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RationaleAlcohol use disorder remains a leading cause of preventable deaths, and current treatments have limited efficacy. Glucagon-like peptide 1 (GLP-1) receptor agonists can reduce alcohol drinking in preclinical studies, but mechanisms are still not fully understood, and data in female subjects are scarce.ObjectivesTo assess whether the GLP-1 receptor agonist exendin-4 could decrease alcohol-seeking behavior in the absence of alcohol consumption or intoxication, to compare the potency and efficacy of exendin-4 in the reduction of alcohol seeking vs. alcohol taking, and to compare effects between male and female mice.MethodsMale and female C57BL/6J mice were trained to self-administer 20% alcohol under an FR 1 schedule of reinforcement. After extinction, systemic exendin-4 (saline, 1.8, and 3.2 μg/kg) was tested in cue-induced reinstatement of alcohol seeking. Effects of exendin-4 on alcohol self-administration were tested in a separate group.ResultsExendin-4 suppressed reinstatement of alcohol seeking to extinction levels, at both doses, in the male mice, but had no effect in the female mice. Both doses of exendin-4 also significantly decreased alcohol self-administration in male mice; females again showed less pronounced effects.ConclusionsIn male mice, exendin-4 appeared more effective at suppressing alcohol seeking in the absence of alcohol relative to alcohol self-administration, consistent with modulation of alcohol reward or inhibitory control, rather than satiety or aversive effects of alcohol. We saw marked sex differences with less effect of exendin-4 in female mice, and it will be important to include both sexes in further investigations into GLP-1 receptor agonists. Rationale: Alcohol use disorder remains a leading cause of preventable deaths, and current treatments have limited efficacy. Glucagon-like peptide 1 (GLP-1) receptor agonists can reduce alcohol drinking in preclinical studies, but mechanisms are still not fully understood, and data in female subjects are scarce. Objectives: To assess whether the GLP-1 receptor agonist exendin-4 could decrease alcohol-seeking behavior in the absence of alcohol consumption or intoxication, to compare the potency and efficacy of exendin-4 in the reduction of alcohol seeking vs. alcohol taking, and to compare effects between male and female mice. Methods: Male and female C57BL/6J mice were trained to self-administer 20% alcohol under an FR 1 schedule of reinforcement. After extinction, systemic exendin-4 (saline, 1.8, and 3.2 μg/kg) was tested in cue-induced reinstatement of alcohol seeking. Effects of exendin-4 on alcohol self-administration were tested in a separate group. Results: Exendin-4 suppressed reinstatement of alcohol seeking to extinction levels, at both doses, in the male mice, but had no effect in the female mice. Both doses of exendin-4 also significantly decreased alcohol self-administration in male mice; females again showed less pronounced effects. Conclusions: In male mice, exendin-4 appeared more effective at suppressing alcohol seeking in the absence of alcohol relative to alcohol self-administration, consistent with modulation of alcohol reward or inhibitory control, rather than satiety or aversive effects of alcohol. We saw marked sex differences with less effect of exendin-4 in female mice, and it will be important to include both sexes in further investigations into GLP-1 receptor agonists.

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

Clinical evidence suggest that men are more sensitive than women to the abuse-related effects of mu-opioid agonists. In contrast, preclinical studies suggest the opposite sex difference. The aim of the present study was to clarify this discrepancy using a fentanyl vs. diluted Ensure® choice procedure to assess sex differences in opioid reinforcement. Sex differences in intravenous (IV) fentanyl self-administration were examined under a fixed-ratio (FR5) schedule, a multi-day progressive-ratio (PR) schedule for behavioral economic analysis, and a concurrent (choice) schedule of fentanyl and diluted Ensure® reinforcement in Sprague–Dawley male and female rats. The fentanyl dose-effect function under the FR5 schedule was significantly shifted upward in females compared to males. Similarly, the reinforcing effectiveness of both fentanyl (3.2 and 10 µg/kg per injection, IV) and diluted Ensure® (18 and 56%) were greater in females than in males as assessed using behavioral economic analysis, irrespective of dose or concentration. However, under a fentanyl vs. foodchoice procedure, males chose 3.2 µg/kg per injection fentanyl injections over 18%, but not 56%, diluted Ensure® at a higher percentage compared to females. Overall, these results suggest that the expression of sex differences in opioid reinforcement depends upon the schedule of reinforcement and that preclinical opioid vs. food choice procedures provide a translationally relevant measure (i.e., behavioral allocation) consistent with the direction of sex differences reported in the clinical literature.

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.

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

The reinforcing effects of cocaine are mediated by the mesolimbic dopamine system. Behavioral and neurochemical studies have shown that the cholinergic muscarinic M4 receptor subtype plays an important role in regulation of dopaminergic neurotransmission. Here we investigated for the first time the involvement of M4 receptors in the reinforcing effects of cocaine using chronic intravenous cocaine self-administration in extensively backcrossed M4 receptor knockout (M4 −/−) mice. We evaluated acquisition of cocaine self-administration in experimentally naive mice. Both cocaine self-administration and food-maintained operant behavior were evaluated under fixed ratio 1 (FR 1) and progressive ratio (PR) schedules of reinforcement. In addition, cocaine-induced dopamine release and cocaine-induced hyperactivity were evaluated. M4 −/− mice earned significantly more cocaine reinforcers and reached higher breaking points than their wild-type littermates (M4 +/+) at intermediate doses of cocaine under both FR 1 and PR schedules of reinforcement. Under the PR schedule, M4 −/− mice exhibited significantly higher response rates at the lowest liquid food concentration. In accordance with these results, cocaine-induced dopamine efflux in the nucleus accumbens and hyperlocomotion were increased in M4 −/− mice compared to M4 +/+ mice. Our data suggest that M4 receptors play an important role in regulation of the reward circuitry and may serve as a new target in the medical treatment of drug addiction.

Dopamine D2-like partial agonists such as aripiprazole have received some attention as potential pharmacotherapies for the treatment of psychostimulant addiction. However, the preclinical evaluations so far have focused on acute effects of aripiprazole.We tested the hypothesis that aripiprazole, both as acute and as chronic treatment, would preferentially decrease cocaine self-administration while sparing behavior maintained by a natural reinforcer, resulting in a shift in the allocation of behavior from cocaine-taking towards the alternative reinforcer.Rats were trained to self-administer intravenous cocaine in a concurrent choice procedure, with a palatable food as the competing reinforcer, under a fixed ratio (FR) 1 FR 5 chain schedule. Aripiprazole was then administered as continuous infusion by osmotic minipumps for 5 days, during which performance in the choice procedure was assessed daily.An intermediate dose of aripiprazole decreased cocaine self-administration and shifted the cocaine choice curve to the right as an acute treatment. However, as a chronic treatment, aripiprazole failed to decrease cocaine self-administration or cocaine choice, despite a dose-dependent decrease in overall response rates and food-maintained behavior.Our results confirm and extend earlier findings and indicate that acute administration of aripiprazole can decrease cocaine self-administration. However, based on the present data, chronic treatment with aripiprazole does not show much promise as a potential pharmacotherapy for cocaine addiction. Both acute and chronic treatment data are in agreement with published clinical findings, suggesting that the concurrent choice procedure in rats has predictive validity of efficacy in humans.

Combining strains to generate mutant mice may obscure conclusions regarding the targeted gene. Specifically, cocaine may have reduced reinforcing effects in 129 substrains compared to the C57BL/6 strain, commonly used for ES cells and breeding, respectively. We tested the hypothesis that reinforcing effects of cocaine differ between the C57BL/6J strain and two substrains of 129, 129X1/SvJ and 129S6/SvEvTac. To assess and reduce performance differences, operant responding was established with liquid food as a reinforcer and evaluated under fixed and progressive ratio schedules. Dose–effect functions for intravenous cocaine self-administration were then determined under both schedules. Finally, reinforced and nonreinforced manipulanda were reversed to assess acquisition of self-administration using a previously nonreinforced response. Relative to C57BL/6J mice, 129X1/SvJ mice showed decreased reinforcing effects of low-magnitude food and cocaine reinforcers. Dose–effect functions for cocaine self-administration were comparable between C57BL/6J and 129S6/SvEvTac mice, despite delayed acquisition of operant behaviors and rightward shifts in the food concentration–effect functions in 129S6/SvEvTac mice. A high cocaine dose clearly served as a positive reinforcer in all three strains in a reversal procedure. Relative to C57BL/6J mice, the reinforcing effects of cocaine were diminished in 129X1/SvJ mice, but only for low cocaine doses, and a similar profile was observed with food reinforcement. 129S6/SvEvTac mice required more extensive operant training than C57BL/6J mice did, but after acquisition, reinforcing effects of cocaine were similar in the two strains. We suggest that comparable phenotypes observed in gene-targeting studies may result from genetic background, whereas more profound or qualitatively different phenotypes may be more confidently attributed to targeted mutations.