Navacaprant, a novel and selective kappa opioid receptor antagonist, has no agonist properties implicated in opioid-related abuse.

Kappa opioid receptors (KORs) are implicated in the pathophysiology of various psychiatric and neurological disorders creating interest in targeting the KOR system for therapeutic purposes. Accordingly, navacaprant (NMRA-140) is a potent, selective KOR antagonist being evaluated as a treatment for major depressive disorder. In the present report, we have extended the pharmacological characterization of navacaprant by further demonstrating its selective KOR antagonist properties and confirming its lack of agonist activity at KORs and related targets involved in opioid-related abuse. Using CHO-K1 cells expressing human KOR, mu (MOR), or delta (DOR) opioid receptors, navacaprant demonstrated selective antagonist properties at KOR (IC50 = 0.029 µM) versus MOR (IC50 = 3.3 µM) and DOR (IC50 > 10 µM) in vitro. In vivo, navacaprant (10-30 mg/kg, i.p.) dose-dependently abolished KOR-agonist induced analgesia in the mouse tail-flick assay. Additionally, navacaprant (10, 30 mg/kg, p.o.) significantly reduced KOR agonist-stimulated prolactin release in mice and rats, confirming KOR antagonism in vivo. Navacaprant showed no agonist activity at any opioid receptor subtype (EC50 > 10 µM) in vitro and exhibited no analgesic effect in the tail-flick assays at doses ≤100 mg/kg, p.o. thereby confirming a lack of opioid receptor agonist activity in vivo. Importantly, navacaprant did not alter extracellular dopamine concentrations in the nucleus accumbens shell of freely-moving rats following doses ≤100 mg/kg, p.o., whereas morphine (10, 20 mg/kg, i.p.) significantly increased dopamine levels. These results demonstrate that navacaprant is a KOR-selective antagonist with no pharmacological properties implicated in opioid-related abuse.

A critical assessment of the abuse, dependence and associated safety risks of naturally occurring and synthetic cannabinoids.

Various countries and US States have legalized cannabis, and the use of the psychoactive1 and non-psychoactive cannabinoids is steadily increasing. In this review, we have collated evidence from published non-clinical and clinical sources to evaluate the abuse, dependence and associated safety risks of the individual cannabinoids present in cannabis. As context, we also evaluated various synthetic cannabinoids. The evidence shows that delta-9 tetrahydrocannabinol (Δ9-THC) and other psychoactive cannabinoids in cannabis have moderate reinforcing effects. Although they rapidly induce pharmacological tolerance, the withdrawal syndrome produced by the psychoactive cannabinoids in cannabis is of moderate severity and lasts from 2 to 6 days. The evidence overwhelmingly shows that non-psychoactive cannabinoids do not produce intoxicating, cognitive or rewarding properties in humans. There has been much speculation whether cannabidiol (CBD) influences the psychoactive and potentially harmful effects of Δ9-THC. Although most non-clinical and clinical investigations have shown that CBD does not attenuate the CNS effects of Δ9-THC or synthetic psychoactive cannabinoids, there is sufficient uncertainty to warrant further research. Based on the analysis, our assessment is cannabis has moderate levels of abuse and dependence risk. While the risks and harms are substantially lower than those posed by many illegal and legal substances of abuse, including tobacco and alcohol, they are far from negligible. In contrast, potent synthetic cannabinoid (CB1/CB2) receptor agonists are more reinforcing and highly intoxicating and pose a substantial risk for abuse and harm. 1 "Psychoactive" is defined as a substance that when taken or administered affects mental processes, e.g., perception, consciousness, cognition or mood and emotions.

Stimulant prodrugs: A pharmacological and clinical assessment of their role in treating ADHD and binge-eating disorder.

In this review, we critically evaluate the contribution of prodrugs to treating two related psychiatric disorders, attention-deficit hyperactivity disorder (ADHD) and binge-eating disorder (BED). ADHD is characterized by inattentiveness, distractibility, impulsiveness, and hyperactivity. BED is also an impulse-control disorder which leads to frequent, compulsive episodes of excessive eating (binges). Lisdexamfetamine (LDX; prodrug of d-amphetamine) is approved to treat both ADHD and BED. Serdexmethylphenidate (SDX; prodrug of d-threo-methylphenidate) is not clinically approved as monotherapy but, in a fixed-dose combination with immediate release d-threo-methylphenidate (Azstarys™), SDX is approved for managing ADHD in children/adolescents. The pharmacological actions of a stimulant mediate both its efficacy and side-effects. Therefore, daily management of ADHD or BED to maintain optimum efficacy and tolerability places highly restrictive requirements on the pharmacokinetic/pharmacodynamic (PK/PD) characteristics of stimulant medications, especially prodrugs. Prodrugs must have good bioavailability and rapid metabolism to provide therapeutic efficacy soon after morning dosing combined with providing stimulant coverage throughout the day/evening. A wide selection of dosages and linear PK for the prodrug and its active metabolite are essential requirements for treatment of these conditions. The proposed neurobiological causes of ADHD and BED are described. The chemical, pharmacological and PK/PD properties responsible for the therapeutic actions of the prodrugs, LDX and SDX, are compared and contrasted. Finally, we critically assess their contribution as ADHD and BED medications, including advantages over their respective active metabolites, d-amphetamine and d-threo-methylphenidate, and also their potential for misuse and abuse.

Adrenoceptors: A Focus on Psychiatric Disorders and Their Treatments.

Research into the involvement of adrenoceptor subtypes in the cause(s) of psychiatric disorders is particularly challenging. This is partly because of difficulties in developing animal models that recapitulate the human condition but also because no evidence for any causal links has emerged from studies of patients. These, and other obstacles, are outlined in this chapter. Nevertheless, many drugs that are used to treat psychiatric disorders bind to adrenoceptors to some extent. Direct or indirect modulation of the function of specific adrenoceptor subtypes mediates all or part of the therapeutic actions of drugs in various psychiatric disorders. On the other hand, interactions with central or peripheral adrenoceptors can also explain their side effects. This chapter discusses both aspects of the field, focusing on disorders that are prevalent: depression, schizophrenia, anxiety, attention-deficit hyperactivity disorder, binge-eating disorder, and substance use disorder. In so doing, we highlight some unanswered questions that need to be resolved before it will be feasible to explain how changes in the function of any adrenoceptor subtype affect mood and behavior in humans and other animals.

Psychedelic drug abuse potential assessment for new drug applications and controlled substance scheduling: A United States perspective.

BACKGROUND: Psychedelics are an increasingly active area of research and pharmaceutical development. This includes abuse potential assessment to better understand their pharmacological mechanisms and effects and guide controlled substance regulation. Psychedelics pose challenges to abuse assessments to ensure valid, reliable, and generalizable outcomes and safe study conduct. FINDINGS: Key nonclinical techniques, for example, receptor binding and functional assays in vitro, and nonclinical physical dependence determinations, are easily adaptable to psychedelics. However, the entactogens (weak reinforcers) and hallucinogens (non-reinforcers) require more flexible approaches than typically recommended by regulatory agencies. Phase 1 pharmacokinetic/pharmacodynamic safety studies and Phases 2/3 efficacy/safety trials with systematic monitoring of abuse-related adverse events are readily applicable to psychedelics. Human abuse trials require modification because supratherapeutic doses may not be safe and procedures, for example, personal monitors to manage serious adverse events, might bias outcomes. RECOMMENDATIONS: Abuse-related studies for psychedelics requiring approval by Food and Drug Administration and other agencies should take into consideration existing knowledge that will vary from extensive, for example, psilocybin, to zero for novel hallucinogens and entactogens. Many abuse assessments can be reasonably applied to animals and humans without compromising scientific integrity. Modification of existing techniques and incorporating a broader range of nonclinical tests should ensure generalizable outcomes. Human abuse studies merit reconsideration and possible modification to ensure safety and validity for psychedelic drug evaluation. Other nonclinical and clinical methods can provide evaluations of the pharmacological equivalence of test drugs to known drugs of abuse to provide context to the abuse assessment and guide drug scheduling.

Psychedelic drug abuse potential assessment research for new drug applications and Controlled Substances Act scheduling.

New medicines containing classic hallucinogenic and entactogenic psychedelic substance are under development for various psychiatric and neurological disorders. Many of these, including psilocybin, lysergic acid diethylamide (LSD), and 3,4-methylenedioxymethamphetamine (MDMA) are Schedule I controlled substances of the United States Controlled Substances Act (US CSA), and similarly controlled globally. The implications of the CSA for research and medicines development, the path to approval of medicines, and their subsequent removal from Schedule I in the US are discussed. This entire process occurs within the framework of the CSA in the US and its counterparts internationally in accordance with international drug control treaties. Abuse potential related research in the US informs the eight factors of the CSA which provide the basis for rescheduling actions that must occur upon approval of a drug that contains a Schedule I substance. Abuse-related research also informs drug product labeling and the risk evaluation and mitigation strategies (REMS) will likely be required for approved medicines. Human abuse potential studies typically employed in CNS drug development may be problematic for substances with strong hallucinogenic effects such as psilocybin, and alternative strategies are discussed. Implications for research, medicinal development, and controlled substance scheduling are presented in the context of the US CSA and FDA requirements with implications for global regulation. We also discuss how abuse-related research can contribute to understanding mechanisms of action and therapeutic effects as well as the totality of the effects of the drugs on the brain, behavior, mood, and the constructs of spirituality and consciousness.

New Drugs to Treat ADHD: Opportunities and Challenges in Research and Development.

Since the landmark MTA (Multimodal Treatment of ADHD) trial unequivocally demonstrated the efficacy of methylphenidate, catecholaminergic drugs, especially stimulants, have been the therapeutic mainstay in treatment of Attention-Deficit Hyperactivity Disorder (ADHD). We review the new drugs which have entered the ADHD formulary. The lessons learned from drug-candidates that have succeeded in clinical trials together with those that have not have also been considered. What emerges confirms and consolidates the hypothesis that clinically effective ADHD drugs indirectly or directly increase catecholaminergic neurotransmission in the prefrontal cortex (PFC). Attempts to enhance catecholaminergic signalling through modulatory neurotransmitter systems or cognitive-enhancing drugs have all failed. New drugs approved for ADHD are catecholaminergic reuptake inhibitors and releasing agents, or selective noradrenaline reuptake inhibitors. Triple reuptake inhibitors with preferential effects on dopamine have not been successful. The substantial number of failures probably accounts for a continued focus on developing novel catecholaminergic and noradrenergic drugs, and a dearth of drug-candidates with novel mechanisms entering clinical development. However, substantial improvements in ADHD pharmacotherapy have been achieved by the almost exclusive use of once-daily medications and prodrugs, e.g. lisdexamfetamine and Azstarys®, which improve compliance, deliver greater efficacy and reduce risks for diversion and abuse.

What pharmacological interventions are effective in binge-eating disorder? Insights from a critical evaluation of the evidence from clinical trials.

Binge-eating disorder (BED) is the commonest eating disorder and an important causal factor in obesity. Lisdexamfetamine is the only approved pharmacological treatment. Many drugs have been clinically evaluated and several were described as potentially promising treatments. A comprehensive reassessment of the evidence from these clinical trials has been performed. The questions to be answered were: (1) Does the evidence support claims of efficacy? (2) What pharmacological mechanisms show promise for developing new BED drugs? (3) What are the clinical implications for treating BED? PubMed and internal database searches identified every available published drug trial in BED. The trials and their results were summarised and reviewed to re-evaluate the evidence. Factors taken into consideration included psychiatric diagnosis, primary endpoint, secondary outcome measures, trial size, blinding and controls, drop-out rates, placebo response rates and weight-loss. Drugs were classified according to their pharmacology and therapeutic indication to determine which mechanisms were effective and to provide insights into the psychopathology of BED. For most drugs, robust evidence of efficacy in BED is insubstantial or absent. Some catecholaminergic drugs developed for ADHD are also effective in BED; other pharmacological mechanisms are weakly efficacious at best. Reducing BED severity has little impact on weight. Conversely, weight-loss from anti-obesity therapy is ineffective in ameliorating the psychopathological drivers of BED. (1) BED is a psychiatric not a metabolic disorder. (2) Weight-loss drugs are generally ineffective in BED. (3) Efficacy in BED is restricted to powerful catecholaminergic drugs. (4) Drugs acting via noradrenaline, 5-HT, GABA, carbonic anhydrase inhibition, opioid receptors and various ion channels are generally minimally effective at best. (5) Efficacy in BED is dependent on treating its core psychopathology; reducing impulsivity and compulsivity and increasing cognitive restraint over eating. (6) Obese subjects with BED may benefit from separate treatments for these two disorders.

Ulotaront, a novel TAAR1 agonist with 5-HT1A agonist activity, lacks abuse liability and attenuates cocaine cue-induced relapse in rats.

BACKGROUND: Ulotaront (SEP-363856) is a trace amine-associated receptor 1 (TAAR1) agonist with 5-hydroxytryptamine type 1A (5-HT1A) agonist activity that is currently in Phase 3 clinical development for the treatment of schizophrenia. Unlike available antipsychotics, the efficacy of ulotaront is not mediated by blockade of dopamine D2 or serotonin 5-HT2A receptors. In a short-term randomized clinical trial, ulotaront has demonstrated significant efficacy in the treatment of adults with an acute exacerbation of schizophrenia. Given ulotaront's novel mechanism of action a series of preclinical studies were performed to evaluate its potential abuse liability. METHODS: A battery of studies were conducted in male and female rats to evaluate whether ulotaront produces behavioral changes suggestive of human abuse potential. In addition, studies were undertaken to probe the potential for ulotaront to block reinstatement of cocaine-seeking behavior in male rats. RESULTS: Ulotaront was not self-administered by rats trained to self-administer amphetamine, cocaine, or heroin. The subjective qualities of ulotaront were distinct from those produced by amphetamine in a drug discrimination procedure. Ulotaront, and buspirone, a non-scheduled anxiolytic with 5-HT1A agonism, partially generalized to the interoceptive cue elicited by 3, 4-methylenedioxymethamphetamine (MDMA). In addition, ulotaront demonstrated a trend to reduce cocaine-primed induced reinstatement, and dose-dependently reduced cue-reinstated responding. CONCLUSION: The current results suggest that the TAAR1/5-HT1A agonist ulotaront is not likely to pose a risk for recreational abuse in humans and may have potential therapeutic utility as a treatment of substance use disorders.

Prospects for new drugs to treat binge-eating disorder: Insights from psychopathology and neuropharmacology.

BACKGROUND: Binge-eating disorder (BED) is a common psychiatric condition with adverse psychological and metabolic consequences. Lisdexamfetamine (LDX) is the only approved BED drug treatment. New drugs to treat BED are urgently needed. METHODS: A comprehensive review of published psychopathological, pharmacological and clinical findings. RESULTS: The evidence supports the hypothesis that BED is an impulse control disorder with similarities to ADHD, including responsiveness to catecholaminergic drugs, for example LDX and dasotraline. The target product profile (TPP) of the ideal BED drug combines treating the psychopathological drivers of the disorder with an independent weight-loss effect. Drugs with proven efficacy in BED have a common pharmacology; they potentiate central noradrenergic and dopaminergic neurotransmission. Because of the overlap between pharmacotherapy in attention deficit hyperactivity disorder (ADHD) and BED, drug-candidates from diverse pharmacological classes, which have already failed in ADHD would also be predicted to fail if tested in BED. The failure in BED trials of drugs with diverse pharmacological mechanisms indicates many possible avenues for drug discovery can probably be discounted. CONCLUSIONS: (1) The efficacy of drugs for BED is dependent on reducing its core psychopathologies of impulsivity, compulsivity and perseveration and by increasing cognitive control of eating. (2) The analysis revealed a large number of pharmacological mechanisms are unlikely to be productive in the search for effective new BED drugs. (3) The most promising areas for new treatments for BED are drugs, which augment noradrenergic and dopaminergic neurotransmission and/or those which are effective in ADHD.

Contrasting effects of the α7 nicotinic receptor antagonist methyllycaconitine in different rat models of heroin reinstatement.

BACKGROUND: α7 Nicotinic acetylcholine receptors are implicated in the reinstatement of drug-seeking, an important component of relapse. We showed previously that the α7 nicotinic acetylcholine receptor antagonist, methyllycaconitine, specifically attenuated morphine-primed reinstatement of conditioned place preference in rodents and this effect was mediated in the ventral hippocampus. AIMS: The purpose of this study was to evaluate α7 nicotinic acetylcholine receptor antagonism in reinstatement of the conditioned place preference for the more widely abused opioid, heroin, and to compare the effect of α7 nicotinic acetylcholine receptor blockade on reinstatement of heroin-seeking and heroin self-administration in an intravenous self-administration model of addictive behaviour. METHODS: Rats were trained to acquire heroin conditioned place preference or heroin self-administration; both followed by extinction of responding. Methyllycaconitine or saline was given prior to reinstatement of drug-primed conditioned place preference, or drug-prime plus cue-induced reinstatement of intravenous self-administration, using two protocols: without delivery of heroin in response to lever pressing to model heroin-seeking, or with heroin self-administration, using fixed and progressive ratio reward schedules, to model relapse. RESULTS: Methyllycaconitine had no effect on acquisition of heroin conditioned place preference or lever-pressing for food rewards. Methyllycaconitine blocked reinstatement of heroin-primed conditioned place preference. Methyllycaconitine did not prevent drug-prime plus cue-induced reinstatement of heroin-seeking, reinstatement of heroin self-administration, or diminish the reinforcing effect of heroin. CONCLUSIONS: The α7 nicotinic acetylcholine receptor antagonist, methyllycaconitine, prevented reinstatement of the opioid conditioned place preference, consistent with a role for α7 nicotinic acetylcholine receptors in the retrieval of associative memories of drug liking. The lack of effect of methyllycaconitine in heroin-dependent rats in two intravenous self-administration models suggests that α7 nicotinic acetylcholine receptors do not play a role in later stages of heroin abuse.

Investigation of the reinforcing potential of samidorphan and naltrexone by fixed and progressive ratio intravenous self-administration testing in heroin-maintained rats.

BACKGROUND: Samidorphan is a novel µ-opioid antagonist with low intrinsic activity at κ- and δ-opioid receptors. AIMS: Because samidorphan is central nervous system-active, we investigated whether samidorphan (13.6, 40.8, 68 µg/kg/injection) served as a positive reinforcer in rats trained to self-administer heroin on a fixed ratio-5 schedule. Samidorphan's relative reinforcing effect was evaluated by progressive ratio/break-point determination. Naltrexone (13.6, 40.8, 68 µg/kg/injection) and heroin (7.5, 15, 25 µg/kg/injection) were comparators. RESULTS: All heroin doses maintained self-administration on fixed ratio-5 and progressive ratio/break-points at levels significantly greater than saline. Samidorphan and naltrexone had similar profiles on fixed ratio-5 with one samidorphan dose serving as a positive reinforcer and one naltrexone dose showing a strong trend ( p=0.053) for positive reinforcement. The numbers of injections of every samidorphan and naltrexone dose were significantly lower than all heroin doses. The numbers of self-administered samidorphan and naltrexone injections/session on fixed ratio-5 were not significantly different from one another. The mean inter-injection intervals for heroin were significantly shorter than for saline, whereas those of samidorphan and naltrexone were not. Progressive ratio break-points for samidorphan and naltrexone were not different from saline except for the highest dose of samidorphan. In addition, the progressive ratio break-points for samidorphan were not significantly different from those of naltrexone and were significantly lower than heroin. The samidorphan unit-doses evaluated in self-administration yielded plasma concentrations ranging between 25-109% and 10-45% of the maximum concentration values in humans. CONCLUSIONS: Overall, the profiles of samidorphan and naltrexone, which has no abuse liability, were similar in this model.

Inhibition of alpha7 nicotinic receptors in the ventral hippocampus selectively attenuates reinstatement of morphine-conditioned place preference and associated changes in AMPA receptor binding.

Recurrent relapse is a major problem in treating opiate addiction. Pavlovian conditioning plays a role in recurrent relapse whereby exposure to cues learned during drug intake can precipitate relapse to drug taking. α7 nicotinic acetylcholine receptors (nAChRs) have been implicated in attentional aspects of cognition and mechanisms of learning and memory. In this study we have investigated the role of α7 nAChRs in morphine-conditioned place preference (morphine-CPP). CPP provides a model of associative learning that is pertinent to associative aspects of drug dependence. The α7 nAChR antagonist methyllycaconitine (MLA; 4 mg/kg s.c.) had no effect on the acquisition, maintenance, reconsolidation or extinction of morphine-CPP but selectively attenuated morphine-primed reinstatement of CPP, in both mice and rats. Reinstatement of morphine-CPP in mice was accompanied by a selective increase in [3 H]-AMPA binding (but not in [3 H]-MK801 binding) in the ventral hippocampus that was prevented by prior treatment with MLA. Administration of MLA (6.7 µg) directly into the ventral hippocampus of rats prior to a systemic priming dose of morphine abolished reinstatement of morphine-CPP, whereas MLA delivered into the dorsal hippocampus or prefrontal cortex was without effect. These results suggest that α7 nAChRs in the ventral hippocampus play a specific role in the retrieval of associative drug memories following a period of extinction, making them potential targets for the prevention of relapse.

Catecholamines: Knowledge and understanding in the 1960s, now, and in the future.

The late 1960s was a heyday for catecholamine research. Technological developments made it feasible to study the regulation of sympathetic neuronal transmission and to map the distribution of noradrenaline and dopamine in the brain. At last, it was possible to explain the mechanism of action of some important drugs that had been used in the clinic for more than a decade (e.g. the first generation of antidepressants) and to contemplate the rational development of new treatments (e.g. l-dihydroxyphenylalanine therapy, to compensate for the dopaminergic neuropathy in Parkinson's disease, and ß1-adrenoceptor antagonists as antihypertensives). The fact that drug targeting noradrenergic and/or dopaminergic transmission are still the first-line treatments for many psychiatric disorders (e.g. depression, schizophrenia, and attention deficit hyperactivity disorder) is a testament to the importance of these neurotransmitters and the research that has helped us to understand the regulation of their function. This article celebrates some of the highlights of research at that time, pays tribute to some of the subsequent landmark studies, and appraises the options for where it could go next.

Evaluating the abuse potential of psychedelic drugs as part of the safety pharmacology assessment for medical use in humans.

Psychedelics comprise drugs come from various pharmacological classes including 5-HT2A agonists, indirect 5-HT agonists, e.g., MDMA, NMDA antagonists and κ-opioid receptor agonists. There is resurgence in developing psychedelics to treat psychiatric disorders with high unmet clinical need. Many, but not all, psychedelics are schedule 1 controlled drugs (CDs), i.e., no approved medical use. For existing psychedelics in development, regulatory approval will require a move from schedule 1 to a CD schedule for drugs with medical use, i.e., schedules 2-5. Although abuse of the psychedelics is well documented, a systematic preclinical and clinical evaluation of the risks they pose in a medical-use setting does not exist. We describe the non-clinical tests required for a regulatory evaluation of abuse/dependence risks, i.e., drug-discrimination, intravenous self-administration and physical dependence liability. A synopsis of the existing data for the various types of psychedelics is provided and we describe our findings with psychedelic drugs in these models. FDA recently issued its guidance on abuse/dependence evaluation of drug-candidates (CDER/FDA, 2017). We critically review the guidance, discuss the impact this document will have on non-clinical abuse/dependence testing, and offer advice on how non-clinical abuse/dependence experiments can be designed to meet not only the expectations of FDA, but also other regulatory agencies. Finally, we offer views on how these non-clinical tests can be refined to provide more meaningful information to aid the assessment of the risks posed by CNS drug-candidates for abuse and physical dependence. This article is part of the Special Issue entitled 'Psychedelics: New Doors, Altered Perceptions'.

Dopamine and µ-opioid receptor dysregulation in the brains of binge-eating female rats - possible relevance in the psychopathology and treatment of binge-eating disorder.

Adult, female rats given irregular, limited access to chocolate develop binge-eating behaviour with normal bodyweight and compulsive/perseverative and impulsive behaviours similar to those in binge-eating disorder. We investigated whether (a) dysregulated central nervous system dopaminergic and opioidergic systems are part of the psychopathology of binge-eating and (b) these neurotransmitter systems may mediate the actions of drugs ameliorating binge-eating disorder psychopathology. Binge-eating produced a 39% reduction of striatal D1 receptors with 22% and 23% reductions in medial and lateral caudate putamen and a 22% increase of striatal µ-opioid receptors. There was no change in D1 receptor density in nucleus accumbens, medial prefrontal cortex or dorsolateral frontal cortex, striatal D2 receptors and dopamine reuptake transporter sites, or µ-opioid receptors in frontal cortex. There were no changes in ligand affinities. The concentrations of monoamines, metabolites and estimates of dopamine (dopamine/dihydroxyphenylacetic acid ratio) and serotonin/5-hydroxyindolacetic acid ratio turnover rates were unchanged in striatum and frontal cortex. However, turnover of dopamine and serotonin in the hypothalamus was increased ~20% and ~15%, respectively. Striatal transmission via D1 receptors is decreased in binge-eating rats while µ-opioid receptor signalling may be increased. These changes are consistent with the attenuation of binge-eating by lisdexamfetamine, which increases catecholaminergic neurotransmission, and nalmefene, a µ-opioid antagonist.

Investigation of impulsivity in binge-eating rats in a delay-discounting task and its prevention by the d-amphetamine prodrug, lisdexamfetamine.

Freely-fed, female, rats were trained in a two-lever, delay-discounting task: one lever delivered a single chocolate-flavoured pellet immediately and the other a three-pellet reward after increasing delay (0, 4, 8, 16, 32 s). Rats were divided into two groups (i.e. binge-eating rats given irregular, limited access to chocolate in addition to normal chow and controls maintained on normal chow). Both groups exhibited increased preference for the immediate reward as the delay interval was lengthened. The discounting rate was significantly greater in binge-eating rats than non-binge-eating controls, especially as the behaviour became more established indicating that increased impulsivity and intolerance of delayed reward are part of the psychopathology of binge-eating. Lisdexamfetamine (0.8 mg/kg, orally ( d-amphetamine base)) reversed the reduced preference of binge-eating rats for larger rewards at delay intervals of 4 s, 8 s and 32 s and across all sessions. Lisdexamfetamine-treated binge-eating rats consumed the same number of pellets as vehicle-treated, binge-eating rats and non-binge-eating controls eliminating the possibility lisdexamfetamine's actions on appetite or satiety mediated its effects on operant responding for food pellets in delay-discounting. In summary, binge-eating rats showed increased impulsive choice compared with non-binge-eating controls that was reversed by lisdexamfetamine, complementing results showing lisdexamfetamine reduced impulsiveness scores in patients with binge-eating disorder.

The NK1R-/- mouse phenotype suggests that small body size, with a sex- and diet-dependent excess in body mass and fat, are physical biomarkers for a human endophenotype with vulnerability to attention deficit hyperactivity disorder.

The abnormal behaviour of NK1R-/- mice (locomotor hyperactivity, inattentiveness and impulsivity in the 5-Choice Serial Reaction-Time Test) is arguably analogous to that of patients with attention deficit hyperactivity disorder (ADHD). Evidence suggests that small body size and increased body weight are risk factors for ADHD. Here, we compared the body size, body mass and body composition of male and female NK1R-/- mice and their wildtypes that had been fed either standard laboratory chow or a high-fat (45%: 'Western') diet. Male NK1R-/- mice from both cohorts were approximately 7% shorter than wildtypes. A similar trend was evident in females. Male NK1R-/- mice fed the normal diet weighed less than wildtypes but the 'body mass index' ('mBMI': weight (mg)/length (cm)(2)) of female NK1R-/- mice was higher than wildtypes. When given the high-fat diet, the mBMI of both male and female NK1R-/- mice was higher than wildtypes. There were no consistent genotype or sex differences in protein, ash or water content of mice from the two cohorts. However, the fat content of male NK1R-/- mice on the Western diet was considerably (35%) higher than wildtypes and resembled that of females from both genotypes. We conclude that a lack of functional NK1R is associated with small body size but increases vulnerability to an increase in mBMI and fat content, especially in males. This phenotype could also be evident in ADHD patients with polymorphism(s) of the TACR1 gene (the human equivalent of Nk1r).