Brain levels of morphine in mice following removal of a morphine pellet and naloxone challenge: no evidence for displacement

Male ICR mice were rendered tolerant to and dependent on morphine by subcutaneous implantation of a 75 mg morphine pellet for 72 hours. At 2, 4, and 6 hours after pellet removal groups of 7–10 mice were challenged with ip saline or naloxone and their brain concentrations of morphine estimated by radioimmunoassay (RIA). The brains were prepared for RIA by either organic or inorganic (0.01 N HC1) extraction and in most experiments the two methods were shown to be equivalent with respect to the final concentration of morphine. There was no difference in brain morphine between saline and naloxone (10 mg/kg) treated groups when they were challenged 4 hours after pellet removal and sacrificed 1, 5, 10, 15, 20, 30, 45, and 60 minutes later. In contrast, when the challenge was administered 6 hours after pellet removal the naloxone treated groups has higher concentrations of brain morphine than the saline controls. Brain levels in mice that received 0.10, 1.0, 10, 100 mg/kg naloxone did not differ consistently from saline controls. We found no consistent evidence that naloxone decreases the concentration of morphine in brain homogenates obtained from mice during the initial 6 hours after pellet removal.