NALOXONE BLOCKS ANXIOLYTIC-LIKE EFFECTS OF BENZODIAZEPINES IN SWISS BUT NOT IN BALB C MICE/

The ability of naloxone to block the effects of the benzodiazepines ch lordiazepoxide and diazepam was evaluated in Swiss and Balb/c mice sub jected to the light/dark choice test of anxiety or to a choice paradig m for measuring spontaneous exploratory behaviour. In Swiss mice, nalo xone (5 or 10 mg/kg) completely or partially suppressed the anxiolytic -like effects of chlordiazepoxide (5 mg/kg) and diazepam (1 mg/kg) in the light/dark test. Naloxone alone was ineffective. None of these com pounds affected locomotion in the free exploratory test. In Balb/c mic e, naloxone did not reduce the anxiolytic-like action of benzodiazepin es in the light/dark test. Moreover, naloxone did not antagonize the d ecrease in neophobia observed after anxiolytic treatment in Balb/c mic e in the free exploratory paradigm. In this strain, benzodiazepines pr oduced an increase of locomotor activity, whereas naloxone decreased i t. The stimulant effects of benzodiazepines on locomotor activity were abolished by naloxone. As naloxone (2 mg/kg) reversed the morphine-in duced hyperthermia both in Swiss and in Balb/c mice, differences in po ssible pharmacokinetic factors between the two strains can be ruled ou t as an explanation for the failure of naloxone to antagonize anxiolyt ic-like effects in Balb/c mice. Therefore, the ability of naloxone to reverse anxiolytic effects does not hold for all strains of mice.