Acid-base dysregulation and chemosensory mechanisms in panic disorder: a translational update

Panic disorder (PD), a complex anxiety disorder characterized by recurrent panic attacks, represents a poorly understood psychiatric condition which is associated with significant morbidity and an increased risk of suicide attempts and completed suicide. Recently however, neuroimaging and panic provocation challenge studies have provided insights into the pathoetiology of panic phenomena and have begun to elucidate potential neural mechanisms that may underlie panic attacks. In this regard, accumulating evidence suggests that acidosis may be a contributing factor in induction of panic. Challenge studies in patients with PD reveal that panic attacks may be reliably provoked by agents that lead to acid-base dysbalance such as CO2 inhalation and sodium lactate infusion. Chemosensory mechanisms that translate pH into panic-relevant fear, autonomic, and respiratory responses are therefore of high relevance to the understanding of panic pathophysiology. Herein, we provide a current update on clinical and preclinical studies supporting how acid-base imbalance and diverse chemosensory mechanisms may be associated with PD and discuss future implications of these findings.

Failure of glucagon-like peptide-1 to induce panic attacks or anxiety in patients with panic disorder

The insulin secretogogue glucagon like peptide-1 (GLP-1), as well as agents which enhance GLP-1 signaling, are being studied as potential treatments for diabetes. Pre-clinical evidence suggests that these agents may have neuropsychiatric side effects; however, there have been no investigations or reports of these effects in humans. We evaluated possible anxiogenic and panicogenic properties of GLP-1 in 9 healthy subjects (age 47+/-8 years) and 7 patients with panic disorder (age 38+/-17 years) using a single-blinded intravenous GLP-1 challenge (2pmol/kg/min over 60min). We assessed the occurrence of panic attacks during and after GLP-1 infusion and the emergence of anxiety or panic symptoms using the Acute Panic Inventory (API). No patient or healthy subject experienced any panic attacks at any point during this study. Moreover, there were no significant changes in API scores following the infusion in either group. These data suggest that in humans, intraveneously administered GLP-1 does not appear to have anxiogenic or panicogenic properties, even in patients at highest risk for such reactions.

Noradrenergic dysfunction and the psychopharmacology of posttraumatic stress disorder

The catecholamine norepinephrine is a critical effector of the mammalian stress response and has been implicated in the pathophysiology of posttraumatic stress disorder (PTSD)-a syndrome intrinsically related to the experience of extraordinary stress. Symptom-linked hypernoradrenergic derangements have been observed in PTSD and several studies have examined the potential therapeutic effects of agents that dampen the centrally hyperactive noradrenergic state. These agents include compounds that decrease norepinephrine release (e.g. centrally acting alpha(2) agonists such as clonidine) and those which block post-synaptic norepinephrine receptors (e.g. centrally acting alpha(1) or beta receptor antagonists such as prazosin or propranolol). In this article, we review studies of central noreadrenergic hyperactivity under both basal and challenge conditions and explore the evidence for these derangements as potential psychopharmacologic targets in patients with PTSD. Given the significant involvement of CNS norepinephrine hyperactivity in PTSD, and its link to intrusive and hyperarousal symptoms, it is not surprising that interventions directed at this system have therapeutic potential in PTSD. The utility of these anti-adrenergics in the clinical treatment of PTSD remains to be determined, though it is possible that they may prove to have primary roles in a disorder that is only modestly responsive to antidepressant treatment.

Pituitary-thyroid state correlates with central dopaminergic and serotonergic activity in healthy humans

Data from lower animals suggest anatomic and physiological interactions between brain dopamine and serotonin (5-hydroxytryptamine, 5-HT) systems and the hypothalamic-pituitary-thyroid axis. However, in humans, investigations of interactions between these central neurochemical systems (especially the dopaminergic system) and thyroid function are rare; in healthy humans they are practically nonexistent. Using cerebrospinal fluid (CSF) and blood samples simultaneously obtained from indwelling subarachnoid and venous catheters in healthy humans, we determined the CSF concentrations of homovanillic acid (HVA) and 5-hydroxyindolacetic acid, the major metabolites of dopamine and 5-HT, and plasma concentrations of TSH, total triiodothyronine (T(3)), free T(3), total thyroxine (T(4)) and free T(4). CSF HVA concentrations were significantly and negatively correlated with plasma TSH and T(3) (free and total), but not with T(4) (free or total). CSF 5-HIAA concentrations were significantly and negatively correlated with plasma TSH and total T(3) but not with free T(3) or T(4) (free or total). These results indicate that CNS monoamine-thyroid interactions are of physiological significance in the normal, euthyroid human.

Intra- and inter-individual relationships between central and peripheral serotonergic activity in humans: a serial cerebrospinal fluid sampling study

Data are lacking concerning the longitudinal covariability and cross-sectional balance between central and peripheral 5-HIAA concentrations in humans and on the possible associations between tobacco smoking or post-traumatic stress disorder (PTSD) and CSF and plasma 5-HIAA concentrations. Using serial cerebrospinal fluid (CSF) and blood sampling, we determined the concentrations of 5-HIAA in CSF and plasma over 6 h, and examined their relationships in healthy volunteers and patients with PTSD-both smokers and nonsmokers. Patients with PTSD and healthy volunteers had very similar CSF 5-HIAA concentrations. Significant and positive correlations between CSF and plasma 5-HIAA levels were observed within individuals, but this CNS-peripheral 5-HIAA relationship was significantly reduced in smokers (nonsmokers: mean r = 0.559 +/- 0.072; smokers: mean r = 0.329 +/- 0.064 p < 0.038). No significant cross-sectional, interindividual correlation of mean CSF and mean plasma 5-HIAA was seen (r = 0.094). These data show that changes in CSF 5-HIAA levels within an individual over time are largely reflected in plasma 5-HIAA, albeit significantly less so in smokers. The present results therefore suggest that clinically, longitudinal determination of plasma 5-HIAA concentrations within an individual patient can be used to make inferences about relative changes in integrated CSF 5-HIAA concentrations. However, plasma 5-HIAA concentrations provide no significant information about absolute levels of the serotonin metabolite in the CSF.

In-use stability of monoamine metabolites in human cerebrospinal fluid

Cerebrospinal fluid (CSF) concentrations of the monoamine metabolites homovanillic acid (HVA) and 5-hydroxyindolacetic acid (5-HIAA) are commonly used to provide information about central nervous system (CNS) dopaminergic and serotonergic activity. However, little attention has been given to the effects of sample handling on the concentrations of these compounds in human CSF. Using high-performance liquid chromatography (HPLC) with electrochemical detection, we observed that, in CSF stored at -80 degrees C, concentrations of the serotonin metabolite 5-HIAA and the dopamine metabolite HVA remained unchanged through six 1-h and six 24-h freeze-thaw cycles. Exposure to bright room light (3 h, 1,230 lux) resulted in a 5-HIAA concentration that was 96.3 +/- 2.0% of the initial and an HVA concentration that was 98.8 +/- 1.03% of initial. The pH of the CSF significantly increased during both freeze-thaw series and while maintained on ice (4 degrees C). These results demonstrate the in-use stability of 5-HIAA and HVA in human CSF under commonly-encountered laboratory conditions.

The effect of feeding on cerebrospinal fluid corticotropin-releasing hormone levels in humans

Corticotropin-releasing hormone (CRH) is a neuropeptide thought to play a role in appetite regulation. In this report, we used a serial cerebrospinal fluid (CSF) sampling technique to examine the relationship between CSF CRH, plasma ACTH and cortisol and perceptions of hunger and satiety in fasting and sated volunteers. CSF was withdrawn continuously from 11:00 AM to 5:00 PM via an indwelling subarachnoid catheter. Blood was withdrawn every 10 min via an antecubital vein catheter. Fed subjects received a meal at 1:00 PM. Subjects who were fed had lower post-prandial ratings on hunger scales and higher ratings on satiety scales. Fed subjects also had slightly lower levels of CSF CRH after feeding. Furthermore, fed subjects had higher ACTH and cortisol concentrations in the first 3 h; by the fourth h the opposite was true. Our findings do not support the hypothesis that CNS CRH is a central satiety factor in the human. Instead our findings of slightly diminished CSF CRH levels after feeding may be accounted for by the rises in glucocorticoids and their associated negative feedback effects on CNS CRH. Alternatively, our findings could also reflect changes in CRH levels associated with feeding in multiple brain areas and in the spinal cord with the net effect being in the negative direction.

The effects of 5-HT on sensory, central and motor neurons driving the abdominal superficial flexor muscles in the crayfish

Serotonin (5-HT) induces a variety of physiological and behavioral effects in crustaceans. However, the mechanisms employed by 5-HT to effect behavioral changes are not fully understood. Among the mechanisms by which these changes might occur are alterations in synaptic drive and efficacy of sensory, interneurons and motor neurons, as well as direct effects on muscles. We investigated these aspects with the use of a defined sensory-motor system, which is entirely contained within a single abdominal segment and consists of a 'cuticular sensory neurons segmental ganglia abdominal superficial flexor motor neurons-muscles' circuit. Our studies address the role of 5-HT in altering (1) the activity of motor neurons induced by sensory stimulation; (2) the inherent excitability of superficial flexor motor neurons; (3) transmitter release properties of the motor nerve terminal and (4) input resistance of the muscle. Using en passant recordings from the motor nerve, with and without sensory stimulation, and intracellular recordings from the muscle, we show that 5-HT enhances sensory drive and output from the ventral nerve cord resulting in an increase in the firing frequency of the motor neurons. Also, 5-HT increases transmitter release at the neuromuscular junction, and alters input resistance of the muscle fibers.