Morphine modulates microvascular leakage dose-dependently in the airway of ovalbumin-sensitized rats

Aim: To investigate the effect of morphine on ovalbumin-evoked airway microvascular leakage in sensitized rats. Materials and methods: Rats were sensitized on clays 0, 14, and 21 with ovalbumin. Intravenous ovalbumin (2 mg/kg) or capsaicin (50 mu g/kg) increased the extravasation of Evans blue dye in trachea, bronchi, and intra-pulmonary tissues of sensitized rats. Results: Morphine (1-10 mg/kg) inhibited ovalbumin-evoked increase in microvascular plasma leakage in a dose-dependent manner; however, it had no significant effect at the doses of 0.1 or 30 mg/kg. In addition, morphine, at the closes of 1-30 mg/kg, abolished microvascular leakage increased by capsaicin. The inhibition caused by morphine was blocked by the peripheral opioid receptor antagonist, naloxone methiodide, in ovalbumin or capsaicin series. Morphine or naloxone methiodide has alone no effect on plasma leakage. Conclusion: These results conclude that morphine inhibits microvascular leakage, maybe mediated by neurogenic inflammation in sensitized rats, via peripheral opioid receptors.