Opioid modulation of facial itch- and pain-related responses and grooming behavior in rats.

Intradermal facial injections of pruritogens or algogens elicit distinct behavioral hindlimb scratch or forelimb wiping responses in rodents. We systematically investigated the parameters and opioid modulation of these evoked behaviors and spontaneous facial grooming in rats. Serotonin (5-HT) elicited hindlimb scratch bouts with few wipes. Scratching was attenuated by the µ-opiate antagonist naltrexone but not morphine. In contrast, cheek injection of mustard oil (allyl-isothiocyanate (AITC)) elicited ipsilateral forelimb wipes but little hindlimb scratching. AITC-evoked wiping was significantly attenuated by morphine but not naltrexone. Spontaneous facial grooming by the forepaws was attenuated by naltrexone, whereas morphine did not affect grooming behavior before or after cheek injections of 5-HT or AITC. These data validate that the rodent "cheek" model discriminates between itch- and pain-related behaviors. Naltrexone sensitivity of facial grooming and 5-HT-evoked scratch-ing suggests a common functionality. Forelimb wipes may represent a nocifensive response akin to rubbing an injury to relieve pain.