Refinement of pain evaluation techniques. The formalin test.

The formalin test is an important means of assessing pain evoked-behaviours induced by moderately intense and long-lasting noxious stimuli, yet the procedures for performing the test have not been standardised, hindering comparisons of data. In rodents, the level of sensitivity to the test and the evoked response have been observed to change depending on a number of variables. Among them, gender is the most important, although many other variables can influence the behavioural response (e.g., the animal species, the strain of mice and rats, housing conditions, and early social experience). In the present work, we provide information on the procedures used to perform the formalin test and the effects of both these procedures and the conditions and manipulation prior to the test on the animal and its behaviour, with the ultimate goal of improving these procedures and conditions and decreasing the variability among laboratories.