5-HT and its receptors in acute cardiovascular responses: from the Bezold-Jarish reflex to heart failure

The contribution of 5-HT to the pathophysiology of the failing heart was suggested many years ago, following the description of the Bezold-Jarish reflex, which is typically intense bradycardia associated with atrioventricular block, 60 minutes after an anterior or posterior myocardial infarction; it has been used as a marker of successful thrombolysis, indicating that it occurs at the time of reperfusion . In addition to bradycardia, this reflex is characterized by a drop in blood pressure, a decrease in cardiac contractility and coronary artery vasodilatation.

The Bezold-Jarish reflex can be reproduced by an intravenous bolus injection of 5-HT. The subsequent response occurs in three phases. The first phase is transient, combining bradycardia and a fall in blood pressure; this is caused by the simulation of 5-HT ₃ receptors localized on afferent parasympathetic nerves that, in turn, block sympathetic neurons at the level of the brainstem. This response can provoke atrioventricular block, and is reversed by atropine. In the second phase, 5-HT triggers an increase in blood pressure as a consequence of 5-HT _2A receptor stimulation in the arterial wall, leading to reflex bradycardia. There is also a reduction in the release of catecholamines by the presynaptic stimulation of 5-HT _1B/1D receptors. Finally, the third phase is a long-lasting reduction in blood pressure resulting from a ganglionic blockade following 5-HT _1B/1D stimulation, and the activation of 5-HT ₇ receptors localized on smooth muscle cells and of 5-HT _2B receptors that drive nitric oxide (NO) release by endothelial cells . Interestingly, continuous 5-HT infusion triggers only the third phase, but reveals tachycardia linked to atrial 5-HT ₄ receptor stimulation in humans. This response could become relevant in the case of heart failure, where the 5-HT ₄ receptor appears to be overexpressed.

All these data raise the question of the origin of cardiac 5-HT. In humans, myocardium contains around 0.4 μg of 5-HT/g of tissue ; its origin is unclear. Local synthesis by tryptophan hydroxylase-2 in autonomic ganglia or capture by SERT from circulating stores have been suggested. Microdialysis experiments in rabbits revealed an increase in myocardial 5-HT during coronary artery occlusion, most of it coming from platelets in a mechanism involving 5-HT _2A receptors . This free 5-HT, acutely released at the time of reperfusion, could explain the first phase of the Bezold-Jarish reflex.

5-HT receptors and hemodynamics in the failing heart

Interestingly, the distribution and functional role of serotonergic receptors in the heart follows that of adrenergic receptors. Similarly to the α1-adrenergic subtype, 5-HT ₂ receptors are classically Gq/diacyl glycerol/inositol trisphosphate-coupled when β1 and 5-HT ₄ receptors are coupled to the Gs/adenylyl cyclase system . In humans, the 5-HT ₄ receptor is expressed in atria and ventricles; as with other serotonergic receptors, its level of expression is quite low in a physiological situation, but can increase markedly in case of ventricular dysfunction . The 5-HT ₄ receptor appears to be a representative of a fetal cardiac gene programme, which is reactivated in heart failure . Without phosphodiesterase inhibition, only sparse effects are obtained following 5-HT ₄ receptor stimulation, probably because of the rapid withdrawal of cyclic adenosine monophosphate . In case of heart failure, this activity is reduced, revealing a role for these receptors; their stimulation provokes an increase in myocardial contractility and relaxation at a similar concentration to isoproterenol ; in some cases it can also trigger arrhythmias . Interestingly, responses involving 5-HT ₄ receptors do not become desensitized, unlike β-adrenoceptors, and might therefore be used as a compensatory mechanism driving hemodynamic support. This question was studied in a rat model of rapidly progressing cardiac hypertrophy followed by heart failure caused by pressure overload . An increase in 5-HT ₄ receptor expression was observed from the early steps of hypertrophy, and was maintained in the failing myocardium. This overexpression was correlated to inotropic responses by the natural agonist 5-HT. Therefore, it was tempting to investigate the effect of chronic 5-HT ₄ receptor blockade in cardiac hypertrophy. The same group investigated the effect of 6 weeks of treatment with the selective 5-HT ₄ receptor antagonist, piboserod, started 3 days after a myocardial infarction induced by coronary artery ligation in rats . The drug improved cardiac hemodynamics, but no survival study was performed. Nevertheless, all the data obtained on 5-HT ₄ receptors and piboserod paved the way for a prospective double-blind parallel-group study in patients receiving conventional therapy, with New York Heart Association class II–IV heart failure and left ventricular ejection fraction ≤ 35%. Patients were randomized to receive either placebo or piboserod 80 mg for 24 weeks, including 4 weeks of up-titration. The primary endpoint was left ventricular ejection fraction measured by cardiac magnetic resonance imaging. Piboserod slightly but significantly increased ejection fraction (+1.7%), with a trend towards a greater effect in patients not treated with beta-blockers. These partly disappointing results slowed down an area of research that could restart with atrial fibrillation, a frequently associated problem. 5-HT _4a and 5-HT _4b receptors are expressed at the level of atria, where they activate adenylyl cyclase through the stimulation of the Gs protein, driving the opening of L-type calcium channels . To our knowledge, all research has now stopped in this field. In the failing heart, part of the positive inotropic response elicited by 5-HT is prevented by the 5-HT _2A receptor antagonist, ketanserin . Part of this response could come from phosphorylation of myosin light chain-2, in a similar way to that triggered by α1-adrenergic stimulation .

5-HT and its receptors in cardiac remodeling

In this research area, most work is related to 5-HT _2A/2B receptors. The 5-HT _2A receptor is expressed in both cardiomyocytes and fibroblasts. In aging rats, its expression is increased in left ventricular hypertrophy and dysfunction because of hypertension . Genetic studies failed to show polymorphisms in the 5-HT _2A receptor gene in patients with hypertrophic cardiomyopathy of genetic origin or caused by hypertension, but its pharmacological blockade can prevent cardiac hypertrophy induced by transverse aortic constriction in mice . Caveolin-3 and the calcineurin/nuclear factor of activated T cells (NFAT) pathways may be involved in these regulations. This receptor, expressed by non-cardiomyocytes, also appears to be involved in cardiac fibrosis; its activation by platelet extracts induces fibroblast proliferation and transdifferentiation to myofibroblasts, and collagen secretion . Recently, the same team identified serotonergic system activation in patients with aortic stenosis, suggesting that this hormone contributes to the pathophysiology of valve fibrosis and adverse ventricular remodeling .

Extensive work has been done on the role of the 5-HT _2B receptor subtype in cardiac hypertrophy. In the failing human heart, the 5-HT _2B receptor is markedly overexpressed compared with in normal tissue, in which the expression level is very low . Interestingly, we found a correlation between the expression of the receptor and plasma cytokines and norepinephrine. These results confirmed previous studies in mice, where chronic β-adrenergic stimulation induced cardiac hypertrophy and secretion of interleukin-6, tumor necrosis factor-α and interleukin-1β, all of which were prevented by simultaneous treatment with a 5-HT _2B receptor blocker or in Htr _2B ^−/− mice ; this prevention was also associated with a marked reduction in myocardial oxidative stress . Of note, similar results were obtained in terms of hypertrophy, cytokines and oxidative stress produced by stimulation of AT1-angiotensinergic receptors, without any hemodynamic effect. Taking into account that the receptor is expressed in both cardiomyocytes and non-cardiomyocytes, we addressed the question of the exact cardiac cell type(s) involved in these protections. Mice expressing the receptor only in cardiomyocytes were generated. These animals were fully protected from the deleterious effects of both isoproterenol and angiotensin II infusions, indicating that non-cardiomyocytes are involved in cardioprotection provided by 5-HT _2B receptor blockade . This work also demonstrated that the 5-HT _2B receptor works as a heterodimer with the AT1-angiotensin receptor .

Nevertheless, a direct effect on cardiomyocytes themselves cannot be fully excluded, as serotonin, via the Gq-coupled 5-HT _2B receptor, has been shown to protect cardiomyocytes against apoptosis by preventing cytochrome C release and the activation of caspases via cross-talks between phosphatidylinositol-3 kinase/Akt and extracellular signal-regulated kinase 1/2 signaling pathways that can activate nuclear factor-κB and regulate the mitochondrial adenine nucleotide translocator, ANT-1 . Furthermore, after 2 weeks of aortic banding, 5-HT _2B receptors were overexpressed at both the messenger ribonucleic acid level and the protein level, and the 5-HT _2B receptor antagonist, SB215505, attenuated the overexpression and cardiac hypertrophy . Moreover, in a cardiomyocyte cell culture, 5-HT _2B receptor blockade prevented 5-HT- and stretch-induced brain natriuretic peptide secretion by blocking nuclear factor-κB nuclear translocation. Based on these results, we investigated the effects of chronic 5-HT _2B receptor blockade by the selective antagonist, RS127445, in aging spontaneously-hypertensive rats showing left ventricular hypertrophy with diastolic dysfunction and a normal ejection fraction . Blocking the 5-HT _2B receptor, however, did not reduce cardiac hypertrophy, even if blood pressure was reduced with the calcium channel antagonist, nicardipine, but amplified subendocardial fibrosis. In fact, we pointed out a crucial role for endothelial 5-HT _2B receptors in maintaining coronary vasodilatation in hypertensive cardiopathy . Therefore, all these data have revealed the complex effect of 5-HT _2B receptors on cardiomyocytes, cardiac fibroblasts and coronary vessels, and their role in regulating cardiac hypertrophy and remodeling in left ventricular dysfunction.