(104) VGLUT3-containing primary muscle afferents are a unique subpopulation that respond to innocuous metabolites

Primary group III and IV muscle afferents respond to varying combinations of mechanical, thermal and chemical stimuli, and each subtype likely plays diverse roles in somatosensory processing from the muscles. While the basic characteristics of dorsal root ganglion (DRG) neurons that respond to specific peripheral stimuli in the muscles have been described, biomarkers for distinct subpopulations have yet to be identified. A previous report has shown that a subset of DRG neurons express the vesicular glutamate transporter 3 (VGLUT3), and these cells have been implicated in the development of mechanical allodynia after inflammation and nerve injury. We have also recently shown that alterations in mechanical and chemical responsiveness in muscle afferents may play an important role in the development of myalgia after ischemia/reperfusion injury. While VGLUT3 neurons have been widely studied in multiple tissues, their role in muscle afferents remains to be elucidated. Therefore, to determine the physiological function of VGLUT3-containing afferents innervating the muscles, we functionally and neurochemically characterized individual muscle sensory neurons using an ex vivo forepaw muscles/median and ulnar nerves/DRG/spinal cord recording preparation in transgenic mice that expresses a td-Tomato reporter protein in VGLUT3-containing cells. We found that 50% of group III/IV muscle afferents which respond to an innocuous metabolite mixture consisting of low ATP and lactic acid at pH 7.0 were VGLUT3 positive. Surprisingly, this was the only population of muscle sensory neurons found to express VGLUT3 no VGLUT3-positive cells responded to mechanical or heat stimuli, and only one of the innocuous metabolite responders also fired to cold. These results suggest that the VGLUT3-positive afferents innervating the muscles may play a distinct role from what has been reported in cutaneous afferents and could be the population of chemosensitive cells that are thought to be crucial in the development of ischemic muscle pain.