The direct demonstration of binding of mu, delta and kappa agonists to digitonin-solubilized opioid receptors from bovine striatum.

Active opioid binding sites, that retain the ability to bind tritiated agonists have been obtained in good yield in digitonin/NaCl/Mg2+ extracts of morphine protected bovine striatal membranes. Ligand protection of binding sites and the presence of Mg ions were found to be absolute requirements for agonist binding in this solubilized opioid receptor preparation. Soluble preparations contained a ratio of mu: delta:kappa similar to that in the membranes.