8. Dopamine reuptake, dopamine degradation

Extracellular dopamine in the synaptic cleft can be broken down by reuptake into dopaminergic neurons or, after uptake into glial cells, by MAO or COMT. Within dopaminergic neurons, dopamine located outside vesicles is broken down by MAO.¹ Furthermore, dopamine can be inactivated through sulfation or glucuronidation, as well as converted into norepinephrine through metabolism. Finally, dopamine can diffuse and is then transported away via the bloodstream.

8.1. Dopamine reuptake (recycling)¶

Transporters are classified into high-affinity transporters with low transport capacity (Uptake-1 transporters: DAT, NET, SERT) and low-affinity transporters with high transport capacity (Uptake-2 transporters: PMAT, OCT). Uptake-1 transporters are located on presynaptic cells and facilitate the reuptake of the neurotransmitter for reuse in release or efflux. Uptake-2 transporters are located on glial cells, which mediate the degradation of neurotransmitters. All transporters are located outside the synaptic cleft.²
In SERT-KO mice, OCT3 (and possibly other Uptake-2 transporters) are upregulated to compensate for serotonin depletion.² Conversely, they could interfere with the effects of SSRIs.

8.1.1. Dopamine reuptake via the dopamine transporter (DAT)¶

DAT (also known as DAT1) is a plasma membrane transport protein responsible for regulating the duration and intensity of dopaminergic signaling. In ADHD and ASD, DAT function is often altered.

DAT reuptake dopamine from the extracellular space and transport it into the cell. This dopamine may have been released by the cell itself (reuptake) or may have originated from neighboring cells.

DAT is expressed within a dopaminergic neuron:³

• in the terminal area
• along the axon
• around the soma and dendrites (somatodendritic region)
• on the outer plasma membranes of small distal dendrites
• primarily located in the perisynaptic region (at the edge of the synapse) within the intracellular membranes of dopaminergic cells

Through reuptake, DATs regulate the temporal availability of freshly released dopamine (and, to a lesser extent, norepinephrine)—which is primarily phasic rather than tonic—in the synaptic cleft or in the presynaptic neuron.⁴⁵ This ensures the fine-tuning of the phasic nature of the dopamine signal,⁶ because only when the synaptic cleft is quickly cleared of previously released dopamine is newly released (signal-coding) dopamine is able to transmit these signals cleanly, unimpeded by dopamine from earlier releases. By removing dopamine from the synaptic cleft, DAT modulates the signal-to-noise ratio of dopamine signaling.⁷
For a discussion of the distinction between tonic and phasic dopamine, see ⇒ Dopamine release (tonic, phasic) and encoding.
The tonic extrasynaptic dopamine level is less affected by reuptake.⁴

DAT is found primarily at the terminals of the nigrostriatal and mesolimbic pathways, but rarely in somatodendritic regions or in the PFC.⁸
DAT enzymes are responsible for the majority of dopamine breakdown in the striatum. In the PFC, on the other hand, where fewer DATs are found, dopamine breakdown is primarily mediated by COMT (60%) and NET, and only to a small extent by DAT (15%).⁹ In the PFC, norepinephrine transporters (NET) significantly reuptake dopamine.
There are five times as many DAT neurons in the nucleus accumbens as in the basolateral amygdala.¹⁰

The binding affinity of DAT is as follows:¹¹

• Dopamine 885 / 2,140 / 5,200 km
• Norepinephrine 17,000 km
  Lower values indicate a higher binding affinity.

During the first few months of life, DATs undergo epigenetic changes in expression due to environmental influences.¹²
The dopamine transporter (DAT) transports not only dopamine but also norepinephrine.¹² In addition, the DAT can transport neurotoxic compounds such as 6-hydroxydopamine, 1-methyl-4-phenylpyridinium, or environmental chemicals such as paraquat, making it a gateway for harmful substances and a potential mechanism for dopaminergic neurodegeneration in Parkinson’s disease.¹³

The DAT gene is located on chromosome 5p15.3. It occurs in various variants, which differ, among other things, in the number of 40-bp repeats (allele repeats), ranging from 3 to 11 repeats (R). The most common variants are the 10R variant with 480 bp (70%) and the 9R variant with 440 bp (27%) in the Caucasian and Hispanic populations, and 72% and 17% in the African population, which, in contrast, showed rare allelic repeat variants significantly more frequently at 12%.¹⁴¹⁵ A study of n = 192 subjects found a distribution of:¹⁶
DAT 9R: 37%
DAT 10R: 49%
DAT 11R: 3%,
DAT 7R: less than 0.3%
DAT 13R: less than 0.3%
Since DAT 10R expresses significantly more dopamine transporters than DAT 9R¹⁷, DAT 10R results in increased dopamine clearance from the synaptic cleft, thereby resulting in reduced tonic (but unaffected or even increased phasic) dopamine, whereas DAT 9R results in reduced dopamine clearance and thus increased tonic and reduced phasic dopamine. DAT 10R is associated with ADHD, DAT 9R with borderline personality disorder.
Children with ASD and a DAT 10R showed (compared to a combined group of all other genotypes)¹⁸

• less severe symptoms of hyperactivity and impulsivity as rated by parents
• less severe language deficits as rated by parents
• more severe social anxiety and tic symptoms, as rated by teachers
• DAT heterozygosity (9-10-repeat genotype) could be a risk or protective factor for AS

A comprehensive overview of the various DAT gene variants and their functional differences can be found at SLC6A3, DAT1, dopamine transporter gene (chromosome 5p15.3; 10-R allele, VNTR) In the article Genetic candidates for ADHD with a plausible mechanism of action on ADHD in the subsection Genetic candidates for ADHD in the section Genetic and epigenetic causes of ADHD - Section in the chapter Development.

DATs are dynamically regulated by a variety of cellular factors.
Their expression in different brain regions or within a specific cell is not static, but highly plastic.

• A deletion of the first 22 amino acids of DAT¹⁹
  • Significantly reduces AMP-induced dopamine efflux
  • Re-enrollment remains unchanged
  • Eliminates the 32P incorporation in DAT in response to PKC activation²⁰
• Mutation of the five N-terminal serines to alanine¹⁹
  • Causes a significant reduction in AMP-induced dopamine efflux
  • Re-enrollment remains unchanged
• Mutation of the five N-terminal serines to aspartate (mimicking phosphorylation)¹⁹
  • Efflux is preserved
  • It is likely that the phosphorylation of one or more of these five N-terminal serines is required for AMP-induced dopamine efflux

8.1.2. Is DAT elevated or reduced in ADHD?¶

It is unclear whether DAT levels in the striatum are elevated or reduced in ADHD.

According to Volkow et al., people with ADHD have fewer dopamine transporters than persons without the condition:

• In the midbrain: 43% less
• In the left caudate nucleus: 20% less
• In the nucleus accumbens: 11% less

Other studies have found elevated levels of DAT in people with ADHD, although the sample sizes were very small.²³²⁴ According to a somewhat larger study, DAT levels in the striatum are elevated in both hemispheres in both ADHD-HI and ADHD-I. Nicotine dependence (smoking) reduced DAT levels in the striatum to the levels seen in non-affected individuals or even below.²⁵
Another study found no elevated DAT levels in people with ADHD who have the homozygous 10-allele polymorphism of the DAT gene (DAT 10R).²⁶
During a delay discounting task performed under fMRI, adults with ADHD showed reduced activation in a number of brain regions (including the dlPFC, the anterior frontal gyrus, the anterior cingulate, the caudate nucleus, and the cerebellum). At the same time, the extent to which people with ADHD discounted delayed rewards was associated with reduced activation of the cerebellum. Consequently, the striatum was underactivated in relation to reward anticipation, and the dlPFC and oPFC were overactivated in relation to reward receipt.²⁷
In a very small fMRI study (16 participants) on the effects of methylphenidate on boys with and without ADHD, people with ADHD showed increased activation of the frontal cortex and reduced activation of the striatum during Go/No-Go tasks prior to taking methylphenidate, compared to people without ADHD. (According to our understanding, this would have required an increased number of DAT receptors due to upregulation.) Methylphenidate leveled out these differences.²⁸

8.1.3. Additional dopaminergic influences of the DAT¶

8.1.3.1. Dopamine release (efflux) via DAT¶

DATs can also release dopamine—at least in the substantia nigra. While the D2 dopamine autoreceptor downregulates dopamine release when extracellular dopamine levels are high, the DAT promotes dopamine release when dopamine levels are low. Unlike methylphenidate, which inhibits dopamine reuptake via DAT, amphetamine is a substrate for DAT that may potentially trigger dopamine release in the substantia nigra.³¹¹¹ According to other reports, MPH also increases DAT efflux (see under MPH).
Only an increase in population activity resulting from inhibition of GABAergic afferents from the pallidum (but not the activation of pedunculopontine inputs, which increases burst firing) increases dopamine efflux in the ventral striatum. However, following dopamine reuptake blockade, dopamine efflux increased three times more in response to enhanced burst firing than to increased population activity.⁴

Dextroamphetamine and methamphetamine also stimulate efflux via DAT,³² whereas MPH does not.
However, there is no evidence that amphetamine increases efflux even at therapeutically relevant doses. All studies showing efflux caused by AMP involve drug doses that are much higher than therapeutic doses.³³
Although amphetamine at therapeutically relevant doses leads to a selective redistribution of the vesicular monoamine transporter 2 (VMAT2), which would be a prerequisite for efflux, However, MPH (which does not cause efflux even at high doses) also leads to this redistribution of VMAT2 at therapeutically relevant doses.³⁴ The plasma concentration of 10 μM AMP used in the study by Støier et al. (2023)³⁵, which showed efflux, is significantly higher than the 0.05 to 0.5 μM typically achievable with medications. However, the study also observes efflux at 1 μM AMP, which is at least closer to a drug dose.

8.1.3.2. DAT Channel Function¶

DAT not only transport dopamine, but also appear to have a channel mode that directly modulates membrane potential and neuronal function.
The depolarizing currents generated by DAT are therefore not solely the result of the high density and rapid turnover rate of a classical transporter, but also stem from genuine channel activity of DAT.³⁶

8.1.4. Drug-binding sites of the DAT¶

The structures of the DAT to which drugs bind differ between outward- and inward-facing states. These states exhibit different binding modes and conformational transitions during substrate transport. While the outward-facing state is stabilized by cocaine, GBR12909 and benzatropine stabilize the dopamine transporter in the inward-facing state.³⁷

8.1.5. Dopamine reuptake via the norepinephrine transporter (NET)¶

The norepinephrine transporter (NET) is commonly found in the PFC and rarely in the striatum, whereas the DAT is rare in the PFC and common in the striatum. The NET has a slightly higher affinity for dopamine than for norepinephrine,³⁸³⁹ so that a significant portion of dopamine degradation/reuptake in the PFC (but not in the striatum) occurs via the NET.
In ADHD, norepinephrine reuptake appears to be reduced in the attentional networks of the right hemisphere.⁴⁰

The binding affinity of NET is as follows:¹¹

• Dopamine 240 / 730 km
• Norepinephrine 539 / 580 km
  Lower values indicate a higher binding affinity.

NET-KO mice (mice lacking the norepinephrine transporter) show no effective dopamine degradation in the PFC.⁴¹

In the DAT-KO mouse, it was found that NET in the striatum barely contributes to dopamine degradation in the striatum. Inhibition of serotonin transporters, norepinephrine transporters, MAOA, or COMT did not alter dopamine degradation in the striatum of the DAT-KO mouse. In the absence of DAT in the striatum, this appears to occur primarily via diffusion.⁴² We do wonder, however, whether the process by which DAT is inactivated in the DAT-KO mouse might also inactivate NET, since NET also takes up dopamine.

8.1.6. Dopamine reuptake inhibitors¶

8.1.6.1. Dopamine reuptake inhibitor¶

Substances that inhibit the dopamine transporter (DAT) are referred to as dopamine reuptake inhibitors. The term thus refers to the transporter being inhibited, rather than the resulting inhibition of neurotransmitter reuptake.
Dopamine reuptake inhibitors are (bold: typical ADHD medications)

• Amineptine
• Amphetamines
• Bupropion
• Bromantan
• CE-158⁴³
• Dasotralin
• Difemetorex
• Difluoropin
• Fencamfamine
• GBR-12909 (1-(2-bis(4-fluorophenyl)methoxy)ethyl)-4-(3-phenylpropyl)piperazine, selective DAT inhibitor)⁴⁴⁴⁵
• Lefetamine
• Levofacetoperan
• Medifoxamine
• Mesocarb
• Methylnaphthidate (HDMP-28)⁴⁶
• Methylphenidate
• Nomifensine (brand names: Alival, Merital, Psyton)
• Pipradrol
• Prolintan
• Pyrovalerone
• Reserpine
• Solriamfetol
• Vanoxerin (in development)

8.1.6.2. Norepinephrine reuptake inhibitor¶

Substances that inhibit the norepinephrine transporter (NET) are referred to as norepinephrine reuptake inhibitors. The term is therefore based on the transporter being inhibited, rather than on the resulting inhibition of neurotransmitter reuptake.
Selective norepinephrine reuptake inhibitors (SNRIs) include, among others,

About ADHD treatment:

• Atomoxetine (Strattera®, Atomoxe®, Agakalin®, generic atomoxetine)
• Viloxazine
  On the treatment of depression:
• Reboxetine (Solvex®, Edronax®)
• Viloxazine (Vivalan®; the manufacturer did not renew the marketing authorization, so the product is no longer available)

Regarding obesity treatment:

• Mazindol

As a skeletal muscle relaxant:

• Orphenadrine (Norflex®)

Research:

• Nisoxetine (never approved)

8.2. Dopamine depletion¶

8.2.1. Dopamine degradation via the dopamine 2 transporter¶

8.2.1.1. Dopamine degradation via plasma membrane monoamine transporters (PMAT)¶

PMAT and OCT are so-called Uptake-2 transporters, as they have a low affinity (Kd = 252 vs. 0.27 μM) combined with a transport capacity approximately 80 times higher than that of Uptake-1 transporters such as DAT, NET, or SERT (Vmax = 100 nmol/min/g tissue vs. 1.22 nmol/min/g tissue). They operate sodium-independently and bidirectionally.⁴⁷ However, Uptake-2 transporters are active not only at very high substrate concentrations, but at any substrate concentration.⁴⁸
Since Uptake-2 transporters are barely found on (dopamine-releasing) dopaminergic neurons but are primarily located on (dopamine-degrading) glial cells, PMAT and OCT 1 through 3 are not involved in reuptake but rather in dopamine degradation.

In addition to DAT and NET, dopamine and norepinephrine are taken up by the plasma membrane monoamine transporter (PMAT). This transporter is also known as human equilibrative nucleoside transporter-4 (hENT4). It is encoded by the SLC29A4 gene. Its binding affinity is lower than that of DAT or NET. It binds dopamine and serotonin, as well as, to a much lesser extent, norepinephrine, epinephrine, and histamine.⁴⁹

PMAT, discovered in 2004 (⁵⁰ ), is widely distributed in the human brain. PMAT (⁵¹ ) is found

• widespread in the brain²
• Amygdala²
• cerebral cortex²
• Hippocampus²
• Striatum²
• on the CSF side of the endothelial cells of the blood-CSF barrier in the choroid plexus⁵²
  • Choroid plexuses are tangle-like clusters of arteriovenous vessels composed of specialized glial cells. They are found in the cerebral ventricles and are responsible for the production of cerebrospinal fluid, the formation of the blood-cerebrospinal fluid barrier, as well as the reabsorption and detoxification of cerebrospinal fluid.⁵³

PMAT-KO mice are viable, fertile, and exhibit normal physiological characteristics.²

PMAT gene polymorphisms associated with reduced transport activity for the monoamines serotonin and dopamine, as well as the neurotoxin 1-methyl-4-phenylpyridinium (MPP(+)), are correlated with autism spectrum disorders (ASD).⁵⁴
PMAT-KO mice (which therefore lack PMAT) show neither significant changes in brain histamine levels nor behavioral abnormalities outside of stressful situations.⁵²⁵¹

PMAT gene variants with reduced activity are thought to be involved in AS.²

8.2.1.2. Dopamine Degradation via Organic Cation Transporters (OCT)¶

Like PMAT, OCTs are so-called Uptake-2 transporters. See above.

Dopamine (albeit to a lesser extent than norepinephrine) is taken up from the extracellular space not only by the uptake transporters DAT and NET, but also, to a lesser extent, by organic cation transporters (in rats: OCT1, OCT2, OCT3; in humans: only OCT2⁵⁵). These Uptake-2 transporters are also referred to as Solute Carrier Family 22 members 1/2/3 or extraneuronal monoamine transporters (EMT). OCT2 and OCT3 are found in nerve cells and astrocytes and bind histamine > norepinephrine and epinephrine > dopamine > serotonin.⁴⁹ Unlike with DAT and NET, uptake does not occur into the presynaptic cell, but rather into glial cells. There, dopamine and norepinephrine are broken down by COMT into methoxytyramine.⁵⁶

The coding genes are:⁵⁷

• OCT1: SLC22A1
• OCT2: SLC22A2
• OCT3: SLC22A3

Strong OCT antagonists include, for example,⁵⁶

• Amantadine
• Memantine

8.2.1.2.1. OCT1¶

OCT1:²

• relatively low expression in the brain⁵⁸
• is found in the brain in astrocytes, but not in neurons.
• OCT1 knockout mice are viable, fertile, and exhibit normal physiological characteristics⁵⁹

OCT1 (SLC22A1) transports with low affinity but relatively high turnover

• endogenous monoamines such as
  • N-1-methylnicotinamide (NMN)
  • Guanidine
  • histamine
• Neurotransmitters such as
  • Dopamine
  • Serotonin
  • Adrenaline
  • Acetylcholine
• Is involved in
  • hepatic uptake of vitamin B1/thiamine
  • Intake of xenobiotics

8.2.1.2.2. OCT2¶

Like OCT3, OCT2 reabsorbs histamine, but it has not been detected in human astrocytes; however, it has been found in the human brain, as h⁶⁰, OCTN1 (SLC22AN1), and OCTN2 (SLC22AN2).⁶¹

OCT2:²

• is widely distributed in the brain, e.g., in
  • limbic regions (e.g., amygdala)
  • cerebral cortex
  • Hippocampus
  • Striatum
• OCT2 is found in neurons and glial cells (astrocytes, microglia)
• OCT2 knockout mice
  • are viable, fertile, and exhibit normal physiological characteristics⁵⁹
  • reduced anxiety⁶²
  • reduced symptoms of depression (reduced immobility time in the forced swim test and the tail suspension test)⁶²
  • 1.9 times higher levels of the stress hormone (corticosterone)⁶³
  • increased susceptibility to depression due to recurring, unpredictable stress⁶³

OCT2 transports:⁶¹

• has an affinity similar to that of OCT1
  • MPP
  • TEA
  • Quinine
  • Metformin
• 4 times as specific as OCT1
  • Acetylcholine
• and so on
  • Choline
  • Agmatine
• Neurotransmitters
  • Dopamine
  • Norepinephrine
  • Adrenaline
  • Serotonin
  • histamine
• Glutamate receptor antagonists
  • Amantadine
  • Memantine
• Histamine H2 receptor antagonists
  • Cimetidine
  • Famotidine
  • Ranitidine
• Cytostatic drug
  • Cisplatin
• Blood pressure medication
  • Debrisoquin

OCT2 is involved in⁶⁴

• Memory
• Pain perception
• Stress management:⁶³
  • OCT2 is expressed in various stress-related circuits in the brain and along the HPA axis
  • OCT2 knockout mice exhibit
    • intensified the hormonal response to acute stress
    • impaired HPA function
    • Unchanged adrenal sensitivity to ACTH
    • a significantly more sensitive response to unpredictable, chronic, mild stress. Changes in:
      • symptoms of depression
      • Self-care
      • spatial memory
      • social interaction
      • stress-sensitive spontaneous behavior
    • Altered glycogen synthase kinase 3β (GSK3β) signaling pathway in the hippocampus
      • The GSK3β signaling pathway responds strongly to acute stress in healthy mice
      • In OCT2-KO mice, it appears that elevated serotonin levels are primarily responsible for disrupting GSK3β signaling in the brain

8.2.1.2.3. OCT3¶

OCT3 exhibits low affinity but high binding capacity for the neurotransmitters norepinephrine, epinephrine, dopamine, serotonin, and histamine.⁶⁵⁶⁶

OCT3 is the most common OCT in the brain. In the brains of rodents, OCT3 is found particularly in:⁶⁷⁵¹

• dopaminergic neurons of the substantia nigra compacta
• non-aminergic neurons of
  • VTA
  • Reticulated substantia nigra
  • Locus coeruleus
  • Hippocampus
  • Cerebellum
  • Cortex
    • Nucleus accumbens¹⁰
    • basolateral amygdala¹⁰
      • approximately as frequently as in the nucleus accumbens
• mainly in neurons
• in astrocytes
  • occasionally in astrocytes in the substantia nigra reticulata, hippocampus, and several hypothalamic nuclei⁶⁷
  • found in large numbers in astrocytes that are adjacent to both the cell bodies and the terminals of dopaminergic midbrain neurons⁶⁸⁶⁰

OCT3 is primarily found as an autoreceptor on histaminergic neurons, and thus inhibits histamine synthesis and release.⁶⁴
Under normal conditions, OCT3 does not appear to affect histamine levels in the brain. In OCT3-knockout mice, cortical histamine levels were elevated following cerebral ischemia, suggesting that the OCT3 transporter contributes to histamine concentrations in the brain.⁵¹

Although all “Uptake-2” transporters are inhibited by corticosterone, their sensitivity to corticosterone varies depending on the species and tissue sample.⁶¹⁶⁹

• OCT3 is more sensitive to corticosteroids than OCT1, OCT2, and PMAT
  • OCR3 exhibits IC50 values within the physiological range for corticosterone
• OCT3 therefore acts as a critical mediator of stress and corticosteroid effects on neuronal and glial physiology and behavior

OCT3 mediates, via the stress-induced increase in glucocorticoids, a strong modulatory effect of stress on the actions of norepinephrine, dopamine, serotonin, and histamine through a rapid, non-genomic mechanism.⁶⁶
Corticosterone inactivates OCT1⁷⁰ and OCT3⁶⁵

• fast
• through direct interaction of corticosterone with the transporter at specific sites
• though probably not via glucocorticoid receptors⁷¹

We are considering whether this mechanism could explain why people with ADHD are able to overcome their usual tendency to procrastinate when under high stress, and why their motivation is (only) then sufficient to complete tasks that are not intrinsically interesting to them. It is at least conceivable, but so far it remains purely a hypothesis.

OCT3 is not affected by cocaine or antidepressants (desipramine).⁶⁵

OCT3 inhibitors that are effective at physiological concentrations include:⁷²

• Azlocillin
• Aztreonam
• Famotidine
• Flufenamic acid
• Meropenem
• Propafenone
• Quinine
• Trazodone
• Trimethoprim

8.2.1.2.4. OCT and ADHD¶

Methylphenidate binds selectively to OCT1 (IC50: 0.36) and does not bind to OCT2, OCT3, or PMAT. Ketamine, on the other hand, binds only to OCT2⁷³, and PMAT.²
d-amphetamine is a highly potent hOCT2 reuptake inhibitor (Ki: 10.5 mM) and a moderately potent hOCT1 reuptake inhibitor (Ki: 202 mM), whereas it did not interact with hOCT3 until concentrations of 100 μM (Ki: 460 mM) (hOCT: human OCT).⁷³⁵⁵
d-amphetamine binds with approximately the same affinity to hOCT2 and hOCT3, and with an affinity that is one order of magnitude (a factor of 10) weaker than that for DAT.⁵⁵

The binding of amphetamine to OCT may contribute to the cellular and behavioral effects of amphetamine.⁵⁵

OCT3 was found in brain regions relevant to ADHD:

• in the striatum⁵⁵
• Nucleus accumbens¹⁰
• Cerebellum⁵¹

OCT2 found:⁵⁵

• in the hippocampus
  • which is involved in dopamine-dependent, reward-dependent behavior and responses to amphetamines
• barely at all in dopaminergic systems

OCT3-30%-KO mice showed:⁷⁴⁷⁵

• unchanged spontaneous motor activity
• Increased motor activity induced by methamphetamine compared to METH in wild-type mice
• increased motor activity with imipramine
• This could suggest that OCT3 contributes to the reuptake of norepinephrine and serotonin
• Anxiety
  • increased in OCT3 30% knockout mice⁶⁷
  • reduced in OCT3 100% knockout mice⁷⁶
• elevated stress levels⁶⁷
• increased sensitivity to psychostimulants⁶⁷

With regard to norepinephrine reuptake inhibitors, it has been hypothesized that drugs that block the OCT2 transporter, such as normetanephrine², when combined with NET reuptake inhibitors, could accelerate the onset of therapeutic benefits in depression.⁷⁷ Preclinical studies support this hypothesis.² For example, in OCT2-KO mice, much lower doses of venlafaxine or reboxetine have an antidepressant effect than in wild-type mice.⁶² OCT2 reuptake inhibitors also have an antidepressant effect.⁷⁸
We believe it is worth considering whether this approach might also support the effects of dopamine reuptake inhibitors in ADHD.
Furthermore, these mechanisms could explain why MPH, which binds only to OCT1, has a weaker antidepressant effect than amphetamine-based medications that inhibit OCT2.

At the same time, we find it striking that ADHD medications almost universally cause an increase in histamine, while OCT2 and OCT3 have a significantly higher affinity for histamine than for norepinephrine and dopamine. An abundant supply of histamine should primarily bind to OCT2 and OCT3, thereby reducing the uptake of dopamine and norepinephrine by OCT2 and OCT3 and, as a result, increasing extracellular norepinephrine and dopamine. According to this mechanism, histamine would act as a dopamine uptake inhibitor.

8.2.2. Dopamine degradation through autooxidation¶

Dopamine is unstable and can be oxidized by enzymes or metal ions, or, in the absence of enzymes or metal ions, it can auto-oxidize.

The oxidation of DA can produce:

• low-molecular-weight ROS⁷⁹
  • can trigger oxidative stress in dopamine neurons through reversible oxidative modification of macromolecules such as proteins, lipids, and nucleic acids
• 3,4-dihydroxyphenylacetaldehyde (DOPAL)⁸⁰
• highly reactive DA quinones (DAQ)⁷⁹
  • can cause vulnerability in dopamine neurons through several toxic mechanisms:
    • irreversible and covalent conjugation with cysteine residues of proteins
      • leads to protein misfolding, inactivation, and aggregation
    • Free DAQ and DAQ-conjugated proteins can undergo redox cycles and generate harmful ROS
    • Endogenous DAQs can cause irreversible inhibition of the ubiquitin-proteasome system
    • DOQ can
      • are converted back into dopamine by reducing agents in the environment
      • undergo further oxidation to form reactive aminochrome (a type of cyclized DAQ)
        • Aminochrome is more stable than DOQ and can be detected, monitored, and characterized
        • DOQ and AM can react and conjugate with many biomolecules, including the amino acids cysteine and tyrosine, which contain sulfhydryl and hydroxyl groups
        • AM is converted by polymerization into neuromelanin (an insoluble granular pigment in the substantia nigra)
          • Neuromelanin
            • prevents the neurotoxicity of DAQs
            • has antioxidant properties
              • binds and inactivates reactive oxygen species (ROS) under normal conditions
            • also produces ROS under oxidative stress conditions
              • could therefore be involved in α-syn-associated damage to dopamine neurons

Enzymes (such as tyrosinase) or metal ions (such as iron species or Mn³⁺) can catalyze the oxidation of dopamine in solutions.
Reducing agents, particularly glutathione, can effectively prevent autooxidation.

Highly reactive DAQs appear to play a more significant pathological role in the degeneration of dopamine neurons in Parkinson’s disease than low-molecular-weight ROS. ⁷⁹

8.2.3. Dopamine breakdown through metabolism (via enzymes)¶

While dopamine transporters facilitate the reuptake of dopamine from the synaptic cleft back into the presynaptic cell, where it is repackaged into vesicles by VMAT2 transporters, dopamine is also broken down through conversion into other substances. The main enzymes involved here are COMT and MAO.

Metabolism is the biochemical conversion or breakdown of a substance by the body’s own enzyme systems into a chemically altered metabolite.⁸¹

Dopamine degradation involves the mitochondria and leads to the formation of reactive oxygen species (ROS).⁸²
Under physiological conditions, DA oxidation proceeds slowly, allowing the cellular antioxidant machinery to cope with the amount of highly reactive products generated by DA oxidation. Higher dopamine levels lead to increased dopamine oxidation and are toxic to the mitochondria of neurons and glial cells.⁸³ Mitochondrial defects impair dopamine degradation and can lead to elevated dopamine levels in the cytosol.⁸⁴ This interaction between dopamine and mitochondria is involved in the pathogenesis of Parkinson’s disease and schizophrenia.⁸⁵

8.2.3.1. Dopamine breakdown by COMT¶

The COMT enzyme primarily metabolizes catecholamines (adrenaline, norepinephrine, dopamine) through O-methylation. S-Adenosyl-L-methionine (SAM) acts as the methyl donor.
COMT breaks down dopamine in receiving nerve terminals, while MAO is active in receiving nerve terminals.Rostain (2015): The Neurobiology of ADHD, Perelman School of Medicine, University of Pennsylvania

8.2.3.1.1. Dopamine breakdown in the PFC occurs via NET and COMT, but barely via DAT; in the striatum, it occurs via DAT, but barely via NET and COMT¶

COMT is not present in nigrostriatal neurons.¹
In the striatum, dopamine is therefore barely broken down by COMT. Instead, there are many DAT receptors in the striatum.^86878889
The PFC has relatively few dopamine transporters (DAT), unlike the striatum.^86878889

Therefore, the PFC requires alternative pathways to break down dopamine (which increases in the PFC during stress). In addition to NETs, it relies particularly on the enzyme catechol-O-methyltransferase (COMT), which deactivates dopamine by adding a methyl group and accounts for 60% of dopamine breakdown in the PFC (and only 15% of dopamine breakdown in the striatum). Another important dopamine-degrading enzyme is monoamine oxidase B (MAO-B).^90919293

However, COMT is found primarily in glial cells, particularly in microglia, and is present barely at all, if at all, in nerve cells.⁹⁴ Apparently, dopamine from the synaptic cleft is also taken up by glial cells.¹

In addition to dopamine, COMT also metabolizes levodopa, thereby inhibiting dopamine synthesis. COMT inhibitors such as entacapone, tolcapone, and opicapone are used in the treatment of Parkinson’s disease.⁹⁵

COMT is regulated by the COMT gene. COMT gene polymorphisms that affect the activity of the COMT gene therefore primarily influence dopamine levels in the PFC and barely influence dopamine levels in other brain regions.

8.2.3.1.2. COMT isoforms: soluble and membrane-bound¶

There are two isoforms of COMT:¹

• Soluble COMT (free COMT)
  • freely moving cytoplasmic form
  • in glial cells
  • in the outskirts
  • tends to metabolize exogenous catecholamines
• Mb-COMT (membrane-bound)
  • membrane-bound isoform
  • predominantly located on the neuronal membrane
  • higher catecholamine affinity
  • primarily metabolizes dopaminergic and noradrenergic catecholamines

While COMT-mediated dopamine degradation in the striatum appears to be carried out by membrane-bound COMT, only soluble COMT may be involved in the PFC. Mb-COMT knockout mice (mice lacking membrane-bound COMT) exhibit elevated dopamine levels in the striatum, but not in the PFC. This suggests that Mb-COMT is involved in dopamine degradation in the striatum, whereas in the PFC, only soluble COMT may be involved.²¹

8.2.3.1.3. COMT gene variants alter dopamine levels in the PFC¶

The homozygous Val158Val polymorphism of the COMT gene results in dopamine breakdown that is four times faster than that of the homozygous COMT Met158Met variant, which causes a less active COMT and thus slower dopamine breakdown.⁹⁶⁹⁷ In terms of catecholamine metabolism, the Val158Met polymorphism lies between the fast-metabolizing Val158Val and the slow-metabolizing Met158Met.
Healthy COMT Met158Met carriers are

• Compared to COMT Val158Val carriers (likely due to higher dopamine levels in the PFC)
  • Mentally more capable (more efficient, not more intelligent)⁹⁸
  • More difficulty switching tasks, reduced mental flexibility
    • Carriers of at least one Met allele exhibited greater task-switching costs (i.e., lower cognitive flexibility) than carriers of the homozygous Val/Val COMT gene. This suggests that low prefrontal dopamine levels correlate with greater cognitive flexibility and fewer task-switching problems.⁹⁹
  • More sensitive to stress
    • High dopamine levels (only) in the PFC even at rest
    • A significant increase in dopamine (only) in the PFC even under mild stress
  • More anxious⁹⁸¹⁰⁰
  • Increased loss aversion¹⁰⁰ (similar to the altered behavior seen in ADHD in response to punishment) and
  • More sensitive to pain.^10110297
  • In addition, they are less susceptible to psychosis and schizophrenia when abusing cannabis.¹⁰³ This is plausible insofar as schizophrenia is associated with elevated dopamine levels in the striatum,¹⁰⁴ and elevated dopamine levels in the PFC reduce dopamine levels in the striatum.¹⁰⁵
  • Social impairments are significantly increased,¹⁰⁰ although this study found no direct effects of the COMT gene on ADHD.
  • Faster reaction times compared to Val/Val¹⁰⁶
  • Müller¹⁰⁷ assigns Met158Met to a phenotype, although the source cited by Müller¹⁰⁸ does not address this issue:
    • Build
      • Slender to gaunt
    • Food intake
      • Can eat large amounts without gaining weight
      • In women, even in cases of unfounded suspicion of anorexia nervosa
    • Capacity
      • Physically
        • Above average
      • Mentally
        • Above average
        • Strong ability to understand and handle complex issues
      • Great stamina
    • Restlessness
      • From agility to restlessness, hecticness, and agitation
      • Inability to recover
        • Yoga, contemplation, and meditation are aversive
        • Should not be expected from a therapeutic standpoint either
      • Relaxation through physical activity
    • Anxiety and panic are more common
    • Increased aggressiveness
    • Sore losers
    • Differential diagnosis
      • Hyperthyroidism can cause similar symptoms
• Compared to COMT Val158Met carriers
  • Less emotional¹⁰⁹
  • Lower extraversion¹⁰⁹
  • Lower novelty seeking¹⁰⁹
  • Less cooperation, less altruism
    • In a study, individuals carrying at least one Val allele—which is associated with high dopamine turnover—exhibited significantly greater cooperativeness and altruism than Met/Met carriers.¹¹⁰

Healthy Val/Val carriers have suboptimally low dopamine levels, whereas Met/Met carriers have nearly optimal dopamine levels at rest.¹¹¹ Val/Val carriers achieve optimal dopamine levels through reduced COMT activity or increased dopamine turnover in the PFC (e.g., acute stress), whereas these changes have the opposite effect in Met/Met carriers.¹¹²

The link between intellectual ability and high sensitivity via the COMT Met158Met polymorphism could be a factor that explains the correlation between giftedness and high sensitivity.¹¹³
COMT-Met158Met is an opportunity-risk gene, along with DRD4-7R and 5HTTPR. In our view, opportunity-risk genes determine performance and vulnerability.
⇒ How ADHD Develops: Genes + Environment

The combination of COMT Val/Val and DAT 10R was associated with increased hyperactivity and ADHD symptoms at age 18 among 11- to 15-year-old boys, but not among girls.¹¹⁴ This can be explained by the fact that COMT Val/Val breaks down dopamine particularly rapidly in the PFC, and DAT 10R is associated with strong dopamine reuptake from the synaptic cleft in the striatum, both of which lead to low dopamine levels, as is typically assumed in ADHD.
This is consistent with the finding that people with ADHD who have the COMT Val/Val genotype respond better to stimulants (which increase dopamine levels in the prefrontal cortex) than people with the COMT Met/Met genotype.¹¹⁵

Surprisingly, another study found improved sustained attention in children with ADHD who carried the Val/Val variant. Children with ADHD and the Val/Met or Met/Met variant showed significantly poorer sustained attention than the norm.¹¹⁶ This would be more conclusive if ADHD were associated with dopaminergic hyperactivity in the PFC, since increased dopamine breakdown would then bring dopamine levels into the moderate range associated with optimal cognitive ability. For both dopamine excess and dopamine deficiency impair performance equally.¹¹⁷ This, however, conflicts with the fact that amphetamine medications, which increase dopamine levels in the PFC, can improve sustained attention in ADHD. 0.25 mg/kg of amphetamine improved physiological efficiency in healthy Val/Val gene carriers (= increased dopamine breakdown) and impaired it in healthy Met/Met gene carriers (slowed dopamine breakdown).¹¹⁸

Individuals with the COMT Val/Val polymorphism, which results in higher COMT synthesis in the PFC—thereby breaking down dopamine more rapidly and leading to lower dopamine levels in the PFC—may have lower tonic and higher phasic dopamine levels in subcortical brain regions.¹¹⁹ However, this hypothesis is not without controversy.¹⁰⁶
A study found that Met carriers had significantly lower connectivity between the right crus I/II and the left dlPFC than Val/Val carriers.¹²⁰

The COMT Met158Met variant results in low dopamine levels in the prefrontal cortex and high dopamine levels in the striatum.
In the PFC, dopamine is broken down by COMT, which inactivates dopamine by adding a methyl group. COMT accounts for approximately 60% of dopamine breakdown in the PFC and only 15% of dopamine breakdown in the striatum.^{12191 92 93} Mice with excess COMT due to the COMT Met158Val gene variant showed reduced dopamine levels in the PFC and increased dopamine levels in the striatum.¹²²

8.2.3.1.4. Regulation of COMT¶

8.2.3.1.4.1. Estrogens reduce COMT activity and thus the breakdown of dopamine by COMT in the PFC¶

Estrogens reduce COMT transcription. Depending on the COMT gene variant, this leads to gender-specific and menstrual-related changes in dopamine levels in the PFC of varying degrees of intensity.⁸⁶
COMT inhibitors (which increase dopamine levels) therefore primarily improve the executive functions of the PFC (in cases of dopamine excess in the PFC), but do not affect the symptoms of hyperactivity or impulsivity originating in the striatum.¹²⁹¹³⁰

Estrogens genetically reduce the activity of COMT, the enzyme that breaks down dopamine.^{131132 133}

Estrogen levels in women are low after menstruation (days 1–9), then rise steadily until they reach their peak at ovulation (days 10–15), drop to one-third of the peak level at ovulation (days 16–17), rise to two-thirds of the peak level by day 24, and then decline until menstrual bleeding begins (day 27).¹³⁴

Shortly before ovulation, as well as (to a slightly lesser extent) approximately 1 week after ovulation, dopamine breakdown in the PFC is therefore significantly reduced (dopamine levels are elevated, potentially leading to a reduced need for ADHD medication). Before menstruation, dopamine breakdown is noticeably increased (dopamine levels are reduced, potentially leading to an increased need for ADHD medication).
As a result, COMT is, on average, 30% less active in women than in men.¹³⁵¹³⁶
Since COMT accounts for at least 60% of dopamine breakdown in the PFC (and a maximum of 15% of dopamine breakdown in the striatum⁹³ ), women in the estrogen-rich phase just before ovulation experience a nearly 20% reduction in dopamine breakdown in the PFC.

The consequences are that some women may require a lower dose of PFC-mediated ADHD medications (such as stimulants or atomoxetine) for PFC-related ADHD symptoms, such as inattention, during periods of high estrogen levels (3 –4 days before ovulation and approximately one week after ovulation) than during periods of low dopaminergic medication levels (days before menstruation).¹³⁷ This depends, among other factors, on the COMT gene variant in individual cases.
This could further explain why women are more sensitive than men, as a slight increase in dopamine levels heightens the intensity of perception.
However, the intensification of PMS symptoms toward the end of the luteal phase (before and during the first few days of menstruation) may be a consequence of the drop in estrogen levels rather than a consequence of their lowest levels. Since estrogen increases serotonin, a drop in estrogen causes a drop in serotonin.¹³⁸ Although estrogen levels drop in the days leading up to menstruation, the lowest estrogen levels are found during menstruation (days 1–4 of the cycle) and in the first days of the proliferative phase (days 5–14 of the cycle). There are no reports of a cycle-related increase in the need for ADHD medication during these periods.
Estrogens also influence dopamine. For more on this, see Estrogens promote dopamine in the striatum and PFC In the article What Regulates Dopamine

Since the COMT Met-158-Met variant—which causes dopamine breakdown in the PFC to be five times slower—is also common in borderline personality disorder, and estrogen further slows COMT-mediated dopamine breakdown, this association could potentially provide a clue to explaining the higher prevalence of borderline personality disorder in women (75% of people with ADHD who have borderline personality disorder are women).¹³⁹

The reduction in dopamine breakdown in the PFC caused by estrogen via COMT means that (mild) stress can have different effects depending on gender.
In males and male animals, the slight increase in dopamine levels in the PFC caused by mild stress enhances cognitive performance compared to the resting state. In females and female animals, however, the slight increase in dopamine in the PFC caused by mild stress (on average) leads to impaired mental performance. This difference appears to be caused by estrogen. The decline in cognitive performance due to mild stress occurs only during the estrogen-rich phase shortly before ovulation. During the estrogen-poor phase before or during menstruation, mild stress enhances cognitive performance in women just as it does in men.^{137140141142143144145}

8.2.3.1.4.2. Hypoxia, vascular occlusion, and traumatic brain injury increase COMT¶

Hypoxia, vascular occlusion, and traumatic brain injury increase COMT expression in hippocampal microglia. This appears to be a compensatory mechanism designed to halt excessive catecholamine signaling in injured brain regions.¹⁴⁶

8.2.3.1.4.3. COMT inhibitors¶

Just as COMT promotes dopamine breakdown in the PFC and estrogen can inhibit dopamine breakdown by reducing COMT activity, other COMT inhibitors should also inhibit dopamine breakdown in the PFC.
Taking these medications could therefore be helpful for ADHD—provided that a dopamine deficiency in the PFC is indeed caused by COMT overactivity.

COMT inhibitors include:

• Serotonin hydrochloride¹⁴⁷
• Tolcapone (Ro 40-7592)¹⁴⁷
• Opicapone¹⁴⁷
• Flopropione (also a 5-HT1A receptor antagonist)¹⁴⁷
• Entacapone sodium salt¹⁴⁷⁵⁶

Salsolinol inhibits COMT.¹⁴⁸ Salsolinol is a tetrahydroisoquinoline.

8.2.3.1.5. COMT genetic test¶

A genetic test can determine which COMT gene variant is present. This can provide clues as to whether symptoms of impaired working memory result from an excess of dopamine or a deficiency of dopamine in the prefrontal cortex.
However, since other genes also influence dopamine levels in the PFC, a test result is only one of several factors. Therefore, it may be advisable to combine this test with testing for other candidate genes that also influence dopamine levels in the PFC, such as the DAT gene or the 5 dopamine receptors.
This information could provide an indication of whether a lack of response to stimulants might result from elevated dopamine or norepinephrine levels in the PFC.
A COMT genetic test is available for around €60 (as of 2019). We do not know whether testing multiple genes at once is more cost-effective.

8.2.4. Dopamine degradation via enzymatic oxidation¶

This presentation is based on Meiser et al.¹

The oxidative deamination of catecholamines by MAO produces hydrogen peroxide. Hydrogen peroxide causes oxidative stress in catecholaminergic cells or cells that break down catecholamines.
All catecholamines—including dopamine—are susceptible to oxidation at their electron-rich catechol moiety. Enzymatic oxidation can be catalyzed by various enzymes:

• MAO
• Cyclooxygenases (COX, prostaglandin H synthase)
• tyrosinase and
• other enzymes

With oxygen acting as an electron acceptor, these reactions produce superoxide anions (OO-2).
The enzymatic oxidation of dopamine or L-DOPA, whether spontaneous or catalyzed by metals (Fe³⁺), produces highly reactive electron-deficient orthoquinones (DOPA quinone and dopamine quinone). DOPA quinone and dopamine quinone readily react with nucleophiles. Quinones, as well as ROS, can react nonspecifically with many cellular components and alter their functionality, which is potentially neurodegenerative.

8.2.4.1. Dopamine degradation by monoamine oxidase (MAO-A, MAO-B)¶

Dopamine is broken down by both MAO isoenzymes, MAO-A and MAO-B. In humans, MAO-A-mediated degradation predominates in vivo (within the overall MAO-mediated degradation process), since dopamine enters astrocytes barely, where it could be degraded by MAO-B.
MAO breaks down dopamine in presynaptic nerve terminals, while COMT is active in postsynaptic nerve terminals.¹⁴⁹

8.2.4.1.1. MAO-A¶

MAO-A is found primarily in nigrostriatal dopaminergic axon terminals.¹⁵⁰
MAO-A breaks down various monoamines:¹⁵⁰

• Dopamine⁵⁶
  • Rats:
    • In the cortex, 60% of dopamine breakdown is caused by MAO-A and 40% by MAO-B¹⁵¹¹⁵²
    • in the striatum:¹⁵³
      • MAO-A, but not MAO-B, is the primary contributor to striatal dopamine degradation in rats
      • MAO-B, but not MAO-A, is responsible for astrocyte-mediated GABAergic tonic inhibitory currents in the rat striatum
      • MAO-B mediates GABA synthesis; upregulation of GABA synthesis can cause motor symptoms of Parkinson’s disease, so MAO-B inhibitors could potentially alleviate these symptoms
  • People:
    • in vitro in the cortex, 30% by MAO-A and 70% by MAO-B¹⁵⁴
    • However: There is barely any DAT in astrocytes, so very little dopamine enters them in vivo, where it could be broken down by MAO-B¹⁵⁰
      • However, astrocytes contain NETs that reuptake roughly as much dopamine as norepinephrine
    • People with different MAO-A gene variants exhibit different symptoms related to dopamine, whereas those with different MAO-B gene variants show barely any such symptoms
• Adrenaline
• Norepinephrine
• Melatonin
• Serotonin

8.2.4.1.2. MAO-B¶

MAO-B is found in astrocytes and serotonergic neurons, while MAO-¹⁵⁰ ly in histaminergic neurons, as well as outside the nervous system in platelets and lymphocytes.¹⁵⁵
MAO-B regulates the breakdown of¹⁵⁰

• Phenylethylamine
• benzylamine
• Dopamine
  • in rats:
    • in the cortex, 40% by MAO-B and 60% by MAO-A¹⁵¹¹⁵²
    • in the striatum: no MAO-B¹⁵³
      • MAO-A, but not MAO-B, is the primary contributor to striatal dopamine degradation in rats
      • MAO-B, but not MAO-A, is responsible for astrocyte-mediated GABAergic tonic inhibitory currents in the rat striatum
      • MAO-B mediates GABA synthesis; upregulation of GABA synthesis can cause motor symptoms of Parkinson’s disease, so MAO-B inhibitors could potentially alleviate these symptoms
  • in humans:
    • in vitro in the cortex, 70% by MAO-B and 30% by MAO-A¹⁵⁴
      • However, there is barely any DAT in astrocytes, so very little dopamine enters them in vivo, where it could be broken down by MAO-B¹⁵⁰
      • On the other hand: Astrocytes (like microglia) can synthesize and metabolize dopamine themselves.¹⁵⁶ Astrocytes and microglia are in direct contact with dopaminergic neurons. It is possible that glial cells play a role in maintaining dopamine levels in the brain in both healthy and pathological states. However, astrocytes are not capable of converting L-DOPA they have taken up into dopamine.
  • In people with Parkinson’s disease, MAO-B is upregulated in the brain
    • resulting in excessive DA breakdown, which contributes to the development of Parkinson’s disease
    • Irreversible MAO-B inhibitors (selegiline, rasagiline) are effective (to a limited extent) in treating Parkinson’s disease
    • MAO-B inhibition also prevents the formation and spread of α-synuclein aggregates¹⁵⁷
  • MAO-B levels increase with age¹⁵⁸

8.2.4.1.3. MAO inhibition¶

Inhibition of MAO indirectly leads to an inhibition of the breakdown of dopamine, norepinephrine, serotonin, and epinephrine, thereby increasing the levels of these neurotransmitters.

MAO inhibitors include:¹⁵⁹

• Rasagiline
• Selegiline
• Safinamide
• Tranylcypromine
• Phenelzine
• Moclobemide
• Isocarboxazid
• Salsolinol¹⁴⁸ Salsolinol is an alkaloid and a tetrahydroisoquinoline.
  Chocolate (cocoa) contains significant amounts of the alkaloids salsonisol (up to 2.5 mg) (1-methyl-6,7-dihydroxy-tetrahydroisoquinoline) and salsolin (1-methyl-6-methoxy-7-hydroxy-tetrahydroisoquinoline).¹⁶⁰
• Lazabemide (development discontinued)
• Harmaline (indole alkaloid; not used as a medicinal substance)

8.2.5. Breakdown of dopamine by dopamine β-hydroxylase (DBH) into norepinephrine¶

Dopamine β-hydroxylase is a copper-dependent monooxygenase, an enzyme that converts dopamine into norepinephrine.
DBH polymorphisms are associated with:¹⁶¹

• ADHD
• Parkinson’s disease
• Alzheimer’s
• Schizophrenia

Dopamine and norepinephrine levels are altered in disorders of copper metabolism. Infants with Menkes disease, which is caused by a defect in the ATP7A gene, exhibit a marked copper deficiency in the brain. In these patients, the impaired copper supply to the CNS is associated with higher dopamine and lower norepinephrine levels in the brain and plasma, which is used in the clinical diagnosis of the disease. It is believed that the alteration in catecholamine levels is caused by the loss of copper incorporation into dopamine β-hydroxylase and, consequently, by its reduced activity. Restoring ATP7A expression in a mouse model of Menkes disease corrects the dopamine-norepinephrine ratio, particularly when combined with additional copper injections.¹⁶²
However, ATP7A has not yet been identified as a candidate gene for ADHD.

Salsolinol inhibits dopamine β-hydroxylase.¹⁴⁸
Bleomycin is a potent inhibitor of dopamine β-hydroxylase.¹⁶³

8.2.6. Dopamine degradation via sulfation (mediated by sulfotransferases)¶

8.2.6.1. Sulfation converts dopamine into dopamine sulfate¶

The sulfation of dopamine results in the conversion of active dopamine into inactive dopamine sulfate.
The sulfation of dopamine and other catecholamines and bioamines serves to modulate and transport them.
It is much more pronounced and important in humans than in rodents.¹⁶⁴ Sulfoconjugation is the primary form of dopamine inactivation in human serum, whereas glucuronidation predominates in rats. Consequently, no ortholog of SULT1A3 is known in rodents.¹⁶⁵
Plasma dopamine sulfate in humans is derived primarily from the sulfoconjugation of dopamine, which is synthesized from L-DOPA in the gastrointestinal tract. Both dietary and endogenous factors influence plasma dopamine sulfate levels. There appears to be an enzymatic gut-blood barrier that detoxifies exogenous dopamine and limits the autocrine/paracrine effects of endogenous dopamine, which is produced in a “third catecholamine system.”¹⁶⁶

The serum level of dopamine sulfate (approximately 5 ng/ml) is 10 to 15 times higher than the levels of free dopamine (0.3 ng/ml), norepinephrine (0.2 ng/ml), or epinephrine (0.05 ng/ml). Dopamine sulfate cannot be detected using routine analytical methods and requires special extraction techniques.¹⁶⁷ After the first meal following fasting, serum dopamine sulfate levels increase 50-fold. Food intake appears to stimulate peripheral dopamine synthesis and/or dopamine sulfoconjugation in the gastrointestinal tract.¹⁶⁷

8.2.6.2. Sulfotransferases¶

The sulfation reaction is catalyzed by sulfotransferases (SULT). Sulfation plays an important role in the homeostasis and regulation of catecholamines, steroids, and iodinated thyroxines, as well as in the detoxification of xenobiotics. There are two SULT enzyme superfamilies:¹⁶⁷

• SULT1 (phenol sulfotransferases, or PST)
  • consists of 6 homodimeric enzymes
  • SULT1A3
    • has high specificity for both catecholamines and catecholestrogens [29]
    • A single amino acid substitution (Glu 146) confers on the enzyme a higher affinity for DA than for NE or Epi [34].
    • tall mirror in
      • Gastrointestinal tract
    • moderate reflection in
      • Liver
      • Lung
      • Pancreas
      • platelets
• SULT2 (steroid sulfotransferases)

SULTs are found in a wide variety of human tissues:

• Liver
• Intestine
• Brain (PST)¹⁶⁸
  • tall mirrors:
    • temporal cortex
    • frontal cortex
  • low mirrors (approx. 1/10):
    • parietal lobe
    • Occipital lobe
    • Amygdala
    • Hypothalamus
    • Hippocampus
  • Lowest levels:
    • Nucleus accumbens
    • Caudate nucleus
    • substantia nigra

8.2.6.3. Sulfotransferases and dopamine¶

PST appears to be an important regulator of dopamine storage and metabolism.
L-Dopa-modified PST in Parkinson’s patients:¹⁶⁷

• significantly reduced in
  • Hypothalamus
  • frontal cortex
  • temporal cortex
  • Amygdala
  • occipital cortex
  • parietal cortex
• slightly decreased in
  • Hippocampus
  • Nucleus accumbens
  • Putamen
  • substantia nigra
• unchanged in
  • Meynert nucleus (basal nucleus)
• doubled in
  • Caudate nucleus

8.2.6.4. Sulfatases convert dopamine sulfate back into dopamine¶

Sulfoconjugation is reversible by sulfatases (unlike glucuronidation, which is irreversible).¹⁶⁹
Sulfatases convert biologically inactive dopamine sulfate into unconjugated, active dopamine.
To date, 17 sulfatases have been identified in humans. These are primarily located in the lysosomes.

• Arylsulfatases
  • Arylsulfatase A (ARSA)
  • Arylsulfatase B (ARSB)
  • Arylsulfatase C (Estrone/Dehydroepiandrosterone Sulfatase; Steroid Sulfatase, STS)
  • Arylsulfatases D through K
• Iduronate-2-sulfatase

8.2.6.5. Sulfation and ADHD¶

Arylsulfatase C (STS) is associated with inattention, cognitive difficulties, and other ADHD symptoms.^170171172173

Arylsulfatase C (STS, steroid sulfatase) cleaves sulfate groups from steroid hormones, thereby altering their biological activity. This also applies to dehydroepiandrosterone sulfate (DHEAS).
Sulfated and unsulfated steroids can affect GABA-A and NMDA receptors in the brain.¹⁷⁴ Both DHEAS and its unsulfated form, DHEA, inhibit the GABA-A receptor and activate the NMDA receptor.¹⁷⁵
Learn more about DHEA at DHEA in the chapter on Neurological Aspects / Hormones in ADHD.

8.2.7. Dopamine degradation via glucuronidation (mediated by glucuronosyltransferases)¶

UDP-glucuronosyltransferases (UGTs) are involved in the detoxification of xenobiotics. They thus help protect the body from harmful chemicals. Detoxification takes place in the liver. Humans have 19 UGT isoforms, which are divided into three subfamilies: UGT1A, UGT2A, and UGT2B.
In the brain, UGTs are mainly expressed in endothelial cells and astrocytes of the blood-brain barrier, but are also found in brain regions without a blood-brain barrier, such as the periventricular region, the pineal gland, the pituitary gland, and the neuro-olfactory tissue.¹⁷⁶ In addition to their key role as a detoxification barrier, UGTs are also involved in maintaining the balance of endogenous compounds such as steroids or DA. Only the UGT isoform UGT1A10 is capable of catalyzing the glucuronidation of dopamine to a significant extent in humans. This process yields dopamine-4-O-glucuronide and dopamine-3-O-glucuronide. However, UGT1A10 is not found in the human brain; only the isoforms 1A, 2A, 2B, and 3A are present. Therefore, based on current knowledge, UGTs are not involved in dopamine metabolism in the human brain.¹⁷⁷

8.3. Dopamine removal by diffusion¶

In the DAT-KO mouse, inhibition of serotonin transporters, norepinephrine transporters, MAO-A, or COMT did not alter dopamine degradation in the striatum. In the absence of DAT in the striatum, this degradation appears to occur primarily via diffusion.⁴²

8.3.1. Dopamine uptake by astrocytes¶

Astrocytes are glial cells that actively help regulate dopaminergic tone. They are also involved in the clearance of dopamine and other monoamines from the synaptic cleft.¹⁷⁸
Astrocytic processes physically surround the synaptic cleft and directly take up dopamine and norepinephrine at its edge via: (see above for references)

• NET (norepinephrine transporters take up roughly as much dopamine as norepinephrine)
• Oct. 1
• Oct. 2
• OCT3
• MAO-B

Because dopamine and norepinephrine are reabsorbed at the edges of the synaptic cleft, there is relatively less dopamine and norepinephrine there than in the center of the synaptic cleft. This concentration difference leads to an increased movement of these neurotransmitters from the center of the synaptic cleft toward the edges. As a result, dopamine and norepinephrine diffuse even faster from the center outward than they would without the astrocytes. In this way, astrocytes actively reduce the dopamine concentration in the synaptic cleft. Furthermore, they thereby limit extracellular dopamine levels and volume transmission.¹⁷⁸

8.4. Conclusions Regarding Medication for ADHD¶

8.4.1. Binding affinity of MPH, AMP, and ATX to DAT, NET, and SERT¶

The active ingredients methylphenidate (MPH), d-amphetamine (d-AMP), l-amphetamine (l-AMP), and atomoxetine (ATX) bind with varying affinities to dopamine transporters (DAT), norepinephrine transporters (NET), and serotonin transporters (SERT). This binding inhibits the activity of the respective transporters.¹¹
The norepinephrine transporter—along with COMT—is responsible for most of the dopamine breakdown in the PFC, while the DAT plays a key role in regulating it in the striatum.

Binding affinity: higher for smaller numbers (KD = Ki)	DAT	NET	SERT
MPH	34 - 200	339	> 10,000
d-AMP (Vyvanse, Attentin)	34–41	23.3–38.9	3,830–11,000
l-AMP	138	30.1	57,000
ATX	1451–1600	2.6–5	48–77

8.4.2. Effects of MPH, AMP, and ATX on dopamine and norepinephrine by brain region¶

The active ingredients methylphenidate (MPH), amphetamine (AMP), and atomoxetine (ATX) affect extracellular dopamine (DA) and norepinephrine (NE) to varying degrees in different regions of the brain. Table modified from Madras,¹¹.

	PFC (	) Striatum (	) Nucleus accumbens
MPH	DA + NE (+)	DA + NE +/- 0	DA + NE +/- 0
AMP	DA + NE +	DA + NE +/- 0	DA + NE +/- 0
ATX	DA + NE +	DA +/- 0 NE +/- 0	DA +/- 0 NE +/- 0

Note: NET binds DA more strongly than NE (only in the PFC), and DAT binds DA much more strongly than NE.