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Intraplantar morphine depresses spinal c‐Fos expression induced by carrageenin inflammation but not by noxious heat

1 We have studied the effects of intraplantar administration of the same doses of morphine on intraplantar carrageenin (6 mg 150 μ***l−1 of saline) and noxious heat (52°C for 15 s) induced spinal c‐Fos expression and inflammation. 2 Intraplantar carrageenin, in awake rats, induced numerous Fos‐like immunoreactive (Fos‐LI) neurones in the dorsal horn of L4‐L5 lumbar segments of the spinal cord and extensive peripheral oedema. At 1 h 30 min, Fos‐LI neurones were preferentially located in the superficial laminae (74 ± 2%) whereas at 3 h, Fos‐LI neurones were observed both in the superficial (45 ± 2%) and deep (37 ± 1%) laminae of the spinal dorsal horn. 3 Intraplantar morphine dose‐dependently reduced c‐Fos expression induced 1 h 30 min after carrageenin (r = 0.605, P < 0.02), these effects were completely blocked by intraplantar methiodide naloxone (20 μg) (121 ± 22% of control carrageenin expression). The systemic injection of the highest dose of intraplantar morphine (50 μg) had no significant effect on the number of Fos‐LI neurones (88 ± 9% of control carrageenin expression). None of the drugs influenced unilateral peripheral oedema observed 1 h 30 min after carrageenin. 4 In the second series of experiments, intraplantar morphine dose‐dependently reduced the number of superficial and deep Fos‐LI neurones induced 3 h after carrageenin (r = 0.794, P < 0.0004 and r = 0.698, P < 0.004, respectively). Furthermore, the effects of the highest dose of intraplantar morphine were completely blocked by co‐administration of intraplantar methiodide naloxone (20 μg). 5 In addition, intraplantar morphine dose‐dependently reduced the ankle (r = 0.747, P < 0.002) and paw (r = 0.682, P < 0.005) oedema observed 3 h after carrageenin, with the effect of the highest dose of intraplantar morphine being completely blocked by co‐administration of methiodide naloxone (98 ± 4% and 102 ± 8% of control paw and ankle oedema, respectively). 6 Brief noxious heat stimulation, in urethane anaesthetized rats, induced, 2 h after the stimulation, numerous Fos‐LI neurones in the dorsal horn of L3‐L4 lumbar segments of the spinal cord but no detectable peripheral oedema. Fos‐LI neurones were preferentially located in superficial laminae (94 ± 2%) of the spinal dorsal horn. None of the drugs influenced the noxious heat induced c‐Fos expression. 7 Such results illustrate that peripheral effects of morphine preferentially occur during inflammatory states and outline the interest of extending clinical investigations of the possible use of local injection of morphine in various inflammatory pain states.