<em>In Vivo</em> Calcium Imaging of Neuronal Ensembles in Networks of Primary Sensory Neurons in Intact Dorsal Root Ganglia

Ca2+ imaging can be used as a proxy for cellular activity, including action potentials and various signaling mechanisms involving Ca2+ entry into the ...

University of Texas Health Science Center at San Antonio

Previous studies have shown that infiltration of capsaicin into the surgical site can prevent incision-induced spontaneous pain like behaviors and heat hyperalgesia. In the present study, we aimed to monitor primary sensory neuron Ca activity in the intact dorsal root ganglia (DRG) using Pirt-GCaMP3 male and female mice pretreated with capsaicin or vehicle before the plantar incision. Intraplantar injection of capsaicin (0.05%) significantly attenuated spontaneous pain, mechanical, and heat hypersensitivity after plantar incision. The Ca response in  DRG and in  spinal cord was significantly enhanced in the ipsilateral side compared with contralateral side or naive control. Primary sensory nerve fiber length was significantly decreased in the incision skin area in capsaicin-pretreated animals detected by immunohistochemistry and placental alkaline phosphatase (PLAP) staining. Thus, capsaicin pretreatment attenuates incisional pain by suppressing Ca response because of degeneration of primary sensory nerve fibers in the skin. Postoperative surgery pain is a major health and economic problem worldwide with ∼235 million major surgical procedures annually. Approximately 50% of these patients report uncontrolled or poorly controlled postoperative pain. However, mechanistic studies of postoperative surgery pain in primary sensory neurons have been limited to  models or small numbers of neurons. Using an innovative, distinctive, and interdisciplinary  populational dorsal root ganglia (DRG) imaging (>1800 neurons/DRG) approach, we revealed increased DRG neuronal Ca activity from postoperative pain mouse model. This indicates widespread DRG primary sensory neuron plasticity. Increased neuronal Ca activity occurs among various sizes of neurons but mostly in small-diameter and medium-diameter nociceptors. Capsaicin pretreatment as a therapeutic option significantly attenuates Ca activity and postoperative pain.

 Calcium Imaging Visualizes Incision-Induced Primary Afferent Sensitization and Its Amelioration by Capsaicin Pretreatment.

Reduced skin blood flow has been reported in neuropathic pain patients as well as various peripheral neuropathic pain model animals. We have previously shown that vasodilators, which improves reduced skin blood flow, correlatively alleviate neuropathic pain in chronic constriction injury (CCI) mice, a model of neuropathic pain from peripheral nerve injury. Here, we sought to elucidate the mechanism underlying the reduced skin blood flow in CCI rats. The skin blood flow of the ipsilateral plantar arteries was significantly reduced compared to that of the contralateral ones 4 weeks after loose ligation of the sciatic nerve. The contraction induced by noradrenaline, serotonin, and U46619, a thromboxane receptor agonist, in the isolated ipsilateral plantar arteries was significantly enhanced compared to that in the contralateral ones. KB-R7943, a Na/Ca exchanger (NCX) inhibitor, shifted the concentration-response curves of noradrenaline to the left in the contralateral arteries but had no effect on the ipsilateral side. There was no significant difference in concentration-response curves of noradrenaline between the ipsilateral and contralateral arteries in the presence of KB-R7943. Amiloride, a non-specific inhibitor of Na channels and transporters, comparably shifted concentration-response curves of noradrenaline to the left in both the contralateral and ipsilateral arteries. One hundred nM of noradrenaline induced intracellular Ca elevation in the ipsilateral arteries, which was significantly larger than that induced by 300-nM noradrenaline in the contralateral arteries. These results suggest that reduced peripheral blood flow after nerve injury is due to Na-dependent inactivation of NCX in the ipsilateral plantar arteries.

Na-dependent inactivation of vascular Na/Ca exchanger responsible for reduced peripheral blood flow in neuropathic pain model.

Trigeminal (TG), dorsal root (DRG), and nodose/jugular (NG/JG) ganglia each possess specialized and distinct functions. We used RNA sequencing of two-cycle sorted Pirt-positive neurons to identify genes exclusively expressing in L3-L5 DRG, T10-L1 DRG, NG/JG, and TG mouse ganglion neurons. Transcription factor Phox2b and Efcab6 are specifically expressed in NG/JG while Hoxa7 is exclusively present in both T10-L1 and L3-L5 DRG neurons. Cyp2f2, Krt18, and Ptgds, along with pituitary hormone prolactin (Prl), growth hormone (Gh), and proopiomelanocortin (Pomc) encoding genes are almost exclusively in TG neurons. Immunohistochemistry confirmed selective expression of these hormones in TG neurons and dural nerves; and showed GH expression in subsets of TRPV1 and CGRP TG neurons. We next examined GH roles in hypersensitivity in the spinal versus trigeminal systems. Exogenous GH produced mechanical hypersensitivity when injected intrathecally, but not intraplantarly. GH-induced thermal hypersensitivity was not detected in the spinal system. GH dose-dependently generated orofacial and headache-like periorbital mechanical hypersensitivity after administration into masseter muscle and dura, respectively. Periorbital mechanical hypersensitivity was reversed by a GH receptor antagonist, pegvisomant. Overall, pituitary hormone genes are selective for TG versus other ganglia somatotypes; and GH has distinctive functional significance in the trigeminal versus spinal systems.

Pituitary hormones are specifically expressed in trigeminal sensory neurons and contribute to pain responses in the trigeminal system.

We introduce cytoNet, a cloud-based tool to characterize cell populations from microscopy images. cytoNet quantifies spatial topology and functional relationships in cell communities using principles of network science. Capturing multicellular dynamics through graph features, cytoNet also evaluates the effect of cell-cell interactions on individual cell phenotypes. We demonstrate cytoNet's capabilities in four case studies: 1) characterizing the temporal dynamics of neural progenitor cell communities during neural differentiation, 2) identifying communities of pain-sensing neurons in vivo, 3) capturing the effect of cell community on endothelial cell morphology, and 4) investigating the effect of laminin α4 on perivascular niches in adipose tissue. The analytical framework introduced here can be used to study the dynamics of complex cell communities in a quantitative manner, leading to a deeper understanding of environmental effects on cellular behavior. The versatile, cloud-based format of cytoNet makes the image analysis framework accessible to researchers across domains.

cytoNet: Spatiotemporal network analysis of cell communities.

As the outermost barrier tissue of the body, the skin harbors a large number of innate lymphoid cells (ILCs) that help maintain local homeostasis in the face of changing environments. How skin-resident ILCs are regulated and function in local homeostatic maintenance is poorly understood. We here report the discovery of a cold-sensing neuron-initiated pathway that activates skin group 2 ILCs (ILC2s) to help maintain thermal homeostasis. In stearoyl-CoA desaturase 1 (SCD1) knockout mice whose skin is defective in heat maintenance, chronic cold stress induced excessive activation of CCR10CD81ST2 skin ILC2s and associated inflammation. Mechanistically, stimulation of the cold-sensing receptor TRPM8 expressed in sensory neurons of the skin led to increased production of IL-18, which, in turn, activated skin ILC2s to promote thermogenesis. Our findings reveal a neuroimmune link that regulates activation of skin ILC2s to support thermal homeostasis and promotes skin inflammation after hyperactivation.

Activation of CD81 skin ILC2s by cold-sensing TRPM8 neuron-derived signals maintains cutaneous thermal homeostasis.

Chemotherapy-induced peripheral neuropathy (CIPN) affects about 68% of patients undergoing chemotherapy, causing debilitating neuropathic pain and reducing quality of life. Cisplatin is a commonly used platinum-based chemotherapeutic drug known to cause CIPN, possibly by causing oxidative stress damage to primary sensory neurons. Metabotropic glutamate receptors (mGluRs) are widely hypothesized to be involved in pain processing and pain mitigation. Meclizine is an H1 histamine receptor antagonist known to have neuroprotective effects, including an anti-oxidative effect. Here, we used a mouse model of cisplatin-induced CIPN using male and female mice to test agonists of mGluR8 and group II mGluR as well as meclizine as interventions to reduce cisplatin-induced pain. We performed behavioral pain tests, and we imaged Ca activity of the large population of DRG neurons  For the latter, we used a genetically-encoded Ca indicator, Pirt-GCaMP3, which enabled us to monitor different drug interventions at the level of the intact DRG neuronal ensemble. We found that CIPN increased spontaneous Ca activity in DRG neurons, increased number of Ca transients, and increased hyper-responses to mechanical, thermal, and chemical stimuli. We found that mechanical and thermal pain caused by CIPN was significantly attenuated by the mGluR8 agonist, (S)-3,4-DCPG, the group II mGluR agonist, LY379268, and the H1 histamine receptor antagonist, meclizine. DRG neuronal Ca activity elevated by CIPN was attenuated by LY379268 and meclizine, but not by (S)-3,4-DCPG. Furthermore, meclizine and LY379268 attenuated cisplatin-induced weight loss. These results suggest that group II mGluR agonist, mGluR8 agonist, and meclizine are promising candidates as new treatment options for CIPN, and studies of their mechanisms are warranted.Chemotherapy-induced peripheral neuropathy (CIPN) is a painful condition that affects most chemotherapy patients and persist several months or longer after treatment ends. Research on CIPN mechanism is extensive but has produced only few clinically useful treatments. Utilizing  GCaMP Ca imaging in live animals over 1800 neurons/DRG at once, we have characterized the effects of the chemotherapeutic drug, cisplatin and three treatments that decrease CIPN pain. Cisplatin increases sensory neuronal Ca activity and develops various sensitization. Metabotropic glutamate receptor agonist, LY379268 or the H1 histamine receptor antagonist, meclizine decreases cisplatin's effects on neuronal Ca activity and reduces pain hypersensitivity. Our results and experiments provide insights into cellular effects of cisplatin and drugs preventing CIPN pain.

Meclizine and metabotropic glutamate receptor agonists attenuate severe pain and Ca activity of primary sensory neurons in chemotherapy-induced peripheral neuropathy.

Chemotherapy-induced cognitive impairment (CICI) is a neurotoxic side effect of chemotherapy that has yet to have an effective treatment.

Yu Shin Kim

University of Texas Health Science Center at San Antonio

In Vivo Calcium Imaging of Neuronal Ensembles in Networks of Primary Sensory Neurons in Intact Dorsal Root Ganglia