Endothelin-1 enhances acid-sensing ion channel currents in rat primary sensory neurons

CCK-8 enhances acid-sensing ion channel currents in rat primary sensory neurons

Cholecystokinin (CCK) is a peptide that has been implicated in pain modulation. Acid sensitive ion channels (ASICs) also play an important role in pain associated with tissue acidification. However, it is still unclear whether there is an interaction between CCK signaling and ASICs during pain process. Herein, we report that a functional link between them in rat dorsal root ganglion (DRG) neurons. Pretreatment with CCK-8 concentration-dependently increased acid-evoked ASIC currents. CCK-8 increased the maximum response of ASICs to acid, but did not changed their acid sensitivity. Enhancement of ASIC currents by CCK-8 was mediated by the stimulation of CCK2 receptor (CCK2R), rather than CCK1R. The enhancement of ASIC currents by CCK-8 was prevented by application of either G-protein inhibitor GDP-β-S or protein kinase C (PKC) inhibitor GF109203×, but not by protein kinase A (PKA) inhibitor H-89 or JNK inhibitor SP600125. Moreover, CCK-8 increased the number of action potentials triggered by acid stimuli by activating CCK2R. Finally, CCK-8 dose-dependently exacerbated acid-induced nociceptive behavior in rats through local CCK2R. Together, these results indicated that CCK-8/CCK2R activation enhanced ASIC-mediated electrophysiological activity in DRG neurons and nociception in rats. The enhancement effect depended on G-proteins and intracellular PKC signaling rather than PKA and JNK signaling pathway. These findings provided that CCK-8/CCK2R is an important therapeutic target for ASIC-mediated pain.

The Involvement of Endothelin Pathway in Chronic Psychological Stress-Induced Bladder Hyperalgesia Through Capsaicin-Sensitive C-Fiber Afferents

Introductions

Interstitial cystitis/bladder pain syndrome (IC/BPS) is a poorly understood chronic disorder characterized by bladder-related pain. Chronic psychological stress plays a key role in the exacerbation and development of IC/BPS via unclear mechanisms. This study aimed to investigate the role of endothelin 1 (ET-1) and its receptors in the development of chronic stress-induced bladder dysfunction.

Methods

Wistar‐Kyoto rats were exposed to chronic (10 days) water avoidance stress (WAS) or sham stress, with subgroups receiving capsaicin pretreatment to desensitize C-fiber afferents. Thereafter, cystometrograms (CMG) were obtained with visceromotor response (VMR) simultaneously during intravesical saline or ET-1 infusion. CMG recordings were analyzed for the first and the continuous voiding cycles, respectively. Endothelin receptor type A (ETAR) expression was examined in the bladder tissues and L6-S1 dorsal root ganglions (DRGs). Toluidine blue staining was to check the bladder inflammation and double-labeling immunofluorescence (IF) staining was to identify the locations of ETAR, respectively.

Results

During saline infusion, WAS rats elicited significant decreases in pressure threshold (PT) and in the ratio of VMR threshold/maximum intravesical pressure (IVPmax), and a significant increase in VMR duration and area under the curve (AUC). ET-1 infusion induced similar alternations in WAS rats, but further significantly diminished the pressure to trigger PT and VMR, together with a more forceful and longer VMR. The sole effect of WAS exposure or ET-1 administration on the micturition reflex could be suppressed by capsaicin pretreatment. WAS exposure significantly induced an increased number of total mast cells in the bladder, while capsaicin pretreatment possibly antagonized them. No significant difference in ETAR expression was found between all groups. IF staining indicated the co-localization of ETAR and calcitonin gene-related peptides in both bladder and DRGs.

Conclusion

The activation of ET-1 receptors could enhance chronic stress-induced bladder hypersensitization and hyperalgesia through capsaicin-sensitive C-fiber afferents. Targeting the endothelin pathway may have therapeutic value for IC/BPS.

Periodontal acidification contributes to tooth pain hypersensitivity during orthodontic tooth movement

Tooth movements associated with orthodontic treatment often cause tooth pain. However, the detailed mechanism remains unclear. Here, we examined the involvement of periodontal acidification caused by tooth movement in mechanical tooth pain hypersensitivity. Elastics were inserted between the first and second molars to move the teeth in Sprague-Dawley rats. Mechanical head-withdrawal reflex threshold to first molar stimulation and the pH of the gingival sulcus around the tooth were measured. The expression of acid-sensing ion channel 3 (ASIC3) in trigeminal ganglion neurons and phosphorylation of ASIC3 in the periodontal tissue were analyzed. The mechanical head-withdrawal reflex threshold to first molar stimulation and pH in the gingival sulcus decreased on day 1 after the elastic insertion. These decreases recovered to the sham level by buffering periodontal acidification. Periodontal inhibition of ASIC3 channel activity reversed the decreased mechanical head-withdrawal reflex threshold to first molar stimulation. On day 1 after elastic insertion, the tooth movement did not change the number of ASIC3 immunoreactive trigeminal ganglion neurons innervating the periodontal tissue but increased phosphorylated-ASIC3 levels in the periodontal tissue. Periodontal acidification induced by tooth movement causes phosphorylation of ASIC3, resulting in mechanical pain hypersensitivity in mechanically forced tooth.

Potentiation of P2X3 receptor mediated currents by endothelin-1 in rat dorsal root ganglion neurons

Endothelin-1 (ET-1), an endogenous vasoconstrictor, has been known as a pro-nociceptive agent involved in multitude of pain. ET-1 acts on endothelin receptors on vascular endothelial cells, sensitizes release of ATP, which then acts on P2X3 receptors on nociceptors and results in mechanical hyperalgesia. Both endothelin receptors and P2X3 receptors are present in primary sensory neuron, where it remains unclear whether there is an interaction between them. Herein, we reported that ET-1 potentiated the electrophysiological activity of P2X3 receptors in rat dorsal root ganglia (DRG) neurons. ET-1 concentration-dependently increased α,β-methylene-ATP (α,β-meATP)-evoked inward currents, which were mediated by P2X3 receptors. ET-1 shifted the α,β-meATP concentration-response curve upwards, with an increase of 34.38 ± 4.72% in the maximal current response to α,β-meATP in the presence of ET-1. ET-1 potentiation of α,β-meATP-evoked currents was voltage-independent. ET-1 potentiated P2X3 receptor-mediated currents through endothelin-A receptors (ET_AR), but not endothelin-B receptors (ET_BR). ET-1 potentiation was supressed by blockade of intracellular G-protein or protein kinase C (PKC) signaling. Moreover, there is a synergistic effect on mechanical allodynia induced by intraplantar injection of ET-1 and α,β-meATP in rats. Pharmacological blockade of P2X3 receptors also alleviated ET-1-induced mechanical allodynia. These results suggested that ET-1 sensitized P2X3 receptors in primary sensory neurons via an ET_AR and PKC signaling pathway. Our data provide evidence that cutaneous ET-1 induced mechanical allodynia not only by increasing the release of ATP from vascular endothelial cells, but also by sensitizing P2X3 receptors on nociceptive DRG neurons.