Xylazine suppresses fentanyl consumption during self-administration and induces a unique sex-specific withdrawal syndrome that is not altered by naloxone in rats

Prescription and illicit opioid use are a public health crisis, with the landscape shifting to fentanyl use. Since fentanyl is 100-fold more potent than morphine, its use is associated with a higher risk of fatal overdose that can be remediated through naloxone (Narcan) administration. However, recent reports indicate that xylazine, an anesthetic, is increasingly detected in accidental fentanyl overdose deaths. Anecdotal reports suggest that xylazine may prolong the fentanyl "high," alter the onset of fentanyl withdrawal, and increase resistance to naloxone-induced reversal of overdose. To date, no preclinical studies have evaluated the impacts of xylazine on fentanyl self-administration (SA; 2.5 μg/kg/infusion) or withdrawal to our knowledge. We established a rat model of xylazine/fentanyl co-SA and withdrawal and evaluated outcomes as a function of biological sex. When administered alone, chronic xylazine (2.5 mg/kg, intraperitoneal) induced unique sex-specific withdrawal symptomatology, whereby females showed delayed onset of signs and a possible enhancement of sensitivity to the motor-suppressing effects of xylazine. Xylazine reduced fentanyl consumption in both male and female rats regardless of whether it was experimenter-administered or added to the intravenous fentanyl product (0.05, 0.10, and 0.5 mg/kg/infusion) when compared to fentanyl SA alone. Interestingly, this effect was dose-dependent when self-administered intravenously. Naloxone (0.1 mg/kg, subcutaneous injection) did not increase somatic signs of fentanyl withdrawal, regardless of the inclusion of xylazine in the fentanyl infusion in either sex; however, somatic signs of withdrawal were higher across time points in females after xylazine/fentanyl co-SA regardless of naloxone exposure as compared to females following fentanyl SA alone. Together, these results indicate that xylazine/fentanyl co-SA dose-dependently suppressed fentanyl intake in both sexes and induced a unique withdrawal syndrome in females that was not altered by acute naloxone treatment. (PsycInfo Database Record (c) 2024 APA, all rights reserved).

Impacts of xylazine on fentanyl demand, body weight, and acute withdrawal in rats: A comparison to lofexidine

The opioid use landscape has recently shifted to include xylazine, a veterinary anesthetic, as an adulterant in the fentanyl supply. The health impacts of xylazine as an emerging fentanyl adulterant has raised alarm regarding xylazine as a public health threat, warranting research on the impacts of xylazine on fentanyl's behavioral effects. No prior studies have evaluated the effects of xylazine on fentanyl consumption at various unit doses, fentanyl demand, or withdrawal as compared to the Food and Drug Administration-approved opioid withdrawal medication, lofexidine (Lucemyra®). This is important because lofexidine and xylazine are both adrenergic α2a (A2aR) agonists, however, lofexidine is not a noted fentanyl adulterant. Here we evaluated xylazine and lofexidine combined with self-administered fentanyl doses in male and female rats and evaluated fentanyl demand, body weight, and acute withdrawal. Consumption of fentanyl alone increased at various unit doses compared to saline. Xylazine but not lofexidine shifted fentanyl consumption downward at a number of unit doses, however, both lofexidine and xylazine suppressed fentanyl demand intensity as compared to a fentanyl alone control group. Further, both fentanyl + lofexidine and fentanyl + xylazine reduced behavioral signs of fentanyl withdrawal immediately following SA, but signs increased by 12 h only in the xylazine co-exposed group. Weight loss occurred throughout fentanyl SA and withdrawal regardless of group, although the xylazine group lost significantly more weight during the first 24 h of withdrawal than the other two groups. Severity of weight loss during the first 24 h of withdrawal was also correlated with severity of somatic signs of fentanyl withdrawal. Together, these results suggest that body weight loss may be an important indicator of withdrawal severity during acute withdrawal from the xylazine/fentanyl combination, warranting further translational evaluation.

Synthetic contraceptive hormones occlude the ability of nicotine to reduce ethanol consumption in ovary-intact female rats

Rates of tobacco and alcohol use in women are rising, and women are more vulnerable than men to escalating tobacco and alcohol use. Many women use hormonal birth control, with the oral contraceptive pill being the most prevalent. Oral contraceptives contain both a progestin (synthetic progesterone) and a synthetic estrogen (ethinyl estradiol; EE) and are contraindicated for women over 35 years who smoke. Despite this, no studies have examined how synthetic contraceptive hormones impact this pattern of polysubstance use in females. To address this critical gap in the field, we treated ovary-intact female rats with either sesame oil (vehicle), the progestin levonorgestrel (LEVO; contained in formulations such as Alesse®), or the combination of EE+LEVO in addition to either undergoing single (nicotine or saline) or polydrug (nicotine and ethanol; EtOH) self-administration (SA) in a sequential use model. Rats preferred EtOH over water following extended EtOH drinking experience as well as after nicotine or saline SA experience, and rats undergoing only nicotine SA (water controls) consumed more nicotine as compared to rats co-using EtOH and nicotine. Importantly, this effect was occluded in groups treated with contraceptive hormones. In the sequential use group, both LEVO alone and the EE+LEVO combination occluded the ability of nicotine to decrease EtOH consumption. Interestingly, demand experiments suggest an economic substitute effect between nicotine and EtOH. Together, we show that chronic synthetic hormone exposure impacts nicotine and EtOH sequential use, demonstrating the crucial need to understand how chronic use of different contraceptive formulations alter patterns of polydrug use in women.

Natural and synthetic estrogens specifically alter nicotine demand and cue-induced nicotine seeking in female rats

Women have more difficulty maintaining smoking cessation than men, and experience greater withdrawal symptomatology as well as higher prevalence of relapse. Further, currently available treatments for smoking cessation, such as the nicotine patch and varenicline, have been shown to be less effective in women. Fluctuations in ovarian hormones across the menstrual cycle can affect craving and smoking relapse propensity. In addition, many women who smoke use some form of oral contraceptives, which most often contain ethinyl estradiol (EE), a synthetic, orally bio-available estrogen that is currently prescribed to women chronically and has been shown to alter smoking reward in women. The current study examined the impact of 17β-estradiol (E2), the prominent endogenous form of the steroid hormone estrogen, as well as EE, on nicotine self-administration, demand, and reinstatement following ovariectomy (OVX) or sham surgery. OVX vehicle-treated female rats consumed less nicotine, had lower intensity of demand, and reinstated less compared to sham vehicle-treated female rats. OVX-E2 and OVX-EE treatment groups showed a rebound of nicotine intake later in training, and Q₀ levels of consumption were partially rescued in both groups. Further, E2 but not EE reversed the abolishment of reinstated nicotine seeking induced by OVX. Taken together, these results demonstrate that natural and synthetic estrogens play a critical role in mediating the neurobehavioral effects of nicotine, and future studies are essential for our understanding of how synthetic hormones contained within oral contraceptives interact with smoking.

Quantifying value-based determinants of drug and non-drug decision dynamics

RATIONALE

A growing body of research suggests that substance use disorder (SUD) may be characterized as disorders of decision making. However, drug choice studies assessing drug-associated decision making often lack more complex and dynamic conditions that better approximate contexts outside the laboratory and may lead to incomplete conclusions regarding the nature of drug-associated value.

OBJECTIVES

The current study assessed isomorphic (choice between identical food options) and allomorphic (choice between remifentanil [REMI] and food) choice across dynamically changing reward probabilities, magnitudes, and differentially reward-predictive stimuli in male rats to better understand determinants of drug value. Choice data were analyzed at aggregate and choice-by-choice levels using quantitative matching and reinforcement learning (RL) models, respectively.

RESULTS

Reductions in reward probability or magnitude independently reduced preferences for food and REMI commodities. Inclusion of reward-predictive cues significantly increased preference for food and REMI rewards. Model comparisons revealed that reward-predictive stimuli significantly altered the economic substitutability of food and REMI rewards at both levels of analysis. Furthermore, model comparisons supported the reformulation of reward value updating in RL models from independent terms to a shared, relative term, more akin to matching models.

CONCLUSIONS

The results indicate that value-based quantitative choice models can accurately capture choice determinants within complex decision-making contexts and corroborate drug choice as a multidimensional valuation process. Collectively, the present study indicates commonalities in decision-making for drug and non-drug rewards, validates the use of economic-based SUD therapies (e.g., contingency management), and implicates the neurobehavioral processes underlying drug-associated decision-making as a potential avenue for future SUD treatment.

Remifentanil-food choice follows predictions of relative subjective value

BACKGROUND

Preclinical studies into drug vs. nondrug choice have emerged to better model and investigate the neurobehavioral mechanisms underlying drug preference. Current literature has suggested that drugs of abuse have inherently low value, thus promoting food preference. Herein, we examined remifentanil vs. food choice to test both the relative value hypothesis and the 'direct effects' (pharmacological effects of drugs on alternative reinforcers) hypothesis of opioid preference.

METHODS

Adult male rats were trained under two choice procedures (controlled vs. uncontrolled reinforcer frequency) for remifentanil vs. food choice. Furthermore, a series of procedural manipulations known to affect drug reinforcement were tested under both choice procedures. Using remifentanil self-administration data, pharmacokinetic profiles were calculated and analyzed to determine if opioid intake was related to opioid preference.

RESULTS

Both choice procedures produced dose-dependent preference. Moreover, procedural manipulations produced comparable changes in remifentanil preference under both choice procedures. In addition, calculated pharmacokinetic data revealed that preference was dissociable from intake under the controlled reinforcer frequency choice procedure.

CONCLUSIONS

When compared to the 'direct effects' hypothesis, remifentanil preference was better predicted by the relative value hypothesis, formalized in generalized matching. Use of a controlled reinforcer frequency schedule successfully removed the drug preference-intake confound found in most drug-choice procedures. Importantly, drug preference under the controlled reinforcer frequency schedule remained sensitive to procedural manipulations known to affect drug reinforcement. Thus, given that differential drug intake itself affects neurobiological measurements, future use of controlled reinforcer frequency schedules may help to better isolate the neurobehavioral mechanisms that mediate opioid preference.

Neuronal activity associated with cocaine preference: Effects of differential cocaine intake

Differences in overall cocaine intake can directly affect neuroadaptations, and this relationship can make it difficult to interpret neurobiological changes seen in drug-choice studies, since drug intake varies between subjects. Herein, a choice procedure that controls for cocaine intake was utilized to explore if neuronal activity, measured as cFos expression in the orbitofrontal cortex (OFC) and nucleus accumbens (NAc), was reflective of preference. Results demonstrated that cFos expression, in both the OFC and NAc, was independent of cocaine preference when cocaine intake was kept constant across individuals. However, when cocaine intake was systematically varied, the expression of cFos associated with cocaine preference was related to overall cocaine intake in the OFC, but not the NAc. Altogether, these results demonstrate that cocaine intake during choice can affect neurobiological outcome measures; thus, the neurobehavioral mechanisms underlying cocaine preference may be better isolated when controlling for cocaine frequency and intake. In all, some caution is warranted when interpreting results from choice studies evaluating the neurobehavioral mechanisms that underlie drug preference when drug frequency and intake are uncontrolled, and future research is needed to determine the role of drug frequency and intake on neurobiological measures associated with drug choice.

Changes in fentanyl demand following naltrexone, morphine, and buprenorphine in male rats

BACKGROUND

Individuals with opioid use disorder (OUD) exhibit high levels of economic demand for opioids, with high levels of consumption and relative insensitivity to changes in price. Because the medications used to treat OUD in medication-assisted therapy (MAT) act as antagonists or agonists at μ opioid receptors, they may alter the relationship between price and opioid intake.

METHODS

This study examined demand for a commonly abused synthetic prescription opioid, fentanyl, in male rats following s.c. pre-treatment with naltrexone (0.1-1.0 mg/kg), morphine (0.3-3.0 mg/kg) or buprenorphine (0.3-3.0 mg/kg). We normalized demand curves to intake at the lowest price and estimated effects on elasticity (sensitivity to changes in price). Rats were first trained to earn fentanyl (5 μg/kg/infusion) on a fixed ratio schedule, then they underwent daily training under a threshold procedure designed to produce within-session demand curve estimates. Rats received 14 threshold sessions before undergoing a series of tests encompassing each drug, at each dose.

RESULTS

Elasticity was increased by pretreatment with naltrexone, morphine or buprenorphine. Morphine also decreased initial intake, when the price for fentanyl was lowest. In contrast, initial intake was increased by naltrexone (according to an inverted-U shaped curve). The effects of naltrexone did not persist after the test session, but morphine and buprenorphine continued affecting demand elasticity 24 h or 48 h after the test, respectively.

CONCLUSIONS

These results indicate that fentanyl demand is sensitive to blockade or activation of opioid receptors by the drug classes used for MAT in humans.

Economic demand analysis of within-session dose-reduction during nicotine self-administration

BACKGROUND

This study determined if a within-session dose-reduction design sufficiently captures elasticity of demand for nicotine in male and female rats using environmental enrichment to manipulate demand elasticity.

METHODS

Male and female Sprague-Dawley rats were trained to self-administer nicotine (60 μg/kg/infusion). In Experiment 1, rats began daily dose-reduction for nine sessions following acquisition. Rats then underwent a minimum of five within-session dose-reduction sessions where each dose was available for 10 min. In Experiment 2, rats were reared in isolated, social, or enriched housing followed by acquisition of nicotine self-administration. Rats then underwent within-session dose-reduction. Housing environments were then switched, followed by additional testing sessions. Consumption was calculated for each dose and exponential demand curves were fit.

RESULTS

No sex differences in acquisition of nicotine self-administration were detected for either experiment. In experiment 1, demand intensity (Q₀; estimated intake if nicotine were freely available), was higher with between- compared to within-session dose-reduction, although elasticity of demand (α; rate of decline in nicotine intake as a function of increasing unit price), was lower. In Experiment 2, animals reared in enrichment had fewer infusions during acquisition compared to animals in isolation. Enriched males had reduced demand intensity compared to both isolated and social males, whereas isolated females had reduced intensity compared to enriched females.

CONCLUSIONS

The within-session dose-reduction procedure for nicotine self-administration replicated effects of environmental enrichment on consumption behaviors. Additionally, this procedure captured differences in nicotine demand due to sex, laying important groundwork for future translational research on mechanisms of nicotine dependence.

Cocaine-associated decision-making: Toward isolating preference

Ever-increasing evidence suggests that substance use disorder is mediated by decision-making processes, and as such, providing nondrug alternatives can shift maladaptive preferences away from drug reinforcers, such as cocaine. Of note, a recent hypothesis suggests that preference for cocaine is simply a byproduct of cocaine intake, such that the 'direct' effects of cocaine weaken the impact of non-drug alternatives while measuring choice. Conversely, existing quantitative theories of decision-making suggest preference is determined by various dimensions of concurrent reinforcers that in turn determine the relative value of available alternatives. Toward teasing apart the conflicting theories above, we developed a novel drug-choice procedure to control for reinforcer frequency and magnitude (two reinforcer dimensions well known to influence preference) that consequently controls for overall cocaine intake. As predicted by quantitative choice theory, results suggest that cocaine intake and preference are dissociable while measuring choice, with reinforcer frequency and magnitude having independent influence on the relative value of choice alternatives. Furthermore, we demonstrate that the choice procedure is sensitive to various manipulations known to alter cocaine reinforcement, all while keeping cocaine intake constant. Finally, the results point to the process of economic substitution as an important avenue of future neurobehavioral investigation toward the improvement of behavioral and pharmacological therapies for substance use disorders. Overall, the proposed choice procedure will allow for improved isolation of the neurobehavioral processes that mediate drug-associated decision-making in future studies.

Environmental enrichment and drug value: a behavioral economic analysis in male rats

Rats raised in an enriched condition (EC) show decreased stimulant self-administration relative to rats reared in an isolated condition (IC). However, few studies have examined the behavioral mechanisms underlying this environment-induced difference in self-administration. Because economic demand for drugs of abuse predicts addiction-like behavior in both humans and animals, we applied a behavioral economic analysis to cocaine self-administration data in EC and IC rats. During cocaine self-administration, the dose decreased across blocks of trials (0.75-0.003 mg/kg/inf), which allowed for a determination of demand intensity and demand elasticity. Demand intensity did not differ between EC and IC rats; however, cocaine was more elastic in EC rats relative to IC rats (i.e. EC rats were less willing to respond for cocaine as the unit price increased). When EC rats were placed in an isolated condition, demand elasticity decreased, whereas elasticity increased for IC rats placed in an enriched condition. Additionally, we applied behavioral economic analyses to previously published self-administration data and found that our results replicate past findings with cocaine and methylphenidate. To determine if differences in demand elasticity are specific to drug reinforcement, a separate group of rats was tested in sucrose or saccharin self-administration. Results showed that sucrose and saccharin were more elastic in EC rats relative to IC rats, and demand intensity was lower for saccharin in EC rats relative to IC rats. Overall, drug and nondrug reinforcers are more elastic in EC rats, which may account for the protective effects of environmental enrichment against stimulant self-administration.

Contribution of cocaine-related cues to concurrent monetary choice in humans

RATIONALE

Theoretical accounts highlight the importance of drug-related cues for the development and persistence of drug-taking behavior. Few studies have evaluated the ability of spatially contiguous drug cues to bias decisions between two concurrently presented non-drug reinforcers.

OBJECTIVE

Evaluate the contribution of spatially contiguous cocaine cues to choice between two concurrently presented monetary reinforcers METHODS: Participants with cocaine use disorder completed a cued concurrent choice task. Two cues (one cocaine and one control image) were presented side-by-side followed by concurrent monetary offers below each image. Concurrent choice was measured for cocaine-side advantageous, equal, and disadvantageous concurrent monetary offers. The primary dependent measure was bias for selecting cocaine-cued monetary reinforcers. Three experiments tested selectivity of cocaine-cued bias in individuals with a cocaine use history (Experiment 1), replication when including additional control trials (Experiment 2), and a potential attentional mechanism evaluated using eye-tracking technology (Experiment 3).

RESULTS

Significant and robust cocaine-cued bias at equal monetary value was observed in three experiments (mean percent choice = 65-77%) and higher Drug Abuse Screening Test (DAST) scores were associated with greater cocaine-choice bias. These experiments demonstrated that cocaine-cued bias was (1) selective to individuals with a cocaine use history, (2) specific to trials involving a cocaine cue, and (3) partially associated with attentional bias.

CONCLUSIONS

These experiments provide evidence that drug-related cues can influence choice and potentially promote maladaptive decision making during concurrent choice events. Future research evaluating prospective associations of drug-cued bias with drug-associated behaviors will help reveal the clinical relevance for substance use disorder.

Differential stimulus control of drug-seeking: multimodal reinstatement

In animal models of substance-use disorder, individuals that repeatedly self-administer drugs of abuse have long-lasting neuronal adaptations that do not occur ostensibly in control animals only exposed to natural reinforcers (e.g. food). Because any treatment for substance-use disorder will be given to individuals with drug-taking histories, adequate dissociation of the specific neurobehavioral mechanisms underlying drug reinforcement, natural reinforcement and their associated cue effects requires an experimental model that exposes individuals to both reinforcer conditions, along with their associated stimuli. Furthermore, contingent stimuli that reinforce drug seeking through second-order relationships may produce reinstatement of drug seeking through different neurobehavioral means than non-contingent exposure to stimuli that signal the availability of a drug reinforcer, effectively producing different modes of stimulus-induced reinstatement. Toward experimental isolation of the relationships mentioned, herein, we used a within-session multiple schedule of reinforcement containing both discriminative (S^D ) and conditioned (CS) stimuli to study stimulus control of drug-taking and food-taking behavior, along with how these functionally distinct cues may differentially reinstate drug-seeking and food-seeking behavior within a single animal. We demonstrate specific stimulus control over drug and food taking; furthermore, we demonstrate that the same stimulus (i.e. cue light) operating as an S^D or CS produced differential reinstatement of drug-taking and food-taking behavior. The results suggest that contingent CSs and non-contingent S^D s produce reinstatement through different neurobehavioral processes and, within-session multiple schedules, can be used to study different modes of specific stimulus control over drug and food seeking in a single animal with both drug-taking and food-taking history.

The role of glutamate signaling in incentive salience: second-by-second glutamate recordings in awake Sprague-Dawley rats

The attribution of incentive salience to reward-predictive stimuli has been shown to be associated with substance abuse-like behavior such as increased drug taking. Evidence suggests that glutamate neurotransmission and sequential N-methyl-D-aspartate (NMDA) activation are involved in the attribution of incentive salience. Here, we further explore the role of second-by-second glutamate neurotransmission in the attribution of incentive salience to reward-predictive stimuli by measuring sign-tracking behavior during a Pavlovian conditioned approach procedure using ceramic-based microelectrode arrays configured for sensitive measures of extracellular glutamate in awake behaving Sprague-Dawley rats. Specifically, we show that there is an increase in extracellular glutamate levels in the prelimbic cortex (PrL) and the nucleus accumbens core (NAcC) during sign-tracking behavior to a food-predictive conditioned stimulus (CS+) compared to the presentation of a non-predictive conditioned stimulus (CS-). Furthermore, the results indicate greater increases in extracellular glutamate levels in the PrL compared to NAcC in response to the CS+, including differences in glutamate release and signal decay. Taken together, the present research suggests that there is differential glutamate signaling in the NAcC and PrL during sign-tracking behavior to a food-predictive CS+.

The role of 'jackpot' stimuli in maladaptive decision-making: dissociable effects of D1/D2 receptor agonists and antagonists

RATIONALE

Laboratory experiments often model risk through a choice between a large, uncertain (LU) reward against a small, certain (SC) reward as an index of an individual's risk tolerance. An important factor generally lacking from these procedures are reward-associated cues that may modulate risk preferences.

OBJECTIVE

We tested whether the addition of cues signaling 'jackpot' wins to LU choices would modulate risk preferences and if these cue effects were mediated by dopaminergic signaling.

METHODS

Three groups of rats chose between LU and SC rewards for which the LU probability of reward decreased across blocks. The unsignaled group received a non-informative stimulus of trial outcome. The signaled group received a jackpot signal prior to reward delivery and blackout on losses. The signaled-light group received a similar jackpot for wins, but a salient loss signal distinct from the win signal.

RESULTS

Presenting win signals decreased the discounting of LU value for both signaled groups regardless of loss signal, while the unsignaled group showed discounting similar to previous research without cues. Pharmacological challenges with D1/D2 agonists and antagonists revealed that D1 antagonism increased and decreased sensitives to the relative probability of reward for unsignaled and signaled groups, respectively, while D2 agonists decreased sensitivities to the relative magnitude of reward.

CONCLUSION

The results highlight how signals predictive of wins can promote maladaptive risk taking in individuals, while loss signals have reduced effect. Additionally, the presence of reward-predictive cues may change the underlying neurobehavioral mechanisms mediating decision-making under risk.

The Winding Road to Relapse: Forging a New Understanding of Cue-Induced Reinstatement Models and Their Associated Neural Mechanisms

In drug addiction, cues previously associated with drug use can produce craving and frequently trigger the resumption of drug taking in individuals vulnerable to relapse. Environmental stimuli associated with drugs or natural reinforcers can become reliably conditioned to increase behavior that was previously reinforced. In preclinical models of addiction, these cues enhance both drug self-administration and reinstatement of drug seeking. In this review, we will dissociate the roles of conditioned stimuli as reinforcers from their modulatory or discriminative functions in producing drug-seeking behavior. As well, we will examine possible differences in neurobiological encoding underlying these functional differences. Specifically, we will discuss how models of drug addiction and relapse should more systematically evaluate these different types of stimuli to better understand the neurobiology underlying craving and relapse. In this way, behavioral and pharmacotherapeutic interventions may be better tailored to promote drug use cessation outcomes and long-term abstinence.

A pilot study of loss aversion for drug and non-drug commodities in cocaine users

BACKGROUND

Numerous studies in behavioral economics have demonstrated that individuals are more sensitive to the prospect of a loss than a gain (i.e., loss aversion). Although loss aversion has been well described in "healthy" populations, little research exists in individuals with substance use disorders. This gap is notable considering the prominent role that choice and decision-making play in drug use. The purpose of this pilot study was to evaluate loss aversion in active cocaine users.

METHODS

Current cocaine users (N=38; 42% female) participated in this within-subjects laboratory pilot study. Subjects completed a battery of tasks designed to assess loss aversion for drug and non-drug commodities under varying risk conditions. Standardized loss aversion coefficients (λ) were compared to theoretically and empirically relevant normative values (i.e., λ=2).

RESULTS

Compared to normative loss aversion coefficient values, a precise and consistent decrease in loss aversion was observed in cocaine users (sample λ≈1). These values were observed across drug and non-drug commodities as well as under certain and risky conditions.

CONCLUSIONS

These data represent the first systematic study of loss aversion in cocaine-using populations and provide evidence for equal sensitivity to losses and gains or loss equivalence. Futures studies should evaluate the specificity of these effects to a history of cocaine use as well as the impact of manipulations of loss aversion on drug use to determine how this phenomenon may contribute to intervention development efforts.

NMDA receptor blockade specifically impedes the acquisition of incentive salience attribution

Glutamatergic signaling plays an important role in learning and memory. Using Pavlovian conditioned approach procedures, the mechanisms that drive stimulus-reward learning and memory have been investigated. However, there are instances where reward-predictive stimuli can function beyond being solely predictive and can be attributed with "motivational value" or incentive salience. Using a Pavlovian conditioned approach procedure consisting of two different but equally predictive stimuli (lever vs. tone) we investigated the role NMDA receptor function has in the attribution of incentive salience. The results revealed that the administration of MK-801, an NMDA receptor antagonist, during acquisition of Pavlovian conditioned approach promoted goal-tracking to a lever stimulus, while control animals learned to sign-track. Moreover, within the same animals, the use of a tone stimulus elicited goal-tracking responses that were unaffected by MK-801 pretreatments. Furthermore, a lever CS that elicited sign-tracking served as a more robust conditioned reinforcer than a tone CS that elicited goal-tracking or a lever CS that elicited goal-tracking via MK-801 pretreatments. Collectively, these results demonstrate that NMDA receptor antagonism can alter the stimulus-reward relationship learned and prevent the attribution of incentive salience, rather than impede general learning.

Suboptimal choice in rats: Incentive salience attribution promotes maladaptive decision-making

Stimuli that are more predictive of subsequent reward also function as better conditioned reinforcers. Moreover, stimuli attributed with incentive salience function as more robust conditioned reinforcers. Some theories have suggested that conditioned reinforcement plays an important role in promoting suboptimal choice behavior, like gambling. The present experiments examined how different stimuli, those attributed with incentive salience versus those without, can function in tandem with stimulus-reward predictive utility to promote maladaptive decision-making in rats. One group of rats had lights associated with goal-tracking as the reward-predictive stimuli and another had levers associated with sign-tracking as the reward-predictive stimuli. All rats were first trained on a choice procedure in which the expected value across both alternatives was equivalent but differed in their stimulus-reward predictive utility. Next, the expected value across both alternatives was systematically changed so that the alternative with greater stimulus-reward predictive utility was suboptimal in regard to primary reinforcement. The results demonstrate that in order to obtain suboptimal choice behavior, incentive salience alongside strong stimulus-reward predictive utility may be necessary; thus, maladaptive decision-making can be driven more by the value attributed to stimuli imbued with incentive salience that reliably predict a reward rather than the reward itself.

Mechanisms of midsession reversal accuracy: Memory for preceding events and timing

The midsession reversal task involves a simultaneous discrimination between 2 stimuli (S1 and S2) in which, for the first half of each session, choice of S1 is reinforced and, for the last half, choice of S2 is reinforced. On this task, pigeons appear to time the occurrence of the reversal rather than using feedback from previous trials, resulting in increased numbers of errors. In the present experiments, we tested the hypothesis that pigeons make so many errors because they fail to remember the last response made and/or the consequence of making that response both of which are needed ideally as cues to respond on the next trial. To facilitate memory, during the 5-s intertrial interval, we differentially lit a houselight correlated with the prior response to S1 or S2 and maintained the hopper light when that response was correct. A control group received uncorrelated houselights and no maintained hopper light. To test for continued use of temporal information, both groups received probe sessions in which the intertrial interval was either halved or doubled. Providing relevant reminder cues of the stimulus chosen and its consequence resulted in improved reversal accuracy and reduced disruption from probe sessions compared with irrelevant cues. Nevertheless, despite the reminder cues, the pigeons in both groups appeared to continue to time the point in the session at which the reversal occurred. (PsycINFO Database Record

Rearing environment differentially modulates cocaine self-administration after opioid pretreatment: A behavioral economic analysis

BACKGROUND

Research has shown that previous experiences during development, especially if stressful, can alter an organism's response to opioids later in life. Given the previous literature on opioid modulation of cocaine self-administration, the current study raised rats in either an enriched condition (EC) or isolated condition (IC) and employed behavioral economics to study the effects of naltrexone and morphine on cocaine self-administration.

METHODS

EC and IC rats were trained to lever press for cocaine using a within-session demand procedure. This procedure measured cocaine consumption under changing cocaine price by decreasing the dose of cocaine earned throughout a session. Rats were able to self-administer cocaine on a FR1; every 10min the cocaine dose was systematically decreased (0.75-0.003mg/kg/infusion cocaine). After reaching stability on this procedure, rats were randomly pretreated with 0, 0.3, 1, or 3mg/kg naltrexone once every 3days, followed by random pretreatments of 0, 0.3, 1, or 3mg/kg morphine once every 3days. Economic demand functions were fit to each rat's cocaine consumption from each pretreatment, and appropriate mathematical parameters were extracted and analyzed.

RESULTS

Naltrexone decreased the essential value of cocaine in IC rats only. However, morphine decreased the essential value of cocaine and the consumption of cocaine at zero price in both EC and IC rats.

CONCLUSION

These results indicate that environmental experiences during development should be considered when determining the efficacy of opioid drugs, especially for the treatment of substance abuse.

Suboptimal Choice in Pigeons: Stimulus Value Predicts Choice over Frequencies

Pigeons have shown suboptimal gambling-like behavior when preferring a stimulus that infrequently signals reliable reinforcement over alternatives that provide greater reinforcement overall. As a mechanism for this behavior, recent research proposed that the stimulus value of alternatives with more reliable signals for reinforcement will be preferred relatively independently of their frequencies. The present study tested this hypothesis using a simplified design of a Discriminative alternative that, 50% of the time, led to either a signal for 100% reinforcement or a blackout period indicative of 0% reinforcement against a Nondiscriminative alternative that always led to a signal that predicted 50% reinforcement. Pigeons showed a strong preference for the Discriminative alternative that remained despite reducing the frequency of the signal for reinforcement in subsequent phases to 25% and then 12.5%. In Experiment 2, using the original design of Experiment 1, the stimulus following choice of the Nondiscriminative alternative was increased to 75% and then to 100%. Results showed that preference for the Discriminative alternative decreased only when the signals for reinforcement for the two alternatives predicted the same probability of reinforcement. The ability of several models to predict this behavior are discussed, but the terminal link stimulus value offers the most parsimonious account of this suboptimal behavior.

New quality measure for SNP array based CNV detection

MOTIVATION

Only a few large systematic studies have evaluated the impact of copy number variants (CNVs) on common diseases. Several million individuals have been genotyped on single nucleotide variation arrays, which could be used for genome-wide CNVs association studies. However, CNV calls remain prone to false positives and only empirical filtering strategies exist in the literature. To overcome this issue, we defined a new quality score (QS) estimating the probability of a CNV called by PennCNV to be confirmed by other software.

RESULTS

Out-of-sample comparison showed that the correlation between the consensus CNV status and the QS is twice as high as it is for any previously proposed CNV filters. ROC curves displayed an AUC higher than 0.8 and simulations showed an increase up to 20% in statistical power when using QS in comparison to other filtering strategies. Superior performance was confirmed also for alternative consensus CNV definition and through improving known CNV-trait associations.

AVAILABILITY AND IMPLEMENTATION

http://goo.gl/T6yuFM CONTACT: zoltan.kutalik@unil.ch or aurelien@mace@unil.chSupplementary information: Supplementary data are available at Bioinformatics online.

Toward isolating the role of dopamine in the acquisition of incentive salience attribution

Stimulus-reward learning has been heavily linked to the reward-prediction error learning hypothesis and dopaminergic function. However, some evidence suggests dopaminergic function may not strictly underlie reward-prediction error learning, but may be specific to incentive salience attribution. Utilizing a Pavlovian conditioned approach procedure consisting of two stimuli that were equally reward-predictive (both undergoing reward-prediction error learning) but functionally distinct in regard to incentive salience (levers that elicited sign-tracking and tones that elicited goal-tracking), we tested the differential role of D1 and D2 dopamine receptors and nucleus accumbens dopamine in the acquisition of sign- and goal-tracking behavior and their associated conditioned reinforcing value within individuals. Overall, the results revealed that both D1 and D2 inhibition disrupted performance of sign- and goal-tracking. However, D1 inhibition specifically prevented the acquisition of sign-tracking to a lever, instead promoting goal-tracking and decreasing its conditioned reinforcing value, while neither D1 nor D2 signaling was required for goal-tracking in response to a tone. Likewise, nucleus accumbens dopaminergic lesions disrupted acquisition of sign-tracking to a lever, while leaving goal-tracking in response to a tone unaffected. Collectively, these results are the first evidence of an intraindividual dissociation of dopaminergic function in incentive salience attribution from reward-prediction error learning, indicating that incentive salience, reward-prediction error, and their associated dopaminergic signaling exist within individuals and are stimulus-specific. Thus, individual differences in incentive salience attribution may be reflective of a differential balance in dopaminergic function that may bias toward the attribution of incentive salience, relative to reward-prediction error learning only.

Buspirone reduces sexual risk-taking intent but not cocaine self-administration

Impulsive sexual decision-making may underlie sexual risk-taking behavior that contributes to the disproportionately high prevalence of HIV infection among cocaine users. Delay-discounting procedures measure impulsive decision-making and may provide insight into the underlying mechanisms of sexual risk-taking behavior. The anxiolytic drug buspirone reduces delay discounting in rats and blunts the reinforcing effects of cocaine in some preclinical studies suggesting that it might have utility in the treatment of cocaine-use disorders. This study determined whether buspirone mitigates impulsive risky sexual decision-making in cocaine users on a sexual delay-discounting procedure. The effects of buspirone maintenance on the abuse-related and physiological effects of cocaine were also tested. Nine (N = 9) current cocaine users completed a repeated-measures, inpatient protocol in which sexual delay discounting was assessed after 3 days of maintenance on placebo and buspirone (30 mg/day) in counterbalanced order. The reinforcing, subject-rated, and physiological effects of placebo and intranasal cocaine (15 and 45 mg) were also assessed during buspirone and placebo maintenance. Buspirone increased the likelihood of condom use for hypothetical sexual partners that were categorized as most likely to have a sexually transmitted infection and least sexually desirable. Cocaine functioned as a reinforcer and increased positive subjective effects ratings, but buspirone maintenance did not impact these effects of cocaine. Buspirone was also safe and tolerable when combined with cocaine and may have blunted some its cardiovascular effects. The results from the sexual delay-discounting procedure indicate that buspirone may reduce preference for riskier sex in cocaine users. (PsycINFO Database Record

Individual differences in impulsive action and dopamine transporter function in rat orbitofrontal cortex

Impulsivity, which can be subdivided into impulsive action and impulsive choice, is implicated as a factor underlying drug abuse vulnerability. Although previous research has shown that dopamine (DA) systems in prefrontal cortex are involved in impulsivity and substance abuse, it is not known if inherent variation in DA transporter (DAT) function contributes to impulsivity. The current study determined if individual differences in either impulsive action or impulsive choice are related to DAT function in orbitofrontal (OFC) and/or medial prefrontal cortex (mPFC). Rats were first tested both for impulsive action in a cued go/no-go task and for impulsive choice in a delay-discounting task. Following behavioral evaluation, in vitro [(3)H]DA uptake assays were performed in OFC and mPFC isolated from individual rats. Vmax in OFC, but not mPFC, was correlated with performance in the cued go/no-go task, with decreased OFC DAT function being associated with high impulsive action. In contrast, Vmax in OFC and mPFC was not correlated with performance in the delay-discounting task. The current results demonstrate that impulsive behavior in cued go/no-go performance is associated with decreased DAT function in OFC, suggesting that hyperdopaminergic tone in this prefrontal subregion mediates, at least in part, increased impulsive action.

16p11.2 Locus modulates response to satiety before the onset of obesity

BACKGROUND

The 600 kb BP4-BP5 copy number variants (CNVs) at the 16p11.2 locus have been associated with a range of neurodevelopmental conditions including autism spectrum disorders and schizophrenia. The number of genomic copies in this region is inversely correlated with body mass index (BMI): the deletion is associated with a highly penetrant form of obesity (present in 50% of carriers by the age of 7 years and in 70% of adults), and the duplication with being underweight. Mechanisms underlying this energy imbalance remain unknown.

OBJECTIVE

This study aims to investigate eating behavior, cognitive traits and their relationships with BMI in carriers of 16p11.2 CNVs.

METHODS

We assessed individuals carrying a 16p11.2 deletion or duplication and their intrafamilial controls using food-related behavior questionnaires and cognitive measures. We also compared these carriers with cohorts of individuals presenting with obesity, binge eating disorder or bulimia.

RESULTS

Response to satiety is gene dosage-dependent in pediatric CNV carriers. Altered satiety response is present in young deletion carriers before the onset of obesity. It remains altered in adolescent carriers and correlates with obesity. Adult deletion carriers exhibit eating behavior similar to that seen in a cohort of obesity without eating disorders such as bulimia or binge eating. None of the cognitive measures are associated with eating behavior or BMI.

CONCLUSIONS

These findings suggest that abnormal satiety response is a strong contributor to the energy imbalance in 16p11.2 CNV carriers, and, akin to other genetic forms of obesity, altered satiety responsiveness in children precedes the increase in BMI observed later in adolescence.

r-bPiDI, an α6β2* Nicotinic Receptor Antagonist, Decreases Nicotine-Evoked Dopamine Release and Nicotine Reinforcement

α6β2* nicotinic acetylcholine receptors (nAChRs) expressed by dopaminergic neurons mediate nicotine-evoked dopamine (DA) release and nicotine reinforcement. α6β2* antagonists inhibit these effects of nicotine, such that α6β2* receptors serve as therapeutic targets for nicotine addiction. The present research assessed the neuropharmacology of 1,10-bis(3-methyl-5,6-dihydropyridin-1(2H)-yl)decane (r-bPiDI), a novel small-molecule, tertiary amino analog of its parent compound, N,N-decane-1,10-diyl-bis-3-picolinium diiodide (bPiDI). bPiDI was previously shown to inhibit both nicotine-evoked DA release and the reinforcing effects of nicotine. In the current study, r-bPiDI inhibition of [(3)H]nicotine and [(3)H]methyllycaconitine binding sites was evaluated to assess interaction with the recognition binding sites on α4β2* and α7* nAChRs, respectively. Further, r-bPiDI inhibition of nicotine-evoked DA release in vitro in the absence and presence of α-conotoxin MII and following chronic in vivo nicotine administration were determined. The ability of r-bPiDI to decrease nicotine self-administration and food-maintained responding was also assessed. Results show that r-bPiDI did not inhibit [(3)H]nicotine or [(3)H]methyllycaconitine binding, but potently (IC50 = 37.5 nM) inhibited nicotine-evoked DA release from superfused striatal slices obtained from either drug naïve rats or from those repeatedly treated with nicotine. r-bPiDI inhibition of nicotine-evoked DA release was not different in the absence or presence of α-conotoxin MII, indicating that r-bPiDI acts as a potent, selective α6β2* nAChR antagonist. Acute systemic administration of r-bPiDI specifically decreased nicotine self-administration by 75 %, and did not alter food-maintained responding, demonstrating greater specificity relative to bPiDI and bPiDDB, as well as the tertiary amino analog r-bPiDDB. The current work describes the discovery of r-bPiDI, a tertiary amino, α-conotoxin MII-like small molecule that acts as a potent and selective antagonist at α6β2* nAChRs to specifically decrease nicotine self-administration in rats, thus, establishing r-bPiDI as a lead compound for development as a treatment for nicotine addiction.

Role of serotonin transporter function in rat orbitofrontal cortex in impulsive choice

Impulsivity is a multi-faceted personality construct that plays a prominent role in drug abuse vulnerability. Dysregulation of 5-hydroxytryptamine (serotonin, 5-HT) systems in subregions of the prefrontal cortex has been implicated in impulsivity. Extracellular 5-HT concentrations are regulated by 5-HT transporters (SERTs), indicating that these transporters may be important molecular targets underlying individual differences in impulsivity and drug abuse vulnerability. The present study evaluated the role of SERT in mediating individual differences in impulsivity. Rats were tested for both impulsive action using the cued go/no-go task and for impulsive choice using a delay discounting task in a counterbalanced design. Following behavioral evaluation, Km and Vmax were obtained from kinetic analysis of [(3)H]5-HT uptake by SERT using synaptosomes prepared from both orbitofrontal cortex (OFC) and medial prefrontal cortex (mPFC) obtained from each individual rat. Vmax for SERT in OFC, but not mPFC, was negatively correlated with mean adjusted delay scores in the delay discounting task. In contrast, Vmax for SERT in OFC and mPFC was not correlated with performance in the cued go/no-go task. To further evaluate the relationship between SERT function and impulsive choice, a selective SERT inhibitor, fluoxetine (0, 15, 50 and 150pmol/side) was microinjected bilaterally into OFC and effects on the delay discounting task determined. Following stabilization of behavior, fluoxetine increased mean adjusted delay scores (decreased impulsivity) in high impulsive rats compared to saline microinjection, but had no effect in low impulsive rats. These ex vivo and in vivo results suggest that enhanced SERT function in OFC underlies high impulsive choice behavior.

Role of ionotropic glutamate receptors in delay and probability discounting in the rat

RATIONALE

Discounting of delayed and probabilistic reinforcement is linked to increased drug use and pathological gambling. Understanding the neurobiology of discounting is important for designing treatments for these disorders. Glutamate is considered to be involved in addiction-like behaviors; however, the role of ionotropic glutamate receptors (iGluRs) in discounting remains unclear.

OBJECTIVES

The current study examined the effects of N-methyl-D-aspartate (NMDA) and α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) glutamate receptor blockade on performance in delay and probability discounting tasks.

METHODS

Following training in either delay or probability discounting, rats (n = 12, each task) received pretreatments of the NMDA receptor antagonists MK-801 (0, 0.01, 0.03, 0.1, or 0.3 mg/kg, s.c.) or ketamine (0, 1.0, 5.0, or 10.0 mg/kg, i.p.), as well as the AMPA receptor antagonist CNQX (0, 1.0, 3.0, or 5.6 mg/kg, i.p.). Hyperbolic discounting functions were used to estimate sensitivity to delayed/probabilistic reinforcement and sensitivity to reinforcer amount.

RESULTS

An intermediate dose of MK-801 (0.03 mg/kg) decreased sensitivity to both delayed and probabilistic reinforcement. In contrast, ketamine did not affect the rate of discounting in either task but decreased sensitivity to reinforcer amount. CNQX did not alter sensitivity to reinforcer amount or delayed/probabilistic reinforcement.

CONCLUSIONS

These results show that blockade of NMDA receptors, but not AMPA receptors, decreases sensitivity to delayed/probabilistic reinforcement (MK-801) and sensitivity to reinforcer amount (ketamine). The differential effects of MK-801 and ketamine demonstrate that sensitivities to delayed/probabilistic reinforcement and reinforcer amount are pharmacologically dissociable.

The 16p11.2 locus modulates brain structures common to autism, schizophrenia and obesity

Anatomical structures and mechanisms linking genes to neuropsychiatric disorders are not deciphered. Reciprocal copy number variants at the 16p11.2 BP4-BP5 locus offer a unique opportunity to study the intermediate phenotypes in carriers at high risk for autism spectrum disorder (ASD) or schizophrenia (SZ). We investigated the variation in brain anatomy in 16p11.2 deletion and duplication carriers. Beyond gene dosage effects on global brain metrics, we show that the number of genomic copies negatively correlated to the gray matter volume and white matter tissue properties in cortico-subcortical regions implicated in reward, language and social cognition. Despite the near absence of ASD or SZ diagnoses in our 16p11.2 cohort, the pattern of brain anatomy changes in carriers spatially overlaps with the well-established structural abnormalities in ASD and SZ. Using measures of peripheral mRNA levels, we confirm our genomic copy number findings. This combined molecular, neuroimaging and clinical approach, applied to larger datasets, will help interpret the relative contributions of genes to neuropsychiatric conditions by measuring their effect on local brain anatomy.

Impulsivity affects suboptimal gambling-like choice by pigeons

Pigeons prefer a low-probability, high-payoff but suboptimal alternative over a reliable low-payoff optimal alternative (i.e., one that results in more food). This finding is analogous to suboptimal human monetary gambling because in both cases there appears to be an overemphasis of the occurrence of the winning event (a jackpot) and an underemphasis of losing events. In the present research we found that pigeons chose suboptimally to the degree that they were impulsive as indexed by the steeper slope of the hyperbolic delay-discounting function (i.e., the shorter the delay they would accept in a smaller-sooner/larger-later procedure). These correlational findings have implications for the mechanisms underlying suboptimal choice by humans (e.g., problem gamblers) and they suggest that high baseline levels of impulsivity can enhance acquisition of a gambling habit.

Isolating the incentive salience of reward-associated stimuli: value, choice, and persistence

Sign- and goal-tracking are differentially associated with drug abuse-related behavior. Recently, it has been hypothesized that sign- and goal-tracking behavior are mediated by different neurobehavioral valuation systems, including differential incentive salience attribution. Herein, we used different conditioned stimuli to preferentially elicit different response types to study the different incentive valuation characteristics of stimuli associated with sign- and goal-tracking within individuals. The results demonstrate that all stimuli used were equally effective conditioned stimuli; however, only a lever stimulus associated with sign-tracking behavior served as a robust conditioned reinforcer and was preferred over a tone associated with goal-tracking. Moreover, the incentive value attributed to the lever stimulus was capable of promoting suboptimal choice, leading to a significant reduction in reinforcers (food) earned. Furthermore, sign-tracking to a lever was more persistent than goal-tracking to a tone under omission and extinction contingencies. Finally, a conditional discrimination procedure demonstrated that sign-tracking to a lever and goal-tracking to a tone were dependent on learned stimulus–reinforcer relations. Collectively, these results suggest that the different neurobehavioral valuation processes proposed to govern sign- and goal-tracking behavior are independent but parallel processes within individuals. Examining these systems within individuals will provide a better understanding of how one system comes to dominate stimulus–reward learning, thus leading to the differential role these systems play in abuse-related behavior.

Effect of environmental enrichment on dopamine and serotonin transporters and glutamate neurotransmission in medial prefrontal and orbitofrontal cortex

Recent studies have reported that rats raised in an enriched condition (EC) have decreased dopamine transporter (DAT) function and expression in medial prefrontal cortex (mPFC), as well as increased d-amphetamine-induced glutamate release in nucleus accumbens compared to rats raised in an isolated condition (IC). In these previous studies, DAT function and expression were evaluated using mPFC pooled from four rats for each condition to obtain kinetic parameters due to sparse DAT expression in mPFC. In contrast, accumbal glutamate release was determined using individual rats. The current study extends the previous work and reports on the optimization of DAT and serotonin transporter (SERT) functional assays, as well as cell surface expression assays using both mPFC and orbitofrontal cortex (OFC) from individual EC or IC rats. In addition, the effect of d-amphetamine on glutamate release in mPFC and OFC of EC and IC rats was determined using in vivo microdialysis. Results show that environmental enrichment decreased maximal transport velocity (Vmax) for [(3)H]dopamine uptake in mPFC, but increased Vmax for [(3)H]dopamine uptake in OFC. Corresponding changes in DAT cell surface expression were not found. In contrast, Vmax for [(3)H]serotonin uptake and cellular localization of SERT in mPFC and OFC were not different between EC and IC rats. Further, acute d-amphetamine (2mg/kg, s.c.) increased extracellular glutamate concentrations in mPFC of EC rats only and in OFC of IC rats only. Overall, these results suggest that enrichment produces long-lasting alterations in mPFC and OFC DAT function via a trafficking-independent mechanism, as well as differential glutamate release in mPFC and OFC. Rearing-induced modulation of DAT function and glutamate release in prefrontal cortical subregions may contribute to the known protective effects of enrichment on drug abuse vulnerability.

Concurrent choice for social interaction and amphetamine using conditioned place preference in rats: effects of age and housing condition

BACKGROUND

Social interaction can serve as a natural reward that attenuates drug reward in rats; however, it is unknown if age or housing conditions alter the choice between social interaction and drug.

METHODS

Individually- and pair-housed adolescent and adult male rats were tested using conditioned place preference (CPP) in separate experiments in which: (1) social interaction was conditioned against no social interaction; (2) amphetamine (AMPH; 1mg/kg, s.c.) was conditioned against saline; or (3) social interaction was conditioned against AMPH.

RESULTS

Social interaction CPP was obtained only in individually-housed adolescents, whereas AMPH CPP was obtained in both individually-housed adolescents and adults; however, the effect of AMPH was not statistically significant in pair-housed adults. When allowed to choose concurrently between compartments paired with either social interaction or AMPH, individually-housed adolescents preferred the compartment paired with social interaction, whereas pair-housed adolescents preferred the compartment paired with AMPH. Regardless of housing condition, adults showed a similar preference for the compartments paired with either social interaction or AMPH.

CONCLUSIONS

Although some caution is needed in interpreting cross-experiment comparisons, the overall results suggest that individually-housed adolescents were most sensitive to the rewarding effect of social interaction, and this hypersensitivity to social reward effectively competed with AMPH reward.

The effect of VMAT2 inhibitor GZ-793A on the reinstatement of methamphetamine-seeking in rats

RATIONALE

The vesicular monoamine transporter 2 (VMAT2) has been identified as a potential target for the treatment of methamphetamine (METH) abuse. GZ-793A is a potent and selective VMAT2 inhibitor that has been shown to block the primary and conditioned reinforcing effects of METH, while demonstrating no abuse liability when given alone.

OBJECTIVES

The aim of the current study was to determine if GZ-793A attenuates METH- or cue-induced reinstatement of METH-seeking after a period of extinction. The effect of acute GZ-793A on locomotor activity also was assessed.

METHODS

After a period of extinction, rats were administered GZ-793A (15 mg/kg, s.c.) 15 min prior to a priming injection of METH or re-exposure to cues associated with METH infusions. GZ-793A also was administered 20 min prior to an injection of METH (0.5 mg/kg, s.c.) or saline to determine its effect on locomotor behavior.

RESULTS

Pretreatment with GZ-793A (15 mg/kg) decreased cue-induced reinstatement, without demonstrating any response suppressive effects when administered in the absence of reinstating stimuli. GZ-793A also decreased methamphetamine-induced reinstatement; however, response suppressant effects of GZ-793A were obtained when the compound was presented alone. In this latter experiment, GZ-793A may have reduced responding for the conditioned reinforcing effects of the contingently available cues rather than having nonspecific effects on baseline responding. GZ-793A had no effect on locomotor activity when administered alone or with METH.

CONCLUSIONS

GZ-793A and related VMAT2 inhibitors may be promising leads for reducing the risk of relapse to METH use following exposure to drug-associated cues.

Escalation of cocaine intake with extended access in rats: dysregulated addiction or regulated acquisition?

RATIONALE

Understanding the neurobehavioral mechanisms underlying dysregulated cocaine intake is important for the development of new cocaine abuse therapies.

OBJECTIVES

The current study determined if cocaine escalation under extended access conditions (6-h access) is regulated by discrimination learning processes.

METHODS

Rats were initially trained on cocaine self-administration (0.1 or 0.25 mg/kg/infusion) using a fixed ratio 1 (FR 1) schedule under 1-h access for 12 sessions. Some rats were then trained to self-administer cocaine under 1-h or 6-h access conditions exclusively for 14 additional sessions, while other rats were trained under both 1- and 6-h access conditions that were cued or noncued for 28 additional sessions (14 sessions for each 1- and 6-h access). Two additional groups of rats were initially trained to self-administer cocaine using an FR 1 schedule under 10-min access for 12 sessions; half of the animals were then switched to 60-min access conditions for 14 additional sessions.

RESULTS

When access conditions were differentially cued, escalation of cocaine intake was evident in animals with both 1- and 6-h access conditions during the escalation phase. Escalation also was evident in animals initially trained with 10-min access and then switched to 60-min access.

CONCLUSIONS

The results demonstrate that dysregulated and regulated intakes can be expressed within the same animal, indicating that escalation is context-dependent. Furthermore, escalated cocaine intake can be expressed under 1-h access conditions. Overall, these results suggest that escalated cocaine intake may be representative of discrimination-dependent regulated intake rather than addiction-like, compulsive intake.

A translational behavioral model of mood-based impulsivity: Implications for substance abuse

BACKGROUND

Laboratory tasks that measure various facets of impulsivity derived from self-report questionnaires are important for elucidating the behavioral consequences of impulsivity in humans and for back-translating these facets to non-human species. Negative urgency, or mood-based rash action, is a self-report facet of impulsivity linked to problem substance use; however, a valid behavioral task is lacking.

METHODS

The current studies were designed to bridge self-report questionnaire and behavioral measures of negative urgency in humans and to determine if this could be back-translated to rats.

RESULTS

Humans scoring high in negative urgency showed greater behavioral responding and increased frustration following unexpected reward omission on a monetary-based task compared to subjects low in negative urgency. Rats also showed elevated responding for either sucrose pellets or intravenous amphetamine following unexpected reward omission.

CONCLUSION

These results suggest that impulsive behavior engendered by unexpected reward omission may represent a valid behavioral model of negative urgency linked to substance abuse.

Genetic testing in patients with obesity

The obesity epidemic is associated with the recent availability of highly palatable and inexpensive caloric food as well as important changes in lifestyle. Genetic factors, however, play a key role in regulating energy balance and numerous twin studies have estimated the BMI heritability between 40 and 70%. While common variants, identified through genome-wide association studies (GWAS) point toward new pathways, their effect size are too low to be of any use in the clinic. This review therefore concentrates on genes and genomic regions associated with very high risks of human obesity. Although there are no consensus guidelines, we review how the knowledge on these "causal factors" can be translated into the clinic for diagnostic purposes. We propose genetic workups guided by clinical manifestations in patients with severe early-onset obesity. While etiological diagnoses are unequivocal in a minority of patients, new genomic tools such as Comparative Genomic Hybridization (CGH) array, have allowed the identification of novel "causal" loci and next-generation sequencing brings the promise of accelerated pace for discoveries relevant to clinical practice.

Social facilitation of d-amphetamine self-administration in rats

The link between social influence and drug abuse has long been established in humans. However, preclinical animal models of drug abuse have only recently begun to consider the role of social influence. Since social factors influence the initiation and maintenance of drug use in humans, it is important to include these factors in preclinical animal models. The current study examined the effects of the presence of a social partner on responding for sucrose pellets under various motivational conditions, as well as on d-amphetamine (AMPH) self-administration. Rats were trained to lever press for either sucrose or AMPH (0.01 or 0.1 mg/kg/infusion unit dose). Following response stability, a novel same-sex conspecific was presented in an adjacent compartment separated by a clear divider, and responding for sucrose or AMPH reward was measured. Rats were allowed to restabilize, and subsequently given an additional partner presentation. Presence of the social partner increased responding only during the first pairing with the AMPH 0.1 mg/kg/infusion unit dose, whereas inhibition of responding was observed during the first pairing during access to the 0.01 mg/kg/infusion unit dose. Under free feed conditions, inhibition of sucrose pellet responding was observed in the presence of the social partner, but this effect was attenuated under food restriction. In contrast, the results demonstrate social facilitation of AMPH self-administration at a high unit dose, thus extending the influence of social factors to an operant conditioning task. This model of social facilitation may have important implications as a preclinical model of social influence on drug abuse.

Environmental enrichment reduces attribution of incentive salience to a food-associated stimulus

Animals reared in an enriched environment are less vulnerable to abuse-like behavior and exhibit less persistent drug seeking, perhaps due to a decrease in the incentive value of stimuli associated with reward. The present study investigated the effects of environmental enrichment on Pavlovian conditioned approach (PCA) performance, a measure of incentive salience attribution. Rats were first reared from postnatal day 21 to postnatal day 51 in either an enriched environment with large cages, social cohorts and novel objects, or in an isolated environment with small, hanging cages, no social cohorts and no novel objects. Rats were then trained on a PCA task for 5 consecutive days, where a retractable lever was predictive of a food reward. Isolated rats predominantly exhibited sign-tracking responses directed toward the reward-predicted lever (indicative of incentive salience attribution), while enriched rats predominantly exhibited goal-tracking responses directed toward the location of food delivery. Both groups learned their respective response type at equal rates. The results indicate that environmental enrichment reduces the readiness to attribute incentive value to reward-associated cues, which may explain the enrichment-induced protection against addiction-like behaviors.

High impulsivity in rats predicts amphetamine conditioned place preference

Stimulants such as d-amphetamine (AMPH) are used commonly to treat attention-deficit hyperactivity disorder (ADHD), but concerns have been raised regarding the use of AMPH due to its reinforcing and potentially addictive properties. The current study examined if individual differences in impulsive choice predict AMPH-induced hyperactivity and conditioned place preference (CPP). Rats were first tested in delay discounting using an adjusting delay procedure to measure impulsive choice and then were subsequently tested for AMPH CPP. High impulsive (HiI) and low impulsive (LoI) rats were conditioned across four sessions with 0.1, 0.5, or 1.5 mg/kg of AMPH. AMPH increased locomotor activity for HiI and LoI rats following 0.5 mg/kg but failed to increase activity following 0.1 and 1.5 mg/kg. CPP was established for HiI rats with both 0.5 and 1.5 mg/kg of AMPH, whereas LoI rats did not develop CPP following any dose of AMPH; HiI and LoI groups differed significantly following 0.5 mg/kg of AMPH. These results indicate that HiI rats are more sensitive to the rewarding effects of AMPH compared to LoI rats, which is consistent with research showing that high impulsive individuals may be more vulnerable to stimulant abuse.

Effect of environmental enrichment on escalation of cocaine self-administration in rats

BACKGROUND

Previous studies found that environmental enrichment protects against the initiation of stimulant self-administration in rats, but it is unclear if enrichment also protects against the escalation of stimulant use with long-term exposure.

OBJECTIVE

The current study examined the effects of environmental enrichment on escalation of cocaine self-administration using an extended access procedure.

METHODS

Rats were raised from 21 days in an enriched condition (EC) with social cohorts and novel objects, a social condition with only social cohorts (SC), a novelty condition (NC) with novel objects in isolated cages, or an isolated condition (IC) without social cohorts or novel objects. In young adulthood, EC, SC, NC, and IC rats were separated into short access (ShA) or long access (LgA) groups that received either 1 or 6 h, respectively, of daily cocaine self-administration (0.1 mg/kg/infusion) for 14 days. In a second experiment, EC and IC rats were used to assess differences in acquisition and escalation of cocaine self-administration at a 0.5 mg/kg/infusion unit dose.

RESULTS

With ShA sessions, EC rats acquired cocaine self-administration at a slower rate than IC rats at both unit doses; however, with extended training, both groups eventually reached similar rates. At the 0.1 mg/kg/infusion dose, only NC and IC rats escalated in amount of intake when switched to the LgA sessions. At the 0.5 mg/kg/infusion dose, rates of cocaine self-administration escalated in LgA groups over 14 days regardless of EC or IC rearing condition; however, EC rats escalated at a faster rate, eventually reaching the same level of intake observed in IC rats.

CONCLUSIONS

Although environmental enrichment protects against escalation of a low unit dose of cocaine, it may not protect against escalation with a higher unit dose. In addition, at a lower unit dose, this protective mechanism appears to be due to the presence of social cohorts rather than novel objects.

meso-Transdiene analogs inhibit vesicular monoamine transporter-2 function and methamphetamine-evoked dopamine release

Lobeline, a nicotinic receptor antagonist and neurotransmitter transporter inhibitor, is a candidate pharmacotherapy for methamphetamine abuse. meso-Transdiene (MTD), a lobeline analog, lacks nicotinic receptor affinity, retains affinity for vesicular monoamine transporter 2 (VMAT2), and, surprisingly, has enhanced affinity for dopamine (DA) and serotonin transporters [DA transporter (DAT) and serotonin transporter (SERT), respectively]. In the current study, MTD was evaluated for its ability to decrease methamphetamine self-administration in rats relative to food-maintained responding. MTD specifically decreased methamphetamine self-administration, extending our previous work. Classical structure-activity relationships revealed that more conformationally restricted MTD analogs enhanced VMAT2 selectivity and drug likeness, whereas affinity at the dihydrotetrabenazine binding and DA uptake sites on VMAT2 was not altered. Generally, MTD analogs exhibited 50- to 1000-fold lower affinity for DAT and were equipotent or had 10-fold higher affinity for SERT, compared with MTD. Representative analogs from the series potently and competitively inhibited [(3)H]DA uptake at VMAT2. (3Z,5Z)-3,5-bis(2,4-dichlorobenzylidene)-1-methylpiperidine (UKMH-106), the 3Z,5Z-2,4-dichlorophenyl MTD analog, had improved selectivity for VMAT2 over DAT and importantly inhibited methamphetamine-evoked DA release from striatal slices. In contrast, (3Z,5E)-3,5-bis(2,4-dichlorobenzylidene)-1-methylpiperidine (UKMH-105), the 3Z,5E-geometrical isomer, inhibited DA uptake at VMAT2, but did not inhibit methamphetamine-evoked DA release. Taken together, these results suggest that these geometrical isomers interact at alternate sites on VMAT2, which are associated with distinct pharmacophores. Thus, structural modification of the MTD molecule resulted in analogs exhibiting improved drug likeness and improved selectivity for VMAT2, as well as the ability to decrease methamphetamine-evoked DA release, supporting the further evaluation of these analogs as treatments for methamphetamine abuse.

The novel pyrrolidine nor-lobelane analog UKCP-110 [cis-2,5-di-(2-phenethyl)-pyrrolidine hydrochloride] inhibits VMAT2 function, methamphetamine-evoked dopamine release, and methamphetamine self-administration in rats

Both lobeline and lobelane attenuate methamphetamine self-administration in rats by decreasing methamphetamine-induced dopamine release via interaction with vesicular monoamine transporter-2 (VMAT2). A novel derivative of nor-lobelane, cis-2,5-di-(2-phenethyl)-pyrrolidine hydrochloride (UKCP-110), and its trans-isomers, (2R,5R)-trans-di-(2-phenethyl)-pyrrolidine hydrochloride (UKCP-111) and (2S,5S)-trans-di-(2-phenethyl)-pyrrolidine hydrochloride (UKCP-112), were evaluated for inhibition of [(3)H]dihydrotetrabenazine binding and [(3)H]dopamine uptake by using a rat synaptic vesicle preparation to assess VMAT2 interaction. Compounds were evaluated for inhibition of [(3)H]nicotine and [(3)H]methyllycaconitine binding to assess interaction with the major nicotinic receptor subtypes. In addition, compounds were evaluated for inhibition of methamphetamine-evoked endogenous dopamine release by using striatal slices. The most promising compound, UKCP-110, was evaluated for its ability to decrease methamphetamine self-administration and methamphetamine discriminative stimulus cues and for its effect on food-maintained operant responding. UKCP-110, UKCP-111, and UKCP-112 inhibited [(3)H]dihydrotetrabenazine binding (K(i) = 2.66 ± 0.37, 1.05 ± 0.10, and 3.80 ± 0.31 μM, respectively) and had high potency inhibiting [(3)H]dopamine uptake (K(i) = 0.028 ± 0.001, 0.046 ± 0.008, 0.043 ± 0.004 μM, respectively), but lacked affinity at nicotinic receptors. Although the trans-isomers did not alter methamphetamine-evoked dopamine release, UKCP-110 inhibited (IC(50) = 1.8 ± 0.2 μM; I(max) = 67.18 ± 6.11 μM) methamphetamine-evoked dopamine release. At high concentrations, UKCP-110 also increased extracellular dihydroxyphenylacetic acid. It is noteworthy that UKCP-110 decreased the number of methamphetamine self-infusions, while having no effect on food-reinforced behavior or the methamphetamine stimulus cue. Thus, UKCP-110 represents a new lead in the development of novel pharmacotherapies for the treatment of methamphetamine abuse.

Novelty seeking, incentive salience and acquisition of cocaine self-administration in the rat

It has been suggested that incentive salience plays a major role in drug abuse and the development of addiction. Additionally, novelty seeking has been identified as a significant risk factor for drug abuse. However, how differences in the readiness to attribute incentive salience relate to novelty seeking and drug abuse vulnerability has not been explored. The present experiments examined how individual differences in incentive salience attribution relate to novelty seeking and acquisition of cocaine self-administration in a preclinical model. Rats were first assessed in an inescapable novelty task and a novelty place preference task (measures of novelty seeking), followed by a Pavlovian conditioned approach task for food (a measure of incentive salience attribution). Rats then were trained to self-administer cocaine (0.3 or 1.0 mg/kg/infusion) using an autoshaping procedure. The results demonstrate that animals that attributed incentive salience to a food-associated cue were higher novelty seekers and acquired cocaine self-administration more quickly at the lower dose. The results suggest that novelty-seeking behavior may be a mediator of incentive salience attribution and that incentive salience magnitude may be an indicator of drug reward.

Methylphenidate as a reinforcer for rats: contingent delivery and intake escalation

Methylphenidate (MPH) is one of the most widely prescribed drugs for treating attention-deficit hyperactivity disorder. Previous research suggested that MPH is a reinforcer for rats, but not all of the manipulations to show that lever pressing is controlled by the contingency to obtain MPH have been examined. In Experiment 1, responding for MPH on a progressive ratio (PR) schedule was assessed. Rats self-administered varying doses of MPH (0.056-1.0 mg/kg/infusion) on a PR schedule of reinforcement, and self-administered more MPH than saline, with maximal responding occurring at a unit dose of 0.56 mg/kg/infusion. Experiment 2 examined if there were differences in responding between contingent and noncontingent MPH (0.56 mg/kg/infusion) on a fixed ratio schedule of reinforcement. Results showed that rats responded for contingent MPH, and that responding was not maintained when MPH was delivered noncontingently. Experiment 3 examined self-administration of MPH (0.1 or 0.3 mg/kg/infusion) during long access (6 hr) compared to short access sessions (1 hr). Results showed that rats given long access to MPH showed an escalation of intake across sessions, with this escalation being more pronounced at the lower unit dose (0.1 mg/kg/infusion); in contrast, rats given short access to MPH did not show an increase in MPH self-administration across sessions at either MPH dose tested. Taken together, these results indicate that MPH is an effective intravenous reinforcer for rats and that, similar to other stimulants such as cocaine, amphetamine and methamphetamine, MPH is subject to abuse as reflected by dysregulated intake across repeated long access sessions.