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

Platelet 3H-imipramine binding in major depressive episodes

Density (Bmax) and affinity constant (Kd) values of tritiated imipramine binding sites were determined on platelets from patients suffering from major depression and from healthy, age-and sex-matched controls. Significantly lower Bmax values were found in the depressed patients, while the Kd values did not differ from those of controls. The results suggest that, in accordance with data from the literature, decrease in 3H-imipramine binding sites may be used as a state-dependent biological marker of depression in clinical practice.

Sex differences in diencephalon serotonin transporter availability in major depression

BACKGROUND

Major depression is more prevalent in women than men. The present study evaluated if previous findings that demonstrated decreased 5-hydroxytryptamine (5-HT) transporter availability in depressed patients would be confirmed in a larger sample and also evaluated sex differences.

METHODS

Depressed (n = 32) and healthy subjects (n = 32), including 16 pairs of women and men, participated in an iodine-123-2 beta-carbomethoxy-3beta-(4-iodophenyltropane) ([(123)I]beta-CIT) single photon emission computed tomography (SPECT) and a magnetic resonance imaging (MRI) scan. Participants were administered [(123)I]beta-CIT (225.7 +/- 3.7 MBq) and imaged 23.0 +/- 1.6 hours later. Statistical analyses included analysis of variance and a regression analysis of the main and interactive effects of age, sex, and depression.

RESULTS

Overall, depressed patients demonstrated 12% lower diencephalon and no change in striatal or brainstem [(123)I]beta-CIT uptake. Significant age by sex, sex by depression, and age by sex by depression interactions were noted due to 22% lower diencephalon [(123)I]beta-CIT uptake in depressed women compared with less than a 1% decrease in depressed men.

CONCLUSIONS

As observed previously, diencephalon 5-HT transporter availability is decreased in depressed patients. However, the decrease appears to be sex-specific and age-dependent. These findings suggest that serotonergic mechanisms mediating depressed mood differ between men and women in an age-dependent manner and may explain why young women respond better to treatment with selective serotonin reuptake inhibitor (SSRI) antidepressants.

Differential changes in brain and platelet 5-HT concentrations after steady-state achievement and repeated administration of antidepressant drugs in mice

The aim of the study was to compare in male NMRI mice the simultaneous evolution of blood serotonin (5-HT) concentrations, which correspond to 99% of platelet 5-HT content, and 5-HT parameters in the dorsal raphe, caudate nucleus and frontal cortex after clomipramine, fluoxetine and moclobemide treatments. After steady-state concentrations of the three compounds were reached, the 5-HT levels were significantly enhanced vs. saline-treated mice in the three brain areas studied. Tryptophan (TRP) levels in the three brain areas were significantly increased with clomipramine and fluoxetine but not with moclobemide. A significant decrease in the metabolite 5-hydroxyindoleacetic acid (5-HIAA) levels was observed only with moclobemide. After 14 days of treatment, 5-HT levels in all areas studied were found to be enhanced only with moclobemide while TRP and 5-HIAA levels were not different under the three drug regimes from those of controls. After 21 days of treatment, 5-HT levels were found enhanced only with moclobemide in the nerve terminal regions. An important depletion in platelet 5-HT content was observed after clomipramine and fluoxetine treatments at day 14 and day 21 and a significant increase was observed after moclobemide treatment at day 14 with a return to initial values after 21 days. Our results show significantly different effects between central and peripheral indices of 5-HT metabolism according to time and to the antidepressant assessed: (i) an enhancement of total tissue 5-HT levels in the three brain areas studied after steady-state achievement of the 3 antidepressants, (ii) the return to initial values of brain 5-HT levels after repeated administration of the two 5-HT re-uptake inhibitors, consistent with the presence of brain adaptative mechanisms, with a concomitant dramatic decrease of platelet 5-HT content and (iii) an apparent gradual attenuation of the brain and periphery MAOI-A effect induced by moclobemide with 5-HT levels remaining elevated only in 5-HT nerve terminal regions.

The cortical serotonin2 receptors studied with positron-emission tomography and [18F]-setoperone during depressive illness and antidepressant treatment with clomipramine

BACKGROUND

Changes in serotonin (5-HT)2 receptor densities were reported in depression by postmortem studies and following treatment with tricyclic antidepressants in animal studies. Here, 5-HT2 receptors were studied in vivo in depressed patients.

METHODS

Cortical 5-HT2 receptors were investigated prospectively using positron-emission tomography and [18F]-setoperone in 7 depressed patients, before and after at least 3 weeks of clomipramine (CMI), 150 mg daily. They were compared to 7 age-matched controls.

RESULTS

There was no significant difference between the untreated patients and the controls, except in the frontal region, where the [18F]-setoperone specific binding was slightly lower in patients. After CMI treatment, depression scores significantly improved and [18F]-setoperone specific binding decreased in cortical regions, suggesting receptor occupancy and/or receptor regulation, by CMI; however, no clinical score correlated with the 5-HT2 receptor measurements either in the untreated or in the treated conditions.

CONCLUSIONS

These data substantiate the view that tricyclic antidepressants such as clomipramine significantly interact with cortical 5-HT2 serotoninergic receptors in actual therapeutic situations.

Plasma melatonin and cortisol in patients with obsessive-compulsive disorder: relationship with axillary temperature, physical activity, and clinical symptoms

BACKGROUND

Many studies have found biological abnormalities in obsessive-compulsive disorder (OCD), although most of them have not been replicated. The investigation of melatonin rhythm may thus provide an indirect clue to neurotransmitter alterations, and allow a biological comparison with depression.

METHODS

The circadian variations of plasma melatonin, plasma cortisol, axillary temperature, motor activity, and obsessive-compulsive symptoms have been documented on a circadian basis in 8 patients with OCD compared to 8 paired healthy volunteers.

RESULTS

The circadian pattern of axillary temperature was slightly different in OCD patients when compared to control subjects. No significant difference between the two groups could be observed for any other variable studied.

CONCLUSIONS

The discrepancies with previous studies are discussed on the basis of the methods used (patients and control subjects samples, biological measurement procedures). An alteration of temperature circadian rhythm hypothesis is suggested.

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

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

Is platelet imipramine binding reduced in depression? A meta-analysis

Although it has been suggested that decreased platelet imipramine binding may be a putative biological marker of depressive illness, a number of studies have not confirmed this finding, including a recent multicenter investigation by the World Health Organization (Mellerup and Langer 1990). We performed a meta-analysis of published reports on imipramine binding in groups of depressed and healthy control subjects and found that there was a highly significant decrease in Bmax (maximal binding) values in the depressed subject groups, which was even greater among those who had been free of medication for 4 weeks at the time of investigation. This finding remained highly significant even when only high affinity binding studies (Kd < 1 nmol/L) were considered, although the absolute size of this decrease was smaller.

A single electroconvulsive treatment affects platelet serotonin uptake in bipolar I patients

We evaluated serotonin (5HT) uptake parameters in platelets of a group of bipolar I patients in both a depressive phase and a mixed state, before (t0) and after (t1) the first electroconvulsive treatment (ECT). The results showed the presence of significant changes in the number of the carrier proteins for 5HT at t1, which in most cases preceded the clinical improvement. Although other mechanisms are certainly involved, these data suggest that ECT may influence the 5HT transport through platelet membranes.

Platelet 5-HT uptake sites in depression: three concurrent measures using [3H] imipramine and [3H] paroxetine

Platelet 5-HT uptake sites were measured in 40 depressed patients and 40 controls using [3H] imipramine binding, defined with desmethylimipramine (DMI) and Na+ dependence, and [3H] paroxetine binding. In control subjects the Bmax of DMI defined [3H] imipramine binding was significantly higher than both Na+ dependent [3H] imipramine (by 30%) and [3H] paroxetine binding (by 22%). The Bmax of Na+ dependent [3H] imipramine and [3H] paroxetine binding did not differ significantly. The Kd of Na+ dependent [3H] imipramine binding was significantly lower than the Kd of DMI defined [3H] imipramine binding. The binding of DMI defined and Na+ dependent [3H] imipramine and [3H] paroxetine did not differ significantly between depressed patients and controls in the total group, in those depressed patients and controls in the total group, in those depressed patients who had never taken antidepressants or in those depressed patients who had been recently withdrawn from antidepressants. This study provides no support for the view that the number of platelet 5-HT uptake sites are reduced in depression.

Mianserin-induced up-regulation of serotonin receptors on normal human platelets in vivo

The antidepressant, mianserin, is a serotonin receptor (5-HT2) antagonist and produces down-regulation of 5-HT2 and 5-HT1c receptors in the cerebral cortex of rats. In preparation for testing the validity of platelets as a model system for changes in 5-HT2 receptors during antidepressant drug treatment, mianserin (40 mg daily), was given to five human volunteers for five days, and platelets were collected on days 0, 1, 6, and 8. 5-HT2 receptor affinity and density were measured by specific binding of 125I-LSD, with and without an excess of spiperone. 5-HT uptake site affinity and density were determined by 3H-paroxetine binding, with and without an excess of fluoxetine. Platelet serotonin content was measured using high pressure liquid chromatography and electrochemical detention. Platelet 5-HT2 receptor density was increased and the ligand affinity was decreased during mianserin administration. In contrast, platelet 5-HT content was not altered significantly by mianserin administration, nor was platelet uptake site density and ligand affinity.

Platelet 3H-imipramine binding during recovery from depression

Decreased binding of tritiated imipramine to platelets has been considered to be a potential biological marker of depression. However, it has been unclear how binding values alter during treatment and recovery. This study investigated imipramine binding parameters and depressive symptoms in 25 patients suffering from major depression at entry to the study and 1, 3 and 6 months later. Although the initial Bmax values were significantly lower in the depressed patients than in healthy subjects, it was not possible to establish a clear relationship between recovery from depression and Bmax. The power of this study to detect an effect of at least 10% of the variance in Bmax due to factors related to recovery from depression was 0.78.

Effects of monoamine uptake inhibitors on extracellular and platelet 5-hydroxytryptamine in rat blood: different effects of clomipramine and fluoxetine

1. The concentration of 5-hydroxytryptamine (5-HT) in rat platelet-free plasma increased significantly 30 min after a single i.p. injection (10 mg kg-1) of each of six inhibitors of the high-affinity 5-HT uptake (fluvoxamine, fluoxetine, alaproclate, paroxetine, sertraline and clomipramine). The increases ranged from 226% to 776% of control values. In contrast, imipramine, desipramine and femoxetine had no significant effect. The increase elicited by paroxetine was dependent on the dose (1, 5 and 10 mg kg-1) and returned to control values after 4 h. That observed after clomipramine was also transient and paralleled the plasma concentration of the drug (Spearman-rank correlation r = 0.43). 2. In vivo, the rat pulmonary vascular endothelium removed trace amounts (8.8 nmol in a bolus) of intravenously injected [14C]-5-HT. Paroxetine pretreatment (10 mg kg-1, 30 min before-hand) reduced this uptake by 73%. 3. Repeated fluoxetine treatments reduced rat whole blood 5-HT concentration (ca. -60% after daily 2 x 5 mg kg-1, i.p. during 14 days). However, plasma (extracellular) 5-HT was not increased. 4. Various repeated treatments with clomipramine (i.p. injections or osmotic minipumps, up to 30 mg kg-1 day-1), failed to decrease rat whole blood 5-HT concentrations. Platelet-free plasma 5-HT was also unchanged, even after treatments yielding plasma clomipramine levels 2.7 times higher than those that increased it acutely. 5. These results indicate that the extracellular pool of 5-HT in rat blood (measured in the platelet-free plasma) is physiologically under the control of high-affinity 5-HT uptake systems.The sustained 5-HT uptake inhibition does not result in an increase of 5-HT in platelet-free plasma, suggesting that adaptative mechanisms are triggered. The distinct long-term effects of the two antidepressants clomipramine and fluoxetine on rat whole blood 5-HT suggest a differential in vivo action on the rat 5-HT uptake.

Rapid changes in 3H-imipramine platelet binding after chronic treatment with amineptine, a selective dopamine uptake blocker, in major depressed patients

The effect of chronic treatment with amineptine (200 mg daily), a tricyclic antidepressant drug selectively blocking dopamine uptake, on 3H-imipramine binding, was investigated in platelets of major depressed patients in conjunction with changes in clinical state. Before treatment, depressed patients had a significantly lower Bmax (P less than 0.01) than age- and gender-matched healthy controls. After only 1 week of amineptine administration, Bmax values increased significantly (P less than 0.01) and reached the control value concomitantly with a large and significant clinical improvement (P less than 0.01). After 1 month, Bmax was still significantly different from the pretreatment value (P less than 0.05), and not significantly different from the control value, while the improvement in clinical status persisted. No significant changes in Kd values were observed during treatment. We also verified that amineptine did not displace 3H-imipramine binding from platelets either in depressed or in control subjects. The results show that the successful treatment with amineptine, an antidepressant drug devoid of affinity for the tritiated imipramine platelet binding site, can rapidly lead to its density normalization.

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

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

Platelet imipramine binding in children and adolescents with impulsive behavior

The serotonergic system has been implicated in the regulation of impulsive aggressive behavior either toward oneself or others. Imipramine binding sites were measured in the platelets of 23 impulsive aggressive children. Subjects ratings of total behavior, externalizing behavior, hostility, and aggression, as measured by the Child Behavior Checklist, were inversely correlated with the platelet imipramine binding. These findings are consistent with previous studies that suggest that decreased serotonergic activity is associated with impulsive aggressive behavior.

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

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

Platelet 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.

3H-imipramine binding to previously frozen platelet membranes from depressed patients, before and after treatment

3H-imipramine binding in 39 drug-free patients with major depression and 44 healthy controls did not differ significantly between the two groups, in male or female subjects or in subgroups of depressed patients divided by endogenicity or dexamethasone suppression test result. 3H-imipramine binding in depressed patients drug-free for less than three weeks did not differ from those drug-free for longer intervals or from controls. A significant seasonal variation of 3H-imipramine Bmax was found, with lower values in summer and autumn. Treatment of depressed patients with imipramine or lofepramine for six weeks increased KD and Bmax. Methodological modification (in preparation and storage of platelets) does not explain the major differences in results between this study (using frozen platelets), a previous one (using freshly prepared platelets) and others in general, although it might contribute to the range of values reported.

Tritiated imipramine binding in obsessive-compulsive volunteers and psychiatrically normal controls

The authors evaluated platelet tritiated imipramine binding in 22 outpatients with obsessive-compulsive disorder (OCD) and 22 psychiatrically normal controls matched for age and gender. Mean maximal binding site density (Bmax) and equilibrium dissociation affinity (Kd) values were not significantly different. In OCD patients, Bmax was positively associated with age but was not associated with age of onset, gender, personality disorder, or five measures of illness severity. Eight patients with OCD were subsequently treated with clomipramine up to 300 mg/day for 10 weeks. Among these 8 patients, Bmax values had a 65% mean decrease from baseline, but Bmax values did not change among 7 OCD patients receiving placebo. The results suggest that a reduced density of tritiated imipramine binding sites may not be associated with OCD.

Changes in platelet 3H-imipramine binding with chronic imipramine treatment are not state-dependent

One month of imipramine treatment increased both the Kd and Bmax of platelet 3H-imipramine binding in 11 endogenous unipolar depressed patients. Continued treatment (13 weeks) of 5 patients subsequently lowered the Bmax values of 2 patients who had initially shown the largest increases, so that binding was no longer significantly elevated after 13 weeks. The observed changes in Kd but not in Bmax, could be explained by the carryover of tightly bound drug to the binding assay, although neither of the measures were correlated with plasma imipramine levels. Posttreatment Bmax (4 weeks) values were inversely related to plasma cortisol levels, although a weak but positive correlation was found before treatment. No significant change was found in plasma cortisol with treatment. Clinical responses were not related to cortisol or Bmax changes, although optimal improvement was associated with extreme values (high and low) of pretreatment Bmax. The present results, obtained with imipramine, and similar results obtained after nortriptyline and electroconvulsive shock by others, suggest that at least some antidepressants may induce transient changes in the Bmax of platelet binding that are independent of affective state.

3H-imipramine binding to freshly prepared platelet membranes in depression

3H-Imipramine binding was measured in freshly prepared platelet membranes from 47 drug-free major depressives and 46 healthy controls. Where possible, platelet binding in depressed subjects was repeated following treatment. A significant negative correlation was found between Bmax and assay protein concentration and Bmax values were corrected for this effect. Adjusted Bmax was significantly lower (by 14%) in female depressed patients than in female control subjects, and the difference was of similar magnitude premenopausally and postmenopausally. No such difference was found in males. Kd did not differ significantly between depressed and control subjects. Multiple regression analysis confirmed significant effects on Bmax of presence of depressive illness, age (positive correlation), and season (higher in summer). Within the depressed sample, Bmax was significantly lower in those subjects with obsessional features. Endogenicity (Research Diagnostic Criteria or Newcastle), dexamethasone suppression test result, drug-free interval, family history of depression, depressive psychosis, suicidal ideation, and past history of suicide attempts were not significantly related to Bmax. Paired comparisons revealed no significant effect on Bmax of 6 weeks' treatment with imipramine, maprotiline, or BRL 14342 or of a course of electroconvulsive therapy.

Platelet 3H-imipramine binding sites in depressed patients and healthy controls: a comparison between morning and afternoon samples

Platelet 3H-imipramine binding was compared between 19 hospitalized, depressed patients and age-matched controls (n = 11). For each individual person blood samples were withdrawn at 8 a.m. and 3 p.m. on two separate occasions (day 0 and day 28). Mean Bmax and Kd values did not differ between depressed patients and healthy controls at any of the four time-points tested, and no significant differences could be established between the different time-points, though there were irregular intraindividual variations.

Increased plasma free serotonin but unchanged platelet serotonin in bipolar patients treated chronically with lithium

The effect of lithium salts administered chronically to bipolar patients on peripheral measures of the serotoninergic system has been studied. Plasma free serotonin (5HT), whole blood 5HT, plasma 5-hydroxyindoleacetic acid (5HIAA) and plasma total tryptophan (TP) have been analyzed in 22 patients treated with lithium carbonate (mean daily dose: 1280 mg, mean serum concentration: 0.73 mmol/l) and compared to 14 healthy controls and 11 patients treated chronically with antipsychotic drugs. Lithium salts induced significant increases in plasma free 5HT (+159% with respect to control values) and in plasma 5HIAA (+39%) without affecting 5HT contained in platelets. Plasma TP was also unchanged by chronic lithium treatment. The ratio between 5HT stored in platelets and 5HT free in plasma, a variable reduced after uptake inhibitors like clomipramine, was decreased in lithium-treated patients (-50%). These results are compatible with an enhanced synthesis of 5HT in the periphery (mainly enterochromaffin cells) as well as with an inhibition of platelet 5HT uptake (or increased 5HT efflux from intracellular stores) induced by lithium. The lack of effect of several antipsychotic drugs upon these variables is consistent with their predominant effect on the dopaminergic system and reinforces the specificity of the effect observed with lithium salts. Taken together, these results support the usefulness of using this "in vivo" 5HT peripheral model for the study of the actions elicited by drugs acting on the presynaptic components of the 5HT system.

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.

Studies on the serotonin transporter in platelets

[3H]-Imipramine and [3H]-paroxetine label with high affinity a recognition site which is associated with the serotonergic transporter in blood platelets. The pharmacological profile of [3H]-imipramine and [3H]-paroxetine binding is highly correlated with the potency of drugs to inhibit the uptake of serotonin. Dissociation kinetic experiments suggest that the substrate recognition site for serotonin may be different from the modulatory site which is labeled with [3H]-imipramine or [3H]-paroxetine. The existence of an endocoid acting on the imipramine receptor to modulate the serotonin transporter has been proposed by several laboratories. In clinical studies most laboratories have reported a decrease in Bmax of [3H]-imipramine binding in platelets from depressed untreated patients when compared with matched healthy volunteers. The Bmax of [3H]-imipramine binding in platelets appears to be a state-dependent biological marker in depression.

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.

Up-regulatory effect of triphasic oral contraceptive on platelet 3H-imipramine binding sites

Triphasic oral contraceptive (Logynon) induced a significant increase (36%) in the maximal binding capacity of platelet membranes for [3H]imipramine. The increase was achieved in the second Logynon cycle as compared to pretreatment and first Logynon cycle binding values. The pill contains a combination of ethinyl estradiol and levonorgestrel, and it is as yet unclear which of the two hormones is responsible for the up-regulatory effect. The increase in the density of platelet imipramine binding sites may reflect a similar alteration in brain. The increase in imipramine binding did not correlate with alteration in mood as assessed by Beck Depression Inventory scores.

Circadian rhythm of the Bmax of [3H]-imipramine binding in rabbit platelets

[3H]-imipramine binding was measured in rabbit blood platelet membranes on a 24 h cycle. Animals were kept on a 14 h light (L) 10 h dark (D) schedule, and blood samples were collected at L + 2, L + 8, D + 2, D + 8 and L + 2 h on a following cycle. Significant differences were found for Bmax values of [3H]-imipramine binding, with highest values during the dark phase and lowest during the light phase. No significant differences were found in Kd values. These results suggest the existence of a circadian rhythm for the Bmax of [3H]-imipramine binding in blood platelets.