DESCENDING MODULATION OF CENTRAL NEURAL PLASTICITY IN THE FORMALIN PAIN TEST

Subcutaneous injection of formalin produces a biphasic profile of pain response: a transient early phase followed by a tonic late phase. A n umber of studies have indicated that the development of the late phase of formalin pain is dependent upon prolonged changes in central neura l function produced by neural activity that is generated during the ea rly phase (i.e. central sensitization). In support of this, the presen t study demonstrates that stimulation- or morphine-produced analgesia derived from the periaqueductal grey (FAG) during the early phase prev ents the development of the late phase. These results suggest that des cending mechanisms of pain inhibition, as reflected by FAG stimulation - and morphine-produced analgesia, can prevent the development of cent ral neural plasticity following injury.