Intense peripheral electrical stimulation differentially inhibits tail vs. limb withdrawal reflexes in the rat

In an on-going study on mechanisms by which activation of sensory afferents regulates nociception, high-intensity, low-frequency electrical stimulation was applied to previously defined meridian and non-meridian points of the hindlimb or forelimb, and the effects measured on the withdrawal reflex of the tail or limb in the lightly anesthetized rat. Withdrawal was evoked by application of noxious radiant heat to the tip of the tail or to the plantar surface of a hindpaw or forepaw. Parameters of conditioning electrical stimulation were 2 ms pulses at 4 Hz for 20 min at 20 x threshold (20-30 mA) where threshold was the minimum intensity which evoked muscle twitch. In experiments on tail withdrawal, stimulation applied to meridian points fengshi (GB-31), femur-futu (ST-32) and zusanli (ST-36) of the hindlimb or to wai-kuan (TH-5) and hoku (LI-4) of the forelimb increased the latency of the withdrawal reflex to 70-100% of the maximum possible inhibition (MPI) during the stimulation. Inhibition persisted for more than 1 h after the end of stimulation. Bilateral stimulation of hindlimb meridian points evoked a greater inhibition during the stimulation (> 95% of the MPI); the inhibition persisted for 40 min. Stimulation of non-meridian sites in hindlimb or forelimb inhibited the withdrawal reflexes by 45-50% of the MPI during the stimulation only. Thus, the evoked inhibition has two components, a brief effect elicited by non-meridian point stimulation and a persistent post-stimulation effect produced only upon stimulation of meridian points. Stimulation produced little effect on nociceptive limb withdrawal reflexes. The results suggest that high-intensity, low-frequency electrical stimulation of meridian points produced a long-lasting, extrasegmental inhibition of the tail withdrawal but not of limb withdrawal reflexes. This differential inhibition may be due to differences in neuronal circuitry and CNS modulatory control mechanisms. The persistent inhibition appears to be dependent on the site of stimulation because it is not evoked by stimulation of sites outside of meridian points.