Naloxone does not attenuate the locomotor effects of ethanol in FAST,SLOW, or two heterogeneous stocks of mice

Rationale Previous studies suggest that some behavioral effects of ethanol and morphine are genetically correlated. For example, mice bred for sensitivity (FAST) or insensitivity (SLOW) to the locomotor stimulant effects of ethanol differ in their locomotor response to morphine. Objective To evaluate a possible common mechanism for these traits, we examined the effect of naloxone, an opioid receptor antagonist, on ethanol- and morphine-induced locomotion in FAST and SLOW mice, as well as on ethanol-induced locomotion in two heterogeneous stocks of mice. Method In experiments 1 and 2, naloxone was given to FAST and SLOW mice 30 min prior to 2 g/kg ethanol or 32 mg/kg morphine, and locomotor activity was measured for 15 min (ethanol) or 30 min (morphine). In experiments 3 and 4, naloxone was administered 30 min prior to 1.25 g/kg ethanol, and locomotor activity was assessed in FAST mice and in a heterogeneous line of mice Withdrawal Seizure Control (WSC)]. Experiment 5 assessed the effect of naloxone on ethanol-induced stimulation in outbred National Institutes of Health (NIH) Swiss mice. Results There was no effect of naloxone on the locomotor response to ethanol in FAST, SLOW, WSC, or NIH Swiss mice. However, naloxone did significantly attenuate the locomotor effects of morphine in FAST and SLOW mice. Conclusions These results suggest that a common opioidergic mechanism is not responsible for the correlated locomotor responses to ethanol and morphine in FAST and SLOW mice, and that activation of the endogenous opioid system is not critical for the induction of ethanol-induced alterations in activity.