Individual differences to repeated ethanol administration may predict locomotor response to other drugs, and vice versa.

Repeated administration of drugs may induce adaptations which affect the behavioral responses to the drug itself or to other drugs. Whether individual characteristics to repeated drug administration predict sensitivity to the effects of another drug is not clear. We evaluated whether or not mice that present higher vs. lower locomotor response after repeated treatment with ethanol display increased or decreased locomotor responses when challenged with methamphetamine or morphine, and vice versa. Mice received daily i.p. 2.2 g/kg ethanol (21 days), 1.0 mg/kg methamphetamine or 10 mg/kg morphine (10 days). According to the response presented during repeated drug treatment, mice were classified as HIGH or LOW activity groups. Locomotor activity was monitored after mice were challenged with saline, and 48 h later with a drug. Ethanol-treated mice were challenged with methamphetamine or morphine, methamphetamine- and morphine-treated animals were challenged with ethanol. After repeated treatment with ethanol or methamphetamine, locomotor sensitization was observed only in HIGH mice, not LOW mice. Ethanol-treated mice with HIGH activity showed sensitized, increased locomotor responses to methamphetamine (p<0.05), but not to morphine. Locomotor responses to ethanol were not affected by a previous history of methamphetamine treatment. Although repeated administration of morphine failed to induce sensitization, morphine-treated mice with HIGH activity presented sensitized locomotor responses after an ethanol challenge. The current experiments confirm important individual differences in response to repeated administration of ethanol, methamphetamine and morphine, which in some cases affected the locomotor response to a second drug challenge, in an asymmetrical pattern.