Bradykinin and prostaglandin E₁ regulate calcitonin gene-related peptide expression in cultured rat sensory neurons

Role of bradykinin and prostaglandin EP4 receptors in regulating TRPV1 channel sensitization in rat dorsal root ganglion neurons

Transient receptor potential vanilloid type-1 (TRPV1) channels play key roles in chronic pain conditions and are modulated by different inflammatory mediators to elicit heat sensitisation. Bradykinin is a 9-amino acid peptide chain that promotes inflammation. The aim of present study is to investigate how bradykinin and prostaglandin receptors (EP3 and EP4 ) modulate the sensitisation of TRPV1-mediated responses. Calcium imaging studies of rat dorsal root ganglion (DRG) neurons were employed to investigate the desensitizing responses of TRPV1 ion channels by capsaicin, and the re-sensitization of TRPV1 by bradykinin, then to explore the role EP3 and EP4 receptors in mediating these bradykinin-dependent effects. Immunocytochemistry was used to study the co-expression and distribution of EP4, TRPV1, COX-1 and B2 in rat DRG neurons. Desensitization was seen upon repeated capsaicin application, we show that bradykinin-mediated sensitization of capsaicin-evoked calcium responses in rat DRG neurons occurs is dependent on COX-1 activity and utilizes a pathway that involves EP4 but not EP3 receptors. Immunocytochemical techniques revealed that EP4, TRPV1, COX-1 and B2 proteins are expressed mainly in small diameter (<1000 μm2 ) cell bodies of rat DRG neurons which are typically nociceptors. The present study provides suggestive evidence for a potential signalling pathway through which bradykinin may regulate TRPV1 ion channel function via EP4 receptors. In addition to confirming existing knowledge, the anatomical distribution and colocalization of these proteins in DRG neurons as revealed by this study offer valuable insight.

Bradykinin receptor expression and bradykinin-mediated sensitization of human sensory neurons

ABSTRACT

Bradykinin is a peptide implicated in inflammatory pain in both humans and rodents. In rodent sensory neurons, activation of B1 and B2 bradykinin receptors induces neuronal hyperexcitability. Recent evidence suggests that human and rodent dorsal root ganglia (DRG), which contain the cell bodies of sensory neurons, differ in the expression and function of key GPCRs and ion channels; whether bradykinin receptor expression and function are conserved across species has not been studied in depth. In this study, we used human DRG tissue from organ donors to provide a detailed characterization of bradykinin receptor expression and bradykinin-induced changes in the excitability of human sensory neurons. We found that B2 and, to a lesser extent, B1 receptors are expressed by human DRG neurons and satellite glial cells. B2 receptors were enriched in the nociceptor subpopulation. Using patch-clamp electrophysiology, we found that acute bradykinin increases the excitability of human sensory neurons, whereas prolonged exposure to bradykinin decreases neuronal excitability in a subpopulation of human DRG neurons. Finally, our analyses suggest that donor's history of chronic pain and age may be predictors of higher B1 receptor expression in human DRG neurons. Together, these results indicate that acute bradykinin-induced hyperexcitability, first identified in rodents, is conserved in humans and provide further evidence supporting bradykinin signaling as a potential therapeutic target for treating pain in humans.

Bradykinin receptor expression and bradykinin-mediated sensitization of human sensory neurons

Bradykinin is a peptide implicated in inflammatory pain in both humans and rodents. In rodent sensory neurons, activation of B1 and B2 bradykinin receptors induces neuronal hyperexcitability. Recent evidence suggests that human and rodent dorsal root ganglia (DRG), which contain the cell bodies of sensory neurons, differ in the expression and function of key GPCRs and ion channels; whether BK receptor expression and function are conserved across species has not been studied in depth. In this study, we used human DRG tissue from organ donors to provide a detailed characterization of bradykinin receptor expression and bradykinin-induced changes in the excitability of human sensory neurons. We found that B2 and, to a lesser extent, B1 receptors are expressed by human DRG neurons and satellite glial cells. B2 receptors were enriched in the nociceptor subpopulation. Using patch-clamp electrophysiology, we found that acute bradykinin increases the excitability of human sensory neurons, while prolonged exposure to bradykinin decreases neuronal excitability in a subpopulation of human DRG neurons. Finally, our analyses suggest that donor’s history of chronic pain and age may be predictors of higher B1 receptor expression in human DRG neurons. Together, these results indicate that acute BK-induced hyperexcitability, first identified in rodents, is conserved in humans and provide further evidence supporting BK signaling as a potential therapeutic target for treating pain in humans.

Alpha-Calcitonin Gene Related Peptide: New Therapeutic Strategies for the Treatment and Prevention of Cardiovascular Disease and Migraine

Alpha-calcitonin gene-related peptide (α-CGRP) is a vasodilator neuropeptide of the calcitonin gene family. Pharmacological and gene knock-out studies have established a significant role of α-CGRP in normal and pathophysiological states, particularly in cardiovascular disease and migraines. α-CGRP knock-out mice with transverse aortic constriction (TAC)-induced pressure-overload heart failure have higher mortality rates and exhibit higher levels of cardiac fibrosis, inflammation, oxidative stress, and cell death compared to the wild-type TAC-mice. However, administration of α-CGRP, either in its native- or modified-form, improves cardiac function at the pathophysiological level, and significantly protects the heart from the adverse effects of heart failure and hypertension. Similar cardioprotective effects of the peptide were demonstrated in pressure-overload heart failure mice when α-CGRP was delivered using an alginate microcapsules-based drug delivery system. In contrast to cardiovascular disease, an elevated level of α-CGRP causes migraine-related headaches, thus the use of α-CGRP antagonists that block the interaction of the peptide to its receptor are beneficial in reducing chronic and episodic migraine headaches. Currently, several α-CGRP antagonists are being used as migraine treatments or in clinical trials for migraine pain management. Overall, agonists and antagonists of α-CGRP are clinically relevant to treat and prevent cardiovascular disease and migraine pain, respectively. This review focuses on the pharmacological and therapeutic significance of α-CGRP-agonists and -antagonists in various diseases, particularly in cardiac diseases and migraine pain.

Molecular mechanisms underlying the actions of arachidonic acid-derived prostaglandins on peripheral nociception

Arachidonic acid-derived prostaglandins not only contribute to the development of inflammation as intercellular pro-inflammatory mediators, but also promote the excitability of the peripheral somatosensory system, contributing to pain exacerbation. Peripheral tissues undergo many forms of diseases that are frequently accompanied by inflammation. The somatosensory nerves innervating the inflamed areas experience heightened excitability and generate and transmit pain signals. Extensive studies have been carried out to elucidate how prostaglandins play their roles for such signaling at the cellular and molecular levels. Here, we briefly summarize the roles of arachidonic acid-derived prostaglandins, focusing on four prostaglandins and one thromboxane, particularly in terms of their actions on afferent nociceptors. We discuss the biosynthesis of the prostaglandins, their specific action sites, the pathological alteration of the expression levels of related proteins, the neuronal outcomes of receptor stimulation, their correlation with behavioral nociception, and the pharmacological efficacy of their regulators. This overview will help to a better understanding of the pathological roles that prostaglandins play in the somatosensory system and to a finding of critical molecular contributors to normalizing pain.

CGRP/CGRP Receptor Antibodies: Potential Adverse Effects Due to Blockade of Neovascularization?

Migraine is a severe neurological disorder in which calcitonin gene-related peptide (CGRP) is a key molecule in pathophysiology. Neuronal system-derived CGRP enhances neovascularization in several important pathological conditions and sends a cue to the vascular system. In 2018, the FDA approved erenumab and fremanezumab, antibodies against CGRP receptor and CGRP, as the first new class of drugs for migraine. Treatment of migraine with these antibodies requires great care because neovascularization-related adverse effects may be induced in some patients. Here, we focus on enhancement of neovascularization by CGRP and discuss possible adverse effects resulting from blocking neovascularization. We also suggest that CGRP antibodies may also be used as novel antitumor agents by suppressing tumor-associated angiogenesis.

Bradykinin Induces TRPV1 Exocytotic Recruitment in Peptidergic Nociceptors

Transient receptor potential vanilloid I (TRPV1) sensitization in peripheral nociceptors is a prominent phenomenon that occurs in inflammatory pain conditions. Pro-algesic agents can potentiate TRPV1 activity in nociceptors through both stimulation of its channel gating and mobilization of channels to the neuronal surface in a context dependent manner. A recent study reported that ATP-induced TRPV1 sensitization in peptidergic nociceptors involves the exocytotic release of channels trafficked by large dense core vesicles (LDCVs) that cargo alpha-calcitonin gene related peptide alpha (αCGRP). We hypothesized that, similar to ATP, bradykinin may also use different mechanisms to sensitize TRPV1 channels in peptidergic and non-peptidergic nociceptors. We found that bradykinin notably enhances the excitability of peptidergic nociceptors, and sensitizes TRPV1, primarily through the bradykinin receptor 2 pathway. Notably, bradykinin sensitization of TRPV1 in peptidergic nociceptors was significantly blocked by inhibiting Ca²⁺-dependent neuronal exocytosis. In addition, silencing αCGRP gene expression, but not substance P, drastically reduced bradykinin-induced TRPV1 sensitization in peptidergic nociceptors. Taken together, these findings indicate that bradykinin-induced sensitization of TRPV1 in peptidergic nociceptors is partially mediated by the exocytotic mobilization of new channels trafficked by αCGRP-loaded LDCVs to the neuronal membrane. Our findings further imply a central role of αCGRP peptidergic nociceptors in peripheral algesic sensitization, and substantiate that inhibition of LDCVs exocytosis is a valuable therapeutic strategy to treat pain, as it concurrently reduces the release of pro-inflammatory peptides and the membrane recruitment of thermoTRP channels.

Different responses of galanin and calcitonin gene-related peptide to capsaicin stimulation on dorsal root ganglion neurons in vitro

Both galanin (Gal) and calcitonin gene-related peptide (CGRP) are sensory neuropeptides which expressed in dorsal root ganglion (DRG) neurons and are involved in nociceptive processing. Capsaicin (CAP) influences nociceptive processing via influencing the expression of sensory neuropeptides in primary sensory neurons. However, little is known about the alterations of Gal and CGRP expression at the same condition stimulated by CAP. In the present study, primary cultured DRG neurons were used to determine the different responses of Gal and CGRP to CAP stimulation. DRG neurons were cultured for 48 hours and then exposed to CAP (2 μmol/L), capsazepine (CPZ) (2 μmol/L) plus CAP (2 μmol/L), or extracellular signal-regulated kinase 1/2 (ERK1/2) inhibitor PD98059 (10 μmol/L) plus CAP (2 μmol/L) for an additional 24hours. The DRG neurons were continuously exposed to culture media as a control. After that, the levels of Gal mRNA and CGRP mRNA of DRG neurons were determined using real time-PCR analysis. Gal and CGRP expression in situ was detected by an immunofluorescent labeling technique. The levels of phosphorylated-ERK1/2 (pERK1/2) protein were detected using a Western blot assay. The results showed that CAP evoked increases of Gal and its mRNA and decreases of CGRP and its mRNA in DRG neurons. Administration of either CPZ or PD98059 blocked the effects of CAP. These data indicate that Gal and CGRP shared different responses to CAP stimulation. Gal and CGRP may have different effects in nociceptive processing during neurogenic inflammation.

Sensory and signaling mechanisms of bradykinin, eicosanoids, platelet-activating factor, and nitric oxide in peripheral nociceptors

Peripheral mediators can contribute to the development and maintenance of inflammatory and neuropathic pain and its concomitants (hyperalgesia and allodynia) via two mechanisms. Activation or excitation by these substances of nociceptive nerve endings or fibers implicates generation of action potentials which then travel to the central nervous system and may induce pain sensation. Sensitization of nociceptors refers to their increased responsiveness to either thermal, mechanical, or chemical stimuli that may be translated to corresponding hyperalgesias. This review aims to give an account of the excitatory and sensitizing actions of inflammatory mediators including bradykinin, prostaglandins, thromboxanes, leukotrienes, platelet-activating factor, and nitric oxide on nociceptive primary afferent neurons. Manifestations, receptor molecules, and intracellular signaling mechanisms of the effects of these mediators are discussed in detail. With regard to signaling, most data reported have been obtained from transfected nonneuronal cells and somata of cultured sensory neurons as these structures are more accessible to direct study of sensory and signal transduction. The peripheral processes of sensory neurons, where painful stimuli actually affect the nociceptors in vivo, show marked differences with respect to biophysics, ultrastructure, and equipment with receptors and ion channels compared with cellular models. Therefore, an effort was made to highlight signaling mechanisms for which supporting data from molecular, cellular, and behavioral models are consistent with findings that reflect properties of peripheral nociceptive nerve endings. Identified molecular elements of these signaling pathways may serve as validated targets for development of novel types of analgesic drugs.