Spinal allografts of adrenal medulla block nociceptive facilitation in thedorsal horn

Transplantation of chromaffin cells into the lumbar subarachnoid space has been found to produce analgesia, most conspicuously against chronic neuropa thic pain. To ascertain the neurophysiological mechanism, we recorded elect rical activity from wide-dynamic-range dorsal horn neurons in vivo, measuri ng the short-lasting homosynaptic facilitatory effect known as windup, whic h is induced by repetitive C-fiber input. Rats were given adrenal medulla a llografts, or, as controls, striated-muscle allografts. The adrenal-transpl anted rats showed analgesia 3-4 wk after transplantation, measured as a red uction in flinching reflexes 30-55 min after subcutaneous formalin injectio n. Recordings were made under halothane anesthesia, 3-7 days following the behavioral testing. The average C-fiber response and subsequent afterdischa rge were facilitated severalfold in control rats by 1-Hz cutaneous electric al stimulation. Such facilitation was essentially absent in adrenal-transpl anted animals and also in the A-fiber response of both preparations. Extirp ation of transplanted tissue several hours prior to recording did not signi ficantly affect this difference. In conclusion, the adrenal transplants blo ck 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.