endomorphin-1 and Neurogenic-Inflammation

Substance P (SP) and calcitonin gene-related peptide (CGRP) released from capsaicin-sensitive sensory nerves induce local neurogenic inflammation in the innervated area. The aim of the present study was to investigate the effects of an endogenous opioid peptide, endomorphin-1, on sensory neuropeptide release in vitro and acute neurogenic and non-neurogenic inflammatory reactions in vivo. Electrical field stimulation (EFS; 40 V, 0.1 ms, 10 Hz, 120 s; 1200 impulses) was performed to evoke SP and CGRP release from peptidergic afferents of the isolated rat tracheae which was determined from the incubation medium with radioimmunoassay. Neurogenic inflammation in the skin of the acutely denervated rat hind paw was induced by topical application of 1% mustard oil and detected by Evans Blue leakage. Mustard oil-induced ear swelling of the mouse was determined with a micrometer during 3 h and myeloperoxidase activity as an indicator of granulocyte accumulation was measured with spectrophotometry at 6 h. EFS evoked about a twofold elevation in the release of both pro-inflammatory sensory neuropeptides. Endomorphin-1 (5 nM-2 microM) diminished the release of SP and CGRP in a concentration-dependent manner, the EC50 values were 39.45 nM and 10.84 nM, respectively. The maximal inhibitory action was about 80% in both cases. Administration of endomorphin-1 (1-100 microg/kg i.p.) dose-dependently inhibited mustard oil-evoked neurogenic plasma protein extravasation in the rat skin as determined by microg Evans Blue per g wet tissue. Repeated i.p. injections of the 10 microg/kg dose three times per day for 10 days did not induce desensitization in this model. Neurogenic swelling of the mouse ear was also dose-dependently diminished by 1-100 microg/kg i.p. endomorphin-1, but non-neurogenic neutrophil accumulation was not influenced. These results suggest that endomorphin-1 is able to inhibit the outflow of pro-inflammatory sensory neuropeptides. Based on this mechanism of action it is also able to effectively diminish neurogenic inflammatory responses in vivo.

Topics: Animals; Calcitonin Gene-Related Peptide; Electric Stimulation; Male; Mice; Mice, Inbred BALB C; Mustard Plant; Neurogenic Inflammation; Neurons, Afferent; Oligopeptides; Plant Oils; Rats; Rats, Wistar; Skin; Substance P

Endomorphin-1 is a selective endogenous ligand for the micro-opioid receptor, and this study investigated the effect of endomorphin-1 on rat knee joint inflammation by examining the ability of the neuropeptide to modulate synovial protein extravasation. Acute joint inflammation was induced by intraarticular injection of 2% kaolin followed by 2% carrageenan and the animals allowed to recover for 3 h. Immunohistochemical examination of these inflamed joints revealed endomorphin-1-like immunoreactive nerves in deep synovium with a proportion of the nerve fibers occurring in close proximity to synovial blood vessels. Perfusion of inflamed knees with exogenous endomorphin-1 across the dose range 10-9-10-6 M produced a significant reduction in synovial vascular permeability with the 10-7M dose producing the greatest fall in protein exudation (approx 55%). These effects were blocked by the specific micro-opioid receptor antagonist CTOP. Destruction of knee joint unmyelinated afferent nerve fibers by capsaicin treatment significantly attenuated the anti-inflammatory effects of endomorphin-1, suggesting that the peptide is acting via a neurogenic mechanism. The findings of this study indicate that endomorphin-1 acts peripherally in knee joints to reduce synovial protein extravasation. These anti-inflammatory effects are mediated by micro-opioid receptors located on capsaicin-sensitive afferent nerves.

Topics: Animals; Anti-Inflammatory Agents; Arthritis; Blood Vessels; Capillary Permeability; Capsaicin; Dose-Response Relationship, Drug; Drug Administration Routes; Knee Joint; Male; Narcotic Antagonists; Nerve Fibers, Unmyelinated; Neurogenic Inflammation; Neurotoxins; Oligopeptides; Perfusion; Peripheral Nerves; Rats; Rats, Wistar; Receptors, Opioid, mu; Sensory Receptor Cells; Synovial Membrane

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.. 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.. 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.. 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.

Topics: Administration, Topical; Analgesics, Opioid; Animals; Blister; Blood Flow Velocity; Electric Stimulation; Male; Naloxone; Neurogenic Inflammation; Oligopeptides; Plasma; Rats; Rats, Sprague-Dawley; Receptors, Opioid, mu; Sciatic Nerve; Skin Diseases, Vesiculobullous; Substance P; Vasodilation