Platelet alpha-adrenergic binding and biochemical responsiveness in depressed patients and controls

Evaluation of peripheral serotonin content and α2-adrenergic receptor function as potential markers for life-long recurrent depressive disorder by using methodological improvements

OBJECTIVE

We aimed to evaluate the role and the relations between peripheral platelet serotonin content, blood plasma serotonin concentration and the function of platelet α₂-adrenergic receptors (α₂-AR) as potential state or trait biomarkers for recurrent depressive disorder (RDD).

METHODS

26 drug-free patients with life-long RDD and 31 healthy controls were included in the study. Several methodological improvements in blood collection and platelet isolation were implemented following the present standards in Haematology and Light transmission aggregometry.

RESULTS

Our results have shown lower platelet serotonin content, higher plasma serotonin concentration and desensitization of platelet α₂-AR in patients with RDD. The variables were found heterogeneous and mainly influenced by the clinical characteristics of the current episode. High amplitude of the α₂-AR correlated with severe anxious symptoms and high platelet serotonin content (as well as low plasma serotonin levels) were associated with psychotic symptoms.

CONCLUSIONS

The evaluated peripheral markers reflect only state (but not trait) abnormalities in patients with current severe episode of RDD. The observed peripheral α₂-AR and serotonin abnormalities are mutually not related and they are probably triggered by different mechanisms.

Cyclic-AMP response element binding protein (CREB) in the neutrophils of depressed patients

Cyclic-AMP response element binding (CREB) protein regulates the expression of many genes involved in the pathophysiology of depression. Increased CREB levels were found in the brain of antidepressant-treated rats and decreased protein and mRNA expression of CREB was reported in the postmortem brain of depressed suicide victims. We determined CREB protein expression, using Western blot technique, and CRE-DNA binding, using gel shift assay, in neutrophils obtained from 22 drug-free patients with major depressive disorder (MDD) and 23 normal control subjects. Diagnosis of patients was based on Diagnostic and Statistical Manual of Mental Disorders DSM-IV criteria; severity of illness was rated by Hamilton Depression Rating Scale (HDRS). We found that the CRE-DNA binding activity and CREB protein expression were significantly decreased in the neutrophils of drug-free MDD patients compared with normal control subjects. Our findings suggest that CREB may play an important role in the pathophysiology of depression and that it may be an important target for the therapeutic action of antidepressant drugs. Neutrophil CREB levels may also serve as a useful biomarker for patients with MDD.

Adenylyl cyclase-cyclicAMP signaling in mood disorders: role of the crucial phosphorylating enzyme protein kinase A

Mood disorders are among the most prevalent and recurrent forms of psychiatric illnesses. In the last decade, there has been increased understanding of the biological basis of mood disorders. In fact, novel mechanistic concepts of the neurobiology of unipolar and bipolar disorders are evolving based on recent pre-clinical and clinical studies, most of which now focus on the role of signal transduction mechanisms in these psychiatric illnesses. Particular investigative emphasis has been given to the role of phosphorylating enzymes, which are crucial in regulating gene expression and neuronal and synaptic plasticity. Among the most important phosphorylating enzyme is protein kinase A (PKA), a component of adenylyl cyclase-cyclic adenosine monophosphate (AC-cAMP) signaling system. In this review, we critically and comprehensively discuss the role of various components of AC-cAMP signaling in mood disorders, with a special focus on PKA, because of the interesting observation that have been made about its involvement in unipolar and bipolar disorders. We also discuss the functional significance of the findings regarding PKA by discussing the role of important PKA substrates, namely, Rap-1, cyclicAMP-response element binding protein, and brain-derived neurotrophic factor. These studies suggest the interesting possibility that PKA and related signaling molecules may serve as important neurobiological factors in mood disorders and may be relevant in target-specific therapeutic interventions for these disorders.

Genetic studies at the receptor level: investigations in human twins and experimental animals

In receptors, as in enzymes, quantitative as well as qualitative genetic variation may exist. Studies in inbred strains of mice have shown for various receptors that the receptor density as determined by Bmax values is under genetic control. In healthy adult twins we have shown that the density of alpha-adrenoceptors on platelets is also influenced by genetic factors, since monozygotic twins were much more similar to one another than dizygotic twins. However, Bmax values are up-regulated and down-regulated by endogenous neurotransmitters and pharmacologically active agents. Thus, receptor densities are under considerable regulatory influences. Bmax values therefore reflect regulatory mechanisms rather than innate characteristics of the receptor protein. In another twin study we failed to find evidence for a genetic influence on the density of imipramine-binding sites on platelets. Since qualitative variation (polymorphism) is well known in enzymes, it may also apply to receptors. Qualitative differences in the receptor protein within one species would be of particular interest because of possible functional implications. As a first approach we examined central benzodiazepine receptors by photoaffinity labelling and sodium dodecyl sulphate-polyacrylamide gel electrophoresis. A comparison of fish, frog, chicken, mouse, rat and calf led to the detection of variation between species. Investigations in five inbred mouse and rat strains have not so far revealed genetic variation in benzodiazepine receptors. Nevertheless variation may be detectable by more sensitive methods such as peptide mapping after limited proteolysis or two-dimensional electrophoresis.

Platelet radioligand binding and neuroendocrine challenge tests in depression

The purpose of this work was to examine the number and function of alpha 2-adrenoceptors and the number of serotonin uptake sites in depressed patients and controls. Platelet alpha 2-adrenoceptors and platelet serotonin uptake sites were labelled with [3H]yohimbine and [3H]imipramine respectively. Central alpha 2-adrenoceptor function was assessed by growth hormone and other responses to challenge with the alpha 2-agonist clonidine. No overall difference in the binding parameters was observed between the control and depressed groups, but the results highlight the importance of drug-free interval, menopausal status and membrane protein concentration within the binding assays in the interpretation of such studies. The growth hormone response to clonidine tended to be blunted in depressed females and was significantly blunted in the subgroup of depressives who failed to suppress plasma cortisol concentrations in response to dexamethasone. Depressed subjects also showed a smaller decrease in diastolic blood pressure and a smaller increase in sedation than control subjects.

Epinephrine- and thrombin-stimulated high-affinity GTPase activity in platelet membranes from patients with psychiatric disorders

Although heterotrimeric guanine nucleotide-binding regulatory (G) proteins have been implicated in the pathophysiology of mental illnesses (especially mood disorders), direct evidence has been scarce. This study was designed to reveal possible abnormalities of receptor-coupled G protein function in platelets in patients with psychiatric disorders such as depression and schizophrenia. The functional status of alpha(2A)-adrenergic receptor-coupled G(i2) and thrombin receptor-coupled G proteins (G(i2)+G(q)) was determined by the increase in high-affinity GTPase activity in response to epinephrine and thrombin, respectively, in platelet membranes from 18 patients with mood disorders (15 unipolar and three bipolar subtype), 13 schizophrenic patients, four neurotic patients and 29 healthy control subjects. Neither alpha(2A)-adrenergic receptor-coupled G(i2) nor thrombin receptor-coupled G(q) was functionally altered in platelets from psychiatric patients compared with control subjects. No significant correlation was observed between these biochemical measures in platelets and severity of psychopathological symptoms. The functional coupling efficiency of G proteins with receptors appears intact, at least between alpha(2A)-adrenergic receptors and G(i2), and between thrombin receptors and G(q), in platelets from patients with psychiatric disorders.

Interaction of lithium with 5-HT(1B) receptors in depressed unipolar patients treated with clomipramine and lithium versus clomipramine and placebo: preliminary results

Lithium is commonly used in combination with antidepressant drugs as a treatment for refractory depression; less often, it is used in non-resistant depression. The aim of this study was to examine the interaction of lithium with 5-HT(1B) receptors in 10 non-resistant unipolar depressed patients treated with clomipramine+lithium (C+L) vs. clomipramine+placebo (C+P). A mediation of the serotonergic system has been proposed in the literature to explain the clinical effect of lithium. Indeed, in a previous study of healthy human blood platelets, we demonstrated the interaction of lithium with adenylate cyclase activity coupled to 5-HT(1B) receptors. The functional activity of these receptors was measured by studying the inhibitory effect of L694,247, a 5-HT(1B) receptor agonist, on the adenylate cyclase activity determined by the production of cAMP. Using the same technique in the present study, we found that lithium significantly reduced the inhibition of adenylate cyclase activity induced by 5-HT(1B) receptor activation. This result confirms the specific interaction of lithium with 5-HT(1B) receptors. Moreover, a correlation between the percentage of 5-HT(1B) receptor-dependent adenylate cyclase inhibition and the clinical benefit of lithium was established, suggesting 5-HT(1B) receptors may be a target for the therapeutic effect of lithium.

The neurobiology of bipolar disorder: focus on signal transduction pathways and the regulation of gene expression

OBJECTIVE

This article presents an overview of signal transduction pathways and reviews the research undertaken to study these systems in clinically relevant samples from patients with bipolar disorder (BD).

METHOD

We reviewed the published findings from studies of postmortem brain tissue and blood samples from patients with BD.

RESULTS

Although the exact biochemical abnormalities have yet to be identified, the presented findings strongly suggest that BD may be due, at least in part, to abnormalities in signal transduction mechanisms. In particular, altered levels or function, or both, of G-protein alpha subunits and effector molecules such as protein kinase A (PKA) and protein kinase C (PKC) have consistently been associated with BD both in peripheral cells and in postmortem brain tissue, while more recent studies implicate disruption in novel second-messenger cascades, such as the ERK/MAPK pathway.

CONCLUSIONS

Despite the difficulties inherent in biochemical studies of clinically relevant tissue samples, numerous investigations have illuminated the signal transduction mechanisms in patients with BD. These studies also suggest that BD may be due to the interaction of many abnormalities. In this context, novel techniques enabling the study of gene expression promise to assist in untangling these complex interactions, through visualizing the end result of these changes at the level of gene transcription.

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.

Signal transduction by platelet adenylate cyclase: alterations in depressed patients may reflect impairment in the coordinated integration of cellular signals (coincidence detection)

BACKGROUND

Adenylate cyclase (AC) responds to distinct but coincident signals from the agonist-stimulated G-protein Gs and the inhibitory G-protein Gi by generating a greater output signal-to-noise ratio--i.e., agonist-stimulated to basal ratio (fold-stimulation)--through coincidence detection than that generated by a single input (Gs) alone. Such coincidence detection by murine brain AC was found to be enhanced during chronic antidepressant treatment with imipramine.

METHODS

We examined and compared the basal, agonist-stimulated, and guanosine 5'-3-O-(thio)triphosphate (GTP gamma S) or AlF4 ion postreceptor-stimulated AC activities in mononuclear leukocytes and platelets from the same blood specimens obtained from depressed patients (n = 27) and control subjects (n = 19).

RESULTS

In all subjects, the differences (delta GTP gamma S or delta AlF4) between postreceptor measures of AC in mononuclear leukocytes (where AC is regulated by Gs but not by Gi) and platelets (where AC is regulated by both Gs and Gi) were highly significant. In controls, the relationships between delta GTP gamma S or delta AlF4 and basal, agonist-stimulated, and the fold-stimulation of agonist-stimulated platelet AC resembled the regulation of AC by Gi in model-membrane systems. Comparable relationships between delta GTP gamma S or delta AlF4 and basal, agonist-stimulated, and the fold-stimulation of agonist-stimulated platelet AC activities were not observed in depressed patients.

CONCLUSIONS

Our results suggest that in controls, platelet AC enzyme activity is determined (in part) by the coordinated integration of signals from Gs and Gi through coincidence detection, while such coincidence detection by platelet AC may be impaired in patients with depressive disorders.

Subsensitivity of adenylyl cyclase-coupled receptors on mononuclear leukocytes from drug-free inpatients with a major depressive episode

Previous studies have demonstrated blunted beta-adrenergic responsivity in leukocytes from depressed patients. We sought to determine if this blunted cyclic adenosine monophosphate (AMP) response is specific for beta-adrenergic receptors (homologous), or whether other adenylyl cyclase-coupled receptors are also involved (heterologous), in order to localize this effect at the level of the receptor versus the coupling protein or the transducer, adenylyl cyclase. We studied adenylyl cyclase-mediated responses in peripheral blood mononuclear cells from 95 drug-free patients with a major depressive episode and 69 healthy controls. We found a similar degree of decrease in the peak cyclic AMP response to activation of the beta-adrenergic receptor (28%) and the prostaglandin receptor (34%) in the depressed patients, which indicated heterologous desensitization. Forskolin cyclic AMP responses were not blunted. Blunting of cyclic AMP responses to isoproterenol did not appear to correlate with levels of plasma norepinephrine and epinephrine or hypothalamic-pituitary-adrenocortical function. The absence of a decrease in the peak forskolin-generated cyclic AMP response, which involves direct activation of adenylyl cyclase, suggests an abnormality at the level of the coupling protein in these adenylyl-coupled receptors in depressed patients. Future studies need to determine whether this leukocyte signal transduction defect in depression also involves brain adenylyl cyclase-coupled receptors.

Forskolin-stimulated platelet adenylyl cyclase activity is lower in persons with major depression

We investigated platelet adenylyl cyclase activity in 17 subjects with a history of major depression ("depressed subjects") and 20 controls. Forskolin was used to directly activate adenylyl cyclase, while guanine nucleotides (Gpp(NH)p) and fluoride ions were used to measure adenylyl cyclase activity modulated through the G proteins. Forskolin-stimulated adenylyl cyclase was significantly lower in the depressed subjects (p < 0.0005). There was a statistically significant difference in basal adenylyl cyclase activity between male depressed subjects and male controls. The basal adenylyl cyclase activity was also lower in female depressed subjects, but this difference did not reach statistical significance (p < 0.2). The adenylyl cyclase activity measured after stimulation with a guanine nucleotide or cesium fluoride did not differ between control and depressed male or female subjects. Severity of current depression and the current use of antidepressant medication were not related to the lower forskolin-stimulated enzyme activity in the depressed subjects. The difference in forskolin-stimulated adenylyl cyclase activity appears to reflect a qualitative difference in the adenylyl cyclase enzyme activity in persons with a history of major depression.

Regulation of alpha 2-adrenergic receptor expression and signaling in pancreatic beta-cells

Activation of alpha 2-adrenergic receptors (alpha 2-AR) in pancreatic beta-cells inhibits insulin secretion in response to various stimuli, and acute or long-term regulation of alpha 2-AR receptor-mediated effects may influence the tissue response to glucose dishomeostasis. As an initial approach to this issue, we determined the effect of various metabolic and hormonal treatments on alpha 2-AR expression and coupling in the pancreatic beta-cell lines HIT-T15 and RIN-5AH. Radioligand binding studies ([3H]RX-821002) and RNA blot analysis indicate that both pancreatic beta-cell lines express the alpha 2A/D-AR subtype [for HIT-T15 the maximum binding (Bmax) = 113 +/- 28; for RIN-5AH Bmax = 93 +/- 18 fmol/mg of cellular protein]. Treatment of HIT-T15 or RIN-5AH cells with glucocorticoids [dexamethasone, hydrocortisone, or prednisolone (1 microM)] increased alpha 2-AR mRNA level and receptor protein density three- to fivefold. The glucocorticoid-induced increase in receptor density in HIT-T15 cells was associated with 1) an increase in the amount of receptors coupled to G protein as determined by analysis of high-affinity 5'-guanylyl imidodiphosphate-sensitive binding of [3H]UK-14304, a selective alpha 2-AR agonist, and 2) a greater inhibition of forskolin-induced elevation of cellular adenosine 3',5'-cyclic monophosphate after receptor activation. Receptor density in HIT-T15 cells was not altered by different growth conditions, insulin (1 microM), phorbol 12-myristate 13-acetate (1 microM), or the sex steroids testosterone and progesterone (1 microM). These data indicate that glucocorticoids upregulate alpha 2-AR expression and signaling in pancreatic beta-cells. Such regulation may operate in a cell-specific manner, allowing discrete modulation of tissue responses to glucose dishomeostasis.

Platelet adenylate cyclase activity in Israeli victims of Iraqi Scud missile attacks with post-traumatic stress disorder

Platelet adenylate cyclase activity was measured in 16 control subjects and 16 patients who developed post-traumatic stress disorder (PTSD) as a result of damage inflicted on their homes during the Iraqi Scud missile attacks on Israel which occurred during the 1991 Gulf War. There were no differences in basal, NaF-stimulated, PGE1-stimulated or forskolin-stimulated activity between controls and PTSD subjects. Epinephrine inhibition of forskolin-stimulated activity, an effect mediated by alpha 2 adrenergic receptors, was slightly but not significantly increased in the PTSD patients compared to the controls, while 5-HT induced inhibition, an effect mediated by putative 5-HT1a-like receptors, was unchanged. The relationship of these activities to measures of anxiety and depression in these patients is discussed.

CNS signal transduction in the pathophysiology and pharmacotherapy of affective disorders and schizophrenia

Until recently, research on the neurochemical basis of affective disorders (AD) and schizophrenia (SCZ) focused on detecting postulated disturbances in presynaptic neurotransmitter release and metabolism, or postsynaptic receptor function. New insights into the molecular mechanisms involved in the propagation of neurotransmitter signals across biological membranes and in the regulation of neuronal responses have allowed the development of novel hypotheses, which may explain the altered postsynaptic neuroreceptor responsivity thought to be integral to the pathophysiology of these disorders. In this review we evaluate evidence from both basic science and clinical research implicating disturbances in postreceptor signal transduction in the pathophysiology and pharmacotherapy of AD and SCZ. Specific findings regarding potential postreceptor sites of pathophysiology are highlighted in each of these disorders, together with the growing body of data on the possible postreceptor loci of psychotropic drug action, especially lithium and antidepressants.

Platelet α2-adrenoreceptors in depression: a critical examination

During the past decade, results from radioligand studies comparing platelet α(2)-adrenoreceptors in depressed patients and healthy volunteers have been inconsistent, especially when related to the known functional characteristics of these receptors. Despite the availability of radioligands for α(2)-adrenoreceptors, inherent methodological problems exist which make data from these studies difficult to interpret. The authors review the overall data from radioligand studies using [(3)H] clonidine and [(3)H] yohimbine of platelet α(2)-adrenoreceptors in depressed patients and healthy volunteers. Theoretical and methodological issues are critically examined in the light of recent findings. Finally, alternative strategies for studying α( 2)-adrenoreceptors in clinical populations are considered.

Mode of action of the triazolobenzodiazepines in the treatment of panic attacks: a hypothesis

Alprazolam (Xanax) or 8-chloro-1-methyl-6-phenyl-4H-S-triazolobenzodiazepine is a potent drug for the treatment of anxiety disorders. The chemical structure differs from the classical benzodiazepines by incorporation of the triazoloring. Due to the triazolo ring, the drug can have additional modes of action than the normal benzodiazepines. The triazolobenzodiazepines are potent inhibitors of the platelet-activating factor. This factor is a potent stimulator of the corticotropin-releasing hormone. This hormone has an effect on the hypothalamo-pituitary-adrenal axis but the corticotropin-releasing hormone is also known to be a stimulator of the locus coeruleus. The corticotropin-releasing hormone in patients with panic attacks is elevated. This could be a result of the hyperactive metabolism which is observed by positron emission tomographic (PET) studies of the right parahippocampal area.

Platelet membrane alpha 2-adrenergic receptors in depression

The platelet membrane was used as a model system to examine alpha 2-adrenergic receptors in 30 depressed patients and 30 healthy control subjects. The number of binding sites and their affinity for 3H-UK 14304 (5-bromo-6-(2-imidazoline-2-ylamino)-quinoxaline), a potent, highly selective alpha 2-adrenergic receptor agonist, was measured. Plasma magnesium and free 3-methoxy-4-hydroxyphenylglycol (MHPG) concentrations were assayed in the same sample. A decreased agonist-receptor affinity was found in depressed patients, whereas receptor density was not significantly altered compared with that in control subjects. In bipolar depressed and dysthymic patients, there was a tendency toward a higher density of alpha 2-adrenergic receptors. This trend was not apparent in unipolar, recurrent depressed subjects. Moreover, a positive correlation between Bmax and Kd values was observed in patients but not in control subjects--a finding that suggests that a compensatory phenomenon occurs in depression. After the patients were treated with antidepressant drugs, an increased affinity (decrease in Kd) was observed, together with a decrease in binding sites. Plasma magnesium concentrations were higher in drug-free depressed patients than in control subjects. In addition, magnesium concentrations were negatively correlated with the density of alpha 2-adrenergic receptor binding sites in depressed patients, both before and during treatment. Lastly, a trend toward a negative correlation between plasma MHPG concentration and the number of binding sites was also observed. These results suggest a complex multifactorial regulation of alpha 2-adrenergic receptors, which are probably hyposensitive in depressive syndromes.

Alpha 2 adrenergic receptor subtypes in depression: a candidate gene study

Alpha 2 adrenergic receptors play an important role in regulating the neuronal release of norepinephrine through presynaptic feedback inhibition in the locus ceruleus. Therefore, alpha 2 adrenergic autoreceptors may underlie some aspects of the pathogenesis and symptomatic expression of depressive illness. We studied two brain-expressed alpha 2 adrenergic receptor genes as genetic markers in linkage analyses in 17 multiplex pedigrees of unipolar depression. Neither of the genes was supportive of linkage to depression. Lod scores of less than -2 were found in both familial pure depressive disease pedigrees and in depression spectrum disease pedigrees. Therefore, we conclude that depression in our pedigrees is not related to mutations in the two alpha 2 adrenergic receptor genes tested.

Platelet adenylate cyclase activity in depression and after clomipramine and lithium treatment: relation to serotonergic function

Adenylate cyclase activity was measured in platelet membranes from 10 healthy controls, 12 depressed patients, and the same patients after treatment with clomipramine (CMI) followed by lithium carbonate (Li) supplementation, in an attempt to determine whether any evidence for an effect on the serotonergic system could be obtained in peripheral cells. There were no differences in basal, NaF-, PGE1-, or forskolin-stimulated activity either between the control subjects and depressed patients or between activities in the patients measured before treatment, after CMI, and after CMI+Li. The degree of inhibition of forskolin-stimulated adenylate cyclase by 5-HT, an effect putatively mediated by a 5-HT1A-like receptor, was not different in the depressed patients compared to controls or affected by CMI treatment, but was significantly reduced after Li supplementation.

Clinical and biochemical aspects of depressive disorders: II. Transmitter/receptor theories

The present document is the second of three parts in a review that focuses on recent data from clinical and animal research concerning the biochemical bases of depressive disorders, diagnosis, and treatment. Various receptor/transmitter theories of depressive disorders are discussed in this section. Specifically, data supporting noradrenergic, serotonergic, cholinergic, dopaminergic, GABAergic, and peptidergic theories, as well as interactions between noradrenergic and serotonergic, or cholinergic and catecholaminergic systems are presented. Problems with the data and future directions for research are also discussed. A previous publication, Part I of this review, dealt with the classification of depressive disorders and research techniques for studying the biochemical mechanisms of these disorders. A future publication, Part III of this review, discusses treatments for depression and some of the controversies in this field.

Platelet alpha-2 adrenergic receptor-mediated phosphoinositide responses in endogenous depression

We have previously indicated that epinephrine stimulates phosphoinositide (PI) hydrolysis by activating alpha-2 adrenergic receptors in human platelets [H. Mori et. al. Life Sci., 741-747 44 (1989)]. This method involves the measurement of the accumulation of [3H]-inositol-1-phosphate (IP-1) as an index of PI hydrolysis; lithium is added to inhibit the metabolism of IP-1, thus giving an enhanced signal. In the present study, we assessed the platelet alpha-2 adrenergic receptor-mediated PI responses in samples from 15 unmedicated patients with endogenous depression and 15 age- and sex-matched control subjects. The responses to epinephrine (10 microM and 100 microM) in the depressed patients were significantly higher than those of the controls, whereas the basal values did not differ significantly. These results support the hypothesis that platelet alpha-2 adrenergic receptors may be supersensitive in patients with endogenous depression.

Platelet alpha 2-adrenergic receptor binding sites in major depressive disorder and borderline personality disorder

Platelet alpha 2-adrenergic receptor binding sites were measured in a group of patients with major depressive disorder (MDD) (n = 23) and in normal controls (n = 25). When all depressed subjects were compared to controls, there were no differences in either Kd (affinity of the ligand) or total binding site (number/platelet), although a significant change in the ratio of high to low affinity states was observed in the depressed group. When the depressed patients were subdivided into those with and without a co-occurring borderline personality disorder (BPD), the BPD group had significantly fewer alpha 2 high affinity binding sites, while the group with depression alone had significantly more binding sites (both low and high affinity) than the control group. The results support the concept that assessment of comorbid diagnoses may be essential to biological studies of depression.

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).

Platelet adrenoceptors and prostaglandin responses in depressed patients

Platelet alpha 2-adrenergic receptor binding and prostaglandin responsivity were measured in depressed patients. Depressed patients had significantly higher platelet 3H-dihydroergocryptine (3H-DHE) binding values than controls. Depressed patients also showed significantly reduced prostaglandin E1-stimulated cyclic adenosine 3',5'-monophosphate (cAMP) production and significantly decreased % inhibition of cAMP production by norepinephrine. These results support the suggestion that there may be a dissociation between alpha 2-adrenergic receptor binding and responsivity in depression. There were no significant correlations between platelet adrenergic variables and other indices of noradrenergic function. However, there was a significant correlation between 3H-DHE binding values and basal plasma levels of cortisol.

[3H]yohimbine binding to platelet alpha 2-adrenoceptors in depression

[3H]Yohimbine binding to platelet alpha 2-adrenoceptors was studied in depressed patients and healthy volunteers. Where possible platelet binding measurement was repeated in depressed patients following treatment. Bmax of [3H]yohimbine binding did not differ significantly between depressed patients and control subjects and did not change with treatment in depressed patients. KD was significantly lower in female depressed patients, particularly in those who were post-menopausal. Multivariate analysis showed significant effects on KD of depression, season of testing and assay protein concentration.

Polyunsaturated fatty acids, prostaglandins, and schizophrenia

Psychotic disorders, particularly schizophrenia, are associated with clinical phenomena that can be explained by disturbances in polyunsaturated fatty acid and prostaglandin metabolism. Previous studies of PUFA, PG synthesis, PGE1 receptor activity and aggregation responses in platelets, and clinical treatment trials suggest a role for PGE in the pathophysiology of schizophrenia. Since a decrease in PGE1 activity can be associated with an increase of dopamine release, a deficiency of PGE1 is consistent with the dopamine hypothesis of schizophrenia. State-of-the-art assay and clinical trial methodology should clarify the role of PUFA metabolism in schizophrenia.

Platelet 3H-clonidine and 3H-imipramine binding and plasma cortisol level in depression

Platelet 3H-clonidine (alpha 2-adrenergic agonist) binding and 3H-imipramine binding were measured and the Dexamethasone Suppression Test performed in 17 normal controls and 14 unmedicated depressed patients in order to clarify the relationship among these three biological markers. Increases in the Bmax and the Kd for 3H-clonidine binding and decreases in the Bmax for 3H-imipramine binding of the platelets from depressed patients were observed when compared with controls. There was a significant positive correlation among 3H-clonidine Bmax, the basal (predexamethasone) plasma cortisol levels, and the severity of depression, as indicated by the Hamilton Depression Rating Scale. On the other hand, no significant correlation was observed in 3H-imipramine binding between the Bmax and the severity of depression or between the Bmax and the basal plasma cortisol levels. There was no statistically significant correlation between the Bmax of 3H-clonidine binding and that of 3H-imipramine binding in depression, but there was a trend toward correlation in normal controls.

Increased 3H-clonidine binding in the platelets of patients with depressive and schizophrenic disorders

To examine whether alpha 2-adrenergic receptor function is altered in affective and schizophrenic disorders, we determined 3H-clonidine binding in platelets obtained from 33 normal control subjects and from 24 patients with depressive, 22 patients with schizophrenic, 18 with bipolar, and 8 patients with schizoaffective disorders during a drug-free period. The maximum number of binding sites (Bmax) and apparent dissociation constant (Kd) for high affinity 3H-clonidine binding was computed by Scatchard analysis. Comparison of the diagnostic groups indicated that the Bmax in depressed, schizophrenic, and schizoaffective patients was significantly higher than in normal controls, but there were no significant Bmax differences between bipolar patients and controls. Comparison of the Kd among the diagnostic groups indicated no significant differences among the groups or between patient diagnostic groups and normal controls. Baseline Bmax in schizophrenic patients was significantly correlated with the decrease in Brief Psychiatric Rating Scale (BPRS) scores after treatment, suggesting a relationship between baseline Bmax and clinical response. Treatment with lithium caused a significant decrease in the baseline Bmax, whereas treatment with desipramine or trifluoperazine did not cause significant changes in the baseline Bmax. Our results thus indicate an increase in the number of alpha 2-adrenergic receptors in depressed and schizophrenic patients as compared to normal controls.

GH responses to growth hormone releasing factor in depression

The growth hormone (GH), thyrotrophin (TSH) and prolactin response to growth hormone releasing factor (GRF) was investigated in 18 patients suffering from major depression with melancholia and in 18 age- and sex-matched normal controls. There was no significant difference in the GH response to GRF stimulation between the patients and controls and in neither subject group was there a demonstrable TSH or prolactin response to GRF. These findings indicate that the pathophysiology underlying the blunted GH response to pharmacological challenge, demonstrated in other studies, must lie at a suprapituitary level.

Alpha 2-adrenergic receptor sensitivity in depressed patients: relation between 3H-yohimbine binding to platelet membranes and clonidine-induced hypotension

Alpha 2-adrenergic receptor changes during antidepressant treatment were studied using 3H-yohimbine binding to human platelet membranes and clonidine-induced hypotension. Twenty-six patients with major depressive disorder (MDD) participated for 4-6 weeks in a trial of imipramine (2.5 mg/kg/day), tyrosine (100 mg/kg/day), or placebo. Alpha 2-adrenergic receptors measured by 3H-yohimbine binding were not significantly changed following antidepressant treatment. Similarly, clonidine-induced hypotension did not differ significantly following treatment. No measure of alpha 2-adrenergic receptor sensitivity was significantly correlated with clinical improvement. The correlation between platelet receptor binding and clonidine-induced hypotension was not statistically significant, even though both tests are considered to be measures of alpha 2-adrenoceptor sensitivity.

Noradrenergic function in generalized anxiety disorder, major depressive disorder, and healthy subjects

Plasma norepinephrine (NE), free 3-methoxy-4-hydroxyphenethylene glycol (MHPG), and binding of tritiated yohimbine to platelet membranes were measured in 14 patients with generalized anxiety disorder (GAD), who were matched for age and sex with 14 patients with unipolar major depressive disorder (MDD) and 14 normal subjects. Plasma NE and MHPG levels were increased and the number of alpha2-adrenoreceptors (Bmax) was decreased in GAD patients compared with MDD and normal subjects. No differences were found between MDD patients and normal subjects for plasma NE, MHPG, and alpha2-adrenoreceptor binding. Plasma NE and MHPG were significantly correlated in MDD patients and tended toward a significant positive correlation in GAD patients. Plasma MHPG and affinity of binding platelet alpha2-adrenoreceptors (Kd) were significantly correlated in normal subjects. Thus, noradrenergic activity seems to be increased in patients with GAD, but not in patients with MDD. In GAD patients, higher levels of catecholamines may lead to a down-regulation of presynaptic alpha2-adrenoreceptors.

Platelet alpha-2-receptor binding and adenylate cyclase activity in panic disorder

The function of the alpha-2-receptor and intracellular effector systems was examined in 39 panic disorder patients and 30 healthy subjects using the platelet as a model system. Alpha-2-receptor density, as reflected by 3H-yohimbine binding, was not different between the two groups. Platelet alpha-2-receptor affinity was decreased (higher Kd) in the panic disorder patients. Other significant abnormalities in the patients included a decreased basal adenylate cyclase activity, reduced EC50 for the epinephrine inhibition of adenylate cyclase activity, and decreased stimulation of adenylate cyclase activity by prostaglandin E1 and sodium fluoride. These results are consistent with a dysfunction in some panic disorder patients at the level of the stimulatory GTP binding regulatory protein, Gs, or the adenylate cyclase catalytic unit. The relationship of these findings to previous studies of noradrenergic function in panic disorder patients is discussed.

Growth hormone (GH) responses to GH-releasing hormone in depression: correlation with GH release following clonidine

Twenty subjects (10 patients with major depressive disorder and 10 controls matched for age, gender, and ovarian status) received 1 microgram/kg synthetic human growth hormone-releasing hormone (GHRH)-44 amide as an i.v. bolus dose. Compared to controls, depressed patients showed a significant attenuation of net growth hormone (GH) responses to GHRH associated with normal basal GH concentrations. The blunted GH responses occurred in the face of significantly higher somatomedin C (Sm-C) concentrations. Comparison of GH responses after GHRH with GH output following the alpha 2-agonist clonidine (CLON) revealed a significant positive correlation. The concordance between GH responses after specific challenges at different levels of the GHRH-GH-somatomedin axis indicates the integrity of the hypothalamic-pituitary-somatotropic system in depression and supports the view that altered GH secretory patterns in depression may primarily be due to a suprapituitary disturbance.

Rapid antidepressant response to alprazolam in depressed patients with high catecholamine output and heterologous desensitization of platelet adenylate cyclase

The present study examined the relationship between 24-hr urinary catecholamine (norepinephrine and epinephrine) output and measures of platelet adenylate cyclase (AC) activity in depressed patients (n = 17) and control subjects (n = 10). In both groups, significant inverse correlations were observed when 24-hr urinary catecholamine levels were examined in relation to measures of both receptor-mediated (prostaglandin D2 and alpha 2-adrenergic) and postreceptor-mediated (NaF) platelet AC enzyme activities, suggesting that circulating catecholamines may regulate platelet AC by heterologous (agonist-nonspecific) desensitization of the AC enzyme complex. Depressed patients who had favorable antidepressant responses to alprazolam had significantly higher pretreatment urinary catecholamine output and lower receptor-mediated platelet AC enzyme activities than control subjects, whereas the nonresponders did not. After 8 days of treatment with alprazolam, urinary catecholamine levels declined significantly. In responders, receptor-mediated measures of platelet AC activity increased significantly by day 8 to values comparable to those in control subjects; but similar changes were not observed in nonresponders. Prior to treatment, responders showed a strict linear relationship between receptor-mediated (prostaglandin D2) and postreceptor-mediated (NaF) stimulation of platelet AC activity through the stimulatory guanine nucleotide regulatory protein (Ns), whereas nonresponders did not. This suggests the presence of two distinct coupling interactions between platelet prostaglandin D2 receptors and the stimulatory guanine nucleotide regulatory protein in responders and nonresponders to the antidepressant effects of alprazolam prior to treatment. The authors propose that catecholamines, possibly acting through prostaglandins, may regulate platelet AC enzyme activity by heterologous desensitization occurring through postreceptor mechanisms.

Platelet research in psychiatry

The platelet is one of the most researched biological markers in psychiatry. Characteristics of MAO activity, 5-HT uptake, imipramine and alpha 2-adrenergic receptor binding, for example, are similar in platelet and CNS. Methodological factors are not negligible, and range from diagnostic specificity and drug effects to the normal physiological variability of age and hormone-related changes, circadian and seasonal rhythms. As yet, there are no clear state or trait platelet markers in affective disorders and schizophrenia that can be unequivocally used to detect vulnerability to the illness, predict therapeutic response, define clinical diagnostic entities or follow the course of the illness. However, platelet markers are increasingly being used in careful studies to monitor psychopharmacological effects (an in vivo assay of all active metabolites), different ligands can be specific markers for certain aspects of a psychiatric illness (e.g. alpha 2-adrenergic receptors and weight loss), and this homogeneous preparation of human cells is an increasingly important tool in studying mechanisms in pathophysiology. More longitudinal studies are required to establish functional relationships between platelet variables and psychopathology.

Uncoupling of the platelet alpha 2-adrenoceptor adenylate cyclase system by N-ethylmaleimide and the resulting effect on receptor density and sensitivity

Preincubation of platelet-rich plasma with N-ethylmaleimide (NEM) attenuated the inhibitory effect of the alpha 2-adrenoceptor agonist UK 14304 on basal and forskolin-stimulated adenylate cyclase activities. NEM also led to concomitant marked reductions of the specific binding of [3H]UK 14304 to platelet membranes and of the primary aggregation response induced by UK 14304. These results indicate that uncoupling of the receptor adenylate cyclase system by NEM induces down-regulation of platelet alpha 2-adrenoceptor density (3H-agonist binding sites) and of the associated functional response (platelet aggregation).

Receptor adaptive responsiveness in disease models: 6-OHDA lesioned and spontaneously hypertensive rats

We have examined the adaptive modifications of brain monoamine receptors in response to pathophysiological processes in animal disease models: 6-OHDA lesioned and spontaneously hypertensive rats (SHR). The two models share a similar increase in D-1 receptor densities, while noradrenergic receptors are affected in different way: alpha-1 and beta are supersensitive in 6-OHDA lesioned rats and only alpha-2 are increased in SHR. S-1 receptors too are up-regulated in SHR. We must notice that though receptor hypersensitivity in the 6-OHDA model is linked to massive decreases in neurotransmitter levels, this mechanism seems not to exist in the SHR model.

Clonidine challenge testing of alpha-2-adrenoceptor function in man: the effects of mental illness and psychotropic medication

The clonidine challenge test is a means of assessing alpha-2-adrenoceptor sen sitivity in man. We review studies which have used this test to investigate central alpha-2- adrenoceptor changes in psychiatric illness, and to determine receptor changes after adminis tration and withdrawal of psychotropic treatments. Patients with severe depression show evidence of reduced alpha-2-adrenoceptor sensitivity, especially a reduced growth hormone response to clonidine. This may delineate a subgroup of patients with severe depressive illness from those with milder depression, and may even provide a trait marker for some depressed patients. Patients with panic disorder show evidence of subsensitivity to some and supersensitivity to other clonidine-induced responses. Other disorders, although less well investigated, may have abnormal test responses which may provide additional information about their cause and treatment. Changes in response after drug treatment have provided important information on the mode of action of antidepressants, and have suggested that noradrenergic function is altered by a variety of different antidepressants.

Platelet alpha 2-adrenergic receptor function in psychiatric disorders

The inhibitory effects of norepinephrine (NE) on the cyclic adenosine-3',5'-monophosphate (cAMP) response to prostaglandin E1 (PGE1), a measure of alpha 2-adrenergic receptor function, have been compared in platelets from drug-free schizophrenic patients, depressive patients, and normal controls. The absolute value of the inhibition by NE of the cAMP response to PGE1 was smaller in platelets from schizophrenic and depressive patients than in controls. However, this result was secondary to the smaller baseline platelet cAMP response to PGE1 in patients with these disorders. Effects of NE on cAMP production did not discriminate between actively ill and remitted patients with either schizophrenia or depression. Platelet alpha 2-receptor sensitivity, as measured by the effects of NE on cAMP production, does not appear to be altered in these psychiatric disorders.

The neurobiology of aging: does it predispose to depression?

This review examines the various research approaches undertaken to investigate possible central nervous system correlates of major depressive illness and relates findings from these studies to the alterations in central nervous system and neuroendocrine function that normally accompany aging in humans. The topics reviewed include: epidemiology of depression and suicide in the elderly; monoamine theories of depression; neuroendocrine disturbances in depression; and imaging studies.

Incorporation of 14C-arachidonic acid into platelet phospholipids of untreated patients with schizophreniform or schizophrenic disorders

The incorporation rate of 14C-labeled arachidonic acid (14C-AA) into membrane phospholipids was measured in a group of untreated (greater than 6 months) psychiatric patients (n = 33) and healthy controls (n = 31). Platelets from controls and from patients with schizophrenia (n = 10), schizophreniform disorder (n = 11), schizoaffective disorder (n = 6), major depression (n = 2), or an atypical psychosis (n = 4), diagnosed according to DSM-III, were incubated with 14C-AA. Platelets from patients with a schizophreniform and a schizoaffective disorder incorporated greater than 50% less 14C-AA than the platelets from controls. The incorporation rates of platelets from schizophrenic patients were slightly (18%), but not significantly, reduced compared to controls. Characterization of variables affecting arachidonic acid and phospholipid metabolism may be helpful in studies focused on the assessment of first-episode psychotic patients and in long-term outcome studies.

Noradrenergic function and hypothalamic-pituitary-adrenal axis activity in primary unipolar major depressive disorder

Plasma levels of cortisol, norepinephrine (NE), and 3-methoxy-4-hydroxyphenylglycol (MHPG) were found to be significantly higher in 16 drug-free patients with primary, unipolar major depressive disorder than in 20 controls. Plasma free MHPG and basal cortisol levels showed a significant positive correlation in the controls, but not in the patients. There were, however, significant positive correlations between cortisol and NE, as well as between NE and free MHPG levels in the patients. No correlations were observed between patient plasma NE levels and platelet alpha 2-adrenoceptor or lymphocyte beta-adrenoceptor Kd or Bmax values. These peripheral measures of noradrenergic function are proposed as useful markers for patients with primary, unipolar major depressive disorder with melancholia.

Alpha 2-adrenergic receptors in platelet membranes of depressed patients: increased affinity for 3H-yohimbine

Specific binding to alpha 2-adrenergic receptors was studied in the platelets of 31 patients with major depressive disorder and 18 normal controls using the selective antagonist 3H-yohimbine. Receptor density for depressed patients (Bmax = 88 +/- SD 45.1 fmoles/mg) was not significantly lower than that for controls (124 +/- SD 78.1 fmoles/mg). The affinity of the receptor for yohimbine was significantly greater in depressed patients (Kd = 1.05 +/- SD 0.47 nM) than in controls (Kd = 1.47 +/- SD 0.63 nM). This is consistent with the hypothesis of increased alpha 2-adrenergic receptor sensitivity in depressive disorders. Past studies of alpha 2-adrenergic receptors on platelets are reviewed, and the importance of designing studies with sufficient statistical power is discussed.