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Drug, alcohol and tobacco use disorders are a global burden affecting millions of people. Despite decades of research, treatment options are sparse or missing, and relapse rates are high. Glucagon-like peptide 1 (GLP-1) is released in the small intestine, promotes blood glucose homeostasis, slows gastric emptying and reduces appetite. GLP-1 receptor agonists approved for treating Type 2 diabetes mellitus and obesity have received attention as a potential anti-addiction treatment. Studies in rodents and non-human primates have demonstrated a reduction in intake of alcohol and drugs of abuse, and clinical trials have been initiated to investigate whether the preclinical findings can be translated to patients. This review will give an overview of current findings and discuss the possible mechanisms of action. We suggest that effects of GLP-1 in alcohol and substance use disorders is mediated centrally, at least partly through dopamine signalling, but precise mechanisms are still to be uncovered. LINKED ARTICLES: This article is part of a themed issue on GLP1 receptor ligands (BJP 75th Anniversary). To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v179.4/issuetoc.

Rationale: After alcohol ingestion, the brain partly switches from consumption of glucose to consumption of the alcohol metabolite acetate. In heavy drinkers, the switch persists after abrupt abstinence, leading to the hypothesis that the resting brain may be “starved” when acetate levels suddenly drop during abstinence, despite normal blood glucose, contributing to withdrawal symptoms. We hypothesized that ketone bodies, like acetate, could act as alternative fuels in the brain and alleviate withdrawal symptoms. Objectives: We previously reported that a ketogenic diet during alcohol exposure reduced acute withdrawal symptoms in rats. Here, our goals were to test whether (1) we could reproduce our findings, in mice and with longer alcohol exposure; (2) ketone bodies alone are sufficient to reduce withdrawal symptoms (clarifying mechanism); (3) introduction of ketogenic diets at abstinence (a clinically more practical implementation) would also be effective. Methods: Male C57BL/6NTac mice had intermittent alcohol exposure for 3 weeks using liquid diet. Somatic alcohol withdrawal symptoms were measured as handling-induced convulsions; anxiety-like behavior was measured using the light-dark transition test. We tested a ketogenic diet, and a ketone monoester supplement with a regular carbohydrate-containing diet. Results: The regular diet with ketone monoester was sufficient to reduce handling-induced convulsions and anxiety-like behaviors in early withdrawal. Only the ketone monoester reduced handling-induced convulsions when given during abstinence, consistent with faster elevation of blood ketones, relative to ketogenic diet. Conclusions: These findings support the potential utility of therapeutic ketosis as an adjunctive treatment in early detoxification in alcohol-dependent patients seeking to become abstinent. Trial registration: clinicaltrials.gov NCT03878225, NCT03255031

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 signaling 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 vs. M4 stimulation, we tested whether the M4 PAM VU0152100 suppressed cocaine self-administration in mice lacking CalDAG-GEFI signaling 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 behavior by distinct mechanisms.

Glucagon-like-peptide-1 (GLP-1)-receptor agonists have been proposed as putative treatment for substance use disorders (SUD). The objective of this systematic review is to characterize the effects of GLP-1-receptor agonists on SUD-related behavioural effects of drugs, nicotine, and alcohol. The review was performed according to Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA). A search was performed in PubMed and EMBASE on June 16, 2018. The inclusion criteria were primary studies investigating the use of GLP-1-receptor agonists on behavioural endpoints related to SUD. Seventeen studies were included, investigating the effect of the GLP-1-receptor agonists exendin-4, fluoro-exendin-4, liraglutide, AC3174 and GLP-1 (7–36) on SUD-related behavioural effects of ethanol, cocaine, amphetamine, and/or nicotine. All studies used rodents as subjects. Nine of the studies dealt with ethanol, six with cocaine, two with amphetamine, and two with nicotine. Most studies investigated acute treatment effects, finding a significant effect in all but one experiment. A few studies investigated more chronic effects, but only on ethanol. All the studies reported sustained effects. Eleven studies tested more than one dose, finding a dose-related response in ten out of thirteen experiments. Six studies report a central effect through intra-cerebral administration or by using mice in which the central GLP-1-receptors had been inactivated. In conclusion, a solid body of evidence documents acute effects of GLP-1-receptor agonist treatment on behavioural effects of alcohol, nicotine, amphetamine and cocaine. Documentation of effect of more chronic GLP-1-receptor stimulation on these behaviours is limited.

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.

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. natural reinforcer 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 four 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 dramatic decreases in cocaine-induced dopamine and glutamate outflow four weeks after VU0364572 treatment, without significant changes in dopamine uptake function. These lasting and dramatic 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.

RATIONALE: Stimulating muscarinic M(1)/M(4) receptors can blunt reinforcing and other effects of cocaine. A hallmark of addiction is continued drug seeking/craving after abstinence, and relapse. OBJECTIVES: We tested whether stimulating M(1) and/or M(4) receptors could facilitate extinction of cocaine seeking, and whether this was mediated via memory consolidation. METHODS: Experimentally naïve C57BL/6J mice were allowed to acquire self-administration of intravenous cocaine (1 mg/kg/infusion) under a fixed-ratio 1 schedule of reinforcement. Then, saline was substituted for cocaine until responding extinguished to £30% of cocaine-reinforced responding. Immediately after each extinction session, mice received saline, the M(1)/M(4) receptor-preferring agonist xanomeline, the M(1) receptor-selective allosteric agonist VU0357017, the M(4) receptor-selective positive allosteric modulator VU0152100, or VU0357017+VU0152100. In additional experiments, xanomeline was administered delayed after the session, or in the home cage before extinction training began. In the latter group, reinstatement of responding by a 10 mg/kg cocaine injection was also tested. RESULTS: Stimulating M(1) + M(4) receptors significantly expedited extinction from 17.2 sessions to 8.3 using xanomeline or 7.8 using VU0357017+VU0152100. VU0357017 alone and VU0152100 alone did not significantly modify rates of extinction (12.6 and 14.6 sessions). The effect of xanomeline was fully preserved when administered delayed after or unpaired from extinction sessions (7.5 and 6.4 sessions). Xanomeline-treated mice showed no cocaine-induced reinstatement. CONCLUSIONS: These findings show that M(1)/M(4) receptor stimulation can decrease cocaine seeking in mice. The effect lasted beyond treatment duration, and was not dependent upon extinction learning. This suggests that M(1)/M(4) receptor stimulation modulated or reversed some neurochemical effects of cocaine exposure.

Glucagon-like peptide-1 (GLP-1) is a gut peptide that regulates food intake and glucose metabolism. GLP-1 is also produced and released in the brain, and GLP-1 receptors are expressed in brain regions important for alcohol and drug reward, and for the development of addiction. GLP-1 receptor agonists can decrease alcohol intake acutely in rodents. However, alcohol use disorder is a chronic condition that requires treatments to be effective in promoting abstinence from excessive alcohol consumption over time. Here, we assessed the effect of daily treatment with the GLP-1 receptor agonist Exendin-4 in an assay of relapse-like drinking in socially housed mice. Male C57BL/6NTac mice were allowed continuous access to alcohol without tastant in the home cage for 37days. Then, alcohol bottles were removed and Exendin-4 (1.5μg/kg/day) or saline was administered subcutaneously for 8days during alcohol deprivation. Treatment continued for 8 additional days after reintroducing access to alcohol. A high-precision automated fluid consumption system was used to monitor intake of alcohol and water, drinking kinetics, and locomotor activity. Exendin-4 prevented the deprivation-induced increase in alcohol intake observed in control mice, without significantly affecting total fluid intake, body weight, or locomotor activity. The reduced alcohol intake was caused by a protracted latency to the first drink of alcohol and a reduced number of drinking bouts, while bout size and duration were not affected. The effect was maintained undiminished throughout the treatment period. These findings support the possible use of GLP-1 receptor agonists in the treatment of alcohol use disorder.

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.

Background Glucagon-like peptide 1 (GLP-1) receptor agonists have been shown to decrease ethanol (EtOH) drinking in rodent assays. The GLP-1 system also powerfully modulates food and fluid intake, gastrointestinal functions, and metabolism. To begin to understand the neurobiological mechanisms by which GLP-1 receptor ligands may be able to control EtOH intake, it is important to ascertain whether they can modulate the direct reinforcing effects of EtOH, without the confound of effects on ingestive behaviors generally. Methods We trained experimentally naive, free-fed C57BL/6J mice to self-administer EtOH intravenously. Once stable EtOH intake was acquired, we tested the effect of acute pretreatment with the GLP-1 receptor agonist Exendin-4. Effect of Exendin-4 on operant behavior reinforced by a palatable liquid food was similarly evaluated as a control. Results Intravenous EtOH functioned as a positive reinforcer in over half the mice tested. In mice that acquired self-administration, EtOH intake was high, indeed, reaching toxic doses; 3.2 μg/kg Exendin-4 decreased intravenous EtOH intake by at least 70%, but had no significant effect on food-maintained operant responding. Conclusions This experiment produced 2 main conclusions. First, although technically challenging and yielding only moderate throughput, the intravenous self-administration procedure in mice is feasible, and sensitive to pharmacological manipulations. Second, GLP-1 receptor agonists can powerfully attenuate voluntary EtOH intake by directly modulating the reinforcing effects of EtOH. These findings support the potential usefulness of GLP-1 receptor ligands in the treatment of alcohol use disorder.

BACKGROUND: Preclinical studies in rodents have demonstrated inhibitory effects of glucagon-like peptide-1 (GLP-1) receptor stimulation on alcohol consumption. The effects of GLP-1 receptor stimulation on alcohol intake in primates have not been investigated. METHODS: We performed placebo-controlled studies on the effects of the GLP-1 receptor agonists exenatide and liraglutide on alcohol consumption in alcohol-preferring male African vervet monkeys. Monkeys selected for voluntary alcohol drinking were observed for at least ten days of baseline drinking and allocated to drug or vehicle (n=11–12 per group) balanced with respect to alcohol intake. Monkeys had access to alcohol 4h/day. In a first study, monkeys were treated with exenatide 0.04 mg/kg or vehicle once weekly for five weeks to obtain steady state plasma levels. In a second study, monkeys were treated daily with liraglutide (increasing dosing, 10 to 50 μg/kg/day) or vehicle over two weeks. In both studies, access to alcohol was suspended during drug up-titration. Then, alcohol was again made available 4h/day and treatment was continued for two weeks, during which alcohol intake was recorded. Observation of alcohol intake was continued for a week of drug washout. RESULTS: Liraglutide and to a lesser extent exenatide significantly reduced alcohol consumption without causing any signs of emesis and with no effect on water intake as compared to vehicle. CONCLUSIONS: The present study demonstrates for the first time that GLP-1 receptor agonists can reduce voluntary alcohol drinking in non-human primates. The data substantiate the potential usefulness of GLP-1 receptor agonists in the treatment of alcohol use disorder.

Background Alcohol use disorder is underdiagnosed and undertreated, and up to 50% of alcohol-abstinent patients diagnosed with alcohol dependence relapse within the first year of treatment. Current treatments for the maintenance of alcohol abstinence in patients with alcohol use disorder have limited efficacy, and there is an urgent need for novel treatment strategies. Decreased cerebral glucose metabolism and increased brain uptake of acetate were recently reported in heavy drinkers, relative to controls. Given the switch of metabolic fuel from glucose to acetate in the alcohol-dependent brain, we investigated the potential therapeutic benefit of a ketogenic diet in managing alcohol withdrawal symptoms during detoxification. Methods Male Sprague Dawley rats fed either ketogenic or regular diet were administered ethanol or water orally, twice daily for 6 days while the diet conditions were maintained. Abstinence symptoms were rated 6, 24, 48, and 72 hours after the last alcohol administration. Results Maintenance on a ketogenic diet caused a significant decrease in the alcohol withdrawal symptoms' “rigidity” and “irritability.” Conclusions Our preclinical pilot study suggests that a ketogenic diet may be a novel approach for treating alcohol withdrawal symptoms in humans.

After publication of this paper, the authors determined an error in Fig. 4. Below is the correct Fig. 4.

Muscarinic cholinergic M(1) and M(4) receptors may participate in schizophrenia's etiology and have been proposed as targets for antipsychotic medications.Here, we investigated the involvement of these receptors in behavioral measures pertinent to schizophrenia using knockout mice lacking M(1) receptors (M(1)-/-), M(4) receptors (M(4)-/-), or both (M(1)-/-M(4)-/-).We measured prepulse inhibition (PPI) of startle without drugs and after treatment with scopolamine (0.32-1.8 mg/kg), xanomeline (3.2 mg/kg), oxotremorine (0.032-0.1 mg/kg), clozapine (1.0-5.6 mg/kg), or haloperidol (0.32-3.2 mg/kg).In female (but not male) mice, combined deletion of both M(1) and M(4) receptors decreased PPI relative to wild-type mice, while knockout of either receptor alone had no significant effect. Scopolamine disrupted PPI in wild-type and M(4)-/- mice, but not in female M(1)-/-M(4)-/- or female M(1)-/- mice. When administered before scopolamine, xanomeline restored PPI in wild-type mice and M(1)-/- mice, but not in M(4)-/- mice. In contrast, pretreatment with oxotremorine increased PPI regardless of genotype. Effects of clozapine and haloperidol on PPI were not hindered by either mutation.Deletion of both M(1) and M(4) receptors can disrupt PPI, suggesting that (at least partially redundant) M(1) and M(4) receptor-dependent functions are involved in sensorimotor gating mechanisms. PPI-disrupting effects of muscarinic antagonists appeared dependent upon M(1) receptor blockade. Our data also suggest that xanomeline exerts antipsychotic-like effects mainly through M(4) receptor stimulation, while stimulation of non-M(1)/M(4) subtypes may also have antipsychotic potential. Finally, our results do not support a role of M(1)/M(4) receptors in mediating antipsychotic-like effects of clozapine.

Purpose To evaluate the feasibility of using manganese-based MR imaging contrast agent EVP-ABD to detect diffuse liver disease in an established rat hepatitis model. Materials and Methods Hepatitis was induced by administration of CCl4 in corn oil vehicle to rats intraperitoneally. MR images were acquired on a 3T scanner using a volume coil ≈36 hours after the administration of CCl4. EVP-ABD was administered via a tail vein at a dose of 10 μmol/kg. Multi-TI turboflash images were acquired to evaluate liver R1 (=1/T1) values before and after the EVP-ABD administration. Eighteen rats received various doses of CCl4 and completed pre- and postcontrast MRI scans and liver histologic evaluation. Results The liver R1 after the EVP-ABD administration and the change of the liver R1 before and after the administration, ΔR1, show significant correlations with the CCl4 dose. A significant correlation was also found between the histologic scores and the CCl4 doses despite known variability in the relationship of CCl4 dose to histology. A significant correlation was found between the histologic score and ΔR1. Conclusion Our results indicate that EVP-ABD-enhanced MRI can detect diffuse liver disease generated by CCl4 based on the significant correlation between proton R1 in liver following EVP-ABD and the CCl4 doses as well as the histologic scores. J. Magn. Reson. Imaging 2008;27:1317–1321. © 2008 Wiley-Liss, Inc.

Chronic intravenous drug self-administration in rodents is a useful procedure for predicting the abuse liability of novel drugs in humans, for evaluating candidate treatments for drug abuse and dependence, and for studying the biological basis of addiction. This unit focuses on recent technical innovations for conducting long-term studies of i.v. drug self-administration behavior in healthy, freely moving rats and mice. Included are protocols for construction, implantation, and maintenance of chronic indwelling jugular catheters, commentary on critical parameters, a troubleshooting guide, and anticipated results.

Schizophrenic patients show decreased measures of sensorimotor gating, such as prepulse inhibition of startle (PPI). In preclinical models, these measures may be used to predict antipsychotic activity. While current antipsychotic drugs act largely at dopamine receptors, the muscarinic acetylcholine receptors offer promising novel pharmacotherapy targets. Of these, the M(5) receptor gene was recently implicated in susceptibility to schizophrenia. Due to the lack of selective ligands, muscarinic receptor knockout mice have been generated to elucidate the roles of the five receptor subtypes (M(1)-M(5)).Here, we used M(5) receptor knockout (M(5)-/-) mice to investigate the involvement of M(5) receptors in behavioral measures pertinent to schizophrenia. We tested the hypothesis that disruption of M(5) receptors affected PPI or the effects of muscarinic or dopaminergic agents in PPI or psychomotor stimulation.We measured PPI in M(5)-/-, heterozygous and wild-type mice without drugs, and with clozapine (0.56-3.2 mg/kg) or haloperidol (0.32-3.2 mg/kg) alone, and as pretreatment to D: -amphetamine. In addition, we evaluated locomotor stimulation by the muscarinic antagonist trihexyphenidyl (0.56-56 mg/kg) and by cocaine (3.2-56 mg/kg).The M(5)-/- mice showed decreased PPI relative to wild-type mice, and clozapine appeared to reduce this difference, while haloperidol increased PPI regardless of genotype. The M(5)-/- mice also showed more locomotor stimulation by trihexyphenidyl than wild-type mice, while cocaine had similar effects between genotypes.These data suggest that disruption of the M(5) receptor gene affected sensorimotor gating mechanisms, increased sensitivity to clozapine and to the psychostimulant effects of muscarinic antagonists without modifying the effect of dopaminergic drugs.

Neuropeptide Y (NPY) induces analgesic-like effects after central administration across diverse pain models in rodents. In spinal pain models, previous studies indicate a prominent role for Y 1 receptors at mediating this effect of NPY. In supraspinal pain models like the hot plate test, the NPY receptors involved have not been thoroughly explored. By intracerebroventricular (i.c.v.) administration of selective NPY receptor ligands, the possible involvement of Y 5 receptors in analgesic-like mechanisms was investigated using the hot plate test in rats. Both NPY and selective Y 5 agonists induced analgesic-like effects as revealed by prolonged hot plate latencies. Further consistent with a role for Y 5 receptors, pretreatment with a selective Y 5 receptor antagonist blocked the Y 5 agonist-induced analgesic-like effect. The present study indicates involvement of Y 5 receptors probably at the supraspinal level in mediation of NPY agonist-induced analgesic-like effects in the hot plate test.

Chronic intravenous drug self-administration in rodents is a useful procedure for predicting the abuse liability of novel drugs in humans, for evaluating candidate treatments for drug abuse and dependence, and for studying the biological basis of addiction. Despite the technical challenge in achieving chronic self-administration behavior in the mouse species, researchers are increasingly using genetically engineered mice to investigate the role of specific genes in abuse-related effects of drugs. This review focuses on recent technical innovations as well as theoretical considerations for comparing intravenous (i.v.) drug self-administration behavior between mouse strains, including mice with targeted mutations. Part I of the present article describes techniques for successfully conducting self-administration studies in mice, including advantages, disadvantages and possible implications of employing various experimental approaches. Part II provides a review of recent data that address how the genetic background on which mutations are expressed may influence results from gene-targeting studies.

The reinforcing effects of cocaine have been related to increased extracellular concentrations of dopamine in the ventral striatum. Several studies suggest that M5muscarinic receptors facilitate striatal dopamine release. We tested the hypothesis that the reinforcing effects of cocaine are decreased in M5receptor-deficient mice using chronic intravenous cocaine self-administration in extensively backcrossed mice. We also assessed whether operant performance generally, rather than cocaine self-administration specifically, was altered in the mutant mice. To this end, we evaluated both food-maintained operant behavior and cocaine self-administration under a fixed ratio 1 and a progressive ratio (PR) schedule of reinforcement. We also evaluated acquisition of self-administration in experimentally naive mice using several doses of cocaine. M5receptor deletion decreased self-administration of low to moderate doses of cocaine under a PR schedule of reinforcement and diminished acquisition of self-administration of a low dose in experimentally naive mice. We found no differences between genotypes in food-maintained behavior. The present study extends our previous findings using backcrossed mice and covering various experimental conditions. Our results indicate that M5receptor deletion diminished the reinforcing effects of low doses of cocaine and identified specific conditions under which this may be observed.