Abstract

Morphine is the prototypic mu-opioid analgesic; however, in certain situations in vitro, morphine behaves as a partial agonist. To assess the relative intrinsic activity of morphine and three other mu-opioid analgesics in vivo, beta-funaltrexamine (beta-FNA), an irreversible antagonist selective for the mu receptor, was used to reduce the effective receptor reserve. By using a stereotaxic device, 1.25 to 20 micrograms of beta-FNA was infused into the lateral ventricle of rats. Twenty-four hours later, animals were tested in the tail-flick assay with cumulative doses of morphine, levorphanol, methadone or fentanyl. Pretreatment with 2.5 micrograms of beta-FNA induced parallel rightward shifts of both the morphine and levorphanol dose-effect curves and 5.0 micrograms of beta-FNA reduced the maximum analgesic effect of these agonists. Methadone surmounted the antagonism of 5.0 micrograms of beta-FNA; 10 micrograms was required to reduce the maximum analgesic effect of methadone. Fentanyl overcame the blockade induced by both 5.0 and 10 micrograms of beta-FNA. Only with a pretreatment dose of 20 micrograms of beta-FNA was the maximum analgesic effect of fentanyl reduced. Thus, when a certain proportion of mu receptors is inactivated, i.e., with 5.0 micrograms of beta-FNA, fentanyl and methadone have the capacity to surmount the blockade, whereas morphine and levorphanol do not. This suggests that fentanyl and methadone have higher intrinsic efficacies than do morphine and levorphanol. Thus, a strategy used widely in vitro was applied successfully in vivo to assess relative intrinsic activities of a series of mu-opioid agonists.