Effects of orbital prefrontal cortex dopamine depletion on inter-temporal choice: a quantitative analysis

Lesions of the orbital prefrontal cortex (OPFC) can cause pathologically impulsive behaviour in humans. Inter-temporal choice behaviour (choice between reinforcers differing in size and delay) has been proposed as a model of "impulsive choice" in animals. We recently found that destruction of the OPFC disrupted inter-temporal choice in rats. It is not known whether the dopaminergic projection to the OPFC contributes to the regulation of inter-temporal choice.

A quantitative method was used to compare inter-temporal choice in rats whose OPFC had been depleted of dopamine with that of sham-lesioned control rats.

Under halothane anaesthesia, rats received injections of 6-hydroxydopamine into the OPFC (2 microg microl(-1), 0.5 microl, two injections in each hemisphere), or sham lesions (injections of the vehicle). They were trained to press two levers (A and B) for sucrose reinforcement (0.6 M) in discrete-trials schedules. In free-choice trials, a press on A resulted in delivery of 50 microl of the sucrose solution after a delay dA; a press on B resulted in delivery of 100 microl of the same solution after a delay dB. dB was increased progressively across successive blocks of six trials in each session, while dA was manipulated systematically across phases of the experiment. The indifference delay, dB(50) (value of dB corresponding to 50% choice of B) was estimated for each rat in each phase. Linear functions of dB(50) versus dA were derived, and the parameters of the function compared between the groups. Concentrations of monoamines in the OPFC were determined by high-performance liquid chromatography at the end of the experiment.

In both groups, dB(50) increased linearly with dA (r2>0.9 in each case). The slope of the function was significantly steeper in the lesioned group than the sham-lesioned group, whereas the intercept did not differ significantly between the groups. When delays of 4 or 8 s were imposed on the smaller reinforcer, the lesioned rats showed greater tolerance of delay to the larger reinforcer (i.e. they exhibited longer values of dB(50)) than the sham-lesioned rats. Dopamine, noradrenaline and 5-hydroxytryptamine levels in the OPFC of the lesioned group were 20, 75 and 98% of those of the sham-lesioned group.

The results indicate that dopaminergic afferents to the OPFC contribute to the regulation of inter-temporal choice behaviour due to their role in determining organisms' sensitivity both to reinforcer size and to delay of reinforcement.