Multimodal Enhancement of Colonic Motility by Acupuncture at ST36 is Mediated by TRPV1+ Cutaneous Sensory Fibers

Background: Acupuncture has been shown to promote gastrointestinal motility. This study explores whether cutaneous transient receptor potential vanilloid 1 (TRPV1)+ fibers at Zusanli (ST36) acupoint can mediate the multimodal effects of acupuncture on colorectal motility, as well as examining their mechanistic role. Methods: C57BL/6 mice were subjected to electroacupuncture (EA), manual acupuncture (MA), 46 °C thermal stimulation, and 1% capsaicin at the ST36 acupoint. Colon motility was quantified via the area under the curve (AUC) and contraction amplitude. Immunofluorescent co-localization of TRPV1 with CGRP, NF200, peripherin, and tyrosine hydroxylase (TH) was conducted in TrpV1Cre mice to determine neural phenotypic subtypes. Furthermore, TrpV1ChR2-eYFP and TrpV1NpHR-eYFP transgenic mice that underwent optogenetic activation or silencing of local TRPV1+ fibers at ST36 were evaluated for acupuncture-like stimulation effects on colorectal AUC and amplitude. Results: All applied stimuli in C57BL/6 mice significantly increased colorectal motility parameters (AUC and amplitude, p < 0.05) compared to baseline. TRPV1+ somatosensory neurons in the dorsal root ganglion (DRG) predominantly co-expressed with peripherin (46.76%) and CGRP (27%), which are markers of unmyelinated peptidergic fibers, but rarely with NF200 (6%) or TH (< 1%). Optogenetic activation (30 mW blue light) of TRPV1+ fibers in TrpV1ChR2-eYFP mice mimicked acupuncture-like stimuli, with significantly enhanced colorectal AUC and amplitude (p < 0.05). In contrast, optogenetic silencing of TRPV1+ fibers with yellow light abolished acupuncture-like stimulation of colorectal motility in TrpV1NpHR-eYFP mice (p < 0.05). Conclusion: Through the use of spatiotemporally precise optogenetic control, our study revealed that TRPV1+ sensory fibers at ST36 are the major convergent pathway for multimodal (electrical/mechanical/thermal/chemical) enhancement of colorectal motility by acupuncture.