The depolarizing action of 5-hydroxytryptamine on rabbit vagal primary afferent and sympathetic neurones and its selective blockade by MDL 72222

Pharmacologically evoked apnoeas. Receptors and nervous pathways involved

This review analyses the knowledge about the incidence of transient apnoeic spells, induced by substances which activate vagal chemically sensitive afferents. It considers the specificity and expression of appropriate receptors, and relevant research on pontomedullary circuits contributing to a cessation of respiration. Insight is gained into an excitatory drive of 5-HT_1A serotonin receptors in overcoming opioid-induced respiratory inhibition.

Aequorin luminescence-based assay for 5-hydroxytryptamine (serotonin) type 3 receptor characterization

The classical electrophysiological method to measure the function of the 5-hydroxytryptamine (serotonin) type 3 (5-HT(3)) receptor, a cation-permeable ligand-gated ion channel, is time-consuming and not suitable for high-throughput screening. Therefore, we have optimized the conditions for a sensitive assay suitable to measure 5-HT(3) receptor responses in cell suspension based on aequorin bioluminescence caused by Ca(2+) influx. The assay, carried out in 96-well plates, was applied for the pharmacological characterization of 5-HT(3) receptors on human embryonic kidney (HEK) 293 cells transiently coexpressing apoaequorin and either the human homopentameric 5-HT(3A) receptor or the human heteromeric 5-HT(3A/B) receptor in the same subset of cells. Thus, the luminescence signal originates exclusively from transfected cells, leading to a high signal/noise ratio, a major advantage compared with fluorescence techniques using Ca(2+)-sensitive dyes. The potencies of two 5-HT(3A) receptor agonists and two antagonists as well as the potency and efficacy of serotonin at the heteromeric 5-HT(3A/B) receptor were comparable to those reported using other functional methods. In conclusion, the aequorin assay described here provides a convenient and highly sensitive method for functional characterization of 5-HT(3) receptors that is well suited for high-throughput screening.

Role of serotonin in thromboxane A2-induced coronary chemoreflex

We reported previously that the thromboxane A(2) (TxA(2)) mimetic U-46619 stimulates cardiac vagal afferent nerves, eliciting a reflex decrease in heart rate (HR) and arterial blood pressure (ABP). The present experiments were designed to test the hypothesis that TxA(2) evokes these changes via the release of serotonin [5-hydroxytryptamine (5-HT)] and activation of the 5-HT(3) receptor. Injections of the 5-HT(3) antagonist tropisetron (1 mg of 3-tropanyl-indole-3-carboxylate or ICS-205-930) attenuated the decreases in HR and ABP induced by left atrial injections of U-46619 (20 microg). Tropisetron administration also eliminated the U-46619-induced increase in impulse frequency in a majority of cardiac, vagal afferent units tested. Measurement of serum 5-HT levels revealed an elevation in serum 5-HT levels after U-46619 injection in those rabbits that displayed a significant HR change following injection of U-46619. These results indicate that although other factors may also contribute to these reflex responses, the release of 5-HT and stimulation of the 5-HT(3) receptor plays a significant role in coronary reflexes induced by TxA(2).

Afferent pathways for cardiac-somatic motor reflexes in rats

The present study used a rat model in which algogenic chemicals were infused into the pericardial sac to evoke spasmlike contractions in paraspinal muscles. The following techniques were used to study the roles of sympathetic (SCA) and vagal cardiac afferents (VCA) in electromyographic (EMG) responses to pericardial algogenic chemicals: chemical stimulation, electrical stimulation, and nerve transection. Activation with bradykinin (n = 46) produced a significantly higher peak response than infusion of an algogenic mixture (n = 53) containing chemicals that also activate VCA. Electrical stimulation of SCA produced bilateral EMG activities (7 of 7). Electrical stimulation of VCA did not evoke EMG activity but inhibited the chemically evoked EMG response (12 of 12). The chemically evoked response was decreased after transection of the left sympathetic chain (n = 22) and was increased after bilateral vagotomy (n = 19). These results suggest an excitatory and inhibitory role for SCA and VCA, respectively. Therefore, in addition to spinothalamic convergence of somatic and visceral afferents, activation of SCA to generate spasmlike muscle contractions could account in part for anginal pain, and VCA activation could attenuate this effect.

Pharmacological characterization of the 5-HT receptor-mediated contraction in the mouse isolated ileum

The pharmacological characterization of a 5-HT receptor-mediated contractile response in the mouse isolated ileum is described. In the presence of methysergide (1 microM), 5-hydroxytryptamine (5-HT, 0.3 - 100 microM) produced phasic concentration-dependent contractions of segments of the mouse isolated ileum with a pEC(50) value of 5.47+/-0.09. The 5-HT(3) receptor selective agonists m-chlorophenylbiguanide (0.3 - 100 microM, pEC(50) 5.81+/-0.04), 1-phenylbiguanide (3 - 100 microM, pEC(50) 5.05+/-0.06) and 2-methyl-5-HT (3 - 100 microM, pEC(50) 5.00+/-0.07) acted as full agonists to induce contractile responses. 5-methoxytryptamine (0.1 - 100 microM), RS 67506 (0.1 - 100 microM) and alpha-methyl-5-HT (0.1 - 100 microM) failed to mimic the 5-HT responses. The contractile response to 5-HT was not antagonized by either 5-HT(2) receptor antagonists ritanserin (0.1 microM) or ketanserin (1 microM) nor the 5-HT(4) receptor antagonist SB 204070 (0.1 microM). The 5-HT(3) receptor selective antagonists granisetron (0.3 - 1 nM), tropisetron (1 - 10 nM), ondansetron (10 nM - 1 microM) and MDL 72222 (10 nM - 1 microM) caused rightward displacement of the concentration-response curves to 5-HT. The lower concentrations of the antagonists caused approximate parallel rightward shifts of the concentration-response curves to 5-HT with apparent pK(B) values for granisetron (9.70+/-0. 39), tropisetron (9.18+/-0.20), ondansetron (8.84+/-0.24) and MDL 72222 (8.65+/-0.35). But higher concentrations of antagonists resulted in a progressive reduction in the maximum responses. The contractile response to 5-HT was abolished by tetrodotoxin (0.3 microM); atropine (0.1 and 1 microM) decreased the maximum response of the 5-HT concentration-response curve by approximately 65%. It is concluded that a neuronally located 5-HT(3) receptor mediates a contractile response to 5-HT in the mouse ileum. The 5-HT(3) receptor in the mouse ileum has a different pharmacological profile to that reported for the guinea-pig ileum.

Multiple receptor subtypes mediate the effects of serotonin on rat subfornical organ neurons

The subfornical organ (SFO) receives significant serotonergic innervation. However, few reports have examined the functional effects of serotonin on SFO neurons. This study characterized the effects of serotonin on spontaneously firing SFO neurons in the rat brain slice. Of 31 neurons tested, 80% responded to serotonin (1-100 microM) with either an increase (n = 15) or decrease (n = 10) in spontaneous activity. Responses to serotonin were dose dependent and persisted after synaptic blockade. Excitatory responses could also be mimicked by the 5-hydroxytryptamine (5-HT)2A/2C receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI; 1-10 microM) and could be blocked by the 5-HT2A/2C-receptor antagonist LY-53,857 (10 microM). LY-53,857 unmasked inhibitory responses to serotonin in 56% of serotonin-excited cells tested. Serotonin-inhibited cells were also inhibited by the 5-HT1A-receptor agonist 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT; 1-10 microM; n = 7). The data indicate that SFO neurons are responsive to serotonin via postsynaptic activation of multiple receptor subtypes. The results suggest that excitatory responses to serotonin are mediated by 5-HT2A or 5-HT2C receptors and that inhibitory responses may be mediated by 5-HT1A receptors. In addition, similar percentages of serotonin-excited and -inhibited cells were also sensitive to ANG II. As such the functional relationship between serotonin and ANG II in the SFO remains unclear.

Multiple 5-HT receptors in the guinea-pig superior cervical ganglion

1. We have studied the pharmacology of the depolarization by 5-hydroxytryptamine (5-HT) of the guinea-pig isolated superior cervical ganglion (SCG) using the grease-gap technique. We studied the effects of selective and non-selective antagonists on the responses to 5-HT and other 5-HT receptor agonists. 2. We have extended the pharmacology of the 5-HT3 receptor in this preparation by studying the effects of granisetron, BRL 46470 and mianserin on the concentration-response curve (CRC) to 2-methyl-5-HT. As with other 5-HT3 receptor antagonists, these compounds exhibited a lower affinity for guinea-pig 5-HT3 receptors than for rat 5-HT3 receptors. 3. We have confirmed that low concentrations of 5-HT (< or = 1 microM) mediate ketanserin-sensitive responses and higher concentrations of 5-HT also recruit 5-HT3 receptors. The responses to low concentrations of 5-HT were antagonized by low concentrations of ketanserin, spiperone, mianserin, DOI and LSD indicating probably mediation by 5-HT2A receptors. At high concentrations, the hallucinogen, DOI, but not LSD, evoked a ketanserin-sensitive depolarization. 4. Although mianserin could bind to the 5-HT2A receptors in this preparation, we could not demonstrate a down-regulation of depolarizations evoked by these receptors after a 10 day oral treatment with mianserin (10 mg kg-1, daily). 5. 5-Carboxamidotryptamine (5-CT) evoked a prolonged depolarization. Although high concentrations of 5-CT (> or = microM) appeared to activate 5-HT2A receptors, lower concentrations of 5-CT evoked a response with a distinct pharmacology. After studying the action of 20 selective and non-selective 5-HT receptor ligands we believe that this response may be mediated by a novel receptor; but its pharmacology is closest to that of receptors in the 5-HT2 receptor family. Like 5-CT, 5-HT (3-300 microM) could evoke an LSD-sensitive response in the presence of the 5-HT2 receptor antagonist, ketanserin and the 5-HT3 receptor antagonist, tropisetron (all 1 microM). 6. We conclude that 5-HT activates three pharmacologically distinct receptors to depolarize the guinea-pig SCG. Low concentrations of 5-HT appear to activate 5-HT2A receptors. Higher concentrations of 5-HT also activate 5-HT3 receptors and a possible novel 5-HT receptor. The novel receptor could be a species homologue of a 5-HT2 receptor or an, as yet, unclassified 5-HT receptor.

Evidence that the 5-HT3 receptors of the rat, mouse and guinea-pig superior cervical ganglion may be different

1. Using grease-gap recordings from the isolated superior cervical ganglion of mouse, rat and guinea-pig, we have compared the depolarization evoked by 5-hydroxytryptamine (5-HT) with that evoked by the selective 5-HT3 receptor agonist 2-methyl-5-HT (2-Me-5-HT). 2. The maximum depolarization induced by 2-Me-5-HT was smaller than that induced by 5-HT in all three species, and particularly in the guinea-pig. 3. The 5-HT2 receptor antagonist ketanserin (1 microM) caused a clear rightward shift of the dose-response curve to 5-HT on the guinea-pig ganglion, but not on the mouse or rat ganglion. Spiperone (0.03 microM) had a quantitatively similar action to ketanserin (0.1 microM) on the 5-HT dose-response curve of the guinea-pig ganglion. Ketanserin had no significant effect on the dose-response curve to 2-Me-5-HT on any of these ganglia. 4. Using 2-Me-5-HT as the agonist, we determined the pA2 values for two 5-HT3 receptor antagonists. The potency of ICS 205-930 varied by approximately 100 fold between the species and that of (+)-tubocurarine varied by over 1000 fold. The differences in the pA2 values of these compounds varied independently among the species. 5. We conclude that 5-HT3 receptors are present on the superior cervical ganglion from the rat, mouse and guinea-pig, but these receptors may be pharmacologically distinct from each other. In addition, the depolarization of the guinea-pig superior cervical ganglion by low concentrations of 5-HT is largely mediated by ketanserin-sensitive receptors.

Transient and long-lasting actions of 5-HT on rat dentate gyrus neurones in vitro

1. The actions of 5-hydroxytryptamine (5-HT) on rat dentate gyrus neurones were measured with conventional intracellular recording techniques in brain slices maintained in vitro at 32 degrees C. 2. Bath application of 5-HT (0.3-100 microM) hyperpolarized the membrane potential and reduced the input resistance; these effects persisted in tetrodotoxin (1 microM) and were abolished by MDL 73,005EF, a 5-HT1A receptor antagonist. 3. Local application of 5-HT via a pressure pipette also elicited a hyperpolarization and a reduction in resistance, and evoked a transient 'burst' of spontaneous inhibitory postsynaptic potentials (IPSPs) which were blocked by tetrodotoxin or bicuculline. 4. The 'burst' of IPSPs was subject to desensitization. It was completely abolished in the presence of the 5-HT3 receptor antagonist dolasetron. 5. In some cells, a longer lasting increase in spontaneous IPSP frequency was observed during application of 5-HT; this effect was blocked by the 5-HT2 receptor antagonist MDL 100,907. 6. 5-HT (30 microM) shortened the decay time constants of the glutamatergic and GABAergic evoked EPSPs and IPSPs without changing their amplitudes. 7. It is concluded that 5-HT hyperpolarizes granule cells via postsynaptic 5-HT1A receptors and increases spontaneous GABA release from inhibitory interneurones via the activation of 5-HT3 receptors and/or 5-HT2 receptors.

Antagonistic properties of McNeil-A-343 at 5-HT4 and 5-HT3 receptors

1. This study describes the in vitro interaction of the muscarinic ligand McNeil-A-343 with two 5-hydroxytryptamine (5-HT) receptor subtypes, the 5-HT4 and 5-HT3 receptors, using functional as well as radioligand binding studies. 2. In the rat oesophageal muscularis mucosae, precontracted with carbachol, McNeil-A-343 was a competitive antagonist (pA2 6.2) of the 5-HT4 receptor which mediates the relaxation induced by 5-HT. The compound per se relaxed the oesophagus at high concentration only (> or = 10 microM), an effect unchanged by desensitization of the 5-HT4 receptor with 10 microM 5-methoxytryptamine. In the same preparation in the absence of tone, McNeil-A-343 displaced the carbachol concentration-response curve to the right, yielding an apparent affinity (pA2) of 4.9 for muscarinic receptors. 3. In the rat isolated superior cervical ganglion preparation, after blockade of muscarinic and nicotinic receptors, McNeil-A-343 caused a concentration-dependent depolarization that was unaffected by 100 nM ondansetron. The concentration-fast depolarization curve to 5-HT, mediated by the 5-HT3 receptor, was displaced to the right by McNeil-A-343, which showed an apparent affinity (pA2) of 4.8 for the 5-HT3 subtype. 4. In binding studies, McNeil-A-343 recognized a single population of 5-HT4 receptors in pig caudate nucleus, with a pKI of 5.9. The binding affinity of McNeil-A-343 for 5-HT3 receptors in NG 108-15 cells was approximately four times lower (pKI 5.3). Binding affinities (pKI) for muscarinic receptor subtypes in rat tissues were 5.3 (M1, cortex), 5.2 (M2, heart) and 4.9 (M3, submandibular glands), respectively. 5. McNeil-A-343 is an antagonist at 5-HT4 and 5-HT3 receptors; the interaction of the compound with these receptor subtypes (notably the 5-HT4) occurs in a range of concentrations which generally overlaps that relevant to the interaction with muscarinic receptors.

Characterization of 5-hydroxytryptamine-induced depolarizations in rat isolated vagus nerve

An additional component of the depolarization induced by 5-hydroxytryptamine (5-HT) in the rat isolated vagus nerve has recently been attributed to activation of 5-HT4 receptors. To confirm and extend this finding, extracellular recordings of D.C. potentials were made using the 'grease-gap' technique during continuous superfusion of the isolated nerve. Beginning at 1 nM, 5-HT induced small depolarizations that displayed a slow onset. At concentrations > or = 1 microM, large depolarizations with rapid onset were elicited. In the presence of the 5-HT3 receptor antagonists, granisetron or ondansetron, 5-HT responses were diminished and exhibited an increased latency to peak. These small, slow depolarization were not reduced by 5-HT1 or 5-HT2 receptor antagonists, but were potently inhibited by the 5-HT4 receptor antagonist GR 113808 (pA2 = 9.3), and mimicked by 5-methoxytryptamine (pEC50 = 5.3). 5-HT4-mediated responses were larger at 37 degrees C than at 31 degrees C, but also showed marked diminution with repeated 5-HT applications at concentrations greater than 1 microM. Conversely, 5-HT3 receptor responses were potentiated at lower temperatures (< or = 31 degrees C). Consistent with the reported positive coupling of 5-HT4 receptors to adenylyl cyclase, forskolin and 8-Br-cAMP produced slowly developing depolarizations which were qualitatively similar to 5-HT4 receptor activation. Pre-depolarization of nerves with 10 microM forskolin or 300 microM 8-Br-cAMP diminished the effect of 5-HT4 receptors. This study has confirmed the presence of 5-HT4 receptors on the vagus nerve of the rat and defined some conditions that optimize their pharmacological isolation.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological characterization of 5-HT receptors in parasympathetic innervation of rat heart

A study was made of the effects of 5-hydroxytryptamine (5-HT) on bradycardia induced in vivo by electrical stimulation of the vagus nerves in pithed rats pretreated with atenolol. 5-HT significantly decreased vagally induced, but not acetylcholine-induced, bradycardia. The first effect was blocked by methiothepin, ketanserin or methiothepin with ketanserin. When 5-HT1 and 5-HT2 receptors were blocked, 5-HT produced an increase in vagally induced bradycardia. Both the inhibition and the potentiation were blocked by simultaneous pretreatment with methiothepin, ketanserin and MDL-72222. The 5-HT2 receptor agonist m-CPP (1-(3-chlorophenyl) piperazine dihydrochloride) caused an inhibition of vagally induced bradycardia whereas the 5-HT3 receptor agonist m-CPBG (1-(m-chlorophenyl)biguanide hydrochloride) produced a significant increase. The data suggest the presence of presynaptic and/or ganglionic 5-HT2 receptors in parasympathetic innervation of the rat heart, stimulation of which inhibits the release of acetylcholine. The presence of 5-HT3 receptors is also suggested, stimulation of which induces the release of acetylcholine.

An electrophysiological investigation of the properties of 5-HT3 receptors of rabbit nodose ganglion neurones in culture

1. The biophysical and pharmacological properties of 5-hydroxytryptamine (5-HT)-evoked currents in rabbit nodose ganglion neurones in culture have been determined by use of the whole-cell and outside-out membrane patch recording modes of the patch-clamp technique. 2. In 49% of cells investigated the bath application of 10(-5) M 5-HT at negative holding potentials elicited an inward current. The whole-cell response to 5-HT reversed in sign (E5-HT) at approximately -2 mV and exhibited inward rectification. 3. The influence of various ion substitutions upon E5-HT established that the 5-HT-evoked current is mainly mediated by a mixed Na+, K+ cation conductance with little or no contribution from Cl- ions. The omission of Ca2+ and Mg2+ from the extracellular solution enhanced the amplitude of the 5-HT-induced current. 4. On isolated outside-out membrane patches, the bath application of 10(-6) M 5-HT induced single channel currents with a chord conductance of approximately 17 pS at -70 mV and an average slope conductance of 19 pS over the range -100 to -40 mV. The 5-HT-induced single channels exhibited modest inward rectification and were reduced in frequency, but not amplitude, by the 5-HT3 receptor antagonist metoclopramide (10(-6) M). 5. The bath application of 5-HT (3 x 10(-7)-3 x 10(-5) M) to whole cells voltage clamped at -60 mV produced dose-dependent inward currents which were mimicked by 2-methyl-5-HT and 1-phenylbiguanide with equipotent molar ratios, relative to 5-HT, of 2.5 and 32 respectively. 6. Whole-cell inward currents produced by the local pressure application of 5-HT (10(-5) M) were unaffected by 10(-6) M methysergide, 10(-6) M ketanserin or 10(-6) M citalopram, but were concentration-dependently antagonized by the selective 5-HT3 receptor antagonists tropisetron (IC50 = 4.6 x 10(-11) M) ondansetron (IC50 = 5.7 x 10(-11) M), and bemesetron (IC50 = 3.3 x 10(-10) M). The response to 5-HT was also blocked by the non-selective antagonists metoclopramide (IC50 = 1.2 x 10(-8) M), cocaine (IC50 = 8.3 x 10(-8) M) and (+)-tubocurarine (IC50 = 1.6 x 10(-7) M).

Increase in serotonin levels in the dog ileum and blood by cisplatin as measured by microdialysis

Involvement of ileal and circulating serotonin (5-HT) levels in cisplatin-induced emesis was examined using a microdialysis technique and an extraction method in dogs. The 5-HT levels in the ileal dialysate were increased to 232-294% of the basal level from 100 to 180 min after cisplatin administration (3 mg/kg, i.v.) and had returned to the basal level 280 min after dosing. The 5-HT levels in the blood dialysate were increased to 424-2165% from 140 to 180 min after dosing. The concentrations of 5-HT determined by HPLC following extraction were increased to 271% in the ileal mucosa and to 478% in plasma 3 hr after dosing. In immunohistochemistry, the number of 5-HT-immunoreactive cells was increased to 166% in the ileal mucosa following cisplatin treatment. These results strongly suggest that increases in the release and synthesis of 5-HT in the gut, probably in the enterochromaffin cells, are intimately involved in cisplatin-induced emesis.

Sympathoinhibitory responses to 2-methylserotonin during changes in sodium intake

The vagal-mediated reflex responses elicited by the selective serotonin type 3 receptor agonist 2-methyl-serotonin were examined by administration (6.25, 12.5, 25, and 50 micrograms/kg i.v.) of 2-methylserotonin to sinoaortic-denervated rats with either intact or sectioned vagi. To study the influence of dietary sodium intake on 2-methylserotonin-induced vagal reflex responses, we performed experiments in rats fed either a high or low sodium diet. Left ventricular end-diastolic pressure was significantly higher in animals on high than low salt diet. However, mean arterial pressure and heart rate were similar in high and low salt groups. In rats with intact vagi, 2-methylserotonin produced a dose-dependent increase in afferent vagal nerve activity and a dose-dependent decrease in efferent renal sympathetic nerve activity, mean arterial pressure, and heart rate. The sympathoinhibitory responses of decreased efferent renal sympathetic nerve activity, mean arterial pressure, and heart rate were abolished by vagotomy and were not affected by changes in dietary sodium intake. We conclude that the sympathoinhibitory effect of 2-methylserotonin is due to stimulation of vagal afferents with inhibitory action on peripheral sympathetic nerve activity and that the sympathoinhibitory responses are unaffected by changes in dietary sodium intake.

A component of 5-HT-evoked depolarization of the rat isolated vagus nerve is mediated by a putative 5-HT4 receptor

This study describes a component of 5-HT-evoked depolarization of the rat isolated vagus nerve which was unaffected by the 5-HT3 receptor antagonist ondansetron. A grease-gap extracellular recording technique was used. Ondansetron (10-100 nmol/l) displaced the 5-HT concentration-response curve to the right yielding a pA2 value of 8.6 (8.5-8.8), consistent with 5-HT3 receptor antagonism, and revealing a component of the 5-HT response which was resistant to ondansetron blockade. In the presence of ondansetron (100 nmol/l) the maximum depolarization in the resistant phase was 15.5 (12.6-19.2)% of the initial maximum response to 5-HT and the pEC50 value was 7.0 (6.7-7.3). The mechanism of the ondansetron-resistant component of the 5-HT response resembled a 5-HT4-receptor-effect in being absent in preparations equilibrated with 5-methoxytryptamine (10 mumol/l) and antagonised by ICS 205930 (tropisetron, pA2 6.4). 5-Methoxytryptamine alone was an agonist in the vagus nerve with a maximum response similar to that of the ondansetron resistant phase of the 5-HT response. Similarly renzapride alone evoked small depolarizations of this preparation but antagonized the ondansetron resistant phase of the 5-HT response (pA2 7.3-7.4). These effects of 5-methoxytryptamine and renzapride are also consistent with a 5-HT4 receptor mechanism. Ketanserin (1 mumol/l) and methysergide (1 mumol/l) had little effect on responses to 5-HT. The depolarization evoked by this putative 5-HT4 receptor mechanism was small but prolonged and appears to mask and after-hyperpolarizing phase of the 5-HT response in this tissue.

A critical review of the afferent pathways and the potential chemical mediators involved in cardiac pain

There is considerable evidence that on the anterior surface of the heart (which is usually supplied by the left anterior descending and the proximal part of the left circumflex coronary arteries), sympathetic efferent reflexes characterized by tachycardia and/or hypertension predominate following experimental or pathological perturbations. These cardiovascular reflexes are accompanied by an increase in presumed nociceptive afferent traffic and, in pathological condition, by pain. In these experiments, there is generally no effect of vagotomy on afferent nerve traffic, and lower cervical and upper thoracic sympathectomies help provide relief from angina. On the other hand, experimental or pathological perturbations involving the inferior-posterior surface of the heart (supplied by the right and distal parts of the left circumflex coronary arteries), are characterized by vagal efferent reflexes, resulting in bradycardia and/or hypotension. These reflexes are accompanied by an increase in vagal afferent nerve traffic and, in pathological conditions, by pain. In these experiments, vagotomy generally abolishes such cardiovascular reflexes, and lower cervical and upper thoracic sympathectomies are not effective in the relief from angina. Although cardiac sympathetic afferents are unquestionably involved in the central transmission of nociceptive information from the heart, it is also likely that there is a contributing role from the vagus in cardiac pain. It is important experimentally to understand the natural stimulus that gives rise to angina. In the clinical situation, a decrease in coronary blood flow or an increase in the metabolic demands of the myocardium due to increased work are obvious precipitating factors which lead to myocardial ischemia. In the experimental situation, occlusion of the coronary arteries is often used as a stimulus which mimics myocardial ischemia. As people who frequently experience angina have varying degrees of coronary artery disease, it is difficult to accept that the state of the coronary arteries of the normal experimental animal bear any resemblance to the state of the coronary arteries under pathological conditions. That is, the gain of homeostatic reflexes, the basal concentrations of neuroactive substances in the plasma, the myocardium and the afferent terminals, the excitability of the afferents, access of chemical mediators (e.g. bradykinin, 5-HT, adenosine, histamine, prostaglandins, potassium, lactate), to afferents, and the overall function of the animal are all significantly different. We have no idea how control mechanisms have been altered in the person with severe coronary artery disease compared to the normal patient or the "normal" experimental animal.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of 5-HT on articular sensory receptors in normal and arthritic rats

1. The effects of intra arterial (i.a.) injections of 5-hydroxytryptamine (5-HT, 1-100 micrograms) on the discharge of (a) identified articular high threshold mechanoreceptors and (b) unidentified chemosensitive receptors in the ankle joint have been studied electrophysiologically in anaesthetized normal and arthritic rats. Recordings were made from a fine branch of the medial plantar nerve. 2. 5-HT increased the mechanical responsiveness of high threshold nociceptive mechanoreceptors with C and A delta fibre afferents in both normal and adjuvant-arthritic rats. Receptors in arthritic joints were more sensitive to 5-HT than were those from normal joints. 3. 5-HT produced a complex response from both types of articular receptors following i.a. injection. Two separate components were identified: (a) a fast transient burst of activity was obtained within 10 s of this injection in 66% of units from normal animals and 45% from arthritics, followed by (b) a delayed slow longer-lasting excitation seen in 62% of the units examined from normals and 77% of units from arthritic rats. 4. Increased mechanoreceptor responsiveness produced by 5-HT was reduced or abolished by the 5-HT3 receptor antagonists studied (MDL 72222, ICS 205-930, or GR 38032F, in single doses of 100 micrograms kg-1, i.a.). 5. Fast excitation showed marked tachyphylaxis and was antagonized by MDL 72222, ICS 205-930 or GR 38032F. It was unaffected by ketanserin (100 micrograms kg-1, i.a.). Delayed excitation was reduced or abolished by ketanserin but was unaffected by the 5-HT3-receptor antagonists. 6. Administration of MDL 72222, ICS 205-930 or GR 38032F caused short lasting (< 5 min) reductions in background activity from both types of unit recorded in arthritic rats, as well as in normal rats in which activity had increased following administration of 5-HT. Ketanserin caused similar reductions in background activity in chemosensitive units, but had no effect on mechanoreceptors. 7. At least two types of receptor are involved in the actions of 5-HT on articular sensory receptors with fine afferent fibres. Increased mechano-responsiveness involves a 5-HT3-receptor as does fast excitation. Delayed excitation probably involves a 5-HT2-receptor. Endogenous 5-HT appears not to play a crucial role in sensitization of high threshold mechanoreceptors in this model of chronic inflammation and arthritis, although its local release may potentiate the actions of other inflammatory mediators on sensory receptors in the ankle joint.

Temperature dependence of angiotensin II-mediated depolarisation of the rat isolated nodose ganglion

The ability of angiotensin II (A II) and 5-hydroxytryptamine (5-HT) to depolarise the rat isolated nodose ganglion preparation was examined. 5-HT depolarised the nodose ganglion, both at room temperature (20-24 degrees C) and at 35-37 degrees C. However, A II depolarised the nodose ganglion only under the latter condition, and these responses were blocked by the A II receptor antagonist saralasin. This study extends previous findings which have demonstrated A II binding sites on the nodose ganglion and axon, and identifies the rat nodose ganglion as a sensitive preparation in which to study the interactions between neuronal A II receptor activation and its blockade by A II receptor antagonists.

Vagal afferent-mediated inhibition of a nociceptive reflex by intravenous serotonin in the rat. I. Characterization

The effect of intravenous (i.v.) serotonin (5-HT) on nociception and blood pressure was examined in male Sprague-Dawley rats. Intravenous 5-HT produced a dose-dependent (6-192 micrograms/kg, i.v.) inhibition of the nociceptive tail-flick (TF) reflex in lightly pentobarbital-anesthetized (ED50 = 40 micrograms/kg) and conscious rats (ED50 = 44 micrograms/kg). In the lightly pentobarbital-anesthetized rat, the blood pressure response to i.v. 5-HT was typically a triphasic response with a marked Bezold-Jarisch reflex-induced decrease in pressure (associated with a brief period of apnea) followed by a pressor phase and a subsequent delayed hypotension. In the conscious rat, the response was typically biphasic with the late hypotensive phase absent. A variety of anatomical and pharmacological manipulations were performed to characterize the 5-HT-induced inhibition of the TF reflex and associated changes in blood pressure. Prevention of 5-HT-induced reflex apnea by artificial ventilation did not affect inhibition of the TF reflex produced by 5-HT. Pharmacological manipulations were performed to mimic, as closely as possible, the acute increases and decreases in blood pressure associated with i.v. 5-HT. Nitroprusside (8 micrograms/kg, i.v.) produced a decrease in blood pressure of similar magnitude and rate as that associated with the Bezold-Jarisch reflex-induced decrease in pressure produced by 72 micrograms/kg 5-HT, but did not change TF latency from baseline. Similarly, acute increases in pressure produced by phenylephrine (8 micrograms/kg, i.v.), intended to mimic the secondary pressor effect of 5-HT, did not change TF latency. The short-acting ganglion blocker trimethaphan (5 mg/kg, i.v.) closely mimicked the late hypotensive phase produced by 5-HT, but again resulted in no change in TF latency. Pretreatment with the ganglion blocker chlorisondamine (2.5 mg/kg) abolished all depressor responses to 72 micrograms/kg 5-HT, but did not significantly affect the TF reflex. These results indicate that acute changes in blood pressure and respiration associated with i.v. 5-HT do not contribute to inhibition of the TF reflex. This conclusion was confirmed in experiments in which bilateral vagotomy abolished approximately 70% of the 5-HT-induced inhibition of the TF reflex (and all depressor responses), and resulted in a significantly greater pressor response. Finally, low thoracic spinal cord transection (T9-10) abolished the inhibition of the TF reflex produced by i.v. 5-HT. Therefore, 5-HT stimulates vagal afferents and inhibits the TF reflex by activating descending inhibitory systems from the brainstem.(ABSTRACT TRUNCATED AT 400 WORDS)

Antagonism of the effects of 5-hydroxytryptamine on the rabbit isolated vagus nerve by BRL 43694 and metoclopramide

Depolarization and reduction in the C fibre compound action potential (C spike) in response to 5-HT were recorded simultaneously from rabbit isolated vagus nerve. 5-HT (0.1-100 mumol/l) was applied cumulatively and EC50 and IC50 values measured from individual concentration-response curves. Blockade of 5-HT responses by the 3-indazole carboxamide, BRL 43694, was investigated and compared with the blocking action of metoclopramide. BRL 43694 was a selective antagonist of 5-HT responses. A concentration of 10 nmol/l BRL 43694, which nearly abolished the depolarization and reduction of the C spike evoked by 5-HT (100 mumol/l), had no effect on similar responses evoked by DMPP (100 mumol/l) or GABA (100 mumol/l). Blockade of 5-HT responses by BRL 43694 (0.3 nmol/l) was slow in onset, a plateau blockade occurring after equilibrium of tissue with antagonist for 2 to 3 h. Metoclopramide induced a blockade of rapid onset. The maximal blockade was apparent within 30 min of application. Full recovery in the responsiveness of the tissue to 5-HT was observed within 30 min of washing out metoclopramide. BRL 43694 at concentrations of 0.3, 1, 3 and 10 nmol/l caused a progressive rightward shift of the concentration-response curves to 5-HT. At the highest concentration of antagonist, there was some depression of the maximal 5-HT response. The apparent pA2 estimated from the Schild equation was 10.03 +/- 0.09 (mean +/- SEM, n = 20) against 5-HT depolarization and 10.31 +/- 0.1 against C spike reduction. Schild plots had slopes not significantly different from 1.0.(ABSTRACT TRUNCATED AT 250 WORDS)

Association of [3H]zacopride with 5-HT3 binding sites

An assay was developed for [3H]zacopride binding to 5-HT3 specific sites in membranes from rabbit ileum muscularis. The binding was rapid, saturable, reversible, salt-insensitive, unaffected by pH between 6.5 and 9.5, and of high affinity (apparent KD = 0.65 +/- 0.15 nM). ICS 205-930, a potent 5-HT3 antagonist that inhibited competitively, was utilized to define 5-HT3 specific binding. Other 5-HT3 antagonists and agonists, although exhibiting marked differences in potency, were also effective inhibitors; whereas, antagonists of other classes of serotonin receptors, guanyl nucleotides and numerous receptor-specific ligands, including peptide hormones, were inactive. Vagus nerve exhibited the greatest amount of 5-HT3 specific binding amongst rabbit tissues and virtually all of the [3H]zacopride was bound to 5-HT3 binding sites. In rabbit, rat and ferret a fairly uniform distribution of 5-HT3 binding sites was observed along the muscularis of the small bowel. [3H]Zacopride is a high-affinity ligand for detecting 5-HT3 binding sites and rabbit small bowel muscularis membranes are a sensitive system for evaluating the potency of 5-HT3 antagonists or agonists.

5-Hydroxytryptamine evokes three distinct responses on the rat superior cervical ganglion in vitro

In addition to the 5-HT3-mediated fast depolarisation, 5-hydroxytryptamine (5-HT) evoked two additional responses on the rat superior cervical ganglion: a hyperpolarisation and a slow depolarisation. These responses appeared to be direct actions on 5-HT receptors since they were present in a low calcium medium containing tetrodotoxin and were not abolished by a variety of non-serotonin antagonists. The hyperpolarisation was not antagonised by 5-HT3 or 5-HT2 antagonists. The 5-HT1 ligands 5-carboxamidotryptamine (5-CT) and 8-OH-DPAT also evoked a hyperpolarisation. The hyperpolarisation was antagonised by six 5-HT1A antagonists including WB-4101 and spiroxatrine. It was therefore concluded to be mediated by a 5-HT1A receptor. The slow depolarisation was only evoked by 5-HT. The receptor involved in this response, however, could not be determined. We conclude that in addition to 5-HT3 receptors the rat superior cervical ganglion possesses 5-HT1A receptors and another uncharacterised 5-HT receptor.

Different receptor subtypes mediate the dual presynaptic effects of 5-hydroxytryptamine on peripheral sympathetic neurones

1. The aim of the present work was to characterize the presynaptic 5-HT receptors that mediate either the facilitation of the responses to nerve stimulation in the nictitating membrane of the cat or the inhibition of the responses to nerve stimulation in the guinea-pig atria. 2. In the nictitating membrane of the cat, the shift to the left in the frequency-response curves produced by 5-HT (0.1 microM) was prevented by the 5-HT3 receptor antagonists, metoclopramide (1 microM) and MDL 72222 (0.01 microM). 3. The facilitatory effect of 5-HT is also prevented by the 5-HT2 receptor antagonist, 0.01 microM ketanserin. Nevertheless, this drug reduced by itself the responses to both nerve stimulation and exogenous NA in the nictitating membrane. 4. In the guinea-pig isolated atria, the inhibitory effect of 5-HT on the chronotropic responses to cardioaccelerans nerve stimulation was mimicked by the mixed 5-HT1A + 5-HT1B + 5-HT1D receptor agonist 5-carboxamidotryptamine (5-CT 0.1 and 1 microM). The 5-HT1A receptor agonist 8-OH-DPAT (0.1 and 1 microM) did not modify the responses of the atria to the nerve stimulation. 5. The 5-HT2 receptor antagonists, ketanserin (0.01 and 0.1 microM) and cyproheptadine (1 microM), did not prevent the inhibitory effect of 5-HT in the guinea-pig atria. 6. The present results suggest that the facilitatory effects of 5-HT in the nictitating membrane of the cat are linked to the activation of 5-HT3 receptors whereas the inhibitory effects observed in the guinea-pig atria are mediated by 5-HT1-like receptors.

5-hydroxytryptamine3 receptors in the human brain: autoradiographic visualization using [3H]ICS 205-930

The distribution of 5-hydroxytryptamine receptors of the 5-hydroxytryptamine3 type was examined in human brain post mortem tissue, using quantitative in vitro autoradiography. The selective and potent 5-hydroxytryptamine3 receptor antagonist [3H]ICS 205-930 [(3 alpha-tropanyl)-1H-indole-3-carboxylic acid ester] was used as ligand. Highest levels of labelling were found in discrete nuclei of the lower brainstem. At all levels of the spinal cord the substantia gelatinosa was also densely labelled. In contrast, specific binding in the forebrain was very low and concentrated in some regions of the limbic system. The enrichment of [3H]ICS 205-930 binding sites in nuclei of the dorsal medulla and spinal cord is in good agreement with the proposed role for 5-hydroxytryptamine in sensory processing. High densities of 5-hydroxytryptamine3 binding sites in the area postrema support a central site of action for 5-hydroxytryptamine in emesis. Finally, the presence of [3H]ICS 205-930 binding sites in the limbic system provides an anatomical substrate for the behavioural effects of 5-hydroxytryptamine3 receptor antagonists.

The depolarizing action of 5-hydroxytryptamine on rabbit isolated preganglionic cervical sympathetic nerves

This study describes a depolarizing action of 5-hydroxytryptamine (5-HT) on rabbit isolated preganglionic cervical sympathetic nerves using an extracellular recording technique. From cumulative concentration-response curves for 5-HT (1 mumol/l-1 mmol/l), the mean maximal depolarization was shown to be 277 +/- 32 microV and EC50 was 9.4 mumol/l (6.5-13.6 mumol/l, geometric mean, 95% confidence limits, n = 42). The responses to 5-HT displayed marked tachyphylaxis. When cumulative concentration-response curves to 5-HT and 2-methyl-5-HT were determined in the same preparations (n = 4), the mean maximal response to 5-HT was 519 +/- 167 microV, EC50 32.2 mumol/l (8.8-118 mumol/l) and the mean maximal response to 2-methyl-5-HT was 317 +/- 63 microV, EC50 35.1 mumol/l (12.9-95.5 mumol/l, geometric means, 95% confidence limits). The action of selective 5-HT antagonists was tested on repeated cumulative concentration-response curves to 5-HT. Neither methiothepin (0.1-1 mumol/l, n = 3) nor ketanserin (0.1-1 mumol/l, n = 3) had an action on 5-HT responses. The selective 5-HT3 antagonists MDL 72222, ICS 205-930 and SDZ 206-830 were all potent antagonists of the 5-HT depolarizations. The action of these antagonists was quantified by determining the apparent pA2 from the dose ratios and a Schild plot.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological properties of GR38032F, a novel antagonist at 5-HT3 receptors

1. This paper describes the pharmacology of the novel 5-hydroxytryptamine3 (5-HT3) receptor antagonist GR38032F. 2. On the isolated vagus nerve and superior cervical ganglion of the rat, R,S-GR38032F behaved as a reversible competitive antagonist of 5-HT-induced depolarization with pKB values of 8.61 +/- 0.08 (n = 19) and 8.13 +/- 0.07 (n = 16), respectively. The resolved R- and S-isomers of GR38032F were approximately equipotent as 5-HT antagonists on the rat vagus nerve: the pKB values were 8.95 +/- 0.05 (n = 16) and 8.63 +/- 0.08 (n = 17), respectively. R,S-GR38032F was also an effective antagonist of 5-HT on the rabbit isolated vagus nerve: in this case the pKB value was 9.40 +/- 0.14 (n = 4). 3. On the rabbit isolated heart, low concentrations of R,S-GR38032F (3 X 10(-11)-1 X 10(-9) M) antagonized the positive chronotropic effect of 5-HT and 2-methyl-5-hydroxytryptamine (2-methyl-5-HT). However, the effects of the compound did not appear consistent with simple reversible competition. 4. On the longitudinal smooth muscle of the guinea-pig ileum, R,S-GR38032F caused concentration-dependent parallel rightward displacement of the 2-methyl-5-HT concentration-contraction response curve; in contrast, a portion of the response to 5-HT appeared resistant to R,S-GR38032F. pKB values estimated from the effects of the compound against 2-methyl-5-HT or the inhibitable portion of the response to 5-HT were 7.31 +/- 0.06 (n = 8) and 7.33 +/- 0.13 (n = 8), respectively. Against 2-methyl-5-HT, R-GR38032F seemed more potent (pKB 7.20 +/- 0.10; n = 6) than S-GR38032F (pKB 6.30 +/- 0.05; n = 6). 5. R,S-GR38032F is highly selective for 5-HT3 receptors, and at concentrations of 3 X 10(-6)-3 X 10(-5) M, had negligible agonist or antagonist activity on other 5-HT or non-5-HT receptor-containing tissues on which it was tested. 6. The potency and duration of action of R,S-GR38032F in blocking 5-HT3 receptors in vivo were assessed by measuring its ability to antagonize the bradycardic response to 5-HT or 2-methyl-5-HT administered intravenously (i.v.) to anaesthetized animals. For i.v. administration to the rat, the ED50 for R,S-GR38032F against 2-methyl-5-HT (100pgkg-1) was 0.4 (95% confidence limits 0.18- 0.87) ygkg-1 (n = 10); the corresponding value for oral administration to this species was 7.0 (3.0- 22.0)pgkg-' (n = 8-10 per dose level). R,S-GR38032F was similarly effective in the anaesthetized cat. 7. The present results are discussed with reference to the postulated existence of subtypes of the 5-HT3 receptor.

Evidence for depressant 5-HT1-like receptors on rat brainstem neurones

1. The technique of microiontophoresis was used to evaluate the contribution of 5-HT1-like, 5-HT2- and 5-HT3-receptors to the depressant effects of 5-hydroxytryptamine (5-HT) on neurones in the midline of the medullary brainstem of the rat in vivo. 2. Depressant responses to 5-HT were resistant to antagonism by the 5-HT2-receptor antagonist ketanserin and the 5-HT3-receptor antagonist MDL 72222 applied either microiontophoretically or administered systemically. 3. Microiontophoretic or systemic administration of the 5-HT antagonist metergoline, which shows nanomolar affinity for the 5-HT1-binding site, also failed to attenuate the depressant responses to 5-HT. 4. Systemic administration of high doses of methysergide (30-40 mg kg-1) attenuated the depressant responses to 5-HT but did not block depressant responses to GABA or excitatory responses to glutamate. 5. The depressant effects of 5-HT were potently mimicked by the 5-HT1-like receptor agonists 5-carboxamidotryptamine and 8-OH-DPAT. 6. These results indicate that neither 5-HT2-receptors nor 5-HT3-receptors are involved in the depressant effects of 5-HT on midline brainstem neurones. The depressant effects of 5-carboxamidotryptamine (5-CT) and 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and blockade of the response to 5-HT by high doses of methysergide suggests the involvement of 5-HT1-like receptors. The lack of effect of metergoline, however, indicates that this receptor may be different from any of the 5-HT1 binding sites yet described.

A comparison of fast and slow depolarizations evoked by 5-HT in guinea-pig coeliac ganglion cells in vitro

1. 5-Hydroxytryptamine (5-HT) was applied by pressure ejection to coeliac ganglion cells of the guinea-pig maintained in vitro and responses measured intracellularly. 2. Cells responded in one of three ways to 5-HT: by (a) a fast, transient depolarization (43%), (b) a fast transient followed by a slow depolarization (biphasic response, 30%) or (c) a slow sustained depolarization (25%). 3. Fast depolarizations (response (a) above] were graded according to the duration of the ejection pulse. Maximal responses had a mean amplitude of 12 +/- 0.8 mV, a duration of 6.4 +/- 1.0 s, a latency of 0.4 +/- 0.1 s, were associated with a fall in membrane input resistance, increased in amplitude by hyperpolarization and probably mediated by an increased conductance to Na and K. The estimated reversal potential was -22.8 +/- 2.4 mV (n = 14). The maximal fast response seen in biphasically-responding cells (b) appeared similar to fast response (a). 4. Fast depolarizations (a) showed marked tachyphylaxis and were abolished by superfusion of the ganglion with 5-HT (100 microM). They were reduced in amplitude by tubocurarine (10-100 microM, pIC50 4.4), MDL 72222 (1-5 microM, pIC50 5.8), quipazine (1 microM reduced responses by 65 +/- 15%, n = 3), ICS 205-930 (1 microM reduced responses by 64 +/- 14%, n = 7) and metoclopramide (10 microM reduced responses by about 45%), but were unafected by methysergide (up to 1 microM) or hexamethonium (up to 1 mM). 5. Slow depolarizations (c) varied in amplitude with the duration of the ejection pulse. Maximal responses had a mean amplitude of 6.4 +/- 0.7 mV, a duration of 62 +/- 6 s, a latency of 3.5 +/- 0.8 s and were reduced in amplitude by methysergide (0.1-1 microM, pIC50 6.5) but not by MDL 72222 (1 microM). The maximal slow component in biphasically-responding cells (b) was similar in amplitude and duration to slow response (c), was partially blocked by methysergide (1-5 microM) in 4 of 6 cells and was enhanced by tubocurarine (50 microM) which reduced the fast component. 6. Slow depolarizations (b,c) were associated with either a small reduction or no change in membrane input resistance depending on the cell studied. Hyperpolarization had variable effects on slow depolarization amplitude. 7. It was concluded that the fast, phasic depolarization is mediated by an ionic mechanism and by receptors both of which are distinct from those involved in the slow depolarization. The receptor mediating the fast depolarization is a 5-HT3 receptor while that mediating the slow depolarization has yet to be identified.

Fast and slow depolarizing responses of guinea-pig coeliac ganglion cells to 5-hydroxytryptamine

Responses to 5-hydroxytryptamine (5-HT) applied by pressure ejection to guinea-pig coeliac ganglion cells were recorded intracellularly in vitro. Three types of responses were observed: a fast depolarization (43% of cells), a slow depolarization (25%) and a biphasic response comprising an initial fast followed by a slow depolarization (30%). Fast depolarizations had a mean duration of 6.4 s and were associated with a decrease in membrane input resistance. On hyperpolarizing the membrane, responses increased in amplitude; the mean reversal potential was -23 mV. Fast responses were nearly eliminated in a Na-free medium but were unaffected by low Ca/high Mg solution or by low Cl medium. Fast responses were depressed by MDL 72222, quipazine, metoclopramide, and by relatively high concentrations of D-tubocurarine, but unaffected by methysergide. Slow responses had a mean duration of 62 s and were associated either with no change or a slight fall in membrane resistance. hyperpolarizing the membrane, slow responses were increased in most but not all cells tested. Slow depolarizations were depressed by methysergide but unaffected by agents that suppressed the fast responses. The biphasic response displayed electrophysiological and pharmacological characteristics of both the fast and slow depolarizations. It is concluded that guinea-pig coeliac ganglion cells display either a rapid depolarization, a slow depolarization or a biphasic response to 5-HT, that the fast depolarization appears to be mediated by a conductance increase to Na and K, while a different mechanism appears to underlie the slow depolarization, and that the two responses are mediated by different kinds of 5-HT receptor.

Reflex inhibition of efferent renal sympathetic nerve activity by 5-hydroxytryptamine and nicotine is elicited by different epicardial receptors

Intrapericardial administration of 5-hydroxytryptamine (5-HT) induced reflex effects consisting in an inhibition of renal sympathetic nerve activity (RSNA), bradycardia and a fall in blood pressure. Nicotine caused the same reflex effects as 5-HT. The reflex effects of both 5-HT and nicotine were abolished by vagotomy. MDL 72222, an antagonist at 5-HT M-receptors, abolished or attenuated the decreases in RSNA, heart rate and blood pressure induced by 5-HT, leaving the reflex effects of nicotine unchanged. In the absence of MDL 72222 the reflex bradycardia partially concealed a positive chronotropic response to 5-HT. After blockade of the bradycardia response by MDL 72222, 5-HT elicited a significant tachycardia, which was not altered by propranolol and phentolamine, but was prevented by phenoxybenzamine. 5-HT probably reaches the sinoatrial node and activates 5-HT receptors that mediate directly the increase in heart rate. The nicotine receptor antagonist hexamethonium selectively abolished or attenuated the reflex effects of nicotine without interfering with those of 5-HT. We conclude that 5-HT and nicotine elicit similar reflex effects in epicardial vagal nerve endings by stimulation of M-receptors or nicotine receptors, respectively.

Further studies on the blockade of 5-HT depolarizations of rabbit vagal afferent and sympathetic ganglion cells by MDL 72222 and other antagonists

The blocking action of MDL 72222 (1 alpha H, 3 alpha, 5 alpha H-tropan-3-yl-3, 5-dichlorobenzoate at 5-hydroxytryptamine (5-HT) receptors on nodose (NG) and superior cervical ganglia (SCG) has been investigated further. The sucrose-gap technique was used to record potential changes from populations of neurones. The surmountable blockade induced by small concentrations of the antagonist was quantified and the blocking potency compared with that of a number of other compounds. In nodose ganglia three 4-5 point dose-response (DR) curves were established, using bolus injections of 5-HT (5-80 nmol). The mean amplitude of the response to 80 nmol was 4.18 +/- 0.53 mV and the ED50 was 18.2 nmol. Second and 3rd dose-response curves showed small displacements to the right, indicating a slight reduction in sensitivity. In superior cervical ganglia responsiveness was less. Amounts of 5-HT ranging from 20 to 320 nmol evoked dose-related depolarizations. The mean amplitude of the response evoked by 320 nmol 5-HT was 1.7 +/- 0.14 mV. Three 4-5 point dose-response curves could be elicited from a single ganglion. The ED50 was 55.8 nmol. Initial, 2nd and 3rd dose-response curves could be superimposed, there being no significant rightward shift. The results confirm that MDL 72222 is a potent, selective antagonist at 5-HT receptors in nodose and superior cervical ganglia. In the nodose ganglion, after equilibration for 1 hr with 10(-8) or 10(-7) M MDL 72222, dose-response curves for 5-HT showed rightward, parallel shifts. In contrast, 10(-6) M MDL 72222 or prolonged exposure (3-4 hr) to 10(-8), 10(-7) or 10(-6) M caused larger rightward shifts of the dose-response curves and depressed the maximum responses. In the superior cervical ganglion, equilibration for 1 hr with concentrations of 10(-8) or 10(-7) M produced effects on the dose-response curves similar to those seen in the nodose ganglion, but longer exposures (3-4 hr) did not depress the maximum. Apparent pA2 values were determined from individual experiments on both the nodose and superior cervical ganglia, where MDL 72222 (10(-7) M or less, for 1 hr) caused parallel or near parallel shifts of dose-response curves. In the nodose ganglion the apparent pA2 was 7.7 +/- 0.1, while in the superior cervical ganglion it was 7.8 +/- 0.1 (mean +/- SEM). The nature of the blockade induced by prolonged exposures or by concentrations greater than 10(-7) M is discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacological characterization of 5-hydroxytryptamine-induced depolarization of the rat isolated vagus nerve

A study has been made of the pharmacology of the 5-hydroxytryptamine (5-HT)-induced depolarization responses that can be recorded extracellularly from the rat isolated cervical vagus nerve. Phenylbiguanide (PBG) and 2-methyl-5-hydroxytryptamine (2-methyl-5-HT) were found to mimic the effects of 5-HT on the vagus nerve. Their EC50 values were respectively 2.0 fold and 3.9 fold greater than that of 5-HT. Metoclopramide behaved as a reversible competitive antagonist of depolarization induced by PBG and 2-methyl-5-HT, with pKB values of 6.48 +/- 0.04, respectively. These agreed well with the pKB value of 6.60 +/- 0.04 obtained previously for metoclopramide against 5-HT on the rat vagus nerve. 5-HT, PBG and 2-methyl-5-HT had no demonstrable agonist effects at non-5-HT receptors on the rat vagus nerve. Tropacaine and m-chlorophenylpiperazine were found to behave as reversible competitive antagonists of 5-HT-induced depolarization of the vagus nerve. The pKB values were 6.29 +/- 0.03 and 6.90 +/- 0.03, respectively. Quipazine, MDL 72222 and ICS 205-930 were also shown to be effective antagonists of 5-HT on the vagus nerve. However, although these compounds were highly potent, they all caused a marked concentration-dependent reduction in the amplitude of the maximum response to 5-HT. This behaviour was not consistent with a simple reversible competitive mechanism. The results are discussed with reference to the current classification of mammalian peripheral neuronal 5-HT receptors.

Peripheral neural serotonin receptors: identification and characterization with specific antagonists and agonists

Serotonin (5-hydroxytryptamine, 5-HT) has been shown to be a neurotransmitter in the enteric nervous system (ENS). Although 5-HT is a mediator of slow excitatory postsynaptic potentials evoked by stimulation of interganglionic connectives, the precise role it plays in the physiology of the gut is unclear. Research has been hampered by an inadequate knowledge of the types of 5-HT receptor in the ENS and thus the lack of well-characterized antagonists. We now report the identification of two classes of enteric neural 5-HT receptor, the effects of activating these receptors on myenteric type II/AH neurons, and their characterization with specific agonists and antagonists. One class, which we propose to call 5-HT1P, is characterized by a high affinity for [3H]5-HT in radioligand binding assays. This class of receptor mediates a slow depolarization of myenteric type II/AH neurons associated with an increase in input resistance. Agonists at this receptor include, in addition to 5-HT (in order of potency), 5- and 6-hydroxyindalpine and 2-methyl-5-HT. 5-HT1P-mediated responses are specifically antagonized by 5-hydroxytryptophyl-5-hydroxytryptophan amide. The other class of 5-HT receptor, which we propose to call 5-HT2P, appears not to have a high affinity for [3H]5-HT. This receptor mediates a brief depolarization of myenteric II/AH neurons associated with a fall in input resistance. 2-Methyl-5-HT, at low concentrations, is a specific agonist at this receptor and ICS 205-930 is a specific antagonist. Binding of [3H]5-HT to enteric membranes is inhibited by 5-HT1P receptor agonists and antagonists but not by the 5-HT2P receptor antagonist ICS 205-930 or by MDL 72222, another compound reported to be an antagonist of 5-HT at peripheral receptors.

Proposals for the classification and nomenclature of functional receptors for 5-hydroxytryptamine

As a result of controversy in the literature regarding the classification and nomenclature of functional receptors for 5-hydroxytryptamine (5-HT), a framework for classification is proposed. The formulation of these proposals has only been made possible by the recent advent of new drug tools. It is considered that there are three main types of 5-HT receptor, two of which have been well characterised pharmacologically, using selective antagonists, and which it is proposed to name 5-HT2 and 5-HT3. These two groups broadly encompass the "D" and "M" receptors, respectively, which Gaddum identified in the guinea-pig ileum (Gaddum and Picarelli, 1957). The 5-HT2 receptor, which mediates a variety of actions of 5-HT, has been definitively shown to correlate with the 5-HT2 binding site in the brain. No binding studies in brain tissue have yet been published with radiolabelled ligands specific for 5-HT3 receptors. A number of other actions of 5-HT appear to be mediated via receptors distinct from 5-HT2 or 5-HT3 receptors. Since selective antagonists are not yet available, these receptors cannot be definitively characterised, although in many cases they do have some similarities with 5-HT1 binding sites, which are a heterogeneous entity. Criteria are proposed for tentatively classifying these receptors as "5-HT1-like" (Table 1). Definitive characterisation of these receptors will await the identification of specific antagonists. This classification of 5-HT receptors into three main groups (Table 1) is based largely, but not exclusively, on data from studies in isolated peripheral tissues where definitive classification is possible. However, it is believed that this working classification will be relevant to functional responses to 5-HT in the central nervous system.

The depolarizing action of 5-hydroxytryptamine on rabbit vagal afferent and sympathetic neurones in vitro and its selective blockade by ICS 205-930

Depolarizing responses to 5-hydroxytryptamine (5-HT) were recorded from rabbit nodose (NG) and superior cervical (SCG) ganglia using the sucrose-gap technique. The antagonist potency and selectivity of ICS 205-930 ([3 alpha-tropanyl]-1H-indole-3-carboxylic acid ester) were investigated. In NG, 5-HT (5 to 80 nmol) evoked depolarizations of graded amplitude. The ED50 was 18.2 (10.9-30.5) nmol (geometric mean, 95% confidence limits). Responses were blocked surmountably by ICS 205-930, 10(-11) and 10(-10) M, the threshold for blockade being below 10(-11) M. Parallel, rightward shifts in dose-response curves were seen with these concentrations of antagonist, but at higher concentrations (10(-9) and 10(-8) M) there was a further rightward shift with reduction in slope and maximum of the curves. In SCG, where 5-HT (20 to 320 nmol) evoked depolarizations of graded amplitude and the ED50 was 55.8 (22.3-139.6) nmol (geometric mean, 95% confidence limits), ICS 205-930 had a similar inhibitory effect to that observed in NG. The apparent pA2 values for the surmountable blockade produced by ICS 205-930 at concentrations of 10(-11) and 10(-10) M were 10.2 +/- 0.2 for NG and 10.4 +/- 0.1 for SCG (means +/- s.e. mean). ICS 205-930 was selective in its action since it had no effect on dimethylphenylpiperazinium (DMPP) responses in either ganglion or on GABA responses in NG. This study provides quantitative evidence on the blocking action of ICS 205-930 at neuronal 5-HT receptors using a technique that allows the depolarizing responses evoked by the amine to be directly recorded.

Blockade of the flare response to intradermal 5-hydroxytryptamine in man by MDL 72.222, a selective antagonist at neuronal 5-hydroxytryptamine receptors

MDL 72.222 is a potent and selective antagonist of excitatory neuronal 5-hydroxytryptamine (5-HT) receptors which has been shown to provide symptomatic benefit in migraine. In the present study, the effects of MDL 72.222 on the flare response to intradermal injection of 5-HT (10(-5), 4 X 10(-5) and 1.6 X 10(-4) mol/l) have been examined in six healthy male volunteers using a double-blind, placebo-controlled, randomized, crossover design. The study consisted of two sessions separated by at least 5 days. Comparison of within and between day responses to 5-HT indicated good reproducibility. Significant attenuation of the flare response to 5-HT, 10(-5) and 4 X 10(-5) mol/l was observed following a slow (4 min) i.v. injection of MDL 72.222, 20 mg. No significant change was observed for the 1.6 X 10(-4) mol/l dose. The investigator was able to guess with a high degree of accuracy the sequence of MDL 72.222 and placebo (100% correct) and the order of 5-HT dose administration (70% correct), providing further evidence of the reproducibility and sensitivity of the method. MDL 72.222 was well tolerated. Thus, a dose of MDL 72.222 previously shown to provide symptomatic relief in migraine attenuates the flare response to intradermal 5-HT in the human forearm. The observation strengthens the view that the beneficial effects of MDL 72.222 in migraine result from blockade of the sensory neuronal stimulant effects of 5-HT and implies a role for 5-HT in the pain production of the acute attack.

Effects on migraine headache of MDL 72,222, an antagonist at neuronal 5-HT receptors. Double-blind, placebo-controlled study

MDL 72,222 (1 alpha H, 5 alpha H-tropan-3 alpha-yl 3,5-dichlorobenzoate), a novel agent with antagonist activity at neuronal 5-HT receptors, was tested as an acute treatment for migraine pain under double-blind, placebo-controlled conditions. Forty-seven patients with common (n = 29) or classical (n = 8) migraine or mixed type with or without a psychogenic component (n = 10) received 20-40 mg MDL 72,222 (n = 24) or placebo (n = 23) intravenously during the headache phase of a migraine attack. MDL 72,222 was consistently superior to placebo in rapidly alleviating the migraine pain. The treatment was remarkably well tolerated. The results are consistent with the hypothesis that the pain of migraine is related to activation of neuronal 5-HT receptors and suggest that compounds such as MDL 72,222, which block neuronal 5-HT receptors, could be useful new therapeutic agents for the management of migraine.