CHARACTERISTICS AND REGIONAL DISTRIBUTIONS OF TWO DISTINCT [ 3 H]NALOXONE BINDING SITES IN THE RAT BRAIN

— Using [ 3 H]naloxone at a concentration of 4.5 n m , the potent opiate agonist etorphine as well as the potent antagonist diprenorphine displace only about 75% of specific naloxone binding P 2 fractions from rat whole forebrain, without additive effect. Several other opiates and antagonists completely displace specific naloxone binding. This indicates that etorphine and diprenorphine specifically bind to one and the same naloxone binding site (type I) while leaving another naloxone binding site (type II) unaffected. Type I binding sites are much more thermo‐labile than type II. [ 3 H]Naloxone binding to type I sites is unaffected by incubation temperature in the range 10 to 25°C. while binding type II sites decreases rapidly with increasing incubation temperature, no specific type II binding being detectable at or above 20°C. The two naloxone receptor types also differ with respect to pH dependence, and affinity for naloxone with types I and II having affinity constants (K d ) of 2 and 16 n m , respectively, at 0°C. The two binding sites have different regional distributions with high relative levels of type II receptors in cerebellum and low relative levels in pons‐medulla and striatum. In whole rat brain there are about 4 times as many type II receptors as type I. These results suggest that naloxone and several other opiate agonists and antagonists bind to two distinct receptor types which are probably not agonist/antagonist aspects of the same receptor.