Central serotonergic projections to the nucleus tractus solitarii: evidence from a double labeling study in the rat

Peripheral Neuroplasticity of Respiratory Chemoreflexes, Induced by Prenatal Nicotinic Exposure: Implication for SIDS

Sudden Infant Death Syndrome (SIDS) occurs during sleep in seemingly healthy infants. Maternal cigarette smoking and hypoxemia during sleep are assumed to be the major causal factors. Depressed hypoxic ventilatory response (dHVR) is observed in infants with high risk of SIDS, and apneas (lethal ventilatory arrest) appear during the fatal episode of SIDS. Disturbance of the respiratory center has been proposed to be involved, but the pathogenesis of SIDS is still not fully understood. Peripherally, the carotid body is critical to generate HVR, and bronchopulmonary and superior laryngeal C-fibers (PCFs and SLCFs) are important for triggering central apneas; however, their roles in the pathogenesis of SIDS have not been explored until recently. There are three lines of recently accumulated evidence to show the disorders of peripheral sensory afferent-mediated respiratory chemoreflexes in rat pups with prenatal nicotinic exposure (a SIDS model) in which acute severe hypoxia leads to dHVR followed by lethal apneas. (1) The carotid body-mediated HVR is suppressed with a reduction of the number and sensitivity of glomus cells. (2) PCF-mediated apneic response is largely prolonged via increased PCF density, pulmonary IL-1β and serotonin (5-hydroxytryptamine, 5-HT) release, along with the enhanced expression of TRPV1, NK₁R, IL1RI and 5-HT₃R in pulmonary C-neurons to strengthen these neural responses to capsaicin, a selective stimulant to C-fibers. (3) SLCF-mediated apnea and capsaicin-induced currents in superior laryngeal C-neurons are augmented by upregulation of TRPV1 expression in these neurons. These results, along with hypoxic sensitization/stimulation of PCFs, gain insight into the mechanisms of prenatal nicotinic exposure-induced peripheral neuroplasticity responsible for dHVR and long-lasting apnea during hypoxia in rat pups. Therefore, in addition to the disturbance in the respiratory center, the disorders of peripheral sensory afferent-mediated chemoreflexes may also be involved in respiratory failure and death denoted in SIDS victims.

Activation of 5-HT3 receptors in the medulla oblongata is involved in the phasic control of urinary bladder

The control of micturition depends on reflex mechanisms, however, it undergoes modulation from cortex, pons and medullary areas. This study investigated if the activation of 5-HT3 receptors in the medulla influences the urinary bladder (UB) regulation in rats. Isoflurane female Wistar rats were submitted to catheterization of the femoral artery and vein for mean arterial pressure (MAP) and heart rate (HR) recordings and injection of drugs, respectively. The UB was cannulated for intravesical pressure (IP) measurement. The Doppler flow probe was placed around the left renal artery for renal conductance (RC) recordings. Phenylbiguanide (PB) and granisetron (GN) were injected into the 4th brain ventricle in rats with guide cannulas implanted 5 days prior to the experiments; or PB and GN were randomly injected intravenously or applied topically (in situ) on the UB. PB injection into 4th V significantly increased IP (68.67 ± 11.70%) and decreased MAP (-29 ± 6 mmHg) compared to saline (0.34 ± 0.64% and -2 ± 2 mmHg), with no changes in the HR and RC. GN injection into the 4th V did not significantly change the IP and RC compared to saline, nevertheless, significantly increased MAP (25 ± 4 mmHg) and heart rate (36 ± 9 bpm) compared to saline. Intravenous PB and GN only produced cardiovascular effects, whilst PB but not GN in situ on the UB evoked increase in IP (111.60 ± 30.36%). Therefore, the activation of 5HT-3 receptors in medullary areas increases the intravesical pressure and these receptors are involved in the phasic control of UB. In contrast, 5-HT3 receptors in the medulla oblongata are involved in the pathways of the tonic control of the cardiovascular system. The activation of 5-HT3 receptors in the bladder cause increase in intravesical pressure and this regulation seem to be under phasic control as the blockade of such receptors elicits no changes in baseline intravesical pressure.

Serotonin Modulates the Correlations between Obsessive-compulsive Trait and Heart Rate Variability in Normal Healthy Subjects: A SPECT Study with [123I]ADAM and Heart Rate Variability Measurement

Objective

The impact of serotonergic system on obsessive-compulsive disorder (OCD) is well studied. However, the correlation between OC presentations and autonomic nervous system (ANS) is still unclear. Furthermore, whether the correlation might be modulated by serotonin is also uncertain.

Methods

We recruited eighty-nine healthy subjects. Serotonin transporter (SERT) availability by [¹²³I]ADAM and heart rate variability (HRV) tests were measured. Symptoms checklist-90 was measured for the OC presentations. The interaction between HRV and SERT availability were calculated and the correlation between HRV and OC symptoms were analyzed after stratified SERT level into two groups, split at medium.

Results

The interactions were significant in the factors of low frequency (LF), high frequency (HF), and root mean square of successive differences (RMSSD). Furthermore, the significantly negative correlations between OC symptoms and the above HRV indexes existed only in subjects with higher SERT availability.

Conclusion

OC symptoms might be correlated with ANS regulations in subjects with higher SERT availability.

Rostral cuneiform nucleus and the defence reaction: Direct and indirect midbrain-medullary 5-HT mechanisms in baroreflex inhibition

BACKGROUND AND PURPOSE

Activation of the defence reaction inhibits the baroreflex response via the intermediate rostro-ventromedial medulla (B3 raphé) and nucleus tractus solitarius (NTS). Our aim was to determine whether and how baroreflex inhibition, induced by the disinhibition of the rostral cuneiform nucleus (part of the defence pathway), involves 5-HT neurons in B3 and 5-HT₃ receptors in the NTS.

EXPERIMENTAL APPROACH

We performed immunohistochemistry and anatomical experiments to determine whether raphé 5-HT cells expressing Fos were directly targeted by the rostral cuneiform nucleus. The effect of blocking raphé 5-HT neurotransmission and NTS 5-HT₃ receptors on cuneiform-induced inhibition of the baroreflex cardiac response were also analysed.

KEY RESULTS

Bicuculline, microinjected into the rostral cuneiform nucleus, induced an increase of double-labelled Fos-5-HT-IR cells in both the lateral paragigantocellular nucleus (LPGi) and raphé magnus. The anterograde tracer Phaseolus vulgaris leucoaggutinin injected into the rostral cuneiform nucleus revealed a dense projection to the LPGi but not raphé magnus. Cuneiform-induced baroreflex inhibition was prevented by B3 injection of 8-OH-DPAT, a selective 5-HT_1A receptor agonist. Cuneiform disinhibition also failed to inhibit the baroreflex bradycardia after NTS microinjection of the 5-HT₃ receptor antagonist granisetron and in 5-HT₃ receptor knockout mice.

CONCLUSION AND IMPLICATIONS

The rostral cuneiform nucleus participates in the defence inhibition of the baroreflex bradycardia via direct activation of the LPGi and via a projection to the raphé magnus to activate NTS 5-HT₃ receptors and inhibit second-order baroreflex neurons. These data bring new insights in primary and secondary mechanisms involved in vital baroreflex prevention during stress.

Central afferents to the nucleus of the solitary tract in rats and mice

The nucleus of the solitary tract (NTS) regulates life-sustaining functions ranging from appetite and digestion to heart rate and breathing. It is also the brain's primary sensory nucleus for visceral sensations relevant to symptoms in medical and psychiatric disorders. To better understand which neurons may exert top-down control over the NTS, here we provide a brain-wide map of all neurons that project axons directly to the caudal, viscerosensory NTS, focusing on a medial subregion with aldosterone-sensitive HSD2 neurons. Injecting an axonal tracer (cholera toxin b) into the NTS produces a similar pattern of retrograde labeling in rats and mice. The paraventricular hypothalamic nucleus (PVH), lateral hypothalamic area, and central nucleus of the amygdala (CeA) contain the densest concentrations of NTS-projecting neurons. PVH afferents are glutamatergic (express Slc17a6/Vglut2) and are distinct from neuroendocrine PVH neurons. CeA afferents are GABAergic (express Slc32a1/Vgat) and are distributed largely in the medial CeA subdivision. Other retrogradely labeled neurons are located in a variety of brain regions, including the cerebral cortex (insular and infralimbic areas), bed nucleus of the stria terminalis, periaqueductal gray, Barrington's nucleus, Kölliker-Fuse nucleus, hindbrain reticular formation, and rostral NTS. Similar patterns of retrograde labeling result from tracer injections into different NTS subdivisions, with dual retrograde tracing revealing that many afferent neurons project axon collaterals to both the lateral and medial NTS subdivisions. This information provides a roadmap for studying descending axonal projections that may influence visceromotor systems and visceral "mind-body" symptoms.

Prefrontal cortex modulates the correlations between brain-derived neurotrophic factor level, serotonin, and the autonomic nervous system

Top-down regulation in the human brain and anatomical connections between the prefrontal cortex (PFC) and specific catecholamine-related regions have been well-studied. However, the way in which the PFC modulates downstream neuro-networks in terms of serotonin and the autonomic nervous system (ANS) by variation in the level of brain-derived neurotrophic factor (BDNF) is still unclear. We recruited sixty-seven healthy subjects. Serotonin transporter (SERT) availability was examined by SPECT with [¹²³I]ADAM analysis; heart rate variability (HRV) testing was performed, and the BDNF level was measured. The Wisconsin card-sorting test (WCST), which assesses PFC activation, was also conducted. The interactions of BDNF level and SERT availability were significant in relation to the HRV indexes of low frequency, high frequency, total power, and mean heart rate range. Moderate to significant positive correlations between SERT availability and the above-mentioned HRV indexes existed only in subjects with a low BDNF level. Furthermore, in the low BDNF level group, only those with high WCST perseveration errors or low category completions exhibited significant positive correlations between SERT availability and HRV indexes. A lower BDNF level and poorer PFC function might modulate the synergistic effects of serotonergic and ANS systems in order to maintain brain physiological and psychological homeostasis.

A hypothalamo-midbrain-medullary pathway involved in the inhibition of the respiratory chemoreflex response induced by potassium cyanide in rodents

Recent studies have demonstrated that a mild stimulation of the dorsomedian nucleus of the hypothalamus (DMH), a defense area, induces the inhibition of the carotid chemoreflex tachypnea. DMH activation reduces the cardiac chemoreflex response via the dorsolateral part of the periaqueductal grey matter (dlPAG) and serotonin receptors (5-HT₃ subtype) in the nucleus tractus solitarius (NTS). The objectives of this study were to assess whether dlPAG and subsequent NTS 5-HT₃ receptors are involved in chemoreflex tachypnea inhibition during mild activation of the DMH. For this purpose, peripheral chemoreflex was activated with potassium cyanide (KCN, 40 μg/rat, i.v.) during electrical and chemical minimal supra-threshold (mild) stimulation of the dlPAG or DMH. In both situations, changes in respiratory frequency (RF) following KCN administration were reduced. Moreover, pharmacological blockade of the dlPAG prevented DMH-induced KCN tachypnea inhibition. Activation of NTS 5-HT₃ receptors also reduced chemoreflex tachypnea in a dose-dependent manner. In addition, blockade of NTS 5-HT₃ receptors with granisetron (2.5 but not 1.25 mM), or the use of mice lacking the 5-HT_3a receptor (5-HT_3a KO), prevented dlPAG-induced KCN reductions in RF. A respiratory hypothalamo-midbrain-medullary pathway (HMM) therefore plays a crucial role in the inhibition of the hyperventilatory response to carotid chemoreflex.

Key role of 5-HT3 receptors in the nucleus tractus solitarii in cardiovagal stress reactivity

Serotonin plays a modulatory role in central control of the autonomic nervous system (ANS). The nucleus tractus solitarii (NTS) in the medulla is an area of viscerosomatic integration innervated by both central and peripheral serotonergic fibers. Influences from different origins therefore trigger the release of serotonin into the NTS and exert multiple influences on the ANS. This major influence on the ANS is also mediated by activation of several receptors in the NTS. In particular, the NTS is the central zone with the highest density of serotonin₃ (5-HT₃) receptors. In this review, we present evidence that 5-HT₃ receptors in the NTS play a key role in one of the crucial homeostatic responses to acute and chronic stress: inhibitory modulation of the parasympathetic component of the ANS. The possible functional interactions of 5-HT₃ receptors with GABA_A and NK₁ receptors in the NTS are also discussed.

5-HT1A receptors of the nucleus tractus solitarii facilitate sympathetic recovery following hypotensive hemorrhage in rats

The role of serotonin in the hemodynamic response to blood loss remains controversial. Caudal raphe serotonin neurons are activated during hypotensive hemorrhage, and their destruction attenuates sympathetic increases following blood loss in unanesthetized rats. Caudal raphe neurons provide serotonin-positive projections to the nucleus tractus solitarii (NTS), and disruption of serotonin-positive nerve terminals in the NTS attenuates sympathetic recovery following hemorrhage. Administration of 5-HT1A-receptor agonists following hemorrhage augments sympathetic-mediated increases in venous tone and tissue hypoxia. These findings led us to hypothesize that severe blood loss promotes activation of 5-HT1A receptors in the NTS, which facilitates sympathetic recovery and peripheral tissue perfusion. Here, we developed an adeno-associated viral vector encoding an efficacious small hairpin RNA sequence targeting the rat 5-HT1A receptor. Unanesthetized rats subjected to NTS injection of the anti-rat 5-HT1A small hairpin RNA-encoding vector 4 wk prior showed normal blood pressure recovery, but an attenuated recovery of renal sympathetic nerve activity (-6.4 ± 12.9 vs. 42.6 ± 15.6% baseline, P < 0.05) 50 min after 21% estimated blood volume withdrawal. The same rats developed increased tissue hypoxia after hemorrhage, as indicated by prolonged elevations in lactate (2.77 ± 0.5 vs. 1.34 ± 0.2 mmol/l, 60 min after start of hemorrhage, P < 0.05). 5-HT1A mRNA levels in the commissural NTS were directly correlated with renal sympathetic nerve activity (P < 0.01) and inversely correlated with lactate (P < 0.05) 60 min after start of hemorrhage. The data suggest that 5-HT1A receptors in the commissural NTS facilitate tissue perfusion after blood loss likely by increasing sympathetic-mediated venous return.

Cardiovascular afferents cause the release of 5-HT in the nucleus tractus solitarii; this release is regulated by the low- (PMAT) not the high-affinity transporter (SERT)

The nucleus tractus solitarii (NTS) integrates inputs from cardiovascular afferents and thus is crucial for cardiovascular homeostasis. These afferents primarily release glutamate, although 5-HT has also been shown to play a role in their actions. Using fast-cyclic voltammetry, an increase in 5-HT concentrations (range 12-50 nm) could be detected in the NTS in anaesthetized rats in response to electrical stimulation of the vagus and activation of cardiopulmonary, chemo- and baroreceptor reflexes. This 5-HT signal was not potentiated by the serotonin transporter (SERT) or the noradrenaline transporter (NET) inhibitors citalopram and desipramine (1 mg kg(-1) ). However, decynium-22 (600 μg kg(-1) ), an organic cation 3 transporter (OCT3)/plasma membrane monoamine transporter (PMAT) inhibitor, increased the 5-HT signal by 111 ± 21% from 29 ± 10 nm. The effectiveness of these inhibitors was tested against the removal time of 5-HT and noradrenaline applied by microinjection to the NTS. Citalopram and decynium-22 attenuated the removal of 5-HT but not noradrenaline, whereas desipramine had the reverse action. The OCT3 inhibitor corticosterone (10 mg kg(-1) ) had no effect. Blockade of glutamate receptors with topical kynurenate (10-50 nm) reduced the vagally evoked 5-HT signal by 50%, indicating that this release was from at least two sources. It is concluded that vagally evoked 5-HT release is under the regulation of the high-capacity, low-affinity transporter PMAT, not the low-capacity, high-affinity transporter SERT. This is the first demonstration that PMAT may be playing a physiological role in the regulation of 5-HT transmission and this could indicate that 5-HT is acting, in part, as a volume transmitter within the NTS.

Peripheral injected cholecystokinin-8S modulates the concentration of serotonin in nerve fibers of the rat brainstem

Serotonin and cholecystokinin (CCK) play a role in the short-term inhibition of food intake. It is known that peripheral injection of CCK increases c-Fos-immunoreactivity (Fos-IR) in the nucleus of the solitary tract (NTS) in rats, and injection of the serotonin antagonist ondansetron decreases the number of c-Fos-IR cells in the NTS. This supports the idea of serotonin contributing to the effects of CCK. The aim of the present study was to elucidate whether peripherally injected CCK-8S modulates the concentration of serotonin in brain feeding-regulatory nuclei. Ad libitum fed male Sprague-Dawley rats received 5.2 and 8.7 nmol/kg CCK-8S (n=3/group) or 0.15M NaCl (n=3-5/group) injected intraperitoneally (ip). The number of c-Fos-IR neurons, and the fluorescence intensity of serotonin in nerve fibers were assessed in the paraventricular nucleus (PVN), arcuate nucleus (ARC), NTS and dorsal motor nucleus of the vagus (DMV). CCK-8S increased the number of c-Fos-ir neurons in the NTS (mean±SEM: 72±4, and 112±5 neurons/section, respectively) compared to vehicle-treated rats (7±2 neurons/section, P<0.05), but did not modulate c-Fos expression in the DMV or ARC. Additionally, CCK-8S dose-dependently increased the number of c-Fos-positive neurons in the PVN (218±15 and 128±14, respectively vs. 19±5, P<0.05). In the NTS and DMV we observed a decrease of serotonin-immunoreactivity 90 min after injection of CCK-8S (46±2 and 49±8 pixel/section, respectively) compared to vehicle (81±8 pixel/section, P<0.05). No changes of serotonin-immunoreactivity were observed in the PVN and ARC. Our results suggest that serotonin is involved in the mediation of CCK-8's effects in the brainstem.

5-hydroxytryptamine 2C receptors tonically augment synaptic currents in the nucleus tractus solitarii

The nucleus tractus solitarii (nTS) is the primary termination and integration point for visceral afferents in the brain stem. Afferent glutamate release and its efficacy on postsynaptic activity within this nucleus are modulated by additional neuromodulators and transmitters, including serotonin (5-HT) acting through its receptors. The 5-HT(2) receptors in the medulla modulate the cardiorespiratory system and autonomic reflexes, but the distribution of the 5-HT(2C) receptor and the role of these receptors during synaptic transmission in the nTS remain largely unknown. In the present study, we examined the distribution of 5-HT(2C) receptors in the nTS and their role in modulating excitatory postsynaptic currents (EPSCs) in monosynaptic nTS neurons in the horizontal brain stem slice. Real-time RT-PCR and immunohistochemistry identified 5-HT(2C) receptor message and protein in the nTS and suggested postsynaptic localization. In nTS neurons innervated by general visceral afferents, 5-HT(2C) receptor activation increased solitary tract (TS)-EPSC amplitude and input resistance and depolarized membrane potential. Conversely, 5-HT(2C) receptor blockade reduced TS-EPSC and miniature EPSC amplitude, as well as input resistance, and hyperpolarized membrane potential. Synaptic parameters in nTS neurons that receive sensory input from carotid body chemoafferents were also attenuated by 5-HT(2C) receptor blockade. Taken together, these data suggest that 5-HT(2C) receptors in the nTS are located postsynaptically and augment excitatory neurotransmission.

Serotonin nerve terminals in the dorsomedial medulla facilitate sympathetic and ventilatory responses to hemorrhage and peripheral chemoreflex activation

Serotonin neurons of the caudal raphe facilitate ventilatory and sympathetic responses that develop following blood loss in conscious rats. Here, we tested whether serotonin projections to the caudal portion of the dorsomedial brain stem (including regions of the nucleus tractus solitarius that receive cardiovascular and chemosensory afferents) contribute to cardiorespiratory compensation following hemorrhage. Injections of the serotonin neurotoxin 5,7-dihydroxytryptamine produced >90% depletion of serotonin nerve terminals in the region of injection. Withdrawal of ∼21% of blood volume over 10 min produced a characteristic three-phase response that included 1) a normotensive compensatory phase, 2) rapid sympathetic withdrawal and hypotension, and 3) rapid blood pressure recovery accompanied by slower recovery of heart rate and sympathetic activity. A gradual tachypnea developed throughout hemorrhage, which quickly reversed with the advent of sympathetic withdrawal. Subsequently, breathing frequency and neural minute volume (determined by diaphragmatic electromyography) declined below baseline following termination of hemorrhage but gradually recovered over time. Lesioned rats showed attenuated sympathetic and ventilatory responses during early compensation and later recovery from hemorrhage. Both ventilatory and sympathetic responses to chemoreceptor activation with potassium cyanide injection were attenuated by the lesion. In contrast, the gain of sympathetic and heart rate baroreflex responses was greater, and low-frequency oscillations in blood pressure were reduced after lesion. Together, the data are consistent with the view that serotonin innervation of the caudal dorsomedial brain stem contributes to sympathetic compensation during hypovolemia, possibly through facilitation of peripheral chemoreflex responses.

Evidence that 5-hydroxytryptamine(7) receptors play a role in the mediation of afferent transmission within the nucleus tractus solitarius in anaesthetized rats

BACKGROUND AND PURPOSE

Central 5-hydroxytryptamine (5-HT)-containing pathways utilizing 5-HT(7) receptors are known to be critical for the mediation of cardiovascular reflexes. The nucleus tractus solitarius (NTS) is a site involved in the integration of cardiovascular afferent information. The present experiments examined the involvement of the 5-HT(7) receptor in the processing of cardiovascular reflexes in the NTS.

EXPERIMENTAL APPROACH

In anaesthetized rats extracellular recordings were made from 104 NTS neurones that were excited by electrical stimulation of the vagus nerve and/or activation of cardiopulmonary afferents. Drugs were applied ionophoretically in the vicinity of these neurones.

KEY RESULTS

The non-selective 5-HT(7) receptor agonist 5-carboxamidotryptamine maleate (5-CT) applied to 78 neurones increased the firing rate in 18 by 59% and decreased it in 38 neurones by 47%. Similarly, the 5-HT(1A) agonist 8-OH-DPAT applied to 20 neurones had an excitatory (8), inhibitory (7) or no effect (5) on the 20 neurones tested. In the presence of the 5-HT(7) antagonist SB 258719 the 5-CT excitation was attenuated. Furthermore, the excitatory response of NTS neurones evoked by electrical stimulation of the vagus nerve or activation of cardiopulmonary afferents with intra atrial phenylbiguanide was attenuated by SB 258719. The inhibitory action of 5-CT was unaffected by SB 258719 and the 5-HT(1A) antagonist WAY-100635. WAY-100635 failed to have any effect on 5-CT and vagal afferent-evoked excitations.

CONCLUSIONS AND IMPLICATIONS

Vagal afferent-evoked excitation of NTS neurones can be blocked by SB 258719, a selective 5-HT(7) antagonist. This observation further supports the involvement of 5-HT neurotransmission in NTS afferent processing.

Neuroinvasion of the 263K scrapie strain after intranasal administration occurs through olfactory-unrelated pathways

The olfactory system has been implicated in the pathogenesis of transmissible spongiform encephalopathies (TSEs). To examine this issue and identify the pattern of TSE agent spread after intranasal administration, we inoculated a high-infectious dose of neurotropic scrapie strain 263K into the nasal cavity of Syrian hamsters. All animals allowed to survive became symptomatic with a mean incubation period of 162.4 days. Analysis at different time points revealed deposition of the pathological prion protein (PrP(TSE)) in nasal-associated lymphoid tissues in the absence of brain involvement from 80 days post-infection (50% of the incubation period). Olfactory-related structures and brainstem nuclei were involved from 100 days post-inoculation (62% of the incubation period) when animals were still asymptomatic. Intriguingly, vagal or trigeminal nuclei were identified as early sites of PrP(TSE) deposition in some pre-symptomatic animals. These findings indicate that the 263K scrapie agent is unable to effectively spread from the olfactory neuroepithelium to the olfactory-related structures and that, after intranasal inoculation, neuroinvasion occurs through olfactory-unrelated pathways.

Repetitive paired stimulation of nasotrigeminal and peripheral chemoreceptor afferents cause progressive potentiation of the diving bradycardia

Hallmarks of the mammalian diving response are protective apnea and bradycardia. These cardiorespiratory adaptations can be mimicked by stimulation of the trigeminal ethmoidal nerve (EN5) and reflect oxygen-conserving mechanisms during breath-hold dives. Increasing drive from peripheral chemoreceptors during sustained dives was reported to enhance the diving bradycardia. The underlying neuronal mechanisms, however, are unknown. In the present study, expression and plasticity of EN5-bradycardias after paired stimulation of the EN5 and peripheral chemoreceptors was investigated in the in situ working heart-brain stem preparation. Paired stimulations enhanced significantly the bradycardic responses compared with EN5-evoked bradycardia using submaximal stimulation intensity. Alternating stimulations of the EN5 followed by paired stimulation of the EN5 and chemoreceptors (10 trials, 3-min interval) caused a progressive and significant potentiation of EN5-evoked diving bradycardia. In contrast, bradycardias during paired stimulation remained unchanged during repetitive stimulation. The progressive potentiation of EN5-bradycardias was significantly enhanced after microinjection of the 5-HT(3) receptor agonist (CPBG hydrochloride) into the nucleus tractus solitarii (NTS), while the 5-HT(3) receptor antagonist (zacopride hydrochloride) attenuated the progressive potentiation. These results suggest an integrative function of the NTS for the multimodal mediation of the diving response. The potentiation or training of a submaximal diving bradycardia requires peripheral chemoreceptor drive and involves neurotransmission via 5-HT(3) receptor within the NTS.

Serotonergic neurons activate chemosensitive retrotrapezoid nucleus neurons by a pH-independent mechanism

Serotonin activates respiration and enhances the stimulatory effect of CO2 on breathing. The present study tests whether the mechanism involves the retrotrapezoid nucleus (RTN), a group of medullary glutamatergic neurons activated by extracellular brain pH and presumed to regulate breathing. We show that the RTN is innervated by both medullary and pontine raphe and receives inputs from thyrotropin-releasing hormone (TRH) and substance P-expressing neurons. Coexistence of serotonin and substance P in terminals within RTN confirmed that lower medullary serotonergic neurons innervate RTN. In vivo, unilateral injection of serotonin into RTN stimulated inspiratory motor activity, and pH-sensitive RTN neurons were activated by iontophoretic application of serotonin or substance P. In brain slices, pH-sensitive RTN neurons were activated by serotonin, substance P, and TRH. The effect of serotonin in slices was ketanserin sensitive and persisted in the presence of glutamate, GABA, glycine, and purinergic ionotropic receptor antagonists. Serotonin and pH had approximately additive effects on the discharge rate of RTN neurons, both in slices and in vivo. In slices, serotonin produced an inward current with little effect on conductance and had no effect on the pH-induced current. We conclude that (1) RTN receives input from multiple raphe nuclei, (2) serotonin, substance P, and TRH activate RTN chemoreceptors, and (3) excitatory effects of serotonin and pH are mediated by distinct ionic conductances. Thus, RTN neurons presumably contribute to the respiratory stimulation caused by serotonergic neurons, but serotonin seems without effect on the cellular mechanism by which RTN neurons detect pH.

Compensatory airway dilation and additive ventilatory augmentation mediated by dorsomedial medullary 5-hydroxytryptamine 2 receptor activity and hypercapnia

5-HT2 receptor activity in the hypoglossal nucleus and hypercapnia is associated with airway dilation. 5-HT neurons in the medullary raphe and hypercapnia are responsible for tidal volume change. In this study, the effects of 5-HT2 receptors in the dorsomedial medulla oblongata (DMM), which receives projections from the medullary raphe, and hypercapnia on airway resistance and respiratory variables were studied in mice while monitoring 5-HT release in the DMM. A microdialysis probe was inserted into the DMM of anesthetized adult mice. Each mouse was placed in a double-chamber plethysmograph. After recovery from anesthesia, the mice were exposed to stepwise increases in CO2 inhalation (5%, 7%, and 9% CO2 in O2) at 8-min intervals with a selective serotonin reuptake inhibitor, fluoxetine, or fluoxetine plus a 5-HT2 receptor antagonist, LY-53857 in the DMM. In response to fluoxetine plus LY-53857 coperfusion, specific airway resistance was increased, and tidal volume and minute ventilation were decreased. CO2 inhalation with fluoxetine plus LY-53857 coperfusion in the DMM largely decreased airway resistance and additively increased minute ventilation. Thus, 5-HT2 receptor activity in the DMM increases basal levels of airway dilation and ventilatory volume, dependent on central inspiratory activity and the volume threshold of the inspiratory off-switch mechanism. Hypercapnia with low 5-HT2 receptor activity in the DMM largely recovers airway dilation and additively increases ventilatory volume. Interaction between 5-HT2 receptor activity in the DMM and CO2 drive may elicit a cycle of hyperventilation with airway dilation and hypoventilation with airway narrowing.

Involvement of central 5-HT7 receptors in modulation of cardiovascular reflexes in awake rats

This study evaluated the role of 5-HT7 receptors within the central nervous system in modulating cardiovascular responses to the activation of chemo-, baro- and cardiopulmonary reflexes and in the regulation of mean arterial pressure and heart rate, using intracisternal (i.c.) application of the selective 5-HT7 receptor antagonist SB-269970 in awake rats. Experiments were performed on male Wistar rats (300-320 g). At 4 days before the experiment, rats were anesthetized and placed in a stereotaxic frame implantation of a guide cannula in the direction of the cisterna magna to be used for microinjection of saline or SB-269970 (100 microg/kg). On the day before the experiments a femoral artery and vein were cannulated to record arterial pressure and heart rate and to inject drugs to activate cardiovascular reflexes, respectively. The chemo-, baro- and cardiopulmonary reflexes were activated in different experimental groups before and after i.c. injection of saline or SB-269970. The antagonism of 5-HT7 receptors reduced: (a) the pressor (50+/-4 vs. 19+/-9 mm Hg) and bradycardic (-247+/-13 vs. -69+/-27 bpm) responses to chemoreflex activation; (b) the fall in MAP (-54+/-4 vs. -20+/-6 mm Hg) and the bradycardia (-294+/-12 vs. -98+/-34 bpm) in response to cardiopulmonary reflex activation; and (c) the gain of the baroreflex (-2.3+/-0.1 to -0.9+/-0.2 bpm/mm Hg). Intracisternal application of SB-269970 increased significantly baseline MAP in those rats previously submitted to the activation of a cardiovascular reflex but in naïve rats produced no changes in the baseline MAP were observed. The fact that cardiovascular responses to all reflexes tested were attenuated by the antagonism of 5-HT7 receptors suggests that brainstem 5-HT7 receptors brainstem facilitate the processing of the autonomic responses to cardiovascular reflex activation and that a 5-HT-containing pathway to the brainstem provides a normalizing input during challenges produced by cardiovascular reflex activation which seems to be mediated by 5-HT7 receptors.

Ventilatory effects of muscimol microdialysis into the rostral medullary raphé region of conscious rats

We hypothesized that inhibition of the rostral medullary raphe region (MRR), a putative central chemoreceptor location, with the GABA(A) receptor agonist muscimol would decrease ventilatory responses to hypercapnia and hypoxia in conscious rats, and that its known effect at this site on body temperature might alter its effect upon these ventilatory responses. At ambient temperatures of 24.5-26.5 degrees C (Cool), microdialysis of 1mM muscimol into the MRR significantly decreased body temperature by approximately 0.5 degrees C, increased the ventilatory response to 7% CO(2) and decreased the response to 10% O(2). At ambient temperatures of 29.5-30.5 degrees C (Warm), 1 mM muscimol microdialysis no longer decreased body temperature and increased the ventilatory response to hypercapnia and to hypoxia. Muscimol did not significantly affect the VE/VO2 ratio at either temperature. Muscimol significantly increased the hypercapnic ventilatory responses in Cool and Warm conditions and the hypoxic response in Warm conditions, which indicates the presence of an inhibitory effect of rostral MRR neurons sensitive to muscimol. In the Cool condition the ventilatory response to hypoxia is inhibited but appropriately so for the lower VO2 .

Modulation of the Hering-Breuer reflex by raphe pallidus in rabbits

Activation of the pulmonary stretch receptors by lung inflation or vagal stimulation evokes Hering-Breuer (HB) reflex, which is characterized by inspiratory inhibition and expiratory prolongation. In this work, whether the HB reflex could be modulated by the serotonergic raphe pallidus (RP) was studied by comparing the strength of this reflex before and after electrical or chemical stimulation of the RP. Experiments were performed on urethane anesthetized adult rabbits. The HB reflex was simulated with electrical stimulation of the central end of cervical vagus nerve. The RP was stimulated electrically or chemically by microinjection of glutamate. We found that after either electrical stimulation or chemical stimulation of the RP, the inspiratory inhibition and expiratory prolongation of the HB reflex were significantly attenuated. This attenuation showed post-stimulation time dependency or short-term memory, as well as RP stimulation intensity dependency. Results of the present study suggested that the serotonergic RP could exert its respiratory effects by modulating the strength of HB reflex.

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.

5-HT3A receptor subunits in the rat medial nucleus of the solitary tract: subcellular distribution and relation to the serotonin transporter

The 5-hydroxytryptamine 3 (5HT3) receptor is a serotonin-gated ion channel implicated in reflex regulation of autonomic functions within the nucleus of the solitary tract (NTS). To determine the relevant sites for 5-HT3 receptor mediated transmission in this region, we used electron microscopic immunocytochemistry to examine the subcellular distribution of the 5HT3 receptor subunit A (5HT3A) in relation to the serotonin transporter (SERT) in the intermediate medial NTS (mNTS) of rat brain. The 5HT3A immunolabeling was detected in many axonal as well as somatodendritic and glial profiles. The axonal profiles included small axons and axon terminals in which the 5HT3A immunoreactivity was localized to membranes of synaptic vesicles and extrasynaptic plasma membranes. In dendrites and glia, the 5HT3A immunoreactivity was located on the plasma membranes or in association with membranous cytoplasmic organelles. The dendritic plasmalemmal 5HT3A labeling was prominent within and near excitatory-type synapses from terminals including those that resemble vagal afferents. The 5HT3A-labeled glial processes apposed 5HT3A-immunoreactive axonal and dendritic profiles, some of which also contained SERT. Terminals containing 5-HT3A and/or SERT were among those providing synaptic input to 5HT3A-labeled dendrites. Thus, 5HT3A has a subcellular distribution consistent with the involvement of 5-HT3 receptors in modulation of both presynaptic release and postsynaptic responses of mNTS neurons, some of which are serotonergic. The results further suggest that the neuronal as well as glial 5HT3 receptors can be activated by release of serotonin from presynaptic terminals or by diffusion facilitated by SERT distribution at a distant from the synapse.

Endogenous 5-HT(1/2) systems and the newborn rat respiratory control. A comparative in vivo and in vitro study

Consequences of 5-HT(1/2) systems blockade by methysergide on newborn rats respiratory drive were evaluated in vivo with unrestrained animals and in vitro using brainstem-spinal cord preparations. A decrease in respiratory frequency until a plateau level was observed under both in vivo (82.8 +/- 0.6% of control values) and in vitro (76.8 +/- 0.8% of control values) conditions whereas an increase in inspiratory amplitude (135.1 +/- 2.1% of control values) was only retrieved in vivo. By the use of the c-fos expression analysis, we correlated these effects with neuronal activity changes, particularly, in vivo in two key structures between the respiratory ponto-medullary network and the peripheral or suprapontine afferences, namely the commissural subnucleus of the nucleus of the solitary tract and the lateral parabrachial nucleus. Thus, peripheral and suprapontine inputs seem to be of a primeval importance in the respiratory influence of endogenous 5-HT. Besides, as 5-HT is involved in the respiratory perturbations that occur in sudden infant death syndrome (SIDS), our results suggest a participation of peripheral and suprapontine inputs in these disorders.

5-HT-mediated inhibition of cardiovagal baroreceptor reflex response during defense reaction in the rat

Previous studies showed that the cardiac response of the baroreceptor reflex (bradycardia) is inhibited during the defense reaction evoked by direct electrical or chemical stimulation of the periaqueductal gray (dPAG) in the rat. Whether central serotonin and nucleus tractus solitarius (NTS) serotonin3 (5-HT3) receptors might participate in this inhibition was investigated in urethane-anesthetized and atenolol-pretreated rats. Our results showed that both electrical and chemical stimulation of the dPAG produced a drastic reduction of the cardiovagal component of the baroreceptor reflex triggered by either intravenous administration of phenylephrine or aortic nerve stimulation. This inhibitory effect of dPAG stimulation on the baroreflex bradycardia was not observed in rats that had been pretreated with p-chlorophenylalanine (300 mg/kg ip daily for 3 days) to inhibit serotonin synthesis. Subsequent 5-hydroxytryptophan administration (60 mg/kg ip), which was used to restore serotonin synthesis, allowed the inhibitory effect of dPAG stimulation on both aortic and phenylephrine-induced cardiac reflex responses to be recovered in p-chlorophenylalanine-pretreated rats. On the other hand, in nonpretreated rats, the inhibitory effect of dPAG stimulation on the cardiac baroreflex response could be markedly reduced by prior intra-NTS microinjection of granisetron, a 5-HT3 receptor antagonist, or bicuculline, a GABAA receptor antagonist. These results show that serotonin plays a key role in the dPAG stimulation-induced inhibition of the cardiovagal baroreceptor reflex response. Moreover, they support the idea that 5-HT3 and GABAA receptors in the NTS contribute downstream to the inhibition of the baroreflex response caused by dPAG stimulation.

Sub-chronic exposure to toluene at 40ppm alters the monoamine biosynthesis rate in discrete brain areas

Effects of long-term exposure to a sub-toxic concentration of toluene (40 ppm, 104 h per week, 16 weeks) have been studied on monoamine biosynthesis rate in rat. The activities of the rate limiting enzymes in catecholamine and 5-hydroxytryptamine biosynthesis tyrosine and tryptophan hydroxylase, respectively, were estimated in vivo by measuring the accumulation of l-dihydroxyphenylalanine and 5-hydroxytryptophan after pharmacological blockade of l-aromatic acid decarboxylases by NSD-1015 (100 mg kg(-1) ip). The sub-chronic exposure to toluene led to a significant and gender dependent alteration in both catecholamine and 5-hydroxytryptamine biosynthesis rate in brainstem catecholaminergic cell groups and hypothalamus. In females exposed to toluene, changes in tryptophan hydroxylation were found in rostral subset of A2C2 (+41%) and in A1C1 (+41%) while a decrease in A5 (-44%) and an increase in A2C2 (+28%) were found in tyrosine hydroxylation. In males, an increase in tryptophane hydroxylation was observed in rostral subset of A2C2 (+35%) while a decrease was observed in ventro-median hypothalamus (-17%). These results suggest that toluene exposure to a dose generally recognized as sub-toxic (40 ppm, no observed adverse effect level) leads to adverse effects on monoaminergic systems. Therefore, the neurotoxicity of toluene should be carefully re-evaluated taking into account not only the exposure level but also the duration.

The 5-hydroxytryptamine1B/1D/1F receptor agonists eletriptan and naratriptan inhibit trigeminovascular input to the nucleus tractus solitarius in the cat

Migraine pain arises in the trigeminovascular system and is often associated with nausea and sometimes with vomiting. In this study, an in vivo cat model of trigeminovascular stimulation was used to determine first whether there is a functional connection between the trigeminovascular system and the nucleus tractus solitarius (NTS), which is involved in regulating vomiting, and second whether anti-migraine drugs have any effect on such a connection. Chloralose-anaesthetised cats (n=16) were prepared for single neuron recording. The superior sagittal sinus (SSS) was isolated and stimulated electrically. The brainstem near the obex was exposed and a metal microelectrode equipped with six glass barrels for microiontophoresis was placed in the NTS. Recordings were made from 44 NTS neurons which responded to SSS stimulation with A-delta latencies. Iontophoretic ejection (50 nA) of eletriptan or naratriptan suppressed the response in 75% (15/20) and 78% (11/14) of cells and caused an average suppression of cell firing of 42+/-5% (n=20) and 54+/-8% (n=14), respectively. This suppression could be antagonized by the concurrent ejection (20-50 nA) of the 5-HT(1B/1D) receptor antagonist GR127935. We conclude that activation of the trigeminovascular system excites cells in the NTS that can be inhibited by eletriptan and naratriptan through activation of 5-HT(1B/1D) receptors. It is possible that in patients having a migraine attack trigeminovascular activation triggers nausea and vomiting, and that the alleviation of these symptoms by anti-migraine compounds may be via an action at 5-HT(1B/1D) receptors in the NTS.

Role of nucleus tractus solitarius 5-HT3 receptors in the defense reaction-induced inhibition of the aortic baroreflex in rats

Different stressful conditions elicit a typical behavior called the defense reaction. Our aim was to determine whether 5-HT3 receptors in the nucleus tractus solitarius (NTS) are involved in 1) the inhibition of the baroreflex bradycardia and 2) the rise in blood pressure, which are known to occur during the defense reaction. In urethane-anesthetized rats, the defense reaction was elicited by electrical stimulation of the dorsomedial nucleus of the hypothalamus (DMH) or the dorsal part of the periaqueductal gray (dPAG). Direct electrical stimulation of the aortic depressor nerve was used to trigger the typical baroreflex responses. Aortic stimulation at high (100-150 microA) and low (50-90 microA) intensity produced a decrease in heart rate of -39 to -44% (relative to baseline, Group 1 responses, n = 113) and -19 to -24% (Group 2 responses, n = 43), respectively. In spontaneously breathing rats, Group 1 and Group 2 bradycardiac responses were inhibited during DMH (-75 +/- 4% and -96 +/- 4%, n = 38 and n = 11, respectively), as well as dPAG (-81 +/- 3% and -95 +/- 4%, n = 36 and n = 10, respectively) stimulation. The aortic baroreflex bradycardia was hardly affected by DMH or dPAG stimulation when bicuculline (5 pmol), a specific GABAA receptor antagonist, had previously been microinjected into the NTS. Likewise, NTS microinjections of granisetron, a specific 5-HT3 receptor antagonist, prevented, in a dose-dependent manner, the baroreflex bradycardia inhibition. In addition, intra-NTS granisetron did not affect the rise in blood pressure induced by either site stimulation. These data show that 5-HT3 receptors in the NTS are involved in the GABAergic inhibition of the aortic baroreflex bradycardia, but not in the rise in blood pressure, occurring during the defense reaction elicited by DMH or dPAG stimulation.

5-HT2A receptors are expressed by catecholaminergic neurons in the rat nucleus tractus solitarii

Within the nucleus tractus solitarii (NTS), serotonin (5-HT) exerts modulatory effects on central mechanisms controlling autonomic functions, notably through the stimulation of 5-HT2 receptors. Using double immunocytochemical labeling with specific anti-5-HT2A receptor antibodies and antibodies directed against the catecholamine (CA) synthesizing enzymes, i.e. tyrosine hydroxylase, dopamine beta-hydroxylase (DBH) and phenylethanolamine N-methyltransferase, we investigated whether 5-HT effects could be mediated by 5-HT2A receptors located on CA perikarya and/or processes within the NTS. A relatively high density of 5-HT2A immunoreactive processes was observed throughout the NTS. 5-HT2A neuronal perikarya were also found within the NTS except in its rostrolateral part. Double immunolabeling experiments revealed many 5-HT2A receptors on CA processes but only few 5-HT2A/TH and 5-HT2A/DBH perikarya. These data support the idea that, within the NTS, 5-HT2A-mediated control of autonomic functions involves CA neurons.

Involvement of NMDA receptors in the pressor response to microinjection of 5-HT3 agonist into the NTS of awake rats

Previous studies have shown that the activation of 5-HT3 receptors in the nucleus tractus solitarii (NTS) increases the baseline mean arterial pressure (MAP). In the present study, we evaluated the possible involvement of NMDA receptors in this pressor response. Four days before the experiments, under tribromoethanol anesthesia, rats received two guide cannulas in the direction of the NTS, and 1 day before the experiments, under tribromoethanol anesthesia, the femoral artery was cannulated for pulsatile arterial pressure (PAP), MAP, and heart rate (HR) measurements. On the day of the experiments, 2-methyl-serotonin, a 5-HT3 agonist, was microinjected into the NTS after microinjection of saline or AP-5, a selective NMDA receptor antagonist. Microinjection of 2-methyl-serotonin (5 nmol/50 nl) into the NTS after the vehicle (saline) produced a significant increase in MAP (+ 20 +/- 5 mm Hg, n = 8) while microinjection of 2-methyl-serotonin after microinjection of AP-5 (10 nmol/50 nl) produced no change in baseline MAP (-1 +/- 3 mm Hg, n = 11). Microinjection of AP-5 into the NTS produced no significant changes in the baseline MAP and HR. The data show that the increase in MAP in response to microinjection of a 5-HT3 agonist into the NTS is dependent on NMDA receptors.

Hyperpolarization-activated current I(h) in nucleus of solitary tract neurons: regional difference in serotonergic modulation

The nucleus of solitary tract (NTS) contains diverse neural circuits responsible for basic vital functions. We examined the effect of serotonin (5-HT) on hyperpolarization-activated current (I(h)) in neurons acutely isolated from caudal, medial and rostral parts of the NTS. Caudal and medial NTS neurons showed a large amplitude of I(h) compared with rostral neurons. In these neurons, perfusion with 5-HT potentiated Ih amplitude in a concentration-dependent manner. The effect of 5-HT was blocked by NAN-190, a 5-HT1A receptor antagonist. Thus, 5-HT1A receptors may regulate I(h) channel activity in caudal and medial NTS neurons.

Sodium appetite and Fos activation in serotonergic neurons

We evaluated serotonergic hindbrain groups of cells for their involvement in the generation and inhibition of sodium appetite. For that purpose, we analyzed the number of Fos-immunoreactive (Fos-ir) cells and double-labeled Fos-serotonin (5-HT)-ir neurons within different nuclei of the hindbrain raphe system and the area postrema (AP). Sodium depletion and sodium appetite were induced by peritoneal dialysis. Twenty-four hours after peritoneal dialysis, a 2% NaCl solution intake test was given to peritoneal dialyzed animals [PD-with access (PD-A) group] and to control dialyzed animals [CD-with access (CD-A) group]. Two additional groups of animals received either peritoneal dialysis or control dialysis but were not given access to the 2% NaCl [CD-no access (CD-NA) group or PD-no access (PD-NA) group]. The number of Fos-ir neurons within different nuclei of the raphe system was increased in spontaneous and induced sodium ingestion of CD-A and PD-A groups compared with the CD-NA and PD-NA groups. The PD-NA group had significantly fewer double-labeled cells along the raphe system compared with the animals in near-normal sodium balance (CD-NA and CD-A) or in the process of restoring sodium balance by consuming NaCl (PD-A). The AP of the PD-A group showed a significant increase in the number of Fos-ir and Fos-5-HT-ir cells compared with the PD-NA and CD groups. Our results suggest that serotonergic pathways with cell bodies in the AP and the raphe system are involved in the control of sodium appetite.

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.

Brain stem control of swallowing: neuronal network and cellular mechanisms

Swallowing movements are produced by a central pattern generator located in the medulla oblongata. It has been established on the basis of microelectrode recordings that the swallowing network includes two main groups of neurons. One group is located within the dorsal medulla and contains the generator neurons involved in triggering, shaping, and timing the sequential or rhythmic swallowing pattern. Interestingly, these generator neurons are situated within a primary sensory relay, that is, the nucleus tractus solitarii. The second group is located in the ventrolateral medulla and contains switching neurons, which distribute the swallowing drive to the various pools of motoneurons involved in swallowing. This review focuses on the brain stem mechanisms underlying the generation of sequential and rhythmic swallowing movements. It analyzes the neuronal circuitry, the cellular properties of neurons, and the neurotransmitters possibly involved, as well as the peripheral and central inputs which shape the output of the network appropriately so that the swallowing movements correspond to the bolus to be swallowed. The mechanisms possibly involved in pattern generation and the possible flexibility of the swallowing central pattern generator are discussed.

In vivo modulation of vagal-identified dorsal medullary neurones by activation of different 5-Hydroxytryptamine(2) receptors in rats

1. In in vivo experiments, DOI (a 5-HT(2) receptor agonist), MK-212 (a 5-HT(2C) receptor agonist), and BW-723C86 (a 5-HT(2B) receptor agonist) were applied by ionophoresis to neurones in the rat nucleus tractus solitarius (NTS) receiving vagal afferent input. 2. The majority of the putative 'monosynaptically' vagal activated cells were inhibited by both MK-212 (4/6) and DOI (2/4), but unaffected by BW-723C86 (12/14). In contrast, 'polysynaptically' activated NTS cells were excited by both BW-723C86 (13/19) and DOI (9/10). Inactive 'intermediate' cells were inhibited by BW-723C86 (9/12), MK-212 (5/6) and DOI (3/4), whilst active cells of this group were excited by BW-723C86 (7/13) and DOI (5/5). 3. The selective 5-HT(2B) receptor antagonist LY-202715 significantly reduced the excitatory actions of BW-723C86 on 'intermediate' and 'polysynaptic' cells (13/13), but not the inhibitory effects observed on inactive Group 2 cells (n=5) whereas the selective 5-HT(2C) receptor antagonist RS-102221 reversed the inhibitory effects of MK-212 and DOI on 'monosynaptic and 'intermediate' neurones. 4. Cardio-pulmonary afferent stimulation inhibited two of four putative 'monosynaptically' activated calls and all four inactive intermediate cells. These were also inhibited by DOI and MK-212. In contrast, cardio-pulmonary afferents excited all five active intermediate cells and all six putative 'polysynaptically' activated NTS cells, while all were also previously excited by BW-723C86 and/or DOI. 5. In conclusion, these data demonstrate that neurones in the NTS are affected differently by 5-HT(2) receptor ligands, in regard of their vagal postsynaptic location, the type of cardio-pulmonary afferent they receive and the different 5-HT(2) receptors activated.

In vivo modulation of nucleus tractus solitarius (NTS) neurones by activation of 5-hydroxytryptamine(2) receptors in rats

In in vivo experiments, 5-hydroxytryptamine (5-HT) and (+/-)-2,5-dimethoxy-4-iodoamphetamine HCl (DOI), a 5-HT(2) receptor agonist, were applied by ionophoresis to rat nucleus tractus solitarius (NTS) neurones identified by their vagal and cardiopulmonary afferent inputs to test whether the response of NTS cells to 5-HT(2) receptor activation was related to whether they received mono- or polysynaptic vagal inputs and their presumed function as defined by their afferent input. Cells were classified on the basis of the variability of the latency of the vagal-evoked spikes: this varied by less than 3 ms for Group 1, from 3 to 5 ms for Group 2, and more than 5 ms for Group 3. Both 5-HT and DOI inhibited most Group 1 cells (16/18) and inactive (without ongoing activity) cells (8/13) in Group 2. Cells inhibited by DOI were also inhibited by cardiopulmonary afferent stimulation, evoked by atrial phenylbiguanide administration. By contrast, application of 5-HT and DOI excited the majority of Group 3 cells (14/19) and Group 2 with ongoing activity (7/9). Cells excited by DOI were also activated by cardiopulmonary stimulation. Both actions of DOI were reversed by application of ketanserin (n=15). In conclusion, these data demonstrate that activation of 5-HT(2) receptors in the NTS produces different effects dependent on whether the neurones received mono- or polysynaptic vagal input and their response to cardiopulmonary afferent stimulation.

Glutamate stimulation of raphe pallidus attenuates the cardiopulmonary reflex in anaesthetised rats

Serotonin has been implicated as having a modulatory action on NTS neurones mediating vagal inputs controlling the cardiorespiratory system. Since raphe pallidus and raphe obscurus both send serotonergic projections to the NTS, we have investigated a neuromodulatory role for these structures on the cardiopulmonary reflex. A multibarrelled microelectrode positioned around the level of the area postrema was placed at varying depths into mid-line brainstem structures and the effect of glutamate stimulation on the cardiopulmonary reflex tested. Excitatory chemical stimulation in the region of raphe pallidus, but not raphe obscurus, attenuated significantly the respiratory and bradycardic components of the cardiopulmonary reflex. This attenuation was reversed by an NTS microinjection of RS-39604, a selective 5-HT, receptor antagonist. We propose that neurones in raphe pallidus that project to the NTS can release serotonin which acts via 5-HT4 receptors to attenuate the reflex phrenic nerve activity and heart rate components of the cardiopulmonary reflex.

Field potential analysis of synaptic transmission in spiking neurons in a sparse and irregular neuronal structure in vitro

Extracellular field potential (FP) recording of dendritic depolarizations evoked by afferent stimulation is widely used as a quantitative measure of excitatory synaptic transmission in brain slices in-vitro for structures with high neuronal density and regularity such as hippocampus, neocortex and cerebellum. On the other hand, FP recordings of somatic depolarizations induced by orthodromic or antidromic stimulation of afferent or efferent nerves have been used in some in-vivo preparations for mapping the central projections of these nerves. In this study, we examined the applicability of somatic FPs as a measure of excitatory synaptic transmission in a sparse and irregular brain structure. Excitatory FPs were induced in nucleus tractus solitarius (NTS) in the dorsal medulla by electrical impulse stimulation of primary afferent fibers in the tractus solitarius (TS) in rat brainstem slices in vitro. The evoked FP was rapid and biphasic, and was stimulus-intensity dependent and saturable. The morphology of these somatic FPs resembled the dendritic FPs found in hippocampal and neocortical slices, with an excitatory postsynaptic component that exhibited similar pharmacological and stimulus frequency-dependent properties as found in NTS cells with intracellular or whole-cell recordings. Simultaneous FP and whole-cell recordings revealed that the postsynaptic component of FP was associated with neuronal firing rather than subthreshold membrane depolarizations. We conclude that somatic FP recording provides a simple and reliable measure of excitatory neurotransmission in the TS-NTS pathway and is a useful alternative or adjunct to intracellular or whole-cell recordings especially for studies of long-term synaptic plasticity in spiking neurons. This technique may also be applicable to other brain regions that lack the regular and dense organization of hippocampal and neocortical structures.

The neurophysiology of sleep and waking: intracerebral connections, functioning and ascending influences of the medulla oblongata

This paper focuses on the successive historical papers related to medulla oblongata (M.O.) intracerebral connections, its activities and ascending influences regulating sleep waking behavior. The M.O. certainly influences the quantitative and qualitative processes of waking. However, its neurophysiological properties are often concealed by those of the upper-situated brain stem structures. The M.O., particularly the solitary tract nucleus, is involved in sleep-inducing processes. This nucleus seem to act as a deactivating system of the above situated reticular formation, but it also impacts directly on the thalamocortical slow wave and spindle-inducing processes. The M.O. is significantly involved in paradoxical sleep mechanisms. Indeed, the mesopontine executive centers are unable to induce paradoxical sleep without the M.O. Moreover, stimulation of the solitary tract nucleus afferents can induce paradoxical sleep, and the M.O. metabolic functioning is specifically disturbed by paradoxical sleep deprivation. Finally. there seems to be a paradoxical sleep Zeitgeber. Our current knowledge shows that this lowest brain stem level is crucial for sleep waking mechanisms. It will undoubtedly be further highlighted by future electrophysiologial and neurochemical studies.

Ultrastructure of glutamate and GABA immunoreactive axon terminals of the rat nucleus tractus solitarius, with a note on infralimbic cortex afferents

The principal fast neurotransmitters in the CNS are glutamate and GABA. Our aim was to provide a baseline account on the ultrastructure of the axon terminals immunoreactive to glutamate or GABA present in the nucleus tractus solitarius (NTS) of the rat. In addition, we wanted to complete our study of cortico-solitary afferents at the electron microscopic level, by analyzing the inputs from the infralimbic cortex. Using post-embedding immunogold, we found that nearly 61% of the axon terminals were glutamatergic, and 36% were GABAergic in the rat visceral NTS. In general, axons making asymmetric synaptic contacts were enriched in glutamate, compared to axons involved in symmetric synapses. In contrast, the vast majority of the GABAergic axon terminals made symmetric synaptic contacts. We could discern five types of glutamatergic and two types of GABAergic axon terminals that differed in their fine structure. Afferents from the infralimbic cortex were small, with clear synaptic vesicles and no dense core vesicles; they made asymmetric contacts with fine dendrites, and were glutamatergic. We conclude that most axon terminals in the NTS use glutamate or GABA as fast transmitters, in addition to being a heterogeneous population of morphological types.

Presynaptic 5-HT3 receptors evoke an excitatory response in dorsal vagal preganglionic neurones in anaesthetized rats

1. Recordings were made from a total of sixty-four vagal preganglionic neurones in the dorsal vagal motor nucleus (DVMN) of pentobarbitone sodium anaesthetized rats. The effects of ionophoretic administration of Mg2+ and Cd2+, inhibitors of neurotransmitter release, and the selective NMDA and non-NMDA receptor antagonists (+/-)-2-amino-5-phosphono-pentanoic acid (AP5) and 6,7-dinitroquinoxaline-2,3-dione (DNQX) on the excitatory actions of the 5-HT3 receptor agonist 1-phenylbiguanide (PBG) were studied. 2. In extracellular recording experiments, PBG (0-40 nA) increased the firing rate of thirty-five of the thirty-nine neurones tested. The PBG-evoked excitation was attenuated by application of Mg2+ (1-10 nA) in sixteen of seventeen neurones or Cd2+ (2-10 nA) in seven of eight neurones tested. At these low ejection currents neither Mg2+ nor Cd2+ altered baseline firing rates and Mg2+ had no effect on the excitations evoked by DL-homocysteic acid (n = 4), NMDA (n = 4) or (AMPA; n = 2). 3. Ionophoresis of AP5 (2-10 nA), at currents which selectively inhibited NMDA-evoked excitations, attenuated PBG-evoked excitations in all eight neurones tested. DNQX (5-20 nA), at currents which selectively inhibited AMPA-evoked excitations, also attenuated PBG-evoked excitations (n = 3). 4. Intracellular activity was recorded in nine DVMN neurones. In six neurones ionophoretic application of PBG (10-200 nA) depolarized the membrane and increased firing rate whilst in the other three neurones, PBG had no effect on membrane potential though it increased synaptic noise (n = 3) and firing rate (n = 2). In all six neurones tested, ionophoresis of Mg2+ (10-120 nA) attenuated the PBG-evoked increases in synaptic noise and firing rate. 5. In conclusion, the data are consistent with the hypothesis that 5-HT3 receptor agonists activate DVMN neurones partly by acting on receptors located at sites presynaptic to the neurones. Activation of these receptors appears to facilitate release of glutamate, which, in turn, acts on postsynaptic NMDA and non-NMDA receptors to activate the neurones.

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

The effects of intracisternal (i.c.) application of the 5-HT3 receptor antagonist granisetron (0.016-0.16 microg kg-1) and the agonist phenylbiguanide (0.3-3 microg kg-1) on reflex bradycardia evoked by injection of phenylbiguanide (i.v.; 10 microg kg-1) were investigated in urethane anesthetized atenolol-pretreated rats. The effect of bilateral microinjection of granisetron (10 nmol per side, 100 nl) into the nucleus tractus solitarius (NTS) on the reflex was also investigated. Intracisternal administration of granisetron dose-dependently (0.016-0.16 microg kg-1) and significantly attenuated the reflex bradycardia whilst the highest dose given i.v. had no significant effect on the reflex bradycardia. Phenylbiguanide given i.c. only caused significant potentiation at the middle dose (1 microg kg-1), having no significant effects at the other doses. Neither granisetron nor phenylbiguanide given i.c. affected resting heart rate or blood pressure. Granisetron microinjected bilaterally into the NTS also significantly attenuated both reflex bradycardia and hypotension. It is concluded that excitation of cardiac vagal motoneurones evoked by cardiopulmonary afferents involves activation of 5-HT3 receptors located in the nucleus tractus solitarius and other brainstem areas.

Activity of tryptophan hydroxylase and content of indolamines in discrete brain regions after a long-term hypoxic exposure in the rat

The influence of long-term hypoxia (10% O2, 14 days) on in vivo activity of tryptophan hydroxylase and on 5-hydroxytryptamine (5-HT) and 5-hydroxyindoleacetic acid (5-HIAA) concentration in discrete brain regions of rats was assessed. The activity of tryptophan hydroxylase was determined through 5-hydroxytryptophan accumulation (5-HTPacc) following the administration of NSD 1015. The 5-HTPacc was significantly decreased in the dorsal and median raphe (56 and 42%, respectively) and in the striatum (62%). Both 5-HTPacc and the ratio of the concentrations of 5-HIAA to 5-HT were decreased in the nucleus raphe magnus (46 and 27%, respectively), the dorsomedian medulla oblongata (52 and 51%), the locus coeruleus (62 and 40%) and the anterior hypothalamic nucleus (30 and 50%). In contrast, 5-HTPacc was increased in the ventrolateral medulla oblongata (55%) and the preoptic area (83%), but the 5-HIAA/5-HT ratio was lower in these two regions. Finally, 5-HIAA/5-HT ratio was also decreased in the periventricular nucleus and in the frontal cortex. Since various patterns of variations in 5-HTPacc and in 5-HIAA/5-HT ratio were observed, the factors affecting serotonin metabolism in hypoxic rats can be different among brain regions. These results show that, in the rat, long-term hypoxia induces changes in in vivo activity of tryptophan hydroxylase and in 5-HT and 5-HIAA content of some brain structures; some of these biochemical changes may be linked to adaptative mechanisms.