Animals
We employed rats in this study because the rat is the preferred species for preclinical addiction research (Homberg et al., 2017). We used SERT-/- rats as model for the human SERT gene polymorphism as has been established previously (Caspi et al., 2010). SERT−/− rats (SLC6A41Hubr) were generated by N-ethyl-N-nitrosurea (ENU) induced mutagenesis (Homberg et al., 2007; Smits et al., 2006) and outcrossed with commercially available Wistar rats (Harlan, Ter Horst, the Netherlands) for at least 15 generations. Male SERT−/− and SERT+/+ offspring were used for experiments as in human psychostimulant addiction has a clear male preponderance (Degenhardt et al., 2014). The rats were bred in Nijmegen and either tested in Nijmegen or transported to Poland. In Nijmegen, rats were housed in groups of 2 rats in enriched Macrolon type III cages (42 x 26 x 15 cm; Techniplast 1291H, Tecnilab-BMI) with corncobs bedding (irradiated, SPPS COB12, Bio Services) under conventional conditions (no filtertops). The animals had access to food (dried pellets of standard chow food (Ssniff RM V1534-703 diet supplied by BioServices) and waterad libitum , except during test phases. The rats were housed under a reversed day and night cycle (lights off at 08:00 AM) in temperature (21±1°C) and humidity (55±5%) controlled rooms. Testing (AMPH self-administration) took place in the dark phase of the light/dark cycle. In Poland, rats were group-housed 4 rats per cage (380 × 200 × 590 mm, Ehret Labor‐ und Pharmatechnik GMBH & Co.KG, Germany) with aspen litter (MIDI LTE E‐002 Abedd, AnimaLab, Poland) and environmental enrichment. The rats had free access to food (VRF1 (P) Special Diets Services, England) and water. Animals were housed in a temperature-controlled (21 ± 1°C) and humidity-controlled (40–50%) colony room under a 12/12 h light/dark cycle (lights on at 06:00 AM). Behavioural testing (locomotor activity test) was performed during the light phase of the light/dark cycle. Testing was always done blindly by an experimenter who was unaware of the genotype of the animals. All procedures were carried out in agreement with the current Directive 2010/63/EU and Dutch and Polish Research Council Guide for the Care and Use of Laboratory Animals and were approved by local and national Institutional Animal Care and Use Committes. The experimental procedures at Radboudumc (Nijmegen, The Netherlands) were performed under a project license from the Central Committee on Animal Experiments (Centrale Commissie Dierproeven, CCD, The Hague, The Netherlands), in full compliance with the legal requirements of Dutch legislation on the use and protection of laboratory animals (Animal Testing Act, WOD). The experimental procedures in Poland were performed at the Maj Institute of Pharmacology, Polish Academy of Science, Krakow, Poland. All efforts were made to reduce the number of animals used and their suffering both in the Netherlands and in Poland.