Intravenous Self-administration Procedure Research Articles

Target deconvolution studies of (2R,6R)-hydroxynorketamine: an elusive search.

The off-label use of racemic ketamine and the FDA approval of (S)-ketamine are promising developments for the treatment of depression. Nevertheless, racemic ketamine and (S)-ketamine are controlled substances with known abuse potential and their use is associated with undesirable side effects. For these reasons, research efforts have focused on identifying alternatives. One candidate is (2R,6R)-hydroxynorketamine ((2R,6R)-HNK), a ketamine metabolite that in preclinical models lacks the dissociative and abuse properties of ketamine while retaining its antidepressant-like behavioral efficacy. (2R,6R)-HNK's mechanism of action however is unclear. The main goals of this study were to perform an in-depth pharmacological characterization of (2R,6R)-HNK at known ketamine targets, to use target deconvolution approaches to discover novel proteins that bind to (2R,6R)-HNK, and to characterize the biodistribution and behavioral effects of (2R,6R)-HNK across several procedures related to substance use disorder liability. We found that unlike (S)- or (R)-ketamine, (2R,6R)-HNK did not directly bind to any known or proposed ketamine targets. Extensive screening and target deconvolution experiments at thousands of human proteins did not identify any other direct (2R,6R)-HNK-protein interactions. Biodistribution studies using radiolabeled (2R,6R)-HNK revealed non-selective brain regional enrichment, and no specific binding in any organ other than the liver. (2R,6R)-HNK was inactive in conditioned place preference, open-field locomotor activity, and intravenous self-administration procedures. Despite these negative findings, (2R,6R)-HNK produced a reduction in immobility time in the forced swim test and a small but significant increase in metabolic activity across a network of brain regions, and this metabolic signature differed from the brain metabolic profile induced by ketamine enantiomers. In sum, our results indicate that (2R,6R)-HNK does not share pharmacological or behavioral profile similarities with ketamine or its enantiomers. However, it could still be possible that both ketamine and (2R,6R)-HNK exert antidepressant-like efficacy through a common and previously unidentified mechanism. Given its pharmacological profile, we predict that (2R,6R)-HNK will exhibit a favorable safety profile in clinical trials, and we must wait for clinical studies to determine its antidepressant efficacy.

Extreme phenotypic diversity in operant response to intravenous cocaine or saline infusion in the hybrid mouse diversity panel.

Cocaine self-administration is a complexly determined trait, with a substantial proportion of individual differences being determined by genetic variation. However, the relevant genetic variants that drive heritable differences in cocaine use remain undiscovered. Cocaine intravenous self-administration (IVSA) procedures in laboratory animals provide opportunities to prospectively investigate neurogenetic influences on the acquisition of voluntary cocaine use. Here, we provide information on cocaine (or saline-as a control) IVSA in 84 members of the hybrid mouse diversity panel (HMDP), an array of genetically distinct classical or recombinant inbred strains. We found cocaine IVSA to be substantially heritable in this population, with strain-level intake ranging for near 0 to >25 mg/kg/session. Though saline IVSA was also found to be heritable, a modest genetic correlation between cocaine and saline IVSA indicates that operant responding for the cocaine reinforcer was influenced, at least in part, by unique genetic variants. Genome-wide association studies (GWAS) of infusions earned in cocaine and saline groups revealed significant quantitative trait loci (QTL) on Chromosomes 3 and 14 for cocaine, but not saline, IVSA. Positional candidates were further prioritized through use of bulk RNA sequencing data that revealed genes with cis-eQTL and genetic correlation to number of infusions. Additionally, these data identify reference strains with extreme cocaine IVSA phenotypes, revealing them as polygenic models of risk and resilience to cocaine reinforcement. This work is part of an ongoing effort to characterize genetic variation that moderates cocaine IVSA that may, in turn, provide a more comprehensive understanding of cocaine risk genetics and neurobiology.

(±)VK4‐40, a novel dopamine D3 receptor partial agonist, attenuates cocaine reward and relapse in rodents

Despite widespread abuse of cocaine, there are no approved treatments for cocaine use disorder. Chronic cocaine use is associated with up-regulated dopamine D3 receptor expression in the brain. Therefore, most D3 -based medication development has focused on D3 antagonists. However, D3 antagonists do not attenuate cocaine intake under "easy" self-administration conditions, when response requirements are low. We evaluated a novel, highly selective and metabolically stable D3 partial agonist, (±)VK4-40, for its efficacy in reducing cocaine intake and relapse to drug seeking. The impact of (±)VK4-40 on cocaine intake and relapse was evaluated using intravenous self-administration procedures under a fixed-ratio 2 reinforcement schedule and cocaine-primed reinstatement conditions in rats. Optogenetic brain-stimulation reward procedures were used to evaluate the interaction of (±)VK4-40 and cocaine in the mesolimbic dopamine system in mice. Sucrose self-administration in rats and a conditioned place preference paradigm in mice were used to evaluate the abuse potential of (±)VK4-40 alone and other unwanted effects. (±)VK4-40 dose-dependently reduced cocaine self-administration and cocaine-primed reinstatement of drug-seeking behaviour. (±)VK4-40 also inhibited cocaine-enhanced brain-stimulation reward caused by optogenetic stimulation of dopamine neurons in the ventral tegmental area. (±)VK4-40 alone decreased brain-stimulation reward but produced neither conditioned place preference nor place aversion. This new D3 partial agonist also failed to alter oral sucrose self-administration. The novel D3 partial agonist, (±)VK4-40 attenuates cocaine reward and relapse in rodents, without significant unwanted effects. These findings support further investigation of D3 partial agonists as putative treatments for cocaine use disorder.

The active heroin metabolite 6-acetylmorphine has robust reinforcing effects as assessed by self-administration in the rat

Previous studies have suggested that at least some of the behavioral effects of heroin might be mediated by its active metabolite 6-acetylmorphine (6-AM). The aim of the present study was to investigate the reinforcing effects of 6-AM and its role in mediating those of heroin. We used an intravenous self-administration procedure in male Sprague-Dawley rats including four phases: acquisition, extinction, reinstatement of drug-seeking, and re-acquisition. Independent groups of rats readily learned to self-administer equimolar doses (0.135 μmol/kg) of either 6-AM (44.3 μg/kg) or heroin (50 μg/kg). Under a fixed ratio 1 (FR1) schedule of reinforcement, the rate of responding was the same for 6-AM and heroin, but it was significantly higher for 6-AM than for heroin under a FR2 schedule. A non-contingent infusion (‘priming’) of 0.068 μmol/kg of either 6-AM or heroin reinstated non-reinforced drug-seeking (relapse). The rats readily re-acquired self-administration behaviour when given access to one of two doses (0.068 and 0.135 μmol/kg) of 6-AM or heroin. Pretreatment with a specific monoclonal antibody (mAb) against 6-AM blocked the priming effect of 6-AM, and modified the rate of lever-pressing on re-acquisition of 6-AM self-administration in a manner compatible with a shift to the right of the dose-effect curve. The mAb did not affect heroin responding. The present results show that 6-AM possesses reinforcing effects similar to those of heroin. The lack of effect of 6-AM mAb on heroin priming and heroin self-administration calls for further studies to clarify the role of heroin and its metabolites in heroin reward.This article is part of the Special Issue entitled ‘New Vistas in Opioid Pharmacology’.

Locomotor and reinforcing effects of pentedrone, pentylone and methylone in rats

The broad diversity of synthetic cathinone psychostimulant drugs that are available to users complicates research efforts to provide understanding of health risks. Second generation cathinones pentedrone and pentylone are distinguished from each other by the 3,4-methylenedioxy structural motif (which distinguishes methamphetamine from 3,4-methylenedioxymethamphetamine) and each incorporates the α-alkyl chain motif contained in the transporter-inhibitor cathinones (3,4-methylenedioxypyrovalerone (MDPV), α-pyrrolidinopentiophenone (α-PVP)) but not in the monoamine releasers (mephedrone, methylone). Studies were conducted in male and female Wistar rats to compare locomotor and thermoregulatory effects of pentedrone, pentylone and methylone using an implanted radiotelemetry system. Reinforcing effects were assessed in female Wistar rats trained in the intravenous self-administration (IVSA) procedure and subjected to dose-substitution (0.025–0.3 m/gkg/inf) under a fixed-ratio 1 response contingency. Pentedrone, pentylone and methylone dose-effect curves were contrasted with those for α-PVP and α-pyrrolidinohexiophenone (α-PHP). Dose dependent increases in locomotion were observed after intraperitoneal injection of pentylone (0.5–10.0 mg/kg), pentedrone (0.5–10.0 mg/kg) or mephedrone (0.5–10.0 mg/kg) in male and female rats. The maximum locomotor effect was similar across drugs but lasted longest after pentedrone. Mean body temperature did not vary systematically more than 0.5 °C after pentedrone or pentylone in either sex. A sustained hyperthermia (0.4–0.8 °C) was observed for four hours after 10.0 mg/kg methylone in male rats. More infusions of pentedrone or pentylone were self-administered compared with methylone, but all three were less potent than α-PVP or α-PHP. These studies support the inference that second generation cathinones pentylone and pentedrone have abuse liability greater than that of methylone.This article is part of the Special Issue entitled ‘Designer Drugs and Legal Highs.’

Effects of the GLP-1 Agonist Exendin-4 on Intravenous Ethanol Self-Administration in Mice.

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. We trained experimentally naïve, 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. 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; 3.2μg/kg Exendin-4 decreased intravenous EtOH intake by at least 70%, but had no significant effect on food-maintained operant responding. 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.

An intravenous self-administration procedure for assessing the reinforcing effects of hallucinogens in nonhuman primates

IntroductionSelf-administration procedures are the gold standard for investigating the reinforcing effects of drugs. The notable exception to good correspondence between laboratory self-administration studies and human drug taking behavior has historically been the classic hallucinogens. MethodThe present study used a well-established daily access procedure, followed by a novel intermittent access procedure, to investigate the reinforcing effects of LSD in baboons. ResultsRates of self-injection in the daily access procedure were minimal. One baboon self-administered 0.001mg/kg and a second baboon self-administered 0.0032mg/kg above vehicle levels, though rates of self-injection were clearly low and neither of the two remaining baboons self-administered any LSD dose tested in the daily access procedure. Rates of self-injection using an intermittent access procedure with discriminative stimuli resulted in two doses of LSD being self-administered above vehicle levels in two of three baboons tested (0.01 and 0.032mg/kg in one baboon; 0.0032 and 0.01mg/kg in a second). In addition, the number of self-injections at these doses was higher (range=3–6 injections) in the intermittent access procedure than in the daily access procedure (range=1–2 injections). DiscussionThe present study is the first to demonstrate LSD self-administration in a laboratory animal, and though the results are limited, they indicate intermittent access procedures with discriminative stimuli may provide a reliable and valid method for investigating the reinforcing effects of IV self-administered hallucinogens in laboratory animals. The usefulness of such procedures should be further evaluated in a larger number of subjects.

Lack of Specific Involvement of (+)-Naloxone and (+)-Naltrexone on the Reinforcing and Neurochemical Effects of Cocaine and Opioids

Effective medications for drug abuse remain a largely unmet goal in biomedical science. Recently, the (+)-enantiomers of naloxone and naltrexone, TLR4 antagonists, have been reported to attenuate preclinical indicators of both opioid and stimulant abuse. To further examine the potential of these compounds as drug-abuse treatments, we extended the previous assessments to include a wider range of doses and procedures. We report the assessment of (+)-naloxone and (+)-naltrexone on the acute dopaminergic effects of cocaine and heroin determined by in vivo microdialysis, on the reinforcing effects of cocaine and the opioid agonist, remifentanil, tested under intravenous self-administration procedures, as well as the subjective effects of cocaine determined by discriminative-stimulus effects in rats. Pretreatments with (+)-naloxone or (+)-naltrexone did not attenuate, and under certain conditions enhanced the stimulation of dopamine levels produced by cocaine or heroin in the nucleus accumbens shell. Furthermore, although an attenuation of either cocaine or remifentanil self-administration was obtained at the highest doses of (+)-naloxone and (+)-naltrexone, those doses also attenuated rates of food-maintained behaviors, indicating a lack of selectivity of TLR4 antagonist effects for behaviors reinforced with drug injections. Drug-discrimination studies failed to demonstrate a significant interaction of (+)-naloxone with subjective effects of cocaine. The present studies demonstrate that under a wide range of doses and experimental conditions, the TLR4 antagonists, (+)-naloxone and (+)-naltrexone, did not specifically block neurochemical or behavioral abuse-related effects of cocaine or opioid agonists.

Coordinated and homologous intravenous cocaine self-administration procedures in monkeys and humans: A translational approach to medications development

Coordinated and homologous intravenous cocaine self-administration procedures in monkeys and humans: A translational approach to medications development

Olfactory bulbectomy increases reinstatement of methamphetamine seeking after a forced abstinence in rats

Drug addiction is commonly associated with depression and comorbid patients also suffer from higher cravings and increased relapse rate. To address this issue preclinically we combined the olfactory bulbectomy (OBX) model of depression and intravenous methamphetamine self-administration procedure in rats to assess differences in relapse-like behavior.Male Sprague-Dawley rats were divided randomly into two groups; in one group the bilateral olfactory bulbectomy (OBX) was performed while the other group was sham operated. After recovery, intracardiac catheter was implanted. Intravenous self-administration procedure was conducted in operant boxes using nose-poke operandi (Coulbourn Instruments, Inc., USA) under fixed ratio 1 schedule of reinforcement. Methamphetamine was available at dose 0.08 mg/kg/infusion. After stable methamphetamine intake was maintained, a period of forced abstinence was initiated and rats were kept in their home-cages for 14 days. Finally, one reinstatement session was conducted in operant boxes with no drug delivery.In the reinstatement session the mean of 138.4 active nose-pokes was performed by the OBX group, while the sham group displayed 41 responses, i.e. 140 % and 48 % of basal nose-poking during maintenance phase in OBX and sham operated group respectively. OBX group also showed significantly more passive nose-pokes indicating hyperactive behavioral traits in bulbectomized rats. However, the % of active operandum preference was equal in both groups.Olfactory bulbectomy model significantly increased reinstatement of methamphetamine seeking behavior. This paradigm can be used to evaluate potential drugs that are able to suppress the drug-seeking behavior.

Enhancing glutamatergic transmission during adolescence reverses early-life stress-induced deficits in the rewarding effects of cocaine in rats

Adolescence marks a critical time when the brain is highly susceptible to pathological insult yet also uniquely amenable to therapeutic intervention. It is during adolescence that the onset of the majority of psychiatric disorders, including substance use disorder (SUDs), occurs. It has been well established that stress, particularly during early development, can contribute to the pathological changes which contribute to the development of SUDs. Glutamate as the main excitatory neurotransmitter in the mammalian CNS plays a key role in various physiological processes, including reward function, and in mediating the effects of psychological stress. We hypothesised impairing glutamatergic signalling during the key adolescent period would attenuate early-life stress induced impaired reward function. To test this, we induced early-life stress in male rats using the maternal-separation procedure. During the critical adolescent period (PND25–46) animals were treated with the glutamate transporter activator, riluzole, or the NMDA receptor antagonist, memantine. Adult reward function was assessed using voluntary cocaine intake measured via intravenous self-administration. We found that early-life stress in the form of maternal-separation impaired reward function, reducing the number of successful cocaine-infusions achieved during the intravenous self-administration procedure as well impairing drug-induced reinstatement of cocaine-taking behaviour. Interestingly, riluzole and memantine treatment reversed this stress-induced impairment. These data suggest that reducing glutamatergic signalling may be a viable therapeutic strategy for treating vulnerable individuals at risk of developing SUDs including certain adolescent populations, particularly those which may have experienced trauma during early-life.

The Reinforcing Effects of Nicotine in Humans and Nonhuman Primates: A Review of Intravenous Self-Administration Evidence and Future Directions for Research.

Cigarette smoking is largely driven by the reinforcing properties of nicotine. Intravenous (IV) self-administration procedures are the gold standard for investigating the reinforcing effects of psychoactive drugs. The goal of this review was to examine the results of published investigations of the reinforcing effects of nicotine measured using IV self-administration procedures in humans and nonhuman primates. The body of literature using nonhuman primate subjects indicates nicotine functions as a positive reinforcer when available for self-administration via IV catheters. However, it can also be difficult to establish IV nicotine self-administration in nonhuman primates and sometimes supplemental strategies have been required (e.g., priming injections or food deprivation) before subjects acquire the behavior. Although the body of literature using human subjects is limited, the evidence indicates nicotine functions as a reinforcer via the IV route of administration in adult cigarette smokers. Rates of nicotine self-injection can be variable across subjects and responding is sometimes inconsistent across sessions in both humans and nonhuman primates. The Family Smoking Prevention and Tobacco Control Act, enacted in 2009, gave the Food and Drug Administration regulatory authority over the manufacture, marketing, and distribution of tobacco products. Research examining the threshold reinforcing doses for initiation and maintenance of nicotine self-administration, comparisons of the reinforcing effects of nicotine in adolescent versus adult subjects, investigations of gender differences in the reinforcing effects of nicotine, and studies of the abuse liability of non-nicotine tobacco product constituents and their ability to alter the reinforcing effects of nicotine will inform potential tobacco regulatory actions.

Abuse liability assessment in preclinical drug development: Predictivity of a translational approach for abuse liability testing using methylphenidate in four standardized preclinical study models

ObjectivesPreclinical abuse liability assessment of novel clinical CNS-active candidates involves several tests, addressing different aspects characteristic for abuse potential, which are considered predictive for substance abuse of these candidates, thus ensuring an appropriate translational approach. To demonstrate how such a strategy could work, a known drug of abuse, methylphenidate was evaluated in a full rodent test battery, comprising four test models, and in accordance with the requirements of the FDA, ICH and EMA guidelines. MethodsMethylphenidate was tested orally at 2.5, 5 or 10mg/kg for its physical dependence potential in a repeated dose non-precipitated withdrawal test, for its drug profiling in a drug discrimination learning procedure (single escalating doses), and for its reinforcing properties in a conditioned place preference test (alternate dosing days) and an intravenous self-administration procedure (0.05 to 1mg/kg/IV infusion during 5 daily 1-h test sessions). The stimulant d-amphetamine served as positive control and was administered subcutaneously at 0.8mg/kg in the first three test models. In the intravenous self-administration procedure rats were habituated to intravenously self-administer d-amphetamine at 0.06mg/kg/IV infusion prior to methylphenidate substitution. ResultsCessation of subchronic dosing up to 10mg/kg methylphenidate led to sustained or even exacerbated effects on locomotion and behavior, body temperature, body weight, food consumption, and alteration of the diurnal rhythm during withdrawal. Clear generalization to d-amphetamine was obtained in the drug discrimination test at 5 and 10mg/kg. Distinct reinforcing properties were present in the conditioned place preference test at 10mg/kg and in the intravenous self-administration study from 0.05mg/kg/IV infusion onwards. The maximum plasma exposure after oral administration of methylphenidate over the dose ranges tested in the present rat studies covered at least 1.9-fold to 18.9-fold the recommended human therapeutic exposure of 10ng/ml, a plasma level that is considered representative of the human efficacious methylphenidate dose. The ratio Cmax Hu/rat calculated from the intravenous self-administration data ranged from 14.9 to 576.5. Consequently the regulatory requirements, stating that preclinical drug abuse liability studies should include high doses that produce plasma levels that are multiples of the therapeutic dose were fulfilled (FDA, EMA, ICH). DiscussionThe presented preclinical models, implemented within a drug development environment, were considered highly predictive to assess the abuse potential of methylphenidate, and in accordance with the regulatory requirements of drug licensing authorities in terms of appropriate methods, dose selection and subsequent plasma exposure.

MGluR5 Antagonist-Induced Psychoactive Properties: MTEP Drug Discrimination, a Pharmacologically Selective Non–NMDA Effect with Apparent Lack of Reinforcing Properties

Fenobam [N-(3-chlorophenyl)-N'-(4,5-dihydro-1-methyl-4-oxo-1H-imidazole-2-yl)urea], a potent metabotropic glutamate mGluR5 receptor antagonist, reported to have analgesic effects in animals and anxiolytic effects in humans, also caused adverse events, including psychostimulant-type effects and "derealization phenomena." Recent electrophysiologic, pharmacologic, and anatomic data show that the mGluR5 antagonists 2-methyl-6-(phenylethynyl)pyridine (MPEP) and (E)-2-methyl-6-styryl-pyridine (SIB-1893) can inhibit NMDA receptor-mediated activity and that mGluR5 receptors are highly expressed in limbic and forebrain regions. The present studies first evaluated the potential of mGluR5 receptor antagonists to cause PCP-like psychoactive effects in a rat drug discrimination procedure and, second, explored and characterized the selective mGluR5 antagonist 3-[(2-methyl-1,3-thiazol-4-yl)ethynyl]pyridine (MTEP) as a discriminative stimulus and compared MTEP with other drugs known to be psychoactive in humans. Additionally, the reinforcing potential of MPEP and MTEP was compared with phencyclidine (PCP) in a rat intravenous self-administration procedure. Dizocilpine [(+)-MK-801] and ketamine caused full PCP-appropriate responding. Memantine and the mGluR5 antagonists caused no or weak partial PCP-appropriate responding. In MTEP-trained rats, MTEP, MPEP, and fenobam caused full and equipotent MTEP-appropriate responding. (+)-MK-801 and memantine caused MTEP-appropriate responding below 70%, whereas PCP, chlordiazepoxide and LSD caused MTEP-appropriate responding below 50%. Δ(9)-Tetrahydrocannabinol, yohimbine, arecoline, and pentylenetetrazole all caused MTEP-appropriate responding below 20%. Rats self-administered PCP but not MPEP or MTEP, indicating a lack of reinforcing effects of the mGluR5 antagonists. These data suggest that the mGluR5 antagonists appear not to have reinforcing properties, that the discriminative effects of mGluR5 antagonists and PCP are dissimilar, and that mGluR5 antagonists may produce psychoactive effects different from NMDA-antagonists and other drugs with known psychotomimetic properties.

Evaluation of WIN 55,212-2 self-administration in rats as a potential cannabinoid abuse liability model

Because Δ9-tetrahydrocannabinol (THC) has been a false negative in rat intravenous self-administration procedures, the evaluation of the abuse potential of candidate cannabinoid medications has proved difficult. One lab group has successfully trained self-administration of the aminoalkylindole WIN55,212-2 in rats; however, their results have not been independently replicated. The purpose of this study was to extend their model by using a within-subjects design, with the goal of establishing a robust method suitable for substitution testing of other cannabinoids. Male Long–Evans rats were trained to self-administer WIN55,212-2 (0.01mg/kg/infusion) on a fixed ratio 3 schedule. Dose–effect curves for WIN55,212-2 were determined, followed by vehicle substitution and a dose–effect curve with THC. WIN55,212-2 self-administration was acquired; however, substitution with THC did not maintain responding above vehicle levels. Dose-dependent attenuation by rimonabant confirmed CB1 receptor mediation of WIN55,212-2's reinforcing effects. Vehicle substitution resulted in a session-dependent decrease in responding (i.e., extinction). While this study provides systematic replication of previous studies, lack of substitution with THC is problematic and suggests that WIN55,212-2 self-administration may be of limited usefulness as a screening tool for detection of the reinforcing effects of potential cannabinoid medications. Clarification of underlying factors responsible for failure of THC to maintain self-administration in cannabinoid-trained rats is needed.

Abuse‐related behavioral effects of ketamine in adult and adolescent rats

The dissociative anesthetic ketamine has been subject to growing rates of abuse worldwide, particularly in adolescent populations. This project compared the behavioral effects of ketamine in place conditioning and intravenous self‐administration procedures in adolescent (PND 28–50) and adult (>;PND72) male Sprague Dawley rats. Cocaine served as a positive control. In the place conditioning study, 4 conditioning sessions following administration of 1, 3 or 10 mg/kg ketamine were counterbalanced with 4 sessions following saline administration. Preference was determined based on change in time spent in the drug‐paired environment. In the self‐administration assay, 0.03, 0.1 or 0.3 mg/kg/infusion of ketamine was evaluated for acquisition of self‐administration behavior and level of drug taking under a progressive ratio schedule. In the place‐conditioning assay, the 3 and 10 mg/kg ketamine produced aversion in the adults whereas adolescents demonstrated place preference for the 1 and 3 mg/kg doses. Conversely both age groups acquired self‐administration behavior at all doses of ketamine and cocaine. These divergent results for ketamine in the adults may reflect inherent differences in the two procedures. However, because cocaine produced evidence of hedonic effects in both age groups, it also suggests unique behavioral characteristics of ketamine and important differences in its effects based on age.

AM404 attenuates reinstatement of nicotine seeking induced by nicotine-associated cues and nicotine priming but does not affect nicotine- and food-taking

Multiple studies suggest a pivotal role of the endocannabinoid system in the regulation of the reinforcing effects of various substances of abuse. Different approaches have been used to modulate endocannabinoid neurotransmission including the use of endogenous cannabinoid anandamide reuptake inhibitors. Previously, the effects of one of them, N-(4-hydroxyphenyl)-arachidonamide (AM404), have been explored in rodents trained to self-administer ethanol and heroin, producing some promising results. Moreover, AM404 attenuated the development and reinstatement of nicotine-induced conditioned place preference (CPP). In this study, we used the nicotine intravenous self-administration procedure to assess the effects of intraperitoneal administration of 0, 1, 3 and 10 mg/kg AM404 on nicotine-taking and food-taking behaviors under fixed-ratio and progressive-ratio schedules of reinforcement, as well as on reinstatement of nicotine-seeking induced by nicotine priming and by presentation of nicotine-associated cues. The ability of AM404 to produce place preference was also evaluated. AM404 did not produce CPP and did not modify nicotine-taking and food-taking behaviors. In contrast, AM404 dose-dependently attenuated reinstatement of nicotine-seeking behavior induced by both nicotine-associated cues and nicotine priming. Our results indicate that AM404 could be a potential promising therapeutic option for the prevention of relapse to nicotine-seeking in abstinent smokers.

Extended access to methamphetamine self-administration affects sensorimotor gating in rats

Disturbed information processing observed in neuropsychiatric disorders is reflected by deficient sensorimotor gating, measured as prepulse inhibition (PPI) of the acoustic startle response (ASR). Long-term, higher dose methamphetamine (METH) abuse patterns are associated with cognitive impairments, mania and/or schizophrenia-like psychosis. The present study investigated in rats METH-induced impairment of sensorimotor gating using an intravenous self-administration (IVSA) escalating dose procedure. In this procedure, rats escalated drug intake during weekly extended access periods to METH IVSA (1, 3, and 6h), where PPI was assessed after each access period and thus at various times of drug exposure. Despite increased drug intake over the course of extended access to METH, disruption of sensorimotor gating was only seen after the access period of 6h. The data suggest that METH-induced impairment of sensorimotor gating in IVSA-tasks is rather attributed to continuous and higher dose exposure than to actual amounts of drug present at the time of testing. IVSA procedures, comprising stepwise stimulant escalation may serve as a useful translational model in rats that approximate important aspects of human abuse pattern in the context of stimulant-induced cognitive and behavioral deficits.

Genetically correlated effects of selective breeding for high and low methamphetamine consumption

Improved prevention and treatment of drug addiction will require deeper understanding of genetic factors contributing to susceptibility to excessive drug use. Intravenous operant self-administration methods have greatly advanced understanding of behavioral traits related to addiction. However, these methods are not suitable for large-scale genetic experiments in mice. Selective breeding of mice can aggregate 'addiction alleles' in a model that has the potential to identify coordinated effects of multiple genes. We produced mouse lines that orally self-administer high (MAHDR) or low (MALDR) amounts of methamphetamine, representing the first demonstration of selective breeding for self-administration of any psychostimulant drug. Conditioned place preference and taste aversion results indicate that MAHDR mice are relatively more sensitive to the rewarding effects and less sensitive to the aversive effects of methamphetamine, compared to MALDR mice. These results validate the oral route of self-administration for investigation of the motivational effects of methamphetamine and provide a viable alternative to intravenous self-administration procedures. Gene expression results for a subset of genes relevant to addiction-related processes suggest differential regulation by methamphetamine of apoptosis and immune pathways in the nucleus accumbens of MAHDR and MALDR mice. In each line, methamphetamine reduced an allostatic state by bringing gene expression back toward 'normal' levels. Genes differentially expressed in the drug-naï ve state, including Slc6a4 (serotonin transporter), Htr3a (serotonin receptor 3A), Rela [nuclear factor kappaB (NFkappaB)] and Fos (cFos), represent candidates whose expression levels may predict methamphetamine consumption and susceptibility to methamphetamine reward and aversion.

A reliable method to study cue-, priming-, and stress-induced reinstatement of cocaine self-administration in mice

Cocaine addiction is a relapsing psychiatric disorder with a high prevalence in developed countries. To date, the reinstatement model has been difficult to implement in mice. The design of an appropriate reinstatement model in mice is required in order to use genetically modified animals with the aim of clarifying the mechanisms involved in cocaine relapse. Our aim was to develop an appropriate model of reinstatement of cocaine-seeking behavior and to investigate the factors that can trigger this reinstatement by using an operant intravenous self-administration procedure in mice. Discrete cues, priming injection of cocaine, and exposure to stress were the stimuli used to reinstate cocaine-seeking behavior. Mice were trained to acquire intravenous self-administration of cocaine (1 mg/kg per infusion) on a fixed ratio 1 (FR1) schedule of reinforcement. After achieving the acquisition criteria, animals were led to extinguish the operant behavior. Subsequently, under extinction conditions, mice were tested after the administration of a cocaine priming injection (10 mg/kg i.p.), the presentation of a light cue associated with cocaine administration, or the exposure to a stressful situation (0.21 mA electric footshock). Under our experimental conditions the three stimuli successfully reinstated an extinguished cocaine-seeking behavior. Reexposure to cocaine effects by a priming injection was revealed as the strongest stimulus, capable of reinstating cocaine-seeking behavior. The effective reinstatement model that we have developed will become a useful tool for future understanding of the neurobiological basis of cocaine addiction and relapse, specifically, with the use of genetically modified mice.