Recently evidence suggests that measures of child temperament may predict ADHD symptoms (Einziger et al., 2018). Temperament is a characteristic of personality (Buss & Plomin, 1984; Crowell, 2016) and refers to individual, neurobiologically-based difference in reactivity, self-regulation and cognition (Eisenberg, 2012).
It has been previously emphasized (Nigg J., 2016), that Hypotalamic-Pituitary-Adrenal (HPA) axis, through the cortisol hormone, may represent a powerful biological measure of behavioural self-regulatory systems, activity level, cognition, temperament (Stadler et al, 2011; Martel et al., 2009; Sonuga-Barke, 2005) and arousal (Snoek, Van Goozen, Matthys, Buitelaar, & Engeland, 2004). Cortisol is released from the surrenal gland by means of the HPA axis activation, in response to catecholaminergic neurotransmitters (Ulrich-Lai & Herman, 2009). Cortisol is involved in the regulation of a wide range of body functions, including emotion processing (Skosnik, Chatterton, Swisher, & Park, 2000), awakening (Fries, Dettenborn, & Kirschbaum, 2009) and stress responses (Chrousos, Kino, & Charmandari, 2009).
Both cortisol and temperament may share self-response regulatory processes (Martel et al., 2008; Nigg, 2016; Ulrich-Lai & Herman, 2009), and one study on 70 healthy pre-schoolers indicates that children with SE temperament has higher morning salivary cortisol levels during their first week of a new primary school year (Davis, Donzella, Krueger, & Gunnar, 1999), hypothesized as a stress-induced effect.
To date, no studies have been conducted to study heterogeneity starting from the self-response regulatory processes between temperament and cortisol in children and adolescents with ADHD.
As for clinical application of HPA-axis and cortisol level in ADHD diagnosis and dimension, available studies are, as yet, either inconsistent (Bonvicini, Faraone, & Scassellati, 2016; Freitag et al, 2009, Corominas et al, 2012) or suggestive, but not significant, upon dimensional stratification of ADHD symptoms (Pinto et al., 2016).
The aims of this study to contribute to the issue of clinical heterogeneity of ADHD, analysing whether ADHD symptoms and co-morbidity traits simultaneously link to both cortisol level and temperament dimensions, as biomarkers of arousal and inhibited behaviour. To pursue our aim we formulated five specific research questions: i) Do temperament dimensions and awakening cortisol level differ between children and adolescents with ADHD and TDC? ii) Do temperament dimensions and awakening cortisol levels correlate with dimension of ADHD core symptoms? iii) Does awakening cortisol level associate with any of the three-temperament dimension? iv) Do parent ratings of the oppositional and defiant traits and anxiety traits affect the association between temperament dimensions and cortisol levels with ADHD core symptoms in the ADHD group?
