β-endorphin differentially affects inflammation in two inbred rat strains

It has been shown that inflammation of rat paws elicits accumulation of opioid peptide β-endorphin-containing immune cells in the inflamed subcutaneous tissue, contributing to immunocyte-produced pain suppression. However, the possible mechanisms involved in the pharmacological application of β-endorphin in rat paw inflammation have not been investigated. The present study was set up to explore the effects of intraplantar injection of β-endorphin on Concanavalin A-induced paw edema in two inbred rat strains, Albino Oxford (AO) and Dark Agouti (DA). Both high dose-induced suppression and low dose-induced potentiation of edema development in AO and DA rats, respectively, were blocked with antagonists specific for δ (naltrindole) and κ (nor-binaltorphimine) opioid receptors. β-endorphin in vitro decreased phagocytosis and increased nitric oxide (NO) production in air pouch granulocytes obtained from AO rats. However, in cells from DA rat strain β-endorphin modulated both phagocytosis and NO production in a concentration-dependent manner. It could be concluded that the strain-dependent opposing effects of β-endorphin on paw inflammation are mediated through δ and κ opioid receptors and probably involve changes in the production of reactive oxygen species by inflammatory cells. Our results point to the importance of genotype for pharmacological manipulations and the development of inflammation.