@article {Lange167, author = {D G Lange and J M Fujimoto and S Roerig and R I Wang}, title = {Enhanced naloxone distribution to the brain by morphine pretreatment in mice.}, volume = {5}, number = {2}, pages = {167--173}, year = {1977}, publisher = {American Society for Pharmacology and Experimental Therapeutics}, abstract = {An increase in the disposition of naloxone to the mouse brain was observed for animals previously exposed to morphine. Compared to controls, mice receiving morphine sulfate (10 mg/kg, sc) 3 hr prior to naloxone had a 28\% increase in naloxone concentration in brain (200 to 260 pmol of naloxone per g of brain) 10 min after 3H-naloxone-HCl (0.4 mg/kg, 11.0 micronCi/kg, sc) administration. Also, if similar morphine-pretreated mice received a second dose of morphine sulfate (1.0 mg/kg, sc) concurrent with 3H-naloxone-HCl, the morphine-induced enhancement of 3H-naloxone concentration in brain was unaltered. This drug-treatment protocol paralleled that used by others in pA2-analgesia assays to demonstrate sensitization to naloxone for morphine-pretreated animals. In prior (3 hr) morphine-treated animals, administration of 3H-naloxone-HCl (0.1 mg/kg, 33.3 micronCi/kg) iv resulted in an 11.0\% increase in 3H-naloxone brain concentration after 1 min. Thus, the enhancement of naloxone brain concentration was independent of the route of naloxone administration. No enhancement of 3H-naloxone brain concentration could be seen 24 hr after morphine sulfate pretreatment (10 mg/kg, sc), a decline in the effect similar to that seen for morphine-induced sensitization to naloxone. Finally, when morphine pellet-implanted mice (75 mg of morphine base, 72 hr) were administered 3H-naloxone-HCl (0.4 mg/kg, 10.0 micronCi/kg, sc), only a 22.5\% enhancement of 3H-naloxone concentration in brain was obtained, as opposed to a reported 8-fold increase in the potency of naloxone. Thus, although a number of similarities exist between the enhancement by morphine of naloxone concentration in brain and its sensitization to the antagonistic activity of naloxone, a quantitative correlation appears to be lacking between the two phenomena.}, issn = {0090-9556}, URL = {https://dmd.aspetjournals.org/content/5/2/167}, eprint = {https://dmd.aspetjournals.org/content/5/2/167.full.pdf}, journal = {Drug Metabolism and Disposition} }