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We have demonstrated that the antagonism of 5-HT2 receptors produces an enhancement of serotonergic sympathoinhibitory effect by 5-HT1D and 5-HT7 activation. The aim of this work was to determine mechanisms involved in the 5-hydroxytriptaminergic inhibitory action on the pressor responses elicited by sympathostimulation in pithed rats treated with a 5-HT2 receptor blocker. The blockade of 5-HT2 receptors was induced by orally sarpogrelate treatment (30 mg/kg/day). Two weeks later, animals were anaesthetized and pithed. A bolus injection of 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ) (10 µg/kg), a guanylyl cyclase inhibitor, or indomethacin (2mg/kg), a non-selective COX inhibitor, prior to the infusion of (2S)(+)-5-(1,3,5-trimethylpyrazol-4-yl)-2-(dimethylamino)tetralin, AS-19 (5 µg/kg/min) were not able to abolish its inhibitory action. However, i.v. administration of glibenclamide (20mg/kg), a blocker of ATP-sensitive K(+) channels, completely reversed AS-19 sympathoinhibitory action. The inhibitory effect of 2-[5-[3-(4-methylsulfonylamino)benzyl-1,2,4-oxadiazol-5-yl]-1H-indol-3-yl]ethanamine, L-694,247 (5 µg/kg/min) was abolished by indomethacin, whereas pretreatment with ODQ had no effect. Nimesulide (3mg/kg), a COX-2 inhibitor, completely reversed the inhibitory action of L-694,247, whereas 1-[[4,5-bis (4-methoxyphenyl)-2-thiazolyl]carbonyl]-4-methylpiperazine hydrochloride (FR122047) (3mg/kg), a COX-1 inhibitor, partially blocked this action. The sympathoinhibition by 5-HT (20 µg/kg/min) could not be elicited after i.v. treatment with indomethacin plus glibenclamide. In conclusion, these results suggest that in chronic sarpogrelate-treated rats, the inhibitory serotonergic effect of the pressor responses induced by electrical stimulation of the sympathetic outflow via 5-HT7 and 5-HT1D receptor activation is mediated by KATP channel-mediated smooth muscle hyperpolarization and the COX pathway, respectively., (Copyright © 2014 Elsevier B.V. All rights reserved.)

We investigated the mechanisms involved in the 5-hydroxytriptaminergic inhibitory action on the pressor responses elicited by sympathostimulation in long-term-diabetic pithed rats. Diabetes was induced in rats by alloxan administration. Eight weeks later, the animals were anaesthetized and pithed. The action and mechanisms of 5-HT were analysed based on the pressor responses induced by sympathostimulation. In 8-week-diabetic animals, 5-HT (20 µg/kg/min) inhibits the pressor effect of sympathostimulation which is reproduced by two selective 5-HT(1A) and 5-HT(2) receptor agonists: 8-hydroxydipropylaminotetralin hydrobromide (8-OH-DPAT, 5 µg/kg/min) and α-methyl-5-HT (5 µg/kg/min). A bolus injection of 1H-[1,2,4] oxadiazolo[4,3-a] quinoxalin-1-one (ODQ, 10 µg/kg), or L-arginine HCl, N(ω)-L-arginine methyl ester hydrochloride (L-NAME, 10 mg/kg), an inhibitor of NO production, prior to the infusion of 8-OH-DPAT (5 µg/kg/min) reversed the inhibitory effect of 8-OH-DPAT. The inhibitory effect of infusion of α-methyl 5-HT (5 µg/kg/min) was abolished in the presence of indomethacin (2 mg/kg), a non-selective cyclooxygenase (COX) inhibitor, or FR 122047 (1.5 mg/kg) or nimesulide (1.5 mg/kg), two selective COX-1 and COX-2 inhibitors, respectively, in long-term-diabetic pithed rats. Our results indicate that 5-HT inhibition of the pressor responses induced by electrical stimulation is mediated both by the NO and COX pathways in long-term-diabetic rats., (Copyright © 2012 S. Karger AG, Basel.)

1. In the present study, we investigated how alloxan-induced diabetes affects the ability of 5-hydroxytryptamine (5-HT) to modulate bradycardia induced in vivo by electrical stimulation of the vagus nerve in pithed rats. We also analysed the type and/or subtype of 5-HT receptors involved. 2. Diabetes was induced in male Wistar rats with a single injection of alloxan (150 mg/kg, s.c.). Four weeks later, rats were anaesthetized, pretreated with atenolol and pithed. Electrical stimulation (3, 6 and 9 Hz) of the vagus nerve resulted in frequency dependent decreases in heart rate (HR). 3. In diabetic rats, intravenous bolus administration of high doses of 5-HT (100 and 200 microg/kg) increased the bradycardia induced by vagal electrical stimulation. Similarly, low doses (10 microg/kg) of the 5-HT(1/7) receptor agonist 5-carboxamidotryptamine (5-CT), increased vagally induced bradycardia. However, at high doses (50, 100 and 150 microg/kg), 5-CT reduced the bradycardia. Attenuation of the vagally induced bradycardia evoked by the higher doses of 5-CT was reproduced by L-694,247 (50 microg/kg), a selective agonist for the non-rodent 5-HT(1B) and 5-HT(1D) receptors. Enhancement of the vagally induced bradycardia elicited by low doses of 5-CT was reproduced by the selective 5-HT(1A) receptor agonist 8-hydroxydipropylaminotretalin hydrobromide (8-OH-DPAT; 50 microg/kg). These stimulatory and inhibitory actions on vagal stimulation-induced bradycardia in diabetic rats were also observed after administration of exogenous acetylcholine. 4. Vagally induced bradycardia in diabetic rats was not affected by administration of the selective 5-HT(2) receptor agonist alpha-methyl-5-HT (150 microg/kg), the selective 5-HT(3) receptor agonist 1-phenylbiguanide (150 microg/kg) or the selective 5-HT(1B) receptor agonist CGS-12066B (50 microg/kg). 5. Enhancement of the electrical stimulation-induced bradycardia in diabetic rats caused by 5-CT (10 microg/kg) or 8-OH-DPAT (50 microg/kg) was abolished by the selective 5-HT(2/7) receptor antagonist mesulergine (1 mg/kg) and the selective 5-HT(1A) receptor antagonist WAY-100,635 (100 microg/kg), respectively. Similarly, pretreatment with the non-selective 5-HT(1) receptor antagonist methiothepin (0.1 mg/kg) blocked the inhibitory effect of 5-CT (50 microg/kg) on the bradycardia induced by vagal electrical stimulation in diabetic rats. BRL-15572 (2 microg/kg), a selective 5-HT(1D) receptor antagonist, inhibited the action of L-694,247 (50 microg/kg), a selective agonist for the non-rodent 5-HT(1B) and 5-HT(1D) receptors, on the vagally induced bradycardia. 6. In conclusion, in the present study, experimental diabetes evoked changes in both the nature and 5-HT receptor types/subtypes involved in vagally induced bradycardia.

1. In the present study, we analysed the effect of different doses of 5-hydroxytryptamine (5-HT; intravenous infusions of 0.001-40 microg/kg per min) in the autoperfused hindquarters of the rat subjected to electrical stimulation (frequencies of 0.5-20 Hz) of the lumbar chains, investigating the relationship between the adrenergic and serotonergic systems in this vascular bed. 2. Because we observed that 5-HT inhibited the increases in perfusion pressure induced by electrical stimulation of the lumbar chains, we used different agonists and antagonists to analyse the mechanism of action of 5-HT. 3. The effect of 5-HT was inhibited by methiothepin (a non-specific 5-HT receptor antagonist), but not by ritanserin (a selective 5-HT2 receptor antagonist). The effects of 5-HT were mimicked by 5-carboxamidotryptamine (a 5-HT1 receptor agonist) and L-694 247 (a selective 5-HT1D receptor agonist), but not by 8-hydroxy-2-dipropylaminotetralin (a 5-HT1A receptor agonist), CGS-12066B (a 5-HT1B receptor agonist), alpha-methyl-5-HT (a 5-HT2 receptor agonist), 1-(3-chlorophenyl) piperazine (a 5-HT2C receptor agonist) or 1-phenylbiguanide (a 5-HT3 receptor agonist). The selective 5-HT1D/1B receptor antagonist BRL 15572 inhibited the effect of the agonist L-694 247. 4. Our data suggest that 5-HT inhibits the increases in perfusion pressure induced by the electrical stimulation of the lumbar chains, acting on presynaptic 5-HT1D receptors and decreasing the release of noradrenaline from the sympathetic nerves in the hindquarter vascular bed of the rat.

In this work we studied the responses and receptors involved in the effects of intra-arterial 5-hydroxytryptamine (5-HT) in the in situ autoperfused hindquarters of spontaneously hypertensive rats (SHR). Intra-arterial administration of the highest doses (50-1,000 ng/kg) produced a vasoconstrictor effect that was inhibited by ritanserin (a selective 5-HT2 receptor antagonist), SB 206553 (a selective 5-HT(2B/2C) receptor antagonist) and spiperone (a nonspecific 5-HT(1/2A) receptor antagonist), and was mimicked by alpha-methyl-5-HT (a selective 5-HT2 receptor agonist) and m-CPP (a selective 5-HT2C receptor agonist), but not by the intra-arterial administration of BW 723C86, a selective 5HT2B receptor agonist. SB 206553 and spiperone inhibited alpha-methyl-5HT-induced vasoconstriction in the hindquarters of SHR. Our data suggest that the vasoconstrictor response induced by 5-HT in the autoperfused hindquarters of SHR is mainly mediated by the activation of 5-HT2A and 5-HT2C receptors., (Copyright 2004 S. Karger AG, Basel)

These studies were conducted to examine the role of the vasoactive mediators nitric oxide (NO) and adrenaline (epinephrine) in the serotonin (5-hydroxytryptamine; 5-HT)-induced vasodilator response in the hindquarter vascular bed of anaesthetized rats. Intra-arterial administration of doses of 5-HT in the range 0.12-25 ng kg(-1) produced a dose-independent vasodilator effect in the hindquarters. The selective 5-HT(1D/1B) receptor agonist, L-694,247 at intra-arterial doses of 0.0012-1000 ng kg(-1), as well as adrenaline (at doses of 0.05-50 ng kg(-1) i.a.), mimicked the dose-independent vasodilator effect induced by intra-arterial administration of 5-HT. Intravenous pre-treatment with the selective beta2-receptor antagonist ICI 118,551 (0.5 mg kg(-1)) blocked the vasodilator effect of 5-HT, adrenaline and L-694,247. Additionally, the inhibitor of NO synthase NG-nitro-L-arginine (L-NAME) (at a dose of 10 mg kg(-1) i.v.) blocked the vasodilator action of acetylcholine 300-3000 ng kg(-1)) but did not modify 5-HT-induced vasodilatation. The vasodilator effect produced by intra-arterial administration of 5-HT in the hindquarters was significantly inhibited both 30 min after denervation of the lumbar sympathetic chains and 1 h after bilateral adrenalectomy. Our data suggest that in the in-situ autoperfused hindquarters of the rat 5-HT-induced vasodilatation is mediated by a local 5-HT(1D) or 5-HT(1D/1B) activation, which in turn mediates the adrenal release of adrenaline, which then produces beta2-activation and vasodilatation.

In the present study we attempted to characterise the responses and receptors involved in the effects of 5-hydroxytryptamine (5-HT, serotonin) in in situ autoperfused rat hindquarters. Intra-arterial administration of the lowest doses of 5-HT used (0.12-12.5 ng/kg) induced vasodilator responses, whereas the highest doses (25-1000 ng/kg) produced vasoconstriction. The vasodilator effect was inhibited by methiothepin (a non-specific 5-HT(1,2,5,6,7) receptor antagonist) and by a 5-HT(1D/1B) receptor antagonist, i.e., 3-[4-(4-chlorophenyl)piperazin-1-yl]-1,1-diphenyl-2-propanolol (BRL 15572), but not by ritanserin (a selective 5-HT(2) receptor antagonist), 5-methyl-1-(3-pyridylcarbamoyl)-1,2,3,5-tetrahydropyrrolo[2,3-f] indole (SB 206553, a selective 5-HT(2B/2C) receptor antagonist) or mesulergine (a non-specific serotonergic antagonist that shows affinity to the 5-HT(7) receptor). This vasodilator effect was mimicked by administration of a selective 5-HT(1) receptor agonist - 5-carboxamidotryptamine (5-CT) - and by 2-[5-[3-(4-methylsulphonylamino)benzyl-1,2,4-1 H-indol-3-yl]ethanamine (L-694,247, a selective 5-HT(1D/1B) receptor agonist). Methiothepin, but not mesulergine, inhibited 5-CT-induced vasodilatation and the selective 5-HT(1D/1B) receptor antagonist (BRL 15572) inhibited the vasodilator action induced by L-694,247. The vasoconstrictor effect of 5-HT was significantly decreased by methiothepin, ritanserin and SB 206553, and was mimicked by alpha-methyl-5-HT (a selective 5-HT(2) receptor agonist) but not by administration of BW 723C86, a selective 5HT(2B) receptor agonist. Ritanserin, SB 206553 and spiperone (a non-specific 5-HT(1/2A) receptor antagonist) inhibited the alpha-methyl-5HT-induced vasoconstriction.Our data suggest that the vasodilator response induced by 5-HT in autoperfused rat hindquarters is mainly mediated by 5-HT(1D/1B) receptors, whereas the vasoconstrictor effect is mainly due to the activation of 5-HT(2A) receptors.

Using a number of agonist and antagonist compounds, we attempted to characterize the responses and receptors involved in the effects of 5-hydroxytryptamine (5-HT) in the in situ blood perfused rat mesentery. An intra-arterial (i.a.) bolus injection of 5-HT increased mesenteric perfusion pressure in a dose-dependent way but did not change the systemic blood pressure. The selective 5-HT(2) receptor agonists alpha-methyl-5-HT, 1-(3-chlorophenyl)piperazine dihydrochloride (m-CPP) and (+/-)-1-(4-iodo-2, 5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI), caused a local vasoconstrictor effect in the autoperfused vascular mesenteric bed. Intra-arterial injection of 5-carboxamidotryptamine (5-CT) and 1-(m-chlorophenyl)-biguanide (m-CPBG) did not modify the mesenteric perfusion pressure. The vasoconstrictor effect elicited by 5-HT and alpha-methyl-5-HT was significantly decreased by ritanserin and by a selective 5-HT(2B/2C) receptor antagonist, N-3-pyridinyl-3, 5-dihydro-5-methyl-benzo[1,2-b:4,5-b']dipyrrole-1(2H)-carboxamide hydrochloride (SB 206553), but was not modified by prazosin, propranolol, indomethacin or enalapril pretreatment. Our data suggest that the vasoconstrictor serotonergic response induced in the in situ autoperfused rat mesenteric vascular bed is mainly mediated by activation of 5-HT(2B) and/or 5-HT(2C) receptors.

1. A study was made of the effects of 5-carboxamidotryptamine (5-CT) on pressor responses induced in vivo by electrical stimulation of the sympathetic outflow from the spinal cord of pithed rats. All animals had been pretreated with atropine. Sympathetic stimulation (0.1, 0.5, 1 and 5 Hz) resulted in frequency-dependent increases in blood pressure. Intravenous infusion of 5-CT at doses of 0.01, 0.1 and 1 mg kg(-1) min(-1) reduced the pressor effects obtained by electrical stimulation. The inhibitory effect of 5-CT was significantly more pronounced at lower frequencies of stimulation. In the present study we characterized the pharmacological profile of the receptors mediating the above inhibitory effect of 5-CT. 2. The inhibition induced by 0.01 microg kg(-1) min(-1) of 5-CT on sympathetically-induced pressor responses was partially blocked after i.v. treatment with methiothepin (10 microg kg(-1)), WAY-100,635 (100 microg kg(-1)) or GR127935T (250 microg kg(-1)), but was not affected by cyanopindolol (100 microg kg(-1)). 3. The selective 5-HT1A receptor agonist 8-OH-DPAT and the selective 5-HT(1B/1D) receptor agonists sumatriptan and L-694,247 inhibited the pressor response, whereas the 5-HT1B receptor agonists CGS-12066B and CP-93,129 and the 5-HT2C receptor agonist m-CPP did not modify the pressor sympathetic responses. 4. The selective 5-HT1A receptor antagonist WAY-100,635 (100 microg kg(-1)) blocked the inhibition induced by 8-OH-DPAT and the selective 5-HT(1B/1D) receptor antagonist GR127935T (250 microg kg(-1)) abolished the inhibition induced either by L-694,247 or sumatriptan. 5. None of the 5-HT receptor agonists used in our experiments modified the pressor responses induced by exogenous noradrenaline (NA). 6. These results suggest that the presynaptic inhibitory action of 5-CT on the electrically-induced pressor response is mediated by both r-5-HT1D and 5-HT1A receptors.

Using a number of agonist and antagonist compounds, we attempted to characterize the responses and receptors involved in the effects of 5-hydroxytryptamine (5-HT) in the in situ autoperfused rat kidney. An intra-arterial (i.a.) bolus injection of 5-HT (0.0125 to 0.1 microg/kg) increased renal perfusion pressure in a dose-dependent way but did not change the systemic blood pressure. The 5-HT2 receptor agonist, (1-(3-chlorophenyl) piperazine), m-CPP, caused a local vasoconstrictor effect in the autoperfused rat kidney. An intra-arterial injection of 5-carboxamidotryptamine, 5-CT and 1-(m-chlorophenyl)-biguanide (m-CPBG) did not modify the renal perfusion pressure. The vasoconstrictor effect elicited by 5-HT and m-CPP was significantly decreased by ritanserin, enalapril and losartan but was not modified by prazosin, propranolol or indomethacin pretreatment. Our data suggest that the vasoconstrictor serotonergic response induced in the in situ autoperfused rat kidney is mediated through angiotensin II activation by a local 5-HT2 receptor mechanism.