Platelet alpha 2-adrenergic receptors in depressed patients and healthy volunteers: the effects of desipramine.

Binding parameters (Bmax and Kd) of alpha 2-adrenergic receptors were studied in platelets from 14 depressed patients and 18 control subjects. Using 3H-clonidine (a partial alpha 2-adrenergic agonist) as the ligand and membranes, prepared from platelets isolated under physiological conditions, we found no significant differences in Bmax and Kd between medication free patients and control subjects. Platelet binding parameters in the depressed patients did not correlate with plasma levels of norepinephrine, epinephrine or MHPG. Age had a significant positive effect on platelet alpha 2-adrenergic receptor Bmax in both groups, and may have masked the patient-control differences. Treatment with desipramine for 28 days had no effect on the binding parameters in depressed patients when compared to pretreatment values. Adding desipramine to platelets of control subjects 'in vitro' did also not affect binding parameters. Our findings suggest that receptor binding studies with a partial alpha 2-adrenergic agonist in platelet membranes are not a useful model to test the hypothesis of a central supersensitive adrenergic system in depression.     

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.     

Comparison of3H-para-aminoclonidine binding to different platelet preparations

Summary The binding to human platelets of³H-para-aminoclonidine (³H-PAC), an alpha₂ adrenoceptor partial agonist, appears to be altered in depressed patients. We observed that the parameters of³H-PAC binding to purified plasma membranes from platelets of normal Red Cross volunteers, compare favorably to those reported for binding to normal human autopsy prefrontal cortical lysates. However, only purified plasma membranes from platelets yielded a close comparison.³H-PAC binding tointact platelets from healthy volunteers was less than 10% displaceable by an alpha₂ adrenoceptor antagonist and was therefore unquantifiable. A low percent of specific binding (approx. 35%) was also observed in washed plateletlysates, and the binding was not of very high affinity (K_D>10 nM). In contrast, the binding of³H-PAC to platelet purifiedplasma membranes from healthy subjects displayed two high affinity binding sites (K_D1=10.6 pM and K_D2=1.2nM). These results are discussed in relation to our recent finding of elevated³H-PAC binding to platelet purified plasma membranes from depressed patients as compared to healthy subjects.     

Adrenergic receptor sensitivity in depression. Effects of clonidine in depressed patients and healthy subjects

Several recent investigations have raised the possibility that the sensitivity of alpha 2-adrenergic receptor may be of etiologic importance in depression. To assess whether abnormalities in presynaptic alpha 2-adrenergic receptor exist in depressed patients not taking drugs, the effects of an alpha 2 agonist, clonidine, on plasma 3-methoxy-4-hydroxyphenelethyleneglycol (MHPG) and on blood pressure (BP) were evaluated in 15 depressed patients and 12 healthy controls of similar age. The ability of clonidine to increase growth hormone (GH) secretion was also assessed. The effect of clonidine on plasma MHPG and BP was not different between the depressed patients and controls. However, the GH response to clonidine was blunted in the depressed patients. These results suggest that in depression (1) the sensitivity of the presynaptic alpha 2-adrenergic receptor is not abnormal, and (2) the sensitivity of postsynaptic adrenergic receptors may be decreased.     

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.     

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 receptors in major depressive disorder. Binding of tritiated clonidine before and after tricyclic antidepressant drug treatment

The specific binding of tritiated (3H)-clonidine, an alpha 2-adrenergic receptor agonist, to platelet membranes was measured in normal subjects and in patients with major depressive disorder. The number of platelet alpha 2-adrenergic receptors from the depressed group was significantly higher than that found in platelets obtained from the control population. Treatment with tricyclic antidepressant drugs led to significant decreases in the number of platelet alpha 2-adrenergic receptors. These results support the hypothesis that the depressive syndrome is related to an alpha 2-adrenergic receptor supersensitivity and that the clinical effectiveness of tricyclic antidepressant drugs is associated with a decrease in the number of these receptors.     

Clinical studies of monoamine receptors in the affective disorders and receptor changes with antidepressant treatment

Pre-clinical and clinical studies suggest that the responsiveness of monoamine and cholinergic receptors may be altered in the affective disorders and that antidepressants may modify the sensitivity of these receptors. The growth hormone response to clonidine is reduced in depressed patients compared to controls according to several independent studies, suggesting that post-synaptic alpha 2-adrenergic receptors may be less responsive in depressed patients. The cortisol response to clonidine is enhanced in depressed patients compared to controls in our study raising the possibility that cortisol hypersecretion in depressed patients may be related to noradrenergic dysfunction. The hypotensive response to clonidine is blunted in patients on chronic antidepressant treatment with either clorgyline or desipramine suggesting that pre-synaptic alpha 2-adrenergic receptors may subsensitize with chronic antidepressant treatment. The prolactin increase in response to fenfluramine is less in depressed patients compared to controls suggesting decreased functional activity of the serotonergic system in depression. Platelet alpha 2-adrenergic receptor number as measured by tritiated dihydroergocriptine (3H-DHE) binding is increased in depressed patients compared to controls, while cyclic 3'-5' adenosine monophosphate (cAMP) production in response to prostaglandin E1 (PGE1) and norepinephrine (NE) inhibition of PGE1-stimulated cAMP production are reduced in the platelets of depressed patients. Thus, it is not clear that increased 3H-DHE binding reflects increased functional responsiveness and might in fact be compensatory to decreases in functional responses of alpha 2-adrenergic receptors.     

Hyperactive phosphoinositide signaling pathway in platelets of depressed patients: effect of desipramine treatment.

There is some evidence to suggest that certain neurotransmitter receptors, such as adrenergic and serotonergic receptors and receptor-linked signaling systems, may be altered in depression. Serotonin(2A) and alpha(2)-adrenergic receptors are linked to the phosphoinositide (PI) signaling system in platelets and brain. To examine if the PI signaling system is altered in depression, we studied thrombin- and sodium fluoride-stimulated inositol phosphate(1) (IP(1)) formation before and during desipramine (DMI) treatment in platelets of depressed patients and normal control subjects. We determined thrombin- and sodium fluoride-stimulated IP(1) formation in platelets obtained from hospitalized depressed patients during a drug-free baseline period and after 6 weeks of DMI treatment, and drug-free non-hospitalized normal control subjects. Depressed subjects were diagnosed according to DSM-IV criteria, and severity of illness was assessed with the Hamilton Depression Rating Scale. We observed that thrombin-stimulated IP(1) formation in platelets of depressed patients was significantly higher compared with that of normal control subjects. There were no significant differences in sodium fluoride-stimulated IP(1) formation between depressed patients and normal control subjects. We also did not find any significant effect of treatment with DMI on either thrombin- or sodium fluoride-stimulated IP(1) formation in platelets of depressed patients, which continued to be significantly higher after 6 weeks of treatment with DMI, compared with normal control values. Our studies found a hyperactive PI signaling system in platelets of depressed patients. This hyperactive system may be related either to an increased number of thrombin receptors or to a generalized overstimulation of this pathway; however, since we did not observe any differences in sodium fluoride-stimulated IP(1) formation, it appears that, although the sites distal to the receptors may be altered, this abnormality is probably not related to the abnormalities in G proteins.     

Platelet membrane fluidity in Alzheimer's disease and major depression

Double-blind fluorescence studies of platelet membrane fluidity were conducted at 37 degrees C for 51 patients with Alzheimer-type dementia, 24 nondemented depressed patients, and 50 neurologically healthy subjects. The fluidity of the hydrocarbon region of platelet membranes from the demented group, as reflected by the steady-state anisotropy of the fluorescent probe 1,6-diphenyl-1,3,5-hexatriene (DPH), was significantly greater than that for the depressed and normal control subjects. Within the demented group, platelet membrane fluidity was significantly correlated with severity of dementia but not with duration of illness or age at onset. Demented patients with "increased" platelet membrane fluidity had an earlier onset, were more severely demented, and deteriorated more rapidly.     

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.     

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.     

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.     

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.     

Alpha 2-adrenergic receptor function in depression. The cortisol response to yohimbine

There is evidence that the abnormalities in hypothalamic-pituitary-adrenal (HPA) axis function observed in patients with depression may be related to changes in central neurotransmitter receptor function. To evaluate this possibility further, the alpha 2-adrenergic receptor antagonist yohimbine hydrochloride, which increases brain norepinephrine turnover, was administered to 40 patients with DSM-III major depression (18 melancholic, 22 nonmelancholic) and 16 healthy controls. Plasma free 3-methoxy-4-hydroxyphenylglycol (MHPG) level was measured as an index of noradrenergic function, and plasma cortisol level was used to assess the HPA response. Baseline cortisol levels were elevated in melancholic depressed patients, but not in nonmelancholic patients, when compared with healthy controls. The cortisol response to yohimbine was significantly greater in depressed patients than in controls, despite similar MHPG responses between groups. Since there is evidence that stimulation of postsynaptic alpha 2-adrenergic receptors inhibits HPA axis function, the abnormally increased cortisol response to the alpha 2-antagonist yohimbine suggests a relative subsensitivity of postsynaptic alpha 2-adrenergic receptors in depression.     

Platelet alpha 2-adrenergic receptor binding and plasma catecholamines. Before and during imipramine treatment in patients with panic anxiety

Specific binding of tritiated clonidine, an alpha 2-adrenergic receptor agonist, and tritiated yohimbine, an alpha 2-adrenergic receptor antagonist, to platelet membranes was measured in persons with panic attacks or major depression and in normal subjects. Plasma catecholamine levels were measured in patients with panic attacks and in normal subjects. The number of binding sites in patients with panic attacks, as measured with tritiated clonidine, was lower than in depressed persons and was the same as in normal subjects. The number in patients with panic attacks, as measured with tritiated yohimbine, was lower than in either depressives or normal subjects. Catecholamine levels were somewhat higher in patients with panic attacks than in normal subjects. Treatment with imipramine hydrochloride decreased the number of sites, as measured with either ligand, in both patient groups and increased catecholamine levels in patients with panic attacks.     

Are disturbances in lipid-protein interactions by phospholipase-A2 a predisposing factor in affective illness?

Current theories of affective disorders do not account for many of the biological markers replicated in patient studies. We link many biological findings in a reasonable physiological relationship, compatible with mechanisms of action of pharmacological and electroshock therapies for depression. We propose that excessive phospholipase-A2 (PLA2) activity disrupts membrane fluidity, composition, and therefore, the activity, of membrane-dependent proteins. Similar disruptions in these proteins are documented in depressed patients and can be accounted for by excessive PLA2 activity. This paradigm accounts for disturbances in the activity of Na-K-ATPase, beta2- and alpha2-adrenergic receptors, MAO, norepinephrine and serotonin uptake, and imipramine binding. Disturbances in other membrane-dependent proteins, tyrosine and tryptophan hydroxylase, can explain the biogenic amine hypothesis. Inhibition of glucocorticoid receptor and TRH receptor binding to their respective ligands by PLA2 may explain patient nonsuppression in the Dexamethasone Suppression Test and poor response in the TRH stimulation test. Physiological regulators of PLA2 activity; calcium, cortisol, estrogen, progesterone, and PGE2 are documented abnormalities in some patients with affective disorders and consistent with excessive PLA2 activity. Thus, postpartum depression and premenstrual tension syndrome may be described in the paradigm. The mechanisms of action of tricyclic antidepressants, lithium, electroconvulsive shock, and some novel antimanic agents can be described in terms of alterations of PLA2 activity. Interestingly, ethanol perturbs membrane fluidity and membrane-bound enzymes in a manner similar to excessive PLA2 activity. A hereditary factor predisposing patients to affective disorders may be a gene defect at either PLA2 or in its regulation.     

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.     

Correlation between platelet alpha(2)-adrenoreceptors and symptom severity in major depression

BACKGROUND: Abnormalities in different parameters of the norepinephrine system have been widely described in major depression. The presence of alpha(2)-adrenoreceptors in blood platelets, similar to those in the brain, prompted us to evaluate them in depressed patients, as compared with healthy controls. METHODS: Fifteen outpatients affected by major depression, according to DSM IV criteria, and 15 comparable healthy control subjects, were included in the study. The alpha(2)-adrenoreceptors were measured by means of the specific binding of [(3)H]rauwolscine, a highly selective antagonist for this receptor subtype. The severity of depression was assessed by means of the Hamilton Rating Scale for Depression (HRSD). RESULTS: The results did not show any difference in [(3)H]rauwolscine binding parameters (B(max) and K(d)) between patients and controls. However, in the patients, a significant and positive correlation between B(max), which measures the density of the receptors, and HRSD total score was detected. CONCLUSIONS: Therefore, although no change in alpha(2)-adrenoreceptors seems to occur in major depression, the density of these receptors would seem to be related to the severity of depressive symptoms.     

Decrease in epinephrine-induced attenuation of platelet adenylate cyclase activity in depressed patients: relation with plasma electrolytes.

We have measured the alpha 2-adrenoceptor-mediated inhibition of platelet membrane adenylate cyclase in depressed patients and control subjects. The results showed a decrease in the forskolin-stimulated adenylate cyclase inhibition of depressed patients compared to the healthy subjects. This suggests a subsensitivity of alpha 2-adrenoceptor in depression. However, this subsensitivity was not correlated to the severity of depression as both severely and moderately depressed patients exhibited the same percent of adenylate cyclase inhibition. The antidepressant drugs treatment induced an increase in the percent of adenylate cyclase inhibition with a trend towards the control values. However, this increase did not equal control value, and moreover both remitted and unremitted patients presented a similar change in their alpha 2-adrenoceptor-mediated adenylate cyclase inhibition. This result raises the question about a simple and direct relation between the clinical status of depression and the power of alpha 2-adrenoceptor-mediated adenylate cyclase inhibition. Plasma magnesium and sodium yielded correlations to this alpha 2-adrenoceptor-mediated adenylate cyclase inhibition suggesting a relation between the platelet adrenergic function and plasma electrolytes.