reduced temporal integration of excitatory postsynaptic potentials at dendritic glutamatergic synapses and impaired the induction of activity-dependent long-term potentiation. CDGI deletion selectively increased psychostimulant-induced repetitive behaviors, disrupted sequence learning, and eliminated M1R blockade of cocaine self-administration. These findings place CDGI as a major, but previously unrecognized, mediator of cholinergic signaling in the striatum. The effects of CDGI deletion on the self-administration of drugs of abuse and its marked alterations in hyperkinetic extrapyramidal disorders highlight CDGI’s therapeutic potential.

Spatiotemporal expression of Rap1 and Ras mediates the acquisition and reinstatement of methamphetamine-induced conditioned place preference in mice via extracellular signal-regulated kinase activation

Drug addiction is a chronic recurrent brain disease characterized by compulsive drug use and a high tendency to relapse. We previously reported that the Ras-extracellular signal-regulated kinase (ERK)-ΔFosB pathway in the caudate putamen (CPu) was involved in methamphetamine-induced behavioral sensitization. Rap1, as an antagonist of Ras originally, was found to participate in neuronal synaptic plasticity recently, but the role of Rap1 in methamphetamine addiction is unclear. First, in this study, we constructed the acquisition, extinction and reinstatement of methamphetamine-induced conditioned place preference (CPP) in mice, respectively. Then, protein levels of Rap1, Ras and pERK/ERK in the prefrontal cortex (PFc), CPu and hippocampus of CPP mice on three phases were detected. We found that protein levels of Rap1, Ras and pERK/ERK in the CPu were significantly increased after repeated methamphetamine administration, as well as Rap1 and pERK/ERK in the hippocampus. However, protein levels of Rap1 and pERK/ERK in the CPu were decreased on the reinstatement of CPP mice. Therefore, Rap1 and Ras in the CPu and Rap1 in the hippocampus may participate in the regulation of the acquisition of methamphetamine-induced CPP in mice by activating ERK. Moreover, Rap1-ERK cascade in the CPu contributes to both the acquisition and reinstatement of methamphetamine-induced CPP in mice.

Acetylcholine Muscarinic M4 Receptors as a Therapeutic Target for Alcohol Use Disorder: Converging Evidence From Humans and Rodents

BACKGROUND

Alcohol use disorder (AUD) is a major socioeconomic burden on society, and current pharmacotherapeutic treatment options are inadequate. Aberrant alcohol use and seeking alters frontostriatal function.

METHODS

We performed genome-wide RNA sequencing and subsequent quantitative polymerase chain reaction and receptor binding validation in the caudate-putamen of human AUD samples to identify potential therapeutic targets. We then back-translated our top candidate targets into a rodent model of long-term alcohol consumption to assess concordance of molecular adaptations in the rat striatum. Finally, we adopted rat behavioral models of alcohol intake and seeking to validate a potential therapeutic target.

RESULTS

We found that G protein-coupled receptors were the top canonical pathway differentially regulated in individuals with AUD. The M4 muscarinic acetylcholine receptor (mAChR) was downregulated at the gene and protein levels in the putamen, but not in the caudate, of AUD samples. We found concordant downregulation of the M4 mAChR, specifically on dopamine D1 receptor-expressing medium spiny neurons in the rat dorsolateral striatum. Systemic administration of the selective M4 mAChR positive allosteric modulator, VU0467154, reduced home cage and operant alcohol self-administration, motivation to obtain alcohol, and cue-induced reinstatement of alcohol seeking in rats. Local microinjections of VU0467154 in the rat dorsolateral striatum reduced alcohol self-administration and cue-induced reinstatement of alcohol seeking.

CONCLUSIONS

Collectively, these results identify the M4 mAChR as a potential therapeutic target for the treatment of AUD and the D1 receptor-positive medium spiny neurons in the dorsolateral striatum as a key site mediating the actions of M4 mAChR in relation to alcohol consumption and seeking.

Effects of muscarinic M1 receptor stimulation on reinforcing and neurochemical effects of cocaine in rats

Cocaine addiction is a chronic illness characterized by maladaptive drug-induced neuroplastic changes that confer lasting vulnerability to relapse. Over several weeks we observed the effects of the M₁ receptor-selective agonist VU0364572 in adult male rats that self-administer cocaine in a cocaine vs. food choice procedure. The drug showed unusual long-lasting effects, as rats gradually stopped self-administering cocaine, reallocating behavior towards the food reinforcer. The effect lasted as long as tested and at least 4 weeks. To begin to elucidate how VU0364572 modulates cocaine self-administration, we then examined its long-term effects using dual-probe in vivo dopamine and glutamate microdialysis in nucleus accumbens and medial prefrontal cortex, and ex vivo striatal dopamine reuptake. Microdialysis revealed marked decreases in cocaine-induced dopamine and glutamate outflow 4 weeks after VU0364572 treatment, without significant changes in dopamine uptake function. These lasting and marked effects of M₁ receptor stimulation reinforce our interest in this target as potential treatment of cocaine addiction. M₁ receptors are known to modulate medium spiny neuron responses to corticostriatal glutamatergic signaling acutely, and we hypothesize that VU0364572 may oppose the addiction-related effects of cocaine by causing lasting changes in this system.

Revealing a compulsive phenotype in cholinergic M4-/- mice depends on the inter-trial interval initiation settings in a five choice serial reaction time task

BACKGROUND

Muscarinic acetylcholine receptor 4 (M₄) modulates dopaminergic neurotransmission and is a target for novel treatments of schizophrenia, cognitive deficits, and addiction. Impulsive and compulsive behaviors are key traits of addiction, yet the importance of M₄ receptor signaling to these traits is poorly understood. We investigated impulsive action and compulsivity by measuring premature and perseverative responses in the five choice serial reaction time task (5CSRTT). Furthermore, we hypothesized that inter-trial interval (ITI) initiation settings affected training durations and test performances in these experiments.

METHODS

M₄^-/- and wildtype mice were trained and tested on two versions of the 5CSRTT with different ITI initiation settings. One setting, the head-in condition, allowed the ITI to start while the mouse's head remained in the reward receptacle (magazine). The other setting, the head-out condition, required the mouse to remove its head from the magazine to initiate the ITI.

RESULTS AND DISCUSSION

We did not observe differences in premature or perseverative responses in M₄^-/- mice in either condition, but found evidence of reward-related compulsive behavior in M₄^-/- mice. In the head-in condition, M₄^-/- mice were slower to acquire the 5CSRTT, had more omissions, and had longer correct response latencies than wildtype mice. In the head-out condition, genotypes did not differ in training, but M₄^-/- mice showed small decreases in accuracy. Our findings demonstrate that ITI initiation settings contribute to different training durations and tested behaviors in M₄^-/- mice, suggesting ITI initiation settings are an important consideration for the general use of the 5CSRTT.

Berberine Facilitates Extinction of Drug-Associated Behavior and Inhibits Reinstatement of Drug Seeking

A high rate of relapse is a major clinical problem among drug-addicted individuals. Persistent traces of drug-associated reward memories contribute to intense craving and often trigger relapse. A number of interventions on drug-associated memories have shown significant benefits in relapse prevention. Among them are pre- or post-extinction pharmacological manipulations that facilitate the extinction of drug-associated behavior. Berberine, a bioactive isoquinoline alkaloid, has been recently reported to provide therapeutic benefits for a number of central nervous system (CNS) disorders, including morphine addiction. The present study aimed to investigate whether berberine could serve as a post-extinction pharmacological intervention agent to reduce risks of reinstatement of drug seeking. We found that an intragastric administration of berberine at doses of 25 and 50 mg/kg during the critical time window significantly facilitated the extinction of morphine-reward related behavior in free access and confined conditioned place preference (CPP) extinction paradigms, and subsequently, it prevented reinstatement and spontaneous recovery of morphine-induced CPP in mice. Intriguingly, the berberine treatment with or without extinction training altered expression of plasticity-related proteins such as brain-derived neurotrophic factor (BDNF), AMPA receptors (GluA1 and GluA2) in the nucleus accumbens (NAc). Moreover, the post-extinction berberine treatment significantly reduced reinstatement of cocaine-induced CPP and operant intravenous self-administration (IVSA) memories in rats. Altogether, our findings suggest that extinction training combined with the post-extinction berberine treatment can facilitate extinction of drug-associated behavior making it an attractive therapeutic candidate in relapse prevention.

Targeting Muscarinic Acetylcholine Receptors for the Treatment of Psychiatric and Neurological Disorders

Muscarinic acetylcholine receptors (mAChR) play important roles in regulating complex behaviors such as cognition, movement, and reward, making them ideally situated as potential drug targets for the treatment of several brain disorders. Recent advances in the discovery of subtype-selective allosteric modulators for mAChRs has provided an unprecedented opportunity for highly specific modulation of signaling by individual mAChR subtypes in the brain. Recently, mAChR allosteric modulators have entered clinical development for Alzheimer's disease (AD) and schizophrenia, and have potential utility for other brain disorders. However, mAChR allosteric modulators can display a diverse array of pharmacological properties, and a more nuanced understanding of the mAChR will be necessary to best translate preclinical findings into successful clinical treatments.

Selective allosteric modulation of muscarinic acetylcholine receptors for the treatment of schizophrenia and substance use disorders

Muscarinic acetylcholine receptor (mAChRs) subtypes represent exciting new targets for the treatment of schizophrenia and substance use disorder (SUD). Recent advances in the development of subtype-selective allosteric modulators have revealed promising effects in preclinical models targeting the different symptoms observed in schizophrenia and SUD. M₁ PAMs display potential for addressing the negative and cognitive symptoms of schizophrenia, while M₄ PAMs exhibit promise in treating preclinical models predictive of antipsychotic-like activity. In SUD, there is increasing support for modulation of mesocorticolimbic dopaminergic circuitry involved in SUD with selective M₄ mAChR PAMs or M₅ mAChR NAMs. Allosteric modulators of these mAChR subtypes have demonstrated efficacy in rodent models of cocaine and ethanol seeking, with indications that these ligand may also be useful for other substances of abuse, as well as in various stages in the cycle of addiction. Importantly, allosteric modulators of the different mAChR subtypes may provide viable treatment options, while conferring greater subtype specificity and corresponding enhanced therapeutic index than orthosteric muscarinic ligands and maintaining endogenous temporo-spatial ACh signaling. Overall, subtype specific mAChR allosteric modulators represent important novel therapeutic mechanisms for schizophrenia and SUD.

Glucagon-Like Peptide-1 Receptor Agonist Treatment Does Not Reduce Abuse-Related Effects of Opioid Drugs

Dependence on opioids and the number of opioid overdose deaths are serious and escalating public health problems, but medication-assisted treatments for opioid addiction remain inadequate for many patients. Glucagon-like pepide-1 (GLP-1) is a gut hormone and neuropeptide with actions in peripheral tissues and in the brain, including regulation of blood glucose and food intake. GLP-1 analogs, which are approved diabetes medications, can reduce the reinforcing and rewarding effects of alcohol, cocaine, amphetamine, and nicotine in rodents. Investigations on effects of GLP-1 analogs on opioid reward and reinforcement have not been reported. We assessed the effects of the GLP-1 receptor agonist Exendin-4 (Ex4) on opioid-related behaviors in male mice, i.e., morphine-conditioned place preference (CPP), intravenous self-administration (IVSA) of the short-acting synthetic opioid remifentanil, naltrexone-precipitated morphine withdrawal, morphine analgesia (male and female mice), and locomotor activity. Ex4 treatment had no effect on morphine-induced CPP, withdrawal, or hyperlocomotion. Ex4 failed to decrease remifentanil self-administration, if anything reinforcing effects of remifentanil appeared increased in Ex4-treated mice relative to saline. Ex4 did not significantly affect analgesia. In contrast, Ex4 dose dependently decreased oral alcohol self-administration, and suppressed spontaneous locomotor activity. Taken together, Ex4 did not attenuate the addiction-related behavioral effects of opioids, indicating that GLP-1 analogs would not be useful medications in the treatment of opioid addiction. This difference between opioids and other drug classes investigated to date may shed light on the mechanism of action of GLP-1 receptor treatment in the addictive effects of alcohol, central stimulants, and nicotine.