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Enduring potentiation of sham active (dark) phase voluntary wheel running by 5 days of systemic morphine. In sham rats, systemic administration of 5 days of morphine (vertical gray bar) resulted in enhanced active phase running distance (a) and maximum running speed (b), relative to sham rats treated with saline (vehicle). This potentiation of voluntary wheel running continued through the 5-week observation after all dosing ceased. n = 5–6 per group
