5-Hydroxytryptamine-induced tachycardia in the pig: possible involvement of a new type of 5-hydroxytryptamine receptor

Cardiac Roles of Serotonin (5-HT) and 5-HT-Receptors in Health and Disease

Serotonin acts solely via 5-HT4-receptors to control human cardiac contractile function. The effects of serotonin via 5-HT4-receptors lead to positive inotropic and chronotropic effects, as well as arrhythmias, in the human heart. In addition, 5-HT4-receptors may play a role in sepsis, ischaemia, and reperfusion. These presumptive effects of 5-HT4-receptors are the focus of the present review. We also discuss the formation and inactivation of serotonin in the body, namely, in the heart. We identify cardiovascular diseases where serotonin might play a causative or additional role. We address the mechanisms which 5-HT4-receptors can use for cardiac signal transduction and their possible roles in cardiac diseases. We define areas where further research in this regard should be directed in the future, and identify animal models that might be generated to this end. Finally, we discuss in what regard 5-HT4-receptor agonists or antagonists might be useful drugs that could enter clinical practice. Serotonin has been the target of many studies for decades; thus, we found it timely to summarise our current knowledge here.

Tegaserod, a 5-hydroxytryptamine type 4 receptor partial agonist, is devoid of electrocardiographic effects

OBJECTIVES

Certain GI prokinetic agents have been shown to affect cardiac repolarization, which may be associated with life-threatening arrhythmias. The selective 5-hydroxytryptamine type 4 receptor partial agonist tegaserod is a novel promotile agent developed for the treatment of functional motility disorders such as irritable bowel syndrome (IBS). The aim of the study was to investigate the cardiac safety profile of tegaserod through analysis of electrocardiographic data from clinical studies conducted in patients with IBS and a study conducted in healthy male subjects.

METHODS

In three randomized, double blind, placebo-controlled, parallel group clinical studies, 2516 IBS patients with symptoms of abdominal pain and constipation received tegaserod 2 or 6 mg b.i.d. (n = 1679) or placebo (n = 837) for 12 wk. In an additional study, 36 healthy male subjects received iv. single doses of tegaserod (0.8 mg to 20 mg) or placebo. Standard 12-lead electrocardiograms were recorded at baseline and during treatment. Baseline values were compared with data collected during the treatment period.

RESULTS

The proportion of patients with prolongation of the QTc interval was the same for placebo and tegaserod, as was the frequency of overall electrocardiographic abnormalities. No ventricular or supraventricular tachycardia was observed. Comparable electrocardiographic results were obtained during placebo and tegaserod treatment. In healthy volunteers, tegaserod at i.v. doses resulting in plasma concentrations up to 100 times those measured after therapeutic doses (6 mg b.i.d.) did not influence electrocardiographic parameters.

CONCLUSIONS

Tegaserod is devoid of electrocardiographic effects and is not expected to adversely influence cardiac function. These data confirm preclinical findings.

A61603-induced vasoconstriction in porcine carotid vasculature: involvement of a non-adrenergic mechanism

It has recently been shown that the pharmacological profile of alpha(1)-adrenoceptors mediating constriction of porcine carotid arteriovenous anastomoses resembles that of alpha(1A)- and alpha(1B)-adrenoceptor subtypes. In an attempt to verify the involvement of alpha(1A)-adrenoceptors, we used the potent alpha(1A)-adrenoceptor agonist N-[5-(4,5-dihydro-1H-imidazol-2yl)-2-hydroxy-5,6,7,8-tetrahydro-naphthalen-1-yl]methane sulphonamide (A61603) and found that intracarotid (i.c.) administration of A61603 (0.3-10 microg kg(-1)) dose-dependently decreased porcine carotid blood flow and vascular conductance. This decrease was exclusively due to a constriction of carotid arteriovenous anastomoses; the capillary blood flow and conductance remained unchanged. Surprisingly, the responses to A61603 were little modified by prior i.v. treatment with 5-methylurapidil (1000 microg kg(-1)), prazosin (100 microg kg(-1)) or a combination of prazosin and rauwolscine (100 and 300 microg kg(-1), respectively). 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 (GR127935; 500 microg kg(-1)) and ketanserin (500 microg kg(-1)) also failed to modify carotid vascular responses to A61603, but, interestingly, methiothepin (3000 microg kg(-1)) proved to be an effective antagonist. Taken together, the present results show that A61603 is a relatively poor agonist at the alpha(1A)-adrenoceptor in anaesthetised pigs and that the carotid vasoconstriction produced by A61603 is mediated by a novel non-adrenergic mechanism.

External carotid effects of 2-(2-aminoethyl)-quinoline (D-1997) in vagosympathectomized dogs

Serotonin (5-hydroxytryptamine; 5-HT) elicits external carotid vasoconstriction in vagosympathectomized dogs via 5-HT1B/1D receptors and a mechanism unrelated to the 5-HT1, 5-HT2, 5-HT3 and 5-HT4 types. In order to further explore the nature of this novel mechanism, the canine external carotid effects of 2-(2-aminoethyl)-quinoline (D-1997), a novel 5-HT1 receptor agonist, were analyzed and compared with those of 5-HT and sumatriptan. Intracarotid (i.c.) infusions of 5-HT, D-1997 and sumatriptan to vagosympathectomized dogs dose-dependently decreased external carotid conductance, the rank order of agonist potency being 5-HT > sumatriptan > D-1997. The effects to D-1997 were resistant to intravenous (i.v.) pretreatment with 5-HT2 and 5-HT3/5-HT4 receptor antagonists. Remarkably, the effects induced by lower (10-100 microg/min), but not higher (300-1000 microg/min), doses of D-1997 were blocked by high doses of methiothepin (1 and 3 mg/kg, i.v.), as previously shown with 5-HT. In addition, GR-127935 (1-10 microg/kg, i.v.), partially and dose-dependently antagonized D-1997-induced responses. However, the effects of D-1997 remained unaltered after blockade of alpha- and beta-adrenoceptors, muscarinic, nicotinic, histamine and dopamine receptors, or inhibition of 5-HT-uptake or cyclo-oxygenase, depletion of biogenic amines or blockade of Ca2+ channels. These results may support our previous contention that lower doses of 5-HT elicit external carotid vasoconstriction in vagosympathectomized dogs by activation of 5-HT1B/1D receptors, whilst higher doses of 5-HT stimulate a novel vasoconstrictor mechanism.

5-HT4 receptors improve social olfactory memory in the rat

Serotonin (5-HT) is involved in a large variety of physiological functions and it appears now that it could play a role in cognitive processes through the activation of 5-HT4 receptors. The present study was conducted to determine the effect of BIMU1, a mixed 5-HT4 agonist/5-HT3 antagonist on social olfactory recognition in rats, a behaviour test which has previously been shown to access short-term memory and to be sensitive to cholinergic drugs. This test is based on the investigation of an unfamiliar juvenile by an adult rat during two distinct 5-min presentations. At a 30-min delay after each presentation adults recognized the juvenile, whereas after a 2-hr delay all the adults had forgotten it. When administered intraperitoneally immediately after the first presentation, BIMU1 (10 mg/kg) enhanced short-term memory (i.e. recognition of the juvenile after a 2-hr delay). Ondansetron (10 and 100 micrograms/kg injected intraperitoneally), a 5-HT3 antagonist, had no significant effect on this form of memory. The effect of BIMU1 was antagonized by intraperitoneal injection of GR 125487, a very selective and potent 5-HT4 antagonist. The antagonistic effect was obtained at 1 and 10 mg/kg of GR 125487, but not at 0.1 mg/kg. It is certainly a specific effect on brain 5-HT4 receptors, since we determined a brain concentration of GR 125487 equal to 3.8 x 10(-7) M after the intraperitoneal injection of 10 mg/kg of this drug. This GR 125487 concentration is certainly sufficient to occupy all the 5-HT4 brain receptors (Kd = 10(-10) M) but not to occupy 5-HT3 receptors (Kd > 10(-6) M). The 5-HT4 specificity of the blockade by GR 125487 is further demonstrated by the fact that a 10-fold lower dose of GR 125487 (1 mg/kg) is also effective to inhibit the BIMU1 effect.

Identification and characterization of the 5-HT4 receptor in the intestinal tract and striatum of the guinea pig

Receptors for 5-hydroxytryptamine (5-HT) of the 5-HT4 type were investigated in the intestinal tract and the striatum in guinea-pig, in binding studies using the 5-HT4 radioligand, [3H]GR113808. In the intestinal tract, specific binding was observed in preparations of the longitudinal muscle with the myenteric plexus (LMMPs) but not in the whole tissue. Saturable binding of [3H]GR113808 was demonstrated (striatum: Kd = 0.054 +/- 0.002 nM, Bmax = 90.25 +/- 10.44 fmol/mg protein, LMMPs of ileum: Kd = 0.077 +/- 0.016 nM, Bmax = 11.95 +/- 3.24 fmol/mg protein). Selective 5-HT4 receptor agonists and antagonists inhibited binding of [3H]GR113808 with high affinities in LMMPs of the ilcum and which correlated well with their actions on the striatum (r = 0.98), as indicated by the rank order of displacement potencies: SDZ205-557 > LY297524 > cisapride = BIMU8 > 5-HT > mosapride > renzapride > 5-hydroxy-N-methyltryptamine(5-HMT) > (+/-) zacopride > alpha-methyl-5-hydroxytryptamine (alpha-M-5-HT) > 5-methyltryptamine(5-MT) > 5-carboxamidotryptamine (5-CT). The number of binding sites of [3H]GR113808 in the intestine was significantly smaller than that in the brain. In the intestine, a larger number of binding sites was noted in the upper part of the intestine, the rank order being duodenum > jcjunum > ilcum > > colon > rectum. Such data are relevant regarding the potential use of the 5-HT4 receptor ligand to modify motility and secretion in the intestine.

Pharmacological profile of the receptors that mediate external carotid vasoconstriction by 5-HT in vagosympathectomized dogs

1. 5-Hydroxytryptamine (5-HT) can produce vasodilatation or vasoconstriction of the canine external carotid bed depending upon the degree of carotid sympathetic tone. Hence, external carotid vasodilatation to 5-HT in dogs with intact sympathetic tone is primarily mediated by prejunctional 5-HT1-like receptors similar to the 5-HT1D subtype, which inhibit the carotid sympathetic outflow. The present investigation is devoted to the pharmacological analysis of the receptors mediating external carotid vasoconstriction by 5-HT in vagosympathectomized dogs. 2. Intracarotid (i.c.) infusions for 1 min of 5-HT (0.3, 1, 3, 10, 30 and 100 micrograms) resulted in dose-dependent decreases in both external carotid blood flow and the corresponding conductance; both mean arterial blood pressure and heart rate remained unchanged during the infusions of 5-HT. These responses to 5-HT were resistant to blockade by antagonists at 5-HT2 (ritanserin) and 5-HT3/5-HT4 (tropisetron) receptors, but were partly blocked by the 5-HT1-like and 5-HT2 receptor antagonist, methiothepin (0.3 mg kg-1); higher doses of methiothepin (1 and 3 mg kg-1) caused little, if any, further blockade. These methiothepin (3 mg kg-1)-resistant responses to 5-HT were not significantly antagonized by MDL 72222 (0.3 mg kg-1) or tropisetron (3 mg kg-1). 3. The external carotid vasoconstrictor effects of 5-HT were mimicked by the selective 5-HT1-like receptor agonist, sumatriptan (3, 10, 30 and 100 micrograms during 1 min, i.c.), which produced dose-dependent decreases in external carotid blood flow and the corresponding conductance; these effects of sumatriptan were dose-dependently antagonized by methiothepin (0.3, 1 and 3 mg kg-1), but not by 5-HT1D-like receptor blocking doses of metergoline (0.1 mg kg-1). 4. The above vasoconstrictor effects of 5-HT remained unaltered after administration of phentolamine, propranolol, atropine, hexamethonium, brompheniramine, cimetidine and haloperidol, thus excluding the involvement of alpha- and beta-adrenoceptors, muscarinic, nicotinic, histamine and dopamine receptors. Likewise, inhibition of either 5-HT-uptake (with fluoxetine) or cyclo-oxygenase (with indomethacin), depletion of biogenic amines (with reserpine) or blockade of calcium channels (with verapamil) did not modify the effects of 5-HT. 5. Taken together, the above results support our contention that the external carotid vasoconstrictor responses to 5-HT in vagosympathectomized dogs are mainly mediated by activation of sumatriptan-sensitive 5-HT1-like receptors. It must be emphasized, notwithstanding, that other mechanisms of 5-HT, including an interaction with a novel 5-HT receptor (sub)type and/or an indirect action that may lead to the release of a known (or even unknown) neurotransmitter substance cannot be categorically excluded.

Serotonin receptors: subtypes, functional responses and therapeutic relevance

Recent, rapid progress in the molecular biology of serotonin (5-HT) receptors requires conceptual re-thinking with respect to receptor classification. Thus, based on operational criteria (agonist and antagonist rank order), as well as transduction mechanisms involved and the structure of the receptor protein, the Nomenclature Committee of the Serotonin Club has proposed the following classification and nomenclature: the main receptor types 5-HT1 to 5-HT4, recombinant receptors (e.g. 5-ht5 to 5-ht7) and 'orphan' receptors. The aim of the present review is to discuss the events leading to this classification, the criteria for and functional responses mediated by various 5-HT receptors, as well as the therapeutic possibilities with 5-HT ligands.

Different pharmacological responses of atrium and ventricle: studies with human cardiac tissue

It has been recently reported that 5-hydroxytryptamine (5-HT) increases force of contraction in atrial tissue but not in ventricular tissue. In the present study with trabeculae obtained from non-diseased human hearts, we investigated whether this difference in the contractile responses is specific for 5-HT or is also observed for other substances: calcitonin gene-related peptide (CGRP), angiotensin II, adenosine, somatostatin and acetylcholine. CGRP (10(-9) to 10(-7) M) and angiotensin II (10(-9) to 10(-5) M) caused concentration-dependent increases in force of contraction in atrial trabeculae (up to 36 +/- 8% and 42 +/- 8% of the response to 10(-5) M noradrenaline, respectively). Similar to 5-HT, no effects were observed with CGRP and angiotensin II in ventricular trabeculae. Adenosine (10(-8) to 10(-5) M) and somatostatin (10(-8) to 10(-6) M) caused concentration-dependent negative inotropic effects on baseline atrial contractility (-54 +/- 17% and -51 +/- 25%, respectively), but no response was found on baseline ventricular contractility. Adenosine, but not somatostatin, reduced force of contraction after pre-stimulation with 10(-5) M noradrenaline in atrial tissue and, to a lesser extent, in ventricular tissue. Acetylcholine exhibited a biphasic concentration-response curve in the atrial tissue, consisting of an initial negative inotropic response (10(-9) to 10(-7) M, from 120 +/- 41 mg at baseline to 48 +/- 16 mg at 10(-7) M), followed by a positive inotropic response (10(-6) to 10(-3) M, from 48 +/- 16 mg at 10(-7) M to 77 +/- 15 mg).(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of serotonin on gastrointestinal electrical activity in the conscious piglet

In conscious piglets with electrodes implanted in the wall of the antrum pylori, duodenum, jejunum and ileum, the influence of intravenous infusion of serotonin (5-HT), 4 micrograms/kg/min for 2 hours, on gastrointestinal electrical activity and arterial pressure was examined. 5-HT was without significant influence on antral electrical activity and arterial pressure. In the small intestinal parts 5-HT provoked a shortening of the interval of the migrating myoelectrical complex (MMC) by reducing the duration of phase I and phase II activities, and an increase in propagation velocity. 5-HT also induced an increase in phase III activity. A possible involvement of 5-HT in the initiation and propagation of the MMC is suggested.

Development of a radioligand binding assay for 5-HT4 receptors in guinea-pig and rat brain

1. The 5-HT4 receptor antagonist, GR113808, has been radiolabelled to a high specific activity with tritium. 2. Characterization of specific [3H]-GR113808 binding in homogenates of guinea-pig striatum and hippocampus revealed a single site of high affinity (Kd values 0.20 and 0.13 nM respectively). 3. [3H]-GR113808 binding was reversible and displayed rapid kinetics such that association and dissociation were complete within 3 min. 4. Specific [3H]-GR113808 binding was potently and stereoselectively inhibited by agonists and antagonists acting at the 5-HT4 receptor but not by compounds selective for other 5-HT receptors or other neurotransmitter receptors. 5. Autoradiographic analysis revealed a discrete localization in both guinea-pig and rat brain with high concentrations of binding in brain areas such as the striatum, substantia nigra and olfactory tubercle. 6. [3H]-GR113808 binding to homogenates of guinea-pig striatum meets the criteria for labelling of the 5-HT4 receptor and, as such, represents the first characterization of this receptor in a radioligand binding assay.

Effects of serotonin on rat ileocolonic transit and fluid transfer in vivo: possible mechanisms of action

The aim was to investigate the action of serotonin (5HT) on function of the ileocolonic junction (ICJ) in vivo. In anaesthetised rats, models were developed to study the effects of intra-aortic (ia) serotonin on ileocolonic and colonic transit, and the effects on transit of a number of 5HT receptor antagonists. In the first series of experiments, a bolus of saline labelled with 99mTc DTPA was instilled 20 cm proximal to the ICJ and transit was assessed three hours later by the geometric centre of the spread of isotope. In the second series, similar techniques were used on the postcaecal colon and transit assessed two hours later. In the third series of experiments, the effects of ia 5HT on ileal net fluid flux was evaluated by standard perfusion experiments with 14C polyethylene glycol (PEG) 4000 as a non-absorbable marker in rat plasma-like electrolyte solution. Compared with ia saline, 5HT accelerated ICJ transit significantly (p < 0.05). This acceleration was comparable with the effect of ia bethanechol. The effects of 5HT on ICJ transit were inhibited by the intraperitoneal (ip) infusion of atropine, the 5HT receptor antagonists, methysergide, ketanserin, zacopride, and the 5HT4 agonist, SC53116. Methysergide, zacopride, and SC53116 given with ia 5HT slowed ICJ transit to rates below those of ia saline alone. When these same agents were given together with ia saline, the ICJ transit was not significantly altered. Serotonin, at the dose that accelerated ICJ transit, did not significantly alter colonic transit or ileal fluid transport. In conclusion, 5HT is a potent pharmacological stimulant of transit across the rat ICJ in vivo; the action of 5HT is mediated partly through muscarinic neurones and several 5HT receptor subtypes.

The novel 5-HT4 receptor antagonist DAU 6285 antagonizes 5-hydroxytryptamine-induced tachycardia in pigs

The 5-HT4 receptor antagonist action of DAU 6285 was investigated in vivo in anesthetized pigs. DAU 6285 (0.3-3 mg/kg i.v.) dose dependently antagonized 5-hydroxytryptamine (5-HT)-induced tachycardiac responses. In contrast, the 5-HT3 receptor antagonist, ondansetron (0.3-3 mg/kg i.v.) did not influence the tachycardia induced by 5-HT. These results indicate that DAU 6285 is a potent antagonist of 5-HT4 receptor-mediated responses in vivo.

5-Hydroxytryptamine-induced increase in left ventricular dP/dtmax does not suggest the presence of ventricular 5-HT4 receptors in the pig

Although 5-hydroxytryptamine (5-HT) increases porcine atrial force and rate via 5-HT4 receptors, its effect on left ventricular contractility is not known. Therefore, using the maximum rate of rise of left ventricular pressure (LVdP/dtmax) as an index of cardiac contractility, we have attempted to analyze the possible role of ventricular 5-HT4 receptors in the anaesthetized pig. The full agonists at 5-HT4 receptors, 5-HT and 5-methoxytryptamine (each 3, 10 and 30 micrograms.kg-1), and the beta-adrenoceptor agonist, isoprenaline (0.01, 0.03 and 0.1 micrograms.kg-1), increased heart rate, LVdP/dtmax and cardiac output. For a given degree of tachycardia, the increase in LVdP/dtmax by isoprenaline was substantially more than that observed with either 5-HT or 5-methoxytryptamine. The 5-HT4 receptor partial agonist, renzapride (3, 10, 30, 100 and 300 micrograms.kg-1), also increased heart rate and LVdP/dtmax dose-dependently. When the heart was paced at 150 beats.min-1, increases in LVdP/dtmax as well as cardiac output (except with the highest doses) by 5-HT, 5-methoxytryptamine and isoprenaline were clearly attenuated. However, the magnitude of attenuation of LVdP/dtmax responses by cardiac pacing was more marked in the case of 5-HT and 5-methoxytryptamine than with isoprenaline. The effects of renzapride (300 micrograms.kg-1) and tropisetron (0.3 and 3 mg.kg-1) on increases in heart rate and LVdP/dtmax by 5-HT, 5-methoxytryptamine and isoprenaline were also studied.(ABSTRACT TRUNCATED AT 250 WORDS)

Impaired vasodilator and chronotropic responses to 5-hydroxytryptamine in two models of hypertension-associated cardiac hypertrophy

OBJECTIVE

In connection with hypertension, research concerning 5-hydroxytryptamine (5-HT) receptors and subtypes in the cardiovascular system has so far been predominantly focused on various vascular tissues. In this study, the effects of 5-HT were investigated in isolated hearts with experimental cardiac hypertrophy.

DESIGN AND METHODS

Cardiac hypertrophy was induced by stenosing the abdominal aorta (ASR) of 5-week-old Wistar rats. The functional response to serotonin was measured in unpaced, ASR hearts (18-20 weeks) and compared with those of "sham" operated SHR and WKY rats.

RESULTS

The ASR, less hypertensive than SHR, showed more pronounced cardiac hypertrophy. The positive chronotropic and coronary vasodilator response to 5-HT was reduced in hypertrophied hearts from SHR and ASR when compared to "sham" operated and normotensive controls. The positive chronotropic effect of 5-HT could be antagonised with ketanserin, without affecting the coronary vasodilation. 5-HT did not induce any change in contractile force.

CONCLUSIONS

Cardiac hypertrophy is associated with impaired coronary vasodilator and chronotropic responsiveness to serotonin. The chronotropic response to 5-HT is mediated by the 5-HT2-receptor subtype.

5-Hydroxytryptamine increases contractile force in porcine right atrium but not in left ventricle

Positive chronotropic as well as inotropic effects of 5-hydroxytryptamine (5-HT) have been observed in pig atrial tissue, but no data are available about the direct effects of 5-HT on ventricular tissue. In the present study we investigated inotropic effects of 5-HT on atrial and ventricular trabeculae obtained from hearts of 3 months old pigs. The baseline isometric contractile force was significantly higher in ventricular (4.14 +/- 1.25 mN) than in atrial tissue (0.47 +/- 0.11 mN). A noradrenaline concentration-response curve (0.01 to 10 mumol/l) was used to check contractile responsiveness of the tissue and all responses were expressed as percentage of the response to 10 mumol/l noradrenaline. Noradrenaline caused a concentration-dependent increase in contractile force in both atrial and ventricular trabeculae. In contrast, though 5-HT (0.01 to 100 mumol/l) did increase force of contraction in atrial tissue (maximum: 72 +/- 20% of the response to noradrenaline 10 mumol/l), the contractility of ventricular trabeculae was not significantly affected (maximum: 12 +/- 6%). The present data show that, in contrast to atrial tissue, contractile force of ventricular tissue could not be significantly affected by 5-HT. To our knowledge, this is the first study to show that an agent which increased force of contraction in the atrium, did not have a corresponding effect on the ventricle. These findings may have important implications for a better understanding of the physiology and pharmacology of cardiac contractility.

Differential cardiovascular and neuroendocrine effects of epinine and dopamine in conscious pigs before and after adrenoceptor blockade

1. The effects of epinine or dopamine (both 1-10 micrograms kg-1 min-1) on systemic haemodynamics and plasma concentrations of catecholamines and prolactin were studied in conscious pigs before and after combined non-selective alpha- and beta-adrenoceptor blockade. 2. The plasma concentrations of the two compounds did not differ from each other over the entire dose-range. 3. Epinine increased aortic blood flow (AoBF, 24 +/- 6%), which was due to an increase in heart rate (HR) for doses less than 10 micrograms kg-1 min-1. At 10 micrograms kg-1 min-1, HR decreased slightly (10 +/- 3%, as compared to the value obtained at 5 micrograms kg-1 min-1) and stroke volume increased up to 15% (P < 0.05). Mean arterial pressure (MAP, 99 +/- 3 mmHg at baseline) decreased dose-dependently (14 +/- 2%, P < 0.05) up to the infusion rate of 5 micrograms kg-1 min-1, but increased by 4.0 +/- 1.8 mmHg during infusion of 10 micrograms kg-1 min-1. Systemic vascular resistance (SVR) decreased up to 23 +/- 3% for doses less than 10 micrograms kg-1 min-1, but did not change further during infusion of the highest dose. LVdP/dtmax increased during the two highest infusion rates up to 22 +/- 6% (P < 0.05). After the infusion was stopped there was an abrupt increase in HR (18 +/- 4%, P < 0.05) and a further decrease in SVR before all parameters returned to baseline.4. Dopamine caused increases in AoBF (27 +/- 3%) similar to epinine, the only difference being that HR continued to increase (32 +/- 5%) and MAP (13 +/- 3%) and SVR continued to decrease (31 +/- 3%) over the entire dose-range. The increase in LVdP/dt,,,, at the highest dose (48 +/- 4%, P <0.05) was more pronounced than with epinine.5. Adrenoceptor blockade inhibited all epinine-induced changes, but did not affect the dopamineinduced changes in AoBF, SVR and MAP, but attenuated the increases in HR and LVdP/dtmax.6. Noradrenaline (NA) and adrenaline (Ad) concentrations did not change during infusion of epinine or dopamine, but NA increased by 50% within 2.5 min after stopping the infusion of epinine. After adrenoceptor blockade NA and Ad concentrations did not change during infusion of dopamine, which contrasted with a decrease of 55 +/- 5% (P<0.05) in NA during infusion of epinine.7. Prolactin concentrations decreased gradually from 480 +/- 40 pg ml-' to 270 +/- 50 pg ml1' (P<0.05) during infusion of epinine, but did not change significantly during dopamine infusion.8. The differential effects of epinine and dopamine on MAP, SVR, plasma NA (before and after adrenoceptor blockade) and prolactin, leads us to conclude that in conscious pigs, epinine is a more potent a, P2 and D2-receptor agonist, but a weaker D,-receptor agonist than dopamine.

From serotonin receptor classification to the antimigraine drug sumatriptan

After the synthetic serotonin 5-hydroxytryptamine (5-HT) became available in the early 1950s, attempts were soon under way to study the nature of 5-HT receptors. Using the guinea-pig isolated ileum, Gaddum and Picarelli (1957) suggested that 5-HT-induced contractions were mediated by a morphine-sensitive "M" receptor located on the parasympathetic ganglion and a dibenzyline-sensitive "D" receptor located on the smooth muscle. Though this classification ws used during the next three decades, it was realized that some effects of serotonin, for example vasoconstriction within the carotid vascular bed, were not mediated by either "M" or "D" receptors. When radioligand binding studies led to the identification of 5-HT1 and 5-HT2 "receptors" in the rat brain membranes, it became increasingly apparent that the two receptor classifications were not identical. Thus, a new framework for serotonin receptor nomenclature and classification was proposed: 5-HT1-like (5-HT1), 5-HT2 (formerly "D") and 5-HT3 (formerly "M") receptors. At the present time, several subtypes of 5-HT1 receptors as well as a 5-HT4 receptor are also recognized. As the serotonin receptor classification was emerging to indicate that carotid vasoconstriction by serotonin is mediated by a subtype of 5-HT1 receptors, on the migraine front it was being suggested that the disease is associated with vasodilation within the cranial extracerebral circulation and deranged serotonin metabolism and that certain antimigraine drugs caused a selective carotid vasoconstriction, probably via serotonin receptors. Therefore, Humphrey and colleagues conceived that synthesis of serotonin derivatives may lead to a compound that would elicit highly selective carotid vasoconstriction and abort migraine attacks. Indeed, via the synthesis of 5-carboxamidotryptamine and AH25086, sumatriptan was designed. The drug acts as an agonist at the vasoconstrictor 5-HT1 receptor subtype and has proved highly effective in the therapy of migraine attacks.

Role of 5-HT1-like receptors in the increase in intragastric pressure induced by 5-hydroxytryptamine in the rat

The study concerned the effects of 5-hydroxytryptamine (5-HT) on intragastric pressure in bilaterally vagotomized spinal rats. Intravenous (i.v.) bolus injections of 5-HT (2.5, 5.0 and 10 micrograms/kg) produced dose-dependent increases in intragastric pressure; these effects were not modified by atropine (up to 0.2 mg/kg) or mepyramine (1 mg/kg), but were blocked by the mixed 5-HT1-like and 5-HT2 receptor antagonists, methiothepin (0.1, 0.3 and 0.5 mg/kg i.v.) and methysergide (0.5, 1 and 2.5 mg/kg i.v.). However, metergoline (0.5, 1 and 2 mg/kg i.v.) did not markedly modify this effect of 5-HT; only the response induced by 5 micrograms/kg 5-HT was significantly antagonized by the highest dose of metergoline. In contrast, neither the 5-HT2 receptor antagonist, ketanserin (0.5, 1 and 1.5 mg/kg i.v.), nor the 5-HT3 receptor antagonist, ICS 205-930 (0.5, 1 and 3 mg/kg i.v.), influenced the 5-HT-induced increase in intragastric pressure. In addition, 5-carboxamidotryptamine (25, 50 and 100 micrograms/kg i.v.) and RU 24969 (50, 100 and 200 micrograms/kg i.v.) mimicked the aforementioned effects of 5-HT but were weaker than 5-HT. These data suggest that the 5-HT-induced increase in intragastric pressure in the spinal and bilaterally vagotomized rat is mediated by an atypical 5-HT1-like receptor, which, based on the low agonist potency of 5-carboxamidotryptamine and RU 24969 and the resistance to blockade by metergoline, does not seem to correspond to either the 5-HT1A, 5-HT1B, 5-HT1C or the 5-HT1D receptor subtypes.

Identification of putative 5-HT4 receptors in guinea-pig ascending colon

Experiments were carried out to characterise pharmacologically a neuronal, non-5-HT3, 5-hydroxytryptamine (5-HT) receptor in guinea-pig isolated ascending colon. In preparations pretreated with methysergide (1 microM) and ondansetron (10 microM), 5-HT (0.003-1 microM) produced repeatable concentration-related contractions of guinea-pig ascending colon with an EC50 value of 29 (20-41) nM. The responses to 5-HT could be antagonised substantially by tetrodotoxin (0.3 microM) and atropine (1 microM) indicating a neuronal cholinergically mediated effect. The 5-HT-induced response was mimicked by 5-methoxytryptamine and alpha-methyl-5-HT with equipotent concentration ratios of 26 and 28, respectively. In contrast, 2-methyl-5-HT, 8-OH-DPAT, sumatriptan, phenylbiguanide and 5-hydroxyindalpine had no agonist activity up to 10 microM. The benzamides, metoclopramide, cisapride, R,S-zacopride and renzapride, mimicked the contractile action of 5-HT, acting like partial agonists. ICS205-930 (3 microM) acted as a competitive antagonist against 5-HT and 5-methoxytryptamine with estimated pKB values of 6.4 and 6.7 respectively. ICS205-930 (1 microM) also antagonised responses to R,S-zacopride and renzapride. Ketanserin (1 microM), phenylbiguanide (10 microM) and sulpiride (1 microM) had no effect on responses to 5-HT. We conclude that the pharmacological characteristics of the receptor, which mediates contraction of guinea-pig ascending colon by activation of cholinergic nerves, are consistent with it being of the putative 5-HT4 receptor type.

Further characterization, by use of tryptamine and benzamide derivatives, of the putative 5-HT4 receptor mediating tachycardia in the pig

1. It has recently been shown that the tachycardic response to 5-hydroxytryptamine (5-HT) in the anaesthetized pig, being mimicked by 5-methoxytryptamine and renzapride and blocked by high doses of ICS 205-930, is mediated by the putative 5-HT4 receptor. In the present investigation we have further characterized this receptor. 2. Intravenous bolus injections of the tryptamine derivatives, 5-HT (3, 10 and 30 micrograms kg-1), 5-methoxytryptamine (3, 10 and 30 micrograms kg-1) and alpha-methyl-5-hydroxytryptamine (alpha-methyl-5-HT; 3, 10, 30 and 100 micrograms kg-1), resulted in dose-dependent increases in heart rate of, respectively, 25 +/- 2, 48 +/- 3 and 68 +/- 3 beats min-1 (5-HT; n = 35); 15 +/- 1, 32 +/- 2 and 57 +/- 3 beats min-1 (5-methoxytryptamine; n = 30); 6 +/- 4, 18 +/- 6, 34 +/- 6 and 64 +/- 11 beats min-1 (alpha-methyl-5-HT; n = 3). 3. The increases in heart rate following i.v. administration of certain substituted benzamide derivatives were genereally less marked and not dose-dependent: 1 + 5, 11 + 3 and 10 + 5 beats min1- after 300, 1000 and 3000,jgkg' of metoclopramide, respectively, (n = 8); 21 + 4, 19 + 2 and 2 + 2 beats min'- after 100, 300 and lOOOIpgkg1- of cisapride, respectively, (n = 5); 6 + 2, 14 + 2, 37 + 6, 43 + 8 and 34 + 10 beats min- after 10, 30, 100, 300 and lOOOjigkg' of zacopride, respectively, (n = 6); and 1 + 1, 2 + 1 and 5 + 2 beats min- 1 after 300, 1000 and 3000 pg kg' of dazopride, respectively, (n = 4). These drugs behaved as partial agonists, antagonizing the responses to 5-HT and 5-methoxytryptamine dosedependently. 4. The 5-HT3 receptor agonist 1-phenyl-biguanide (100, 300 and lOOOpgkg-1) induced only slight increases in heart rate of 1 + 1, 6 + 2 and 11 + 1 beats min 1, respectively, (n = 3). These effects were not antagonized by the selective 5-HT3 receptor antagonist granisetron (3mgkg-1). In addition, 1-phenylbiguanide (1000,pg kg- 1) did not modify the tachycardia induced by either 5-HT- or 5- methoxytryptamine. 5. High doses (3mg kg- 1) of ICS 205-930, a 5-HT3 receptor antagonist with an indole group and devoid of effects on porcine heart rate per se, antagonized the stimulatory effects of 5-HT, 5-methoxytryptamine, alpha-Me-5-HT, metoclopramide, cisapride, zacopride, dazopride and 1-phenyl-biguanide. However, the 5-HT2 receptor antagonist ketanserin (0.5 mg kg- 1), the 5-HT3 receptor antagonists granisetron (3mg kg- 1) and MDL 72222 (3mg kg- ') and the dopamine D2 receptor antagonist domperidone (3 mg kg- 1) had no antagonist activity. 6. The above results support our contention that 5-HT, 5-methoxytryptamine, alpha-Me-5-HT and the substituted benzamide derivatives increase porcine heart rate by a direct action on the cardiac pacemaker, via the activation of a putative 5-HT4 receptor. The pharmacological profile of this novel 5-HT receptor is similar (neurones from mouse brain colliculi and human heart) or, perhaps, even identical (guinea-pig cholinergic neurones) to other putative 5-HT4 receptors.

Piglet sinoatrial 5-HT receptors resemble human atrial 5-HT4-like receptors

5-Hydroxytryptamine (5-HT), 5-carboxamidotryptamine (5-CT) and the gastrointestinal kinetic benzamides renzapride and cisapride caused tachycardia in spontaneously beating right atria of piglet in the presence of 400 nmol/l (+/-)-propranolol and 6 mumol/l cocaine. The maximum tachycardia caused by agonists, compared to that evoked by 200 mumol/l (-)-isoprenaline, was 63% for 5-HT, 50% for 5-CT, 50% for renzapride and 28% for cisapride. The rank order of potency was 5-HT greater than renzapride greater than cisapride greater than 5-CT. The effects of the agonists, but not those of (-)-isoprenaline, were antagonised by 3 alpha-tropanyl-1H-indole-3-carboxylic acid (ICS 205930); the pKB of ICS 205930 (vs 5-HT) was 6.9. These characteristics suggest that piglet sinoatrial 5-HT receptors are similar to "so-called" 5-HT4 receptors previously described in mouse colliculi neurons. Piglet sinoatrial 5-HT4-like receptors resemble the human atrial 5-HT receptors that mediate positive inotropic effects of 5-HT.

Mediation of 5-hydroxytryptamine-induced tachycardia in the pig by the putative 5-HT4 receptor

Intravenous bolus injections of 5-hydroxytryptamine (5-HT; 3, 10 and 30 micrograms kg-1), 5-methoxytryptamine (5-MeO-T; 3, 10 and 30 micrograms kg-1), renzapride (BRL 24924; 3, 10, 30 and 100 micrograms kg-1) and isoprenaline (0.03, 0.1 and 0.3 micrograms kg-1) to anaesthetized pigs increased heart rate by, respectively, 22 +/- 3, 44 +/- 3 and 65 +/- 4 beats min-1 (5-HT; n = 17); 12 +/- 1, 26 +/- 2 and 44 +/- 4 beats min-1 (5-MeO-T; n = 15), 5 +/- 2, 11 +/- 2, 18 +/- 4 and 37 +/- 5 beats min-1 (renzapride; n = 8) and 17 +/- 2, 46 +/- 3 and 75 +/- 3 beats min-1 (isoprenaline; n = 13). The responses to 5-HT, 5-MeO-T and renzapride were antagonized by ICS 205-930 (1 and 3 mg kg-1, i.v.), which did not modify the increases in heart rate by isoprenaline. Renzapride showed tachyphylaxis and attenuated the responses to 5-HT. These findings indicate that 5-HT elicits tachycardia in the pig by acting on a novel receptor, either similar or identical to the 5-HT4 receptor identified in mouse brain colliculi.

A 5-hydroxytryptamine receptor in human atrium

1. The effects of 5-hydroxytryptamine (5-HT) were investigated on right atrial appendages obtained from patients treated with beta-adrenoceptor blocking agents who were undergoing open heart surgery. Atrial strips were paced under isometric conditions. 2. 5-HT increased contractile force to approximately one half of the force produced by a saturating concentration of (-)-isoprenaline. Both 5-HT and (-)-isoprenaline accelerated the onset of relaxation, as indicated by an abbreviation of time to peak force. 3. The effects of 5-HT were resistant to blockade by 0.4 microM (+/-)-propranolol, 1 microM (-)-pindolol, 0.4 microM methiothepin, 4 microM yohimbine, 0.4 microM ketanserin, 10 microM phenoxybenzamine, 1 microM methysergide, 2 microM MDL 72222 and 20 microM granisetron. 4. Cocaine 6 microM potentiated the effects of 5-HT, increasing the pEC50 from 6.6 to 7.4. The inotropic potency of 5-HT is five times greater than that of (-)-noradrenaline. 5. ICS 205930 antagonized competitively the effects of 5-HT with a pKB of 6.7. 6. In the presence of 0.4 microM (+/-)-propranolol, 10 microM 5-HT increased both adenosine 3':5' cyclic-monophosphate (cyclic AMP) levels and cyclic AMP-dependent protein kinase activity by approximately one half and two thirds respectively, of the corresponding effects of 200 microM (-)-isoprenaline. 7. Both the increase in cyclic AMP levels and the stimulation of protein kinase activity are consistent with the inotropic effects of 5-HT being mediated by cyclic AMP-dependent phosphorylation of Ca2+ channels and of proteins involved in contraction and relaxation. 8. The human atrial 5-HT receptor resembles the neuronal 'so called' 5-HT4 receptor of rodents both in increasing cyclic AMP levels and in its affinity for ICS 205930.

Pharmacodynamic effects of serotonin (5-HT) receptor ligands in pigs: stimulation of 5-HT2 receptors induces malignant hyperthermia

In pigs, the serotonin-2 (5-HT2) receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), 0.8 mg/kg, induced "psychotic" behaviour (e.g., grimacing, backward locomotion, blank stare) and a muscular syndrome, which is known as malignant hyperthermia (MH) in pigs and humans. This syndrome is characterized by generalized skeletal muscle rigidity, leading to an increase in body temperature, marked acidosis, hyperkaliaemia, cyanosis and elevation of lactate, carbon dioxide and the muscle enzyme creatine kinase (CK) in plasma. In pigs which were selectively bred for susceptibility to MH induction by known triggering agents, such as halothane, the administration of DOI was fatal in 3 out of 5 animals. In genetically susceptible pigs, MH was also induced by 5-methoxy-N,N-dimethyltryptamine (5-MeO-DMT), 0.5-1.8 mg/kg, and D-lysergic acid diethylamide (LSD), 60-110 micrograms/kg. Furthermore, 5-MeO-DMT and LSD induced head shakes in the animals, which had not been observed after DOI and could not be blocked by 5-HT2-antagonists, ketanserin (0.5-5 mg/kg) and ritanserin (1-2.5 mg/kg). The psychotomimetic effects of 5-MeO-DMT could be blocked by ketanserin or ritanserin, which, depending on the dose, also reduced or totally prevented the hyperthermia and metabolic changes induced by 5-MeO-DMT in pigs. Administration of 5-MeO-DMT, 1.8 mg/kg, was fatal in 4 of 5 MH-susceptible pigs, whereas pigs injected with this dosage after pretreatment with ketanserin (0.5-5 mg/kg) or ritanserin (1-2.5 mg/kg) did not die. In pigs from MH-resistant littermates, administration of 5-MeO-DMT was not fatal. Comparison of metabolic changes in susceptible and non-susceptible pigs suggested that the marked increase in plasma potassium, which arises principally from damaged muscle cells, is primarily responsible for the fatal effect of DOI and 5-MeO-DMT in genetically susceptible individuals. In MH-susceptible pigs, which were anesthetized, relaxed and artificially ventilated, 5-MeO-DMT did not induce hyperthermia, thus substantiating that the marked hyperthermia observed in conscious pigs was a result of muscle activation and not due to effects on thermoregulation or blood pressure. The results indicate that hallucinogenic drugs with 5-HT2 agonistic effects trigger a life-threatening syndrome, MH, in genetically susceptible pigs. 5-HT2 antagonists, such as ketanserin or ritanserin, are capable of counteracting the fatality of this syndrome.

Reduction of cephalic arteriovenous shunting by ergotamine is not mediated by 5-HT1-like or 5-HT2 receptors

1. The potent, antimigraine drug ergotamine has affinity for both 5-HT1 and 5-HT2 binding sites and constricts arteriovenous anastomoses. Since 5-HT also constricts arteriovenous anastomoses (mainly via 5-HT1-like receptors), this study investigates the involvement of 5-HT receptors in the ergotamine-induced reduction of arteriovenous shunting in the carotid circulation of the cat and pig. 2. In the cat, ergotamine (3, 10 and 30 micrograms kg-1, i.v.) reduced carotid blood flow, predominantly by a reduction in arteriovenous anastomotic blood flow. Pretreatment with ketanserin (0.5 mg kg-1, i.v.) or methiothepin (1 mg kg-1, i.v.) did not antagonize the effects of ergotamine. 3. In the pig, ergotamine (2.5, 5, 10 and 20 micrograms kg-1, i.v.) also reduced carotid blood flow and arteriovenous shunting, which was not affected by pretreatment with methiothepin (1 mg kg-1, i.v.). 4. These results suggest that the reduction by ergotamine in the shunting of carotid arterial blood via cephalic arteriovenous anastomoses is not mediated by 5-HT1-like or 5-HT2 receptors.

Inhibition of noradrenaline release in the pig coronary artery via a novel serotonin receptor

In pig coronary artery preincubated with [3H]noradrenaline, the effects of serotonin (5-HT) receptor agonists and antagonists on the electrically evoked (0.66 Hz) tritium overflow were determined. Tritium overflow was inhibited by 5-HT, 5-aminotryptamine, N,N-dimethyl-5-hydroxytryptamine, 5-hydroxytryptamine, 5-methoxy-3(1,2,3,6-tetrahydropyridin-4-yl)-1H-indole (RU 24969) and tryptamine. The maximum inhibition obtainable with 5-HT was by about 35%, its pIC20 value was 7.85. 8-Hydroxy-di(n-propylamino)tetralin, urapidil, ipsapirone, 5-carboxamidotryptamine, 4-hydroxytryptamine, 5-methoxytryptamine and alpha-methyl-5-hydroxytryptamine did not decrease 3H overflow. The inhibitory effect of 5-HT was not antagonized by ketanserin, mesulergine, metitepine, propranolol, (3 alpha-tropanyl)-1H-indole-3-carboxylic acid ester (ICS 205-930) and yohimbine. Additionally, it was not altered by indomethacin. We conclude from the present data that the sympathetic nerves of the pig coronary artery are endowed with inhibitory presynaptic 5-HT receptors which do not belong to the 5-HT1, 5-HT2 or 5-HT3 receptor type but seem to represent a so far unknown receptor class.

Cardiovascular effects from stimulation of 5-hydroxytryptamine receptors

The cardiovascular effects of 5-hydroxytryptamine (5-HT), consisting of bradycardia or tachycardia, hypotension or hypertension, and vasodilatation or vasoconstriction, are mediated by three main types of receptors called 5-HT1-like, 5-HT2, and 5-HT3. In intact animals 5-HT elicits a short-lasting bradycardia, accompanied by hypotension, via stimulation of 5-HT3 receptors located on sensory vagal nerve endings in the heart (Bezold-Jarisch reflex). The nature of 5-HT receptors mediating tachycardiac responses is species-dependent. Myocardial 5-HT1-like and 5-HT2 receptors subserve tachycardia in the cat and rat, respectively. Tachycardia in the dog and rabbit is due to a release of catecholamines effected via the 5-HT2 receptors on the adrenal medulla and the 5-HT3 receptors on postganglionic cardiac sympathetic nerve fibres, respectively. The receptors mediating tachycardia in the pig are unique as they do not resemble any of the three 5-HT receptors characterized so far. The blood pressure response to 5-HT is usually triphasic: initial short-lasting hypotension due to reflex bradycardia (via 5-HT3 receptors), a middle pressor phase (via 5-HT2 receptors), and a longer-lasting hypotension (via 5-HT1-like receptors). Vascular contraction by 5-HT is generally mediated by 5-HT2 receptors (located primarily on the large conducting vessels), though in some instances (e.g., dog saphenous vein, dog and human basilar artery, and porcine arteriovenous anastomoses) the contractile response is (also) mediated via 5-HT1-like receptors. Venous dilatation and arteriolar dilatation (leading to increased capillary ['nutrient'] blood flow) occur via 5-HT1-like receptors located mainly on the vascular smooth muscles but also on the endothelium; the smooth muscle and endothelial 5-HT1-like receptors seem to be heterogeneous. In addition, 5-HT can elicit vasodilatation and hypotension as a result of decreased sympathetic nervous tone by acting within the central nervous system and by inhibiting noradrenaline release by a presynaptic action. Both these effects also involve 5-HT1-like receptors that do not appear to be identical. Last, knowledge of the cardiovascular effects of 5-HT and the nature of the receptors involved should be helpful in developing 5-HT-related compounds that may be useful in the treatment of hypertension, migraine, and peripheral vascular diseases.

Study on the mechanism of 5-HT-induced tachycardia in the pithed rat

1. Intravenous infusion of serotonin (5-HT) (2.5, 5, 10 and 20 micrograms/kg/min) in pithed rats induced a dose-dependent sustained tachycardia. 2. Pretreatment by phentolamine or diltiazem did not modify the chronotropic response to 5-HT. In contrast, atenolol antagonized this tachycardia and the 5-HT antagonists methysergide, ketanserin and MDL 72222 reduced it. 3. The 5-HT-induced tachycardia was abolished by desipramine and was not affected by fluvoxamine, a specific 5-HT uptake inhibitor. Surrenalectomy did not change the response to 5-HT but catecholamine depletion by reserpine markedly inhibited it. 4. Infusion of 5-HT increased the ratio of noradrenaline (NA) in the heart to NA in plasma, from 1.70 in control group to 2.76 in treated group (P less than 0.05). Desipramine inhibited this effect. 5. It was concluded that the tachycardia induced by an infusion of 5-HT in pithed rat results from a complex mechanism involving mainly the release of NA from the cardiac sympathetic nerves and a less important direct 5-HT2 mechanism.

Analysis of the tachycardiac response to 5-hydroxytryptamine in the spinal guinea-pig

The mechanism of the increase in heart rate caused by 5-hydroxytryptamine (5-HT) in the spinal guinea-pig was investigated. Administration of 5-HT (15, 30, 60 and 120 micrograms.kg-1 i.v.) elicited dose dependent increases in heart rate. The responses to 5-HT were not modified by methiothepin (0.5 and 1.5 mg.kg-1), ketanserin (0.5-4.5 mg.kg-1) or MDL 72222 (0.5-4.5 mg.kg-1) but were antagonized by the beta-adrenoceptor antagonists, propranolol (0.1-1 mg.kg-1) or atenolol (0.5-4.5 mg.kg-1). Indalpine, which is known to interfere with the uptake of 5-HT by nerve terminals and blood platelets, significantly reduced the effects of 5-HT at a dose (5 mg.kg-1) that also affected the tachycardia elicited by tyramine. The increase in heart rate caused by tyramine in reserpinized animals was attenuated and, unlike normal animals where the responses to 5-HT remained constant after repeated administration, there was a quickly developing tachyphylaxis to 5-HT. These results show that the increase in heart rate elicited by 5-HT in spinal guinea-pigs is not mediated by any of the currently characterized 5-HT receptors ('5-HT1-like', 5-HT2 and 5-HT3), and that a major part of the tachycardia seems to be mediated by a release of catecholamines by a mechanism similar, though perhaps not identical, to that for tyramine. Another as yet unidentified mechanism could be involved besides though perhaps not identical, to that for tyramine. Another as yet unidentified mechanism could be involved besides this action.