Platelet research in psychiatry

Platelets: A possible glance into brain biological processes in schizophrenia

Schizophrenia is a severe mental disorder, characterized by behavioral, emotional and cognitive disturbances, which commonly follows a chronic course. Diagnostic accuracy, management plans, treatment evaluation and prognosis are dependent on relatively subjective assessments. Despite extensive research and improvement in imaging technology, as well as modern genetic and molecular methodologies, the biological basis of this disease is still unclear. Therefore, there is a need for objective and valid biological markers. Platelets have often been used as a model in neurobiological research. The accessibility of platelets and their similarities with neurons turns them into an attractive candidate to search for biological markers for diagnosis and for unraveling pathophysiological processes relevant to the etiology of brain disorders, including schizophrenia. The present review addresses the main changes in platelet physiology observed in schizophrenia and its response to antipsychotic medication. We summarize numerous studies demonstrating impaired metabolism, uptake and receptor kinetics of schizophrenia-relevant neurotransmitters, abnormalities in membrane derived phospholipids and polyunsaturated fatty acids, as well as dysfunctions in the mitochondria. These changes fit with the various hypotheses raised for the etiology of schizophrenia, including the dopamine-glutamate hypothesis, the autoimmune hypothesis, the polyunsaturated fatty acid hypothesis and the impaired energy metabolism hypothesis. Despite extensive research in platelets, no conclusive reliable biomarker has been identified yet. This review suggests that the clinical heterogeneity and the biological complexity of schizophrenia lead to the inevitable conclusion that biomarkers will be identified only for subgroups characterized according to the different diagnostic criteria. Moreover, any biomarker would have to be an array of interrelated factors or even a set of several such arrays.

Regulation of monoamine transporters: Role of transporter phosphorylation

Presynaptic biogenic amine transporters mediate reuptake of released amines from the synapse, thus regulating serotonin, dopamine and norepinephrine neurotransmission. Medications utilized in the treatment of depression, attention deficit-hyperactivity disorder and other psychiatric disorders possess high affinity for amine transporters. In addition, amine transporters are targets for psychostimulants. Altered expression of biogenic amine transporters has long been implicated in several psychiatric and degenerative disorders. Therefore, appropriate regulation and maintenance of biogenic amine transporter activity is critical for the maintenance of normal amine homoeostasis. Accumulating evidence suggests that cellular protein kinases and phosphatases regulate amine transporter expression, activity, trafficking and degradation. Amine transporters are phosphoproteins that undergo dynamic control under the influence of various kinase and phosphatase activities. This review presents a brief overview of the role of amine transporter phosphorylation in the regulation of amine transport in the normal and diseased brain. Understanding the molecular mechanisms by which phosphorylation events affect amine transporter activity is essential for understanding the contribution of transporter phosphorylation to the regulation of monoamine neurotransmission and for identifying potential new targets for the treatment of various brain diseases.

[Psychosomatic concept of hyperthyroidism-Graves type--behavioral and biochemical characteristics]

INTRODUCTION

In this study we test the hypothesis that specific behavioral and biochemical vulnerabilities characterize individuals with hyperthyroidism-Graves type, one of the classically cited "psychosomatic disorders".

MATERIAL AND METHODS

The sample included 24 subjects with Graves disease and 34 controls. All participants were evaluated for personality and temperament characteristics and for platelet MAO activity. A smaller group of panic disorder patients was tested with the same set of measures to ensure a validity of the study, especially regarding results on personality tests.

RESULTS

Individuals with hyperthyroidism had lower platelet MAO activity and higher scores on histrionic (Hy), depressive (D) and hypochondriac (H) subscale on the MMPI-201 than normal controls. Their TPQ temperament scores were characterized by high Harm Avoidance, whereas other temperament traits were average. Platelet MAO activity was inversely correlated with the MMPI-201 psychopatic deviance scale (Pd) and positively correlated with the TPQ Reward Dependence scale.

CONCLUSIONS

Our results provide support for the psychosomatic concept of Graves' disease. Personality features, temperament traits, and platelet MAO activity of hyperthyroid individuals are different from those in normal controls and correspond to those observed in anxiety disorders. We propose that the observed behavioral and biochemical similaritites between hyperthyroid and anxiety disorder patients represent an equicausality phenomenon, where the same underlying heritable factors, such as variable central monoaminergic activity coupled with temperament-related susceptibility to stress, facilitate phenotypic manifestation of a number of psychosomatic and psychialric disorders--including Graves disease. The observed correlations between personality traits and MAO activity provide support for the hypothesized functional relationship between the underlying central monoaminergic activity and temperament traits associated with anxiety, depression, and impulsivity.

Platelet serotonin uptake in drug-naïve depressive patients before and after treatment with citalopram

We investigated the kinetic parameters of serotonin (5-HT) uptake into platelets in a group of 26 drug-naïve patients suffering from major depression before and after 3-7 weeks of treatment with citalopram. The degree of depression was rated using the Hamilton Depression Rating Scale (HDRS). The 5-HT uptake characteristics in untreated depressive patients were not significantly different from those of normal subjects. The apparent Michaelis constant (K(M)) was significantly increased, the apparent maximal velocity (V(max)) was not different from baseline, and the uptake efficiency (V(max)/K(M)) was significantly decreased after citalopram treatment. A significantly positive correlation between K(M) and V(max) was found in all groups. There was a significantly lower V(max) and V(max)/K(M) in the female compared with the male depressed patients before citalopram treatment; a hypothesis was supported that lowered 5-HT uptake may reflect a gender-linked vulnerability to a serotonin-related depression. A significant negative correlation between 5-HT uptake efficiency and the initial HDRS score suggests that platelet 5-HT uptake can be used as a marker of effective depressive disorder pharmacotherapy. The initial severity of depression was significantly negatively correlated with V(max), which supported a hypothesis that the initial severity of depressive disorder could be related to the lower V(max).

Cerebral glucose utilization and platelet mitochondrial complex I activity in schizophrenia: A FDG-PET study

Altered cerebral energy metabolism and mitochondrial dysfunction in periphery and in brain are implicated in the pathophysiology of schizophrenia. This study investigated whether cerebral glucose metabolism (rCGM) abnormalities are linked to altered mitochondrial complex I activity in the periphery, in schizophrenia. Sixteen schizophrenic patients, 8 with total positive PANSS score >or=20 (high positive schizophrenics; HPS), and 8 with total positive score <or=12 (low positive schizophrenics; LPS), and 8 healthy subjects, were analyzed for their complex I activity in platelets mitochondria and underwent FDG-PET scans at rest. Complex I activity was significantly increased only in HPS and was positively correlated with positive PANSS scores. Images were spatially normalized to an SPM template, their intensities normalized based on average brain activity. Hypermetabolism was observed in the basal ganglia, thalamus, amygdala, and brainstem of both patient groups compared with controls, and in LPS patients extended to parts of cerebellum, left and right cingulate gyrus, parietal and frontal lobes. rCGM in basal ganglia and thalamus significantly and positively correlated with complex I activity in the HPS. In the LPS, a negative correlation was identified in the cerebellum and brainstem. In the control group, however, no areas demonstrated significant positive or negative correlation. These results suggest that the correlation between peripheral complex I activity and rCGM in regions implicated in schizophrenia, could be a pathological factor that is differentially expressed in subgroups of schizophrenic patients.

Possible involvement of alpha-2A adrenergic receptors in attention deficit hyperactivity disorder: radioligand binding and polymorphism studies

Neuropharmacological and genetic association studies have implicated norepinephrine and adrenergic receptors in the pathogenesis of ADHD. The purpose of this study was to compare genetic association studies of three polymorphisms of the alpha-2A adrenergic receptor gene (ADRA2A) with radioligand binding studies of the alpha-2A adrenergic receptor protein in platelets from a sample of children without or with ADHD. The pediatric subjects ranged from 6 to 18 years of age. A thorough clinical assessment of each child resulted in one of the following DSM-IV ADHD diagnoses: inattentive, hyperactive/impulsive, combined, or no ADHD. No significant linkage was found between the ADRA2A polymorphisms (MspI, HhaI, and DraI) and any of the phenotypes tested. Association analysis, however, did detect significant linkage disequilibrium for the DraI polymorphism. Association was also evaluated considering the three ADRA2A single nucleotide polymorphisms as haplotypes. The HhaI-DraI and the MspI-HhaI-DraI haplotypes were significantly associated with ADHD. The platelet alpha-2 adrenergic receptor density did not differ between children without or with ADHD. The affinity of the receptor for the radioligand however, differed significantly between those without and with ADHD. In addition, there were some significant correlations between binding parameters and severity of ADHD in this well-characterized clinical population, and significant association was found between these measures of receptor function and MspI and DraI polymorphisms. Thus, both the genetic and the binding studies indicate that the alpha-2 adrenergic receptor may play a role in ADHD.

Effect of D-serine on the serotonin receptors of human platelets

Recent literature and our previous observations indicated the presence of both NMDA and serotonin type 3 receptors in human platelets with very similar ionic currents to that of cultured mammalian neuronal receptors. Baseline electrophysiological data shows similar profile for platelets from both normal and schizophrenic subjects, whereas serotonin receptor studies exhibited the presence of 5-hydroxytryptamine type-3 (5-HT3) currents in both normal and schizophrenic platelets significantly different from each other. The two major differences observed were: first, 5-HT3 receptors present in the platelets of schizophrenic patients were four times more sensitive to serotonin than those present in the platelets of normal subjects and, second, that D: -serine in micro molar concentrations dampens this effect in platelets from schizophrenic patients but increases the sensitivity of serotonin for platelet 5-HT3 receptors of normal subjects. Patch clamp technique was used to measure the whole cell currents passing through serotonin receptors in these two types of human platelets. The currents were found to be 5-HT3 receptor currents as they were abolished by 10 microM D-tubocurarine. Similarly, micromolar concentrations of D: -serine increased the sensitivity of 5HT3 receptor currents in the normal human platelets but decreased it in the platelets of the schizophrenic patients. This effect was reversed when D-amino acid oxidase (0.3 microM) was co applied with 100 microM of D-serine, raising the possibility that D-serine by itself may act as a modulator for platelet 5-HT3 receptor channel currents. These observations raised exciting new questions about the role of platelet serotonin receptors and their regulation by D-serine.

Human recombinant monoamine oxidase B as reliable and efficient enzyme source for inhibitor screening

Interest in inhibitors of monoamine oxidase type B (MAO B) has grown in recent years, due to their therapeutic potential in aging-related neurodegenerative diseases, such as Parkinson's disease and Alzheimer's disease. This study is devoted to the use of human recombinant MAO B obtained from a Baculovirus expression system (Supersomes MAO B, BD Gentest, MA, USA) as reliable and efficient enzyme source for MAO B inhibitor screening. Comparison of inhibition potencies (pIC50 values) determined with human cloned and human platelet MAO B for the two series of MAO B inhibitors, coumarin and 5H-indeno[1,2-c]pyridazin-5-one derivatives, showed that the difference between pIC50 values obtained with the two enzyme sources was not significant (P>0.05, Student's t-test). Hence, recombinant enzyme is validated as convenient enzyme source for MAO B inhibitor screening.

Evidence for biphasic effects of protein kinase C on serotonin transporter function, endocytosis, and phosphorylation

The serotonin transporter (SERT) regulates 5-hydroxytryptamine (serotonin) (5-HT) neurotransmission and is a high-affinity target for antidepressants and psychostimulants. In the present study, we investigated the mechanisms that contribute to a previously unidentified biphasic regulation of endogenous SERTs expressed in the platelets. Treatment of rat platelets with beta-phorbol 12-myristate 13-acetate (PMA) for 5 min or less resulted in a rapid inhibition of SERT involving changes in intrinsic activity of the transporter (increased K(m) and decreased V(max)). beta-PMA treatment for 30 min or more produced a sustained inhibition of SERT with a decrease only in the V(max). Whereas inhibition of SERT activity was detected from 1 to 45 min after phorbol ester addition, the decrease in surface SERT required at least 30 min of phorbol ester incubation. Increased endocytosis of SERT accounted for the decrease in surface SERT at the later point. Protein kinase C (PKC)-mediated phosphorylation of SERT occurs on the plasma membrane during the initial phase of rapid transporter inhibition, and later, the phosphorylated SERT enters the intracellular pool. beta-PMA-induced phosphorylation of SERT occurs initially on serine residues(s) and then on threonine residue(s). The initial serine phosphorylation corresponded to the first phase of rapid inhibition mediated by changes in intrinsic activity and/or silencing of SERT. The later phosphorylation on threonine residue(s) corresponded to the later phase of sustained inhibition mediated by an enhanced endocytosis of SERT. Together, these data reveal that in platelets, SERT function is regulated by PKC in a biphasic manner involving both trafficking-dependent and independent mechanisms and that these two events occur at distinct phases of transporter phosphorylation.

Initial platelet serotonin (5-HT) transport kinetics predict nortriptyline treatment outcome

According to the hypothesis of initial conditions, drug response may be determined by different initial states of neurotransmitter protein recognition systems. Platelet serotonin (5-HT) transport kinetics were studied as initial-conditions predictors of antidepressant response in 24 depressed patients before and after 3 weeks of treatment with nortriptyline (75 mg). The initial affinity of the 5-HT transporter (5-HTT) correctly predicted 71% of the outcome. The pretreatment affinity constant ( Km) correlated (r = 0.61; p < 0.002) with that measured after 3 weeks of treatment (Kapp). Responding patients had a significantly higher initial Km before treatment and a significantly higher Kapp after treatment. Nonresponders had an initial Km significantly lower than that of 24 controls. Nortriptyline plasma levels were not statistically different between response groups. These results are consistent with two previously published observations, which indicate that the initial affinity of the 5-HTT predicted response to fluvoxamine or fluoxetine in the same way. Insofar as all three drugs increase the apparent affinity of the 5-HTT, it appears that a better response is related to those cases where the initial affinity is already higher before treatment.

Serotonergic function in major depression and effect of sertraline and paroxetine treatment

We investigated platelet [14C]serotonin (5-HT) uptake and lysergic acid diethylamide [N-methyl-3H] ([3H]LSD)- and phenyl-6'-paroxetine ([3H]paroxetine) binding in 30 patients with major depression at baseline and after 6 months of treatment with either paroxetine or sertraline. The study was of a double-blind design. Baseline data was compared with an age- and gender-matched group of healthy volunteers. Baseline Vmax was significantly lower in patients than in controls. Bmax for [3H]paroxetine binding were similar in patients and controls, but patients who suffered their first depression had significantly lower Bmax for [3H]paroxetine binding than patients who had suffered multiple depressions. Twenty-three patients (76%) (13 in the paroxetine group and 10 in the sertraline group) responded to treatment as judged by a 50% or more reduction in Montgomery-Asberg Depression Rating Scale (MADRS) scores after 6 months of treatment. There were no significant differences between the paroxetine and sertraline treated groups. Both paroxetine and sertraline caused a significant reduction in Vmax and a significant increase in Km. There was a strong correlation between Km and plasma drug concentration in patients who experienced their first depression but not in patients who had suffered multiple episodes. Bmax for [3H]paroxetine binding increased after paroxetine treatment while the opposite occurred after sertraline treatment. There was a significant interaction between the impact of drug and earlier depressions. All patients included in the study had been drug free for at least 2 months. Earlier antidepressant treatment may have long withstanding effects on the serotonin uptake machinery but it cannot be excluded that the sensitivity of the uptake mechanism may become more resistant to change in patients with recurrent depressive episodes.

In vitro metabolism of citalopram by monoamine oxidase B in human blood

The metabolism of the antidepressant citalopram (CIT) by monoamine oxidase B (MAO-B) was studied in vitro. In incubations with blood of nine healthy volunteers R-(P=0.015) and S-(P=0.0034) CIT propionic acid (CITPROP) production was correlated with the number of platelets. S-CITPROP production was 5.6 times higher than R-CITPROP production and in incubations containing the MAO-B inhibitor deprenyl, racemic CITPROP production was diminished to 9.1%. To our knowledge, this is the first time that MAO-B activity in blood is shown with an antidepressant as substrate. As MAO is strongly expressed in human brain, this observation suggests that this enzymatic system may be implicated in drug metabolism in the CNS.

Decreased reuptake of serotonin in human platelets after surgery

We have documented earlier a decrease in platelet serotonin and a concurrent increase in plasma serotonin, 5-hydroxytryptamin (5-HT) after various forms of stress, suggesting a disturbed platelet 5-HT reuptake mechanism following stress. In order to further elucidate these findings, we have studied platelet 5-HT reuptake kinetics (Vmax and Km) in nine patients before and 4 days after major, uncomplicated abdominal surgery. We found a significant decrease in the maximal 5-HT reuptake velocity (Vmax) after surgery and changes in Km, verifying alterations in the affinity of the platelet 5-HT transport system. The present results thus confirm the hypothesis that 5-HT reuptake kinetics are altered following adrenergic hyperactivity. A decrease in platelet 5-HT reuptake may bear implications for our understanding of poststress adaptive changes in the cardiovascular system as well as in the central nervous system (CNS) serotonergic neurones following stressful stimulation.

Platelet alpha2A-adrenoceptor function in major depression: Gi coupling, effects of imipramine and relationship to treatment outcome

Studies suggest alpha2A-adrenoceptors (alpha(2A)AR) dysregulation in major depressive disorder (MDD). Platelet alpha(2A)ARs exist in high- and low-conformational states that are regulated by Gi protein. Although alpha(2A)AR coupling to Gi protein plays an important role in signal transduction and is modulated by antidepressants, it has not been previously investigated. Alpha2AR density in the high- and low-conformational states, agonist affinity and coupling efficiency were investigated in 27 healthy control subjects, 23 drug-free MDD patients and 16 patients after imipramine treatment using [3H]yohimbine saturation and norepinephrine displacement of [3H]yohimbine binding experiments. Coupling measures were derived from NE-displacement experiments. Patients had significantly higher alpha(2A)AR density, particularly in the high-conformational state, than control subjects. Coupling indices were normal in patients. High pre-treatment agonist affinity to the receptor in the high-conformational state and normal coupling predicted positive treatment outcome. Decreased coupling to Gi predicted a negative treatment outcome. Imipramine induced uncoupling (-11%) and redistribution of receptor density in treatment responders only, but had no effect on alpha(2A)AR coupling or density in treatment non-responders. Increased alpha(2A)AR density may represent a trait marker in MDD. The results provide indirect evidence for abnormal protein kinase A (PKA) and protein kinase C (PKC) in MDD which may be pursued in future investigations.

Effects of serotonin transporter promoter genotype on platelet serotonin transporter functionality in depressed children and adolescents

OBJECTIVE

To investigate possible associations between serotonin transporter (5-HTT) promoter genotypic variants (l/l, l/s, and s/s) and differential regulation of platelet 5-HTT functionality parameters in a group of drug-naive depressed children and adolescents and healthy controls.

METHOD

Children and adolescents with major depression (n = 18) defined by DSM-III-R criteria and normal controls (n = 21) were assessed both for platelet serotonin functionality and for genotypic variants on 5-HTT promoter region. Four parameters were considered: (1) serotonin uptake rate (Vmax); (2) serotonin dissociation constant (K(m)); (3) paroxetine binding and density of site (Bmax); and (4) paroxetine dissociation constant (Kd).

RESULTS

Depressed children had lower Vmax and K(m). Control subjects with l/l genotype had significantly higher Vmax than control subjects with l/s and s/s genotype. Control subjects with l/l genotype also had significantly higher Vmax than their depressed homologs. In contrast, Vmax was not significantly different between depressed and nondepressed subjects who carried the other 2 genotypes. The 5-HTT promoter genotype, diagnoses, or their interaction had no effect on the other serotonin parameters.

CONCLUSIONS

While showing a significant decrease of Vmax and K(m) in a group of drug-naive depressed children and adolescents, these data suggest that l/l genotype has a substantial effect on the decrease of Vmax during a depressive episode.

Characteristics of agonist displacement of [3H]ketanserin binding to platelet 5-HT2A receptors: implications for psychiatric research

Brain 5-HT(2A) receptors exist in two agonist affinity states as a function of their coupling to Gq protein. This has not yet been shown in platelets. We examined [3H]ketanserin's saturable binding to platelet 5-HT2A receptors and characteristics of agonist displacement curves of [3H]ketanserin binding in healthy control subjects. [3H]ketanserin saturation curves showed a trend for a two-site model, reflecting two independent binding sites. At low [3H]ketanserin concentrations, agonist displacement curves were flat and best fit a two-site model, indicating the existence of two agonist affinity states. Guanylyl 5'-imidotriphosphate [Gpp(NH)p] induced a significant rightward shift in agonist displacement curves to fit a one-site model. Platelet membrane 5-HT2A receptors exist in two agonist affinity states that are regulated by Gq protein. Platelet 5-HT2A receptors provide an accessible model for examining possible dysregulation in the agonist affinity or coupling efficiency to the phosphoinositide system in psychiatric disorders and their modulation by psychotropic medications.

MAO inhibitory side effects of neuroleptics and platelet serotonin content in schizophrenic patients

In order to study the putative monoamine oxidase (MAO) inhibitory side effect of neuroleptics and simultaneous changes in platelet serotonin content both MAO-B activity and serotonin (5-HT) content in platelets of 30 healthy volunteers and 50 schizophrenic patients treated with neuroleptics were investigated. Our results have shown significantly lower MAO-B activity (15.26 +/- 6.81 S.D. vs. 8.63 +/- 3.82 mmol/hour/10(9) platelets) and higher platelet 5-HT content (906.19 +/- 285.33 vs. 1,727.85 +/- 947.40 ng/10(9) platelets) in the schizophrenic group. Platelet MAO-B activity was considerably lower in paranoid and residual schizophrenics compared with other patients, however, no difference was found in platelet 5-HT content between different subtypes of schizophrenia. Various neuroleptic treatments did not produce different effects either on platelet serotonin content or platelet MAO-B activity.

Platelet serotonergic indices in major depression: up-regulation of 5-HT2A receptors unchanged by antidepressant treatment

This study examined, in the largest sample of major depressives reported so far, platelet serotonergic parameters (5-HT uptake, [3H]paroxetine binding and 5-HT2A receptors measured by [3H]LSD binding) in 60 antidepressant-free depressed patients and 40 age- and gender-matched control subjects before treatment, and in 45 major depression patients during treatment with antidepressants. We found that, at baseline, the density (Bmax) of 5-HT2A receptors was significantly higher (by 39%) in depressed patients than in controls. Suicidal patients had significantly higher Bmax values than controls or non-suicidal patients. The rate of serotonin uptake (Vmax), but not the uptake at a single concentration, was significantly higher in depressed patients, particularly in females. There was no significant difference between the Kd or Bmax of [3H]paroxetine binding in control and depressed subjects. Treatment with antidepressant drugs of different pharmacological profile had no significant effect on the density of 5-HT2A receptors, nor did the receptor number predict the response to treatment. The affinity of serotonin uptake site for 5-HT and [3H]paroxetine significantly decreased during treatment with antidepressants, particularly SSRIs. Suppression of 5-HT uptake correlated with decreases in Hamilton depression (HAMD) scores. Our data suggest that the increased density of platelet 5-HT2A receptors may be associated with untreated major depression in antidepressant-free depressed patients, in particular those with suicidal thoughts. The persistence after antidepressant treatment and clinical improvement would suggest that up-regulation of 5-HT2A receptors is a trait rather than state phenomenon. Correlation of 5-HT uptake suppression with decreases in HAMD scores suggests that serotonin uptake inhibition is a relevant factor in antidepressant drug effect and clinical improvement.

[3H]paroxetine binding to platelets of patients with social phobia: comparison to patients with panic disorder and healthy volunteers

Recent studies suggest that serotonergic functioning may be aberrant in patients with social phobia. Capacity of the serotonin (5-HT) transporter, as determined by 3H-paroxetine binding, was measured in 18 drug-free patients with generalized social phobia and compared to 15 drug-free patients with panic disorder and 23 healthy control subjects. The density (Bmax) and affinity (1/Kd) of 3H-paroxetine binding sites was similar in all three groups. To the extent that the serotonin transporter in platelets and neurons is comparable, these findings suggest that this aspect of serotonergic function is normal in patients with social phobia.

Phencyclidine inhibits epinephrine-stimulated platelet aggregation independently of high affinity N-methyl-D-aspartate (NMDA)-type glutamatereceptors

The psychotomimetic analgesic phencyclidine (PCP), which binds to a high affinity site on the neuronal N-methyl-D-aspartate (NMDA)-sensitive glutamate receptor, has previously been found to bind to platelets with high affinity and to specifically delay the onset of epinephrine-stimulated platelet aggregation (Jamieson et al. (1992) Biochem. J. 285, 35-39). We have now shown that the rank order of binding affinities of 14 synthetic PCP analogs at the high affinity binding site on platelets does not parallel the rank order of their affinities in binding to rat brain membranes, indicating that the high affinity PCP binding sites in platelets is distinct from the neuronal NMDA receptor. The order of potency of six of these analogs in delaying the onset of epinephrine-stimulated platelet aggregation also did not parallel the rank order of their binding affinities for platelet or brain binding sites. These data indicate that the ability of PCP analogs to inhibit epinephrine-stimulated aggregation is not related to their ability to bind to the high affinity platelet PCP binding site. Furthermore, (+)MK-801, which binds to the same high affinity binding site in neurons as does PCP, failed to inhibit epinephrine-stimulated platelet aggregation, further suggesting that the site at which PCP acts in platelets is not related to the NMDA-type glutamate receptor. Further studies showed that 5-HT2 receptors and effects on platelet secretion are not involved in PCP-mediated inhibition of epinephrine-induced platelet aggregation.

Electric-foot-shock induced the suppression of fibrinolytic activity in rats

Electric-foot-shock was given to rats to initiate constant mental stress and its effect on fibrinolytic activity was analyzed. After the termination of electric-foot-shock which was given for an hour, euglobulin clot lysis time in the stressed group significantly prolonged than those in the control group. tissue plasminogen activator activity was also significantly lower in the stressed group. These effects lasted at least for an hour and returned to the control values 24 hours after the stress. Whole blood serotonin levels, which mainly show serotonin contents in platelets, were higher in the stressed group. A negative correlation between whole blood serotonin and tPA activity in the stressed group was obtained. These results suggest that prolonged mental stress impairs fibrinolysis by decreasing tPA activity with a concomitant increase of serotonin contents in platelets.

Stress and/or tranylcypromine treatment affects serotonergic measures in blood and brain in rats

Since stress can alter serotonin (5-hydroxytryptamine, 5-HT) turnover in the brain and the periphery, the effects of different types of acute stress on serotonin and related substances in the whole blood and various brain areas in rats pretreated with tranylcypromine (TCP) were studied. TCP administered alone caused a rise in 5-HT, a fall in its metabolite (5-hydroxyindoleacetic acid, 5-HIAA) in the whole blood and in every part of the brain analyzed relative to controls. In rats given TCP and subjected to footshock or water-immersion restraint stress similar changes, but to a different extent, were observed. 5-HT level remained essentially constant except in the blood and the limbic system, whereas 5-HIAA level was found to be increased in the blood and the brain, mainly in the limbic system and the brainstem following footshock. Water-immersion restraint stress caused an increase in 5-HT only in the limbic system without any changes in 5-HT and 5-HIAA in the blood. Relative to controls, an increase in total tryptophan concentration in the whole blood and in every part of the brain was found only after footshock application with or without pretreatment with TCP. In conclusion, responses to stress in rats may depend upon the type of stimulus applied as well as of a concurrent administration of TCP. Some regional differences may account for an altered in vivo efficacy of this drug.

The use of platelets as models for neurons: possible applications to the investigation of eating disorders

The pathogenesis of eating disorders is still unclear. However, there is evidence that neurotransmitters may be involved in the control of satiety/appetite. Investigating these disorders could be facilitated if a readily available human cell model were to reflect central nervous system (CNS) function. Platelets may satisfy, at least partially, this requirement. We discuss here the use of platelets as neuronal models with an emphasis on eating disorders.

Altered serotonergic activity in women with dysphoric premenstrual syndromes

OBJECTIVE

Dysphoric Premenstrual Syndromes (PMS) are quite prevalent and in some women they are severe enough to warrant treatment. Their pathophysiology is still unknown, despite increased interest and research. Here we review the possible role of serotonin in the multidimensional interactive pathophysiology of PMS.

METHOD

Over 170 articles are reviewed. An extensive library search has been conducted and articles are included because of their relevance to: 1) the phenomenology of PMS; 2) the putative association of serotonergic (5-HT) activity with syndromes that occur premenstrually; 3) changes in 5-HT activity along the menstrual cycle, especially the late luteal phase; 4) influence of gonadal hormones on serotonergic functions; 5) endocrine strategies for assessment of 5-HT abnormalities; and 6) treatment studies of PMS with serotonergic agonists.

RESULTS AND CONCLUSIONS

The data presented here suggest that post-synaptic serotonergic responsivity might be altered during the late-luteal-premenstrual phase of the menstrual cycle. Some serotonergic functions of women with PMS might be altered during the entire cycle and be associated with a vulnerability trait. It is hypothesized that gonadal hormones might cause changes in levels of activity of 5-HT systems as part of a multidimensional interactive system. Strategies to evaluate 5-HT activities in the context of the menstrual cycle are discussed--leading to the conclusion that the most promising approach is active stimulation with specific post-synaptic serotonin agonists. Treatment outcome studies of some imperfect compounds that are currently applied as a symptomatic treatment of PMS support the notion that 5-HT is involved in the pathophysiology of these syndromes.

Platelet [3H]paroxetine binding to 5-HT uptake sites in Alzheimer's disease

Platelet serotonin (5-hydroxytryptamine, 5-HT) uptake sites were studied in a control group (n = 30) and an Alzheimer group (n = 40) using [3H]paroxetine as radioligand. The maximum number of binding sites (Bmax) for control (1250 +/- 60 fmol/mg protein) was not different from the Alzheimer group (1280 +/- 40 fmol/mg protein). There were no differences in apparent binding affinity (Kd): 0.046 (0.024-0.062) nM for control and 0.040 (0.027-0.061) nM for Alzheimer. Thus even though several previous studies have demonstrated marked atrophy of 5-HT containing neurites and 5-HT uptake sites in Alzheimer's disease, these findings are not found in the periphery on platelets. The platelet 5-HT uptake site cannot be used as a peripheral marker of Alzheimer's disease.

Elevated platelet intracellular calcium concentration in bipolar depression

Baseline and thrombin-stimulated free intracellular calcium concentrations in blood platelets were significantly higher in untreated depressed bipolar patients than in untreated unipolar depressed patients or controls. Platelet intracellular calcium ion concentrations in euthymic-treated bipolar patients were equivalent to control values, suggesting but not proving a state-dependent change in intracellular calcium ion dynamics in bipolar depression. Unipolar and some subsets of bipolar patients appear not to exhibit this change.

Effects of clomipramine treatment on cerebrospinal fluid monoamine metabolites and platelet 3H-imipramine binding and serotonin uptake and concentration in major depressive disorder

In an open study of 12 inpatients who met the DSM-III criteria for a major depressive episode, the effects of clomipramine (CI) on the monoamine metabolites 5-hydroxyindoleacetic acid (5-HIAA), homovanillic acid (HVA), 4-hydroxy-3-methoxyphenyl glycol (HMPG) in cerebrospinal fluid (CSF) were measured simultaneously with the effects on 3H-imipramine binding, serotonin (5-HT) uptake and 5-HT concentration in platelets after 3 and 6 weeks of treatment. Drug (CI and desmethylclomipramine) plasma concentrations were determined. The concentrations of 5-HIAA and HMPG decreased substantially, and the concentration of HVA remained unchanged. There was also a large and significant reduction of the number of imipramine binding sites (Bmax) and of the platelet 5-HT concentration. The 5-HT uptake was not measurable after 3 weeks of treatment. None of the parameters changed significantly between weeks 3 and 6. There were no significant correlations between antidepressant effect (measured by the Montgomery-Asberg Depression Rating Scale) and plasma drug concentrations, although a tendency to a significant correlation between antidepressant effect and CI was observed at 3 weeks. There were no significant intercorrelations between the different 5-HT parameters and no other significant correlations between the biochemical measures and clinical outcome.

Influence of age, sex and diurnal variability on imipramine receptor binding and serotonin uptake in platelets of normal subjects

Imipramine (IMI) binding and serotonin (5-HT) uptake were determined in platelets of 98 healthy volunteers; and their association with age, sex and circadian rhythm were evaluated. A large interindividual variability was found for both IMI and 5-HT parameters. There was a negative correlation of IMI affinity constant (Kd) and binding (Bmax) with age, but no such correlation of 5-HT affinity constant (Km) or uptake (Vmax). Significant age-related diurnal variability was found for 5-HT Km in the whole group as well as for IMI Kd in males, but not in females. There was no significant correlation between 5-HT Vmax and IMI Bmax. Our results underscore a cautious approach to the interpretation of platelet serotonergic studies. In light of the multiple variables influencing the results, the usefulness of IMI or 5-HT as clinical markers should be re-evaluated.

Effects of fluoxetine treatment of platelet 3H-imipramine binding, 5-HT uptake and 5-HT content in major depressive disorder

Platelet 3H-imipramine binding, serotonin (5-HT) uptake and 5-HT concentrations were studied in 14 hospitalized patients with depressive disorder following 6 weeks of treatment with a selective 5-HT uptake blocker, fluoxetine. After 3 weeks of treatment there was a significant decrease in Bmax of 3H-imipramine binding and a significant increase in Kd. A highly significant decrease in Vmax of 5-HT uptake was seen after 3 weeks of treatment which was accompanied by a slight increase in Km. At the same time the platelet 5-HT content was significantly reduced by about 90% of its original level. The platelet 5-HT content continued to decrease with further treatment while there was a tendency for Vmax to return to pretreatment levels. The affinity of the 5-HT uptake carrier continued to decrease significantly. There was no further significant change in Bmax of 3H-imipramine binding during further treatment, although there was an increase in Bmax in the majority of patients. The changes in Bmax and Vmax were closely associated throughout the treatment. In some cases the changes in different platelet parameters correlated with the changes in depression rating scores during treatment, but this correlation did not reach statistical significance.

Platelet serotonin level in rats is under genetic control

It has recently been shown that platelet serotonin (5-hydroxytryptamine, 5HT) levels (PSL) in Wistar rats represent an individually stable biological parameter (neither subject to periodic oscillations nor markedly influenced by sex and age) that shows a unimodal frequency distribution within the population (range: 1.2-2.2 micrograms 5HT/mg platelet protein). To investigate the genetic background of PSL, selective breeding for the extreme values of this trait was performed. In the fourth generation, two discrete sublines of animals (statistically different from the unselected population) with constitutionally high or low PSL could be discerned: one with congenitally low PSL (1.1-1.6 micrograms 5HT/mg platelet protein; approximately normal distribution) and the other with congenitally high PSL (1.6-2.9 micrograms 5HT/mg platelet protein; irregular distribution). No difference in the pattern of inheritance between sexes could be discerned. The results demonstrated a marked heritable component underlying the expression of individual values of PSL in rats, suggesting that this parameter is a trait characteristic.

Comorbidity of panic disorder and major depressive disorder: effects on platelet alpha 2 adrenergic receptors

Adrenergic receptor dysregulation has been described as occurring in both major depressive disorder (MDD) and panic disorder. Measurements of platelet alpha 2 adrenergic receptors in these patients may be confounded by the coexistence or comorbidity of both diagnoses in the same patient. To explore this possibility, we measured platelet alpha 2 adrenergic receptors (3H-clonidine and 3H-yohimbine binding) in 3 groups of patients (MDD only, panic disorder only, and those showing comorbidity of MDD and panic) and normal controls. Patients with comorbidity of MDD and panic disorder had significantly lower agonist binding (3H-clonidine).

Accumulation of diacylgylcerol in platelet phosphoinositide turnover in schizophrenia: a biological marker of good prognosis?

Phosphoinositide (PI) turnover was examined by measuring phosphatidic acid (PA) and diacylglycerol (DG) production following thrombin stimulation in platelet membranes from 20 schizophrenic patients, 10 patients with other mental disorders, and 9 normal controls. In 6 of 13 acute schizophrenic patients, DG was not transformed into phosphatidic acid (PA), but accumulated in the platelets instead. The abnormal findings persisted for at least 2 months, but then reversed over a long period. Three years later, the patients with abnormal PI turnover had a significantly better outcome than other acute schizophrenic patients.

Platelet serotonin uptake and 3H-imipramine binding in panic disorder

Platelet serotonin uptake and 3H-imipramine binding were measured in eight patients with panic disorders and nine controls. The Vmax of serotonin uptake was significantly elevated in patients compared to controls (77 +/- 14 vs. 50 +/- 4 pmol/10(8) platelets/min; P less than 0.05) while Km values were not different (1.46 +/- 0.41 vs. 1.24 +/- 0.20 microM). 3H-Imipramine binding to ruptured platelet membranes was not significantly different between patients and controls for either Bmax (395 +/- 71 vs. 412 +/- 107 fmol/mg protein) or Kd (0.90 +/- 0.18 vs. 1.09 +/- 0.30 nM). The implications for a serotonergic dysfunction in panic disorders are discussed.

Platelet serotonin levels and gonadal hormones in rats

The role of gonadal hormones in the control of platelet serotonin levels was studied by evaluating the effect of sexual maturation in rats of both sexes and the time-course of changes following gonadectomy performed either prepubertally or on sexually mature animals. In males, platelet serotonin levels remained fairly stable during sexual maturation as well as during the whole postgonadectomy period monitored (four months). In females, somewhat higher values of platelet serotonin levels in adult than in sexually immature animals were found (9%, p less than 0.001, N = 34). A slight decrease of platelet serotonin (10-18%, p less than 0.05) was observed following ovariectomy of sexually mature females, but it was of transient nature. When females were ovariectomized prepubertally a tendency towards permanently lower platelet serotonin levels was noticed. These results suggest that gonadal hormones have no major role in the control of platelet serotonin levels in rats, although a subtle hormonal modulation of this platelet variable in females may exist.

Physiological characteristics of platelet serotonin in rats

Physiological characteristics of platelet serotonin (5-HT) levels in rats of Wistar origin were investigated by use of a recently developed method. By comparison of populations of male and female rats (N = 281) similar unimodal frequency distributions, with a tendency to higher values in females (1.61 vs. 1.70 micrograms 5HT/mg platelet protein; p less than 0.01), were found. For a group of 55 animals, monitored twice for this parameter within a week interval, a remarkable intraindividual constancy in time, the mean difference between two determinations being 5.5%, was shown. No age-dependence could be demonstrated for platelet serotonin concentrations in 5- to 30-week-old rats, nor were there significant circadian or seasonal oscillations.