229 Psychiatry Research, 33~229-242 Elsevier

Platelet lmipramine Binding in Endogenous Depressed Patients and Controls: Relationship to Platelet MAO and 5HT Uptake During Successful lmipramine Treatment

Javier Quintana

Received July 7, 1989; first revised version received November 30, 1989; second revised version received February 8, 1990; accepted March 18, 1990.

Abstract. 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 (B,,). 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.

Key Words. Endogenous depression, imipramine binding, platelets, serotonin uptake, monoamine oxidase, long-term imipramine therapy.

Platelet imipramine binding has been the focus of many studies on affective disorders in recent years, ever since it was first described (Briley et al., 1979; Paul et al., 1980) and soon thereafter found altered in depressive patients (Asarch et al., 1980; Briley et al., 1980; Paul et al., 1981; Raisman et al., 1981). Imipramine binding (IB) was at first considered closely related or even identical to the serotonin (Shy- droxytryptamine; SHT) uptake site (Langer et al., 1980; Paul et al., 1981); however, 5HT uptake and IB may be independently regulated (Arora et al., 1983; Barbaccia et al., 1983). Their recovery time courses after changes produced by chlorimipramine administration are different (Poirier et al., 1987). A dissociation between the two parameters has been demonstrated in depression (Raisman et al., 1982; Wood et al., 1983; Suranyi-Cadotte et al., 19853) as well as in several other diseases (Ahtee et al., 1981; Wood et al., 1983; Kamal et al., 1984; Weizman et al., 1986). Discrepancies exist as to the pharmacological significance of the IB site: its affinity is much stronger than the potency of the drug in inhibiting 5HT uptake (Langer et al., 1980),

Javier Quintana, M.D., Ph.D., is Visiting Assistant Professor, Department of Neurology, School of Medicine, University of California at Los Angeles. (Reprint requests to Dr. J. Quintana, UCLA Neuropsychiatric Institute, 760 Westwood Plaza, Los Angeles, CA 90024, USA.)

01651781/90/$03.50 @ 1990 Elsevier Scientific Publishers Ireland Ltd.

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although this fact has been shown to be due to the temperature-dependence of the drug interaction with the 5HT transporter (Segonzac et al., 1987). It has been suggested that the IB site is distinct from but allosterically coupled to the 5HT transporter (Meyerson et al., 1987). Notwithstanding, the IB site is currently regarded as a component of the 5HT transporter complex (Segonzac et al., 1987). The platelet binding site appears to be similar to the corresponding binding site in the brain except for some regulatory aspects (Abel et al., 1985).

Several studies have replicated the initial finding of a decrease in the number of IB sites (B,,,) in platelets from depressed patients (for recent reviews, see Beth et al., 1988; Gentsch et al., 1989). Another study, however, found an increase in the number of binding sites (Mellerup et al., 1982), and still others found no statistically signifi- cant differences in this parameter between controls and depressives (Berrettini et al., 1982; Baron et al., 1983, 1986; Egrise et al., 1983, 1986; Muscettola et al., 1984, 1986; Rehavi et al., 1984; Whitaker et al., 1984; Gentsch et al., 1985, 1989; Hrdina et al., 1985; Tang and Morris, 1985; Braddock et al., 1986; Carstens et al., 1986; Desmedt et al., 1987; Kanof et al., 1987; see also detailed references in Beth et al., 1988). Further- more, recent preliminary results from a multisite World Health Organization Colla- borative standardized study (Beth et al., 1988; Gentsch et al., 1989) indicate that there are no significant differences in platelet IB between depressed patients and controls. 1.

The effect of antidepressant treatment and clinical recovery on platelet IB has also been studied and found controversial. Early reports (Asarch et al., 1980; Briley et al., 1980; Paul et al., 1981; Raisman et al., 1981,; Langer et al., 1982) suggested that platelet IB is a “trait” marker or parameter (i.e., it does not change with shifting symptomatology but, rather, indicates individual, inherited or not, predisposition). Later, this was confirmed (Baron et al., 1986) but also contradicted (Suranyi- Cadotte et al., 1982, 19856; Marazziti et al., 1988) in reports that showed a recovery of platelet IB levels paralleling clinical improvement throughout tricyclic anti- depressant (TCA) therapy (which would suggest that the binding site is a “state” indicator). Similar platelet IB values have been found in euthymic bipolar patients and normal controls (Berrettini et al., 1982; Mellerup et al., 1982; Suranyi-Cadotte et al., 1982; Goodnick et al., 1984) as well as in comparisons of children and adole- scents with major depressive disorders with their respective controls (Rehavi et al., 1984), suggesting that the site is not a trait marker for depression. The answer to the question of whether IB can be considered as a trait or as a state marker for depression could vary when dealing with different subsets of affective disorders (Lewis and McChesney, 1985; Baron et al., 1986; Carstens et al., 1986).

There might be several explanations for the conflicting results summarized above. Methodological differences among the different studies, the presence of circannual and circadian variations in platelet IB in controls and depressives, and the demo- graphic and diagnostic composition of the study group have been shown to influence IB parameters. The medication status of the patients (e.g., the type of previous drug intake and the “washout” period allowed before the first biological measurement is made) may be a source of variability between studies. Moreover, plasmatic factors, which could vary among studies, have been shown to inhibit IB to platelet mem- branes (Barkai et al., 1986).

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Thus, there are several unresolved issues concerning platelet IB. The present study, using a careful methodological design, was intended to address some of them, particularly those referring to the question of trait versus state marker, the changes of platelet IB with treatment and clinical recovery, and their relation to platelet 5HT transport and monoamine oxidase (MAO) activity. To do so, platelet IB was measured in unmedicated depressed patients and controls, and again in patients after 3 weeks and 2 months of imipramine treatment, at the end of which they were clinically recovered from depression. Both platelet MAO (Quintana, 1988) and platelet 5HT uptake (Quintana, 1989) were studied in parallel in the same popula- tion. Thus, an additional interest in this study was to correlate platelet IB changes with dynamic changes occurring concurrently in the platelets of the same subjects.

Methods

Subjects. Twenty-five depressed patients (age range 32-64 years, mean = 54, SD = 9.3; 60% women), and 25 sex- and age-matched drug-free controls were studied. Diagnoses of major depressive disorder, endogenous subtype, were established by two psychiatrists according to Research Diagnostic Criteria (RDC; Spitzer et al., 1978). The patients were admitted to the Adult Psychiatry Division, Bellvitge Hospital, University of Barcelona, Spain. Once admitted to the study, the patients were kept free of medication for 6 days; none of them had taken any drug for the previous 7 days. The control group consisted of 25 healthy volunteers having no personal or familial history of major psychiatric disorders, and included laboratory personnel and ambulatory control visits. They were screened for any medication taken during the 2 weeks before the blood extraction. After the first sample was taken, the patients began oral imipramine therapy (125175 mg/day). Two other samples were taken, after 3 weeks and 2 months of treatment; between these samples, most of the patients were discharged from the hospital due to their apparent clinical improvement. At the time of the third and last sampling, during an ambulatory visit, only 18 patients were available. Clinical change was verified by interviews or the Hamilton Rating Scale for Depression (HRSD; Hamilton, 1960).

Isolation of Blood Platelets. Blood samples (5-8 ml) were collected from patients and controls between 8 and 9 a.m. after overnight fasting. The samples from controls were obtained the same day as those from an age- and sex-matched unmedicated patient. Platelets, isolated from the samples following procedures described elsewhere (Quintana 1988, 1989), were resuspended in Krebs Ringer bicarbonated media (NaCl 120 mM, KC1 4.93 mM, KH,PO, 1.24 mM, MgSO, 1.23 mM, NaHCO,-stirred with CO, for 1 hour-l .98 mM, final pH of the medium 7.4) containing 2.64 mM CaCl, up to a final density of 500,000 platelets/p1 (Thrombocounter C, Coulter Electronics Ltd., Harpenden, Herts.), and immediately frozen at -60 “C. The protein concentration in the final suspension was determined according to the method of Lowry et al. (1951).

lmipramine binding. Within 7 days of their collection, the samples from patients and controls were processed simultaneously to determine platelet IB. They were thawed, frozen, and thawed again. Aliquots of the suspensions (average membrane protein, 89 pg) were incubated for 1 hour over an ice-cold bath at 0 “C with 0.5 ml of Krebs buffer containing 2.64 mM CaCl, and one of six different concentrations of 3H-imipramine (23 Ci/mmol, Amersham, England), ranging from 0.25 to 3 nA4. Parallel series were processed identically except for the addition of 100 PM desipramine in each tube. After the incubation, the content of each tube was quickly and individually filtered under vacuum through Whatman GF/B glass fiber filters. The filters were washed two times with 5 ml of the ice-cold buffer and, once dry, placed in 10 ml of UNISOLVE I (Koch Light Laboratories) for liquid scintillation counting (Beckman LS-7000). The specific binding for each imipramine concentration,

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defined as the difference in binding between the corresponding samples in the two series (total minus nonspecific binding), was used to construct Scatchard plots (Scatchard, 1949). Both the density of binding sites (B,,,,,) and the apparent affinity constant (I(d) were determined by linear regression analysis. Statistical comparisons between groups were made using a two- tailed Student’s t test. Correlations between biological parameters or between them and the patients’ clinical status (HRSD scores) were evaluated using linear regression analyses (Pearson’s product-moment correlation coefficient) or Spearman’s rank-order tests, respectively.

Results

Platelet lmipramine Binding. Only one type of platelet IB site was observed by Scatchard analysis of the binding curves of samples from unmedicated patients and controls. No significant differences in platelet IB were found between control subjects and unmedicated patients (Table 1). Both the Kd and the B,,, were similar in the two groups. However, a significant difference emerged after 3 weeks of imipramine treatment: in the medicated patients, the Kd increased and the B,, decreased (p < 0.01) with respect to controls. Such changes held after 2 months of therapy, and even showed a tendency to progress; however, no statistically signifi- cant differences were seen between Kd parameters in medicated patients after 3 weeks and 2 months of treatment.

Table 1. Platelet imipramine binding in unmedicated control subjects and depressed patients, and in the same patients during imipramine treatment and clinical recovery (mean f SD)

Depressed Depressed patients patients treated treated

Unmedicated 3 weeks 2 months depressed with with

Controls patients imipramine imipramine (n = 25) (n=25 (n = 24) (n = 18)

Kd (nM) 0.45 f 0.40 0.65 k 0.38 0.93 * 0.71' 1.89 + 2.33'1'

B,., (fmol/mg

protein) 1169.30 zt 524.79 1091.21 f 194.43 659.73 + 124.78- 516.69 k 98.75'."

1. Statistically significant differences @ < 0.01) when compared to corresponding control value (2-tailed Student t test). 2. Statistically significant differences (p < 0.01; p < 0.05 for Kd values) when compared to corresponding value in unmedicated (pretreatment) patients.

Correlations Between Time-Course Changes in Platelet IB, MAO Activity, 5HT Transport, and Clinical Evolution. Individual platelet IB parameters and HRSD scores were not significantly correlated, at any given treatment stage, in the patients studied (Pretreatment: KJHRSD, n = 25, r = 0.05, p = 0.810, df = 23; B,,,/HRSD, n = 25, r = -0.16, p = 0.452, df = 23. Three-week treatment: Kd/HRSD,n =24,r=0.32,p=O.l32,df=22; B,,,/HRSD,n=24,r=-0.02,

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p = 0.931, df = 22. Two-month treatment: KJHRSD, n = 16, r = 0.20, p = 0.462, df = 14; B,,,,,/HRSD, n = 16, r = 0.32, p = 0.224, df = 14). The magnitude of clinical improvement between the pretreatment and the 3-week treatment measure- ments (as represented by the difference in individual HRSD scores) was not correlated with the magnitude of change in either Kd or B,, of the platelet IB during that same interval (HRSD/ Kd changes correlation, n = 24, r = 0.11, p = 0.620, df = 22; HRSD/ B,,, changes correlation, n = 24, r = 0.01, p = 0.978; df = 22). Such lack of correlation was also true for the interval between pretreatment and 2 months of therapy (HRSD/ Kd changes correlation, n = 16, r = 0.18, p = 0.495, df = 14;

HRSD/ B,,, changes correlation, n = 16, p = 0.16, p = 0.544, df = 14) and for that between 3 weeks and 2 months of imipramine treatment (HRSD/K, changes correlation, n = 16, r = 0.02,~ = 0.952, df = 14; HRSD/ B,,, changes correlation, n = 16, r = 0.15, p = 0.570, df = 14). However, a within-subject rank correlation between platelet IB parameters and HRSD scores throughout treatment, performed separately for each individual, indicated, in all but three (n = 13) of the patients from whom a complete set of data was available (n = 16), a significant correlation (positive for B,,,/ HRSD, negative for Kd/ HRSD). The individual HRSD scores at pretreatment and at 2-week treatment stages were significantly correlated (n = 24, r = 0.69, p < 0.0001, df = 22).

Additional blood samples were drawn from the same subjects, on the same days, to measure platelet MAO activity (Quintana, 1988) and platelet 5HT transport (Quintana, 1989). Because the patients and the controls, as well as their medication and clinical status, were common to all the studies, one of the most interesting possibilities of such a design was to correlate the changes in biological parameters with changes in clinical status throughout the therapy. Fig. 1 shows the trends of concurrent changes with respect to control levels of each biological parameter (group mean values) throughout treatment. Platelet MAO levels decreased, but were still higher than normal control values, after 3 weeks of treatment, when a noticeable but incomplete clinical recovery had begun (Quintana, 1988). The kinetic characteristics of the two platelet 5HT uptake affinity states found altered in unmedicated depressives appeared statistically normalized after 3 weeks of imipramine therapy (Quintana, 1989), except for the V,, of the highest affinity system which, however, showed at that time a statistically significant change in the direction of normalization with respect to the corresponding unmedicated (pretreatment) depressed patients’ value. After 2 months of therapy, when the patients showed a full clinical recovery, the platelet 5HT transport kinetics were similar in controls and patients (Quintana, 1989). When individual results, instead of group values, were considered, neither the values of any of the biological parameters measured (platelet MAO, 5HT uptake, and platelet IB) nor the magnitude of their changes were significantly correlated at any given stage or interval of the study, respectively.

Thus, at least for the sample, methods, and conditions of this and parallel studies, endogenous depression is associated with increased platelet MAO levels, with dynamic changes in platelet 5HT uptake, and with normal platelet IB values. Clinical recovery is associated with normalized platelet MAO levels and 5HT transport kinetics. Imipramine administration induces platelet IB changes and clinical

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Fig. 1. Platelet biological parameters in endogenous depressed patients during imipramine Geatnient

400 ‘1

*

-100 J Km1 Km2 Vmaxl VmaxP Kd

MAO

t *

Bmax

5-HT UPTAKE IMIPRAMINE

BINDING

E4 UNMEDICATED DEPRESSED PATIENTS

Ia THREE-WEEK IMIPRAMINE TREATED PATIENTS

0 TWO-MONTH IMIPRAMINE TREATED PATIENTS

(CLINICALLY RECOVERED)

Results represent percentages of increase or decrease of each group’s mean values (standard deviation not considered) over mean values in nonmedicated normal controls for the different parameters. * = statistically significant difference (2-tailed t test) between the mean value for the group and the parameter indicated and the corresponding control’s mean. See text for statistical correlations.

recovery. In the patients considered as a group, clinical status changes parallel changes in platelet MAO and platelet 5HT transport kinetics. However, the magnitude of individual clinical changes is not correlated with that of biological parameters, and the magnitude of individual changes in any two biological parameters (platelet MAO, kinetic parameters of platelet 5HT uptake, or platelet IB kinetic parameters) is not correlated either. During imipramine therapy, the time course of clinical, platelet MAO, and platelet 5HT uptake changes in depressed patients is not identical.

Discussion

The present results suggest that platelet IB is not altered in endogenous depression. Such results, which agree with several previous studies (Berrettini et al., 1982; Baron

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et al., 1983, 1986; Egrise et al., 1983, 1986; Muscettola et al., 1984, 1986; Rehavi et al., 1984; Whitaker et al., 1984; Gentsch et al., 1985, 1989; Hrdina et al., 1985; Tang and Morris, 1985; Braddock et al., 1986; Carstens et al., 1986; Desmedt et al., 1987; Kanof et al., 1987; Beth et al., 1988), were obtained from a diagnostically homo- geneous sample of patients, using methods designed to cope with the methodological differences previously blamed as one of the sources of variability between studies. Platelet membranes were obtained using a harvesting method that reportedly ensures (Corash, 1980; Baron et al., 1983, 1986) a representation of the whole platelet population. They were resuspended in Krebs Ringer media, a buffer commonly used in cellular physiology and membrane studies, to match the conditions of parallel studies on 5HT uptake (Quintana, 1989) in the platelets of the same patients (preliminary experiments showed that using either Tris or Krebs buffers made no difference with respect to the platelet IB parameters; Quintana, 1984). The protein content of the samples was determined using the Lowry method (Lowry et al., 1951), and found to be in the same range as that in other studies that attempted to address methodological problems (Mellerup et al., 1982; Anderson et al., 1984; Muscettola et al., 1986). The assays were carried out within a week of the samples’ collection, after the platelets had been stored at -60 ‘C, a method that produces no change in platelet IB kinetics (Anderson et al., 1984; Muscettola et al., 1986). The samples were stored as buffer suspensions in a manner similar to the majority of other studies not using fresh samples; preliminary experiments showed that either using fresh platelets or storing the isolated platelets resuspended in buffer at -60 “C to measure platelet IB gave similar results (Quintana, 1984). Those preliminary experiments also showed that sonication of the samples (Soniprep 150, MSE, 10 set at 0 ‘C) by itself or after a series of freezing-thawing steps did not change platelet IB characteristics as compared with the freezing-thawing procedure alone, which was finally used to lyse the platelets before the platelet IB assay was performed. Nonspecific platelet IB was measured in the presence of lOO,~Mdesipramine, as in the vast majority of published studies on the subject; under such conditions, the specific platelet IB at substrate concentrations close to the Kd of the binding constitutes between 75% and 90% of the total binding. The assays were carried out in silicon-covered glass tubes, as recom- mended by others (Muscettola et al., 1986). Problems with demographic variables were addressed by using age- and sex-matched controls for each patient; the blood samples of the corresponding control-patient pairs were withdrawn the same day, and processed in parallel to avoid the introduction of circannual or circadian variations in the results. Such precautions, obviously, would not deal with differences in circannual or circadian patterns of platelet IB between the control and depressed populations, a possibility that was not the object of this study but perhaps a good ground for future research. However, the available data regarding circannual (Egrise et al., 1983, 1986; Mendlewicz, 1984; Whitaker et al., 1984; Tang and Morris, 1985; Carstens et al., 1986; Galzin et al., 1986; Kanof et al., 1987) or circadian (Baron et al., 1983; Wirz-Justice et al., 1983; Gentsch et al., 1985, 1989; Nankai et al., 1986) variations in platelet IB are controversial and inconclusive.

An important point in deciding whether platelet IB values, altered or not, are a true marker for depression is the medication status of the patients of the sample. In several studies in which platelet IB was found altered in unmedicated patients,

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treatment with tricyclic antidepressants normalized IB levels (Suranyi-Cadotte et al., 1985b; Marazziti et al., 1988). Other studies have reached similar conclusions, but have pointed out the necessity of long-term antidepressant treatment for biological normalization that coincides with full clinical recovery (Asarch et al., 1980; Briley et al., 1980; Raisman et al., 1981; Suranyi-Cadotte et al., 1985b). In my experiments, the first sample in depressives was taken after at least a 13-day “washout” period from previous medication. A similar or even shorter period has been used in many studies (Lewis and McChesney, 1985; Baron et al., 1986; Kanof et al., 1987). Some studies that compared the results from patients in different medication states con- cluded that drug intake was unlikely to be a factor in the results (Gentsch et al., 1985, 1989; Baron et al., 1986; Braddock et al., 1986; Nankai et al., 1986). Even in the case that, in the present study, some patients had taken tricyclic antidepressants during the weeks immediately before the 2-week washout period, this factor could hardly account for the results obtained. For one thing, the patients showed an increase in Kd and a decrease in B,, after imipramine treatment began; any previous medication would have likely produced the same effect, but the IB kinetic values of unmedicated patients in the present study were not different from those of the controls. It still could be argued that a previous treatment (before the 2-week washout period) could have obscured a change in opposite direction in the kinetic parameters of depressed patients. However, when the variable clinical recovery is absent, the data in the literature point to a decrease or no change in platelet IB kinetic parameters following administration of tricyclic antidepressants both in experimental animals (Kinnier et al., 1980; Raisman et al., 1980; Briley et al., 1982; Plenge and Mellerup, 1982; Barbaccia et al., 1983; Lee et al., 1983; Racagni et al., 1983; Gentsch et al., 1984) and in healthy volunteers (Poirier et al., 1984, 1987; Gentsch et al., 1985; Suranyi- Cadotte et al., 1985~) although it was found in normal subjects that amitriptyline (Braddock et al., 1984) and desipramine (Cowen et al., 1986) produced an increase in IB binding. Thus, the available data generally indicate that the drug intake factor alone produces a decrease in kinetic parameters, and that clinical recovery produces an increase in such parameters: neither of those two arguments can account for the lack of differences between controls and unmedicated depressives in the present study, for the patients showed a full depressive syndrom, while their platelet IB parameters were not altered with respect to control subjects. Moreover, an indirect proof of the lack of effects of previous medication intake in those patients is that imipramine lowers the platelet MAO activity as measured in them (Quintana, 1984, 1988), but the unmedicated patients showed highly significant increased MAO levels (Quintana, 1988). An increase in the number of platelet IB sites in depressed patients treated chronically with antidepressants has been observed in several studies (Gentsch et al., 1985; Suranyi-Cadotte et al., 1985b; Marazziti et al., 1988). While these findings are in disagreement with studies on the effects of tricyclic antidepressants on platelet IB in healthy individuals, it is possible that the effects of tricyclic antidepressants on platelet IB differ in controls and depressives (Poirier et

’ al., 1984, 1987; Cowen et al., 1986). In any event, the possible effects of treatment with tricyclic antidepressants on platelet IB have often been found to be con- tradictory and, by themselves, cannot account for the lack of platelet IB differences between controls and depressives in the present study.

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The changes in platelet IB kinetics seen in this study after imipramine therapy might be related not only to the presence of the drug in the blood stream, but also to the parallel clinical recovery from depression seen in those patients, in agreement with the conclusions of other authors (Marazziti et al., 1988). To determine whether the binding parameters are changed by one of the two variables, both variables, or neither variable, we have to take into account the time course of both the clinical evolution and the biological changes. Patients in this study showed a progressive clinical improvement from the beginning to the end of therapy: a noticeable (and statistically significant according to HRSD score changes) difference in clinical status was observed between 3 weeks and 2 months of treatment. However, platelet IB parameters changed after 3 weeks of therapy with respect to pretreatment values, but they did not show any difference between 3 weeks and 2 months of imipramine treatment. A daily imipramine dose of around 200 mg is sufficient to maintain therapeutic plasma imipramine levels of an order of 10-r M to 10m6 M (Glassman et al., 1977); therefore, blood tests for platelet IB in subjects taking imipramine reflect a displacement of the actual IB (for it is calculated from the ratio of labeled imipramine concentration in the incubation bath and the labeled imipramine bound to the platelets). It is known that it takes approximately 6 days for imipramine to be almost completely excreted (Crammer et al., 1968). From these data we can conclude that the putative effect of steady plasma concentrations of imipramine on platelet IB must be already manifested after 3 weeks of therapy, and be constant between that point and any measurement thereafter (excluding any receptor subsensitivity or supersensitivity process that might take a longer time to develop). Thus, the time course of platelet IB changes in this study parallels the expected effects of plasma imipramine rather than the patients’clinical recovery, which might suggest that those platelet IB changes are the consequence of the drug’s presence in the blood stream. An additional argument supporting this point is that the magnitude of clinical improvement did not correlate with that of platelet IB changes during any interval of the present study. Because the time courses of clinical and biological (platelet MAO, 5HT uptake, and platelet IB) parameter changes reported in this and parallel studies are not identical, it is tempting to speculate that changes in biological parameters during imipramine therapy are more the result of the drug’s presence in the blood than a direct cause or consequence of clinical improvement. However, because both clinical and biological changes occur in depression and subside after recovery, some relationship, although indirect, must exist between the two.

Since the binding site for imipramine has been considered to be functionally and structurally related to the 5HT uptake system (Langer et al., 1980; Paul et al., 1981; Segonzac et al., 1987), one purpose of the present study was to determine whether the existence of two mechanisms of 5HT in platelets of depressives (Quintana, 1989) was paralleled by that of two different IB sites. There was no evidence that a double IB site population was present in the subjects sampled, either before or after imipramine treatment. There is evidence for a functional independence of the platelet 5HT transport and the IB site (Weizman et al., 1986), and a recent study suggested that platelet 5HT is under genetic control whereas platelet IB is not (Meltzer and Arora, 1988). The presence of a single binding site population structurally associated with the 5HT transport system might indicate that only one type of 5HT transport

238

site exists. The finding of two different transport mechanisms (Quintana, 1989) would then mean two different states of a single system, rather than a dual uptake site. If, as the results of this and a previous study (Quintana, 1989) indicate, unmedicated depressed patients show changes in the I’,,, of platelet 5HT uptake but not in the B,,, of platelet IB, such uptake changes must be interpreted as conformational changes in the transporter complex. This dynamic view of the 5HT transport mechanism favors the idea of adaptive changes occurring in the synaptic membrane that lead both to the onset of and the recovery from depression.

Increases in the & of the binding site observed in the present study during treatment with tricyclic antidepressants agree with other reports which either describe such a tendency in both experimental animals (Briley et al., 1982) and human subjects (Muscettola et al., 1984), or clearly significant increases in the latter (Plenge and Mellerup, 1982; Braddock et al., 1984, 1986; Gentsch et al., 1984; Suranyi-Cadotte et al., 1985a, 1985b; Cowen et al., 1986; Marazzitti et al., 1988). It has been well documented (Plenge and Mellerup, 1982; Gentsch et al., 1984; Suranyi- Cadotte et al., 1985~) that the effect of long-term imipramine administration on the affinity of binding is transient and due to residual drug concentrations in membrane preparations.

If platelet IB sites are part of the 5HT transporter system, several conclusions are apparent when considering the present results. First, since only one platelet IB site was found, as compared to two 5HT transporters, either imipramine is not a selective marker (by means of different binding characteristics) for different 5HT uptake sites, or the two 5HT platelet uptake mechanisms are actually two different conditions of the same membrane system rather than two physically different sites. Second, the long-term imipramine action slowly modifies the characteristics of the 5HT transport system by being constantly present in the vicinity of that system, rather than by a lasting modification of an imipramine receptor coupled to the 5HT trans- port mechanism. And third, platelet IB changes cannot be a trait marker for depres- sion, for they depend on the presence of the drug in the blood; those changes may modify the 5HT uptake, but platelet IB is not lastingly modified by either the drug itself or the clinical condition of the patient.

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