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Neuroscience and Biobehavioral Reviews
journal homepage: www.elsevier.com/locate/neubiorev
The pharmacology of amphetamine and methylphenidate: Relevance to the
neurobiology of attention-deficit/hyperactivity disorder and other
psychiatric comorbidities
Stephen V. Faraone
a,b,⁎
a
Departments of Psychiatry and of Neuroscience and Physiology, State University of New York (SUNY) Upstate Medical University, Syracuse, NY, United States
b
K.G. Jebsen Centre for Research on Neuropsychiatric Disorders, University of Bergen, Bergen, Norway
ARTICLE INFO
Keywords:
Amphetamine
Attention-deficit/hyperactivity disorder
Methylphenidate
Pharmacology
ABSTRACT
Psychostimulants, including amphetamines and methylphenidate, are first-line pharmacotherapies for in-
dividuals with attention-deficit/hyperactivity disorder (ADHD). This review aims to educate physicians re-
garding differences in pharmacology and mechanisms of action between amphetamine and methylphenidate,
thus enhancing physician understanding of psychostimulants and their use in managing individuals with ADHD
who may have comorbid psychiatric conditions. A systematic literature review of PubMed was conducted in
April 2017, focusing on cellular- and brain system–level effects of amphetamine and methylphenidate. The
primary pharmacologic effect of both amphetamine and methylphenidate is to increase central dopamine and
norepinephrine activity, which impacts executive and attentional function. Amphetamine actions include do-
pamine and norepinephrine transporter inhibition, vesicular monoamine transporter 2 (VMAT-2) inhibition, and
monoamine oxidase activity inhibition. Methylphenidate actions include dopamine and norepinephrine trans-
porter inhibition, agonist activity at the serotonin type 1A receptor, and redistribution of the VMAT-2. There is
also evidence for interactions with glutamate and opioid systems. Clinical implications of these actions in in-
dividuals with ADHD with comorbid depression, anxiety, substance use disorder, and sleep disturbances are
discussed.
1. Introduction
Attention-deficit/hyperactivity disorder (ADHD) was initially iden-
tified in children (Lahey et al., 1994) but is now understood to persist
into adulthood in about two thirds of cases (Barkley et al., 2002;Weiss
et al., 1985;Faraone et al., 2006). In a 2007 meta-analysis that included
more than 100 studies, the estimated worldwide prevalence of ADHD in
individuals < 18 years old was 5.29% (Polanczyk et al., 2007). The
estimated prevalence in adults was 4.4% in a national survey in the
United States (Kessler et al., 2006), 3.4% in a 10-nation survey (Fayyad
et al., 2007), and 2.5% in a meta-regression analysis of 6 studies (Simon
et al., 2009).
A large body of evidence suggests that multiple neurotransmitters
and brain structures play a role in ADHD (Purper-Ouakil et al., 2011;
Cortese, 2012;Faraone et al., 2015). Although a substantial amount of
research has focused on dopamine (DA) and norepinephrine (NE),
ADHD has also been linked to dysfunction in serotonin (5hydro-
xytryptamine [5-HT]), acetylcholine (ACH), opioid, and glutamate
(GLU) pathways (Cortese, 2012;Maltezos et al., 2014;Blum et al.,
2008;Potter et al., 2014;Elia et al., 2011). The alterations in these
neurotransmitter systems affect the function of brain structures that
moderate executive function, working memory, emotional regulation,
and reward processing (Fig. 1)(Faraone et al., 2015).
Individuals with ADHD are often diagnosed with additional psy-
chiatric comorbidities, including anxiety, mood, substance use, sleep
disturbances, and antisocial personality disorders (Mao and Findling,
2014;Kooij et al., 2012;Konofal et al., 2010). Importantly, the neu-
robiological substrates that mediate behaviors associated with ADHD
share commonalities to some extent with those involved in these co-
morbid disorders (Farb and Ratner, 2014;Sternat and Katzman, 2016;
Schwartz and Kilduff, 2015). Genetic studies have also identified shared
genetic risk factors between ADHD and associated comorbid disorders
(Sharp et al., 2014;Carey et al., 2016). As such, comorbidities need to
be taken into account when considering pharmacotherapy in an in-
dividual with ADHD.
Although stimulants (including amphetamine [AMP]–based and
https://doi.org/10.1016/j.neubiorev.2018.02.001
Received 2 November 2017; Received in revised form 25 January 2018; Accepted 5 February 2018
⁎
Correspondence to: Departments of Psychiatry and of Neuroscience and Physiology, State University of New York (SUNY) Upstate Medical University, Syracuse, NY 13210, United
States.
E-mail address: sfaraone@childpsychresearch.org.
Neuroscience and Biobehavioral Reviews 87 (2018) 255–270
Available online 08 February 2018
0149-7634/ © 2018 The Author. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/BY-NC-ND/4.0/).
T
methylphenidate [MPH]–based agents) and nonstimulants (e.g., ato-
moxetine, clonidine, and guanfacine) are approved for the treatment of
ADHD (Thomas et al., 2013), stimulants are considered first-line
therapy in children, adolescents, and adults with ADHD because of their
greater efficacy (Atkinson and Hollis, 2010;Subcommittee on
Attention-Deficit/Hyperactivity Disorder et al., 2011;Rostain, 2008;
Bolea-Alamanac et al., 2014;Kooij et al., 2010;Canadian Attention
Deficit Hyperactivity Disorder Resource Alliance (CADDRA), 2011).
AMP and MPH have been shown to exhibit comparable efficacy in 2
meta-analyses (Faraone and Glatt, 2010;Catala-Lopez et al., 2017),
with other analyses reporting that AMP has moderately greater effects
than MPH (Faraone and Buitelaar, 2010;Joseph et al., 2017;Stuhec
et al., 2015). The tolerability and safety profiles of AMP and MPH in
terms of adverse events, treatment discontinuation, and cardiovascular
effects are also generally comparable (Duong et al., 2012;Vaughan and
Kratochvil, 2012;Martinez-Raga et al., 2017), although weight loss and
insomnia have been reported to be more common with AMP than with
MPH (Catala-Lopez et al., 2017). Additional work is planned that will
further compare the efficacy, tolerability, and safety profiles of different
pharmacologic interventions in children, adolescents, and adults with
ADHD (Cortese et al., 2017).
Although increased synaptic availability of DA and NE is a key result
of exposure to both AMP and MPH (dela Pena et al., 2015;Zhu and
Reith, 2008), differences in the specific cellular mechanisms of action of
AMP and MPH may influence their effects on the neurobiological sub-
strates of ADHD and response to treatment in individuals with ADHD as
well as their effects on common comorbidities, such as depression and
anxiety.
The objective of this review is to educate physicians about the
mechanisms of action of AMP and MPH and the implications of these
actions on the management of ADHD and its comorbidities. To achieve
this goal, a systematic review of the published literature was conducted
to obtain articles describing the cellular- and brain system–level effects
of AMP and MPH. The results of relevant studies are described and
interpreted in the context of the treatment of ADHD and in light of the
comorbidities associated with ADHD.
2. Methods
A systematic literature review of PubMed was conducted on April
24, 2017; no limits were included for publication year or the language
of publication. The search consisted of titles and abstracts and used the
following search string: (amphetamine [MESH term] OR methylpheni-
date [MESH term]) AND (cellular OR receptor binding OR
Fig. 1. Brain Mechanisms in ADHD*.
(a) The cortical regions (lateral view) of the brain have a role in attention-deficit/hyperactivity disorder (ADHD). The dorsolateral prefrontal cortex is linked to working memory, the
ventromedial prefrontal cortex to complex decision making and strategic planning, and the parietal cortex to orientation of attention. (b) ADHD involves the subcortical structures
(medial view) of the brain. The ventral anterior cingulate cortex and the dorsal anterior cingulate cortex subserve affective and cognitive components of executive control. Together with
the basal ganglia (comprising the nucleus accumbens, caudate nucleus, and putamen), they form the frontostriatal circuit. Neuroimaging studies show structural and functional ab-
normalities in all of these structures in patients with ADHD, extending into the amygdala and cerebellum. (c) Neurotransmitter circuits in the brain are involved in ADHD. The dopamine
system plays an important part in planning and initiation of motor responses, activation, switching, reaction to novelty, and processing of reward. The noradrenergic system influences
arousal modulation, signal-to-noise ratios in cortical areas, state-dependent cognitive processes, and cognitive preparation of urgent stimuli. (d) Executive control networks are affected in
patients with ADHD. The executive control and cortico-cerebellar networks coordinate executive functioning (i.e., planning, goal-directed behavior, inhibition, working memory, and the
flexible adaptation to context). These networks are underactivated and have lower internal functional connectivity in individuals with ADHD compared with individuals without the
disorder. (e) ADHD involves the reward network. The ventromedial prefrontal cortex, orbitofrontal cortex, and ventral striatum are at the center of the brain network that responds to
anticipation and receipt of reward. Other structures involved are the thalamus, the amygdala, and the cell bodies of dopaminergic neurons in the substantia nigra, which, as indicated by
the arrows, interact in a complex manner. Behavioral and neural responses to reward are abnormal in ADHD. (f) The alerting network is impaired in ADHD. The frontal and parietal
cortical areas and the thalamus intensively interact in the alerting network (indicated by the arrows), which supports attentional functioning and is weaker in individuals with ADHD than
in controls. (g) ADHD involves the default-mode network (DMN). The DMN consists of the medial prefrontal cortex and the posterior cingulate cortex (medial view) as well as the lateral
parietal cortex and the medial temporal lobe (lateral view). DMN fluctuations are 180° out of phase with fluctuations in networks that become activated during externally oriented tasks,
presumably reflecting competition between opposing processes for processing resources. Negative correlations between the DMN and the frontoparietal control network are weaker in
patients with ADHD than in people who do not have the disorder.
*
Reprinted with permission from Macmillan Publishers Ltd: [NAT REV DIS PRIMERS] (Faraone et al., 2015), copyright 2015.
S.V. Faraone Neuroscience and Biobehavioral Reviews 87 (2018) 255–270
256
neuroimaging OR FMRI OR SPECT OR PET OR positron emission to-
mography OR magnetic resonance OR tomography OR spectroscopy)
AND (dopamine OR serotonin OR norepinephrine OR acetylcholine OR
glutamate OR opioid OR opiate) NOT (methamphetamine OR MDMA
OR ecstasy OR addiction OR abuse). The literature search included both
animal and human studies.
Additional articles of interest were obtained via an assessment of the
relevant articles obtained through the literature review and based on
author knowledge. A publication was excluded if it did not specifically
focus on the mechanism of action or pharmacologic effects of AMP or
MPH or if it was an imaging study in individuals who were other than
healthy (i.e., studies in those with psychiatric disorders were not in-
cluded).
3. Results
The literature search yielded 673 articles (Fig. 2). Of these, 137
were considered relevant to the topic of this review. Additional articles
(n = 13) were identified based on author knowledge and assessment of
the reference sections of relevant citations. The articles included in this
review are summarized in Table 1.
3.1. Preclinical studies
3.1.1. Amphetamine
The main mechanism of action of AMP is to increase synaptic ex-
tracellular DA and NE levels (Avelar et al., 2013;Covey et al., 2013;
Finnema et al., 2015;Floor and Meng, 1996;Jedema et al., 2014;Joyce
et al., 2007;Kuczenski and Segal, 1997;May et al., 1988;Mukherjee
et al., 1997;Pum et al., 2007;Ren et al., 2009;Schiffer et al., 2006;Xiao
and Becker, 1998;Young et al., 2011;Wall et al., 1995). This effect is
mediated by inhibition of DA transporters (DAT) and NE transporters
(NET) (Avelar et al., 2013;Covey et al., 2013;Easton et al., 2007bb),
which reduces the reuptake of these molecules from the synapse. In
wild-type mice, AMP initially increases surface trafficking of DAT and
DA uptake, but continued AMP exposure results in decreased surface
expression of the DAT and decreases in DA uptake (Chen et al., 2009).
In a dose-dependent and region-specific manner, AMP also increases
vesicular DA release via inhibition of the vesicular monoamine trans-
porter 2 (VMAT-2), which releases DA from vesicular storage, and the
concomitant release of cytosolic DA via reverse transport by the DAT
(Easton et al., 2007bb; Riddle et al., 2007;Sulzer et al., 1995). Fur-
thermore, AMP inhibits monoamine oxidase (MAO) activity (Miller
et al., 1980;Robinson, 1985), which decreases cytosolic monoamine
breakdown. A wide array of studies using positron emission tomo-
graphy (PET) or single-photon emission computed tomography (SPECT)
have demonstrated that AMP produced reductions in the binding po-
tential of ligands for DA receptors (al-Tikriti et al., 1994;Carson et al.,
1997;Castner et al., 2000;Chou et al., 2000;Dewey et al., 1993;
Drevets et al., 1999;Gallezot et al., 2014;Ginovart et al., 1999;Howlett
and Nahorski, 1979;Laruelle et al., 1997;Le Masurier et al., 2004;Lind
et al., 2005;Mukherjee et al., 1997;Pedersen et al., 2007;Saelens et al.,
1980;Schiffer et al., 2006;Seneca et al., 2006;Sun et al., 2003;Tomic
et al., 1997;Tomic and Joksimovic, 2000;van Berckel et al., 2006) and
NE receptors (Finnema et al., 2015;Landau et al., 2012), which is an
indirect indicator of increased competition for binding sites resulting
from increased extracellular DA or NE. The striatum, which contains
most of the DATs in the brain (Volkow et al., 1996bb; Fischman et al.,
1997), appears to be a principal site of action of AMP (Avelar et al.,
2013;Kilbourn and Domino, 2011), but direct effects in the cortex and
the ventral tegmental area have also been reported (Pum et al., 2007;
Ren et al., 2009;Schwarz et al., 2007bb). The effects of AMP extend to
and are modulated by other neurotransmitter systems (Choe et al.,
2002;Duttaroy et al., 1992;Inderbitzin et al., 1997;Konradi et al.,
1996;Liu et al., 2003;Pum et al., 2007;Quelch et al., 2014;Ritz and
Kuhar, 1989;Shaffer et al., 2010;Smith et al., 2005;Yin et al., 2010;Yu
et al., 2003), including ACH, 5-HT, opioid, and GLU, either directly
through enhanced release from presynaptic terminals or via down-
stream effects.
In other studies, AMP has been shown to produce changes at a more
global level. In studies of cerebral blood flow (CBF), AMP increased
whole brain CBF in rats and baboons (Chen et al., 2008;Kashiwagi
Fig. 2. PRISMA Flow Diagram of the Literature Search.
*Publications were excluded if they did not focus on the mechanism of action or pharmacologic effects of amphetamine or methylphenidate or if the publications were imaging studies in
individuals who were not healthy (i.e., studies in those with psychiatric disorders were not included).
S.V. Faraone Neuroscience and Biobehavioral Reviews 87 (2018) 255–270
257
Table 1
Studies of the Actions of AMP and MPH.
Preclinical studies
Amphetamine
al-Tikriti et al. (1994) •AMP increased the washout rate of [
123
I]IBF (a D
2
receptor antagonist) from the striatum of baboons, as measured using SPECT
Annamalai et al. (2010) •AMP-stimulated downregulation of the NET was linked to the PKC-resistant T258/S259 structural motif and mediated by reduced plasma
membrane insertion and enhanced endocytosis
Avelar et al. (2013) •AMP increased electrically evoked DA levels, inhibited DA uptake, and upregulated DA vesicular release in rat striatum
Bjorklund et al. (2008) •Adenosine A
3
receptor knockout mice exhibited reduced locomotor responses to AMP, suggesting potential alterations in monoaminergic
(DA, 5-HT, or NE) systems
Carson et al. (1997) •AMP reduced [
11
C]raclopride (a D
2
receptor antagonist) binding in the striatum of rhesus monkeys, as measured by PET
Castner et al. (2000) •AMP reduced [
123
I]IBZM (a D
2
receptor antagonist) binding in the striatum of rhesus monkeys, as measured by SPECT
Chen et al. (2008) •AMP-induced increases in whole-brain relative cerebral blood volume were attenuated by electroacupuncture (electrical paw stimulation) in
rats
Chen et al. (2009) •Rapid AMP-induced increases in striatal surface DAT levels and DA release in mice were not observed in PKC-βknockout mice
Choe et al. (2002) •AMP increased phosphorylation of CREB in rats; intrastriatal administration of PHCCC (an mGluR type 1 antagonist) and systemic
administration of MPEP (an mGluR type 5 antagonist) attenuated this effect
Chou et al. (2000) •AMP reduced [
11
C]FLB 457 (a D
2
receptor antagonist) binding in the thalamus, cortex, and striatum of cynomolgus monkeys, as measured by
PET
Covey et al. (2013) •AMP increased DA release in rat striatum when administered with a short-duration electrical stimulus and decreased DA release when
administered with a long-duration electrical stimulus
Dewey et al. (1993) •AMP decreased [
11
C]raclopride (a D
2
receptor antagonist) binding in the striatum of baboons, as measured by PET
Dixon et al. (2005) •AMP caused widespread increases in fMRI BOLD signal intensity in subcortical structures containing rich DA innervation, with decreases in
BOLD signal observed in the superficial layers of the cortex in rats
Drevets et al. (1999) •AMP-induced reductions in [
11
C]raclopride (a D
2
receptor antagonist) binding in baboons were greater in the anteroventral striatum than in
the dorsal striatum, as measured by PET
Duttaroy et al. (1992) •Chronic AMP exposure blocked naloxone-induced supersensitivity of μ- and δ-opioid receptors without altering naloxone-induced
upregulation of these receptors in mice
Easton et al. (2007a) •AMP isomers increased DA release (basal and electrically stimulated) in the nucleus accumbens, medial entorhinal cortex, colliculi,
hippocampal area CA
1
, and thalamic nuclei of rats
Easton et al. (2007b) •AMP inhibited accumulation of DA or NE in rat brain synaptosomes and vesicles, which was mitigated by inhibitors of the DAT and NET
Finnema et al. (2015) •AMP reduced binding of [
11
C]ORM-13070 (an adrenergic α
2c
receptor antagonist) in the striatum of cynomolgus monkeys, as measured by
PET, and increased rat striatal NE and DA concentrations
Floor and Meng (1996) •Low AMP concentrations increased DA release from rat brain synaptic vesicles independent of vesicular alkalinization; high-AMP-
concentration DA release was reduced as a function of the proton gradient
Gallezot et al. (2014) •AMP reduced [
11
C]raclopride (a D
2
receptor antagonist) and [
11
C]PHNO (a D
2
/D
3
receptor agonist) binding in the striatum, globus pallidus,
and substantia nigra of rhesus monkeys, as measured by PET
Ginovart et al. (1999) •Chronic AMP administration (14 days) was associated with an increased [
11
C]raclopride (a D
2
receptor antagonist) binding affinity after 1
day of drug withdrawal and with binding after 7 and 14 days of withdrawal in cynomolgus monkeys, as measured by PET
Hartvig et al. (1997) •AMP increased DA biosynthesis in the brain in nonhuman primates, as measured by PET
Helmeste and Seeman (1982) •Mice with high D
2
receptor density in the striatum and olfactory tubercle exhibited hypolocomotion in response to low-dose AMP, whereas
mice with low D
2
receptor density were nonresponsive to AMP
Howlett and Nahorski (1979) •AMP exposure increased [
3
H]spiperone (a D
2
/D
3
receptor antagonist) binding affinity and density after 4 days of exposure and decreased
binding density after 20 days of exposure in rat striatum
Inderbitzin et al. (1997) •AMP increased preprodynorphin mRNA expression and κ-opioid receptor binding in the basal ganglia of rats
Jedema et al. (2014) •AMP increased DA release in the caudate and prefrontal cortex of rhesus macaques, as measured using microdialysis, with DA levels
remaining elevated for a longer duration in the prefrontal cortex than in the caudate
Joyce et al. (2007) •Administration of a combination of mixed amphetamine salts directly into the striatum of rats stimulated DA release
Kashiwagi et al. (2015) •AMP increased relative cerebral blood volume in awake and anesthetized rats, with changes correlated to changes in striatal DA
concentration
Konradi et al. (1996) •AMP-induced activation of immediate early genes in dissociated rat striatal cultures was blocked by MK-801 (an NMDA receptor antagonist)
Kuczenski and Segal (1997) •AMP increased DA and 5-HT efflux in the caudate and NE efflux in the hippocampus of rats following systemic administration
Lahti and Tamminga (1999) •AMP increased D
2
receptor occupancy in the cortex and caudate of rats, as measured by receptor autoradiography
Landau et al. (2012) •AMP decreased binding of [
11
C]yohimbine (an adrenergic ɑ
2
receptor antagonist) in the thalamus, cortex, and caudate of pigs, as measured
by PET
Laruelle et al. (1997) •AMP reduced [
123
I]IBZM and [
123
I]IBF (D
2
receptor antagonists) in vervets (nonhuman primates), as measured by SPECT, with binding
changes correlating with peak DA release as measured by microdialysis
Le Masurier et al. (2004) •AMP-reduced [
11
C]raclopride (a D
2
receptor antagonist) binding in the striatum of rats, as measured by PET, was attenuated by pretreatment
with a tyrosine-free amino acid mixture
Lind et al. (2005) •AMP decreased [
11
C]raclopride (a D
2
receptor antagonist) binding in the caudate, putamen, and ventral striatum of pigs, as measured by PET
Liu et al. (2003) •AMP increased cytosolic calcium concentrations and NE release in a dose-dependent and extracellular calcium-dependent manner in bovine
adrenal chromaffin cells; nicotinic receptor antagonists suppressed these effects
•AMP acted as a nicotinic receptor agonist to induce calcium increases and NE release in bovine adrenal chromaffin cells
May et al. (1988) •AMP stimulated DA release in the caudate nucleus of rats, as measured by in vivo fast-scan cyclic voltammetry
Miller et al. (1980) •AMP inhibited MAO type A activity
Mukherjee et al. (1997) •AMP reduced [
18
F]fallypride (a D
2
receptor antagonist) binding in the striatum of rats and monkeys, as measured by PET
Patrick et al. (1981) •Stimulation of DA synthesis in rat brain striatal synaptosomes produced by the depolarizing agent veratridine markedly reduced AMP,
suggesting AMP altered interactions between tyrosine hydroxylase and the synaptosomes regulating catecholamine formation
Pedersen et al. (2007) •AMP-induced decreases in [
11
C]raclopride (a D
2
receptor antagonist) binding in the dorsal and ventral striatum of rats were not altered by
pretreatment with pargyline, as measured using PET
Preece et al. (2007) •AMP reduced BOLD signal intensity in the nucleus accumbens and prefrontal cortex and increased signal intensity in the motor cortex of rats
as measured by fMRI; the signal intensity changes in the nucleus accumbens and caudate (but not in the motor cortex) were attenuated by
pretreatment with a tyrosine-free amino acid mixture
Price et al. (2002) •After administration of AMP, a general increase in CBF that gradually declined toward baseline values was observed using a bolus injection
PET method in baboons
Pum et al. (2007) •AMP dose-dependently increased DA and 5-HT levels in the perirhinal, entorhinal, and prefrontal cortices
(continued on next page)
S.V. Faraone Neuroscience and Biobehavioral Reviews 87 (2018) 255–270
258
Table 1 (continued)
Preclinical studies
Quelch et al. (2014) •AMP reduced [
3
H]carfentanil (a μ-opioid receptor antagonist) binding in rat total brain homogenate, as measured by subcellular
fractionation, and in the superior colliculi, hypothalamus, and amygdala of rats, as measured by in vivo receptor binding
Ren et al. (2009) •AMP increased DA release in the caudate/putamen and cAMP activity in the caudate/putamen, nucleus accumbens, and medial prefrontal
cortex in rats
Riddle et al. (2007) •Administration of low-dose AMP altered VMAT-2 distribution within nerve terminals selectively in monoaminergic neurons
Ritz and Kuhar (1989) •AMP exhibited high affinity for DA, NE, and 5-HT reuptake sites and for α
2
adrenergic receptor sites, as measured using in vivo binding in
rats
Robinson (1985) •AMP enantiomers (dand l) inhibited MAO type A and MAO type B activity in rat liver mitochondria
Saelens et al. (1980) •AMP increased [
3
H]spiroperidol (a D
2
receptor antagonist) binding in regions containing DA terminals (e.g., the septum, nucleus accumbens,
cortex, caudate-putamen) or dendrites (e.g., ventral tegmental area, substantia nigra), as measured by in vivo receptor binding
Schiffer et al. (2006) •AMP elevated extracellular DA in the striatum but to a greater degree than MPH; reductions in [
11
C]raclopride (a D
2
receptor antagonist)
binding were similar with AMP and MPH
Schwarz et al. (2007a) •AMP produced widespread increases in regional CBF in multiple brain region clusters that are involved in primary DA pathways in rats
Schwendt et al. (2006) •AMP decreased RGS4 mRNA in the caudate-putamen and cortex and RGS4 protein levels in the caudate-putamen of rats but did not modify
the effects of SCH 23390 or eticlopride (D
2
receptor antagonists) on RGS4 mRNA or protein levels in these same brain regions
Seneca et al. (2006) •AMP reduced binding of [
11
C]MNPA and [
11
C]raclopride (D
2
receptor antagonists) in the striatum of cynomolgus monkeys, as measured by
PET
Shaffer et al. (2010) •AMP reduced striatal and increased medial prefrontal cortical protein expression of mGluR type 5 in rats
Skinbjerg et al. (2010) •AMP-induced decreases in [
18
F]fallypride (a D
2
receptor antagonist) binding were associated with receptor internalization, as assessed using
PET in knockout mice lacking the ability to internalize D
2
receptors
Smith et al. (2005) •Melanin-concentrating hormone type 1 receptor knockout mice exhibited an increased locomotor response to AMP but did not exhibit
altered DA, NE, or 5-HT release in the striatum in response to AMP
Sulzer et al. (1995) •AMP produced a rapid release of DA from vesicles, resulting in redistribution to the cytosol
Sulzer and Rayport (1990) •AMP reduced the pH gradient in rat chromaffin cells, resulting in reduced DA uptake
Sun et al. (2003) •AMP decreased binding of [
3
H]raclopride but not of [
3
H]spiperone (D
2
receptor antagonists) in rat striatum
Tomic et al. (1997) •AMP decreased [
3
H]SCH 23390 (a D
1
receptor antagonist) binding in the caudate and nucleus accumbens, increased [
3
H]SCH 23390 binding
in the substantia nigra, and decreased [3H]spiperone (a D
2
receptor antagonist) binding in the striatum and nucleus accumbens in rats, as
measured using autoradiography
Tomic and Joksimovic (2000) •Acute treatment with AMP reduced [
3
H]SCH 23390 (a D
1
receptor antagonist) and [
3
H]spiperone (a D
2
receptor antagonist) binding in the
striatum and nucleus accumbens in rats, as measured by autoradiography
van Berckel et al. (2006) •AMP-induced decrease in [
3
H]raclopride (a D
2
receptor antagonist) binding in the striatum of baboons was enhanced by LY354740 (an
mGluR type 2/3 receptor agonist), as measured by PET
Xiao and Becker (1998) •AMP-induced striatal DA release in rats was increased by catechol estrogens, as measured using in vivo microdialysis
Yin et al. (2010) •AMP produced time-dependent changes in levels of GABA, 5-HT, and NE in the cerebellar vermis of mice
Young et al. (2011) •AMP increased DA concentrations in the striatum, but not in the cortex or ventral tegmental area, in prairie voles
Yu et al. (2003) •AMP enhanced the cellular response of cortical and hippocampal neurons to CHPG (an mGluR type 5 agonist) in rats
Methylphenidate
Andrews and Lavin (2006) •Intracortical MPH increased cortical cell excitability in rats, an effect that was lost following catecholamine depletion and was mediated via
stimulation of α
2
adrenergic receptors but not D
1
receptors
Bartl et al. (2010) •MPH exerted diverse cellular effects including increased neurotransmitter levels, downregulated synaptic gene expression, and enhanced cell
proliferation in rat pheochromocytoma cells
Ding et al. (1994) •MPH reduced binding of [
11
C]dl-threo-MPH in the striatum of baboons, as measured by PET
Ding et al. (1997) •MPH reduced binding of [
11
C]d-threo-MPH (but not [
11
C]l-threo-MPH) in the striatum of baboons, as measured by PET
Dresel et al. (1999) •MPH reduced [
99
Tc]TRODAT-1 (a DAT/SERT ligand) binding to DAT in the striatum, but not to the SERT in the midbrain/hypothalamus in
baboons, as measured by PET
Easton et al. (2007b) •MPH inhibited accumulation of DA or NE in rat brain synaptosomes and vesicles, with greater potency observed for synaptosomal inhibition
Federici et al. (2005) •MPH reduced spontaneous firing of DA neurons, as measured by electrophysiologic recordings in rat brain slices, via blockade of the DAT
Gamo et al. (2010) •MPH increased performance on a spatial working memory task in rhesus monkeys, an effect that was blocked by the α
2
adrenergic antagonist
idazoxan
Gatley et al. (1995) •[
11
C]d-threo-MPH exhibited high affinity for the DAT that was insensitive to competition with endogenous DA in baboons, as measured by
PET; the highest concentrations of [
11
C]d-threo-MPH binding in mouse brain were found in striatum
Gatley et al. (1999) •MPH reduced [
3
H]cocaine (a DAT agonist) binding in the olfactory tubercle and striatum in mice, as measured by PET
Gatley et al. (1996) •MPH exhibited higher affinities for the DAT and NET than for the serotonin transporter in rat brain
Kuczenski and Segal (1997) •MPH increased DA efflux in the caudate and NE efflux in the hippocampus of rats following systemic administration
Markowitz et al. (2009) •MPH acted as an agonist at the 5-HT
1A
receptor in guinea pig ileum
Markowitz et al. (2006) •MPH exhibited affinity at the NET and DAT and at 5-HT
1A
and 5-HT
2B
receptors
Michaelides et al. (2010) •MPH differentially altered metabolism in the prefrontal cortex and cerebellar vermis as a function of DA D
4
receptor functionality in mice
Nikolaus et al. (2005) •MPH reduced [
123
I]FP-CIT (a DAT ligand) binding in rat striatum, as measured by SPECT
Nikolaus et al. (2007) •MPH dose-dependently reduced [
123
I]FP-CIT (a DAT ligand) binding in rat striatum, as measured by SPECT
Nikolaus et al. (2011) •MPH decreased [
123
I]IBZM (a D
2
receptor antagonist) binding in the striatum of rats, as measured by SPECT
Riddle et al. (2007) •Administration of low-dose MPH altered VMAT-2 distribution within nerve terminals selectively in monoaminergic neurons
Sandoval et al. (2002) •MPH increased vesicular DA uptake and binding of VMAT-2 and altered VMAT-2 cellular distribution
Schiffer et al. (2006) •MPH elevated extracellular DA in the striatum but to a lesser degree than AMP; reductions in [
11
C]raclopride (a D
2
receptor antagonist)
binding were similar with MPH and AMP
Somkuwar et al. (2013) •MPH administered during the adolescent period in rats produced strain-dependent alterations in cortical DAT activity at adulthood
Volkow et al. (1999a) •MPH reduced [
11
C]raclopride (a D
2
receptor antagonist) binding potential but not binding affinity in baboons, as measured by PET
Wall et al. (1995) •MPH caused little or no change in DA, NE, or 5-HT efflux, although it blocked uptake mediated by both NET and DAT in transfected cell lines
Human neuroimaging studies
Amphetamine
Aalto et al. (2009) •AMP did not alter [
11
C]FLB 457 (a D
2
/D
3
receptor ligand) binding in the cortex of healthy adults, as measured by PET
Boileau et al. (2007) •AMP decreased [
11
C]raclopride (a D
2
receptor antagonist) binding in ventral striatum and putamen of healthy adults in response to
conditioned stimuli, as measured by PET
Buckholtz et al. (2010) •AMP-induced reductions in [
18
F]fallypride (a D
2
receptor antagonist) binding in the striatum of healthy adults were associated with
increased trait impulsivity
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259
Table 1 (continued)
Preclinical studies
Cardenas et al. (2004) •AMP produced decreases in [
11
C]raclopride (a D
2
receptor antagonist) binding that were sustained for up to 6 h in healthy adults, as
measured using PET studies
Colasanti et al. (2012) •AMP reduced [
11
C]carfentanil (a μ-opioid receptor antagonist) binding in the caudate, putamen, frontal cortex, thalamus, insula, and
anterior cingulate cortex of healthy adult males, as measured by PET
Cropley et al. (2008) •AMP reduced [
18
F]fallypride (a D
2
receptor antagonist) binding in the substantia nigra, medial prefrontal and orbital cortices, and caudate of
healthy adults, as measured by PET
Devous et al. (2001) •AMP increased regional CBF to the prefrontal cortex, inferior orbital frontal cortex, ventral tegmentum, amygdala, and anterior thalamus,
and decreased regional CBF to the motor and visual cortices, fusiform gyrus, and regions of the temporal lobe of healthy adults, as measured
by SPECT
Drevets et al. (2001) •AMP-induced reductions in [
11
C]raclopride (a D
2
receptor antagonist) binding in the anteroventral striatum of healthy adults, as measured
by PET, were negatively correlated with feelings of euphoria
Garrett et al. (2015) •AMP increased fMRI-based BOLD signal variability in healthy adults, with the effects being more pronounced in older adults (60–70 years
old) than younger adults (20–30 years old)
Guterstam et al. (2013) •AMP had no effect on [
11
C]carfentanil (a μ-opioid receptor antagonist) binding in the striatum, cortex, amygdala, or hippocampus of healthy
adult males, as measured by PET
Hariri et al. (2002) •AMP potentiated responses of the right amygdala to angry and fearful facial expressions of healthy adults, as measured by fMRI, without
producing changes in performance of an emotional recognition task or a sensorimotor control task
Kegeles et al. (1999) •AMP decreased [
123
I]IBZM (a D
2
receptor antagonist) binding in the striatum of healthy adults, as measured by SPECT
Knutson et al. (2004) •AMP exerted an equalizing influence on activity in the ventral striatum by enhancing tonic activity over phasic activity during anticipation of
positive and negative incentives in healthy adults, as measured by fMRI
Laruelle et al. (1995) •AMP decreased [
123
I]IBZM (a D
2
receptor antagonist) binding in the striatum of healthy adults, as measured by SPECT
Laruelle and Innis (1996) •AMP decreased [
123
I]IBZM (a D
2
receptor antagonist) binding in the striatum of healthy adults, as measured by SPECT
Leyton et al. (2002) •AMP-decreased [
11
C]raclopride (a D
2
receptor antagonist) binding in the ventral striatum of healthy adults, as measured by PET, was
correlated with increases in drug wanting and novelty seeking
Leyton et al. (2004) •AMP-induced decreases in [
11
C]raclopride (a D
2
receptor antagonist) binding in the ventral striatum of healthy adults, as measured by PET,
were attenuated by acute depletion of phenylalanine and tyrosine
Martinez et al. (2003) •AMP decreased [
11
C]raclopride (a D
2
receptor antagonist) binding in healthy adults, as measured by PET, with larger reductions observed in
the limbic (ventral) and the sensorimotor (postcommissural) striatal regions than associative (caudate and precommissural) striatal regions
Mick et al. (2014) •AMP reduced [
11
C]carfentanil (a μ-opioid receptor antagonist) binding in the caudate, putamen, frontal lobe, thalamus, nucleus accumbens,
insula, amygdala, and anterior cingulate cortex of healthy adults, as measured by PET
Narendran et al. (2009) •AMP decreased [
11
C]FLB 457 (a D
2
/D
3
receptor ligand) and [
11
C]fallypride (a D
2
receptor antagonist) binding in the medial temporal lobe,
anterior cingulate cortex, dorsolateral and medial prefrontal cortices, and parietal cortex of healthy adults, as measured by PET
Narendran et al. (2010) •AMP decreased [
11
C]NPA (a D
2
/D
3
receptor agonist) and [
11
C]raclopride (a D
2
receptor antagonist) binding in the striatum of healthy adults,
as measured by PET, with reductions of [
11
C]NPA binding being greater than those of [
11
C]raclopride
Oswald et al. (2005) •AMP-induced reductions in [
11
C]raclopride (a D
2
receptor antagonist) binding in the striatum of healthy adults, as measured by PET, were
correlated with increased cortisol release and positive subjective ratings of AMP
Oswald et al. (2015) •AMP decreased [
11
C]raclopride (a D
2
receptor antagonist) binding in the striatum of healthy adults, with lower binding in the right dorsal
caudate being associated with lower winnings in the Iowa Gambling Task
Riccardi et al. (2006a) •AMP reduced [
18
F]fallypride (a D
2
receptor antagonist) binding in healthy adults, as measured by PET, with the reductions across brain
regions differing as a function of gender
Riccardi et al. (2006b) •AMP reduced [
18
F]fallypride (a D
2
receptor antagonist) binding in the striatum, substantia nigra, amygdala, temporal cortex, and thalamus
of healthy adults, as measured by PET
Riccardi et al. (2011) •AMP reduced [
18
F]fallypride (a D
2
receptor antagonist) binding in healthy adults, as measured by PET, with the reductions in selected baring
regions being correlated with Stroop test scores, Stroop test interference, and affective state
Rose et al. (2006) •AMP increased CBF in the cerebellum, brainstem, temporal lobe, striatum, and prefrontal and parietal cortices of healthy adults, as measured
by MRI
Schouw et al. (2013) •AMP-induced increases in regional CBF in the striatum, anterior cingulate cortex, thalamus, and cerebellum of healthy adults, as measured
by MRI, were not correlated with reductions in [
123
I]IBZM receptor binding in the striatum, as measured by SPECT
Shotbolt et al. (2012) •AMP induced reductions in [
11
C]−(+)-PHNO (a D
2
/D
3
receptor agonist) binding in the ventral pallidum, ventral striatum, thalamus, and
putamen of healthy adults, as measured by PET, which were greater than the reductions in [
11
C]raclopride (a D
2
receptor antagonist) binding
Silverstone et al. (2002) •AMP increased myo-inositol and phosphomonoester concentrations in the temporal lobe of healthy adults, as measured by MRS
Slifstein et al. (2010) •AMP reduced [
18
F]fallypride (a D
2
receptor antagonist) binding in the striatum, globus pallidus, midbrain, hippocampus, and amygdala of
healthy adults, as measured by PET
Vollenweider et al. (1998) •AMP increased regional cerebral glucose metabolism in the anterior and posterior cingulate cortex, caudate nucleus, putamen, and thalamus
of healthy adults, as measured by PET with [
18
F]-FDG
Wand et al. (2007) •AMP-induced reductions in [
11
C]raclopride (a D
2
receptor antagonist) binding in striatum of healthy adults, as measured by PET, were
associated with stress-induced cortisol levels
Willeit et al. (2008) •AMP reduced [
11
C]–(+)-PHNO (a D
2
/D
3
receptor agonist) binding in the striatum but not in the globus pallidus of healthy adults, as
measured by PET
Wolkin et al. (1987) •AMP decreased regional cerebral glucose metabolism in the frontal cortex, temporal cortex, and striatum of healthy adults, as measured by
PET
Woodward et al. (2011) •AMP-induced reductions in [
18
F]fallypride (a D2 receptor antagonist) binding in the striatum of healthy adults, as measured by PET, were
positively correlated with overall schizotypal traits
Methylphenidate
Booij et al. (1997) •MPH reduced [
123
I]IBZM (a D
2
receptor antagonist) binding in the striatum of healthy adults, as measured by SPECT
Clatworthy et al. (2009) •MPH reduced [
11
C]raclopride (a D
2
receptor antagonist) binding in the putamen, ventral striatum, and postcommissural caudate of healthy
adults, as measured by PET, with reductions in the postcommissural caudate being negatively correlated with reversal learning performance
Costa et al. (2013) •MPH increased neuronal activation in the putamen of healthy adults as measured by fMRI, during a go/no-go task when a response inhibition
error occurred but not when a response was successfully inhibited
Ding et al. (1997) •MPH reduced binding of [
11
C]d-threo-MPH to a greater degree than [
11
C]l-threo-MPH in the striatum of healthy adults, as measured by PET
Hannestad et al. (2010) •MPH dose-dependently reduced [11C]MRB (a NET ligand) binding across the brain (e.g., locus coeruleus, raphe nucleus, hypothalamus,
thalamus) of healthy adults, as measured using PET
Kasparbauer et al. (2015) •MPH increased BOLD signal during successful go/no-go trials in healthy adult carriers of the SLC6A3 9R allele of the DAT gene but a decrease
in 10/10 allele homozygotes, as measured by fMRI
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S.V. Faraone Neuroscience and Biobehavioral Reviews 87 (2018) 255–270
260
et al., 2015;Price et al., 2002;Schwarz et al., 2007aa), with changes in
CBF being correlated with striatal DA concentration (Kashiwagi et al.,
2015). In a functional magnetic resonance imaging (fMRI) study in rats,
AMP caused widespread increases in blood-oxygen-level–dependent
(BOLD) signal intensity in subcortical structures with rich DA in-
nervation and with decreases in BOLD signal in the superficial layers of
the cortex (Dixon et al., 2005). In another fMRI study in rats, AMP
reduced BOLD signal intensity in the nucleus accumbens and prefrontal
cortex and increased signal intensity in the motor cortex of rats, with
signal intensity changes in the nucleus accumbens and caudate (but not
in the motor cortex) being attenuated by pretreatment with a tyrosine-
free amino acid mixture (Preece et al., 2007).
3.1.2. Methylphenidate
The direct effects of MPH include inhibition of the DAT and NET
(Dresel et al., 1999;Federici et al., 2005;Gatley et al., 1996;Markowitz
et al., 2006;Nikolaus et al., 2007;Wall et al., 1995), an affinity for and
agonist activity at the 5-HT
1A
receptor (Markowitz et al., 2009;
Markowitz et al., 2006), and redistribution of VMAT-2 (Riddle et al.,
2007;Sandoval et al., 2002). As a consequence of these interactions,
MPH elevates extracellular DA and NE levels (Easton et al., 2007bb;
Kuczenski and Segal, 1997;Schiffer et al., 2006;Young et al., 2011).
The enhanced efflux of DA and NE associated with MPH exposure re-
sults in increased availability of DA and NE to bind to their respective
transporters (i.e., the DAT or NET) or to DA or NE receptors, as evi-
denced by reductions in ligand binding in PET and SPECT studies (Ding
et al., 1997;Dresel et al., 1999;Gatley et al., 1999;Nikolaus et al.,
2005,Nikolaus et al., 2007, 2011; Volkow et al., 1999a).
Although increases in extracellular levels of striatal DA in rats
measured using microdialysis are less pronounced with MPH than with
AMP, both compounds have been shown to exhibit similar magnitude of
effects with regard to reductions in DA binding potential as measured
Table 1 (continued)
Preclinical studies
Moeller et al. (2014) •MPH improved Stroop color-word task performance and concurrently reduced dorsal anterior cingulate cortex responses of healthy adults, as
measured by fMRI
Montgomery et al. (2007) •MPH reduced [
11
C]FLB 457 (a D
2
/D
3
receptor ligand) binding in the frontal cortex, anterior cingulate cortex, temporal cortex, and thalamus
of healthy adults, as measured by PET
Mueller et al. (2014) •MPH increased connectivity strength between the dorsal attention network and thalamus, with the left and right frontoparietal networks and
the executive control networks also showing increased connectivity to sensory-motor and visual cortex regions, and decreased connectivity
to cortical and subcortical components of cortico-striato-thalamo-cortical circuits in healthy adults, as measured by fMRI
Ramaekers et al. (2013) •MPH reduced functional connectivity between the nucleus accumbens and the basal ganglia, medial prefrontal cortex, and temporal cortex
without changing functional connectivity between the medial dorsal nucleus and the limbic circuit, as measured by fMRI
Ramasubbu et al. (2012) •MPH decreased oxy-hemoglobin levels, an index of decreased neural activation, in the right lateral prefrontal cortex of healthy adults, as
measured by fNIRS
Schabram et al. (2014) •MPH decreased DA turnover in the caudate and putamen of healthy adult females, as measured by PET using [
18
F]FDOPA
Schlosser et al. (2009) •Processing of uncertain information was associated with higher activation of the parietal association cortex and posterior cingulate cortex
after placebo relative to MPH, but with higher left and right parahippocampal and cerebellar activation after MPH relative to placebo, as
measured by fMRI
Schweitzer et al. (2004) •MPH improved Paced Auditory Serial Addition Test performance, reduced regional CBF in the prefrontal cortex, and increased regional CBF
in the right thalamus and precentral gyrus of healthy adults, as measured by PET
Spencer et al. (2006) •Both short-acting and long-acting MPH formulations decreased [
11
C]altropane (a DAT ligand) binding in the striatum of healthy adults, as
measured by PET
Spencer et al. (2010) •Plasma MPH concentrations were correlated with [
11
C]altropane (a DAT ligand) binding in the striatum of healthy adults, as measured by
PET
Tomasi et al. (2011) •MPH increased the activation of the parietal and prefrontal cortices and increased the deactivation of the insula and posterior cingulate
cortex in healthy adults, as measured by fMRI during visual attention and working memory tasks
Udo de Haes et al. (2005) •MPH reduced [
11
C]raclopride (a D
2
receptor antagonist) binding in healthy adults, as measured by PET, with binding changes in the dorsal
striatum correlating with MPH-induced increases in euphoria
Volkow et al. (1994) •MPH decreased [
11
C]raclopride (a D
2
receptor antagonist) binding in the striatum of healthy adults, as measured by PET
Volkow et al. (1996a) •MPH decreased [
11
C]d-threo-MPH binding in the striatum of healthy adults, as measured by PET
Volkow et al. (1997) •MPH produced variable changes in brain metabolism in healthy adults, as measured by PET using [
18
F]FDG, but produced consistent
increases in cerebellar metabolism and decreases in the basal ganglia
Volkow et al. (1998) •MPH dose-dependently reduced [
11
C]cocaine (a DAT ligand) binding in the striatum of healthy adults, as measured by PET, with effects
observed at therapeutic doses used for ADHD
Volkow et al. (1999a,b) •MPH dose-dependently reduced [
11
C]cocaine (a DAT ligand) binding in the striatum of healthy adults, as measured by PET, with self-reports
of MPH-induced “high”and “rush”being significantly correlated with [
11
C]cocaine binding
Volkow et al. (2001) •MPH reduced [
11
C]raclopride (a D
2
receptor antagonist) binding in the striatum of healthy adults, as measured by PET
Volkow et al. (2002a) •MPH reduced [
11
C]cocaine (a DAT ligand) binding in the striatum of healthy adults, as measured by PET
Volkow et al. (2002b) •A review of PET studies suggested that mechanism of action of MPH was related to binding of MPH to the DAT levels sufficient to increase
signal-to-noise ratios and to increase the salience of stimuli, with variability in MPH response being related to differences in DAT level and
DA release across individuals
Volkow et al. (2004) •MPH reduced [
11
C]raclopride (a D
2
receptor antagonist) binding in the striatum of healthy adults in a context-dependent manner, as
measured by PET, with reductions observed when a remunerated mathematical task was performed but not when scenery cards were
passively viewed
Wang et al. (1999) •MPH reduced [
11
C]raclopride (a D
2
receptor antagonist) binding in the striatum of healthy adults, as measured by PET, with changes being
reproducible in individuals at 1- to 2-week intervals
5-HT = 5-hydroxytryptamine (serotonin); ADHD = attention-deficit/hyperactivity disorder; AMP = amphetamine; BOLD = blood-oxygen-level dependent; cAMP = cyclic adenosine
monophosphate; CBF = cerebral blood flow; CHPG = 2-chloro-5-hydroxyphenylglycine; CREB = cAMP-responsive element binding protein; DA = dopamine; DAT = dopamine trans-
porter; FDG = 2-deoxy-2-fluoro-D-glucose; FDOPA = fluorodopamine; fMRI =functional magnetic resonance imaging; fNIRS = functional near infrared spectroscopy; FP-CIT = N-ω-
fluoropropyl-2β-carbomethoxy-3β-(4-iodophenyl)-nortropane; GABA = gamma-aminobutyric acid; GLU = glutamate; IBF = iodobenzofuran; IBZM = iodobenzamide;
MAO = monoamine oxidase; mGluR = metabotropic glutamate receptor; MNPA = (R)-2-CH
3
O-N-n-propylnorapomorphine; MPEP = 2-methyl-6-(phenylethynyl)pyridine hydro-
chloride; MPH = methylphenidate; MRB = methylreboxetine; MRI = magnetic resonance imaging; mRNA = messenger ribonucleic acid; MRS = magnetic resonance spectroscopy;
NE = norepinephrine; NET = norepinephrine transporter; NMDA = N-methyl-D-aspartate; NPA = 2-methoxy-N-propyl-norapomorphine; PET = positron emission tomography;
PHCCC = N-phenyl-7-(hydroxyimino)cyclopropa[b]chromen-1a-carboxamide; PHNO = (+)-4-propyl-3,4,4a,5,6,10b-hexahydro-2H-naphtho[1,2-b][1,4]oxazin-9-ol; PKC = protein ki-
nase C; RGS4 = regulator of G-protein signaling 4; SERT = serotonin transporter; SPECT = single-photon emission computed tomography; TRODAT = ([2-[2-[3-(4-chlorophenyl)-8-
methyl-8-azabicyclo[3,2,1]oct-2-yl]methyl](2-mercaptoethyl)amino]ethyl-amino-ethan-etio-lato-(3-)-oxo-[1R-(exo-exo)]; VMAT-2 = vesicular monoamine transporter 2.
S.V. Faraone Neuroscience and Biobehavioral Reviews 87 (2018) 255–270
261
by PET in rodents and nonhuman primates (Schiffer et al., 2006).
Multiple studies have demonstrated that MPH also directly interacts
with adrenergic receptors (Andrews and Lavin, 2006;Gamo et al.,
2010;Markowitz et al., 2009, 2006). Through activation of α
2
adre-
nergic receptors, MPH has been demonstrated to stimulate cortical
excitability (Andrews and Lavin, 2006). Further evidence for the in-
teraction of MPH with α
2
adrenergic receptors comes from data in-
dicating that the procognitive effects of MPH in a working memory task
are blocked by the α
2
adrenergic antagonist idazoxan (Gamo et al.,
2010). The effects of MPH on α
2
adrenergic receptors are notable given
that two α
2
adrenergic receptor agonist drugs (extended-release forms
of guanfacine and clonidine) are indicated for the treatment of ADHD
(Jain and Katic, 2016).
3.2. Human neuroimaging studies
3.2.1. Amphetamine
Reductions in ligand binding in PET (Boileau et al., 2007;Buckholtz
et al., 2010;Cardenas et al., 2004;Cropley et al., 2008;Drevets et al.,
2001;Leyton et al., 2002,2004;Martinez et al., 2003;Narendran et al.,
2009,2010;Oswald et al., 2005,2015;Riccardi et al., 2006aa,b, 2011;
Shotbolt et al., 2012;Slifstein et al., 2010;Wand et al., 2007;Willeit
et al., 2008;Woodward et al., 2011) and SPECT (Kegeles et al., 1999;
Laruelle et al., 1995;Laruelle and Innis, 1996;Schouw et al., 2013)
studies in healthy humans indicate that AMP increases DA release
across multiple brain regions, including the dorsal and ventral striatum,
substantia nigra, and regions of the cortex. AMP also has been shown to
alter regional CBF to areas of the brain with DA innervation, including
the striatum, anterior cingulate cortex, prefrontal and parietal cortex,
inferior orbital cortex, thalamus, cerebellum, and amygdala (Devous
et al., 2001;Rose et al., 2006;Schouw et al., 2013;Vollenweider et al.,
1998;Wolkin et al., 1987). The effects of AMP on regional CBF appear
to be dependent on the dose, with lower doses decreasing rates of blood
flow in the frontal and temporal cortices and in the striatum (Wolkin
et al., 1987) and higher doses increasing blood flow in the anterior
cingulate cortex, caudate nucleus, putamen, and thalamus
(Vollenweider et al., 1998). In fMRI studies in healthy adults, AMP
increased BOLD signal variability (Garrett et al., 2015) and exerted an
“equalizing”effect on ventral striatum activity during incentive pro-
cessing (Knutson et al., 2004). In addition, AMP was shown to
strengthen amygdalar responses during the processing of angry and
fearful facial expressions (Hariri et al., 2002).
Changes in neuronal activity have been shown to correlate with
various behavioral traits (Buckholtz et al., 2010;Drevets et al., 2001;
Leyton et al., 2002;Woodward et al., 2011). In PET studies, changes in
the binding potential of [
11
C]raclopride (a D
2
receptor antagonist) in
regions of the ventral striatum of healthy adults associated with AMP
binding have been reported to be negatively correlated with changes in
AMP-associated euphoria (Drevets et al., 2001) and with increases in
drug wanting and novelty seeking (Leyton et al., 2002).
3.2.2. Methylphenidate
Methylphenidate has also been shown to increase striatal DA
availability, as measured by reductions in ligand binding potential in
PET studies (Booij et al., 1997;Clatworthy et al., 2009;Montgomery
et al., 2007;Spencer et al., 2006,2010;Udo de Haes et al., 2005;
Volkow et al., 1994,2001,2004;Wang et al., 1999), with evidence to
indicate that this effect is related to binding to the DAT (Volkow et al.,
1998,Volkow et al., 1999a,b,2002a). MPH-induced reductions in
striatal [
11
C]raclopride binding were associated with MPH-induced
changes in euphoria and anxiety and were correlated to age (Udo de
Haes et al., 2005;Volkow et al., 1994). In addition, NE systems have
been implicated as key targets for MPH, with MPH dose-dependently
blocking the NET in the thalamus and other NET-rich regions; the es-
timated occupancy of the NET at therapeutic doses of 0.35–0.55 mg/kg
MPH is 70%–80% (Hannestad et al., 2010).
Assessments of functional activity using fMRI have provided evi-
dence for the widespread functional effects of MPH (Costa et al., 2013;
Moeller et al., 2014;Mueller et al., 2014;Ramaekers et al., 2013;
Schlosser et al., 2009;Tomasi et al., 2011). Using fMRI, it has been
shown that MPH increases activation of the parietal and prefrontal
cortices and increases deactivation of the insula and posterior cingulate
cortex during visual attention and working memory tasks (Tomasi et al.,
2011). Another fMRI study reported MPH-induced activation in the
putamen during a go/no-go task when a response inhibition error oc-
curred but not when a response was successfully inhibited (Costa et al.,
2013), suggesting that the effects of MPH are context dependent. Fur-
thermore, MPH exposure altered connectivity strength across various
cortical and subcortical networks (Mueller et al., 2014) and shifted
brain activation under conditions of uncertainty to higher levels of
activation in left and right parahippocampal regions and cerebellar
regions (Schlosser et al., 2009). Lastly, MPH-associated decreases in
task-related errors on the Stroop color-word task were associated with
concurrent decreases in anterior cingulate cortex activity (Moeller
et al., 2014). MPH has also been shown to reduce regional CBF in the
prefrontal cortex and increase regional CBF in the thalamus and pre-
central gyrus (Schweitzer et al., 2004). In another study that used
functional near-infrared spectroscopy, MPH-associated improvements
in the performance of a working memory task corresponded with de-
creased oxy-hemoglobin levels in the right lateral prefrontal cortex,
which is a surrogate for decreased neural activation (Ramasubbu et al.,
2012).
4. Discussion
Although their mechanisms of action differ, the primary central
nervous system effects of AMP and MPH within the brain include in-
creased catecholamine availability in striatal and cortical regions, as
evidenced in preclinical (Avelar et al., 2013;Covey et al., 2013;Easton
et al., 2007bb; Finnema et al., 2015;Floor and Meng, 1996;Jedema
et al., 2014;Joyce et al., 2007;Kuczenski and Segal, 1997;May et al.,
1988;Mukherjee et al., 1997;Pum et al., 2007;Ren et al., 2009;
Schiffer et al., 2006;Xiao and Becker, 1998;Young et al., 2011;Wall
et al., 1995) and human (Boileau et al., 2007;Buckholtz et al., 2010;
Cardenas et al., 2004;Cropley et al., 2008;Drevets et al., 2001;Kegeles
et al., 1999;Laruelle et al., 1995;Laruelle and Innis, 1996;Leyton et al.,
2002,2004;Martinez et al., 2003;Narendran et al., 2009,2010;
Oswald et al., 2005,2015;Riccardi et al., 2006aa,b, 2011;Schouw
et al., 2013;Shotbolt et al., 2012;Slifstein et al., 2010;Wand et al.,
2007;Willeit et al., 2008;Woodward et al., 2011;Booij et al., 1997;
Clatworthy et al., 2009;Montgomery et al., 2007;Spencer et al., 2006,
2010;Udo de Haes et al., 2005;Volkow et al., 1994,2001,2004;Wang
et al., 1999) studies. These increases in DA and NE availability affect
corticostriatal systems that subserve behaviors related to cognition and
executive function (Moeller et al., 2014;Schlosser et al., 2009;Tomasi
et al., 2011), risky decision making (Oswald et al., 2015), emotional
responsivity (Hariri et al., 2002), and the regulation of reward pro-
cesses (Haber, 2016). Importantly, ADHD has been associated with
structural and functional alterations in regions of the brain where AMP
and MPH have been shown to alter DA and NE activity.
4.1. Structural alterations in ADHD
A meta-analysis of imaging data from individuals with ADHD across
all age groups revealed altered white matter integrity in diverse brain
areas, including the striatum and the frontal, temporal, and parietal
lobes (van Ewijk et al., 2012). In a meta-analysis of imaging data from
children and adults (Nakao et al., 2011), global gray matter volume was
significantly smaller in those with ADHD, especially in basal ganglia
structures integral to executive function. In adults with ADHD, reduced
gray matter volume in the caudate and parts of the dorsolateral pre-
frontal cortex, inferior parietal lobe, anterior cingulate cortex,
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262
putamen, and cerebellum were observed; increased volume was noted
in other parts of the dorsolateral prefrontal cortex and inferior parietal
lobe (Seidman et al., 2011). A meta-analysis of 1713 persons with
ADHD and 1529 controls found volumetric reductions in the ac-
cumbens, amygdala, caudate, hippocampus, and putamen (Hoogman
et al., 2017).
In a meta-analysis of 9 PET and SPECT studies, which included 169
patients with ADHD and 173 healthy controls, it was reported that
striatal DAT density in patients with ADHD was 14% higher than in
healthy controls (Fusar-Poli et al., 2012). Meta-regression analysis
further revealed that previous exposure to ADHD medication influenced
striatal DAT density, with lower DAT density being associated with a
lack of medication exposure (Fusar-Poli et al., 2012). As the correlation
between medication exposure and striatal DAT density accounted for
48% of the variance across studies (Fusar-Poli et al., 2012), it was
suggested that the higher striatal DAT density in individuals with ADHD
was a neuroadaptive response to stimulant exposure.
4.2. Functional alterations in ADHD
A meta-analysis of imaging data focusing specifically on timing
function, which is important for impulsiveness in ADHD, showed con-
sistent deficits in the left inferior prefrontal, parietal, and cerebellar
regions of individuals with ADHD (Hart et al., 2012). A meta-analysis of
24 task-related fMRI studies coupled with functional decoding based on
the BrainMap database reported hypoactivation in the left putamen,
inferior frontal gyrus, temporal pole, and right caudate of individuals
with ADHD (Cortese et al., 2016). When examining these deficits in
regard to the BrainMap database, it was suggested that individuals with
ADHD may exhibit deficits in the cognitive aspects of music, perception
and audition, speech and language, and executive function (Cortese
et al., 2016).
In high-functioning, drug-naive young adults with ADHD, resting-
state fMRIs revealed altered connectivity in the orbitofrontal-temporal-
occipital and frontal-amygdala-occipital networks (relating to in-
attentive and hyperactive/impulsive symptoms, respectively) compared
with matched controls; these abnormalities were not related to devel-
opmental delays, impaired cognition, or use of pharmacotherapy
(Cocchi et al., 2012). Imaging studies have also reported hypoactivity in
the prefrontal cortex and weak connections to other brain regions in
individuals with ADHD (Arnsten, 2009). A review of progress in neu-
roimaging indicated that the initial focus on frontostriatal dysfunction
has given way to a broader understanding of the complex interactions
of various regions of the brain in which alterations may contribute to
ADHD symptoms across the life span (Cortese and Castellanos, 2012).
A substantial body of literature has examined the role of DA systems
in the neurobiology of ADHD and ADHD-related symptoms and beha-
viors. Among treatment-naive adolescents with ADHD, DAT density
shows a significant inverse relationship with blood flow in the cingulate
cortex, the frontal and temporal lobes, and the cerebellum, brain re-
gions which are involved in modulating attention (da Silva et al., 2011).
In a PET study of treatment-naive men with ADHD and men without
ADHD, more pronounced reductions in AMP-induced reductions in
striatal [
11
C]raclopride binding were associated with worse response
inhibition, and those with ADHD had the highest-magnitude reductions
and AMP-induced reductions in striatal [
11
C]raclopride binding
(Cherkasova et al., 2014). Two SPECT studies and 1 PET study showed
that adults with ADHD had higher DAT concentrations than adults
without ADHD (Dresel et al., 2000;Krause et al., 2000;Spencer et al.,
2007), with the SPECT studies further reporting that MPH reduced DAT
availability in adults with ADHD (Dresel et al., 2000;Krause et al.,
2000). Furthermore, studies have shown significant correlations be-
tween global clinical improvement in ADHD symptoms following MPH
treatment and striatal DAT availability (Krause et al., 2005;la Fougere
et al., 2006), and the MPH-induced increases in DA availability in the
ventral striatum are associated with improved ADHD symptomology in
adults with ADHD (Volkow et al., 2012). In another study, in adults
with ADHD, long-term MPH treatment increased striatal DAT avail-
ability (Wang et al., 2013). In another PET study in adults with ADHD,
decreased DAT and D
2
/D
3
receptor availability in the nucleus ac-
cumbens and midbrain compared with individuals without ADHD was
reported, and reduced availability of DAT and D
2
/D
3
receptor avail-
ability was significantly correlated with lower indices of motivation
only in those with ADHD (Volkow et al., 2011). Also, a PET study in
young male adults with ADHD has also reported dysfunctional DA
metabolism in the putamen, amygdala, and dorsal midbrain relative to
healthy controls regardless of treatment status (naive vs previously
treated with MPH) and that a history of MPH treatment resulted in a
down-regulation of DA turnover (Ludolph et al., 2008). Despite the
neuroimaging evidence that supports a role for the DAT in adult ADHD,
a systematic literature review examining the pharmacogenetics of adult
ADHD found that only 1 of 5 identified studies reported finding a
polymorphism at the DAT gene associated with ADHD (Contini et al.,
2013).
Studies of NET availability have not consistently reported altered
NET availability in individuals with ADHD (Sigurdardottir et al., 2016;
Vanicek et al., 2014), but one study reported genotype-dependent in-
creases in NET binding in the thalamus and cerebellum of adults with
ADHD compared with controls; this effect was largely due to the effect
of NET gene polymorphisms on NET binding potential (Sigurdardottir
et al., 2016). Another study reported no NET differences across multiple
brain regions, including the hippocampus, thalamus, and midbrain, in
individuals with ADHD compared with controls (Vanicek et al., 2014).
Beyond the changes observed in DA and NE systems in individuals
with ADHD, there is evidence that GLU, 5-HT, ACH, and opioid systems
play a role in ADHD. Studies examining neurometabolism using proton
MRIs have reported glutamatergic deficits in the frontal cortical and
striatal regions in individuals with ADHD that may be related to cog-
nitive control and symptom severity (Maltezos et al., 2014;Dramsdahl
et al., 2011;Arcos-Burgos et al., 2012). Regarding the 5-HT systems, it
has been reported that increased methylation of the 5-HT transporter is
associated with worse clinical presentation and reduced cortical
thickness in children with ADHD (Park et al., 2015). In adults, sig-
nificant differences in 5-HT transporter interregional correlations be-
tween the precuneus and hippocampus have been reported in adults
with ADHD compared with controls without ADHD (Vanicek et al.,
2017). Functionally, it has been reported that decreased levels of acti-
vation are observed in the precuneus of adolescents with ADHD com-
pared with individuals without ADHD; however, there is a significantly
greater increase in the activation of the precuneus following the ad-
ministration of fluoxetine in adolescents with ADHD compared with
controls (Chantiluke et al., 2015). At present, there are no published
neuroimaging studies of endogenous opioid or ACH systems in in-
dividuals with ADHD. However, altered function in both systems has
been implicated in ADHD (Blum et al., 2008;Potter et al., 2014).
4.3. Co-occurring psychiatric conditions and ADHD
Attention-deficit/hyperactivity disorder is often associated with
comorbid psychiatric disorders (Mao and Findling, 2014;Kooij et al.,
2012;Konofal et al., 2010), such as anxiety and mood disorders. Im-
portantly, the same brain regions and neurotransmitter systems that
underlie ADHD are also implicated in the psychiatric disorders that are
frequently comorbid with ADHD (Farb and Ratner, 2014;Sternat and
Katzman, 2016;Schwartz and Kilduff, 2015). Thus, it is important to
understand how ADHD therapies might influence psychiatric co-
morbidities.
4.3.1. Anxiety disorders
In healthy human volunteers, AMP has been reported to potentiate
amygdalar activity in response to the processing of angry and fearful
facial expressions (Hariri et al., 2002). These data provide a potential
S.V. Faraone Neuroscience and Biobehavioral Reviews 87 (2018) 255–270
263
neurobiologic basis for the anxiogenic effects of AMP (Hariri et al.,
2002). However, it has been theorized that stimulant-associated aug-
mentation of serotonergic drive could ameliorate the comorbid anxiety
associated with ADHD (Heal et al., 2013). In practice, the effects of
stimulants on anxiety can be complex, with acute administration of
MPH reducing anxiety in adults and chronic treatment during early life
increasing anxiety during adulthood (Sanchez-Perez et al., 2012).
4.3.2. Depressive disorders
Psychostimulants have been used in the treatment of major de-
pressive disorder (MDD) since the early 1950s, when the use of MPH
was first examined for MDD (Robin and Wiseberg, 1958). The rationale
for examining the potential utility of psychostimulants in depressive
disorders is based on preclinical and clinical evidence implicating DA in
depressive symptomatology (Treadway and Zald, 2011). However, the
rapid onset of action of psychostimulants suggests the mechanisms by
which they may influence depressive symptoms is likely to differ from
that of antidepressants (Malhi et al., 2016).
Based on the published literature, the effects of psychostimulants on
depressive symptoms appear equivocal. Although one meta-analysis
published in 2008 based on 3 short-term trials reported statistically
significant improvements favoring monotherapy with psychostimulants
compared with placebo for depressive symptoms (Candy et al., 2008), a
systematic review of augmentation therapy for MDD published in 2009
based on MPH (2 studies) and modafinil (2 studies) reported that nei-
ther augmentation strategy was clinically superior to antidepressant
monotherapy in reducing depressive symptoms (Fleurence et al., 2009).
In addition, 2 large phase 3 studies and a phase 2 study of lisdex-
amfetamine dimesylate augmentation in adults with MDD and in-
adequate response to antidepressant therapy failed to meet their pri-
mary efficacy endpoint versus placebo (Richards et al., 2016;Richards
et al., 2017). The lack of treatment effect in these recently published
studies, taken together with inconsistent findings based on meta-ana-
lyses and systematic reviews, suggests that psychostimulants are in-
effective in treating the undifferentiated symptoms of depression. Psy-
chostimulants have also been examined in other mood disorders,
including treatment-resistant depression, bipolar depression, and de-
pression associated with specific medical conditions (Dell’Osso and
Ketter, 2013). For bipolar depression, there is some evidence sup-
porting the efficacy of psychostimulant augmentation, but the quality
and quantity of studies does not allow for a strong evidence-based re-
commendation for their use to be put forth (Dell’Osso and Ketter,
2013).
Despite inconclusive evidence regarding the efficacy of psychosti-
mulants in treating depressive symptoms, the continued publication of
review articles on this topic (Malhi et al., 2016;McIntyre et al., 2017;
Hegerl and Hensch, 2017) demonstrates continued interest in this area
of research. These review articles hypothesize that the lack of consistent
clinical efficacy of psychostimulant augmentation could be due to
poorly defined psychopathology and that psychostimulant effects may
be more pronounced in selected symptom domains (McIntyre et al.,
2017;Hegerl and Hensch, 2017) or that the effects of psychostimulants
are short-lasting (Malhi et al., 2016). It has also been speculated that
the use of clinician-rated scales (rather than patient-rated scales) in
some studies (Richards et al., 2016, 2017) may not have adequately
captured the effects of psychostimulant treatment.
It should also be noted that the use of stimulants as augmentation
agents in combination with tricyclic antidepressants and MAO in-
hibitors is controversial, with issues concerning the possible develop-
ment of an adrenergic crisis, the emergence of serotonin syndrome, or a
hypertensive crisis being raised (Stotz et al., 1999). In fact, both MPH-
based agents and AMP-based agents are contraindicated in individuals
taking an MAO inhibitor (currently or within the preceding 2 weeks)
because a hypertensive crisis may result (Thomas et al., 2015).
4.3.3. Substance use disorders
The neurobiology of reward and addiction and the key role of me-
solimbic DA systems have been described in great detail (Volkow and
Morales, 2015;Koob, 2006). Although associations have been made
between ADHD and substance abuse, their relationship is complex.
Several reviews have emphasized that substance use disorders can be
comorbid with ADHD (Mao and Findling, 2014;Kooij et al., 2012). For
example, in a study of 208 adults diagnosed with ADHD and treated
with psychostimulants as youths, the relative risk of having a diagnosis
of substance use disorder or alcohol abuse, respectively, compared with
the general population was 7.7 or 5.2 (Dalsgaard et al., 2014). Fur-
thermore, a 2012 review noted that there was evidence for increased
rates of substance abuse in individuals with ADHD treated with psy-
chostimulants (Nelson and Galon, 2012). Given the known abuse lia-
bility of psychostimulants and data indicating that psychostimulant
medications are associated with misuse and diversion (Rabiner et al.,
2009;Garnier et al., 2010), it is not surprising that psychostimulant
medications approved for use in ADHD are schedule II medications with
black box warnings for potential drug dependence (Panagiotou et al.,
2011). Some treatment guidelines suggest that nonstimulant alter-
natives be considered as therapies for ADHD when issues related to
abuse and dependence are a concern (Atkinson and Hollis, 2010;Bolea-
Alamanac et al., 2014;Pliszka and AACAP Work Group on Quality
Issues, 2007).
However, multiple studies have provided evidence that psychosti-
mulant treatment in individuals with substance use disorders and
ADHD is not associated with a significant worsening of substance abuse
(Konstenius et al., 2010;Levin et al., 2015). In a study that examined
the efficacy of extended-release mixed AMP salts in the treatment of
ADHD symptoms and cocaine abuse in 126 cocaine-dependent adults
with ADHD (Levin et al., 2015), significantly greater reductions in
ADHD symptoms and higher abstinence from cocaine use were ob-
served with extended-release mixed AMP salts than placebo. In another
study, osmotic-controlled release oral delivery system (OROS) MPH did
not produce significantly greater reductions in ADHD symptoms than
placebo in 24 AMP-dependent adults with newly diagnosed ADHD.
However, OROS MPH treatment also was not associated with evidence
of increased AMP abuse, as measured by self-reported days of AMP use
or craving for AMP, time to relapse, or cumulative abstinence duration
(Konstenius et al., 2010).
4.3.4. Sleep disturbances
The neurobiologic substrates of sleep are diverse and distributed
throughout the brain, with monoaminergic systems playing an im-
portant role in wakefulness (Schwartz and Kilduff, 2015). Substantial
literature exists regarding the sleep disturbances associated with ADHD,
which include insomnia, disordered sleep, difficulty falling asleep, sleep
apnea, daytime somnolence, and increased nocturnal motor activity
(see (Konofal et al., 2010;Cohen-Zion and Ancoli-Israel, 2004;
Lecendreux and Cortese, 2007;Snitselaar et al., 2017) for reviews).
Evidence suggests that impaired and/or disordered sleep is present in
individuals not being treated with psychostimulants (Konofal et al.,
2010). For example, in a study of the effects of MPH on sleep in children
with ADHD, parents reported that approximately 10% of study parti-
cipants had sleep problems before starting their medication (Becker
et al., 2016). However, in regard to the reported effects of psychosti-
mulants on sleep in individuals with ADHD, there are some dis-
crepancies. In a meta-analysis of 9 articles, the use of psychostimulant
medication was associated with longer sleep latency, worse sleep effi-
ciency, and shorter sleep duration (Kidwell et al., 2015). A review of the
safety and tolerability of ADHD medications noted that insomnia was
one of the most commonly reported adverse events associated with
psychostimulant treatment (Duong et al., 2012). In contrast, some
studies have shown that psychostimulants have no significant negative
impact on sleep (Becker et al., 2016;Owens et al., 2016;Surman and
Roth, 2011). A post hoc analysis of the effects of lisdexamfetamine or
S.V. Faraone Neuroscience and Biobehavioral Reviews 87 (2018) 255–270
264
SHP465 mixed amphetamine salts in adults with ADHD demonstrated
that the proportions of participants exhibiting a worsening of sleep
during treatment, as measured by the Pittsburgh Sleep Quality Index,
did not differ from that of placebo (Surman and Roth, 2011). Dis-
crepancies in the effects of stimulants on sleep in individuals with
ADHD might be attributable to various factors, including sleep quality
prior to treatment, the stimulant formulation, the length of treatment,
and the method of sleep assessment (Cohen-Zion and Ancoli-Israel,
2004;Becker et al., 2016;Kidwell et al., 2015). For example, in a study
of MPH in children with ADHD, 23% of participants without preexisting
sleep problems developed sleep problems while taking MPH, whereas
68.5% of those with preexisting sleep problems no longer experienced
sleep problems after taking MPH (Becker et al., 2016).
4.4. Other safety concerns
In addition to considering the potential effect of AMP and MPH in
individuals with other comorbid psychiatric disorders, the peripheral
effects of AMP and MPH related to their pharmacology need to be
considered, particularly in regard to their effects on cardiovascular
function. Increased heart rate and blood pressure are among the most
frequent treatment-emergent adverse events reported with psychosti-
mulant treatment (Duong et al., 2012;Vaughan and Kratochvil, 2012);
increases in pulse and blood pressure are also frequently reported
(Duong et al., 2012;Vaughan and Kratochvil, 2012;Santosh et al.,
2011). There is also a safety concern related to the potential for adverse
cardiovascular outcomes, including ischemic attacks, myocardial in-
farction, and stroke (Westover and Halm, 2012). In a self-controlled
case series analysis, treatment with MPH was associated with an in-
creased overall risk of arrhythmia and with an increased risk of myo-
cardial infarction from 1 week to 2 months after treatment initiation in
children and adolescents with ADHD (Shin et al., 2016). Another study
reported that the risk for an emergency department visit for cardiac-
related reasons among youths did not differ between those being
treated with AMP versus MPH (Winterstein et al., 2009). However,
these findings should be considered in light of data from 2 large studies
that reported no significant increase in risk for cardiovascular events in
current users of ADHD medications compared with nonusers (Cooper
et al., 2011;Habel et al., 2011).
The neurotransmitter systems responsible for stimulant-associated
adverse events and safety concerns are in large part related to stimu-
lation of peripheral NE activity (Duong et al., 2012;Westover and
Halm, 2012). Based on these concerns, package inserts for stimulants
include black box warnings regarding the potential for serious adverse
cardiovascular events (Panagiotou et al., 2011). Assessments of the
risks and benefits of stimulant therapy for ADHD should be made on an
individual basis, and individuals on psychostimulant treatment should
be monitored.
Another issue of potential concern is the possible neuroin-
flammatory effects of psychostimulants. In rats, MPH administration
has been reported to produce neuroinflammation and oxidative stress in
the hippocampus and cerebral cortex, as measured by the inflammatory
markers tumor necrosis factor αand interleukin 1β(Motaghinejad
et al., 2017,2016). In a systematic review of 14 studies (Anand et al.,
2017), no study assessed the relationship between psychostimulant
treatment and neuroinflammation so the relevance of preclinical
models of neuroinflammation to psychostimulant treatment in in-
dividuals with ADHD is unknown. The same review did find evidence
suggesting a role for inflammation in the pathogenesis of ADHD (Anand
et al., 2017), which is consistent with a meta-analysis finding elevated
levels of oxidative stress in patients diagnosed with ADHD (Joseph
et al., 2015). Oxidative stress has also been implicated in the lower
brain volumes seen in ADHD patients (Hess et al., 2017).
5. Conclusions
Based on the published literature, the primary pharmacologic ef-
fects of both AMP and MPH are related to increased central DA and NE
activity in brain regions that include the cortex and striatum. These
regions are involved in the regulation of executive and attentional
function (Faraone et al., 2015). In ADHD, dysfunction in the DA and NE
systems, which are critical to proper cortical and striatal function, likely
account for some of the pathophysiology of ADHD (Cortese, 2012;
Faraone et al., 2015;Arnsten, 2009). Although it is a limitation of the
review that the only database searched was PubMed, it is unlikely that
important studies were not captured.
It has been speculated that the moderately greater efficacy of AMP-
based agents compared with MPH-based agents in ADHD may be re-
lated to differences in their molecular actions (Faraone and Buitelaar,
2010), but to date there is no conclusive clinical evidence to support
this speculation. Furthermore, there is no conclusive clinical evidence
supporting a prospective choice for an AMP-based agent over an MPH-
based agent (or vice versa) based on the mechanisms of action of these
drug classes. As such, the current understanding of differences in the
mechanisms of action of AMP and MPH has not led to clinical guide-
lines regarding their use in specific patient populations. Furthermore, it
is possible that differences in the pharmacologic profile between AMP
and MPH, in combination with the complexities associated with the
etiology of ADHD (Faraone et al., 2015), contribute to individual dif-
ferences in treatment response to AMP-based agents or MPH-based
agents in individuals with ADHD. Interactions among these factors
might explain why some patients have a differential response to these
drugs.
When contemplating pharmacotherapy for ADHD, in addition to
taking into account the potential for adverse cardiovascular outcomes
(Westover and Halm, 2012), the presence of comorbid psychiatric dis-
orders should be considered. Multiple psychiatric comorbidities, in-
cluding depression and anxiety (Mao and Findling, 2014;Kooij et al.,
2012), are thought to be mediated in part by shared neurobiological
pathways that are also implicated in the pathophysiology of ADHD
(Farb and Ratner, 2014;Sternat and Katzman, 2016). As such, the effect
of psychostimulant treatment on the symptoms of these disorders and
the potential interactions with medications used to treat these disorders
need to be considered.
Author contribution
Dr. Faraone designed the systematic literature review, is responsible
for the content of the manuscript, and approved the final draft.
Disclosures
In the past year, Dr. Faraone received income, potential income,
travel expenses, continuing education support, and/or research support
from Otsuka, Lundbeck, Kenpharma, Rhodes, Arbor, Ironshore, Shire,
Akili Interactive Labs, CogCubed, Alcobra, VAYA, Sunovion, Genomind,
and NeuroLifeSciences. With his institution, he holds US patent
US20130217707 A1 for the use of sodium-hydrogen exchange in-
hibitors in the treatment of ADHD.
Acknowledgments
Shire Development LLC (Lexington, MA) provided funding to
Complete Healthcare Communications, LLC (CHC; West Chester, PA),
an ICON plc company, for support in writing and editing this manu-
script. Under the direction of the author, writing assistance was pro-
vided by Madhura Mehta, PhD, and Craig Slawecki, PhD, employees of
CHC. Editorial assistance in the form of proofreading, copyediting, and
fact checking was also provided by CHC. The author exercised full
control over the content throughout the development and had final
S.V. Faraone Neuroscience and Biobehavioral Reviews 87 (2018) 255–270
265
approval of the manuscript for submission.
Dr. Faraone is supported by the K. G. Jebsen Centre for Research on
Neuropsychiatric Disorders, University of Bergen, Bergen, Norway, the
European Union’s Seventh Framework Programme for research, tech-
nological development, and demonstration under grant agreement no
602805, the European Union’s Horizon 2020 research and innovation
programme under grant agreement No 667302, and NIMH grants
5R01MH101519 and U01 MH109536-01.
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