Affinities of opiate agonists and antagonists for the enkephalin receptors of rat brain.

The binding of 3H-enkephalin (methionine-enkephalin) to membranes of rat brain is inhibited by enkephalin and levorphanol with IC50 values of 5 and 1.6 nM, respectively. Dextrorphan, the optical isomer of levorphanol, is over 6,000 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. However, 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 or greater than their affinities for the receptors of 3H-opiates.