In Vivo Calcium Imaging Visualizes Incision-Induced Primary Afferent Sensitization and Its Amelioration by Capsaicin...

Previous studies have shown that infiltration of capsaicin into the surgical site can prevent incision-induced spontaneous pain like behaviors and heat hyperalgesia. In the present study, we aimed to monitor primary sensory neuron Ca²⁺ activity in the intact dorsal root ganglia (DRG) using Pirt-GCaMP3 male and female mice pretreated with capsaicin or vehicle before the plantar incision. Intraplantar injection of capsaicin (0.05%) significantly attenuated spontaneous pain, mechanical, and heat hypersensitivity after plantar incision. The Ca²⁺ response in in vivo DRG and in in situ spinal cord was significantly enhanced in the ipsilateral side compared with contralateral side or naive control. Primary sensory nerve fiber length was significantly decreased in the incision skin area in capsaicin-pretreated animals detected by immunohistochemistry and placental alkaline phosphatase (PLAP) staining. Thus, capsaicin pretreatment attenuates incisional pain by suppressing Ca²⁺ response because of degeneration of primary sensory nerve fibers in the skin.SIGNIFICANCE STATEMENT Postoperative surgery pain is a major health and economic problem worldwide with ∼235 million major surgical procedures annually. Approximately 50% of these patients report uncontrolled or poorly controlled postoperative pain. However, mechanistic studies of postoperative surgery pain in primary sensory neurons have been limited to in vitro models or small numbers of neurons. Using an innovative, distinctive, and interdisciplinary in vivo populational dorsal root ganglia (DRG) imaging (>1800 neurons/DRG) approach, we revealed increased DRG neuronal Ca²⁺ activity from postoperative pain mouse model. This indicates widespread DRG primary sensory neuron plasticity. Increased neuronal Ca²⁺ activity occurs among various sizes of neurons but mostly in small-diameter and medium-diameter nociceptors. Capsaicin pretreatment as a therapeutic option significantly attenuates Ca²⁺ activity and postoperative pain.