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

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 Ca 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 Ca response in DRG and in 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 Ca response because of degeneration of primary sensory nerve fibers in the skin. 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 models or small numbers of neurons. Using an innovative, distinctive, and interdisciplinary populational dorsal root ganglia (DRG) imaging (>1800 neurons/DRG) approach, we revealed increased DRG neuronal Ca activity from postoperative pain mouse model. This indicates widespread DRG primary sensory neuron plasticity. Increased neuronal Ca activity occurs among various sizes of neurons but mostly in small-diameter and medium-diameter nociceptors. Capsaicin pretreatment as a therapeutic option significantly attenuates Ca activity and postoperative pain.