The effect of antinociceptive drugs tested at different times after nerve injury in rats

Abstract

Given the evolving nature of anatomical and functional changes in the nervous system that are involved in the development of neuropathic pain, it is possible that the differing time course after injury underlies the inconsistent efficacy of drugs in neuropathic pain patients. In the current study, we evaluated the behavioral effects of two standard drugs used clinically for neuropathic pain, the anticonvulsant gabapentin and antidepressant imipramine, in rats at different times after peripheral nerve injury. Rats that underwent the spared nerve injury procedure responded to an innocuous mechanical stimulus (von Frey filament) 2, 4, and 8 wk after injury. Gabapentin dose-dependently suppressed mechanical sensitivity at all time points tested but the potency of gabapentin was three-fold less 4 wk postinjury (135 mg/kg) compared with 2 and 8 wk postinjury (41 and 44 mg/kg, respectively). In contrast, imipramine lacked significant efficacy at 2 and 8 wk postinjury but slightly attenuated mechanical hypersensitivity at 4 wk postinjury. The results show that drug effects may change over time in the neuropathic state, which should be an important consideration in the evaluation of drugs in preclinical animal pain models and has implications for temporal approaches to therapy in the clinic.