Cocaine sensitization in periadolescent and adult rats.

Periadolescent rats have been reported to be affected differentially by catecholaminergic agents when compared with younger or adult animals. The present study evaluated the behavioral responsivity of periadolescent (34- to 39-day-old) and adult (60- to 70-day-old) Sprague-Dawley rats of both sexes to i.p. cocaine (Coc) administration (0, 10 or 20 mg/kg, once daily for 4 days). All animals received injections of both saline and Coc every day paired with a different context, with one-half of the animals receiving the drug in the home cage (Coc-Home) and the other half in the testing chamber (Coc-Test). Forty-eight hours after the last drug injection, all animals were challenged with 10 mg/kg i.p. of Coc, and their behavior in the test chamber was scored. As expected, acute Coc induced a prominent increase in a number of behaviors, and this response profile was less marked in periadolescent relative to adult animals. In contrast, Coc-Test animals of both ages showed a clear behavioral sensitization relative to the chronic saline group. No evidence of carry-over effects was found in Coc-Home animals. Females were in general more sensitive than males to acute Coc effects. The development of behavioral sensitization to Coc was a function of age-specific alterations in sensitivity to psychostimulants. Periadolescent rats of both sexes showed sensitization to the locomotor activating effects (matrix crossings) of Coc, whereas a consistent sensitization profile for both stereotyped head scanning and focused sniffing activities were found in adults but not in periadolescents. Chronic Coc reduced body weight and food consumption, particularly in adult males, whereas it did not affect periadolescent patterns. No evidence of sensitization to Coc was found in the hormonal parameters considered.