Regional variations in binding capacities at μ-, δ- and κ-opioid sites in membrane suspensions from rabbit brain

The highest maximum binding capacities at the μ-sites of rabbit brain are in the striatum, with intermediate levels in the diencephalon, mesencephalon, cerebellum and cortex and low level in the pons-medulla and hippocampus. For the δ-site the highest maximum binding capacity is also in the striatum; there are almost equally low levels in the other brain regions. At the κ-sites the maximum binding capacities are highest in the diencephalon; there are intermediate levels in the cortex and striatum, and low levels in the mesencephalon, cerebellum, hippocampus and pons-medulla. The K_D values lack reproducibility; there are no regional variations at the κ-site as estimated with [³H](-)-bremazocine, but the possibility cannot be excluded that there are regional variations in the K_D values for [³H][D-Ala², MePhe⁴, Gly-ol⁵]enkephalin at the μ-site or for [³H][D-Ala²,D-Leu⁵]enkephalin at the δ-site. It may be important to use saturation analysis in future investigations of the distributions of the binding sites.