AFFINITIES OF OPIATE AGONISTS AND ANTAGONISTS FOR ENKEPHALIN RECEPTORS OF RAT-BRAIN

Abstract

The binding of 3H-enkephalin (methionine-enkephalin) to membranes of rat brain is inhibited by enkephalin and levorphanol with IC50 [median inhibitory concentration dose] values of 5 and 1.6 nM, respectively. Dextrorphan, the optical isomer of levorphanol, is over 6000 times less effective. Opiate agonists morphine, dihydromorphine and l-methadone, and opiate antagonists naltrexone and naloxone are less effective inhibitors of 3H-enkephalin than in 3H-dihydromorphine and 3H-naloxone binding. Opiates without the oxygen bridge as in the structure of morphine, such as levorphanol and the benzomorphans, show affinities for the receptors of 3H-enkephalin equal to or greater than their affinities for the receptors of 3H-opiates.