On the presence of serotonin in mammalian cardiomyocytes

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.

Monoamine Oxidase A Contributes to Serotonin-But Not Norepinephrine-Dependent Damage of Rat Ventricular Myocytes

Serotonin effects on cardiac hypertrophy, senescence, and failure are dependent either on activation of specific receptors or serotonin uptake and serotonin degradation by monoamine oxidases (MAOs). Receptor-dependent effects are specific for serotonin, but MAO-dependent effects are nonspecific as MAOs also metabolize other substrates such as catecholamines. Our study evaluates the role of MAO-A in serotonin- and norepinephrine-dependent cell damage. Experiments were performed in vivo to study the regulation of MAOA and MAOB expression and in vitro on isolated cultured adult rat ventricular cardiomyocytes (cultured for 24 h) to study the function of MAO-A. MAOA but not MAOB expression increased in maladaptive hypertrophic stages. Serotonin and norepinephrine induced morphologic cell damage (loss of rod-shaped cell structure). However, MAO-A inhibition suppressed serotonin-dependent but not norepinephrine-dependent damages. Serotonin but not norepinephrine caused a reduction in cell shortening in nondamaged cells. Serotonin induced mitochondria-dependent oxidative stress. In vivo, MAOA was induced during aging and hypertension but the expression of the corresponding serotonin uptake receptor (SLC6A4) was reduced and enzymes that reduce either oxidative stress (CAT) or accumulation of 5-hydroxyindolacetaldehyde (ALDH2) were induced. In summary, the data show that MAO-A potentially affects cardiomyocytes' function but that serotonin is not necessarily the native substrate.

Importance of Mitochondria in Cardiac Pathologies: Focus on Uncoupling Proteins and Monoamine Oxidases

On the one hand, reactive oxygen species (ROS) are involved in the onset and progression of a wide array of diseases. On the other hand, these are a part of signaling pathways related to cell metabolism, growth and survival. While ROS are produced at various cellular sites, in cardiomyocytes the largest amount of ROS is generated by mitochondria. Apart from the electron transport chain and various other proteins, uncoupling protein (UCP) and monoamine oxidases (MAO) have been proposed to modify mitochondrial ROS formation. Here, we review the recent information on UCP and MAO in cardiac injuries induced by ischemia-reperfusion (I/R) as well as protection from I/R and heart failure secondary to I/R injury or pressure overload. The current data in the literature suggest that I/R will preferentially upregulate UCP2 in cardiac tissue but not UCP3. Studies addressing the consequences of such induction are currently inconclusive because the precise function of UCP2 in cardiac tissue is not well understood, and tissue- and species-specific aspects complicate the situation. In general, UCP2 may reduce oxidative stress by mild uncoupling and both UCP2 and UCP3 affect substrate utilization in cardiac tissue, thereby modifying post-ischemic remodeling. MAOs are important for the physiological regulation of substrate concentrations. Upon increased expression and or activity of MAOs, however, the increased production of ROS and reactive aldehydes contribute to cardiac alterations such as hypertrophy, inflammation, irreversible cardiomyocyte injury, and failure.

Cardiac Roles of Serotonin (5-HT) and 5-HT-Receptors in Health and Disease

Serotonin acts solely via 5-HT₄-receptors to control human cardiac contractile function. The effects of serotonin via 5-HT₄-receptors lead to positive inotropic and chronotropic effects, as well as arrhythmias, in the human heart. In addition, 5-HT₄-receptors may play a role in sepsis, ischaemia, and reperfusion. These presumptive effects of 5-HT₄-receptors are the focus of the present review. We also discuss the formation and inactivation of serotonin in the body, namely, in the heart. We identify cardiovascular diseases where serotonin might play a causative or additional role. We address the mechanisms which 5-HT₄-receptors can use for cardiac signal transduction and their possible roles in cardiac diseases. We define areas where further research in this regard should be directed in the future, and identify animal models that might be generated to this end. Finally, we discuss in what regard 5-HT₄-receptor agonists or antagonists might be useful drugs that could enter clinical practice. Serotonin has been the target of many studies for decades; thus, we found it timely to summarise our current knowledge here.

The serotonergic system dysfunction in diabetes mellitus

Most peripheral serotonin (5-HT) is synthesized in enterochromaffin cells, and most circulating 5-HT is stored in platelets. As a monoamine, 5-HT has several functions in various non-neuronal and neuronal systems. In the central nervous system, it functions as a neurotransmitter to modulate feeding behavior and mood. Numerous clinical trials have focused on increasing 5-HT activation in the central nervous system, including those involving anti-obesity drugs currently in the market, although severe side effects on peripheral system can lead to the withdrawal of certain drugs. Recent studies have revealed that both the peripheral and central serotonergic systems play a vital role in diabetes and its complications. This review summarizes the roles of the serotonergic system in blood glucose regulation, diabetic macroangiopathy, diabetic peripheral neuropathy, and diabetic encephalopathy, indicating its potential clinical significance as a therapeutic target for the treatment of diabetes and its complications.

Functional interaction of H2-receptors and 5HT4-receptors in atrial tissues isolated from double transgenic mice and from human patients

In the past, we generated transgenic mice that overexpress the human histamine 2 (H₂)-receptor (H₂-TG) or that overexpress the human serotonin 4 (5-HT₄)-receptor (5-HT₄-TG) in the heart. Here, we crossbred these lines of mice to generate double transgenic mice that overexpress both receptors (DT). This was done to study a conceivable interaction between these receptors in the mouse heart as a model for the human heart. When in left atria, initially, force of contraction was elevated maximally with 1 µM serotonin, and subsequently, histamine was cumulatively applied; a biphasic effect of histamine was noted: the force of contraction initially decreased, maximally at 10 nM histamine, and thereafter, the force of contraction increased again at 1 µM histamine. Notably, functional interaction between 5-HT and histamine was also identified in isolated electrically stimulated trabeculae carneae from human right atrium (obtained during cardiac surgery). These functional and biochemical data together are consistent with a joint overexpression of inotropically active H₂-receptors and 5-HT₄-receptors in the same mouse heart. We also describe an antagonistic interaction on the force of contraction of both receptors in the mouse atrium (DT) and in the human atrial muscle strips. We speculate that via this interaction, histamine might act as a "brake" on the cardiac actions of 5-HT via inhibitory GTP-binding proteins acting on the activity of adenylyl cyclase.

Carrier-mediated serotonin efflux induced by pharmacological anoxia in the rat heart in vivo

Serotonin (5-HT) accumulates in the heart during myocardial ischaemia and induces deleterious effects on the cardiomyocytes. We aimed to investigate whether carrier-mediated 5-HT efflux contributed to the increase in interstitial 5-HT level during ischaemia. Using microdialysis technique applied to the heart of anaesthetised Wistar rats, myocardial interstitial concentration of 5-HT was measured by electro-chemical detection coupled with high-performance liquid chromatography (HPLC-ECD) while simultaneously various pharmacological agents, which create a similar condition to ischaemia, were locally administered by reverse-microdialysis. Sodium cyanide-induced chemical anoxia increased dialysate 5-HT concentration. A similar increase in dialysate 5-HT concentration was induced by ouabain, an inhibitor of sodium-potassium ATPase and reserpine, an inhibitor of vesicular monoamine transporter. Fluoxetine, a selective serotonin reuptake inhibitor raised the baseline level of 5-HT, and neither sodium cyanide nor the combination of ouabain and reserpine induced further increase in 5-HT in the presence of fluoxetine. The results indicate that reverse transport of 5-HT via SERT, which is caused by an impaired ion gradient, contributes to the rise in interstitial level of 5-HT during ischaemia suggesting carrier-mediated 5-HT efflux occurs in the heart in vivo.

Influence of Serotonin 5-HT4 Receptors on Responses to Cardiac Stressors in Transgenic Mouse Models

The current study aimed to deepen our knowledge on the role of cardiac 5-HT4 receptors under pathophysiological conditions. To this end, we used transgenic (TG) mice that overexpressed human 5-HT4a receptors solely in cardiac myocytes (5-HT4-TG mice) and their wild-type (WT) littermates that do not have functional cardiac 5-HT4 receptors as controls. We found that an inflammation induced by lipopolysaccharide (LPS) was detrimental to cardiac function in both 5-HT4-TG and WT mice. In a hypoxia model, isolated left atrial preparations from the 5-HT4-TG mice went into contracture faster during hypoxia and recovered slower following hypoxia than the WT mice. Similarly, using isolated perfused hearts, 5-HT4-TG mice hearts were more susceptible to ischemia compared to WT hearts. To study the influence of 5-HT4 receptors on cardiac hypertrophy, 5-HT4-TG mice were crossbred with TG mice overexpressing the catalytic subunit of PP2A in cardiac myocytes (PP2A-TG mice, a model for genetically induced hypertrophy). The cardiac contractility, determined by echocardiography, of the resulting double transgenic mice was attenuated like in the mono-transgenic PP2A-TG and, therefore, largely determined by the overexpression of PP2A. In summary, depending on the kind of stress put upon the animal or isolated tissue, 5-HT4 receptor overexpression could be either neutral (genetically induced hypertrophy, sepsis) or possibly detrimental (hypoxia, ischemia) for mechanical function. We suggest that depending on the underlying pathology, the activation or blockade of 5-HT4 receptors might offer novel drug therapy options in patients.

Histamine can be Formed and Degraded in the Human and Mouse Heart

Histamine is metabolized by several enzymes in vitro and in vivo. The relevance of this metabolism in the mammalian heart in vivo is unclear. However, histamine can exert positive inotropic effects (PIE) and positive chronotropic effects (PCE) in humans via H₂-histamine receptors. In transgenic mice (H₂-TG) that overexpress the human H₂ receptor in cardiomyocytes but not in wild-type littermate mice (WT), histamine induced PIE and PCE in isolated left or right atrial preparations. These H₂-TG were used to investigate the putative relevance of histamine degrading enzymes in the mammalian heart. Histidine, the precursor of histamine, increased force of contraction (FOC) in human atrial preparations. Moreover, histamine increased the phosphorylation state of phospholamban in human atrium. Here, we could detect histidine decarboxylase (HDC) and histamine itself in cardiomyocytes of mouse hearts. Moreover, our data indicate that histamine is subject to degradation in the mammalian heart. Inhibition of the histamine metabolizing enzymes diamine oxidase (DAO) and monoamine oxidase (MAO) shifted the concentration response curves for the PIE in H₂-TG atria to the left. Moreover, activity of histamine metabolizing enzymes was present in mouse cardiac samples as well as in human atrial samples. Thus, drugs used for other indication (e.g. antidepressants) can alter histamine levels in the heart. Our results deepen our understanding of the physiological role of histamine in the mouse and human heart. Our findings might be clinically relevant because we show enzyme targets for drugs to modify the beating rate and force of the human heart.

Role of serotonin in regulation of pancreatic and mesenteric arterial function in diabetic mice

The aim of this study was to investigate the reaction of pancreatic and mesenteric artery to 5-hydroxytryptamine (5-HT, serotonin) and the mechanism of nitric oxide in diabetes. Diabetic mice were induced by streptozotocin through intraperitoneal injection. The vascular tension of the pancreatic, mesenteric and brain basilar arteries in diabetic and control mice were measured by myograph in the applications of angiotensin II, 5-HT, 5-HT_2A receptor agonist 2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI), 5-HT_1B/1D receptor agonist sumatriptan, 5-HT_2B receptor agonist BW723C86, 5-HT_1D receptor antagonist Palonosetron and 5-HT₂ receptor antagonist Sarpogrelate. The effect of 5-HT on arteries pretreated with L-NAME and sodium nitroprusside (SNP) on arteries pretreated with norepinephrine were measured. The mRNA expressions of eNOS, 5-HT_1B, 5-HT_1D, 5-HT_2A and 5-HT_2B in pancreatic and mesenteric arteries were measured by Real-time PCR. The concentration of 5-HT in plasma and eNOS in pancreatic and mesenteric arteries were tested. Our results showed that the tension of pancreatic and mesenteric arteries in diabetic mice impaired to 5-HT, but not Ang II, and to DOI and sumatriptan, but normalized by incubation with L-NAME. Pancreatic and mesenteric arteries showed no differences to SNP after pretreated with NE between diabetic and control mice. The mRNA of eNOS and 5-HT receptors in pancreatic and mesenteric artery showed no difference between control and diabetic mice. We conclude that the effect of 5-HT on the tension of pancreatic and mesenteric arteries decrease in diabetic mice. It may due to the decreased activity of 5-HT receptors and the activation of eNOS, which causes nitric oxide to release more and makes the tension of vessels decreased.

Monoamine oxidases in age-associated diseases: New perspectives for old enzymes

Population aging is one of the most significant social changes of the twenty-first century. This increase in longevity is associated with a higher prevalence of chronic diseases, further rising healthcare costs. At the molecular level, cellular senescence has been identified as a major process in age-associated diseases, as accumulation of senescent cells with aging leads to progressive organ dysfunction. Of particular importance, mitochondrial oxidative stress and consequent organelle alterations have been pointed out as key players in the aging process, by both inducing and maintaining cellular senescence. Monoamine oxidases (MAOs), a class of enzymes that catalyze the degradation of catecholamines and biogenic amines, have been increasingly recognized as major producers of mitochondrial ROS. Although well-known in the brain, evidence showing that MAOs are also expressed in a variety of peripheral organs stimulated a growing interest in the extra-cerebral roles of these enzymes. Besides, the fact that MAO-A and/or MAO-B are frequently upregulated in aged or dysfunctional organs has uncovered new perspectives on their roles in pathological aging. In this review, we will give an overview of the major results on the regulation and function of MAOs in aging and age-related diseases, paying a special attention to the mechanisms linked to the increased degradation of MAO substrates or related to MAO-dependent ROS formation.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Deoxycholic acid activates colonic afferent nerves via 5-HT3 receptor-dependent and -independent mechanisms

Increased bile acids in the colon can evoke increased epithelial secretion resulting in diarrhea, but little is known about whether colonic bile acids contribute to abdominal pain. This study aimed to investigate the mechanisms underlying activation of colonic extrinsic afferent nerves and their neuronal cell bodies by a major secondary bile acid, deoxycholic acid (DCA). All experiments were performed on male C57BL/6 mice. Afferent sensitivity was evaluated using in vitro extracellular recordings from mesenteric nerves in the proximal colon (innervated by vagal and spinal afferents) and distal colon (spinal afferents only). Neuronal excitability of cultured dorsal root ganglion (DRG) and nodose ganglion (NG) neurons was examined with perforated patch clamp. Colonic 5-HT release was assessed using ELISA, and 5-HT immunoreactive enterochromaffin (EC) cells were quantified. Intraluminal DCA increased afferent nerve firing rate concentration dependently in both proximal and distal colon. This DCA-elicited increase was significantly inhibited by a 5-HT₃ antagonist in the proximal colon but not in the distal colon, which may be in part due to lower 5-HT immunoreactive EC cell density and lower 5-HT levels in the distal colon following DCA stimulation. DCA increased the excitability of DRG neurons, whereas it decreased the excitability of NG neurons. DCA potentiated mechanosensitivity of high-threshold spinal afferents independent of 5-HT release. Together, this study suggests that DCA can excite colonic afferents via direct and indirect mechanisms but the predominant mechanism may differ between vagal and spinal afferents. Furthermore, DCA increased mechanosensitivity of high-threshold spinal afferents and may be a mechanism of visceral hypersensitivity.NEW & NOTEWORTHY Deoxycholic acid (DCA) directly excites spinal afferents and, to a lesser extent, indirectly via mucosal 5-HT release. DCA potentiates mechanosensitivity of high-threshold spinal afferents independent of 5-HT release. DCA increases vagal afferent firing in proximal colon via 5-HT release but directly inhibits the excitability of their cell bodies.

Cardiovascular effects of cisapride and prucalopride on human 5-HT4 receptors in transgenic mice

Cisapride and prucalopride act as 5-HT₄ receptor agonists. As a part of our ongoing effort to study the utility of a transgenic (TG) mouse model overexpressing cardiac 5-HT₄ receptors, we assessed the extent to which we could recapitulate cisapride and prucalopride agonists. Contractile studies were performed using isolated left and right atrial preparations of TG mice showing cardiac-specific human 5-HT_4a receptor expression and those of their wild-type (WT) littermates. 5-Hydroxytryptamine (5-HT), cisapride, and prucalopride exerted concentration-dependent positive inotropic effects in the left atrial preparations of TG mice. Moreover, 5-HT induced concentration-dependent arrhythmias in the right atrial preparations of TG mice starting from 10-nM concentration. However, cisapride induced arrhythmias not only in the right atrial preparations of TG mice but also in the right atrial preparations of WT mice. For instance, 10 μM cisapride induced arrhythmias in the right atrial preparations of TG and WT mice to the same extent. Prucalopride did not exert concentration-dependent proarrhythmic effects in the isolated atrial preparations (left or right, WT or TG). Furthermore, cisapride and prucalopride increased the contractility and beating rate in vivo in TG mice, as assessed by performing echocardiography and surface electrocardiography. In summary, our results indicate that cisapride and prucalopride increase contractility and beating rate in the isolated atrial preparations of TG mice or in intact TG mice. Moreover, 5-HT induced arrhythmias in the isolated right atrial preparations of TG mice in a concentration-dependent manner. Furthermore, cisapride induced arrhythmias in the isolated right atrial preparations of both TG and WT mice. In contrast, prucalopride did not induce arrhythmias in the atrial preparations (left or right) of both WT and TG mice. We suggest that the present TG mouse model might be useful to predict at least some important cardiac effects of 5-HT₄ receptor agonists in the human heart.

Pharmacological and physiological assessment of serotonin formation and degradation in isolated preparations from mouse and human hearts

Using transgenic (TG) mice that overexpress the human serotonin (5-HT)4a receptor specifically in cardiomyocytes, we wanted to know whether 5-HT can be formed and degraded in the mammalian heart and whether this can likewise lead to inotropic and chronotropic effects in this TG model. We noted that the 5-HT precursor 5-hydroxy-tryptophan (5-HTP) can exert inotropic and chronotropic effects in cardiac preparations from TG mice but not from wild-type (WT) mice; similar results were found in human atrial preparations as well as in intact TG animals using echocardiography. Moreover, by immunohistochemistry we could detect 5-HT metabolizing enzymes and 5-HT transporters in mouse hearts as well as in human atria. Hence, in the presence of an inhibitor of aromatic l-amino acid decarboxylase, the positive inotropic effects of 5-HTP were absent in TG and isolated human atrial preparations, and, moreover, inhibitors of enzymes involved in 5-HT degradation enhanced the efficacy of 5-HT in TG atria. A releaser of neurotransmitters increased inotropy in the isolated TG atrium, and this effect could be blocked by a 5-HT4a receptor antagonist. Fluoxetine, an inhibitor of 5-HT uptake, elevated the potency of 5-HT to increase contractility in the TG atrium. In addition, inhibitors of organic cation and monoamine transporters apparently reduced the positive inotropic potency of 5-HT in the TG atrium. Hence, we tentatively conclude that a local production and degradation of 5-HT in the mammalian heart and more specifically in mammalian myocytes probably occurs. Conceivably, this formation of 5-HT and possibly impaired degradation may be clinically relevant in cases of unexplained tachycardia and other arrhythmias.

NEW & NOTEWORTHY The present work suggests that inotropically active serotonin (5-HT) can be formed in the mouse and human heart and probably by cardiomyocytes themselves. Moreover, active degradation of 5-HT seems to occur in the mammalian heart. These findings may again increase the interest of researchers for cardiac effects of 5-HT.

Evidence for a functional role of calsequestrin 2 in mouse atrium

AIM

Several genetically modified mice models were studied so far to investigate the role of cardiac calsequestrin (CSQ2) for the contractile function of the ventricle and for the occurrence of ventricular tachycardia. Using a CSQ2 knockout mouse, we wanted to study also the atrial function of CSQ2.

METHODS

The influence of CSQ2 on atrial function and, for comparison, ventricular function was studied in isolated cardiac preparations and by echocardiography as well as electrocardiography in mice with deletion of CSQ2.

RESULTS

Using deletion of exon 1, we have successfully generated a constitutive knockout mouse of the calsequestrin 2 gene (CSQ2^-/- ). CSQ2 protein was absent in the heart (atrium, ventricle), but also in oesophagus and skeletal muscle of homozygous knockout mice. In 6-month-old CSQ2^-/- mice, relative left atrial weight was increased, whereas relative heart weight was unchanged. The staircase phenomena in paced left atrial preparations on force of contraction and the post-rest potentiation were different between wild type and CSQ2^-/- indicative for a decreased sarcoplasmic Ca²⁺ load and supporting an important role of CSQ2 also in the atrium. The incidence of arrhythmias was increased in CSQ2^-/- . In 2-year-old CSQ2^-/- mice, cardiac hypertrophy and heart failure were noted possibly as a result of chronically increased cytosolic Ca²⁺ levels.

CONCLUSION

These data suggest a functional role of CSQ2 not only in the ventricle but also in the atrium of mammalian hearts. Loss of CSQ2 function can cause not only arrhythmias, but also cardiac hypertrophy and heart failure.

In vivo Monitoring of Serotonin by Nanomaterial Functionalized Acupuncture Needle

Acupuncture treatment is amazing but controversial. Up to now, the mechanism of treating diseases by acupuncture and moxibustion is still unclear, especially the occurrence of the molecular events in local acupoints. Herein, we report an extremely stable microsensor by modifying carbon nanotube (CNT) to the tip surface of acupuncture needle and applying this CNT-modified acupuncture needle for real time monitoring of serotonin (5-HT) in vivo. To stabilize CNT modification on the needle tip surface, poly(3,4-ethylenedioxythiophene)(PEDOT) was employed as glue water to stick CNT on the needle. The detection limit of the CNT-modified needle was found to be approximately 50 nM and 78 nM in the PBS and the cell medium, respectively. In addition, the needle showed good selectivity to some inflammatory mediators and some electroactive molecules. For the first time, the CNT-modified needle could be directly probed into rat body for real time monitoring of 5-HT in vivo, showing a great potential for better understanding the mechanism of acupuncture treatment.

Serotonin as a New Therapeutic Target for Diabetes Mellitus and Obesity

Serotonin (5-hydroxytryptamine [5-HT]) is a monoamine that has various functions in both neuronal and non-neuronal systems. In the central nervous system, 5-HT regulates mood and feeding behaviors as a neurotransmitter. Thus, there have been many trials aimed at increasing the activity of 5-HT in the central nervous system, and some of the developed methods are already used in the clinical setting as anti-obesity drugs. Unfortunately, some drugs were withdrawn due to the development of unwanted peripheral side effects, such as valvular heart disease and pulmonary hypertension. Recent studies revealed that peripheral 5-HT plays an important role in metabolic regulation in peripheral tissues, where it suppresses adaptive thermogenesis in brown adipose tissue. Inhibition of 5-HT synthesis reduced the weight gain and improved the metabolic dysfunction in a diet-induced obesity mouse model. Genome-wide association studies also revealed genetic associations between the serotonergic system and obesity. Several genetic polymorphisms in tryptophan hydroxylase and 5-HT receptors were shown to have strong associations with obesity. These results support the clinical significance of the peripheral serotonergic system as a therapeutic target for obesity and diabetes.

Myocardial interstitial serotonin and its major metabolite, 5-hydroxyindole acetic acid levels determined by microdialysis technique in rat heart

AIMS

The aim of this study was to elucidate myocardial interstitial serotonin (5-HT) kinetics in the heart, including 5-HT reuptake and enzymatic degradation to 5-hydroxyindole acetic acid (5-HIAA) via monoamine oxidase (MAO).

MAIN METHODS

Using microdialysis technique in anesthetized rats, we simultaneously monitored myocardial interstitial levels of 5-HT and its major metabolite, 5-HIAA, in the left ventricle and examined the effects of local administration of a MAO inhibitor, pargyline, or a 5-HT uptake inhibitor, fluoxetine.

KEY FINDINGS

Pargyline increased dialysate 5-HT concentration from 1.8±0.3 at baseline to 3.9±0.5nM but decreased dialysate 5-HIAA concentration from 20.7±1.0 at baseline to 15.8±1.4nM at 60-80min of administration. Fluoxetine increased dialysate 5-HT concentration from 1.9±0.4 at baseline to 6.5±0.9nM at 60-80min of administration, but did not change dialysate 5-HIAA concentration. Local administration of ADP (100mM) increased dialysate 5-HT and 5-HIAA concentrations. Pargyline did not affect ADP-induced increase in dialysate 5-HT concentration but suppressed ADP-induced increase in dialysate 5-HIAA concentration during 60min of ADP administration. Fluoxetine increased dialysate 5-HT concentration at 40-60min of ADP administration, but did not affect ADP-induced increase in dialysate 5-HIAA concentration.

SIGNIFICANCE

Simultaneous monitoring of myocardial interstitial 5-HT and 5-HIAA levels provides valuable information on 5-HT kinetics including reuptake and enzymatic degradation by MAO, which play a role in the regulation of myocardial interstitial 5-HT levels at baseline and when 5-HT levels are elevated.

Distinct signaling pathways activated by "extracellular" and "intracellular" serotonin in heart valve development and disease

Cardiac valve diseases are often due to developmental anomalies that progressively lead to the abnormal distribution and organization of extracellular matrix proteins overtime. Whereas mechanisms underlying adult valvulopathies are unknown, previous work has shown a critical involvement of the monoamine serotonin in disease pathogenesis. In particular, the interaction of serotonin with its receptors can activate transforming growth factor-β1 (TGF-β1) signaling, which in turn promotes extracellular matrix gene expression. Elevated levels of circulating serotonin can lead to aberrant TGF-β1 signaling with significant effects on cardiac valve structure and function. Additional functions of serotonin have recently been reported in which internalization of serotonin, through the serotonin transporter SERT, can exert important cytoskeletal functions in lieu of simply being degraded. Recent findings demonstrate that intracellular serotonin regulates cardiac valve remodeling, and perturbation of this pathway can also lead to heart valve defects. Thus, both extracellular and intracellular mechanisms of serotonin action appear to be operative in heart valve development, functionality, and disease. This review summarizes some of the salient aspects of serotonin activity during cardiac valve development and disease pathogenesis with an understanding that further elaboration of intracellular and extracellular serotonin pathways may lead to beneficial treatments for heart valve disease.

Comprehensive analysis of neurotransmitters from regenerating planarian extract using an ultrahigh-performance liquid chromatography/mass spectrometry/selected reaction monitoring method

RATIONALE

Absolute quantification of neurotransmitters (NTs) from biological systems is imperative to track how changes in concentration of active neurochemicals may affect biological behavior. A sensitive method for the absolute quantification of multiple NTs in a single method is highly needed.

METHODS

A stable-isotope dilution ultrahigh-performance liquid chromatography/mass spectrometry/selected reaction monitoring (UHPLC/MS/SRM) assay has been developed for a sensitive and quantitative assessment of NTs in planaria. We used this method for the simultaneous quantification of 16 NTs. All analytes showed a linear relationship between concentrations (0.78-50 ng/mL), regression coefficients higher than 0.97, accuracy (91-109%) and low coefficients of variation (CVs). The inter-day CVs for the lowest quality controls (1.56 ng/mL) were in the range between 2-11%.

RESULTS

The levels of most of the NTs were similar in both sexual and asexual planarians except for glutamic acid, which was about two-fold higher in asexual compared to sexual planarians. We identified high levels of serotonin and failed to detect tryptamine suggesting that the pathway essential for the conversion of tryptophan into tryptamine is absent in planarians. Interestingly, we also found high levels of dopamine and L-DOPA in regenerating planarians suggesting their possible role in regeneration.

CONCLUSIONS

For the first time, we developed novel methodology based on UHPLC/MS/SRM and quantified 16 NTs with high sensitivity and specificity from sexual and asexual strains of planarian Schmidtea mediterranea. This method will also have great application in quantifying various NTs with great precision in different model systems.

Human 5-HT₄receptor stimulation in atria of transgenic mice

In human atrium, serotonin (5-HT) exerts pleiotropic effects, which are thought to be mediated via 5-HT4 receptors. Here, we used transgenic mice (TG) that overexpress the human 5-HT4(a) receptor under control of the heart-specific α-myosin heavy chain promoter in the atria (and ventricles). Contractile studies were performed in isolated electrically driven left atrial preparations and spontaneously beating right atrial preparation of TG and littermate control mice (wild type (WT)). 5-HT increased force of contraction and phospholamban phosphorylation on serine 16 only in left atrial preparations from TG but not from WT. In contrast, β-adrenoceptor stimulation of left atrial preparations by isoprenaline increased force of contraction with similar pEC50 values and to a similar maximum extent in both TG and WT. The contractile effects of 5-HT in left atrial preparations from TG could be blocked by the 5-HT4 receptor-specific antagonists GR125487 or GR113808. In right atrial preparations from WT and TG, the β-adrenoceptor agonist isoprenaline exerted a positive chronotropic effect with similar pEC50 values and similar maximum effects. Only in right atrial preparations from TG but not WT, 5-HT exerted a positive chronotropic effect that could be attenuated by 5-HT4 receptor-specific antagonists. Finally, in left atrial preparations of TG, a higher incidence of arrhythmias was noted compared to WT. The present data indicate that the human 5-HT4 receptors expressed in mouse atria are functional. This is the first transgenic model to study this human receptor in the atrium ex vivo or in vivo.