title: Chronic running wheel activity attenuates the antinociceptive actions of morphine and morphine-6-glucouronide administration into the periaqueductal gray in rats.
abstract: Chronic exercise in a running wheel increases baseline pain sensitivity while attenuating the antinociceptive effects of peripherally administered opiate agonists in laboratory rodents. To determine if these effects are due to exercise-induced changes in the central nervous system (CNS) or an artifact of exercise-induced alterations in peripheral physiology, the present study evaluated the antinociceptive actions of centrally administered opiate agonists in active and inactive female rats. Rats were implanted with cannula into the right periaqueductal gray (PAG) area of the midbrain. After the completion of the surgery, the animals were allowed ad libitum access to running wheels or housed in standard cages for three weeks. Pain sensitivity was measured on the tail flick test before and immediately following microinjections of either morphine (0, 2.5, 5.0, 10.0, 20.0 microg/rat) or the more potent morphine metabolite, morphine-6-glucuronide (M6G) (0, 0.03, 0.1, 0.3, 1.0 microg/rat). Baseline tail flick latencies were significantly shorter in active than in inactive rats. Additionally, active animals were less sensitive to the antinociceptive effects of morphine and M6G than inactive rats. These findings provide evidence for the involvement of the CNS in exercise-mediated alterations in pain sensitivity and opiate drug actions.
authors.affiliation: Department of Psychology, Tufts University, 490 Boston Avenue, Medford, MA 02155, USA. wendy.mathes@tufts.edu
authors.forename: Wendy Foulds
authors.initials: WF
authors.lastname: Mathes
authors.forename: Robin B
authors.initials: RB
authors.lastname: Kanarek
mesh_terms: D000701:Analgesics, Opioid / Q000494:pharmacology*; D000818:Animals; D004305:Dose-Response Relationship, Drug; D004971:Estrus / Q000187:drug effects; D005260:Female; D009020:Morphine / Q000494:pharmacology*; D009022:Morphine Derivatives / Q000494:pharmacology*; D017288:Pain Threshold; D010487:Periaqueductal Gray / Q000187:drug effects* / Q000502:physiology; D051381:Rats; D020318:Rats, Long-Evans; D011930:Reaction Time; D012420:Running
chemical_list: D000701:Analgesics, Opioid; D009022:Morphine Derivatives; C035349:morphine-6-glucuronide; D009020:Morphine