[Binding characteristics of [3H]etorphine with kappa receptors in the golden hamster brain].

In previous work, we have reported that the predominant kappa opioid receptor subtype existed in the golden hamster brain. Some more properties of [3H]etorphine binding to kappa sites in the hamster brain have been examined in this paper. After blockade of mu and delta sites by unlabelled 30 nmol/L morphine and 100 nmol/L (D-Ala2, D-Leu5) enkephalin (DA- DLE), [3H]etorphine was still able to bind to residual sites with high affinity (Kd = 0.52±0.17 nmol/L, Bmax = 34±7 pmol/g protein). The specific binding was completely abolished by 5μmol/L DADLE, suggesting that [3H]etorphine didn’t bind to DADLE insensitive sites which correspond to the kappa1 sites. Competition binding studies indicated that benzomorphan and oripavine drugs showed high affinity to the kappa sites whereas dynorphin A (1-13) did not. Morphine displayed a lower affinity and DADLE had a KI value of 940 nmol/L. On the other hand, delta ligand, (D- Ser2, L-Leu5) enkephalyl-Thr6 weakly competed for the binding sites. These results suggest that the kappa sites which interacted with [3H]etorphine differ from the classical kappa sites, and the former sites may be the representatives of kappa2 sites.