Identification of novel high affinity opiate receptor binding in rat brain

SPECIFIC opiate receptor binding sites^1-3, occur in the brains of numerous vertebrates. In our initial studies^4-6 binding seemed to involve a homogeneous population of receptors, although the biphasic degradation of receptor binding by trypsin and chymotrypsin suggested more than one site⁵. We have used ³H-naloxone and ³H- dihydromorphine of very high specific activity, and now report evidence for a new high affinity opiate-binding site in addition to the already reported binding site of lower affinity. The high and low affinity binding site may reflect interactions of opiates with two conformations of the opiate receptor which determine agonist and antagonist actions of the drugs.