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

Area of Science:

• Neuroscience
• Behavioral Economics
• Pharmacology

Background:

• Ascending serotonergic (5-HTergic) pathways are implicated in impulse control.
• Central 5-HT depletion in rats increases impulsive choice (smaller, immediate vs. larger, delayed reinforcers).
• The precise mechanism (altered time discounting vs. altered reward sensitivity) remains unclear.

Purpose of the Study:

• To investigate the effect of central 5-HT depletion on time discounting.
• To differentiate effects on time discounting from effects on reinforcer size sensitivity using a quantitative model.

Main Methods:

• Rats underwent 5,7-dihydroxytryptamine lesions of dorsal and median raphe nuclei or sham procedures.
• A discrete-trials adjusting-delay schedule assessed inter-temporal choice between one immediate pellet and two delayed pellets.
• The multiplicative hyperbolic model quantified time discounting and reinforcer size sensitivity.

Main Results:

• Both groups exhibited linear indifference functions, consistent with the multiplicative hyperbolic model.
• 5-HT depleted rats showed shorter indifference delays, indicating accelerated time discounting.
• Lesioned rats had reduced brain 5-HT and 5-HIAA, with unaltered noradrenaline and dopamine levels.

Conclusions:

• Central 5-HT depletion uniquely alters time discounting, not sensitivity to reward size.
• These findings demonstrate that serotonin depletion accelerates the rate of time discounting for food rewards.