Expression of opioid receptors during peripheral inflammation

Opioid receptors (OR) and their mRNA are present in the central and peripheral nervous system of mammals. In this review we examine the behavioral effects of opioids and the expression of their receptors during peripheral inflammation in two experimental models: the rat paw and the mouse intestine. Inflammation increased the antinociceptive (paw) and the inhibitory effects of opioids in the gut (transit, permeability and plasma extravasation) by interaction with OR located at peripheral sites. Based on agonist efficacy, μ > δ ≫ κ-OR mediate the antinociceptive and antitransit effects of opioids during inflammation. Intestinal permeability is modulated by δ = μ ≫ κ-OR, while κ > δ ≫ μ-OR are involved in the inhibition of plasma extravasation. Intestinal inflammation increased the transcription of μ and δ-OR (but not κ) genes in the gut, thus explaining the enhanced antitransit and antisecretory effects of μ and δ-OR agonists; however, the increased inhibitory effects of κ-OR agonists on plasma extravasation could result from post-transcriptional regulation of the receptor. Similarly, the increased expression of peripheral μ-OR observed in the rat paw during inflammation, occurs at post-transcriptional levels and is related to an increased axonal transport from the dorsal root ganglia to peripheral terminals. The sites and mechanisms implicated in the increased transcription of μ and δ-OR during intestinal inflammation are under investigation.