Longevity of the expression of behavioral sensitization to cocaine in preweanling rats.

The influence of the treatment-to-test interval on the expression of behavioral sensitization to cocaine was assessed following chronic cocaine exposure during the late preweanling period. From postnatal day 14 (P14) to P20, Sprague-Dawley rat pups received a daily intraperitoneal injection of either 30 mg/kg/2 cc cocaine or an equivalent volume of saline that was paired with placement in the treatment/test context for 30 min. Animals were challenged with 15 mg/kg cocaine in this context on the test day following drug-free intervals of 1, 3, 7, 14, or 21 days. Behavioral sensitization was evident following the preweanling cocaine regimen in terms of both matrix crossings and stereotypy. For matrix crossings, there was no evidence that this sensitization decreased across the time intervals examined, and sensitization was evident with stereotypy even at the 21-day injection test interval. The expression of sensitization, however, was partially overshadowed by an overall reduced sensitivity to cocaine seen in testing during the periadolescent period (i.e., at P34 and P41). Thus, behavioral sensitization to cocaine can be expressed for weeks following chronic treatment during the late preweanling period, although the magnitude of behavioral expression may be influenced by age-related neurobehavioral alterations.