The efficacy of Dynorphin fragments at the κ, μ and δ opioid receptor in transfected HEK cells and in an animal model of unilateral peripheral inflammation

Abstract

BackgroundDynorphin 1–17 is an endogenous peptide that is released at sites of inflammation by leukocytes, binding preferentially to κ-opioid receptors (KOP) to mediate nociception. We have previously shown that dynorphin 1–17 is rapidly biotransformed to smaller peptide fragments in inflamed tissue homogenate. This study aimed to determine the efficacy and potency of selected dynorphin fragments produced in an inflamed environment at the KOP, μ and δ-opioid receptors (MOP and DOP respectively) and in a model of inflammatory pain. Functional activity of Dynorphin 1–17 and fragments (1–6, 1–7 and 1–9) were screened over a range of concentrations against forskolin stimulated human embryonic kidney 293 (HEK) cells stably transfected with one of KOP, MOP or DOP. The analgesic activity of dynorphin 1–7 in a unilateral model of inflammatory pain was subsequently tested. Rats received unilateral intraplantar injections of Freund’s Complete Adjuvant to induce inflammation. After six days rats received either dynorphin 1–7, 1–17 or the selective KOP agonist U50488H and mechanical allodynia determined. Dynorphin 1–7 and 1–9 displayed the greatest activity across all receptor subtypes, while dynorphin 1–7, 1–9 and 1–17 displaying a potent activation of both KOP and DOP evidenced by cAMP inihibition. Administration of dynorphin 1–7 and U50488H, but not dynorphin 1–17 resulted in a significant increase in paw pressure threshold at an equimolar dose suggesting the small peptide dynorphin 1–7 mediates analgesia. These results show that dynorphin fragments produced in an inflamed tissue homogenate have changed activity at the opioid receptors and that dynorphin 1–7 mediates analgesia.