Rapid cleavage of IL-1β in DRG neurons produces tissue injury-induced pain hypersensitivity.

Background: IL-1β plays a critical role in the pathophysiology of neuroinflammation. The presence of cleaved IL-1β (cIL-1β) in the neurons of the dorsal root ganglion (DRG) implicates its function in biological signaling arising from the sensory neuron. This study was conducted to analyze the role of IL-1β in nociceptive transduction after tissue injury. Methods: A plantar incision was made in C57BL/6 mice, following which immunohistochemistry and RNA scope in situ hybridization were performed at various time points to analyze cIL-1β, caspase-1, and IL-1 receptor 1 (IL-1R1) expression in the DRG. The effect of intrathecal administration of a caspase-1 inhibitor or regional anesthesia using local anesthetics on cIL-1β expression and pain hypersensitivity was analyzed by immunohistochemistry and behavioral analysis. ERK phosphorylation was also analyzed to investigate the effect of IL-1β on the activity of spinal dorsal horn neurons. Results: cIL-1β expression was significantly increased in caspase-1-positive DRG neurons 5 min after the plantar incision. Intrathecal caspase-1 inhibitor treatment inhibited IL-1β cleavage and pain hypersensitivity after the plantar incision. IL-1R1 was also detected in the DRG neurons, although the majority of IL-1R1-expressing neurons lacked cIL-1β expression. Regional anesthesia using local anesthetics prevented cIL-1β processing. Plantar incision-induced phosphorylation of ERK was inhibited by the caspase-1 inhibitor. Conclusion: IL-1β in the DRG neuron undergoes rapid cleavage in response to tissue injury in an activity-dependent manner. Cleaved IL-1β causes injury-induced functional activation of sensory neurons and pain hypersensitivity. IL-1β in the primary afferent neurons is involved in physiological nociceptive signal transduction.