Desensitization of the human 5-HT4 receptor in isolated atria of transgenic mice

Cardiac effects of two hallucinogenic natural products, N,N-dimethyl-tryptamine and 5-methoxy-N,N-dimethyl-tryptamine

It is unclear whether hallucinogenic tryptamine derivatives namely N,N-dimethyl-tryptamine (DMT) and 5-methoxy-N,N-dimethyl-tryptamine (5-MeO-DMT) exert positive inotropic effects in the human heart. Therefore, we measured the inotropic effects of these drugs in isolated left and right atrial preparations of mice that overexpress human 5-HT4 receptors (5-HT4-TG) and preparations from wild type mice (WT). Moreover, we measured force of contraction in isolated right atrial preparations from adult patients, obtained in the process of open heart surgery due to severe coronary heart disease. DMT and 5-MeO-DMT augmented the force of contraction in isolated paced (1 Hz) left atrial preparations from 5-HT4-TG and raised the spontaneous beating rate of right atrial preparations from 5-HT4-TG. The drugs elevated force of contraction in paced (1 Hz) human right atrial muscle preparations. The maximum inotropic effects of DMT and 5-MeO-DMT were smaller at 10 µM (about 65%) than that of 1 µM 5-HT on the left atria from 5-HT4-TG. The maximum increase in the beating rate due to DMT and 5-MeO-DMT amounted 40 ± 5% of the effect of 5-HT on right atrial preparations from 5-HT4-TG (n = 5-6). DMT and 5-MeO-DMT were inactive in WT. The potency of 5-MeO-DMT to increase force of contraction could be increased by pre-treatment of human atrial preparations by the phosphodiesterase inhibitor cilostamide (1 µM). 5-MeO-DMT increased the phosphorylation state of phospholamban at serine 16 in isolated left atrial muscle strips of 5-HT4-TG. In summary, DMT and 5-MeO-DMT acted as partial agonists on human 5-HT4 receptors.

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.

Cardiovascular effects of bufotenin on human 5-HT4 serotonin receptors in cardiac preparations of transgenic mice and in human atrial preparations

It is unclear whether bufotenin (= N,N-dimethyl-serotonin = 5-hydroxy-N,N-dimethyl-tryptamine), a hallucinogenic drug, can act on human cardiac serotonin 5-HT₄ receptors. Therefore, the aim of the study was to examine the cardiac effects of bufotenin and for comparison tryptamine in transgenic mice that only express the human 5-HT₄ receptor in cardiomyocytes (5-HT₄-TG), in their wild-type littermates (WT) and in isolated electrically driven (1 Hz) human atrial preparations. In 5-HT₄-TG, we found that both bufotenin and tryptamine enhanced the force of contraction in left atrial preparations (pD2 = 6.77 or 5.5, respectively) and the beating rate in spontaneously beating right atrial preparations (pD2 = 7.04 or 5.86, respectively). Bufotenin (1 µM) increased left ventricular force of contraction and beating rate in Langendorff perfused hearts from 5-HT₄-TG, whereas it was inactive in hearts from WT animals, as was tryptamine. The positive inotropic and chronotropic effects of bufotenin and tryptamine were potentiated by an inhibitor of monoamine oxidases (50 µM pargyline). Furthermore, bufotenin concentration- (0.1-10 µM) and time-dependently elevated force of contraction in isolated electrically stimulated musculi pectinati from the human atrium and these effects were likewise reversed by tropisetron (10 µM). We found that bufotenin (10 µM) increased the phosphorylation state of phospholamban in the isolated perfused hearts, left and right atrial muscle strips of 5-HT₄-TG but not from WT and in isolated human right atrial preparations. In summary, we showed that bufotenin can increase the force of contraction via stimulation of human 5-HT₄ receptors transgenic mouse cardiac preparations but notably also in human atrial preparations.

Chronic stimulation of the serotonergic 5-HT4 receptor modulates amyloid-beta-related impairments in synaptic plasticity and memory deficits in male rats

Alzheimer's disease (AD) is a neurodegenerative disease characterized by memory decline and impaired hippocampal synaptic plasticity. The serotonin 5-HT4 receptor is involved in learning and memory processes. This study explored the effects of chronic stimulation of 5-HT4R on cognition, memory, long-term potentiation (LTP), paired-pulse ratio (PPR), and neuronal apoptosis in a rat model of amyloid-beta (Aβ)-induced AD. Thirty-five male Wistar rats were randomly divided into three groups as follows: the sham, Aβ, and Aβ + BIMU8 groups. Aβ (6 µg/µl) was administrated by intracerebroventricular (icv) injection. The animals were treated with BIMU8 (1 μg/μL, ICV) as a 5-HT4R agonist for 30 days. Memory and behavioral changes were assessed by the passive avoidance learning, novel object recognition, open field, and elevated plus maze tests. Hippocampal synaptic plasticity was evaluated in the dentate gyrus (DG) in response to the stimulation applied to the perforant pathway. Furthermore, neuronal apoptosis was measured in the hippocampus. Data were analyzed by SPSS version 19 using one-way ANOVA, followed by Tukey's post hoc test. Aβ induced memory deficits and neuronal loss and inhibited LTP induction. Aβ also increased the normalized PPR. BIMU8 enhanced the slope of the field excitatory postsynaptic potential in LTP and improved cognition behavior. Paired-pulse inhibition or facilitation was not affected by LTP induction in Aβ animals receiving the BIMU8. It can be concluded that the stimulation of the 5-HT₄ receptor modulated the Aβ-induced cognition and memory deficits, probably via a decrease in the hippocampal apoptotic neurons and an improvement in the hippocampal synaptic functions without involving its inhibitory interneurons.

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.

The proarrhythmic effects of hypothermia in atria isolated from 5-HT4-receptor-overexpressing mice

We investigated whether hypothermia would be arrhythmogenic in mice that overexpress the human 5-HT₄ receptor only in their cardiac myocytes (5-HT₄-TG). Contractile studies were performed in isolated, electrically driven (1 Hz) left and spontaneously beating right atrial preparations of 5-HT₄-TG and littermate wild-type control mice (WT). Hypothermia (23 °C) decreased the force of contraction in the mouse right and left atrial preparations. Moreover, the concentration-dependent positive inotropic effects of 5-HT were blunted but still shifted to lower 5-HT concentrations in the left 5-HT₄-TG atria in hypothermia compared to normothermia (37 °C). Furthermore, hypothermia increased the incidence of right atrial arrhythmias in 5-HT₄-TG more than in WT mice. In contrast, at 37 °C, lowering the potassium concentration from 5.2 to 2.0 mM also induced arrhythmias in the right atrium, but with a similar incidence in WT and 5-HT₄-TG mice. In contrast, 10 μM d,l-sotalol and 300 μM erythromycin did not induce arrhythmias. Hypothermia was accompanied by the increased expression of heat shock protein 70 (HSP70) in WT but not in 5-HT₄-TG mice. We concluded that without the stimulation of 5-HT₄-receptors by exogenous agonists, a simple temperature reduction can increase arrhythmias in 5-HT₄-TG mice. It is tempting to speculate that in human patients, 5-HT₄ receptors might contribute to potentially deadly hypothermia-induced arrhythmias.

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.

Cardiovascular effects of metoclopramide and domperidone on human 5-HT4-serotonin-receptors in transgenic mice and in human atrial preparations

It is unclear whether metoclopramide and domperidone act on human cardiac serotonin 5-HT₄-receptors. Therefore, we studied transgenic mice that only express the human 5-HT₄ receptor in cardiomyocytes in the atrium and in the ventricle (5-HT₄-TG), their wild type-littermates (WT) and isolated human atrial preparations. We found that only metoclopramide but not domperidone enhanced the force of contraction in left atrial preparations (pEC₅₀ = 6.0 ± 0.1; n = 7) from 5-HT₄-TG, isolated spontaneously beating right atrial preparations (pEC₅₀ = 6.1 ± 0.1; n = 7) from 5-HT₄-TG, Langendorff perfused hearts from 5-HT₄-TG, living 5-HT₄-TG and human right atrial muscle preparations obtained during bypass surgery of patients suffering from coronary heart disease. The maximum inotropic effect of metoclopramide was smaller (81 ± 2%) than that of 5-HT on the left atria from 5-HT₄-TG. The maximum increase in the beating rate due to metoclopramide was 93 ± 2% of effect of 5-HT on right atrial preparations from 5-HT₄-TG. Metoclopramide and domperidone were inactive in WT. We found that metoclopramide but not domperidone increased the phosphorylation state of phospholamban in the isolated perfused hearts or muscle strips of 5-HT₄-TG, but not in WT. Metoclopramide, but not domperidone, shifted the positive inotropic or chronotropic effects of 5-HT in isolated left atrial and right atrial preparations from 5-HT₄-TG dextrally, resp., to higher concentrations: the pEC₅₀ of 5-HT for increase in force was in the absence of metoclopramide 8.6 ± 0.1 (n = 5) versus 8.0 ± 0.3 in the presence of 1 μM metoclopramide (n = 5; P < 0.05); and the beating rate was 7.8 ± 0.2 (n = 7) in the absence of metoclopramide versus 7.2 ± 0.1 in the presence of 1 μM metoclopramide (n = 6; P < 0.05). These results suggested that metoclopramide had an antagonistic effect on human cardiac 5-HT₄ receptors. In summary, we showed that metoclopramide, but not domperidone, was a partial agonist at human cardiac 5-HT₄-receptors.

Amitriptyline functionally antagonizes cardiac H2 histamine receptors in transgenic mice and human atria

We have previously shown that histamine (2-(1H-imidazol-4-yl)ethanamine) exerted concentration-dependent positive inotropic effects (PIE) or positive chronotropic effects (PCE) on isolated left and right atria, respectively, of transgenic (H₂R-TG) mice that overexpress the human H₂ histamine receptor (H₂R) in the heart; however, the effects were not seen in their wild-type (WT) littermates. Amitriptyline, which is still a highly prescribed antidepressant drug, was reported to act as antagonist on H₂Rs. Here, we wanted to determine whether the histamine effects in H₂R-TG were antagonized by amitriptyline. Contractile studies were performed on isolated left and right atrial preparations, isolated perfused hearts from H₂R-TG and WT mice and human atrial preparations. Amitriptyline shifted the concentration-dependent PIE of histamine (1 nM-10 μM) to higher concentrations (rightward shift) in left atrial preparations from H₂R-TG. Similarly, in isolated perfused hearts from H₂R-TG and WT mice, histamine increased the contractile parameters and the phosphorylation state of phospholamban (PLB) at serine 16 in the H₂R-TG mice, but not in the WT mice. However, the increases in contractility and PLB phosphorylation were attenuated by the addition of amitriptyline in perfused hearts from H₂R-TG. In isolated electrically stimulated human atria, the PIE of histamine that was applied in increasing concentrations from 1 nM to 10 μM was reduced by 10-μM amitriptyline. In summary, we present functional evidence that amitriptyline also acts as an antagonist of contractility at H₂Rs in H₂R-TG mouse hearts and in the human heart which might in part explain the side effects of amitriptyline.

Phosphodiesterases 2, 3 and 4 can decrease cardiac effects of H2-histamine-receptor activation in isolated atria of transgenic mice

Histamine exerts cAMP-dependent positive inotropic effects (PIE) and positive chronotropic effects (PCE) on isolated left and right atria, respectively, of transgenic mice which overexpress the human H₂-receptor in the heart (=H₂-TG). To determine whether these effects are antagonized by phosphodiesterases (PDEs), contractile studies were done in isolated left and right atrial preparations of H₂-TG. The contractile effects of histamine were tested in the additional presence of the PDE-inhibitorserythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride (EHNA, 1 μM, PDE2-inhibitor) or cilostamide (1 μM, PDE3-inhibitor), rolipram (10 μM, a PDE4-inhibitor), and their combinations. Cilostamide (1 μM) and EHNA (1 μM), rolipram (1 μM), and EHNA (1 μM) and the combination of rolipram (0.1 μM) and cilostamide (1 μM) each increased the potency of histamine to elevate the force of contraction (FOC) in H₂-TG. Cilostamide (1 μM) and rolipram (10 μM) alone increased and EHNA (1 μM) decreased alone, and their combination increased the potency of histamine to increase the FOC in H₂-TG indicating that PDE3 and PDE4 regulate the inotropic effects of histamine in H₂-TG. The PDE inhibitors (EHNA, cilostamide, rolipram) alone did not alter the potency of histamine to increase the heart beat in H₂-TG whereas a combination of rolipram, cilostamide, and EHNA, or of rolipram and EHNA increased the potency of histamine to act on the beating rate. In summary, the data suggest that the PCE of histamine in H₂-TG atrium involves PDE 2 and 4 activities, whereas the PIE of histamine are diminished by activity of PDE 3 and 4.

Role of Cardiac A2A Receptors Under Normal and Pathophysiological Conditions

This review presents an overview of cardiac A2A-adenosine receptors The localization of A2A-AR in the various cell types that encompass the heart and the role they play in force regulation in various mammalian species are depicted. The putative signal transduction systems of A2A-AR in cells in the living heart, as well as the known interactions of A2A-AR with membrane-bound receptors, will be addressed. The possible role that the receptors play in some relevant cardiac pathologies, such as persistent or transient ischemia, hypoxia, sepsis, hypertension, cardiac hypertrophy, and arrhythmias, will be reviewed. Moreover, the cardiac utility of A2A-AR as therapeutic targets for agonistic and antagonistic drugs will be discussed. Gaps in our knowledge about the cardiac function of A2A-AR and future research needs will be identified and formulated.

Initial Characterization of Transgenic Mice Overexpressing Human Histamine H2 Receptors

In an integrative approach, we studied the role of histamine H2 receptors in the mouse heart. We noted that histamine, added cumulatively to the organ bath, failed to affect the force of contraction in left atrial preparations and did not change spontaneous heart rate in right atrial preparations from wild-type mice. By contrast, in the same preparations from mice that overexpressed the human H2 receptor in a cardiac-specific way, histamine exerted concentration- and time-dependent positive inotropic and positive chronotropic effects. Messenger RNA of the human H2 receptor was only detected in transgenic mice. Likewise, immunohistology and autoradiography only gave signals in transgenic but not in wild-type cardiac preparations. Similarly, a positive inotropic and positive chronotropic effect was observed with histamine in echocardiography of living transgenic mice and isolated perfused hearts (Langendorff preparation). Phosphorylation of phospholamban was increased in atrial and ventricular preparations from transgenic mice, but not in wild-type animals. The effects of histamine were mimicked by dimaprit and amthamine and antagonized by cimetidine. In summary, we generated a new model to study the physiologic and pathophysiologic cardiac role of the human H2 receptor.

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.