Binding of [3H]naloxonazine to rat brain membranes

[3H]Naloxonazine binds to opioid-binding sites in rat brain homogenates. Prior administration of either morphine or D-Ala2-D-Leu5-enkephalin to the homogenates inhibits in a concentration-dependent manner the specific binding of [3H]naloxonazine. Most important, all the binding competed by unlabeled naloxonazine at 1 microM is also competed by morphine and D-Ala2-D-Leu5-enkephalin. [3H]Naloxonazine binding is linear with tissue up to 10 mg/ml wet weight of tissue, is temperature dependent, and has a pH maximum of approximately 7.7. Maximal binding is reached within 90 min at 25 degrees. The affinity of [3H]naloxonazine for its binding sites is quite high with half-maximal binding obtained at a concentration of approximately 2 nM. Approximately 40% of the total specific binding of [3H]naloxonazine is resistant to multiple washes and to displacement by levallorphan (1 microM) added 60 min after the [3H]naloxonazine, suggesting that a portion of [3H]naloxonazine binding is not freely reversible. The percentage of total [3H] naloxonazine binding which is not freely reversible varies 3-fold between regions, with the hypothalamus (60%) being the highest and the brainstem (18%) the lowest.