Authors: Diana Sporici, Lucia
INTRODUCTION
Breathing carbon dioxide considerably increases anxiety and fear and may cause panic attacks. This phenomenon was known to neurologists and psychiatrists since the beginning of 20th century, but the biologic mechanism that regulates this reaction has been a mystery up to now.
But today the research team of University of Iowa leaded by the neuroscientist Adam E.Ziemann, has discovered that the effect of carbon dioxide on nervous system is controlled by a protein named with the acronym AISC 1a. This protein is particularly abundant in the amygdala, a region of the brain part of limbic system, i.e. the set of encephalic structures that regulates psychic states such as emotions and behavior. In particular, several studies have already demonstrated the direct implication of amygdala in the management of panic and anxious states. Thanks to the trials on mice, scientists observed that in the amygdala ASIC 1a activates as a response to the increase of cerebral acidity, causing behaviors typical of fear. On the contrary, obstructing the activity of such protein, these behaviors are not shown. So the correlation between this molecule and panic attacks seems to be explained considering that the inhalation of carbon dioxide significantly increases the acidity of brain.
Le Scienze - Panico da CO2
PANIC ATTACK
Panic attacks are periods of intense fear or apprehension that are of sudden onset and of variable duration from minutes to hours. Panic attacks usually begin abruptly, may reach a peak within 10 to 20 minutes, but may continue for hours in some cases. Often, those afflicted will experience significant anticipatory anxiety and limited symptom attacks in between attacks, in situations where attacks have previously occurred.
The effects of a panic attack are various like fear of dying or heart attack, flashing vision, faintness or nausea, numbness throughout the body, heavy breathing, hyperventilation, loss of bodily control.
Some people also suffer from tunnel vision, mostly due to blood flow leaving the head to more critical parts of the body in defense. At least four of these symptoms must be present for a person to be considered having a full blown panic attack. These feelings may provoke a strong urge to escape or flee the place where the attack began that is a consequence of the sympathetic "fight-or-flight response". This response floods the body with hormones, particularly adrenaline, that aid it in defending against harm.
In fact a panic attack is a response of the sympathetic nervous system.
The most common symptoms may include:
- dyspnea
- heart palpitations
- chest pain
- hot or cold flashes
- sweating
- dizziness or slight vertigo
- hyperventilation
- tingling sensations
- sensations of choking or smothering
Often, the onset of shortness of breath and chest pain are the predominant symptoms.
While the various symptoms of a panic attack may cause the person to feel that their body is failing, it is in fact protecting itself from harm. The various symptoms of a panic attack can be understood as follows: there is frequently the sudden onset of fear with little provoking stimulus that leads to a release of adrenaline which brings about the fight-or-flight response wherein the person's body prepares for strenuous physical activity. This leads to an increased heart rate, rapid breathing which may be perceived as shortness of breath, and sweating which increases grip and aids heat loss. The release of adrenaline during a panic attack causes vasoconstriction resulting in slightly less blood flow to the head which causes dizziness and lightheadedness. A panic attack can cause blood sugar to be drawn away from the brain and towards the major muscles. It is also possible for the person experiencing such an attack to feel as though they are unable to catch their breath, and they begin to take deeper breaths, which also acts to decrease carbon dioxide levels in the blood.
Panic attacks are distinguished from other forms of anxiety by their intensity and their sudden and episodic nature. They are often experienced in conjunction with anxiety disorders and other psychological conditions, although panic attacks are not usually indicative of a mental disorder.
A person with panic disorder (PD) may have repeated panic attacks (at least several a month) and feel severe anxiety about having another attack.
Panic disorder is a chronic, debilitating condition that can have a devastating impact on a person's family, work, and social life. Typically, the first attack strikes without warning. Pounding heart, sweating palms, and an overwhelming feeling of impending doom are common features. While the attack may last only seconds or minutes, the experience can be profoundly disturbing. A person who has had one panic attack typically worries that another one may occur at any time.
As the fear of future panic attacks deepens, the person begins to avoid situations in which panic occurred in the past. In severe cases of panic disorder, the victim refuses to leave the house for fear of having a panic attack. This fear of being in exposed places is often called agoraphobia.
Wikipedia - Panic attack
PubMed - Stress and anxiety
Medical News Today - All about anxiety
THE AMYGDALA
The amygdalae are almond-shaped set of nuclei located deep and medially within the temporal lobes of the brain. Shown in research to play a key role in the processing of memory and emotional reactions, the amygdalae are considered part of the limbic system.
The amygdala is known to have 13 nuclei, which can be categorized into lateral, basal and central subregions.
In humans, nuclei of the amygdala are usually grouped as:
- laterobasal subgroup including both lateral and basal nuclei
- centromedial subgroup
- cortical subgroup
These subgroups deal with fear in a process which can be exemplified as follows: the lateral subgroup receives information from the cortical and subcortical areas, the basal subgroup inter-connects the lateral and central subgroups, and sends the output to the cortical areas, and the central subgroup pass on the information to the brain regions like hypothalamus and periaqueductal gray.
The figure shows in basic terms inputs and outputs of the amygdala, which have been reported to be associated with PD pathogenesis. Hypothetically, disruption in any of these brain areas and connections along these areas, or any lack of balance in the network can cause maladaptive and overwrought fear responses such as panic attacks, increased nervousness, and immoderate worrying.
Particularly, the basolateral amygdala contains ASIC 1a and receives most of the sensory inputs that designate fear associations and careful activation of neurons within this nucleus is able to associate the incoming sensory information with unrestricted fear responses.
The central nucleus of the amygdala is widely considered the main output regulator for mediating fear responses.
For instance, the role of the amygdala in associative learning of fear appears to be ubiquitous across species.
Studies in well-defined larger samples, with more sophisticated research designs and advanced technologies would promise a better understanding on the role of the amygdala in the pathophysiology of PD.
Wikipedia - The amygdala
PubMed - An acid-sensing channel sows fear and panic
PubMed - The role of the amygdala in the pathophysiology of panic disorder: evidence from neuroimaging studies
ACID SENSIG ION CHANNEL (ASIC 1a)
Acid-sensing ion channels (ASICs) are voltage-independent, proton-activated receptors that belong to the epithelial sodium channel/degenerin family of ion channels and are implicated in perception of pain, ischaemic stroke, mechanosensation, learning and memory.
The structure
The structure of ASIC1 is characterized by subunit of the chalice-shaped homotrimer that is formed by a short amino and carboxy termini, two transmembrane helices, a bound chloride ion and a disulphide-rich, multidomain extracellular region enriched in acidic residues and carboxyl-carboxylate pairs within 3.0Å, suggesting that at least one carboxyl group bears a proton. Electrophysiological studies on aspartate-to-asparagine mutants confirm that these carboxyl-carboxylate pairs participate in proton sensing. Between the acidic residues and the transmembrane pore lies a disulphide-rich ‘thumb’ domain poised to couple the binding of protons to the opening of the ion channel, thus demonstrating that proton activation involves long-range conformational changes.
PubMed - Structure of an Acid-sensing Ion Channel 1 at 1.9 Å Resolution and LOW PH
ASICs: background
Significant progress has been made in understanding the structure and function of ASICs at the molecular level. Studies aimed at clarifying their physiological importance have suggested roles for ASICs in pain, neurological and psychiatric disease, in fact recent findings link these channels to physiology and disease.
Acid-evoked currents were first observed in neurons in the early 1980s. In 1997, a protein producing a similar acid-gated current was cloned and identified as an acid-sensing ion channel (ASIC). This protein was closely related to a previously cloned member of the degenerin–epithelial Na+ channel family (DEG–ENaC family). This and other related channel family members were subsequently found to be pH sensitive and renamed ASICs to reflect their related structure, function and pH sensitivity. We now know that the ASIC family of channel subunits (TABLE 1) is largely responsible for the acid-evoked currents observed in neurons.
- In peripheral sensory neurons, ASICs have been found on cell bodies and sensory terminals, where they have been suggested to be important for nociception and mechanosensation.
- In central neurons, ASICs have been found on the cell body, dendrites and at dendritic spines and have been suggested to contribute to synaptic plasticity.
ASICs are permeable to cations and are activated by extracellular acidosis. They are subject to modulation by extracellular alkalosis, intracellular pH and various other factors. Much of what we know about these channels’ properties comes from expressing recombinant ASIC subunits in heterologous cells. Channels are formed by combinations of ASIC subunits in homotrimeric or heterotrimeric complexes, with different subunits conferring distinct properties (TABLE 1). The amino acid sequences of ASIC subunits are well conserved between species, in fact the mouse ASIC 1a and the human ASIC 1a share over 99% of their amino acid sequence identity. The recently described crystal structure of the chicken ASIC1 homomultimeric channel has shed light on the subunit interactions and, along with sequence homology analyses, has driven numerous structure–function experiments that are revealing how the channels respond to pH and other stimuli. In addition to the non-covalent inter-subunit interactions, disulphide bonds between trimers may also create higher-order complexes and alter channel function.
The channels are activated by protons and other endogenous or exogenous chemicals, it has also been sug¬gested that ASICs respond to mechanical stimuli.
Compared with ASIC 1a, much less is known about the other ASIC subunits in the brain: it is known that ASIC 2a and ASIC 2b are expressed in the brain, but unlike ASIC 1a these subunits are not required for acid-evoked currents in central neurons. However, in the absence of ASIC 1a, ASIC 2 subunits produced a small amount of current in brain neurons in response to very acidic pH (pH 4.0). ASIC 2a and ASIC 2b interact with ASIC 1a and can shift the pH sensitivity, desensitization kinetics and ion selectivity of acid-evoked currents.
The precise mechanisms by which ASICs are activated in the brain remain uncertain, but one potential mechanism might involve protons released during neurotransmis¬sion by acidic (~pH 5.5) neurotransmitter-containing vesicles, also protons generated by other sources, such as localized energy metabolism, might also contribute to ASIC activation.
ASICs most probably influence neuronal function through membrane depolarization, Ca++ entry and through a number of downstream signalling cascades.
PubMed - Acid-sensing ion channels in pain and disease
ASICs and fear – related behaviours
The synaptic localization of ASICs and their prominent expres¬sion in brain structures underlying emotion, cognition and behaviour, including the amygdala, bed nucleus of the stria terminalis, habenula, nucleus accumbens and periaqueductal grey, suggest that ASICs are well positioned to influence psychiatric symptoms. The possibility that ASICs have a role in brain function and behaviour is supported by initial studies of the effects of ASIC 1a disruption in mice on neu¬rophysiology and models of anxiety and depression. ASIC 1a knockout mice exhibited deficits in cued and contextual fear conditioning as well as in unconditioned fear behaviours, such as predator odour-evoked freezing, open-field centre-avoidance and acoustic startle responses.
Another role for ASICs in fear-related behaviours was recently identified, which linked ASIC 1a more closely to brain pH changes in vivo. CO2 inhalation rapidly lowers pH in the brain and has long been known to trigger fear and panic attacks in humans.
One might also speculate that ASICs in the amyg¬dala might help to prevent suffocation by inducing active defence responses. Rising CO2 heralds the potential threat of suffocation. Thus, the ability to detect CO2 and elicit prompt defensive action could be life-saving. From an evolutionary perspective, it is interesting to imagine that this might be a key role for ASICs.
Also genetic studies have evalu¬ated the potential link between ASICs and human psy¬chiatric illness, the largest of these studies examined the relationship between SNPs in ASIC1 with anxiety disorders and depression.
A quantitative trait locus for anxiety-related behaviours in mice was found to be homologous to the human chromosomal region 12q13, which con¬tains ASIC1. Interestingly, this same region was linked to panic disorder in humans.
Img.C: ASICs are activated by extracellular protons H+ and possibly other ligands, and are modulated by a number of other factors. ASIC 1° is permeable to cations, primarly Na+ and to lasser degree Ca++. Upon activation, an invar current depolarizes the cel membrane, which activates voltage-gaes Ca++ channels and voltage-gated Na+ channels and may contribute to NMDA receptor activation through the release of the voltage-dependent Mg++ blockade. Thus, Na+ and Ca++ influì contributes to membrane depolarization, the generation of dendritic spikes and action potentials, Ca++/Calmodulin-dependent protein kinase II activation and possibly influence other second-messenger pathways. A number of intracellular proteins have been suggested to regulate ASICs.
CO2 AND PANIC DISORDER
CO2 is regularly produced in the brain and throughout the body as a final product of carbohydrate metabolism. CO2 crosses cell membranes and the blood-brain barrier. In a reaction catalyzed by carbonic anhydrase, CO2 is hydrolyzed to carbonic acid (H2CO3), which then dissociates into HCO3– and H+.
This reaction leads up to acidosis which is thought to be responsible for most of the physiological effects of CO2, including stimulating acid-activated respiratory chemoreceptors in the brain stem.
These chemoreceptors stimulate breathing to expel CO2 and regulate systemic pH.
Inhaling CO2 raises the partial pressure of CO2 in the blood and lowers pH throughout the body. Consequently, the CO2 provocation challenges used in psychiatric research are likely to acutely and transiently acidify brain pH.
Increasing evidence suggests that pH may be abnormally regulated in panic disorder, in fact brain pH is largely controlled by the CO2/HCO3– buffering system, which is acutely regulated by breathing.
Many researchers have reported irregular breathing in panic disorder, involving greater tidal volume variability, which may be resulted from more frequent sighing. In accord with a persistent breathing irregularity, panic disorder patients show a chronically low end-tidal CO2 and a compensatory decline in serum bicarbonate.
Panicogenens and chemosensitivity in the CNS
The possible connection between panic disorder, the action of panicogens, and brain pH begs the question of how the brain normally responds to pH change.
The majority of research on chemosensitivity in the CNS has focused on respiratory control.
Hence, understanding how pH regulates breathing could provide comprehension into panic disorder. Breathing rate and volume are sensitive to CO2 in the blood, chiefly through interstitial pH and activation of pH-sensitive chemoreceptors.
Although the precise sites of CO2-mediated ventilatory control are uncertain, they are thought to lie in the brain stem (medulla and pons).
It is known that the amygdala integrates sensory input from other brain structures to orchestrate fear behavior; Ziemann et al suspected this possibility after observing that the ASIC 1a was abundantly expressed in the basolateral amygdala and other fear circuit structures, and it was found that breathing 10% CO2 lowered pH to levels sufficiently to activate ASIC 1a in amygdala neurons.
With this background, Ziemann et al hypothesized that inhaling CO2 would decrease brain pH and that the acidosis would provoke ASIC channels in the amygdala to bring out fear. While CO2 is known to stimulate chemosensors in the periphery and brainstem, if CO2 and hence acidosis were to activate the amygdala, it would suggest that the amygdala is an important chemosensory structure.
PubMed - The biology of fear
ASIC 1a and CO2 induced fear behavior
The research developed four paradigms to test CO2-triggered fear in mice:
1) CO2-evoked freezing
2) CO2 effects in the open-field test
3) CO2 aversion
4) CO2-potentiated context fear conditioning%
1) To value the response to CO2, the study analyzes freezing, which is used as a correlate of fear and panic in mice. In humans with panic disorder, breathing 5% CO2 evokes panic attacks but it rarely evokes panic in control subjects. Consistent with this, 5% CO2 produced little or no freezing in mice.
In humans who do not have panic disorder, breathing higher CO2 concentrations is more likely to evoke panic. Likewise, breathing 10% CO2 leads to significant freezing in control mice (Fig. 1A).
However, disrupting the ASIC 1a gene essentially unpointed the response. Moreover, acutely inhibiting ASIC 1a with either psalmotoxin (PcTx1) or A-317567 also reduced CO2-evoked freezing in wild-type mice (Fig. 1B).
Neither inhibitor reduced freezing in ASIC1a −/− animals, suggesting that their effects were ASIC 1a-specific. Thus, CO2 produced fear-like freezing behavior that depended on ASIC 1a.
2) The tendency of mice to avoid the center of an open field is used as a measure of their fear and anxiety of open spaces.
In wild-type mice, breathing CO2 concentrations greater than 5% reduced the amount of center activity in an open field (Fig. 1C).
However this effect was much attenuated in ASIC 1a−/− mice.
3) The study examined the aversive effect of CO2 on behavior by allowing mice to move between two chambers, one with 15% CO2 and one with <2% CO2.
During a 10 min assay, ASIC 1a+/+ mice only briefly sampled the CO2 chamber, spending > 90% of the time in the air chamber.
In contrast, ASIC 1a−/− mice spent similar amounts of time in both chambers.
Failure to avoid CO2 did not generalize to other aversive stimuli as both genotypes avoided moving air infused into one of the chambers; ASIC 1a+/+ mice spent 69 ± 7% and ASIC 1a−/− mice 82 ± 4% of time in the non-air-infused chamber (n = 7 per group; p > 0.15; Wilcoxon rank sum).
4) Context fear conditioning is often used to measure fear and anxiety, and earlier work showed that disrupting or inhibiting ASIC 1a impaired fear conditioning.
During training with a series of footshocks, wild-type mice froze (Fig. 1E), as previously described. However, when training was conducted in the presence of 10% CO2, mice began to freeze before receiving footshocks and froze even more once the footshocks were delivered. The following day, it was possible to return the mice to the original context (minus footshocks and minus CO2) and it was found that wild-type mice trained in CO2 showed more freezing than those not exposed to CO2 (Fig. 1F).
In contrast, CO2 had no significant effect on context fear conditioning in ASIC 1a−/− mice.
To learn whether CO2 itself might have served as an unconditioned stimulus, wild-type mice delivered CO2 or air but no footshocks on the training day.
On the testing day, neither group froze in response to the context (Fig. 1F, inset). Thus, CO2 by itself did not act as an unconditioned stimulus, but it enhanced fear memory when coupled with footshocks.
Thus, results in four different models suggest that CO2 evokes fear-like responses in wild-type mice and that those behaviors depend on ASIC 1a.
PubMed - Neurobiology of panic and pH chemosensation in the brain
PubMed - The amygdala is a chemosensor that detects carbon dioxide and acidosis to elicit fear behavior
AMYGDALA pH
The study then turned the attention to the amygdala because of the possible association between the amygdala and panic, and because of the robust ASIC 1a expression in the basolateral amygdala. It was measured pH in the basolateral amygdala of anesthetized mice and found that breathing CO2 reduced pH of both genotypes (+/+ and /) likewise. Baseline pH in the mice breathing air was less than the expected range (pH ~7.29–7.39), due to respiratory defeat during anesthesia. In accord with this idea, PaCO2 in the anesthetized mice was elevated (65 ± 3 mmHg) (Fig. 2A) compared to levels (39 – 45 mm Hg) reported for awake or mechanically ventilated animals. To determine if these changes were sufficient to activate ASICs, current in cultured amygdala neurons have been measured: in wild-type neurons, lowering pH to 7.2 induced current in 21% of cells, and further reductions generated current in a greater percentage of neurons and evoked larger currents.
The acidosis did not brought out current in amygdala neurons from ASIC 1a −/− mice, consistent with data from neurons of other brain areas.
Supporting a role for pH in panic pathophysiology, correcting blood gas abnormalities through breathing control or pharmacology has been suggested to produce clinical improvement.
CURIOSITIES: HIPERVENTILATION AND PAPER BAG
Hyperventilation syndrome is a nonmedical cause of shortness of breath. Hyperventilation syndrome is very scary, but not life-threatening.
Hyperventilation syndrome is usually associated with panic disorders. It is a psychological or emotional condition that causes victims to breathe too much. Breathing too deep and too fast causes the body to lose carbon dioxide (CO2), the byproduct of metabolism in our exhaled air. While CO2 is a byproduct, we still need a minimum amount in the bloodstream to maintain the proper pH balance in our bodies.
When there is a lost of a significant amount of CO2, some body tissues start to malfunction: numbness develops in certain areas, typically the lips, fingers and toes. After a while, the muscles of the hands and feet begin to cramp.
Paper bags have been used for years to treat hyperventilation syndrome. The idea is that rebreathing the air we exhale makes us inhale more CO2 and helps us to quickly add the CO2 back into our bloodstreams, in fact breathing into a paper bag has been shown to increase CO2 levels in the blood.
The problem with paper bags is not that true hyperventilation syndrome patients are at risk from the treatment. On the contrary, while it has not been shown to really help hyperventilation syndrome patients, it has not been shown to hurt them, either. What paper bags do hurt are the dangerous medical conditions that look like hyperventilation: heart attacks and asthma.
Breathing into a paper bag restricts the fresh air you are able to get. Without fresh air, too little oxygen is in the air you're inhaling. So, breathing into a paper bag dangerously lowers the amount of oxygen in your bloodstream.
To make matters worse, several studies now show a link between high concentrations of CO2 and panic attacks, which means that artificially increasing CO2 in inhaled air is likely to trigger more feelings of panic in patients who suffer from anxiety.
The best treatment of hyperventilation syndrome is to stay calm and practice breathing slowly and not too deeply. Calmness and breathing exercises have just as much success as paper bag breathing, and no one is going to die from staying calm.
FirstAid - Can I treat hyperventilation syndrome by breathing into a paper bag?
CONCLUSIONS
Since the inhalation of CO2 substantially increases the acidity of the brain, and then activates ASIC 1a, it seems to be explained the correlation between this molecule and panic attacks. According to the researchers, the discovery could provide also a valid interpretation of the idea that the control of breathing has anti-anxiety effects.
In addition to these information it is known that there is not a single brain structure for processing fear, neither a small set of necessary and sufficient structures has emerged. Anyway this study has opened new perspective in therapeutic field and also reveals another important point of view: it seems demonstrated that evolution has provided our organism with susceptible sampling equipment of outer dangers such as carbon dioxide which is potentially lethal.
This sensor is able to active effective defense reaction, like fear, in a very short time.
