Aversive property of opioid receptor blockade in drug-naive mice.

The motivational effect of naloxone administration in the non-dependent laboratory mouse was examined with taste and place conditioning procedures. Thus, male CD1 mice without any history of drug exposure avoided a cue paired with three SC injections of as little as 0.1 mg/kg naloxone HCl. The aversive effect of naloxone was also seen in DBA/2 and C57BL/6 mice. In addition, it only occurred with the minus isomer and not the plus isomer, and it was potentiated by implantation, 3 days prior to training, of a morphine-containing (37.5 mg) but not a placebo pellet. Naloxone injection, therefore, acts as an aversive stimulus in naive mice and this is probably produced by decreases in activity of endogenous opioid peptide systems. Together with other data, the present results support the conclusion that the aversive effect of opioid receptor blockade in the opiate non-dependent organism may be general to a wide range of species including primates. The importance of training and testing variables for observing the naloxone aversive effect is discussed. Advantages of studying preference conditioning with mice are also given.