Premium
Blocking Peripheral Opioid Receptors with Naloxone Methiodide Prevents Acute and Chronic Training‐Induced Analgesia in a Rat Model of Fibromyalgia
Author(s) -
Williams Emily R,
Ford Caleb M,
Simonds J. Grant,
Leal Anna K
Publication year - 2017
Publication title -
the faseb journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.709
H-Index - 277
eISSN - 1530-6860
pISSN - 0892-6638
DOI - 10.1096/fasebj.31.1_supplement.1020.16
Subject(s) - fibromyalgia , medicine , (+) naloxone , naloxone hydrochloride , opioid , hyperalgesia , peripheral , endocrinology , opioid receptor , receptor , chronic pain , anesthesia , nociception , physical therapy
Data from our lab and others has shown that chronic treadmill training decreases mechanical hyperalgesia in a rat model of fibromyalgia. Opioid receptors have been shown to contribute to this training effect. Our study investigated the contribution of peripheral opioid receptors to acute and chronic exercise‐induced analgesia using naloxone methiodide, a nonspecific opioid receptor antagonist that does not cross the blood‐brain barrier. Fibromyalgia, characterized by chronic widespread pain, was induced in female Sprague Dawley rats (n=10) by acidic saline injections into the right gastrocnemius muscle (0.1 mL; pH 4). Trained + treated rats received injections of naloxone methiodide (120 μg) into both gastrocnemius muscles prior to moderate‐intensity treadmill running (4 weeks; 5 days/week; 30 min/day). Hindpaw pain withdrawal threshold (PWT), a measure of mechanical hyperalgesia, was quantified with von Frey filaments before and after each running bout. PWT was also measured daily in a subgroup of sedentary control rats. Selectively blocking peripheral opioid receptors prior to running prevented acute and chronic training‐induced increases in PWT (p > .05). Further, PWT was significantly lower in the trained + treated group (week 1: 30 ± 6 mN; week 2: 26 ± 4 mN; week 3: 22 ± 4 mN; week 4: 21 ± 3 mN) compared to sedentary rats (week 1: 64 ± 6 mN; week 2: 57 ± 5 mN; week 3: 51 ± 4 mN; week 4: 40 ± 5 mN; all p < .05). Results from sedentary rats suggest that preventing endorphin release by limiting physical activity maintains increased pain sensitivity. Results from trained + treated rats also show peripheral opioid receptors contribute to acute and chronic exercise‐induced analgesia.