Differences of chronic pain behaviors between child and adult rats

Abstract

Clinically, it is known that the incidence of chronic pain is much lower in children than in adults. This low-incidence may disclose some important factors to develop chronic pain in the adulthood. In this study, we examined whether chronic pain would be induced in child rats, using a chronic pain animal model we previously developed. Lipopolysaccharide (LPS: 2 mg/kg) and 6% hypertonic saline were injected into the unilateral gastrocnemius muscle of rats. The treatments were done in 3-week-old rats (T-3w) and 9-week-old rats (T-adult). We measured changes in the pain behaviors (von Frey test: VFT) at the bilateral plantar surface, the circumference of the calves, and the body weights. Non-treated age-matched control group was also measured. Pain behavior increased and lasted over 10 weeks in T-adult. On the contrary, in T-3w, pain behaviors did not last and decayed after post-treated two weeks. The responses to LPS were smaller in T-3w than in T-adult. In the normal condition, sensitivity to VFT was higher in 3-week-old rats than in 9-week-old rats. In child rats, it was indicated that chronic pain was hard to occur. It is suggested that the developments of nervous and immune systems may be important in onset and maintenance of chronic pain. [J Physiol Sci. 2006;56 Suppl:S181]