Activity-induced plasticity in brain stem pain modulatory circuitry after inflammation

Brain stem descending pathways modulate spinal nociceptive transmission. In a lightly anesthetized rat preparation, we present evidence that such desc ending modulation undergoes time-dependent changes following persistent hin dpaw inflammation. There was an initial decrease and a subsequent increase in the excitability of neurons in the rostral ventromedial medulla (RVM) in volving facilitation and inhibition. These changes were most robust after s timulation of the inflamed paw although similar findings were seen on the n on-inflamed paw and tail. The enhanced descending modulation appeared to be mediated by changes in the activation of the NMDA excitatory amino acid re ceptor. These findings demonstrate the dynamic plasticity of the pain modul ating pathways in response to persistent tissue injury. NeuroReport 11:1915 -1919 (C) 2000 Lippincott Williams & Wilkins.