ATTENUATION OF FORMALIN PAIN RESPONSES IN THE RAT BY ADRENAL-MEDULLARY TRANSPLANTS IN THE SPINAL SUBARACHNOID SPACE

Previous reports have indicated that the implantation of adrenal medul lary chromaffin cells into the spinal subarachnoid space can reduce bo th acute and chronic pain in several animal models. Recent findings su ggest that acute and chronic pain alleviation may be mediated by disti nct mechanisms. Since the formalin response is composed of an acute an d tonic phase which can be pharmacologically distinguished, the abilit y of adrenal medullary implants to alter these responses was assessed. In rats with adrenal medullary transplants, both phases of the formal in response were attenuated, in contrast to control implanted animals. Suppression of the acute phase by adrenal medullary implants was reve rsed by the opiate antagonist naloxone, and partially reversed by the a-adrenergic antagonist phentolamine, suggesting that opioid peptides and catecholamines released by the implanted chromaffin cells contribu te to the observed antinociception. However, neither antagonist altere d the antinociceptive effects of adrenal medullary implants on the ton ic phase of the formalin response. These results indicate that adrenal medullary implants in the spinal subarachnoid space alleviate acute a nd tonic pain via distinct pharmacologic mechanisms. (C) 1997 Internat ional Association for the Study of Pain.