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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 stress systems that 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.
A fundamental 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
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, in acute stress the amygdala via the hypothalamus and midbrain (mesencephalon, part of the brain stem) increases the dopamine level5 and noradrenaline level to such an extent that the 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,9cAMP 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 intensify 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 to 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 the tonic dopamine levels in the PFC are reduced in ADHD due to chronic stress, a stronger cortisol stress response, which increases the dopamine levels, should result in a less pronounced dopamine reduction 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 a slightly lower 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 overall symptom severity than those who reported being ADHD-HI/ADHD-C. ADHD-HI had consistently more severe symptoms than ADHD-I in all symptom areas, including those outside of hyperactive/impulsive symptoms. In particular, ADHD-HI sufferers also reported thinking blocks and decision-making difficulties more frequently or more strongly than ADHD-I sufferers.
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 movement impairment (restraints) of rodents (8 out of 24 hours on 5 out of 7 days) caused a loss of dopaminergic cells in the substantia nigra of up to 61 % after 16 weeks, which increased with duration. 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
1.1.2.2.1. Reduction of noradrenergic cells in the nucleus coeruleus due to chronic stress¶
Chronic restraint of rodents (8 out of 24 hours on 5 out 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 long-lasting stressor, as used to induce PTSD, caused the following 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
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 845
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.
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 attention 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.
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
The working memory
Working memory is tested using tests in which information received must be retained during a delay in order to make a decision after the end of the delay. Monkeys are asked to remember the position of a briefly presented stimulus on a screen and then move their eyes to focus on that position. Rodents are asked to remember which arm of a T-shaped maze they previously visited and then visit the opposite arm on the following trial. The tasks are repeated tens or hundreds of times, so that during the delay not only the “signal” (i.e. the correct choice) must be remembered, but also the “noise” (information from previous trials) must be suppressed
Certain nerve cells of the PFC are only active during this delay.56 Lesions of the PFC affect the accuracy of decisions only in relation to tasks associated with a delay, and the longer the delay, the more severely.57 Consequently, the PFC is not involved in the motor or motivational parts of the task. Working memory function requires moderate neurotransmitter levels in the PFC, which are altered by stress.18
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 drug after MPH and AMP for ADHD, is an α2-adrenoceptor agonist. ⇒ Guanfacine for ADHD
1.2.2.2. Stress deactivates the working memory via dopamine¶
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 memory747576
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.80Protein 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 hardly ever been used to treat ADHD
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.
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 PFC8226
Stimulants have the same effect on ADHD-HI as on ADHD-I.
As shown above, the functionality of the PFC is equally impaired if 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 problems of working memory in ADHD therefore appear to result from an 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 decades-long 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.
ADHD sufferers who live for several weeks in an extremely low-stimulus environment (mountain 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 when the stress systems are calmed in an extremely low-stimulus environment.
A report on a mountain 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.
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
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 overeating88899091
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 the symptoms of ADHD.
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 the dopamine level depends 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
Under stress, the amygdala increases noradrenaline levels
The amygdala moderates anxiety and fear conditioning by means of high noradrenaline levels.100
While conditioned fear is generated by phasically activated neurons, anxiety is generated by permanently activated neurons.101
ADHD correlates significantly with increased anxiety and comorbid anxiety disorders. Furthermore, untreated ADHD significantly increases the risk of later anxiety disorders.
4. 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.
4.1. 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.102103104 One study was only able to reproduce this for certain analysis methods. A recent review summarizes the current status of alpha asymmetry.105106
In humans, a 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.107
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.108
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.109 Infants at 10 months react to positive faces with stronger activity in left frontal brain regions.110
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:111
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 nerve cells 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 susceptibility to emotional stress.
4.3. Acute stress even once increases sensitivity to stress¶
Even a single (severe) acute stress can cause long-lasting neuroplastic changes in dopaminergic cells of the ventral tegmentum (VTA) (similar to drug abuse) 113114115
VTA dopamine neurons are relatively depolarized in the baseline state and thus typically at or very close to the action potential threshold.113
Acute stress causes dopamine neurons in the VTA113114
Induced long-term potentiation at glutamatergic (= exitatory) synapses
Increases calcium permeability and changes calcium dynamics
This means that subliminal stimulation can induce robust long-term potentiation119
Thereby increasing the future excitability of the postsynaptic nerve cell116
AMPA and NMDA receptors in the VTA mediate increased dopamine release in the mPFC in response to acute pain stress120
Blockade of AMPA and NMDA receptors in the VTA prevents dopamine release in the mPFC in response to acute pain stress120
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).120
Blocked long-term potentiation at GABAergic (= inhibitory) synapses113114
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.112