Background & Aims

Chronic pain is a complex neuroimmune pathology in which reactive microglia is known to be a key player. Spinal microglial reactivity has been linked to abnormal peripheral activity coming from primary sensory neurons (i.e. nociceptors and non-nociceptors). Our aim is to further investigate the contribution of nociceptive and non-nociceptive inputs to spinal microglial reactivity in the context of chronic pain.

Methods

We first used electrical stimulation of the left sciatic nerve of CX3CR1-eGFP mice to observe if this activation of nociceptive and non-nociceptive neurons would induce pain hypersensitivity and spinal microglial reactivity (proliferation and increase in number of cells) two days after stimulation. We then used optogenetics to activate the different subpopulations. We activated all sensory neurons (i.e nociceptive and non-nociceptive) using the Advillin-Cre/ChR2-TdTomato line, nociceptors alone using the SNS-Cre/ChR2-TdTomato line and non-nociceptors using a NetrinG1-Cre/ChR2-TdTomato line.

Results

Dorsal horn spinal microglia reactivity, assessed by an increased proliferation and number of cells, was induced following electrical stimulation of the sciatic nerve. In addition, this activation of all sensory neurons induced increased hypersensitivity to thermal and to mechanical stimuli lasting up to four days. Proliferation of dorsal horn spinal microglia was also observed after optogenetic activation of nociceptors alone and of all sensory neurons. No proliferation was observed after activation of non-nociceptors alone. However, a significant increase in number of microglial cells at two days after optogenetic stimulation was only observed after activation of all sensory neurons using the Advillin-Cre/ChR2-TdTomato line. Finally, thermal and mechanical hypersensitivities were observed after optogenetic activation of all sensory neurons but not after activation of nociceptive or non-nociceptive neurons alone.

Conclusions

Our data shows that electrically induced neuronal activity from both nociceptive and non-nociceptive neurons is sufficient to induce spinal microglial reactivity and pain. A similar microglial reactivity and pain sensitivity is observed after optogenetic activation of both nociceptors and non-nociceptors using the Advillin-Cre/ChR2-TdTomato line. Activation of nociceptors alone using the SNS-Cre/ChR2-TdTomato line induced a smaller response of spinal microglial cells and activation of non-nociceptors using the NetrinG1-ChR2-TdTomato failed to induce any reactivity in spinal microglia. This suggests that a combined activity from both populations of sensory neurons triggers a stronger reactivity of spinal microglia.

References

1.Hathway, G. J.; Vega-Avelaira, D.; Moss, A.; Ingram, R.; Fitzgerald, M., Brief, low frequency stimulation of rat peripheral C-fibres evokes prolonged microglial-induced central sensitization in adults but not in neonates. Pain 2009, 144, (1-2), 110-8.
2.Suter, M. R.; Siegenthaler, A.; Decosterd, I.; Ji, R. R., Perioperative nerve blockade: clues from the bench. Anesthesiology research and practice 2011, 2011, 124898.
3.Suter, M. R.; Berta, T.; Gao, Y. J.; Decosterd, I.; Ji, R. R., Large A-fiber activity is required for microglial proliferation and p38 MAPK activation in the spinal cord: different effects of resiniferatoxin and bupivacaine on spinal microglial changes after spared nerve injury. Molecular pain 2009, 5, 53.
4.Suter, M. R., Microglial role in the development of chronic pain. Current Opinion in Anaesthesiology 2016, 29, (5), 584-589.

Presenting Author

Paul Chu Sin Chung

Poster Authors

Paul Chu Sin Chung

PhD

Anesthesiology Department, Pain Center – Research laboratory

Lead Author

Manon Isler

Pain center, Lausanne University Hospital (CHUV)

Lead Author

Guylène Kirschmann

Pain center, Lausanne University Hospital (CHUV)

Lead Author

Ezgi Gozlugol

Pain center, Lausanne University Hospital (CHUV)

Lead Author

Isabelle Decosterd

Pain center, Lausanne University Hospital (CHUV)

Lead Author

Marc Suter

Lausanne University Hospital

Lead Author

Topics

  • Mechanisms: Biological-Systems (Physiology/Anatomy)