Naloxone and Rimonabant Reduce the Reinforcing Properties of Exercise in Rats

Naloxone and rimonabant block neurotransmitter action of some drugs of abuse (such as ethanol, opiates, and nicotine), and thereby reduce drug seeking and self-administration by suppressing the drugs' reinforcing properties. The present study represents an attempt to elucidate whether these drugs may also reduce rewarding properties of other events, in this case, activity-based reinforcement. In Experiment 1, 10 obese and 10 lean Zucker rats pressed a locked door under a progressive ratio schedule of reinforcement that, when unlocked, provided access to a running wheel for 2-min intervals. After baseline breakpoints were established, doses of naloxone (0.3-10 mg/kg) were administered prior to experimental sessions. Obese rats exhibited lower baseline breakpoints for wheel activity, lower response rates, and fewer revolutions compared to lean rats. Naloxone decreased revolutions and response rates for lean and obese rats, but did not reduce breakpoints. In Experiment 2, five Long-Evans rats pressed a door to unlock a wheel for 20 s of wheel activity. Doses of rimonabant (1-10 mg/kg) were administered before some experimental sessions. The highest dose of rimonabant suppressed breakpoints and response rates, but did not affect revolutions. These data suggest that both drugs reduce the reinforcing properties of wheel running, but do so in different manners: naloxone may suppress wheel-based activity (consummatory behavior), but not seeking (appetitive behavior), and rimonabant does the converse. The data also support the role of endocannabinoids in the reinforcing properties of exercise, an implication that is important in terms of CB1 antagonists as a type of pharmacotherapy.