Differential Itch- and Pain-related Behavioral Responses and μ-opioid Modulation in Mice

Intradermal microinjection of the pruritogen histamine, or the algogen capsaicin, in the mouse cheek differentially elicits mainly hindlimb scratching or ipsilateral forelimb wiping, respectively We investigated the dose-dependency of thew responses elicited by various pruritogens and algogens, and mu-opioid modulation Histamine, 5-hydro-xytryptamine (5-HT) and agonists of protease-activated receptors PAR-2 and PAR-4, all elicited dose-related hindlimb scratching bouts with little forelimb wiping In contrast, capsaicin, allyl isothiocyanate and bradykinin elicited dose-related forelimb wiping with little scratching Morphine reduced capsaicin-evoked wiping but not pruritogen-evoked scratching The la-antagonist naltrexone decreased pruritogen-evoked scratching but not capsaicin-evoked wiping A cowhage spicule inserted intradermal elicited equivalent scratching and wiping, while inactivated cowhage spicules loaded with histamine or capsaicin elicited significantly more scratching or wiping, respectively The mouse cheek injection model appears to be a useful behavioral test that distinguishes between Itch and pain