MK-801, an NMDA receptor antagonist, in the rostroventromedial medulla attenuates development of neuropathic symptoms in the rat.

Segmental ligation of spinal nerves in the rat induces a long-lasting hyperalgesia and allodynia that mimicks neuropathic conditions in humans. In the present study we attempted to determine whether supraspinal NMDA receptors contribute to the induction of the long-lasting hypersensitivity to noxious and innocuous mechanical stimulation following segmental ligation of spinal nerves in the rat. MK-801, an NMDA receptor antagonist, was microinjected into the rostroventromedial medulla (RVM) 15 min before or 25 min after the ligation of spinal nerves and mechanical hypersensitivity was assessed at various time points following surgery by determining the hindlimb withdrawal threshold to noxious and innocuous mechanical stimulation. A single dose of MK-801 administered prior to nerve ligation into the RVM significantly attenuated the development of mechanical hypersensitivity throughout the 2 week postoperative observation period, whereas corresponding administration of MK-801 immediately after the nerve ligation attenuated the development of mechanical hypersensitivity only during the first postoperative day but not later. The results indicate that NMDA receptors in the RVM are involved in triggering the enhanced sensitivity to mechanical stimulation induced by a nerve injury.