Application of the formalin test to the study of orofacial pain in the rat.

A modification of the formalin test for assessing pain and analgesia in the orofacial region of the rat is described. A formalin solution (5%) was subcutaneously injected into the upper lip, then the length of time the animal spent rubbing the injected zone was recorded. Two distinct periods of intensive rubbing activity were identified: an early phase between 0 and 3 min after the injection and a late phase between 18 and 42 min after the injection. Acetylsalicylic acid, paracetamol and morphine all had an antinociceptive effect during the two phases although incomplete during the early phase. Our results indicate that this orofacial formalin test is a valid technique for the study of orofacial pain.