Pharmacological profile of the 5-HT-induced inhibition of cardioaccelerator sympathetic outflow in pithed rats: correlation with 5-HT1 and putative 5-ht5A/5B receptors

Serotonergic Modulation of Neurovascular Transmission: A Focus on Prejunctional 5-HT Receptors/Mechanisms

5-Hydroxytryptamine (5-HT), or serotonin, plays a crucial role as a neuromodulator and/or neurotransmitter of several nervous system functions. Its actions are complex, and depend on multiple factors, including the type of effector or receptor activated. Briefly, 5-HT can activate: (i) metabotropic (G-protein-coupled) receptors to promote inhibition (5-HT₁, 5-HT₅) or activation (5-HT₄, 5-HT₆, 5-HT₇) of adenylate cyclase, as well as activation (5-HT₂) of phospholipase C; and (ii) ionotropic receptor (5-HT₃), a ligand-gated Na⁺/K⁺ channel. Regarding blood pressure regulation (and beyond the intricacy of central 5-HT effects), this monoamine also exerts direct postjunctional (on vascular smooth muscle and endothelium) or indirect prejunctional (on autonomic and sensory perivascular nerves) effects. At the prejunctional level, 5-HT can facilitate or preclude the release of autonomic (e.g., noradrenaline and acetylcholine) or sensory (e.g., calcitonin gene-related peptide) neurotransmitters facilitating hypertensive or hypotensive effects. Hence, we cannot formulate a specific impact of 5-HT on blood pressure level, since an increase or decrease in neurotransmitter release would be favoured, depending on the type of prejunctional receptor involved. This review summarizes and discusses the current knowledge on the prejunctional mechanisms involved in blood pressure regulation by 5-HT and its impact on some vascular-related diseases.

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.

International Union of Basic and Clinical Pharmacology. CX. Classification of Receptors for 5-hydroxytryptamine; Pharmacology and Function

5-HT receptors expressed throughout the human body are targets for established therapeutics and various drugs in development. Their diversity of structure and function reflects the important role 5-HT receptors play in physiologic and pathophysiological processes. The present review offers a framework for the official receptor nomenclature and a detailed understanding of each of the 14 5-HT receptor subtypes, their roles in the systems of the body, and, where appropriate, the (potential) utility of therapeutics targeting these receptors. SIGNIFICANCE STATEMENT: This review provides a comprehensive account of the classification and function of 5-hydroxytryptamine receptors, including how they are targeted for therapeutic benefit.

Role of peripheral 5-HT5A receptors in 5-HT-induced cardiac sympatho-inhibition in type 1 diabetic rats

5-HT inhibits cardiac sympathetic neurotransmission in normoglycaemic rats, via 5-HT1B, 5-HT1Dand 5-HT5Areceptor activation. Since type 1 diabetes impairs the cardiac sympathetic innervation leading to cardiopathies, this study aimed to investigate whether the serotonergic influence on cardiac noradrenergic control is altered in type 1 diabetic rats. Diabetes was induced in male Wistar rats by streptozotocin (50 mg/kg, i.p.). Four weeks later, the rats were anaesthetized, pithed and prepared for producing tachycardic responses by electrical preganglionic stimulation (C7-T1) of the cardioaccelerator sympathetic outflow or i.v. noradrenaline bolus injections. Immunohistochemistry was performed to study 5-HT1B, 5-HT1Dand 5-HT5Areceptor expression in the stellate ganglion from normoglycaemic and diabetic rats. In the diabetic group, i) i.v. continuous infusions of 5-HT induced a cardiac sympatho-inhibition that was mimicked by the 5-HT1/5Aagonist 5-carboxamidotryptamine (without modifying noradrenaline-induced tachycardia), but not by the agonists indorenate (5-HT1A), CP 93,129 (5-HT1B), PNU 142633 (5-HT1D), or LY344864 (5-HT1F); ii) SB 699551 (5-HT5Aantagonist; i.v.) completely reversed 5-CT-induced cardiac sympatho-inhibition; and iii) 5-HT5Areceptors were more expressed in the stellate ganglion compared to normoglycaemic rats. These results show the prominent role of the peripheral 5-HT5Areceptors prejunctionally inhibiting the cardiac sympathetic drive in type 1 diabetic rats.

Role of peripheral 5-HT5A receptors in 5-HT-induced cardiac sympatho-inhibition in type 1 diabetic rats

5-HT inhibits cardiac sympathetic neurotransmission in normoglycaemic rats, via 5-HT_1B, 5-HT_1D and 5-HT_5A receptor activation. Since type 1 diabetes impairs the cardiac sympathetic innervation leading to cardiopathies, this study aimed to investigate whether the serotonergic influence on cardiac noradrenergic control is altered in type 1 diabetic rats. Diabetes was induced in male Wistar rats by streptozotocin (50 mg/kg, i.p.). Four weeks later, the rats were anaesthetized, pithed and prepared for producing tachycardic responses by electrical preganglionic stimulation (C₇-T₁) of the cardioaccelerator sympathetic outflow or i.v. noradrenaline bolus injections. Immunohistochemistry was performed to study 5-HT_1B, 5-HT_1D and 5-HT_5A receptor expression in the stellate ganglion from normoglycaemic and diabetic rats. In the diabetic group, i) i.v. continuous infusions of 5-HT induced a cardiac sympatho-inhibition that was mimicked by the 5-HT_1/5A agonist 5-carboxamidotryptamine (without modifying noradrenaline-induced tachycardia), but not by the agonists indorenate (5-HT_1A), CP 93,129 (5-HT_1B), PNU 142633 (5-HT_1D), or LY344864 (5-HT_1F); ii) SB 699551 (5-HT_5A antagonist; i.v.) completely reversed 5-CT-induced cardiac sympatho-inhibition; and iii) 5-HT_5A receptors were more expressed in the stellate ganglion compared to normoglycaemic rats. These results show the prominent role of the peripheral 5-HT_5A receptors prejunctionally inhibiting the cardiac sympathetic drive in type 1 diabetic rats.

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.

Characterization of binding, functional activity, and contractile responses of the selective 5-HT1F receptor agonist lasmiditan

BACKGROUND AND PURPOSE

Triptans are 5-HT1B/1D receptor agonists (that also display 5-HT1F receptor affinity) with antimigraine action, contraindicated in patients with coronary artery disease due to their vasoconstrictor properties. Conversely, lasmiditan was developed as an antimigraine 5-HT1F receptor agonist. To assess the selectivity and cardiovascular effects of lasmiditan, we investigated the binding, functional activity, and in vitro/in vivo vascular effects of lasmiditan and compared it to sumatriptan.

EXPERIMENTAL APPROACH

Binding and second messenger activity assays of lasmiditan and other serotoninergic agonists were performed for human 5-HT1A , 5-HT1B , 5-HT1D , 5-ht1E , 5-HT1F , 5-HT2A , 5-HT2B , and 5-HT7 receptors, and the results were correlated with their potency to constrict isolated human coronary arteries (HCAs). Furthermore, concentration-response curves to lasmiditan and sumatriptan were performed in proximal and distal HCA, internal mammary, and middle meningeal arteries. Finally, anaesthetized female beagle dogs received i.v. infusions of lasmiditan or sumatriptan in escalating cumulative doses, and carotid and coronary artery diameters were measured.

KEY RESULTS

Lasmiditan showed high selectivity for 5-HT1F receptors. Moreover, the functional potency of the analysed compounds to inhibit cAMP increase through 5-HT1B receptor activation positively correlated with their potency to contract HCA. In isolated human arteries, sumatriptan, but not lasmiditan, induced contractions. Likewise, in vivo, sumatriptan decreased coronary and carotid artery diameters at clinically relevant doses, while lasmiditan was devoid of vasoconstrictor activity at all doses tested.

CONCLUSIONS AND IMPLICATIONS

Lasmiditan is a selective 5-HT1F receptor agonist devoid of vasoconstrictor activity. This may represent a cardiovascular safety advantage when compared to the triptans.

Functional Characterization of the Prejunctional Receptors Mediating the Inhibition by Ergotamine of the Rat Perivascular Sensory Peptidergic Drive

Calcitonin gene-related peptide (α-CGRP) released from perivascular sensory nerves induces decreases in diastolic blood pressure (DBP). Experimentally, this can be shown by spinal thoracic (T₉-T₁₂) electrical stimulation of these afferent fibers. Because ergotamine inhibits these neurogenic vascular responses and displays affinity for monoaminergic receptors that inhibit neurotransmitter release, we investigated whether this ergotamine-induced inhibition results from activation of serotonin 5-HT_1B/1D, dopamine D₂-like, and α₂-adrenergic receptors. Wistar rats were pithed and, under autonomic ganglion blockade, received intravenous infusions of methoxamine followed by ergotamine (0.1-3.1 μg kg^-1 min^-1). Thoracic T₉-T₁₂ electrical stimulation or an intravenous bolus of α-CGRP resulted in decreases in DBP. Ergotamine inhibited the electrically induced, but not α-CGRP-induced, responses. The vasodilator sensory inhibition by 3.1 μg of ergotamine kg^-1 min^-1 was resistant to simultaneous blockade of 5-HT_1B/1D, D₂-like, and α₂-adrenergic receptors upon addition of antagonists GR127935, haloperidol, and rauwolscine. Moreover, the inhibition by 0.31 μg of ergotamine kg^-1 min^-1 was unaltered by GR127935 and haloperidol, partly blocked by GR127935 and rauwolscine or rauwolscine and haloperidol, and abolished by GR127935, haloperidol, and rauwolscine. These findings imply that prejunctional 5-HT_1B/1D, D₂-like, and α₂-adrenergic receptors mediate the sensory inhibition induced by 0.31 μg of ergotamine kg^-1 min^-1, whereas larger doses may involve other receptors. Thus, ergotamine's ability to inhibit the perivascular sensory peptidergic drive may result in facilitation of its systemic vasoconstrictor properties.

Chronic 5-HT2 receptor blockade unmasks the role of 5-HT1F receptors in the inhibition of rat cardioaccelerator sympathetic outflow

Serotonin (5-hydroxytryptamine; 5-HT) inhibits the rat cardioaccelerator sympathetic outflow by 5-HT1B/1D/5 receptors. Because chronic blockade of sympatho-excitatory 5-HT2 receptors is beneficial in several cardiovascular pathologies, this study investigated whether sarpogrelate (a 5-HT2 receptor antagonist) alters the pharmacological profile of the above sympatho-inhibition. Rats were pretreated for 2 weeks with sarpogrelate in drinking water (30 mg/kg per day; sarpogrelate-treated group) or equivalent volumes of drinking water (control group). Animals were pithed and prepared for spinal stimulation (C7-T1) of the cardioaccelerator sympathetic outflow or for intravenous (i.v.) bolus injections of noradrenaline. Both procedures produced tachycardic responses remaining unaltered after saline. Continuous i.v. infusions of 5-HT induced a cardiac sympatho-inhibition that was mimicked by the 5-HT receptor agonists 5-carboxamidotryptamine (5-CT; 5-HT1/5A), CP 93,129 (5-HT1B), or PNU 142633 (5-HT1D), but not by indorenate (5-HT1A) in both groups; whereas LY344864 (5-HT1F) mimicked 5-HT only in sarpogrelate-treated rats. In sarpogrelate-treated animals, i.v. GR 127935 (310 μg/kg; 5-HT1B/1D/1F receptor antagonist) attenuated 5-CT-induced sympatho-inhibition and abolished LY344864-induced sympatho-inhibition; while GR 127935 plus SB 699551 (1 mg/kg; 5-HT5A receptor antagonist) abolished 5-CT-induced inhibition. These results confirm the cardiac sympatho-inhibitory role of 5-HT1B, 5-HT1D, and 5-HT5A receptors in both groups; nevertheless, sarpogrelate treatment specifically unmasked a cardiac sympatho-inhibition mediated by 5-HT1F receptors.

Role of 5-HT5A and 5-HT1B/1D receptors in the antinociception produced by ergotamine and valerenic acid in the rat formalin test

Sumatriptan, dihydroergotamine and methysergide inhibit 1% formalin-induced nociception by activation of peripheral 5-HT1B/1D receptors. This study set out to investigate the pharmacological profile of the antinociception produced by intrathecal and intraplantar administration of ergotamine (a 5-HT1B/1D and 5-HT5A/5B receptor agonist) and valerenic acid (a partial agonist at 5-HT5A receptors). Intraplantar injection of 1% formalin in the right hind paw resulted in spontaneous flinching behavior of the injected hindpaw of female Wistar rats. Intrathecal ergotamine (15nmol) or valerenic acid (1 nmol) blocked in a dose dependent manner formalin-induced nociception. The antinociception by intrathecal ergotamine (15nmol) or valerenic acid (1nmol) was partly or completely blocked by intrathecal administration of the antagonists: (i) methiothepin (non-selective 5-HT5A/5B; 0.01-0.1nmol); (ii) SB-699551 (selective 5-HT5A; up to 10nmol); (iii) anti-5-HT5A antibody; (iv) SB-224289 (selective 5-HT1B; 0.1-1nmol); or (v) BRL-15572 (selective 5-HT1D; 0.1-1nmol). Likewise, antinociception by intraplantar ergotamine (15nmol) and valerenic acid (10nmol) was: (i) partially blocked by methiothepin (1nmol), SB-699551 (10nmol) or SB-224289 (1nmol); and (ii) abolished by BRL-15572 (1nmol). The above doses of antagonists (which did not affect per se the formalin-induced nociception) were high enough to completely block their respective receptors. Our results suggest that ergotamine and valerenic acid produce antinociception via 5-HT5A and 5-HT1B/1D receptors located at both spinal and peripheral sites. This provides new evidence for understanding the modulation of nociceptive pathways in inflammatory pain.

Further evidence for the role of histamine H3, but not H1, H2 or H4, receptors in immepip-induced inhibition of the rat cardioaccelerator sympathetic outflow

Since histamine H3 and H4 receptors are coupled to heterotrimeric Gi/o proteins, a signal transduction pathway associated with inhibition of neurotransmitter release, the present study has investigated the inhibition of the rat cardioaccelerator sympathetic outflow induced by the H3/H4 receptor agonist immepip by using antagonists for histamine H1 (ketotifen), H2 (ranitidine), H3 (thioperamide) and H4 (JNJ7777120) receptors. For this purpose, 102 male Wistar rats were pithed, artificially ventilated and prepared for either preganglionic spinal (C7-T1) stimulation of the cardioaccelerator sympathetic outflow (n=90) or i.v. bolus injections of noradrenaline (n=12). This approach resulted in frequency-dependent and dose-dependent tachycardic responses, respectively. I.v. continuous infusions of immepip (3 and 10 μg/kg min), but not of saline (0.02 ml/min), dose-dependently inhibited the sympathetically-induced tachycardic responses. Moreover, the cardiac sympatho-inhibition induced by 10 μg/kg min immepip (which failed to affect the tachycardic responses to i.v. noradrenaline) was: (i) unaltered after i.v. treatment with 1 ml/kg vehicle, 100 μg/kg ketotifen, 3000 μg/kg ranitidine, 30 μg/kg thioperamide or 300 μg/kg JNJ7777120; and (ii) abolished after 100 μg/kg thioperamide (i.v.). These doses of antagonists, which did not affect per se the sympathetically-induced tachycardic responses, were high enough to block their respective receptors. In conclusion, the cardiac sympatho-inhibition induced by 10 μg/kg.min immepip involves histamine H3 receptors, with further pharmacological evidence excluding the involvement of H1, H2 and H4 receptors.

5-Hydroxytryptamine does not reduce sympathetic nerve activity or neuroeffector function in the splanchnic circulation

Infusion of 5-hydroxytryptamine (5-HT) in conscious rats results in a sustained (up to 30 days) fall in blood pressure. This is accompanied by an increase in splanchnic blood flow. Because the splanchnic circulation is regulated by the sympathetic nervous system, we hypothesized that 5-HT would: 1) directly reduce sympathetic nerve activity in the splanchnic region; and/or 2) inhibit sympathetic neuroeffector function in splanchnic blood vessels. Moreover, removal of the sympathetic innervation of the splanchnic circulation (celiac ganglionectomy) would reduce 5-HT-induced hypotension. In anaesthetized Sprague-Dawley rats, mean blood pressure was reduced from 101±4 to 63±3mm Hg during slow infusion of 5-HT (25μg/kg/min, i.v.). Pre- and postganglionic splanchnic sympathetic nerve activity were unaffected during 5-HT infusion. In superior mesenteric arterial rings prepared for electrical field stimulation, neither 5-HT (3, 10, 30nM), the 5-HT1B receptor agonist CP 93129 nor 5-HT1/7 receptor agonist 5-carboxamidotryptamine inhibited neurogenic contraction compared to vehicle. 5-HT did not inhibit neurogenic contraction in superior mesenteric venous rings. Finally, celiac ganglionectomy did not modify the magnitude of fall or time course of 5-HT-induced hypotension when compared to animals receiving sham ganglionectomy. We conclude it is unlikely 5-HT interacts with the sympathetic nervous system at the level of the splanchnic preganglionic or postganglionic nerve, as well as at the neuroeffector junction, to reduce blood pressure. These important studies allow us to rule out a direct interaction of 5-HT with the splanchnic sympathetic nervous system as a cause of the 5-HT-induced fall in blood pressure.

Pharmacological evidence that dopamine inhibits the cardioaccelerator sympathetic outflow via D2-like receptors in pithed rats

It has been suggested that N,N-di-n-propyl-dopamine (dopamine analogue) decreased heart rate in rats through stimulation of dopamine receptors. Nevertheless, the role of prejunctional dopamine D1/2-like receptors or even α2-adrenoceptors to mediate cardiac sympatho-inhibition induced by dopamine remains unclear. Hence, this study identified the pharmacological profile of the cardiac sympatho-inhibition to dopamine in pithed rats. Male Wistar rats were pithed and prepared to stimulate the cardiac sympathetic outflow or to receive i.v. bolus of exogenous noradrenaline. I.v. continuous infusions of dopamine (endogenous ligand) or quinpirole (D2-like agonist) dose-dependently inhibited the tachycardic responses to sympathetic stimulation, but not those to exogenous noradrenaline. In contrast, SKF-38393 (100 μg/kg∙min, D1-like agonist) failed to modify both of these responses. The sympatho-inhibition to dopamine (1.8 μg/kg∙min) or quinpirole (100 μg/kg∙min): i) remained unaltered after saline or the antagonists SCH-23390 (D1-like, 300 μg/kg) and rauwolscine (α2-adrenoceptors, 300 μg/kg); and ii) was significantly antagonized by raclopride (D2-like, 300 μg/kg). These antagonists, at the above doses, failed to modify the sympathetically-induced tachycardic responses. The above results suggest that the inhibition of the cardiac sympathetic outflow to dopamine and quinpirole is primarily mediated by prejunctional D2-like receptors but not D1-like receptors or α2-adrenoceptors.

The role of dopamine D2, but not D3 or D4, receptor subtypes, in quinpirole-induced inhibition of the cardioaccelerator sympathetic outflow in pithed rats

BACKGROUND AND PURPOSE

Quinpirole (a dopamine D2-like receptor agonist) inhibits the cardioaccelerator sympathetic outflow in pithed rats by sympathoinhibitory D2-like receptors. The present study was designed to identify pharmacologically the specific D2-like receptor subtypes (i.e. D2 , D3 and D4) involved in this sympathoinhibition by quinpirole.

EXPERIMENTAL APPROACH

One hundred fourteen male Wistar rats were pithed, artificially ventilated with room air and prepared for either preganglionic spinal (C7-T1) stimulation of the cardioaccelerator sympathetic outflow (n = 102) or i.v. bolus injections of exogenous noradrenaline (n = 12). This approach resulted in frequency-dependent and dose-dependent tachycardic responses, respectively, as previously reported by our group.

KEY RESULTS

I.v. continuous infusions of quinpirole (0.1-10 μg kg(-1)  min(-1)), but not of saline (0.02 mL min(-1)), dose-dependently inhibited the sympathetically induced tachycardic responses. Moreover, the cardiac sympathoinhibition induced by 3 μg kg(-1)  min(-1) quinpirole (which failed to affect the tachycardic responses to i.v. noradrenaline) was: (i) unchanged after i.v. injections of the antagonists SB-277011-A (D3 ; 100-300 μg kg(-1)) or L-745,870 (D4 ; 30-100 μg kg(-1)); and (ii) markedly blocked and abolished by, respectively, 100 and 300 μg kg(-1) of the D2 preferring receptor subtype antagonist L-741,626. These doses of antagonists, which did not affect per se the sympathetically induced tachycardic responses, were high enough to completely block their respective receptors.

CONCLUSIONS AND IMPLICATIONS

The cardiac sympathoinhibition induced by 3 μg kg(-1)  min(-1) quinpirole involves the dopamine D2 receptor subtype, with no evidence for the involvement of the D3 or D4 subtypes. This provides new evidence for understanding the modulation of the cardioaccelerator sympathetic outflow.

Effect of central serotonin depletion on 5-HT receptor-mediated vasomotor responses in the middle meningeal artery of anaesthetized rats

1 It has been hypothesized that craniovascular 5-HT receptors mediating dilatation of cranial vessels undergo sensitization on decreased serotonergic transmission in migraine. This study analysed the effect of chemical lesion of the 5-HT system in the brain with 5,7-dihydroxytryptamine (5,7-DHT) on 5-HT receptor-mediated dilator responses to 5-carboxamidotryptamine (5-CT) in the middle meningeal artery of anaesthetized rats. 5-CT has recently been shown to elicit dilator responses in this cranial vessel via 5-HT(7) receptors and, to a much lesser extent, 5-HT(1B/1D) receptors. 2 Pretreatment with 5,7-DHT produced a drastic and selective decrease of 5-HT levels in the brain (78 +/- 6% and 94 +/- 2% in dorsal raphe and hypothalamic paraventricular nuclei, respectively) compared with controls (1% ascorbic acid). 3 Topical application of 5-CT (1-1000 microm) to exposed dura mater encephali produced concentration-dependent decreases in diastolic blood pressure and dilator responses in the middle meningeal artery that were similar in vehicle- and 5,7-DHT-pretreaed animals. 4 Hypotensive and meningeal dilator responses to 5-CT were unaltered by the 5-HT(1B/1D) receptor antagonist, GR-127935 (1 mg kg(-1), i.v.), but were strongly inhibited by the 5-HT(7) receptor antagonist, SB-269970 (1 mg kg(-1), i.v.), with similar efficacy, in both groups of animals. Treatment with GR-127935 + SB-269970 (1 mg kg(-1), i.v. each), produced a stronger inhibitory effect than individual treatments on hypotensive but not on meningeal responses to 5-CT. Meningeal 5-HT(7) receptor-mediated responses (i.e. in GR-127935-pretreated animals) were unchanged by 5,7-DHT pretreatment. 5 Results suggest that the sensitivity of craniovascular 5-HT(7) receptors mediating dilatation is unaffected by a decrease of 5-HT levels in the brain. A neuronal involvement of 5-HT in migraine seems more likely, therefore.

Pharmacological profile of the clonidine-induced inhibition of vasodepressor sensory outflow in pithed rats: correlation with alpha(2A/2C)-adrenoceptors

BACKGROUND AND PURPOSE

Resistance blood vessels are innervated by sympathetic and primary sensory nerves, which modulate vascular tone through the release of noradrenaline and calcitonin gene-related peptide (CGRP), respectively. Moreover, electrical stimulation of the perivascular sensory outflow in pithed rats results in vasodepressor responses which are mainly mediated by CGRP release. The present study has investigated the role of alpha(2)-adrenoceptors in the inhibition of these vasodepressor responses.

EXPERIMENTAL APPROACH

144 pithed male Wistar rats were pretreated with hexamethonium (2 mg kg(-1) min(-1)) followed by i.v. continuous infusions of either methoxamine (15 and 30 microg kg(-1) min(-1)) or clonidine (3, 10 and 30 microg kg(-1) min(-1)). Under these conditions, electrical stimulation (0.56-5.6 Hz; 50 V and 2 ms) of the spinal cord (T(9)-T(12)) resulted in frequency-dependent decreases in diastolic blood pressure.

KEY RESULTS

The infusion of clonidine (10 microg kg(-1) min(-1)), as compared to those of methoxamine (15 or 30 microg kg(-1) min(-1)), inhibited the vasodepressor responses to electrical stimulation without affecting those to i.v. bolus injections of alpha-CGRP (0.1-1 microg kg(-1)). This inhibition by clonidine was: (i) antagonized by 300 microg kg(-1) rauwolscine (alpha(2A/2B/2C)), 300 and 1000 microg kg(-1) BRL44408 (alpha(2A)), or 10 and 30 microg kg(-1) MK912 (alpha(2C)); and (ii) unaffected by 1 ml kg(-1) saline, 100 microg kg(-1) BRL44408, 3000 and 10,000 microg kg(-1) imiloxan (alpha(2B)) or 3 microg kg(-1) MK912.

CONCLUSIONS AND IMPLICATIONS

The inhibition produced by 10 microg kg(-1) min(-1) clonidine on the vasodepressor (perivascular) sensory outflow in rats may be mainly mediated by prejunctional alpha(2A)/alpha(2C)-adrenoceptors.

Cardiovascular responses produced by 5-hydroxytriptamine:a pharmacological update on the receptors/mechanisms involved and therapeutic implications

The complexity of cardiovascular responses produced by 5-hydroxytryptamine (5-HT, serotonin), including bradycardia or tachycardia, hypotension or hypertension, and vasodilatation or vasoconstriction, has been explained by the capability of this monoamine to interact with different receptors in the central nervous system (CNS), on the autonomic ganglia and postganglionic nerve endings, on vascular smooth muscle and endothelium, and on the cardiac tissue. Depending, among other factors, on the species, the vascular bed under study, and the experimental conditions, these responses are mainly mediated by 5-HT(1), 5-HT(2), 5-HT(3), 5-HT(4), 5-ht(5A/5B), and 5-HT(7) receptors as well as by a tyramine-like action or unidentified mechanisms. It is noteworthy that 5-HT(6) receptors do not seem to be involved in the cardiovascular responses to 5-HT. Regarding heart rate, intravenous (i.v.) administration of 5-HT usually lowers this variable by eliciting a von Bezold-Jarisch-like reflex via 5-HT(3) receptors located on sensory vagal nerve endings in the heart. Other bradycardic mechanisms include cardiac sympatho-inhibition by prejunctional 5-HT(1B/1D) receptors and, in the case of the rat, an additional 5-ht(5A/5B) receptor component. Moreover, i.v. 5-HT can increase heart rate in different species (after vagotomy) by a variety of mechanisms/receptors including activation of: (1) myocardial 5-HT(2A) (rat), 5-HT(3) (dog), 5-HT(4) (pig, human), and 5-HT(7) (cat) receptors; (2) adrenomedullary 5-HT(2) (dog) and prejunctional sympatho-excitatory 5-HT(3) (rabbit) receptors associated with a release of catecholamines; (3) a tyramine-like action mechanism (guinea pig); and (4) unidentified mechanisms (certain lamellibranch and gastropod species). Furthermore, central administration of 5-HT can cause, in general, bradycardia and/or tachycardia mediated by activation of, respectively, 5-HT(1A) and 5-HT(2) receptors. On the other hand, the blood pressure response to i.v. administration of 5-HT is usually triphasic and consists of an initial short-lasting vasodepressor response due to a reflex bradycardia (mediated by 5-HT(3) receptors located on vagal afferents, via the von Bezold-Jarisch-like reflex), a middle vasopressor phase, and a late, longer-lasting, vasodepressor response. The vasopressor response is a consequence of vasoconstriction mainly mediated by 5-HT(2A) receptors; however, vasoconstriction in the canine saphenous vein and external carotid bed as well as in the porcine cephalic arteries and arteriovenous anastomoses is due to activation of 5-HT(1B) receptors. The late vasodepressor response may involve three different mechanisms: (1) direct vasorelaxation by activation of 5-HT(7) receptors located on vascular smooth muscle; (2) inhibition of the vasopressor sympathetic outflow by sympatho-inhibitory 5-HT(1A/1B/1D) receptors; and (3) release of endothelium-derived relaxing factor (nitric oxide) by 5-HT(2B) and/or 5-HT(1B/1D) receptors. Furthermore, central administration of 5-HT can cause both hypotension (mainly mediated by 5-HT(1A) receptors) and hypertension (mainly mediated by 5-HT(2) receptors). The increasing availability of new compounds with high affinity and selectivity for the different 5-HT receptor subtypes makes it possible to develop drugs with potential therapeutic usefulness in the treatment of some cardiovascular illnesses including hypertension, migraine, some peripheral vascular diseases, and heart failure.

Pharmacological evidence that 5-HT(1B/1D) receptors mediate hypotension in anesthetized rats

5-Carboxamidotryptamine (5-CT; 0.003-310 microg/kg, i.v.) produced dose-dependent hypotensive responses which were blocked in a complex manner by the 5-HT(7) receptor antagonist, (R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl) ethyl] pyrrolidine (SB-269970; 1 mg/kg, i.v.), in anesthetized vagosympathectomized rats. Interestingly, the 5-HT(1B/1D) receptor antagonist, N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl) [1,1-biphenyl]-4-carboxamide hydrochloride monohydrate GR-127935 (1 mg/kg, i.v.), also inhibited 5-CT-induced hypotension but the effect was clearly noncompetitive. Finally, the combination of GR-127935+SB-269970 (1 mg/kg, i.v., each) produced a further decreased of 5-CT-induced responses as compared to the effect of individual treatments. These data suggest that, in addition to 5-HT(7) receptors, 5-HT(1B/1D) receptors may also mediate hypotension in rats.

5-HT7 receptor-mediated dilatation in the middle meningeal artery of anesthetized rats

Topical administration of 5-carboxamidotryptamine (5-CT; 0.01-1000 microM) to the exposed dura mater encephali of anesthetized rats produced decreases in blood pressure and dilatation in the middle meningeal artery. Pretreatment with the 5-HT(1B/1D) receptor antagonist, N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl) [1,1-biphenyl]-4-carboxamide hydrochloride monohydrate (GR-127935; 1 mg/kg, i.v.), unmasked meningeal dilator responses to lower concentrations of 5-CT, and attenuated those to higher concentrations; GR-127935 also inhibited 5-CT-induced hypotension. The 5-HT7 receptor antagonist, (R)-1-{(3-hydroxyphenyl)sulfonyl}-2-{2-(2-(4-methyl-1-piperidinyl) ethyl} pyrrolidine (SB-269970; 1 mg/kg, i.v.), strongly inhibited dilator and hypotensive responses to 5-CT; the combination of GR-127935+SB-269970 (1 mg/kg, i.v., each) further inhibited meningeal and hypotensive responses. Thus, 5-CT may produce dilatation in the middle meningeal artery via 5-HT7 receptors; complex effects appear to involve 5-HT(1B/1D) receptors.

Potential vascular α1-adrenoceptor blocking properties of an array of 5-HT receptor ligands in the rat

This study set out to analyse the potential ability of some 5-hydroxytryptamine (5-HT) receptor ligands widely used in cardiovascular experimental models to interact with vascular alpha1-adrenoceptors in the pithed rat. These ligands included: methiothepin, methysergide and metergoline (5-HT(1)/5-HT2); WAY-100635, buspirone, ipsapirone and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (5-HT(1A)); GR127935 (5-HT(1B/1D)); ketanserin, ritanserin, spiperone and pizotifen (5-HT2); granisetron and metoclopramide (5-HT3); tropisetron (5-HT3/5-HT4); ergotamine (5-HT(1B/1D), 5-ht(5A/5B)); clozapine (5-HT6/5-HT7); as well as LY215840 and mesulergine (5-HT2/5-HT7). For this purpose, the increases in diastolic blood pressure produced by the selective alpha1-adrenoceptor agonist, phenylephrine, were analysed before and after the above antagonists or saline. The adrenoceptor antagonist properties of prazosin (alpha1) and yohimbine (alpha2) were also analysed for comparison. Thus, the phenylephrine-induced vasopressor responses were dose-dependently antagonised with the following apparent rank order of potency by: prazosin > or = methiothepin > ketanserin > clozapine > or = lisuride >> buspirone; this potency correlates with the affinity of these compounds for alpha1-adrenoceptor binding sites. In contrast, the other compounds were either devoid of any blocking effect on--or even potentiated (i.e. lisuride, methysergide, 8-OH-DPAT, granisetron and GR127935)--the responses to phenylephrine. These results show that methiothepin, ketanserin, clozapine, lisuride and buspirone can block alpha1-adrenoceptors in the rat systemic vasculature.

Further characterization of the 5-HT1 receptors mediating cardiac sympatho-inhibition in pithed rats: pharmacological correlation with the 5-HT1B and 5-HT1D subtypes

Serotonin (5-hydroxytryptamine; 5-HT) is capable of inhibiting the tachycardic responses elicited by sympathetic stimulation, but not by exogenous noradrenaline, in pithed rats pre-treated with desipramine. More recently, it has been shown that this cardiac sympatho-inhibitory response to 5-HT, mediated by prejunctional 5-HT1 receptors as well as putative 5-ht5A/5B receptors, is mimicked dose-dependently by the agonists CP 93,129 (r5-HT1B), sumatriptan (5-HT1B/1D) and PNU-142633 (5-HT1D). This study analysed further the pharmacological profile of the above 5-HT1 receptors. Continuous i.v. infusions of CP 93,129, sumatriptan or PNU-142633 (30 micro g kg(-1)min(-1) each) failed to modify the tachycardic responses to exogenous noradrenaline but inhibited those elicited by preganglionic (C7-T1) stimulation of the cardiac sympathetic outflow. These sympatho-inhibitory responses were unaltered after i.v. administration of physiological saline (1 ml kg(-1)) or the 5-HT1A receptor antagonist WAY 100635 (10 micro g kg(-1)). In contrast, the antagonist GR 127935 (5-HT1B/1D; 100 micro g kg(-1), i.v.) abolished the responses to CP 93,129, sumatriptan and PNU-142633, whilst SB224289 (5-HT1B; 300 micro g kg(-1), i.v.) abolished the responses to CP 93,129 without affecting those to sumatriptan and PNU-142633. Interestingly, BRL15572 (5-HT1D; 300 micro g kg(-1), i.v.) abolished the responses to PNU-142633 and attenuated those to sumatriptan, but not those to CP 93,129. WAY 100635, GR 127935, SB224289 and BRL15572, given alone at the above doses, failed to modify the sympathetically induced tachycardic responses. The 5-HT1 receptors producing cardiac sympatho-inhibition in pithed rats thus display the pharmacological profile of the 5-HT1B and 5-HT1D receptor subtypes.