Animals
Animal experiments were approved by the State animal care committee
(Landesamt für Gesundheit und Soziales, Berlin, Germany) and followed
the ARRIVE guidelines (Kilkenny et al., 2010). Male Wistar rats (200-300
g; Janvier, Le Genest-Saint-Isle, France) were housed in groups of 1-2
per cage in a 12 h light/dark cycle, lined with ground corncob bedding
and with free access to standard laboratory rodent chow and tap water.
Room temperature was 22 ± 0.5◦C and humidity was 60–65%. We
investigated males only because the current evidence on gender- or
sex-differences in opioid analgesia is not strong enough to warrant
gender- or sex-specific therapeutic interventions (Bruehl et al., 2013;
Fullerton et al., 2018).