THE OROFACIAL FORMALIN TEST IN RATS - EFFECTS OF DIFFERENT FORMALIN CONCENTRATIONS

In this study of the orofacial formalin test in rats, the effects of d ifferent formalin concentrations (0.2%, 0.5%, 1.5%, 2.5%, 5% and 10%) on the behavioural nociceptive response (face rubbing) was investigate d. The histological responses of the skin were also evaluated. Increas ing the concentration of formalin caused a parallel aggravation of his tological signs of tissue inflammation and injury. All concentrations provoked an early phase of nociceptive response, but its intensity was not concentration-dependent. The 2nd phase of response to formalin on ly occurred for concentrations of 1.5% and higher. A positive relation ship between the formalin concentration and the amplitude of the rubbi ng activity measured between 12 and 45 min after injection could be ob served until 2.5% but with the highest concentrations (5 and 10%), the amplitude of the response decreased. Our findings indicate that the o rofacial formalin test should be carried out using concentration betwe en 0.5 and 2.5%. This is essential to assess increase as well as decre ase in pain intensity. Moreover, this will have the effect of minimizi ng the suffering of the experimental animal.