OPIATE AND H1 ANTAGONIST EFFECTS ON HISTAMINE-INDUCED PRURITUS AND ALLOKNESIS

Itching is a well known side-effect of opiate therapy. To gain insight into the possible contribution of opiate receptors to itching we comp ared the antipruritic effect of naltrexone (Nemexin(R)), an opiate ant agonist, to an Hi-receptor antagonist and to placebo. In a double blin d cross-over study on 15 healthy volunteers, 25 mg naltrexone or place bo was orally given 60 min prior to a histamine stimulus. In a second, otherwise identical experiment, 10 mg cetirizine, an H1 blocker, or p lacebo was orally given 12 h before the experiment to the same group o f volunteers. Histamine was applied iontophoretically to the forearm s kin and the following parameters were assessed thereafter: weal and fl are size, itch intensity and the extension of the area of alloknesis ( 'itchy skin') around the application site. Naltrexone had no effect on the vascular histamine reactions 'weal' and 'flare', whereas cetirizi ne abolished the weal reactions and greatly diminished the flare react ions. Both naltrexone and cetirizine significantly diminished histamin e induced itching. In contrast to placebo and cetirizine, naltrexone a bolished alloknesis completely in four of 15 volunteers and in the oth ers alloknesis was greatly reduced after naltrexone. Since vascular re actions to histamine are of peripheral origin, whereas alloknesis depe nds on central nervous mechanisms, our findings suggest a pronounced c entrally mediated action of naltrexone on histamine induced pruritus. (C) 1997 International Association for the Study of Pain. Published by Elsevier Science B.V.