Transition from ethanol-induced sensitization to tolerance across early and late infancy in the rat

Drugs of abuse, as cocaine or amphetamine, induce locomotor sensitization during infancy and adulthood of the rat. This effect during the preweanling period is observed only after a short interval of time between training and testing. We recently reported short-term locomotor sensitization induced by ethanol in pups chronically exposed to the drug during the second postnatal week of life. The present series of experiments was designed to explore the persistence of the sensitization effect across the preweanling period. Pups were chronically exposed to ethanol in five consecutive days during the second or the third postnatal weeks, and their locomotor activity was evaluated in an open field 3, 8 or 15 days later. Our results showed that, contrarily to what has been observed with other drugs during infancy, sensitization to ethanol persisted at least 8 days in rats exposed to the drug during the second postnatal week. Surprisingly, in older pups, the same procedure induced tolerance instead sensitization. This ontogenetic model offers a potentially interesting tool for studying within the same species, how tolerance and sensitization are interrelated, and how these effects affect ethanol-mediated reinforcement and ethanol intake during ontogeny.