Nociceptors in the dorsal root ganglia (DRG) and trigeminal ganglion (TG) play an essential role in initiating pain by detecting painful stimuli through their peripheral axons and sending signals to the spinal cord via their central axons [1]. Pathological conditions such as inflammation and nerve injury can sensitize nociceptors, causing heightened pain sensitivity and often leading to chronic pain conditions like TMJ disorders. Despite its importance in understanding the mechanism of nociceptor sensitization, monitoring neuronal activity of nociceptors in tissue explants or in live animals is still technically challenging due to the interference of the surrounding tissues. Recently, we have developed a novel approach to directly monitor neuronal activity and hyperactivity after injury and revealed the contribution of central terminal sensitization of primary nociceptive neurons to molecular mechanisms underlying the maintenance of trigeminal neuropathic pain. We generated Pirt-GCaMP3 mice in which GCaMP3, a genetic-encoded Ca2+-sensitive indicator [2], is specifically expressed in >95% of all DRG and TG neurons under the Pirt promoter[3]. Because of the specific expression of the Ca2+ sensor (i.e., only in DRG and TG and not in skin cells or spinal cord neurons), we detected robust neuronal hyperexcitability in TG explants and TG’s axons in the skin explants and trigeminal brain-stem slices of animals with nerve injury compared with naïve or sham-treated mice. In addition, we are developing techniques to image DRG neuronal activity in live mice in response to various sensory stimuli applied to sensory peripheral receptive fields. The advantages of the functional imaging using Pirt-GCaMP3 mice include simple tissue preparation and imaging procedures, intact sensory somatotopic organization, and simultaneously monitoring a large population of neurons and nerves. Previous and ongoing studies using this technique have revealed new mechanisms underlying chronic pain conditions including orofacial pain.
Disclosures
Dr. Caterina is an inventor on a patent on the use of products related to TRPV1, which is licensed through UCSF and Merck, and may be entitled to royalties related to these products. He is on the Scientific Advisory Board for Hydra Biosciences, which develops products related to TRP channels. These conflicts are being managed by the Johns Hopkins Office on Policy Coordination.
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Acknowledgements
We thank Dr. Loren Looger at Howard Hughes Medical Institute, Janelia Farm for providing us GCaMP3 cDNA and Yixun Geng for technical assistance. We thank Chip Hawkins and Holly Wellington of Transgenic Mouse Core at Johns Hopkins University School of Medicine for assistance with Pirt-GCaMP3 mouse generation. This work was supported by National Institutes of Health Grants (R01DE022750 and R01GM087369 to X.D.; R01DE018573 to F.W.), Johns Hopkins University Brain Science Institute grant, and T32 (T32NS070201 to Y.S.K.) Johns Hopkins University Pain Fellowship.
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