Age-Related Differences in Neuropathic Pain Behavior and Spinal Microglial Activity after L5 Spinal Nerve Ligation in Male Rats

Introduction: Several studies have reported the involvement of age-related changes in the&nbsp;development of neuropathic pain behaviors. However, limited data are available on the role of age in&nbsp;establishing and maintaining chronic neuropathic pain after peripheral nerve injury. Methods: In the present study, we examined age-related neuropathic behavior among rats in 4 age&nbsp;groups: pups (4 weeks old; weight, 60&ndash;80 g), juvenile rats (6 weeks old; weight, 120&ndash;140 g), and&nbsp;mature rats (10&ndash;12 weeks old; weight, 200&ndash;250 g). Because the exact contribution of spinal microglia&nbsp;and its association with the development of neuropathic pain remains unknown, we also evaluated the&nbsp;expression of spinal Iba1, a microglial marker, by using western blotting before and 5 days after spinal&nbsp;nerve ligation (SNL) as well as after the daily IP administration of minocycline (30 mg/kg). Results: Our results showed that SNL-induced mechanical allodynia but not thermal hyperalgesia&nbsp;in mature rats but not in pups (P<0.05 and P<0.01, respectively). The expression of spinal Iba1&nbsp;in the juvenile rats was significantly lower than that in pups and mature rats (P<0.01). Moreover,&nbsp;administration of minocycline decreased the expression of spinal Iba1 in the pup rats more than in&nbsp;juvenile rats (P<0.001) and in the juvenile rats more than in the mature rats (P<0.05). Conclusion: These data suggest that the development of neuropathic behaviors and microglial&nbsp;activation after SNL could be age dependent.