Intradermal administration of formalin to the cheek induces itch as well as pain behaviors in rats
Objective · Formalin is a classic and most widely used algogenic substance, but its itchy effect is not clear. The present study aims to explore the hypothesis that formalin may induce itch as well as pain. Methods · Flinching, as well as licking and forelimb wiping of the site of injection were counted as pain responses, whereas biting and hind paw scratching of the cheek were counted as itchy responses. To discriminate formalin-induced sensations in rats, the irritant (saline as control) was injected, and then pain and itchy responses were recorded. Results · Intraplantar injection of formalin elicited biphasic behavior responses characterized as flinching, as well as biting or licking of the hind paw without significant gender differences. Following intradermal administration of formalin to the cheek, rats exhibited episodic forelimb wiping of the cheek, representative of pain. No gender difference was noticed for this type of behavior. In addition, episodes of hind paw scratches of the cheek, representative of pruritoceptive responses, also occurred. Interestingly, hind paw scratches appeared to be more pronounced in male than in female rats. Conclusion · Intradermal administration of formalin elicits pruritoceptive as well as nociceptive responses in rats.