Platelet [3H]paroxetine binding in panic disorder

Platelet studies in panic disorder: A review

Objective: The blood platelet has been proposed as a model of central neurons and may therefore be used as a peripheral marker of psychiatric illness. One method of investigating serotonin function in panic disorder has relied on the use of the platelet as a model of serotonergic neurons. This article reviews the studies of platelet function in panic disorder.

Method: A literature search and review of relevant papers was undertaken.

Result: Studies examining platelet serotonin uptake and concentration in panic disorder patients have to date yielded conflicting results, with some investigators reporting increased serotonin uptake, others reduced uptake. Similarly studies of platelet 3H-imipramine binding have also yielded conflicting results. Two studies of platelet 3H-paroxetine binding have shown a reduction in the density of binding sites (Bmax) in patients with panic disorder. Platelet monoamine oxidase activity in anxiety disorders has been reported to be increased by some investigators but decreased by others. Methodological considerations may have been responsible for these differences. Finally, studies of α2 adrenoceptor density have also produced contrasting findings.

Conclusion: The findings of these studies indicate that platelet function is altered in panic disorder. Such changes may allow an insight into the biochemical aetiology of the illness. Further studies are required to delineate the role of serotonin and non-adrenaline in panic disorder.

Platelet serotonin transporter function after short-term paroxetine treatment in patients with panic disorder

Dysfunctions in serotonin neurotransmission have been implicated in some psychiatric disorders, and in particular, altered serotonin transporter function has been noted in panic disorder. In this study, the authors compared platelet [(3)H]serotonin uptake parameters, including maximum velocities (V(max)) and affinity constants (K(m)), in patients with panic disorder not undergoing treatment (n=21) and in healthy subjects (n=20). V(max) and K(m) values were re-examined after 12 weeks of paroxetine treatment. Values of V(max) and K(m) were lower in panic disorder patients at baseline than in healthy subjects. After treatment, K(m) normalized in panic patients, whereas V(max) did not change. A significant inverse correlation was found between increased K(m) and changes in anxiety levels. These results support a hypothesis of serotonergic transporter abnormalities in panic disorder, and suggest that increased K(m) values of platelet serotonin transporters parallel clinical improvement after short-term pharmacotherapy in panic disorder.

Reduced brain serotonin transporter binding in patients with panic disorder

There is strong evidence for the importance of the serotonin (5-HT) system in the neurobiology of panic disorder (PD); however, the exact role of this system remains unclear. The 5-HT transporter (5-HTT) is a key element in 5-HT neurotransmission. The current study aimed to investigate the binding of 5-HTT in the brain of patients with PD. We used single-photon emission computed tomography with a radioligand that specifically labels the 5-HTT, [(123)I]nor-beta-CIT. Subjects comprised eight patients with current PD, eight patients with PD in remission, and eight healthy control subjects. The patients with current PD showed a significant decrease in 5-HTT binding in the midbrain, in the temporal lobes and in the thalamus in comparison to the controls. The binding of 5-HTT in patients with PD in remission was similar to findings in the control group in the midbrain and in the temporal lobes, but lower in the thalamus. Regional 5-HTT binding significantly and negatively correlated with the severity of panic symptoms. These findings point to a dysregulation of the 5-HT system in PD patients. Altered function of 5-HTT appears to be related to the clinical status of patients. Clinical improvement in the patients in remission is associated with normalization of 5-HTT binding.

Serotonin transporter gene polymorphisms and platelet [3H] paroxetine binding in premenstrual dysphoria

The purpose of this study was to investigate if women with premenstrual dysphoria differ from controls with respect to the number of platelet serotonin transporters, and with respect to three polymorphisms in the gene coding for the serotonin transporter: a 44 base pair insertion/deletion in the promoter region, a variable number of tandem repeats in the second intron, and a single nucleotide polymorphism in the 3' untranslated region. Also, the possible relationship between the three polymorphisms and platelet serotonin transporter density was analyzed. The density of platelet [(3)H]paroxetine binding sites was significantly lower in women with premenstrual dysphoria than in controls, but patients and controls did not differ with respect to allele or genotype frequency for any of the three polymorphisms examined. A significant association between the number of platelet serotonin transporters and the promoter polymorphism was observed, subjects being homozygous for the short (deletion) variant having higher platelet serotonin transporter density than subjects carrying the long (insertion) allele. The results support the assumption that serotonin-related psychiatric disorders-such as premenstrual dysphoria-may be associated with a reduction in platelet [(3)H]paroxetine binding, but argue against the notion that this reduction is due to certain variants of the serotonin transporter gene being more common in patients than in controls.

Differences in platelet serotonin transporter sites between African-American tobacco smokers and non-smokers

RATIONALE

The serotonin transporter (5HTT) regulates the magnitude and duration of serotonergic neurotransmission. Although nicotine and other constituents of tobacco smoke may influence serotonin turnover among animals, few studies have examined whether smoking is associated with alteration in 5HTT in humans.

OBJECTIVE

We investigated whether tobacco smokers and non-smokers differed in platelet tritiated paroxetine binding, a measure of 5HTT sites, and whether severity of nicotine dependence (ND) was related to 5HTT measures.

METHODS

Tritiated paroxetine binding sites on platelets were assayed in 26 African-American smokers and 30 non-smokers. Severity of smoking was assessed using the Fagerstrom Test for Nicotine Dependence (FTND). Relationships between FTND scores and maximum number of transporter sites (B(max)) and affinity constant (K(d)) of paroxetine binding were determined.

RESULTS

B(max) values showed a significant negative correlation with FTND scores (rho=-0.28, P<0.01). Notably, smokers with higher ND had significantly lower B(max) compared to those with lower ND and non-smokers; the latter two groups did not differ in B(max) ( F=3.92, P<0.05). Smokers scored higher on impulsivity than non-smokers, however, behavioral variables did not influence the relationship of smoking with B(max). Age, gender and K(d) values were not associated with smoking or B(max).

CONCLUSIONS

Smoking, in particular higher nicotine dependence, appears to be correlated with decreased density of platelet 5HTT sites in African-Americans. The nature of the relationship and whether similar changes occur in the brain merit further investigation.

Decreased platelet 3H-paroxetine binding in untreated panic disorder patients

Different findings support the involvement of the serotonin (5-HT) system in panic disorder. The presence of the 5-HT transporter in blood platelets similar to that in presynaptic serotonergic neurons, permits the investigation of this structure in periphery. We therefore evaluated the binding of 3H-paroxetine, a selective 5-HT reuptake inhibitor which is considered the ligand of choice for labelling the 5-HT transporter, in platelets of 20 drug-free patients with panic disorder. The same measurement was repeated after one year's treatment with different drugs. The results showed patients to have a lower number of 3H-paroxetine sites than a group of age- and sex-matched controls, thus suggesting the involvement of the 5-HT transporter in panic disorder. This abnormality reverted after one year of treatment with specific drugs that provoked the symptom remission in all cases, which would suggest a link with the clinical improvement.

Regulation of the platelet serotonin transporter by protein kinase C in the young and elderly

BACKGROUND

Some data show that different factors may influence the serotonin (5-HT) uptake rate. Our study aimed at evaluating the possible role of a protein kinase C (PKC) activator, i.e., 4-beta-12-tetradecanoylphorbol-13-acetate (beta-TPA) on the platelet 5-HT uptake of young and elderly subjects, through the measurement of the 5-HT uptake itself and 3H-paroxetine ([3H]PAR) binding sites, which correspond to the transporter protein.

METHODS

Human platelets and 5-HT uptake were evaluated according to the method of Arora and Meltzer, while [3H]PAR binding was performed following the Marazziti et al method.

RESULTS

The results showed that beta-TPA reduced significantly the maximal velocity (Vmax) of 5-HT uptake, with no change in the Michaelis constant or in [3H]PAR binding parameters, in platelets of both young and elderly subjects. Although this last group of subjects had a significantly lower Vmax than the other, the degree of inhibition was almost the same (75%) in both.

CONCLUSIONS

These findings indicate that PKC decreases the 5-HT uptake rate by modifying the phosphorylation state of the transporter and with no change in the number of [3H]PAR binding sites. The responsiveness of this pathway is identical in both young and elderly subjects.

Cardiovascular, neuroendocrine, and monoaminergic responses to psychological stressors: possible differences between remitted panic disorder patients and healthy controls

Both clinical symptomatology and stress research suggest that panic attacks might be partially attributable to exaggerated psychophysiological responses to environmental stressors. In the present study, we aimed to explicitly test this idea by measuring the physiological responses to a mild psychological stressor in both healthy controls (n = 8) and fully remitted, medication-free panic disorder patients (n = 8). One hour before the stressor, former patients, compared to healthy controls, exhibited higher diastolic blood pressure. From a blood sample taken 30 min before the stressor, patients, compared to controls, had lower paroxetine platelet binding site densities. During the stressor, patients, compared to controls, had greater increases in plasma levels of cortisol. These preliminary findings suggest that remitted panic disorder patients might have disturbed physiological responses to mild psychological stressors. These disturbances might be related to the development of future episodes.