Regional interactions of opioid peptides at mu and delta sites in rat brain.

Opiate binding sites in five brain regions were labeled with the mu and delta markers, 3H-morphine and 3H-[D-Ala2,D-leu5]enkephalin, respectively. The highest densities of both 3H-morphine and 3H-DADLE labeled sites are found in striatum and frontal cortex. Hypothalamus and midbrain contain predominantly 3H-morphine labeled sites. The selectivity of the opioid peptides [D-Ala2,D-leu5]enkephalin, beta-endorphin and dynorphin(1-13) for the two opiate sites was investigated by comparing the potency of these unlabeled compounds against the mu and delta markers in different brain regions. This determination has the effect of controlling for the breakdown of peptides within each region. While the enkephalin analogue shows a preference for the delta binding site and beta-endorphin is more nearly equipotent towards the two binding sites, dynorphin(1-13) shows a high affinity and selective preference for the mu binding site over the delta site. The potency of the opioid peptides in displacing the mu and delta markers varies from region to region according to the relative densities of the two opiate binding site populations.