Intrathecal minocycline attenuates peripheral inflammation‐induced hyperalgesia by inhibiting p38 MAPK in spinal microglia

Abstract

Activation of p38 mitogen-activated protein kinase (p38) in spinal microglia is implicated in spinal nociceptive processing. Minocycline, a tetracycline derivative, displays selective inhibition of microglial activation, a function that is distinct from its antibiotic activity. In the present study we examined antinociceptive effects of intrathecal (IT) administration of minocycline in experimental models of inflammation-evoked hyperalgesia in addition to the effect of minocycline on stimulation-induced activation of p38 in spinal microglia. Intrathecal minocycline produced a dose-dependent reduction of formalin-evoked second-phase flinching behaviour in rats, and prevented thermal hyperalgesia induced by carrageenan injection into the paw. In contrast, systemic delivery (intraperitoneally) of minocycline inhibited the first but not the second phase of formalin-induced flinching, and it had no effect on carrageenan-induced hyperalgesia. Centrally mediated hyperalgesia induced by IT delivery of N-methyl-d-aspartate was completely blocked by IT minocycline. An increase in phosphorylation (activation) of p38 (P-p38) was observed in the dorsal spinal cord after carrageenan paw injection, assessed by both Western blotting and immunohistochemistry. The increased P-p38 immunoreactivity was seen primarily in microglia but also in a small population of neurons. Minocycline, at the IT dose that blocked carrageenan-induced hyperalgesia, also attenuated the increased P-p38 in microglia. In addition, minocycline suppressed lipopolysaccharide-evoked P-p38 in cultured spinal microglial cells. Taken together, these findings show that minocycline given IT produces a potent and consistent antinociception in models of tissue injury and inflammation-evoked pain, and they provide strong support for the idea that this effect is mediated by direct inhibition of spinal microglia and subsequent activation of p38 in these cells.

Mitochondria and calcium flux as targets of neuroprotection caused by minocycline in cerebellar granule cells

“…Minocycline failed to modify the intracellular levels of Ca 2+ in resting conditions, and did not attenuate a Ca 2+ increase following cellular depolarization with 50 mM KCl. Our data also revealed that induced by intrathecal NMDA [46], or shed some light on the favourable effects of minocycline in a therapeutic window between 6 and 24 hours after a stroke [47].…”

Do glial cells control pain?

“…Another groups using the same model also shows p38 activation in spinal microglia with slightly delayed time course (Tsuda et al, 2004). Other groups also demonstrate p38 activation in spinal microglia in different pain conditions (Hains & Waxman, 2006;Hua et al, 2005;Kim et al, 2002;Svensson et al, 2003;Svensson et al, 2005b;Xu et al, 2007a;Xu et al, 2007b). In the SNI model of neuropathic pain, the microglia activation of p38 is mostly seen in the medial part of the dorsal horn, where the injured tibial and peroneal nerves terminate .…”

“…Upregulation of OX-42 or Iba1 is indicative of microglia activation. In inflammatory pain models that do not involve nerve injury, such as carrageenan model (Hua et al, 2005) and zymosan model (Clark et al, 2007a;Sweitzer et al, 1999), there is only a very moderate increase of OX42-immunoreactivity (IR) in the spinal cord. Whereas other models such as complete Freund's adjuvant model (Clark et al, 2007a;Zhang et al, 2003) and mustard oil model (Molander et al, 1997) fail to demonstrate any increase of OX-42-IR.…”

“…p38 MAPK inhibitors show high efficacy in different animal models, but they may produce side effects when high doses or long-term treatment are employed. The beta isoform of p38 MAPK (p38β) could be a valid target due to its relatively restricted expression compared to p38α isoform (Svensson 2005;Xu et al, unpublished observation). Third, those drugs, such as minocycline, propentofylline, and methotrexate, which have already been tested clinically for other indications, are certainly easier to get approved for pain treatment than new compounds.…”

Possible role of spinal astrocytes in maintaining chronic pain sensitization: review of current evidence with focus on bFGF/JNK pathway

“…Intrathecal injection of ATP-activated microglia induces mechanical allodynia (a nociceptive response to normally innocuous mechanical stimulation) that requires microglial production of BDNF (Tsuda et al, 2003;Coull et al, 2005). A non-specific microglial inhibitor minocycline prevents/delays pain development (Raghavendra et al, 2003;Ledeboer et al, 2005;Hua et al, 2005). Notably, studies from different laboratories have demonstrated that p38 mitogen-activated protein kinase (MAPK) is activated in spinal microglia under different chronic pain conditions, and that blocking this kinase attenuates pain hypersensitivity (Table 1 ( Jin et al, 2003;Schafers et al, 2003;Tsuda et al, 2004;Boyle et al, 2006;Hains and Waxman, 2006).…”