Day-by-day maturation of the long-term expression of cocaine sensitization acquired before weaning in the rat.

This study aimed to identify the ontogenetic period during which long-term expression of behavioral sensitization to cocaine begins to emerge. Rat pups aged 4, 8, 12, or 16 days received a pretreatment of 4 daily injections of 15 mg/kg sc cocaine paired with the test chamber for 45 min. Pups were then tested for sensitization in that context after abstinence intervals ranging from 2 to 10 days. On test days, pups were videotaped, and their behavior was scored later. Sensitization was detected after intervals of 2, 4, 5, or 9 days in pups aged 4-7, 8-11, 12-15, or 16-19 days during pretreatment, respectively. These results suggest that the mechanisms for long-term retention of sensitization mature incrementally in the rat, starting to emerge gradually after the 1st week of age, whereas those relevant to short-term retention and initiation of sensitization are present earlier.