Acidification of rat TRPV1 alters the kinetics of capsaicin responses. Mol Pain 1:28

Neuroscience Research, Global Pharmaceutical Research and Development, Abbott Laboratories, R04PM, AP9A, Abbott Park, IL 60064-6123, USA.
Molecular Pain (Impact Factor: 3.65). 02/2005; 1(1):28. DOI: 10.1186/1744-8069-1-28
Source: PubMed


TRPV1 (vanilloid receptor 1) receptors are activated by a variety of ligands such as capsaicin, as well as by acidic conditions and temperatures above 42 degrees C. These activators can enhance the potency of one another, shifting the activation curve for TRPV1 to the left. In this study, for example, we observed an approximately 10-fold shift in the capsaicin EC50 (640 nM to 45 nM) for rat TRPV1 receptors expressed in HEK-293 cells when the pH was lowered from 7.4 to 5.5. To investigate potential causes for this shift in capsaicin potency, the rates of current activation and deactivation of whole-cell currents were measured in individual cells exposed to treatments of pH 5.5, 1 microM capsaicin or in combination. Acidic pH was found to both increase the activation rate and decrease the deactivation rate of capsaicin-activated currents providing a possible mechanism for the enhanced potency of capsaicin under acidic conditions. Utilizing a paired-pulse protocol, acidic pH slowed the capsaicin deactivation rate and was readily reversible. Moreover, the effect could occur under modestly acidic conditions (pH 6.5) that did not directly activate TRPV1. When TRPV1 was maximally activated by capsaicin and acidic pH, the apparent affinity of the novel and selective capsaicin-site competitive TRPV1 antagonist, A-425619, was reduced approximately 35 fold. This shift was overcome by reducing the capsaicin concentration co-applied with acidic pH. Since inflammation is associated with tissue acidosis, these findings enhance understanding of TRPV1 receptor responses in inflammatory pain where tissue acidosis is prevalent.

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    • "We reviewed literature regarding EC50 of capsaicin and acid for TRPV1. Under in vitro conditions at 37°C, EC50 for capsaicin at pH 7.4 is 640 nM and EC50 for acid stimulation is pH 5.35 [70]. The ratio for capsaicin used over capsaicin’s EC50 is approximately 2.6 × 104, whereas the ratio for acid saline used over its EC50 is much smaller (2.2–2.2 × 102). "
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    ABSTRACT: Acupuncture is a therapy that involves applying mechanical stimulation to acupoints using needles. Although acupuncture is believed to trigger neural regulation by opioids or adenosine, still little is known about how physical stimulation is turned into neurological signaling. The transient receptor potential vanilloid receptors 1 and 4 (TRPV1 and TRPV4) and the acid-sensing ion channel 3 (ASIC3) are regarded as mechanosensitive channels. This study aimed to clarify their role at the Zusanli acupoint (ST36) and propose possible sensing pathways linking channel activation to neurological signaling. First, tissues from different anatomical layers of ST36 and the sham point were sampled, and channel expressions between the two points were compared using western blotting. Second, immunofluorescence was performed at ST36 to reveal distribution pattern of the channels. Third, agonist of the channels were injected into ST36 and tested in a mouse inflammatory pain model to seek if agonist injection could replicate acupuncture-like analgesic effect. Last, the components of proposed downstream sensing pathway were tested with western blotting to determine if they were expressed in tissues with positive mechanosensitive channel expression. The results from western blotting demonstrated an abundance of TRPV1, TRPV4, and ASIC3 in anatomical layers of ST36. Furthermore, immunofluorescence showed these channels were expressed in both neural and non-neural cells at ST36. However, only capsaicin, a TRPV1 agonist, replicated the analgesic effect of acupuncture when injected into ST36. Components of calcium wave propagation (CWP, the proposed downstream sensing pathway) were also expressed in tissues with abundant TRPV1 expression, the muscle and epimysium layers. The results demonstrated mechanosensitive channel TRPV1 is highly expressed at ST36 and possibly participated in acupuncture related analgesia. Since CWP was reported by other to occur during acupuncture and its components were shown here to express in tissues with positive TRPV1 expression. These findings suggest TRPV1 might act as acupuncture-responding channel by sensing physical stimulation from acupuncture and conducting the signaling via CWP to nerve terminals. This study provided a better understanding between physical stimulation from acupuncture to neurological signaling.
    Full-text · Article · Mar 2014 · BMC Complementary and Alternative Medicine
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    • "Therefore, we have assumed that a single injection of pH 4.0 saline would not extremely decrease the tissue pH, and further considered that temporary acidic pH tissue condition induced by pH 4.0 saline could mimic the pathophysiological state shown in peripheral ischemic condition. Collectively, the acidic environment is easily buffered by physiological buffering systems [15,21,43], it may be difficult to activate TRPV1 receptors directly, since they only respond under relatively low tissue pH (pH < 5–6) [44-46]. Therefore, there is the strong possibility that the interaction of the acidic environment and ATP indirectly activate TRPV1 via P2Y1 receptors which up-regulates intracellular signaling cascades, but this only occurs in an ischemic environment in which tissue acidosis is present. "
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    ABSTRACT: We previously developed a thrombus-induced ischemic pain (TIIP) animal model, which was characterized by chronic bilateral mechanical allodynia without thermal hyperalgesia (TH). On the other hand we had shown that intraplantar injection of acidic saline facilitated ATP-induced pain, which did result in the induction of TH in normal rats. Because acidic pH and increased ATP are closely associated with ischemic conditions, this study is designed to: (1) examine whether acidic saline injection into the hind paw causes the development of TH in TIIP, but not control, animals; and (2) determine which peripheral mechanisms are involved in the development of this TH. Repeated intraplantar injection of pH 4.0 saline, but not pH 5.5 and 7.0 saline, for 3 days following TIIP surgery resulted in the development of TH. After pH 4.0 saline injections, protein levels of hypoxia inducible factor-1alpha (HIF-1alpha) and carbonic anhydrase II (CA II) were elevated in the plantar muscle indicating that acidic stimulation intensified ischemic insults with decreased tissue acidity. At the same time point, there were no changes in the expression of TRPV1 in hind paw skin, whereas a significant increase in TRPV1 phosphorylation (pTRPV1) was shown in acidic saline (pH 4.0) injected TIIP (AS-TIIP) animals. Moreover, intraplantar injection of chelerythrine (a PKC inhibitor) and AMG9810 (a TRPV1 antagonist) effectively alleviated the established TH. In order to investigate which proton- or ATP-sensing receptors contributed to the development of TH, amiloride (an ASICs blocker), AMG9810, TNP-ATP (a P2Xs antagonist) or MRS2179 (a P2Y1 antagonist) were pre-injected before the pH 4.0 saline. Only MRS2179 significantly prevented the induction of TH, and the increased pTRPV1 ratio was also blocked in MRS2179 injected animals. Collectively these data show that maintenance of an acidic environment in the ischemic hind paw of TIIP rats results in the phosphorylation of TRPV1 receptors via a PKC-dependent pathway, which leads to the development of TH mimicking what occurs in chronic ischemic patients with severe acidosis. More importantly, peripheral P2Y1 receptors play a pivotal role in this process, suggesting a novel peripheral mechanism underlying the development of TH in these patients.
    Full-text · Article · Jan 2014 · Molecular Pain
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    • " activating effect of low pH o on TRPV1 activity . Most in vitro studies have not investigated this possibility , because they were designed to study the effects of acidosis on TRPV1 activity using a whole - cell patch - clamp configuration with a Hepes ( 5 – 10 mM ) - containing pipette solution ( Tominaga et al . 1998 ; Ryu et al . 2003 , 2007 ; Neelands et al . 2005 ) . Hepes is a potent acid – base buffer , so pH i changes induced by extracellular acidosis can be artificially prevented by a Hepes - containing pipette solution . Therefore , to determine the effects of extracellular acidosis on TRPV1 activity more precisely , it is essential to examine the effect of extracellular acidosis on TRPV1 i"
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    ABSTRACT: Transient receptor potential V1 (TRPV1) has been suggested to play an important role in detecting decreases in extracellular pH (pH(o)). Results from recent in vivo studies, however, have suggested that TRPV1 channels play less of a role in sensing a moderately acidic pH(o) (6.0 < pH < 7.0) than predicted from the in vitro experiments. A clear explanation for this discrepancy between the in vitro and in vivo data has not yet been provided. We report here that intracellular acidification induced by a moderately low pH(o) (6.4) almost completely inhibited the effect of extracellular acidosis on TRPV1 activity. In our experiments, sodium acetate (20 mm), which was used to induce intracellular acidosis, attenuated the capsaicin-evoked TRPV1 current (I(CAP)) in a reversible manner in whole-cell patch-clamp mode and shifted the concentration-response curve to the right. Likewise, the concentration-response curve was significantly shifted to the right by lowering the pH of the pipette solution from 7.2 to 6.5. In addition, application of an acidic bath solution (pH 6.4) to the intracellular side also significantly suppressed I(CAP) in inside-out patch mode. In cell-attached patch mode, the single-channel activity of i(CAP) was significantly attenuated by intracellular acidosis that was induced by a decrease in pH(o) (6.4). These results suggested that intracellular acidification induced by a low pH(o) inhibited TRPV1 activity. When studied in perforated patch mode or by acidifying the intracellular pipette solution, potentiation or activation of TRPV1 by extracellular acidosis (pH 6.4) at 37 °C was almost completely inhibited. Likewise, enhancement of neuronal excitability by a moderately acidic pH(o) (6.4) at a physiological temperature (37 °C) was attenuated by lowering the pH of the pipette solution to 6.5 or using perforated patch mode. Taken together, these results suggest that extracellular acidosis of moderate intensity may not significantly modulate TRPV1 activity in physiological conditions at which intracellular pH can be readily affected by pH(o), and this phenomenon is due to attenuation of TRPV1 channel activity by low-pH(o)-induced intracellular acidification.
    Full-text · Article · Sep 2011 · Experimental physiology
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