Nociceptive inhibition of withdrawal reflex responses increases over time in spinalized rats.

Time-dependent changes of nociceptive inhibition of hindlimb withdrawal reflex (WR) pathways were investigated in acute decerebrate spinal rats. Electromyographic recordings of WR of peroneus longus and extensor digitorum longus muscles were made. For each muscle, both short and long latency WR evoked by subcutaneous electrical stimulation and WR evoked by noxious pinch could be inhibited by noxious CO2 laser stimulation of a specific skin area of the ipsilateral hindlimb. The inhibitory effects and size of the inhibitory receptive fields increased spontaneously between two- and three-fold during the first 12 h after spinalization. The functional and possible clinical significance of the strengthened nociceptive inhibitory control and mechanisms underlying the changes of this control after spinal transection are discussed.