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

Serotonergic platelet markers of suicidal behavior--do they really exist?

BACKGROUND

Abnormal changes in platelets used as peripheral markers of central serotonergic functions are said to be associated with suicidality and depression, but this association has not been supported by consistent findings.

AIM

This review based on selected, often quoted publications in this area focuses firstly, on obviously contradictory findings and, secondly, on potential methodological flaws explaining these discrepancies.

RESULTS

The platelet 5-HT transporter has been found to have an inconsistent association with suicidality; furthermore, the specificity of imipramine for the 5-HT transporter is most likely low, since the number of platelet impramine binding sites has not been reliably associated with platelet serotonin uptake (Vmax). Significant changes of platelet serotonin content in suicidal individuals, as described in various studies, are most likely due to washout periods that are too short to eliminate the effects of a previous drug intake, or, in violent suicide attempters, due to blood loss and dilution. Similar methodological shortcomings might account for an often-reported elevated number of platelet 5-HT(2) receptor binding sites in suicidal individuals. In many studies, the results have not been sufficiently controlled for drug effects on platelet 5-HT(2) receptors, and associations of platelet 5-HT(2) binding with selective classifications of suicidal behavior are often generalized as further evidence for an association of platelet 5-HT(2) receptors with 'suicidality'. Finally, changes in platelet MAO-B-activity in suicidal patients have not been reproducibly found, and the impact of smoking on MAO-B activity has not been controlled in any studies.

CONCLUSIONS

Methodological flaws such as small sample sizes, insufficient matching criteria for controls, use of inadequate ligands in binding experiments, nonconsideration of comorbidity etc. and considerable methodological differences between studies limit their validity and comparability. It does not seem possible, at present, to integrate published findings and concepts into a plausible biological model of suicidality.

Seasonal variability in blood platelet 3H-imipramine binding in healthy controls: age and gender effects

Binding of 3H-imipramine to blood platelet membranes was determined four times (once each season) in 26 healthy volunteers (11 men and 15 women), over the course of 1 year to determine possible seasonal variations. Blood platelets were obtained in April-May, July-August, October-November, and January-February. Significant seasonal variations in the maximum number of binding sites were found in women but not in men, with circannual peak in summer and a nadir in spring. The pattern of seasonal variations was not the same in men and women. The present results highlight the importance of monitoring for gender and season in binding studies. We found no significant correlation between 3H-imipramine binding parameters and age.

Changes in platelet 3-H-imipramine binding: influences of protein concentration of varying proportions of cytosol or intact platelets and displacing agents used

Platelet 3-H-imipramine binding exhibits considerable variation, both interindividually and between several groups. The aim of this study was to measure 3-H-imipramine binding, simultaneously in platelet membranes vs. intact platelets vs. cytosol or intracytosolic protein in order to determine their effect on Bmax and Kd values. 3-H-imipramine binding was carried out at different protein concentrations. Our results indicate that the affinity constant is heavily influenced by the presence of cytosol and intact platelets in membrane preparations. Finally, we demonstrate a negative correlation between Bmax and protein concentration. Only perfect analytical conditions will allow platelet 3-H-imipramine binding to be a biological marker for affective disorders.

Phenomenology and neurobiology of cocaine withdrawal: are they related?

1. The presence of mood disturbances and platelet 3H-imipramine binding, a putative peripheral serotonergic marker, were evaluated in a group of 27 cocaine users three days after drug withdrawal. 2. Parameters of cocaine use and the linkage between cocaine withdrawal and "post-cocaine depression" were also investigated. In a subgroup of 10 patients, both psychopathological and biological measurements were repeated after 5 or 6 weeks. 3. Interpretation of the data by Pearson's analysis showed a statistically significant and positive correlation between Hamilton Rating Scale for Depression (HAM-D) scores and period of use. A trend towards a negative correlation, which however did not reach the statistical significance, was found between 3H-Imipramine binding and period of cocaine use, number of days of abstinence and HAM-D scores 4. When compared with normal volunteers at baseline, patients had significantly lower Bmax and Kd values which returned towards normal values after 5 or 6 weeks of cocaine withdrawal. 5. These results indicate the presence of a decreased platelet imipramine binding during cocaine withdrawal which may be due to the effect of the drug or alternatively, a result of concomitant depression which may be primary or secondary in origin. The decreased imipramine binding is a reversible phenomenon, since it increases with the time, in parallel with the improvement of depressive symptoms.

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.

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.

Peripheral markers of serotonin and dopamine function in obsessive-compulsive disorder

In an attempt to clarify the possible role of a serotonergic and dopaminergic dysfunction in obsessive-compulsive disorder (OCD), we measured platelet 3H-imipramine (3H-IMI) binding, serotonin uptake, and platelet sulfotransferase activity in 17 drug-free OCD patients and an equal number of healthy controls. Serotonin uptake and 3H-IMI binding sites in platelets have been shown to constitute peripheral markers of those present in presynaptic serotonergic neurons. Sulfotransferase, an enzyme involved in the catabolism of phenolic compounds and of cathecholamines such as dopamine, has similar kinetic characteristics in brain and platelets. Our results showed a lower number of 3H-IMI binding sites and a higher level of sulfotransferase activity in OCD patients compared with those in controls. These preliminary results suggest involvement of both the serotonin and dopamine systems in OCD.

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.

Binding of some antidepressants to the 5-hydroxytryptamine transporter in brain and platelets

Antidepressant agents with properties to inhibit 5-hydroxytryptamine (5-HT, serotonin) uptake in brain tissue and platelets bind with high affinities to neuronal and platelet membranes. [3H]Imipramine, [3H]paroxetine and [3H]citalopram label specific binding sites related to the 5-HT transporter. [3H]Paroxetine and [3H]citalopram appear to be better ligands than [3H]imipramine. The former label a homogenous population of binding sites, whereas the displaceable binding of [3H]imipramine is heterogenous. Recent observations in several laboratories, which have taken the heterogeneity of [3H]imipramine binding into account, indicate that the binding of antidepressants to the 5-HT transporter probably occurs to the same site that binds 5-HT for transport and not to a separate site as previously suggested. Additional bonds to subsites in close vicinity to the 5-HT recognition site may contribute to the binding. No convincing evidence has been presented of the existence of an endogenous ligand other than 5-HT itself that binds to the [3H]imipramine binding site. Recent studies also suggest that repeated treatment of rats with antidepressant agents does not produce any alterations of the binding of [3H]imipramine or [3H]paroxetine to membranes of cerebral cortex. It is also doubtful whether the density of the 5-HT uptake site in platelets measured with these ligands is decreased in affective disorders as first reported.

Influence of protein concentration on platelet 3H-imipramine binding

1. The effects of protein concentration in the assay mixture on platelet 3H-imipramine binding were studied in normal controls. 2. Increasing protein concentrations (76-926 micrograms/ml) were found to alter estimates of binding affinity (Kd) but not the number of binding sites (Bmax). 3. Increasing Kd estimates were protein dependent at concentrations in excess of ca. 200 micrograms/ml but were not protein dependent at lower concentrations. 4. A comparison of the present results with previous reports suggests that widespread interlaboratory differences in reported Bmax values for normal controls cannot be attributed to differences in the protein content of incubated samples. Rather these differences may be due to the inclusion of non-membrane protein in the assayed material.