Loss of Serotonin Transporter Function Alters ADP-mediated Glycoprotein αIIbβ3 Activation through Dysregulation of the 5-HT2A Receptor

Endophenotype trait domains for advancing gene discovery in autism spectrum disorder

Autism spectrum disorder (ASD) is associated with a diverse range of etiological processes, including both genetic and non-genetic causes. For a plurality of individuals with ASD, it is likely that the primary causes involve multiple common inherited variants that individually account for only small levels of variation in phenotypic outcomes. This genetic landscape creates a major challenge for detecting small but important pathogenic effects associated with ASD. To address similar challenges, separate fields of medicine have identified endophenotypes, or discrete, quantitative traits that reflect genetic likelihood for a particular clinical condition and leveraged the study of these traits to map polygenic mechanisms and advance more personalized therapeutic strategies for complex diseases. Endophenotypes represent a distinct class of biomarkers useful for understanding genetic contributions to psychiatric and developmental disorders because they are embedded within the causal chain between genotype and clinical phenotype, and they are more proximal to the action of the gene(s) than behavioral traits. Despite their demonstrated power for guiding new understanding of complex genetic structures of clinical conditions, few endophenotypes associated with ASD have been identified and integrated into family genetic studies. In this review, we argue that advancing knowledge of the complex pathogenic processes that contribute to ASD can be accelerated by refocusing attention toward identifying endophenotypic traits reflective of inherited mechanisms. This pivot requires renewed emphasis on study designs with measurement of familial co-variation including infant sibling studies, family trio and quad designs, and analysis of monozygotic and dizygotic twin concordance for select trait dimensions. We also emphasize that clarification of endophenotypic traits necessarily will involve integration of transdiagnostic approaches as candidate traits likely reflect liability for multiple clinical conditions and often are agnostic to diagnostic boundaries. Multiple candidate endophenotypes associated with ASD likelihood are described, and we propose a new focus on the analysis of "endophenotype trait domains" (ETDs), or traits measured across multiple levels (e.g., molecular, cellular, neural system, neuropsychological) along the causal pathway from genes to behavior. To inform our central argument for research efforts toward ETD discovery, we first provide a brief review of the concept of endophenotypes and their application to psychiatry. Next, we highlight key criteria for determining the value of candidate endophenotypes, including unique considerations for the study of ASD. Descriptions of different study designs for assessing endophenotypes in ASD research then are offered, including analysis of how select patterns of results may help prioritize candidate traits in future research. We also present multiple candidate ETDs that collectively cover a breadth of clinical phenomena associated with ASD, including social, language/communication, cognitive control, and sensorimotor processes. These ETDs are described because they represent promising targets for gene discovery related to clinical autistic traits, and they serve as models for analysis of separate candidate domains that may inform understanding of inherited etiological processes associated with ASD as well as overlapping neurodevelopmental disorders.

Platelets bridging the gap between gut dysbiosis and neuroinflammation in stress-linked disorders: A narrative review

In this narrative review, we examine the association between gut dysbiosis, neuroinflammation, and stress-linked disorders, including depression, anxiety, and post-traumatic stress disorder (PTSD), and investigate whether tryptophan (TRP) metabolism and platelets play a role in this association. The mechanisms underlying the aetiology of stress-linked disorders are complex and not yet completely understood. However, a potential link between chronic inflammation and these disorders may potentially be found in TRP metabolism and platelets. By critically analysing existing literature on platelets, the gut microbiome, and stress-linked disorders, we hope to elicit the role of platelets in mediating the effects on serotonin (5-HT) levels and neuroinflammation. We have included studies specifically investigating platelets and TRP metabolism in relation to inflammation, neuroinflammation and neuropsychiatric disorders. Alteration in microbial composition due to stress could contribute to increased intestinal permeability, facilitating the translocation of microbial products, and triggering the release of pro-inflammatory cytokines. This causes platelets to become hyperactive and secrete 5-HT into the plasma. Increased levels of pro-inflammatory cytokines may also lead to increased permeability of the blood-brain barrier (BBB), allowing inflammatory mediators entry into the brain, affecting the balance of TRP metabolism products, such as 5-HT, kynurenic acid (KYNA), and quinolinic acid (QUIN). These alterations may contribute to neuroinflammation and possible neurological damage. Furthermore, platelets can cross the compromised BBB and interact with astrocytes and neurons, leading to the secretion of 5-HT and pro-inflammatory factors, exacerbating inflammatory conditions in the brain. The mechanisms underlying neuroinflammation resulting from peripheral inflammation are still unclear, but the connection between the brain and gut through the bloodstream could be significant. Identifying peripheral biomarkers and mechanisms in the plasma that reflect neuroinflammation may be important. This review serves as a foundation for further research on the association between the gut microbiome, blood microbiome, and neuropsychiatric disorders. The integration of these findings with protein and metabolite markers in the blood may expand our understanding of the subject.

Serotonin and the serotonin transporter in the adrenal gland

The adrenal glands are key components of the mammalian endocrine system, helping maintain physiological homeostasis and the coordinated response to stress. Each adrenal gland has two morphologically and functionally distinct regions, the outer cortex and inner medulla. The cortex is organized into three concentric zones which secrete steroid hormones, including aldosterone and cortisol. Neural crest-derived chromaffin cells in the medulla are innervated by preganglionic sympathetic neurons and secrete catecholamines (epinephrine, norepinephrine) and neuropeptides into the bloodstream, thereby functioning as the neuroendocrine arm of the sympathetic nervous system. In this article we review serotonin (5-HT) and the serotonin transporter (SERT; SLC6A4) in the adrenal gland. In the adrenal cortex, 5-HT, primarily sourced from resident mast cells, acts as a paracrine signal to stimulate aldosterone and cortisol secretion through 5-HT4/5-HT7 receptors. Medullary chromaffin cells contain a small amount of 5-HT due to SERT-mediated uptake and express 5-HT1A receptors which inhibit secretion. The atypical mechanism of the 5-HT1A receptors and interaction with SERT fine tune this autocrine pathway to control stress-evoked catecholamine secretion. Receptor-independent signaling by SERT/intracellular 5-HT modulates the amount and kinetics of transmitter release from single vesicle fusion events. SERT might also influence stress-evoked upregulation of tyrosine hydroxylase transcription. Transient signaling via 5-HT3 receptors during embryonic development can limit the number of chromaffin cells found in the mature adrenal gland. Together, this emerging evidence suggests that the adrenal medulla is a peripheral hub for serotonergic control of the sympathoadrenal stress response.

Serotonin sensing by microglia conditions the proper development of neuronal circuits and of social and adaptive skills

The proper maturation of emotional and sensory circuits requires fine-tuning of serotonin (5-HT) level during early postnatal development. Consistently, dysfunctions of the serotonergic system have been associated with neurodevelopmental psychiatric diseases, including autism spectrum disorders (ASD). However, the mechanisms underlying the developmental effects of 5-HT remain partially unknown, one obstacle being the action of 5-HT on different cell types. Here, we focused on microglia, which play a role in brain wiring refinement, and we investigated whether the control of these cells by 5-HT is relevant for neurodevelopment and spontaneous behaviors in mice. Since the main 5-HT sensor in microglia is the 5-HT2B receptor subtype, we prevented 5-HT signaling specifically in microglia by conditional invalidation of the Htr2b gene in these cells. We observed that abrogating the serotonergic control of microglia during early postnatal development affects the phagolysosomal compartment of these cells and their proximity to dendritic spines and perturbs neuronal circuits maturation. Furthermore, this early ablation of microglial 5-HT2B receptors leads to adult hyperactivity in a novel environment and behavioral defects in sociability and flexibility. Importantly, we show that these behavioral alterations result from a developmental effect, since they are not observed when microglial Htr2b invalidation is induced later, at P30 onward. Thus, a primary alteration of 5-HT sensing in microglia, during a critical time window between birth and P30, is sufficient to impair social and flexibility skills. This link between 5-HT and microglia may explain the association between serotonergic dysfunctions and behavioral traits like impaired sociability and inadaptability to novelty, which are prominent in psychiatric disorders such as ASD.

Cardiovascular serotonergic system: Evolution, receptors, transporter, and function

The serotonergic system, serotonin (5HT), serotonin transporter (SERT), and serotonin receptors (5HT-x), is an evolutionarily ancient system that has clear physiological advantages to all life forms from bacteria to humans. This review focuses on the role of platelet/plasma serotonin and the cardiovascular system with minor references to its significant neurotransmitter function. Platelets transport and store virtually all plasma serotonin in dense granules. Stored serotonin is released from activated platelets and can bind to serotonin receptors on platelets and cellular components of the vascular wall to augment aggregation and induce vasoconstriction or vasodilation. The vascular endothelium is critical to the maintenance of cardiovascular homeostasis. While there are numerous ligands, neurological components, and baroreceptors that effect vascular tone it is proposed that serotonin and nitric oxide (an endothelium relaxing factor) are major players in the regulation of systemic blood pressure. Signals not fully defined, to date, that direct serotonin binding to one of the 15 identified 5HT receptors versus the transporter, and the role platelet/plasma serotonin plays in regulating hypertension within the cardiovascular system remain important issues to better understand many diseases and to develop new drugs. Also, expanded research of these pathways in lower life-forms may serve as important model systems to further our understanding of the evolution and mechanisms of action of serotonin.

Characterization of the spontaneous degenerative mitral valve disease in FVB mice

BACKGROUND

The development of new non-surgical treatments dedicated to mitral valve degeneration is limited by the absence of relevant spontaneous and rapidly progressing animal experimental models.

ANIMALS

We characterized the spontaneous mitral valve degeneration in two inbred FVB mouse strains compared to C57BL/6J and investigated a contribution of the serotonergic system.

METHODS

Males and females FVB/NJ and FVB/NRj were compared to the putative C57BL/6J control at 12, 16, 20 and 24 weeks of age. Body weight, systolic blood pressure, heart rate, urinary 5-hydroxyindoleacetic acid (5-HIAA), whole blood and plasma serotonin, tail bleeding time, blood cell count, plasma TGF-β1 and plasma natriuretic peptide concentrations were measured. Myocardium and mitral valves were characterized by histology. mRNA mitral expression of 5-HT2A and 5-HT2B receptors was measured in the anterior leaflet. Cardiac anatomy and function were assessed by echocardiography.

RESULTS

Compared to C57BL/6J, FVB mice strains did not significantly differ regarding body weight increase, arterial blood pressure and heart rate. A progressive augmentation of plasma pro-ANP was observed in FVB mice. Nevertheless, no cardiac hypertrophy or left-ventricular fibrosis were observed. Accordingly, plasma TGF-β1 was not different among the three strains. Conversely, FVB mice demonstrated a high prevalence of fibromyxoid highly cellularized and enriched in glycosaminoglycans lesions, inducing major mitral leaflets thickening without increase in length. The increased thickness was correlated with urinary 5-HIAA and blood platelet count. Whole blood serotonin concentration was similar in the two strains but, in FVB, a reduction of plasma serotonin was observed together with an increase of the bleeding time. Finally, echocardiography identified left atrial and left ventricular remodeling associated with thickening of both mitral leaflets and mitral insufficient in 30% of FVB mice but no systolic protrusion of mitral leaflets towards the atrium.

CONCLUSION

The FVB mouse strain is highly prone to spontaneous mitral myxomatous degeneration. A contribution of the peripheral serotonergic system is suggested.

Female serotonin transporter-knockout rat: A potential model of irritable bowel syndrome

Irritable bowel syndrome (IBS) is a common functional gastrointestinal disease. Although visceral hypersensitivity (VH) and disturbed gastrointestinal motility are typical pathophysiological features of IBS, the pathological mechanisms underlying this disease remain unclear. Serotonin system abnormalities are considered to play an important role in the pathomechanisms of IBS. Here, we hypothesize that similar alterations, including VH and colonic motility, induced by serotonin transporter (SERT) knockout result from altered serotonin signaling. We sought to determine the molecular mechanism underlying VH and colonic dysmotility induced by SERT knockout. We found that female SERT (slc6a4)-knockout (KO; ie, slc6a4^-/- ) rats exhibited lower pain pressure thresholds (PPTs) than wild-type (WT; ie, slc6a4^+/+ ) rats in response to colorectal distension (CRD). Significantly increased fecal pellet output and reduced concentration of serum tryptophan were observed in the female SERT KO rats. The concentrations of 5-hydroxytryptamine (5-HT) in platelet-rich plasma (PRP) and serum in SERT KO rats were lower than those in WT rats, but the numbers of enterochromaffin cells (ECs) and the concentrations of 5-HT in colon of SERT KO rats were higher than those of WT rats. Finally, increased expression levels of 5-HT_1B receptors, 5-HT_2C receptors, 5-HT_3A receptors, 5-HT_3B receptors, 5-HT₆ receptors, 5-HT₇ receptors, and glycosylated dopamine transporters (DATs) were found in the female SERT KO rats. We concluded that alterations in the serotonin system induced by the knockout of slc6a4 might result in VH and accelerated gastrointestinal motility in female SERT KO rats, which can be used as an animal model of IBS.

Genetic deletion of Rgs12 in mice affects serotonin transporter expression and function in vivo and ex vivo

BACKGROUND

Regulator of G protein Signaling (RGS) proteins inhibit G protein-coupled receptor (GPCR) signaling, including the signals that arise from neurotransmitter release. We have shown that RGS12 loss diminishes locomotor responses of C57BL/6J mice to dopamine transporter (DAT)-targeting psychostimulants. This diminution resulted from a brain region-specific upregulation of DAT expression and function in RGS12-null mice. This effect on DAT prompted us to investigate whether the serotonin transporter (SERT) exhibits similar alterations upon RGS12 loss in C57BL/6J mice.

AIMS

Does RGS12 loss affect (a) hyperlocomotion to the preferentially SERT-targeting psychostimulant 3,4-methylenedioxymethamphetamine (MDMA), (b) SERT expression and function in relevant brain regions, and/or (c) serotonergically modulated behaviors?

METHODS

Open-field and spontaneous home-cage locomotor activities were quantified. 5-HT, 5-HIAA, and SERT levels in brain-region homogenates, as well as SERT expression and function in brain-region tissue preparations, were measured using appropriate biochemical assays. Serotonergically modulated behaviors were assessed using forced swim and tail suspension paradigms, elevated plus and elevated zero maze tests, and social interaction assays.

RESULTS

RGS12-null mice displayed no hyperlocomotion to 10 mg/kg MDMA. There were brain region-specific alterations in SERT expression and function associated with RGS12 loss. Drug-naïve RGS12-null mice displayed increases in both anxiety-like and anti-depressive-like behaviors.

CONCLUSION

RGS12 is a critical modulator of serotonergic neurotransmission and serotonergically modulated behavior in mice; lack of hyperlocomotion to low dose MDMA in RGS12-null mice is related to an alteration of steady-state SERT expression and 5-HT uptake.

Depression and Cardiovascular Disease: The Viewpoint of Platelets

Depression is a major cause of morbidity and low quality of life among patients with cardiovascular disease (CVD), and it is now considered as an independent risk factor for major adverse cardiovascular events. Increasing evidence indicates not only that depression worsens the prognosis of cardiac events, but also that a cross-vulnerability between the two conditions occurs. Among the several mechanisms proposed to explain this interplay, platelet activation is the more attractive, seeing platelets as potential mirror of the brain function. In this review, we dissected the mechanisms linking depression and CVD highlighting the critical role of platelet behavior during depression as trigger of cardiovascular complication. In particular, we will discuss the relationship between depression and molecules involved in the CVD (e.g., catecholamines, adipokines, lipids, reactive oxygen species, and chemokines), emphasizing their impact on platelet activation and related mechanisms.

Demonstration of an action pathway in mouse platelets leading to prolongation of bleeding time by fluoxetine

Fluoxetine is one of SSRIs commonly used as first-line antidepressants. It also induces adverse effects, including bleeding events. This study clarified the bleeding effect of fluoxetine and explored the action cascade of this drug leading to a longer bleeding time. A total of 48 male adult mice were evenly distributed into four groups and given fluoxetine in saline at 0, 4, 8, or 16 mg/kg, for 14 days. On day 15, tail bleeding time of 6 mice/group was measured, and their blood samples were collected for analyses of relevant platelet functions. The remained mice were allowed to survive for another 14 days without fluoxetine, and subjected to the same analyses on day 29. A significant effect of fluoxetine was reveled on bleeding time (F _(3,20) = 16.842, P < 0.01) and intraplatelet serotonin (F _(3,20) = 90.967, P < 0.01). Moreover, fluoxetine effectively inhibited platelet aggregation (F_{(3, 20)} = 30.247, P < 0.01), decreased amount of GPIbα (F_{(3, 20)} = 23.855, P < 0.01), suppressed GPIIb/IIIa activation (F_{(3, 20)} = 89.441, P < 0.01), and lowered P-selectin (F_{(3, 20)} = 7.960, P < 0.01) on platelet surface. Negative correlations existed between bleeding time and the aforementioned four indices, whereas correlations between intraplatelet serotonin and the same indices were positive. All changes returned to same levels as Control group after fluoxetine withdrawal. These data suggest an action pathway of fluoxetine starting at binding to serotonin transporter, followed by decreased intraplatelet serotonin, increased GPIbα shedding, suppressed GPIIb/IIIa activation, and inhibited α-granule release, and concluding with prolonged bleeding time in mice.

Use of serotonin reuptake inhibitors is not associated with increased bleeding after CABG

OBJECTIVES

Selective serotonin reuptake inhibitors (SSRIs) and serotonin noradrenaline reuptake inhibitors (SNRIs) are the most commonly prescribed antidepressants worldwide. Studies suggest that SSRI/SNRIs can increase bleeding following different surgical procedures, including open heart surgery, but results are conflicting. The objective of this study was to analyse their effects on bleeding after coronary artery bypass grafting (CABG).

METHODS

Of 1237 patients that underwent CABG in Iceland in 2007-2016, 97 (7.8%) used SSRIs/SNRIs preoperatively and were compared to a reference group (n = 1140). Bleeding was assessed using 24-h chest-tube output, number of RBC units transfused and reoperation for bleeding. Thirty-day mortality rates and incidence of complications were also compared.

RESULTS

The two groups were comparable with respect to preoperative and operative variables, with the exception of BMI being significantly higher in the SSRI/SNRI group (30.2 vs. 28.3 kg/m², p < 0.001). No significant differences were observed between groups in 24-h chest-tube output [815 (SSRI/SNRI) vs. 877 ml (reference), p = 0.26], number of RBC units transfused (2.2 vs. 2.2, p = 0.99) or the rate of reoperation for bleeding (4.1% vs. 6.0%, p = 0.61). The incidences of complications and 30-day mortality rate were also similar.

CONCLUSIONS

Using three different criteria, preoperative use of SSRIs/SNRIs was not shown to increase bleeding after CABG. Furthermore, short-term complications as well as 30-day mortality rates did not differ from those of controls. Thus, temporary cessation of SSRI/SNRI treatment prior to CABG to decrease the risk of bleeding is unwarranted.

Platelets: emerging facilitators of cellular crosstalk in rheumatoid arthritis

Rheumatoid arthritis (RA) is an autoimmune disease in which a variety of circulating pro-inflammatory cells and dysregulated molecules are involved in disease aetiology and progression. Platelets are an important cellular element in the circulation that can bind several dysregulated molecules (such as collagen, thrombin and fibrinogen) that are present both in the synovium and the circulation of patients with RA. Platelets not only respond to dysregulated molecules in their environment but also transport and express their own inflammatory mediators, and serve as regulators at the boundary between haemostasis and immunity. Activated platelets also produce microparticles, which further convey signalling molecules and receptors to the synovium and circulation, thereby positioning these platelet-derived particles as strategic regulators of inflammation. These diverse functions come together to make platelets facilitators of cellular crosstalk in RA. Thus, the receptor functions, ligand binding potential and dysregulated signalling pathways in platelets are becoming increasingly important for treatment in RA. This Review aims to highlight the role of platelets in RA and the need to closely examine platelets as health indicators when designing effective pharmaceutical targets in this disease.

Whole Blood Serotonin Levels and Platelet 5-HT2A Binding in Autism Spectrum Disorder

Elevated whole blood serotonin (WB5-HT) is a well-replicated biomarker in autism spectrum disorder (ASD). Decreased platelet serotonin receptor 5-HT_2A binding has been reported in ASD. WB5-HT levels and platelet 5-HT_2A specific binding were obtained from 110 individuals with ASD and 18 controls. Individuals with ASD had significantly higher WB5-HT levels than controls. There was no difference in the platelet 5-HT_2A specific binding between groups. Multiple regression analyses revealed that platelet 5-HT_2A binding significantly predicted WB5-HT in the control sample but not in the ASD sample. These results indicate that the relationship between WB5-HT and platelet 5-HT_2A binding differs depending on ASD diagnosis, suggesting differences in platelet 5-HT system regulation in ASD.

Micromolar concentrations of citalopram or escitalopram inhibit glycoprotein VI-mediated and integrin αIIbβ3-mediated signaling in human platelets

Collagen and convulxin induce platelet aggregation through glycoprotein VI (GPVI)-FcRγ-Syk signaling pathway. In addition, fibrinogen induces platelet activation through integrin αIIbβ3-FcγRIIa-Syk signaling pathway. We previously reported that high concentrations of selective serotonin reuptake inhibitors (SSRI) reduce platelet aggregation induced by collagen. We further investigated the effects of SSRI on GPVI- and αIIbβ3-mediated signaling pathway. Citalopram and escitalopram, two relatively pure SSRI, were used in this study. Both citalopram and escitalopram concentration-dependently inhibited convulxin-induced platelet aggregation, serotonin (5-HT) release and the activation of αIIbβ3. 5-HT concentration in washed platelets was unchanged after short-term treatment with citalopram. The additional 5-HT failed to fully rescue the inhibitory effect of citalopram on convulxin-induced aggregation. Convulxin-induced phosphorylation of Syk, LAT, and Akt was inhibited by citalopram and escitalopram. Citalopram inhibited the interaction between FcRγ and Syk, whereas the phosphorylation of FcRγ in response to convulxin remained unaltered. Further, citalopram inhibited the increase of the interaction between serotonin transporter and Syk induced by convulxin. In the presence of Mn²⁺, escitalopram inhibited the formation of lamellipodia on immobilized fibrinogen. Escitalopram did not influence the binding of fibrinogen to platelets. It inhibited the phosphorylation of Syk and PAK triggered by the adhesion on fibrinogen. Our data demonstrate that micromolar concentrations of citalopram and escitalopram inhibit GPVI- and αIIbβ3-mediated platelet functions. The mechanism of the inhibitory effect of citalopram or escitalopram is not the influence on the activation of GPVI or the interaction between fibrinogen and αIIbβ3, but the interaction between Syk and its upstream molecules.

Oral reserpine administration in horses results in low plasma concentrations that alter platelet biology

BACKGROUND

Reserpine is a popular drug in the equine industry for long-term tranquilisation. Clinical observations revealed that blood from horses receiving oral reserpine was hypercoagulable. No studies have documented the pharmacokinetics of orally administered reserpine nor the effects of reserpine on platelets in horses.

OBJECTIVES

To evaluate the pharmacokinetics of oral reserpine in horses and the effects of clinically relevant concentrations of reserpine on platelet functionality in vitro.

STUDY DESIGN

Experimental controlled study.

METHODS

The pharmacokinetics of oral reserpine (2.5 mg/horse, once) were determined in six healthy adult horses. Plasma samples were collected and concentrations of reserpine were determined by UPLC-MS/MS. Using this data, the in vitro effects of reserpine on platelets were examined. Aggregation, adhesion and releasate assays for serotonin and thromboxane B₂ were performed on platelets exposed to varying concentrations of reserpine (0.01-10 ng/mL), aspirin (negative control) and saline (unexposed control).

RESULTS

Oral reserpine administration demonstrated low plasma concentrations with a C_max of 0.2 ± 0.06 ng/mL and a prolonged half-life of 23.6 ± 6.24 h. Simulations over a dose range of 2-8 μg/kg predicted C_max at steady state between 0.06-0.9 ng/mL. Platelets exposed to these reserpine concentrations in vitro displayed increased aggregation and adhesion compared to unexposed or aspirin-exposed platelets as well as compared to higher concentrations of reserpine. These functional changes correlated with lower concentrations of serotonin and higher concentrations of thromboxane B₂ in the platelet suspension supernatant.

MAIN LIMITATIONS

This study used a small number of horses and only in vitro platelet experiments.

CONCLUSIONS

Oral reserpine demonstrates low plasma concentrations and a prolonged half-life in horses. At these concentrations, reserpine causes significant changes in platelet function, most likely due to serotonin release and re-uptake which primes platelets for activation and thromboxane B₂ release. These findings suggest that clinicians should harvest blood for biological processing prior to the onset of reserpine administration.

Adrenal serotonin derives from accumulation by the antidepressant-sensitive serotonin transporter

Adrenal chromaffin cells comprise the neuroendocrine arm of the sympathetic nervous system and secrete catecholamines to coordinate the appropriate stress response. Deletion of the serotonin (5-HT) transporter (SERT) gene in mice (SERT^-/- mice) or pharmacological block of SERT function in rodents and humans augments this sympathoadrenal stress response (epinephrine secretion). The prevailing assumption is that loss of CNS SERT alters central drive to the peripheral sympathetic nervous system. Adrenal chromaffin cells also prominently express SERT where it might coordinate accumulation of 5-HT for reuse in the autocrine control of stress-evoked catecholamine secretion. To help test this hypothesis, we have generated a novel mouse model with selective excision of SERT in the peripheral sympathetic nervous system (SERT^ΔTH), generated by crossing floxed SERT mice with tyrosine hydroxylase Cre driver mice. SERT expression, assessed by western blot, was abolished in the adrenal gland but not perturbed in the CNS of SERT^ΔTH mice. SERT-mediated [³H] 5-HT uptake was unaltered in midbrain, hindbrain, and spinal cord synaptosomes, confirming transporter function was intact in the CNS. Endogenous midbrain and whole blood 5-HT homeostasis was unperturbed in SERT^ΔTH mice, contrasting with the depleted 5-HT content in SERT^-/- mice. Selective SERT excision reduced adrenal gland 5-HT content by ≈ 50% in SERT^ΔTH mice but had no effect on adrenal catecholamine content. This novel model confirms that SERT expressed in adrenal chromaffin cells is essential for maintaining wild-type levels of 5-HT and provides a powerful tool to help dissect the role of SERT in the sympathetic stress response.

Citalopram inhibits platelet function independently of SERT-mediated 5-HT transport

Citalopram prevents serotonin (5-HT) uptake into platelets by blocking the serotonin reuptake transporter (SERT). Although some clinical data suggest that selective serotonin reuptake inhibitors (SSRIs) may affect haemostasis and thrombosis, these poorly-characterised effects are not well understood mechanistically and useful in vitro data is limited. We sought to determine whether the inhibitory effects of citalopram on platelets are mediated via its pharmacological inhibition of 5-HT transport. We quantified the inhibitory potency of (RS)-, (R)- and (S)-citalopram on platelet function. If SERT blockade is the primary mechanism for citalopram-mediated platelet inhibition, these potencies should show quantitative congruence with inhibition of 5-HT uptake. Our data show that citalopram inhibits platelet aggregation, adhesion and thromboxane production with no difference in potency between (R)- and (S)-isomers. By contrast, citalopram had a eudysmic ratio of approximately 17 (S > R) for SERT blockade. Furthermore, nanomolar concentrations of citalopram inhibited 5-HT uptake into platelets but had no effect on other platelet functions, which were inhibited by micromolar concentrations. Our data indicate that citalopram-induced inhibition of platelets in vitro is not mediated by blockade of 5-HT transport. This raises a new question for future investigation: by what mechanism(s) does citalopram inhibit platelets?