The rat paw formalin test: comparison of noxious agents

A comparison was made of the spontaneous nociceptive behaviors elicited by s.c. injection into the rat hind paw of the following 8 irritants: acetic acid, carrageenan, formalin, kaolin, platelet-activating factor, mustard oil (given topically), serotonin, and yeast. Two distinct quantifiable behaviors indicative of pain were identified: flinching/shaking of the paw and hindquarters and licking/biting of the injected paw. These behaviors were prolonged and intense after formalin and acetic acid. Formalin-induced flinching was biphasic across time, a finding potentially useful for the study of both acute and tonic pain. Of the remaining test agents, only yeast caused significant spontaneous behavioral activity, which was of low intensity but long duration. Different time-courses for nociceptive behavior and development of edema were demonstrated for formalin, acetic acid and yeast. It is therefore unlikely that these endpoints are causally related. Overall, the present data strongly support the use of formalin as a noxious stimulus in tonic pain research.