Long-lasting sensitization of reward-directed behavior by amphetamine

Long-term effects of THC exposure on reward learning and motivated behavior in adolescent and adult male rats

RATIONALE

The endocannabinoid system makes critical contributions to reward processing, motivation, and behavioral control. Repeated exposure to THC or other cannabinoid drugs can cause persistent adaptions in the endocannabinoid system and associated neural circuitry. It remains unclear how such treatments affect the way rewards are processed and pursued.

OBJECTIVE AND METHODS

We examined if repeated THC exposure (5 mg/kg/day for 14 days) during adolescence or adulthood led to long-term changes in rats' capacity to flexibly encode and use action-outcome associations for goal-directed decision making. Effects on hedonic feeding and progressive ratio responding were also assessed.

RESULTS

THC exposure had no effect on rats' ability to flexibly select actions following reward devaluation. However, instrumental contingency degradation learning, which involves avoiding an action that is unnecessary for reward delivery, was augmented in rats with a history of adult but not adolescent THC exposure. THC-exposed rats also displayed more vigorous instrumental behavior in this study, suggesting a motivational enhancement. A separate experiment found that while THC exposure had no effect on hedonic feeding behavior, it increased rats' willingness to work for food on a progressive ratio schedule, an effect that was more pronounced when THC was administered to adults. Adolescent and adult THC exposure had opposing effects on the CB1 receptor dependence of progressive ratio performance, decreasing and increasing sensitivity to rimonabant-induced behavioral suppression, respectively.

CONCLUSIONS

Our findings reveal that exposure to a translationally relevant THC exposure regimen induces long-lasting, age-dependent alterations in cognitive and motivational processes that regulate the pursuit of rewards.

Cognitive mechanisms underlying decision making involving risk of explicit punishment in male and female rats

Individuals engage in the process of risk-based decision making on a daily basis to navigate various aspects of life. There are, however, individual differences in this form of decision making, with some individuals exhibiting preference for riskier choices (risk taking) and others exhibiting preference for safer choices (risk aversion). Recent work has shown that extremes in risk taking (e.g., excessive risk taking or risk aversion) are not only cognitive features of neuropsychiatric diseases, but may in fact predispose individuals to the development of such diseases. To better understand individual differences in risk taking, and thus the mechanisms by which they confer disease vulnerability, the current study investigated the cognitive contributions to risk taking in both males and females. Rats were first behaviorally characterized in a decision-making task involving risk of footshock punishment and then tested on a battery of cognitive behavioral assays. Individual variability in risk taking was compared with performance on these tasks. Consistent with prior work, females were more risk averse than males. With the exception of the Set-shifting Task, there were no sex differences in performance on other cognitive assays. There were, however, sex-dependent associations between risk taking and specific cognitive measures. Greater risk taking was associated with better cognitive flexibility in males whereas greater risk aversion was associated with better working memory in females. Collectively, these findings reveal that distinct cognitive mechanisms are associated with risk taking in males and females, which may account for sex differences in this form of decision making.

Sustained inhibitory transmission but dysfunctional dopamine D2 receptor signaling in dorsal striatal subregions following protracted abstinence from amphetamine

Behavioral sensitization to amphetamine is a complex phenomenon that engages several neurotransmitter systems and brain regions. While dysregulated signaling in the mesolimbic dopamine system repeatedly has been linked to behavioral sensitization, later research has implicated dorsal striatal circuits and GABAergic neurotransmission in contributing to behavioral transformation elicited by amphetamine. The aim of this study was thus to determine if repeated amphetamine exposure followed by abstinence would alter inhibitory neurotransmission in dorsal striatal subregions. To this end, male Wistar rats received amphetamine (2.0 mg/kg) in an intermittent manner for a total of five days. Behavioral sensitization to amphetamine was measured in locomotor-activity boxes, while neuroadaptations were recorded in the dorsolateral (DLS) and dorsomedial striatum (DMS) using ex vivo electrophysiology at different timepoints of amphetamine abstinence (2 weeks, 4-5 weeks, 10-11 weeks). Data show that repeated drug-exposure produces behavioral sensitization to the locomotor-stimulatory properties of amphetamine, which sustains for at least ten weeks. Electrophysiological recordings demonstrated a long-lasting suppression of evoked population spikes in both striatal subregions. Furthermore, following ten weeks of abstinence, the responsiveness to a dopamine D2 receptor agonist was significantly impaired in brain slices from rats previously receiving amphetamine. However, neither the frequency nor the amplitude of spontaneous inhibitory currents was affected by treatment at any of the time points analyzed. In conclusion, passive administration of amphetamine initiates long-lasting neuroadaptations in brain regions associated with goal-directed behavior and habitual performance, but these transformations do not appear to be driven by changes in GABAergic neurotransmission.

Effects of repeated adolescent exposure to cannabis smoke on cognitive outcomes in adulthood

BACKGROUND

Cannabis (marijuana) is the most widely used illicit drug in the USA, and consumption among adolescents is rising. Some animal studies show that adolescent exposure to delta 9-tetrahydrocannabinol or synthetic cannabinoid receptor 1 agonists causes alterations in affect and cognition that can persist into adulthood. It is less clear, however, whether similar alterations result from exposure to cannabis via smoke inhalation, which remains the most frequent route of administration in humans.

AIMS

To begin to address these questions, a rat model was used to determine how cannabis smoke exposure during adolescence affects behavioral and cognitive outcomes in adulthood.

METHODS

Adolescent male Long-Evans rats were assigned to clean air, placebo smoke, or cannabis smoke groups. Clean air or smoke exposure sessions were conducted daily during adolescence (from P29-P49 days of age ) for a total of 21 days, and behavioral testing began on P70.

RESULTS

Compared to clean air and placebo smoke conditions, cannabis smoke significantly attenuated the normal developmental increase in body weight, but had no effects on several measures of either affect/motivation (open field activity, elevated plus maze, instrumental responding under a progressive ratio schedule of reinforcement) or cognition (set shifting, reversal learning, intertemporal choice). Surprisingly, however, in comparison to clean air controls rats exposed to either cannabis or placebo smoke in adolescence exhibited enhanced performance on a delayed response working memory task.

CONCLUSIONS

These findings are consistent with a growing body of evidence for limited long-term adverse cognitive and affective consequences of adolescent exposure to relatively low levels of cannabinoids.

Persistence of Operant Responding for Food After Prior Cocaine Exposure in Fischer 344 But Not Lewis Rats

BACKGROUND AND OBJECTIVES

Chronic cocaine exposure has differential neural effects in Fischer 344 (F344) vs Lewis inbred rats that may explain strain-dependent differences during acquisition vs maintenance of cocaine self-administration. We assessed whether prior cocaine exposure alters operant responding for food across various phases (acquisition, maintenance, extinction, spontaneous recovery, reinitiation) in these strains.

METHODS

Lewis and F344 rats (N = 12) were administered three cocaine (15 mg/kg) or saline injections at hourly intervals for 3 consecutive days. Beginning the next day for 24 days, rats had access to operant chambers in which one lever depression resulted in the delivery of a food pellet. Then, four extinction sessions were conducted in which food was no longer available, but other stimulus conditions remained the same. After a 2-day break, spontaneous recovery was assessed over four sessions. Food delivery was then restored for 3 days to test reinitiation followed by a progressive ratio session.

RESULTS

Lewis rats acquired the operant faster than F344 rats. F344 rats showed lower maintenance rates than Lewis rats but higher spontaneous recovery responding. Cocaine exposure caused persistence of responding during extinction in F344 but not Lewis rats. All groups reinitiated responding when food was available again and did not differ in final ratios completed under the progressive ratio schedule.

DISCUSSION AND CONCLUSIONS

That prior cocaine exposure led to persistence of responding in F344 rats during extinction may reflect heightened contextual conditioning that interferes with the ability to extinguish responding.

SCIENTIFIC SIGNIFICANCE

Results have implications for the genetic contribution to relapse-like behaviors. (Am J Addict 2021;00:00-00).

Dissociation between wanting and liking for coffee in heavy drinkers

BACKGROUND

There is an ongoing discussion about the addictive strength of caffeine. According to the incentive-sensitization theory, the development and the maintenance of drug addiction is the result of a selective sensitization of brain regions that are relevant for wanting without a corresponding increase in liking. Dissociations of wanting and liking have been observed with a wide range of drugs in animals. For human subjects, results are inconclusive, which is possibly due to invalid operationalizations of wanting and liking.

AIM

The present study examined dissociations of wanting and liking for coffee in heavy and low/non-consumers with newly developed and validated response time-based assessment procedures for wanting and liking.

METHODS

For this study 24 heavy and 32 low/non-consumers of coffee completed two versions of the Implicit-Association Test (IAT), one of which has been developed and validated recently to assess wanting for coffee, whereas the other reflects an indicator of liking for coffee.

RESULTS

Results revealed a significant interaction between group (heavy vs. low/non-consumers) and IAT type (wanting vs. liking) indicating that heavy coffee drinkers differed from low/non-consumers by displaying increased wanting but not liking for coffee.

INTERPRETATION

These data confirm that heavy coffee consumption is associated with strong wanting despite low liking for coffee, indicating that wanting becomes independent from liking through repeated consumption of caffeine. This dissociation provides a possible explanation for the widespread and stable consumption of caffeine-containing beverages.

Repeated cocaine exposure dysregulates cognitive control over cue-evoked reward-seeking behavior during Pavlovian-to-instrumental transfer

Drug-paired cues acquire powerful motivational properties, but only lead to active drug-seeking behavior if they are potent enough to overwhelm the cognitive control processes that serve to suppress such urges. Studies using the Pavlovian-to-instrumental transfer (PIT) task have shown that rats pretreated with cocaine or amphetamine exhibit heightened levels of cue-motivated food-seeking behavior, suggesting that exposure to these drugs sensitizes the incentive motivational system. However, the PIT testing protocol can also create conflict between two competing behavioral responses to the reward-paired cue: active reward seeking (e.g., lever pressing) and passive conditioned food-cup approach behavior. We therefore investigated whether repeated cocaine exposure alters the way in which rats use cue-based reward expectations to resolve such conflict. In-depth analysis of previously published and new research confirmed that when drug-naïve rats are given a cue that signals the timing of a delayed noncontingent reward, they adaptively transition from reward seeking to conditioned approach behavior, facilitating efficient collection of the predicted reward. In contrast, cocaine-exposed rats exhibit pronounced behavioral dysregulation, increasing, rather than suppressing, their reward-seeking behavior over time, disrupting their ability to passively collect reward. Such findings speak to the important and sometimes overlooked role that cognitive control plays in determining the motivational impact of cues associated with drug and nondrug rewards.

Decline of prefrontal cortical-mediated executive functions but attenuated delay discounting in aged Fischer 344 × brown Norway hybrid rats

Despite the fact that prefrontal cortex (PFC) function declines with age, aged individuals generally show an enhanced ability to delay gratification, as evident by less discounting of delayed rewards in intertemporal choice tasks. The present study was designed to evaluate relationships between 2 aspects of PFC-dependent cognition (working memory and cognitive flexibility) and intertemporal choice in young (6 months) and aged (24 months) Fischer 344 × brown Norway F1 hybrid rats. Rats were also evaluated for motivation to earn rewards using a progressive ratio task. As previously reported, aged rats showed attenuated discounting of delayed rewards, impaired working memory, and impaired cognitive flexibility compared with young. Among aged rats, greater choice of delayed reward was associated with preserved working memory, impaired cognitive flexibility, and less motivation to work for food. These relationships suggest that age-related changes in PFC and incentive motivation contribute to variance in intertemporal choice within the aged population. Cognitive impairments mediated by PFC are unlikely, however, to fully account for the enhanced ability to delay gratification that accompanies aging.

Dissociation between wanting and liking for alcohol and caffeine: A test of the Incentive Sensitisation Theory

Limited human studies have directly tested the dissociation between wanting and liking with human substance users, a core tenet of the Incentive Sensitisation Theory (IST). The aim of this study is to test the dissociation between wanting and liking in humans across two commonly used licit substances, alcohol and caffeine. The STRAP-R (Sensitivity To Reinforcement of Addictive and other Primary Rewards) questionnaire was administered to 285 alcohol users (mean age=33.30, SD= 8.83) and 134 coffee users (mean age=33.05, SD=8.10) ranging in their levels of substance use to assess wanting and liking. Findings showed that in high risk alcohol users wanting may drive alcohol consumption more so than liking, compared with low risk alcohol users. However, wanting and liking did not significantly dissociate as alcohol consumption increased. These findings partially support IST. Additionally, IST was not supported in coffee users. It is possible that caffeine functions differently at the neurological level compared with alcohol, perhaps explaining the lack of dissociation emerging in coffee users as caffeine use increased. Nevertheless, the current study makes several contributions to IST research. Future studies should focus on utilising the STRAP-R with a clinically dependent sample to test the dissociation between wanting and liking.

Behavioral sensitization of the reinforcing value of food: What food and drugs have in common

Sensitization is a basic property of the nervous system whereby repeated exposure to a stimulus results in an increase in responding to that stimulus. This increase in responding contributes to difficulty with treatment of drug abuse, as stimuli associated with substance use become signals or triggers for drug craving and relapse. Our work over the past decade has applied the theoretical framework of incentive sensitization to overeating. We have shown, in several studies, that lean adults do not commonly demonstrate behavioral sensitization after repeated exposure to snack food, but a subset of obese adults reliably does. This review will discuss this change in behavioral response to repeated consumption of snack food in obese individuals and apply the theoretical framework of incentive sensitization to drugs of abuse to high fat/high sugar snack foods. We will also show data that suggest that behavioral sensitization to repeated administration of snack food is predictive of weight gain, which may enhance its utility as a diagnostic tool for identifying at-risk individuals for obesity. Finally, we will discuss the future directions of this line of research, including studying the phenomenon in children and adolescents and determining if similar principles can be used to increase motivation to eat healthier food. A combination of reductions in unhealthy food intake and increases and healthy food intake is necessary to reduce obesity rates and improve health.

Behavioral and transcriptomic profiling of mice null for Lphn3, a gene implicated in ADHD and addiction

Background

The Latrophilin 3 (LPHN3) gene (recently renamed Adhesion G protein‐coupled receptor L3 (ADGRL3)) has been linked to susceptibility to attention deficit/hyperactivity disorder (ADHD) and vulnerability to addiction. However, its role and function are not well understood as there are no known functional variants.

Methods

To characterize the function of this little known gene, we phenotyped Lphn3 null mice. We assessed motivation for food reward and working memory via instrumental responding tasks, motor coordination via rotarod, and depressive‐like behavior via forced swim. We also measured neurite outgrowth of primary hippocampal and cortical neuron cultures. Standard blood chemistries and blood counts were performed. Finally, we also evaluated the transcriptome in several brain regions.

Results

Behaviorally, loss of Lphn3 increases both reward motivation and activity levels. Lphn3 null mice display significantly greater instrumental responding for food than wild‐type mice, particularly under high response ratios, and swim incessantly during a forced swim assay. However, loss of Lphn3 does not interfere with working memory or motor coordination. Primary hippocampal and cortical neuron cultures demonstrate that null neurons display comparatively enhanced neurite outgrowth after 2 and 3 days in vitro. Standard blood chemistry panels reveal that nulls have low serum calcium levels. Finally, analysis of the transcriptome from prefrontal cortical, striatal, and hippocampal tissue at different developmental time points shows that loss of Lphn3 results in genotype‐dependent differential gene expression (DGE), particularly for cell adhesion molecules and calcium signaling proteins. Much of the DGE is attenuated with age, and is consistent with the idea that ADHD is associated with delayed cortical maturation.

Conclusions

Transcriptome changes likely affect neuron structure and function, leading to behavioral anomalies consistent with both ADHD and addiction phenotypes. The data should further motivate analyses of Lphn3 function in the developmental timing of altered gene expression and calcium signaling, and their effects on neuronal structure/function during development.

Chronic cocaine administration induces long-term impairment in the drive to obtain natural reinforcers in high- but not low-demanding tasks

Repeated drug exposure induces short- and long-term neuroadaptations in brain reward circuitries that are normally involved in the regulation of motivation. Hence, repeated drug exposure has been suggested to also affect the drive to acquire natural reinforcers. Here, we tested how chronic exposure of rats to cocaine, as well as a subsequent withdrawal period, affects acquisition of natural reinforcers in high- and low-demanding tasks (HD and LD tasks, respectively). We chronically administered cocaine (i.p., 15 mg/kg once daily, or saline in control) for 30 days, followed by a 30-day withdrawal period. We tested the effect of this treatment on the acquisition of two natural appetitive reinforcers, namely self-administering a 10% sucrose solution and mounting a receptive female, under LD and HD conditions. During the cocaine exposure period, behavioral testing took place 18 hours after cocaine injection, namely after the acute pharmacologic effect of the drug dissipated. We show that chronic i.p. cocaine exposure decreased procurement of both reinforcers in HD but not in LD tasks. The effect was observed throughout the administration period with partial recovery after withdrawal. Taken together, we present empirical evidence that chronic exposure to a constant dose of cocaine is sufficient to reduce natural reinforcement, and that this decrease can outlast drug exposure. Importantly, such effects are observed only when high demands are opposing the consumption of the natural reinforcer.

Dissociable roles for the basolateral amygdala and orbitofrontal cortex in decision-making under risk of punishment

Several neuropsychiatric disorders are associated with abnormal decision-making involving risk of punishment, but the neural basis of this association remains poorly understood. Altered activity in brain systems including the basolateral amygdala (BLA) and orbitofrontal cortex (OFC) can accompany these same disorders, and these structures are implicated in some forms of decision-making. The current study investigated the role of the BLA and OFC in decision-making under risk of explicit punishment. Rats were trained in the risky decision-making task (RDT), in which they chose between two levers, one that delivered a small safe reward, and the other that delivered a large reward accompanied by varying risks of footshock punishment. Following training, they received sham or neurotoxic lesions of BLA or OFC, followed by RDT retesting. BLA lesions increased choice of the large risky reward (greater risk-taking) compared to both prelesion performance and sham controls. When reward magnitudes were equated, both BLA lesion and control groups shifted their choice to the safe (no shock) reward lever, indicating that the lesions did not impair punishment sensitivity. In contrast to BLA lesions, OFC lesions significantly decreased risk-taking compared with sham controls, but did not impair discrimination between different reward magnitudes or alter baseline levels of anxiety. Finally, neither lesion significantly affected food-motivated lever pressing under various fixed ratio schedules, indicating that lesion-induced alterations in risk-taking were not secondary to changes in appetitive motivation. Together, these findings indicate distinct roles for the BLA and OFC in decision-making under risk of explicit punishment.

Impact of repeated intravenous cocaine administration on incentive motivation depends on mode of drug delivery

The incentive sensitization theory of addiction posits that repeated exposure to drugs of abuse, like cocaine, can lead to long-term adaptations in the neural circuits that support motivated behavior, providing an account of pathological drug-seeking behavior. Although pre-clinical findings provide strong support for this theory, much remains unknown about the conditions that support incentive sensitization. The current study examined whether the mode of cocaine administration is an important factor governing that drug's long-term impact on behavior. Separate groups of rats were allowed either to self-administer intravenous cocaine or were given an equivalent number and distribution of unsignaled cocaine or saline infusions. During the subsequent test of incentive motivation (Pavlovian-to-instrumental transfer), we found that rats with a history of cocaine self-administration showed strong cue-evoked food seeking, in contrast to rats given unsignaled cocaine or saline. This finding indicates that the manner in which cocaine is administered can determine its lasting behavioral effects, suggesting that subjective experiences during drug use play a critical role in the addiction process. Our findings may therefore have important implications for the study and treatment of compulsive drug seeking.

Phasic mesolimbic dopamine signaling encodes the facilitation of incentive motivation produced by repeated cocaine exposure

Drug addiction is marked by pathological drug seeking and intense drug craving, particularly in response to drug-related stimuli. Repeated psychostimulant administration is known to induce long-term alterations in mesolimbic dopamine (DA) signaling that are hypothesized to mediate this heightened sensitivity to environmental stimuli. However, there is little direct evidence that drug-induced alteration in mesolimbic DA function underlies this hypersensitivity to motivational cues. In the current study, we tested this hypothesis using fast-scan cyclic voltammetry to monitor phasic DA signaling in the nucleus accumbens core of cocaine-pretreated (6 once-daily injections of 15 mg/kg, i.p.) and drug-naive rats during a test of cue-evoked incentive motivation for food-the Pavlovian-to-instrumental transfer task. We found that prior cocaine exposure augmented both reward seeking and DA release triggered by the presentation of a reward-paired cue. Furthermore, cue-evoked DA signaling positively correlated with cue-evoked food seeking and was found to be a statistical mediator of this behavioral effect of cocaine. Taken together, these findings provide support for the hypothesis that repeated cocaine exposure enhances cue-evoked incentive motivation through augmented phasic mesolimbic DA signaling. This work sheds new light on a fundamental neurobiological mechanism underlying motivated behavior and its role in the expression of compulsive reward seeking.

Luteolin inhibits behavioral sensitization by blocking methamphetamine-induced MAPK pathway activation in the caudate putamen in mice

Goal

To investigate the effect of luteolin on methamphetamine (MA)-induced behavioral sensitization and mitogen-activated protein kinase (MAPK) signal transduction pathway activation in mice.

Methods

Mice received a single dose of MA to induce hyperactivity or repeated intermittent intraperitoneal injections of MA to establish an MA-induced behavioral sensitization mouse model. The effect of luteolin on the development and expression of MA-induced hyperactivity and behavioral sensitization was examined. The expression and activity of ΔFosB and the levels of phosphorylated extracellular signal-regulated kinase 1/2 (pERK1/2), phosphorylated c-Jun N-terminal kinase (pJNK), and phosphorylated p38 mitogen-activated protein kinase (pp38) in the caudate putamen (CPu) were measured by western blot.

Results

Luteolin significantly decreased hyperactivity as well as the development and expression of MA-induced behavioral sensitization in mice. ΔFosB, pERK1/2, and pJNK levels in the CPu were higher in MA-treated mice than in control mice, whereas the pp38 level did not change. Injection of luteolin inhibited the MA-induced increase in ΔFosB, pERK1/2, and pJNK levels, but did not affect the pp38 level.

Conclusions

Luteolin inhibits MA-induced hyperactivity and behavioral sensitization in mice through the ERK1/2/ΔFosB pathway. Furthermore, the JNK signaling pathway might be involved in MA-induced neurodegeneration in the CPu, and luteolin inhibits this process.

Food consumption and weight gain after cessation of chronic amphetamine administration

Cessation of drug use often coincides with increased food consumption and weight gain in recovering addicts. However, it is not known whether this phenomenon (particularly the weight gain) is uniquely human, or whether it represents a consequence of drug cessation common across species. To address this issue, rats (n = 10/group) were given systemic injections of D-amphetamine (3 mg/kg) or an equal volume of saline vehicle for 9 consecutive days. Beginning 2 days after the final injection, rats were given free access to a highly palatable food mixture (consisting of sugar and butter) along with their standard chow diet, and food consumption and body weight were measured every 48 h for 30 days. Consistent with clinical observations, amphetamine-treated rats showed a greater increase in body weight over the course of the 30 days relative to vehicle-treated rats. Surprisingly, there was no difference in highly palatable food consumption between amphetamine- and vehicle-treated groups, but the amphetamine-treated group consumed significantly more standard chow than the control group. The finding that a history of chronic amphetamine exposure increases food consumption is consistent with previous work in humans showing that withdrawal from drugs of abuse is associated with overeating and weight gain. The current findings may reflect amphetamine-induced sensitization of mechanisms involved in reward motivation, suggesting that weight gain following drug cessation in humans could be due to similar mechanisms.

Repeated cocaine exposure facilitates the expression of incentive motivation and induces habitual control in rats

There is growing evidence that mere exposure to drugs can induce long-term alterations in the neural systems that mediate reward processing, motivation, and behavioral control, potentially causing the pathological pursuit of drugs that characterizes the addicted state. The incentive sensitization theory proposes that drug exposure potentiates the influence of reward-paired cues on behavior. It has also been suggested that drug exposure biases action selection towards the automatic execution of habits and away from more deliberate goal-directed control. The current study investigated whether rats given repeated exposure to peripherally administered cocaine would show alterations in incentive motivation (assayed using the Pavlovian-to-instrumental transfer (PIT) paradigm) or habit formation (assayed using sensitivity to reward devaluation). After instrumental and Pavlovian training for food pellet rewards, rats were given 6 daily injections of cocaine (15 mg/kg, IP) or saline, followed by a 10-d period of rest. Consistent with the incentive sensitization theory, cocaine-treated rats showed stronger cue-evoked lever pressing than saline-treated rats during the PIT test. The same rats were then trained on a new instrumental action with a new food pellet reward before undergoing a reward devaluation testing. Although saline-treated rats exhibited sensitivity to reward devaluation, indicative of goal-directed performance, cocaine-treated rats were insensitive to this treatment, suggesting a reliance on habitual processes. These findings, when taken together, indicate that repeated exposure to cocaine can cause broad alterations in behavioral control, spanning both motivational and action selection processes, and could therefore help explain aberrations of decision-making that underlie drug addiction.

Differential influence of morphine sensitization on accumbens shell and core dopamine responses to morphine- and food-conditioned stimuli

RATIONALE

Sensitization of the incentive and dopamine (DA) stimulant properties of drug-conditioned stimuli (CSs) by repeated exposure to drugs of abuse has been assigned an important role in the genesis of drug addiction.

OBJECTIVE

To test in rats if morphine-induced sensitization potentiates incentive and DA-releasing properties in the nucleus accumbens (NAc) shell and core elicited by presentation of a morphine-conditioned stimulus(CS) and if this property generalizes to a non-drug-(palatable food, Fonzies)-CS.

METHODS

Controls and rats previously sensitized by morphine were trained via three daily sessions consisting of a 10-min presentation of CS (Fonzies filled box, FB) followed by s.c. saline and morphine (1 mg/kg) or by standard food and Fonzies. Rats were implanted with microdialysis probes and the next-day incentive reactions and NAc shell and core DA were monitored during CS presentation and subsequent morphine (1 mg/kg) administration or Fonzies feeding.

RESULTS

Morphine sensitization increased incentive and NAc shell and core DA responses to morphine-CS. Morphine conditioning per se increased incentive reactions and NAc shell but not core DA responses to FB presentation. Morphine sensitization potentiated incentive responses but did not affect NAc shell and core DA responses to Fonzies-CS. Fonzies conditioning increased incentive reactions and NAc core but not shell DA responses to FB presentation.

CONCLUSIONS

These observations confirm the prediction of the incentive sensitization theory in the case of drug-CS but not of non-drug-CS. NAc DA might be differentially involved in the expression of incentive sensitization of drug- and non-drug-CSs, thus providing a clue for the abnormal incentive properties of drug CSs.

The effects of amphetamine exposure on outcome-selective Pavlovian-instrumental transfer in rats

RATIONALE

Repeated exposure to psychostimulants alters behavioral responses to reward-related cues; however, the motivational underpinnings of this effect have not been fully characterized.

OBJECTIVES

The following study was designed to examine how amphetamine sensitization affects performance in rats on a series of Pavlovian and operant tasks that distinguish between general-incentive and outcome-selective forms of conditioned responses.

METHODS

Adult male rats underwent Pavlovian and instrumental training for food pellet rewards. Following training, rats were sensitized to D-amphetamine (2 mg/kg for 7 days). Rats were subsequently tested on an outcome-selective Pavlovian-instrumental transfer (PIT) task, an outcome-reinstatement task, and an outcome devaluation task. Additionally, in a separate experiment, PIT was assessed in amphetamine-sensitized and control rats using a Pavlovian backward-conditioned stimulus.

RESULTS

Repeated amphetamine exposure sensitized locomotor activity to acute amphetamine challenge. Amphetamine altered responses to CS presentations by increasing conditioned approach. During tests of PIT, amphetamine-treated rats showed no outcome-selectivity in their responding, responding to a CS whether or not it shared a common outcome with the instrumental response. No effect of amphetamine sensitization was observed on tests of outcome-selective reinstatement by outcome delivery or action selection based on outcome value. Amphetamine-sensitized rats showed impaired outcome-selective PIT to a backward CS but were unaltered in conditioned approach.

CONCLUSIONS

Amphetamine sensitization prevents outcome-selective responding during PIT, which is dissociable from amphetamine's effects on conditioned approach. These data suggest fundamental alterations in how stimuli motivate action in addiction.

People newly in love are more responsive to positive feedback

Passionate love is associated with increased activity in dopamine-rich regions of the brain. Increased dopamine in these regions is associated with a greater tendency to learn from reward in trial-and-error learning tasks. This study examined the prediction that individuals who were newly in love would be better at responding to reward (positive feedback). In test trials, people who were newly in love selected positive outcomes significantly more often than their single (not in love) counterparts but were no better at the task overall. This suggests that people who are newly in love show a bias toward responding to positive feedback, which may reflect a general bias towards reward-seeking.

Dopaminergic modulation of risky decision-making

Many psychiatric disorders are characterized by abnormal risky decision-making and dysregulated dopamine receptor expression. The current study was designed to determine how different dopamine receptor subtypes modulate risk-taking in young adult rats, using a "Risky Decision-making Task" that involves choices between small "safe" rewards and large "risky" rewards accompanied by adverse consequences. Rats showed considerable, stable individual differences in risk preference in the task, which were not related to multiple measures of reward motivation, anxiety, or pain sensitivity. Systemic activation of D2-like receptors robustly attenuated risk-taking, whereas drugs acting on D1-like receptors had no effect. Systemic amphetamine also reduced risk-taking, an effect which was attenuated by D2-like (but not D1-like) receptor blockade. Dopamine receptor mRNA expression was evaluated in a separate cohort of drug-naive rats characterized in the task. D1 mRNA expression in both nucleus accumbens shell and insular cortex was positively associated with risk-taking, while D2 mRNA expression in orbitofrontal and medial prefrontal cortex predicted risk preference in opposing nonlinear patterns. Additionally, lower levels of D2 mRNA in dorsal striatum were associated with greater risk-taking. These data strongly implicate dopamine signaling in prefrontal cortical-striatal circuitry in modulating decision-making processes involving integration of reward information with risks of adverse consequences.

Effects of developmental nicotine exposure in rats on decision-making in adulthood

Exposure to tobacco smoke during pregnancy is associated with a range of adverse outcomes in offspring, including cognitive deficits and increased incidence of attention deficit-hyperactivity disorder, but there is a considerable controversy with regard to the causal role of tobacco smoke in these outcomes. To determine whether developmental exposure to the primary psychoactive ingredient in tobacco smoke, nicotine, may cause long-lasting behavioral alterations analogous to those in attention deficit-hyperactivity disorder, male Sprague-Dawley rats underwent a chronic neonatal nicotine administration regimen, which models third-trimester human exposure. Male rat pups were administered nicotine (6 mg/kg/day) by oral gastric intubation on postnatal days 1-7. In adulthood, rats were tested in two decision-making tasks (risky decision-making and delay discounting) as well as in free-operant responding for food reward and the elevated plus maze. Chronic neonatal nicotine attenuated weight gain during nicotine exposure, but there were no effects on performance in the decision-making task, and only a modest decrease in arm entries in the elevated plus maze in one subgroup of rats. These data are consistent with previous findings that developmental nicotine exposure has no effect on delay discounting, and they extend these findings to risky decision-making as well. They further suggest that at least some neurocognitive alterations associated with prenatal tobacco smoke exposure in humans may be due to genetic or other environmental factors, including non-nicotine components of tobacco smoke.

A single administration of methamphetamine to mice early in the light period decreases running wheel activity observed during the dark period

Repeated intermittent administration of amphetamines acutely increases appetitive and consummatory aspects of motivated behaviors as well as general activity and exploratory behavior, including voluntary running wheel activity. Subsequently, if the drug is withdrawn, the frequency of these behaviors decreases, which is thought to be indicative of dysphoric symptoms associated with amphetamine withdrawal. Such decreases may be observed after chronic treatment or even after single drug administrations. In the present study, the effect of acute methamphetamine (METH) on running wheel activity, horizontal locomotion, appetitive behavior (food access), and consummatory behavior (food and water intake) was investigated in mice. A multi-configuration behavior apparatus designed to monitor the five behaviors was developed, where combined measures were recorded simultaneously. In the first experiment, naïve male ICR mice showed gradually increasing running wheel activity over three consecutive days after exposure to a running wheel, while mice without a running wheel showed gradually decreasing horizontal locomotion, consistent with running wheel activity being a positively motivated form of natural motor activity. In experiment 2, increased horizontal locomotion and food access, and decreased food intake, were observed for the initial 3h after acute METH challenge. Subsequently, during the dark phase period decreased running wheel activity and horizontal locomotion were observed. The reductions in running wheel activity and horizontal locomotion may be indicative of reduced dopaminergic function, although it remains to be seen if these changes may be more pronounced after more prolonged METH treatments.

Stress-induced cross-sensitization to amphetamine is related to changes in the dopaminergic system

Repeated stress engenders behavioral sensitization. The mesolimbic dopamine system is critically involved in drug-induced behavioral sensitization. In the present study we examined the differences between adolescent and adult rats in stress-induced behavioral sensitization to amphetamine and changes in dopamine (DA) and its metabolite levels in the mesolimbic system. Adolescent or adult rats were restrained for 2 h, once a day, for 7 days. Three days after the last exposure to stress, the animals were challenged with saline or amphetamine (1.0 mg/kg i.p.) and amphetamine-induced locomotion was recorded for 40 min. Immediately after the behavioral tests, rats were decapitated and the nucleus accumbens (NAcc), ventral tegmental area (VTA) and amygdala (AM) were removed to measure tissue levels of DA and its metabolites by HPLC. Exposure to repeated restraint stress promoted behavioral sensitization to amphetamine in both adult and adolescent rats. In adult rats, amphetamine administration increased DA levels in both the stress and control groups in the NAcc and VTA. In adolescent rats, amphetamine increased DA levels in the NAcc in rats exposed to stress. Furthermore, in the AM of adolescent rats in the control group, amphetamine increased the DA levels; however, amphetamine reduced this neurotransmitter in the rats that were exposed to stress. No alteration was observed in the dopamine metabolite levels. Therefore, stress promoted behavioral sensitization to amphetamine and this may be related to changes in DA levels in the mesolimbic system. These changes appear to be dependent on ontogeny.

Selective breeding for magnitude of methamphetamine-induced sensitization alters methamphetamine consumption

RATIONALE

Genetically determined differences in susceptibility to drug-induced sensitization could be related to risk for drug consumption.

OBJECTIVES

Studies were performed to determine whether selective breeding could be used to create lines of mice with different magnitudes of locomotor sensitization to methamphetamine (MA). MA sensitization (MASENS) lines were also examined for genetically correlated responses to MA.

METHODS

Beginning with the F2 cross of C57BL/6J and DBA/2J strains, mice were tested for locomotor sensitization to repeated injections of 1 mg/kg MA and bred based on magnitude of sensitization. Five selected offspring generations were tested. All generations were also tested for MA consumption, and some were tested for dose-dependent locomotor-stimulant responses to MA, consumption of saccharin, quinine, and potassium chloride as a measure of taste sensitivity, and MA clearance after acute and repeated MA.

RESULTS

Selective breeding resulted in creation of two lines [MA high sensitization (MAHSENS) and MA low sensitization (MALSENS)] that differed in magnitude of MA-induced sensitization. Initially, greater MA consumption in MAHSENS mice reversed over the course of selection so that MALSENS mice consumed more MA. MAHSENS mice exhibited greater sensitivity to the acute stimulant effects of MA, but there were no significant differences between the lines in MA clearance from blood.

CONCLUSIONS

Genetic factors influence magnitude of MA-induced locomotor sensitization and some of the genes involved in magnitude of this response also influence MA sensitivity and consumption. Genetic factors leading to greater MA-induced sensitization may serve a protective role against high levels of MA consumption.

Life stress and kindling in bipolar disorder: review of the evidence and integration with emerging biopsychosocial theories

Most life stress literature in bipolar disorder (BD) fails to account for the possibility of a changing relationship between psychosocial context and episode initiation across the course of the disorder. According to Post's (1992) influential kindling hypothesis, major life stress is required to trigger initial onsets and recurrences of affective episodes, but successive episodes become progressively less tied to stressors and may eventually occur autonomously. Subsequent research on kindling has largely focused on unipolar depression (UD), and the model has been tested in imprecise and inconsistent ways. The aim of the present paper is to evaluate evidence for the kindling model as it applies to BD. We first outline the origins of the hypothesis, the evidence for the model in UD, and the issues needing further clarification. Next, we review the extant literature on the changing relationship between life stress and bipolar illness over time, and find that evidence from the methodologically strongest studies is inconsistent with the kindling hypothesis. We then integrate this existing body of research with two emerging biopsychosocial models of BD: the Behavioral Approach System dysregulation model, and the circadian and social rhythm theory. Finally, we present therapeutic implications and suggestions for future research.

Effects of chronic administration of drugs of abuse on impulsive choice (delay discounting) in animal models

Drug-addicted individuals show high levels of impulsive choice, characterized by preference for small immediate over larger but delayed rewards. Although the causal relationship between chronic drug use and elevated impulsive choice in humans has been unclear, a small but growing body of literature over the past decade has shown that chronic drug administration in animal models can cause increases in impulsive choice, suggesting that a similar causal relationship may exist in human drug users. This article reviews this literature, with a particular focus on the effects of chronic cocaine administration, which have been most thoroughly characterized. The potential mechanisms of these effects are described in terms of drug-induced neural alterations in ventral striatal and prefrontal cortical brain systems. Some implications of this research for pharmacological treatment of drug-induced increases in impulsive choice are discussed, along with suggestions for future research in this area.