Neurophysiological correlates of stress

Stress correlates with specific neurophysiological patterns. The changes in neurotransmitters and other messenger substances explain behavioral changes caused by stress.

Our hypothesis is that ADHD causes a disorder of the stress regulation systems in which the stress systems are permanently activated (ADHD-HI, ADHD-C) or overreact (ADHD-I) without the presence of an adequate stressor. At the very least, ADHD mediates its symptoms in the same or a very similar way to chronic stress, namely through a reduced dopamine and noradrenaline effect.
We consider almost all ADHD symptoms to be functional stress symptoms, i.e. regular Consequences of the activation of the stress systems, as they were useful for survival in a life-threatening situation (in the last millions of years).

If our hypothesis that ADHD causes a permanent activation of the stress systems is correct, the same neurophysiological changes in the brain should be found in the specialist literature on (severe) stress reactions as on ADHD symptoms. In order to verify this, in this article we collect the neurophysiological correlates of stress as they emerge from the relevant scientific literature on stress.

As a result, we can conclude that the neurophysiological correlates of ADHD are impressively similar to the neurophysiological correlates of chronic stress, especially chronic social stress in adolescence
Acute short-term stress, on the other hand, has different neurophysiological patterns.

Genes, epigenetic changes and environmental factors determine which stressors trigger which (neuronal/neurobiological) stress reactions in an individual.¹
Consequences of stress vary greatly depending on the duration, type of stressor, whether it is an existing or feared stressor, gender and age of the person with ADHD.
More on this at ⇒ Time of stress determines type of mental disorder And ⇒ Type of stress determines location of neurological changes in the article ⇒ Stress damage due to early / prolonged stress.

This article was originally based on an article by Arnsten (2009).²

• 1. PFC
  • 1.1. Types of stress
    • 1.1.1. Acute severe stress
    • 1.1.2. Chronic severe stress
      • 1.1.2.1. Dopamine reduction due to chronic stress
        • 1.1.2.1.1. Dopamine reduction in the PFC due to chronic stress
        • 1.1.2.1.2. Dopamine reduction in the striatum due to chronic stress
        • 1.1.2.1.3. Reduction of dopaminergic cells in substantia nigra due to chronic stress
      • 1.1.2.2. Further effects of chronic stress
        • 1.1.2.2.1. Reduction of noradrenergic cells in the nucleus coeruleus due to chronic stress
        • 1.1.2.2.2. Altered phosphorylation and oxidative stress
    • 1.1.3. Mild stress
  • 1.2. Behavioral effects of stress in the PFC
    • 1.2.1. Attention
    • 1.2.2. Working memory
      • 1.2.2.1. Acute severe stress deactivates working memory via noradrenaline
      • 1.2.2.2. Stress deactivates the working memory via dopamine
        • 1.2.2.2.1. Acute stress and dopamine in the PFC
        • 1.2.2.2.2. Chronic stress and dopamine in the PFC
    • 1.2.3. Interaction of noradrenaline and dopamine in the stress response
    • 1.2.4. Neurophysiological correlates of acute stress and ADHD
      • 1.2.4.1. Dopamine and noradrenaline receptor antagonists for the treatment of ADHD
      • 1.2.4.2. ADHD: low DA level in the PFC.
  • 1.3. Learning behavior: automatic reactions instead of controlled conclusions
  • 1.4. Impulsiveness due to loss of control
  • 1.5. Addiction due to loss of control
• 2. Striatum
• 3. Amygdala
• 4. Hippocampus
• 5. Neurophysiological correlates of stress sensitivity and stress resilience
  • 5.1. Asymmetrical activity of the brain hemispheres
  • 5.2. Neurotransmitters and stress sensitivity
  • 5.3. Acute stress even once increases sensitivity to stress
• 6. Dopamine and stress
• 7. Endocannabinoids and stress

1. PFC¶

A basic distinction must be made between the consequences of acute and chronic stress. While acute stress increases dopamine and noradrenaline levels in the PFC, chronic stress reduces dopamine levels in the PFC. Both increases and decreases in dopamine and noradrenaline levels impair working memory.

Early childhood stress can reduce or increase dopamine in the PFC.³

1.1. Types of stress¶

1.1.1. Acute severe stress¶

Stress impairs processes of the PFC that support goal-directed behavior, but not processes of the oPFC⁴ such as reversal learning.
While the PFC can control its own noradrenaline and dopamine levels in a non-stressful state so that it functions optimally, during acute stress the amygdala via the hypothalamus and midbrain (mesencephalon, part of the brain stem) increases dopamine levels⁵ and noradrenaline levels to such an extent that PFC function is impaired.²⁶⁷

During acute stress, the dopamine release of the VTA is increased, which increases the dopamine level in the PFC, basolateral amygdala and nucleus accumbens. The mPFC inhibits the nucleus accumbens, while the amygdala inhibits the PFC and activates the nucleus accumbens. If the inhibitory effect of the PFC on the nucleus accumbens is lost due to excessive dopamine levels, severe stress symptoms occur.⁸

If D1 receptors are activated too strongly (e.g. during acute stress), this increases cAMP, which inhibits the entire task-oriented firing of the nerve cells in all directions of the PFC network and impairs working memory.⁸ This can be prevented by appropriate administration of D1 antagonists,⁹ cAMP inhibitors or PKA inhibitors¹⁰ in the PFC. Dopamine and noradrenaline receptors are closely linked to the cAMP-PKA pathway.¹¹
High doses of D1/D5 antagonists or D1/D5 agonists impair working memory equally, while mild doses improve working memory.¹²

According to one account, the dopamine level between the PFC and the striatum is said to be rigidly inversely correlated: a high dopamine level in the PFC correlates with a low dopamine level in the striatum and vice versa.¹³¹⁴ According to another account, a dopamine deficiency in the right mPFC also leads to a reduced dopamine level in the nucleus accumbens of the striatum.¹⁵ This model could better explain ADHD, which is associated with reduced dopamine levels in the PFC and the striatum.

Stress changes the brain’s response patterns from slow, reasoned control by the PFC to reflexive and rapid emotional control by the amygdala and related subcortical structures.²

Acute, uncontrollable stress impairs PFC-mediated cognitive functions in humans and animals and shifts behavioral and emotional control to more primitive posterior brain circuits.²

Glucocorticoid and dopamine receptors work together in relation to stress in the PFC. During stress, glucocorticoids are released by the HPA axis, which cross the blood-brain barrier and address glucocorticoid receptors throughout the brain.¹⁶

• Glucocorticoids administered directly into the PFC increase the dopamine level in the PFC and increase stress-related impairments of executive functions of the PFC. Conversely, glucocorticoid antagonists administered into the PFC reduce stress-induced dopamine levels in the PFC.¹⁷ Similarly, activation of the HPA axis stimulates the ventral tegmentum, which like the PFC has many glucocorticoid receptors, and correlates with an increase in dopamine in the PFC.¹⁸ Since glucocorticoid administration into the ventral tegmentum does not increase the dopamine level in the PFC during stress, this does not appear to be directly controlled by the ventral tegmentum.
  A high level of cortisol in the blood also impairs the working memory, but not the declarative memory.¹⁹
• The glucocorticoid corticosterone blocks the dopaminergic OTC2 transporters located on glial cells in the PFC, which remove dopamine from the synaptic cleft. Corticosterone thereby increases the extracellular dopamine level.²⁰²¹

As a result, glucocorticoids released by stress can cause overstimulation of the D1 receptors in the PFC and thus contribute to the shutdown of the PFC.

If, as we assume, the cortisol stress response is often excessive in ADHD-I (without hyperactivity) and the cortisol stress response is rather flattened in ADHD-HI (with hyperactivity), this should result in working memory problems being somewhat less pronounced in ADHD-I.
Since tonic dopamine levels in the PFC are reduced in ADHD due to chronic stress, a stronger cortisol stress response, which increases dopamine levels, should result in a lesser overall reduction of dopamine in the PFC. In other words, the exaggerated phasic dopamine and noradrenaline stress response might compensate a little for the tonic dopamine and noradrenaline deficit - although the timing must be taken into account, as the cortisol stress response occurs approximately 20 minutes after the stressor. As a result, ADHD-I would be expected to show slightly less impairment of executive functions than ADHD-HI.
At least this is consistent with the results of our online test. Out of around 1400 participants, those who reported having ADHD-I had a slightly lower symptom severity overall than those who reported being ADHD-HI/ADHD-C. ADHD-HI had consistently more severe symptoms than ADHD-I in all symptom areas, including outside of hyperactive/impulsive symptoms. In particular, people with ADHD-HI also reported thinking blocks and decision-making difficulties more frequently or more strongly than people with ADHD-I.

1.1.2. Chronic severe stress¶

1.1.2.1. Dopamine reduction due to chronic stress¶

1.1.2.1.1. Dopamine reduction in the PFC due to chronic stress¶

Chronic stress over 4 weeks caused reduced dopamine levels in the PFC in rats, which impaired working memory.²²
The changes triggered by 4 weeks of chronic stress (depressive behavior, negative feedback resistance in the dexamethasone suppression test, decrease in extracellular dopamine in the PFC) persisted for 3 months afterwards.²³ Only the decrease in extracellular serotonin in the PFC had already regressed by then
Numerous other studies confirm that chronic stress reduces the dopamine level in the PFC. Chronic stress in adolescence reduces the dopamine level in the PFC not only in adolescence, but also throughout adulthood.²⁴

Chronic stress increased noradrenaline transporters in the PFC, while noradrenaline levels remained unchanged.²⁵
Noradrenaline transporters primarily reabsorb dopamine in the PFC. Increased noradrenaline transporters thus cause a reduced dopamine level.²⁶

Stress during birth appears to reduce basal dopamine levels in the PFC and at the same time increase them in the nucleus accumbens and striatum,²⁷ for example oxygen deprivation during birth.²⁸²⁹

Prenatal stress can influence the dopamine level in the PFC laterally.³⁰ In ADHD, the basal dopamine level in the right hemisphere of the PFC is reduced, while in the experiment mentioned above it was increased in the right hemisphere of the PFC.

If rat mothers were administered glucocorticoids in the last trimester of pregnancy, the offspring showed reduced dopamine levels in the nucleus accumbens and other mesolimbic structures, an altered ratio of dopamine D1 to D2 receptors and a tendency towards addiction.³ Another study also found an addictive tendency after prenatal stress.³¹

Repeated social stress in adolescence comparable to bullying caused decreased dopamine levels in the PFC in rodents in adulthood due to increased D2 reuptake and increased dopamine degradation.³² Social stress in youth increased the activity of the DAT, which breaks down dopamine.³³ Repeated social stress in adolescence decreased D2 receptors in adulthood,³⁴ while social isolation in adolescence increased D2 receptors in adulthood.³⁵ Repeated social stress over 5 days in juvenile rodents decreased dopamine levels in the mPFC in adulthood³⁶³⁷ as well as increased norepinephrine and serotonin levels in the ventral dentate gyrus and decreased norepinephrine levels in the dorsal raphe nuclei.³⁶
The reduction in mPFC dopamine levels in adulthood may also be caused by repeated pharmacological activation of D2 autoreceptors in the mPFC during the same juvenile period.³⁷

Chronic defense stress in mice showed two differently sensitive groups. Increased expression of D2S, D2L and D2R receptor dimers in the PFC was observed in both groups. D2S receptor mRNA expression was significantly decreased in the amygdala.³⁸

1.1.2.1.2. Dopamine reduction in the striatum due to chronic stress¶

Prolonged (single) stress, as used to provoke PTSD in rodents, decreased dopamine levels and D2 receptors in the striatum and increased DAT in the striatum. D1 receptors in the striatum remained unchanged.³⁹ It further decreased dopamine in the infralimbic cortex.⁴⁰

1.1.2.1.3. Reduction of dopaminergic cells in substantia nigra due to chronic stress¶

Chronic restraint of rodents (8 out of 24 hours on 5 out of 7 days) caused an increasing loss of dopaminergic cells in the substantia nigra of up to 61 % after 16 weeks. The noradrenergic cells in the locus coeruleus were also reduced. At the same time, there was a significant increase in the activation of microglia and an increase in nitrotyrosine in the substantia nigra and locus coeruleus. This indicates oxidative stress, which could trigger the reduction in dopaminergic and noradrenergic cells.⁴¹

1.1.2.2. Further effects of chronic stress¶

1.1.2.2.1. Reduction of noradrenergic cells in the nucleus coeruleus due to chronic stress¶

Chronic movement impairment (restraint) of rodents (8 of 24 hours on 5 of 7 days) reduced noradrenergic cells in the locus coeruleus, as well as a loss of dopaminergic cells in the substantia nigra that increased with duration.⁴¹

A single prolonged stressor, as used to induce PTSD, in rats in the noradrenergic cells of the locus coeruleus⁴²

• Lower spontaneous activity but higher evoked responses, leading to an increased signal-to-noise ratio of locus coeruleus neurons
• Impaired recovery after stimulation inhibition.
• Excessive tyrosine hydroxylase mRNA expression in the locus coeruleus

A comprehensive study of several types of stress as single or long-term stressors was conducted in rats:⁴³

• One-off stressors had different effects depending on the stressor
  • Immobilization stress
    • Increased tyrosine hydroxylase mRNA expression
      • In brainstem A1, A2, A5 and locus coeruleus
    • Unmodified NET mRNAs and VMAT2 mRNAs
  • Glycolysis inhibition by 2-deoxy-D-glucose
    • Increased tyrosine hydroxylase mRNA expression
      • In brainstem A1, A2, A5 and locus coeruleus
    • Unmodified NET mRNAs and VMAT2 mRNAs
  • Cold
    • Increased tyrosine hydroxylase mRNA expression
      • In brainstem A2 and locus coeruleus
    • Unmodified NET mRNAs and VMAT2 mRNAs
  • Insulin
    • Unchanged tyrosine hydroxylase mRNA expression
    • Unmodified NET mRNAs and VMAT2 mRNAs
• Chronic stress had different effects depending on the stressor
  • Immobilization stress, 2 hours a day for 41 days
    • Increased tyrosine hydroxylase mRNA expression
      • Same increase as through one-time immobilization
        • In brainstem A1, A2, A5 and
      • Increase lower than through one-time immobilization
        • In Locus coeruleus
      • Further increase of the increased tyrosine hydroxylase mRNA expression by single cold stress or 2-deoxy-D-glucose
        • Only in Locus coeruleus
      • No change due to new one-time immobilization
    • Increased NET mRNAs and VMAT2 mRNAs
      • Only in brainstem A1 and A2
  • Glycolysis inhibition by 2-deoxy-D-glucose
    • Increased tyrosine hydroxylase mRNA expression
      • In brainstem A1, A2, A5 and locus coeruleus
  • Cold
    • Increased tyrosine hydroxylase mRNA expression
      • Only in brainstem A2 and nucleus coeruleus
  • Insulin
    • No increased tyrosine hydroxylase mRNA expression

1.1.2.2.2. Altered phosphorylation and oxidative stress¶

Chronic stress (electric foot shocks) in mice

• Reduced the phosphorylation of extracellular-signal regulated kinase (ERK1 / 2) and
• Reduced the phosphorylation of cyclic AMP-responsive element binding protein (CREB-1),
• Increased the phosphorylation of the N-methyl-d-aspartate (NMDA) receptor (type 1) in the hippocampus

These effects were prevented by administering (-)-sesamin (a polyphenol found in sesame oil, among other things) before the stress.⁴⁴

1.1.3. Mild stress¶

Mild acute stress does not impair memory consolidation by the hippocampus and amygdala and can even improve it. The vmPFC regulates fear responses mediated by the amygdala ^{8 45}
More severe acute stressors, on the other hand, impair the functions of the hippocampus, but continue to strengthen the emotional motor functions of the amygdala and striatum.

Mild stress, e.g. when the mother returns after a short separation, promotes stress processing in adulthood and reduces impulsivity and the basal stress hormones of the HPA axis in the blood.⁴⁶. Rat pups that were only briefly and repeatedly removed from their mother or held in her hand showed a lower HPA axis response to stress in adulthood by increasing the expression of glucocorticoid receptors (GR) in the PFC and hippocampus. GR are the receptors through which cortisol (corticosterone in rodents) shuts down the HPA axis at the end of the stress response. In contrast, prolonged maternal deprivation, endotoxin administration or trauma, which trigger severe or chronic stress, increased the sensitivity of the HPA axis in adulthood⁴⁷ and reduced GR expression in the dlPFC (and - to a much lesser extent - in the ventrolateral PFC).⁴⁸ The mineralocorticoid receptors (MR) (which have a much higher corticoid affinity and control the daily routine) remained unchanged, which worsened the ratio of MR to GR. If the stress did not occur until adulthood, the expression of GR in CA1 of the hippocampus and of MR in the ventrolateral PFC decreased instead.

Chronic mild stress is associated by the majority of literature with depressive response patterns linked to underactivity of the mesolimbic dopamine system and increased binding to cortical beta-adrenergic receptors. For a smaller group, an aberrant response to chronic mild stress is described, which is associated with increased activity of the mesolimbic dopamine system and decreased binding to cortical beta-adrenergic receptors.⁴⁹ It would be interesting to find out whether these are the subtypes of melancholic depression and atypical depression.

1.2. Behavioral effects of stress in the PFC¶

1.2.1. Attention¶

Stress changes the control of attention.² Attention regulation changes from top-down control by the PFC, prioritized according to relevance for goal achievement, to stimulus-prioritized bottom-up control by the sensory cortices. The control of attention thus changes from voluntary control by the PFC to an automatic reaction to stimuli by posterior brain regions.⁵⁰
This corresponds quite closely to the change in attentional control in ADHD.

Stress only impaired selective attention in an animal study when it was uncontrollable.²

Stress impairs attentional control and connectivity within a frontoparietal network that mediates attentional switching (task switching).⁵¹
Task changes are controlled by the mPFC.⁵²

Stress impairs task metabolism, which correlates with a decrease in apical dendritic spines in the mPFC.⁵³

In ADHD, attention control is altered in precisely this specific way: in ADHD, neither attention itself nor the directability of attention itself is impaired, but rather attention control is subject to its own specific pattern: task switching is more difficult, distraction is easier. Attention is increasingly subject to intrinsic control.

1.2.2. Working memory¶

Stress impairs working memory in the dlPFC² and increases conditioning for negative, but not for positive stimuli⁵⁴ and the activity of the default mode network.⁵⁵

1.2.2.1. Acute severe stress deactivates working memory via noradrenaline¶

Working memory requires a medium level of noradrenaline for optimal function. Noradrenaline levels that are too low or too high impair working memory. This control is achieved by means of noradrenaline receptors with different affinities: the high-affinity α2-adrenergic receptors and the low-affinity α1- and β-adrenergic receptors.^{58596061626364} This control mechanism corresponds to the deactivation of the HPA axis after a stress response by cortisol via the high-affinity mineralocorticoid receptors and the low-affinity glucocorticoid receptors.
The absence of noradrenaline thus calms the PFC and enables sleep. Medium doses of noradrenaline activate working memory and increase cognitive performance. High noradrenaline levels during stress impair working memory and thus cognitive flexibility. Stimulation of α1- and β-adrenergic receptors by very high noradrenaline levels not only impairs (spatial) working memory, but also increases the activity of posterior and subcortical functions that take over behavioral control instead of the PFC during stress.⁵⁸⁶⁵ α1-receptor antagonists prevent the impairment of the PFC by stress.⁶⁶ In practice, α1-adrenoceptor antagonists are helpful in the treatment of PTSD.⁶⁷⁶⁸ Similarly, β-adrenoceptor antagonists prevent stress-induced impairment of the PFC,⁶⁹ e.g. propanolol.⁷⁰
Guanfacine, which is used as a third choice of medication for ADHD after MPH and AMP, is an α2-adrenoceptor agonist. ⇒ Guanfacine for ADHD

1.2.2.2. Stress deactivates the working memory via dopamine¶

1.2.2.2.1. Acute stress and dopamine in the PFC¶

Dopamine has similar effects on the PFC as noradrenaline. Acute stress induces very high dopamine levels in the PFC. Both very high dopamine levels (via D1 receptors) and very low dopamine levels impair the function of the PFC, in particular (spatial) working memory.⁷¹⁷²⁷³ The impairment of working memory caused by stress can be prevented by D1 antagonists.⁶

As with D1, moderate D2 receptor stimulation in the PFC strengthens working memory, while very strong D2 receptor stimulation in the PFC impairs working memory^{74 7576}

The same effects of dopamine are already known from various COMT gene polymorphisms. COMT controls dopamine degradation in the PFC. Those COMT variants that degrade dopamine more slowly are associated with an increased susceptibility to stress- or stimulation-induced impairments of working memory.⁷⁷ See also ⇒ COMT gene variant influences gender-specific perception of stress

1.2.2.2.2. Chronic stress and dopamine in the PFC¶

Chronic stress reduces the dopamine level in the PFC.⁷⁸

In contrast, the change in dopamine or noradrenaline levels in response to acute stress after 5 weeks of chronic cold stress in rats did not change fundamentally. Previous chronic stress only slightly reduced the dopamine increase caused by acute stress, while the noradrenaline increase in response to acute stress was almost doubled with previous chronic stress.⁷⁹

1.2.3. Interaction of noradrenaline and dopamine in the stress response¶

Dopamine and noradrenaline complement each other in mediating stress reactions and stress symptoms
At medium levels, noradrenaline increases signal strength to all inputs via α2A receptors,⁶⁴ while high levels of noradrenaline reduce firing
Dopamine, on the other hand, improves signal quality (reduced noise) by reducing neuronal activation/addressing of non-preferred inputs via D1 receptors.⁷¹ However, high D1 receptor stimulation suppresses neuron firing for all directions. As a result, the neuron loses both its spatial orientation and its responsiveness.⁷¹

Protein kinase C also affects the PFC.⁸⁰ Protein kinase γ and ε play a role as stress sensors in the brain.⁸¹

This impairment of working memory is a quantitative change in memory functions.

Working memory is impaired in ADHD.

1.2.4. Neurophysiological correlates of acute stress and ADHD¶

1.2.4.1. Dopamine and noradrenaline receptor antagonists for the treatment of ADHD¶

Antagonists of D1 or D2 dopamine receptors have barely been used to treat ADHD to date
Guanfacine, which is used in particular in children and adolescents with ADHD who do not respond to stimulants, acts as an α2-adrenoceptor agonist. Guanfacine thus covers the high-affinity α2-adrenoceptors so that less noradrenaline is required to activate the PFC - or, after completely covering the α2-adrenoceptor, to deactivate the PFC via the α1- or β-adrenoceptors. As a result, guanfacine acts as an indirect α1- and β-adrenoceptor agonist.

1.2.4.2. ADHD: low DA level in the PFC.¶

In ADHD, the dopamine level in the PFC (and in the striatum) is reduced.
Methylphenidate and amphetamine drugs also increase the noradrenaline and dopamine levels in the PFC ^{82 26}
Stimulants have the same effect on ADHD-HI as on ADHD-I.

As shown above, the functionality of the PFC is equally impaired when noradrenaline and dopamine levels are too low or too high. Too little noradrenaline or dopamine does not activate the PFC sufficiently to fully activate working memory.

The working memory problems in ADHD therefore appear to result from underactivation of the PFC, which can be remedied by appropriately dosed stimulants.

Acute stress is characterized by an increase in dopamine (and noradrenaline) in the PFC.

Chronic stress, on the other hand, can trigger reduced dopamine levels in the PFC, which also impair working memory.⁸³ Since ADHD is a decade- to lifelong disorder, it is not surprising that the Consequences of chronic stress are present.

The fact that the dopamine levels in the PFC change drastically in chronic stress compared to acute stress corresponds to the changes in cortisol levels in chronic, prolonged stress: while cortisol rises in response to stress in acute stress, especially in healthy people who are not pre-exposed to stress, chronic stress (depending on the stress phenotype) shows comparatively flattened or excessive cortisol stress responses. The basal cortisol level (the daily cortisol curve) is reduced in chronic stress in both stress phenotypes
In this context, it is theoretically conceivable that glucocorticoids, which contribute to the increase in dopamine levels in the PFC during stress, are no longer high enough to trigger an increase in dopamine in the PFC, especially in the externalizing stress phenotype (ADHD-HI/ADHD-C), which has a flattened cortisol stress response.

People with ADHD who live for several weeks in an extremely low-stimulus environment (alpine hut without internet) are said to lose their ADHD symptoms, even if this only lasts as long as the lack of stimuli persists. Even normal everyday life without particular stressors restores the ADHD symptoms afterwards.
It would be interesting to find out whether stimulant administration then worsens the symptoms in this state. This could indicate that a return to a normal stress response and a normalization of dopamine levels in the PFC would occur if the stress systems were calmed in an extremely low-stimulus environment.

A report on an alpine hut event for children with ADHD is not very positive.⁸⁴

1.3. Learning behavior: automatic reactions instead of controlled conclusions¶

Stress (e.g. fear) changes the learning behavior of mammals in such a way that learning controlled by the hippocampus is replaced by stimulus-response learning controlled by the striatum.⁸⁵⁸⁶
This change in the memory networks involved is a qualitative change in memory functions.

Stress causes further qualitative changes, e.g. in relation to visual memory.⁸⁵

Stress reduces the levels of BDNF and other neurotrophic factors. These are necessary for the neuroplasticity of the brain, i.e. the formation of new synapses, particularly in the hippocampus - known as “memory formation” or “learning”.

ADHD is characterized by learning problems and correlates with reduced levels of BDNF and other neurotrophic factors. ADHD medications normalize the levels of neurotrophic factors.

1.4. Impulsiveness due to loss of control¶

The PFC is a control instance for impulsive and emotionally driven behavior. Stress reduces this control of the PFC and thus increases impulsivity.
Impulsivity is one of the central symptoms of ADHD.⁸⁷

1.5. Addiction due to loss of control¶

The PFC is a controller of impulsive and emotionally driven behavior. An impaired impulse control by the PFC due to stress correlates, for example, with drug addiction, smoking, alcohol consumption and overeating^88899091

In addition to the PFC, the anterior cingulate cortex (ACC), the amygdala and the striatum are involved in addiction symptoms.⁸⁸

ADHD correlates with significantly increased addiction problems and the brain regions mentioned are all involved in relation to the symptoms of ADHD.

2. Striatum¶

The mPFC also appears to control stress responses by regulating the stress response of the mesoaccumbal dopamine system. Rats under 240 minutes of restraint stress showed¹⁴

• Initially
  • A brief increase in noradrenaline in the mPFC
  • A brief increase in dopamine in the nucleus accumbens (part of the striatum)
• Subsequently
  • A sustained increase in dopamine in the mPFC
  • A sustained drop in dopamine in the nucleus accumbens, to below resting levels
• Selective removal (depletion) of noradrenaline in the PFC
  • Prevented the increase in noradrenaline in the mPFC and
  • Prevented the increase in dopamine in the mPFC and the nucleus accumbens
• Selective removal (depletion) of mesocortical dopamine
  • Eliminated the increase in dopamine in the mPFC and
  • Eliminated the reduction of dopamine in the nucleus accumbens
  • The basal catecholamines remained unaffected

Accordingly, the opposing influences of noradrenaline and dopamine in the mPFC determine the stress-induced reaction of dopamine in the nucleus accumbens.

Repeated alcohol exposure during adolescence reduced dopamine levels in the nucleus accumbens (striatum) in adult rodents.⁹² and monkeys.⁹³

The dopamine level in the nucleus accumbens is also altered by prolonged stress. Depending on whether the stress is controllable or uncontrollable, there is an increase or decrease in the dopamine level. Controllable stress causes a tonic increase in dopamine, uncontrollable stress causes a tonic decrease in dopamine.^94959697

The dopamine stress response in the nucleus accumbens appears to be biphasic. A short-term increase in dopamine is followed by a second phase in which dopamine levels depend on control over the stressor.⁹⁸ While short-term stress is accompanied by a mesolimbic increase in dopamine, long-term stress is characterized by a decrease in dopamine.⁹⁹

3. Amygdala¶

The basolateral nucleus of the amygdala (basolateral amygdala, BLA) receives, integrates and processes aversive/stressful stimuli from several regions of the brain. The basolateral amygdala is strongly activated during stressful events¹⁰⁰ during the acquisition and consolidation of fearful experiences¹⁰¹¹⁰² as well as during the deletion of memories¹⁰³.
The basolateral nucleus projects to the central nucleus and the medial nucleus, which in turn project to other brain regions that are responsible for triggering adaptive behavior during stress.¹⁰⁰
During stress, the amygdala increases the noradrenaline level. The amygdala moderates anxiety and fear conditioning by means of high noradrenaline levels.¹⁰⁴ While conditioned fear is generated by phasically activated neurons, anxiety is generated by permanently activated neurons.¹⁰⁵

The amygdala is regulated by inhibitory projections from the infralimbic mPFC. These can be weakened after stressful situations, leading to a lack of inhibition of the BLA by the PFC and subsequently to hyperactivation of the amygdala¹⁰⁶, e.g. as a result of toxins¹⁰⁷. Depression or PTSD correlate with an overactivated BLA¹⁰⁸

ADHD correlates significantly with increased anxiety. Untreated ADHD significantly increases the risk of anxiety disorders later in life.

4. Hippocampus¶

The hippocampus is closely connected to the amygdala and PFC and consists of:¹⁰⁸

• Dentate gyrus
• Subiculum
• Ammonshorn (regions CA1 to CA4)

CA1 has several outputs, including to¹⁰⁸

• prelimbic PFC
• infralib mPFC
• basloatoral amygdala

Functions of the hippocampus:¹⁰⁸

• Memory acquisition
• Distinguishing aversive aversive contexts
• Inhibition of the HPA axis with excessive activity during stress response

Stress affects the hippocampus:¹⁰⁸

• Atrophy
  • also for anxiety disorders, PTSD and depression
  • reduced hippocampus volume is a risk factor for PTSD¹⁰⁹
• reduced neuronal plasticity

Consequences of this include:¹⁰⁸

• Fear extinction impaired
• Fear generalization
• anxiety-like behavior
• depressive behavior

5. Neurophysiological correlates of stress sensitivity and stress resilience¶

Just as certain states and processes in the brain correlate with the development of stress and stress symptoms, there are also neurophysiological images of stress sensitivity and stress resilience.

5.1. Asymmetrical activity of the brain hemispheres¶

A stronger activation of brain regions correlates with a more intense perception of the emotions represented in this brain region. The right hemisphere of the brain tends to represent negative perceptions and feelings, while the left hemisphere tends to represent positive ones. People with a stronger left than right brain activity perceive positive emotions more intensely and people with a stronger right than left brain activity perceive negative emotions more intensely.^110111112 One study was only able to reproduce this for certain analysis methods. A recent review summarizes the current status of alpha asymmetry.¹¹³¹¹⁴
In humans, one study found lower left-sided anterior and lower right-sided posterior alpha activation in formerly depressed subjects than in never-depressed subjects. This pattern is similar to that of acutely depressed patients.¹¹⁵
Studies on rhesus monkeys have shown that increased right frontal brain activity correlates with anxiety behavior and increased blood plasma cortisol levels. Monkeys with increased right frontal brain activity have - stable over years - increased CRH levels in the cerebrospinal fluid.¹¹⁶

At the same time, the relative distribution of brain activity between the left and right hemispheres is said to represent a fairly constant personality trait.¹¹⁷ Infants at 10 months react to positive faces with stronger activity in left frontal brain regions.¹¹⁸

5.2. Neurotransmitters and stress sensitivity¶

One study compared mice that showed pronounced stress responses to chronic stress with mice whose responses to chronic stress were indistinguishable from those of unstressed control mice
Stress-resilient mice showed:¹¹⁹

• Lower noradrenaline levels in the ventral tegmentum than stress-prone mice
• Unchanged noradrenaline levels in limbic areas, in the nucleus accumbens and in the PFC
• Noradrenaline levels in the ventral tegmentum correlated with social interaction
• Unchanged excitability of dopaminergic neurons in the ventral tegmentum compared to controls

Stress-prone mice showed:

• Increased dopaminergic response in the ventral tegmentum
• Higher dopamine levels in the nucleus accumbens
• Unchanged dopamine levels in the PFC

This indicates a higher release of dopamine from the ventral tegmentum, which projects dopaminergically into the nucleus accumbens.

Stress-prone mice showed:

• Reduced expression of the transcription factor c-fos in the locus coeruleus compared to controls and stress-resilient mice. This was fully explained by noradrenergic neurons projecting to the ventral tegmentum. However, the number of non-NE-activated cells (TH-negative) that projected to the ventral tegmentum and NE-activated cells (TH-positive) that did not project to the ventral tegmentum remained unchanged.
• The reduced number of activated noradrenergic cells in the locus coeruleus projecting to the ventral tegmentum correlated with reduced social interaction time.

This suggests that both a reduced activation state of noradrenergic cells in the locus coeruleus projecting to the ventral tegmentum and a reduced amount of noradrenaline released into the ventral tegmentum correlate with vulnerability to emotional stress.

5.3. Acute stress even once increases sensitivity to stress¶

Illustration according to Holly, Miczek.¹²⁰

Even a single (severe) acute stress can cause long-lasting neuroplastic changes in dopaminergic cells of the ventral tegmentum (VTA) (similar to drug abuse) ^{121122 123}

VTA dopamine neurons are relatively depolarized in the baseline state and thus typically at or very close to the action potential threshold.¹²¹

• Acute stress causes dopamine neurons in the VTA¹²¹¹²²
  • Induced long-term potentiation at glutamatergic (= exitatory) synapses
    • Incorporation of new AMPA receptors¹²⁴
    • Thereby increasing the ratio of AMPA to NMDA^{121123 124}
      • Where:
        • Only in medial VTA dopamine neurons that project into the mPFC (toxin as a stressor)¹²⁵
        • Not in VTA dopamine neurons that project into the NAc envelope (toxin as a stressor)¹²⁵
      • How long:
        • Occurs within 2 h after stress¹²⁶
        • Exists for at least 24 h¹²⁶
      • Consequences:
        • Increases calcium permeability and changes calcium dynamics
          • This means that subliminal stimulation can induce robust long-term potentiation¹²⁷
          • Thereby increasing the future excitability of the postsynaptic nerve cell¹²⁴
    • AMPA and NMDA receptors in the VTA mediate increased dopamine release in the mPFC in response to acute pain stress¹²⁸
      • Blockade of AMPA and NMDA receptors in the VTA prevents dopamine release in the mPFC in response to acute pain stress¹²⁸
      • Blockade of the glucocorticoid receptors in the mPFC interrupts the signaling circuit that mediates the increased glutamate level in the VTA and the increased dopamine level in the mPFC (in response to acute pain stress).¹²⁸
  • Blocked long-term potentiation at GABAergic (= inhibitory) synapses¹²¹¹²²

This loss of long-term potentiation at inhibitory synapses on VTA dopamine neurons could remove the brake on the system and, in combination with the induced long-term potentiation at excitatory (glutamatergic) synapses, increase the responsiveness of VTA dopamine neurons to future stress or reward stimuli.¹²⁰

6. Dopamine and stress¶

More on this in detail at Dopamine and stress In the section Dopamine in the chapter Neurological aspects.

7. Endocannabinoids and stress¶

More on this in detail at Cannabinoids for ADHD and stress In the section Dopamine in the chapter Neurological aspects.