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-HT_1/5/7 agonist) reduced electrically induced ΔRPP. Only L-694,247 (5-HT_1D 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-HT_1D 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-HT_1D receptors, via the nitric oxide pathway, inhibit noradrenergic-induced vasoconstrictions 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.

The vascular 5-HT sympatho-modulation may involve inhibitory or potentiating pathways: nitric oxide (NO), endothelium-dependent hyperpolarization (EDH)-K⁺ channels, prostanoids, angiotensin II (Ang-II), or endothelin. Compared to males, female rats show differences in the serotonergic sympatho-regulation; therefore, we aimed to study the involvement of indirect pathways via 5-HT_1D-mediated inhibition and 5-HT_2A/3-mediated potentiation of vascular noradrenergic neurotransmission in females. An i.v. bolus of different inhibitors/blockers of modulators/mediators (NO, K⁺ channels, prostanoids, Ang-II, or endothelin) was administered prior to the infusion of the agonists, L-694,247 (5-HT_1D), TCB-2 (5-HT_2A), or 1-PBG (5-HT₃), in female pithed rats. In these conditions, the vascular sympathetic outflow was electrically stimulated to assess the vasopressor responses. The L-694,247 vascular sympatho-inhibition was abolished by a non-selective K⁺ channel blocker, tetraethylammonium. The 1-PBG sympatho-excitatory vascular effect was not modified by any of the inhibitors tested, whereas TCB-2 sympatho-potentiation was blocked solely by losartan (Ang-II type 1 receptor antagonist). Moreover, Ang-II levels were increased after TCB-2 infusion in females. The EDH pathway mediates the 5-HT_1D-induced sympatho-inhibition, while the 5-HT_2A-evoked sympatho-excitatory effect is associated with Ang-II. In contrast, the 5-HT₃ sympatho-potentiation does not involve any indirect pathway. These findings advance current understanding of the complex interactions between 5-HT and vascular homeostasis in female rats.

Background/Objectives: Adenosine 5′-O-(β-thio)-diphosphate (ADPβS) is a stable analogue of adenosine diphosphate (ADP). In pithed rats, ADPβS produces (i) inhibition of both the cardioaccelerator sympathetic tone (via prejunctional P2Y₁₂/P2Y₁₃ receptors) and the vasodepressor sensory CGRPergic tone (via prejunctional P2Y₁ receptors) and (ii) vasodepressor/vasodilator responses via endothelial P2Y₁ receptors, and vasopressor/vasoconstrictor responses via P2Y₁/P2Y₁₂/P2Y₁₃ receptors. This study pharmacologically investigates whether P2Y₁, P2Y₁₂, and/or P2Y₁₃ receptors modulate the vasopressor sympathetic tone using ADPβS. Methods: A total of 126 male Wistar rats were pithed and classified into two groups to produce vasopressor responses by (i) spinal (T₇–T₉) electrical stimulation of the sympathetic vasopressor tone (0.03–3 Hz; n = 102) and (ii) intravenous (i.v.) boluses of noradrenaline (0.03–3 μg/kg; n = 24). Results: Continuous i.v. infusions of ADPβS (5.6 and 10 μg/kg·min) significantly inhibited the vasopressor responses to sympathetic electrical stimulation, without affecting those induced by exogenous noradrenaline. This sympathoinhibition by ADPβS (5.6 μg/kg·min) remained unchanged after vehicle (1 mL/kg, i.v.) or the antagonists PSB 0739 (P2Y_12; 300 μg/kg, i.v.) and MRS 2211 (P2Y_13; 1000 and 3000 μg/kg, i.v.). Conversely, it was abolished after MRS 2500 (P2Y_1; 300 μg/kg, i.v.) or glibenclamide (K_ATP channel blocker; 20 mg/kg, i.v.). Conclusions: ADPβS inhibits the vasopressor sympathetic tone via prejunctional P2Y₁ receptors through K_ATP channel-dependent mechanisms.