Supplementary methods
Detailed description of delayed reward task
Training of rats occurred in 32 identical rat five-hole nose poke operant chambers (Med Associates, St. Albans, VT, USA). Only the middle three holes were used for this experiment. The white houselight was located above the food receptacle on the wall opposite to the five nose-poke holes. During all phases of training and testing animals were trained once daily from Monday until Friday between 8.00 AM and 1.00 PM. Training occurred in 5 steps, taking 28 sessions, stable baseline behavior in the final procedure was acquired within 28 sessions.
Habituation Rats were habituated to the training environment for 15 minutes. The houselight was illuminated, and the pellet dispenser contained four pellets. Next, there were two sessions of 100 pellet deliveries, randomly delivered with an average interval of 15 seconds. During these sessions, the houselight was on.
Training phase one All training sessions ended after 75 rewards or 30 minutes, whichever came first. The houselight was illuminated during the entire session. During the first phase the three cue lights in the nose-poke holes were turned on during the entire session and every nose poke into one of these holes was rewarded with a food pellet. Within 3 sessions all rats received the maximum number of rewards within 30 minutes.
Training phase two Every trial started by turning on the middle cue light. Once a nose poke into this hole was made this light was switched of, and left and right cue-lights were illuminated. When a nose poke into one of those holes was made cue lights were extinguished, a food pellet was delivered and a new trial commenced. All rats acquired the maximum number of rewards within 3 sessions.
Training phase three Training continued by introducing the choice between 1 and 4 pellets. Sessions contained 5 blocks of 12 trials. The first two trials of each block were forced choice trials. A trial started by illumination of the middle cue light. After a nose-poke was made, the left or right cue light was illuminated. A nose-poke into this illuminated hole was rewarded. One side was associated with 1 pellet, while a nose-poke on the other side resulted in a reward of 4 pellets. The position of the small and large reward was always the same for one animal, but counterbalanced between animals. The forced trials were followed by 10 choice trials. These trials started with the illumination of the middle light, after a nose-poke was made, left and right cue-lights were illuminated. In this way rats could choose to nose-poke for 1 pellet on one side, or 4 pellets on the other side. Once a nose-poke in an illuminated hole was made, cue lights were switched off and the corresponding reward was delivered. If the rat did not respond within the 10 seconds of illumination, the cue light was switched off and the inter trial interval commenced. Nose-pokes into non-illuminated holes were recorded, but without consequences. All rats acquired stable preference (of at least 85%) for the large reward within 11 sessions.
Last training phase In this phase the delay for the large reward was introduced. There were again 5 blocks of 12 trials, including 2 forced and 10 choice trials. The forced trials indicated the length of the delay for the large reward. All trials were the same as during training phase three except that the large reward was provided after a specific delay. The small rewards were always delivered immediately, but the large reward came with a delay. For the first two sessions this delay increased over blocks as follows: 0, 2, 4, 6, and 8 seconds. During the following three sessions the delay increased from 0, 2, 4, and 8, to 16 seconds. Another three sessions were provided including delays of 0, 4, 8, 16 and 32 seconds. Inter trial intervals of the whole session were adapted to the length of the longest delay of that session. In this way it was always more advantageous to choose for the larger reward.
Final procedure The entire procedure was identical to the last phase of training except for the delays between the nose-poke and the delivery of the large reward. The delay for the large reward of the final procedure was increasing over blocks as follows: 0, 5, 10, 20, and 40 seconds. Rats acquired stable preference levels within 28 sessions.
Selection of animals in experimental design 1 The 25% most and 25% least impulsive rats were selected for self administration. The middle 50% of rats was used for other experiments, which are not mentioned in this paper. Selection was based on the average indifference point of the last 3 days of the final DRT procedure. The equation of Mazur (1987) was used to determine the indifference point of each rat: Indifference point = Preference at delay 0 / (1 + k*delay). Rats that showed less than 50% preference for the large reward at delay 0, or that showed an increase in preference over increasing delays were excluded from selection. The 25% least and 25% most impulsive rats were retrained to baseline levels after the vena jugularis surgery. From the start of self administration procedures DRT training occurred once a week.
Acute challenges in DRT
As part of experiment 1, the effect of acute cocaine injections on impulsive choice were tested. In between context-induced reinstatement and context extinction, all animals underwent one week of daily DRT training. On the Wednesday and Friday of this week rats received an injection of either cocaine (7.5 mg/kg) or saline in counterbalanced order, in a volume of 1 ml/kg. This challenge was performed to investigate whether cocaine intake would differentially affect the sensitivity towards an acute challenge of cocaine in high and low impulsive rats. In addition, these changes in impulsive choice and cocaine-induced reinstatement could be related.
In experiment 2, the acute challenge with SCH-23390 in the DRT was performed before and after SA in order to determine whether cocaine intake would change the effect of the preferential dopamine D1 antagonist SCH-23390 on impulsive choice. The challenge after cocaine SA was described in the main text of this paper. The challenge before cocaine SA was performed after establishment of stable baseline performance, but before catheter placement. Within one week of daily DRT training rats received on Wednesday and Friday in counterbalanced order an injection of either SCH-23390 (10 μg/kg) or saline, subcutaneously in a volume of 1 mg/kg bodyweight.
Supplementary Results
Acute cocaine challenge in DRT
Compared to acute injections of saline, the acute challenge with cocaine (7.5 mg/kg) decreased impulsive choice in both HI [F(1,24)=4.70, p<0.05] and LI rats [F(1,22)=6.48, p<0.05], with no difference between saline or cocaine SA pre-treatment (supplementary figure 1). Moreover, this acute cocaine challenge did not change the number of omissions, indicating that cocaine intake did not influence the sensitivity to an acute challenge of cocaine. Moreover, the reduction in impulsive choice induced by the acute challenge of cocaine was not correlated with the cocaine-induced reinstatement test, the latter being described in the main text of the paper [r=-.06 , ns].
Suppl fig 1. Effects of acute cocaine challenge on impulsive choice Acute challenges with 7.5 mg/kg cocaine decreased impulsive choice to an equal extent for both high (HI) and low impulsive (LI) animals after cocaine (A) or saline self-administration (B). *p<0.05 compared to respective saline challenge.
Acute SCH-23390 challenge in DRT
As shown in supplementary figure 2, the pharmacological challenges with SCH-23390 increased impulsive choice both before [F(4,100)=8.46, p<0.001] and following the cocaine SA paradigm (see main text). SCH-23390 did not alter the number of omissions before, but increased the number of omissions after the cocaine SA (see main text). There was, however, no difference in the number of omissions of the two tests [F(1,23)=2.55, ns]. These results indicate that cocaine intake did not change the sensitivity to an acute challenge with SCH-23390.
Suppl fig 2. Effects of acute SCH-23390 challenge on impulsive choice The acute challenge with 0.01 mg/kg SCH-23390 increased impulsive choice to an equal extent before or after cocaine self-administration (A). The number of omissions was only increased after cocaine SA (B). *p<0.05 compared to saline challenge.
Supplementary figures
Suppl fig 3 The acquisition, motivation, sensitivity to self-administer cocaine and context extinction from experiment 1 The number of active and inactive responses during 13 FR1 sessions in the acquisition phase (A). The first 5 sessions delivered 0.25 and the remaining 8 sessions 0.5 mg/kg/infusion cocaine. Over sessions there was an increasing preference for the active over the inactive lever (*p<0.05). Motivation to self-administer cocaine was measured during 3 progressive ratio sessions (B). The number of active responses leading to one infusion was increased along the following equation: response ratio = 5 * e(0.2*infusion number) – 5. There was no significant difference between HI and LI animals. In the cocaine dose response curve (C), the number of active responses during six 1 hour sessions with doses decreasing over sessions from 500 to 250, 125, 62.5, 31.25 and 15.625 μg/kg cocaine. The inverted U-shape of responding was not different between HI and LI animals. **p<0.001, main effect of cocaine dose. The number of active responses during context extinction decreased over sessions and HI animals were more resistant **p<0.001, main session effect and #p<0.05 HI vs LI.
Suppl fig 4 The acquisition and extinction curves from experiment 2. Number of active and inactive responses during 19 FR1 sessions (A). The first 5 sessions delivered 0.25 and the remaining 14 sessions 0.5 mg/kg/infusion cocaine. The first 7 days only afternoon cocaine self-administration (SA) sessions were taken, the remaining 12 days consisted of morning saline SA and afternoon cocaine SA sessions. Over sessions there was an increasing preference for the active over the inactive lever and for cocaine over saline. **p<0.001, main effect of lever and ##p<0.001, main effect of SA drug. The number of active and inactive responses during 16 extinction sessions (B). Active responding for cocaine decreased over the first six sessions. **p<0.001 main effect lever, within cocaine SA.
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