The protective effects of free wheel-running against cocaine psychomotor sensitization persist after exercise cessation in C57BL/6J mice

Previous literature suggests that free access to a running wheel can attenuate the behavioral responsiveness to addictive drugs in rodents. In a few studies, wheel-running cessation accentuated drug responsiveness. Here, we tested whether free wheel-running cessation is followed by (1) an accentuation or (2) an attenuation of cocaine psychomotor sensitization, knowing that no cessation of (continuous) wheel-running is associated with an attenuation of cocaine responsiveness. Male C57BL/6J mice, aged 35 days, were housed singly either with (exercising mice) or without (non-exercising mice) a running wheel. At the end of a period of 36 days, half of the exercising mice were deprived of their wheel whereas the other half of exercising mice kept their wheel until the end of experimentation (which lasted 85 days). The non-exercising mice were housed without wheel throughout experimentation. Testing took place 3 days after exercise cessation. After 2 once-daily drug-free test sessions, mice were tested for initiation of psychomotor sensitization over 13 once-daily injections of 8 mg/kg cocaine. Post-sensitization conditioned activation (saline challenge) and long-term expression of sensitization were assessed 2 or 30 days after the last sensitizing injection (same treatments as for initiation of sensitization), respectively. Exercising mice and mice undergoing wheel-running cessation exhibited comparable degrees of attenuation of all cocaine effects in comparison with the continuously non-exercising mice, which showed the greatest effects. Thus, the efficaciousness of wheel-running at attenuating cocaine sensitization not only resisted to exercise cessation but also was unambiguously persistent (an important effect rarely reported in previous literature)