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.