Activation of hippocampal microglia in a murine model of cancer-induced pain
Received 23 October 2018
Accepted for publication 23 January 2019
Published 18 March 2019 Volume 2019:12 Pages 1003—1016
Checked for plagiarism Yes
Review by Single-blind
Peer reviewers approved by Dr Andrew Yee
Peer reviewer comments 3
Editor who approved publication: Dr Katherine Hanlon
Tanya Miladinovic,1,2 Manu Sharma,1,2 Andy Phan,1,2 Hana Geres,1,2 Robert G Ungard,1,2 Katja Linher-Melville,1,2 Gurmit Singh1,2
1Michael G. DeGroote Institute for Pain Research and Care, Medicine, McMaster University, Hamilton, ON L8S 4M1, Canada; 2Department of Pathology and Molecular Medicine, McMaster University, Hamilton, ON L8S 4M1, Canada
Introduction: Pain is a common and debilitating comorbidity of metastatic breast cancer. The hippocampus has been implicated in nociceptive processing, particularly relating to the subjective aspect of pain. Here, a syngeneic mouse model was used to characterize the effects of peripheral tumors on hippocampal microglial activation in relation to cancer-induced pain (CIP).
Materials and methods: Mice were systemically treated with the colony-stimulating factor 1 receptor inhibitor Pexidartinib prior to intrafemoral (IF) or subcutaneous 4T1 carcinoma cell inoculation. Spontaneous and evoked nociceptive responses were quantitated throughout tumor development, and contralateral hippocampi were collected via endpoint microdissection for RNA analysis. Additionally, IF tumor-bearing animals were sacrificed on days 5, 10, 15, and 20 post 4T1 cell inoculation, and brain sections were immunofluorescently stained for Iba1, a marker of activated microglia.
Results: Ablation of these neuroimmune cells with the CSF1R inhibitor Pexidartinib delayed the onset and severity of cancer-induced nociceptive behaviors in IF tumor-bearing animals, adding to the body of literature that demonstrates microglial contribution to the development and maintenance of CIP. Furthermore, in untreated IF tumor-bearing mice, nociceptive behaviors appeared to progress in parallel with microglial activation in hippocampal regions. Immunofluorescent Iba1+ microglia increased in the dentate gyrus and cornu ammonis 1 hippocampal regions in IF tumor-bearing animals over time, which was confirmed at the mRNA level using relevant microglial markers.
Conclusion: This is the first experimental evidence to demonstrate the effects of peripheral tumor-induced nociception on hippocampal microglial activation. The increase in hippocampal microglia observed in the present study may reflect the emotional and cognitive deficits reported by patients with CIP.
Keywords: nociception, hippocampus, cancer-induced bone pain, breast cancer, Iba1
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