Evidence for behavioral sensitization to cocaine in preweanling rat pups.

While chronic intermittent administration of stimulants often induces behavioral sensitization in adulthood, stimulant sensitization has rarely been reported prior to weaning [around postnatal day (P) 21]. Consistent pairing of drug administration with the test context often facilitates sensitization in adults, yet young animals have been typically returned to the home cage immediately post-injection. To determine whether promoting context-dependent sensitization might facilitate expression of sensitization in preweanlings, Sprague-Dawley rats were injected daily from P14 to P20 with 0, 5, 15, or 30 mg/kg cocaine HCl and placed for 30 min in either the experimental chamber or home cage. On P21 (test day), subjects were challenged with either 15 mg/kg cocaine or saline prior to placement in the experimental chamber. Significant sensitization of cocaine-induced stereotyped head movements was evident in animals given 15 or 30 mg/kg chronically in the experimental chamber, but not when these same doses were given in the home cage. Less consistent evidence for cocaine-induced sensitization was seen when examining locomotion, although trends for sensitization of this behavior were seen in animals chronically injected in either the test chamber or home cage. Thus, preweanlings can exhibit cocaine sensitization, particularly in terms of stereotypy, when tested shortly after the chronic exposure period, with expression of this sensitization being facilitated by pairing the chronic injections with the test context.