2009 Volume 24 Issue 1 Pages 37-44
Clinically, it is known that the incidence of chronic pain is much lower in children than in adults. The low-incidence may disclose some important factors to develop chronic pain in the adulthood. In this study, we examined whether the treatment to young rats would induce the chronic pain behavior, using the myopathic chronic pain animal model we previously developed.
Lipopolysaccharide (LPS: 2 µg/kg) and hypertonic saline (6%) were injected into the uni-lateral gastrocnemius muscle of rats of postnatal three weeks (young) and nine weeks (adult). The treatment to adult rats produced long-persisting pain behaviors as the previous reported, on the contrary, the treatment to young rats did not develop chronic pain behaviors.
Also we examined the developmental changes in the sensitivity to noxious stimuli in normal rats. The withdrawal responses to mechanical and heat stimuli were significantly higher in young rats than in adult rats. As the rats grew up, the responses to mechanical stimuli to the hind-paws by von Frey filaments gradually decreased and became almost stable after postnatal nine weeks.
These results suggest that some immaturity in pain pathway may produce a high nociceptive sensitivity in young rats and the factors in a developmental process would be required for the onset of persisting pain behaviors in the myopathic chronic pain model.
