Abundant expression and functional participation of TRPV1 at Zusanli acupoint (ST36) in mice: mechanosensitive TRPV1 as an “acupuncture-responding channel”

Abstract

BackgroundAcupuncture 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.MethodsFirst, 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.ResultsThe 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.ConclusionsThe 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.

Peripheral and Spinal Mechanisms of Pain and Dry Needling Mediated Analgesia: A Clinical Resource Guide for Health Care Professionals

“…Interestingly, TRPv1 receptors, TRPv4 receptors and ASIC3 channels are sensitive to changes in PH and/or temperature, but these channels are also responsive to mechanical stimulation [81,82]. The mechanical stimulation provided by DN may therefore have the ability to influence the very same channels responsible for sensing pain and inflammation and mediating hypersensitivity.…”

“…The mechanical stimulation provided by DN may therefore have the ability to influence the very same channels responsible for sensing pain and inflammation and mediating hypersensitivity. Wu et al used immunofluorescence to demonstrate the presence of more TRPv1 receptors and ASIC3 channels at ST36 (proximal tibialis anterior) compared to a nonacupoint location [82]. However, pharmaceutical stimulation revealed that only the TRPv1 agonist reproduced the analgesic effects of DN at ST36 [82].…”

“…Of the 4 primary channels/receptors that play a role in peripheral hyperalgesia, TRP receptors are arguably the most intriguing given their possible role in DN-mediated analgesia [81,82]. Interestingly, TRPv1 receptors, TRPv4 receptors and ASIC3 channels are sensitive to changes in PH and/or temperature, but these channels are also responsive to mechanical stimulation [81,82].…”

“…The noxious stimulus-induced analgesia (NSIA) from DN also likely stimulates the nucleus accumbens, resulting in opioid release. The subsequent stimulation of μ and κ opioid receptors on interneurons in the dorsal horn results in the release of GABA and glycine, further inhibiting the transmission of pain information from sensory afferents to second order dorsal horn neurons [82,88] Thus, the stimulation of TRPv1 receptors via tissue mechanotransduction may help explain the physiologic mechanism responsible for DN-mediated peripheral analgesia. Given this mechanism, it is perhaps relevant to point out that most acupuncturists consider de qi to be a crucial parameter in effective needling treatments for pain [89,90].…”

“…According to Wu et al [82] the mechanotransduction of tissue stimulates TRPv1 receptors on both neural and non-neural cells, resulting in intracellular calcium wave propagation (CWP). The CWP causes the release of ATP from pannexin-1 hemi-channels.…”