Inhibition of 5-hydroxytryptamine reuptake by amitryptyline an zimelidine and its relationship to their therapeutic action

Systematic review of studies using platelet serotonin content to assess bioeffect of serotonin reuptake inhibitors at the serotonin transporter

RATIONALE

Assessment of the bioeffect of serotonin reuptake inhibitors (SRIs, including both selective serotonin reuptake inhibitors (SSRIs) and serotonin-norepinephrine reuptake inhibitors (SNRIs)) at the serotonin transporter (SERT) in patients and healthy controls can have important theoretical and clinical implications.

OBJECTIVES

Bioeffect at SERT has been assessed by neuroimaging of brain SERT occupancy, through in vitro measurements of platelet serotonin (5-HT) uptake, and by measuring platelet 5-HT content pre- and post-initiation of SRI administration. Studies of platelet 5-HT content were reviewed in order to (1) determine the overall apparent bioeffect of SRIs; (2) compare bioeffect across types of SRIs; (3) compare the three approaches to assessing SRI bioeffect; and (4) determine how the findings might inform clinical practice.

METHODS

We performed a systematic review of the published studies that measured platelet 5-HT content to assess SRI bioeffect at the platelet SERT. Studies using neuroimaging and in vitro platelet 5-HT uptake to assess SRI bioeffect were reviewed for comparison purposes.

RESULTS

Clinical doses of SRIs typically resulted in 70-90% reductions in platelet 5-HT content. The observed bioeffect at the platelet SERT appeared similar among different SSRIs and SNRIs. The bioeffect estimations based on platelet 5-HT content were consistent with those obtained using neuroimaging to assess brain SERT occupancy and those based on the in vitro measurement of platelet 5-HT uptake.

CONCLUSIONS

In general, excellent agreement was seen in the apparent SRI bioeffect (70-90% inhibition) among the platelet 5-HT content studies and across the three bioeffect approaches. Theoretical and practical clinical implications are discussed.

An intercalation mechanism as a mode of action exerted by psychotropic drugs: results of altered phospholipid substrate availabilities in membranes?

Patients respond differently to psychotropic drugs, and this is currently a controversial theme among psychiatrists. The effects of 16 psychotropics on cell membrane parameters have been reported. These drugs belong to three major groups used in therapeutic psychiatry: antipsychotics, antidepressants, and anxiolytic/hypnotics. Human platelets, lacking dopamine (D(2)) receptors (proposed targets of most psychotropics), have been used as a cell model. Here we discuss the effects of these drugs on three metabolic phenomena and also results from Langmuir experiments. Diazepam, in contrast to the remaining drugs, had negligible effects on metabolic phenomena and had no effects in Langmuir experiments. Psychotropic drugs may work through intercalation in membrane phospholipids. It is possible that the fluidity of membranes, rich in essential fatty acids, the content being influenced by diet, could be a contributing factor to the action of psychotropics. This might in turn explain the observed major differences in therapeutic response among patients.

Initial platelet serotonin (5-HT) transport kinetics predict nortriptyline treatment outcome

According to the hypothesis of initial conditions, drug response may be determined by different initial states of neurotransmitter protein recognition systems. Platelet serotonin (5-HT) transport kinetics were studied as initial-conditions predictors of antidepressant response in 24 depressed patients before and after 3 weeks of treatment with nortriptyline (75 mg). The initial affinity of the 5-HT transporter (5-HTT) correctly predicted 71% of the outcome. The pretreatment affinity constant ( Km) correlated (r = 0.61; p < 0.002) with that measured after 3 weeks of treatment (Kapp). Responding patients had a significantly higher initial Km before treatment and a significantly higher Kapp after treatment. Nonresponders had an initial Km significantly lower than that of 24 controls. Nortriptyline plasma levels were not statistically different between response groups. These results are consistent with two previously published observations, which indicate that the initial affinity of the 5-HTT predicted response to fluvoxamine or fluoxetine in the same way. Insofar as all three drugs increase the apparent affinity of the 5-HTT, it appears that a better response is related to those cases where the initial affinity is already higher before treatment.

Relationship between parameters of serotonin transport and antidepressant plasma levels or therapeutic response in depressive patients treated with paroxetine and amitriptyline

In a double-blind clinical study, antidepressant plasma levels, parameters of platelet serotonin (5-HT) transport (Km, Vmax and basal platelet 5-HT content) and therapeutic response were measured in depressive patients treated with either paroxetine (30 mg/day) or amitriptyline (150 mg/day) for 6 weeks. No correlation could be found between paroxetine plasma levels and therapeutic outcome after 2, 4 and 6 weeks of treatment. In contrast to the amitriptyline group, a marked increase in Km from baseline to week 2 was determined in paroxetine-treated patients, with Km increase being correlated with paroxetine plasma levels at week 2. However, no significant relationship could be found between 5-HT transport parameters and any of the outcome measures in either treatment group.

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.

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.

Seasonal variation of platelet serotonin uptake and 3H-imipramine binding in normal and depressed subjects

Density of 3H-imipramine binding sites and serotonin (5-HT) uptake in blood platelets were repeatedly recorded in normal controls (n = 9) and depressed patients (n = 7 for the imipramine binding assay and n = 4 for the serotonin uptake) over a 1-year period. The study demonstrated a striking seasonal variation of both parameters in both groups, with lower values in winter and spring than in summer and fall. No difference in the density of 3H-imipramine binding sites was found between the two populations throughout the year, but serotonin uptake was significantly decreased in depressed patients in May and September. These results underscore the importance of studying controls and patients at the same time of the year.

Comparison between a serotonin and a noradrenaline reuptake blocker in the treatment of depressed outpatients. Biochemical aspects

Seventy-five outpatients with major depressive disorder (RDC) were randomly referred to treatment with a dominant 5-HT reuptake blocker (zimeldine, 100 mg b.i.d.) or a dominant NA reuptake blocker (maprotiline 75 mg b.i.d.). Pretreatment biochemical, pharmacodynamic and pharmacokinetic variables were studied and related to the treatment outcome with the two drugs. Female responders to the dominant 5-HT reuptake blocker were characterized by low pretreatment accumulation of 14C-5-HT in rat synaptosomes, when incubated in patient plasma. Among zimeldine responders there was a relationship between antidepressive effect and steady-state concentrations of zimeldine and norzimeldine. These findings support the hypothesis of a subgroup of depression characterized by serotonin disturbance.

Serotoninergic mechanisms in human feeding: the pharmacological evidence

This paper reviews the evidence of serotoninergic mechanisms in human feeding by considering the effects of 5-HT agonists, precursors and receptor antagonists on hunger, food intake and weight change in normal volunteers, obese people and psychiatric patients. Although there is compelling evidence for a serotonin (5-HT) mechanism being involved, the paper highlights the considerable individual variation in response to pharmacological manipulation of 5-HT. Such variation may reflect differences in the bio-availability of the drugs used. Subtle psychological factors may also play a role in blurring the pharmacological evidence for 5-HT involvement in the highly complex activity of human feeding.

Imipramine treatment differentially affects platelet 3H-imipramine binding and serotonin uptake in depressed patients

Uptake of serotonin and 3H-imipramine binding in platelets of depressed patients were investigated simultaneously with changes in clinical state. Both Vmax for serotonin uptake and Bmax for 3H-imipramine binding were significantly lower in unmedicated depressed patients with respect to normal subjects. Successful treatment with imipramine led to a significant increase in Bmax for 3H-imipramine binding, without significant change in Vmax for serotonin uptake. Bmax values increased to the normal range following complete, rather than partial clinical improvement. These data indicate that successful antidepressant treatment may increase the density of 3H-imipramine binding sites on platelets by a process which is independent of the uptake of serotonin.

Alaproclate--an open clinical study in depressive illness

This open study of alaproclate points towards an antidepressant effect in a a relatively chronic and drug-resistant group of depressives. Five patients had an average improvement of more than 21 points on the Hamilton Rating Scale for Depression and six had an average improvement of seven points. Several patients had anticholinergic side effects, abnormal results in liver functional tests and faecal occult blood, but none were bad enough to require being taken off the drug and most side effects improved before the end of the trial. The biochemical results suggested that the responders and non-responders constituted two distinct biological groups. In the patients who responded well to treatment there were increases in Km values consistent with treatment. The Km and 5-HT values correlated strongly with plasma drug values. There was a strong correlation between Hamilton Rating scores in the 4th week and Km values in the 4th week, although there were only five cases. However, there were no significant relationships between improvement and any pretreatment value. These results are sufficiently promising to suggest that a controlled clinical trial would yield information on alaproclate as a therapeutic aid.

A double-blind comparison of zimelidine and desipramine in endogenous depression

Zimelidine, a specific 5HT uptake inhibitor (final dose 225 mg), and desipramine, mainly a noradrenaline uptake inhibitor (final dose 150 mg), were given in random order to 24 in- and out-patients fulfilling the Research Diagnostic Criteria for Major Depressive Disorder, definite or probable endogenous type, for a 3-week treatment period. Nonresponders were crossed over to the other drug for another 3 weeks. There was a nonsignificant trend towards more overall improvement on desipramine. Some patients in both groups showed very little change during 3 weeks, indicating a bimodal distribution of response to either drug. Several nonresponders improved markedly upon direct crossing over to the other drug. There were few and mild side effects on both drugs, with no significant difference between them. No significant correlation was found between improvement and plasma concentrations of zimelidine, norzimelidine, or desipramine, whereas a significant positive correlation was found between improvement and platelet serotonin uptake inhibition (measured in fresh platelets incubated in diluted plasma from the patients) in zimelidine-treated patients.

Effects of amoxapine on serotonin uptake in human blood platelets of depressed patients and normal controls

The effect of amoxapine and imipramine on the serotonin (5-HT) uptake of blood platelets from depressed patients and normal controls was studied ex vivo or in vitro, respectively. Amoxapine was approximately one-tenth as potent as imipramine in inhibiting 5-HT uptake in blood platelets from normal controls in vitro. Both drugs inhibited 5-HT uptake in a competitive manner. However, ex vivo studies demonstrated that imipramine produced a mixed inhibition and amoxapine, a competitive inhibition of 5-HT uptake. There was no relationship between the change in the platelet affinity for 5-HT after treatment with amoxapine and clinical response to amoxapine.

Drug plasma levels and platelet 5-HT uptake inhibition during long-term treatment with fluvoxamine or lithium in patients with affective disorders

1 Plasma levels and platelet 5-HT uptake characteristics were determined at baseline and at various times during a long-term study comparing fluvoxamine and lithium prophylaxis. 2 In the fluvoxamine-treated patients after 12 and 24 weeks of treatment, Km was significantly increased and y, the mean overall uptake, significantly decreased compared to the patients' own baseline values and to lithium treatment. No significant change was noted for Vmax. 3 For fluvoxamine, significant negative relationships were established between the Vmax of platelet 5-HT uptake and plasma levels. 4 The trial had to be discontinued prematurely for some patients due to apparent lack of efficacy, unwanted effects or a combination of both in the fluvoxamine-treated patients. Low plasma levels of fluvoxamine may have contributed to the apparent lack of prophylactic efficacy of the drug. 5 Reference is made to the activity of receptor systems in patients receiving lithium and the consequent changes occurring after the administration of a potent 5-HT re-uptake inhibitor.

Biological classification of depressive illness

1 A number of biological criteria have been used for refining clinical classification to identify patients who will respond to antidepressant treatment. 2 Subtypes of depressive illness have been postulated to occur depending on whether there is a relative deficiency of central noradrenaline or 5-hydroxytryptamine. However, antidepressants having selective effects on amine systems have similar therapeutic effects and inhibition of re-uptake of these amines does not correlate with clinical outcome. 3 An abnormal response to thyrotrophin-releasing hormone has not been confirmed as specific for depressive illness. Failure to control for age and sex may account for some of the discordant findings. 4 It has been suggested that depressed patients secrete less growth hormone to a variety of stimuli. It appears that the predictive value of a growth hormone response for the diagnosis of endogenous depression is 53%. 5 The dexamethasone suppression test appears to have high sensitivity for the diagnosis of endogenous depression but low specificity and should therefore be used only in conjunction with clinical assessments.

Blood noradrenaline and 5-HT levels in depressed women during amitriptyline or lithium treatment

Noradrenaline levels and platelet and free serotonin concentrations were studied in depressed women in-patients (n=78) before and during amitriptyline (n=41) or lithium treatment (n=37). Pronounced monthly differences in platelet serotonin level have been shown in these subjects before treatment. In all clinical subgroups (neurotic, involutional, manic-depressive patients) a significant fall in platelet serotonin level was observed with amitriptyline medication while an increase was noted with lithium. No significant correlations between serotonin concentrations and clinical outcome were found. Amitriptyline treatment also produced a decrease in peripheral noradrenaline concentration in all subgroups, while an increase was observed with lithium. Some correlations between noradrenaline level and degree of depression were noted in patients treated with amitriptyline or lithium. A more extended analysis of blood amine levels could supply meaningful information on the peripheral action of antidepressive drugs on noradrenaline and serotonin concentrations in depression.

Effects of antidepressant drugs on different receptors in the brain

Radioligand receptor binding techniques were used to characterize the effects of different structural types of antidepressant drugs on neurotransmitter receptors. The tricyclic antidepressants more or less potently inhibited the binding to rat brain preparations of several different radiolabelled ligands [3H]WB4101, [3H]QNB, [3H]-d-LSD, [3H]mepyramine). The potency of the nontricyclic antidepressants varied greatly. Mianserin, potently displaced [3H]mepyramine, [3H]d-LSD and [3H]WB4101 while it was very weak on [3H]QNB-binding. Nomifensine and the specific 5-HT uptake inhibitors zimelidine and alaproclate had very low affinity for these receptors. All the antidepressants tested were practically devoid of activity on [3H]DHA binding, [3H]spiroperidol binding, [3H]flunitrazepam binding, [3H]muscimol binding and [3H]naloxone binding. The implications of these findings for biogenic amine theories of affective disorders are discussed.

Du 24565, a quipazine derivative, a potent selective serotonin uptake inhibitor

Du 24565, 6-nitro,2-(1-piperazinyl)quinoline, is a potent and selective inhibitor of the synaptosomal uptake of serotonin (5-HT). At concentrations at least 10(3)-fold higher it affects the uptake of norepinephrine (NA) and dopamine (DA), The IC50 values are: 5-HT: 4 x 10(-8) M; NA: 6 x 10(-5) M and DA: 4 x 10(-5) M. Uptake of 5-HT by rat blood platelets is also strongly inhibited (Ki approximately 5 x 10(-8) M); the inhibition is probably noncompetitive. In vivo, DU 24565 is active at low oral doses: the 5-HT depletion in rat brain caused by p-chloroamphetamine is antagonized by DU 24565 (oral ED50 0.7 mg/kg). The decrease in the 5-HT content caused by 4, alpha-dimethyl-m-tyramine (H 77/77) is antagonized by DU 24565 at 1 mg/kg orally, without any effect on the depletion of catecholamines. 5-HT turnover, measured by the probenecid method, is reduced by the same dose of DU 24565. Other tests confirmed the activity and selectivity of DU 24565: it potentiated the behavioural affects of the 5-HT precursor 5-hydroxytryptophan (5-HTP) in mice (ED50 1.5 mg/kg orally); it potentiated the temperature increases caused by 5-HTP in the rabbit; it had low activity or no effect at all in NA potentiation tests. This new compound is more potent and selective than the known 5-HT uptake inhibitors. It is a potential antidepressant and can be useful as a pharmacological tool to study the role of 5-HT in the central nervous system.

Plasma levels of zimelidine and norzimelidine in endogenous depression

A new serotonin uptake inhibitor zimelidine was studied in 16 endogenously depressed inpatients, who received 150 mg/kg orally during 3--6 weeks in a phase II-type study. Plasma concentrations of zimelidine and its main metabolite norzimelidine were determined twice a week. Ten patients obtained a well-defined steady-state plasma level within 1--2 weeks, while three patients still had increasing concentrations of both substances or only norzimelidine within the investigation period. In two patients, biochemical affection of the liver could be demonstrated during the treatment; one associated with moderate clinical symptoms (dizziness and fever), the other without clinical symptoms. Both patients recovered upon cessation of the zimelidine treatment. In the former patient, very high concentrations of zimelidine at the time of hepatic symptoms were demonstrated, while the latter patient was within the average concentration range. Other adverse reactions were mild and few, particularly with respect to anticholinergic effects. With the applied, probably suboptimal, dosage the therapeutic response was only satisfactory in five cases.

Urinary 4-hydroxy-3-methoxyphenylglycol is not a predictor for clinical response to amitriptyline in depressive illness

The urinary excretion of 4-hydroxy-3-methoxyphenylglycol was compared in a group of 23 depressive patients and 27 control subjects of similar age. There was no difference between patients and controls although female controls excreted less than males. After 6 weeks' treatment with 150 mg daily of amitriptyline there was no correlation between therapeutic response and pretreatment urinary excretion value.