INTENSE PERIPHERAL ELECTRICAL-STIMULATION EVOKES BRIEF AND PERSISTENTINHIBITION OF THE NOCICEPTIVE TAIL WITHDRAWAL REFLEX IN THE RAT

In a study of modulation of nociception by sensory inputs, electrical stimulation was applied to specific sites in the hindlimb and effects on the nociceptive tail withdrawal reflex were monitored in the lightl y anaesthetized rat. Stimulation was applied to previously defined sit es in the hindlimb, meridian points femur-futu (ST-32), fengshi (GB-31 ) and zusanli (ST-36). It consisted of a 4 Hz train of 2 ms square pul ses given for 20 min at 20 X the threshold intensity required for musc le twitch. Tail withdrawal was provoked by application of a noxious he at stimulus applied to the tip of the tail. Results were expressed as a percentage of the maximal possible inhibition which is achieved when the post-treatment latency is 2 X the pre-treatment latency otherwise known as the cut off. During stimulation, the latency of the withdraw al increased to similar to 70% of the maximal possible inhibition. Fol lowing stimulation, the inhibition persisted for > 1 h. Stimulation at 2 or 6 Hz elicited similar effects but stimulation at 8 Hz evoked inh ibition during the stimulation only. Stimulation applied to sites away from defined meridian points inhibited tail withdrawal during the sti mulation; no post-stimulation effect was produced. In acutely transect ed animals (less than or equal to 48 h), stimulation of meridian point s elicited a small, brief increase in latency but during stimulation o nly. At 7 and 14 days after spinal transection, this response during s timulation was greater in magnitude and a brief post-stimulation incre ase was also observed. The return of the of this latter effect was coi ncident with the return of bladder function. These data suggest that h igh intensity, low frequency electrical stimulation of hindlimb meridi an points in the lightly anaesthetized rat produces both brief and per sistent inhibitory effects on the nociceptive tail withdrawal reflex. These effects appear to be elicited by different mechanisms. The persi stent effect may represent a plastic change in central inhibitory mech anisms. Data from spinal animals indicate a major participation of sup raspinal structures but that spinal mechanisms are also capable of sus taining both types of effect. (C) 1997 Elsevier Science B.V.