Neuropathic pain in aged rats: behavioral responses and astrocytic activation

We used the Bennett and Xie (1988) model of chronic neuropathic pain to stu dy the effect of age on thermal and tactile sensitivity and on astrocytic a ctivation in the dorsal horn of the spinal cord after nerve injury. Fischer 344 FBNFI hybrid rats in three age groups, 4-6, 14-16, and 24-26 months, w ere studied. Rats were either unligated (day 0, control) or the left sciati c nerve was loosely ligated to cause a chronic constriction injury (CCI). C CI causes a neuropathic pain condition characterized by tactile allodynia a nd thermal hyperalgesia. Rats were behaviorally assessed for tactile and th ermal sensitivity of their ligated and unligated hind paws up to 35 days po stligation. Rats were sacrificed before or at various days postligation, an d activated astrocytes were identified at the L4-L5 levels of their spinal cords by use of an antibody to glial fibrillary acid protein (GFAP). The nu mber of GFAP-ir astrocytes in the dorsal horn of the spinal cord in the con trol, uninjured condition decreased with age (P less than or equal to0.001) but increased after CCI in all three age groups. After CCI, astrocytic act ivation in the cord was less robust in aged rats than in younger ones (P le ss than or equal to0.01). Not all the CCI rats displayed hyperalgesia to to uch and to heat. Rats with an increased sensitivity to heat had increased l evels of GFAP-ir in their cords; however, rats with decreased thermal sensi tivity also displayed increased GFAP-ir. Thus the presence of activated ast rocytes was not correlated with a single behavioral manifestation of neurop athic pain.