David Centurión

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-HT1, 5-HT2, 5-HT3, 5-HT4, 5-ht5A/5B, and 5-HT7 receptors as well as by a tyramine-like action or unidentified mechanisms. It is noteworthy that 5-HT6 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-HT3 receptors located on sensory vagal nerve endings in the heart. Other bradycardic mechanisms include cardiac sympatho-inhibition by prejunctional 5-HT1B/1D receptors and, in the case of the rat, an additional 5-ht5A/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-HT2A (rat), 5-HT3 (dog), 5-HT4 (pig, human), and 5-HT7 (cat) receptors; (2) adrenomedullary 5-HT2 (dog) and prejunctional sympatho-excitatory 5-HT3 (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-HT1A and 5-HT2 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-HT3 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-HT2A 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-HT1B receptors. The late vasodepressor response may involve three different mechanisms: (1) direct vasorelaxation by activation of 5-HT7 receptors located on vascular smooth muscle; (2) inhibition of the vasopressor sympathetic outflow by sympatho-inhibitory 5-HT1A/1B/1D receptors; and (3) release of endothelium-derived relaxing factor (nitric oxide) by 5-HT2B and/or 5-HT1B/1D receptors. Furthermore, central administration of 5-HT can cause both hypotension (mainly mediated by 5-HT1A receptors) and hypertension (mainly mediated by 5-HT2 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.

Migraine: Pathophysiology, Pharmacology, Treatment and Future Trends

Migraine treatment has evolved into the scientific arena, but it seems still controversial whether migraine is primarily a vascular or a neurological dysfunction. Irrespective of this controversy, the levels of serotonin (5-hydroxytryptamine; 5-HT), a vasoconstrictor and a central neurotransmitter, seem to decrease during migraine (with associated carotid vasodilatation) whereas an i.v. infusion of 5-HT can abort migraine. In fact, 5-HT as well as ergotamine, dihydroergotamine and other antimigraine agents invariably produce vasoconstriction in the external carotid circulation. The last decade has witnessed the advent of sumatriptan and second generation triptans (e.g. zolmitriptan, rizatriptan, naratriptan), which belong to a new class of drugs, the 5-HT1B/1D/1F receptor agonists. Compared to sumatriptan, the second-generation triptans have a higher oral bioavailability and longer plasma half-life. In line with the vascular and neurogenic theories of migraine, all triptans produce selective carotid vasoconstriction (via 5-HT1B receptors) and presynaptic inhibition of the trigeminovascular inflammatory responses implicated in migraine (via 5-HT1D/5-ht1F receptors). Moreover, selective agonists at 5-HT1D (PNU-142633) and 5-ht1F (LY344864) receptors inhibit the trigeminovascular system without producing vasoconstriction. Nevertheless, PNU-142633 proved to be ineffective in the acute treatment of migraine, whilst LY344864 did show some efficacy when used in doses which interact with 5-HT1B receptors. Finally, although the triptans are effective antimigraine agents producing selective cranial vasoconstriction, efforts are being made to develop other effective antimigraine alternatives acting via the direct blockade of vasodilator mechanisms (e.g. antagonists at CGRP receptors, antagonists at 5-HT7 receptors, inhibitors of nitric oxide biosynthesis, etc). These alternatives will hopefully lead to fewer side effects.

Cardiovascular Alterations After Spinal Cord Injury: An Overview

The recent developments in the management of spinal cord injury (SCI) have led to a reduction in mortality and in the consequences, resulting from incomplete spinal cord damage in those who survive. In this respect, it is noteworthy that SCI not only results in paraplegia or tetraplegia, but also in systemic, cardiovascular and metabolic alterations secondary to autonomic dysfunction. After SCI there is a decrease in sympathetic discharge and an increase in parasympathetic drive, resulting in profound changes in arterial blood pressure and heart rate. When SCI is induced in experimental animals, an immediate hypotension occurs (acute phase) which has been attributed to an autonomic imbalance involving a predominance of parasympathetic activity. Subsequently, an episodic hypertension may develop (chronic phase) as a part of a condition denominated autonomic dysreflexia. This hypertension is caused by afferent stimulation below the level of injury and can be so severe that sometimes may lead to cerebral haemorrhage, seizures, and death. In the light of the above lines of evidence, experimental SCI may provide an ideal model to study the nature of cardiovascular mechanisms following traumatic injury. Thus, the present review will deal with an update of the possible cardiovascular complications associated to SCI (including spinal shock, autonomic dysreflexia, deep venous thrombosis, and risk for coronary heart disease). This will be discussed within the context of the development of drugs with potential therapeutic usefulness in the acute and chronic stages of SCI.

Canine external carotid vasoconstriction to methysergide, ergotamine and dihydroergotamine: role of 5-HT1B/1D receptors and α2-adrenoceptors

The antimigraine drugs methysergide, ergotamine and dihydroergotamine (DHE) produce selective vasoconstriction in the external carotid bed of vagosympathectomized dogs anaesthetized with pentobarbital and artificially respired, but the receptors involved have not yet been completely characterized. Since the above drugs display affinity for several binding sites, including α‐adrenoceptors and several 5‐HT1 and 5‐HT2 receptor subtypes, this study has analysed the mechanisms involved in the above responses. Intracarotid (i.c.) infusions during 1 min of methysergide (31–310 μg min−1), ergotamine (0.56–5.6 μg min−1) or DHE (5.6–31 μg min−1) dose‐dependently reduced external carotid blood flow (ECBF) by up to 46±4, 37±4 and 49±5%, respectively. Blood pressure and heart rate remained unchanged. The reductions in ECBF by methysergide were abolished and even reversed to increases in animals pre‐treated with GR127935 (10 μg kg−1, i.v.). The reductions in ECBF by ergotamine and DHE remained unchanged in animals pre‐treated (i.v.) with prazosin (300 μg kg−1), but were partly antagonized in animals pre‐treated with either GR127935 (10 or 30 μg kg−1) or yohimbine (1000 μg kg−1). Pre‐treatment with a combination of GR127935 (30 μg kg−1) and yohimbine (1000 μg kg−1) abolished the responses to both ergotamine and DHE. The above doses of antagonists were shown to produce selective antagonism at their respective receptors. These results suggest that the external carotid vasoconstrictor responses to methysergide primarily involve 5‐HT1B/1D receptors, whereas those to ergotamine and DHE are mediated by 5‐HT1B/1D receptors as well as α2‐adrenoceptors.

Mediation of 5‐HT‐induced external carotid vasodilatation in GR 127935‐pretreated vagosympathectomized dogs by the putative 5‐HT7 receptor

The vasodilator effects of 5‐hydroxytryptamine (5‐HT) in the external carotid bed of anaesthetized dogs with intact sympathetic tone are mediated by prejunctional sympatho‐inhibitory 5‐HT1B/1D receptors and postjunctional 5‐HT receptors. The prejunctional vasodilator mechanism is abolished after vagosympathectomy which results in the reversal of the vasodilator effect to vasoconstriction. The blockade of this vasoconstrictor effect of 5‐HT with the 5‐HT1B/1D receptor antagonist, GR 127935, unmasks a dose‐dependent vasodilator effect of 5‐HT, but not of sumatriptan. Therefore, the present study set out to analyse the pharmacological profile of this postjunctional vasodilator 5‐HT receptor in the external carotid bed of vagosympathectomized dogs pretreated with GR 127935 (20 μg kg−1, i.v.). One‐minute intracarotid (i.c.) infusions of 5‐HT (0.330 μg min−1), 5‐carboxamidotryptamine (5‐CT; 0.010.3 μg min−1), 5‐methoxytryptamine (1100 μg min−1) and lisuride (31000 μg min−1) resulted in dose‐dependent increases in external carotid blood flow (without changes in blood pressure or heart rate) with a rank order of agonist potency of 5‐CT>>5‐HT5‐methoxytryptamine>lisuride, whereas cisapride (1001000 μg min−1, i.c.) was practically inactive. Interestingly, lisuride (mean dose of 85±7 μg kg−1, i.c.), but not cisapride (mean dose of 67±7 μg kg−1, i.c.), specifically abolished the responses induced by 5‐HT, 5‐CT and 5‐methoxytryptamine, suggesting that a common site of action may be involved. In contrast, 1 min i.c. infusions of 8‐OH‐DPAT (33000 μg min−1) produced dose‐dependent decreases, not increases, in external carotid blood flow and failed to antagonize (mean dose of 200±33 μg kg−1, i.c.) the agonist‐induced vasodilator responses. The external carotid vasodilator responses to 5‐HT, 5‐CT and 5‐methoxytryptamine were not modified by intravenous (i.v.) pretreatment with either saline, (±)‐pindolol (4 mg kg−1) or ritanserin (100 μg kg−1) plus granisetron (300 μg kg−1), but were dose‐dependently blocked by i.v. administration of methiothepin (10 and 30 μg kg−1, given after ritanserin plus granisetron), mesulergine (10 and 30 μg kg−1), metergoline (1 and 3 mg kg−1), methysergide (1 and 3 mg kg−1) or clozapine (0.3 and 1 mg kg−1). Nevertheless, the blockade of the above responses, not significant after treatment with the lower of the two doses of metergoline and mesulergine, was nonspecific after administration of the higher of the two doses of methysergide and clozapine. Based upon the above rank order of agonist potencies and the antagonism produced by a series of drugs showing high affinity for the cloned 5‐ht7 receptor, our results indicate that the 5‐HT receptor mediating external carotid vasodilatation in GR 127935‐pretreated vagosympathectomized dogs is operationally similar to the putative 5‐HT7 receptor mediating relaxation of vascular and non‐vascular smooth muscles (e.g. rabbit femoral vein, canine coronary artery, rat systemic vasculature and guinea‐pig ileum) as well as tachycardia in the cat.

Characterization of putative 5-HT7 receptors mediating tachycardia in the cat

It has been suggested that the tachycardic response to 5‐hydroxytryptamine (5‐HT) in the spinal‐transected cat is mediated by ‘5‐HT1‐like’ receptors since this effect, being mimicked by 5‐carboxamidotryptamine (5‐CT), is not modified by ketanserin or MDL 72222, but it is blocked by methiothepin, methysergide or mesulergine. The present study was set out to reanalyse this suggestion in terms of the IUPHAR 5‐HT receptor classification schemes proposed in 1994 and 1996. Intravenous (i.v.) bolus injections of the tryptamine derivatives, 5‐CT (0.01, 0.03, 0.1, 0.3, 1, 3, 10 and 30μgkg−1), 5‐HT (3, 10 and 30μgkg−1) and 5‐methoxytryptamine (3, 10 and 30μgkg−1) as well as the atypical antipsychotic drug, clozapine (1000 and 3000μgkg−1) resulted in dose‐dependent increases in heart rate, with a rank order of agonist potency of 5‐CT >>5‐HT >5‐methoxytryptamine >>clozapine. The tachycardic effects of 5‐HT and 5‐methoxytryptamine were dose‐dependently antagonized by i.v. administration of lisuride (30 and 100μgkg−1), ergotamine (100 and 300μgkg−1) or mesulergine (100, 300 and 1000μgkg−1); the highest doses of these antagonists used also blocked the tachycardic effects of 5‐CT. Clozapine (1000 and 3000μgkg−1) did not affect the 5‐HT‐induced tachycardia, but attenuated, with its highest dose, the responses to 5‐methoxytryptamine and 5‐CT. However, these doses of clozapine as well as the high doses of ergotamine (300μgkg−1) and mesulergine (300 and 1000μgkg−1) also attenuated the tachycardic effects of isoprenaline. In contrast, 5‐HT‐, 5‐methoxytryptamine‐ and 5‐CT‐induced tachycardia were not significantly modified after i.v. administration of physiological saline (0.1 and 0.3mlkg−1), the 5‐HT1B/1D receptor antagonist, GR127935 (500μgkg−1) or the 5‐HT3/4 receptor antagonist, tropisetron (3000μgkg−1). Intravenous injections of the 5‐HT1 receptor agonists, sumatriptan (30, 100 and 300μgkg−1) and indorenate (300 and 1000μgkg−1) or the 5‐HT4 receptor (partial) agonist cisapride (300 and 1000μgkg−1) were devoid of effects on feline heart rate per se and failed to modify significantly 5‐HT‐induced tachycardic responses. Based upon the above rank order of agonist potency, the failure of sumatriptan, indorenate or cisapride to produce cardioacceleration and the blockade by a series of drugs showing high affinity for the cloned 5‐ht7 receptor, the present results indicate that the 5‐HT receptor mediating tachycardia in the cat is operationally similar to other putative 5‐HT7 receptors mediating vascular and non‐vascular responses (e.g. relaxation of the rabbit femoral vein, canine external carotid and coronary arteries, rat systemic vasculature and guinea‐pig ileum). Since these responses represent functional correlates of the 5‐ht7 gene product, the 5‐HT7 receptor appellation is reinforced. Therefore, the present experimental model, which is not complicated by the presence of other 5‐HT receptors, can be utilized to characterize and develop new drugs with potential agonist and antagonist properties at functional 5‐HT7 receptors.

Experimental migraine models and their relevance in migraine therapy.

Although the understanding of migraine pathophysiology is incomplete, it is now well accepted that this neurovascular syndrome is mainly due to a cranial vasodilation with activation of the trigeminal system. Several experimental migraine models, based on vascular and neuronal involvement, have been developed. Obviously, the migraine models do not entail all facets of this clinically heterogeneous disorder, but their contribution at several levels (molecular, in vitro, in vivo) has been crucial in the development of novel antimigraine drugs and in the understanding of migraine pathophysiology. One important vascular in vivo model, based on an assumption that migraine headache involves cranial vasodilation, determines porcine arteriovenous anastomotic blood flow. Other models utilize electrical stimulation of the trigeminal ganglion/nerve to study neurogenic dural inflammation, while the superior sagittal sinus stimulation model takes into account the transmission of trigeminal nociceptive input in the brainstem. More recently, the introduction of integrated models, namely electrical stimulation of the trigeminal ganglion or systemic administration of capsaicin, allows studying the activation of the trigeminal system and its effect on the cranial vasculature. Studies using in vitro models have contributed enormously during the preclinical stage to characterizing the receptors in cranial blood vessels and to studying the effects of several putative antimigraine agents. The aforementioned migraine models have advantages as well as some limitations. The present review is devoted to discussing various migraine models and their relevance to antimigraine therapy.

The 5-HT1-like receptors mediating inhibition of sympathetic vasopressor outflow in the pithed rat: operational correlation with the 5-HT1A, 5-HT1B and 5-HT1D subtypes.

It has been suggested that the inhibition of sympathetically‐induced vasopressor responses produced by 5‐hydroxytryptamine (5‐HT) in pithed rats is mediated by 5‐HT1‐like receptors. The present study has re‐analysed this suggestion with regard to the classification schemes recently proposed by the NC‐IUPHAR subcommittee on 5‐HT receptors. Intravenous (i.v.) continuous infusions of 5‐HT and the 5‐HT1 receptor agonists, 8‐OH‐DPAT (5‐HT1A), indorenate (5‐HT1A), CP 93,129 (5‐HT1B) and sumatriptan (5‐HT1B/1D), resulted in a dose‐dependent inhibition of sympathetically‐induced vasopressor responses. The sympatho‐inhibitory responses induced by 5‐HT, 8‐OH‐DPAT, indorenate, CP 93,129 or sumatriptan were analysed before and after i.v. treatment with blocking doses of the putative 5‐HT receptor antagonists, WAY 100635 (5‐HT1A), cyanopindolol (5‐HT1A/1B) or GR 127935 (5‐HT1B/1D). Thus, after WAY 100635, the responses to 5‐HT and indorenate, but not to 8‐OH‐DPAT, CP 93,129 and sumatriptan, were blocked. After cyanopindolol, the responses to 5‐HT, indorenate and CP 93,129 were abolished, whilst those to 8‐OH‐DPAT and sumatriptan (except at the lowest frequency of stimulation) remained unaltered. In contrast, after GR 127935, the responses to 5‐HT, CP 93,129 and sumatriptan, but not to 8‐OH‐DPAT and indorenate, were abolished. In additional experiments, the inhibition induced by 5‐HT was not modified after 5‐HT7 receptor blocking doses of mesulergine. The above results suggest that the 5‐HT1‐like receptors, which inhibit the sympathetic vasopressor outflow in pithed rats, display the pharmacological profile of the 5‐HT1A, 5‐HT1B and 5‐HT1D, but not that of 5‐HT7, receptors.

Current and prospective pharmacological targets in relation to antimigraine action.

Migraine is a recurrent incapacitating neurovascular disorder characterized by unilateral and throbbing headaches associated with photophobia, phonophobia, nausea, and vomiting. Current specific drugs used in the acute treatment of migraine interact with vascular receptors, a fact that has raised concerns about their cardiovascular safety. In the past, α-adrenoceptor agonists (ergotamine, dihydroergotamine, isometheptene) were used. The last two decades have witnessed the advent of 5-HT1B/1D receptor agonists (sumatriptan and second-generation triptans), which have a well-established efficacy in the acute treatment of migraine. Moreover, current prophylactic treatments of migraine include 5-HT2 receptor antagonists, Ca2+ channel blockers, and β-adrenoceptor antagonists. Despite the progress in migraine research and in view of its complex etiology, this disease still remains underdiagnosed, and available therapies are underused. In this review, we have discussed pharmacological targets in migraine, with special emphasis on compounds acting on 5-HT (5-HT1–7), adrenergic (α1, α2, and β), calcitonin gene-related peptide (CGRP1 and CGRP2), adenosine (A1, A2, and A3), glutamate (NMDA, AMPA, kainate, and metabotropic), dopamine, endothelin, and female hormone (estrogen and progesterone) receptors. In addition, we have considered some other targets, including gamma-aminobutyric acid, angiotensin, bradykinin, histamine, and ionotropic receptors, in relation to antimigraine therapy. Finally, the cardiovascular safety of current and prospective antimigraine therapies is touched upon.

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

Continuous infusions of 5‐hydroxytryptamine (5‐HT) inhibit the tachycardiac responses to preganglionic (C7‐T1) sympathetic stimulation in pithed rats pretreated with desipramine. The present study identified the pharmacological profile of this inhibitory action of 5‐HT. The inhibition induced by intravenous (i.v.) continuous infusions of 5‐HT (5.6 μg kg−1 min−1) on sympathetically induced tachycardiac responses remained unaltered after i.v. treatment with saline or the antagonists GR 127935 (5‐HT1B/1D), the combination of WAY 100635 (5‐HT1A) plus GR 127935, ritanserin (5‐HT2), tropisetron (5‐HT3/4), LY215840 (5‐HT7) or a cocktail of antagonists/inhibitors consisting of yohimbine (α2), prazosin (α1), ritanserin, GR 127935, WAY 100635 and indomethacin (cyclooxygenase), but was abolished by methiothepin (5‐HT1/2/6/7 and recombinant 5‐ht5A/5B). These drugs, used in doses high enough to block their respective receptors/mechanisms, did not modify the sympathetically induced tachycardiac responses per se. I.v. continuous infusions of the agonists 5‐carboxamidotryptamine (5‐CT; 5‐HT1/7 and recombinant 5‐ht5A/5B), CP 93,129 (r5‐HT1B), sumatriptan (5‐HT1B/1D), PNU‐142633 (5‐HT1D) and ergotamine (5‐HT1B/1D and recombinant 5‐ht5A/5B) mimicked the above sympatho‐inhibition to 5‐HT. In contrast, the agonists indorenate (5‐HT1A) and LY344864 (5‐ht1F) were inactive. Interestingly, 5‐CT‐induced cardiac sympatho‐inhibition was abolished by methiothepin, the cocktail of antagonists/inhibitors, GR 127935 or the combination of SB224289 (5‐HT1B) plus BRL15572 (5‐HT1D), but remained unchanged when SB224289 or BRL15572 were given separately. Therefore, 5‐HT‐induced cardiac sympatho‐inhibition, being unrelated to 5‐HT2, 5‐HT3, 5‐HT4, 5‐ht6, 5‐HT7 receptors, α1/2‐adrenoceptor or prostaglandin synthesis, seems to be primarily mediated by (i) 5‐HT1 (probably 5‐HT1B/1D) receptors and (ii) a novel mechanism antagonized by methiothepin that, most likely, involves putative 5‐ht5A/5B receptors.