Norbinaltorphimine, a selective kappa-opioid receptor antagonist, induces an itch-associated response in mice.

We examined the possibility that scratching induced by norbinaltorphimine, a selective kappa-opioid receptor antagonist, is due to an itch sensation, using compound 48/80 as control pruritogenic agent. When norbinaltorphimine was injected s.c. into the rostral back, mice scratched the skin around the injection site with their hind paws. Although the intensity of the scratching could not be compared because the dose and injection route were different, the character and time course of the scratching behavior induced by compound 48/80 injected i.d. were similar to those with norbinaltorphimine. The scratching behavior induced by norbinaltorphimine was dose-dependently and significantly inhibited by pretreatment with chlorpheniramine. Compound 48/80-induced scratching was also dose-dependently and significantly inhibited by p.o. pretreatment with chlorpheniramine. The scratching behavior induced by norbinaltorphimine was dose-dependently and significantly inhibited by pretreatment with U-50,488H (trans-(+/-)-2-(3,4-dichlorophenyl)-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl] acetamide methansulfonate), a kappa-opioid receptor agonist. Unexpectedly, the scratching behavior induced by compound 48/80 was also dose-dependently and significantly reduced by pretreatment with U-50,488H. These results suggest that the injection of norbinaltorphimine into the rostral back of the mouse elicited scratching, which may be an itch-associated response. Furthermore, the scratching behavior produced by norbinaltorphimine may be due in part to the release of histamine followed by antagonism of kappa-opioid receptors.