Direct vagus nerve stimulation: A new tool to control allergic airway inflammation through α7 nicotinic acetylcholine receptor.

BACKGROUND AND PURPOSE: Asthma is characterized by airway inflammation, mucus hypersecretion, and airway hyperresponsiveness. The use of nicotinic agents to mimic the cholinergic anti-inflammatory pathway (CAP) controls experimental asthma. Yet, the effects of vagus nerve stimulation (VNS)-induced CAP on allergic inflammation remain unknown. EXPERIMENTAL APPROACH: BALB/c mice were sensitized and challenged with house dust mite (HDM) extract and treated with active VNS (5 Hz, 0.5 ms, 0.05-1 mA). Bronchoalveolar lavage (BAL) fluid was assessed for total and differential cell counts and cytokine levels. Lungs were examined by histopathology and electron microscopy. KEY RESULTS: In the HDM mouse asthma model, VNS at intensities equal to or above 0.1 mA (VNS 0.1) but not sham VNS reduced BAL fluid differential cell counts and alveolar macrophages expressing α7 nicotinic receptors (α7nAChR), goblet cell hyperplasia, and collagen deposition. Besides, VNS 0.1 also abated HDM-induced elevation of type 2 cytokines IL-4 and IL-5 and was found to block the phosphorylation of transcription factor STAT6 and expression level of IRF4 in total lung lysates. Finally, VNS 0.1 abrogated methacholine-induced hyperresponsiveness in asthma mice. Prior administration of α-bungarotoxin, a specific inhibitor of α7nAChR, but not propranolol, a specific inhibitor of ß2-adrenoceptors, abolished the therapeutic effects of VNS 0.1. CONCLUSION AND IMPLICATIONS: Our data revealed the protective effects of VNS on various clinical features in allergic airway inflammation model. VNS, a clinically approved therapy for depression and epilepsy, appears to be a promising new strategy for controlling allergic asthma.

Higher baseline heart rate variability in CCHS patients with progestin-associated recovery of hypercapnic ventilatory response.

After a fortuitous observation of two cases of chemosensitivity recovery in women with congenital central hypoventilation syndrome (CCHS) who took desogestrel, we aimed to evaluate the ventilatory response to hypercapnia of five CCHS patients with or without treatment consisting of desogestrel (DESO) or levonorgestrel (LEVO). Only two patients became responsive to hypercapnia under treatment, according to their basal vagal heart rate variability. These results suggest that heart rate variability may be promising tool to discriminate patients susceptible to become responsive to hypercapnia under DESO-LEVO treatment.Clinical Trials Identifier NCT01243697.

Screening of obstructive sleep apnea syndrome by the deep breathing technique.

STUDY OBJECTIVES: Obstructive sleep apnea syndrome (OSAS) is associated with alterations in heart rate variability (HRV) in relation to chronic autonomic dysfunction. We tested the ability of the deep breathing technique-a simple way to evaluate HRV-to identify patients with OSAS. METHODS: Consecutive patients referred for suspected OSAS (without obesity, diabetes, and heart diseases) were included. They underwent a measure of HRV at rest and of heart rate oscillations during expiration vs inspiration (DeltaHRDB) when breathing deeply at the resonant frequency of 6 cycles per minute (deep breathing technique) while sitting awake, followed by a nighttime polysomnography. We measured DeltaHRDB and performed temporal and spectral HRV analysis. RESULTS: Of 31 included participants (77% male), 14 had mild to moderate OSAS (apnea-hypopnea index median [IQR]: 18 [12]) and 17 had no OSAS. The conventional HRV analysis did not reveal any difference between the groups with vs without OSAS. However, the DeltaHRDB was lower in those with than without OSAS. Lower DeltaHRDB correlated with higher apnea-hypopnea index, arousal index, and desaturation degree. A DeltaHRDB below 11 beats per minute (bpm) predicted OSAS with a sensitivity of 100% and specificity of 86%. CONCLUSIONS: The deep breathing technique accurately identifies a reduction in cardiac changes in patients with mild to moderate OSAS. It could be used as a simple screening tool to select patients for polysomnography. CITATION: Onanga M, Joanny S, Rivals I, et al. Screening of obstructive sleep apnea syndrome by the deep breathing technique. J Clin Sleep Med. 2023;19(2):293-302.

Heart rate variability and slow-paced breathing:when coherence meets resonance.

Clinical research on the beneficial effects induced by slow-paced breathing has been increasingly extended in the past twenty years. Improvements in cardiovascular functioning, executive functions, or stress management appear to be among the most prominent observations in these studies. However, the mechanisms underlying these effects are multiple and complex. This review will focus on the importance of reducing breathing rate at the resonant frequency (~ 0.1 Hz), which increases cardiac oscillations, thus reflecting improved vagally-mediated heart rate variability and baroreflex sensitivity. These effects are achieved through temporal coherence of respiratory, blood pressure, and cardiac phases, which are the origin of multiple peripheral benefits. In return, vagal afferents, which send inputs to interoceptive areas, are stimulated for longer and more intensely than when breathing spontaneously. In limbic areas, which may also be stimulated through larger cerebral blood flow oscillations and increases in oxygen delivery, interoceptive activation produces a cascade of neural activations that may be at the origin of the central benefits of deep and slow-paced breathing.

Experience from five Asia-Pacific countries during the first wave of the COVID-19 pandemic: Mitigation strategies and epidemiology outcomes.

BACKGROUND: With no vaccines or specific treatments, non-pharmaceutical interventions are the only tools for controlling the human-to-human transmission of the COVID-19 disease, which appeared in Wuhan, China last December and has spread globally since. Here we describe and compare the first-wave mitigation strategies and epidemiology of five Asia-Pacific countries that responded rapidly to the epidemic. METHODS: From January to April 2020, mitigation measures and epidemiological data for Singapore, South Korea, Japan, Taiwan, Hong Kong were screened from official local government websites and a review of investigational studies was conducted. Daily case reports and mitigation measures information were extracted. Epidemiological estimates were calculated and compared between countries. RESULTS: All five countries combined measures, focusing on contact tracing, testing, isolation efforts and healthcare management. Epidemiological data varied temporally and geographically: incubation period ranged 3.9-7.1 days, effective reproduction number at time t (Rt) ranged 0.48-1.5, with intensive care admissions 1-3% of hospitalised patients, and case fatality rates were 0.1-3%. Extrinsic estimates to the virus were lower than global estimates. CONCLUSION: Implemented mitigation strategies in these countries allowed a rapid and successful control or delay of the first COVID-19 pandemic wave. These are valuable examples to inform subsequent waves.

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-HT1A 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.

Social defeat: Vagal reduction and vulnerability to ventricular arrhythmias.

Previously, a sub-population of defeated anesthetized rats (Dlow) was characterized by persistent low blood levels of brain-derived neurotrophic factor (BDNF) at day 29 and autonomic alteration at day 30 after social challenge, while the other population (Dhigh) was similar to non-defeated (ND) animals. The aims of this study were to determine the time-course of autonomic dysfunction in awake animals, and whether Dhigh and/or Dlow were vulnerable to cardiac events. Defeated animals were exposed to four daily episodes of social defeats from day 1 to day 4. At day 30, anesthetized Dlow displayed decreased experimental and spontaneous reflex responses reflecting lower parasympathetic efficiency. In addition, Dlow but not Dhigh were characterized by left ventricular hypertrophy at day 30. Telemetric recordings revealed that Dlow had increased low frequency-to-high frequency ratio (LF/HF) and diastolic (DBP) and systolic (SBP) blood pressure, associated with decreased HF and spontaneous baroreflex responses (BRS) from day 3 to day 29. LF/HF, DBP and SBP recovered at day 5, and HF and BRS recovered at day 15 in Dhigh. Ventricular premature beats (VPBs) occurred in Dlow and Dhigh animals from day 5. Time course of VBP fluctuations in Dhigh mirrored that of HF and BRS, but not that of LF/HF, DBP and SBP. These results suggest that a psychosocial stress associated to low serum BDNF levels can lead to vulnerability to persistent autonomic dysfunction, cardiac hypertrophy and ventricular ectopic beats. The parasympathetic recovery seen in Dhigh may provide protection against cardiac events in this population.

Combined Treatment with Peptide-Conjugated Phosphorodiamidate Morpholino Oligomer-PPMO and AAV-U7 Rescues the Severe DMD Phenotype in Mice.

Duchenne muscular dystrophy (DMD) is a devastating neuromuscular disease caused by an absence of the dystrophin protein, which is essential for muscle fiber integrity. Among the developed therapeutic strategies for DMD, the exon-skipping approach corrects the frameshift and partially restores dystrophin expression. It could be achieved through the use of antisense sequences, such as peptide-conjugated phosphorodiamidate morpholino oligomer (PPMO) or the small nuclear RNA-U7 carried by an adeno-associated virus (AAV) vector. AAV-based gene therapy approaches have potential for use in DMD treatment but are subject to a major limitation: loss of the AAV genome, necessitating readministration of the vector, which is not currently possible, due to the immunogenicity of the capsid. The PPMO approach requires repeated administrations and results in only weak cardiac dystrophin expression. Here, we evaluated a combination of PPMO- and AAV-based therapy in a mouse model of severe DMD. Striking benefits of this combined therapy were observed in striated muscles, with marked improvements in heart and diaphragm structure and function, with unrivalled extent of survival, opening novel therapeutic perspectives for patients.

Vulnerability to stress consequences induced by repeated social defeat in rats: Contribution of the angiotensin II type 1 receptor in cardiovascular alterations associated to low brain derived neurotrophic factor.

After social stress, rats become vulnerable to depression, and this state is characterized by persistent low blood levels of brain-derived neurotrophic factor (BDNF). The aim of this study was to determine whether low BDNF levels are associated with long term autonomic changes. Defeated animals were subjected to four daily episodes of social defeats. Twenty five days later, defeated rats with low BDNF levels (Dlow) still displayed elevated sympathetic tone (as indicated by an elevated low frequency to high frequency ratio (LF/HF) in heart rate) and elevated blood pressure, as well as reduced baroreflex sensitivity (BRS). In contrast, those with higher BDNF levels (Dhigh) similar to controls, did not. Dlow animals persistent cardiovascular changes were abolished by acute inhibition of the dorsomedial nucleus of the hypothalamus (DMH). These cardiovascular changes were also prevented by chronic sub-cutaneous osmotic infusion of losartan, an angiotensin II type 1 receptor (AT1) receptor antagonist, started immediately after social defeat. In conclusion, the results show that greater vulnerability to stress consequences following a traumatic event is associated with an elevated LF/HF ratio, a persistent high blood pressure and a low BRS, all due to an AT1 receptor activation.

Respiratory chemoreflex response inhibition by dorsomedian hypothalamic nucleus activation in rats.

Recent observations from our group seem to indicate that repeated stress-evoked dorsomedian hypothalamic nucleus (DMH) activation in rats can lead to persistent bradypnea. One possibility was that respiratory responses to peripheral chemoreceptor activation were reduced by DMH stimulation. In the present study, we therefore investigated the effect of minimal supra-threshold DMH stimulation on respiratory carotid chemoreflex responses. For this purpose, the chemoreflex was activated by potassium cyanide (KCN, 40µg/rat, i.v.) during electrical and chemical stimulation of the DMH. In both situations, changes in breathing frequency but not tidal volume responses to KCN administration were reduced. These findings suggest that low DMH neurotransmission negatively affects respiratory chemoreflex responses and may be involved in stress-induced bradypnea.

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-HT3a receptor (5-HT3a 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.

Experimental dyspnea as a stressor: differential cardiovegetative responses to inspiratory threshold loading in healthy men and women.

Dyspnea is associated with an emotional reaction that involves limbic activation. The inspiratory threshold load (ITL) is known to elicit a dyspneic response in healthy subjects. Laboratory-induced stress conditions have been shown to elicit sex-related differences in cardiovascular responses. The aim of this study was to evaluate how healthy men (n = 8) and women (n = 9) react and adapt to 5-min periods of ITL at three levels (low, medium, and high) in terms of heart rate (HR), temporal (RMSSD) and spectral (LF, HF, LF/HF ratio) HRV indexes, and rating of breathing discomfort. HR increased with low, medium, and high ITL in men, whereas it increased only with high ITL in women. LF/HF ratio increased at low ITL in both men and women. Modifications appear to depend essentially on increased LF in men and on reduced HF in women. In addition, HRV modifications differ between men and women, following the order of presentation of ITLs. Our results show a continuous and sustained stress in men (increased HR, LF, and LF/HF ratio across ITL presentation) and a stress adaptation in women. Subjective responses of breathing discomfort were not correlated with sympatho-vagal balance modifications for a subgroup of subjects (n = 10). Breathing against the ITL induced autonomic modifications that are different between men and women, i.e., driven by sympathetic mediated responses in men, whereas women showed a greater parasympathetic modulation of cardiovascular activity. These results highlight the role of the mechanical inspiratory load in the heart rate variability seen in chronic obstructive pulmonary disease.NEW & NOTEWORTHY Breathing against the ITL induced autonomic modifications driven by sympathetic mediated responses in men, whereas women showed a greater parasympathetic modulation of cardiovascular activity, even for low load. A stress circuit could be at the origin of autonomic modifications induced by ITL. Our results would underline the role of the mechanic inspiratory load in the abnormalities in heart rate variability seen in COPD patients.

Response of Htr3a knockout mice to antidepressant treatment and chronic stress.

BACKGROUND AND PURPOSE: It has recently been suggested that 5-HT3 receptor blockade enhances the efficacy of selective 5-HT (serotonin) reuptake inhibitor (SSRI) antidepressants and may reverse stress-induced deficits in rodents. EXPERIMENTAL APPROACH: To further explore this hypothesis, we used mice lacking the 5-HT3 receptor (Htr3a KO) and their wild-type (WT) controls to assess their response in behavioural paradigms relevant to anxiety and depression. Mice were studied under basal, antidepressant treatments and chronic social defeat stress (CSDS) conditions. KEY RESULTS: In basal conditions, Htr3a KO mice displayed anxiolytic- and antidepressant-like behaviours in the elevated plus maze, the social interaction and the forced swim tests (FST), but behaved as WT mice in response to acute citalopram in the FST. However, the effects of fluoxetine were blunted in Htr3a KO mice in these same tests. In an in vitro electrophysiological paradigm, a low-dose citalopram treatment triggered 5-HT1A receptor desensitization only in the dorsal raphe nucleus of Htr3a KO, although a high dose desensitized 5-HT1A autoreceptor function equally in Htr3a KO and WT mice, suggesting that citalopram may become effective at lower doses when 5-HT3 receptors are inactivated. In addition, Htr3a deletion blocked CSDS-induced modification in the cortical expression of two genes involved in oxidative stress, CaMKIIa and SOD1. CONCLUSIONS AND IMPLICATIONS: Taken together, these data show that Htr3a deletion promotes SSRI efficacy and prevents the occurrence of stress-induced deleterious effects, suggesting that the 5-HT3 receptor may represent an interesting target for the treatment of stress-related disorders.

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 GABAA and NK1 receptors in the NTS are also discussed.

Long-lasting bradypnea induced by repeated social defeat.

Repeated social defeat in the rat induces long-lasting cardiovascular changes associated with anxiety. In this study, we investigated the effects of repeated social defeat on breathing. Respiratory rate was extracted from the respiratory sinus arrhythmia (RSA) peak frequency of the ECG in rats subjected to social defeat for 4 consecutive days. Respiratory rate was recorded under anesthesia 6 days (D+10) or 26 days (D+30) after social defeat. At D+10, defeated (D) rats spent less time in the open arms of the elevated plus maze test, had heavier adrenal glands, and displayed bradypnea, unlike nondefeated animals. At D+30, all signs of anxiety had disappeared. However, one-half of the rats still displayed bradypnea (DL rats, for low respiratory rate indicated by a lower RSA frequency), whereas those with higher respiratory rate (DH rats) had recovered. Acute blockade of the dorsomedial hypothalamus (DMH) or nucleus tractus solitarii (NTS) 5-HT3 receptors reversed bradypnea in all D rats at D+10 and in DL rats at D+30. Respiratory rate was also recorded in conscious animals implanted with radiotelemetric ECG probes. DH rats recovered between D+10 and D+18, whereas DL rats remained bradypneic until D+30. In conclusion, social stress induces sustained chronic bradypnea mediated by DMH neurons and NTS 5-HT3 receptors. These changes are associated with an anxiety-like state that persists until D+10, followed by recovery. However, bradypnea may persist in one-half of the population up until D+30, despite apparent recovery of the anxiety-like state.

Respiratory sinus arrhythmia as a surrogate measure of respiratory frequency: validity and robustness to activity in rats.

Recording of breathing frequency is a basic requirement for respiratory physiology. Usual techniques are invasive and constraining. Respiratory sinus arrhythmia (RSA) has recently been demonstrated to be a simple way to obtain respiration frequency at rest. In this study, we investigated whether this correlation is also observed during activity. We first compared RSA to the respiration frequency obtained in anesthetized rats using a pneumotachograph connected to the trachea (TRF). Data analyses using Passing and Bablok regression confirmed the absence of bias and proportional differences. Accordingly, the Bland-Altman plot did not show any significant differences in data sets. In a second experiment, we compared RSA to the respiration frequency obtained in freely moving rats using a subpleurally inserted telemetric catheter (PRF). Comparisons between RSA and PRF revealed no significant difference in determination of respiratory rate with the two methods, although the bias and confidence interval were greater when activity increased. This was, however, not the case during short episodes of sniffing-like tachypnea, during which no matching RSA peaks were observed. In conclusion, RSA frequency reflected regular respiration frequency independently of the level of activity and appears to be a good surrogate to usual techniques.

Involvement of the dorsomedial hypothalamus and the nucleus tractus solitarii in chronic cardiovascular changes associated with anxiety in rats.

Anxiety disorders in humans reduce both the heart rate variability (HRV) and the sensitivity of the cardiac baroreflex (BRS). Both may contribute to sudden death. To elucidate the mechanisms underlying these alterations, male rats were subjected to social defeat sessions on four consecutive days. Five days later, the rats were found to be in an anxiety-like state. At this time point, we analysed HRV and BRS in the defeated rats, with or without treatment with the anxiolytic chlordiazepoxide (CDZ). HRV was reduced after social defeat, due to changes in the autonomic balance favouring the sympathetic over the parasympathetic component. Spontaneous and pharmacological baroreflex gains were also reduced. CDZ abolished anxiety-like symptoms as well as HRV and BRS alterations. Inhibition of the dorsomedial hypothalamus (DMH) with muscimol reversed all cardiovascular alterations, whereas blockade of the nucleus tractus solitarii (NTS) 5-HT3 receptor by the local or systemic administration of granisetron restored only baroreflex gains and the parasympathetic component of HRV. In conclusion, repeated social defeat in the rat lead to an anxiety-like state that was associated with lasting reduction in HRV and baroreflex gains. The DMH and the NTS were responsible for these chronic cardiovascular alterations. These regions may therefore constitute new therapeutic targets for reducing cardiac dysfunction and fibrillation in anxiety disorders.

Noxious stimulation excites serotonergic neurons: a comparison between the lateral paragigantocellular reticular and the raphe magnus nuclei.

The present study was designed to record electrophysiological responses to graded noxious thermal stimuli of serotonergic and nonserotonergic neurons in the lateral paragigantocellular reticular (LPGi) and the raphe magnus (RMg) nuclei in rats. All of the neurons recorded were juxtacellularly filled with neurobiotin and identified with double immunofluorescent labeling for both neurobiotin and serotonin. Under halothane anesthesia (0.75%), noxious thermal stimuli ⩾48°C activated almost all (88%) of the serotonergic neurons located within the LPGi (n=16). The increase in firing was clear (3.4±0.3spike/s: mean of responses above the population median) and sustained during the whole application of strong thermal noxious stimuli, with a high mean threshold (48.3±0.3°C) and large receptive fields. Recording of serotonergic neurons in the RMg (n=21) demonstrated that the proportion of strongly activated (>2spike/s) neurons (19% vs 59% for the LPGi) as well as the magnitude of the activation (2.1±0.4spike/s: mean of responses above the population median) to thermal noxious stimuli were significantly lower than in the LPGi (P<.05). Within the boundaries of both the LPGi and the RMg (B3 group), nonserotonergic neurons were also predominantly excited (75%) by noxious stimuli, and the resulting activation (7.9±1.2spike/s) was even greater than that of serotonergic neurons. Thermal noxious stimuli-induced activation of LPGi serotonergic cells probably plays a key role in serotonin-mediated modulations of cardiac baroreflex and transmission of nociceptive messages occurring under such intense noxious conditions.

Brain circuits mediating baroreflex bradycardia inhibition in rats: an anatomical and functional link between the cuneiform nucleus and the periaqueductal grey.

Defence responses triggered experimentally in rats by stimulation of the dorsomedial nucleus of the hypothalamus (DMH) and the dorsolateral periaqueductal grey matter (PAG) inhibit the cardiac baroreflex response (i.e. bradycardia). It has also been proposed that the midbrain cuneiform nucleus (CnF) is involved in active responses. Our aim was to identify the neurocircuitry involved in defence-induced baroreflex inhibition, with a particular focus on the link between DMH, CnF and dorsolateral PAG. Microinjection of the anterograde tracer Phaseolus vulgaris leucoaggutinin into the CnF revealed a dense projection to the dorsolateral PAG. Moreover, activation of neurons in the CnF induced increased expression of Fos protein in the dorsolateral PAG. Inhibition of neurons of the CnF or dorsolateral PAG prevented the inhibition of baroreflex bradycardia induced by DMH or CnF stimulation, respectively. These results provide a detailed description of the brain circuitry underlying acute baroreflex modulation by neurons of the DMH. Our data have shown for the first time that the CnF plays a key role in defence reaction-associated cardiovascular changes; its stimulation, from the DMH, activates the dorsolateral PAG, which, in turn, inhibits baroreflex bradycardia.

Inhibition of cardiac baroreflex by noxious thermal stimuli: a key role for lateral paragigantocellular serotonergic cells.

The present study was designed to identify the neuronal mechanisms causing cardiac baroreflex inhibition associated with thermal nociception in rats. Under urethane-anesthesia, noxious thermal stimuli > or = 48 degrees C were found to inhibit the cardiac baroreflex, whereas noxious stimuli < or = 46 degrees C had no effect. Using double immunohistochemical labeling, noxious stimuli > or = 48 degrees C were found to evoke primarily a strong expression of Fos protein (Fos) encoded by c-fos gene in serotonergic neurons of lateral paragigantocellular reticular nucleus (LPGi). Noxious stimuli < or = 46 degrees C did not evoke Fos expression in any serotonergic neurons of the brainstem. Local blockade of neuronal activity by bilateral microinjections of fluorescent muscimol (a GABA(A) receptor agonist tagged with a fluorophore that allowed visualization of the injections) into both the LPGi and the raphe magnus nucleus prevented the inhibitory effect of noxious stimuli > or = 48 degrees C on the cardiac baroreflex. Bilateral microinjections of granisetron (a 5-HT(3) antagonist) within the nucleus tractus solitarius also prevented the inhibition of cardiac baroreflex elicited by noxious stimuli > or = 48 degrees C. These results show that activation of serotonergic cells in the LPGi is critical to trigger nucleus tractus solitarius-mediated cardiac baroreflex inhibition elicited by intense thermal noxious stimuli.