Antinociception mediated by α₂-adrenergic activation involves increasing tumor necrosis factor α (TNFα) expression and restoring TNFα and α₂-adrenergic inhibition of norepinephrine release

The central component that establishes chronic pain from peripheral nerve injury is associated with increased tumor necrosis factor-α (TNFα) production in the brain. This study examined TNFα and its reciprocally permissive role with α₂-adrenergic activation during peak and progressive decline of thermal hyperalgesia in sciatic nerve chronic constriction injury (CCI). Accumulation of TNFα mRNA (in situ hybridization) increases in the hippocampus and locus coeruleus during the onset of neuropathic pain and persists as hyperalgesia abates. Activation of α₂-adrenergic receptors in control rats decreases TNFα mRNA accumulation in these brain regions. In contrast, during hyperalgesia, α₂-adrenergic activation enhances TNFα mRNA accumulation. Whether this enhanced TNFα production is associated with changes in the regulation of norepinephrine (NE) release was tested. Hippocampal slices were electrically depolarized to evaluate α₂-adrenergic and TNFα regulation of NE release. While inhibition of NE release by TNFα is maximal during peak hyperalgesia, it subsequently transforms to facilitate NE release. In addition, α₂-adrenergic receptor activation with clonidine (0.2 mg/kg, i.p.) in CCI rats experiencing hyperalgesia restores TNFα and α₂-adrenergic inhibition of NE release. While TNFα directs the development of hyperalgesia, it also directs its resolution. Transformed sensitivity to α₂-adrenergic agonists during hyperalgesia demonstrates a mechanism for therapy.