Serotonin-mediated thrombosis and selective serotonin re-uptake inhibition

Abstract

Thromb Haemost 2009; 102: 424–425 Since the classification of platelet agonists into strong and weak began, controversy has surrounded the role of serotonin in the amplification of the generation of platelet-rich thrombi in arterial disease. Interest in serotonin-mediated platelet thrombosis stems from the detailed investigations of de Clerck and the Janssen group (1, 2). The paper by Galan et al. (3) in this issue of Thrombosis and Haemostasis is a systematic investigation of serotonin-mediated mechanisms of platelet aggregation and vasoconstriction reaching into fibrin-rich thrombi. The authors thoroughly assessed the influences in vitro of a selective serotonin reuptake inhibitor (SSRI), citalopram, on platelet aggregation and fibrin deposition, demonstrating differentiated effects of low-shear on fibrin deposition and high-shear on platelet-rich thrombus formation. They emphasise the potency of serotonin on thrombus formation and challenge the notion that serotonin is a weak agonist of platelet-mediated thrombosis. Serotonin potentiated ADP stimulation both under acalcaemic and normocalcaemic conditions. Incubation of the cells with citalopram reduced the response to ADP as well as ADP plus serotonin. Flow cytometric studies showed that this agonist combination increased surface expression of P-selectin, binding of annexin V and factor V/Va release; the first two but not the last were inhibited by prior exposure to citalopram. Thrombin generation was promoted by serotonin but was not inhibited by pre-incubation with citalopram; this suggests a possible serotonin 2A receptor-mediated effect, rather than a serotonin transporter (SERT) effect. Perfusion studies confirmed the importance of high shear conditions upon the platelet aggregatory effects of serotonin in the presence of collagen. That fibrin deposition is promoted by serotonin at low-shear is interesting and suggests a mechanism whereby thrombolysis is incompletely achieved. The fact that SSRI-pretreatment decreased serotonin-induced thrombus propagation supports previous studies in which dispersion of established thrombi was achieved with serotonin 2A receptor antagonism (4, 5). Examination of clots confirmed the importance of Ca2+ for serotonin-mediated fibrin deposition with shortened clotting time and concomitant increase in clot firmness. These data chime well with clinical data that depressed patients, under SSRI treatment, have lower risk of myocardial infarction than those on other anti-depressants. Inhibition of thromboxane (TXA2) production by aspirin did not affect platelet aggregation by serotonin, ADP, or both together. This is interesting because aspirin is ineffective at in vivo critical stenoses where serotonin assumes dominance and may be assisted by red cell-derived ADP. This reinforces human studies in which serotonergic inhibition has demonstrated reduced platelet aggregation when challenged by sub-maximal collagen concentrations (about one third the concentration used in the study under discussion) (6). Extracellular calcium concentration is important when relating experimental data to clinical findings, which the investigators have attended to. The absence of bleeding time derangement seen with serotonin 2A receptor inhibition, sufficient to inhibit intracoronary thrombosis (clinically and experimentally), may have led investigators to underestimate the effectiveness of these drugs on platelet-rich thrombosis despite the large number of reports affirming this property (4, 5). SSRIs are reported to have a slightly increased risk of upper gastro-intestinal haemorrhage (7), the mechanism of which is obscure but may be due to down-regulation or shedding of glycoprotein GP-Ia (8) which suggests another mechanism whereby SSRIs reduce platelet aggregability under conditions of high shear. SERT may be related to mechanisms involved in GPIIb-IIIa activation (9) and investigators have demonstrated two-way associations between the function of SERT and GPIIb-IIIa (10). What effects do SSRIs have on serotonin 2A receptors? Chronic SSRI administration to the rodent central nervous system reported decreased serotonin 2A receptor expression by up to 38% (11), although others found a higher number of serotonin 2A receptors in suicidal patients (12). These receptors are downregulated by almost all anti-depressants and by almost all antipsychotic drugs as well (13). Neither fluoxetine nor citalopram changed the number of serotonin 2A receptors (14), although their number correlated strongly with suicidal ideation. Sub-