Salivary Neurosteroid Levels and Behavioural Profiles of Children with Attention-Deficit/Hyperactivity Disorder During Six Months of Methylphenidate Treatment

Abstract

Objective:

This prospective study aimed to investigate the relationships between salivary levels of neurosteroids, including dehydroepiandrosterone (DHEA), cortisol, and DHEA/cortisol ratios, and behavioral symptoms in patients with attention-deficit/hyperactivity disorder (ADHD) during treatment with methylphenidate (MPH).

Methods:

Fifty-eight ADHD patients (48 boys and 10 girls) were included in the study initially. Forty patients (mean age: 7.77±1.64 years; 32 boys and 8 girls) who completed the study received treatment with oral MPH with a dose range of 5–15 mg/day (mean dose: 12.47±7.74 mg/day.) for 6 months at the discretion of the psychiatrist. DHEA and cortisol levels were determined from saliva samples collected at 0800 h at baseline and 6 months from baseline. ADHD symptoms were evaluated with the Child Behavior Checklist (CBCL).

Results:

Salivary DHEA levels (mean difference=9.05 pg/mL, p=0.027) and DHEA/cortisol ratios (mean difference=32.42, p=0.007) in ADHD patients were significantly increased, but the cortisol levels did not change significantly. During a 6 month follow-up, all behavioral problems assessed using the CBCL improved significantly. Changes in salivary DHEA levels were positively correlated with changes in salivary cortisol levels (r=0.44, p=0.004); however, changes in salivary levels of DHEA, cortisol, and the DHEA/cortisol ratio were not significantly correlated with change in any subscales of the CBCL. Mean doses of MPH were not significantly correlated with changes in neurosteroid levels and behavioral symptoms.

Conclusions:

These findings provide evidence that MPH administration might affect DHEA levels and DHEA/cortisol ratios. Whether levels of neurosteroids are directly associated with brain function or behavioral problems in ADHD patients warrants further investigation.

Introduction

Attention deficit/hyperactivity disorder (ADHD) is a common childhood and adolescent neuropsychiatric disorder, with a reported prevalence of ∼3–10% among school-age children worldwide (Polanczyk et al. 2007). The core symptoms of ADHD are inattention, hyperactivity, and impulsivity, which have a significantly negative impact on global aspects of functioning (American Psychiatric Association 2000; Spencer et al. 2007). The underlying pathophysiological mechanisms of ADHD are complex and multidimensional. In recent years, the potential role of neurosteroids has been implicated in the pathogenesis of ADHD (Dubrovsky 2005; Golubchik et al. 2007; Trent et al. 2012, 2013).

Dehydroepiandrosterone (DHEA) and its sulfated form (DHEA-S) are important neurosteroid substrates; they play several vital neuropsychiatric roles and are affected by various physiological processes (Maninger et al. 2009). DHEA(S) may play crucial roles in guiding cortical projections to appropriate targets, and, therefore, is important for the regulation of neurodevelopment (Golubchik et al. 2007). DHEA(S) also exerts stimulatory or antagonic effects at γ-aminobutyric acid (GABA_A) receptors and facilitates N-methyl-d-aspartate (NMDA) activity (Maayan et al. 2003). DHEA has been reported to be associated with the clinical characteristics of ADHD (Strous et al. 2001; Wang et al. 2011b). Dysregulation of the hypothalamic–pituitary–adrenal (HPA) axis, which is typically measured by cortisol levels, has also been proposed to be associated with ADHD (Talge et al. 2007; Ma et al. 2011). Low cortisol levels generally reflect underarousal or an elevated threshold for the detection of stressors (Isaksson et al. 2012). Therefore, baseline salivary cortisol concentrations might serve as a biomarker for ADHD (Scassellati et al. 2012). Moreover, the DHEA/cortisol ratio is a widely utilized index of the relative activity of the two steroids, and has been implicated in analyzing the function of neurosteroids in psychiatric disorders (Goodyer et al. 2001). DHEA/cortisol ratios have shown an inverse correlation with ADHD symptoms (Strous et al. 2001), and a positive correlation with neuropsychological performance in ADHD patients (Wang et al. 2011b).

Methylphenidate (MPH), the most therapeutically efficient drug for ADHD, exerts its pharmacological effects via increasing the levels of dopamine and norepinephrine in the synaptic cleft (Overtoom et al. 2003; Wilens 2008). MPH exerts treatment effects on both behavioral and cognitive dimensions in ADHD patients (Hellwig-Brida et al. 2011; Huang and Tsai 2011). Studies have generally and consistently indicated that MPH administration would lead to increases in the levels of DHEA(S) (Maayan et al. 2003; Lee et al. 2008; Wang et al. 2011a). By contrast, the trends in changes in the cortisol levels of ADHD patients during MPH treatment have not been consistent between studies (Lee et al. 2008; Chen et al. 2012; Wang et al. 2012). Nevertheless, there is as yet no study that has investigated changes in DHEA/cortisol ratios in ADHD patients under MPH treatment. Numerous rating scales are available for measuring the behavioral symptoms associated with ADHD (Collett et al. 2003). A broadband scale measure, such as the Child Behavior Checklist (CBCL) (Achenbach 1991), is capable of establishing a more complete assessment of the changes in behavioral symptoms associated with ADHD among patients. However, research on the use of the CBCL to investigate the relationship between neurosteroid levels and behavior profiles of patients with ADHD during MPH treatment remains scarce.

Therefore, the present study aimed to investigate the changes in salivary levels of DHEA, cortisol, and the DHEA/cortisol ratio in ADHD patients during a 6 month treatment with MPH, and also sought to elucidate the relationship between changes in the salivary levels of neurosteroids and in the behavioral profiles of patients with ADHD.

Methods

Participants

This study was approved by the Institutional Review Board of Chang Gung Memorial Hospital, and was conducted at the Child and Adolescent Psychiatry Outpatient Department of Chang Gung Memorial Hospital, Keelung. Case recruitment and baseline data of the participants are described elsewhere in detail (Wang et al. 2011a, 2012). In brief, only patients aged between 6 and 12 years of age, who met the Diagnostic and Statistical Manual of Mental Disorders, 4th ed. (DSM-IV), criteria for ADHD (American Psychiatric Association 2000) using the Chinese-language version of the Kiddie Schedule for Affective Disorders and Schizophrenia-Epidemiologic version (K-SADS-E) were recruited. Patients with a history of major physical or additional psychiatric diseases were excluded. The patients were either newly diagnosed with ADHD or had an existing diagnosis but had been drug-free for longer than 6 months.

Study procedures

This investigation comprised a 24 week, nonrandomized, observational, prospective study. At baseline (pretreatment), saliva samples of ADHD patients were collected at 0800h in the outpatient department, using the passive drool method; patients had been instructed to avoid excessive levels of physical activity during the preceding 24 h and to maintain an overnight fast prior to saliva collection.

The social and behavioral competence of the ADHD patients was evaluated with the CBCL, which the parents of the patients completed. After the research procedures were performed, we counseled the parents about the treatment program. Subjects were prescribed oral MPH at a dose range of 5–15 mg/day. Concomitant medications were not permitted. Patient care was left to the discretion of the psychiatrist, and modification of the MPH dose was allowed as with the usual practice. After 6 months of treatment from baseline, the patients were administered MPH at 0730h and saliva samples were collected at 0800h. The patients were evaluated with the CBCL again.

Laboratory tests for neurosteroid levels

Saliva was collected into collecting tubes and immediately placed on ice; it was stored at −30°C until assayed. DHEA was quantified with a DHEA Luminescence immunoassay (RE62051), and cortisol with a Cortisol ELISA (RE52611) (IBL Gesellschaft Für Immunchemie Und Immunbiologie MBH, Hamburg, Germany). The range of detection of these methods for DHEA and cortisol was 3–3000 pg/mL and 0.015–4 μg/dL, respectively. The intra- and inter-assay coefficients of variation were 5.4–14.9% and 9.5–15.2% for DHEA and 3.2–6.4% and 6.2–9.1% for cortisol, respectively.

Behavior measurement

The CBCL is an instrument completed by parents or caregivers that evaluates the social and behavioral competence of children between 4 and 16 years of age over the preceding 6 months (Achenbach 1991). The CBCL consists of 113 items and uses a three point Likert scale. The CBCL contains eight narrowband syndromes (i.e., Withdrawn, Anxious/Depressed, Somatic Complaints, Social Problems, Thought Problems, Attention Problems, Aggressive Behavior, and Delinquency) and two broadband syndromes (i.e., Internalizing Problems and Externalizing Problems). A T score of 50 in each subscale indicated average functioning in reference to the other children of the same age and gender. The Chinese-language version of the CBCL has been found to have high test–retest reliability and validity (Yang et al. 2000; Leung et al. 2006;).

Statistical analyses

Data were analyzed using the Statistical Package for the Social Sciences for Windows (version 16.0; SPSS, Inc., Chicago, IL). Variables are presented as either mean±standard deviation (SD) or frequency. Because data on the DHEA levels and the DHEA/cortisol ratio exhibited significant positive skews and violated normal distribution, nonparametric statistics were used to analyze the data.

The paired-samples t test or Wilcoxon signed rank test was used to compare the change in variables at baseline and at end-point. Pearson's correlation coefficient or Spearman's correlation coefficient was used to investigate associations between the changes in variables during the 6 month treatment. Two tailed p values <0.05 were considered statistically significant.

Results

Fifty-eight ADHD patients (48 boys and 10 girls) were included in the study, with mean ages of 7.72±1.63 years, respectively. Of the 58 ADHD patients at the initial visit, 40 completed the study (mean age: 7.77±1.64 years; 32 boys and 8 girls). The reasons for premature discontinuation were adverse events with MPH (n=3), noncompliance (n=3), withdrawal of consent (n=2), and being lost-to-follow-up (n=10). Mean dose of MPH over the 6 month period was 12.47±7.74 mg/day. There were no significant differences in sex, age, neurosteroid levels, and severity of behavioral symptoms between the remaining patients and the dropout patients.

After the 6 month treatment with MPH (Table 1), salivary DHEA levels (mean difference=9.05 pg/mL, p=0.027) and the DHEA/cortisol ratio (mean difference=32.42, p=0.007) were significantly increased, but salivary cortisol levels (mean difference=0.04 μg/dL, p=0.529) did not change significantly. The T scores of all subscales in the CBCL were significantly decreased, including Anxious/Depressed (mean difference=−4.00, p<0.001), Somatic Complaints (mean difference=−5.60, p<0.001), Social Problems (mean difference=−5.69, p<0.001), Thought Problems (mean difference=−4.44, p=0.007), Attention Problems (mean difference=−3.08, p=0.015), Aggressive Behavior (mean difference=−9.48, p<0.001), Delinquency (mean difference=−7.73, p<0.001), Internalizing Problems (mean difference=−5.48, p<0.001), and Externalizing Problems (mean difference=−5.95, p<0.001).

Table 1.

Salivary Levels of DHEA and Cortisol, the DHEA/Cortisol Ratio, and Behavior Profiles in the Child Behavior Checklist (CBCL) of ADHD Patients (n=40) During a 6 Month Follow-up

	Baseline	6 months later	Statistical value
	(Mean±SD)	(Mean±SD)	(t or Z)	p value
Neurosteroids
DHEA levels (pg/mL)	16.68±29.19	25.72±33.25	2.20^a	0.027^*
Cortisol levels (μg/dL)	0.50±0.26	0.54±0.33	0.64	0.529
DHEA/cortisol ratio	29.43±35.06	61.86±97.69	2.68^a	0.007^**
CBCL subscales
Anxiety/Depression	54.35±8.88	50.35±6.75	4.18	<0.001^***
Somatic Complaints	60.18±10.25	54.58±8.90	4.32	<0.001^***
Social Problems	57.66±10.09	51.97±9.06	3.86	<0.001^***
Thought Problems	59.62±10.12	55.19±10.16	2.90	0.007^**
Attention Problems	54.80±10.18	51.72±9.97	2.55	0.015^*
Aggressive Behavior	61.55±12.63	52.08±10.02	5.64	<0.001^***
Delinquency	67.10±10.33	59.38±10.09	6.10	<0.001^***
Internalizing Problems	63.05±8.96	57.58±8.70	4.44	<0.001^***
Externalizing Problems	64.18±10.69	58.22±9.74	4.79	<0.001^***

Statistical analyses were performed using Wilcoxon signed rank test.

p<0.05; ^** p<0.01; ^*** p<0.001.

ADHD, attention-deficit/hyperactivity disorder; DHEA, dehydroepiandrosterone.

The changes in salivary DHEA levels and salivary cortisol levels showed a significant positive correlation (r=0.44, p=0.004) for ADHD patients after 6 months of MPH treatment (Fig. 1). However, changes in salivary levels of DHEA, cortisol, and the DHEA/cortisol ratio were not significantly correlated with changes in any of the subscales of the CBCL (p>0.05). Mean doses of MPH were not significantly correlated with changes in neurosteroid levels and behavioral symptoms (p>0.05).

FIG. 1.

Relationship between the change in salivary dehydroepiandrosterone (DHEA) levels and the change in salivary cortisol levels (r=0.44, p=0.004) of ADHD patients (n=40) under a 6 month treatment with methylphenidate.

Discussion

Consistent with findings in previous studies (Maayan et al. 2003; Lee et al. 2008; Wang et al. 2011a), we found that MPH increased the DHEA levels of ADHD patients during a 6 month treatment. The dopaminergic and adrenergic agonistic activity of MPH may affect the activity of the neuroendocrine system, either through its influence on the secretion and feedback control of the HPA and hypothalamic–pituitary–gonadal (HPG) axes, or through its indirect influence on attenuating or potentiating the impact of environmental stress on the HPA axis and HPG activity (Hibel et al. 2007). DHEA and cortisol could be simultaneously regulated by the adrenocorticotropic hormone (ACTH) (Goodyer et al. 2001). The results of our study showed a significant correlation between change in salivary DHEA levels and change in cortisol levels under MPH administration. However, the levels of cortisol did not elevate significantly during a 6 month period, which is comparable to findings in other studies (Maayan et al. 2003; Lee et al. 2008). In contrast, several researchers suggested that MPH administration may increase cortisol levels, but that the increment may not last >4 weeks (Weizman et al. 1987; Chen et al. 2012; Wang et al. 2012). As no measurement was obtained between baseline and end-point in this study, whether a significant increment of cortisol appeared during this time cannot be known. With regard to the DHEA/cortisol ratio, DHEA (S) appears to antagonize the action of cortisol at the cellular level, and the DHEA/cortisol ratio might be associated with neural functioning (Goodyer et al. 2001). To the best of our knowledge, this report is the first to provide evidence that the DHEA/cortisol ratios of ADHD patients are significantly elevated with MPH treatment.

There are some methodological issues in this study that warrant concern. First, the patients with ADHD were drug-free at baseline; administration of MPH started after the initial visit. Patients with ADHD usually took MPH at ∼0730h daily before attending school. To evaluate the ordinary neurosteroids levels after treatment, the patients were instructed to be administered MPH at 0730h as usual. We then collected saliva samples at 0800h, 30 minutes after drug administration, after 6 months of treatment from baseline. The procedure was to avoid the confounding effect of MPH withdrawal. However, it does limit the ability to distinguish whether the changes in DHEA levels and DHEA/cortisol ratios were derived from acute MPH challenge or the long-term effect of MPH treatment. Second, cortisol levels often increase sharply following awakening, which is referred to as the cortisol-awakening response (CAR) (Elder et al. 2013). Children with ADHD have demonstrated lower CAR than healthy controls (Blomqvist et al. 2007). We measured the levels of cortisol in the ADHD children at a fixed time in the morning, but their waking time was not precisely identified. Therefore, variations in the waking time of individual patients might have affected the study results. Furthermore, changes in DHEA levels and DHEA/cortisol ratios were not associated with the mean dose of MPH. This may imply that the changes in neurosteroid levels might not be dose related. However, the dosage of MPH was flexible in this study; therefore, the relationship between MPH dosage and neurosteroid levels requires further investigation.

During a 6 month follow-up, we found that all behavioral problems assessed using the CBCL had significantly improved. However, none of the behavioral profiles in the CBCL was significantly associated with changes in neurosteroid levels. Longitudinal investigations have also revealed that the levels of DHEA or cortisol did not correlate with the clinical, social, or behavioral symptoms of ADHD patients (Maayan et al. 2003; Lee et al. 2008). Nevertheless, our previous reports demonstrated a positive correlation between salivary DHEA levels and performance on a neuropsychological test (Wang et al. 2011a,b). The CBCL, the major assessment tool used in the current study, covers a broad range of behavioral profiles. Because of the open-label design of this study, the large improvement in CBCL-measured symptoms might be inflated by a placebo effect and the natural development of the patients. In addition, behavioral assessment might be disadvantaged and vulnerably influenced by rater bias, compared with neuropsychological tests (Madaan et al. 2008). Therefore, a study consisting of comprehensive neurocognitive assessments might be helpful in elucidating the relationships between neurosteroids and the clinical manifestations of ADHD.

There are some limitations to our study. First, the case number was small, especially that of female patients. In addition, there was a certain amount of ADHD patient dropout during the follow-up. This issue may have reduced the statistical power, and made the examination of the potential gender effects on neurosteroids and behavioral changes difficult. Second, the MPH treatment procedure was not standardized, and the variation of the MPH dosage and drug adherence may have influenced the estimation of the results. In addition, all ADHD patients enrolled in our study received MPH after the first visit. The behavioral change in ADHD patients may have been the result of parental expectation following treatment, and their endocrinological data may be confounded by age effects or experience. Although we found that the salivary DHEA levels and DHEA/cortisol ratios increased significantly during the 6 months, we could not determine the trends in neurosteroid levels of ADHD patients who were not on medication. Third, the neurosteroid levels in this study were measured using saliva samples. However, these peripheral levels may not actually represent the concentration and activity of neurosteroids in the brain. Furthermore, this study did not include neuropsychological instruments to assess the attention function of the patients.

Conclusions

We suggest that MPH increased morning DHEA levels and DHEA/cortisol ratios, but not cortisol levels, in the 6 month treatment course for ADHD patients. However, the changes in neurosteroid levels were not associated with changes in behavior. Future research is warranted to investigate whether neurosteroids actually play a role in the underlying biological pathogenesis of ADHD, and how neurosteroids influence the function of the brain and behaviors in the real-life settings of ADHD patients.

Clinical Significance

MPH administration could influence the salivary neurosteroid levels in the 6 month treatment course of ADHD patients. Changes in levels of neurosteroids were not significantly correlated with change in behavioral symptoms.

Footnotes

Acknowledgments

The authors thank Prof. Wei-Tsun Soong for granting the use of the Chinese version of the K-SADS, and Dr. Yu-Shu Huang, Dr. Cheng-Cheng Hsiao, and Prof. Chih-Ken Chen for assisting with the study design and protocol development.

Disclosures

No competing financial interests exist.

References

Achenbach

: Manual for the Revised Child Behavior Checklist. Burlington: University of Vermont, Department of Psychiatry; 1991.

American Psychiatric Association: Diagnostic and Statistical Manual of Mental Disorders, 4th ed. Revised. Washington, DC: American Psychiatric Association; 2000.

Blomqvist

, Holmberg

, Lindblad

, Fernell

, Ek

, Dahllof

: Salivary cortisol levels and dental anxiety in children with attention deficit hyperactivity disorder. Eur J Oral Sci, 115:1–6, 2007.

Chen

, Lin

, Chen

, Liu

, Lin

, Wei

, Hong

: The change of the cortisol levels in children with ADHD treated by methylphenidate or atomoxetine. J Psychiatr Res, 46:415–416, 2012.

Collett

, Ohan

, Myers

: Ten-year review of rating scales. V: Scales assessing attention-deficit/hyperactivity disorder. J Am Acad Child Adolesc Psychiatry, 42:1015–1037, 2003.

Dubrovsky

: Steroids, neuroactive steroids and neurosteroids in psychopathology. Prog Neuropsychopharmacol Biol Psychiatry, 29:169–192, 2005.

Elder

, Wetherell

, Barclay

, Ellis

: The cortisol awakening response – Applications and implications for sleep medicine. Sleep Med Reviews, 2013. [Epub ahead of print]

Golubchik

, Lewis

, Maayan

, Sever

, Strous

, Weizman

: Neurosteroids in child and adolescent psychopathology. Eur Neuropsychopharmacol, 17:157–164, 2007.

Goodyer

, Park

, Netherton

, Herbert

: Possible role of cortisol and dehydroepiandrosterone in human development and psychopathology. Br J Psychiatry, 179:243–249, 2001.

10.

Hellwig–Brida

, Daseking

, Keller

, Petermann

, Goldbeck

: Effects of methylphenidate on intelligence and attention components in boys with attention-deficit/hyperactivity disorder. J Child Adolesc Psychopharmacol, 21:245–253, 2011.

11.

Hibel

, Granger

, Cicchetti

, Rogosch

: Salivary biomarker levels and diurnal variation: associations with medications prescribed to control children's problem behavior. Child Dev, 78:927–937, 2007.

12.

Huang

, Tsai

: Long-term outcomes with medications for attention-deficit hyperactivity disorder: current status of knowledge. CNS Drugs, 25:539–554, 2011.

13.

Isaksson

, Nilsson

, Nyberg

, Hogmark

, Lindblad

: Cortisol levels in children with attention-deficit/hyperactivity disorder. J Psychiatr Res, 46:1398–1405, 2012.

14.

Lee

, Yang

, Ko

, Han

, Kim

, Joe

, Jung

: Effects of methylphenidate and bupropion on DHEA-S and cortisol plasma levels in attention-deficit hyperactivity disorder. Child Psychiatry Hum Dev, 39:201–209, 2008.

15.

Leung

, Kwong

, Tang

, Ho

, Hung

, Lee

, Hong

, Chiu

, Liu

: Test–retest reliability and criterion validity of the Chinese version of CBCL, TRF, and YSR. J Child Psychol Psychiatry, 47:970–973, 2006.

16.

, Chen

, Liu

, Wang

: The function of hypothalamus–pituitary–adrenal axis in children with ADHD. Brain Res, 1368:159–162, 2011.

17.

Maayan

, Yoran–Hegesh

, Strous

, Nechmad

, Averbuch

, Weizman

, Spivak

: Three-month treatment course of methylphenidate increases plasma levels of dehydroepiandrosterone (DHEA) and dehydroepiandrosterone-sulfate (DHEA-S) in attention deficit hyperactivity disorder. Neuropsychobiology, 48:111–115, 2003.

18.

Madaan

, Daughton

, Lubberstedt

, Mattai

, Vaughan

, Kratochvil

: Assessing the efficacy of treatments for ADHD: Overview of methodological issues. CNS Drugs, 22:275–290, 2008.

19.

Maninger

, Wolkowitz

, Reus

, Epel

, Mellon

: Neurobiological and neuropsychiatric effects of dehydroepiandrosterone (DHEA) and DHEA sulfate (DHEAS). Front Neuroendocrinol, 30:65–91, 2009.

20.

Overtoom

, Verbaten

, Kemner

, Kenemans

, van Engeland

, Buitelaar

, van der Molen

, van der Gugten

, Westenberg

, Maes

, Koelega

: Effects of methylphenidate, desipramine, and L-dopa on attention and inhibition in children with attention deficit hyperactivity disorder. Behav Brain Res, 145:7–15, 2003.

21.

Polanczyk

, de Lima

, Horta

, Biederman

, Rohde

: The worldwide prevalence of ADHD: A systematic review and metaregression analysis. Am J Psychiatry, 164:942–948, 2007.

22.

Scassellati

, Bonvicini

, Faraone

, Gennarelli

: Biomarkers and attention-deficit/hyperactivity disorder: A systematic review and meta-analyses. J Am Acad Child Adolesc Psychiatry, 51:1003–1019 e20, 2012.

23.

Spencer

, Biederman

, Mick

: Attention-deficit/hyperactivity disorder: Diagnosis, lifespan, comorbidities, and neurobiology. J Pediatr Psychol, 32:631–642, 2007.

24.

Strous

, Spivak

, Yoran–Hegesh

, Maayan

, Averbuch

, Kotler

, Mester

, Weizman

: Analysis of neurosteroid levels in attention deficit hyperactivity disorder. Int J Neuropsychopharmacol, 4:259–264, 2001.

25.

Talge

, Neal

, Glover

: Antenatal maternal stress and long-term effects on child neurodevelopment: How and why?. J Child Psychol Psychiatry, 48:245–261, 2007.

26.

Trent

, Dean

, Veit

, Cassano

, Bedse

, Ojarikre

, Humby

, Davies

: Biological mechanisms associated with increased perseveration and hyperactivity in a genetic mouse model of neurodevelopmental disorder. Psychoneuroendocrinology, 38:1370–1380, 2013.

27.

Trent

, Dennehy

, Richardson

, Ojarikre

, Burgoyne

, Humby

, Davies

: Steroid sulfatase-deficient mice exhibit endophenotypes relevant to attention deficit hyperactivity disorder. Psychoneuroendocrinology, 37:221–229, 2012.

28.

Wang

, Hsiao

, Huang

, Chiang

, Ree

, Chen

, Wu

, Shang

, Chen

: Association of salivary dehydroepiandrosterone levels and symptoms in patients with attention deficit hyperactivity disorder during six months of treatment with methylphenidate. Psychoneuroendocrinology, 36:1209–1216, 2011a.

29.

Wang

, Huang

, Hsiao

, Chen

: The trend in morning levels of salivary cortisol in children with ADHD during 6 months of methylphenidate treatment. J Atten Disord, 2012. [Epub ahead of print]

30.

Wang

, Huang

, Hsiao

, Chiang

, Wu

, Shang

, Chen

: Salivary dehydroepiandrosterone, but not cortisol, is associated with attention deficit hyperactivity disorder. World J Biol Psychiatry, 12:99–109, 2011b.

31.

Weizman

, Dick

, Gil–Ad

, Weitz

, Tyano

, Laron

: Effects of acute and chronic methylphenidate administration on beta-endorphin, growth hormone, prolactin and cortisol in children with attention deficit disorder and hyperactivity. Life Sci, 40:2247–2252, 1987.

32.

Wilens

: Effects of methylphenidate on the catecholaminergic system in attention-deficit/hyperactivity disorder. J Clin Psychopharmacol, 28:S46–53, 2008.

33.

Yang

, Soong

, Chiang

, Chen

: Competence and behavioral/emotional problems among Taiwanese adolescents as reported by parents and teachers. J Am Acad Child Adolesc Psychiatry, 39:232–239, 2000.