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1. Neurotransmitters that activate the stress systems¶
The most important neurotransmitters that activate the stress systems of the CNS include1
from the locus coeruleus (A1/A2) and in the autonomic nervous system
Noradrenaline
from the hypothalamus
AVP
CRH
peptides derived from pro-opiomelanocortin
a-melanocyte stimulating hormone
ß-endorphin
from the raphe nuclei in the midbrain
Serotonin
from the posterior hypothalamic histaminergic system
Histamine
2. Disorders of the dopamine system during stress¶
Stress directly activates the dopaminergic system in the brain (CNS),2 which is centrally impaired in ADHD.
A meta-analysis of a large number of studies showed that dopamine levels and dopamine metabolism increase during acute stress, particularly in the PFC, but less so in subcortical areas.34
In stress responses, dopamine is mainly projected from the ventral tegmentum to the PFC and nucleus accumbens, with projection to the PFC being particularly stress-sensitive.674
Dopamine plays a role in the hedonic and reward aspects of stress.
The effects of stress on sexual activity and appetite as well as on the affinity for drug abuse are likely to be mediated by the dopamine system.
Dopamine increases the ability of neuronal information processing and thus the learning and information processing in relation to the stressor that has occurred.
The amygdala (the central nucleus) influences dopamine neurotransmission in the PFC. Lesions of the central amygdala block stress-induced dopamine release in the PFC. An infusion of AMPA into the central nucleus of the amygdala causes a rapid increase in dopamine in the PFC and (as a result) increased arousal.89 This is consistent with the role of the amygdala in coordinating neuronal systems to regulate behavior under stress.
Acute stress activates nigrostriatal dopamine neurons in two ways:10
- dopamine release through glutamatergic input to the dopamine cell bodies, which increases the firing rate of dopamine neurons
- dopamine synthesis is accelerated locally at the level of the dopamine terminal, which replaces the used dopamine
Thus, endogenous glutamate does not appear to influence dopamine release in the neostriatum, but glutamatergic projections influence dopamine synthesis via a direct cortico-striatal pathway.
Stress increases dopamine synthesis in the neostriatum. Striatal administration of NMDA or AMPA/kainate receptor antagonists attenuated this increase11, but administration into the substantia nigra did not.10
Stress inhibits all “uptake 2” transporters via the released corticosteroids (secured: by corticosterone). The uptake 2 transporters (PMAT, OCT 1 to OCT3) have a higher dopamine reuptake capacity than the uptake 1 transporters DAT and NET, with a lower affinity. Stress thus increases extracellular dopamine (and noradrenaline) through reduced uptake-2 transporter reuptake.
The uptake 2 transporters differ in their sensitivity to corticosterone depending on the species and tissue preparation.1213
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 the stress-induced increase in glucocorticoid hormones in a rapid, non-genomic manner.14
The deactivation of OCT115 and OCT316 by corticosterone occurs
fast
through direct interaction of corticosterone with the transporter at specific sites
Chronic stress causes an increased increase in extracellular dopamine and noradrenaline in the PFC in response to a new acute stressor,10171819 but not in the nucleus accumbens or neostriatum.
These findings are consistent with
our finding that ADHD symptoms resemble those of chronic severe stress
our assumption of dopaminergic hyperfunction in the PFC with simultaneous dopaminergic hypofunction in the striatum and nucleus accumbens in ADHD
2.1. Different types of stress cause different dopamine effects¶
Not all stress is the same. Depending on the type of stress, different effects are triggered on the dopamine system.
The types of stress differ according to:
Duration and intensity of stress
Mild stress slightly increases the dopamine level (as well as the noradrenaline level) in the PFC and thus improves cognitive performance. Severe stress increases the dopamine level and the noradrenaline level in the PFC extremely and causes the PFC to shut down. Behavior control is outsourced to other parts of the brain.
Type of stressor Each stressor has its own specific effects on the neurotransmitters. Different stressors are, for example
Psychological stress
Physical pain
Injuries
Cold
Heat
Diseases
All stress symptoms have their own neurophysiological correlates.
A neurophysiological correlate means that a specific activity or change occurs in a certain area of the brain together with the symptom.
Low stress levels are primarily processed in the mesoprefrontal system. Other ascending dopaminergic systems are not influenced by this.204 This could be due to a significantly lower number of inhibitory D2 autoreceptors in the mesoprefrontal area and extensive excitatory signals to the ventral tegmentum.
Mild stress increases dopamine, serotonin and noradrenaline metabolism21 in the mPFC22
Serotonin influences
The hypothalamus (part of the HPA axis / stress regulation axis)
The amygdala, which activates the HPA axis
The hippocampus, which inhibits the HPA axis.
Mild (not too prolonged) stress causes slightly increased noradrenaline and dopamine levels in the PFC.
Slightly elevated noradrenaline and dopamine levels increase the activity of the PFC and thus its cognitive and executive performance.
Highly elevated dopamine and/or noradrenaline levels switch off the PFC and shift behavioral control to other areas of the brain.
Short-term intense stress massively increases the dopamine level in the PFC
Low to moderate levels of stress increase extracellular23dopamine levels in the nucleus accumbens2425 , but only in the NAc shell, not in the NAc nucleus2627 and PFC2425 , while high levels of stress (intense, chronic or unpredictable) decrease dopamine levels2829 . The increase in dopamine levels is greater in the PFC than in the striatum; within the striatal complex, it is greatest in the NAc shell.3023
Most stressors increase extracellular dopamine through an increase in dopamine efflux, an increase in neuronal activity in total firing rate and/or bursts:23
Food restriction/withdrawal
Bondage stress
Social defeats
Cold swimming
Chronic stressors (chronic cold exposure, chronic mild stress) have been shown to decrease population activity, i.e. the number of active neurons, but only in the medial and central VTA, not in the lateral VTA, and without decreasing the firing frequency. Bursts were slightly increased during chronic cold exposure.
Stressors that increase dopamine firing also increase the risk of addiction and addiction relapses, which are prevented by blocking dopamine receptors.
Chronic early childhood stress reduces the dopamine level in the nucleus accumbens through downregulation.31
Dopamine in the mPFC normally suppresses mesolimbic dopamine transmission. However, this is no longer successful under extreme or unpredictable stress. Dopamine innervation also appears to be important for stress-induced activation of neurons in the stria terminalis (anterolateral BNST).324 which are involved in both the activation of higher-order stress-dependent circuits and the generation of coping behavior.
Increased dopamine levels in the mPFC lead to a reduction in dopamine levels in the nucleus accumbens in the striatum (reinforcement center), which could lead to the long-term overactivation of the dopamine transporters there by means of upregulation, which is a major problem in ADHD.
Chronic stress leads to a reduction in the dopamine level in the PFC via downregulation (increase in the number of dopamine transporters and dopamine receptors).
In chronic stress, the reduced dopamine level in the PFC after downregulation is nevertheless associated with
With overexcitation of the PFC
With a reduction in dopamine levels in the nucleus accumbens in the striatum
Chronic early childhood stress (daily hand-holding in rats, handling) leads to increased dopamine metabolism in the nucleus accumbens in adulthood. This results from a loss of inhibitory control by the right mPFC due to a dopamine deficiency found there. The dopamine deficiency in turn correlates with an increase in the number of dopamine transporters.33
The long-term nature (chronification) of stress and the degree of control over the stressor changes dopamine-dependent behaviors and the activation of afferents to the nucleus accumbens.344
While stress-induced dopamine release in the neostriatum is mediated by a glutamate effect on the dopamine cell body, stress-induced dopamine synthesis in the neostriatum is mediated by a glutamate effect on the dopamine nerve terminals10
Acute stress increases dopamine metabolism and dopamine release more in the PFC (+ 90 %) than in subcortical areas (nucleus accumbens + 40 %, neostriatum + 30 %)10
Previous chronic stress intensifies the reaction to an acute new stressor10
Only in mesocortical dopamine neurons
Not in the subcortical areas
Stress increases DOPAC in tissue35 and c-fos-expressing neurons36
Increases dopamine in the mPFC and in the nucleus accumbens (mesolimbic dopamine system)44 and acetylcholine in the hippocampus.45
The dopamine increase in the mPFC and nucleus accumbens as well as the acetylcholine increase in the hippocampus also occur after the subsequent release, which is why this could be a correlate of emotional arousal due to a sudden change in environmental influences.4544
Increases the concentrations of the dopamine metabolite DOPAC in the PFC and nucleus accumbens46
Induces Fos immunoreactivity in dopamine neurons of the ventral tegmentum (VTA), but not in the substantia nigra47
Oxygen deprivation during birth leads to increased dopamine metabolism in the nucleus accumbens in adulthood. This results from a loss of inhibitory control by the right medial prefrontal cortex (PFC) due to a dopamine deficiency found there. The dopamine deficiency in turn correlates with an increase in the number of dopamine transporters.33
Acute conditioned stress should only increase the noradrenaline level in the mPFC, but not the dopamine level.54
2.2. Dopaminergic neurophysiological correlates of various stress reactions¶
Different stress reactions have different dopaminergic neurological correlates.
Jumpiness
Is controlled by increased dopamine in the dorsal striatum and by stimulation of the (dopamine-producing) substantia nigra pars compacta.2
Dopamine release in the mesolimbic system (nucleus accumbens = ventral striatum) through electrical stimulation of the ventral tegmentum promotes **aversively motivated learning
Learning from stressful experiences
Drug blockade of dopamine receptors in the amygdala prevents this.2
In the CNS, stress is primarily modulated by noradrenaline59
Moderate noradrenaline levels
Strengthen the function of the PFC
High noradrenaline levels
Switch off the PFC (which impairs analytical thinking)
Strengthen the sensorimotor and affective regions of the brain (which intensifies perception and emotion)
Acute stress has a primary noradrenergic effect on the postsynaptic response and reduces the phasic release of noradrenaline60
The activation of microglia by stress appears to be mediated by noradrenaline via β1- and β2-adrenoceptors, but not via β1-AR β3-adrenoceptors or α-adrenoceptors.61
Second graders showed increased cortisol levels on exam days and simultaneously decreased adrenaline and noradrenaline levels. The individual differences in secreted hormones were significantly related to personality variables observed in the classroom as well as the effects of academic stress:62
Social approach behavior correlated with higher cortisol and adrenaline levels
Fidgeting correlated with low adrenaline levels
Aggressiveness correlated with high noradrenaline levels
Inattention correlated with low noradrenaline levels
This section is based on the work of Belujon and Grace63
The locus coeruleus - noradrenaline system (LC-NE system)
is decisively involved in
Regulation of behavioral states
Regulation of stress reactions
Promotion of physiological stress reactions
Amplification of arousal states
Purpose: Adaptation to challenging situations
is activated by many stressors, e.g:
Bondage
Foot shocks
social stress
A stress load increases
the activity of locus coeruleus neurons
noradrenaline turnover in regions into which LC neurons project
is involved in the processing of contextual information
processes the context of a stress load, which is important for effective adaptation
LC regulates inhibition or activation of vSub, which can support stress adaptation
vSub innervates limbic forebrain structures such as
PFC
Amygdala
PFC and amygdala project to paraventricular hypothalamus
as a result, vSub has an upstream influence on limbic stress integration
vSub and BLA inputs show reciprocal activation
Dysfunctional stress integration, as observed in psychiatric disorders, could be associated with dysregulation in the noradrenergic system, as stressors cause morphological changes in the hippocampus and BLA, e.g.
Hippocampus
dendritic atrophy (persistent / repeated stress)
BLA
Increase in dendrite and spine density (prolonged / repeated stress)
with strong correlation between synaptic plasticity and morphological changes of the spines
Increase in adrenergically regulated long-term potentiation (acute stress)
PFC, stress and noradrenaline:
mPFC another crucial component in stress response
is selectively activated by psychological and social stressors
modulates neuroendocrine function during stress via the LC-NE system
inevitable stress inhibits
Long-term potentiation in the BLA-PFC signal pathway
interferes with synaptic plasticity in the PFC-BLA signaling pathway
The ascending serotonergic pathways, which originate in the midbrain (nuclei raphe), accompany the central stress response derived from the locus ceruleus by releasing serotonin.1
There is a connection between serotonin and sensitivity to stress. However, the results are heterogeneous and the causes and correlations are still unclear.66
There is a connection between serotonin and cortisol levels.
Stress increases serotonin levels in healthy people67 as well as noradrenaline, dopamine and cortisol levels68. Acute stress, on the other hand, is said to reduce serotonin production in the dorsal raphe nuclei, while fluoxetine stimulates serotonin production.69
Severe, life-threatening stress appears to increase the function and expression of serotonin 2-A receptors, as found in PTSD. Paradoxically, the PTSD medication 3,4-methylenedioxymethamphetamine acts as a serotonin 2-A receptor agonist.70
The adrenal cortex is removed so that cortisol can no longer be released,
This changes the release of serotonin in the dorsal raphe nuclei (DRN)
Not for nominal conditions
However, it decreased under stress
Stimulation of the glucocorticoid receptors in the DRN then prevents the stress-induced serotonin blockade.66
In the development of which the mineralocorticoid receptor is involved
Which is closely linked to cell proliferation in the hippocampus
This increases serotonin levels and TPH2 expression in the hippocampus in response to chronic unpredictable stress.71
Repeated stress increases serotonin production more than single stress72 and leads to apical dendrite reduction in the medial PFC, which reduces the number of excitatory postsynaptic events mediated by serotonin and orexin/hypocretin. Cortisol did not lead to this consequence. A GR antagonist given before stress avoided the reduction of serotonin-mediated excitatory postsynaptic events, but not those mediated by orexin/hypocretin.73
Chronic stress increases cortisol levels through the release of vasopressin rather than CRH.72
Cortisol increases the serotonin level in the amygdala and in the PFC74 as well as in the hippocampus.72 This is probably due to activation of the glucocorticoid receptors. This is because inhibition of monoamine oxidase increases the serotonin level, while a reduction in the cortisol level prevents this increase in serotonin (caused by monooxidase inhibition).75 This effect of cortisol lasts a long time (as with SSRIs) and presumably occurs through desensitization of the serotonin 1-A autoreceptor.76 However, the desensitization of the serotonin 1-A autoreceptor caused by SSRIs such as fluoxetine appears to act independently of the glucocorticoid receptor.77
Removal of the adrenal gland (in whose “cortex” cortisol is produced) causes66
Unchanged serotonin transporter expression in the dorsal raphe nuclei (where serotonin is produced)
Unchanged [3H]cyano-imipramine binding to serotonin transporters in the dorsal raphe nuclei
Unchanged [3H]citalopram binding to serotonin transporters of the mesencephalon (midbrain)
Reduced serotonin reuptake in the mesencephalon (midbrain)
No change in serotonin transports in the dorsal raphe nuclei, medial raphe nuclei or in the mesencephalon with simultaneous long-term administration of MR- and GR-binding corticoids
Serotonin deficiency via deprivation of the serotonin precursor tryptophan activates the HPA axis in the same way as another stressor, but together with this it did not cause any synergistic stress axis effects.7980
SSRI administration reduced PTSD symptom severity in children and adults in one study.81
Serotonin deficiency is clinically evident in connection with78
That the group of 5-HTTLPR short genotypes (SS, SLG, LGLG, SLA, LGLA) correlated with a greater frequency of early childhood stress experiences in the first 5 years of life compared to 5-HTTLPR long/long (LALA) in younger adults, but not in children.85
Twins who had suffered bullying had a higher serotonin transporter methylation at the age of 10 than their twin siblings without bullying experience. Twins with later (!) bullying experience already showed an increase in methylation at the age of 5, i.e. before this (!) bullying experience, compared to their non-bullied twin siblings. Children with higher serotonin transporter methylation levels showed a flattened cortisol stress response.86 This could be related to the fact that people with impairments (such as ADHD) are more likely to be victims of violence. ADHD increased the probability 2.7-fold according to one study.87
That the group of 5-HTTLPR short genotypes (SS, SLG, LGLG, SLA, LGLA) in combination with many early childhood stress experiences in the first 5 years of life
Correlates with a high cortisol stress response to the TSST.85 Similar results were found in several other studies.888990
That 5-HTTLPR long/long (LALA) in combination with few early childhood stress experiences in the first 5 years of life
Correlates with a high cortisol stress response to the TSST88
Which another study only found in younger adults85
CRH and cortisol are not neurotransmitters, but hormones that are produced by the hypothalamus as the first stage of the HPA axis (CRH) and the adrenal cortex as the last stage of the HPA axis (cortisol).
As the HPA axis is essential for understanding stress and ADHD, we refer here to the detailed description at ⇒ The HPA axis / stress regulation axis and ⇒ Cortisol in ADHD.
Older adults
With a low number of stressful life experiences in the first 15 years of life showed the highest cortisol stress response85
With a high number of stressful life experiences in the first 15 years of life showed the lowest cortisol stress response85
While some authors85 regard a low cortisol stress response as a measure of a healthy reaction, we ask ourselves whether a medium cortisol stress response is not healthy and whether a particularly low cortisol stress response, as well as an excessive cortisol stress response, is a sign of a stress system imbalance, as is also the case with the cortisol stress response.
7. Stress/ADHD symptoms due to too high or too low catecholamine levels¶
7.1. Optimal neurotransmitter levels = optimal information transmission¶
Optimal transmission of information between brain synapses requires an optimal level of the respective neurotransmitters. A neurotransmitter level that is too low leads to an almost identical signal transmission disorder as a neurotransmitter level that is too high (reversed-U theory).91929394939596979899100101102
For optimal signaling, the pyramidal cells of the PFC require moderate stimulation of D1 receptors by dopamine and α2A receptors by norepinephrine. Dopamine binding to D1 receptors reduces the noise of the input signal in the PFC by reducing signals from unneeded external sources, while noradrenaline amplifies the incoming signal from external sources via α2A receptors.103
Increased DA and NE levels cause additional occupancy of receptors, which reduces attention. Reduced DA and NE levels result in all incoming signals being identical, which reduces concentration on individual tasks.
A DA and/or NE level that is too high or too low therefore leads to very similar symptoms due to suboptimal signal transmission in the PFC.104
Therefore, a medication that increases neurotransmitter levels and works well at low doses can, at higher doses, cause the very symptoms that it avoids at low doses. It is therefore a mistake to start medication for ADHD at the desired target dose or to dose it quickly. It is better to start the titration phase (medication phase) particularly slowly and low than too quickly and too high.
Example:
Adult non-smokers were treated with nicotine patches in a small study.
Those with poor concentration improved, while those with good concentration worsened.105
Nicotine has a similar effect to stimulants, only cholinergic instead of dopaminergic; it therefore increases the level of the neurotransmitter acetylcholine. Too low an acetylcholine level causes concentration problems.
Nicotine patches are potentially effective medication for ADHD. ⇒ Nicotine for ADHD
7.2. Stress/ADHD symptoms due to increased catecholamine levels (DA / NE)¶
7.2.1. Acute stress increases dopamine levels in the mPFC, striatum and nucleus accumbens¶
The mild stress reactions of the autonomic nervous system are mediated by acetylcholine and adrenaline.
In the central nervous system (brain), slight increases in dopamine and/or noradrenaline levels cause increased performance of the PFC (except in carriers of the COMT Met158Met gene polymorphism).109110111112113
If this does not solve the problem (the stressor is not eliminated), dopamine and noradrenaline levels continue to rise. High noradrenaline levels activate the HPA axis (stress axis), which only comes into action when stress is difficult to cope with.
7.2.3. Severe acute stress = strong increase in NE/DA and cortisol = reduced cognitive performance¶
In contrast to mild increases in noradrenaline, which stimulate the PFC, strong increases in noradrenaline switch off the PFC and shift behavioral control to posterior brain regions.59114115116117118119
This should correspond to the effect described by Dietrich120 with reference to Mobbs et al121 as posteriorization.
High cortisol levels, which occur in particular in ADHD-I and SCT during acute stress, additionally stimulate the noradrenaline α1 receptors in the PFC, via which noradrenaline already impairs the function of the PFC and working memory. The simultaneous addressing of these receptors by noradrenaline and cortisol intensifies this effect.122
In addition, the shift of control from cognitive brain regions (PFC and hippocampus) to more behavioral brain regions (such as aymgdala and dorsal striatum) is regulated by the cortisolergic mineralocorticoid receptors (MR) and glucocorticoid receptors (GR).123
Cortisol, which is often elevated as a stress response in ADHD-I and presumably also SCT, blocks the retrieval of declarative (explicit) memory via the glucocorticoid receptors (GR) in the PFC and hippocampus. The non-declarative (implicit, intuitive) memory is not affected.124 This could explain the thinking and memory blocks often associated with ADHD-I and also why ADHD-I sufferers are often said to have a higher level of intuition. In any case, it would stand to reason that the shift in the focus of memory skills leads to a shift in problem-solving patterns. Trappmann-Korr calls this “holistic” perception. However, our own data collection has so far shown that a self-assessment of being intuitive is present in 69% of ADHD-HI sufferers and only 60% of ADHD-I sufferers. (n = 1,100, as of August 2019)
It is likely that not only the retrieval (remembering), but also the acquisition (learning) and memory consolidation (long-term storage) of information is impaired. Consolidation occurs particularly during sleep in the first half of the night, which is characterized by particularly low basal cortisol levels. Consolidation can be prevented by low cortisol levels.124
Cortisol stress response does not correlate with mental blocks
Our hypothesis that thinking blocks would occur less frequently in ADHD-HI than in ADHD-I was not confirmed by the analysis of around 1700 data sets from the ADxS online symptom test. According to our data, thinking blocks occurred with approximately the same frequency in ADHD-HI as in ADHD-I.
There is evidence that high noradrenaline levels switch off the PFC via α1 receptors.
We had assumed that ADHD-HI sufferers (due to a reduced noradrenaline stress response in parallel to the reduced cortisol stress response) would suffer less frequent blockages of the PFC and the associated thinking and decision-making problems, while ADHD-I sufferers (without hyperactivity/impulsivity), due to an increased phasic cortisol stress response and an associated increased phasic noradrenaline release to acute stress, would suffer a frequent short-term exaggerated stress response and a more frequent shutdown of the PFC (by noradrenaline and cortisol), which could trigger more frequent thinking blocks. ⇒ Neurotransmitters during stress
Since the intensity of norepinephrine release stimulates the intensity of cortisol release, we hypothesized that cortisol and norepinephrine stress responses would run in parallel. Since several data suggest that ADHD-I correlates with increased cortisol stress responses, if cortisol and norepinephrine stress responses were correlated, it would have been logical that the increased cortisol stress responses typical of ADHD-I would be associated with increased stress-induced release of norepinephrine and resulting increased α1-adrenergic receptor activation.
However, the equal frequency of thought blocks in ADHD-HI and ADHD-I sufferers suggests that this hypothesis is not correct.
Since the PFC controls the HPA axis, it is additionally disinhibited by the loss of control by the PFC.
Other voices distinguish between short-term stress, which increases the cognitive performance of the PFC, and long-term stress, which reduces it,125 which should be the same result.
Slightly elevated catecholamine levels activate postsynaptic alpha2A adrenoceptors (by noradrenaline) and D1 receptors (by dopamine) and thus improve the prefrontal regulation of behavior and attention, while strongly elevated catecholamine levels worsen prefrontal functions by stimulating noradrenergic alpha1 adrenoceptors and (excessively) dopaminergic D1 receptors.12698
Alpha1 adrenoceptors are less sensitive than alpha2A adrenoceptors and therefore only respond to higher noradrenaline levels. If the noradrenaline level is so high that it can activate not only the alpha2a but also the alpha1 adrenoceptors, the alpha1 adrenoceptors inhibit the cognitive performance of the PFC.127116128129
See also the description of adrenoceptors = noradrenaline receptors at ⇒ Noradrenaline.
Physiological stressors such as traumatic brain injury130 or hypoxia131 appear to trigger similar physiological effects in the PFC as psychological stress. The physical stressors also induce the release of catecholamines in the mPFC and activate the same intracellular signaling events (e.g. activation of the cAMP-PKA pathway) that are associated with loss of dendritic spines and impairment of working memory. Apparently, various stressors (physical as well as psychological) can impair the structure and function of the PFC.126
Alpha1-adrenoceptor antagonists (blockers) are used to treat PTSD.
Increases in cortisol are associated with stress-induced release of noradrenaline and α1-adrenergic receptor activation.132133
The increase in cortisol levels following stress is mediated by activation of the adrenergic system and the α1-adrenergic receptors, in that a strong increase in noradrenaline levels activates alpha1-adrenoceptors in the hypothalamus and thus leads to the release of the stress hormone CRH, which activates the further stages of the HPA axis (release of ACTH and cortisol).132134133135 CRH reduces the performance of the PFC (especially working memory) in a dose-dependent manner. CRH antagonists neutralize this effect.136137
The activation of alpha1-adrenoceptors by high noradrenaline levels thus causes high cortisol levels and attention problems.138
The noradrenaline level in the OFC and in the amygdala correlates with the activation of the HPA axis in healthy people. In severely overweight people, however, this correlation is inverted.135
The activity of the PFC is inverse to the activity of the amygdala. An active PFC correlates with a less active amygdala and vice versa.139
It is known that anxiety and depression occur more frequently in people who internalize stress, i.e. eat stress into themselves (internalizing, ADHD-I subtype) rather than acting it out (externalizing, ADHD-HI/ADHD-C). In the latter, externalizing disorders such as aggression disorders (oppositional defiant disorder; social behaviour disorder, borderline) predominate.
With this in mind, the fact that ADHD-I has a higher incidence of disorders associated with an activated amygdala, such as anxiety and depression, suggests that the PFC is more frequently deactivated in ADHD-I than in ADHD-HI. Together with the fact that elevations in cortisol are associated with stress-induced release of norepinephrine and α1-adrenergic receptor activation,132133 this leads us to hypothesize that in ADHD-I, norepinephrine release in response to acute stress is likely to be very frequently excessive, analogous to cortisol release, which causes a more frequent shutdown of the PFC and a shift of behavioral control to subcortical brain regions, while in ADHD-HI, which is often associated with a reduced release of cortisol in response to acute stress, there should be a correlating reduced release of noradrenaline, which less frequently (and perhaps even too rarely in view of the inability to recover) leads to a downregulation of the PFC.
DAT knockout mice, which have almost no dopamine transporters (DAT) (i.e. represent a kind of neurological anti-model of ADHD in which too many DAT are present), have some of the same symptoms as ADHD sufferers:140
Hyperactive
Learning problems
Memory problems
The disorders that often occur comorbidly with ADHD
Conduct disorder (CD)
Oppositional defiant behavior (ODD)
Psychosis
Bipolar
are typically associated with extremely elevated dopamine levels in some areas of the brain.104
7.3. Stress/ADHD symptoms due to low catecholamine levels (DA / NE)¶
Massive dopamine deficiency in the striatum leads to a massive disruption of drive. The interest In pleasure is reduced, while the pleasureability itself is not impaired.
However, dopamine deficiency is only one way of causing the symptoms mentioned. Excess dopamine causes largely identical symptoms, as the main factor is a deviation from an optimal dopamine level for signal transmission (see 1.1. and 1.2. above).
Rats whose ascending dopaminergic pathways were almost completely destroyed, resulting in 99% less dopamine being available, subsequently lacked the drive to consume their previously preferred sugar solution. This phenomenon was therefore caused by a lack of dopamine in the reinforcement center of the brain (striatum). The animals’ ability to perceive pleasure when they were given the sugar solution remained unchanged, as evidenced by the typical tongue movements that rats make in response to foods they find pleasant. This pleasure response could also be enhanced by hedonically activating substances (e.g. benzodiazepines) and weakened by simultaneous unpleasant stimuli.141142
The neurotoxin 6-hydroxydopamine selectively destroys dopaminergic neurons. Animals treated in this way develop hyperactive behavior143
According to other accounts, 6-hydroxydopamine has a more noradrenergic effect.144 Noradrenaline is also significantly involved in ADHD.
Dopamine level disturbances caused by 6-hydroxydopamine showed a great importance of the D4 receptors in the caudate nucleus (but not of D2 receptors) in the development of hyperactivity.145
Those affected by the encephalitis epidemic of 1914 to 1917 developed typical ADHD symptoms. Children developed hyperactive motor skills, adults Parkinson’s symptoms. Encephalitis destroys the cells in the substantia nigra that produce dopamine. This cause could be reproduced in animal experiments as the trigger for the symptoms. The symptoms are therefore the result of dopamine deficiency.146 When diagnosing ADHD, encephalitis must still be clarified as a differential diagnosis.
Perinatal hypoxia, which leads to early childhood brain damage (FKHS), causes the destruction of dopaminergic cells in the striatum, which reduces the dopamine level in the striatum by up to 70 %.
In Parkinson’s sufferers, the cells of the substantia nigra are damaged, which reduces the synthesis of dopamine by up to 90 percent. This causes motor impairments such as rigor, tremor and akinesia. Depression is many times more common in Parkinson’s sufferers, which is also likely to be due to the dopamine deficiency.147
Cocaine or amphetamine abuse causes a downregulation of the body’s own dopamine synthesis. After stopping the cocaine intake, hyperactivity occurs as a withdrawal symptom due to the now too low dopamine level.148
Nicotine, which is consumed earlier and more frequently by ADHD sufferers,149 increases the release of dopamine in nigrostriatal and mesolimbic areas and thus improves attention.150151
Toxins such as polychlorinated biphenyls, which inhibit the synthesis of dopamine and the storage of dopamine in the vesicles and its release, thereby causing dopamine levels to be too low, also cause hyperactivity and impulsivity (in rats even at sub-toxic doses).152
Dysphoria is caused by a lack of dopamine (according to Wender-Utah, dysphoria during inactivity is a core symptom of ADHD in adults).153
The fact that dopamine deficiency is involved in the mediation of ADHD symptoms is shown by the very good effect of drugs that increase dopamine levels or mediate an improved response to dopamine. Stimulants (methylphenidate, amphetamine drugs) and atomoxetine act as dopamine reuptake inhibitors (which increase the availability of dopamine in the synaptic cleft) and stimulate dopamine production.
However, not all medications that increase dopamine levels are helpful for ADHD. The dopamine agonists L-dopa (levodopa), amantadine and piribidel, for example, have no positive effects on ADHD despite their dopamine-increasing effect.154
Levodopa is a precursor of dopamine (prodrug) that can cross the blood-brain barrier and is metabolized to dopamine in the brain.155 Levodopa is helpful for Parkinson’s disease and restless legs syndrome, both of which are characterized by a lack of dopamine, but is not effective for ADHD.
Amantadine is a weak glutamate receptor antagonist of the NMDA receptor, increases the release of dopamine and acts as a dopamine reuptake inhibitor. Its effect in Parkinson’s disease is controversial. In some cases, a weak activating effect on arousal is reported.156
Piribedil is a piperazine derivative and therefore a non-ergot dopamine agonist.
Piribedil is an agonist of the D2 and D3 dopamine receptors and an antagonist of the α2 adrenoreceptor subtypes α2A and α2C. It is used to treat Parkinson’s disease, also in combination with levodopa.
While short-term stress without ADHD leads to an excess of catecholamines (dopamine and noradrenaline) in the PFC,140 early long-term stress leads to a downregulation of the dopamine and noradrenaline systems. For example, chronic early childhood stress reduces dopamine levels in the nucleus accumbens.31
Exercise-induced stress in rats causes a later downregulation of dopamine in the ventral tegmentum via noradrenaline at beta-adrenoceptors of the amygdala.157
Whether there is too little or too much (tonic = long-term) catecholamine in ADHD is the subject of intense debate.158
The disagreement among scientists indicates that both variants occur. It is possible that the subtypes and individual symptom combinations of those affected differ. It is undisputed that many ADHD sufferers have reduced dopamine levels in the PFC and striatum.
According to current knowledge, we assume that ADHD is caused by a deficiency of dopamine and noradrenaline in the dlPFC, striatum and probably also the cerebellum.
The typical ADHD medications (stimulants and atomoxetine act as dopamine and noradrenaline reuptake inhibitors. Stimulants increase the DA and NE levels in the PFC and striatum, atomoxetine only in the PFC) increase the availability of these neurotransmitters in the synaptic cleft.
Conversely, this should mean that stimulants do not work in the case of stress-induced “sham ADHD” symptoms, as they further increase the dopamine level, which is already above the optimum, and thus even further away from the functional level. While dopamine and noradrenaline levels (or the DA / NE effect) are reduced in ADHD, people (with acute but not chronic long-term stress) without ADHD do not have reduced but rather increased levels of dopamine and noradrenaline. Therefore, a further increase in DA and NA levels in non-affected people should tend to worsen the symptoms, whereas they are helpful in ADHD.
Some studies suggest that these considerations may be justified:
Only low doses of methylphenidate cause an improvement in attention and executive abilities even in non-stressed healthy individuals, while higher doses have a negative effect.98 This corresponds to the slight increase in DA and NE during mild stress, which increases cognitive abilities, and the strong increase in DA and NE during severe stress, which switches off the PFC.
However, many ADHD sufferers only respond to some ADHD medications, meaning that in practice no diagnostic conclusions can be drawn from the non-effect of medication alone. This is due to the major differences described above in terms of which stress has caused downregulation in which areas of the brain in each affected person.
Marinelli M (2007): Dopaminergic reward pathways and the effects of stress. In: Al’Absi M, editor. Stress and Addiction: Biological and Psychological Mechanisms. Academic Press; Burlington: 2007. pp. 41–84 ↥
Trott, Wirth (2000): die Pharmakotherapie der hyperkinetischen Störungen; in: Steinhausen (Herausgeber) Hyperkinetische Störungen bei Kindern, Jugendlichen und Erwachsenen, 2. Aufl., Seite 215 ↥
Trott, Wirth (2000): die Pharmakotherapie der hyperkinetischen Störungen; in: Steinhausen (Herausgeber) hyperkinetischen Störungen bei Kindern, Jugendlichen und Erwachsenen, 2. Aufl., Seite 214, mwNw. ↥