Characterization of rat platelet serotonin receptors with tryptamine agonists and the antagonists: ketanserin and SCH 23390

Serotonin Receptors in Myocardial Infarction: Friend or Foe?

Acute myocardial infarction (AMI) is one of the leading causes of death worldwide and treatment costs pose a major burden on the global health care system. Despite the variety of treatment options, individual recovery can be still poor and the mortality rate, especially in the first few years after the event, remains high. Therefore, intense research is currently focused on identifying novel target molecules to improve the outcome following AMI. One of the potentially interesting targets is the serotonergic system (5-HT system), not at least because of its connection to mental disorders. It is known that patients suffering from AMI have an increased risk of developing depression and vice versa. This implicates that the 5-HT system can be affected in response to AMI and might thus represent a target structure for patients' treatment. This review aims to highlight the importance of the 5-HT system after AMI by describing the role of individual serotonin receptors (5-HTR) in the regulation of physiological and pathophysiological responses. It particularly focuses on the signaling pathways of the serotonin receptors 1, 2, 4, and 7, which are expressed in the cardiovascular system, during disease onset, and the following remodeling process. This overview also emphasizes the importance of the 5-HT system in AMI etiology and highlights 5-HTRs as potential treatment targets.

Pharmacometabolomics reveals that serotonin is implicated in aspirin response variability

While aspirin is generally effective for prevention of cardiovascular disease, considerable variation in drug response exists, resulting in some individuals displaying high on-treatment platelet reactivity. We used pharmacometabolomics to define pathways implicated in variation of response to treatment. We profiled serum samples from healthy subjects pre- and postaspirin (14 days, 81 mg/day) using mass spectrometry. We established a strong signature of aspirin exposure independent of response (15/34 metabolites changed). In our discovery (N = 80) and replication (N = 125) cohorts, higher serotonin levels pre- and postaspirin correlated with high, postaspirin, collagen-induced platelet aggregation. In a third cohort, platelets from subjects with the highest levels of serotonin preaspirin retained higher reactivity after incubation with aspirin than platelets from subjects with the lowest serotonin levels preaspirin (72 ± 8 vs. 61 ± 11%, P = 0.02, N = 20). Finally, ex vivo, serotonin strongly increased platelet reactivity after platelet incubation with aspirin (+20%, P = 4.9 × 10⁻⁴, N = 12). These results suggest that serotonin is implicated in aspirin response variability.

Serotonin stimulates platelet receptor shedding by tumor necrosis factor-alpha-converting enzyme (ADAM17)

BACKGROUND

Peripheral serotonin (5-hydroxytryptamine, 5-HT) is transported by platelets and released upon stimulation. In the platelet cytoplasm, 5-HT is transamidated to small GTPases, promoting alpha-granule release and primary hemostasis.

OBJECTIVE

We hypothesized that 5-HT could also stimulate platelet receptor shedding after binding to the membrane 5-HT receptor (5-HT2AR).

METHODS

Western blot and flow cytometry were used to determine levels of the adhesion receptor glycoprotein (GP)Ibalpha on platelets or its shed fragment glycocalicin in plasma and serum from wild-type mice, Tph1(-/-) mice lacking peripheral 5-HT, and mice lacking functional tumor necrosis factor-alpha-converting enzyme (TACE, ADAM17). Flow chamber experiments and intravital microscopy were used to examine the adhesive properties of platelets after stimulation of 5-HT2AR.

RESULTS

Glycocalicin was significantly reduced in Tph1(-/-) plasma and serum. In isolated platelets, 5-HT induced shedding of GPIbalpha, which was increased to 60% when 5-HT uptake was inhibited by the selective serotonin reuptake inhibitor fluoxetine. Specific 5-HT2AR agonism and antagonism suggested activation of this receptor. The shedding could not be induced in TACE(DeltaZn/DeltaZn) platelets, suggesting that activated TACE mediated the shedding of GPIbalpha. Intracellular signaling involved phosphorylation of p38 mitogen-activated protein kinase rather than G-protein signaling. 5-HT2AR stimulation decreased platelet adhesion to collagen-bound von Willebrand factor under arterial shear (1500 s(-1)) and incorporation into FeCl3-induced thrombi in mesenteric arterioles.

CONCLUSIONS

Stimulation of 5-HT2AR on platelets induces TACE-mediated shedding of GPIbalpha, the key adhesion molecule under high shear conditions. Our observations demonstrate a new pathway through which 5-HT could modulate cardiovascular disease.

Serotonylation of small GTPases is a signal transduction pathway that triggers platelet alpha-granule release

Serotonin is a neurotransmitter in the central nervous system. In the periphery, serotonin functions as a ubiquitous hormone involved in vasoconstriction and platelet function. Serotonin is synthesized independently in peripheral tissues and neurons by two different rate-limiting tryptophan hydroxylase (TPH) isoenzymes. Here, we show that mice selectively deficient in peripheral TPH and serotonin exhibit impaired hemostasis, resulting in a reduced risk of thrombosis and thromboembolism, although the ultrastructure of the platelets is not affected. While the aggregation of serotonin-deficient platelets in vitro is apparently normal, their adhesion in vivo is reduced due to a blunted secretion of adhesive alpha-granular proteins. In elucidating the mechanism further, we demonstrate that serotonin is transamidated to small GTPases by transglutaminases during activation and aggregation of platelets, rendering these GTPases constitutively active. Our data provides evidence for a receptor-independent signaling mechanism, termed herein as "serotonylation," which leads to alpha-granule exocytosis from platelets.

Influence of a selective 5HT1-receptor agonist GR43175 on platelet responsiveness

The possible interaction of sumatriptan, a selective 5HT1-receptor agonist, with platelet responsiveness has been investigated. Stimulation of platelet rich plasma with sumatriptan (1-100 microM) did not induce shape change, aggregation or modification of intraplatelet cytosolic calcium levels. Total inhibition of aggregation induced by 20 microM 5HT was observed in platelets preincubated for 20 min with 100 microM sumatriptan. In the same model, platelet stimulation with 4 microM adenosine 5'-diphosphate (ADP), concentration known to induce an irreversible single-phase curve, determined a decrease of aggregatory response. Concentrations from 1 microM to 50 microM of sumatriptan did not influence the aggregatory response induced by 5HT and ADP. These effects appear not to be determined by modifications of platelet calcium homeostasis. The possibility to modulate platelet responsiveness by sumatriptan offers a further approach for evaluating the probable link between platelet behaviour and pathophysiology of migraine.

Role of alpha 1-adrenoceptors and 5-HT2 receptors in serotonin-induced contraction of rat prostate: autoradiographical and functional studies

Urinary obstruction from benign prostatic hyperplasia is a common clinical problem possibly associated with excessive prostatic constriction around the urethra. These studies compared adrenergic and serotonergic functional activity to specific alpha 1 and serotonin (5-hydroxytryptamine; 5-HT) binding sites in the rat prostate. Isolated, left ventral lobes of the rat prostate were removed and examined for in vitro contraction. Norepinephrine-induced contraction of the rat prostate was competitively blocked by prazosin with an apparent antagonist dissociation constant (pKB) of 8.13. 5-HT also contracted the rat prostate. However, in the presence of prazosin, maximum 5-HT contraction was reduced by half suggesting that high concentrations of 5-HT can activate alpha 1 receptors in the prostate. The concentration-response curve to 5-HT in the presence of 1 microM prazosin was competitively inhibited by the 5-HT2 receptor antagonist LY53857 (6-methyl-1-(1-methylethyl)ergoline-8-carboxylic acid 2-hydroxyl-1-methylpropylester (Z)-2-butenedioate (1:1)) (pKB = 9.02). Autoradiographic studies with [125I]LSD (2-iodo-lysergic acid diethylamide) documented the presence of 5-HT2 receptors since significant displacement of the radioligand occurred with 5-HT and LY53857, but not with prazosin. The alpha 1-adrenoceptor ligand [125I]HEAT ([beta-(4-hydroxy-3-iodophenyl)ethylaminomethyl]-tetralone) confirmed the presence of alpha 1-adrenoceptors in the rat prostate since significant displacement of the radioligand occurred with prazosin, but not 5-HT or LY53857. The inability of prazosin to displace [125I]LSD and the inability of 5-HT to displace [125I]HEAT suggest that 5-HT cannot directly interact with alpha 1-adrenoceptors in the prostate.(ABSTRACT TRUNCATED AT 250 WORDS)