Alterations in nociception following adrenal medullary transplants into the rat periaqueductal gray.

Adrenal medullary chromaffin cells were transplanted to the midbrain periaqueductal gray, a region known to play a primary role in the modulation of nociception. Chromaffin cells were chosen for transplantation since they contain several neuroactive substances (e.g. catecholamines, opioid peptides, other neuropeptides) whose release can be stimulated by pharmacological agents such as nicotine. Both dissected adult rat adrenal medullary tissue and bovine chromaffin cells served as graft tissue in the periaqueductal gray region of adult rats. When stimulated with a low dose of nicotine, potent analgesia was induced in these animals as assessed by tail flick, paw pinch and hot plate tests. The bovine chromaffin cell implants were more effective in inducing this response. The analgesia induced by nicotine stimulation in rats with adrenal medullary implants was partially attenuated by both opiate antagonist naloxone, and adrenergic antagonist phentolamine. These results suggest that the implantation of cells which release neuroactive substances can produce reductions in pain sensitivity.