Neurogenic plasma extravasation: inhibition by morphine in guinea pig airways in vivo

Opioid drugs have been shown to inhibit neurogenic plasma exudation in skin by a presynaptic mechanism. We determined whether a similar inhibitory effect operates in the airways of anesthetized guinea pigs in vivo with the use of Evans blue dye as a marker of plasma leakage. Stimulation of the vagus nerve significantly increased leakage of dye in trachea and main bronchi (by approximately 300 and 600%, respectively). Similar increases in leakage were seen in the presence of atropine and propranolol. Morphine (1–30 mg/kg iv) inhibited leakage in a dose-related manner with complete inhibition in the trachea at a dose of 30 mg/kg. The inhibition was blocked by the opioid receptor-antagonist naloxone (1 mg/kg iv). Intravenous substance P significantly increased leakage but was not inhibited by morphine. We conclude that morphine inhibits neurogenic plasma leakage by presynaptic inhibition of release of neuropeptides from sensory nerve endings. If similar mechanisms are operative in human airways, inhibition of neurogenic plasma leakage by opioid drugs devoid of central effects may be of value in the therapy of asthma.