Day-by-day maturation of the long-term expression of cocaine sensitizationacquired 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 pup s aged 4, 8, 12, or 16 days received a pretreatment of 4 daily injections o f 15 mg/kg sc cocaine paired with the test chamber for 45 min. Pups were th en tested for sensitization in that context after abstinence intervals rang ing from 2 to 10 days. On test days, pups were videotaped, and their behavi or 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 ret ention of sensitization mature incrementally in the rat, starting to emerge gradually after the I st week of age, whereas those relevant to short-term retention and initiation of sensitization are present earlier.