Binding capacities and affinities AT μ ∂ and k-opioid sites in membrane suspensions from Guinea-pig brain regions

Binding capacities and affinities (K_D) of ligands at μ ∂ and k-opioid binding sites were determined by selective labelling techniques together with analysis of saturation curves in seven regions of the guinea-pig brain. The k-sites predominated over the other sites in most regions and were 90% of the total in the cerebellum; binding capacities at ∂sites were highest in the cortex, intermediate in the cerebellum, striatum and mesencephalon and lowest in the diencephalon, hippocampus and pons-medulla. At the μsites, binding capacities were highest in the diencephalon and mesencephalon, with intermediate levels in the pons-medulla, cortex and striatum, and low levels in the hippocampus and cerebellum. The highest binding capacity at the ∂sites was in the striatum, intermediate in the cortex, diencephalon and hippocampus, low in the mesencephalon and pons-medulla and not detectable in the cerebellum. No regional differences in binding affinities were found at μ ∂ and k-sites with [³H]-[D-Ala², MePhe⁴, Gly-ol⁵]enkephalin (K_D = 1.10-2,61 nM), [₃H]-[D-Ala₂, D-Leu₅]enkephalin (K_D = 0.81-1.94 nM) and [₃H]-(-)-bremazocine (K_D = 0.083-0.185 nM). Thus in guinea-pig brain there are regional differences in opioid binding capacity and in the distribution of μ ∂ and k-sites, but not in binding affinities.