Mechanosensitive TRPV4 Channel-Induced Extracellular ATP Accumulation at the Acupoint Mediates Acupuncture Analgesia of Ankle Arthritis in Rats

Abstract

(1) Background: Acupuncture (AP) is a safe and effective analgesic therapy. Understanding how fine needles trigger biological signals can help us optimize needling manipulation to improve its efficiency. Adenosine accumulation in treated acupoints is a vital related event. Here, we hypothesized that extracellular ATP (eATP) mobilization preceded adenosine accumulation, which involved local activation of mechanosensitive channels, especially TRPV4 protein. (2) Methods: AP was applied at the injured-side Zusanli acupoint (ST36) of acute ankle arthritis rats. Pain thresholds were assessed in injured-side hindpaws. eATP in microdialysate from the acupoints was determined by luminescence assay. (3) Results: AP analgesic effect was significantly suppressed by pre-injection of GdCl3 or ruthenium red in ST36, the wide-spectrum inhibitors of mechanosensitive channels, or by HC067047, a specific antagonist of TRPV4 channels. Microdialysate determination revealed a needling-induced transient eATP accumulation that was significantly decreased by pre-injection of HC067047. Additionally, preventing eATP hydrolysis by pre-injection of ARL67156, a non-specific inhibitor of ecto-ATPases, led to the increase in eATP levels and the abolishment of AP analgesic effect. (4) Conclusions: These observations indicate that needling-induced transient accumulation of eATP, due to the activation of mechanosensitive TRPV4 channels and the activities of ecto-ATPases, is involved in the trigger mechanism of AP analgesia.

Activation of Subcutaneous Mast Cells in Acupuncture Points Triggers Analgesia

“…TRPV4 channels are mostly considered as pain sensors [ 55 , 56 ] to maintain mechanical allodynia via its mechanosensitivity [ 57 ]. In line with these findings, pharmacological inhibition of TRPV4 channels at the inflammatory site can relieve tactile allodynia, but not thermal hyperalgesia ( Figure 9 a) [ 54 ]. Interestingly, blockage of TRPV4 channels at the treated ST36 acupoint prevents acupuncture intervention to play anti-nociceptive function ( Figure 9 b) [ 54 ].…”

“…Nonlytic release of ATP can be mediated by some mechanosensitive channels, for example, Piezo channels [ 75 , 76 , 77 ] and TRPA1 channels [ 78 ], and also including TRPV4 channels [ 63 , 78 ], mentioned in Section 3.1.3 . In ankle AA rats, pharmacological inhibition of TRPV4 channels in treated ST36 in advance can suppress acupuncture-induced eATP increase ( Figure 15 ), which is accompanied by reduced analgesic effect, as we demonstrated in Figure 9 b [ 54 ].…”

“…In contrast to TRPV1 and 2, TRPV4 is not a thermo receptor [ 52 ]. The main characteristic of TRPV4 is its sensitivity to mechanical stress [ 53 ], which also makes TRPV4 a candidate to being activated by acupuncture [ 54 ]. Besides MCs (see above), TRPV4 mRNA also presents in the skin tissue of rat acupoints, rather than in the skeletal muscle layer [ 54 ].…”

“…The main characteristic of TRPV4 is its sensitivity to mechanical stress [ 53 ], which also makes TRPV4 a candidate to being activated by acupuncture [ 54 ]. Besides MCs (see above), TRPV4 mRNA also presents in the skin tissue of rat acupoints, rather than in the skeletal muscle layer [ 54 ]. TRPV4 channels are mostly considered as pain sensors [ 55 , 56 ] to maintain mechanical allodynia via its mechanosensitivity [ 57 ].…”

“…Rt-PCR determination in rats’ acupoints confirms the presences of multiple types of ecto-nucleotidases [ 54 , 74 ]. Among them, NTPDase 1 and Nt5e are the major nucleotide-hydrolyzing enzymes, which degrades ATP to AMP with intermediate of ADP and hydrolyzes AMP into Ado, respectively [ 84 ].…”