... We place special focus on the serotonin system and its 2A receptor (5-HT2AR) subtype in particular, which has been shown to be particularly implicated in biology x environment interactions (Chang et al., 2017;Dressler et al., 2016;Fiocco et al., 2007;Jiang et al., 2016;Jokela et al., 2007;Lebe et al., 2013;Mellman et al., 2009;Parade et al., 2017;Salo et al., 2011). Various acute stressors appear to be reliable and robust inducers of serotonin release (Adell et al., 1997;Amat et al., 1998Amat et al., , 2005Bastani et al., 2017;Beitia et al., 2005;Bland et al., 2003;Cohen et al., 2015;Ferres-Coy et al., 2013;Fuenmayor and Garcia, 1984;Fujino et al., 2002;Gardner et al., 2005;Hale et al., 2011;Higuchi et al., 2019;Ishida et al., 1997;Ishiwata and Greenwood, 2018;Johnson et al., 2005;Keeney et al., 2006;Kelly et al., 2011;Li et al., 2015;Myers and Beleslin, 1971;Nakajima et al., 2009;Neugebauer, 2020;Paul et al., 2011;Rex et al., 2005;Yoshioka et al., 1995) and different types of stressor, such as sleep deprivation (Elmenhorst et al., 2012;Maple et al., 2015;Zhao et al., 2019), hypoxia (Anju et al., 2011), chronic tryptophan depletion (Cahir et al., 2007), inflammation (Zhang et al., 2001), tonic pain (Kupers et al., 2009), repeated forced swim (Takao et al., 1995), repeated shock (Dwivedi et al., 2005), administration of stress hormones (Jitsuiki et al., 2000;Kuroda et al., 1992Kuroda et al., , 1994, amygdala stimulation (Kalynchuk et al., 2006), time-dependent stress , novelty stress (Aloyo and Dave, 2007), maternal separation (Benekareddy et al., 2010(Benekareddy et al., , 2011Godar et al., 2019;Vazquez et al., 2000; although see Ohta et al., 2014), isolation rearing (Preece et al., 2004;Rilke et al., 1998), isolation housing (Günther et al, 2008;Schiller et al., 2003;although see Bibancos et al., 2007;Schiller (Berton et al., 1997;McKittrick et al., 1995; although see Visser et al., 2014) all appear to upregulate 5-HT2AR expression, particularly in the cerebral cortex (Anju et al., 2011;Benekareddy et al., 2010Benekareddy et al., , 2011Berton et al., 1997;Cahir et al., 2007;Dwivedi et al., 2005;Elmenhorst et al., 2012;Godar et al., 2019;Günther et al, 2008;Harvey et al., 2003;Jitsuiki et al., 2000;Kalynchuk et al., 2006;Kupers et al., 2009;Kuroda et al., 1992Kuroda et al., , 1994Maple et al., 2015;McKittrick et al., 1995;Ossowska et al., 2001;Preece et al., 2004;Rilke et al., 1998;Takao et al., 1995;Vazquez et al., 2000;Zhao et al., 2019; see also Table 1), which is well known to be massively expanded in humans. Social isolation and defeat also reliably sensitize behavioural responses to 5-HT2AR agonists (Benekareddy et al., 2010;Brotto et al., 1998;Gorzalka et al., 1998;Sakaue et al., 2002;Sood et al., 2018;Wright et al., 1991; see also Table 1). ...