Cortical astrocytes rewire somatosensory cortical circuits for peripheral neuropathic pain
Sun Kwang Kim, Hideaki Hayashi, Tatsuya Ishikawa, Keisuke Shibata, Eiji Shigetomi, Youichi Shinozaki, Hiroyuki Inada, Seung Eon Roh, Sang Jeong Kim, Gihyun Lee, Hyunsu Bae, Andrew J. Moorhouse, Katsuhiko Mikoshiba, Yugo Fukazawa, Schuichi Koizumi, Junichi Nabekura
Sun Kwang Kim, Hideaki Hayashi, Tatsuya Ishikawa, Keisuke Shibata, Eiji Shigetomi, Youichi Shinozaki, Hiroyuki Inada, Seung Eon Roh, Sang Jeong Kim, Gihyun Lee, Hyunsu Bae, Andrew J. Moorhouse, Katsuhiko Mikoshiba, Yugo Fukazawa, Schuichi Koizumi, Junichi Nabekura
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Research Article

Cortical astrocytes rewire somatosensory cortical circuits for peripheral neuropathic pain

Abstract

Long-term treatments to ameliorate peripheral neuropathic pain that includes mechanical allodynia are limited. While glial activation and altered nociceptive transmission within the spinal cord are associated with the pathogenesis of mechanical allodynia, changes in cortical circuits also accompany peripheral nerve injury and may represent additional therapeutic targets. Dendritic spine plasticity in the S1 cortex appears within days following nerve injury; however, the underlying cellular mechanisms of this plasticity and whether it has a causal relationship to allodynia remain unsolved. Furthermore, it is not known whether glial activation occurs within the S1 cortex following injury or whether it contributes to this S1 synaptic plasticity. Using in vivo 2-photon imaging with genetic and pharmacological manipulations of murine models, we have shown that sciatic nerve ligation induces a re-emergence of immature metabotropic glutamate receptor 5 (mGluR5) signaling in S1 astroglia, which elicits spontaneous somatic Ca2+ transients, synaptogenic thrombospondin 1 (TSP-1) release, and synapse formation. This S1 astrocyte reactivation was evident only during the first week after injury and correlated with the temporal changes in S1 extracellular glutamate levels and dendritic spine turnover. Blocking the astrocytic mGluR5-signaling pathway suppressed mechanical allodynia, while activating this pathway in the absence of any peripheral injury induced long-lasting (>1 month) allodynia. We conclude that reawakened astrocytes are a key trigger for S1 circuit rewiring and that this contributes to neuropathic mechanical allodynia.

Authors

Sun Kwang Kim, Hideaki Hayashi, Tatsuya Ishikawa, Keisuke Shibata, Eiji Shigetomi, Youichi Shinozaki, Hiroyuki Inada, Seung Eon Roh, Sang Jeong Kim, Gihyun Lee, Hyunsu Bae, Andrew J. Moorhouse, Katsuhiko Mikoshiba, Yugo Fukazawa, Schuichi Koizumi, Junichi Nabekura

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Figure 6

Increased extracellular glutamate and astrocytic mGluR5 signaling in S1 cortex following PSL injury mediates Ca2+-dependent TSP-1 release.

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Increased extracellular glutamate and astrocytic mGluR5 signaling in S1 ...
(A) Extracellular glutamate levels in S1 cortex transiently increased after PSL injury (n = 5 mice/group), but not after sham operation (n = 3 mice). *P < 0.05 and **P < 0.01 versus pre-injury, by 1-way ANOVA. (B) 100 μM glutamate-mediated increases in TSP-1 expression in cultured cortical astrocytes, as measured with Western blots, were inhibited by the mGluR5 antagonists MCPG and MPEP and by BAPTA-AM, but were unaffected by AMPA and NMDA antagonists (CNQX and MK-801). *P < 0.05 versus control; #P < 0.05 versus glutamate, by Kruskal-Wallis test. Note that the MCPG and MPEP data were run on separate gels, as indicated by the vertical lines. (C) Levels of extracellular TSP-1 released from cultured cortical astrocytes as measured by ELISA. Glutamate application significantly increased TSP-1 release, which was blocked by MPEP. **P < 0.01, by Kruskal-Wallis test. (D) Local MPEP infusion in vivo blocked the increase in TSP-1 levels in the S1 cortex following PSL injury. Representative Western blots demonstrated a significant increase in S1 cortex TSP-1 levels following PSL injury, which was inhibited by MPEP administration into S1 cortex (n = 4 mice/group). **P < 0.01, by 1-way ANOVA. Error bars in A–D represent the mean ± SEM. (E) Representative electron micrographs of contralateral (left) S1 cortex (layer I) labeled for mGluR5 in PSL-injured and sham control mice. Immunogold labeling for mGluR5 was found both in neuronal compartments (dend., dendrite; sp., spine) and glia, with the frequency of glia labeling increasing after PSL injury.