A combined activity of peripheral nociceptive and nonnociceptive neurons is necessary to trigger spinal microglial reactivity and sustained pain.

Abnormal activity in injured nerve fibers triggers spinal microglial reactivity, a critical step in the development of chronic postoperative pain. It remains unclear whether spinal microglia respond to inputs from nociceptive fibers alone or require combined activity from nociceptors and nonnociceptors. Using electrical and optogenetic approaches, we selectively activated different fiber subtypes without causing neuronal injury. Our findings demonstrate that only simultaneous activation of both nociceptive and nonnociceptive fibers induces sustained hypersensitivity and a spinal microglia response. This response is characterized by increased cell proliferation, altered electrophysiological profile with hyperpolarized membrane potential, increased potassium currents, and a less ramified morphology. In contrast, activation of nociceptive or nonnociceptive fibers alone does not induce these changes. In addition, inhibiting spinal microglia with minocycline prevents those changes. Overall, our study suggests that both types of peripheral sensory input are necessary to elicit microglial reactivity in the spinal cord and the subsequent development of sustained pain-related behavior.