Analgesic tolerance of opioid agonists in mutant mu-opioid receptors expressed in sensory neurons following intrathecal plasmid gene delivery

Activation of GDNF-ERK-Runx1 signaling contributes to P2X3R gene transcription and bone cancer pain

Bone cancer pain is a common symptom in cancer patients with bone metastases and its underlying mechanisms remain unknown. Here, we report that Runx1 directly upregulates the transcriptional activity of P2X3 receptor (P2X3R) gene promoter in PC12 cells. Knocking down Runx1 in dorsal root ganglion (DRG) neurons suppresses the functional upregulation of P2X3R, attenuates neuronal hyperexcitability and pain hypersensitivity in tumor-bearing rats, whereas overexpressing Runx1 promotes P2X3R gene transcription in DRG neurons, induces neuronal hyperexcitability and pain hypersensitivity in naïve rats. Activation of GDNF-GFRα1-Ret-ERK signaling is required for Runx1-mediated P2X3R gene transcription in DRG neurons, and contributes to neuronal hyperexcitability and pain hypersensitivity in tumor-bearing rats. These findings indicate that the Runx1-mediated P2X3R gene transcription resulted from activation of GDNF-GFRα1-Ret-ERK signaling contributes to the sensitization of DRG neurons and pathogenesis of bone cancer pain. Our findings identify a potentially targetable mechanism that may cause bone metastasis-associated pain in cancer patients.

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Runx1 directly upregulates the transcriptional activity of P2X3R gene promoter

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Upregulation of Runx1-mediated P2X3R gene transcription underlies bone cancer pain

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Involvement of GDNF-Ret-ERK signaling in Runx1-mediated P2X3R gene transcription

Dynamic Opioid Receptor Regulation in the Periphery

Opioids serve a vital role in the current analgesic array of treatment options. They are useful in acute instances involving severe pain associated with trauma, surgery, and terminal diseases such as cancer. In the past three decades, multiple receptor isoforms and conformations have been reported throughout literature. Most of these studies conducted systemic analyses of opioid receptor function, often generalizing findings from receptor systems in central nervous tissue or exogenously expressing immortalized cell lines as common mechanisms throughout physiology. However, a culmination of innovative experimental data indicates that opioid receptor systems are differentially modulated depending on their anatomic expression profile. Importantly, opioid receptors expressed in the peripheral nervous system undergo regulation uncommon to similar receptors expressed in central nervous system tissues. This distinctive characteristic begs one to question whether peripheral opioid receptors maintain anatomically unique roles, and whether they may serve an analgesic advantage in providing pain relief without promoting addiction.

Decreased abundance of TRESK two-pore domain potassium channels in sensory neurons underlies the pain associated with bone metastasis

Bone metastasis–associated VEGF suppresses neuronal K + channels and increases pain in rats.

Chronic cervical radiculopathic pain is associated with increased excitability and hyperpolarization-activated current ( Ih) in large-diameter dorsal root ganglion neurons

Cervical radiculopathic pain is a very common symptom that may occur with cervical spondylosis. Mechanical allodynia is often associated with cervical radiculopathic pain and is inadequately treated with current therapies. However, the precise mechanisms underlying cervical radiculopathic pain-associated mechanical allodynia have remained elusive. Compelling evidence from animal models suggests a role of large-diameter dorsal root ganglion neurons and plasticity of spinal circuitry attached with Aβ fibers in mediating neuropathic pain. Whether cervical radiculopathic pain condition induces plastic changes of large-diameter dorsal root ganglion neurons and what mechanisms underlie these changes are yet to be known. With combination of patch-clamp recording, immunohistochemical staining, as well as behavioral surveys, we demonstrated that upon chronic compression of C7/8 dorsal root ganglions, large-diameter cervical dorsal root ganglion neurons exhibited frequent spontaneous firing together with hyperexcitability. Quantitative analysis of hyperpolarization-activated cation current (I_h) revealed that I_h was greatly upregulated in large dorsal root ganglion neurons from cervical radiculopathic pain rats. This increased I_h was supported by the enhanced expression of hyperpolarization-activated, cyclic nucleotide-modulated channels subunit 3 in large dorsal root ganglion neurons. Blockade of I_h with selective antagonist, ZD7288 was able to eliminate the mechanical allodynia associated with cervical radiculopathic pain. This study sheds new light on the functional plasticity of a specific subset of large-diameter dorsal root ganglion neurons and reveals a novel mechanism that could underlie the mechanical allodynia associated with cervical radiculopathy.

Epac-protein kinase C alpha signaling in purinergic P2X3R-mediated hyperalgesia after inflammation

Protein kinase C alpha plays a major role in mediating Epac-dependent enhancement of purinergic P2X3R activity in dorsal root ganglion neurons after inflammation.

Sensitization of purinergic P2X3 receptors (P2X3Rs) is a major mechanism contributing to injury-induced exaggerated pain responses. We showed in a previous study that cyclic adenosine monophosphate (cAMP)–dependent guanine nucleotide exchange factor 1 (Epac1) in rat sensory dorsal root ganglia (DRGs) is upregulated after inflammatory injury, and it plays a critical role in P2X3R sensitization by activating protein kinase C epsilon (PKCε) inside the cells. protein kinase C epsilon has been established as the major PKC isoform mediating injury-induced hyperalgesic responses. On the other hand, the role of PKCα in receptor sensitization was seldom considered. Here, we studied the participation of PKCα in Epac signaling in P2X3R-mediated hyperalgesia. The expression of both Epac1 and Epac2 and the level of cAMP in DRGs are greatly enhanced after complete Freund adjuvant (CFA)–induced inflammation. The expression of phosphorylated PKCα is also upregulated. Complete Freund adjuvant (CFA)–induced P2X3R-mediated hyperalgesia is not only blocked by Epac antagonists but also by the classical PKC isoform inhibitors, Go6976, and PKCα-siRNA. These CFA effects are mimicked by the application of the Epac agonist, 8-(4-chlorophenylthio)-2 -O-methyl-cAMP (CPT), in control rats, further confirming the involvement of Epacs. Because the application of Go6976 prior to CPT still reduces CPT-induced hyperalgesia, PKCα is downstream of Epacs to mediate the enhancement of P2X3R responses in DRGs. The pattern of translocation of PKCα inside DRG neurons in response to CPT or CFA stimulation is distinct from that of PKCε. Thus, in contrast to prevalent view, PKCα also plays an essential role in producing complex inflammation-induced receptor-mediated hyperalgesia.

Upregulation of Ih expressed in IB4-negative Aδ nociceptive DRG neurons contributes to mechanical hypersensitivity associated with cervical radiculopathic pain

Cervical radiculopathy represents aberrant mechanical hypersensitivity. Primary sensory neuron’s ability to sense mechanical force forms mechanotransduction. However, whether this property undergoes activity-dependent plastic changes and underlies mechanical hypersensitivity associated with cervical radiculopathic pain (CRP) is not clear. Here we show a new CRP model producing stable mechanical compression of dorsal root ganglion (DRG), which induces dramatic behavioral mechanical hypersensitivity. Amongst nociceptive DRG neurons, a mechanically sensitive neuron, isolectin B4 negative Aδ-type (IB4⁻ Aδ) DRG neuron displays spontaneous activity with hyperexcitability after chronic compression of cervical DRGs. Focal mechanical stimulation on somata of IB4^- Aδ neuron induces abnormal hypersensitivity. Upregulated HCN1 and HCN3 channels and increased I_h current on this subset of primary nociceptors underlies the spontaneous activity together with neuronal mechanical hypersensitivity, which further contributes to the behavioral mechanical hypersensitivity associated with CRP. This study sheds new light on the functional plasticity of a specific subset of nociceptive DRG neurons to mechanical stimulation and reveals a novel mechanism that could underlie the mechanical hypersensitivity associated with cervical radiculopathy.

Endogenous opiates and behavior: 2013

This paper is the thirty-sixth consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2013 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.