Pharmacologic effect of imipramine, paroxetine, and sertraline on 35% carbon dioxide hypersensitivity in panic patients: a double-blind, random, placebo-controlled study

Effect of Paroxetine or Quetiapine Combined With Oxycodone vs Oxycodone Alone on Ventilation During Hypercapnia: A Randomized Clinical Trial

IMPORTANCE

Opioids can cause severe respiratory depression by suppressing feedback mechanisms that increase ventilation in response to hypercapnia. Following the addition of boxed warnings to benzodiazepine and opioid products about increased respiratory depression risk with simultaneous use, the US Food and Drug Administration evaluated whether other drugs that might be used in place of benzodiazepines may cause similar effects.

OBJECTIVE

To study whether combining paroxetine or quetiapine with oxycodone, compared with oxycodone alone, decreases the ventilatory response to hypercapnia.

DESIGN, SETTING, AND PARTICIPANTS

Randomized, double-blind, crossover clinical trial at a clinical pharmacology unit (West Bend, Wisconsin) with 25 healthy participants from January 2021 through May 25, 2021.

INTERVENTIONS

Oxycodone 10 mg on days 1 and 5 and the following in a randomized order for 5 days: paroxetine 40 mg daily, quetiapine twice daily (increasing daily doses from 100 mg to 400 mg), or placebo.

MAIN OUTCOMES AND MEASURES

Ventilation at end-tidal carbon dioxide of 55 mm Hg (hypercapnic ventilation) using rebreathing methodology assessed for paroxetine or quetiapine with oxycodone, compared with placebo and oxycodone, on days 1 and 5 (primary) and for paroxetine or quetiapine alone compared with placebo on day 4 (secondary).

RESULTS

Among 25 participants (median age, 35 years [IQR, 30-40 years]; 11 female [44%]), 19 (76%) completed the trial. The mean hypercapnic ventilation was significantly decreased with paroxetine plus oxycodone vs placebo plus oxycodone on day 1 (29.2 vs 34.1 L/min; mean difference [MD], -4.9 L/min [1-sided 97.5% CI, -∞ to -0.6]; P = .01) and day 5 (25.1 vs 35.3 L/min; MD, -10.2 L/min [1-sided 97.5% CI, -∞ to -6.3]; P < .001) but was not significantly decreased with quetiapine plus oxycodone vs placebo plus oxycodone on day 1 (33.0 vs 34.1 L/min; MD, -1.2 L/min [1-sided 97.5% CI, -∞ to 2.8]; P = .28) or on day 5 (34.7 vs 35.3 L/min; MD, -0.6 L/min [1-sided 97.5% CI, -∞ to 3.2]; P = .37). As a secondary outcome, mean hypercapnic ventilation was significantly decreased on day 4 with paroxetine alone vs placebo (32.4 vs 41.7 L/min; MD, -9.3 L/min [1-sided 97.5% CI, -∞ to -3.9]; P < .001), but not with quetiapine alone vs placebo (42.8 vs 41.7 L/min; MD, 1.1 L/min [1-sided 97.5% CI, -∞ to 6.4]; P = .67). No drug-related serious adverse events were reported.

CONCLUSIONS AND RELEVANCE

In this preliminary study involving healthy participants, paroxetine combined with oxycodone, compared with oxycodone alone, significantly decreased the ventilatory response to hypercapnia on days 1 and 5, whereas quetiapine combined with oxycodone did not cause such an effect. Additional investigation is needed to characterize the effects after longer-term treatment and to determine the clinical relevance of these findings.

TRIAL REGISTRATION

ClinicalTrials.gov Identifier: NCT04310579.

Effect of serotonin transporter genotype on carbon dioxide-induced fear-related behavior in mice

BACKGROUND

Inhaling 35% carbon dioxide induces an emotional and symptomatic state in humans closely resembling naturally occurring panic attacks, the core symptom of panic disorder. Previous research has suggested a role of the serotonin system in the individual sensitivity to carbon dioxide. In line with this, we previously showed that a variant in the SLC6A4 gene, encoding the serotonin transporter, moderates the fear response to carbon dioxide in humans. To study the etiological basis of carbon dioxide-reactivity and panic attacks in more detail, we recently established a translational mouse model.

AIM

The purpose of this study was to investigate whether decreased expression of the serotonin transporter affects the sensitivity to carbon dioxide.

METHODS

Based on our previous work, wildtype and serotonin transporter deficient (+/-, -/-) mice were monitored while being exposed to carbon dioxide-enriched air. In wildtype and serotonin transporter +/- mice, also cardio-respiration was assessed.

RESULTS

For most behavioral measures under air exposure, wildtype and serotonin transporter +/- mice did not differ, while serotonin transporter -/- mice showed more fear-related behavior. Carbon dioxide exposure evoked a marked increase in fear-related behaviors, independent of genotype, with the exception of time serotonin transporter -/- mice spent in the center zone of the modified open field test and freezing in the two-chamber test. On the physiological level, when inhaling carbon dioxide, the respiratory system was strongly activated and heart rate decreased independent of genotype.

CONCLUSION

Carbon dioxide is a robust fear-inducing stimulus. It evokes inhibitory behavioral responses such as decreased exploration and is associated with a clear respiratory profile independent of serotonin transporter genotype.

Panic-like escape response elicited in mice by exposure to CO2, but not hypoxia

Exposure to elevated concentrations of CO₂ or hypoxia has been widely used in psychiatric research as a panic provoking stimulus. However, the use of these respiratory challenges to model panic-like responses in experimental animals has been less straightforward. Little data is available, from behavioral and endocrine perspectives, to support the conclusion that a marked aversive situation, such as that experienced during panic attacks, was evoked in these animals. We here compared the behavioral responses of male CB57BL/6 mice during exposure to 20% CO₂ or 7% O₂ and its consequence on plasma levels of corticosterone. We also evaluated whether clinically-effective panicolytic drugs affect the behavioral responses expressed during CO₂ exposure. The results showed that whereas hypoxia caused a marked reduction in locomotion, inhalation of CO₂-enriched air evoked an active escape response, characterized by bouts of upward leaps directed to the border of the experimental cage, interpreted as escape attempts. Corticosterone levels were increased 30min after either of the respiratory challenges used, but it was higher in the hypoxia group. Chronic (21days), but not acute, treatment with fluoxetine or imipramine (5, 10 or 15mg/kg) or a single injection of alprazolam (0.025, 0.05 or 0.1mg/kg), but not of the anxiolytic diazepam (0.025, 0.05 or 0.1 and 1mg/kg) reduced the number of escape attempts, indicating a panicolytic-like effect. Altogether, the results suggest that whereas hypoxia increased anxiety, exposure to 20% CO₂ evoked a panic-like state. The latter condition/test protocol seems to be a simple and validated model for studying in mice pathophysiological mechanisms and the screening of novel drugs for panic disorder.

Effects of 35% carbon dioxide (CO2) inhalation in patients with post-traumatic stress disorder (PTSD): A double-blind, randomized, placebo-controlled, cross-over trial

BACKGROUND

In patients with post-traumatic stress disorder (PTSD) two open pilot studies about the effects of 35% carbon dioxide (CO₂) exist. One shows an augmented panicogenic and anxiogenic response (Muhtz et al., 2011), the other does not (Talesnik et al. 2007). We further characterized the CO₂ reactivity in PTSD using for the first time placebo-controlled and double-blind conditions.

METHODS

In 20 patients with PTSD we assessed panic, anxiety, dissociative and PTSD symptoms after a single vital capacity inhalation of 35% CO₂ compared to a placebo gas condition in a within-participant cross-over, placebo-controlled, double-blind and randomized design.

RESULTS

Inhalation of 35% CO₂ versus placebo provoked significantly increased panic, anxiety, dissociative and PTSD symptoms. The reaction to placebo gas was minimal. Order of inhalation, patients' sex or age did not influence the results. The panic and anxiety response under CO₂ was considerably higher in the PTSD patients than in healthy controls from our previous open study.

CONCLUSIONS

The results corroborate that our preceding findings of an increased CO₂ reactivity in patients with PTSD are not false positive due to the open design or the lack of placebo control. Replication in a larger number of PTSD patients and matched control subjects is needed. The potential role of childhood traumatisation, psychiatric comorbidity, psychotropic medication and trait dissociation in prior contradictory reports should be clarified.

Panic Disorder-A Malfunction of Multiple Transmitter Control Systems within the Midbrain Periaqueductal Gray Matter?

The clinical and psychopharmacological profile of panic disorder in human patients shows a remarkable similarity to the defensive behavioral response evoked in experimental animals by activation of neurons in the dorsal part of the midbrain periaqueductal gray matter (PAG). Studies of the neural circuitry within the PAG indicate that a subpopulation of GABAergic neurons in the dorsolateral sector may act as an amplifying stage that potentiates inhibitory serotonergic input to the "defense area" within the PAG. These neurons may function as a gain-control system that sets the level of excitability of efferent output neurons, which mediate the autonomic and somatomotor components of panic behavior. Dysfunctional activity within the dorsolateral PAG leading to a destabilization of this control system may be a factor underlying panic behavior and predisposes to the development of panic disorder in susceptible persons. NEUROSCIENTIST 6:48-59, 2000

Panic disorder and the respiratory system: clinical subtype and challenge tests

INTRODUCTION

Respiratory changes are associated with anxiety disorders, particularly panic disorder (PD). The stimulation of respiration in PD patients during panic attacks is well documented in the literature, and a number of abnormalities in respiration, such as enhanced CO2 sensitivity, have been detected in PD patients. Investigators hypothesized that there is a fundamental abnormality in the physiological mechanisms that control breathing in PD.

METHODS

The authors searched for articles regarding the connection between the respiratory system and PD, more specifically papers on respiratory challenges, respiratory subtype, and current mechanistic concepts.

CONCLUSIONS

Recent evidences support the presence of subclinical changes in respiration and other functions related to body homeostasis in PD patients. The fear network, comprising the hippocampus, medial prefrontal cortex, amygdala and its brainstem projections, may be abnormally sensitive in PD patients, and respiratory stimulants like CO2 may trigger panic attacks. Studies indicate that PD patients with dominant respiratory symptoms are particularly sensitive to respiratory tests compared to those who do not manifest dominant respiratory symptoms, representing a distinct subtype. The evidence of changes in several neurochemical systems might be the expression of the complex interaction among brain circuits.

Experimental panic provocation in healthy man-a translational role in anti-panic drug development?

Experimental neurochemical provocation of panic attacks in susceptible human subjects has considerably expanded our knowledge of the pathophysiology and psychopharmacology of panic disorder. Some panicogens also elicit short-lived panic-like states in healthy man. This offers the opportunity to assess the anti-panic action of drugs in proof-of-concept studies. However, from current data it is still unclear whether experimental panic in healthy man is a valid translational model. Most such studies in healthy volunteers have been performed using a cholecystokinin tetrapeptide (CCK-4) challenge. While CCK-4 panic was blocked by alprazolam pretreatment, escitalopram showed negative results in healthy man. Preliminary findings on novel investigational drugs and a few problematic results will be reviewed. Small sample sizes in many panic provocation studies, lack of dose-response aspects, and still-insufficient knowledge about the biological underpinning of experimental and spontaneous panic limit the interpretation of existing findings and should inspire further research.

Sensitivity and specificity of the Geriatric Anxiety Inventory and the Hospital Anxiety and Depression Scale in the detection of anxiety disorders in older people with chronic obstructive pulmonary disease

BACKGROUND

Anxiety and depression are prevalent in patients with chronic obstructive pulmonary disease (COPD). This study evaluates the sensitivity and specificity of two self-administered anxiety rating scales in older people with COPD. The Geriatric Anxiety Inventory (GAI) and the Hospital Anxiety and Depression Scale (HADS) are established useful screening tools but they have not been previously validated in this population.

METHODS

Older people with COPD completed the GAI and the HADS along with a structured diagnostic psychiatric interview, the Mini International Neuropsychiatric Interview (MINI). The outcomes of both rating scales were compared against the diagnosis of anxiety disorders based on the MINI. Receiver operating characteristic (ROC) curves were used to identify the optimal diagnostic cut points for each scale.

RESULTS

Fourteen (25.5%) of the 55 participants, were diagnosed with an anxiety disorder. Mean GAI and HADS-anxiety subscale scores were significantly higher in subjects with an anxiety disorder than those without the diagnosis (p = 0.002 and 0.005 respectively). Both scales demonstrated moderate diagnostic value (area under the ROC curve was 0.83 for GAI and 0.79 for HADS). Optimal cut points were ≥3 (GAI) and ≥4 (HADS-anxiety subscale). At these cut-points, the GAI had a sensitivity of 85.7%, specificity of 78.0% and the HADS had a sensitivity of 78.6%, specificity 70.7%.

CONCLUSION

Our results support the use of the GAI and HADS as screening instruments for anxiety disorders in older people with COPD. The optimal cut points in this population were lower than previously recommended for both rating scales. The results of this study should be replicated before these cut points can be recommended for general use in older people with COPD.

Effects of tryptophan depletion and tryptophan loading on the affective response to high-dose CO2 challenge in healthy volunteers

RATIONALE

It has been reported that in panic disorder (PD), tryptophan depletion enhances the vulnerability to experimentally induced panic, while the administration of serotonin precursors blunts the response to challenges.

OBJECTIVES

Using a high-dose carbon dioxide (CO(2)) challenge, we aimed to investigate the effects of acute tryptophan depletion (ATD) and acute tryptophan loading (ATL) on CO(2)-induced panic response in healthy volunteers.

METHODS

Eighteen healthy volunteers participated in a randomized, double-blind placebo-controlled study. Each subject received ATD, ATL, and a balanced condition (BAL) in separate days, and a double-breath 35% CO(2) inhalation 4.5 h after treatment. Tryptophan (Trp) manipulations were obtained adding 0 g (ATD), 1.21 g (BAL), and 5.15 g (ATL) of l-tryptophan to a protein mixture lacking Trp. Assessments consisted of a visual analogue scale for affect (VAAS) and panic symptom list. A separate analysis on a sample of 55 subjects with a separate-group design has also been performed to study the relationship between plasma amino acid levels and subjective response to CO(2).

RESULTS

CO(2)-induced subjective distress and breathlessness were significantly lower after ATD compared to BAL and ATL (p < 0.05). In the separate-group analysis, ΔVAAS scores were positively correlated to the ratio Trp:ΣLNAA after treatment (r = 0.39; p < 0.05).

CONCLUSIONS

The present results are in line with preclinical data indicating a role for the serotonergic system in promoting the aversive respiratory sensations to hypercapnic stimuli (Richerson, Nat Rev Neurosci 5(6):449-461, 2004). The differences observed in our study, compared to previous findings in PD patients, might depend on an altered serotonergic modulatory function in patients compared to healthy subjects.

Selective serotonin-reuptake inhibitors in the treatment of panic disorder: a systematic review of placebo-controlled studies

The selective serotonin-reuptake inhibitors are widely used in clinical practice in the treatment of panic disorder (PD). This article undertakes an up-to-date, systematic review of the published double-blind, placebo-controlled, randomized, short-term studies with currently available selective serotonin-reuptake inhibitors in the treatment of PD. Sertraline, paroxetine, citalopram, escitalopram, fluoxetine and fluvoxamine have all been proven to be superior to pill-placebo, although the placebo effect has been shown to be extremely important in patients with PD. The authors also explore the anxiolytic mechanism of action of this antidepressant drug class and the preclinical studies that are being developed to clarify the etiopathogenic mechanisms of PD and, more precisely, the role of the serotoninergic system in this pathogenesis. These steps are considered fundamental for the improvement of pharmacological treatment of PD.

Respiratory manifestations of panic disorder: causes, consequences and therapeutic implications

Multiple respiratory abnormalities can be found in anxiety disorders, especially in panic disorder (PD). Individuals with PD experience unexpected panic attacks, characterized by anxiety and fear, resulting in a number of autonomic and respiratory symptoms. Respiratory stimulation is a common event during panic attacks. The respiratory abnormality most often reported in PD patients is increased CO2 sensitivity, which has given rise to the hypothesis of fundamental abnormalities in the physiological mechanisms that control breathing in PD. There is evidence that PD patients with dominant respiratory symptoms are more sensitive to respiratory tests than are those who do not manifest such symptoms, and that the former group constitutes a distinct subtype. Patients with PD tend to hyperventilate and to panic in response to respiratory stimulants such as CO2, triggering the activation of a hypersensitive fear network. Although respiratory physiology seems to remain normal in these subjects, recent evidence supports the idea that they present subclinical abnormalities in respiration and in other functions related to body homeostasis. The fear network, composed of the hippocampus, the medial prefrontal cortex, the amygdala and its brain stem projections, might be oversensitive in PD patients. This theory might explain why medication and cognitive-behavioral therapy are both clearly effective. Our aim was to review the relationship between respiration and PD, addressing the respiratory subtype of PD and the hyperventilation syndrome, with a focus on respiratory challenge tests, as well as on the current mechanistic concepts and the pharmacological implications of this relationship.

Respiratory, autonomic, and experiential responses to repeated inhalations of 20% CO₂ enriched air in panic disorder, social phobia, and healthy controls

Inhalation of carbon dioxide (CO₂) enriched air triggers anxiety in panic disorder (PD) patients, which is often interpreted as a sign of biological vulnerability. However, most studies have not measured respiration in these tasks. We compared patients with PD (n=20) and social phobia (SP, n=19) to healthy controls (n=18) during eight inhalations of 20% CO₂, preceded and followed by two inhalations of room air, while continuously measuring subjective anxiety and dyspnea as well as autonomic and respiratory variables. PD patients showed increased reactivity and delayed recovery during CO₂ inhalations for most measures. Unlike both other groups, the PD group's tidal volume responses did not habituate across CO₂ inhalations. However, PD patients did not differ from SP patients on most other measures, supporting a continuum model of CO₂ sensitivity across anxiety disorders. Both patient groups showed continued reactivity during the last air inhalations, which is unlikely to be due to a biological sensitivity.

Effects of escitalopram on anxiety and respiratory responses to carbon dioxide inhalation in subjects at high risk for panic disorder: a placebo-controlled, crossover study

BACKGROUND

Individuals with panic disorder and those at high risk for panic disorder due to a positive family history seem to have elevated sensitivities to the panicogenic effects of carbon dioxide (CO2) inhalation as well as abnormal ventilatory responses to low doses of CO2 exposure. Many reports have described a reduction in anxiety responses in individuals with panic disorder after a brief treatment with a variety of medications effective in the treatment of panic disorder, but few of these have used placebo control.

METHODS

Thirty-two unmedicated individuals who had either a past history of panic attacks or a family history of treated panic disorder and who demonstrated, at baseline, a panic attack after either a single breath of 35% CO2 or 3 minutes of exposure to 5% CO2 or a decline in minute volume during 3 minutes of exposure to 5% CO2 were randomized to 2-week trials of escitalopram at 10 mg daily or placebo and, after a 1-week washout, to a 2-week trial of the alternate condition.

RESULTS

Placebo and escitalopram conditions did not differ by changes in anxiety responses to high-dose, single-breath, or low-dose continuous-breathing CO2 exposures. Ventilatory abnormalities were no more likely to resolve with escitalopram than with placebo. Self-ratings of anxiety and depressive symptoms showed little change over time with either escitalopram or placebo.

CONCLUSIONS

Escitalopram did not produce greater changes than placebo in panic responses or in ventilatory abnormalities seen during CO2 exposure. Future studies concerning the effects of antidepressants on responses to CO2 challenge should use a placebo-controlled design.

GABA-A receptors and the response to CO(2) inhalation - a translational trans-species model of anxiety?

The mechanisms by which the inhalation of carbon dioxide (CO(2)) produces anxiety and panic are not fully understood, although more recently there is evidence to suggest the involvement of a neural 'fear circuit'. We have suggested that this neural fear circuit is partly mediated by the brain noradrenaline network [Bailey, J.E., Argyropoulos, S.V., Lightman, S.L. and Nutt, D.J., (2003) Does the brain noradrenaline network mediate the effects of the CO(2) challenge? J Psychopharmacol 17(3): 252-259.]. However, we now review evidence that GABA-A may also play an important role in the modulation of CO(2)-induced anxiety. The review of this evidence starts with a key publication showing that 1 min of 35% CO(2)/65% air produced anxiogenic effects in a rat model of anxiety, to a similar extent to the anxiogenic betacarboline derivative FG7142, a benzodiazepine receptor inverse agonist. The effects of both anxiogenic stimuli were abolished with pre-treatment with alprazolam (0.5 mg/kg), but only those of FG7142, not CO(2), was blocked by a benzodiazepine antagonist [Cuccheddu, T., Floris, S., Serra, M., Porceddu, L., Sanna, E., Biggio, G., (1995) Proconflict effect of carbon dioxide inhalation in rats. Life Sci 56: PL 321-324.]. Although the evidence from this study did not conclusively prove that CO(2) had an action to reduce GABA function, it was an experiment designed to be translational to compare what was known about CO(2)-induced anxiety in patients, and to also to explore if GABA mechanisms are involved. Additional evidence from the literature is found in the association between GABA and chemoreceptors, both in laboratory and human studies and GABA and anxiety disorders. Evidence of this association is found across species from stress-induced change in GABA levels in plants and insects to humans, where there is now much evidence of abnormalities in GABA/benzodiazepine receptors in anxiety and other psychiatric disorders. This paper reviews some of the evidence and attempts to relate and compare these findings across species from the human to the Drosophila.

Carbon dioxide inhalation induces dose-dependent and age-related negative affectivity

Background

Carbon dioxide inhalation is known to induce an emotion similar to spontaneous panic in Panic Disorder patients. The affective response to carbon dioxide in healthy subjects was not clearly characterized yet.

Methodology/Principal Findings

Sixty-four healthy subjects underwent a double inhalation of four mixtures containing respectively 0, 9, 17.5 and 35% CO₂ in compressed air, following a double blind, cross-over, randomized design. Affective responses were assessed according to DSM IV criteria for panic, using an Electronic Visual Analogue Scale and the Panic Symptom List. It was demonstrated that carbon dioxide challenges induced a dose dependent negative affect (p<0.0001). This affect was semantically identical to the DSM IV definition of panic. Older individuals were subjectively less sensitive to Carbon Dioxide (p<0.05).

Conclusions/Significance

CO₂ induced affectivity may lay on a continuum with pathological panic attacks. Consistent with earlier suggestions that panic is a false biological alarm, the affective response to CO₂ may be part of a protective system triggered by suffocation and acute metabolic distress.

Anxiety responses to CO2 inhalation in subjects at high-risk for panic disorder

BACKGROUND

A number of reports have shown that patients with panic disorder have greater anxiety responses to the inhalation of enhanced carbon dioxide mixtures than do well controls or patients with other psychiatric illnesses. Three earlier studies have shown that well individuals who have first-degree relatives with panic disorder also experience more anxiety following CO(2) than do controls without such a family history. The following was undertaken to confirm and extend these findings.

METHODS

Well subjects at high risk for panic disorder (HR-P, n=132) had a first-degree family member with treated panic disorder but no personal history of panic attacks. Low-risk subjects (LR-C, n=85) had no such family history. All underwent a diagnostic interview with the SADS-LA and completed a battery of self-rating scales before undergoing two CO(2) challenges. One involved a single vital capacity breath of air and then of 35% CO(2) and the other 5 min of air and then 5 min of 5% CO(2).

RESULTS

In comparison to the LR-C group, HR-P subjects had higher scores on various self-ratings of anxiety and depression and were more likely to have a lifetime diagnosis of MDD or of an anxiety disorder. NEO neuroticism and a history of MDD were the most important of these measures in separating the high-risk and low-risk groups. As predicted, the HR-P subjects experienced more anxiety following 35% CO(2) exposure. The removal of individuals with lifetime diagnosis of MDD or of an anxiety disorder eliminated the relationship of neuroticism to CO(2)-induced anxiety and strengthened the relationship between the CO(2) response and a family history of panic disorder. Five minutes of 5% CO(2) produced much lower increases in anxiety than did the 35% exposure, but a dose by group interaction suggested that increasing exposure increased anxiety preferentially in the high-risk subjects.

CONCLUSION

The results confirm earlier findings in indicating that a family history of panic disorder conveys a liability to experience anxiety with CO(2) exposure. They also suggest that this anxiety may reflect several discrete diatheses of relevance to the heritability of panic disorder.

Serotonin function in panic disorder: important, but why?

The essential role of serotonin (5-hydroxytryptamine (5-HT)) system in the neurobiology and pharmacotherapy of panic disorder (PD) continues to be a topic of intensive interdisciplinary research. Interest in the involvement of 5-HT in PD has been fuelled by clinical studies demonstrating that medications increasing the synaptic availability of 5-HT, such as selective 5-HT re-uptake inhibitors, are effective in the treatment of PD. Rival theories of 5-HT deficiency vs excess have attempted to explain the impact of 5-HT function in PD. In the past decade, knowledge of the role of 5-HT in the neurobiology of PD has expanded dramatically due to much new research including experimental, treatment, brain-imaging, and genetic studies. The current review attempts to summarize the new data and their implications. The challenge and treatment studies generally confirm the specific inhibitory influence of 5-HT on panicogenesis. The brain-imaging studies in PD patients demonstrate functional and clinically relevant alterations in various elements of 5-HT system affecting the neurocircuitry of panic. The findings of genetic association studies suggest that certain 5-HT-related genes may contribute to the susceptibility to PD; however, these data are rather limited and inconsistent. It appears that, even if not the primary etiological factor in PD, the 5-HT function conveys important vulnerability, as well as adaptive factors. A better understanding of these processes may be critical in achieving progress in the treatment of patients suffering from PD.

Anxiety and medical disorders

Anxiety symptoms and disorders are associated with a range of general medical disorders. This association may be a physiologic consequence of the general medical disorder, a psychologic reaction to the experience of having a medical illness, a side effect of treatment, or a chance occurrence. This article briefly reviews the associations of panic disorder with seizure disorder, Klüver-Bucy syndrome, mitral valve prolapse, and respiratory disorders; of generalized anxiety disorder with chronic obstructive airway disease and cardiovascular and endocrine disorders; of social anxiety disorder with Parkinson's disease; of obsessive-compulsive disorder with striatal disorders; and of posttraumatic stress disorder with head injury and pain. Such associations provide important clues for understanding the neurobiology of anxiety disorders.

Challenge studies in anxiety disorders

In psychiatry, the use of pharmacological challenges in panic disorder is unique in that the clinical phenomenon of central interest (i.e., the panic attack) can be provoked readily and assessed in the clinical laboratory setting. During the past 20 years pharmacological challenge studies have increased our knowledge concerning the neurobiology of panic disorder remarkably and may ultimately result in novel and more causal treatment strategies. Moreover, the differences in sensitivity to certain panicogens such as serotonergic agents, lactate, carbon dioxide and cholecystokinin tetrapeptide are likely to be fruitful in serving as biological markers of subtypes of panic disorders and should be a major focus of research, as the identification of reliable endophenotypes is currently one of the major rate-limiting steps in psychiatric genetic studies.

The effects of tianeptine or paroxetine on 35% CO2 provoked panic in panic disorder

Antidepressants that inhibit the reuptake of serotonin (5-HT) are particularly effective in the treatment of panic disorder. Evidence suggests that increased 5-HT availability is important for the anti-panic effect of serotonergic drugs and in maintaining the response to selective serotonin reuptake inhibitors (SSRIs). Tianeptine is an antidepressant with 5-HT reuptake enhancing properties (i.e. the opposite pharmacological profile to that of SSRIs). Therefore, no effect would be expected in panic disorder. The aim of the present study was to compare the effect of tianeptine with that of paroxetine, a selective 5-HT reuptake inhibitor with demonstrated efficacy in panic disorder, on the vulnerability to a laboratory panic challenge in panic disorder patients. Twenty panic disorder patients were treated with either tianeptine or paroxetine for a period of 6 weeks, in a randomized, double-blind, separate group design. The reaction to a 35% CO(2) panic challenge was assessed at baseline and after treatment. Improvement on several clinical scales was also monitored. Tianeptine, as well as paroxetine, showed a significant reduction in vulnerability to the 35% CO(2) panic challenge. In spite of their opposite influence on 5-HT uptake, both tianeptine and paroxetine appeared to reduce the reaction to the panic challenge. These results raise questions about the necessity of 5-HT uptake for the therapeutic efficacy of anti-panic drugs.

Modulation of hyperreactivity to 35% CO2 after one week of treatment with paroxetine and reboxetine: a double-blind, randomized study

The effects of short treatments (7 days) with paroxetine and with reboxetine on the reactivity to inhalations of 35% carbon dioxide (CO2)/65% oxygen (O2) were compared in 28 patients with Panic Disorder who had positive responses to 35% CO2 inhalations. A double-blind, randomized design was applied. Each patient was given the 35% CO2 challenge on days 0 (before starting the treatment) and 7. Anxiety reactivity to CO2 decreased significantly with both drugs but the decrease was significantly stronger in the group treated with paroxetine. The rate of patients whose reactivity has reduced of at least 50% after 7 days was significantly higher in the group treated with paroxetine (10/14, 71.5%) than in the one treated with reboxetine (3/14, 21.5%). These results indicate that the modulation of the serotonergic system is more relevant for CO2 hyperreactivity than the modulation of the noradrenergic one.

The effects of anxious responding on mental arithmetic and lexical decision task performance

Anxiety-related responding and skill deficits historically are associated with performance-based problems such as mathematics anxiety, yet the relative contribution of these variables to substandard performance remains poorly understood. Utilizing a 7% carbon dioxide (CO2) gas to induce anxiety, the present study examined the impact of anxious responding on two performance tasks, mental arithmetic and lexical decision. Independent variables included math anxiety group, gender, and gas condition. Dependent variables included task performance and physiological and self-report indices of anxiety. A total of 64 university undergraduate students participated. Physiological and verbal-report measures of anxiety supported the utility of 7% carbon dioxide-enriched air as an anxiety-inducing stimulus. Behavioral disruption on performance tasks, however, did not differ as a function of carbon dioxide inhalation. Performance did differ as a function of math anxiety. High math anxious individuals generally exhibited higher error rates on mathematical tasks, particularly on tasks designed to measure advanced math skill and those requiring working memory resources. These findings are discussed with reference to processing efficiency theory, discordance among anxiety response systems, and the intricacies associated with skill measurement.

Symptom profiles of natural and laboratory panic attacks

BACKGROUND

Little accurate information is available about the symptomatology of real-life panic attacks and about how well they are reproduced by an experimental model such as the 35% CO2 challenge.

METHOD

Real-life panic symptoms were assessed in a group of 67 panic disorder patients, using daily life monitoring. Panic symptoms elicited by a 35% CO2 challenge were assessed in 61 panic disorder patients, and their frequency was compared with the real-life symptoms.

RESULTS

The most frequent real-life symptoms were palpitations, dizziness and trembling. The 35% CO2 challenge reproduced well the majority of real-life symptoms.

CONCLUSION

The findings suggest that the 35% CO2 challenge is a marker for spontaneous panic attacks, which are considered the core of panic disorder.

Comparison of 35% carbon dioxide reactivity between panic disorder and eating disorder

Patients with panic disorder (PD) are hyperreactive to carbon dioxide (CO(2)), but the specificity of this characteristic to PD is controversial. Anxiety and phobic symptomatology are common to both panic and eating disorders (ED). To investigate the specificity of CO(2) hyperreactivity to PD, the responses to inhalation of a 35% CO(2) and 65% oxygen (O(2)) gas mixture were assessed. Reactions to 35% CO(2) challenge were compared among three groups of age- and sex-matched subjects: 14 patients with ED, 14 patients with PD, and 14 healthy controls (HC). A double-blind, randomized, crossover design was used. Only patients with PD showed a strong reaction to 35% CO(2), while patients with ED and HC did not react significantly. The results support the specificity of CO(2) hyperreactivity to PD.

Does the brain noradrenaline network mediate the effects of the CO2 challenge?

The inhalation of carbon dioxide (CO2) is commonly used in patients and volunteers as a means of producing anxiety or panic. It is generally believed that patients with panic disorder are more vulnerable to the effects of CO2 than patients with other anxiety disorders or healthy volunteers and there is speculation and debate as to the mechanism for this apparent sensitivity. Recent work from our group has shown that a single inhalation of 35% CO2 activates the hypothalamic-pituitary-adrenocortical (HPA) axis, increases blood pressure (BP) and increases subjective fear responses in healthy volunteers. Correlation analyses reveal a relationship between the changes in BP and the cortisol increase. These findings led us to postulate that a common mechanism may mediate these and the subjective responses to inhalation of CO2. We propose that the noradrenergic system, particularly the locus coeruleus (LC), but including the A1 and A2 cell groups, may be a key mediator of these responses. This article examines the evidence and discusses the results of studies from our laboratory in relation to a neuroanatomical model centring on the LC.

Carbon dioxide test as an additional clinical measure of treatment response in panic disorder

OBJECTIVE

We aim to determine if a treatment with a dose of clonazepam--2 mg/day, for 6 weeks, blocks spontaneous panic attacks and the ones induced by the inhalation of 35% carbon dioxide (CO2) in panic disorder (PD) patients. The CO2 challenge-test may be a useful addition tool for measuring the pharmacological response during the initial phase (6 weeks) in the treatment of PD.

METHOD

Eighteen PD patients drug free for a week participated in a carbon dioxide challenge test. Fourteen had a panic attack and were openly treated for a 6-week period with clonazepam. At the end of the 6-week period they were submitted again to the CO2 challenge test.

RESULTS

After 6 weeks of treatment with clonazepam, 12 of 14 PD patients (85.7%) did not have a panic attack after the CO2 challenge test. Just 2 of 14 patients (14.3%) had a panic attack after the CO2 challenge test. Ten of 14 (71.4%) PD patients had panic free status after clonazepam treatment. The 2 patients who had a panic attack in the sixth week, after the CO2 test, did not have panic free status after the treatment with clonazepam.

CONCLUSION

The CO2-test may be a valid tool for testing and predicting the drug response.

Antipanic drug modulation of 35% CO2 hyperreactivity and short-term treatment outcome

Carbon dioxide (CO2) inhalation induces acute anxiety and panic attacks in patients with Panic Disorder (PD). Anti-panic drugs decrease CO2 reactivity after the first days of treatment; however, the clinical meaning of this finding has not yet been established. This study investigated the effects of treatment with tricyclic antidepressants and selective serotonin re-uptake inhibitors (SSRIs) on CO2 reactivity and compared the relationships between 35% CO2 hyperreactivity modulation and short-term clinical outcome. One hundred twenty-three patients with PD with or without agoraphobia who were hyperreactive to CO2 were randomly assigned to treatment groups with imipramine, clomipramine, paroxetine, sertraline, or fluvoxamine. A double-blind, randomized design was applied. Each patient received the 35% CO2 challenge on days 0, 7, and 30. The severity of clinical symptomatology was measured on days 0 and 30. Decreased hyperreactivity to 35% CO2 in all five treatment groups was already evident after the first week. The decrease in CO2 reactivity at the end of treatment was proportional to the degree of clinical improvement. Multiple regression analyses showed that the decrease in CO2 reactivity after the first week was a significant predictor for good clinical outcome after one month. The results of this study confirm evidence that psychoactive drugs effective in the treatment of PD decrease CO2 hyperreactivity. They also suggest that precocious modulation of CO2 reactivity might fairly reliably predict short-term clinical outcome in patients with "respiratory" PD.

The 35% CO2 hyperreactivity and clinical symptomatology in patients with panic disorder after 1 week of treatment with citalopram: an open study

The effect of a short treatment (7 days) with citalopram on the reactivity to inhalations of 35% CO2 and 65% oxygen and on clinical symptomatology was investigated in 15 patients with panic disorder who had a positive response to 35% CO2 inhalation. An open study design was applied. On day 0, before starting drug treatment, and after 1 week of treatment, each patient underwent the 35% CO2 challenge, and clinical symptomatology was evaluated with psychometric scales. The results showed a significant reduction of CO2 reactivity and of scores on the anticipatory anxiety subscale of Panic Associated Symptoms Scale. These results confirm that the serotonergic system plays an important role in the modulation of CO2 hyperreactivity and suggest an early anxiolytic effect of citalopram in patients who have panic disorder and are hyperreactive to CO2.

Ataques de pânico provocados pelo dióxido de carbono: estudo clínico-fenomenológico

OBJETIVOS: Verificar a sensibilidade de pacientes com transtorno de pânico (TP) ao teste de indução de ataques de pânico com dióxido de carbono (CO2) a 35% e analisar a intensidade, a duração e a sintomatologia dos ataques de pânico produzidos por esse agente em laboratório, comparando-os com os ataques de pânico espontâneos nesses pacientes. MÉTODOS: Foram selecionados 31 pacientes com TP com ou sem agorafobia (DSM-IV). Após uma semana sem medicação, os pacientes realizavam duas inalações de capacidade vital: uma de mistura carbogênica (CO2 35% e O2 65%) e outra de ar atmosférico comprimido ("placebo"), ordenadas ao acaso e separadas por um intervalo de 20 minutos. Essas inalações eram repetidas após duas semanas. Nesse período, os pacientes não recebiam nenhuma medicação psicotrópica. RESULTADOS: Dos pacientes, 22 (71,0%) apresentaram ataque de pânico em pelo menos um dos testes com CO2. Os sintomas relatados por eles com maior freqüência foram: dificuldade de respirar (n=20, 91,0%), sensação de sufocação/asfixia (n=18, 81,8%), tontura (n=18, 81,8%), estremecimento (n=14, 63,6%), palpitações (n=13, 59,0%) e medo de enlouquecer (n=12, 54,5%). Desse grupo, 11 pacientes (50,0%) consideraram os ataques de pânico experimentados no laboratório mais intensos, comparados aos ataques de pânico espontâneos, quatro (18,2%) consideraram haver a mesma intensidade entre os dois, e sete (31,8%) consideraram o ataque de pânico no laboratório mais leve. CONCLUSÃO: Pacientes com TP têm elevada sensibilidade ao CO2. A inalação de mistura gasosa com 35% de CO2 produz sintomas semelhantes aos ataques de pânico espontâneos, em pacientes com TP. Esse teste pode ser considerado um bom modelo laboratorial para o TP.

Double-blind acute clonazepam vs. placebo in carbon dioxide-induced panic attacks

The inhalation of 35% carbon dioxide has consistently been shown to provoke panic attacks in panic disorder patients. We aim to determine if an acute dose of clonazepam (2 mg) attenuates the panic attacks induced by an inhalation of 35% carbon dioxide in panic disorder. Twenty-two panic disorder patients who had been drug-free for 1 week participated in a carbon dioxide challenge test 1 h after a dose of either 2 mg of clonazepam or placebo with a randomized double-blind method. Also in a double-blind design during the tests the patients inhaled either atmospheric compressed air ('placebo control') or the carbon dioxide mixture. All patients participated in both tests which were done with a 20-min interval. Immediately before and after the inhalation, the anxiety levels and the symptoms of panic were always assessed. In the clonazepam group (n=11) two patients (18.2%) had a mild panic attack and in the placebo group (n=11) nine patients (81.8%) had a moderate to severe panic attack in the CO(2) challenge test. No patient had panic attacks during inhalation of atmospheric compressed air although anticipatory anxiety levels tended to be higher than in the CO(2) tests. After the CO(2) test anxiety levels were significantly greater in the CO(2) group (three-way ANOVA with Geisser-Greenhouse adjustments, F(31.92,1.86)=17.15, d.f.=7, P=0.013). Although a small sample was studied, the findings suggest the efficacy of an acute dose of clonazepam in attenuating panic attacks induced by carbon dioxide inhalation.

Effect of acute tryptophan depletion on CO2-induced anxiety in patients with panic disorder and normal volunteers

BACKGROUND

Uncertainties remain about the role of serotonin in the aetiology and treatment of panic disorder.

AIMS

To investigate the effect of reducing brain serotonin function on anxiety at rest, and following 5% CO2 provocation in normal controls and patients with panic disorder.

METHOD

Twenty drug-free patients with DSM-III-R panic disorder and 19 controls received a tryptophan-free amino acid drink on one occasion and a control drink on the other in a double-blind, balanced protocol. 5% CO2 was given as a panic challenge after 270 minutes.

RESULTS

Plasma tryptophan fell by more than 80% both patients and controls after the tryptophan-free drink. Tryptophan depletion did not alter resting anxiety. In patients alone, tryptophan depletion caused a greater anxiogenic response and an increased rate of panic attacks (9 v. 2, P < 0.05) after 5% CO2 challenge. No normal volunteers panicked.

CONCLUSIONS

Serotonin may directly modulate panic anxiety in patients with panic disorder. This may underlie the efficacy of serotonergic antidepressants in treating panic disorder.

The 35% CO2 inhalation procedure: test-retest reliability

The subjective response to a single-breath, 35% carbon dioxide challenge test shows promise as a tool for the study of panic disorder and may comprise a trait marker for that disorder. Little has been done to measure the reliability of test results, however. Subjects took a single breath at 35% CO2 and completed a self-rating of anxiety symptoms immediately thereafter. This procedure was repeated after a mean interval of 29 days. One group, considered at high risk for panic disorder, consisted of well, first-degree relatives of individuals treated for panic disorder. The control group included well subjects at high risk for affective disorder and subjects who had family histories negative for both affective disorder and panic disorder. On both testing occasions, subjects at high risk for panic disorder had symptom scores that were significantly higher than those of control subjects. Group differences in the portions who experienced a panic attack were dependent on the symptom threshold used to define an attack. A lower threshold was optimal with the second testing and a single, positive test result appeared to be more meaningful than a single negative result. The majority of individual symptom ratings were highly correlated across tests. Ratings for "smothering sensations," in particular, correlated highly across tests and consistently discriminated high-risk from control subjects. The sources of test result variability are unclear and warrant more investigation before the tests can be clinically useful. Research efforts should seek optimal thresholds to define positive test results within given data sets.

Experimental pathophysiology of panic

In this article, we review how the knowledge of the pathophysiology of panic disorder has expanded, with special emphasis on laboratory models using lactate and carbon dioxide challenges. Experiments in the late 1960s revealed that lactate infusion can induce panic attacks. A prominent feature of these attacks is hyperventilation. Because lactate infusion induces a metabolic alkalosis, one would rather expect a compensatory hypoventilation. For years hyperventilation was thought to be causally linked to panic, but it has since been proven to be a symptom rather than a cause of panic attacks. Similarly, it is not hypocapnia but hypercapnia that has proven to be capable of provoking panic attacks. Carbon dioxide challenges are comparable to lactate infusion in the degree to which they meet the criteria for an ideal model of panic disorder. Experiments with carbon dioxide in first-degree relatives of panic disorder patients and in monozygotic twins support the idea of a constitutional predisposition to panic disorder. Of the various other agents that have been used to trigger panic attacks, cholecystokinin seems particularly promising as a valid laboratory model of panic disorder and may provide valuable data regarding the mechanism of panic attacks. The false suffocation alarm theory, proposed by Klein, is an integrative hypothesis that may account for a large number of the laboratory as well as clinical observations.

Comorbid depressive disorder increases vulnerability to the 35% carbon dioxide (CO2) challenge in panic disorder patients

BACKGROUND

The hypothesis of this study was that panic disorder patients with a comorbid depressive disorder would be less vulnerable to the 35% CO2 panic provocation challenge than panic disorder patients without a comorbid depressive disorder. This hypothesis was based on findings from ventilatory response studies in depressive patients.

METHODS

Twelve panic disorder patients with and 23 panic disorder patients without a comorbid depressive disorder were challenged.

RESULTS

Panic disorder patients with a comorbid depressive disorder scored significantly higher on ratings of subjective anxiety and panic symptoms induced by the challenge.

CONCLUSIONS

A comorbid depressive disorder appeared to increase the vulnerability of panic disorder patients to this panic provocation.

LIMITATION

We did find significant differences, but these differences did not confirm the original hypothesis.

CLINICAL RELEVANCE

Our results support clinical data that show that a comorbid depressive disorder correlates with an increased severity of panic disorder.

Reliability of the 35% carbon dioxide panic provocation challenge

The objective of this study was to determine the test-retest reliability of the 35% carbon dioxide (CO2) panic provocation challenge. Thirty patients with panic disorder were included in this study. Twenty-four patients were challenged twice, with 1 week between the two challenges. Six patients dropped out after the first test. The 35% CO2 challenge appeared to have a good test-retest reliability; both on induced subjective anxiety, measured on a Visual Analogue Scale for Anxiety (VAS-A), and induced panic symptoms, measured with a Panic Symptom List. Assessing the state of anxiety immediately after the challenge gave the most reliable results. Calculating increase in anxiety from the pre- and post-scores on the VAS-A rendered less reliable scores. This study completes a series of studies in which the criteria for an ideal model of panic are tested for the 35% CO2 challenge. Apart from an absolute specificity for panic disorder, the challenge meets these criteria.

Modification of 35% carbon dioxide hypersensitivity across one week of treatment with clomipramine and fluvoxamine: a double-blind, randomized, placebo-controlled study

The effects of short treatments (7 days) with clomipramine and fluvoxamine on the reactivity to inhalations of 35% CO2/65% O2 were compared in 39 panic patients who had positive responses to 35% CO2 inhalations. A double-blind, randomized, placebo-controlled design was applied. Each patient was given the 35% CO2 challenge on days 0 (before starting treatment), 3, and 7. Patients on placebo did not report any significant changes in their reactivity to 35% CO2 across the three sessions, whereas patients on clomipramine and fluvoxamine reported a significant attenuation of the reactivity on day 7. These results indicate that treatments with clomipramine and fluvoxamine decrease hypersensitivity to 35% CO2 after a few days, suggesting a relevant role of the modulation of CO2 sensitivity by the serotonergic system in antipanic properties of these compounds.