Effects of naloxone and naltrexone on locomotor activity in C57BL/6 and DBA/2 mice

Administration of naloxone or naltrexone in DBA/2 (DBA) mice was followed by dose related depressant effects. The locomotor activity of the C57BL/6 (C57) mice injected with naltrexone was also depressed. Low doses of naloxone induced a decrease in activity in the C57 strain. This effect gradually disappeared at intermediate doses and recurred again at higher doses. The results are discussed in terms of differences in type number and/or distribution of the receptors influenced by naloxone and naltrexone in the two strains of mice tested.