Multiple opiate binding sites in the central nervous system of the rabbit. Large predominance of a mu subtype in the cerebellum and characterization of a kappa subtype in the thalamus.

We have compared the binding characteristics of [3H]etorphine, a nonselective mu-, delta-, and kappa-opiate agonist, with those of [3H]Tyr-D-Ala-Gly-MePhe-NH(CH2)2OH ([3H]DAGO), a selective mu-agonist, in rabbit cerebellar and thalamic membranes. We have also examined the ability of various unlabeled opioid ligands to compete with the binding of [3H]etorphine in the two preparations. In cerebellar membranes, [3H]DAGO(Kd = 0.7 nM) labels slightly fewer sites than does [3H]etorphine (Kd = 0.06 nM): 0.18 versus 0.24 pmole/mg of protein. In addition, competition studies indicate that up to 75% of the [3H]etorphine binding sites in this preparation display (a) high apparent affinity for unlabeled DAGO and (b) higher apparent affinity for morphine, the prototypical mu-agonist, than for Tyr-D-Ala-Gly-Phe-D-Leu (DADL), a delta-agonist. Together, these results suggest that the rabbit cerebellum contains a very high proportion (0.7-0.8) of mu-opiate binding sites. In thalamic membranes, [3H]DAGO (Kd = 1.1 nM) labels considerably fewer sites than does [3H]etorphine (Kd = 0.08 nM): 0.09 versus 0.27 pmole/mg of protein. In this preparation, the competition curves of DAGO and of DADL resolve binding of [3H]etorphine into two components. The first component accounts for 40-50% of total binding and reflects the interaction of [3H]etorphine with mu-opiate binding sites. The second component (up to 50% of total binding) is unaffected in the presence of DADL at concentrations (1-10 microM) that rule out binding of [3H]etorphine to mu- and delta-opiate binding sites. It disappears readily in the presence of very low concentrations (Ki less than 1 nM) of benzomorphan opiates (bremazocine, cyclazocine, and ethylketocyclazocine) yet it is relatively insensitive to inhibition by mu- and delta-agonists. This second component may therefore reflect [3H]etorphine's interaction with a kappa-opiate binding site. The kappa-opiate binding site is assayed for as that site which binds [3H]etorphine (0.5 nM) in the presence of either DADL (2 microM) or 10 microM of another enkephalin: Tyr-D-Ser-Gly-Phe-Leu-Thr. We find that, in the rabbit central nervous system, the thalamus, followed by frontal cortex and caudate nucleus, shows the highest content of kappa-opiate binding sites.