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Renal vasculature, which is highly innervated by sympathetic fibers, contributes to cardiovascular homeostasis. This renal sympathetic outflow is inhibited by 5-HT in normoglycaemic rats. Considering that diabetes induces cardiovascular complications, we aimed to determine whether diabetic state modifies noradrenergic input at renal level and its serotonergic modulation in rats. Alloxan diabetic rats were anaesthetized (pentobarbital; 60 mg/kg i.p.) and prepared for in situ autoperfusion of the left kidney to continuously measure systemic blood pressure (SBP), heart rate (HR), and renal perfusion pressure (RPP). Electrical stimulation of renal sympathetic outflow induces frequency-dependent increases (Δ) in RPP (23.9 ± 2.1, 59.5 ± 1.9, and 80.5 ± 3.5 mm Hg at 2, 4, and 6 Hz, respectively), which were higher than in normoglycaemic rats, without modifying HR or SBP. Intraarterial bolus of 5-HT and 5-CT (5-HT1/5/7 agonist) reduced electrically induced ΔRPP. Only L-694,247 (5-HT1D agonist) reproduced 5-CT inhibition on sympathetic-induced vasoconstrictions, whereas it did not modify exogenous noradrenaline-induced ΔRPP. 5-CT inhibition was exclusively abolished by i.v. bolus of LY310762 (5-HT1D antagonist). An inhibitor of guanylyl cyclase, ODQ (i.v.), completely reversed the L-694,247 inhibitory effect. In conclusion, diabetes induces an enhancement in sympathetic-induced vasopressor responses at the renal level. Prejunctional 5-HT1D receptors, via the nitric oxide pathway, inhibit noradrenergic-induced vasoconstrictions in diabetic rats. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

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‐HT1B/1D/5 receptors. Interestingly, when 5‐HT2 receptors are chronically blocked with sarpogrelate, the additional role of cardiac sympatho‐inhibitory 5‐HT1F 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. [ABSTRACT FROM AUTHOR]

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5-hydroxytryptamine (5- HT) modulates noradrenergic activity in different cardiovascular territories, but its effect on the mesenteric vasopressor outflow has not yet been clarified. This study investigated the in vivo serotonergic influence, characterizing 5- HT receptors implicated, in sympathetic innervation of mesenteric vasculature. Wistar rats were anaesthetised and prepared for the in situ autoperfused rat mesentery, monitoring systemic blood pressure ( SBP), heart rate ( HR) and mesenteric perfusion pressure ( MPP). Electrical stimulation of mesenteric sympathetic nerves resulted in frequency-dependent increases in MPP (9 ± 1.6, 25.7 ± 3.9 and 60.2 ± 5 mmHg for 2, 4 and 8 Hz, respectively), without altering SBP or HR. 5- HT (1-25 μg/kg), 5-carboxamidotryptamine (5- HT1/7 agonist; 25 μg/kg) or L-694,247 (5-HT1D agonist; 1-25 μg/kg) i.a. bolus inhibited vasopressor responses by mesenteric nerves electrical stimulation, unlike i.a. bolus of agonists 8- OH- DPAT (5- HT1A), CGS-12066B (5- HT1B), BRL 54443 (5- HT1e/1F), α-methyl-5- HT (5- HT2), 1- PBG (5- HT3), cisapride (5- HT4) or AS-19 (5- HT7) (25 μg/kg each). Interestingly, i.a. L-694,247 (25 μg/kg) also reduced the exogenous norepinephrine-induced vasoconstrictions. Pretreatment with selective 5- HT1D receptor antagonist, LY310762 (1 mg/kg, i.v.), completely abolished L-694,247- and 5- HT-induced mesenteric sympathoinhibition. Furthermore, ELISA analysis confirmed 5- HT1D receptors expression in mesenteric artery. These findings suggest that serotonergic mechanisms-induced sympathoinhibition of mesenteric noradrenergic outflow is mediated by pre and/or postjunctional 5- HT1D receptors. [ABSTRACT FROM AUTHOR]

In vivo stimulation of cardiac vagal neurons induces bradycardia by acetylcholine ( ACh) release. As vagal release of ACh may be modulated by autoreceptors (muscarinic M2) and heteroreceptors (including serotonin 5-HT1), this study has analysed the pharmacological profile of the receptors involved in histamine-induced inhibition of the vagal bradycardic out-flow in pithed rats. For this purpose, 180 male Wistar rats were pithed, artificially ventilated and pre-treated (i.v.) with 1 mg/kg atenolol, followed by i.v. administration of physiological saline (1 ml/kg), histamine (10, 50, 100 and 200 μg/kg) or the selective histamine H1 (2-pyridylethylamine), H2 (dimaprit), H3 (methimepip) and H4 (VUF 8430) receptor agonists (1, 10, 50 and 100 μg/kg each). Under these conditions, electrical stimulation (3, 6 and 9 Hz; 15 ± 3 V and 1 ms) of the vagus nerve resulted in frequency-dependent bradycardic responses, which were (i) unchanged during the infusions of saline, 2-pyridylethylamine, dimaprit or VUF 8430; and (ii) dose-dependently inhibited by histamine or methimepip. Moreover, the inhibition of the bradycardia caused by 50 μg/kg of either histamine or methimepip (which failed to inhibit the bradycardic responses to i.v. bolus injections of acetylcholine; 1-10 μg/kg) was abolished by the H3 receptor antagonist JNJ 10181457 (1 mg/kg, i.v.). In conclusion, our results suggest that histamine-induced inhibition of the vagal bradycardic out-flow in pithed rats is mainly mediated by pre-junctional activation of histamine H3 receptors, as previously demonstrated for the vasopressor sympathetic out-flow and the vasodepressor sensory CGRPergic (calcitonin gene-related peptide) out-flow. [ABSTRACT FROM AUTHOR]