774 RI of 120 s on the seeking link (ST(RI-120)). Self- administration training started with the acquisition of the taking response under an FR-1 schedule of reinforcement. During acquisition sessions, only the taking lever was present and pressing this lever immediately resulted in the infusion of 0.25 mg cocaine in 0.1 ml saline delivered over 5.6 s, the illumination of the cue light above the taking lever for 5.6 s, the retraction of the lever, and the switching off of the house light. After a 20 s time-out period, the taking lever was reintroduced and the house light illumi- nated, signaling the start of a new cycle. Once animals had acquired cocaine self-administration, they were gradually introduced to the seeking–taking chain schedule, starting with a schedule with an RI requirement of 2 s on the seeking link. ST(RI) sessions started with the introduction of the “seeking lever” and the illumination of the house light. The first press on the seeking lever initiated the RI and pressing this lever was without consequences until the RI had elapsed. When the RI had elapsed, pressing the seeking lever resulted in retraction of the seeking lever and insertion of the taking lever. Next, responding on the taking lever (FR-1) resulted in an infusion with cocaine, illumina- tion of the cue light, retraction of the taking lever, and the switching off of the house light. This was followed by a 10 min time-out period to minimize the influence of cocaine-induced locomotor effects on responding for the next reward. After the time-out period, a new cycle started by the reintroduction of the seeking lever and the illumination of the house light. When the rats had acquired the task under an RI of 2 s, the RI was progressively increased between sessions until animals had acquired the task under an RI of 120 s. The program automatically ended after 2 h or if the animals had obtained 10 rewards, whichever occurred first. After rats had acquired stable seeking response rates under the training dose of cocaine (0.25 mg/infusion), we performed a between-session dose–response curve for different unit doses of cocaine (0.063, 0.125, 0.25, or 0.5 mg cocaine per infusion). Stable seeking was defined as less than 15% variation in presses per min over three consecutive sessions and no up- or downward trend. Animals were allowed to self-administer each unit dose until they reached stable seeking rates for this dose. Each animal self-administered all unit doses according to a Latin square design. In a second group of animals, we examined the effect of the dopamine receptor antagonist, α-flupenthixol, on the performance under the ST(RI) cocaine schedule of reinforcement. Rats were trained to self-administer 0.25 mg of cocaine per infusion under the ST(RI-120) schedule, and after acquiring stable response rates, the rats received α-flupenthixol (0, 0.05, 0.25, or 0.5 mg/kg, i.p.) 30 min before the start of the session. α-Flupenthixol injections were administered according to a Latin square Psychopharmacology (2012) 220:771–785 design, and test sessions were separated by at least two sessions without treatment. The omission and reacquisition experiment was performed with 15 rats, 10 of these rats were previously tested in the dose–response curve experiment and 5 rats were previously tested for the effects of α-flupenthixol. Preceding reward omission sessions, rats were trained to self- administer 0.25 mg of cocaine per infusion. Next, rats received 10 reward omission sessions. Procedures during reward omission sessions were similar as during cocaine sessions, except that animals received an infusion with saline instead of cocaine after pressing the taking lever. After 10 reward omission sessions, cocaine (0.25 mg/infusion) was reintroduced, to assess reacquisition of responding for cocaine. ST(PR) schedule with cocaine reinforcement Rats were trained to lever press for cocaine under a heterogeneous ST chain schedule of reinforcement with a PR on the seeking link (ST(PR)). Experimental procedures were similar to responding for cocaine under the ST(RI) schedule, with the following exceptions. After acquisition of cocaine taking under an FR-1 schedule, an ST(PR) schedule was introduced. Under the ST(PR) schedule, animals had to meet a response requirement on the seeking lever that progressively increased after every earned reward (1, 2, 4, 9, 12, 15, 20, 25, etc.; Richardson and Roberts 1996) in order to get access to the taking lever. A session continued until the animal failed to meet the response requirement on the seeking lever within 1 h. After acquiring a stable seeking response rate for 0.25 mg/infusion of cocaine, rats were allowed to self- administer either 0.125 or 0.5 mg of cocaine per infusion. Under this schedule, stable levels of responding were defined as less than two reward variations over three consecutive sessions and no up- or downward trend. When individual rats showed stable response rates under this dose, they were switched to the other dose. In an additional group of animals, we tested the effects of α-flupenthixol (0, 0.05, 0.25, or 0.5 mg/kg i.p., 30 min before the start of the session according to a Latin square design). After comple- tion of testing the effects of α-flupenthixol, these rats received at least two more ST(PR) training sessions before they went through a reward omission period of three sessions. After three reward omission sessions, cocaine was reintroduced, to assess reacquisition of responding for cocaine. ST(RI) schedule with sucrose reinforcement Rats were trained to lever press for sucrose under a heterogeneous ST chain schedule of reinforcement with an RI requirement of 120 s on the seeking link (ST(RI-120)).