Evidence that 5-HT3 receptors in the nucleus tractus solitarius and other brainstem areas modulate the vagal bradycardia evoked by activation of the von Bezold-Jarisch reflex in the anesthetized rat

Serotonin and vasovagal syncope

PURPOSE

The goal of this manuscript was to review the biological and clinical evidence that serotonin neurotransmission might play an important role in the  physiology and treatment of vasovagal syncope.

METHODS

The authors reviewed PubMed and handsearches of secondary sources for papers related to the Bezold-Jarisch reflex and serotonin, the plausible involvement of the Bezold-Jarisch reflex in vasovagal syncope, and three lines of clinical evidence involving serotonin and the syncope.

RESULTS

The Bezold-Jarisch reflex was first described following the infusion of veratrum alkaloids into animals in the 19th century. The reflex is triggered by serotonin stimulation chemoreceptors and mechanoreceptors in the the left ventricle. The afferent component of the reflex is carried by unmyelinated type C vagal nerve fibers, which results in parasympathetic efferent stimulation that causes bradycardia. The similarity of the combination of hypotension and bradycardia in the Bezold-Jarisch reflex and in vasovagal syncope led to the suggestion that the reflex was the cause of the syndrome.  Three lines of evidence implicate the serotonin 5HT3 receptors in the heart in the reflex. There is genetic and physiologic evidence for the serotonin 5HT1A and 5HT3 receptors and the serotonin reuptake transporter (SERT). Acute blockade of SERT induces vasovagal syncope in humans undergoing head-up tilt table testing, and SERT inhibition reduces hypotension and bradycardia during spinal anaesthesia. Finally, three randomized clinical trials of SERT inhibitors uniformly reported that they significantly reduce the likelihood of vasovagal syncope recurrences.

CONCLUSION

Multiple lines of evidence implicate serotonin neurotransmission in the cause of vasovagal syncope.

Hypothyroidism and sinus dysfunction associated with lithium-paliperidone combination therapy for bipolar disorder with psychotic symptoms: a case report

Background and aim

Lithium is considered to be the first-line treatment for bipolar disorder, and paliperidone was approved for the treatment of schizophrenia and acute bipolar manic/mixed episodes. However, both agents have been associated with thyroid dysfunction and cardiovascular adverse effects like subclinical hypothyroidism, bradycardia, and sinus arrest, even at therapeutic doses.

Case presentation

Here, we reported a case of a 17-year-old Han Chinese female who developed symptomatic hypothyroidism, sinus bradycardia, and sinus arrest while being treated with lithium and paliperidone for bipolar disorder with psychotic features including auditory hallucinations. Her workup suggested that these adverse effects might be related to the combined lithium and paliperidone treatment, although other causes could not be ruled out. After discontinuing both medications, her thyroid function and heart rhythm normalized over 20 days.

Conclusion

To our knowledge, hypothyroidism, sinus bradycardia, and sinus arrest associated with the combined use of lithium and paliperidone had not been reported previously. Further research is warranted to elucidate the potential risks and benefits of this combination therapy for bipolar disorder with psychotic symptoms.

Role of central vagal 5-HT3 receptors in gastrointestinal physiology and pathophysiology

Vagal neurocircuits are vitally important in the co-ordination and modulation of GI reflexes and homeostatic functions. 5-hydroxytryptamine (5-HT; serotonin) is critically important in the regulation of several of these autonomic gastrointestinal (GI) functions including motility, secretion and visceral sensitivity. While several 5-HT receptors are involved in these physiological responses, the ligand-gated 5-HT3 receptor appears intimately involved in gut-brain signaling, particularly via the afferent (sensory) vagus nerve. 5-HT is released from enterochromaffin cells in response to mechanical or chemical stimulation of the GI tract which leads to activation of 5-HT3 receptors on the terminals of vagal afferents. 5-HT3 receptors are also present on the soma of vagal afferent neurons, including GI vagal afferent neurons, where they can be activated by circulating 5-HT. The central terminals of vagal afferents also exhibit 5-HT3 receptors that function to increase glutamatergic synaptic transmission to second order neurons of the nucleus tractus solitarius within the brainstem. While activation of central brainstem 5-HT3 receptors modulates visceral functions, it is still unclear whether central vagal neurons, i.e., nucleus of the tractus solitarius (NTS) and dorsal motor nucleus of the vagus (DMV) neurons themselves also display functional 5-HT3 receptors. Thus, activation of 5-HT3 receptors may modulate the excitability and activity of gastrointestinal vagal afferents at multiple sites and may be involved in several physiological and pathophysiological conditions, including distention- and chemical-evoked vagal reflexes, nausea, and vomiting, as well as visceral hypersensitivity.

Afferent vagal nerve stimulation resets baroreflex neural arc and inhibits sympathetic nerve activity

It has been established that vagal nerve stimulation (VNS) benefits patients and/or animals with heart failure. However, the impact of VNS on sympathetic nerve activity (SNA) remains unknown. In this study, we investigated how vagal afferent stimulation (AVNS) impacts baroreflex control of SNA. In 12 anesthetized Sprague–Dawley rats, we controlled the pressure in isolated bilateral carotid sinuses (CSP), and measured splanchnic SNA and arterial pressure (AP). Under a constant CSP, increasing the voltage of AVNS dose dependently decreased SNA and AP. The averaged maximal inhibition of SNA was ‐28.0 ± 10.3%. To evaluate the dynamic impacts of AVNS on SNA, we performed random AVNS using binary white noise sequences, and identified the transfer function from AVNS to SNA and that from SNA to AP. We also identified transfer functions of the native baroreflex from CSP to SNA (neural arc) and from SNA to AP (peripheral arc). The transfer function from AVNS to SNA strikingly resembled the baroreflex neural arc and the transfer functions of SNA to AP were indistinguishable whether we perturbed ANVS or CSP, indicating that they likely share common central and peripheral neural mechanisms. To examine the impact of AVNS on baroreflex, we changed CSP stepwise and measured SNA and AP responses with or without AVNS. AVNS resets the sigmoidal neural arc downward, but did not affect the linear peripheral arc. In conclusion, AVNS resets the baroreflex neural arc and induces sympathoinhibition in the same manner as the control of SNA and AP by the native baroreflex.

Afferent vagal nerve stimulation resets the baroreflex neural arc and inhibits sympathetic nerve activity.

Roles of arterial baroreceptor reflex during bezold-jarisch reflex

Among the many cardiopulmonary reflexes, this review specifically examines the roles of the arterial baroreflex during the Bezold-Jarisch reflex (BJR). Activation of cardiopulmonary vagal afferent C-fibers induces hypotension, bradycardia, and apnea, which are known collectively as the BJR; myocardial ischemia and infarction might induce the BJR. Arterial baroreflex has been established as an important negative feedback system that stabilizes arterial blood pressure against exogenous pressure perturbations. Therefore, understanding the functions of the arterial baroreflex during the BJR is crucial for elucidating its pathophysiological implications. The main central pathways of the BJR and the baroreflex are outlined herein, particularly addressing the common pathway between the reflexes. Furthermore, the pathophysiological roles of the arterial baroreflex during the BJR are described along with a brief discussion of pathophysiological merits and shortcomings of the reflexes.

Central 5-HT(2A) receptors modulate the vagal bradycardia in response to activation of the von Bezold-Jarisch reflex in anesthetized rats

Activation of 5-hydroxytryptamine (5-HT) 5-HT(1A), 5-HT(2C), 5-HT(3), and 5-HT(7) receptors modulates the excitability of cardiac vagal motoneurones, but the precise role of 5-HT(2A/2B) receptors in these phenomena is unclear. We report here the effects of intracisternal (ic) administration of selective 5-HT(2A/2B) antagonists on the vagal bradycardia elicited by activation of the von Bezold-Jarisch reflex with phenylbiguanide. The experiments were performed on urethane-anesthetized male Wistar rats (250-270 g, N = 7-9 per group). The animals were placed in a stereotaxic frame and their atlanto-occipital membrane was exposed to allow ic injections. The rats received atenolol (1 mg/kg, iv) to block the sympathetic component of the reflex bradycardia; 20-min later, the cardiopulmonary reflex was induced with phenylbiguanide (15 µg/kg, iv) injected at 15-min intervals until 3 similar bradycardias were obtained. Ten minutes after the last pre-drug bradycardia, R-96544 (a 5-HT(2A) antagonist; 0.1 µmol/kg), SB-204741 (a 5-HT(2B) antagonist; 0.1 µmol/kg) or vehicle was injected ic. The subsequent iv injections of phenylbiguanide were administered 5, 20, 35, and 50 min after the ic injection. The selective 5-HT(2A) receptor antagonism attenuated the vagal bradycardia and hypotension, with maximal effect at 35 min after the antagonist (pre-drug = -200 ± 11 bpm and -42 ± 3 mmHg; at 35 min = -84 ± 10 bpm and -33 ± 2 mmHg; P < 0.05). Neither the 5-HT(2B) receptor antagonists nor the vehicle changed the reflex. These data suggest that central 5-HT(2A) receptors modulate the central pathways of the parasympathetic component of the von Bezold-Jarisch reflex.

Glucose increases synaptic transmission from vagal afferent central nerve terminals via modulation of 5-HT3 receptors

Acute hyperglycemia has profound effects on vagally mediated gastrointestinal functions. We have reported recently that the release of glutamate from the central terminals of vagal afferent neurons is correlated directly with the extracellular glucose concentration. The present study was designed to test the hypothesis that 5-HT(3) receptors present on vagal afferent nerve terminals are involved in this glucose-dependent modulation of glutamatergic synaptic transmission. Whole-cell patch-clamp recordings were made from neurons of the nucleus tractus solitarius (NTS) in thin rat brainstem slices. Spontaneous and evoked glutamate release was decreased in a concentration-dependent manner by the 5-HT(3) receptor selective antagonist, ondansetron. Alterations in the extracellular glucose concentration induced parallel shifts in the ondansetron-mediated inhibition of glutamate release. The changes in excitatory synaptic transmission induced by extracellular glucose concentration were mimicked by the serotonin uptake inhibitor, fenfluramine. These data suggest that glucose alters excitatory synaptic transmission within the rat brainstem via actions on tonically active 5-HT(3) receptors, and the number of 5-HT(3) receptors on vagal afferent nerve terminals is positively correlated with the extracellular glucose concentration. These data indicate that the 5-HT(3) receptors present on synaptic connections between vagal afferent nerve terminals and NTS neurons are a strong candidate for consideration as one of the sites where glucose acts to modulate vagovagal reflexes.

Inhibitory modulation of chemoreflex bradycardia by stimulation of the nucleus raphe obscurus is mediated by 5-HT3 receptors in the NTS of awake rats

Several studies demonstrated the involvement of 5-hydroxytryptamine (5-HT) and its different receptor subtypes in the modulation of neurotransmission of cardiovascular reflexes in the nucleus tractus solitarii (NTS). Moreover, anatomic evidence suggests that nucleus raphe obscurus (ROb) is a source of 5-HT-containing terminals within the NTS. In the present study we investigated the possible changes in the cardiovascular responses to peripheral chemoreceptor activation by potassium cyanide (KCN, i.v.) following ROb stimulation with L-glutamate (10 nmol/50 nL) and also whether 5-HT3 receptors in the caudal commissural NTS are involved in this neuromodulation. The results showed that stimulation of the ROb with L-glutamate in awake rats (n=15) produced a significant reduction in the bradycardic response 30 s after the microinjection (-182+/-19 vs -236+/-10 bpm; Wilcoxon test) but no changes in the pressor response to peripheral chemoreceptor activation (43+/-4 vs 51+/-3 mmHg; two-way ANOVA) in relation to the control. Microinjection of 5--HT3 receptors antagonist granisetron (500 pmol/50 nL), but not the vehicle, into the caudal commissural NTS bilaterally prevented the reduction of chemoreflex bradycardia in response to microinjection of L-glutamate into ROb. These data indicate that 5-HT-containing projections from ROb to the NTS play an inhibitory neuromodulatory role in the chemoreflex evoked bradycardia by releasing 5-HT and activating 5-HT3 receptors in the caudal NTS.

General Anesthetic-Induced Channel Gating Enhancement of 5-Hydroxytryptamine Type 3 Receptors Depends on Receptor Subunit Composition

5-Hydroxytryptamine (serotonin) (5-HT) type 3 (5-HT(3)) receptors are members of an anesthetic-sensitive superfamily of Cys-loop ligand-gated ion channels that can be formed as homomeric 5-HT(3A) or heteromeric 5-HT(3AB) receptors. When the efficacious agonist 5-HT is used, the inhaled anesthetics halothane and chloroform (at clinically relevant concentrations) significantly reduce the agonist EC(50) for 5-HT(3A) receptors but not for 5-HT(3AB) receptors. In the present study, we used dopamine (DA), a highly inefficacious agonist for 5-HT(3) receptors, to determine whether the difference in sensitivity between 5-HT(3A) and 5-HT(3AB) receptors to the potentiating effects of halothane and chloroform is due to differential modulation of agonist affinity, channel gating, or both. Using the two-electrode voltage-clamp technique with 5-HT(3A) and 5-HT(3AB) receptors expressed in Xenopus oocytes, we found that chloroform and halothane enhanced currents evoked by receptor-saturating concentrations of DA for both receptor subtypes in a concentration-dependent manner but that the magnitude of enhancement was substantially greater for 5-HT(3A) receptors than for 5-HT(3AB) receptors. Isoflurane induced only a small enhancement of currents evoked by receptor-saturating concentrations of DA for 5-HT(3A) receptors and no enhancement for 5-HT(3AB) receptors. For both receptor subtypes, none of the three test anesthetics significantly altered the agonist EC(50) for DA, implying that these anesthetics do not affect agonist binding affinity. Our results show that chloroform, halothane, and (to a much lesser degree) isoflurane enhance channel gating for 5-HT(3A) receptors and that the incorporation of 5-HT(3B) subunits to produce heteromeric 5-HT(3AB) receptors markedly attenuates the ability of these anesthetics to enhance channel gating.

Effect of 5-HT depletion on cardiovascular vagal reflex sensitivity in awake and anesthetized rats

Antagonism of central 5-HT1A and 5-HT7 receptors inhibits reflex-evoked vagal bradycardias indicating that 5-HT is released during these reflexes. The present experiments examined the effect of 5-HT depletion with para-chlorophenylalanine (p-CPA) on the cardiac vagal baroreflex and cardiopulmonary reflex in awake and anesthetized rats. Immunocytochemistry and neurochemical detection showed that p-CPA depleted the brainstem of 5-HT, but not of norepinephrine or dopamine. Depletion of 5-HT was associated with an increase in mean arterial pressure (MAP) in awake rats. This difference was abolished by anesthesia, which reduced MAP in both groups of animals. The baroreflex gain, whether calculated from the rise in pressure induced by phenylephrine or the fall in pressure evoked by sodium nitroprusside, was significantly attenuated in depleted rats compared to controls. This attenuation of the baroreflex gain was unaffected by subsequent anesthesia. 5-HT depletion also attenuated the cardiopulmonary reflex vagal bradycardias but this only reached statistical significance when the rats were anesthetized. The data support the view that 5-HT is released in the reflex activation of the cardiac vagal pathway.

The role of central 5-HT3 receptors in vagal reflex inputs to neurones in the nucleus tractus solitarius of anaesthetized rats

Brainstem 5-hydroxytryptamine (5-HT, serotonin)-containing neurones modulate cardiovascular reflex responses but the differing roles of the many 5-HT receptors have not been thoroughly investigated. The present experiments on anaesthetized rats investigated the role of 5-HT3 receptors in modulating vagal afferent evoked activity of nucleus tractus solitarius (NTS) neurones. Recordings were made from 301 NTS neurones receiving an input at long (> 20 ms) minimum onset latency from stimulation of the vagus nerve. These included 140 neurones excited by activating non-myelinated cardiopulmonary afferents by right atrial injection of phenylbiguanide (PBG). Ionophoretic application of PBG, a highly selective 5-HT3 receptor agonist, significantly increased activity (from 2.4 +/- 0.4 to 5.5 +/- 0.8 spikes s(-1)) in 96 of 106 neurones tested and in all 17 neurones tested the increase in activity (3.4 +/- 1.1 to 7.0 +/- 1.9 spikes s(-1)) was significantly attenuated (3.0 +/- 0.9 to 3.8 +/- 1.1 spikes s(-1)) by the selective 5-HT3 receptor antagonist granisetron. Ionophoretic application of PBG potentiated responses to vagus nerve and cardiopulmonary afferent stimulation, and granisetron significantly attenuated this cardiopulmonary input (20.2 +/- 5.7 to 10.6 +/- 4.1 spikes burst(-1)) in 9 of 10 neurones tested. Ionophoretic application of AMPA and NMDA also excited NTS neurones and these excitations could be selectively antagonized by the non-NMDA and NMDA receptor antagonists DNQX and AP-5, respectively. At these selective currents, DNQX and AP-5 also attenuated PBG- and cardiopulmonary input-evoked increases in NTS activity. These data are consistent with the hypothesis that vagal inputs, including non-myelinated cardiopulmonary inputs to the NTS, utilize a 5-HT-containing pathway which activates 5-HT3 receptors. This excitatory response to 5-HT3 receptor activation may be partly a direct postsynaptic action but part may also be due to facilitation of the release of glutamate which in turn acts on either non-NMDA or NMDA receptors to evoke excitation.

Role of central 5-HT3 receptors in the control of blood pressure in stressed and non-stressed rats

The aim of the present study was to investigate the participation of central 5-HT(3) receptors in the control of blood pressure and heart rate (HR) of non-stressed and stressed rats. The pharmacological stimulation of brain 5-HT(3) receptors by third ventricle injections of the selective 5-HT(3) receptor agonist m-CPBG induced a significant decrease in blood pressure in non-stressed rats and impaired the hypertensive response induced by restraint stress. The blockade of brain 5-HT(3) receptors by the central administration of the selective 5-HT(3) antagonist ondansetron elicited a significant increase in blood pressure in non-stressed rats. Conversely, the hypertensive response induced by restraint stress was not affected by central administration of ondansetron. Additionally, baroreflex-mediated bradycardia during phenylephrine-induced hypertensive response was preserved in non-stressed animals receiving third ventricle injections of m-CPBG, while the baroreflex-mediated tachycardia that occurs during the hypotensive response induced by the administration of sodium nitroprusside was impaired. It is concluded that the serotoninergic component represented by the brain 5-HT(3) receptors exerts a tonic inhibitory influence on the central control of blood pressure in non-stressed rats, probably by a sympathoinhibitory-related mechanism. On the other hand, during stress, this central 5-HT(3)-dependent inhibitory drive is overwhelmed by the different neurochemical systems that harmonically trigger and sustain the hypertensive response.

Effect of serotonin on respiration, cerebral circulation, and blood pressure in rats

The effects of intravenous serotonin on respiration, cerebral circulation, and blood pressure were examined in narcotized rats. Serotonin rapidly decreased local cerebral blood flow (by almost 30%) and blood pressure. Hemodynamic phenomena were accompanied by sharp changes in the respiration pattern: short-term apnea in all cases. The mechanism of this apnea was related to initial stages in blood pressure changes and had a neurogenic nature.

Bezold-Jarisch Reflex Blunts Arterial Baroreflex via the Shift of Neural Arc toward Lower Sympathetic Nerve Activity

Although the Bezold-Jarisch (BJ) reflex is potentially evoked during acute myocardial ischemia or infarction, its effects on the static characteristics of the arterial baroreflex remain to be analyzed in terms of an equilibrium diagram between the neural and peripheral arcs. The neural arc represents the static input-output relationship between baroreceptor pressure input and efferent sympathetic nerve activity (SNA), whereas the peripheral arc represents that between SNA and arterial pressure (AP). In 8 anesthetized rabbits, we increased carotid sinus pressure stepwise from 40 to 160 mmHg in increments of 20 mmHg at one-minute intervals while measuring renal SNA and AP under control conditions and during the activation of the BJ reflex by intravenous administration of phenylbiguanide (PBG, 100 microg.kg(-1).min(-1)). The neural arc approximated a sigmoid curve whereas the peripheral arc approximated a straight line. PBG decreased AP at the operating point from -91.3 +/- 2.4 to -71.7 +/- 3.1 mmHg (P < 0.01), and attenuated the total loop gain at the operating point from -1.31 +/- 0.44 to -0.51 +/- 0.14 (P < 0.05). The equilibrium diagram indicated that PBG caused a parallel shift of the neural arc toward lower SNA such that the maximum SNA was reduced to approximately 60% of control. PBG decreased neural and peripheral arc gains at the operating point to approximately 43% and 77%, respectively. In conclusion, the BJ reflex blunts arterial baroreflex via the shift of the neural arc toward lower SNA.

Bezold-Jarisch reflex attenuates dynamic gain of baroreflex neural arc

Although acute myocardial ischemia or infarction may induce the Bezold-Jarisch (BJ) reflex through the activation of serotonin receptors on vagal afferent nerves, the mechanism by which the BJ reflex modulates the dynamic characteristics of arterial pressure (AP) regulation is unknown. The purpose of this study was to examine the effects of the BJ reflex induced by intravenous phenylbiguanide (PBG) on the dynamic characteristics of the arterial baroreflex. In seven anesthetized rabbits, we perturbed intracarotid sinus pressure (CSP) according to a white noise sequence while renal sympathetic nerve activity (RSNA), AP, and heart rate (HR) were recorded. We estimated the transfer function from CSP to RSNA (neural arc) and from RSNA to AP (peripheral arc) before and after 10 min of intravenous administration of PBG (100 microg. kg-1. min-1). The intravenous PBG decreased mean AP from 84.5 +/- 4.0 to 68.2 +/- 4.7 mmHg (P < 0.01), mean RSNA to 76.2 +/- 7.0% (P < 0.05), and mean HR from 301.6 +/- 7.7 to 288.4 +/- 9.0 beats/min (P < 0.01). The intravenous PBG significantly decreased neural arc dynamic gain at 0.01 Hz (1.06 +/- 0.08 vs. 0.59 +/- 0.17, P < 0.05), whereas it did not affect that of the peripheral arc (1.20 +/- 0.12 vs. 1.18 +/- 0.41). In six different rabbits without intravenous PBG, the neural arc transfer function did not change between two experimental runs with intervening interval of 10 min, excluding the possibility that the cumulative effects of anesthetics had altered the neural arc transfer function. In conclusion, excessive activation of the BJ reflex during acute myocardial ischemia may exert an adverse effect on AP regulation, not only by sympathetic suppression, but also by attenuating baroreflex dynamic gain.

New benzimidazole derivatives: selective and orally active 5-HT3 receptor antagonists

The synthesis of new 5-HT(3) receptor antagonists is an interesting field of research because of their wide therapeutic use. The aim of this work is to functionally characterise a new series of benzimidazole derivatives previously described. These compounds bind to 5-HT(3) receptors and have been evaluated using in vitro (rat tunica muscularis mucosae) and in vivo tests (Bezold-Jarisch reflex in rat and gastrointestinal motility and spontaneous motility in mice). Ondansetron and 1-[4-amino-5-chloro-2-(3,5-dimethoxyphenil)methyloxy]-3-[1-[2-methylsulfonylamino]piperidin-4-yl]propan-1-one hydrochloride (RS 39604) were used as well known 5-HT(3) and 5-HT(4) receptor antagonists. These benzimidazole derivatives have proved to be 5-HT(3) receptor antagonists. Interestingly, they are as active as ondansetron when they are intraperitoneally (i.p.) or orally (p.o.) administered and, in mice, they seem to induce fewer behavioural changes at similar effective doses than does ondansetron. The present results confirm the usefulness of the previously proposed pharmacophore and justify the interest in these new benzimidazole derivatives.

Differential distribution of 5HT2A and NMDA receptors in single cells within the rat medial nucleus of the solitary tract

Activation of serotonin (5-hydroxytryptamine; 5HT) receptors of the 2A subtype (5HT2A) in the intermediate portion of the medial nucleus tractus solitarius (mNTS) produces marked hypotension and bradycardia. This portion of the mNTS receives major input from glutamatergic baroreceptor afferents. Thus, the cardiorespiratory effects of 5HT2A agonists may be attributed, in part, to interactions involving the glutamatergic target neurons, some of which express N-methyl-D-aspartate (NMDA) glutamate receptors. To determine the functional sites for activation of 5HT2A receptors and their relationship to NMDA receptors in this region, we used electron microscopic immunocytochemistry for the localization of antipeptide antisera selectively recognizing each receptor protein in the intermediate mNTS in rat brain. Of 1,052 5HT2A-labeled profiles, 38% were dendrites and dendritic spines, 27% were unmyelinated axons, 14% were axon terminals, and 11% were glial processes. These 5HT2A-labeled profiles frequently contained NR1 gold particles with dendrites comprising 68% of the total dual-labeled profiles. In dendrites, the 5HT2A immunoreactivity was localized to cytoplasmic organelles or discretely distributed on synaptic or extrasynaptic segments of the plasma membrane. In contrast, NR1 immunoreactivity was prominently localized to postsynaptic junctions and these were distinct from the 5HT2A receptor labeling when coexpressed in the same dendrites. Dendrites containing both receptors composed 56% (224/399) of the total 5HT2A-labeled dendrites and 34% (224/659) of the total NR1-labeled dendrites. Our results provide the first ultrastructural evidence that in the intermediate mNTS, 5HT2A receptor agonists may affect the postsynaptic excitability of many of the same neurons that show NMDA-evoked responses to glutamate.

Central cardiovascular regulation and 5-hydroxytryptamine receptors

Evidence is provided to support the view that central 5-HT(1A) and 5-HT(2) receptors are the major receptor subtypes important in cardiovascular regulation. Data are also provided to implicate 5-HT(1B/1D/1F) receptors in central cardiovascular regulation. Activation of 5-HT(2) receptors generally causes sympathoexcitation and a rise in blood pressure and this is mainly mediated by 5-HT(2A) receptors. However, presympathetic vasomotor neurones located in the hindbrain (RVLM), controlling sympathetic outflow to the heart, are not activated in the same way as other presympathetic vasomotor neurones, although activation of 5-HT(2) receptors located in the midbrain can activate sympathetic outflow to the heart. Furthermore, at least in the rat, these midbrain 5-HT(2A) receptors are also responsible for the release of vasopressin by activation of a central angiotensinergic pathway. The ability of vasopressin directly and/or indirectly to modify renal sympathetic outflow involves the activation of central 5-HT(2B) receptors, which in turn, when activated via the i.c.v. route, can cause selective renal sympathoexcitation. Evidence is also provided which indicates that the reflex control of parasympathetic outflow to the heart and to other organs involves central 5-HT(1A) receptors located in the vicinity of these preganglionic vagal neurones. Finally, 5-HT(3) receptors are implicated in the afferent regulation of central sympathetic and parasympathetic tone.

The role of central 5-HT(1A) receptors in the control of B-fibre cardiac and bronchoconstrictor vagal preganglionic neurones in anaesthetized cats

1. Experiments were performed to determine whether 5-HT(1A) receptors (a) modulate the activity of cardiac and bronchoconstrictor vagal preganglionic neurones (CVPNs and BVPNs) in the nucleus ambiguus (NA) and (b) are involved in pulmonary C-fibre afferent-evoked excitation of CVPNs, by right-atrial injections of phenylbiguanide (PBG). These experiments were carried out on alpha-chloralose-anaesthetized, artificially ventilated and atenolol (1 mg kg(-1))-pretreated cats. 2. The ionophoretic application of 8-OH-DPAT (a selective 5-HT(1A) receptor agonist) influenced the activity of 16 of the 19 CVPNs tested. 8-OH-DPAT tended to cause inhibition at low currents (40 nA) and excitation at high currents (120 nA). The activity of 15 of these neurones increased in response to the application of 8-OH-DPAT. In six of the CVPNs tested, this excitatory action of 8-OH-DPAT was attenuated by co-application of the selective 5-HT(1A) receptor antagonist WAY-100635. 3. The pulmonary C-fibre afferent-evoked excitation of eight CVPNs was attenuated by ionophoretic application of WAY-100635. 4. In three out of four CVPNs, the ionophoretic application of PBG caused excitation. 5. In five out of the nine identified BVPNs that were tested with ionophoretic application of 8-OH-DPAT, excitation was observed that was attenuated by WAY-100635. 6. WAY-100635 (I.V. or intra-cisternally) also reversed bradycardia, hypotension and the decrease in phrenic nerve activity evoked by the I.V. application of 8-OH-DPAT (42 microg kg(-1)). 7. In conclusion, the data indicate that 5-HT(1A) receptors located in the NA play an important role in the reflex activation of CVPNs and BVPNs, and support the view that overall, these receptors play a fundamental role in the reflex regulation of parasympathetic outflow.

The broad-spectrum anti-emetic activity of AS-8112, a novel dopamine D2, D3 and 5-HT3 receptors antagonist

The anti-emetic and pharmacological profile of AS-8112 ((R)-5-bromo-N-(1-ethyl-4-methylhexahydro-1H-1,4-diazepin-6-yl)-2-methoxy-6-methylamino-3-pyridinecarboxamide.2 fumarate), a novel and potent dopamine D2, D3 and 5-hydroxytryptamine-3 (5-HT3) receptors ligand, was investigated in the present study. In guinea-pig isolated colon, AS-8112 produced a rightward shift of the concentration-response curves of 2-methyl-5HT, a 5-HT3 receptor agonist (pA2 value of 7.04). Other 5-HT3 receptor antagonists also produced such a shift in the following antagonistic-potency order: granisetron> ondansetron=AS-8112>>metoclopramide. In mice, AS-8112 (1.0 - 3.0 mg kg(-1) s.c.) potently inhibited hypothermia induced by the dopamine D3 receptor agonist; R(+)-7-OH-DPAT (R(+)-7-hydroxy-2-(N,N-di-n-propylamino)tetraline) (0.3 mg kg(-1) s.c.). Domperidone and haloperidol, which have affinity for dopamine D3 receptor, also inhibited R(+)-7-OH-DPAT-induced hypothermia. In ferrets or dogs, AS-8112 dose-dependently inhibited emesis induced by R(+)-7-OH-DPAT, apomorphine, morphine or cisplatin with ID50 values of 2.22 microg kg(-1) s.c., 10.5 microg kg(-1) s.c., 14.2 microg kg(-1) i.v. and 17.6 microg kg(-1) i.v., respectively. Moreover, oral administration of AS-8112 significantly inhibited emesis induced by these emetogens. AS-8112 (0.3 mg kg(-1) i.v.) significantly inhibited emesis induced by cyclophosphamide and doxorubicin. In conclusion, AS-8112 is a potent dopamine D2, D3 and 5-HT3 receptors antagonist, and a novel anti-emetic agent with a broad-spectrum of anti-emetic activity. These results suggest that this compound is worthy of clinical investigation.

Microinjection of a 5-HT3 receptor agonist into the NTS of awake rats inhibits the bradycardic response to activation of the von Bezold-Jarisch reflex

In the present study we investigated the effects of bilateral microinjection into the lateral commissural nucleus tractus solitarius (NTS) of 2-methyl-5-HT, a 5-HT3 receptor agonist, on the bradycardic response of the von Bezold-Jarisch reflex of awake rats. We evaluated mainly the bradycardic response because in previous studies we documented that the hypotensive response of the von-Bezold-Jarisch reflex in awake rats is secondary to the intense bradycardic response. The Bezold-Jarisch reflex was activated by intravenous injection of serotonin (8 microg/kg) in awake rats before and 1, 3, 10, 20 and 60 min after bilateral microinjection of 2-methyl-5-HT (5 nmol/50 nl, n = 8) into the NTS. Microinjections of 2-methyl-5-HT into the NTS produced a significant increase in basal mean arterial pressure [(MAP), 97 +/- 4 vs. 114 +/- 4 mmHg), no changes in basal heart rate and a significant reduction in bradycardic (-78 +/- 19; -94 +/- 24 and -107 +/- 21 bpm) and hypotensive (-16 +/- 4; -10 +/- 5 and -17 +/- 4 mmHg) responses to activation of the von Bezold-Jarisch reflex at 3, 10 and 20 min, respectively, when compared with the control value (-231 +/- 13 bpm and -43 +/- 4 mmHg). The data of the present study suggest that serotonin acting on 5-HT3 receptors in the NTS may play an important inhibitory neuromodulatory role in the bradycardic response to activation of the von Bezold-Jarisch reflex.

Ondansetron modulates pharmacodynamic effects of ketamine on electrocardiographic signals in rhesus monkeys

Electrocardiographic signal dynamics were examined in rhesus monkeys (Macaca mulatta) before and after treatment with ketamine and/or ondansetron. Ketamine exerts differential pharmacodynamic effects on behavior in animals stratified according to a measure of central serotonergic turnover. We hypothesized that measures of serotonergic turnover might explain some of the variance in the electrocardiographic (ECG) response to ketamine. Electrocardiographic recordings of animals were obtained at baseline, after administration of either saline or ondansetron (0.125 mg/kg), and after administration of ketamine (15 mg/kg). Electrocardiographic signal dynamics were measured using an algorithm that extracts the Hurst parameter (H) of the interbeat interval (IBI) time-series. H decreased after ketamine administration, (mean+/-S.E.M.), 0.33+/-0.04 vs. 0.12+/-0.02, P</=0. 001, n=10. Cerebrospinal fluid 5-hydroxyindole-3-acetic acid (5-HIAA) concentrations, a measure of serotonergic turnover, predicted the monkeys' response to ketamine, H=0.001 (5-HIAA, pmol/ml)-0.130, R=0.66, P</=0.003, n=18. Ondansetron attenuated the response to ketamine, 0.14+/-0.02 vs. 0.08+/-0.02, P</=0.05, n=8, ondansetron vs. saline. These data provide evidence that naturally occurring differences in serotonin function alter the ECG response of the animals to ketamine and that activation of the serotonin type-3 receptor by ketamine is involved.

Cardiopulmonary reflex impairment in experimental diabetes in rats

The aim of the present study was to evaluate the sensitivity of the cardiopulmonary receptors in experimental diabetes induced by streptozotocin by the use of 2 different methods: (1) administration of increasing doses of serotonin to analyze peak changes of arterial pressure and heart rate for each given dose in conscious intact normal and diabetic rats; (2) expanding blood volume with the use of dextran (6%) to produce similar increases in left ventricular end-diastolic pressure to quantify the arterial pressure, heart rate, and renal sympathetic nerve activity in sinoaortic, denervated, anesthetized normal and diabetic rats. Blood samples were collected to measure blood glucose. Diabetic rats showed hyperglycemia (22+/-0. 7 versus 7+/-0.2 mmol/L), reduced body weight (226+/-12 versus 260+/-4 g) and heart rate (294+/-14 versus 350+/-10 bpm), and similar arterial pressure (104+/-4 versus 113+/-4 mm Hg) when compared with control rats. Serotonin induced significant bradycardia and hypotension, which were similar and proportional to the dose injected in both groups. Mean arterial pressure and heart rate decreases in response to volume overload were significantly lower in diabetic than in control rats. The reflex reduction of the renal sympathetic nerve activity as expressed by percentage changes in nerve activity in response to increasing left end-diastolic pressure was abolished in diabetic animals (1.9+/-0.8% versus -14+/-4%/mm Hg in controls). These results showed an impairment of cardiopulmonary reflex control of circulation in diabetes during acute volume expansion. The normal responses to serotonin administration indicated that the cardiopulmonary reflex is still preserved in diabetic rats.

Ultrastructural localization of the serotonin transporter in limbic and motor compartments of the nucleus accumbens

Extracellular levels of serotonin [5-hydroxytryptamine (5-HT)] in the nucleus accumbens (NAc) can influence both cognitive and motor functions involving extensive connections with the frontal cortex. The 5-HT levels reflect vesicular release and plasmalemmal reuptake through the serotonin transporter (SERT). We used electron microscopic immunocytochemistry to determine the sites for SERT activation in the limbic shell and motor-associated core of the rat NAc. Of the SERT-immunoreactive profiles in each region, >90% were serotonergic axons and axon terminals; the remainder were nonserotonergic dendrites and glia. Axonal SERT immunogold labeling was seen mainly at nonsynaptic sites on plasma membranes and often near 5-HT-containing large dense core vesicles (DCVs). SERT-labeled axonal profiles were larger and had a higher numerical density in the shell versus the core but showed no regional differences in their content of SERT immunogold particles. In contrast, immunoreactive dendrites had a lower numerical density in the shell than in the core. SERT labeling in dendrites was localized to segments of plasma membrane near synaptic contacts from unlabeled terminals and/or dendritic appositions. Our results suggest that in the NAc (1) reuptake into serotonergic axons is most efficient after exocytotic release from DCVs, and (2) increased 5-HT release without concomitant increase in SERT expression in individual axons may contribute to higher extracellular levels of serotonin in the shell versus the core. These findings also indicate that SERT may play a minor substrate-dependent role in serotonin uptake or channel activity in selective nonserotonergic neurons and glia in the NAc.