This paper describes a novel method for studying the bio-behavioral basis of addiction to food. This method combines the surgical component of taste reactivity with the behavioral aspects of operant self-administration of drugs. Under very brief general anaesthesia, rats are implanted with an intraoral (IO) cannula that allows delivery of test solutions directly in the oral cavity. Animals are then tested in operant self-administration chambers whereby they can press a lever to receive IO infusions of test solutions. IO self-administration has several advantages over experimental procedures that involve drinking a solution from a spout or operant responding for solid pellets or solutions delivered in a receptacle. Here, we show that IO self-administration can be employed to study self-administration of high fructose corn syrup (HFCS). Rats were first tested for self-administration on a progressive ratio (PR) schedule, which assesses the maximum amount of operant behavior that will be emitted for different concentrations of HFCS (i.e. 8%, 25%, and 50%). Following this test, rats self-administered these concentrations on a continuous schedule of reinforcement (i.e. one infusion for each lever press) for 10 consecutive days (1 session/day; each lasting 3 hr), and then they were retested on the PR schedule. On the continuous reinforcement schedule, rats took fewer infusions of higher concentrations, although the lowest concentration of HFCS (8%) maintained more variable self-administration. Furthermore, the PR tests revealed that 8% had lower reinforcing value than 25% and 50%. These results indicate that IO self-administration can be employed to study acquisition and maintenance of responding for sweet solutions. The sensitivity of the operant response to differences in concentration and schedule of reinforcement makes IO self-administration an ideal procedure to investigate the neurobiology of voluntary intake of sweets.

Responding on variable-ratio (VR) schedules of wheel-running and sucrose reinforcement was assessed using a within-session procedure. Six female Long Evans rats were exposed to schedules ranging from VR 3 to VR 40 in ascending and descending order within a session with sucrose (0.1 ml of 15 % solution) and wheel-running reinforcement (30 s). Within a session, 10 reinforcers were obtained on each schedule, with a 120 s inter-component interval between schedules. Results showed that local response rates generated by sucrose were higher and that the pattern of local response rates across the schedules differed substantively between reinforcer types. With sucrose, local rates decreased linearly as the ratio requirement increased. With wheel running, local rates did not differ except to decline on the highest ratio. Postreinforcement pauses were longer with wheel-running reinforcement and longer on the highest ratio when this ratio occurred at the end of a session. In contrast, wheel-running rates were lower on the smallest ratio schedule when this schedule occurred at the beginning of a session. The implications of the marked difference in local response rates across these schedules were discussed.

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