Dose-dependent characterization of the rewarding and stimulant properties of cocaine following intraperitoneal and intravenous administration in rats

Dose-dependent differences in the rewarding and stimulant properties of cocaine administered intravenously (IV) and intraperitoneally (IP) were compared. Six 2-day conditioning trials were conducted over consecutive days. Rats received cocaine and were placed into a compartment on one day of the trial, and were directly placed into a different compartment without drug on the other day. Rats were exposed to the compartments for either 20 or 40 min. The effects of cocaine on stimulant behaviors, including locomotion and stereotypies, were compared following the first and last injection. After conditioning, three tests were given with 1 rest day intervening each: (l) conditioned place preference (CPP) was measured as an increase in the amount of time animals spent in the injection compartment relative to the noninjection compartment when given access to both, (2) conditioned activity (CA) was measured as an increase in stimulant behaviors in cocaine-treated animals relative to saline controls following an injection of saline in the injection compartment and (3) context-independent sensitization was measured as an increase in stimulant behaviors following an injection of cocaine in the noninjection compartment relative to the animals' behavior following the first injection. Cocaine did not reliably produce sensitization of locomotion under any of the conditions examined. Cocaine produced sensitization of headbobbing that was more robust following IP administration than it was following IV administration. In both cases, sensitization of headbobbing involved a context-independent component. Cocaine produced CPP and CA with both routes of administration. CPP was established more readily with 40-min relative to 20-min exposures following IV administration, whereas CA was more prevalent with 20-min relative to 40-min exposures. This study provides a thorough characterization of the behavioral effects of cocaine administered IV and a new efficient method for assessing the effects of cocaine on conditioned and unconditioned behaviors following repeated administration.