Binding capacities and affinities at mu-, delta- and kappa-opioid sites in membrane suspensions from guinea-pig brain regions.
Binding capacities and affinities (KD) of ligands at mu-, delta- and kappa-opioid binding sites were determined by selective labelling techniques together with analysis of saturation curves in seven regions of the guinea-pig brain. The kappa-sites predominated over the other sites in most regions and were 90% of the total in the cerebellum; binding capacities at kappa-sites were highest in the cortex, intermediate in the cerebellum, striatum and mesencephalon and lowest in the diencephalon, hippocampus and pons-medulla. At the mu-sites, 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 delta-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 mu-, delta- and kappa-sites with [3H]-[D-Ala2,MePhe4,Gly-ol5]enkephalin (KD = 1.10-2,61 nM), [3H]-[D-Ala2,D-Leu5]enkephalin (KD = 0.81-1.94 nM) and [3H]-(-)-bremazocine (KD = 0.083-0.185 nM). Thus in guinea-pig brain there are regional differences in opioid binding capacity and in the distribution of mu-, delta- and kappa-sites, but not in binding affinities.