alpha 2-Adrenergic receptors in platelet membranes of depressed patients: no change in number of 3H-yohimbine affinity

Is depression a disorder of a receptor superfamily? A critical review of the receptor theory of depression and the appraisal of a new heuristic model

The monoamine hypothesis of depression and its direct derivation, the receptor theory, have constituted for several years a frame of reference for researchers working in the field of biological psychiatry. Although most of the data are derived from animal findings and must be considered inconclusive in view of various controversies, some guidelines may be identified: these would suggest that changes in postsynaptic beta-adrenoreceptors, presynaptic alpha 2-adrenoreceptors, as well as in type 2 serotonin receptors and dopaminergic autoreceptors may be involved in the mode of action of antidepressant drugs and, consequently, in the pathophysiology of depression. Nowadays, any attempt to correlate depression with the dysfunction of a single neurotransmitter or receptor is no longer tenable, since it is clear that depression is a heterogeneous disorder which involves abnormalities in the interactive relationships between neurotransmitters and receptors. If, on the one hand, this new model has opened up new fields of research and has led to the investigation of new systems,egthe GABAergic and GABA B receptors, on the other hand, it has been strongly limited by the lack of research tools and reliable peripheral CNS models forin vivostudies. A possible approach to this unresolved dilemma may be provided by molecular biology techniques, which have permitted the identification of the genes and sequencing of the primary structure of several membrane receptors. It is now established that receptors may be grouped into four superfamilies; in depression, there exists compelling evidence of alterations mainly in receptors belonging to the G-protein-coupled family: it is plausible that depression may be related to a disorder of the G-protein-coupled receptor superfamily. Such an hypothesis would represent an attempt to unify the different receptor abnormalities found in depression or following antidepressant treatments, and to shift from the monoamine paradigm to a new heuristic model. In addition, it would accommodate the various dysfunctions likely to be encountered and would open up new theoretical perspectives in the treatment of depression.

Biological correlates of personality: considerations on the possible usefulness of central nervous system peripheral markers

Although a great deal of biological research has been carried out on several psychiatric disorders, it is disappointing to see how little progress has been made in the field of the biology of personality. The authors underline the methodological problems that arise in the investigation of biological substrates of human personality and review both currently available and putative peripheral markers of the central nervous system that might be used in further human studies.

Beta adrenergic receptor function in depression and the effect of antidepressant drugs

It has been suggested that alterations of monoamine receptor sensitivity in the central nervous system may be associated with some forms of affective illness. It has been observed by several investigators that chronic treatment with antidepressant drugs causes down regulation of NE receptor coupled adenylate cyclase and beta adrenergic receptor binding in rat brain. This observation has led to the suggestion that the therapeutic effects of antidepressant drugs may be related to the changes in the responsivity of beta adrenergic receptors. In order to examine if depressive illness may be associated with altered beta adrenergic function, we studied adenylate cyclase and its responsiveness to norepinephrine and isoproterenol in the leukocytes obtained from patients with psychiatric illness and normal controls as an index of beta adrenergic receptor function. We also studied the effects of antidepressant drugs, in vitro, on isoproterenol sensitive leukocyte adenylate cyclase. We observed that norepinephrine and isoproterenol sensitive leukocyte adenylate cyclase in depressed patients are significantly decreased as compared to normal controls. Our results appear to have been replicated by another group of investigators. We also observed that certain antidepressant drugs potentiate isoproterenol stimulated accumulation of cyclic AMP in human leukocytes. This potentiation was most pronounced in the case of iprindole. These results thus indicated a decreased beta adrenergic receptor function in patients with depressive illness. Whether or not such decreased receptor function is associated with depressive illness or is a manifestation of some other changes unrelated to the illness is not clear. Our results also indicate that some antidepressant drugs may enhance adrenergic transmission by potentiating the effects of neurotransmitters on beta adrenergic receptors.

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.

Brain alpha-adrenoceptors in depressed suicides

alpha1-Adrenoceptors and alpha2-adrenoceptors were measured by radioligand binding to homogenates of brain samples obtained at post-mortem from suicides with a retrospective diagnosis of depression, and age and gender-matched controls. Suicides were subdivided into those who had been free of antidepressant drugs for at least three months, and those in whom prescription of antidepressant drugs was clearly documented. The number of alpha1-adrenoceptors (or alpha1A + alpha1D-adrenoceptors) did not differ significantly between antidepressant-free or antidepressant-treated suicides and controls. In antidepressant-free suicides, the number of alpha2-adrenoceptors was significantly higher in temporal cortex (Ba 21/22). alpha2A-Adrenoceptors did not differ significantly from controls in this brain region, suggesting the involvement of other alpha2-adrenoceptor subtypes. In antidepressant-treated suicides, significantly lower numbers of alpha2-adrenoceptors were found in occipital cortex and hippocampus (and for alpha2A-adrenoceptors in caudate and amygdala) compared to controls.

Effects of electroconvulsive therapy on peripheral adrenoceptors, plasma, noradrenaline, MHPG and cortisol in depressed patients

BACKGROUND

The mechanism of the antidepressant action of electroconvulsive therapy (ECT) remains unknown. Based on previous work with antidepressant drugs and their effects on the noradrenergic system, we undertook this study to further determine the effects of ECT on selected indices of peripheral adrenoceptor function in depressed patients.

METHODS

Binding parameters (Bmax and Kd) of platelet alpha 2- and leukocyte beta 2-adrenoceptors, plasma noradrenaline (NA), 3-methoxy-4-hydroxy-phenylglycol (MHPG) and cortisol levels were determined in 18 patients, prior to treatment and 14 days after the last of a series of ECTs, and compared with samples obtained from 18 matched control subjects.

RESULTS

Platelet alpha 2-adrenoceptor sites were significantly elevated in untreated patients compared with controls (P < 0.03), but leukocyte beta 2-adrenoceptor numbers did not differ. Treatment with ECT led to a significant reduction in platelet alpha 2-adrenoceptor numbers, whereas leukocyte beta 2-adrenoceptor densities increased. Pre-ECT plasma NA, MHPG, and cortisol levels were elevated in patients, compared with controls, and decreased following ECT, but these differences were not statistically significant. Post-ECT plasma NA and beta 2-adrenoceptor numbers were significantly, negatively correlated (P < 0.05).

CONCLUSIONS

These results suggest that platelet alpha 2-adrenoceptors are supersensitive in depressed patients and treatment with ECT results in down-regulation of these receptors, which may be interpreted as a primary therapeutic, "normalising' effect. The post-ECT changes in leukocyte beta 2-adrenoceptors are probably only secondary to the lower circulating plasma NA levels.

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.

The adrenergic-cholinergic imbalance hypothesis of depression: a review and a perspective

The catecholamine deficiency hypothesis of depression was essentially based on the incidental detection of iproniazide and imipramine. However, current findings favor noradrenergic overactivity, at least in the periphery. The incidental observation of acute behavioral inhibition by centrally active cholinomimetics like physostigmine suggested a cholinergic-adrenergic balance involved in the regulation of drive and mood. Indeed, cholinomimetics seem to have acute depressiogenic and antimanic properties and, conversely, anticholinergics some acute euphoriant activity. However, time course and dose-response relationships of drugs influencing mood and drive do not favor simple concepts of too much or too little activity of one or the other transmitter system. Cholinomimetics and psychostimulants show an acute mutual antagonism, the mechanism of which is obscure. In healthy volunteers clonidine and the putative antidepressant brofaromine did not influence the effects of physostigmine. Patients with mood disorders respond supersensitively to a cholinergic challenge in terms of behavior, neuroendocrine regulation and REM sleep induction. Thus, the anticholinergic properties of tricyclics might be relevant to their antidepressant activity. However, adjunctive treatment with the cholinolytic biperiden as compared to placebo did not enhance the antidepressant efficacy of mianserin or viloxazine. This is incompatible with cholinergic overactivity contributing to the depressive state. Physostigmine induces autonomous and endocrine responses reminiscent of stress reactions. Findings in healthy volunteers suggest relationships between the sensitivity to physostigmine and personality traits like irritability and emotional lability and passive stress coping strategies. Thus, the cholinergic supersensitivity in mood disorders might be related to some personality dimension like stress intolerance rather than the depressive state itself.

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.

Serotonin but not norepinephrine-induced calcium mobilization of platelets is enhanced in affective disorders

Intracellular calcium concentrations ([Ca++]i) in blood platelets from 11 depressed patients and 11 healthy controls were investigated. The resting [Ca++]i of platelets from depressed patients was 69.4 +/- 2.9 nM while that from controls was 74.6 +/- 4.0 nM. Serotonin (5-HT), at a concentration of 10 microM, increased [Ca++]i by 129.2 +/- 3.9 nM in platelets from depressed patients, which was significantly greater than that found in platelets from control subjects (105.2 +/- 6.0 nM). Norepinephrine (NE) 100 microM increased [Ca++]i by 46.1 +/- 7.1 nM in platelets from depressed patients, and by 38.6 +/- 6.1 nM in platelets from controls, respectively. These results indicate that 5-HT2 receptor function in platelets of depressed patients is enhanced, and support the hypothesis of hypersensitivity of 5-HT2 receptors in affective disorders.

Platelet alpha 2-adrenoceptors, defined with agonist and antagonist ligands, in depressed patients, prior to and following treatment

Saturation binding of the alpha 2-adrenoceptor antagonist, 3H-yohimbine, and displacement of 3H-yohimbine with the alpha 2-adrenoceptor agonist, UK-14,304, were performed concurrently in platelet membranes obtained from drug-free depressed patients and healthy volunteers. Where possible platelet binding was repeated in depressed patients following treatment. The number and affinity of 3H-yohimbine binding sites did not differ between controls and depressed patients, or when depressed patients were divided on the basis of endogenicity (Newcastle or RDC criteria) or dexamethasone test result. The proportion of alpha 2-adrenoceptor binding sites with high affinity for UK-14,304 and KD values for the two states of the receptor did not differ in the total sample of depressed patients compared to controls. The KD for both states of the receptor and the proportion of sites with high affinity for UK-14,304 was lower in RDC non-endogenous patients than RDC endogenous patients. Treatment did not alter the total number of alpha 2-adrenoceptors or the proportion of sites with high affinity for UK-14,304, but reduced the KD for 3H-yohimbine and the KD of UK-14,304 for the low affinity state of the alpha 2-adrenoceptor.

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.

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.

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.

A comparison of methodologies for the study of functional transmitter neurochemistry in human brain

A number of different approaches to the study of functional neurochemistry in human brain are discussed. The advantages and disadvantages of three main techniques are contrasted: (i) using animal tissue preparations as models of the human brain; (ii) using human peripheral tissue preparations as models of dynamic CNS processes; and (iii) studying human tissue, obtained postmortem, directly. Animal models are often readily obtained and reliable, and the high degree of inbreeding of common laboratory animals ensures that they usually yield consistent results. However, there are a number of human disorders for which animal models are either poor or unavailable, and species differences make extrapolation from the animal to the human case difficult. Human peripheral tissue models rely on a degree of homology between peripheral and CNS processes; in most cases, the evidence for such homologies derives from animal, rather than human, studies. Moreover, several examples are known where a peripheral process mimics the equivalent glial cell activity more closely than the neuronal, which can be a serious drawback for studies of neurotransmission. The use of postmortem human brain tissue presents a number of obvious difficulties, resulting from variations in the patient's age, agonal state, sex, preterminal medication, postmortem delay, etc. Human beings are genetically and nutritionally heterogeneous, so that data variability is usually greater here than when using tissue from laboratory animals. However, it is possible to control for a number of these factors, for example, by matching samples for basal metabolic rate and tissue integrity, and recently developed tissue freezing and storage techniques permit the use of within-subject experimental designs to help reduce experimental variation. A range of neurotransmitter functions are well retained in such tissue samples, so that regional variations, differential transmitter activities, drug effects, etc., can be studied in normal tissue samples, as well as in samples taken from cases of neurological and psychiatric disease. This allows, for example, changes in neuroanatomical indices to be correlated with localised alterations in a specific neurotransmitter function. A systematic approach to the analysis and matching of tissue samples is advocated. The three approaches should be considered to be complementary, especially for the study of human brain diseases.

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.

Super high affinity 3H-para-aminoclonidine binding to platelet adrenoceptors in depression

1. An assay was developed using sucrose gradient purified platelet plasma membranes which allowed detection, for the first time in patients, of both super-high affinity (KD = 17 pM) and high affinity (KD = 1.7 nM) binding sites. 2. Limited Scatchard plot analyses were performed on platelet membranes from depressed patients and controls using 10 pM-2.5 nM 3H-p-aminoclonidine (3H-PAC). 3. Patients (n = 9) were age-paired with healthy control subjects for simultaneous blood drawing, platelet preparation and analysis. 4. All patients were endogenous depressives with Hamilton-Depression scores ranging from 19 to 30 at the time of pre-treatment. Seven of the nine patients were analyzed again at six weeks of treatment with antidepressant medication. 5. Using 60 pM 3H-PAC (a concentration determined to bind predominantly to the super-high affinity receptor state) pre-treatment patient values were higher then paired controls (p = 0.06). Post-treatment analysis of seven of the patients and paired controls showed no differences (p = 0.5) suggesting a normalization of receptor binding following treatment. 6. No differences were observed in platelet yield or morphology or in the percent of other blood cell contaminants in the platelet preparations between patients at pre-treatment and controls. However, the platelet yield was significantly lower in patients post-treatment (p = 0.06). 7. These results are in agreement with two previous studies showing elevated 3H-clonidine binding to high affinity sites from depressed patients. The data presented herein suggest that there is a modest 1.25-fold elevated super-high affinity platelet adrenoceptor binding in depressed patients pre-treatment. Receptor binding becomes normal post-treatment.

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.

Dieting and weight loss in volunteers increases the number of alpha 2-adrenoceptors and 5-HT receptors on blood platelets without effect on [3H]imipramine binding

Platelet monoamine receptor binding was determined in normal male and female subjects before and at the end of a 2- to 3-week weight reducing diet (1200 kcal daily). Dieting was associated with an increase in platelet binding sites for both [3H]yohimbine and [125I]iodolysergic acid diethylamide (iodoLSD). The affinity at the platelet [125I]iodoLSD binding site was reduced. In contrast, [3H]imipramine binding was unchanged. These results have important implications for studies that employ platelet binding as a peripheral marker of neurotransmitter function in psychiatric illness.

[3H] UK-14,304, a new agonist ligand of alpha 2-adrenoceptors: a comparative study with human and rat tissue

[3H] UK-14,304 was used to investigate alpha 2-adrenoceptors in rat brain and human platelets. Receptor pharmacology revealed that the ligand binds with high affinity to alpha 2-adrenoceptors. Psychoactive substances like neuroleptics, antidepressants, and beta-carbolines displace [3H] UK-14,304 from its binding sites in the lower micromolar range. A Hill number around 2 for most neuroleptics suggests a positive cooperativity with the alpha 2-adrenoceptors. Comparative studies with [3H] UK-14,304 and [3H] clonidine utilizing platelet membranes from human volunteers demonstrated that the former ligand is more suitable to investigate possible changes of alpha 2-adrenoceptors; [3H] UK-14,304 labels more receptors with a lower standard deviation, whereby the volume of the blood sample amounted to 35 ml instead of 50 ml required for [3H] clonidine as ligand. No sex differences of binding constants were detected, however an inverse correlation of maximum number of binding sites and affinity was found for female subjects with both ligands. No age-dependent changes of Bmax and KD-values were observed in the range of 24 to 59 years.

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.

Enhanced binding of [3H] (-) adrenaline to platelets of depressed patients with melancholia: effect of long-term clomipramine treatment

The specific binding of the full agonist [3H] (-) adrenaline to platelet membranes was measured in 14 drug-free depressed patients with melancholia. There was an increased number of binding sites (Bmax) with the same apparent affinity (KD) for the radioligand, indicating that platelet alpha 2-adrenoceptor density in the high affinity state was increased in depressed patients. Long-term administration of clomipramine was associated with decreases in platelet alpha 2-adrenoceptor densities which correlated with the duration of treatment. The results support the hypothesis that in endogenous depression there is a dysfunction of the alpha 2-adrenoceptor and that antidepressant treatment might involve a time-dependent desensitization process of this receptor.

Evaluation of studies on platelet alpha 2 adrenoreceptors in depressive illness

Discrepant results have been reported from at least ten laboratories regarding the status of platelet alpha 2 adrenoreceptors in depressed patients. Using a statistical test to combine those studies which utilized radioligand binding techniques, we find the overall data support an elevation in density of platelet alpha 2 adrenoreceptors from drug-free depressed patients (p less than 0.05) and suggest a normalization to lower binding values following antidepressant drug treatment (0.05 less than p less than 0.10). However, these positive results are attributable to highly significant findings by only three laboratories. Much of the discrepancy may be attributable to numerous methodological variables which distinguish the studies. Foremost amongst these variables are the use of different platelet size populations, the use of different medium, and the choice of radioactive ligand and competitor (non-radioactive ligand) in the assay. We present a rationale for the proper choice of each methodological condition used in the clinical assessment of platelet alpha 2 adrenoreceptor status, hoping that improved experimental designs will resolve the current controversy.

Binding of yohimbine and imipramine to platelets in depressive illness

Radioligand binding to intact platelets was carried out in antidepressant-free patients and the 1 mg dexamethasone suppression test (DST) was performed. There were no differences in binding characteristics between patients and controls for either [3H]yohimbine or [3H]imipramine. There were no differences in binding between patients classified as endogenous using the Newcastle Scale, compared with non-endogenous patients, and no difference between DST suppressors and non-suppressors. The severity of depression did not affect binding values. After 4-6 weeks antidepressant treatment [3H]yohimbine binding was significantly reduced but [3H]imipramine binding was unaffected.

Alpha 2-adrenoceptor levels on platelets of patients with major depressive disorders

3H-p-Aminoclonidine binding to platelets of patients with primary, unipolar major depressive disorder was compared to that of a normal healthy control population. The variances of platelet alpha 2-adrenoceptor Kd and Bmax values in patients were significantly greater than in the control group. No significant difference could be demonstrated between the Kd values of the two different groups, but the Bmax value of the depressed group was significantly lower than that of controls. We propose that an abnormal platelet alpha 2-adrenoceptor density may be used as a biological marker for major depressive disorder.

Alpha 2-adrenergic receptor function in patients with unipolar and bipolar affective disorders

Alpha 2-adrenergic receptor function was measured in platelets from unipolar (UP) depressed, bipolar (BP) depressed, and bipolar euthymic patients and normal control subjects. Only the platelets from UP depressed patients were different from control in having an increased number of alpha 2-receptors, a decreased percent norepinephrine inhibition of prostaglandin E1 (PGE1)-stimulated cyclic AMP (cAMP) production, and a decrease in PGE1 stimulation of cAMP production. Platelets from BP patients, depressed or euthymic, were not significantly different from control subjects. These preliminary data suggest that alpha 2-adrenergic receptor function and PGE1 stimulation of cAMP production are diminished in UP patients.

Peripheral adrenoceptors and serotonin receptors in depression. Changes associated with response to treatment with trazodone or amitriptyline

Changes in platelet and lymphocyte adrenoceptor densities, platelet serotonin uptake and aggregatory response to serotonin were assessed in a group of moderately depressed patients before and during treatment with either trazodone or amitriptyline. Platelet serotonin receptor activity and uptake were lower before the start of treatment in all patients than in those patients responding to treatment. The densities of alpha 2- and beta-adrenoceptors tended to be higher in the patients before treatment and returned to control values after effective therapy. There were no major differences in the biochemical changes between the patients treated with trazodone or amitriptyline. When the biochemical data was correlated with the clinical history of the patients, it was found that only endogenously depressed patients, and not those with non-endogenous depression, had a significantly reduced platelet serotonin uptake rate. In addition, female depressives had a slightly lower platelet 5-HT aggregatory response than males irrespective to the type of depression.

Adrenergic receptors in depression. Effects of electroconvulsive therapy

Platelet alpha 2-and lymphocyte beta 2-adrenoceptor densities, plasma noradrenaline and serum cortisol were measured before, during and one week after a course of EEG-monitored electroconvulsive therapy, in nine depressed patients. A 50% fall in Hamilton Depression Rating scores occurred after a fairly consistent total seizure time, regardless of the amount of ECT given. Platelet alpha 2-adrenoceptor densities showed a statistically significant fall after three ECTs, but were unchanged after the full course of ECT and were independent of clinical change. Lymphocyte beta 2-adrenoceptor densities were unaltered. Plasma noradrenaline concentrations were initially high, and fell with ECT in a manner paralleling clinical recovery. Plasma noradrenaline may be a more useful index of central changes during antidepressant treatment than peripheral blood cell receptor densities.

Neurochemical aspects of depression: the past and the future?

The role of aberrant neurochemical substrates in the etiology of depression and the neurochemical mechanisms of antidepressant therapies have been the subjects of many hypotheses in the last 30 years. Pharmacological studies of early antidepressant drugs indicated that brain monoamines were significantly affected by these drugs and these led to the formulation of the biogenic amine hypothesis of depression. Although this hypothesis has been of heuristic value in the study of drug mechanisms and has provided a basis for screening drugs for antidepressant potential, deficiencies in it have become apparent. Neuroanatomical and neurochemical considerations favour the view that brain noradrenaline and serotonin systems may serve as bias adjusting systems for each other and numerous other neural systems. As a consequence of such a relationship, a primary defect in some other neural system would appear amplified in measurements of serotonin or noradrenaline. A possible site for this primary defect may be in membrane composition and function. Recent studies have found that typical and other antidepressant therapies have a pronounced effect on membrane lipids. Thus, in view of the important functions of membrane lipids and the fact that they have been linked to the initiation and development of a number of other disease processes, it is now suggested that consideration be given to them as playing primary causal roles in the etiology of depression and as a site of action for antidepressant drugs.

Enhanced signal transduction by adenylate cyclase in platelet membranes of patients showing antidepressant responses to alprazolam: preliminary data

The triazolobenzodiazepine, alprazolam, was administered to 11 depressed patients over a period of six weeks, and six patients showed a favorable antidepressant response. There were no significant differences between responders and nonresponders in age, pretreatment Hamilton Depression Rating Scores, 4 p.m. postdexamethasone plasma cortisol levels, or platelet monoamine oxidase activities. Blood levels of alprazolam were not meaningfully different in responders and nonresponders when measured on treatment day 8. However, on treatment day 8, significantly enhanced prostaglandin D2-stimulated platelet adenylate cyclase activity, greater suppression of prostaglandin D2-stimulated platelet adenylate cyclase activity by epinephrine, and enhanced sodium fluoride-stimulated platelet adenylate cyclase activity were seen in the six patients who went on to respond to alprazolam, but not in the five nonresponders. In contrast, there were no significant changes in prostaglandin D2, (-)-isoproterenol, or fluoride ion-stimulated leukocyte adenylate cyclase activity in responders or nonresponders. No meaningful changes were observed in the mean densities of either the high-affinity platelet alpha 2-adrenergic receptor (for 3H-p-aminoclonidine) or the leukocyte beta-adrenergic receptor (for 3H-dihydroalprenolol) in responders or nonresponders. The present findings, taken in conjunction with findings from other recent studies, suggest that enhanced coupling between certain neurotransmitter or hormone receptors and adenylate cyclase through the guanine nucleotide regulatory proteins may help explain the antidepressant effects of alprazolam and possibly other forms of antidepressant treatment.

Platelet alpha 2-adrenoceptors in major depression: relationship with urinary 4-hydroxy-3-methoxyphenylglycol and age at onset

Platelet alpha 2-adrenoceptor number and affinity were measured in 31 drug-free patients with major depressive illness utilizing 3H-clonidine as ligand. A significant negative correlation was found between number of alpha 2-adrenoceptors, baseline urinary 4-hydroxy-3-methoxyphenylglycol (MHPG) excretion, present age and age at onset of the disease. Kd did not correlate with any of these variables not with the Bmax of platelet alpha 2-adrenergic binding. Multiple regression analysis, with MHPG and age at onset as independent variables, explained variance for alpha 2-adrenoceptor density better than single regression (from 19% for MHPG and 30% for age at onset to 40%), with the addition of both these variables being significant.

Characterization of platelet alpha 2 adrenoceptors and measurement in control and depressed subjects

The alpha-adrenoceptor on platelets has been characterized using 3H-yohimbine, 3H-dihydroergocriptine, and 3H-clonidine. The receptor, which exhibits the characteristics of an alpha 2-type, has a Bmax for dihydroergocriptine of 330 fmoles/mg protein, for yohimbine of 178 fmoles/mg protein, and for clonidine of 38 fmoles/mg protein. Clonidine, but not yohimbine binding, is decreased by the presence of K+, Na+, or Li+. Adenosine triphosphate (ATP) and the guanosine triphosphate (GTP) analogue, Gpp(NH)p, reduce the affinity of clonidine for its binding site. Acute exercise, such as playing squash, does not apparently alter the Bmax or Kd of 3H-yohimbine binding to platelet membranes, and in vitro studies, with intact platelets or platelet membranes, show that incubation with adrenalin (10 microM) does not induce alterations in Bmax or Kd. In the present study, no correlation was found between age and alpha 2-adrenoceptor numbers. There was no significant difference in the Bmax for 3H-yohimbine binding to platelet membranes from control and depressed subjects, although the mean value for the depressed group was some 10% lower than that for the corresponding control group. There were no overall significant and consistent effects of desipramine (DMI) treatment. After 2-3 days of treatment, the Bmax was reduced by 20%, after 7 days by 14%, and after 21 days it was 8% above the control value. When an additional group of patients on DMI (7 days of treatment) was analyzed using one supramaximal concentration of 3H-yohimbine, there was a significant decrease (25%) in binding.

3H-clonidine binding to membranes of platelets prepared under physiological conditions

A reproducible binding assay has been established to measure alpha 2-adrenoreceptors on membranes of human platelets prepared under physiological conditions. Washed platelet suspensions were obtained from fresh blood by a modified method (Mustard et al., 1972) and membranes were prepared by mechanical homogenization. 3H-clonidine, a selective alpha 2-adrenoreceptor agonist was selected as the ligand in a concentration range of 2-64 nM. Specific binding of 3H-clonidine was defined as the difference between total bound radioactivity and that not displaced by excess cold clonidine (6.4 X 10(-5) M). Platelets from 14 healthy young male volunteers were analysed using this technique, and one high affinity binding site, with KD and Bmax values in the reported range was found (KD: 10.14 +/- 1.95 nM; Bmax: 428 +/- 53 fmol/mg protein (mean +/- S.E.M.)). This technique will be used in future studies that will examine alpha 2-adrenoreceptor changes in specific subgroups of depressed patients.

Alzheimer's disease: isoproterenol and prostaglandin E1-stimulated cyclic AMP accumulation in lymphocytes

Reduced lymphocyte beta-adrenergic receptor activity was observed in patients with Alzheimer's disease and in aged controls; a parallel decline in lymphocytic prostaglandin E1 receptor activity was seen in the aged controls. In the Alzheimer patients, however, such lymphocytic prostaglandin E1 receptor activity was significantly raised and correlated with a rating scale for severity of dementia.

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

In a study of platelet alpha 2-adrenergic receptor number in depressed patients, binding of tritiated dihydroergocriptine (3H-DHE) to platelet membranes was measured in 23 depressed patients and 51 controls. To examine the functional responsiveness of the platelet alpha 2-adrenergic receptor, basal cyclic adenosine 3',5'-monophosphate (cAMP) production, prostaglandin E1 (PGE1) stimulation of cAMP production, and norepinephrine (NE) inhibition of PGE1-stimulated cAMP production were measured in 23 depressed patients and 53 control subjects. Finally, plasma NE concentration was measured in 20 patients to explore the possible relationship between this endogenous agonist and platelet alpha 2-adrenergic receptor function. 3H-DHE binding to platelet membranes was significantly increased in the depressed patients compared to control subjects. Both the PGE1-stimulated cAMP response and the inhibition of this response by NE were significantly reduced in the depressed patients compared to the control subjects. Thus, an apparent dissociation between alpha 2-adrenergic receptor binding and functional responsiveness was observed. Plasma NE concentrations were neither significantly different in the depressed patients than in the controls nor correlated with any of the measures of cAMP responsiveness. They were, however, significantly negatively correlated with 3H-DHE binding in depressed patients with adequate PGE1 stimulation of cAMP production.

Platelet alpha 2-adrenergic receptor sensitivity in major depressive disorder

We have investigated the functioning of alpha 2-adrenergic receptors in patients with major depressive disorder by measuring the specific binding of 3H-yohimbine, an alpha 2-adrenergic receptor antagonist, to platelet membranes. Bmax and Kd values for platelet 3H-yohimbine binding were normal in unmedicated patients with major depressive disorder, and did not correlate with scores on the Hamilton rating scale for depression. Platelet alpha 2-adrenergic antagonist sites were also unchanged in number or affinity in depressed patients after long-term treatment with a variety of antidepressant medications.

A comparison of alpha 2-adrenoreceptor binding characteristics of intact human platelets identified by [3H]-yohimbine and [3H]-dihydroergocryptine

The binding characteristics of [3H]-yohimbine on intact human platelets were investigated and compared to those of [3H]-dihydroergocryptine. A significant correlation (r = 0.92, P less than 0.01, n = 7) was observed between the inhibition of [3H]-yohimbine binding and of noradrenaline-induced platelet aggregation, indicating that [3H]-yohimbine binds to the alpha 2-adrenoreceptor responsible for noradrenaline-induced platelet aggregation. The binding capacity of [3H]-yohimbine in 20 young male volunteers was 39 +/- 2.2 fmol/10(8) platelets (mean +/- s.e.m.). This is considerably lower than the binding capacity of [3H]-dihydroergocryptine previously reported. Simultaneous analysis of [3H]-yohimbine and [3H]-dihydroergocryptine binding in 10 subjects showed a significantly lower binding capacity for [3H]-yohimbine (P less than 0.001, paired t-test). This difference was not present in every subject, and was consistent within individuals on repeated testing. Possible causes of this difference in radioligand binding are discussed.

3H-Rauwolscine binding in platelets from depressed patients and healthy volunteers

3H-Rauwolscine binds specifically and with high affinity to alpha 2-adrenoceptors in human platelets. In a study comparing the binding of 3H-rauwolscine in platelets obtained from 26 control volunteers with 19 hospitalised, untreated, severely depressed patients, the mean maximal binding (Bmax) and mean dissociation constant (Kd) of 3H-rauwolscine binding were found to be identical in both groups. After 7-12 days, treatment with different tricyclic antidepressant drugs there was a significant improvement in the depressive symptoms but no change in the 3H-rauwolscine binding. After an average of 23 days treatment with tricyclic antidepressants, and when the Hamilton Depression Rating Scores had returned to normal, the Kd and Bmax of 3H-rauwolscine binding were still unchanged.