Uptake of 14C-5-hydroxytryptamine by human and rat platelets and its pharmacological inhibition. A comparative kinetic analysis

Mechanism of the inhibitory effect of imipramine on the Na+-dependent transport of L-glutamic acid in rat intestinal brush-border membrane

The mechanism of the inhibitory effect of imipramine, a lipophilic organic cation on the Na+-dependent transport of L-glutamic acid across intestinal brush-border membrane was investigated. The uptake of L-glutamic acid by intestinal brush-border membrane vesicles was dependent on the concentration of Na+. Fitting of the uptake data in the presence of various concentrations of Na+ using Hill equation yielded a Hill coefficient of 2.18. This result suggest that the carrier system of L-glutamic acid has at least two sites for Na+-binding. By the analysis of double reciprocal plot and Dixon-type plot, it was found that imipramine inhibits the transport of L-glutamic acid by interacting competitively with the binding sites of Na+, but not inhibit L-glutamic acid binding site. Moreover, the effect of imipramine on the transport of L-alanine and D-glucose which are co-transported with only one Na+ molecule was also suggestive of interaction with the Na+-binding sites on the carrier. These results indicate that the mechanism of the inhibitory effect of imipramine on the Na+-dependent carrier systems is common for all systems regardless of the stoichiometry or substrates.

Pharmacological profile of antidepressants and related compounds at human monoamine transporters

Using radioligand binding assays, we determined the equilibrium dissociation constants (KD's) for 37 antidepressants, three of their metabolites (desmethylcitalopram, desmethylsertraline, and norfluoxetine), some mood stabilizers, and assorted other compounds (some antiepileptics, Ca2+ channel antagonists, benzodiazepines, psychostimulants, antihistamines, and monoamines) for the human serotonin, norepinephrine, and dopamine transporters. Among the compounds that we tested, mazindol was the most potent at the human norepinephrine and dopamine transporters with KD's of 0.45 +/- 0.03 nM and 8.1 +/- 0.4 nM, respectively. Sertraline (KD = 25 +/- 2 nM) and nomifensine (56 +/- 3 nM) were the two most potent antidepressants at the human dopamine transporter. We showed significant correlations for antidepressant affinities at binding to serotonin (R = 0.93), norepinephrine (R = 0.97), and dopamine (R = 0.87) transporters in comparison to their respective values for inhibiting uptake of monoamines into rat brain synaptosomes. These data are useful in predicting some possible adverse effects and drug-drug interactions of antidepressants and related compounds.

Protein kinase C activation regulates human serotonin transporters in HEK-293 cells via altered cell surface expression

Antidepressant- and cocaine-sensitive serotonin (5-hydroxytryptamine, 5-HT) transporters (SERTs) dictate clearance of extracellular 5-HT after release. To explore protein kinase C-mediated SERT regulation, we generated a stable human SERT (hSERT)-expressing cell line (293-hSERT) and evaluated modulation of 5-HT activity via studies of 5-HT flux, hSERT-mediated currents under voltage clamp, and surface distribution of SERT protein. 293-hSERT cells exhibit saturable, high-affinity, and antidepressant-sensitive 5-HT uptake as well as hSERT-dependent whole-cell currents. In these cells, the protein kinase C activator beta-PMA caused a time-dependent reduction in 5-HT uptake capacity (Vmax) after acute application and a reduction in SERT-mediated currents. Effects of beta-PMA were mimicked by the phorbol ester beta-PDBu, were not observed with the inactive alpha-isomers, and could be blocked by treatment of cells with the protein kinase C inhibitor staurosporine. Biotinylation/immunoblot analyses showed that activity reductions are paralleled by a staurosporine-sensitive loss of surface SERT protein. These data indicate that altered surface abundance, rather than reduced catalytic transport efficiency, mediates acute PKC-dependent modulation of 5-HT uptake.

Pharmacological characterization of the cloned human 5-hydroxytryptamine transporter

We performed an extensive pharmacological study of the 5-hydroxytryptamine (5-HT) transporter polypeptide cloned from human placenta. Transient expression of this 630 amino acid polypeptide in HeLa cells led to saturable 5-HT uptake activity (Km = 858 nM). This 5-HT uptake was blocked by selective 5-HT inhibitors, such as citalopram, litoxetine, sertraline, and indalpine, with Ki values in the low nanomolar range, and it exhibited a pharmacological profile similar to that found in rat brain. [3H]Citalopram binding to membrane preparations of the transfected cells occurred to a single class of high-affinity binding sites (Kd = 5.3 nM) and was potently inhibited by selective 5-HT uptake inhibitors. The pharmacological profile of [3H]citalopram binding to these transfected cells showed a good correlation with that of [3H]paroxetine binding to the rat cerebral cortical 5-HT transporter (r = 0.79). These data confirm that the full pharmacological characteristics of the 5-HT transport system are conferred by the expression of the 630 amino acid human placental 5-HT transporter polypeptide. [3H]Citalopram should, therefore, provide a useful probe for more insights at a molecular level into this cloned 5-HT transport system.

Naftidrofuryl inhibits the release of 5-hydroxytryptamine and platelet-derived growth factor from human platelets

Angioplasty and bypass-grafting are associated with restenosis which limits their efficacy. Platelet-rich thrombus formation is the predominant cause of acute occlusion whereas platelet release products with proliferating properties, e.g. 5-hydroxytryptamine (5-HT) and platelet-derived growth factor (PDGF), may contribute to late restenosis. Naftidrofuryl (NAF), a drug for the treatment of peripheral vascular disease, was shown previously to inhibit platelet shape change and aggregation. This study establishes whether NAF inhibits the release of 5-HT and PDGF from platelets obtained from healthy subjects. Platelets stimulated with agonists aggregated less and released less 5-HT/PDGF when pre-incubated with NAF. Indomethacin (INDO), a cyclooxygenase inhibitor, alone inhibited aggregation and PDGF/5-HT release; NAF enhanced the inhibitory effects of INDO. The effect of NAF, on its own or in combination with a cyclooxygenase inhibitor, may therefore confer protection against graft occlusion.

Platelet of Alzheimer patients: increased counts and subnormal uptake and accumulation of [14C]5-hydroxytryptamine

Platelets are the main source of 5-hydroxytryptamine (5-HT) and amyloid precursor protein (APP) found in plasma. We studied a possible correlation between platelet markers and the clinical diagnosis of Alzheimer disease (AD). Our results indicate that in AD patients: (a) platelets are elevated, (b) their ability to accumulate 5-HT decreases and, (c) the kinetic parameters of 5-HT uptake are altered (decreased Km and Vmax), compared to non-demented healthy individuals. An aged Down syndrome patient presents even more deviant alterations. Our findings supports the idea that platelets may provide a systemic marker of AD, and eventually be useful for the clinical diagnosis of the disease.

Pharmacological comparison of the rat and guinea-pig cortical high affinity 5-hydroxytryptamine uptake system

The pharmacological characteristics of the high affinity [3H]5-hydroxytryptamine ([3H]5-HT) uptake system were investigated in the cerebral cortex of the rat and guinea-pig. In crude cortical synaptosomal preparations from the rat and guinea-pig, [3H]5-HT accumulated with high affinity (Km, 72 +/- 12 and 57 +/- 14 nM for rat and guinea-pig cortical synaptosomal preparation, respectively, mean +/- SEM, N = 5) and with a comparable maximum activity (Vmax, 1.22 +/- 0.21 and 0.90 +/- 0.19 pmol/min/mg protein for rat and guinea-pig cortical synaptosomal preparation, respectively, mean +/- SEM, N = 5). Competition studies employing a range of structurally diverse competing compounds showed that the [3H]5-HT uptake was pharmacologically similar in both preparations. However, citalopram possessed approximately 10-fold weaker affinity to prevent [3H]5-HT uptake in the guinea-pig preparation when compared to the rat and all of the tricyclic antidepressants assessed in the present studies (amitriptyline, nortriptyline, desipramine and imipramine) displayed higher affinity in the guinea-pig preparation when compared to the rat. It is concluded that the high affinity 5-HT uptake systems in the rat and guinea-pig cortex are similar but may not be identical.

Relationship between seasonal patterns of platelet serotonin uptake and 3H-imipramine binding in depressed patients and normal controls

1. Significant seasonal variations were found in the velocity of serotonin (Vmax) uptake and the density of 3H-imipramine binding sites (Bmax) in blood platelets from normal controls. 2. Peak 3H-imipramine (3H-IMI) binding was found in February whereas peak serotonin (5HT) uptake was found in June and these measures were not correlated in paired comparisons. 3. Both Vmax and Bmax values of depressed patients deviated from the normal seasonal pattern with lower uptake and binding in the patient group. 4. A comparison of Vmax and Bmax deviations from the normal patterns of uptake and binding revealed a significant correlation between these measures such that patients with low Vmax values were the same as those with low Bmax values. 5. The results support previous claims that the 3H-IMI binding site may be closely associated with, or identical to, a 5HT transport carrier. 6. A significant correlation between uptake and binding further suggests that a common defect may be responsible for observed decreases in Vmax and Bmax values of depressed patients.

Two affinity states for [3H]imipramine binding to the human platelet 5-hydroxytryptamine carrier: an explanation for the allosteric interaction between 5-hydroxytryptamine and imipramine

5-Hydroxytryptamine (5-HT) showed a biphasic effect on the dissociation rate of [3H]imipramine from human platelet membranes: At low concentrations (EC50, approximately 2.5 microM), 5-HT stimulated the rate, as expected for mutually exclusive binding of 5-HT and imipramine; at higher concentrations (EC50, approximately 40 microM), 5-HT reduced this stimulated rate, a result consistent with 5-HT binding at a site that is physically distinct from both the imipramine binding site and the 5-HT transport recognition site of the 5-HT carrier. This modulatory effect could be mimicked by tryptamine, was saturable and independent of Na+ concentration, and could also be demonstrated for detergent-solubilized carriers. Monophasic association kinetics for [3H]imipramine binding were found. Heat stability experiments showed biphasic thermal inactivation curves. These results are consistent with [3H]imipramine binding to two classes of binding sites at the 5-HT carrier on human platelet membranes, with affinities three- to fivefold different. 5-HT can convert the lower-affinity state into the higher-affinity state.

Kinetics of the interaction of sertraline with the human platelet plasma membrane 5-hydroxytryptamine carrier

Sertraline, a new selective 5-HT uptake inhibitor showed a mixed pattern of inhibition of human platelet 5-HT uptake with a Ki value of 2.5 nM and K'i value of 25 nM. Imipramine and alaproclate were found to be fully competitive inhibitors of 5-HT uptake with Ki values of 8 and 130 nM respectively. Sertraline was a fully competitive inhibitor of high-affinity [3H]imipramine binding to platelet membranes with a Ki value of 1.3 nM, as was alaproclate and 5-HT with Ki values of 170 and 800 nM respectively. Both sertraline and imipramine, at a concentration of 10 microM caused a fast monophasic dissociation of [3H]imipramine from platelet membranes in contrast to serotonin which caused a slow monophasic dissociation.

Allosteric interaction between the site labeled by [3H]imipramine and the serotonin transporter in human platelets

The nature of interaction between the site labeled by [3H]imipramine (IMI) and the 5-hydroxytryptamine (5-HT, serotonin) transporter in human platelets was examined. The sulfhydryl characterizing agent N-ethylmaleimide (NEM) differentially affected [3H]5-HT uptake and [3H]IMI binding in human platelet preparations. Concentrations of NEM that completely abolished [3H]5-HT uptake only minimally reduced [3H]IMI binding. Examining the effect of IMI on the kinetics of human platelet [3H]5-HT uptake revealed significant reductions in maximal velocity (Vmax) without altering affinity (Km). IC50 values for selected uptake blockers on [3H]IMI binding and [3H]5-HT uptake were determined. IC50 values of these compounds for uptake and binding revealed that agents such as IMI, chlorpromazine, amitriptyline, and nisoxetine were preferential inhibitors of [3H]IMI binding whereas fluoxetine, CL 216, 303, pyrilamine, and bicifadine were preferential [3H]5-HT uptake blockers. 5-HT was a weak displacer of [3H]IMI binding (IC25 = 3.0 microM) and exhibited a rather low Hill coefficient (nH app = 0.46). Results reported herein support the notion of an allosteric interaction between the [3H]IMI binding site and the 5-HT transporter complex in human platelets.

Normal serotonin uptake by blood platelets and brain synaptosomes but selective impairment of platelet serotonin storage in mice with Chediack-Higashi syndrome

The Chediack-Higashi syndrome (CHS) is an autosomal recessive disorder reported in man and in several animal species including the "beige mice" (bg/bg). Among several manifestations of this genetic trait, deficiency of secretable substances - including serotonin - normally stored in platelet dense granules is a characteristic feature. The animal model of Chediak-Higashi syndrome used in the present study provides a unique opportunity to compare the kinetics of serotonin (5-hydroxytryptamine, 5-HT) uptake in platelets and brain synaptosomes in conditions of selective reduction of 5HT concentration in the platelets. The kinetics of 5HT uptake, as measured in the present study, was normal in synaptosomes and platelets from the same animals. The lower intraplatelet 5HT levels in bg/bg animals as compared to normal synaptosomes levels in the presence of normal uptake offer an indirect proof that the 5HT defect described in the CHS is due to an impaired 5HT storage mechanism. This is supported by the observation that spontaneous release of 5HT was markedly increased in platelets from CH5 mice but was normal in synaptosomes from the same animals. Thus platelets are a reliable model to study 5HT uptake, but not 5HT storage and release in brain synaptosomes.

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.

Serotonin uptake inhibitors differentially modulate high affinity imipramine dissociation in human platelet membranes

The influence of selective serotonin uptake inhibitors on the dissociation rate of 3H-imipramine from its high affinity binding site in human platelet membranes was studied. Of the uptake inhibitors tested, one group of compounds attenuated dissociation, another group accelerated it, while a third group had little effect on the dissociation process. Drugs unrelated to the serotonin uptake system were ineffective. Removal of sodium ions markedly increased imipramine dissociation. Dose response curves of the active compounds indicated that micromolar concentrations were required to exert an effect on imipramine dissociation. These results can be adequately explained by an allosteric model which includes effector binding sites and distinct conformational states of the high affinity imipramine binding site.

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.

The uptake and metabolism of 5-hydroxytryptamine by tissue slices of the cestode Hymenolepis diminuta

The in vitro uptake of [3H]5HT was investigated in tissue slices of the cestode Hymenolepis diminuta. A concentrative, sodium sensitive, high affinity uptake mechanism (Km 1.43 X 10(-6) M; Vmax 222 fmoles/mg wet wt/min), together with a sodium insensitive component (linear up to 5 X 10(-6) M) were present. In the presence of 2-nitroimipramine the sodium sensitive component was significantly suppressed (Vmax 33 fmoles/mg/wet wt/min) although the Km (1.37 X 10(-6) M) was not affected. Nitroimipramine showed an IC50 of approximately 2 X 10(-6) M. The sodium insensitive component was not affected by nitroimipramine. Biogenic amines and related indoleamines were weak inhibitors of the sodium sensitive and sodium insensitive components of 5HT uptake. The tricyclic antidepressants and fluoxetine were effective inhibitors of the sodium sensitive component of 5-HT uptake; receptor ligands were weak inhibitors or without effect. The metabolism of [3H]5HT in tissue slices of H. diminuta was examined by HPLC. The role of the sodium sensitive uptake and metabolism of 5HT in terms of inactivation and recycling of neurally released 5HT and the possible importance of exogenous recruitment of 5HT are discussed.

Prolongation of rat tail bleeding time by ketanserin: mechanisms of action

Serotonin (5-HT) may play a regulatory role in platelet-vessel wall interaction. This can be reliably investigated by measuring bleeding time. Ketanserin is a recently developed selective 5-HT2 receptor antagonist, reportedly effective against both platelet and vascular 5-HT activation. Ketanserin (5-10 mg/kg) significantly prolonged tail bleeding time measured in conscious rats by two different techniques. While mianserin (a 5-HT2 receptor antagonist exhibiting alpha-adrenolytic activity) also prolonged bleeding time, methysergide, metergoline and cyproheptadine did not. All three compounds acted as 5-HT2 receptor antagonists with appreciable affinity for 5-HT1 receptors. On the other hand, bleeding time was prolonged by either prazosin (a selective alpha 1-adrenoceptor antagonist) or labetalol (an alpha 1- and beta-receptor antagonist). In contrast it was not affected by phentolamine or nicergoline (alpha 1-alpha 2-receptor antagonists) nor by propranolol (a beta-receptor antagonist). The effect of prazosin was significantly increased by combining it with either ketaserin or metergoline. Depletion of platelet serotonin by reserpine did not result in any modification of bleeding time, unless reserpine was combined with an inhibitor of 5-HT synthesis. Platelet activation by 5-HT was neither potentiated by norepinephrine nor prevented by prazosin or phentolamine whereas ketanserin and methysergide were equally effective inhibitors. These findings argue against a role of platelet and/or vascular 5-HT2 receptors in the antihemostatic effect of ketanserin in rats. This drug prolongs bleeding time by antagonising vascular adrenoceptors (prazosin-like effect) and/or by preventing a synergistic interaction between 5-HT and catecholamines at the vascular level.

Blood platelets in psychopharmacological research

Blood platelets are a very good model for nerve endings, for study of serotonin uptake and imipramine binding. The efficiency of various drugs, mainly antidepressants, can be evaluated using blood platelets instead of synaptosomes. A few neuronal active compounds are effective in inducing platelet secretion and shape change reaction. Platelet MAO and c-AMP levels are not yet established as useful in psychopharmacological studies.

Platelet function after shipment of room temperature platelet concentrates

Platelet concentrates stored for 48 or 72 h were shipped up to 10 h at 22 or 4 degrees C. Thereafter platelet function was compared with platelet concentrates stored under optimal blood bank conditions with continuous agitation and strict temperature control. Platelet function measured by hypotonic shock response, aggregation and serotonin uptake is better maintained at 22 than at 4 degrees C. The only difference between shipping and continuous agitation is seen in the hypotonic shock response after a total storage time of 72 h. Shipped platelets showed a marked drop in hypotonic-shock response. The data indicate that shipment of platelet concentrates does not significantly impair platelet function.

Methodological aspects of studies on the 5-HT uptake mechanism in normal platelets

The 5-HT uptake mechanism in human platelets was studied in undiluted platelet-rich plasma (PRP). The kinetic data obtained, which were based on 5-HT concentrations ranging from 0.17 to 1.7 mumol/l, were analyzed according to Lineweaver-Burk, Eadie-Hofstee, Scatchard and Sips. In undiluted PRP all the analyses revealed a strict linearity between 5-HT concentrations and initial uptake velocities. Only one site for active transport seemed to be involved and no passive diffusion was present. The apparent Km and Vmax in a 95 per cent confidence interval (n = 25) were 1.20 +/- 0.14 mumol/l and 1.40 +/- 0.15 pmol/10(6) platelets and minutes, respectively. Dilution of PRP with autologous platelet-poor plasma or Ringer's solution markedly influenced the active 5-HT transport mechanism. The addition of minute amounts of supernatant from lysed autologous platelets to PRP totally impaired the active 5-HT uptake. It is concluded that relevant studies on the active 5-HT transport can be performed only in fresh undiluted PRP. The commonly used Lineweaver-Burk analysis does not discriminate sufficiently between fully saturable and non-saturable transport mechanism and therefore cannot be used for the detection of irregularities in the 5-HT transport.

Arachidonic acid-induced malondialdehyde formation in rat platelets. Kinetic aspects and inhibition by acetylsalicylic acid and indomethacin

Malondialdehyde (MDA) is a stable product of arachidonic acid metabolism, catalyzed by the enzyme cyclo-oxygenase. The experimental conditions for measuring the kinetic of MDA formation in rat platelet-rich plasma were defined. In platelets stimulated with arachidonic acid MDA formation was almost linear for a limited period of time (between 0 and 2 min) and was concentration-dependent with saturation kinetics. The hyperbolic curves obtained were recast in a linear plot (according to transformation S/V versus S) and straight lines fitting all experimental points were obtained. The apparent Km value of arachidonic acid was 0.49 +/- 0.09 mM and Vmax 1.44 +/- 0.06 nmoles MDA/1.4 x 10(9) platelets/min. The apparent type of inhibition and the relative potency of acetylsalicylic acid and indomethacin on MDA formation were also investigated. Inhibition by both drugs was concentration-dependent, and was much stronger when either drug was preincubated with platelets for 10 min than for 1 min. Analysis of the data by Dixon plots (1/V versus inhibitor concentrations) revealed an apparently competitive type of inhibition. It is suggested that both acetylsalicylic acid and indomethacin inhibit cyclo-oxygenase in platelet-rich plasma by a similar mechanism, not involving covalent binding of either drug to the enzyme.

Decreased uptake of 5-hydroxytryptamine in blood platelets from patients with endogenous depression

5-Hydroxytryptamine (5-HT) uptake was studied by using blood platelets from 13 patients with endogenous depression (Hamilton rating scale 33 +/- 7) and 13 healthy volunteers. An improved method with a short incubation time and low substrate concentration was used, and the incubation was performed in Krebs-Henseleit buffer (pH 7.4) at 37 degrees C. A clear difference in 5-HT uptake by blood platelets was noted: The Vmax of the reaction in patients was 39, and in controls 71 pmol per 2 x 10(7) platelets in 5 min. There was no significant difference in the Km. After a 4-week treatment with imipramine, a competitive inhibition of 5-HT uptake with an increased Km was seen; after a similar treatment with amoxapine there was little change in 5-HT uptake. Amoxapine was inferior to imipramine as an inhibitor of 5-HT uptake, also in vitro. There was no difference in clinical recovery in these treatment groups. These results may be of importance so as to understand the potential biological differences between depressed patients and normal persons.

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.

Subcellular distribution of [14C]5-hydroxytryptamine in the blood platelets of rabbits: effects of imipramine and and haloperidol

Long-term (10 days) administration of imipramine [20 mg/(kg X d)] to rabbits significantly increases the Km value (4.0 micron) of 5-hydroxytryptamine uptake in their platelets compared to those of saline- (0.7 micron) or haloperidol- (0.4 micron) treated rabbits. Administration of haloperidol inhibits the 5-hydroxytryptamine uptake non-competitively, and in vitro it had an ID50 value of 22 micron. Intravenous injections of [14C]5-hydroxytryptamine were given to the animals 1 h before blood collection. After isolation of platelets, their sonicates were subjected to 30-60% continuous sucrose gradient centrifugation. The subcellular distribution of [14C]5-hydroxytryptamine indicates that imipramine treatment, in contrast to the control and haloperidol treatment, led to a shift in the exogenous 5-hydroxytryptamine peak from within the granular zone (d 1.18) to the extragranular cytoplasm (d 1.15). Compared to control values, the imipramine treatment caused 63% inhibition in the platelet Na-K-ATPase activity.

Behavioural despair in rats: a new model sensitive to antidepressant treatments

Rats when forced to swim in a cylinder from which they cannot escape will, after an initial period of vigorous activity, adopt a characteristic immobile posture which can be readily identified. Immobility was reduced by various clinically effective antidepressant drugs at doses which otherwise decreased spontaneous motor activity in an open field. Antidepressants could thus be distinguished from psychostimulants which decreased immobility at doses which increased general activity. Anxiolytic compounds did not affect immobility whereas major tranquilisers enhanced it. Immobility was also reduced by electroconvulsive shock, REM sleep deprivation and "enrichment" of the environment. It was concluded that immobility reflects a state of lowered mood in the rat which is selectively sensitive to antidepressant treatments. Positive findings with atypical antidepressant drugs such as iprindole and mianserin suggest that the method may be capable of discovering new antidepressants hitherto undetectable with classical pharmacological tests.

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.

Effect of two non tricyclic antidepressant drugs on [14C]5-hydroxytryptamine uptake by rat platelets

The uptake of 14C-5-HT by rat blood platelets was examined in vitro in experimental conditions which allowed measurement of the initial velocity and excluded other passive processes across the cell membrane. In these conditions, the effect of two non tricyclic antidepressant drugs (Lilly 110140 and trazodone) was investigated. Lilly 110140 was as active as chlorimipramine and several times more active than imipramine as an inhibitor of 14C-5-HT uptake. Like chlorimipramine, Lilly 110140 appeared to be either a non-competitive or an uncompetitive inhibitor, according to the concentration of drug used. Trazodone also inhibited 14C-5-HT uptake by platelets but to a lesser extent than chlorimipramine, imipramine or Lilly 110140. m-Chlorophenylpiperazine, a possible metabolite of trazodone, was about 3 times more potent an inhibitor than the parent molecule. Both compounds acted non-competitively. Compared with published data on the effect of Lilly 110140 and trazodone on brain 5-HT, the present results support the suggestion that rat platelets are a useful pharmacological model of serotoninergic nerve endings.