Sensing acidosis: nociception or sngception?

Abstract

Background: Sensing tissue acidosis is an important function of the somatosensory nervous system to response to noxious stimuli.

Main body: In the pain clinic, acid or soreness sensation is a characteristic sensory phenotype of various acute and chronic pain syndromes, such as delayed onset muscle soreness, fibromyalgia, and radicular pain. However, soreness sensation is a sign of successful analgesia for acupuncture and noxipoint therapy. Thus, the nature of acid or soreness sensation is not always nociceptive (or painful) and could be anti-nociceptive. To facilitate the investigation of the molecular and neurobiological mechanisms of soreness sensation, we propose a concept called "sngception (sng- ception)" to describe the response of the somatosensory nervous system to sense tissue acidosis and to distinguish it from nociception. "Sng" is a Taiwanese word that represents the state of soreness while at the same time imitates the natural vocalization of humans feeling sore.

Conclusion: Here we propose sngception as a specific somatosensory function that transmits the acid sensation from the peripheral to the central nervous system. Sngception could partially overlap with nociception, but it could also transmit antinociception, proprioception, and pruriception.

Keywords: ASIC3; Acidosis; Nociception; Pain; Sngception; Soreness.

Fig. 1

Molecular determinants that sense tissue acidosis. Proton-sensing ion channels and/or receptors expressed in nociceptors include members of acid-sensing ion channels (ASICs), transient receptor potential (TRP) channels, two-pore potassium channels (K2Ps), and protein-sensing G-protein-coupled receptors such as G2A, GRP4, OGR1, and TDAG8

Fig. 2

Soreness hypothesis. Tissue acidosis simultaneously evokes a pro-nociceptive signaling via ASIC3/TRPV1 activation and an anti-nociception via non-ASIC3, non-TRPV1 acid signaling. In most cases, pain associated with tissue acidosis is the balanced result of pro-nociceptive and anti-nociceptive acid signaling. In physical therapy, soreness sensation induced by acupuncture and/or noxipoint therapy is a sign of successful analgesia to balance the pro-nociceptive signaling. Fibromyalgia patients might have soreness phenotypes due to an imbalance of acid signaling in muscle nociceptors, especially with impaired non-ASIC3, non-TRPV1 acid signaling