Abstract
Response-contingent injections of opioids have been shown to control behavior in various species. To determine whether s.c. injections of etonitazene (ETZ) could maintain behavior in rats when administered under a single fixed-interval schedule. Rats were trained to lever press for eight 45-mg food pellets under a single fixed-interval (FI) 10-min schedule of reinforcement: following passage of the 10-min interval, each lever press resulted in a pellet delivery until eight pellets were obtained. Delivery of the reinforcer was signaled by a change in visual stimulus conditions. Once stable responding for the food pellets under the FI 10-min schedule was established, a s.c. injection of 3.2 micrograms/kg ETZ was administered to the rat by the investigator following schedule completion and delivery of the food pellets. After receiving the drug injection, rats were returned to the experimental chamber for 30 min and exposed to the same stimulus conditions that accompanied food reinforcement. Across sessions, the number of food pellets was decreased until rats were responding solely for the drug. Responding for the s.c. administered drug stabilized and persisted across sessions. When saline vehicle injections were substituted for the drug injections, responses diminished across sessions to levels below that of the drug baseline. Subsequent alternating blocks of ETZ and vehicle injections produced respective increases and decreases in responding. This study demonstrates that response-contingent s.c. injections of a drug can control behavior in rats, systematically replicating a previous experiment that used the i.p. route. Since all pertinent operant behavior is emitted prior to the administration of drug, this procedure can be used for testing the reinforcing effects of a drug without interference from any direct (rate-altering) drug effects. The present findings also extend the conditions under which drugs of abuse may reinforce behavior.
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