Differences between sodium-dependent and desipramine-defined [3H]imipramine binding in intact human platelets

Measurements of sodium-dependent [3H]imipramine binding to intact human platelets from 20 human volunteers were made and compared to desipramine-defined binding, a method commonly employed in population studies of platelet [3H]imipramine sites. The density (Bmax) of sodium-dependent [3H]imipramine sides in platelets was significantly lower (449 +/- 36 sites/platelet) and the affinity (Kd) significantly higher (1.15 +/- 0.12 nM) than those obtained when excess desipramine was used to define specific binding (Bmax 654 +/- 33 sites/platelet, p less than 0.001; Kd 1.52 +/- 0.11 nM, p less than 0.001). There was no significant correlation between the density (Bmax) of sodium-dependent and desipramine-defined binding in individual subjects, suggesting that a different proportion of sites are labeled under the two assay conditions. No age-dependent variation was found in either Kd or Bmax values of sodium-dependent or desipramine-defined [3H]imipramine binding. The results suggest determination of sodium-dependent [3H]imipramine binding to intact platelets may be a useful measure for the estimation of [3H]imipramine recognition sites relevant to the serotonin uptake in studies of patients with affective disorders.     

High-affinity imipramine binding and serotonin uptake in platelets of eight adolescent and ten adult obsessive-compulsive patients

The authors evaluated high-affinity [3H]imipramine binding and [3H]serotonin uptake to platelets in eight adolescent and 10 adult patients who met DSM-III criteria for obsessive-compulsive disorder in comparison with those of normal control subjects of similar ages. The maximal binding of [3H]imipramine was significantly lower in adults and adolescents with obsessive-compulsive disorder than in the control subjects. No differences between groups in the affinity of [3H]imipramine to its binding sites or in serotonin uptake kinetic measures were detected. The lower density of [3H]imipramine binding sites in platelet membrane in patients with obsessive-compulsive disorder might implicate involvement of the serotonergic system or might represent an adaptive response to a chronic disease.     

Effects of antidepressants on monoamine transporters

1. Using [3H]antidepressants, high affinity binding sites associated with the neuronal transporter for serotonin, noradrenaline, dopamine and adrenaline have been identified. 2. The association of high affinity [3H]imipramine binding with the serotonin transporter in brain and platelets is well established. Although the exact relationship between the [3H]imipramine recognition site and the serotonin transporter remains to be elucidated, it appears that the [3H]imipramine labelled component of the serotonin transporter represents a novel receptor that functions to modulate serotonin uptake. 3. Most data available to date support the hypothesis that [3H]imipramine binding to platelet represents a biological marker in depression. The majority of studies indicate that the Bmax of platelet [3H]imipramine binding is lower in depressed, untreated patients than in the control population and that this finding is relatively specific to depression. 4. Among the [3H]antidepressant binding sites associated with the other monoaminergic transporters, the recent identification of [3H]desipramine binding to the neuronal transporter for adrenaline offers novel perspectives. Thus, given the high affinity for [3H]desipramine binding to the adrenaline transporter in the frog heart for not only desipramine but also imipramine and the atypical antidepressants mianserin and iprindol, it is possible that an interaction with the adrenaline transporter is of significance to the clinical effects of antidepressant drugs.     

Reduced inhibitory effect of imipramine on radiolabeled serotonin uptake into platelets in geriatric depression

Tritiated imipramine binding, uptake of radiolabeled serotonin, and inhibition of uptake by imipramine in vitro were studied in platelets obtained from four groups of subjects: (1) normal controls 50 years of age or younger, (2) patients with major depression 50 years of age or younger, (3) normal controls 60 years of age or older, and (4) patients with major depression 60 years of age or older. Depression in both age groups was associated with a substantial decrease in the number of [3H]imipramine binding sites; the elderly depressed patients exhibited a small but significant (p less than 0.05) reduction in platelet [3H]serotonin uptake. However, the inhibition of serotonin uptake into platelets by imipramine was markedly reduced only in the elderly depressed patients. This reduced sensitivity to imipramine may explain the reduced responsiveness of patients with geriatric depression to the therapeutic effects of imipramine and other tricyclic antidepressants.     

Platelet [3H]imipramine binding in generalized anxiety disorder, panic disorder, and agoraphobia with panic attacks

The density of platelet [3H]imipramine binding sites is reported to be decreased in unipolar depression and, hence, is a putative biological marker. There is considerable evidence for a phenomenological and biological relationship of panic disorder with affective disorder. We studied platelet [3H]imipramine binding site density in unmedicated subjects with generalized anxiety disorder (GAD; n = 55), panic disorder (PD) with and without agoraphobia (n = 52), and normal controls (n = 26) in order to determine whether or not patients with panic disorder differed from controls in this biological assay. We found no differences in binding site density (Bmax) or affinity (Kd) among the PD, PD with agoraphobia, GAD, and control groups. Nor did we find a relationship between Bmax or Kd and the severity of depressive symptoms or the presence of a family history of affective disorder. In view of two conflicting prior studies, the use of [3H]imipramine binding in panic disorder remains problematic.     

Lack of seasonal variation in platelet [3H]imipramine binding in humans

The Bmax of [3H]imipramine (IMI) binding has been reported to be reduced in platelets of depressed untreated patients as compared with normal controls. However, it has also been suggested that this difference could be related to the failure to take into account seasonal variations in the binding parameters for [3H]IMI recognition sites in platelets. For this reason, [3H]IMI binding was studied throughout 1 year in platelet membranes from 11 control volunteers, with blood samples collected once a month. The Bmax and Kd values of [3H]IMI binding showed no significant variation throughout the 12-month period of the study. These results indicate that in the control population, the platelet [3H]IMI binding parameters remain stable, and that the decrease in Bmax observed in depressed untreated patients reflects a genuine difference, which may be considered to be a biological marker in depression.     

Platelet [3H]imipramine binding in patients with panic disorder

[3H]imipramine binding to platelets was measured in 17 drug-free panic disorder patients and 14 healthy controls. No difference in Bmax or Kd values was found between the two groups. Patients with a past history of major melancholic depression or severe agoraphobia had similar binding parameters as panic disorder patients without a history of depression or severe agoraphobia.     

Binding of [3H]paroxetine to serotonin uptake sites and of [3H]lysergic acid diethylamide to 5-HT2A receptors in platelets from women with premenstrual dysphoric disorder during gonadotropin releasing hormone treatment

Changes in serotonergic parameters have been reported in psychiatric conditions such as depression but also in the premenstrual dysphoric disorder (PMDD). In addition, hormonal effects on serotonergic activity have been established. In the present study, binding of [3H]paroxetine to platelet serotonin uptake sites and binding of [3H]lysergic acid diethylamide ([3H]LSD) to platelet serotonin (5-HT)2A receptors were studied in patients with PMDD treated with a low dose of a gonadotropin releasing hormone (GnRH) agonist (buserelin) or placebo and compared to controls. The PMDD patients were relieved of premenstrual symptoms like depression and irritability during buserelin treatment. The number of [3H]paroxetine binding sites (Bmax) were significantly higher in the follicular phase in untreated PMDD patients compared to controls. When treated with buserelin the difference disappeared. No differences in [3H]LSD binding between the three groups were shown. The present study demonstrated altered platelet [3H]paroxetine binding characteristics in women with PMDD compared to controls. Furthermore, [3H]paroxetine binding was affected by PMDD treatment with a low dose of buserelin. The results are consistent with the hypothesis that changes in serotonergic transmission could be a trait in the premenstrual dysphoric disorder.     

Decreased serotonin transporter binding in unaffected relatives of manic depressive patients.

BACKGROUND: There are numerous reports of decreased binding to platelet serotonin transporter (5-HTT) in depression, suggesting that it might be considered a trait marker of depression. To further investigate whether reduced 5-HTT function could be an endophenotype in manic depressive illness, we looked for abnormalities of platelet 5-HTT among subjects who are potential carriers of genetic vulnerability to manic depressive illness (MDI). METHODS: Blood samples were obtained from 20 unaffected relatives from families with at least two individuals with bipolar disorder and from 19 control participants. Plasma 5-HIAA, platelet 5-HT, and [3H] imipramine binding were measured. RESULTS: Unaffected relatives manifested lower platelet 5-HTT function than control participants as revealed both by reduced number and diminished affinity of imipramine binding sites and diminished platelet 5-HT content. CONCLUSIONS: These preliminary results suggest that reduced 5-HTT function could be considered a trait marker or an endophenotype in MDI.     

Serotonergic markers in platelets of patients with major depression: upregulation of 5-HT2 receptors.

The uptake of [3H]5-HT and the density (Bmax) as well as affinity (Kd) of 5-HT uptake sites labelled with [3H]paroxetine and of 5-HT2 receptors labelled by [3H]LSD were determined in platelets from 25 medication-free patients with major depression and 20 normal controls. The density (Bmax) of 5-HT2 receptors was found to be significantly increased (by 52%) in platelets from depressed patients, particularly females. No changes were found in the affinity (Kd) of 5-HT2 receptors and in 5-HT uptake or [3H]paroxetine binding parameters. Density of 5-HT2 receptors positively correlated with that of [3H]paroxetine sites in control but not in depressed subjects. No correlation was found between the HAMD scores and Bmax of [3H]LSD binding. The results suggest that upregulation of platelet 5-HT2 receptors is a useful biological marker in major depression, particularly in females.     

High affinity [3H]imipramine and [3H]paroxetine binding sites in suicide brains

Summary Specific binding of [3H]imipramine and [3H]paroxetine was simultaneously examined in human brains (frontal cortex, temporal cortex, cingulate cortex, hypothalamus, hippocampus and amygdala) from 11 controls and 11 depressed suicide victims. A single saturable high affinity site was obtained for both radioligands. Age was not related to significant changes in [3H]imipramine and [3H]paroxetine binding parameters, which indicates the stability of the brain serotonergic system with increasing age.A major finding of the present study concerns the existence of a significant decrease in the maximum number (Bmax) of [3H]imipramine binding sites in hippocampus from depressed suicides as compared with the control group, without changes in the binding affinity (Kd). In contrast, when [3H]paroxetine was used as radioligand, no changes in either Bmax or Kd were detected in any of the brain regions studied. These findings suggest that [3H]imipramine may be a better marker than [3H]paroxetine when alterations in the presynaptic serotonergic uptake site are to be detected.     

Upregulation of imipramine binding and serotonin uptake by estradiol in female rat brain

This report describes the effect of chronic estradiol treatment on the serotonin transporter in the female rat brain. Both [3H]imipramine binding and [3H]serotonin uptake increased by 20-30% in the frontal cortex and hypothalamus of ovariectomized rats after 12 days of 17 beta-estradiol treatment. No differences were observed in the binding and uptake parameters as a function of the rats' estrous cycle or in untreated ovariectomized rats, as compared to controls. Estradiol in vitro, inhibits [3H]imipramine binding as well as serotonin uptake in rat brain and human platelets. Like serotonin, estradiol decreases the dissociation rate in vitro of [3H]imipramine from its binding site in a dose-dependent manner.     

Platelet imipramine binding in endogenous depressed patients and controls: relationship to platelet MAO and 5HT uptake during successful imipramine treatment

Platelet imipramine binding was measured in 25 unmedicated depressed patients and 25 age- and sex-matched healthy controls. In the patients, the measurement was repeated after 3 weeks and 2 months of imipramine treatment leading to clinical recovery. No significant differences in imipramine binding were found between controls and unmedicated patients. In the latter, imipramine administration produced a progressive change in the binding characteristics, increasing the apparent Kd and decreasing the number of binding sites (Bmax). The results suggest that platelet imipramine binding is not altered in depression and that changes in this parameter are the consequence of the presence of imipramine in the blood stream. However, such changes accompany changes in other biological parameters, such as platelet monoamine oxidase and serotonin uptake, seen in the same patients throughout imipramine treatment, suggesting that the drug acts on a wide range of normal or altered serotonin-related cellular mechanisms while it accelerates the clinical recovery from depression.     

Decreased platelet alpha-2 adrenoceptor density in major depression: effects of tricyclic antidepressants and fluoxetine.

BACKGROUND: It has been suggested that major depression is accompanied by a subsensitivity of central alpha 2-adrenoceptors (alpha 2-ARs) and, consequently, by an impaired negative feedback on the presynaptic catecholaminergic neuron, which, in turn, may induce a disinhibition of noradrenergic output and norepinephrine release in response to any activation. METHODS: The maximum number of platelet binding sites (Bmax) and their affinity for [3H]-rauwolscine, a selective alpha 2-AR antagonist, were measured in unmedicated and medicated major depressed patients and in normal volunteers. Specific binding was defined as that inhibited by idazoxan, another alpha 2-AR antagonist. RESULTS: Unmedicated major depressed patients had significantly decreased platelet [3H]-rauwolscine binding Bmax values compared to normal volunteers. [3H]-rauwolscine binding Kd values did not differ significantly between unmedicated major depressed patients and normal controls. [3H]-rauwolscine binding Kd values were significantly higher in depressed patients treated with tricyclic antidepressants than in unmedicated patients. Subchronic treatment with fluoxetine did not significantly alter either [3H]-rauwolscine binding Bmax or Kd values. [3H]-rauwolscine binding Bmax values were significantly greater in men than in women. CONCLUSIONS: The results suggest that i) major depression is accompanied by decreased platelet alpha 2-AR density; and that ii) subchronic treatment with tricyclic antidepressants, but not fluoxetine, results in a decreased affinity of rauwolscine for platelet alpha 2-ARs.     

Platelet [3H]-imipramine binding and leukoencephalopathy in geriatric depression.

We examined the relationship between platelet [3H]-imipramine binding and leukoencephalopathy as assessed by 1.5 Tesla Magnetic Resonance Imaging (MRI) in 21 elderly depressed patients who satisfied DSM-III criteria for major depression. Both drug-free platelet [3H]-imipramine binding and brain MRI studies were obtained during the same episode of depression. Our findings show a significant inverse relationship between frequency of subcortical hyperintensity (SCH) and the number (Bmax) of platelet [3H]-imipramine binding sites. Patients with Bmax less than 850 fmol/mg protein had significantly larger SCH compared with patients with a higher Bmax. These data provide further support to the potential use of platelet [3H]-imipramine binding studies and brain MR imaging as diagnostic adjuncts in geriatric depression and suggest, moreover, that these two biological markers may be linked in geriatric patients with depression.     

Imipramine binding sites on platelets of patients with major depressive disorder

3H-Imipramine binding to platelets of patients with primary, unipolar major depressive disorder was investigated and compared to that of a normal, healthy control population. No significant differences could be demonstrated between either the Kd or the Bmax values of the two groups. A negative correlation was observed between imipramine Bmax values and the Hamilton anxiety ratings of the depressed patients. Patients who displayed psychomotor retardation tended to have lower platelet imipramine Bmax values than patients with psychomotor agitation. It is suggested that platelet imipramine Bmax values may be a biological marker for subtypes of depression.     

Changes in [3H]5-HT uptake and [3H]imipramine binding in platelets after chlorimipramine in healthy volunteers. Comparison with maprotiline and amineptine

In the platelets of normal healthy volunteers (n = 8) taking chlorimipramine (50 mg/day) for 1 week, the saturable uptake of [3H]5-hydroxytryptamine (5-HT) was fully inhibited at the end of the week, but returned to control values after 2 weeks washout. The Bmax of [3H]imipramine binding was decreased by 63% at the end of the treatment and remained significantly decreased below control values after 1 week washout, whereas the Kd values were increased at the end of the treatment, but had returned to baseline values after 1 week washout. The time course of recovery following the administration of chlorimipramine showed some variation between subjects, but it was necessary to wait up to 4 weeks of washout before the Bmax of [3H]imipramine returned to baseline levels. In contrast, neither 1-week treatment with maprotiline (50 mg/day) nor with amineptine (100 mg/day) changed the parameters of [3H]5-HT uptake or [3H]imipramine binding in platelets from healthy volunteers. These results support the following conclusions. (1) [3H]Imipramine binding in platelets can be down-regulated by relatively low, subtherapeutic doses of chlorimipramine. (2) It is possible to dissociate [3H]imipramine binding parameters from [3H]5-HT uptake because the time course of recovery was clearly different, indicating that [3H]imipramine labels a site linked with, but different from, the 5-HT recognition site in the transporter complex. (3) A washout of antidepressants of 4 weeks may be needed when studying the parameters of [3H]imipramine binding in platelets from depressed patients if the previous medication involved chlorimipramine. For antidepressants like maprotiline or amineptine, that act through mechanisms other than inhibition of 5-HT uptake, the time of washout appears to be less critical, although it is not possible to rule out the existence of some secondary modifications influencing the 5-HT transporter complex.     

Platelet [3H]imipramine binding in affective disorders: trait versus state characteristics

Platelet [3H]imipramine binding (Bmax) was determined in 67 patients with major affective illness (33 euthymic bipolar, 34 depressed unipolar) and 58 normal control subjects. Bipolar patients had significantly lower Bmax values than did control subjects. The mean Bmax in the unipolar patients was lower than in the control subjects, but the difference was not statistically significant. Dissociation constant (Kd) values did not distinguish patients in either category from control subjects. The significantly lower Bmax in euthymic bipolar patients and the apparent state independence of Bmax in some but not all unipolar patients suggest that platelet imipramine binding may be a trait marker in a subset of affective disorders.     

Imipramine binding in depressive patients diagnosed according to different criteria

3H-imipramine binding to platelet membranes, Bmax and KD, was measured in depressed patients, who were divided into endogenous and non-endogenous depression according to three different criteria, the ICD-9, the Newcastle I and the Newcastle II rating scales. Two groups served as controls, a group of healthy volunteers and a group of psychiatric patients suffering from schizophrenia or senile dementia. No significant differences were found in either Bmax or in KD among the different groups of patients and the control groups.     

Platelet [3H]-imipramine binding according to DSM-III subtypes of depression

The question of the previously reported changes in the density of high-affinity binding sites for [3H]-imipramine (IMI) in platelets from depressed patients was reexamined among the different diagnostic subtypes of depression according to the DSM-III classification and taking into account the possible influence of the low-affinity binding site. Using a least-square computer-assisted analysis, a precise determination of the [3H]-IMI binding parameters exclusively in relationship to the high-affinity site was performed in 46 untreated depressed patients and compared to 35 healthy controls. The results revealed a clear and highly significant 22% decrease in the maximal density (Bmax) of [3H]-IMI binding in all of the depressed patients compared to controls with no change in affinity values. Considering the diagnostic subgroups, we found that all the bipolar patients, the depressed as well as the euthymic or manic ones, had very low Bmax values and that some of them also exhibited an unusual low-affinity binding. Mean Bmax of the unipolar and dysthymic patients significantly decreased when compared to controls, although the Bmax values showed a large variability. Only dysthymic patients presented Bmax values which were significantly associated to symptom severity as assessed by the Hamilton Depression Rating Scale scores. Our results confirm the lower density of platelet [3H]-IMI binding in affective disorders, particularly in bipolar patients, and also suggest that this biological parameter is a trait marker in bipolar depression and a state marker in dysthmic disorder.