Cerebellar opiate receptors in lagomorphs. Demonstration, characterization and regional distribution

The cerebellum of lagomorphs (pika, rabbit, hare) binds 100--200 femtomoles of [3H]etorphine per milligram of protein. This is very high in comparison with the 10--15 fmol/mg found in the cerebellum of rodents (mouse, hamster, rat). In the rabbit cerebellum, the etorphine sites have binding properties indistinguishable from those of genuine opiate receptor sites in brain. They exhibit a high affinity for [3H]etorphine (KD = 1 X 10(-10) M), [3H]naloxone (KD = 9 X 10(-10) M), morphine and levorphanol but not for dextrophan. Moreover, sodium ions enhance binding of naloxone (antagonist response) and diminish binding of etorphine, morphine and levorphanol (agonist response) to cerebellum homogenates. The regional distribution of [3H]etorphine binding sites in the rabbit cerebellum points toward concentrations higher in the neocerebellum (hemispheres) than in the archecerebellum (lingula and flocculonodular lobe). Finally, the specific concentration of opiate receptor sites in the isolated molecular layer is at least two times that in the isolated granular layer and ten times that in white matter.