Involvement of central μ-opioid system in the scratching behavior in mice, and the suppression of it by the activation of κ-opioid system

Abstract

The role of central μ- and κ-opioid receptors in the regulation of itch sensation was examined using pruritogen-induced mouse scratching behavior model. Intracerebroventricular administration of β-funaltrexamine, a selective μ-opioid receptor antagonist, inhibited the scratching behavior induced by intradermal substance P, but subcutaneous administration of β-funaltrexamine did not. Similarly, the scratching inhibitory activity of subcutaneously administered TRK-820, (−)-17-(cyclopropylmethyl)-3, 14β-dihydroxy-4, 5α-epoxy-6β-[ N-methyl-trans-3-(3-furyl) acrylamido] morphinan hydrochloride, a κ-opioid receptor agonist, was antagonized by intracerebroventricular administration of nor-binaltorphimine (10 μg/site), a κ-opioid receptor antagonist, but was not by subcutaneous administration of nor-binaltorphimine. In addition, the scratching induced by the direct activation of central μ-opioid receptor by intracisternal morphine was significantly and dose-dependently inhibited by subcutaneous administration of TRK-820. Taken all together, it is suggested that the central μ-opioid receptors play a role in the processing of itch sensation, and the activation of central κ-opioid receptors antagonize the central μ-opioid receptor mediated itch processing, thereby suppressing itch sensation.