Effects of central 5-hydroxytryptamine depletion on sensitivity to delayed and probabilistic reinforcement

Rationale: The ascending 5-hydroxytryptaminergic (5-HTergic) pathways are believed to be involved in impulse control. Rats whose 5-HTergic pathways have been destroyed are more liable than intact rats to select a smaller, immediate reinforcer rather than a larger, delayed reinforcer (impulsive choice), and recent evidence indicates that this effect of central 5-HT depletion reflects a change in the rate of time discounting (i.e. a change in the rare at which reinforcers become devalued as a function of delay). Delay of reinforcement and uncertainty of reinforcer delivery are believed to have equivalent effects on choice behaviour. However, it is not known whether central 5-HT depletion affects choice between probabilistic reinforcers. Objective: We examined the effects of central 5-HT depletion on choice behaviour in two experiments: In experiment I, rats chose between a smaller immediate reinforcer and a larger delayed reinforcer, in experiment 2, rats chose between a smaller certain reinforcer and a larger probabilistic reinforcer. Methods: Rats received in jections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei or sham lesions. They were rrained to press two levers for food-pellet reinforcers in discrete-trials schedules. In free-choice trials, selection of lever A resulted in immediate delivery of one food pellet: selection of lever B resulted in delivery of 2 pellets, either following a delay (d(B)) (experiment 1) or with a probability (p(B)) less than 1 (experiment 2). Results: In experiment 1, both groups showed declining choice of lever B (%B) as a function of nk. The lesioned group showed shorter indifference delays (D-50: the value of d(B) corresponding to %B=50) than the sham-lesioned group. In experiment 2. both groups showed declining choice of lever B as a function of the odds against delivery of the two-pellet reinforcer, theta (B) (theta (B) = [1/p(B)]-1). There was no difference between the indifference odds (theta (50): the value of theta (B) corresponding to %B=50) between the two groups. In both experiments, the levels: of 5-HT and 5-hydroxyindoleacetic acid were reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not altered. Conclusions: These results provide additional evidence that central 5-HTergic mechanisms are involved in time discounting, but provide no evidence fur a similar role of 5-HT in rats' sensitivity to probabilistic reinforcement.