8. Dopamine reuptake, dopamine degradation

Extracellular dopamine in the synaptic cleft can be degraded by reuptake into dopaminergic neurons or, after uptake into glial cells, by MAO or COMT. Within dopaminergic neurons, dopamine located outside of vesicles is degraded by MAO.¹ Furthermore, dopamine can be deactivated by sulfation or glucuronidation and converted into noradrenaline by metabolism. Finally, dopamine can diffuse and is then transported away with the blood.

• 8.1. Dopamine reuptake (recycling)
  • 8.1.1. Dopamine reuptake through dopamine transporters (DAT)
  • 8.1.2. Further dopaminergic influences of the DAT
    • 8.1.2.1. Dopamine release (efflux) through DAT
    • 8.1.2.2. Channel function of the DAT
  • 8.1.3. Drug binding centers of the DAT
  • 8.1.4. Dopamine reuptake by noradrenaline transporters (NET)
  • 8.1.5. Substances that inhibit the reuptake of dopamine (reuptake inhibitors)
    • 8.1.5.1. Dopamine reuptake inhibitors
    • 8.1.5.2. Noradrenaline reuptake inhibitors
• 8.2. Dopamine degradation
  • 8.2.1. Dopamine degradation through uptake-2 transporters
    • 8.2.1.1. Dopamine degradation by plasma membrane monoamine transporter (PMAT)
    • 8.2.1.2. Dopamine degradation by organic cation transporters (OCT)
      • 8.2.1.2.1. OCT1
      • 8.2.1.2.2. OCT2
      • 8.2.1.2.3. OCT3
      • 8.2.1.2.4. OCT and ADHD
  • 8.2.2. Dopamine degradation through autooxidation
  • 8.2.3. Dopamine degradation through metabolization (by means of enzymes)
    • 8.2.3.1. Dopamine degradation through COMT
      • 8.2.3.1.1. Dopamine degradation in PFC by NET and COMT, barely by DAT; in striatum by DAT, barely by NET and COMT
      • 8.2.3.1.2. COMT isoforms: soluble and membrane-bound
      • 8.2.3.1.3. COMT gene variants alter dopamine levels in the PFC
      • 8.2.3.1.4. Regulation of COMT
      • 8.2.3.1.4.1. Oestrogen reduces COMT and thus dopamine degradation by COMT in the PFC
      • 8.2.3.1.4.2. Hypoxia, vascular occlusion and traumatic brain injury increase COMT
      • 8.2.3.1.4.3. COMT inhibitors
      • 8.2.3.1.5. COMT genetic test
  • 8.2.4. Dopamine degradation through enzymatic oxidation
    • 8.2.4.1. Dopamine degradation by monoamine oxidase (MAO-A, MAO-B)
      • 8.2.4.1.1. MAO-A
      • 8.2.4.1.2. MAO-B
      • 8.2.4.1.3. Inhibition of the MAO
  • 8.2.5. Dopamine degradation by dopamine β-hydroxylase (DBH) to noradrenaline
  • 8.2.6. Dopamine degradation through sulfation (using sulfotransferases)
    • 8.2.6.1. Sulfation deactivates dopamine to dopamine sulfate
    • 8.2.6.2. Sulfotransferases
    • 8.2.6.3. Sulfotransferases and dopamine
    • 8.2.6.4. Sulfatases reactivate dopamine sulfate to dopamine
    • 8.2.6.5. Sulphation and ADHD
  • 8.2.7. Dopamine degradation through glucuronidation (by means of glucuronosyltransferases)
• 8.3. Dopamine removal by diffusion
• 8.4. Conclusions for the medication of ADHD
  • 8.4.1. Binding affinity of MPH, AMP, ATX to DAT / NET / SERT
  • 8.4.2. Effect of MPH, AMP, ATX on dopamine / noradrenaline per brain region

8.1. Dopamine reuptake (recycling)¶

Transporters are divided 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 cause reuptake of the neurotransmitter for reuse for release or efflux. Uptake 2 transporters are located on glial cells, which degrade the neurotransmitter. All transporters are located outside the synaptic cleft²
In SERT-KO mice, OCT3 (and possibly other uptake-2 transporters) are upregulated to compensate for serotonin degradation.² Conversely, they could impair the effect of SSRIs.

8.1.1. Dopamine reuptake through dopamine transporters (DAT)¶

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

DAT (re)absorb dopamine from the extracellular space and transport it into the cell. This can be dopamine that has been released by the cell itself (reuptake) or dopamine that comes from neighboring cells.
DAT are located near the synapses. With the reuptake, the DAT regulate the temporal availability of primarily phasic and less tonic³ freshly released dopamine (and to a lesser extent noradrenaline) in the synaptic cleft or in the presynaptic nerve cell.⁴ This ensures fine-tuning of the phasic nature of the dopamine signal,⁵ because only when the synaptic cleft is quickly cleared of previously released dopamine is newly released (signal-encoding) dopamine able to transmit these signals cleanly, unimpaired by dopamine present from previous releases. By removing dopamine from the synaptic cleft, the DAT modulates the signal-to-noise ratio of dopamine signaling.⁶
For the distinction between tonic and phasic dopamine, see ⇒ Dopamine release (tonic, phasic) and coding.
The tonic extrasynaptic dopamine level is less affected by the reuptake³

DAT cause most of the dopamine degradation in the striatum. In contrast, fewer DAT are present in the PFC, where dopamine degradation is mainly carried out by COMT (60 %) and NET and only slightly via DAT (15 %).⁷ In the PFC, noradrenaline transporters (NET) significantly reabsorb dopamine.
There are 5 times as many DAT in the nucleus accumbens as in the basolateral amygdala.⁸

The binding affinity of DAT is for:⁹

• Dopamine 885 / 2140 / 5,200 Km
• Noradrenaline 17,000 Km
  Lower values mean a higher affinity for commitment.

DAT are subject to epigenetic changes in expression within the first few months of life due to environmental influences. In addition to dopamine, the dopamine transporter (DAT) also transports noradrenaline.¹⁰ 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 possible mechanism for dopaminergic neurodegeneration in Parkinson’s disease.¹¹

The DAT is located on chromosome 5p15.3 and occurs in different variants that differ in the number of 40 bp repeats (allele repeats), which range from 3 to 11 repeats (R). The most common are the 10R variant with 480 bp with 70 % and the 9R variant with 440 bp with 27 % in the Caucasian and Hispanic population and with 72 % and 17 % in the African population, which showed significantly more rare allele repeat variants with 12 %.¹²¹³
While DAT 10R causes increased dopamine degradation from the synaptic cleft, resulting in decreased tonic (but unaffected or even increased phasic dopamine), DAT 9R causes decreased dopamine degradation, resulting in increased tonic and decreased phasic dopamine. DAT 10R is associated with ADHD, DAT 9R with borderline.

DAT are expressed within a dopaminergic neuron:¹⁴

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

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

• A deletion of the first 22 amino acids of DAT¹⁵
  • Strongly reduces AMP-induced dopamine efflux
  • (Re)admission remains unchanged
  • Eliminates 32P incorporation in DAT in response to PKC activation¹⁶
• Mutation of the five N-terminal serines to alanine¹⁵
  • Causes a strong reduction in AMP-induced dopamine efflux
  • (Re)admission remains unchanged
• Mutation of the five N-terminal serines to aspartate (mimicry of phosphorylation)¹⁵
  • Efflux remains intact
  • Phosphorylation of one or more of these five N-terminal serines is probably required for AMP-induced dopamine efflux

8.1.2. Further dopaminergic influences of the DAT¶

8.1.2.1. Dopamine release (efflux) through DAT¶

DAT can also release dopamine - at least in the substantia nigra. While the D2 dopamine autoreceptor downregulates the release of dopamine when extracellular dopamine is high, the DAT promotes the release of dopamine when dopamine is low. Unlike methylphenidate, which inhibits dopamine reuptake by the DAT, amphetamine is a substrate for the DAT that may trigger dopamine release in the substantia nigra.¹⁹⁹ According to another account, MPH also increases DAT efflux (see under MPH).
Only the increase in population activity by inhibiting GABAergic afferents from the pallidum (but not the activation of pedunculopontine inputs, which increases burst firing) increases dopamine efflux in the ventral striatum. However, after blockade of dopamine reuptake, increased burst firing increased dopamine efflux three times more than increased population activity³

AMP and the drug METH also stimulate efflux through DAT,²⁰ while MPH does not.

8.1.2.2. Channel function of the DAT¶

DAT not only transport dopamine, but also appear to have a channel mode that directly modulates membrane potential and neuronal function.
The depolarizing currents caused by DAT thus result not only from the high density and fast neurotransmitter turnover rate of a classical transporter, but also from a genuine channel behavior of DAT.²¹

8.1.3. Drug binding centers of the DAT¶

The structures of the DAT to which drugs bind are divided into outwardly and inwardly directed states. These states show different binding modes and conformational transitions during substrate transport. While the outward-facing state is stabilized by cocaine, GBR12909 and benztropine stabilize the dopamine transporter in the inward-facing state.²²

8.1.4. Dopamine reuptake by noradrenaline transporters (NET)¶

The noradrenaline transporter (NET) is common in the PFC and rare in the striatum, while the DAT is rare in the PFC and common in the striatum. The NET is slightly more affine to dopamine than to noradrenaline,²³²⁴ so that a relevant part of dopamine degradation / dopamine reuptake in the PFC (but not in the striatum) is carried out by the NET.
The noradrenaline transporter appears to be reduced in the attention networks of the right hemisphere of the brain in ADHD.²⁵

The binding affinity of NET is for:⁹

• Dopamine 240 / 730 Km
• Noradrenaline 539 / 580 Km
  Lower values mean a higher affinity for commitment.

NET-KO mice (mice without noradrenaline transporter) show no effective dopamine degradation in the PFC²⁶

In the DAT-KO mouse, NET in the striatum was shown to possibly barely contribute to dopamine degradation in the striatum. Inhibition of serotonin transporters, noradrenaline 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 more by diffusion.²⁷ However, we wonder whether the process by which the DAT is deactivated in the DAT-KO mouse could also deactivate the NET, since the NET also takes up dopamine.

8.1.5. Substances that inhibit the reuptake of dopamine (reuptake inhibitors)¶

8.1.5.1. Dopamine reuptake inhibitors¶

Dopamine reuptake inhibitors are substances that inhibit the dopamine transporter (DAT). The term therefore refers to the inhibited transporter and not the inhibition of neurotransmitter reuptake.
Dopamine reuptake inhibitors are (in bold: typical ADHD medications)

• Amineptin
• Amphetamines
• Bupropion
• Bromantane
• CE-158²⁸
• Dasotraline
• Difemetorex
• Difluoropin
• Fencamfamine
• Lefetamine
• Levophacetoperane
• Medifoxamine
• Mesocarb
• Methyl naphthidate (HDMP-28)²⁹
• Methylphenidate
• Nomifensin (trade names: Alival, Merital, Psyton)
• Pipradrol
• Prolintan
• Pyrovalerone
• Reserpine
• Solriamfetol
• Vanoxerin (in development)

8.1.5.2. Noradrenaline reuptake inhibitors¶

Noradrenaline reuptake inhibitors are substances that inhibit the noradrenaline transporter (NET). The term is therefore based on the inhibited transporter and not on the inhibition of neurotransmitter reuptake.
Selective noradrenaline reuptake inhibitors (SNRIs) include

For ADHD treatment:

• Atomoxetine (Strattera®, Atomoxe®, Agakalin®, Atomoxetine generics)
• Viloxazine
  For the treatment of depression:
• Reboxetine (Solvex®, Edronax®)
• Viloxazine (Vivalan®; approval was no longer extended by the manufacturer, preparation therefore no longer available)

For obesity treatment:

• Mazindol

As a skeletal muscle relaxant:

• Orphenadrine (Norflex®)

Research:

• Nisoxetine (never approved)

8.2. Dopamine degradation¶

8.2.1. Dopamine degradation through uptake-2 transporters¶

8.2.1.1. Dopamine degradation by plasma membrane monoamine transporter (PMAT)¶

PMAT and OCT are so-called uptake-2 transporters, as they have a low affinity (Kd = 252 compared to 0.27 μM) with a transport capacity that is around 80 times higher than uptake-1 transporters such as DAT, NET or SERT (Vmax = 100 nmol/min/g tissue compared to 1.22 nmol/min/g tissue). They work independently of sodium and bidirectionally.³⁰ However, uptake-2 transporters are not only active at very high substrate concentrations, but at any substrate concentration.³¹
Since uptake 2 transporters are barely located on (dopamine-releasing) dopamine neurons, but primarily on (dopamine-degrading) glial cells, PMAT and OCT 1 to 3 are not used for reuptake, but for dopamine degradation.

In addition to the DAT and NET, dopamine and noradrenaline are taken up by the plasma membrane monoamine transporter (PMAT). This is also known as human equilibrative nucleoside transporter-4 (hENT4). It is encoded by the gene SLC29A4. Its binding affinity is lower than that of DAT or NET. It binds dopamine and serotonin and, to a much lesser extent, noradrenaline, adrenaline and histamine.³²

Discovered in 2004,³³ PMAT is widespread in the human brain.³⁴ PMAT is found

• widely distributed in the brain²
• Amygdala²
• Cerebral cortex²
• Hippocampus²
• Striatum²
• on the CSF side of the endothelial cells of the blood-CFS barrier in the choroid plexus³⁵
  • Choroid plexuses are tangle-like arteriovenous vascular bundles made up of specialized glial cells. They are found in the ventricles of the brain and are responsible for the production of cerebrospinal fluid, the formation of the blood-cerebrospinal fluid barrier and the resorption and detoxification of cerebrospinal fluid.³⁶

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

PMAT gene polymorphisms with reduced transport activity for the monoamines serotonin and dopamine as well as the neurotoxin 1-methyl-4-phenylpyridinium (MPP(+)) correlate with Autism Spectrum Disorders (ASD).³⁷
PMAT-KO mice (which therefore have a PMAT deficiency) show neither a strong change in brain histamine levels nor behavioral abnormalities outside of stressful situations.³⁵³⁴

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

8.2.1.2. Dopamine degradation by organic cation transporters (OCT)¶

Like PMAT, OCT are so-called uptake-2 transporters. See above.

Dopamine (although weaker than noradrenaline) is transported from the extracellular area not only by the uptake transporters DAT and NET, but also, albeit to a lesser extent, by the organic cation transporters (in rats: OCT1, OCT2, OCT3; in humans: only OCT2³⁸ Uptake does not take place in the presynaptic cell as with DAT and NET, but in glial cells. There, dopamine and noradrenaline are degraded by COMT to methoxytyramine.³⁹

The coding genes are:⁴⁰

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

Strong OCT antagonists are e.g.³⁹

• Amantadine
• Memantine

8.2.1.2.1. OCT1¶

OCT1:²

• relatively low expression in the brain⁴¹
• is found in astrocytes in the brain, but not in neurons.
• OCT1-KO mice are viable, fertile and show normal physiological characteristics⁴²

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
  • Adrenalin
  • Acetylcholine
• Is involved in
  • hepatic absorption of vitamin B1/thiamine
  • Intake of xenobiotics

8.2.1.2.2. OCT2¶

Like OCT3, OCT2 reuptakes histamine, but has not been found in human astrocytes,⁴³ but in the human brain, as do OCT1 (SLC22A1), OCTN1 (SLC22AN1) and OCTN2 (SLC22AN2).⁴⁴

OCT2:²

• is widespread 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-KO mice
  • are viable, fertile and show normal physiological characteristics⁴²
  • reduced anxiety behavior⁴⁵
  • reduced depression symptoms (reduced immobile time in the forced swim test and in the tail suspension test)⁴⁵
  • 1.9-fold stress hormone level (corticosterone)⁴⁶
  • increased susceptibility to depression due to recurring unpredictable stress⁴⁶

OCT2 is transported:⁴⁴

• similar affinity as OCT1
  • MPP
  • TEA
  • Quinine
  • Metformin
• 4 times as affine as OCT1
  • Acetylcholine
• and further
  • Choline
  • Agmatine
• Neurotransmitters
  • Dopamine
  • Noradrenaline
  • Adrenalin
  • Serotonin
  • Histamine
• Glutamate receptor antagonists
  • Amantadine
  • Memantine
• Histamine H2 receptor antagonists
  • Cimetidine
  • Famotidine
  • Ranitidine
• Cytostatic drug
  • Cisplatin
• Antihypertensives
  • Debrisoquine

OCT2 is involved in⁴⁷

• Memory
• Pain perception
• Stress processing:⁴⁶
  • OCT2 is expressed in various stress-related circuits in the brain and along the HPA axis
  • OCT2-KO mice show
    • increased the hormonal response to acute stress
    • impaired HPA function
    • unchanged sensitivity of the adrenal glands to ACTH
    • much more sensitive reaction to unpredictable chronic mild stress. Changes in:
      • depressive symptoms
      • Self-care
      • spatial memory
      • social interaction
      • stress-sensitive spontaneous behavior
    • Glycogen synthase kinase-3β (GSK3β) signaling pathway altered in the hippocampus
      • GSK3β signaling pathway responds strongly to acute stress in healthy mice
      • in OCT2-KO mice, increased serotonin tone appears to primarily interfere with GSK3β signaling in the brain

8.2.1.2.3. OCT3¶

OCT3 shows a low affinity but high capacity for the neurotransmitters noradrenaline, adrenaline, dopamine, serotonin and histamine.⁴⁸⁴⁹

OCT3 is the most common OCT in the brain. OCT3 is found in the rodent brain in particular in ⁵⁰³⁴

• dopaminergic neurons of the substantia nigra compacta
• non-aminergic neurons of
  • VTA
  • Substantia nigra reticulata
  • Locus coeruleus
  • Hippocampus
  • Cerebellum
  • Cortex
    • Nucleus accumbens⁸
    • basolateral amygdala⁸
      • about the same frequency as in nucleus accumbens
• mainly in neurons
• in astrocytes
  • occasionally in astrocytes in substantia nigra reticulata, hippocampus and several hypothalamic nuclei⁵⁰
  • in high numbers in astrocytes adjacent to both the soma and the terminals of dopaminergic midbrain neurons⁵¹⁴³

OCT3 is primarily found as an autoreceptor on histaminergic neurons, i.e. it inhibits histamine synthesis and histamine release.⁴⁷
Under normal conditions, OCT3 does not appear to affect brain histamine levels. In OCT3-KO mice, cortex histamine levels were increased after cerebral ischemia, indicating a contribution of the OCT3 transporter to histamine concentration in the brain.³⁴

Although all “uptake-2” transporters are inhibited by corticosterone, they differ in their sensitivity to corticosterone depending on the species and tissue preparation.⁴⁴⁵²

• OCT3 is more sensitive to corticosteroids than OCT1, OCT2 and PMAT
  • OCR3 shows IC50 values in 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 a strong modulatory influence of stress on the effects of noradrenaline, dopamine, serotonin and histamine via stress-induced glucocorticoid elevation in a rapid, non-genomic manner.⁴⁹
The deactivation of OCT1⁵³ and OCT3⁴⁸ by corticosterone occurs

• fast
• through direct interaction of corticosterone with the transporter at specific sites
• but probably not via glucocorticoid receptors⁵⁴

We are considering whether this mechanism could explain why people with ADHD can overcome their otherwise existing procrastination when under high stress and why their motivation is (only) then sufficient to get things done that are not intrinsically interesting. It would at least be conceivable, but so far it is purely hypothetical.

OCT3 remains unaffected by cocaine or antidepressants (desipramine).⁴⁸

OCT3 inhibitors that are effective in physiological concentrations are:⁵⁵

• 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 neither to OCT2, OCT3 or PMAT. Ketamine, on the other hand, only binds to OCT2⁵⁶ and PMAT.²
d-Amphetamine is a highly effective hOCT2 reuptake inhibitor (Ki: 10.5 mM) and moderately effective hOCT1 reuptake inhibitor (Ki: 202 mM), while it only interacted with hOCT3 from 100 μM (Ki: 460 mM) (hOCT: human OCT) ⁵⁶⁵⁷
d-Amphetamine binds approximately equally strongly to hOCT2 and hOCT3 and to these by an order of magnitude (factor 10) weaker than to DAT⁵⁷

Binding of amphetamine to OCT may contribute to cellular and behavioral effects of amphetamine.⁵⁷

OCT3 were found in brain regions relevant for ADHD:

• in the striatum⁵⁷
• Nucleus accumbens⁸
• Cerebellum³⁴

OCT2 was found at ⁵⁷

• in the hippocampus
  • involved in dopamine-dependent reward-related behavior and responses to amphetamines
• barely in dopaminergic systems

OCT3-30%-KO mice showed ⁵⁸⁵⁹

• unchanged spontaneous movement activity
• increased locomotor activity by methamphetamine compared to METH in wild-type mice
• increased motor activation to imipramine
• this could indicate a contribution of OCT3 to noradrenaline and serotonin reuptake
• Anxiety
  • increased in OCT3-30%-KO mice⁵⁰
  • reduced in OCT3-100%-KO mice⁶⁰
• increased stress values⁵⁰
• increased sensitivity to psychostimulants⁵⁰

With regard to noradrenaline reuptake inhibitors, it has already been hypothesized that drugs that block uptake 2 transporters, such as normetanephrine², in combination with NET reuptake inhibitors could accelerate the onset of therapeutic benefit in depression.⁶¹ Preclinical studies support this hypothesis.² For example, even much lower doses of venlaflaxine or reboxetine have an antidepressant effect in OCT2-KO mice than in wild-type mice.⁴⁵ OCT2 reuptake inhibitors also have an antidepressant effect.⁶²
We think it is worth considering whether this approach could also support the effect of dopamine reuptake inhibitors in ADHD.
These correlations could also explain why MPH, which only binds to OCT1, has less of an antidepressant effect than amphetamine drugs, which inhibit OCT2.

At the same time, in our view, it is striking that ADHD medications almost universally cause an increase in histamine, while OCT2 and OCT3 are also significantly more affine for histamine than for noradrenaline and dopamine. An abundant supply of histamine should primarily bind OCT2 and OCT3, thereby reducing the uptake of dopamine and noradrenaline by OCT2 and OCT3 and resulting in an increase in extracellular noradrenaline and dopamine. According to this mechanism, histamine would serve 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, auto-oxidize.

The oxidation of DA can produce:

• low molecular weight ROS⁶³
  • can trigger oxidative stress in dopamine neurons through reversible oxidative modification of macromolecules such as proteins, lipids and nucleic acids
• 3,4-Dihydroxyphenylacetaldehyde (DOPAL)⁶⁴
• highly reactive DA quinones (DAQ)⁶³
  • can induce the susceptibility of dopamine neurons via 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 DAQ can cause irreversible inhibition of the ubiquitin-proteasome system
    • DOQ can
      • are converted back to dopamine by reducing agents from the environment
      • are further oxidized 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 protein residues cysteine and tyrosine with sulfhydryl and hydroxyl groups
        • AM is polymerized to neuromelanin (an insoluble granular pigment in the substantia nigra)
          • Neuromelanin
            • prevents the neurotoxicity of DAQs
            • has an antioxidant effect
              • binds and inactivates radical species (ROS) under normal conditions
            • also generates ROS under oxidative stress conditions
              • could thus be involved in the α-syn-associated damage of dopamine neurons

Enzymes (such as tyrosinase) or metal ions (such as iron species or Mn3+) can mediate the oxidation of dopamine in solutions.
Reducing agents, especially glutathione, can effectively prevent auto-oxidation.

Highly reactive DAQ appears to play a more important pathological role than low molecular weight ROS in the degeneration of dopamine neurons in Parkinson’s disease. ⁶³

8.2.3. Dopamine degradation through metabolization (by means of enzymes)¶

While dopamine transporters cause the reuptake of dopamine from the synaptic cleft back into the sending cell, where it is stored in vesicles again by VMAT2 transporters, dopamine is also broken down by conversion into other substances. COMT and MAO are the most important of these.

Metabolization is the biochemical conversion / breakdown of a substance by the body’s own enzyme systems into a chemically altered metabolite.⁶⁵

Dopamine degradation occurs with the involvement of the mitochondria and leads to the formation of reactive oxygen species (ROS)⁶⁶
Under physiological conditions, DA oxidation proceeds slowly so that the cellular antioxidant machinery can cope with the amount of highly reactive products from DA oxidation. Higher dopamine levels lead to increased dopamine oxidation and are toxic to the mitochondria of neurons and glial cells.⁶⁷ Mitochondrial defects impair dopamine degradation and can lead to increased dopamine levels in the cytosol.⁶⁸ This interaction between dopamine and mitochondria is involved in the pathogenesis of Parkinson’s disease and schizophrenia.⁶⁹

8.2.3.1. Dopamine degradation through COMT¶

The enzyme COMT mainly metabolizes catecholamines (adrenaline, noradrenaline, dopamine) by O-methylation. S-adenosyl-L-methionine (SAM) acts as a methyl donor.
COMT breaks down dopamine in receiving nerve endings, while MAO is active in receiving nerve endings.Rostain (2015): The Neurobiology of ADHD, Perelman School of Medicine, University of Pennsylvania

8.2.3.1.1. Dopamine degradation in PFC by NET and COMT, barely by DAT; in striatum by DAT, barely by NET and COMT¶

No COMT is found in nigrostriatal neurons.¹
In the striatum, dopamine is therefore barely degraded by COMT. However, there are many DAT in the striatum.^70717273
The PFC has comparatively few dopamine transporters (DAT), unlike the striatum.^70717273

The PFC therefore needs other ways to break down dopamine (which is increased in the PFC under stress). In addition to NETs, it uses the enzyme catechol-O-methyltransferase (COMT), which deactivates dopamine by adding a methyl group and which causes 60 % of dopamine degradation in the PFC (and only 15 % of dopamine degradation in the striatum). The other important dopamine degradation enzyme is monoamine oxidase B (MAO-B).^74757677

However, COMT is predominantly found in glial cells, especially in microglia, and barely or not at all in nerve cells.⁷⁸ Apparently, dopamine from the synaptic cleft is also taken up in glial cells.¹

In addition to dopamine, COMT also metabolizes levodopa and thus inhibits dopamine synthesis. COMT inhibitors such as entacapone, tolcapone and opicapone are used in the treatment of Parkinson’s disease.⁷⁹

COMT is controlled by the COMT gene. COMT gene polymorphisms that influence the activity of the COMT gene therefore primarily affect the dopamine level of the PFC and barely affect the dopamine level 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 mobile cytoplasmic form
  • in glial cells
  • in the periphery
  • metabolizes rather exogenous catecholamines
• Mb-COMT (Membrane-bound)
  • membrane-bound isoform
  • predominant on the neuron membrane
  • higher catecholamine affinity
  • metabolizes mainly dopaminergic and noradrenergic catecholamines

While COMT dopamine degradation in the striatum appears to be mediated by membrane-bound COMT, only soluble COMT may be involved in the PFC. Mb-COMT knockout mice (mice without membrane-bound COMT) show increased dopamine levels in the striatum, but not in the PFC. This suggests that Mb-COMT is involved in dopamine degradation in the striatum, while only soluble COMT may be involved in the PFC.¹⁷

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

The homozygous Val158Val polymorphism of the COMT gene causes 4 times faster dopamine degradation than the homozygous COMT-Met158Met variant, which causes a more inactive COMT and thus slower dopamine degradation.⁸⁰⁸¹ The Val158Met polymorphism lies between the rapidly degrading Val158Val and the slowly degrading Met158Met with regard to catecholamine metabolization.
Healthy COMT-Met158Met carriers are

• Compared to COMT-Val158Val carriers (probably due to the higher dopamine level in the PFC)
  • Mentally more powerful (more efficient, not more intelligent)⁸²
  • More task switching problems, less mental flexibility
    • Carriers of at least one Met allele showed 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 higher cognitive flexibility and lower task switching problems.⁸³
  • More sensitive to stress
    • High dopamine level (only) in the PFC even at rest
    • Significant increase in dopamine (only) in the PFC even under mild stress
  • More anxious⁸²⁸⁴
  • Increased loss aversion⁸⁴ (comparable to the altered behavior in ADHD in relation to punishment) and
  • More sensitive to pain.⁸⁵⁸⁶⁸¹
  • In addition, they have a lower susceptibility to psychosis and schizophrenia with cannabis abuse.⁸⁷ This is plausible insofar as schizophrenia is associated with an increased dopamine level in the striatum,⁸⁸ and increased dopamine levels in the PFC reduce the dopamine level in the striatum.⁸⁹
  • Social impairments are significantly increased,⁸⁴ although this study did not find any direct influences of the COMT gene on ADHD.
  • Faster response times compared to Val/Val⁹⁰
  • Müller⁹¹ assigns Met158Met to a phenotype, although the source cited by Müller⁹² does not comment on this:
    • Physique
      • Slim to lean
    • Food intake
      • Can consume large quantities without weight problems
      • In women up to the unjustified suspicion of anorexia nervosa
    • Performance capacity
      • Physical
        • Above average
      • Spiritual
        • Above average
        • Good ability to understand and deal with complex issues
      • High endurance
    • Restlessness
      • Agility to restlessness, hectic, restlessness
      • Inability to recover
        • Yoga, contemplation, meditation are aversive
        • Should also not be expected therapeutically
      • Relaxation through physical activity
    • Anxiety and panic more common
    • Increased aggressiveness
    • Bad losers
    • Differential diagnosis
      • Hyperthyroidism can cause similar symptoms
• Compared to COMT-Val158Met carriers
  • Less emotionality⁹³
  • Lower extraversion⁹³
  • Lower novelty seeking⁹³
  • Less cooperativeness, less altruism
    • In a study, carriers of at least one Val allele, which stands for a strong dopamine degradation, showed significantly higher cooperativeness and higher altruism than Met/Met carriers.⁹⁴

Healthy Val/Val carriers have suboptimally low dopamine levels, while Met/Met carriers have almost optimal dopamine levels in the baseline state.⁹⁵ Val/Val carriers achieve optimal dopamine levels due to 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.⁹⁶

The link between intellectual performance and high sensitivity via the COMT-Met158Met polymorphism could be an element that could explain the correlation between giftedness and high sensitivity.⁹⁷
COMT-Met158Met is an opportunity-risk gene alongside DRD 4 7R and 5HTTPR. In our opinion, risk-reward genes determine performance and vulnerability.
⇒ How ADHD develops: genes + environment

COMT Val/Val and DAT 10R in combination correlated with increased hyperactivity and increased ADHD symptoms at age 18 in 11- to 15-year-old boys, but not in girls.⁹⁸ This can be explained by the fact that COMT VAL/VAL degrades dopamine in the PFC particularly quickly and DAT 10R stands for strong dopamine reuptake from the synaptic cleft in the striatum, both of which lead to low dopamine effects, as is typically assumed in ADHD.
This correlates with the fact that people with ADHD with COMT Val/Val respond better to stimulants (which increase dopamine levels in the PFC) than people with COMT Met/Met.⁹⁹

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 the Met/Met variant showed significantly poorer sustained attention than the norm.¹⁰⁰ This would be more conclusive if ADHD were associated with dópaminergic hyperfunction in the PFC, as increased dopamine depletion would then bring the dopamine level into the mid-range associated with optimal cognitive ability. This is because dopamine excess and dopamine deficiency are equally impairing.¹⁰¹ However, this clashes with the fact that amphetamine drugs, which increase dopamine levels in the PFC, can improve sustained attention in ADHD. 0.25 mg/kg amphetamine improved physiological efficiency in healthy Val/Val gene carriers (= increased dopamine degradation) and worsened it in healthy Met/Met gene carriers (slowed dopamine degradation).¹⁰²

Carriers of the COMT Val/Val polymorphism, which synthesizes more COMT in the PFC, which degrades dopamine faster, thus leading to lower dopamine levels in the PFC, may have lower tonic and increased phasic dopamine levels in subcortical brain regions.¹⁰³ However, this hypothesis is not uncontroversial.⁹⁰
One study found significantly lower connectivity of the right Crus I/II with the left dlPFC in Met carriers than in Val/Val carriers.¹⁰⁴

COMT-Met158Met causes low dopamine levels in the PFC and high dopamine levels in the striatum.
In the PFC, dopamine is degraded by COMT, which deactivates dopamine by adding a methyl group. COMT causes about 60 % of dopamine degradation in the PFC and only 15 % of dopamine degradation in the striatum.^105757677 Mice with an excess of COMT due to the COMT Met158Val gene variant showed a reduced dopamine level in the PFC and an increased dopamine level in the striatum.¹⁰⁶

8.2.3.1.4. Regulation of COMT¶

8.2.3.1.4.1. Oestrogen reduces COMT and thus dopamine degradation by COMT in the PFC¶

Oestrogen reduces COMT transcription. Depending on the COMT gene variant, this causes varying degrees of sex-related and menstruation-dependent changes in the dopamine level of the PFC.⁷⁰
COMT inhibitors (which increase the dopamine level) therefore primarily improve the executive abilities of the PFC (with excess dopamine in the PFC), but not the symptoms of hyperactivity or impulsivity in the striatum.¹¹³¹¹⁴
The PFC reacts to even small reductions in the availability of the dopamine precursor tyrosine with a significant decrease in dopamine, unlike other areas of the brain, e.g. the striatum, which remains unaffected.¹¹⁵ However, this is only relevant in phenylketonuria (PKU) and not in ADHD.
As oestrogen is a female sex hormone, women should more frequently have an elevated dopamine level in the PFC.

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

Hypoxia, vascular occlusion and traumatic brain injury increase the expression of COMT in hippocampal microglia. This appears to be a compensatory mechanism to terminate excessive catecholamine signaling in injured brain regions.¹¹⁶

8.2.3.1.4.3. COMT inhibitors¶

Just as COMT promotes dopamine degradation in the PFC and estrogen can inhibit dopamine degradation by reducing COMT, other COMT inhibitors should also inhibit dopamine degradation in the PFC.
Taking these medications could therefore be helpful for ADHD - if a dopamine deficiency in the PFC is actually triggered by COMT overactivity.

COMT inhibitors are among others:

• Serotonin hydrochloride¹¹⁷
• Tolcapone (Ro 40-7592)¹¹⁷
• Opicapone¹¹⁷
• Flopropion (also a 5-HT1A receptor antagonist)¹¹⁷
• Entacapone sodium salt¹¹⁷³⁹

Salsolinol inhibits COMT.¹¹⁸ Salsolinol is a tetrahydroisoquinoline.

8.2.3.1.5. COMT genetic test¶

A genetic test can show which COMT gene variant is present. This can indicate whether corresponding symptoms of impaired working memory result from an excess of dopamine or a lack of dopamine in the PFC.
However, as other genes also have an influence on the dopamine level of the PFC, a test result is only one of several factors. Therefore, a combination with testing of other gene candidates that also have an influence on the dopamine level of the PFC, e.g. the DAT gene or the 5 dopamine receptors, could be indicated.
This knowledge could provide an indication of whether non-responding to stimulants could possibly result from increased dopamine or noradrenaline levels in the PFC.
A COMT gene test is available for around €60 (as of 2019). We do not know whether testing several genes together is cheaper.

8.2.4. Dopamine degradation through enzymatic oxidation¶

This presentation is based on Meiser et al.¹

The oxidative deamination of catecholamines by MAO produces hydrogen peroxide. Hydrogen peroxide generates oxidative stress in catecholaminergic or catecholamine-degrading cells.
All catecholamines - including dopamine - are susceptible to oxidation at their electron-rich catechol part. Enzymatic oxidation can occur through various enzymes:

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

With oxygen as the electron acceptor, these reactions generate superoxide radical anions (OO-⋅2).
The enzymatic oxidation of dopamine or L-dopa, spontaneously or by metal catalysis (Fe 3+), produces highly reactive electron-poor orthoquinones (DOPA-quinone and dopamine-quinone). DOPA quinone and dopamine quinone react easily with nucleophiles. Both quinones and ROS can react non-specifically 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 degraded by both isoenzymes of MAO, MAO-A and MAO-B. In humans, degradation by MAO-A predominates (within the degradation by MAO) in vivo, as barely any dopamine reaches astrocytes, where it could be degraded by MAO-B.
MAO breaks down dopamine in sending nerve endings, while COMT is active in receiving nerve endings.¹¹⁹

8.2.4.1.1. MAO-A¶

MAO-A occurs mainly in nigrostriatal dopaminergic axon terminals.¹²⁰
MAO-A breaks down various monoamines¹²⁰

• Dopamine³⁹
  • Rats:
    • 60% of dopamine degradation in the cortex by MAO-A, 40% by MAO-B¹²¹¹²²
    • in the striatum¹²³
      • MAO-A, but not MAO-B, contributes mainly to striatal dopamine degradation in rats
      • MAO-B, but not MAO-A, is responsible for astrocytic GABA-mediated tonic inhibitory currents in the rat striatum
      • MAO-B mediates GABA synthesis, upregulation of which can cause Parkinson’s motor symptoms, so MAO-B inhibitors could address Parkinson’s symptoms in this way
  • People:
    • in vitro in the cortex 30 % through MAO-A, 70 % through MAO-B¹²⁴
    • but: there is barely any DAT in astrocytes, so little dopamine reaches them in vivo, where it could be degraded by MAO-B¹²⁰
    • People with different MAO-A gene variants show different symptoms related to dopamine, while people with different MAO-B gene variants barely show any symptoms at all
• Adrenalin
• Noradrenaline
• Melatonin
• Serotonin

8.2.4.1.2. MAO-B¶

MAO-B is found in astrocytes and serotonergic neurons,¹²⁰ in histaminergic neurons and outside the nervous system in blood platelets and lymphocytes.¹²⁵
MAO-B controls the breakdown of¹²⁰

• Phenylethylamine
• Benzylamine
• Dopamine
  • in rats:
    • in the cortex 40 % through MAO-B, 60 % through MAO-A¹²¹¹²²
    • in the striatum: no MAO-B¹²³
      • MAO-A, but not MAO-B, contributes mainly to striatal dopamine degradation in rats
      • MAO-B, but not MAO-A, is responsible for astrocytic GABA-mediated tonic inhibitory currents in the rat striatum
      • MAO-B mediates GABA synthesis, upregulation of which can cause Parkinson’s motor symptoms, so MAO-B inhibitors could address Parkinson’s symptoms in this way
  • in humans:
    • in vitro in the cortex 70 % through MAO-B, 30 % through MAO-A¹²⁴
      • but: there is barely any DAT in astrocytes, so little dopamine reaches them in vivo, where it could be degraded by MAO-B¹²⁰
      • on the other hand: Astrocytes (like microglia) can synthesize and metabolize dopamine themselves.¹²⁶ Astrocytes and microglia are in direct contact with dopaminergic neurons. It is possible that glial cells are involved in maintaining dopamine levels in the brain in both healthy and pathological conditions. However, astrocytes are not able to convert L-DOPA that they absorb into dopamine.
  • MAO-B is upregulated in the brain of people with ADHD
    • resulting in excessive DA degradation, which contributes to the development of Parkinson’s disease
    • irreversible MAO-B inhibitors (selegiline, rasagiline) are (limitedly) effective in Parkinson’s disease
    • mAO-B inhibition also prevents the formation and transfer of α-synuclein aggregates¹²⁷
  • mAO-B increases with age¹²⁸

8.2.4.1.3. Inhibition of the MAO¶

Inhibiting the MAO indirectly leads to an inhibition of the breakdown of dopamine, noradrenaline, serotonin and adrenaline and thus increases these neurotransmitters.

MAO inhibitors are:¹²⁹

• Rasagiline
• Selegiline
• Safinamide
• Tranylcypromine
• Phenelzine
• Moclobemide
• Isocarboxazid
• Salsolinol¹¹⁸ Salsolinol is an alkaloid and tetrahydroisoquinoline.
  Chocolate (cocoa) contains significant amounts of the alkaloids salsonisole (up to 2.5 mg) (1-methyl-6,7-dihydroxy-tetrahydroisoquinoline) and salsoline (1-methyl-6,-methoxy-7-hydroxy-tetrahydroisoquinoline).¹³⁰
• Lazabemide (development discontinued)
• Harmaline (indole alkaloid, not used as a medicinal substance)

8.2.5. Dopamine degradation by dopamine β-hydroxylase (DBH) to noradrenaline¶

Dopamine β-hydroxylase is a copper-dependent mono-oxygenase, an enzyme that converts dopamine into noradrenaline.
DBH polymorphisms are associated with:¹³¹

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

Dopamine and noradrenaline levels are altered in Disorders of copper metabolism. Infants with Menkes disease, which is caused by an ATP7A gene defect, exhibit a pronounced copper deficiency in the brain. The impaired copper supply to the CNS in these patients is associated with higher levels of dopamine and lower levels of noradrenaline in the brain and plasma, which is used in the clinical diagnosis of the disease. The change in catecholamine levels is thought to be caused by the loss of copper incorporation into dopamine β-hydroxylase and thus its reduced activity. Restoration of ATP7A expression in the mouse model of Menkes disease corrects the dopamine-norepinephrine ratio, especially when accompanied by additional copper injections.¹³²
However, ATP7A has not yet been identified as a gene candidate for ADHD.

Salsolinol inhibits dopamine β-hydroxylase.¹¹⁸
Bleomycin is a potent inhibitor of dopamine β-hydroxylase.¹³³

8.2.6. Dopamine degradation through sulfation (using sulfotransferases)¶

8.2.6.1. Sulfation deactivates dopamine to dopamine sulfate¶

The sulfation of dopamine causes the active dopamine to break down 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.¹³⁴ Sulfoconjugation is the most important form of dopamine inactivation in human serum, while glucuronidation predominates in rats. Thus, no ortholog of SULT1A3 is known in rodents¹³⁵
Human plasma dopamine sulphate is mainly derived from the sulphoconjugation of dopamine, which is synthesized from L-DOPA in the gastrointestinal tract. Both dietary and endogenous factors affect plasma adopamine sulfate. There appears to be an enzymatic gut-blood barrier to detoxify exogenous dopamine and limit the autocrine/paracrine effects of endogenous dopamine formed in a “third catecholamine system”.¹³⁶

The dopamine sulphate serum level (approx. 5 ng/ml) is 10 to 15 times higher than the levels of free dopamine (0.3 ng/ml), noradrenaline (0.2 ng/ml) or adrenaline (0.05 ng/ml). Dopamine sulphate is not detectable by routine analytical methods and requires special extraction procedures.¹³⁷ After the first meal after fasting, serum dopamine sulfate levels increase 50-fold. Food intake appears to stimulate peripheral dopamine synthesis and/or dopamine sulfoconjugation in the gastrointestinal tract.¹³⁷

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 iodothyronines as well as in the detoxification of xenobiotics. There are two SULT enzyme superfamilies:¹³⁷

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

SULT are found in a variety of human tissues:

• Liver
• Intestine
• Brain (PST)¹³⁸
  • high mirrors:
    • temporal cortex
    • frontal cortex
  • low levels (approx. 1/10):
    • Parietal lobe
    • Occipital lobe
    • Amygdala
    • Hypothalamus
    • Hippocampus
  • lowest mirror:
    • Nucleus accumbens
    • Caudate nucleus
    • Substantia nigra

8.2.6.3. Sulfotransferases and dopamine¶

PST appears to be an important regulator of dopamine storage and dopamine metabolism.
L-dopa altered PST in Parkinson’s patients¹³⁷

• greatly reduced in
  • Hypothalamus
  • frontal cortex
  • temporal cortex
  • Amygdala
  • occipital cortex
  • parietal cortex
• slightly reduced in
  • Hippocampus
  • Nucleus accumbens
  • Putamen
  • Substantia nigra
• unchanged in the
  • Meynert nucleus (nucleus basalis)
• doubled in the
  • Caudate nucleus

8.2.6.4. Sulfatases reactivate dopamine sulfate to dopamine¶

Sulfoconjugation is reversible by sulfatases (unlike glucuronidation, which is irreversible).¹³⁹
Sulfatases convert the biologically inactive dopamine sulfate into an unconjugated, active dopamine.
To date, 17 sulphatases are known in humans. These are mainly located in the lysosomes.

• Arylsulfatases
  • Arylsulfatase A (ARSA)
  • Arylsulfatase B (ARSB)
  • Arylsulfatase C (estrone/dehydroepiandrosterone sulfatase; steroid sulfatase, STS)
  • Arylsulfatases Dbis K
• Iduronate-2-sulfatase

8.2.6.5. Sulphation and ADHD¶

Arylsulfatase C (STS) is associated with inattention, cognitive problems and other ADHD symptoms.^140141142143

Arylsulfatase C (STS, steroid sulfatase) cleaves sulfate groups from steroid hormones, which alters their biological activity. This also affects dehydroepiandrosterone sulphate (DHEAS).
Sulphated and non-sulphated steroids can affect GABA-A and NMDA receptors in the brain.¹⁴⁴ Both DHEAS and its non-sulphated form DHEA inhibit the GABA-A receptor and activate the NMDA receptor.¹⁴⁵
More about DHEA at DHEA in the chapter Neurological aspects / Hormones in ADHD.

8.2.7. Dopamine degradation through glucuronidation (by means of glucuronosyltransferases)¶

UDP-glucuronosyltransferases (UGTs) are involved in the detoxification of xenobiotics. They therefore help to protect the organism from dangerous 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 circumventricle, pineal gland, pituitary gland and neuro-olfactory tissue.¹⁴⁶ 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 able to catalyze the glucuronidation of dopamine to a significant extent in humans. This produces 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. Therefore, according to current knowledge, UGTs are not involved in dopamine metabolism in the human brain.¹⁴⁷

8.3. Dopamine removal by diffusion¶

In the DAT-KO mouse, inhibition of serotonin transporters, noradrenaline transporters, MAO-A or COMT did not alter dopamine degradation in the striatum. In the absence of DAT in the striatum, this appears to occur more by diffusion.²⁷

8.4. Conclusions for the medication of ADHD¶

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

The active ingredients methylphenidate (MPH), d-amphetamine (d-AMP), l-amphetamine (l-AMP) and atomoxetine (ATX) bind with different affinities to dopamine transporters (DAT), noradrenaline transporters (NET) and serotonin transporters (SERT). The binding causes an inhibition of the activity of the respective transporters.⁹
The noradrenaline transporter - along with COMT - is responsible for most dopamine degradation in the PFC, while the DAT regulates this mainly in the striatum.

Binding affinity: stronger with smaller number (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. Effect of MPH, AMP, ATX on dopamine / noradrenaline per brain region¶

The active ingredients methylphenidate (MPH), amphetamine (AMP) and atomoxetine (ATX) alter extracellular dopamine (DA) and noradrenaline (NE) to different degrees in different regions of the brain. Table based on Madras,⁹ modified.

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