Differential effects on natural reward processing in rats after repeated heroin

Aberrations in Incentive Learning and Responding to Heroin in Male Rats After Adolescent or Adult Chronic Binge-Like Alcohol Exposure

BACKGROUND AND PURPOSE

Binge drinking is a serious problem among adolescents and young adults despite its adverse consequences on the brain and behavior. One area that remains poorly understood concerns the impact of chronic intermittent ethanol (CIE) exposure on incentive learning.

METHODS

Here, we examined the effects of CIE exposure during different developmental stages on conditioned approach and conditioned reward learning in rats experiencing acute or protracted withdrawal from alcohol. Two or 21 days after adolescent or adult CIE exposure, male rats were exposed to pairings of a light stimulus (CS) and food pellets for 3 consecutive daily sessions (30 CS-food pellet pairings per session). This was followed by conditioned approach testing measuring responses (food trough head entries) to the CS-only presentations and by conditioned reward testing measuring responses on a lever producing the CS and on another producing a tone. We then measured behavioral sensitization to repeated injections of heroin (2 mg/kg/d for 9 days).

RESULTS

Adolescent and adult alcohol-treated rats showed significantly impaired conditioned reward learning regardless of withdrawal period (acute or prolonged). We found no evidence of changes to conditioned approach learning after adolescent or adult exposure to CIE. Finally, in addition to producing long-term impairments in incentive learning, CIE exposure enhanced locomotor activity in response to heroin and had no effect on behavioral sensitization to heroin regardless of age and withdrawal period.

CONCLUSIONS

Our work sets a framework for identifying CIE-induced alterations in incentive learning and inducing susceptibility to subsequent opioid effects.

Differential motivational profiles following adolescent sucrose access in male and female rats

Adolescents are the highest consumers of sugar sweetened drinks. Excessive consumption of such drinks is a likely contributor to the development of obesity and may be associated with enduring changes in the systems involved in reward and motivation. We examined the impact of daily sucrose consumption in young male and female rats (N=12 per group) across the adolescent period on the motivation to perform instrumental responses to gain food rewards as adults. Rats were or were not exposed to a sucrose solution for 2 h each day for 28 days across adolescence [postnatal days (P) 28-56]. They were then trained as adults (P70 onward) to lever press for a palatable 15% cherry flavored sucrose reward and tested on a progressive ratio (PR) schedule to assess motivation to respond for reinforcement. Female rats exposed to sucrose had higher breakpoints on the PR schedule than controls, whereas male rats exposed to sucrose had lower breakpoints than controls. These results show that consumption of sucrose during adolescence produced sex-specific behavioral changes in responding for sucrose as adults.

The effects of nicotine dependence and acute abstinence on the processing of drug and non-drug rewards

RATIONALE

Drug addiction may be characterised by a hypersensitivity to drug rewards and a hyposensitivity to non-drug rewards. This imbalance may become further polarised during acute abstinence.

OBJECTIVES

(i) Examine the differences between dependent and occasional smokers in choices for, motivation for and self-reported wanting and liking of cigarette and non-drug rewards. (ii) Examine the effects of 12-h nicotine abstinence on these metrics.

METHODS

Dependent (n = 20) and occasional, non-dependent smokers (n = 20) were tested after ad libitum smoking and ≥12-h of nicotine abstinence. A novel task was developed (Drug, Reward and Motivation-Choice (DReaM-Choice)) in which different rewards (cigarettes, music and chocolate) could be won. In each trial, participants chose between two rewards and then could earn the chosen reward via repeated button-pressing. Participants subsequently 'consumed' and rated subjective liking of the rewards they had won.

RESULTS

Compared with occasional smokers, dependent smokers made more choices for (p < 0.001), pressed more for (p = 0.046) and reported more wanting (p = 0.007) and liking (p < 0.001) of cigarettes, and also made fewer choices for chocolate (p = 0.005). There were no differences between the groups on button-pressing for chocolate or music. However, the balance between drug and non-drug reward processing was different between the groups across all metrics. Twelve-hour nicotine abstinence led to more cigarette choices (p < 0.001) and fewer music choices (p = 0.042) in both groups.

CONCLUSIONS

Nicotine dependence was associated with a hypersensitivity to cigarette rewards, but we found little evidence indicating a hyposensitivity to non-drug rewards. Our findings question the moderating influence of dependence on how acute nicotine abstinence affects reward processing.

Exercise decreases speedball self-administration

AIMS

Epidemiological studies report that individuals who exercise are less likely to abuse drugs. Preclinical studies report that exercise, in the form of treadmill or wheel running, reliably decreases the self-administration of psychomotor stimulants and opioids. To date, preclinical studies have only examined the effects of exercise on responding maintained by individual drugs and not by combinations of multiple drugs. This limits the translational appeal of these studies because polydrug abuse is common among substance abusing populations. The purpose of this study was to examine the effects of exercise on the self-administration of speedball, a combination of cocaine and heroin that is frequently encountered in intravenous drug abusing populations.

MAIN METHODS

Female rats were obtained at weaning and assigned to sedentary or exercising conditions. Sedentary rats were housed in standard cages that permitted no exercise beyond normal cage ambulation; exercising rats were housed in similar cages with an activity wheel. After 6weeks, rats were implanted with intravenous catheters and trained to self-administer cocaine, heroin, and dose combinations of cocaine and heroin (i.e., speedball) on a progressive ratio schedule of reinforcement.

KEY FINDINGS

Doses of speedball maintained greater levels of responding than corresponding doses of cocaine and heroin alone. Importantly, responding maintained by cocaine, heroin, and speedball was lower in exercising rats than sedentary rats.

SIGNIFICANCE

These data indicate that exercise decreases the self-administration of speedball and suggest that exercise may reduce the abuse of drug combinations that have traditionally been resistant to treatment.