Minocycline blocks lipopolysaccharide induced hyperalgesia by suppression of microglia but not astrocytes.

Systemic injection of lipopolysaccharide (LPS) induces a robust immune response as well as thermal and mechanical hyperalgesia. Spinal and peripheral glial cells have been implicated as important mediators in this hyperalgesia but the specific contributions of microglia versus astrocytes are not entirely clear. To better define these mechanisms, this study examined the febrile response, nociceptive sensitivity, glial cell reactivity and cytokine production in the dorsal root ganglion (DRG) and spinal cord in rats following systemic treatment with LPS and the effects of minocycline in countering these responses. Intraperitoneal LPS injection resulted in an increase in core body temperature and produced hyperalgesia to heat and mechanical stimuli. Western blot studies revealed increased expression of microgial cell, macrophage and satellite cell markers in DRG and microglial and astrocyte markers in spinal cord following LPS treatment. Real-time RT-PCR indicated that LPS treatment increased cytokine mRNA expression levels in both the DRG and the spinal cord. Minocycline suppressed all LPS-induced behavioral effects but not the febrile response. Moreover, minocycline prevented LPS-induced microglia/macrophage activation and cytokine responses in spinal cord and DRG, but did not affect the activation of astrocytes/satellite cells. These data demonstrate that LPS-induced changes in nociceptive sensitivity are likely mediated by activation of microglial cells and/or macrophages in the spinal cord and DRG.