THE FORMALIN TEST - SCORING PROPERTIES OF THE FIRST AND 2ND PHASES OFTHE PAIN RESPONSE IN RATS

The formalin test is increasingly used as a model of injury-produced p ain but there is no generally accepted method of pain rating. To exami ne the properties of various pain rating methods we established dose-r esponse relations for formalin injected in the plantar surface of one hind paw, and the analgesic effects of morphine and amphetamine using the most frequently reported behavioural measures of pain (favouring, lifting, licking and flinching/shaking of the injured paw) and combina tions of these. Licking, elevation and favouring of the injected paw s howed a biphasic response at all formalin doses. Flinching varied in f orm across the time course of formalin, and the biphasic nature of the behaviour was not as apparent. In untreated rats all these behaviours were infrequent. Flinching and favouring were increased after injecti on of local anaesthetic into the paw but remained negligible relative to the effect of formalin. Grooming other than that directed to the in jected paw was elevated in a dose-dependant manner by formalin. Interc orrelations between the behaviours were different for the initial resp onse and the second phase. Correlational analysis indicated that no si ngle behavioural measure was a strong predictor of formalin, morphine and amphetamine dose. A simple sum of time spent licking plus elevatin g the paw, or the weighted pain score of Dubuisson and Dennis (1977), were superior to any single measure (r ranging from 0.75 to 0.86). Add ition of flinching and favouring to the combined pain score using mult iple regression did not increase variance explained. Depending on the measure used, a sedative dose of pentobarbital produced apparent analg esia, hyperalgesia or no effect. The interphase depression of pain, as well as the analgesic effects of morphine and amphetamine, were all a ssociated with increased motor activation. Power analysis indicated th at using a moderate dose of formalin and a combined pain score gave th e greatest power to detect differences in pain. It was also found that pain scores increase with ambient temperature and that rat strains ma y differ in formalin pain sensitivity.