Rationale

The ability of tobacco harm reduction strategies to produce significant reductions in toxin exposure is limited by compensatory increases in smoking behavior. Characterizing factors contributing to the marked individual variability in compensation may be useful for understanding this phenomenon and assessing the feasibility of harm reduction interventions.

Objective

To use an animal model of human compensatory smoking that involves a decrease in unit dose supporting nicotine self-administration (NSA) to examine potential contributors to individual differences in compensation.

Methods

Rats were trained for NSA during daily 23 hr sessions at a unit dose of 0.06 mg/kg/inf until responding was stable. The unit dose was then reduced to 0.03 mg/kg/inf for at least 10 sessions. Following reacquisition of NSA at the training dose and extinction, single-dose nicotine pharmacokinetic parameters were determined.

Results

Decreases in nicotine intake following dose reduction were proportionally less than the decrease in unit dose, indicating partial compensation. Compensatory increases in infusion rates were observed across the course of the 23 hr sessions. The magnitude of compensation differed considerably between rats. Rats exhibiting the highest baseline infusion rates exhibited the lowest levels of compensation. Nicotine pharmacokinetic parameters were not significantly correlated with compensation. Infusion rates immediately returned to pre-reduction levels when baseline conditions were restored.

Conclusions

These findings provide initial insights into correlates of individual differences in compensation following a reduction in nicotine unit dose. The present assay may be useful for characterizing mechanisms and potential consequences of the marked individual differences in compensatory smoking observed in humans.

Rationale

Compensatory smoking may represent an adverse consequence of smoking reduction or the use of reduced nicotine tobacco products. Factors contributing to individual variability in compensation are poorly understood.

Objective

To examine whether severity of nicotine withdrawal as measured by elevated intracranial self-stimulation (ICSS) thresholds is related to individual differences in compensatory nicotine self-administration (NSA) following unit dose reduction.

Methods

Rats were trained for ICSS and NSA (0.06 mg/kg/inf). After stabilization, effects of reducing the nicotine unit dose to 0.03 mg/kg/inf were examined. Following reacquisition of NSA (0.06 mg/kg/inf), effects of antagonist-precipitated withdrawal and saline extinction (spontaneous withdrawal) were examined.

Results

Reducing the NSA unit dose produced partial compensation as indicated by increased infusion rates but a 35% mean decrease in daily nicotine intake. Magnitude of compensation varied considerably among rats. Dose reduction did not elicit withdrawal in rats as a group, although there were substantial increases in ICSS thresholds in some animals. Intracranial self-stimulation thresholds were consistently elevated during precipitated and spontaneous withdrawal, confirming that rats were nicotine-dependent. Individual differences in compensation were not correlated with changes in ICSS thresholds during dose reduction, precipitated withdrawal, or spontaneous withdrawal. In a secondary analysis, greater precipitated withdrawal severity predicted greater initial nicotine-seeking during extinction.

Conclusions

Severity of nicotine withdrawal was not related to the degree of compensation in this protocol. These data do not support a role for nicotine withdrawal in individual differences in compensation during reduced nicotine exposure, but do suggest that withdrawal may contribute to nicotine-seeking during early abstinence.

Rationale

Several neurotransmitter systems have been hypothesized to be involved in the in vivo effects of volatile anesthetic. Drug discrimination may represent a novel procedure to explore the neurochemical systems underlying the sub-anesthetic behavioral effects of these compounds.

Objectives

The purpose of the present study was to examine the contribution of GABAA and NMDA receptors to the discriminative stimulus effects of a behaviorally-active sub-anesthetic concentration of isoflurane vapor.

Methods

Sixteen B6SJLF1/J mice were trained to discriminate 10 min of exposure to 6000 ppm isoflurane vapor from air. Substitution tests were conducted with volatile anesthetics, abused vapors, GABAA positive modulators, NMDA antagonists and nitrous oxide.

Results

The volatile anesthetics, enflurane and halothane as well as the abused vapors toluene and 1,1,1-trichloroethane fully substituted for isoflurane. The GABAA positive modulators, pentobarbital, midazolam and zaleplon but not the direct GABAA agonist, muscimol, produced high levels of partial substitution for isoflurane. The anticonvulsant, valproic acid fully substituted for isoflurane but a second, tiagabine, did not substitute. The competitive NMDA antagonist, CGS-19755, fully and the non-competitive NMDA antagonist, dizocilpine, partially substituted for isoflurane. The glycine site NMDA antagonist, L-701,324 did not substitute for isoflurane. GHB and nitrous oxide gas also failed to substitute for isoflurane.

Conclusions

The discriminative stimulus effects of subanesthetic concentrations of isoflurane vapor are shared by other vapor anesthetics and abused inhalants. The discriminative stimulus effects of isoflurane vapor appear to be mediated by both positive allosteric modulation of GABAA receptors as well as antagonism of NMDA receptors.

RATIONALE

Tobacco smoke contains nicotine and many other compounds that act in concert on the brain reward system. Therefore, animal models are needed that allow the investigation of chronic exposure to the full spectrum of tobacco smoke constituents.

OBJECTIVES

The aim of these studies was to investigate if exposure to tobacco smoke leads to nicotine dependence in rats.

METHODS

The intracranial self-stimulation procedure was used to assess the negative affective aspects of nicotine withdrawal. Somatic signs were recorded from a checklist of nicotine abstinence signs. Nicotine self-administration sessions were conducted to investigate if tobacco smoke exposure affects the motivation to self-administer nicotine. Nicotinic receptor autoradiography was used to investigate if exposure to tobacco smoke affects central α7 nicotinic acetylcholine receptor (nAChR) and non-α7 nAChR levels (primarily α4β2 nAChRs).

RESULTS

The nAChR antagonist mecamylamine dose-dependently elevated the brain reward thresholds of the rats exposed to tobacco smoke and did not affect the brain reward thresholds of the untreated control rats. Furthermore, mecamylamine induced more somatic withdrawal signs in the smoke exposed rats than in the control rats. Nicotine self-administration was decreased 1 day after the last tobacco smoke exposure sessions and was returned to control levels 5 days later. Tobacco smoke exposure increased the α7 nAChR density in the CA2/3 area and the stratum oriens and increased the non-α7 nAChR density in the dentate gyrus.

CONCLUSION

Tobacco smoke exposure leads to nicotine dependence as indicated by precipitated affective and somatic withdrawal signs and induces an upregulation of nAChRs in the hippocampus.

Rationale

Long-term smoking can lead to changes in autonomic function, including decreased vagal tone and altered stress responses. One index of the inability to adapt to stress may be blunted vagal reactivity. Stress is a primary mechanism involved in relapse to smoking, but mechanisms leading to stress-precipitated relapse are not well understood.

Objectives

Using an experimental paradigm of stress-precipitated smoking behavior, we examined whether autonomic reactivity mediates the relationship between stress and smoking. High-frequency heart rate variability (HF-HRV), a putative measure of vagal tone and the ratio of low-to-high frequency HRV (LF/HF), a measure of sympathovagal balance were assessed.

Methods

Using a within-subjects design, 32 nicotine dependent 15-hour abstinent smokers (a subgroup from McKee et al. (2011)) were exposed to individualized script-driven imagery of stressful and relaxing scenarios and assessed on the ability to resist smoking and subsequent ad-lib smoking. HRV was monitored throughout each laboratory session (maximum 60 min following imagery).

Results

As expected, stress and ad-lib smoking additively decreased HF-HRV and increased LF/HF. Blunted stress-induced HF-HRV responses reflecting decreased vagal reactivity were associated with less time to initiate smoking and increased craving relief and reinforcement from smoking. These relationships were specific to HF-HRV following stress as neither baseline HF-HRV, HF-HRV following relaxing imagery, or LF/HF predicted smoking behavior.

Conclusions

The current findings are the first to experimentally demonstrate that stress-precipitated decreased vagal reactivity predicts the ability to resist smoking. Findings suggest that strategies that normalize vagal reactivity in early abstinent smokers may lead to improved smoking cessation outcomes.

Rationale

Most studies of the reinforcing effects of stimulants have focused on the drugs’ capacity to induce positive mood (i.e., euphoria). However, recent findings suggest drugs may also alter emotional reactivity to external stimuli, and that this may occur independently of direct effects on mood.

Objectives

We aimed to examine effects of d-amphetamine, a prototypic stimulant, on self-reported and psychophysiological reactivity to emotional stimuli as well as overall subjective mood. We predicted that amphetamine would enhance reactivity to pleasant stimuli, particularly, stimuli with social content and that these effects would be independent of the drug’s direct effects on mood.

Methods

Over three sessions, 36 healthy normal adults received placebo, d-amphetamine 10 and 20 mg under counterbalanced double-blind conditions. At each session, emotional reactivity to standardized positive, neutral, and negative pictures with and without social content was measured in self-reports and facial muscles sensitive to emotional state. Drug effects on cardiovascular variables and subjective mood were also measured.

Results

Amphetamine produced euphoria, feelings of drug effect, and increased blood pressure. Most notably, amphetamine enhanced self-reported positive reactions to all pictures and psychophysiological reactions to positive pictures. These effects were not significantly related to drug-induced mood changes. Contrary to our hypothesis, effects of amphetamine on emotional reactivity were not moderated by social content.

Conclusions

This study demonstrates a previously unexamined and potentially reinforcing effect of stimulant drugs in humans, distinct from more typically measured euphorigenic effects, and suggests new areas of research in stimulant abuse risk and adaptations occurring during drug dependence.

Rationale

Evidence for a relationship between cigarette smoking and Attention-Deficit/Hyperactivity Disorder (ADHD) has prompted investigations into nicotinic treatments for this disorder. Impulsivity is a hallmark of ADHD and is measured in the laboratory as behavioral inhibition (BI) using the Stop Signal Task (SST). Acute nicotine improves SST performance in adolescents and young adults with ADHD and impaired baseline SST performance, raising questions about the role of nicotinic acetylcholine receptor function in BI. The specificity of this effect to those with ADHD, the component processes of the SST affected by nicotine, and the effects of nicotinic antagonism are yet unknown.

Objectives

This study investigated the effects of both a nicotinic receptor agonist and antagonist on the SST and choice reaction time task (CRT) in highly impulsive (HI) and control (CTRL) subjects.

Methods

This was a within-subjects, double blind study of: 7 mg transdermal nicotine, 20 mg oral mecamylamine and placebo. Subjects were recruited into HI (n=11) and CTRL (n=14) groups based on both SST and clinical criteria.

Results

BI was significantly improved by nicotine compared to placebo in the HI group and impaired by mecamylamine in the CTRL group. Reaction time on the SST was improved by nicotine compared to placebo in the CTRL group and was unchanged in both groups on the CRT.

Conclusions

These findings demonstrate nicotinic modulation of BI in subjects with both normal and disordered baseline performance. The effects on BI are consistent with cholinergic enhancement of signal detection processes and/or modulation of noradrenaline by nicotine.

Rationale

The endocannabinoid signaling system (ECS) has been targeted for developing novel therapeutics since ECS dysfunction has been implicated in various pathologies. Current focus is on chemical modifications of the hexahydrocannabinol (HHC) nabilone (Cesamet®).

Objective

To characterize the novel, high-affinity cannabinoid receptor 1 (CB1R) HHC-ligand AM2389 [9β-hydroxy-3-(1-hexyl-cyclobut-1-yl)-hexahydrocannabinol in two rodent pre-clinical assays.

Materials and methods

CB1R mediation of AM2389-induced hypothermia in mice was evaluated with AM251, a CB1R-selective antagonist/inverse agonist. Additionally, two groups of rats discriminated the full cannabinergic aminoalkylindole AM5983 (0.18 and 0.56 mg/kg) from vehicle 20 min post-injection in a two-choice operant conditioning task motivated by 0.1% saccharin/water. Generalization/substitution tests were conducted with AM2389, AM5983, and Δ9-tetrahydrocannabinol (Δ9-THC).

Results

Δ9-THC (30 mg/kg)-induced hypothermia exhibited a faster onset and shorter duration of action compared with AM2389 (0.1 and 0.3 mg/kg). AM251 (3 and 10 mg/kg) attenuated/blocked hypothermia induced by 0.3 mg/kg AM2389. In drug discrimination, the order of potency was AM2389>AM5983>Δ9-THC with ED50 values of 0.0025, 0.0571, and 0.2635 mg/kg, respectively, in the low-dose condition. The corresponding ED50 values in the high-dose condition were 0.0069, 0.1246, and 0.8438 mg/kg, respectively. Onset of the effects of AM2389 was slow with a protracted time-course; the functional, perceptual in vivo half-life was approximately 17 h.

Conclusions

This potent cannabinergic HHC exhibited a slow onset of action with a protracted time-course. The AM2389 chemotype appears well suited for further drug development, and AM2389 currently is used to probe behavioral consequences of sustained ECS activation.

Rationale

The abuse potential of opioids may be due to their reinforcing and rewarding effects, which may be attenuated by neurokinin-1 receptor (NK1R) antagonists.

Objective

To measure the effects of opioid and neurokinin-1 (NK1R) receptor blockade on the potentiation of brain stimulation reward (BSR) by morphine using the intracranial self-stimulation (ICSS) method.

Methods

Adult male C57BL/6J mice (n = 15) were implanted with unipolar stimulating electrodes in the lateral hypothalamus and trained to respond for varying frequencies of rewarding electrical stimulation. The BSR threshold (θ0) and maximum response rate (MAX) were determined before and after intraperitoneal administration of saline, morphine (1.0 - 17.0 mg/kg), or the NK1R antagonists L-733,060 (1.0 - 17.0 mg/kg) and L-703,606 (1.0 - 17.0 mg/kg). In morphine antagonism experiments, naltrexone (0.1 – 1.0 mg/kg) or 10.0 mg/kg L-733,060 or L-703,606 was administered 15 minutes before morphine (1.0 - 10.0 mg/kg) or saline.

Results

Morphine dose-dependently decreased θ0 (maximum effect = 62% of baseline) and altered MAX when compared to saline. L-703,606 and L-733,060 altered θ0 without affecting MAX. 10.0 mg/kg L-733,060 and L-703,606, which did not affect θ0 or MAX, attenuated the effects of 3.0 and 10.0 mg/kg morphine. 1.0 and 0.3 mg/kg naltrexone blocked the effects of 10.0 mg/kg morphine. Naltrexone given before saline did not affect θ0 or MAX.

Conclusions

The decrease in θ0 by morphine reflects its rewarding effects, which were attenuated by NK1R and opioid receptor blockade. These results demonstrate the importance of substance P signaling during limbic reward system activation by opioids.

Rationale

Previous studies have shown that adenosine A2A receptors are colocalized with dopamine D2 receptors on striatal neurons. Activation of these two receptors has antagonistic effects under a number of conditions suggesting that stimulation of adenosine A2A receptors may have behavioral effects resembling those produced by blockade of dopamine D2 receptors, but this possibility has been investigated in a limited number of situations.

Objective

We compared the effects of the adenosine A2A agonist CGS-21680 and the preferential D2 dopamine antagonist haloperidol in a situation in which dopamine blockade produces a distinctive pattern of behavioral effects.

Materials and methods

Six rats were trained to lever press for food reward on a fixed ratio 15 schedule of reinforcement and then tested after being injected with various doses of CGS-21680 (0.064, 0.128, and 0.25 mg/kg) and haloperidol (0.25 and 0.1 mg/kg).

Results

Haloperidol produced a dose-dependent suppression of lever pressing with mean response rates declining across the duration of the test session. CGS-21680 also produced a dose-dependent suppression of responding, but this effect was not temporally graded, and responding was equivalently suppressed across the duration of the session. Additionally, CGS-21680 increased post-reinforcement pause duration to a much greater extent than did haloperidol.

Conclusions

On this task, the behavioral effects of CGS-21680 do not resemble those produced by haloperidol. Several explanations of this discrepancy are possible, the most likely being that the observed behavioral effects of CGS-21680 result from an action at a site other than D2 receptor-expressing striatal neurons.

Rationale

Attention dysfunction is the hallmark of cognitive deficits associated with major psychiatric illnesses including schizophrenia. Cognitive deficits of schizophrenia have been attributed to reduced function of the N-methyl-D-aspartate (NMDA) receptor or reduced expression of the gamma-aminobutyric acid (GABA)-synthesizing enzyme glutamic acid decarboxylase-67, which presumably leads to attenuated neurotransmission at GABAA receptors.

Objective

The present study used a rodent model to compare the inhibition of NMDA and GABAA receptors, and GAD activity on attention. We tested the impact of inhibiting these proteins brain wide or in the anterior cingulate cortex (ACC), a prefrontal cortex region critical for attentional processing.

Methods

Rats were trained on the three choice serial reaction time task (3-CSRT), an attention test. The impact of systemic or intra-ACC injection of drugs on performance was measured in well-trained rats.

Results

Reducing GABAA receptor function within the ACC with the direct antagonist SR95531 (1 or 3 ng/side) or brain wide using systemic injection of the benzodiazepine inverse agonist FG7142 (5 mg/kg) impaired accuracy and increased omissions. Systemic or intra-ACC inhibition of NMDA receptors using MK-801 (at 3 mg/kg or 3 μg, respectively) also impaired performance. Inhibition of GAD with 3-mercaptopropionic acid, even at high doses, had no effect on 3-CSRT accuracy or omissions when administered systemically or within the ACC.

Conclusions

These data demonstrate that, while tonic stimulation of NMDA and GABAA receptors within the ACC are critical for attentional performance, reduction in GAD activity may have little functional significance and is not indicative of reduced GABA neurotransmission.

Rationale

Abuse and neglect are highly prevalent in children and have enduring neurobiological effects. Stressful early life environments perturb the hypothalamic–pituitary–adrenal (HPA) axis, which in turn may predispose to psychiatric disorders in adulthood. However, studies of childhood maltreatment and adult HPA function have not yet rigorously investigated the differential effects of maltreatment subtypes, including physical abuse.

Objective

In this study, we sought to replicate our previous finding that childhood maltreatment was associated with attenuated cortisol responses to stress and determine whether the type of maltreatment was a determinant of the stress response.

Methods

Salivary cortisol response to the Trier Social Stress Test (TSST) was examined in a non-clinical sample of women (n=110). Subjects had no acute medical problems and were not seeking psychiatric treatment. Effects of five maltreatment types, as measured by the Childhood Trauma Questionnaire, on cortisol response to the TSST were investigated. To further examine the significant (p<0.005) effect of one maltreatment type, women with childhood physical abuse (PA) (n=20) were compared to those without past PA (n=90).

Results

Women reporting childhood PA displayed a significantly blunted cortisol response to the TSST compared with subjects without PA, after controlling for estrogen use, age, other forms of maltreatment, and other potential confounds. There were no differences between PA and control groups with regard to physiological arousal during the stress challenge.

Conclusions

In a non-clinical sample of women with minimal or no current psychopathology, physical abuse is associated with a blunted cortisol response to a psychosocial stress task.

Rationale

Individuals vary considerably in the extent to which they attribute incentive salience to food-associated cues.

Objectives

We asked whether individuals prone to attribute incentive salience to a food cue are also prone to attribute incentive properties to a stimulus associated with a drug of abuse - cocaine.

Methods

We first identified those rats that attributed incentive salience to a food cue by quantifying the extent to which they came to approach and engage a food cue. We then used a conditioned place preference procedure to pair an injection of 10 mg/kg cocaine (i.p.) with one distinct floor texture (grid or holes) and saline with another. Following 8 days of conditioning, each rat was given a saline injection and placed into a chamber that had both floors present. We measured the time spent on each floor, and also 50-kHz ultrasonic vocalizations, which have been associated with positive affective states.

Results

Rats that vigorously engaged the food cue (“sign-trackers”) expressed a preference for the cocaine-paired floor, compared to those that did not (“goal-trackers”). In addition, sign-trackers made substantially more frequency modulated 50-kHz vocalizations when injected with cocaine and when later exposed to the cocaine cue.

Conclusions

Rats prone to attribute incentive salience to a food cue are also prone to attribute incentive motivational properties to a tactile cue associated with cocaine. We suggest that individuals prone to attribute incentive salience to reward cues will have difficulty resisting them, and therefore, may be especially vulnerable to develop impulse control disorders, including addiction.

Rationale

Treatment of attention-deficit/hyperactivity disorder (ADHD) has for many years relied on psychostimulants, particularly various formulations of amphetamines and methylphenidate. These are central nervous system stimulants and are scheduled because of their abuse potential. Atomoxetine (atomoxetine hydrochloride; Strattera®) was approved in 2002 for treatment of ADHD, and was the first nonstimulant medication approved for this disorder. It was classified as an unscheduled medication indicating a low potential for abuse. However, the abuse potential of atomoxetine has not been reviewed.

Objectives

In this article, we review the evidence regarding abuse potential of atomoxetine, a selective inhibitor of the presynaptic norepinephrine transporter, which is unscheduled/unrestricted in all countries where it is approved.

Methods

Results from receptor binding, in vitro electrophysiology, in vivo microdialysis, preclinical behavioral, and human laboratory studies have been reviewed.

Results

Atomoxetine has no appreciable affinity for, or action at, central receptors through which drugs of abuse typically act, i.e., dopamine transporters, GABAA receptors, and opioid μ receptors. In behavioral experiments in rodents, atomoxetine does not increase locomotor activity, and in drug discrimination studies, its profile is similar to that of drugs without abuse potential. Atomoxetine does not serve as a reinforcer in monkey self-administration studies, and human laboratory studies suggest that atomoxetine does not induce subjective effects indicative of abuse.

Conclusion

Neurochemical, preclinical, and early clinical studies predicted and supported a lack of abuse potential of atomoxetine, which is consistent with the clinical trial and postmarketing spontaneous event data in the past 10 years.

Rationale

Early parental loss has been associated with neuroendocrine dysregulation in youth; however, the form of cortisol dysregulation varies widely. Identifying risk and protective factors that influence physiological regulation has important implications for understanding the development of mental health problems in parentally bereaved youth.

Objectives

The current study investigated the prospective effects of positive parenting on the relation between recent negative life events and cortisol activity in adolescents/young adults several years after bereavement.

Methods

Positive parenting was assessed an average of 18.5 months following parental death. Six years later, adolescents/young adults (N=55) reported on exposure to recent negative life events, and salivary cortisol was assessed before and after a conflict discussion task with their caregiver. The interaction between positive parenting and exposure to recent negative events was used to predict total cortisol output and response to the task.

Results

Multilevel modeling and the probing of the interaction effect demonstrated that total cortisol output increased with greater exposure to recent negative events among those with lower levels of past positive parenting. These relations were significant over and above current internalizing and externalizing symptoms.

Conclusions

The current results highlight the need to consider the interactive influence of proximal and distal factors on neuroendocrine functioning for youth exposed to early parental loss.

Rationale

Given evidence for age-related differences in the effects of drugs of abuse, surprisingly few preclinical studies have explored effects of opioids in adolescents (versus adults).

Objectives

This study compared the motor stimulating, ataxic, and hypothermic effects of morphine in adolescent, late-adolescent and adult mice. Plasma and brain levels of morphine were assessed to examine possible pharmacokinetic differences among the age groups.

Methods

Locomotion was measured as occlusions of horizontal infrared light beams, ataxia as failing the horizontal wire test, body temperature by rectal probe, and morphine levels by HPLC-UV.

Results

Morphine (3.2 – 56 mg/kg, i.p.) increased locomotion along an inverted U-shaped dose-response curve in adolescent, late adolescent, and adult male C57BL/6J mice. Its potency to stimulate locomotion was similar in all age groups. However, maximal stimulation was higher in adolescents than in late adolescents, and higher in late adolescents than in adults. In contrast, adolescents showed less ataxia than adults when given morphine (5.6 – 100 mg/kg, i.p.). The hypothermic effects of morphine did not differ among the age groups. Morphine levels, which peaked in plasma at 15 min after i.p. injection and in brain at 45 min, did not show age-related differences.

Conclusions

The finding that adolescents are not generally more sensitive to morphine than adults, but differ in their sensitivity to effects involving nigrostriatal/mesolimbic dopamine systems, is consistent with evidence of overactivity of these dopamine systems during adolescence relative to adulthood. The age-related differences observed here are unlikely due to pharmacokinetic factors.

Rationale and Objectives

Drug addiction is not just the repeated administration of drugs, but compulsive drug use maintained despite the accumulation of adverse consequences for the user. In an attempt to introduce adverse consequences of drug seeking to laboratory animals, we have developed the ‘conflict model’ in which the access of rats to a reinforcing lever allowing self-administration requires passing of an electrified grid floor. In this model, the current intensity leading to complete abstinence from drug seeking can be measured individually. The present study was designed to evaluated whether reinstatement of drug or natural reward seeking, despite the presence of the electrical barrier, can be achieved by presentation of discrete cues that were associated with the reward, and whether prolonged home-cage confinement can facilitate such reinstatement in this model.

Methods

The ‘conflict model’ was used to test cue-induced reinstatement in the presence of the electrical barrier, after 1 or 14 days of home-cage confinement, in groups of rats that were previously trained to self-administer cocaine or sucrose.

Results

Although similar shock intensity was required to suppress sucrose or cocaine self-administration, subjects exhibited significantly lower response to sucrose-, as compared to cocaine-, associated cues, during the reinstatement test. Importantly, cue-induced reinstatement of cocaine seeking was attenuated following 14 days of home-cage confinement.

Conclusions

The incorporation of aversive consequence in the self-administration model enable detection of what can be interpreted as a compulsive component unique to drug reinforcers. Moreover, the effect of the aversive consequence seems to increase following home-cage confinement.

Rationale and Objectives

Our knowledge about genes involved in the control of basal motor activity that may contribute to the pathology of the hyperactivity disorders, e.g. attention deficit hyperactivity disorder (ADHD), is limited. Disruption of monoamine neurotransmitter signaling through G protein-coupled receptors (GPCR) is considered to be a major contributing factor to the etiology of the ADHD. Genetic association evidence and functional data, suggest that regulators of G protein signaling proteins of the R7 family (R7 RGS) that form obligatory complexes with type 5 G protein beta subunit (Gβ5) and negatively regulate signaling downstream from monoamine GPCRs, may play a role in controlling hyperactivity.

Methods

To test this hypothesis, we conducted behavioral, pharmacological and neurochemical studies using a genetic mouse model that lacked Gβ5, a subunit essential for the expression of the entire R7 RGS family.

Results

Elimination of Gβ5-RGS complexes led to a striking level of hyperactivity that far exceeds activity levels previously observed in animal models. This hyperactivity was accompanied by motor learning deficits and, paradoxical behavioral sensitization to a novel environment. Neurochemical studies indicated that Gβ5-RGS deficient mice had higher sensitivity of inhibitory GPCR signaling and deficits in basal levels, release and reuptake of dopamine. Surprisingly, pharmacological treatment with monoamine reuptake inhibitors failed to alter hyperactivity. In contrast, blockade of NMDA receptors reversed the expression of hyperactivity in Gβ5-RGS deficient mice.

Conclusions

These findings establish that Gβ5-RGS complexes are critical regulators of monoamine-NMDA receptor signaling cross-talk and link these complexes to disorders that manifest as hyperactivity, impaired learning and motor dysfunctions.

Rationale

We have previously described a model in which adult outbred male Sprague-Dawley rats are classified as either low or high cocaine responders (LCRs or HCRs, respectively) based on acute cocaine-induced open-field activation. This model revealed important individual differences in cocaine’s effects, including that LCRs exhibited greater responding than HCRs on a progressive-ratio schedule of cocaine reinforcement. However, no LCR/HCR differences in acquisition of cocaine self-administration (0.25 mg/kg/12 s infusion) were observed under these conditions.

Objectives

To determine if LCRs and HCRs differ in the effectiveness of cocaine to function as a reinforcer under a broader range of conditions, the present study assessed the acquisition of cocaine self-administration (fixed ratio 1 schedule of reinforcement) as a function of i.v. cocaine dose (0.1875, 0.375, 0.5, 1, or 1.5 mg/kg/6 s infusion).

Results

LCRs and HCRs did not differ significantly on any measure of acquisition examined, including day to meet acquisition criterion, percent acquired, and cocaine intake. The effect of dose on percent acquired and rate of acquisition peaked at the 1 mg/kg/infusion dose of cocaine. In contrast, the effect of dose on cocaine intake was linear, with the highest rate of intake occurring at the 1.5 mg/kg/infusion dose of cocaine.

Conclusions

LCRs and HCRs do not appear to differ in their acquisition of cocaine reinforced operant responding across a range of cocaine doses, including conditions that lead to high levels of cocaine intake.

Rationale

Subtypes of 50-kHz ultrasonic vocalizations (USVs) in rats are thought to reflect positive affect and occur with cocaine or amphetamine delivery. In contexts predicting forthcoming cocaine, pre-drug anticipatory USVs are initially minimal during daily sessions but gradually escalate over several weeks, presumably as the animal learns to expect and look forward to impending drug access. To gain more insight into motivational aspects of cocaine intake in animal models of drug dependence studies, it is important to compare experience-dependent changes in lever response rate, USVs and locomotion during cocaine conditioning and extinction trials.

Objective

To address whether cocaine-induced increases in lever responding and locomotor activity correspond with USV production. The study also determined whether short-term cocaine and context deprivation effects could be detected during conditioning or extinction.

Methods

Rats underwent 20 days of 60-min sessions of self- or yoked administration of cocaine (0.75 mg/kg/infusion, i.v.), followed by 19 days of extinction training (8 weeks total, weekends off).

Results

Lever responding for cocaine and cocaine-induced locomotor activity increased across conditioning sessions. In contrast, the number of frequency modulated (FM) 50-kHz USVs evoked in response to cocaine infusion decreased with cocaine experience, suggesting perhaps tolerance to the rewarding properties of the drug. In addition, USVs but not lever pressing or locomotion are affected after brief periods of drug and/or drug context abstinence.

Conclusions

Except for initial drug exposure, increased cocaine seeking during cocaine delivery could reflect either enhanced drug motivation or the development of drug tolerance, but not enhanced positive affect.

Rationale

Delta-opioid agonists enhance the antinociceptive efficacy of methadone and other mu-opioid agonists. However, relatively little is known about the degree to which delta agonists might enhance the abuse-related effects of mu agonists.

Objective

This study used a behavioral economic approach to examine effects of the delta agonist SNC80 [(+)-4-[(αR)-α-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxy-benzyl]-N,N-diethylbenzamide] on the reinforcing effects of methadone in a drug self-administration assay. Interactions between SNC80 and cocaine were also examined for comparison.

Methods

Rhesus monkeys (n=4), surgically implanted with indwelling intravenous catheters, were tested in two phases. In phase 1, drug self-administration dose-effect curves for methadone (0.0032–0.1 mg/kg/injection (inj)) and cocaine (0.0032–0.32 mg/kg/inj) alone were determined under a fixed-ratio 10 (FR 10) schedule of reinforcement. In phase 2, FR values were increased every 3 days (FR 1–FR 1800) during availability of methadone alone (0.032 mg/kg/inj) and in combination with varying proportions of SNC80 (0.1:1, 0.3:1, and 0.9:1 SNC80/methadone) or of cocaine alone (0.032 mg/kg/inj) and in combination with varying proportions of SNC80 (0.33:1, 1:1, and 3:1 SNC80/ cocaine). Demand curves related drug intake to FR price, and measures of reinforcement were derived.

Results

Methadone and cocaine alone each functioned as a reinforcer. SNC80 did not alter measures of reinforcement for either methadone or cocaine.

Conclusions

SNC80 at proportions previously shown to enhance methadone-induced antinociception did not enhance the abuse-related effects of methadone. These results support the proposition that delta agonists may selectively enhance mu agonist analgesic effects without enhancing mu agonist abuse liability.

Rationale and objectives

Previous work has shown that wheel running reduced the maintenance of cocaine self-administration in rats. In the present study, the effect of wheel running on extinction and reinstatement of cocaine seeking was examined. Female rats were trained to run in a wheel during 6-h sessions, and they were then catheterized and placed in an operant conditioning chamber where they did not have access to the wheel but were allowed to self-administer iv cocaine. Subsequently, rats were divided into four groups and were tested on the extinction and reinstatement of cocaine seeking while they had varying access to a wheel in an adjoining compartment. The four groups were assigned to the following wheel access conditions: (1) wheel running during extinction and reinstatement (WER), (2) wheel running during extinction and a locked wheel during reinstatement (WE), (3) locked wheel during extinction and wheel running during reinstatement (WR), and (4) locked wheel during extinction and reinstatement (WL). WE and WR were retested later to examine the effect of one session of wheel access on cocaine-primed reinstatement.

Results

There were no group differences in wheel revolutions, in rate of acquisition of cocaine self-administration, or in responding during maintenance when there was no wheel access. However, during extinction, WE and WER responded less than WR and WL. WR and WER had lower cocaine-primed reinstatement than WE and WL. One session of wheel exposure in WE also suppressed cocaine-primed reinstatement.

Conclusions

Wheel running immediately and effectively reduced cocaine-seeking behavior, but concurrent access to running was necessary. Thus, exercise is a useful and self-sustaining intervention to reduce cocaine-seeking behavior.

Rationale

In schizophrenia patients, optimal treatment with antipsychotics requires weeks to months of sustained drug therapy. However, single administration of antipsychotic drugs can reverse schizophrenia-like behavioral alterations in rodent models of psychosis. This raises questions about the physiological relevance of such antipsychotic-like activity.

Objective

This study evaluates the effects of chronic treatment with clozapine on the cellular and behavioral responses induced by the hallucinogenic serotonin 5-HT2A receptor agonist lysergic acid diethylamide (LSD) as a mouse model of psychosis.

Method

Mice were treated chronically (21 days) with 25 mg/kg/day clozapine. Experiments were conducted 1, 7, 14, and 21 days after the last clozapine administration. [3H]Ketanserin binding and 5-HT2A mRNA expression were determined in mouse somatosensory cortex. Head-twitch behavior, expression of c-fos, which is induced by all 5-HT2A agonists, and expression of egr-1 and egr-2, which are LSD-like specific, were assayed.

Results

Head-twitch response was decreased and [3H]ketanserin binding was downregulated in 1, 7, and 14 days after chronic clozapine. 5-HT2A mRNA was reduced 1 day after chronic clozapine. Induction of c-fos, but not egr-1 and egr-2, was rescued 7 days after chronic clozapine. These effects were not observed after short treatment (2 days) with clozapine or chronic haloperidol (1 mg/kg/day).

Conclusion

Our findings provide a murine model of chronic atypical antipsychotic drug action and suggest downregulation of the 5-HT2A receptor as a potential mechanism involved in these persistent therapeutic-like effects.

Rationale

Alteration of dopamine neurotransmission in the prefrontal cortex, especially hypofunction of dopamine D1 receptors, contributes to psychotic symptoms and cognitive deficit in schizophrenia. D1 receptors signal through the cAMP/PKA second messenger cascade, which is modulated by phosphodiesterase (PDE) enzymes that hydrolyze and inactivate cyclic nucleotides. Though several PDEs are expressed in cortical neurons, the PDE4 enzyme family (PDE4A-D) has been implicated in the control of cognitive function. The best studied isoform, PDE4B, interacts with a schizophrenia susceptibility factor, disrupted in schizophrenia 1 (DISC1).

Objectives

We explore the control of mouse frontal cortex dopamine D1 receptor signaling and associated behavior by PDE4.

Results

Inhibition of PDE4 by rolipram induced activation of cAMP/PKA signaling in cortical slices and in vivo, leading to the phosphorylation of DARPP-32 and other postsynaptic and presynaptic PKA-substrates. Rolipram also enhanced DARPP-32 phosphorylation invoked by D1 receptor activation. Immunohistochemical studies demonstrated PDE4A, PDE4B and PDE4D expression in DARPP-32-positive neurons in layer VI of frontal cortex, most likely in D1 receptor-positive, glutamatergic corticothalamic pyramidal neurons. Furthermore, the ability of rolipram treatment to improve the performance of mice in a sensorimotor gating test was DARPP-32-dependent.

Conclusions

PDE4, which is co-expressed with DARPP-32 in D1 receptor-positive cortical pyramidal neurons in layer VI, modulates the level of D1 receptor signaling and DARPP-32 phosphorylation in the frontal cortex, likely influencing cognitive function. These biochemical and behavioral actions of PDE4 inhibitors may contribute to the hypothesized antipsychotic actions of this class of compounds.

Rationale

Methamphetamine (mAMPH) administration in animals can lead to a variety of cognitive and behavioral deficits. We previously reported non-acute reversal learning impairments after a single-day administration of mAMPH, providing evidence of this drug’s selective effects on inhibitory control. Effortful decision-making (i.e., how much effort to invest in rewards) is an aspect of cognition that has not yet been explored after mAMPH.

Objectives

Given that frontostriatal circuitry mediating this type of choice is vulnerable to the effects of mAMPH, we tested the hypothesis that mAMPH may also affect decision-making involving effort, another form of cognitive flexibility.

Methods

We examined the non-acute effects of an experimenter-administered single day of mAMPH on effort discounting. In this task, rats previously treated with mAMPH or saline (SAL) could select a high reward at the cost of climbing over a tall barrier or a low reward with no barrier impeding its procurement.

Results

Following treatment, mAMPH rats were more work-averse than SAL rats. A control task showed there were no treatment group differences when the high and low rewards involved equal work: all rats chose the high reward preferentially. There were no significant treatment group differences in [125I]RTI-55 binding to dopamine and serotonin transporters (DAT, SERT) in any of the regions assayed; however, there were significant correlations of accumbens DAT and cingulate SERT with post-treatment performance.

Conclusions

These findings suggest that even modest dose mAMPH exposure has long-lasting effects on effortful decision-making and may do so through influences on forebrain monoaminergic systems.