The effect of morphine dependence on impulsive choice in rats

Effects of fentanyl self-administration on risk-taking behavior in male rats

RATIONALE

Individuals with opioid use disorder (OUD) exhibit impaired decision making and elevated risk-taking behavior. In contrast to the effects of natural and semi-synthetic opioids, however, the impact of synthetic opioids on decision making is still unknown.

OBJECTIVES

The objective of the current study was to determine how chronic exposure to the synthetic opioid fentanyl alters risk-based decision making in adult male rats.

METHODS

Male rats underwent 14 days of intravenous fentanyl or oral sucrose self-administration. After 3 weeks of abstinence, rats were tested in a decision-making task in which they chose between a small, safe food reward and a large food reward accompanied by variable risk of footshock punishment. Following testing in the decision-making task, rats were tested in control assays that assessed willingness to work for food and shock reactivity. Lastly, rats were tested on a probabilistic reversal learning task to evaluate enduring effects of fentanyl on behavioral flexibility.

RESULTS

Relative to rats in the sucrose group, rats in the fentanyl group displayed greater choice of the large, risky reward (risk taking), an effect that was present as long as 7 weeks into abstinence. This increased risk taking was driven by enhanced sensitivity to the large rewards and diminished sensitivity to punishment. The fentanyl-induced elevation in risk taking was not accompanied by alterations in food motivation or shock reactivity or impairments in behavioral flexibility.

CONCLUSIONS

Results from the current study reveal that the synthetic opioid fentanyl leads to long-lasting increases in risk taking in male rats. Future experiments will extend this work to females and identify neural mechanisms that underlie these drug-induced changes in risk taking.

Oxycodone: A Current Perspective on Its Pharmacology, Abuse, and Pharmacotherapeutic Developments

Oxycodone, a semisynthetic derivative of naturally occurring thebaine, an opioid alkaloid, has been available for more than 100 years. Although thebaine cannot be used therapeutically due to the occurrence of convulsions at higher doses, it has been converted to a number of other widely used compounds that include naloxone, naltrexone, buprenorphine, and oxycodone. Despite the early identification of oxycodone, it was not until the 1990s that clinical studies began to explore its analgesic efficacy. These studies were followed by the pursuit of several preclinical studies to examine the analgesic effects and abuse liability of oxycodone in laboratory animals and the subjective effects in human volunteers. For a number of years oxycodone was at the forefront of the opioid crisis, playing a significant role in contributing to opioid misuse and abuse, with suggestions that it led to transitioning to other opioids. Several concerns were expressed as early as the 1940s that oxycodone had significant abuse potential similar to heroin and morphine. Both animal and human abuse liability studies have confirmed, and in some cases amplified, these early warnings. Despite sharing a similar structure with morphine and pharmacological actions also mediated by the μ-opioid receptor, there are several differences in the pharmacology and neurobiology of oxycodone. The data that have emerged from the many efforts to analyze the pharmacological and molecular mechanism of oxycodone have generated considerable insight into its many actions, reviewed here, which, in turn, have provided new information on opioid receptor pharmacology. SIGNIFICANCE STATEMENT: Oxycodone, a μ-opioid receptor agonist, was synthesized in 1916 and introduced into clinical use in Germany in 1917. It has been studied extensively as a therapeutic analgesic for acute and chronic neuropathic pain as an alternative to morphine. Oxycodone emerged as a drug with widespread abuse. This article brings together an integrated, detailed review of the pharmacology of oxycodone, preclinical and clinical studies of pain and abuse, and recent advances to identify potential opioid analgesics without abuse liability.

Differential effects of acute and prolonged morphine withdrawal on motivational and goal-directed control over reward-seeking behavior

Opioid addiction is a relapsing disorder marked by uncontrolled drug use and reduced interest in normally rewarding activities. The current study investigated the impact of spontaneous withdrawal from chronic morphine exposure on emotional, motivational, and cognitive processes involved in regulating the pursuit and consumption of natural food rewards in male rats. In Experiment 1, rats experiencing acute morphine withdrawal lost weight and displayed somatic signs of drug dependence. However, hedonically-driven sucrose consumption was significantly elevated, suggesting intact and potentially heightened emotional reward processing. In Experiment 2, rats undergoing acute morphine withdrawal displayed reduced motivation when performing an effortful response for palatable food reward. Subsequent reward devaluation testing revealed that acute withdrawal also disrupted their ability to exert flexible goal-directed control over their reward-seeking behavior. Specifically, morphine-withdrawn rats displayed insensitivity to reward devaluation both when relying on prior action-outcome learning and when given direct feedback about the consequences of their actions. In Experiment 3, rats tested after prolonged morphine withdrawal displayed heightened rather than diminished motivation for food rewards and retained their ability to engage in flexible goal-directed action selection. However, brief re-exposure to morphine was sufficient to impair motivation and disrupt goal-directed action selection, though in this case insensitivity to reward devaluation was only observed in the presence of morphine-paired context cues and in the absence of response-contingent feedback. We suggest that these opioid-withdrawal induced deficits in motivation and goal-directed control may contribute to addiction by interfering with the pursuit of adaptive alternatives to drug use.

Examining the effects of psychoactive drugs on complex behavioral processes in laboratory animals

Behavioral pharmacology has been aided significantly by the development of innovative cognitive tasks designed to examine complex behavioral processes in laboratory animals. Performance outcomes under these conditions have provided key metrics of drug action which serve to supplement traditional in vivo assays of physiologic and behavioral effects of psychoactive drugs. This chapter provides a primer of cognitive tasks designed to assay different aspects of complex behavior, including learning, cognitive flexibility, memory, attention, motivation, and impulsivity. Both capstone studies and recent publications are highlighted throughout to illustrate task value for two distinct but often interconnected translational strategies. First, task performance in laboratory animals can be utilized to elucidate how drugs of abuse affect complex behavioral processes. Here, the expectation is that adverse effects on such processes will have predictive relevance to consequences that will be experienced by humans. Second, these same task outcomes can be used to evaluate candidate therapeutics. In this case, the extent to which drug doses with medicinal value perturb task performance can contribute critical information for a more complete safety profile appraisal and advance the process of medications development. Methodological and theoretical considerations are discussed and include an emphasis on determining selectivity in drug action on complex behavioral processes.

The Neuropharmacology of Impulsive Behaviour, an Update

Neuropharmacological interventions in preclinical translational models of impulsivity have tremendously contributed to a better understanding of the neurochemistry and neural basis of impulsive behaviour. In this regard, much progress has been made over the last years, also due to the introduction of novel techniques in behavioural neuroscience such as optogenetics and chemogenetics. In this chapter, we will provide an update of how the behavioural pharmacology field has progressed and built upon existing data since an earlier review we wrote in 2008. To this aim, we will first give a brief background on preclinical translational models of impulsivity. Next, recent interesting evidence of monoaminergic modulation of impulsivity will be highlighted with a focus on the neurotransmitters dopamine and noradrenaline. Finally, we will close the chapter by discussing some novel directions and drug leads in the neuropharmacological modulation of impulsivity.

Naloxone-precipitated withdrawal ameliorates impairment of cost-benefit decision making in morphine-treated rats: Involvement of BDNF, p-GSK3-β, and p-CREB in the amygdala

Several studies indicated that morphine administration impairs cognitive brain functions. Therefore, in the current study, we investigated the effect of subchronic exposure to morphine and its withdrawal on effort- and/or delay-based forms of cost-benefit decision making and alterations in p-CREB/CREB ratio, p-GSK3β/GSK3β ratio, and BDNF level during decision making in the amygdala. Our data displayed an impairment of both forms of cost-benefit decision making following subchronic exposure to morphine. However, preference of high reward/high effort and/or high delay reward increased after naloxone injection. In molecular section, levels of BDNF and p-CREB/CREB ratio increased during cost-benefit decision making while p-GSK3β/GSK3β ratio decreased in both forms of decision making. In morphine-treated rats, level of BDNF and p-CREB/CREB ratio reduced during both forms of decision making while p-GSK3β/GSK3β ratio increased during delay-based and did not have a significant difference with the control group during effort-based decision making. On the withdrawal day, BDNF level raised while p-GSK3β/GSK3β ratio attenuated compared to morphine-treated group in both form of decision making. In addition, p-CREB/CREB ratio increased only during delay-based decision making on the withdrawal day. In conclusion, our data revealed that subchronic exposure to morphine interferes with the cost-benefit decision making may be via changes in level of BDNF, p-CREB/CREB and p-GSK3β/GSK3β ratio in the amygdala.

Visual and Auditory Reaction Times of Patients with Opioid Use Disorder

OBJECTIVE

Reaction time is defined as the time from the start of a stimulus to the start of the voluntary movement. Time plays an important role in undertaking daily living activities. Reaction time is an important factor in respect of both quality of life and of capabilities demonstrated in the work environment. Alcohol and some addictive substances have effect on RT. The aim of this study was to compare the visual and auditory reaction times of patients with opioid use disorder with healthy control subjects.

METHODS

The study was applied to two groups as the opioid use disorder group and the control group. A Sociodemographic and Clinical Data Form was prepared for each patient including age, gender, marital status and education level. Using a computer program the response to visual screen color change (red/blue) and to an auditory 'beep' sound of the computer system were recorded. The Student's t-test was applied as a statistical method.

RESULTS

The results showed longer reaction times in the patients with opioid use disorder.

CONCLUSION

To add improving reaction time approaches in opioid use disorder treatment may contribute to treatment by increasing quality of life and work performance.

Effects of morphine/CP55940 mixtures on an impulsive choice task in rhesus monkeys

µ-Opioid receptor agonists are commonly used to treat pain despite their adverse effects. In preclinical studies, cannabinoid receptor agonists increase the potency of opioids for producing antinociceptive but not reinforcing effects. It is unknown whether other adverse effects of these drugs, such as impairment of complex behavior, are enhanced by their co-administration. This study characterized the effects of morphine (µ-opioid receptor agonist; 0.32-5.6 mg/kg, subcutaneously) and CP55940 (CB1/CB2 cannabinoid receptor agonist; 0.0032-0.32 mg/kg, subcutaneously), alone and in mixtures, in monkeys (n=3) choosing between one pellet delivered immediately and two pellets delivered after a delay. Two consecutive choices of the immediate or delayed reward decreased or increased, respectively, the delay. The median adjusted delay, indicating indifference between the immediate and delayed reinforcers, was increased by morphine (3.2 mg/kg) and CP55940 (0.01-0.032 mg/kg). Performance after administration of morphine (0.32 and 1 mg/kg)/CP55940 (0.0032-0.032 mg/kg) mixtures was not different from performance after CP55940 alone. Neither morphine, CP55940, nor mixtures decreased the median adjusted delay (i.e. increased impulsivity). These findings failed to confirm previous studies showing that morphine increases impulsivity, perhaps because of procedural differences among studies. Treatment of pain often requires repeated drug administration; thus, it remains to be determined whether the present findings predict the effects of chronically administered morphine/CP5540 mixtures on impulsive choice.

Adolescent morphine exposure induces immediate and long-term increases in impulsive behavior

RATIONALE

Adolescence in humans represents a unique and critical developmental time point associated with increased risk-taking behavior. Converging clinical and epidemiological studies report a peak of drug use during adolescence, leading to the hypothesis that the developing adolescents brain is at risk to lose control over drug intake. Both adolescence and drug abuse are associated with significant cognitive and psychological changes such as lack of impulse control. A simple definition for impulsive behavior is the tendency to act prematurely without foresight. Increase in impulsivity is evident in acute morphine consumption, but to date, little is known with respect to subchronic morphine administration in impulsive behavior, particularly comparing time-dependent effects in adults, young adults, and adolescents.

METHODS

To evaluate this, adult, young adult, and adolescent rats were treated with a subchronic regimen of morphine or saline during 5 days (s.c.). Thereafter, we examined impulsive behavioral effects of morphine administration, 24 h and 25 days after administration in rats, while responding under a five-choice serial reaction time task (5-CSRTT).

RESULTS

Subchronic morphine administration increased premature responding 24 h after the last injection of morphine in adult, young adult, and adolescent rats without increasing motor activity but a significant change in motivation in adult and young adult rats only. After 25 days of abstinence, premature responses were significantly increased in comparison with baseline in adolescent rats but not in adults and young adults.

CONCLUSION

The main conclusion of this study is that morphine exposure in adolescents has a long-term profound effect on motor impulsive behavior later in adulthood. An implication of our findings might be that we should be especially careful about consuming and prescribing opioid drugs in adolescents.

Effect of daily morphine administration and its discontinuation on delay discounting of food in rhesus monkeys

Opioid abusers discount delayed reinforcers more rapidly than nonusers; however, it is unclear whether chronic drug administration or its discontinuation impacts discounting. This study examined the impact of daily morphine administration and its discontinuation on delay discounting of food in rhesus monkeys. Responding on one lever delivered one food pellet immediately; responding on another lever delivered two food pellets either immediately or after a delay (30-120 s) that increased within the session. Monkeys (n=3) responded for the large reinforcer when both reinforcers were delivered immediately and more for the smaller, immediately available reinforcer as the delay to delivery of the large reinforcer increased. When administered acutely, morphine (0.032-5.6 mg/kg) increased trial omissions and had variable effects on choice, with small doses decreasing and large doses increasing choice of the large delayed reinforcer. Chronic morphine administration (0.1 mg/kg/day to 3.2 mg/kg twice daily) reduced choice of the large delayed reinforcer in two monkeys, while increasing choice in a third monkey. Despite the development of tolerance to some effects (i.e. rightward shifts in dose-effect curves for the number of trials omitted) and evidence of mild opioid dependence (e.g. decrease in the number of trials completed, as well as body weight), discontinuation of treatment did not appear to systematically impact discounting. Overall, these results suggest that repeated opioid administration causes persistent effects on choice under a delay discounting procedure; however, differences in the direction of effect among individuals suggest that factors other than, or in addition to, changes in discounting might play a role.

Choice between delayed food and immediate oxycodone in rats

RATIONALE

The choice to seek immediate drug effects instead of more meaningful but delayed rewards is a defining feature of addiction.

OBJECTIVES

To develop a rodent model of this behavior, we allowed rats to choose between immediate intravenous delivery of the prescription opioid oxycodone (50 μg/kg) and delayed delivery of palatable food pellets.

RESULTS

Rats preferred food at delays up to 30 s, but they chose oxycodone and food equally at 60-s delay and preferred oxycodone over food at 120-s delay. Comparison of food-drug choice, food-only, and drug-only conditions indicated that food availability decreased drug intake, but drug availability increased food intake. In the food-only condition, food was effective as a reinforcer even when delayed by 120 s. Pre-session feeding with chow slowed acquisition of food and drug self-administration, but did not affect choice. To establish procedures for testing potential anti-addiction medications, noncontingent pre-treatment with oxycodone or naltrexone (analogous to substitution and antagonist therapies, respectively) were tested on a baseline in which oxycodone was preferred over delayed food. Naltrexone pre-treatment decreased drug intake and increased food intake. Oxycodone pre-treatment decreased drug intake, but also produced extended periods with no food or drug responding.

CONCLUSIONS

These findings show that the contingencies that induce preference for drugs over more meaningful but less immediate rewards in humans can be modeled in rodents, and they suggest that the model could be useful for assessing the therapeutic potential of treatments and exploring the underlying behavioral and neural mechanisms involved in addiction.

Multifaceted impairments in impulsivity and brain structural abnormalities in opioid dependence and abstinence

BACKGROUND

Chronic opioid exposure, as a treatment for a variety of disorders or as drug of misuse, is common worldwide, but behavioural and brain abnormalities remain under-investigated. Only a small percentage of patients who receive methadone maintenance treatment (MMT) for previous heroin misuse eventually achieve abstinence and studies on such patients are rare.

METHOD

The Cambridge Neuropsychological Test Automated Battery and T1 weighted magnetic resonance imaging (MRI) were used to study a cohort of 122 male individuals: a clinically stable opioid-dependent patient group receiving MMT (n = 48), an abstinent previously MMT maintained group (ABS) (n = 24) and healthy controls (n = 50).

RESULTS

Stable MMT participants deliberated longer and placed higher bets earlier in the Cambridge Gambling Task (CGT) and showed impaired strategic planning compared with healthy controls. In contrast, ABS participants showed impairment in choosing the least likely outcome, delay aversion and risk adjustment on the CGT, and exhibited non-planning impulsivity compared with controls. MMT patients had widespread grey matter reductions in the orbitomedial prefrontal cortex, caudate, putamen and globus pallidus. In contrast, ABS participants showed midbrain-thalamic grey matter reductions. A higher methadone dose at the time of scanning was associated with a smaller globus pallidus in the MMT group.

CONCLUSIONS

Our findings support an interpretation of heightened impulsivity in patients receiving MMT. Widespread structural brain abnormalities in the MMT group and reduced brain structural abnormality with abstinence suggest benefit of cessation of methadone intake. We suggest that a longitudinal study is required to determine whether abstinence improves abnormalities, or patients who achieve abstinence have reduced abnormalities before methadone cessation.

Daily morphine administration increases impulsivity in rats responding under a 5-choice serial reaction time task

BACKGROUND AND PURPOSE

Repeated administration of a μ opioid receptor agonist can enhance some forms of impulsivity, such as delay discounting. However, it is unclear whether repeated administration alters motor impulsivity.

EXPERIMENTAL APPROACH

We examined the effects of acute administration of morphine and amphetamine prior to and during daily morphine administration in rats responding under a five-choice serial reaction time task. Rats (n = 5) were trained to detect a brief flash of light presented randomly in one of five response holes; responding in the target hole delivered food, whereas responding in the wrong hole or responding prior to illumination of the target stimulus (premature response) initiated a timeout. Premature responding served as an index of motor impulsivity.

KEY RESULTS

Administered acutely, morphine (0.1-10 mg·kg(-1) , i.p.) increased omissions and modestly, although not significantly, premature responding without affecting response accuracy; amphetamine (0.1-1.78 mg·kg(-1) , i.p.) increased premature responding without changing omissions or response accuracy. After 3 weeks of 10 mg·kg(-1) ·day(-1) morphine, tolerance developed to its effects on omissions whereas premature responding increased approximately fourfold, compared with baseline. Effects of amphetamine were not significantly affected by daily morphine administration.

CONCLUSIONS AND IMPLICATIONS

These data suggest that repeated administration of morphine increased effects of morphine on motor impulsivity, although tolerance developed to other effects, such as omissions. To the extent that impulsivity is a risk factor for drug abuse, repeated administration of μ opioid receptor agonists, for recreational or therapeutic purposes, might increase impulsivity and thus the risk for drug abuse.

Inflated reward value in early opiate withdrawal

Through incentive learning, the emotional experience of a reward in a relevant need state (e.g. hunger for food) sets the incentive value that guides the performance of actions that earn that reward when the need state is encountered again. Opiate withdrawal has been proposed as a need state in which, through experience, opiate value can be increased, resulting in escalated opiate self-administration. Endogenous opioid transmission plays anatomically dissociable roles in the positive emotional experience of reward consumption and incentive learning. We, therefore, sought to determine if chronic opiate exposure and withdrawal produces a disruption in the fundamental incentive learning process such that reward seeking, even for non-opiate rewards, can become maladaptive, inconsistent with the emotional experience of reward consumption and irrespective of need. Rats trained to earn sucrose or water on a reward-seeking chain were treated with morphine (10-30 mg/kg, s.c.) daily for 11 days prior to testing in withdrawal. Opiate-withdrawn rats showed elevated reward-seeking actions, but only after they experienced the reward in withdrawal, an effect that was strongest in early (1-3 days), as opposed to late (14-16 days), withdrawal. This was sufficient to overcome a negative reward value change induced by sucrose experience in satiety and, in certain circumstances, was inconsistent with the emotional experience of reward consumption. Lastly, we found that early opiate withdrawal-induced inflation of reward value was blocked by inactivation of basolateral amygdala mu opioid receptors. These data suggest that in early opiate withdrawal, the incentive learning process is disrupted, resulting in maladaptive reward seeking.

The effect of acute morphine on delay discounting in dependent and non-dependent rats

RATIONALE

Chronic opiate use is associated with increased impulsivity in both humans and animals, and previous studies suggest that acute morphine can increase impulsivity in non-dependent rats. However, the extent to which chronic opiate usage modulates the effect of acute morphine is unknown.

METHODS

Rats were trained to delay discount 20 % sucrose solution and then randomly assigned to either a dependent group that received a nightly 30 mg/kg subcutaneous dose of morphine or a non-dependent group that received a nightly saline injection. Once dependence was established, rats were then assigned to one of four acute morphine doses (0, 1.25, 2.5, 5 mg/kg). For 5 days, delay discounting curves were determined 22.5 h after maintenance doses and 1 h after their prescribed acute injections.

RESULTS

In non-dependent rats, 2.5 and 5 mg/kg doses of morphine caused decreased preference for the large reward at all delays. Acute morphine had no effect on discounting curves in dependent rats.

CONCLUSIONS

Morphine dependence can cause tolerance to the effects of acute morphine on delay discounting.

Do the adjusting-delay and increasing-delay tasks measure the same construct: delay discounting?

Delay discounting describes the subjective devaluation of a reward when it is delayed. In animals, the adjusting-delay (AD) and increasing-delay (ID) tasks often are used to assess individual differences in, and drug effects on, delay discounting. No study to date, however, has compared systematically the measures of discounting produced in these tasks. The current study examined the correlation between measures of delay discounting derived from AD and ID procedures. Twenty rats completed 30 sessions under each task (order counterbalanced across rats). Quantitative measures of delay discounting produced by the two tasks were positively correlated, suggesting that the AD and ID tasks measure the same underlying facet of impulsive choice (i.e. individual or conjoint sensitivities to reward delay and magnitude). The measures derived from either task, however, depended on the sequences in which the tasks were experienced. That is, pre-exposure to one task decreased discounting of delayed rewards in the second task. Consistent with other published findings, exposure to delayed consequences during the initial discounting assessment might explain this effect. Despite the observed correlation between ID and AD indifference delays, we suggest that the ID procedure might be a more appropriate procedure for pharmacological studies.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

Recent Translational Findings on Impulsivity in Relation to Drug Abuse

Impulsive behavior is strongly implicated in drug abuse, as both a cause and a consequence of drug use. To understand how impulsive behaviors lead to and result from drug use, translational evidence from both human and non-human animal studies is needed. Here, we review recent (2009 or later) studies that have investigated two major components of impulsive behavior, inhibitory control and impulsive choice, across preclinical and clinical studies. We concentrate on the stop-signal task as the measure of inhibitory control and delay discounting as the measure of impulsive choice. Consistent with previous reports, recent studies show greater impulsive behavior in drug users compared with non-users. Additionally, new evidence supports the prospective role of impulsive behavior in drug abuse, and has begun to identify the neurobiological mechanisms underlying impulsive behavior. We focus on the commonalities and differences in findings between preclinical and clinical studies, and suggest future directions for translational research.

Delay discounting of oral morphine and sweetened juice rewards in dependent and non-dependent rats

RATIONALE

Opioid-dependent humans are reported to show accelerated delay discounting of opioid rewards when compared to monetary rewards. It has been suggested that this may reflect a difference in discounting of consumable and non-consumable goods not specific to dependent individuals. Here, we evaluate the discounting of similar morphine and non-morphine oral rewards in dependent and non-dependent rats

METHODS

We first tested the analgesic and rewarding effects of our morphine solution. In a second experiment, we assigned rats randomly to either dependent or non-dependent groups that, 30 min after daily testing, received 30 mg/kg subcutaneous dose of morphine, or saline, respectively. Delay discounting of drug-free reward was examined prior to initiation of the dosing regimen. We tested discounting of the morphine reward in half the rats and retested the discounting of the drug-free reward in the other half. All tests were run 22.5 h after the daily maintenance dose.

RESULTS

Rats preferred the morphine cocktail to the drug-free solution and consumed enough to induce significant analgesia. The control quinine solution did not produce these effects. Dependent rats discounted morphine rewards more rapidly than before dependence and when compared to discounting drug-free rewards. In non-dependent rats both reward types were discounted similarly.

CONCLUSIONS

These results show that morphine dependence increases impulsiveness specifically towards a drug reward while morphine experience without dependence does not.

Acupuncture Stimulation Attenuates Impaired Emotional-Like Behaviors and Activation of the Noradrenergic System during Protracted Abstinence following Chronic Morphine Exposure in Rats

The purpose of this study was to evaluate whether acupuncture stimulation attenuates withdrawal-induced behaviors in the rats during protracted abstinence following chronic morphine exposure. To do this, male rats were first exposed to morphine gradually from 20 to 100 mg/kg for 5 days, and subsequently naloxone was injected once to extend despair-related withdrawal behaviors for 4 weeks. Acupuncture stimulation was performed once at the SP6 (Sanyinjiao) acupoint on rat's; hind leg for 5 min during protracted abstinence from morphine. The acupuncture stimulation significantly decreased despair-like behavior deficits in the forced swimming test and low sociability in the open-field test as well as increased open-arm exploration in the elevated plus maze test in the last week of 4-week withdrawal period. Also the acupuncture stimulation significantly suppressed the increase in the hypothalamic corticotropin-releasing factor (CRF) expression, the decrease in the tyrosine hydroxylase expression in the locus coeruleus, and the decrease in the hippocampal brain-derived neurotrophic factor mRNA expression, induced by repeated injection of morphine. Taken together, these findings demonstrate that the acupuncture stimulation of SP6 significantly reduces withdrawal-induced behaviors, induced by repeated administration of morphine in rats, possibly through the modulation of hypothalamic CRF and the central noradrenergic system.

Impulsivity and drugs of abuse: a juice-reinforced operant procedure for determining within-session delay discounting functions in rhesus monkeys

INTRODUCTION

Impulsivity is a behavioral trait that is thought to contribute to a variety of disorders, including drug abuse. Efficient, sensitive procedures are needed for studying drug effects on impulsivity (e.g., delay discounting) in nonhumans.

METHODS

Three monkeys responded under an operant choice procedure whereby responses on one lever resulted in immediate delivery of 0.15 ml of juice [Hawaiian Punch®] and responses on another lever resulted in delivery of 0.75 ml of juice, either immediately or after a delay (3.75-40 s). The delay to the larger reinforcer increased within-session across discrete blocks allowing for generation of delay-discounting functions within sessions.

RESULTS

Without delay, monkeys chose the larger reinforcer nearly exclusively. With increasing delay, monkeys progressively switched their choice from the larger to the smaller reinforcer in a delay-dependent manner. In 2 monkeys, acute administration of morphine (0.1 and 0.32 mg/kg) or ketamine (0.1 and 0.32 mg/kg) but not diazepam (0.1-1.0 mg/kg) dose-dependently shifted the delay-discounting functions to the left, indicating increased discounting. In one monkey, daily morphine treatment (0.32 mg/kg/day, 3-hour pretreatment) produced a rapid, sustained leftward shift in the delay-discounting function; the curve returned to the pre-drug position 6 days after discontinuation of treatment.

DISCUSSION

This delay discounting procedure is sensitive to both behavioral and pharmacological manipulations and appears to be particularly sensitive to chronic drug treatment and drug withdrawal. Given the importance of drug dependence and withdrawal in the initiation, maintenance of, and relapse to drug taking, this procedure should be useful to study one aspect of this process.