Abstract.
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 rate 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 1, 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 injections of 5,7-dihydroxytryptamine into the dorsal and median raphe nuclei or sham lesions. They were trained 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 d B . 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, θ B (θ B =[1/p B ]–1). There was no difference between the "indifference odds" (θ 50 : the value of θ 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 for a similar role of 5-HT in rats' sensitivity to probabilistic reinforcement.
Author information
Authors and Affiliations
Additional information
Electronic Publication
Rights and permissions
About this article
Cite this article
Mobini, S., Chiang, TJ., Ho, MY. et al. Effects of central 5-hydroxytryptamine depletion on sensitivity to delayed and probabilistic reinforcement. Psychopharmacology 152, 390–397 (2000). https://doi.org/10.1007/s002130000542
Received:
Accepted:
Issue Date:
DOI: https://doi.org/10.1007/s002130000542