Chronic Sarpogrelate Treatment Reveals 5-HT7 Receptor in the Serotonergic Inhibition of the Rat Vagal Bradycardia

Fluoxetine Treatment Decreases Cardiac Vagal Input and Alters the Serotonergic Modulation of the Parasympathetic Outflow in Diabetic Rats

Comorbid diabetes and depression constitutes a major health problem, worsening associated cardiovascular diseases. Fluoxetine's (antidepressant) role on cardiac diabetic complications remains unknown. We determined whether fluoxetine modifies cardiac vagal input and its serotonergic modulation in male Wistar diabetic rats. Diabetes was induced by alloxan and maintained for 28 days. Fluoxetine was administered the last 14 days (10 mg/kg/day; p.o). Bradycardia was obtained by vagal stimulation (3, 6 and 9 Hz) or i.v. acetylcholine administrations (1, 5 and 10 μg/kg). Fluoxetine treatment diminished vagally-induced bradycardia. Administration of 5-HT originated a dual action on the bradycardia, augmenting it at low doses and diminishing it at high doses, reproduced by 5-CT (5-HT_1/7 agonist). 5-CT did not alter the bradycardia induced by exogenous acetylcholine. Decrease of the vagally-induced bradycardia evoked by high doses of 5-HT and 5-CT was reproduced by L-694,247 (5-HT_1D agonist) and blocked by prior administration of LY310762 (5-HT_1D antagonist). Enhancement of the electrical-induced bradycardia by 5-CT (10 μg/kg) was abolished by pretreatment with SB269970 (5-HT₇ receptor antagonist). Thus, oral fluoxetine treatment originates a decrease in cardiac cholinergic activity and changes 5-HT modulation of bradycardic responses in diabetes: prejunctional 5-HT₇ receptors augment cholinergic-evoked bradycardic responses, whereas prejunctional 5-HT_1D receptors inhibit vagally-induced bradycardia.

Dopamine D4 receptor subtype activation reduces the rat cardiac parasympathetic discharge

The dopaminergic system influences the heart rhythm by inhibiting the rat cardiac sympathetic and parasympathetic neurotransmissions through activation of D₂-like receptors (encompassing the D₂, D₃, and D₄ subtypes). Whereas D₂ receptor subtype activation results in cardiac sympatho-inhibition, the dopamine receptor subtypes involved in rat cardiac vago-inhibition remain unknown. Hence, this study investigated the specific functional role of the D₂-like receptor subtypes (D₂, D₃, and/or D₄) inhibiting the rat heart cholinergic drive. For this purpose, male Wistar rats were pithed and prepared for cardiac vagal stimulation. Bradycardic responses were obtained by electrical stimulation of vagal fibres (3, 6, 9 Hz; n = 100) or i.v. acetylcholine (ACh; 1, 5, 10 μg/kg; n = 15). Expression of D₂, D₃, and D₄ receptors was studied in left and right atrium samples by PCR (n = 4). Intravenous injections of quinpirole (D₂-like agonist; 1-30 μg/kg), but not of SFK-38393 (D₁-like agonist; 1-30 μg/kg), dose-dependently inhibited the vagally induced bradycardia. The vago-inhibition induced by quinpirole (which failed to affect the bradycardia to i.v. ACh) was unchanged after i.v. injections of the antagonists L-741,626 (D₂; 100 μg/kg) or SB-277011-A (D₃; 100 μg/kg), but it was abolished by L-745,870 (D₄; 100 μg/kg). mRNA levels of D₂, D₃, and D₄ receptor subtype were detected in the left and right rat atria. Our results suggest that the quinpirole-induced vagolytic effect involves prejunctional D₄ receptor subtypes, located in the left and right atria. This provides new evidence on the relevance of D₄ receptor modulating the heart parasympathetic control.

Blockade of 5-HT2 receptors with sarpogrelate uncovers 5-HT7 receptors inhibiting the tachycardic sympathetic drive in pithed rats

Our group has previously shown in pithed rats that the cardiac sympathetic drive, which produces tachycardic responses, is inhibited by 5-HT via the activation of prejunctional 5-HT_1B/1D/5 receptors. Interestingly, when 5-HT₂ receptors are chronically blocked with sarpogrelate, the additional role of cardiac sympatho-inhibitory 5-HT_1F receptors is unmasked. Although 5-HT₂ receptors mediate tachycardia in rats, and the chronic blockade of 5-HT₂ receptors unmasked 5-HT₇ receptors mediating cardiac vagal inhibition, the role of 5-HT₇ receptors in the modulation of the cardiac sympathetic tone remains virtually unexplored. On this basis, male Wistar rats were pretreated during 14 days with sarpogrelate (a 5-HT₂ receptor antagonist) in drinking water (30 mg/kg/day; sarpogrelate-pretreated group) or equivalent volumes of drinking water (control group). Subsequently, the rats were pithed to produce increases in heart rate by either electrical preganglionic spinal (C₇ -T₁ ) stimulation of the cardiac sympathetic drive or iv administration of exogenous noradrenaline. The iv continuous infusion of AS-19 (a 5-HT₇ receptor agonist; 10 µg/kg/min) (i) inhibited the tachycardic responses to sympathetic stimulation, but not those to exogenous noradrenaline only in sarpogrelate-pretreated rats. This inhibition was completely reversed by SB258719 (a selective 5-HT₇ receptor antagonist; 1 mg/kg, iv) or glibenclamide (an ATP-sensitive K⁺ channel blocker; 20 mg/kg, iv). These results suggest that chronic 5-HT₂ receptor blockade uncovers a cardiac sympatho-inhibitory mechanism mediated by 5-HT₇ receptors, involving a hyperpolarization due to the opening of ATP-sensitive K⁺ channels. Thus, these findings support the role of 5-HT₇ receptors in the modulation of the cardiac sympathetic neurotransmission.

Fluoxetine oral treatment discloses 5-HT1D receptor as vagoinhibitor of the cardiac cholinergic neurotransmission in rat

Although depression and cardiovascular diseases are related, the role of antidepressants such as fluoxetine (increasing serotonin levels) within cardiac regulation remains unclear. We aimed to determine whether fluoxetine modifies the pharmacological profile of serotonergic influence on vagal cardiac outflow. Rats were treated with fluoxetine (10 mg/kg per day; p.o.) for 14 days or equivalent volumes of drinking water (control group); then, they were pithed and prepared for vagal stimulation. Bradycardic responses were obtained by electrical stimulation of the vagal fibers (3, 6, and 9 Hz) or i.v. acetylcholine (ACh; 1, 5, and 10 μg/kg). The i.v. administration of 5-hydroxytryptamine (5-HT; 10 and 50 μg/kg) inhibited the vagally induced bradycardia. 5-CT (5-HT1/7 agonist) and L-694,247 (5-HT1D agonist) mimicked the serotonin inhibitory effect while α-methyl-5-HT (5-HT2 agonist) was devoid of any action. SB269970 (5-HT7 antagonist) did not abolish 5-CT inhibitory action on the electrically induced bradycardia. Pretreatment with LY310762 (5-HT1D antagonist) blocked the effects induced by L-694,247 and 5-CT. 5-HT and 5-CT failed to modify the bradycardia induced by exogenous ACh. Our outcomes suggest that fluoxetine treatment modifies 5-HT modulation on heart parasympathetic neurotransmission in rats, evoking inhibition of the bradycardia via prejunctional 5-HT1D in pithed rats.

Pharmacology and Therapeutic Potential of the 5-HT7 Receptor

It is well-documented that serotonin (5-HT) exerts its pharmacological effects through a series of 5-HT receptors. The most recently identified member of this family, 5-HT₇, was first identified in 1993. Over the course of the last 25 years, this receptor has been the subject of intense investigation, and it has been demonstrated that 5-HT₇ plays an important role in a wide range of pharmacological processes. As a result of these findings, modulation of 5-HT₇ activity has been the focus of numerous drug discovery and development programs. This review provides an overview of the roles of 5-HT₇ in normal physiology and the therapeutic potential of this interesting drug target.