Sensitization of pain-modulating neurons in the rostral ventromedial medulla after peripheral nerve injury.

Nerve injury can lead to mechanical hypersensitivity in both humans and animal models, such that innocuous touch produces pain. Recent functional studies have demonstrated a critical role for descending pain-facilitating influences from the rostral ventromedial medulla (RVM) in neuropathic pain, but the underlying mechanisms and properties of the relevant neurons within the RVM are essentially unknown. We therefore characterized mechanical responsiveness of physiologically characterized neurons in the RVM after spinal nerve ligation, a model of neuropathic pain that produces robust mechanical hyperalgesia and allodynia. RVM neurons were studied 7-14 d after spinal nerve ligation, and classified as "on-cells," "off-cells," or "neutral cells" using standard criteria of changes in firing associated with heat-evoked reflexes. On-cells are known to promote nociception, and off-cells to suppress nociception, whereas the role of neutral cells in pain modulation remains an open question. Neuronal and behavioral responses to innocuous and noxious mechanical stimulation were tested using calibrated von Frey filaments (4-100 g) applied to the hindpaws ipsilateral and contralateral to the injury, and in sham-operated and unoperated control animals. On- and off-cells recorded in nerve-injured animals exhibited novel responses to innocuous mechanical stimulation, and enhanced responses to noxious mechanical stimulation. Neuronal hypersensitivity in the RVM was correlated with behavioral hypersensitivity. Neutral cells remained unresponsive to cutaneous stimulation after nerve injury. These data demonstrate that both on- and off-cells in the RVM are sensitized to innocuous and noxious mechanical stimuli after nerve injury. This sensitization likely contributes to allodynia and hyperalgesia of neuropathic pain states.