Solubilization of High‐Affinity, Guanine Nucleotide‐Sensitive μ‐Opioid Receptors from Rat Brain Membranes

: High‐affinity μ‐opioid receptors have been solubilized from rat brain membranes. In most experiments, rats were treated for 14 days with naltrexone to increase the density of opioid receptors in brain membranes. Occupancy of the membrane‐associated receptors with morphine during solubilization in the detergent 3‐[(3‐cholamidopropyl)dimethyl]‐1‐propane sulfonate appeared to stabilize the μ‐opioid receptor. After removal of free morphine by Sephadex G50 chromatography and adjustment of the 3‐[(3‐cholamidopropyl)dimethyl]‐1‐propane sulfonate concentration to 3 mM, the solubilized opioid receptor bound [3H][d‐Ala2,N‐Me‐Phe4,Gly‐ol5]‐enkephalin ([3H]DAMGO), a μ‐selective opioid agonist, with high affinity (KD = 1.90 ± 0.93 nM; Bmax = 629 ± 162 fmol/mg of protein). Of the membrane‐associated [3H]‐DAMGO binding sites, 29 ± 7% were recovered in the solubilized fraction. Specific [3H]DAMGO binding was completely abolished in the presence of 10 µM guanosine 5′‐O‐(3‐thiotriphosphate). The solubilized receptor also bound [3H]diprenorphine, a nonselective opioid antagonist, with high affinity (KD = 1.4 ± 0.39 nM, Bmax = 920 ± 154 fmol/mg of protein). Guanosine 5′‐O‐(3‐thiotriphosphate) did not diminish [3H]diprenorphine binding. DAMGO at concentrations between 1 nM and 1 µM competed with [3H]diprenorphine for the solubilized binding sites; in contrast, [d‐Pen2,d‐Pen5]‐enkephalin, a δ‐selective opioid agonist, and U50488H, a κ‐selective opioid agonist, failed to compete with [3H]diprenorphine for the solubilized binding sites at concentrations of