Age differences in the response to the formalin test in rats

We report the results of a study designed to assess age differences in the response to the formalin test, a model of tissue injury and inflammation, w hile controlling for differences in weight and motoric abilities in three g roups of adult male Long-Evans rats: young (3 months old), middle-aged (18 months old), and old (24 months old). The first part of the study assessed initial differences in responsivity and found that the middle-aged group sh owed the greatest response, whereas the young and old groups did not differ from each other. In the second part of the study, the young and middle-age d animals were followed for a 4-month period. The formalin test was repeate d at 2-month intervals. These results indicate that there may he an age-ass ociated change in the sensitivity to tonic pain and that this sensitivity m ay peak at mid-life. (C) 2000 Elsevier Science Inc. All rights reserved.