The inhibition of human platelet 5-hydroxytryptamine uptake by tricyclic antidepressive drugs. The relation between structure and potency

Maternal stress and placental function; ex vivo placental perfusion studying cortisol, cortisone, tryptophan and serotonin

Background

Exposure to maternal stress during pregnancy can have adverse effects on the fetus, which has potential long-term effects on offspring´s development and health. We investigated the kinetics and metabolism of the hormones and amino acids: cortisol, cortisone, tryptophan and serotonin in the term placenta in an ex vivo human placental perfusion model. The placentas used in the experiments were donated from families participating in the Maternal Stress and Placental Function project with a known maternal stress background.

Method

Cortisol, cortisone, tryptophan and serotonin were added simultaneously to the maternal side in the 6 hour ex vivo term human recirculating placental perfusion model, in four experimental set-ups: without inhibitors, with carbenoxolone -that inhibits cortisol metabolism into cortisone, with fluoxetine that inhibits the serotonin transporter, and with PCPA that inhibits metabolism of tryptophan into serotonin. The concentration of cortisol and cortisone, and tryptophan and serotonin were quantified using UPLC and HPLC-MS respectively.

Results

Cortisol was rapidly metabolized into cortisone in the placenta, to a somewhat lesser degree when adding the inhibitor carbenoxolone, resulting in higher fetal exposure to cortisol. Serotonin was also rapidly metabolized in the placenta. When adding fluoxetine a peak of fetal serotonin levels was seen in the first hour of the perfusion. No effect was seen of the maternal stress levels on placental transport kinetics in this study.

Conclusion

Inhibiting the metabolism of cortisol in the placenta increased fetal exposure to cortisol as expected. Unexpectedly we saw an increased fetal exposure to serotonin when inhibiting the serotonin transporter, which may be related to the increased serotonin concentration on the maternal side of the placenta. No effect on placental kinetics were evident on maternal stress levels during the pregnancy as the majority of participating mothers had normal stress levels.

Azidobupramine, an Antidepressant-Derived Bifunctional Neurotransmitter Transporter Ligand Allowing Covalent Labeling and Attachment of Fluorophores

The aim of this study was to design, synthesize and validate a multifunctional antidepressant probe that is modified at two distinct positions. The purpose of these modifications was to allow covalent linkage of the probe to interaction partners, and decoration of probe-target complexes with fluorescent reporter molecules. The strategy for the design of such a probe (i.e., azidobupramine) was guided by the need for the introduction of additional functional groups, conveying the required properties while keeping the additional moieties as small as possible. This should minimize the risk of changing antidepressant-like properties of the new probe azidobupramine. To control for this, we evaluated the binding parameters of azidobupramine to known target sites such as the transporters for serotonin (SERT), norepinephrine (NET), and dopamine (DAT). The binding affinities of azidobupramine to SERT, NET, and DAT were in the range of structurally related and clinically active antidepressants. Furthermore, we successfully visualized azidobupramine-SERT complexes not only in SERT-enriched protein material but also in living cells stably overexpressing SERT. To our knowledge, azidobupramine is the first structural analogue of a tricyclic antidepressant that can be covalently linked to target structures and further attached to reporter molecules while preserving antidepressant-like properties and avoiding radioactive isotopes.

Neutrophil proteinase cathepsin G is proteolytically active on the human platelet glycoprotein Ib-IX receptor: characterization of the cleavage sites within the glycoprotein Ib alpha subunit

The proteolytic activity of the neutrophil serine-proteinase cathepsin G (CG) on platelet adherence receptors, the glycoprotein (GP) Ib-IX complex and the integrin alpha IIb beta 3, has been investigated. In the range 50 to 200 nmol/l, CG is a potent platelet agonist which induces shape change, granule exocytosis and aggregation. Investigation of the proteolysis of the receptors' subunits during the course of platelet activation by CG was performed by immunoblot analysis of platelet proteins using a panel of specific antibodies. Exposure of platelets for 3 min at 37 degrees C to CG at a concentration that induces full cell activation resulted in an extensive cleavage of the N-terminal region of the extracellular domain of GPIb alpha, the largest (relative molecular mass, M(r), 143,000) of the three subunits constituting the GPIb-IX complex. In contrast, no detectable proteolytic modification of the two other subunits, GPIb beta and GPIX, was detected. Similarly, we observed that neither of the two subunits of the alpha IIb beta 3 receptor were proteolytically modified by CG. Cleavage of GPIb alpha by CG leaves a remnant of the polypeptide chain with M(r) approx. 106,000 in the plasma membrane, while releasing into the extracellular milieu the N-terminal domain with M(r) in the range 40,000 to 46,000. N-terminal sequencing of the CG-derived fragments of GPIb alpha indicated that the Leu275-Tyr276 peptide bond was the primary cleavage site for this proteinase. Proteolysis of GPIb alpha was already detectable at concentrations of CG as low as 25 nmol/l, while with 200 nmol/l the cleavage was detected as soon as 10 s after exposure of platelets to the proteinase. Comparison of the kinetics and concentration dependency for the proteolysis of GPIb alpha and for the activation of platelets by CG showed that cleavage of the GPIb-IX receptor is an early event that accompanies exocytosis and aggregation. Quantitative evaluation of the conversion of GPIb alpha into its membrane fragment indicated that, under optimal conditions, a maximum of approx. 50% of the total GPIb alpha can be affected by proteolysis. However, this proteolysis was > 90% complete when platelets were in the presence of the potent antagonist prostacyclin, suggesting that cellular redistribution of the GPIb-IX receptor may also occur during activation by CG. These results thus indicate that the very early phase of platelet activation by CG is accompanied by extensive modifications in the structure and expression of the GPIb-IX receptor, an effect that might be of functional significance for the interaction of platelets with the vessel wall.

Kinetic effects of desmethylimipramine treatment on platelet serotonin uptake in depressed patients: a comparison with imipramine

The kinetic effects of desmethylimipramine (DMI) on platelet serotonin (5HT) uptake were compared to those of imipramine (IMI) in eight DMI-treated depressed patients and seven IMI-treated depressed patients, and compared to values after patients were off drug for 19 (+/- 8 SD) and 33 (+/- 15) days. As expected, IMI was a stronger inhibitor of 5HT uptake than DMI during treatment, with the mean apparent Km in treated patients being elevated nearly threefold over that of the drug-free condition. In DMI-treated patients, the mean Km was elevated nearly twofold over that of the drug-free condition. Although DMI is considered a preferential norepinephrine uptake inhibitor, the results suggest the following: (1) Significant decreases in the apparent platelet 5HT affinity are achieved with DMI; (2) the inhibition kinetics in depressed patients are competitive; (3) there was a significant relationship between Km change and depression outcome with DMI discontinuation; and (4) DMI, as a metabolite, appears to contribute to the 5HT uptake inhibition of IMI in vivo.

Imipramine and 5HT re-uptake inhibition: a narrative

In 1960, the first paper on the pharmacology of imipramine, the 'original' antidepressive drug, was published in the British Journal of Pharmacology (Marshall et al. , 1960). It came from the Department of Clinical Research of the Crichton Royal Hospital, Dumfries, Scotland and not as might have been expected from a university department or the laboratories of a pharmaceutical firm. Under the circumstances, it seems relevant to record for the benefit of scientific historians, the somewhat tortuous chain of events that led to its publication.

Differential release of histamine and 5-hydroxytryptamine from rat mast cells: the contribution of amine uptake to the apparent pattern of secretion

Despite the fact that mast cells isolated from rat mesentery and lung tissue contain 5-HT in excess of histamine, the pattern of amine output, in response to compound 48/80, mirrors that in peritoneal mast cells where histamine is in excess of 5-HT, such that this secretagogue induces a greater percentage release of histamine than 5-HT. Mast cells are capable of taking up both these amines, although uptake of 5-HT is in preference to that of histamine. With concentrations of clomipramine and fluoxetine that inhibited 5-HT uptake, the net percentage release of 5-HT increased in a corresponding manner, and the concentration-effect curves in response to compound 48/80 ran in a collinear fashion with that of control histamine output (measured in the absence of the drugs). Both drugs had no effect upon the uptake or secretion of histamine. Thus, observed differences in the apparent pattern of histamine and 5-HT secretion from rat mast cells may be due to selective reuptake of amines rather than a reflection of differential amine release.

The activity of amitriptyline as a differential inhibitor of amine secretion from rat peritoneal mast cells: the contribution of amine uptake

Amitriptyline, at a concentration of 10(-4) M, significantly inhibited the release of histamine from purified peritoneal rat mast cells in response to compound 48/80, but was without effect upon the output of 5-HT. The reduction was not extensive, and concentrations of the drug above or below 10(-4) M were without effect. Amitriptyline (10(-8)-10(-4) M) inhibited uptake of exogenous 5-HT in a concentration dependent manner. Paradoxically, however, histamine uptake was significantly increased in the presence of this drug, but only at a concentration of 10(-4) M. This effect was observed at 4 degrees C for both amines. Concentrations of amitriptyline in excess of 10(-4) M caused lysis of the mast cells. These results suggest that the observed selective inhibitory effect of amitriptyline upon histamine output may be a function of altered amine uptake rather than differential inhibition and 5-HT secretion.

Loss of low-affinity serotonin receptors upon storage of human platelets

Platelets actively accumulate virtually all plasma serotonin within their dense granules. As a readily isolated, homogeneous cell type, platelets have served as a model for serotonin uptake into neurological tissue, in addition to defining the role of serotonin in hemostasis. The number of serotonin receptor types on the platelet membrane and the function of these receptors has not been conclusively demonstrated. The presence of different receptor types that may be altered or lost in disease or upon aging (in vitro storage or in vivo) could have significant physiological effects on platelet function. This report demonstrates that at least two receptor types are present on freshly prepared human platelets. However, after 3 to 4 days of storage in autologous plasma, the low-affinity, high-capacity serotonin receptor appears to be lost. This phenomenon probably accounts for some of the discrepancies reported in the literature. The high-affinity receptor present in both freshly isolated and stored platelets binds about 9 x 10(3) serotonin molecules per platelet. Binding can be completely blocked by imipramine; however, some passive diffusion appears to occur even at the low level of extracellular serotonin concentrations employed in these studies (nanomolar range). The influx of serotonin into platelets appears to be poorly reversible, even in reserpine-treated cells, where the extravesicular cytoplasmic concentration would be high. The loss of the low-affinity serotonin receptor type reported in these studies may be directly or indirectly associated with the reduced responsiveness observed in stored platelets.

The effects of citalopram (Lu 10-171) on the serotonin (5-HT) uptake kinetics in platelets from endogenously depressed patients

The uptake kinetics of serotonin (5-HT) in platelets from eight patients with endogenous depression was determined in platelet rich plasma (PRP). From the uptake data the Scatchard correlation coefficient (r[S] ), Sips' index of heterogeneity (alpha), Km and Vmax (Lineweaver-Burk) were calculated. The measurements were performed before and after two weeks treatment with Citalopram, 1-(3-dimethylaminopropyl)-1-(p-fluorophenyl)-5-phtalan carbonitrile (Lu 10-171), a selective 5-HT uptake inhibitor. After two weeks treatment with Citalopram (5, 25 or 50 mg/day) a lowered platelet count in whole blood and PRP was observed and none of the patients had a measurable 5-HT uptake for which kinetic parameters (Km and Vmax, Lineweaver-Burk) could be calculated. However, all the patients but one improved significantly in Global score.

Platelet uptake of serotonin following repeated administration of 3-cyano-imipramine to healthy volunteers

In a double blind, placebo-controlled study the effects of daily oral administration of 3-cyano-imipramine on the 3H-serotonin uptake capacity of platelets were investigated in healthy volunteers. The initial dose was 1 mg, rising to 3 mg daily for 7 days. A rapid and profound inhibition of 3H-serotonin uptake was observed in platelets isolated from the treated subjects. During repeated administration, uptake was reduced to less than 10% of pre-drug values. Five days after the final dose uptake had only partially recovered, to 53% of pre-drug values. A similar inhibition profile was observed when serum from the treated subjects was incubated with normal platelets and 3H-serotonin. The results establish 3-cyano-imipramine as a potent inhibitor of platelet serotonin uptake in humans.

Assessment of the antidepressant activity of dothiepin and its metabolites by preclinical tests

The affinities of dothiepin and its principal metabolites northiaden, dothiepin sulphoxide and northiaden sulphoxide for [3H]imipramine binding sites in the rat cortical homogenates, and for [3H]spiperone and [3H]serotonin receptor sites in preparations from the rat frontal cortex and hippocampus were studied. As inhibitors of [3H]imipramine binding, the strengths of the drugs are, in terms of their IC50 (concentration corresponding to 50% inhibition): dothiepin 2.8 X 10(-6) M, northiaden 5.0 X 10(-6) M, northiaden sulphoxide 4.0 X 10(-5) M and dothiepin sulphoxide 3.2 X 10(-5) M. The potencies of the drugs in inhibiting serotonergic binding followed a similar trend. Using frontal cortical tissue suspensions and [3H]spiperone, the IC50 values were determined to be: dothiepin 4.2 X 10(-6) M, northiaden 5.0 X 10(-6) M, northiaden sulphoxide 1.6 X 10(-4) M and dothiepin sulphoxide 1.6 X 10(-4) M; whereas in hippocampal suspensions and using [3H]serotonin, the IC50 values were 2.5 X 10(-6) M, northiaden 4.0 X 10(-5) M, dothiepin sulphoxide 2.5 X 10(-4) M and northiaden sulphoxide greater than 10(-3) M. The influence of the drugs on the uptake of [14C]serotonin into human platelets was also investigated. All had an inhibitory effect upon the uptake, the order of potency being dothiepin greater than northiaden greater than northiaden sulphoxide greater than dothiepin sulphoxide. Plots of 1/v versus 1/s showed that the inhibition was competitive for all four compounds.

Ligand binding and platelet uptake studies of loxapine, amoxapine and their 8-hydroxylated derivatives

Loxapine, amoxapine and their 8-hydroxylated derivatives were studied by means of [3H]imipramine binding to rat cortical membranes, [3H]spiperone binding to rat striatal membranes, and the inhibition of serotonin uptake by human platelets. As inhibitors of [3H]imipramine binding: amoxapine greater than hydroxyamoxapine greater than loxapine = hydroxyloxapine; as inhibitors of platelet serotonin uptake: hydroxyamoxapine greater than amoxapine greater than hydroxyloxapine greater than loxapine; and as inhibitors of [3H]spiperone binding: loxapine greater than amoxapine greater than hydroxyamoxapine greater than hydroxyloxapine. The antipsychotic properties of loxapine and amoxapine were supported by the binding results, which also indicated the probable antipsychotic activities of the metabolites. All 4 compounds may possess dual action of antidepressant effect as well as antipsychotic effect.

Correlation between serotonin uptake and imipramine binding in schizophrenic patients

1. The active uptake of serotonin by platelets of schizophrenic patients and a healthy control group was compared with imipramine binding to these platelets. 2. Significant correlation was observed between the Vmax of serotonin uptake and the imipramine binding. 3. Serotonin uptake by patients was much lower than that of control group.

Human platelet imipramine recognition sites: biochemical and pharmacological characterization

The influence of the membrane environment on the integrity of the human platelet [3H]-imipramine recognition site was examined. When platelet membranes were isolated in a buffer containing enzyme inhibitors (EDTA, EGTA and antiprotease) a significantly greater number of high affinity [3H]-imipramine binding sites was observed. A calcium-stimulated degradation of imipramine sites was also demonstrated. This degradation occurred in vitro over physiologically relevant time periods. Furthermore, inactivation of imipramine binding was achieved by very low concentrations (IC50 = 5 microgram/ml) of phospholipase A2. Specific serotonin reuptake inhibitors were potent displacers of [3H]-imipramine binding; histamine (H1), alpha-adrenergic (alpha 1), and muscarinic agents were much less active. The receptor was shown to be proteinaceous in nature due to its sensitivity to protease, heat denaturation and chemical modification with N-ethylmaleimide. From these results it is proposed that membrane lipid perturbations, catalyzed by calcium, may control expression of platelet [3H]-imipramine sites. The relation of this recognition site to aminergic systems and the possible relevancy to the action of antidepressants are addressed.

A modified assay method for determining serotonin uptake in human platelets

Effects of various experimental conditions on serotonin (5-HT) uptake in human platelets were examined. The experimental design allowed the evaluation of the effect of diffusion and other non-saturable processes on the affinity and maximum activity of the membrane pump for 5-HT uptake. Total 5-HT uptake was determined by incubating platelet-rich plasma (PRP) with increasing concentrations of serotonin at 37 degrees C for 4 min. The passive uptake was measured by the addition of various 5-HT concentrations to PRP in buffer at 37 degrees C, followed by immediate transfer to an ice-cold water bath. The difference between the total and passive uptake was linear for 6 min. The affinity (Km) for active platelet serotonin uptake was 0.45 +/- 0.09 mumol/l and maximal rate of uptake (V) was 10.7 +/- 2.1 pmol/10(7) platelets/min. The described method provides a convenient and reliable measure of active 5-HT uptake suitable for clinical investigation. The effect of passive diffusion on kinetic parameters is discussed.

Blood platelets and psychiatry

Blood platelets appear to have a somewhat schizoid existence. Haematologically they are intimately connected with the process of clot formation (for review, see Zucker, 1980) and, at least from a research standpoint, this aspect of their being is largely separate from their use as models of monoaminergic neurones (for reviews, see Maynert and Isaac, 1968; Abrams and Solomon, 1969; Lingjaerde, 1977; Stahl, 1977; Pletscher, 1978).

Clinical import of serotonin activity in the bladder and urethra

It has been observed in animal studies that 5-hydroxytryptamine produces bladder contractions. The role of 5-hydroxytryptamine in human vesicourethral function was studied by administering 75 mg. clomipramine daily for 4 days to 8 patients with neurogenic bladder dysfunction. Clomipramine inhibits the uptake of serotonin into 5-hydroxytryptamine containing presynaptic nerve endings and, thus, makes more neurotransmitter available to the postsynaptic site. The intravesical volume, at which first detrusor reflex activity was observed, showed a decrease from 329.5 to 178.5 ml. after clomipramine therapy. The residual urine also decreased from 111.25 to 30.0 ml. No side effect was observed. Further investigations involving a larger number of patients are indicated to study the possible use of serotonin activity in the bladder for promoting bladder emptying.

The effect of multiple rising doses of Ro 11-2465 (serotonin uptake inhibitor) on serotonin content of human platelets

Ro 11-2465, a cyanide derivative of imipramine with serotonin uptake inhibitory properties, was investigated in six healthy volunteers for its effect on serotonin concentration in blood platelets. The initial dose was 1 mg daily, the maximum dose of 3 mg being reached on day 3 and maintained for 7 days. A significant decrease in the platelet serotonin concentration was not observed until 3 days after the start of drug administration, after which depletion was rapid. After 5 days of treatment, the reduction was about 80% compared to pre-drug level. Serotonin restoration after drug withdrawal was very slow, and 5 days after discontinuation, it was still 70% below its baseline level.

Pre- and postsynaptic serotonergic manipulations in an animal model of depression

Rats working on a food-reinforced operant schedule and exhibiting behavioral depression following administration of D,L-5-hydroxytryptophan (5-HTP) were pretreated with one of three drugs: methysergide, fluoxetine, or amitriptyline. The former two drugs were used to establish a basis for distinguishing between pre- and postsynaptic events. We found that methysergide, a known postsynaptic blocker of serotonin, almost completely abolished the depressive effect of 5-HTP, whereas fluoxetine, a known specific uptake blocker of serotonin, potentiated the depressive effect of the 5-hydroxytryptamine (5-HT) precursor. Amitriptyline, one of the commonly prescribed antidepressive drugs, reduced the behavioral depression following 5-HTP by approximately 50%. These data indicate that amitriptyline can act as an antagonist of 5-HT at the postsynaptic receptor. The results of this study, as well as those recently reported from CNS membrane binding studies, suggest that the therapeutic effects of some antidepressive drugs may be explained by their postsynaptic rather than presynaptic properties at central serotonergic receptors. Thus, these studies support the hypothesis that some types of human depression may be primarily due to an excess of free 5-HT acting at postsynaptic receptors.

Effect of acute and chronic treatment of tandamine, a new heterocyclic antidepressant, on biogenic amine metabolism and related activities

The effects of tandamine, a clinically effective heterocyclic antidepressant, administered either acutely (10 mg/kg i.p.) or chronically (10 mg/kg i.p. daily for 21 days) on biogenic amine uptake and metabolism in the rat were determined and a comparison with desipramine was made. Tandamine, similarly to desipramine, blocked norepinephrine (NE) uptake in rat brain and heart following both acute and chronic administration. No effect of tandamine on dopamine (DA) or serotonin (5-HT) uptake was observed. Both drugs lowered endogenous brain NE when given chronically but not acutely. In contrast, no such effect on brain DA and 5-HT or heart NE was observed. Tandamine, like desipramine, administered chronically prior to an intraventricular injection of 3H-NE, produced increases in the decline of 3H-NE as indicated by decreased 3H-NE with increased levels of 3H-normetanephrine in brain stem of rats, suggesting an increased turnover of NE. No such effect was observed following acute treatment. Both drugs increased the behavioural effects of L-Dopa following and acute oral administration, with tandamine appearing superior to desipramine at the lower dose examined (10 mg/kg). Tandamine was 57--833 times less effective in binding to rat brain muscarinic receptors than desipramine, imipramine, butriptyline and amitriptyline, respectively. Thus, tandamine affects biogenic amine mechanism following either acute or chronic administration in a fashion similar to desipramine, but unlike desipramine, it exhibits relatively little anticholinergic properties, a further indication of the potential use of tandamine in the treatment of human depression, particularly where an increase in drive is desired.

Inhibition of 5-HT uptake into neurons and platelets in mice treated chronically with chlorimipramine and femoxetine

A single treatment with 5-HT uptake inhibitors potentiates the hypermotility in mice produced by the MAO-inhibitor nialamide. The effect of nialamide on motility was studied in mice after 4 weeks of feeding with a normal diet and diets containing various concentrations of the 5-HT uptake inhibitors chlorimipramine and femoxetine. Chronic treatment with the two substances enhanced the motor effects of nialamide about equally, which indicates a preservation of the neuronal 5-HT uptake inhibition during such treatment. The effect of chlorimipramine and femoxetine was obtained at plasma levels equivalent to or lower than the steady-state plasma concentrations found in patients treated with two 5-HT uptake inhibitors. Determination of decreased blood 5-HT after the 4 weeks of treatment was used as an in vivo test for inhibition of 5-HT uptake into platelets. Femoxetine was a much weaker depletor of blood 5-HT than chlorimipramine. These results indicate that blockade of neuronal 5-HT uptake is obtained at lower doses of femoxetine than blockade of 5-HT uptake into platelets. In contrast, chlorimipramine presumably inhibits 5-HT uptake into neurons and platelets at about the same dose.

Reduction of whole blood serotonin in depressed patients treated with a new, selective serotonin-uptake inhibitor, femoxetine

Femoxetine, a phenylpiperidine derivative with potent serotonin (5-HT)-uptake inhibitory properties, was investigated in 12 depressive patients for its clinical effect as well as its effect on 5-HT concentration in whole blood. The global evaluation after 6 weeks showed a good effect in six patients. The 5-HT concentration was found to be reduced from a mean value of 0.21 to about 0.05 micrograms/ml, indicating a total depletion of 5-HT from the thrombocytes. The steady-state concentrations of femoxetine resulted in reduction of 5-HT to the same level in all patients. No correlation between degree of 5-HT reduction and therapeutic effect was found.

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

Amitriptyline and zimelidine significantly reduce the uptake of 5-HT into the blood platelets of depressive patients. The degree of inhibition is significantly correlated with the plasma drug level of amitriptyline. No significant relationship could be detected between the degree of inhibition of uptake and therapeutic outcome. The accepted mode of action of tricyclic antidepressant drugs and the deficiency hypotheses of affective disorders are discussed.

Effect of a specific 5-HT uptake inhibitor, citalopram (Lu 10-171), on 3H-5-HT uptake in rat brain synaptosomes in vitro

The uptake of 3H-5-HT in synaptosomes from rat brains was investigated. Addition of DA or NA had only a slight or no effect on the uptake. When the uptake into NA and DA neurons was inhibited by the addition of high concentrations of NA and DA, the uptake of 3H-5-HT was unchanged. This was also found after destruction of NA and DA neurons by 6-hydroxydopamine treatment. Furthermore, the uptake of 3H-5-HT was almost equal in different brain parts containing NA and DA in very different amounts. These observations show that the uptake measured with 3H-5-HT is specific for 5-HT neurons. The present study revealed that citalopram and chlorimipramine inhibited uptake competitively, and in this respect the two drugs were equipotent. Compared with a series of tricyclic thymoleptics, the two drugs were the most potent inhibitors of 5-HT uptake, about 20 to 35 times more active than imipramine and amitriptyline. The metabolites of citalopram were also rather potent. The results obtained in the present study correlate closely with those obtained using inhibition of 14C-5-HT uptake in blood platelets, or using the inhibition of H 75/12-induced 5-HT depletion in rat brain. When rats were treated orally with citalopram or chlorimipramine, the inhibition of 3H-5-HT uptake in synaptosomes derived from these rats was two times greater after citalopram than after chlorimipramine.

Drowsiness, impaired performance and tricyclic antidepressants drugs

1. The effects of amitriptyline, protriptyline, protriptyline, and a chemically related potential antidepressant, BW247, on performance tests and subjective ratings were studied. 2. Two groups of twelve healthy subjects received drugs and lactose dummy in identical capsules at weekly intervals according to a balanced design, under double-blind conditions, and with standarized tests and environment. 3. Amitriptyline produced the most marked effects, with significant (P less than 0.05) impairment in auditory vigilance after 6.25 mg. Auditory reaction time, tapping rate, arithmetic, and digit symbol substitutions were impaired by amitriptyline 12.5 and 25 mg and all doses produced increased ratings of mental sedation. The effects began 1.5 h after drug and lasted approximately 5 h. 4. Nortriptyline produced fewer effects which were later in onset. Tapping at 1.8 h and auditory vigilance at 3.5 to 4.5 h were impaired by nortriptyline 25 mg whereas reaction time was prolonged by both doses at 5 h. No change in rating of mental sedation occurred. 5. No significant change in performance or subjective ratings followed protriptyline 10 mg or BW247, 12.5 and 25 mg. 6. The findings are discussed in relation to the presence of secondary and tertiary amines on the side chain of the compounds, and their relative abilities to block neuronal uptake of noradrenaline and 5-hydroxytryptamine.

5-Hydroxytryptamine and dopamine transport by rat and human blood platelets

1 Uptake of 5-hydroxytryptamine (5-HT) by rat platelets in plasma was very rapid and diffusion did not contribute significantly at substrate concentrations that did not saturate the active transport.2 Under conditions which allowed measurement of initial rates of uptake, kinetic analysis revealed a high affinity uptake mechanism for 5-HT (K(m) = 0.7 muM).3 Uptake of dopamine was relatively slow and involved a lower affinity (K(m) = 70 muM) active transport process. Diffusion contributed significantly at concentrations that did not saturate the active transport.4 5-HT competitively inhibited uptake of dopamine, and vice versa; K(i) values for both amines were similar to their respective K(m) values for uptake.5 Chlorimipramine, desmethylimipramine and benztropine were tested as uptake inhibitors. Each was equipotent against 5-HT and dopamine, although the absolute potency of the drugs varied greatly. Chlorimipramine was the most potent (K(i)## 100 nM), and kinetic analysis revealed that the inhibition was competitive against both 5-HT and dopamine.6 Similar results were obtained in studies with human platelets: K(m) values for 5-HT and dopamine were about 1 muM and 100 muM respectively. Activity profiles of inhibitors were also similar: each compound tested was equipotent against 5-HT and dopamine, and the two amines each competitively inhibited uptake of the other.7 We conclude that dopamine is actively transported by platelets via the 5-HT uptake mechanism, but with a much lower affinity. There is no high-affinity dopamine-specific mechanism corresponding to that in the corpus striatum. Consequently although platelets may be valid models of transport in 5-hydroxytryptaminergic neurones, they should not be regarded as models for the dopamine transport mechanism found in dopaminergic neurones.

Fluvoxamine, a specific 5-hydroxytryptamine uptake inhibitor

1. On the basis of both in vitro and in vivo experiments fluvoxamine has been characterized as a potential anti-depressant drug with almost exclusively 5-hydroxytryptamine (5-HT) uptake inhibiting properties. 2. Fluvoxamine is effective in inhibiting 5-ht uptake by blood platelets and brain synaptosomes. Due to inhibition of the membrane pump the compound prevents 5-HT depletion by the tyramine-derivatives H 75/12 and H 77/77. As a result of the interference with the neuronal re-uptake mechanism for 5-HT, fluvoxamine produces a decreased 5-HT turnover in the brain. Effects of 5-hydroxytryptophan (5-HTP) are potentiated in mice and in combination with pargyline, fluvoxamine induces 5-HT-like behavioural effects. 3. In contrast to tricyclic antidepressants, noradrenaline uptake processes are either unaffected or only slightly inhibited by fluvoxamine. The noradrenaline depleting effects of tyramine derivates are not influenced by fluvoxamine. Reserpine effects, such as ptosis are affected only at very high doses of the test compound. The antagonism by fluvoxamine of the reserpine-induced lowering of the pentamethylenetetrazole convulsive threshold can be regarded as due to an effect upon 5-HT uptake. In contrast to the effects of desmethylimipramine and imipramine, no stimulatory effects are found in rats when rapidly acting reserpine-like compounds are given following a dose of fluvoxamine.