Search

Background Mechanisms underlying neuropathic pain relief by the neurosteroid allopregnanolone remain uncertain. We investigated if allopregnanolone attenuates glial extracellular signal-regulated kinase (ERK) activation in the cuneate nucleus (CN) concomitant with neuropathic pain relief in median nerve chronic constriction injury (CCI) model rats. Methods We examined the time course and cellular localization of phosphorylated ERK (p-ERK) in CN after CCI. We subsequently employed microinjection of a mitogen-activated protein kinase kinase (ERK kinase) inhibitor, PD98059, to clarify the role of ERK phosphorylation in neuropathic pain development. Furthermore, we explored the effects of allopregnanolone (by mouth), intra-CN microinjection of γ-aminobutyric acid type A receptor antagonist (bicuculline) or γ-aminobutyric acid type B receptor antagonist (phaclofen) plus allopregnanolone, and allopregnanolone synthesis inhibitor (medroxyprogesterone; subcutaneous) on ERK activation and CCI-induced behavioral hypersensitivity. Results At 7 days post-CCI, p-ERK levels in ipsilateral CN were significantly increased and reached a peak. PD98059 microinjection into the CN 1 day after CCI dose-dependently attenuated injury-induced behavioral hypersensitivity (withdrawal threshold [mean ± SD], 7.4 ± 1.1, 8.7 ± 1.0, and 10.3 ± 0.8 g for 2.0, 2.5, and 3.0 mM PD98059, respectively, at 7 days post-CCI; n = 6 for each dose). Double immunofluorescence showed that p-ERK was localized to both astrocytes and microglia. Allopregnanolone significantly diminished CN p-ERK levels, glial activation, proinflammatory cytokines, and behavioral hypersensitivity after CCI. Bicuculline, but not phaclofen, blocked all effects of allopregnanolone. Medroxyprogesterone treatment reduced endogenous CN allopregnanolone and exacerbated nerve injury-induced neuropathic pain. Conclusions Median nerve injury-induced CN glial ERK activation modulated the development of behavioral hypersensitivity. Allopregnanolone attenuated glial ERK activation and neuropathic pain via γ-aminobutyric acid type A receptors. Reduced endogenous CN allopregnanolone after medroxyprogesterone administration rendered rats more susceptible to CCI-induced neuropathy.

Background Neuroprotective effects of hypothermia on peripheral nerve injury remain uncertain. This study investigated the efficacy of hypothermia in attenuating neuropathic pain and glial activation in the cuneate nucleus in a median nerve chronic constriction injury (CCI) model. Methods Sprague-Dawley rats (n = 246) that underwent median nerve ligature at the elbow received various degrees of regional and whole-body hypothermia 15 min before CCI and 5 h, 1, 3, and 5 days after CCI. Hypothermia was maintained for 4 h. Seven days after CCI, behavioral and electrophysiological testings were conducted. Immunohistochemistry, immunoblotting, and enzyme-linked immunosorbent assay were used for qualitative and quantitative analysis of glial activation and measuring pro-inflammatory cytokines, respectively. Results Mild (32°C) and deep (28°C) regional hypothermia administered preinjury and 5 h postinjury attenuated neuropathic pain and glial activation. Application of whole-body hypothermia preinjury and 5 h postinjury provided a similar therapeutic effect. However, whole-body hypothermia, but not regional hypothermia, applied 1, 3, and 5 days postinjury attenuated glial activation and neuropathic pain. Similarly, on days 1, 3, and 5 postinjury, only whole-body hypothermia was effective in decreasing proinflammatory cytokine levels. The increase in injury discharge observed after CCI could be suppressed by regional or whole-body hypothermia at different stages of nerve injury. Conclusions At the early stage following nerve injury, regional and whole-body hypothermia suppresses ectopic discharges, and consequently inhibits glial activation and neuropathic pain. At the later stage, pain processing is mediated mainly by cytokines released from activated microglia; therefore, only whole-body hypothermia is effective in modulating pain.