Objectives: Neurogenic inflammation is mediated via sensory peptides released from the peripheral terminals of sensory nerves and can be modulated by locally released opioid peptides at the site of injury. Endomorphins are recently discovered endogenous opioid peptides with high selectivity and affinity for the mu-opioid receptor. The aim of this study was to examine the ability of endomorphin-1 (EM-1) to modulate the inflammatory response under different injury conditions.
Methods: A vacuum-induced blister model in anaesthetised rats (nembutal 60 mg/kg i.p.) was used to examine the effect of EM-1 on the acute inflammatory response induced by; (1) electrical stimulation (ES) of the distal portion of the exposed/cut sciatic nerve at 20 V, 5 Hz, 2 ms for 1 min or; (2) superfusion of substance P (SP) over the blister base. In addition, the effect of EM-1 on the inflammatory response to SP was examined under acute, recurrent (repeated blister induction) and chronic (chronic sciatic nerve lesion) injury conditions.
Results: Prior and concomitant perfusion of EM-1 (100 microM) significantly inhibited the vascular response to ES by 58% compared to controls. EM-1 also inhibited the inflammatory response to SP (both vasodilatation and plasma extravasation) in a dose-dependent manner. Significant inhibition was achieved at 100 microM and 1 mM concentrations of EM-1. Naloxone (1 mg/kg i.v.) reversed the inhibitory effect of EM-1 on the inflammatory response to SP. EM-1 (100 microM) was equally potent in inhibiting the inflammatory response to SP under acute (34% inhibition) recurrent (39%) and chronic (42%) injury conditions.
Conclusions: The current results demonstrate a greater inhibitory effect of EM-1 on the inflammatory response to electrical nerve stimulation (58% inhibition) compared to SP (34% inhibition) suggesting the involvement of both pre- and post-terminal mechanisms in the inhibitory modulatory actions of EM-1. Evidence is provided for the involvement of opioid receptors in this inhibitory effect. The results also suggest that EM-1 is equipotent in inhibiting the inflammatory response under different injury conditions.