Dopamine modulates effort-based decision making in rats

Abstract

Previous research has implicated dopamine as a modulating factor in choice behavior based on effort. The purpose of the present study was to determine the individual contribution of different dopamine receptors to effort-based decision making in rats. Rats were trained in a T-maze to choose a large-reward arm that contained 8 pellets of food over a small-reward arm that contained 2 pellets of food. The rats then were trained to climb progressively higher barriers to obtain the food from the large-reward arm. Using a discounting procedure on each test day, it was found that rats were more likely to choose the small-reward arm after treatment with the D1 antagonist, SCH-23390, or the D2 antagonist, haloperidol. The dopamine agonist, D-amphetamine, biased the rats toward choosing the large-reward arm and blunted the effects of SCH-23390 or haloperidol. Treatment with the D3 receptor antagonist, U99194, or the D3 receptor agonist, 7-OH-DPAT, did not alter choice behavior. These data indicate that D1 and D2 receptors are required for decisions based on effort.

Figure 1

A. Timeline of training procedures. B. Diagram of discounting procedure.

Figure 2

Effects of the barrier procedure on arm choice and time to complete the trial. A. Rats were significantly less likely to choose the arm containing eight pellets (LR arm) on the first day of training in comparison to the third day as indicated by the single asterisk. Placing a barrier in the LR arm significantly reduced the % of LR arm choices in comparison to the third day of training without the barrier, as indicated by the double asterisks. Rats made significantly more LR arm choices after the third day of training with the barrier in comparison to their choice behavior on the first day of training with the barrier. However, they also made significantly fewer LR choices as compared to the third day of training without the barrier, as indicated by the triple asterisks. In B. as indicated by the single asterisk, rats took significantly longer to complete the trial on the first day of barrier training in comparison to all other training days, except the first one without the barrier. Data represent mean ± S.E.M. n = 9.

Figure 3

Effects of the discounting procedure on arm choice and time to complete the trial. A. Rats took significantly fewer trials to choose the small-reward (SR), non-barrier arm that contained two pellets when tested under the discounting versus the non-discounting procedure, as indicated by the asterisk. B. Rats also took longer to complete the trial during the Discount test days as opposed to the No Discount test days, as indicated by the asterisk. Data were obtained from three sessions under each condition and represent mean ± S.E.M. n = 9.

Figure 4

Effects of the D₁ antagonist, SCH-23390, and D-amphetamine on choice behavior and time to complete a trial. A. Pretreatment with SCH-23390 (0.0125 mg/kg) and treatment with saline decreased the number of trials to choose the small-reward (SR) arm in comparison to all pretreatment/treatment regimens, as indicated by the asterisk. B. Pretreatment with SCH-23390 and treatment with saline significantly increased the time to complete a trial in comparison to all pretreatment/treatment regimens, as indicated by the single asterisk. Pretreatment with SCH-23390 and treatment with D-amphetamine (0.75 mg/kg) significantly increased the time to complete a trial in comparison to pretreatment with saline, as indicated by the two asterisks. Data represent mean ± S.E.M. n = 9.

Figure 5

Effects of haloperidol and D-amphetamine on choice behavior and time to complete a trial. A. Pretreatment with saline and treatment with D-amphetamine (0.75 mg/kg) increased the number of trials to choose the SR arm in a comparison with all other regimens, as indicated by the single asterisk. Pretreatment with haloperidol (0.1 mg/kg) and treatment with saline decreased the number of trials to choose the SR arm in comparison to all other pretreatment/treatment regimens as indicated by the two asterisks. B. Pretreatment with haloperidol significantly increased the time to complete each trial in comparison to pretreatment with saline, as indicated by the asterisk. Data represent mean ± S.E.M. n = 9.

Figure 6

Effects of the D₃ agonist, 7-OH-DPAT (A.) and the D₃ antagonist, U-99194 (6.25 mg/kg) (B.) on choice behavior. There were no significant treatment effects in either experiment. Data represent mean ± S.E.M. n = 9.