Potentiated expression of FOS protein in the rat spinal cord following bilateral noxious cutaneous stimulation.

A noxious mechanical or chemical stimulus to the ventral skin of one hindpaw induced the expression of FOS proteins ipsilaterally in the spinal dorsal horn neurons in the rat. The number of FOS-labelled cells reached a maximum at 2-3 h, and decayed to basal levels within 6 h after the stimulus. When a first noxious stimulus was applied to the contralateral hindpaw 1-1.5 h prior to this stimulus, the number of FOS-labelled cells increased, over all laminae, to 153% (mechanical) and 164% (chemical) compared to the number produced by a single stimulus. This effect of a prior stimulus in increasing the number of FOS-labelled cells produced by a contralateral stimulus persisted for several hours after the first stimulus. The results are interpreted as a sensitization of dorsal horn neurons induced by peripheral noxious stimuli, which is manifest at the molecular biological level.