Transplantation of chromaffin cells into the lumbar subarachnoid space has been found to produce analgesia, most conspicuously against chronic neuropathic pain. To ascertain the neurophysiological mechanism, we recorded electrical activity from wide-dynamic-range dorsal horn neurons in vivo, measuring the short-lasting homosynaptic facilitatory effect known as windup, which is induced by repetitive C-fiber input. Rats were given adrenal medulla allografts, or, as controls, striated-muscle allografts. The adrenal-transplanted rats showed analgesia 3-4 wk after transplantation, measured as a reduction in flinching reflexes 30-55 min after subcutaneous formalin injection. Recordings were made under halothane anesthesia, 3-7 days following the behavioral testing. The average C-fiber response and subsequent afterdischarge were facilitated severalfold in control rats by 1-Hz cutaneous electrical stimulation. Such facilitation was essentially absent in adrenal-transplanted animals and also in the A-fiber response of both preparations. Extirpation of transplanted tissue several hours prior to recording did not significantly affect this difference. In conclusion, the adrenal transplants block short-term spinal nociceptive facilitation, probably by stimulating some persistent cellular process that may be an important determinant, but not the only one, of their analgesic effect.
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