Effects of central 5-hydroxytryptamine depletion on sensitivity to delayed and probabilistic reinforcement.
BACKGROUND
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 (dB) (experiment 1) or with a probability (pB) less than 1 (experiment 2).


RESULTS
In experiment 1, both groups showed declining choice of lever B (%B) as a function of dB. The lesioned group showed shorter indifference delays (D50: the value of dB 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, thetaB (thetaB=[1/pB]-1). There was no difference between the "indifference odds" (theta50: the value of thetaB 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.
Mobini
S
S
Division of Psychiatry, University of Nottingham, Medical School, Queen's Medical Centre, UK.
Chiang
T J
TJ
Ho
M Y
MY
Bradshaw
C M
CM
Szabadi
E
E
D000818:Animals; D002755:Choice Behavior / Q000502:physiology*; D005260:Female; D006897:Hydroxyindoleacetic Acid / Q000378:metabolism; D007175:Impulsive Behavior / Q000378:metabolism / Q000523:psychology; D011336:Probability; D051381:Rats; D017208:Rats, Wistar; D012054:Reinforcement, Psychology; D012701:Serotonin / Q000172:deficiency / Q000378:metabolism* / Q000502:physiology
D012701:Serotonin; D006897:Hydroxyindoleacetic Acid