Primary afferent depolarization evoked by electroacupuncture in the lumbar cord of the cat

In precollicular decerebrate cats, experiments were performed to ascertain the presence of primary afferent depolarization at the slowly conducting fiber terminals of the sural nerve, in an attempt to substantiate our previous postulation of a possible presynaptic mechanism underlying acupuncture analgesia in the spinal cord. A well correlated temporal course has been observed to exist between the negativity of dorsal root potential, suppression of sural polysynaptic reflexes, and increased excitability of sural primary afferent terminals, under the influence of the same electroacupuncture to the left tsusanli point in the hindlimb. Furthermore, by a collision test and conduction velocity measurement, the acupuncture-evoked primary afferent depolarization thus indicated was found to occur solely at the terminals of the slowly conducting fibers of the sural nerve, fibers believed to transmit “pain” impulses. As primary afferent depolarization has powerful inhibitory actions and its existence is well demonstrated in the spinal cord and trigeminal system, we suggest that acupuncture can also utilize this well established mechanism in modulating “pain” information at the primary afferent level in the spinal cord. This spinal presynaptic inhibitory mechanism, however, is thought to be only a part of an overall process underlying the production of acupuncture analgesia.