Reduced Bmax of [3H]-imipramine binding to platelets of depressed patients free of previous medication with 5HT uptake inhibitors

Antidepressant-binding sites in brain and platelets

[3H]Imipramine and [3H]paroxetine label with high affinity a site associated with the serotonin transporter in brain and platelets. The maximum binding capacity (Bmax) of [3H]imipramine in platelets is reduced in untreated depressed patients, and it may represent a useful biological marker in depression. The existence of an endogenous ligand acting on the [3H]imipramine-recognition site to modulate the serotonin transporter has been proposed by several laboratories. 5-Methoxytryptoline inhibits [3H]imipramine binding and [3H]serotonin uptake in the nanomolar range. This compound has been reported to occur in the pineal gland, but probably only in trace amounts. While the physiological relevance of 5-methoxytryptoline or a close analogue remains an open question, the possibility exists that the 'endocoid' for the [3H]imipramine-recognition site plays a role in the pathogenesis of depression.

Imipramine binding in depression and other psychiatric conditions

30 studies of platelet 3H-imipramine binding in depressed patients are reviewed. 19 of these studies found that depressed patients had lower binding than control persons, whereas 11 studies found no difference. This discrepancy is discussed and it is concluded that methodological problems may account for a substantial part of the low imipramine binding in depressed patients.

Methodologic pitfalls in measurement of 5-hydroxytriptamine uptake transporters in human platelets by [3H]-paroxetine binding assay

BACKGROUND

Studies in platelet of 5-HT uptake transporters have been performed using binding assay methodology designed for ligand-receptor interactions; however, uptake transporters present requirements that may question the validity of these particular binding assays.

METHODS

To explore methodologic aspects that may be crucial to the validity of these assays, we studied the binding of [3H]-paroxetine to platelet membranes of healthy subjects under different conditions of time, temperature, and protein concentrations.

RESULTS

A correlation between protein concentration in incubation media and percentage of specific binding of [3H]-paroxetine was found: the lower the protein concentrations (10 and 20 microg/mL) in incubation media, the lower the percentage of specific [3H]-paroxetine binding. Moreover, low specificity in [3H]-paroxetine binding affected Bmax values obtained in saturation binding experiments.

CONCLUSIONS

The use of low protein concentrations could affect Bmax values in binding assays of 5-HT uptake transporters. This may induce confusing interpretation of data in clinical experiments that use human platelets to explore the participation of serotonin in depressed patients.

Cerebrospinal fluid biogenic amine metabolites, plasma-rich platelet serotonin and [3H]imipramine reuptake in the primary fibromyalgia syndrome

BACKGROUND

Primary fibromyalgia syndrome (PFS) is a chronic disorder commonly seen in rheumatological practice. The pathophysiological disturbances of this syndrome, which was defined by the American College of Rheumatology in 1990, are poorly understood. This study evaluated, in 30 patients, the hypothesis that PFS is a pain modulation disorder induced by deregulation of serotonin metabolism.

OBJECTIVES

To compare platelet [(3)H]imipramine binding sites and serotonin (5-HT) levels in plasma-rich platelets (PRP) of PFS patients with those of matched healthy controls and to compare the levels of biogenic amine metabolites in the cerebrospinal fluid (CSF) of PFS patients with those of matched controls.

METHODS

Platelet [(3)H]imipramine binding sites were defined by two criteria, B(max) for their density and K(d) for their affinity. PRP 5-HT and CSF metabolites of 5-HT (5-hydroxyindoleacetic acid, 5-HIAA), norepinephrine (3-methoxy, 4-hydroxy phenylglycol, MHPG) and dopamine (homovanillic acid, HVA) were assayed by reversed-phase high-performance liquid chromatography with coulometric detection.

RESULTS

[(3)H]Imipramine platelet binding was similar (P=0.43 for B(max) and P=0.30 for K(d)) in PFS patients (B(max)=901+/-83 fmol/mg protein, K(d)=0.682+/-0.046) and in matched controls (B(max)=1017+/-119 fmol/mg protein, K(d)=0.606+/-0.056). PRP 5-HT was significantly higher (P=0.0009) in PFS patients (955+/-101 ng/10(9) platelets) than in controls (633+/-50 ng/10(9) platelets). When adjusted for age, the levels of all CSF metabolites were lower in PFS patients. The CSF metabolite of norepinephrine (MHPG) was lower (P:=0.003) in PFS patients (8.33+/-0.33 ng/ml) than in matched controls (9.89+/-0.31 ng/ml) and 5-HIAA was lower (P=0.042) in PFS female patients (22.34+/-1.78 ng/ml) than in matched controls (25.75+/-1.75 ng/ml). For HVA in females, the difference between PFS patients (36.32+/-3.20 ng/ml) and matched controls (38.32+/-2.90 ng/ml) approached statistical significance (P=0.054).

CONCLUSION

Changes in metabolites of CSF biogenic amines appear to be partially correlated to age but remained diagnosis-dependent. High levels of PRP 5-HT in PFS patients were associated with low CSF 5-HIAA levels in female patients but were not accompanied by any change in serotonergic uptake as assessed by platelet [(3)H]imipramine binding sites. These findings do not allow us to confirm that serotonin metabolism is deregulated in PFS patients.

Chronic amitriptyline administration increases serotonin transporter binding sites in the hippocampus of aged rats

The effects of ageing and of chronic antidepressant treatment upon 5-HT transporter sites ([3H]paroxetine binding) in the rat hippocampus was examined. [3H]paroxetine binding to transporter sites was decreased with ageing in the hippocampus of control rats (38% decrease in dentate gyrus and CA4). Amitriptyline (10 mg/kg, i.p.) had no significant effect on [3H]paroxetine binding in 10 months old rats, but increased binding sites in 24 months rats in all hippocampal subregions (greatest increase of 109% in CA1 compared to saline controls). These data indicate an age-related decrease in hippocampal serotonin transporter sites and upregulation of these sites following 10 weeks of amitriptyline. The observed increase in transporter sites following amitriptyline may contribute to the general lower effectiveness of tricyclic antidepressants with ageing.

Altered [3H]imipramine and 5-HT2 but not [3H]paroxetine binding sites in platelets from depressed patients

BACKGROUND

Serotonergic system alterations were studied in 51 depressed patients classified according to DSM-III-R criteria for major depression with melancholia compared to 31 healthy controls.

METHOD

[3H]Imipramine and [3H]paroxetine binding sites and the 5HT2 receptor were simultaneously determined in blood platelet membranes.

RESULTS

A significantly lower maximum binding in [3H]imipramine binding was observed in depressed patients compared to controls (1134+/-74 vs. 1712+/-106 fmol/mg protein, P<0.0001) without changes in the equilibrium dissociation constant (1.10+0.05 vs. 1.25-/+0.09 nM). [3H]Paroxetine binding did not differ between the two groups (Bmax, 1441+/-55 vs. 1280+/-81 fmol/mg protein; Kd, 0.060+/-0.002 vs. 0.062+/-0.002 nM). The K(d) value of 5HT2 binding was lower in depressed patients than controls (0.95+/-0.04 vs. 1.15+/-0.09 nM, P<0.039) without changes in maximum binding (140+/-11 vs. 127+/-14 fmol/mg protein).

CONCLUSIONS

Taken together, these results suggest that [3H]imipramine and 5HT2 receptors may be good biological markers for serotonergic dysfunction in depressive disorders.

Sex steroid control of mood, mental state and memory

1. Sex steroid hormones exert profound effects on mood and mental state. Thus, in women, oestrogen is thought to protect against depression and delay the onset of schizophrenia and Alzheimer's disease. 2. Our studies in the female rat show that oestradiol, in its positive feedback mode for gonadotrophin release, increases the expression of genes for the 5-hydroxytryptamine 5-HT2A receptor and the serotonin transporter (SERT) in the dorsal raphe nucleus and the density of 5-HT2A receptor and SERT sites in regions of the forebrain that, in the human, are concerned with cognition, mental state, emotion and memory. 3. In the male rat, castration decreases while oestrogen and testosterone, but not 5 alpha-dihydrotestosterone (5 alpha-DHT), increase the density of 5-HT2A receptors in forebrain. The fact that 5 alpha-DHT has no effect suggests that the action of testosterone depends on its conversion to oestradiol by aromatase. 4. In intact rats, the density of 5-HT2A receptors in cerebral cortex is significantly higher in pro-oestrous female than in male and dioestrous female rats, showing that the spontaneous, preovulatory surge of oestradiol that reaches a peak at 12.00 h of pro-oestrus also increases the density of 5-HT2A receptors in cortex. 5. Oestrogen and testosterone (by way of its conversion to oestrogen) also stimulate the expression of the arginine vasopressin gene in the bed nucleus of the stria terminalis of the rodent, a mechanism that plays a key role in olfactory memory. 6. These actions of sex steroid hormones are discussed in the context of genomic versus non-genomic mechanisms, the recent discovery that there are two oestradiol receptors with different distributions in brain, the significance of our findings for our understanding of the control of mood, mental state and memory and the mechanism by which oestrogen stimulation of the 5-HT2A receptor could delay the onset of Alzheimer's disease.

Opposite modulatory effects of ovarian hormones on rat brain dopamine and serotonin transporters

The present study was designed to investigate the modulatory effect of gonadal steroids on brain dopamine (DA) and serotonin (5-HT) presynaptic transporters in female and male rats. Female and male rats were castrated and treated with either vehicle or gonadal hormones. The pharmacodynamic characteristics of the DA and 5-HT transporters were analyzed by [3H]BTCP and [3H]imipramine binding respectively. Ovariectomy (OVX) resulted in an upregulation of the striatal DA transporter and this alteration was prevented by estradiol (E2) or E2 + progesterone (P) treatment but not by P alone. In contrast to the DA transporter, the hypothalamic 5-HT transporter was down-regulated by OVX in female rats and this decrease was reversed by the administration of E2, P or their combination. The striatal DA transporter and the hypothalamic 5-HT transporter in male rat were not affected by orchidectomy or by administration of testicular hormone. Our findings indicate that ovarian, but not testicular, steroid hormones may play an important role in the regulation of brain DA and 5-HT transporters. It appears that ovarian hormones modulate rat brain 5-HT and DA transporters in opposite directions. These interactions between ovarian steroids and presynaptic transporters may be relevant to DA- and 5-HT-related neuropsychiatric disorders.

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.

Clinical and biochemical aspects of depressive disorders: II. Transmitter/receptor theories

The present document is the second of three parts in a review that focuses on recent data from clinical and animal research concerning the biochemical bases of depressive disorders, diagnosis, and treatment. Various receptor/transmitter theories of depressive disorders are discussed in this section. Specifically, data supporting noradrenergic, serotonergic, cholinergic, dopaminergic, GABAergic, and peptidergic theories, as well as interactions between noradrenergic and serotonergic, or cholinergic and catecholaminergic systems are presented. Problems with the data and future directions for research are also discussed. A previous publication, Part I of this review, dealt with the classification of depressive disorders and research techniques for studying the biochemical mechanisms of these disorders. A future publication, Part III of this review, discusses treatments for depression and some of the controversies in this field.

Platelet 3H-paroxetine binding to the serotonin transporter is insensitive to changes in central serotonergic innervation in the rat

The serotonin transporter labeled in platelets by 3H-imipramine or 3H-paroxetine binding has been suggested to be a peripheral marker for changes in serotonin uptake in the brain that may be related to depression. The present study was designed to determine whether major changes in central serotonergic innervation modify the platelet serotonin transporter as labeled by 3H-paroxetine binding. Fifteen days after the intracerebroventricular administration of 5,7-dihydroxytryptamine (250 micrograms/animal) to rats to lesion central serotonergic neurons, serotonergic innervation was reduced by 82% in the cortex and 98% in the hippocampus as determined by endogenous serotonin levels. The maximum binding of 3H-paroxetine was reduced by 55% in the cortex and was undetectable in the hippocampus. Serotonin levels and 3H-paroxetine binding in platelets were not, however, significantly modified in the same animals. Thus, following a major serotonergic lesion in the brain, changes in the platelet serotonin transporter do not parallel serotonergic changes in the brain. The hypothesis that binding to the platelet serotonin transporter is a state-dependent marker of brain serotonergic activity therefore appears to be unlikely.

Platelet [3H]imipramine binding in migraine and tension headache in relation to depression

Platelet [3H]imipramine binding was measured in 40 migrainous (7 classical and 33 common) and 17 tension headache patients and in 28 normal controls. A significant reduction in Bmax was found in migraine compared with controls (p less than 0.05) but not in tension headache. In migraine, there was no significant relationship between Bmax and depression or anxiety score on the self-rating Hospital Anxiety and Depression (HAD) Scale, suggesting that the reduction in Bmax is a concomitant of migraine itself rather than a manifestation of associated depression. Preliminary evaluation using the Schedule of Affective Disorders and Schizophrenia-Lifetime Version (SADS-L) tended to confirm this conclusion.

Influence of age, sex and diurnal variability on imipramine receptor binding and serotonin uptake in platelets of normal subjects

Imipramine (IMI) binding and serotonin (5-HT) uptake were determined in platelets of 98 healthy volunteers; and their association with age, sex and circadian rhythm were evaluated. A large interindividual variability was found for both IMI and 5-HT parameters. There was a negative correlation of IMI affinity constant (Kd) and binding (Bmax) with age, but no such correlation of 5-HT affinity constant (Km) or uptake (Vmax). Significant age-related diurnal variability was found for 5-HT Km in the whole group as well as for IMI Kd in males, but not in females. There was no significant correlation between 5-HT Vmax and IMI Bmax. Our results underscore a cautious approach to the interpretation of platelet serotonergic studies. In light of the multiple variables influencing the results, the usefulness of IMI or 5-HT as clinical markers should be re-evaluated.

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.

Neuroendocrine aspects of the serotonergic hypothesis of depression

This review examines the role of serotonin (5-HT) in depression. Dysfunction of serotonergic neurons has been implicated as one of the causes of endogenous depression. Since serotonergic neurons innervate the hypothalamus and these neurons send collaterals to several other brain areas, it is possible that hypothalamic sites which control hormone secretion receive the same serotonergic afferents that innervate other limbic areas in the brain. Several investigators have devised neuroendocrine challenge tests measuring the effect of 5-HT agonists on plasma cortisol and prolactin in depressed patients. These tests help to identify dysfunctional 5-HT neurons, and are a "window into the brain." The secretion of cortisol and prolactin is increased predominantly by 5-HT1 receptors. However, changes in 5-HT2 receptors have also been implicated in depression. Results from our laboratory and by others suggest that brain serotonergic neurons stimulate renin and vasopressin secretion by activation of 5-HT2 receptors. Therefore, the renin and vasopressin response to 5-HT agonists should be included in neuroendocrine tests of serotonergic function in affective disorders. Since antidepressants produce a decrease in the density of 5-HT2 receptors, renin and vasopressin could be used to evaluate the antidepressant potential of new drugs.

3H-imipramine binding in the frontal cortex of suicides

Imipramine binding (desipramine- and serotonin-sensitive) was determined in the frontal cortex of suicide victims and nonpsychiatric controls who died due to medical disease or accidents. There were no differences in Kd or Bmax of imipramine binding between controls and suicides. The Kd and Bmax values of serotonin-sensitive imipramine binding were significantly lower than desipramine-sensitive imipramine binding, both in controls and suicides. There were significant correlations between Kd and Bmax of serotonin-sensitive imipramine binding and desipramine-sensitive imipramine binding in suicides but not in controls.

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 is not modified in Alzheimer's disease

Platelet [3H]-imipramine binding was studied in patients with Alzheimer's disease and control subjects matched to the patients for age and sex. There were no differences in the binding parameters of [3H]-imipramine on platelet membranes from patients with Alzheimer's disease, when compared with the control group. These results suggest that [3H]-imipramine binding could be a useful tool to discriminate between demented and depressive patients in elderly populations.

Increased serotonin2 (5-HT2) receptor binding as measured by 3H-lysergic acid diethylamide (3H-LSD) in the blood platelets of depressed patients

3H-Lysergic acid diethylamide (3H-LSD) binding, a putative measure of 5-HT2 receptor binding, was studied in the blood platelets of 29 depressed patients and 24 normal controls. The Bmax (maximum number of 3H-LSD binding sites) in the blood platelets of depressed patients was significantly greater than that of normal volunteers. This increase in Bmax was due to an increase in female depressed patients only. Bmax was significantly lower in female compared to male normal controls but there was no difference between male and female depressed patients. There was also no difference in Kd (an inverse measure of affinity of 3H-LSD binding to its sites) between normal controls and depressed patients. The correlations between Bmax of 3H-LSD binding and the Bmax of the 3H-imipramine binding site or the Vmax of 5-HT uptake sites were not significant. The role of serotonergic processes in the psychobiology of depression is discussed.

Platelet 3H-imipramine binding in psychogenic pain patients

High-affinity binding of 3H-imipramine was analyzed in platelet membranes from patients with chronic psychogenic pain. The patients who in addition to the pain also showed affective symptoms such as depression and anxiety had lower binding than the pain patients without these symptoms.

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.

Effect of desipramine treatment on 3H-imipramine binding in the blood platelets of depressed patients

Platelet imipramine binding (IB) was studied in depressed patients before and after treatment with desipramine for 17-28 days. Administration of desipramine was associated with a significant increase in Bmax. There was a trend for an increase in Kd, but it did not reach statistical significance. The net result of the changes in Bmax and Kd was an increase in IB. There were significant correlations between the change in depression ratings and pretreatment Kd, as well as the change in Kd during treatment. These results suggest that decreased IB is not a trait-dependent marker, but a state-dependent marker for 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.

Short-term lithium administration to healthy volunteers produces long-lasting pronounced changes in platelet serotonin uptake but not imipramine binding

Platelet [3H]-5HT uptake, [3H]-imipramine binding and endogenous 5HT levels were measured in healthy volunteers during short-term (20 days) administration of lithium, and following its withdrawal. The Vmax of [3H]-5HT uptake was significantly decreased during lithium treatment. Following lithium withdrawal, platelet [3H]-5HT uptake (Vmax) remained decreased and was followed by a pronounced rebound effect in some of the subjects for up to 3 months. The affinity constant (Km) of [3H]-5HT uptake was not modified. Binding of tritiated imipramine during the same period and platelet 5HT levels measured till 14 days after withdrawal was not affected by lithium treatment. As lithium is devoid of in vitro effects on both 5HT uptake and imipramine binding, it is concluded that the effects of lithium on the 5HT transporter do not reflect a direct effect on the transporter complex. Our results indicate that lithium-induced changes at the level of 5HT uptake in platelets are not correlated with concomitant variations in platelet 5HT content and can be dissociated from modifications at the level of imipramine binding sites within the macromolecular complex of the 5HT transporter. Moreover, platelet 5HT uptake is apparently modulated by lithium, with a similar pattern in healthy volunteers and in manic-depressive patients.

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.

Pitfalls in demonstrating an endogenous ligand of imipramine recognition sites

The recognition sites for the 5-hydroxytryptamine (5-HT) uptake inhibitors imipramine and paroxetine may represent receptors for a presently unknown endogenous ligand, whose function would be to modulate 5-HT uptake. Attempts to isolate such a factor from rat brain tissue are described, following a published procedure. It is shown that chromatographic fractions found to inhibit the binding of [3H]imipramine and [3H]paroxetine to rat brain membranes consisted of material essentially unretained by the reverse-phase HPLC column, and they were of high osmolarity. Thus, the activity was probably unspecific in nature, and the presence in rat brain of the factor has not been unequivocally demonstrated.

Monoamine transport in depression: kinetics and dynamics

There appears to be an abnormality in monoamine transport in major depressive disorders that may be specific to these disorders, although it may not be primarily an aminergic lesion. Kinetic factors alone are insufficient to explain the observed effects of antidepressants on transport mechanisms, or changes in uptake with therapeutic response, suggesting dynamic influences on uptake processes that deserve further exploration. The investigation of these influences may indicate that platelet 5-HT uptake offers diagnostic and theoretical possibilities not adverted to at present and another rationale for the use of platelets in research on mental disorders.

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.

Association of [3H]-imipramine and [3H]-paroxetine binding with the 5HT transporter in brain and platelets: relevance to studies in depression

[3H]-Imipramine and [3H]-paroxetine label with high affinity a site which is associated with the serotonergic transporter in brain and platelets. The pharmacological profile of inhibition by drugs of [3H]-imipramine and [3H]-paroxetine binding is highly correlated with the potency of the drugs to inhibit the uptake of 5HT. Denervation of serotonergic neurons by electrolytic lesions or with 5,7-dihydroxytryptamine produces marked decreases in the density of [3H]-imipramine as well as [3H]-paroxetine binding. Dissociation kinetic experiments support the view that the substrate recognition site for 5HT is different from the modulatory site which is labelled by [3H]-imipramine or [3H]-paroxetine. The existence of an endogenous ligand acting on the [3H]-imipramine recognition site to modulate the 5HT transporter was proposed by several laboratories. [3H]-Imipramine binding in platelets appears to be a biological marker in depression. Studies carried out in several laboratories report a significant decrease in the Bmax of platelet [3H]-imipramine binding without changes in Kd, when severely depressed untreated patients are compared with healthy volunteers matched for age and sex. The Bmax of platelet [3H]-imipramine binding appears to be a state-dependent biological marker in depression. It is tempting to speculate that the endocoid of the [3H]-imipramine recognition site may play a role in the pathogenesis of depression.

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.