Mechanical allodynia is more strongly manifested in older rats in an experimental model of peripheral neuropathy

Partial peripheral nerve injury often leads to chronic neuropathic pain characterized by symptoms such as allodynia. In the present study, employing a rat model of experimental neuropathy produced by partial denervation of the tail, we examined whether peripheral nerve injury-induced mechanical and thermal allodynia were affected by the animal's age at the time of the injury. The motive of this study was the demonstration in other neuropathy models of the age effects on the manifestation of neuropathic pain symptoms following partial peripheral nerve injury. We compared two groups of young (n = 23, 7-8 weeks old, 150-200 g) and old rats (n = 14, 16-18 months old, 550-800 g). We found that the older rats exhibited more vigorously the behavioral signs of mechanical allodynia during the first week after the nerve injury. With respect to thermal (cold or warm) allodynia, however, we detected no significant difference between young and old rat groups. The results of the present study, as those of previous studies, support the idea that the age at the time of partial peripheral nerve injury affects the severity of certain neuropathic pain symptoms appearing after the injury. However, the present results argue against the suggestion from previous studies that younger subjects are more vulnerable to partial peripheral nerve injury-induced neuropathic pain symptoms.