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Psychopharmacology

, Volume 149, Issue 3, pp 313–318 | Cite as

Effect of central 5-hydroxytryptamine depletion on inter-temporal choice: a quantitative analysis

  • S. Mobini
  • T.-J. Chiang
  • A. S. A. Al-Ruwaitea
  • M.-Y. Ho
  • C. M. Bradshaw
  • E. Szabadi
Original Investigation

Abstract 

Rationale: It has been proposed that the ascending 5-hydroxytryptaminergic (5-HTergic) pathways are involved in ”impulse control”. Previous experiments have shown that 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). However, it remains unclear whether 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) or a change in sensitivity to reinforcer size. Objective: We examined the effect of central 5-HT depletion on time discounting using a quantitative model of inter-temporal choice (multiplicative hyperbolic model), which enables effects on time discounting to be differentiated from effects on sensitivity to reinforcer size. 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 a discrete-trials adjusting-delay schedule. In free-choice trials, selection of lever A resulted in a brief fixed delay (d A ) followed by delivery of one pellet; selection of lever B resulted in a longer variable delay (d B ) followed by delivery of two pellets; d B was adjusted in accordance with the subject’s choices. The value of d A was varied (0.5–8.0 s) in successive phasesof the experiment, and the indifference value of d B was determined in each case. Results: In both groups, the indifference value of d B was linearly related to the value of d A ,in accordance with the multiplicative hyperbolic model. The lesioned group showed shorter indifference delays than the sham-lesioned group, this being reflected in a parallel displacement of the linear indifference function. In both experiments, the levels of 5-HT and 5-hydroxyindole-acetic acid were reduced in the brains of the lesioned rats, but the levels of noradrenaline and dopamine were not altered. Conclusions: According to the multiplicative hyperbolic model, parallel displacement of the linear indifference function uniquely specifies a change in time discounting. Thus these results indicate that central 5-HT depletion results in an increase in the rate of time discounting for food reinforcers.

Key words 5-HT 7-Dihydroxytryptamine Operant behaviour Inter-temporal choice 

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Copyright information

© Springer-Verlag Berlin Heidelberg 2000

Authors and Affiliations

  • S. Mobini
    • 1
  • T.-J. Chiang
    • 1
  • A. S. A. Al-Ruwaitea
    • 1
  • M.-Y. Ho
    • 1
  • C. M. Bradshaw
    • 1
  • E. Szabadi
    • 1
  1. 1.Psychopharmacology Section, Division of Psychiatry, University of Nottingham, Room B 109, Medical School, Queen’s Medical Centre, Nottingham NG7 2UH, UKGB

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