Multimodal Enhancement of Colonic Motility by Acupuncture at ST-36 is Mediated by TRPV1+ Cutaneous Sensory Fibers

This experimental study investigated the mechanisms by which acupuncture at the ST-36 (Zusanli) point improves colorectal motility, specifically focusing on the role of TRPV1 (Transient Receptor Potential Vanilloid 1) receptors. The ST-36 point is traditionally used in Chinese medicine to treat gastrointestinal disorders, but the underlying neural mechanisms remained poorly understood. The researchers used an innovative approach combining optogenetic techniques with transgenic animal models to investigate, with temporal and spatial precision, the role of TRPV1+ sensory fibers in mediating the effects of acupuncture. The study used 21 mice divided into different strains: C57BL/6 mice as control, TrpV1Cre mice for molecular characterization, TrpV1ChR2-eYFP for selective optogenetic activation, and TrpV1NpHR-eYFP for optogenetic inhibition.

The methodology included four types of stimulation at the ST-36 point: electroacupuncture (1 mA, 10 Hz), manual acupuncture, thermal stimulation (46°C), and capsaicin application (1%). Colorectal motility was quantified through area under the curve (AUC) and contraction amplitude using an intracolonic balloon system connected to pressure transducers. The results demonstrated that all stimulation modalities significantly increased colorectal motility in C57BL/6 mice. Electroacupuncture showed the most robust effects, with a 376% increase in AUC (p = 0.0174), followed by manual acupuncture (243%, p = 0.0313), thermal stimulation (200%, p = 0.0313), and capsaicin (201%, p = 0.0313).

Immunohistochemical characterization revealed that TRPV1+ neurons in the dorsal root ganglion predominantly coexpress markers of unmyelinated nociceptive fibers: 27% with CGRP (calcitonin gene-related peptide) and 47% with peripherin, but rarely with NF200 (6%) or tyrosine hydroxylase (<1%). This establishes TRPV1 as a specific marker for small- to medium-diameter nociceptive neurons. Selective optogenetic activation of TRPV1+ fibers with blue light (473 nm, 30 mW) in TrpV1ChR2-eYFP mice mimicked the effects of acupuncture, producing significant increases in colorectal motility (p < 0.05). Conversely, optogenetic inhibition with yellow light (593 nm, 30 mW) in TrpV1NpHR-eYFP mice significantly attenuated the effects of all stimulation modalities, reducing efficacy by 65-82% depending on the type of stimulus.

Quantitative analysis showed that TRPV1 contributes differentially to the effects of each modality: electroacupuncture (65% contribution), manual acupuncture (65%), thermal stimulation (78%), and capsaicin (82%). The clinical implications are substantial. The study provides mechanistic evidence that TRPV1+ sensory fibers constitute a convergent neural pathway for multiple modalities of acupuncture stimulation. This suggests that the therapeutic efficacy of acupuncture for functional gastrointestinal disorders can be optimized through targeted modulation of these receptors.

The finding that different stimulation modalities have varying contributions from TRPV1 may inform therapeutic personalization strategies. Limitations include the use of isoflurane anesthesia, which may influence autonomic tone, the limited penetration of transcutaneous optical stimulation, and the absence of direct electrophysiological recordings to quantify the efficiency of optogenetic inhibition. Future studies should employ orthogonal approaches such as selective antagonists and chemogenetic models for additional validation, in addition to investigating complete neural circuits in awake models.