Exercise decreases speedball self-administration

Abstract

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.

Keywords: Cocaine; Exercise; Heroin; Progressive ratio; Rat; Self-administration; Speedball.

Figure 1

Wheel running in exercising rats. Left axis depicts exercise output expressed as the mean number of wheel revolutions per day (rev/day); horizontal axis depicts time expressed in “weeks” of 5- to 9-day intervals. Reference line after week 6 (vertical broken line extending from abscissa) indicates catheter implantation and the beginning of behavioral testing. Vertical lines surrounding data points represent the SEM; where not indicated, the SEM fell within the data point.

Figure 2

Responding maintained by cocaine, heroin, and speedball in sedentary (left panels) and exercising (right panels) rats. Vertical axes depict breakpoints expressed as the number of infusions obtained (upper panels) or active lever responses (x 100; lower panels). Horizontal axes depict doses of cocaine. Points above “0” depict the effects of saline and individual doses of heroin when administered alone. Vertical lines surrounding data points represent the SEM; where not indicated, the SEM fell within the data point.

Figure 3

Session lengths during testing with cocaine, heroin, and speedball in sedentary (left panels) and exercising (right panels) rats. Vertical axes depict session lengths expressed in minutes (min). Horizontal axes depict doses of cocaine. Points above “0” depict the effects of saline and individual doses of heroin when administered alone. Vertical lines surrounding data points represent the SEM; where not indicated, the SEM fell within the data point.

Figure 4

Inactive lever responding during testing with cocaine, heroin, and speedball in sedentary (left panels) and exercising (right panels) rats. Vertical axes depict inactive lever responses (x 10). Horizontal axes depict doses of cocaine. Points above “0” depict the effects of saline and individual doses of heroin when administered alone. Vertical lines surrounding data points represent the SEM; where not indicated, the SEM fell within the data point.