The effects of anorexic drugs on free-fed rats responding under a second-order FI15-min (FR10:S) schedule for high incentive foods

Abstract

Many similarities exist between the overconsumption of food, which results in obesity, and drug addiction. The present study investigated the effects of anorectic drugs on responding maintained by high incentive, but nutritionally unnecessary, food reinforcers using an FI15(fixed-ratio 10:S) schedule of reinforcement, similar to that used in studies on the incentive properties of drugs of abuse. Rats were trained to respond on a lever to gain access to two high incentive foods--chocolate chip cookies and cheese. Under the FI15(FR10:S) schedule, every 10th response (fixed-ratio 10) delivered a tone and light conditioned stimulus. The first ratio completed 15 min after the start of the session produced the conditioned stimulus and opened a door to give access to a piece of cookie. After 5 min to consume the high incentive food, a second 15-min interval was started, terminating in access to a second reinforcer, cheese. Once trained, the rats were given free access to laboratory chow in the home cage. They continued to work for the high incentive foods for a period of over 1 year, showing a pattern of responding appropriate to an FI(fixed-ratio) schedule. Naloxone (1.0 mg/kg), fenfluramine (1 and 2 mg/kg), D-amphetamine (0.25 and 0.5 mg/kg), and rimonabant (3 mg/kg) significantly reduced responding, especially in the second interval. In contrast, complete removal of the high incentive food from the test procedure did not immediately reduce the rate of responding, tending to increase it in the second of the intervals. Apparently, the drugs did not reduce responding by reducing the experienced magnitude of the high incentive food, but more probably by reducing the animals' motivation.