Characterization and distribution of mu opioid receptors in rabbit cerebellum.

The binding characteristics and distributions of [3H]ohmefentanyl ([3H]OMF), [3H]etorphine, [3H] N-methyl-N-[7-(I-pyrrolidinyl)-1-oxaspiro(4,5)dec-7-yl] benzeneacetamide [3H]U-69593), and [3H] (D-penicillamine2, D-penicillamine5) enkephalin ([3H]DPDPE) in rabbit cerebellum were studied. In saturation experiment, [3H]OMF bound to a single population of sites with Kd = 2.2 +/- 1.3 nmol.L-1 and Bmax = 69 +/- 13 fmol/mg protein. The results of [3H]etorphine (Kd = 1.0 +/- 0.4 nmol.L-1 and Bmax = 16 +/- 6 fmol/mg protein) were similar to those of [3H]OMF. There were no specific bindings of [3H]U-69593 and [3H]DPDPE in rabbit cerebellar membranes. In autoradiographic study, the gray scales of specific binding for [3H]OMF, [3H]etorphine, [3H]U-69593, and [3H]DPDPE were 20.2 +/- 4.6, 8.5 +/- 2.7, 2.0 +/- 0.7, and 3.2 +/- 4.3, respectively. The gray matter of rabbit cerebellum was intensely labeled by [3H]OMF and [3H]etorphine, but not by [3H]U-69593 or [3H]DPDPE. These results suggested that the rabbit cerebellum contained mu opioid receptors.