Long-lasting changes in rostral ventral medulla neuronal activity after inflammation

Activity-dependent synaptic plasticity occurs in many regions of the central nervous system. It is known that spinal sensory transmission undergoes long-lasting changes after tissue injury and inflammation, but much less is known about descending modulation. In the present study, we demonstrate that tissue injury causes long-lasting changes within the rostral ventral medulla, a region critical for descending modulation of spinal sensory transmission. Subcutaneous formalin injections induced changes in the activity of rostral ventral medulla neurons, particularly during phase 2 (10 to 55 minutes after injection). The activity of neutral cells, which showed no response to acute noxious stimuli, was significantly decreased after formalin injection. Furthermore, 2 "silent" cells became active after the formalin injection. To demonstrate directly that descending biphasic modulation from the rostral ventral medulla was affected after the inflammation, we investigated descending modulation of a spinal nociceptive reflex produced by focal electrical stimulation in the rostral ventral medulla and found that both facilitation and inhibition were significantly decreased. These data suggest that rostral ventral medulla circuits modulating spinal sensory transmission undergo profound and long-lasting changes after tissue injury and inflammation. This may contribute to the pathological modification of nociceptive processing in chronic pain states. (C) 2002 by the American Pain Society.