Age-related differences in response to pain stimuli remain a controversial issue. The purpose of this study was to evaluate the effect of age on pain response using the formalin treated rats and streptozotocin-induced diabetic rats tested for mechnical, cold allodynia and thermal hyperalgesia.

We divided Sprague-Dawley rats into 3 groups by age, 5 weeks old (n = 10), 8 weeks old (n = 10), 12 weeks old (n = 10). Each group was divided into 2 subgroups (n = 5). One was formalin tested and the other was injected with streptozotocin 75 mg intraperitoneally for succesive two days. On the 3rd day after injection, we examined mechnical allodynia using a von Frey filament, and tested thermal hyperalgesia using a tail immersion test in 50℃ or 5℃ water.

In the formalin test, pain response was higher in the younger age group (P < 0.05) in phase 2. In the streptozotocin-induced diabetic rats, no difference in mechanical allodynia was observed between the subgroups. In the 50℃ water thermal hyperalgesia test, withdrawal latency decreased in each group after streptozotocin injection (P < 0.05) and 5-week animals had a lower withdrawal latency than the 12-week animals (P < 0.05). In the 5℃ water cold allodynia test, the 5-week animals showed a lower withdrawal latency than the other age groups (P < 0.05).

In this study, the effect of age on the response to a pain stimulus depends upon the stimulus. This is important when designing the pain models. The mechnisms involved require further investigation.