Arachidonic acid potentiates acid-sensing ion channels in rat sensory neurons by a direct action.
ABSTRACT Acid-sensing ion channels (ASICs) are activated by a decrease in extracellular pH. ASICs are expressed in nociceptive sensory neurons, and several lines of evidence suggest that they are responsible for signaling the pain caused by extracellular acidification, but little is understood of the modulation of ASICs by pro-inflammatory factors. Using whole-cell patch clamp we demonstrate that low pH evokes three distinct inward currents in rat dorsal root ganglion neurons: a slowly inactivating transient current, a rapidly inactivating transient current, and a sustained current. All three currents were potentiated by arachidonic acid (AA), to 123%, 171%, and 264% of peak current, respectively. Membrane stretch had no effect on proton-gated currents, implying that AA is unlikely to act via local membrane deformation. The current carried by heterologously expressed ASIC1a and ASIC3 was also potentiated by AA. AA potentiates ASIC activation by a direct mechanism, because inhibition of AA metabolism had no effect on potentiation, and potentiation of single ASIC2a channels could be observed in cell-free patches. Potentiation by lipids with the same chain length as AA increased as the number of double bonds was increased. AA is known to be released in inflammation and the results suggest that AA may be an important pro-inflammatory agent responsible for enhancing acid-mediated pain.
Article: ASIC3 channels integrate agmatine and multiple inflammatory signals through the nonproton ligand sensing domain.[show abstract] [hide abstract]
ABSTRACT: Acid-sensing ion channels (ASICs) have long been known to sense extracellular protons and contribute to sensory perception. Peripheral ASIC3 channels represent natural sensors of acidic and inflammatory pain. We recently reported the use of a synthetic compound, 2-guanidine-4-methylquinazoline (GMQ), to identify a novel nonproton sensing domain in the ASIC3 channel, and proposed that, based on its structural similarity with GMQ, the arginine metabolite agmatine (AGM) may be an endogenous nonproton ligand for ASIC3 channels. Here, we present further evidence for the physiological correlation between AGM and ASIC3. Among arginine metabolites, only AGM and its analog arcaine (ARC) activated ASIC3 channels at neutral pH in a sustained manner similar to GMQ. In addition to the homomeric ASIC3 channels, AGM also activated heteromeric ASIC3 plus ASIC1b channels, extending its potential physiological relevance. Importantly, the process of activation by AGM was highly sensitive to mild acidosis, hyperosmolarity, arachidonic acid (AA), lactic acid and reduced extracellular Ca2+. AGM-induced ASIC3 channel activation was not through the chelation of extracellular Ca2+ as occurs with increased lactate, but rather through a direct interaction with the newly identified nonproton ligand sensing domain. Finally, AGM cooperated with the multiple inflammatory signals to cause pain-related behaviors in an ASIC3-dependent manner. Nonproton ligand sensing domain might represent a novel mechanism for activation or sensitization of ASIC3 channels underlying inflammatory pain-sensing under in vivo conditions.Molecular Pain 01/2010; 6:88. · 3.53 Impact Factor
Article: Acid-sensing ion channel 3 mediates peripheral anti-hyperalgesia effects of acupuncture in mice inflammatory pain.[show abstract] [hide abstract]
ABSTRACT: Peripheral tissue inflammation initiates hyperalgesia accompanied by tissue acidosis, nociceptor activation, and inflammation mediators. Recent studies have suggested a significantly increased expression of acid-sensing ion channel 3 (ASIC3) in both carrageenan- and complete Freund's adjuvant (CFA)-induced inflammation. This study tested the hypothesis that acupuncture is curative for mechanical hyperalgesia induced by peripheral inflammation. Here we used mechanical stimuli to assess behavioral responses in paw and muscle inflammation induced by carrageenan or CFA. We also used immunohistochemistry staining and western blot methodology to evaluate the expression of ASIC3 in dorsal root ganglion (DRG) neurons. In comparison with the control, the inflammation group showed significant mechanical hyperalgesia with both intraplantar carrageenan and CFA-induced inflammation. Interestingly, both carrageenan- and CFA-induced hyperalgesia were accompanied by ASIC3 up-regulation in DRG neurons. Furthermore, electroacupuncture (EA) at the ST36 rescued mechanical hyperalgesia through down-regulation of ASIC3 overexpression in both carrageenan- and CFA-induced inflammation. In addition, electrical stimulation at the ST36 acupoint can relieve mechanical hyperalgesia by attenuating ASIC3 overexpression.Journal of Biomedical Science 11/2011; 18:82. · 2.01 Impact Factor
Article: Prokineticin 2 potentiates acid-sensing ion channel activity in rat dorsal root ganglion neurons.[show abstract] [hide abstract]
ABSTRACT: Prokineticin 2 (PK2) is a secreted protein and causes potent hyperalgesia in vivo, and is therefore considered to be a new pronociceptive mediator. However, the molecular targets responsible for the pronociceptive effects of PK2 are still poorly understood. Here, we have found that PK2 potentiates the activity of acid-sensing ion channels in the primary sensory neurons. In the present study, experiments were performed on neurons freshly isolated from rat dorsal root ganglion by using whole-cell patch clamp and voltage-clamp recording techniques. PK2 dose-dependently enhanced proton-gated currents with an EC₅₀ of 0.22 ± 0.06 nM. PK2 shifted the proton concentration-response curve upwards, with a 1.81 ± 0.11 fold increase of the maximal current response. PK2 enhancing effect on proton-gated currents was completely blocked by PK2 receptor antagonist. The potentiation was also abolished by intracellular dialysis of GF109203X, a protein kinase C inhibitor, or FSC-231, a protein interacting with C-kinase 1 inhibitor. Moreover, PK2 enhanced the acid-evoked membrane excitability of rat dorsal root ganglion neurons and caused a significant increase in the amplitude of the depolarization and the number of spikes induced by acid stimuli. Finally, PK2 exacerbated nociceptive responses to the injection of acetic acid in rats. These results suggest that PK2 increases the activity of acid-sensing ion channels via the PK2 receptor and protein kinase C-dependent signal pathways in rat primary sensory neurons. Our findings support that PK2 is a proalgesic factor and its signaling likely contributes to acidosis-evoked pain by sensitizing acid-sensing ion channels.Journal of Neuroinflammation 05/2012; 9:108. · 3.83 Impact Factor