Delay discounting of the μ-opioid receptor agonist remifentanil in rhesus monkeys

Prenatal exposure to opioids: Associations between the caregiving environment and externalizing behaviors

Maternal opioid use during pregnancy is a rapidly growing public health crisis and is associated with a range of adverse developmental outcomes including externalizing behaviors among exposed children. Recent work has highlighted the role of indirect pathways from prenatal opioid exposure to behavioral outcomes through aspects of the caregiving environment, including parenting. This review highlights maternal sensitivity and related aspects of the caregiving environment that may impact the development of externalizing behaviors among children with a history of prenatal exposure to opioids. We conclude by providing suggestions for future directions in research examining development among children with prenatal opioid exposure.

Reinforcing effects of opioid/cannabinoid mixtures in rhesus monkeys responding under a food/drug choice procedure

OBJECTIVE

Cannabinoid receptor agonists such as delta-9-tetrahydrocannabinol (Δ⁹-THC) enhance the antinociceptive potency of mu opioid receptor agonists such as morphine, indicating that opioid/cannabinoid mixtures might be effective for treating pain. However, such enhancement will be beneficial only if cannabinoids do not also enhance adverse effects of opioids, including those related to abuse. In rhesus monkeys, cannabinoids fail to enhance and often decrease self-administration of the mu opioid receptor agonist heroin, suggesting that opioid/cannabinoid mixtures do not have greater reinforcing effects (abuse potential) compared with opioids alone. Previous studies on the self-administration of opioid/cannabinoid mixtures used single-response procedures, which do not easily differentiate changes in reinforcing effects from other effects (e.g., rate decreasing).

METHODS

In this study, rhesus monkeys (n = 4) responded under a choice procedure wherein responding on one lever delivered sucrose pellets and responding on the other lever delivered intravenous infusions of the mu opioid receptor agonist remifentanil (0.032-1.0 μg/kg/infusion) alone or in combination with either Δ⁹-THC (10-100 μg/kg/infusion) or the synthetically derived cannabinoid receptor agonist CP55940 (3.2-10 μg/kg/infusion).

RESULTS

Remifentanil dose-dependently increased choice of drug over food, whether available alone or in combination with a cannabinoid, and the potency of remifentanil was not significantly altered by coadministration with a cannabinoid. Mixtures containing the largest doses of cannabinoids decreased response rates in most subjects, confirming that behaviorally active doses were studied.

CONCLUSION

Overall, these results extend previous studies to include choice behavior and show that cannabinoids do not substantially enhance the reinforcing effects of mu opioid receptor agonists.

Pharmacotherapies for decreasing maladaptive choice in drug addiction: Targeting the behavior and the drug

Drug addiction can be conceptualized as a disorder of maladaptive decision making in which drugs are chosen at the expense of pro-social, nondrug alternatives. The study of decision making in drug addiction has focused largely on the role of impulsivity as a facilitator of addiction, in particular the tendency for drug abusers to choose small, immediate gains over larger but delayed outcomes (i.e., delay discounting). A parallel line of work, also focused on decision making in drug addiction, has focused on identifying the determinants underlying the choice to take drugs over nondrug alternatives (i.e., drug vs. nondrug choice). Both tracks of research have been valuable tools in the development of pharmacotherapies for treating maladaptive decision making in drug addiction, and a number of common drugs have been studied in both designs. However, we have observed that there is little uniformity in the administration regimens of potential treatments between the designs, which hinders congruence in the development of single treatment strategies to reduce both impulsive behavior and drug choice. The current review provides an overview of the drugs that have been tested in both delay-discounting and drug-choice designs, and focuses on drugs that reduced the maladaptive choice in both designs. Suggestions to enhance congruence between the findings in future studies are provided. Finally, we propose the use of a hybridized, experimental approach that may enable researchers to test the effectiveness of therapeutics at decreasing impulsive and drug choice in a single design.

Remifentanil maintains lower initial delayed nonmatching-to-sample accuracy compared to food pellets in male rhesus monkeys

Emerging human laboratory and preclinical drug self-administration data suggest that a history of contingent abused drug exposure impairs performance in operant discrimination procedures, such as delayed nonmatching-to-sample (DNMTS), that are hypothesized to assess components of executive function. However, these preclinical discrimination studies have exclusively used food as the reinforcer and the effects of drugs as reinforcers in these operant procedures are unknown. The present study determined effects of contingent intravenous remifentanil injections on DNMTS performance hypothesized to assess 1 aspect of executive function, working memory. Daily behavioral sessions consisted of 2 components with sequential intravenous remifentanil (0, 0.01-1.0 μg/kg/injection) or food (0, 1-10 pellets) availability in nonopioid dependent male rhesus monkeys (n = 3). Remifentanil functioned as a reinforcer in the DNMTS procedure. Similar delay-dependent DNMTS accuracy was observed under both remifentanil- and food-maintained components, such that higher accuracies were maintained at shorter (0.1-1.0 s) delays and lower accuracies approaching chance performance were maintained at longer (10-32 s) delays. Remifentanil maintained significantly lower initial DNMTS accuracy compared to food. Reinforcer magnitude was not an important determinant of DNMTS accuracy for either remifentanil or food. These results extend the range of experimental procedures under which drugs function as reinforcers. Furthermore, the selective remifentanil-induced decrease in initial DNMTS accuracy is consistent with a selective impairment of attentional, but not memorial, processes. (PsycINFO Database Record

Choice between delayed food and immediate opioids in rats: treatment effects and individual differences

RATIONALE

Addiction involves maladaptive choice behavior in which immediate drug effects are valued more than delayed nondrug rewards.

OBJECTIVES AND METHODS

To model this behavior and extend our earlier work with the prescription opioid oxycodone, we allowed rats to choose between immediate intravenous delivery of the short-acting opioid remifentanil and delayed delivery of highly palatable food pellets. Treatment drugs were tested on a baseline where remifentanil was preferred over food.

RESULTS

Treatment with a high dose of the opioid antagonist naltrexone decreased but did not reverse the preference for remifentanil. Treatment with the serotonin 5-HT_2C agonist lorcaserin decreased remifentanil and food self-administration nonselectively. Across conditions in which the alternative to delayed food was either a moderate dose of oxycodone, a moderate or high dose of remifentanil, a smaller more immediate delivery of food, or timeout with no primary reinforcement, choice was determined by both the length of the delay and the nature of the alternative option. Delayed food was discounted most steeply when the alternative was a high dose of remifentanil, which was preferred over food when food was delayed by 30 s or more. Within-subject comparisons showed no evidence for trait-like impulsivity or sensitivity to delay across these conditions.

CONCLUSIONS

Choice was determined more by the current contingencies of reinforcement than by innate individual differences. This finding suggests that people might develop steep delay-discounting functions because of the contingencies in their environment, and it supports the use of contingency management to enhance the relative value of delayed nondrug reinforcers.

Shallow discounting of delayed cocaine by male rhesus monkeys when immediate food is the choice alternative

Huskinson et al. (2015) recently examined delay discounting in monkeys choosing between an immediate drug (cocaine) reinforcer and a delayed nondrug (food) reinforcer. The present experiment examined the reverse situation: choice between immediate nondrug (food) and delayed drug (cocaine) reinforcers. Whereas the former choice situation exemplifies drug abuse from a delay-discounting perspective, our interest in the latter choice situation is derived from the observation that drug abusers, who characteristically are associated with impulsive choice, typically must devote considerable time to procuring drugs, often at the expense of immediate nondrug alternatives. Accordingly, we analyzed 3 male rhesus monkeys' choices between immediate food and delayed cocaine (0.1 and 0.2 mg/kg/injection) using a hyperbolic model that allowed us to compare discounting rates between qualitatively different reinforcers. Choice of immediate food increased with food amount, and choice functions generally shifted leftward as delay to cocaine increased, indicating a decrease in the subjective value of cocaine. Compared with our previous delay-discounting experiment with immediate cocaine versus delayed food, both doses of delayed cocaine were discounted at a shallow rate. The present results demonstrate that rhesus monkeys will tolerate relatively long delays in an immediate-food versus delayed-drug situation, suggesting that in intertemporal choices between cocaine and food, the subjective value of cocaine is less affected by the delay until reinforcement than is the subjective value of delayed food. More generally, the present findings suggest that although drug abusers may choose impulsively when immediate drug reinforcement is available, they exercise self-control in the acquisition of a highly preferred, delayed drug reinforcer. (PsycINFO Database Record

Determinants of choice, and vulnerability and recovery in addiction

Addiction may be viewed as choice governed by competing contingencies. One factor impacting choice, particularly as it relates to addiction, is sensitivity to delayed rewards. Discounting of delayed rewards influences addiction vulnerability because of competition between relatively immediate gains of drug use, e.g. intoxication, versus relatively remote gains of abstinence, e.g. family stability. Factors modifying delay sensitivity can be modeled in the laboratory. For instance, increased delay sensitivity can be similarly observed in adolescent humans and non-human animals. Similarly, genetic factors influence delay sensitivity in humans and animals. Recovery from addiction may also be viewed as choice behavior. Thus, reinforcing alternative behavior facilitates recovery because reinforcing alternative behavior decreases the frequency of using drugs. How reinforcing alternative behavior influences recovery can also be modeled in the laboratory. For instance, relapse risk decreases as abstinence duration increases, and this decreasing risk can be modeled in animals using choice procedures. In summary, addiction in many respects can be conceptualized as a problem of choice. Animal models of choice disorders stand to increase our understanding of the core processes that establish and maintain addiction and serve as a proving ground for development of novel treatments.