Blood Pressure Regulation by the Rostral Ventrolateral Medulla in Conscious Rats: Effects of Hypoxia, Hypercapnia, Baroreceptor Denervation, and Anesthesia

Spinal projecting neurons in rostral ventromedial medulla co-regulate motor and sympathetic tone

Many behaviors require the coordinated actions of somatic and autonomic functions. However, the underlying mechanisms remain elusive. By opto-stimulating different populations of descending spinal projecting neurons (SPNs) in anesthetized mice, we show that stimulation of excitatory SPNs in the rostral ventromedial medulla (rVMM) resulted in a simultaneous increase in somatomotor and sympathetic activities. Conversely, opto-stimulation of rVMM inhibitory SPNs decreased both activities. Anatomically, these SPNs innervate both sympathetic preganglionic neurons and motor-related regions in the spinal cord. Fiber-photometry recording indicated that the activities of rVMM SPNs correlate with different levels of muscle and sympathetic tone during distinct arousal states. Inhibiting rVMM excitatory SPNs reduced basal muscle and sympathetic tone, impairing locomotion initiation and high-speed performance. In contrast, silencing the inhibitory population abolished muscle atonia and sympathetic hypoactivity during rapid eye movement (REM) sleep. Together, these results identify rVMM SPNs as descending spinal projecting pathways controlling the tone of both the somatomotor and sympathetic systems.

[Repeated mild traumatic brain injury in the parietal cortex inhibits expressions of NLG-1 and PSD-95 in the medulla oblongata of mice]

OBJECTIVE

To assess the effects of repeated mild traumatic brain injury (rmTBI) in the parietal cortex on neuronal morphology and synaptic plasticity in the medulla oblongata of mice.

METHODS

Thirty-two male ICR mice were randomly divided into sham operation group (n=8) and rmTBI group (n=24). The mice in the latter group were subjected to repeated mild impact injury of the parietal cortex by a free-falling object. The mice surviving the injuries were evaluated for neurological deficits using neurological severity scores (NSS), righting reflex test and forced swimming test, and pathological changes of the neuronal cells in the medulla oblongata were observed with HE and Nissl staining. Western blotting and immunofluorescence staining were used to detect the expressions of neuroligin 1(NLG-1) and postsynaptic density protein 95(PSD-95) in the medulla oblongata of the mice that either survived rmTBI or not.

RESULTS

None of the mice in the sham-operated group died, while the mortality rate was 41.67% in rmTBI group. The mice surviving rmTBI showed significantly reduced NSS, delayed recovery of righting reflex, increased immobility time in forced swimming test (P < 0.05), and loss of Nissl bodies; swelling and necrosis were observed in a large number of neurons in the medulla oblongata, where the expression levels of NLG-1 and PSD-95 were significantly downregulated (P < 0.05). The mice that did not survive rmTBI showed distorted and swelling nerve fibers and decreased density of neurons in the medulla oblongina with lowered expression levels of NLG-1 and PSD-95 compared with the mice surviving the injuries (P < 0.01).

CONCLUSION

The structural and functional anomalies of the synapses in the medulla oblongata may contribute to death and neurological impairment following rmTBI in mice.

Hindbrain Adrenergic/Noradrenergic Control of Integrated Endocrine and Autonomic Stress Responses

Hindbrain adrenergic/noradrenergic nuclei facilitate endocrine and autonomic responses to physical and psychological challenges. Neurons that synthesize adrenaline and noradrenaline target hypothalamic structures to modulate endocrine responses while descending spinal projections regulate sympathetic function. Furthermore, these neurons respond to diverse stress-related metabolic, autonomic, and psychosocial challenges. Accordingly, adrenergic and noradrenergic nuclei are integrative hubs that promote physiological adaptation to maintain homeostasis. However, the precise mechanisms through which adrenaline- and noradrenaline-synthesizing neurons sense interoceptive and exteroceptive cues to coordinate physiological responses have yet to be fully elucidated. Additionally, the regulatory role of these cells in the context of chronic stress has received limited attention. This mini-review consolidates reports from preclinical rodent studies on the organization and function of brainstem adrenaline and noradrenaline cells to provide a framework for how these nuclei coordinate endocrine and autonomic physiology. This includes identification of hindbrain adrenaline- and noradrenaline-producing cell groups and their role in stress responding through neurosecretory and autonomic engagement. Although temporally and mechanistically distinct, the endocrine and autonomic stress axes are complementary and interconnected. Therefore, the interplay between brainstem adrenergic/noradrenergic nuclei and peripheral physiological systems is necessary for integrated stress responses and organismal survival.

Selective transduction and photoinhibition of pre-Bötzinger complex neurons that project to the facial nucleus in rats affects nasofacial activity

The pre-Bötzinger complex (preBötC), a key primary generator of the inspiratory breathing rhythm, contains neurons that project directly to facial nucleus (7n) motoneurons to coordinate orofacial and nasofacial activity. To further understand the identity of 7n-projecting preBötC neurons, we used a combination of optogenetic viral transgenic approaches to demonstrate that selective photoinhibition of these neurons affects mystacial pad activity, with minimal effects on breathing. These effects are altered by the type of anesthetic employed and also between anesthetized and conscious states. The population of 7n-projecting preBötC neurons we transduced consisted of both excitatory and inhibitory neurons that also send collaterals to multiple brainstem nuclei involved with the regulation of autonomic activity. We show that modulation of subgroups of preBötC neurons, based on their axonal projections, is a useful strategy to improve our understanding of the mechanisms that coordinate and integrate breathing with different motor and physiological behaviors. This is of fundamental importance, given that abnormal respiratory modulation of autonomic activity and orofacial behaviors have been associated with the development and progression of diseases.

Wireless Self-Powered Optogenetic System for Long-Term Cardiac Neuromodulation to Improve Post-MI Cardiac Remodeling and Malignant Arrhythmia

Autonomic imbalance is an important characteristic of patients after myocardial infarction (MI) and adversely contributes to post-MI cardiac remodeling and ventricular arrhythmias (VAs). A previous study proved that optogenetic modulation could precisely inhibit cardiac sympathetic hyperactivity and prevent acute ischemia-induced VAs. Here, a wireless self-powered optogenetic modulation system is introduced, which achieves long-term precise cardiac neuromodulation in ambulatory canines. The wireless self-powered optical system based on a triboelectric nanogenerator is powered by energy harvested from body motion and realized the effective optical illumination that is required for optogenetic neuromodulation (ON). It is further demonstrated that long-term ON significantly mitigates MI-induced sympathetic remodeling and hyperactivity, and improves a variety of clinically relevant outcomes such as improves ventricular dysfunction, reduces infarct size, increases electrophysiological stability, and reduces susceptibility to VAs. These novel insights suggest that wireless ON holds translational potential for the clinical treatment of arrhythmia and other cardiovascular diseases related to sympathetic hyperactivity. Moreover, this innovative self-powered optical system may provide an opportunity to develop implantable/wearable and self-controllable devices for long-term optogenetic therapy.

Renal sympathetic activity: A key modulator of pressure natriuresis in hypertension

Hypertension is a complex disorder ensuing necessarily from alterations in the pressure-natriuresis relationship, the main determinant of long-term control of blood pressure. This mechanism sets natriuresis to the level of blood pressure, so that increasing pressure translates into higher osmotically driven diuresis to reduce volemia and control blood pressure. External factors affecting the renal handling of sodium regulate the pressure-natriuresis relationship so that more or less natriuresis is attained for each level of blood pressure. Hypertension can thus only develop following primary alterations in the pressure to natriuresis balance, or by abnormal activity of the regulation network. On the other hand, increased sympathetic tone is a very frequent finding in most forms of hypertension, long regarded as a key element in the pathophysiological scenario. In this article, we critically analyze the interplay of the renal component of the sympathetic nervous system and the pressure-natriuresis mechanism in the development of hypertension. A special focus is placed on discussing recent findings supporting a role of baroreceptors as a component, along with the afference of reno-renal reflex, of the input to the nucleus tractus solitarius, the central structure governing the long-term regulation of renal sympathetic efferent tone.

Peripheral chemoreflex activation induces expiratory but not inspiratory excitation of C1 pre-sympathetic neurones of rats

AIMS

Stimulation of peripheral chemoreceptors, as during hypoxia, increases breathing and respiratory-related sympathetic bursting. Activation of catecholaminergic C1 neurones induces sympathoexcitation, while its ablation reduces the chemoreflex sympathoexcitatory response. However, no study has determined the respiratory phase(s) in which the pre-sympathetic C1 neurones are recruited by peripheral chemoreceptor and whether C1 neurone activation affects all phases of respiratory modulation of sympathetic activity. We addressed these unknowns by testing the hypothesis that peripheral chemoreceptor activation excites pre-sympathetic C1 neurones during inspiration and expiration.

METHODS

Using the in situ preparation of rat, we made intracellular recordings from baroreceptive pre-sympathetic C1 neurones during peripheral chemoreflex stimulation. We optogenetically activated C1 neurones selectively and compared any respiratory-phase-related increases in sympathetic activity with that which occurs following stimulation of the peripheral chemoreflex.

RESULTS

Activation of peripheral chemoreceptors using cytotoxic hypoxia (potassium cyanide) increased the firing frequency of C1 neurones and both the frequency and amplitude of their excitatory post-synaptic currents during the phase of expiration only. In contrast, optogenetic stimulation of C1 neurones activates inspiratory neurones, which secondarily inhibit expiratory neurones, but produced comparable increases in sympathetic activity across all phases of respiration.

CONCLUSION

Our data reveal that the peripheral chemoreceptor-mediated expiratory-related sympathoexcitation is mediated through excitation of expiratory neurones antecedent to C1 pre-sympathetic neurones; these may be found in the Kölliker-Fuse nucleus. Despite peripheral chemoreceptor excitation of inspiratory neurones, these do not trigger C1 neurone-mediated increases in sympathetic activity. These studies provide compelling novel insights into the functional organization of respiratory-sympathetic neural networks.

The Effects of Acidosis on eNOS in the Systemic Vasculature: A Focus on Early Postnatal Ontogenesis

The activity of many vasomotor signaling pathways strongly depends on extracellular/intracellular pH. Nitric oxide (NO) is one of the most important vasodilators produced by the endothelium. In this review, we present evidence that in most vascular beds of mature mammalian organisms metabolic or respiratory acidosis increases functional endothelial NO-synthase (eNOS) activity, despite the observation that direct effects of low pH on eNOS enzymatic activity are inhibitory. This can be explained by the fact that acidosis increases the activity of signaling pathways that positively regulate eNOS activity. The role of NO in the regulation of vascular tone is greater in early postnatal ontogenesis compared to adulthood. Importantly, in early postnatal ontogenesis acidosis also augments functional eNOS activity and its contribution to the regulation of arterial contractility. Therefore, the effect of acidosis on total peripheral resistance in neonates may be stronger than in adults and can be one of the reasons for an undesirable decrease in blood pressure during neonatal asphyxia. The latter, however, should be proven in future studies.

Adrenergic C1 neurons monitor arterial blood pressure and determine the sympathetic response to hemorrhage

Hemorrhage initially triggers a rise in sympathetic nerve activity (SNA) that maintains blood pressure (BP); however, SNA is suppressed following severe blood loss causing hypotension. We hypothesized that adrenergic C1 neurons in the rostral ventrolateral medulla (C1^RVLM) drive the increase in SNA during compensated hemorrhage, and a reduction in C1^RVLM contributes to hypotension during decompensated hemorrhage. Using fiber photometry, we demonstrate that C1^RVLM activity increases during compensated hemorrhage and falls at the onset of decompensated hemorrhage. Using optogenetics combined with direct recordings of SNA, we show that C1^RVLM activation mediates the rise in SNA and contributes to BP stability during compensated hemorrhage, whereas a suppression of C1^RVLM activity is associated with cardiovascular collapse during decompensated hemorrhage. Notably, re-activating C1^RVLM during decompensated hemorrhage restores BP to normal levels. In conclusion, C1 neurons are a nodal point for the sympathetic response to blood loss.

Putative Roles of Astrocytes in General Anesthesia

General anesthetics are a mainstay of modern medicine, and although much progress has been made towards identifying molecular targets of anesthetics and neural networks contributing to endpoints of general anesthesia, our understanding of how anesthetics work remains unclear. Reducing this knowledge gap is of fundamental importance to prevent unwanted and life-threatening side-effects associated with general anesthesia. General anesthetics are chemically diverse, yet they all have similar behavioral endpoints, and so for decades, research has sought to identify a single underlying mechanism to explain how anesthetics work. However, this effort has given way to the 'multiple target hypothesis' as it has become clear that anesthetics target many cellular proteins, including GABAA receptors, glutamate receptors, voltage-independent K+ channels, and voltagedependent K+, Ca2+ and Na+ channels, to name a few. Yet, despite evidence that astrocytes are capable of modulating multiple aspects of neural function and express many anesthetic target proteins, they have been largely ignored as potential targets of anesthesia. The purpose of this brief review is to highlight the effects of anesthetic on astrocyte processes and identify potential roles of astrocytes in behavioral endpoints of anesthesia (hypnosis, amnesia, analgesia, and immobilization).

Adaptive cardiorespiratory changes to chronic continuous and intermittent hypoxia

This chapter reviews cardiorespiratory adaptations to chronic hypoxia (CH) experienced at high altitude and cardiorespiratory pathologies elicited by chronic intermittent hypoxia (CIH) occurring with obstructive sleep apnea (OSA). Short-term CH increases breathing (ventilatory acclimatization to hypoxia) and blood pressure (BP) through carotid body (CB) chemo reflex. Hyperplasia of glomus cells, alterations in ion channels, and recruitment of additional excitatory molecules are implicated in the heightened CB chemo reflex by CH. Transcriptional activation of hypoxia-inducible factors (HIF-1 and 2) is a major molecular mechanism underlying respiratory adaptations to short-term CH. High-altitude natives experiencing long-term CH exhibit blunted hypoxic ventilatory response (HVR) and reduced BP due to desensitization of CB response to hypoxia and impaired processing of CB sensory information at the central nervous system. Ventilatory changes evoked by long-term CH are not readily reversed after return to sea level. OSA patients and rodents subjected to CIH exhibit heightened CB chemo reflex, increased hypoxic ventilatory response, and hypertension. Increased generation of reactive oxygen species (ROS) is a major cellular mechanism underlying CIH-induced enhanced CB chemo reflex and the ensuing cardiorespiratory pathologies. ROS generation by CIH is mediated by nontranscriptional, disrupted HIF-1 and HIF-2-dependent transcriptions as well as epigenetic mechanisms.

Rostral ventrolateral medulla, retropontine region and autonomic regulations

The rostral half of the ventrolateral medulla (RVLM) and adjacent ventrolateral retropontine region (henceforth RVLMRP) have been divided into various sectors by neuroscientists interested in breathing or autonomic regulations. The RVLMRP regulates respiration, glycemia, vigilance and inflammation, in addition to blood pressure. It contains interoceptors that respond to acidification, hypoxia and intracranial pressure and its rostral end contains the retrotrapezoid nucleus (RTN) which is the main central respiratory chemoreceptor. Acid detection by the RTN is an intrinsic property of the principal neurons that is enhanced by paracrine influences from surrounding astrocytes and CO₂-dependent vascular constriction. RTN mediates the hypercapnic ventilatory response via complex projections to the respiratory pattern generator (CPG). The RVLM contributes to autonomic response patterns via differential recruitment of several subtypes of adrenergic (C1) and non-adrenergic neurons that directly innervate sympathetic and parasympathetic preganglionic neurons. The RVLM also innervates many brainstem and hypothalamic nuclei that contribute, albeit less directly, to autonomic responses. All lower brainstem noradrenergic clusters including the locus coeruleus are among these targets. Sympathetic tone to the circulatory system is regulated by subsets of presympathetic RVLM neurons whose activity is continuously restrained by the baroreceptors and modulated by the respiratory CPG. The inhibitory input from baroreceptors and the excitatory input from the respiratory CPG originate from neurons located in or close to the rhythm generating region of the respiratory CPG (preBötzinger complex).

Monoaminergic Receptors as Modulators of the Perivascular Sympathetic and Sensory CGRPergic Outflows

Blood pressure is a highly controlled cardiovascular parameter that normally guarantees an adequate blood supply to all body tissues. This parameter is mainly regulated by peripheral vascular resistance and is maintained by local mediators (i.e., autacoids), and by the nervous and endocrine systems. Regarding the nervous system, blood pressure can be modulated at the central level by regulating the autonomic output. However, at peripheral level, there exists a modulation by activation of prejunctional monoaminergic receptors in autonomic- or sensory-perivascular fibers. These modulatory mechanisms on resistance blood vessels exert an effect on the release of neuroactive substances from the autonomic or sensory fibers that modify blood pressure. Certainly, resistance blood vessels are innervated by perivascular: (i) autonomic sympathetic fibers (producing vasoconstriction mainly by noradrenaline release); and (ii) peptidergic sensory fibers [producing vasodilatation mainly by calcitonin gene-related peptide (CGRP) release]. In the last years, by using pithed rats, several monoaminergic mechanisms for controlling both the sympathetic and sensory perivascular outflows have been elucidated. Additionally, several studies have shown the functions of many monoaminergic auto-receptors and hetero-receptors expressed on perivascular fibers that modulate neurotransmitter release. On this basis, the present review: (i) summarizes the modulation of the peripheral vascular tone by adrenergic, serotoninergic, dopaminergic, and histaminergic receptors on perivascular autonomic (sympathetic) and sensory fibers, and (ii) highlights that these monoaminergic receptors are potential therapeutic targets for the development of novel medications to treat cardiovascular diseases (with some of them explored in clinical trials or already in clinical use).

Loss of putative GABAergic neurons in the ventrolateral medulla in multiple system atrophy

STUDY OBJECTIVES

Multiple system atrophy (MSA) is associated with disturbances in cardiovascular, sleep and respiratory control. The lateral paragigantocellular nucleus (LPGi) in the ventrolateral medulla (VLM) contains GABAergic neurons that participate in control of rapid eye movement (REM) sleep and cardiovagal responses. We sought to determine whether there was loss of putative GABAergic neurons in the LPGi and adjacent regions in MSA.

METHODS

Sections of the medulla were processed for GAD65/67 immunoreactivity in eight subjects with clinical and neuropathological diagnosis of MSA and in six control subjects. These putative GABAergic LPGi neurons were mapped based on their relationship to adjacent monoaminergic VLM groups.

RESULTS

There were markedly decreased numbers of GAD-immunoreactive neurons in the LPGi and adjacent VLM regions in MSA.

CONCLUSIONS

There is loss of GABAergic neurons in the VLM, including the LPGi in patients with MSA. Whereas these findings provide a possible mechanistic substrate, given the few cases included, further studies are necessary to determine whether they contribute to REM sleep-related cardiovagal and possibly respiratory dysregulation in MSA.

Medullary and Hypothalamic Functional Magnetic Imaging During Acute Hypoxia in Tracing Human Peripheral Chemoreflex Responses

[Figure: see text].

Neuroplasticity in N-methyl-d-aspartic acid receptor signaling in subregions of the rat rostral ventrolateral medulla following sedentary versus physically active conditions

The rostral ventrolateral medulla (RVLM) is a brain region involved in normal regulation of the cardiovascular system and heightened sympathoexcitatory states of cardiovascular disease (CVD). Among major risk factors for CVD, sedentary lifestyles contribute to higher mortality than other modifiable risk factors. Previous studies suggest excessive glutamatergic excitation of presympathetic neurons in the RVLM occurs in sedentary animals. Therefore, the purpose of this study was to examine neuroplasticity in the glutamatergic system in the RVLM of sedentary and physically active rats. We hypothesized that relative to active rats, sedentary rats would exhibit higher expression of glutamate N-methyl-d-aspartic acid receptor subunits (GluN), phosphoGluN1, and the excitatory scaffold protein postsynaptic density 95 (PSD95), while achieving higher glutamate levels. Male Sprague-Dawley rats (4 weeks old) were divided into sedentary and active (running wheel) conditions for 10-12 weeks. We used retrograde tracing/triple-labeling techniques, western blotting, and magnetic resonance spectroscopy. We report in sedentary versus physically active rats: 1) fewer bulbospinal non-C1 neurons positive for GluN1, 2) significantly higher expression of GluN1 and GluN2B but lower levels of phosphoGluN1 (pSer896) and PSD95, and 3) higher levels of glutamate in the RVLM. Higher GluN expression is consistent with enhanced sympathoexcitation in sedentary animals; however, a more complex neuroplasticity occurs within subregions of the ventrolateral medulla. Our results in rodents may also indicate that alterations in glutamatergic excitation of the RVLM contribute to the increased incidence of CVD in humans who lead sedentary lifestyles. Thus, there is a strong need to further pursue mechanisms of inactivity-related neuroplasticity in the RVLM.

Depletion of C1 neurons attenuates the salt-induced hypertension in unanesthetized rats

High salt intake is able to evoke neuroendocrine and autonomic responses that include vasopressin release and sympathoexcitation resulting in increasing in the arterial blood pressure (BP). The C1 neurons are a specific population of catecholaminergic neurons located in the RVLM region and they control BP under homeostatic imbalance. Thus, here we hypothesized that the ablation of C1 neurons mitigate the high blood pressure induced by high-salt intake. To test this hypothesis, we injected anti-DβH-SAP saporin at the RVLM and monitored the BP in unanesthetized animals exposed to high salt intake of 2% NaCl solution for 7 days. The injection of anti-DβH-SAP into the RVLM depleted 80% of tyrosine hydroxylase-positive neurons (TH⁺ neurons) in the C1, 38% in the A5, and no significant reduction in the A1 region, when compared to control group (saline as vehicle). High salt intake elicited a significant increase in BP in the control group, while in the anti-DβH-SAP group the depletion of TH⁺ neurons prevents the salt-induced hypertension. Moreover, the low frequency component of systolic BP and pulse interval were increased by high-salt intake in control animals but not in anti-DβH-SAP group, which indirectly suggests that the increase in the BP is mediated by increase in sympathetic activity. In conclusion, our data show that hypertension induced by high-salt intake is dependent on C1 neurons.

Off-Peak 594-nm Light Surpasses On-Peak 532-nm Light in Silencing Distant ArchT-Expressing Neurons In Vivo

For large brain volume manipulations using optogenetics, both effective opsin excitation and efficient light delivery with minimal light absorption are required to minimize the illuminating light intensity and concomitant off-target effects. ArchT, a widely used potent inhibitory opsin, is commonly activated by 532-nm light, which lies on its in vitro excitation peak. However, 532-nm light also lies on a peak range of the hemoglobin absorption spectrum. Therefore, we predicted that 594-nm light is superior in suppressing distant ArchT-expressing neurons, which is slightly off the ArchT-excitation-plateau and largely off the peak of the hemoglobin absorption spectrum. We quantitatively tested this prediction by the electrophysiological recording of the rat cortex in vivo. At illumination distances greater than 500 μm, 594-nm light was more effective than 532-nm light. Its superiority increased with distance. These results validate our prediction and highlight the significance of excitation-absorption trade-off in selecting illumination wavelength for optogenetics in vivo.

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Wavelength-dependency of optogenetic neuronal control was directly measured in vivo

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Off-peak light silence 1-mm-distant ArchT-neuron twice more than on-peak light in vivo

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Superiority of off-peak light at distance arose from its less absorption of light

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Simulation of light propagation supported unexpectedly large effect of hemoglobin

Neuronal Networks in Hypertension: Recent Advances

Neurogenic hypertension is associated with excessive sympathetic nerve activity to the kidneys and portions of the cardiovascular system. Here we examine the brain regions that cause heightened sympathetic nerve activity in animal models of neurogenic hypertension, and we discuss the triggers responsible for the changes in neuronal activity within these regions. We highlight the limitations of the evidence and, whenever possible, we briefly address the pertinence of the findings to human hypertension. The arterial baroreflex reduces arterial blood pressure variability and contributes to the arterial blood pressure set point. This set point can also be elevated by a newly described cerebral blood flow-dependent and astrocyte-mediated sympathetic reflex. Both reflexes converge on the presympathetic neurons of the rostral medulla oblongata, and both are plausible causes of neurogenic hypertension. Sensory afferent dysfunction (reduced baroreceptor activity, increased renal, or carotid body afferent) contributes to many forms of neurogenic hypertension. Neurogenic hypertension can also result from activation of brain nuclei or sensory afferents by excess circulating hormones (leptin, insulin, Ang II [angiotensin II]) or sodium. Leptin raises blood vessel sympathetic nerve activity by activating the carotid bodies and subsets of arcuate neurons. Ang II works in the lamina terminalis and probably throughout the brain stem and hypothalamus. Sodium is sensed primarily in the lamina terminalis. Regardless of its cause, the excess sympathetic nerve activity is mediated to some extent by activation of presympathetic neurons located in the rostral ventrolateral medulla or the paraventricular nucleus of the hypothalamus. Increased activity of the orexinergic neurons also contributes to hypertension in selected models.

Baroreceptor denervation reduces inflammatory status but worsens cardiovascular collapse during systemic inflammation

Beyond the regulation of cardiovascular function, baroreceptor afferents play polymodal roles in health and disease. Sepsis is a life-threatening condition characterized by systemic inflammation (SI) and hemodynamic dysfunction. We hypothesized that baroreceptor denervation worsens lipopolysaccharide (LPS) induced-hemodynamic collapse and SI in conscious rats. We combined: (a) hemodynamic and thermoregulatory recordings after LPS administration at a septic-like non-lethal dose (b) analysis of the cardiovascular complexity, (c) evaluation of vascular function in mesenteric resistance vessels, and (d) measurements of inflammatory cytokines (plasma and spleen). LPS-induced drop in blood pressure was higher in sino-aortic denervated (SAD) rats. LPS-induced hemodynamic collapse was associated with SAD-dependent autonomic disbalance. LPS-induced vascular dysfunction was not affected by SAD. Surprisingly, SAD blunted LPS-induced surges of plasma and spleen cytokines. These data indicate that baroreceptor afferents are key to alleviate LPS-induced hemodynamic collapse, affecting the autonomic control of cardiovascular function, without affecting resistance blood vessels. Moreover, baroreflex modulation of the LPS-induced SI and hemodynamic collapse are not dependent of each other given that baroreceptor denervation worsened hypotension and reduced SI.

Identifying Increases in Activity of the Human RVLM Through MSNA-Coupled fMRI

AIM

We initially developed concurrent recording of muscle sympathetic nerve activity (MSNA) and functional magnetic resonance imaging (fMRI) of the brain to functionally identify the human homolog of the rostral ventrolateral medulla (RVLM). Here we summarize the cortical and subcortical connections to the RVLM, as identified using MSNA-coupled fMRI.

METHODS

MSNA was recorded via tungsten microelectrodes inserted into the peroneal nerve. Gradient echo, echo-planar fMRI was performed at 3T (Philips Achieva). 200 volumes (46 axial slices (TR = 8 s, TE = 4 s, flip angle = 90°, raw voxel size = 1.5 × 1.5 × 2.75 mm) were collected in a 4 s-ON, 4 s-OFF sparse sampling protocol and MSNA measured in each 1 s epoch in the 4-s period between scans. Blood oxygen level dependent (BOLD) signal intensity was measured in the corresponding 1 s epoch 4 s later to account for peripheral neural conduction and central neurovascular coupling delays.

RESULTS

BOLD signal intensity was positively related to bursts of MSNA in the RVLM, dorsomedial hypothalamus (DMH), ventromedial hypothalamus (VMH), insula, dorsolateral prefrontal cortex (dlPFC), posterior cingulate cortex (PCC), and precuneus, and negatively related in the caudal ventrolateral medulla (CVLM), nucleus tractus solitarius (NTS), and the midbrain periaqueductal gray (PAG). During physiological increases in MSNA (tonic muscle pain), MSNA-coupled BOLD signal intensity was greater in RVLM, NTS, PAG, DMH, dlPFC, medial prefrontal cortex (mPFC), precuneus, and anterior cingulate cortex (ACC) than at rest. During pathophysiological increases in MSNA [obstructive sleep apnoea (OSA)] signal intensity was also higher in dlPFC, mPFC, ACC, and precuneus than in controls. Conversely, signal intensity was lower in RVLM in OSA than in controls, which we interpret as reflecting a withdrawal of active inhibition of the RVLM.

CONCLUSION

These results suggest that multiple cortical and subcortical areas are functionally coupled to the RVLM, which in turn is functionally coupled to the generation of spontaneous bursts of MSNA and their augmentation during physiological and pathophysiological increase in vasoconstrictor drive.

Hypergravity-induced plastic alteration of the vestibulo-sympathetic reflex involves decrease in responsiveness of CAMK2-expressing neurons in the vestibular nuclear complex

The vestibular system contributes to not only eye movement and posture but also the sympathetic response. Plastic alteration of the vestibulo-sympathetic reflex is induced by hypergravity load; however, the mechanism remains unknown. Here, we examined 2 g-induced changing in responsiveness of CAMK2-expressing neurons in the vestibular nucleus complex using optogenetic tools. The excitatory photostimulation of the CAMK2-expressing neurons in the unilateral vestibular nuclear complex induced body tilt to the contralateral side, while inhibitory photostimulation showed the opposite response. Photoactivation of either cell body or the axonal terminal in the rostral ventrolateral medulla showed sympathoexcitation followed by the pressor response. Furthermore, this response was significantly attenuated (49.8 ± 4%) after the 1st day of 2 g loading, and this value was further reduced by the 5th day (22.4 ± 3%), suggesting that 2 g-induced attenuation of the vestibulo-sympathetic reflex involves at least decrease in responsiveness of CAMK2-expressing neurons in the vestibular nuclear complex.

2019 Ludwig Lecture: Rhythms in sympathetic nerve activity are a key to understanding neural control of the cardiovascular system

This review is based on the Carl Ludwig Distinguished Lecture, presented at the 2019 Experimental Biology Meeting in Orlando, FL, and provides a snapshot of >40 years of work done in collaboration with the late Gerard L. Gebber and colleagues to highlight the importance of considering the rhythmic properties of sympathetic nerve activity (SNA) and brain stem neurons when studying the neural control of autonomic regulation. After first providing some basic information about rhythms, I describe the patterns and potential functions of rhythmic activity recorded from sympathetic nerves under various physiological conditions. I review the evidence that these rhythms reflect the properties of central sympathetic neural networks that include neurons in the caudal medullary raphe, caudal ventrolateral medulla, caudal ventrolateral pons, medullary lateral tegmental field, rostral dorsolateral pons, and rostral ventrolateral medulla. The role of these brain stem areas in mediating steady-state and reflex-induced changes in SNA and blood pressure is discussed. Despite the common appearance of rhythms in SNA, these oscillatory characteristics are often ignored; instead, it is common to simply quantify changes in the amount of SNA to make conclusions about the function of the sympathetic nervous system in mediating responses to a variety of stimuli. This review summarizes work that highlights the need to include an assessment of the changes in the frequency components of SNA in evaluating the cardiovascular responses to various manipulations as well as in determining the role of different brain regions in the neural control of the cardiovascular system.

Breathing regulation and blood gas homeostasis after near complete lesions of the retrotrapezoid nucleus in adult rats

KEY POINTS

The retrotrapezoid nucleus (RTN) drives breathing proportionally to brain PCO2 but its role during various states of vigilance needs clarification. Under normoxia, RTN lesions increased the arterial PCO2 set-point, lowered the PO2 set-point and reduced alveolar ventilation relative to CO₂ production. Tidal volume was reduced and breathing frequency increased to a comparable degree during wake, slow-wave sleep and REM sleep. RTN lesions did not produce apnoeas or disordered breathing during sleep. RTN lesions in rats virtually eliminated the central respiratory chemoreflex (CRC) while preserving the cardiorespiratory responses to hypoxia; the relationship between CRC and number of surviving RTN Nmb neurons was an inverse exponential. The CRC does not function without the RTN. In the quasi-complete absence of the RTN and CRC, alveolar ventilation is reduced despite an increased drive to breathe from the carotid bodies.

ABSTRACT

The retrotrapezoid nucleus (RTN) is one of several CNS nuclei that contribute, in various capacities (e.g. CO₂ detection, neuronal modulation) to the central respiratory chemoreflex (CRC). Here we test how important the RTN is to PCO₂ homeostasis and breathing during sleep or wake. RTN Nmb-positive neurons were killed with targeted microinjections of substance P-saporin conjugate in adult rats. Under normoxia, rats with large RTN lesions (92 ± 4% cell loss) had normal blood pressure and arterial pH but were hypoxic (-8 mmHg PaO₂ ) and hypercapnic (+10 mmHg ). In resting conditions, minute volume (V_E ) was normal but breathing frequency (f_R ) was elevated and tidal volume (V_T ) reduced. Resting O₂ consumption and CO₂ production were normal. The hypercapnic ventilatory reflex in 65% FiO₂ had an inverse exponential relationship with the number of surviving RTN neurons and was decreased by up to 92%. The hypoxic ventilatory reflex (HVR; FiO₂ 21-10%) persisted after RTN lesions, hypoxia-induced sighing was normal and hypoxia-induced hypotension was reduced. In rats with RTN lesions, breathing was lowest during slow-wave sleep, especially under hyperoxia, but apnoeas and sleep-disordered breathing were not observed. In conclusion, near complete RTN destruction in rats virtually eliminates the CRC but the HVR persists and sighing and the state dependence of breathing are unchanged. Under normoxia, RTN lesions cause no change in V_E but alveolar ventilation is reduced by at least 21%, probably because of increased physiological dead volume. RTN lesions do not cause sleep apnoea during slow-wave sleep, even under hyperoxia.

Neuroimmune interactions and kidney disease

The autonomic nervous system plays critical roles in maintaining homeostasis in humans, directly regulating inflammation by altering the activity of the immune system. The cholinergic anti-inflammatory pathway is a well-studied neuroimmune interaction involving the vagus nerve. CD4-positive T cells expressing β2 adrenergic receptors and macrophages expressing the alpha 7 subunit of the nicotinic acetylcholine receptor in the spleen receive neurotransmitters such as norepinephrine and acetylcholine and are key mediators of the cholinergic anti-inflammatory pathway. Recent studies have demonstrated that vagus nerve stimulation, ultrasound, and restraint stress elicit protective effects against renal ischemia-reperfusion injury. These protective effects are induced primarily via activation of the cholinergic anti-inflammatory pathway. In addition to these immunological roles, nervous systems are directly related to homeostasis of renal physiology. Whole-kidney three-dimensional visualization using the tissue clearing technique CUBIC (clear, unobstructed brain/body imaging cocktails and computational analysis) has illustrated that renal sympathetic nerves are primarily distributed around arteries in the kidneys and denervated after ischemia-reperfusion injury. In contrast, artificial renal sympathetic denervation has a protective effect against kidney disease progression in murine models. Further studies are needed to elucidate how neural networks are involved in progression of kidney disease.

Cognitive impairment in heart failure is associated with altered Wnt signaling in the hippocampus

Age represents the highest risk factor for death due to cardiovascular disease. Heart failure (HF) is the most common cardiovascular disease in elder population and it is associated with cognitive impairment (CI), diminishing learning and memory process affecting life quality and mortality in these patients. In HF, CI has been associated with inadequate O₂ supply to the brain; however, an important subset of HF patients displays CI with almost no alteration in cerebral blood flow. Importantly, nothing is known about the pathophysiological mechanisms underpinning CI in HF with no change in brain tissue perfusion. Here, we aimed to study memory performance and learning function in a rodent model of HF that shows no change in blood flow going to the brain. We found that HF rats presented learning impairments and memory loss. In addition, HF rats displayed a decreased level of Wnt/β-catenin signaling downstream elements in the hippocampus, one pathway implicated largely in aging diseases. Taken together, our results suggest that in HF rats CI is associated with dysfunction of the Wnt/β-catenin signaling pathway. The mechanisms involved in the alterations of Wnt/β-catenin signaling in HF and its contribution to the development/maintenance of CI deserves future investigations.

Neuroinflammation in the NTS is associated with changes in cardiovascular reflexes during systemic inflammation

BACKGROUND

Lipopolysaccharide (LPS)-induced systemic inflammation (SI) is associated with neuroinflammation in the brain, hypotension, tachycardia, and multiple organs dysfunctions. Considering that during SI these important cardiovascular and inflammatory changes take place, we measured the sensitivity of the cardiovascular reflexes baroreflex, chemoreflex, and Bezold-Jarisch that are key regulators of hemodynamic function. We also evaluated neuroinflammation in the nucleus tractus solitarius (NTS), the first synaptic station that integrates peripheral signals arising from the cardiovascular and inflammatory status.

METHODS

We combined cardiovascular recordings, immunofluorescence, and assays of inflammatory markers in male Wistar rats that receive iv administration of LPS (1.5 or 2.5 mg kg-1) to investigate putative interactions of the neuroinflammation in the NTS and in the anteroventral preoptic region of the hypothalamus (AVPO) with the short-term regulation of blood pressure and heart rate.

RESULTS

LPS induced hypotension, tachycardia, autonomic disbalance, hypothermia followed by fever, and reduction in spontaneous baroreflex gain. On the other hand, during SI, the bradycardic component of Bezold-Jarisch and chemoreflex activation was increased. These changes were associated with a higher number of activated microglia and interleukin (IL)-1β levels in the NTS.

CONCLUSIONS

The present data are consistent with the notion that during SI and neuroinflammation in the NTS, rats have a reduced baroreflex gain, combined with an enhancement of the bradycardic component of Bezold-Jarisch and chemoreflex despite the important cardiovascular impairments (hypotension and tachycardia). These changes in the cardiac component of Bezold-Jarisch and chemoreflex may be beneficial during SI and indicate that the improvement of theses reflexes responsiveness though specific nerve stimulations may be useful in the management of sepsis.

Bioelectronic Approaches to Control Neuroimmune Interactions in Acute Kidney Injury

Recent studies have shown renal protective effects of bioelectric approaches, including ultrasound treatment, electrical vagus nerve stimulation, and optogenetic brainstem C1 neuron stimulation. The renal protection acquired by all three modalities was lost in splenectomized mice and/or α7 subunit of the nicotinic acetylcholine receptor-deficient mice. C1 neuron-mediated renal protection was blocked by β2-adrenergic receptor antagonist. These findings indicate that all three methods commonly, at least partially, activate the cholinergic anti-inflammatory pathway, a well-studied neuroimmune pathway. In this article, we summarize the current understanding of neuroimmune axis-mediated kidney protection in preclinical models of acute kidney injury by these three modalities. Examination of the differences among these three modalities might lead to a further elucidation of the neuroimmune axis involved in renal protection and is of interest for developing new therapeutic approaches.

Cholinergic neurons in the pedunculopontine tegmental nucleus modulate breathing in rats by direct projections to the retrotrapezoid nucleus

KEY POINTS

Cholinergic projections from the pedunculopontine tegmental nucleus (PPTg) to the retrotrapezoid nucleus (RTN) are considered to be important for sleep-wake state-dependent control of breathing. The RTN also receives cholinergic input from the postinspiratory complex. Stimulation of the PPTg increases respiratory output under control conditions but not when muscarinic receptors in the RTN are blocked. The data obtained in the present study support the possibility that arousal-dependent modulation of breathing involves recruitment of cholinergic projections from the PPTg to the RTN.

ABSTRACT

The pedunculopontine tegmental nucleus (PPTg) in the mesopontine region has important physiological functions, including breathing control. The PPTg contains a variety of cell types, including cholinergic neurons that project to the rostral aspect of the ventrolateral medulla. In addition, cholinergic signalling in the retrotrapezoid nucleus (RTN), a region that contains neurons that regulate breathing in response to changes in CO₂ /H⁺ , has been shown to activate chemosensitive neurons and increase inspiratory activity. The present study aimed to identify the source of cholinergic input to the RTN and determine whether cholinergic signalling in this region influences baseline breathing or the ventilatory response to CO₂ in conscious male Wistar rats. Retrograde tracer Fluoro-Gold injected into the RTN labelled a subset of cholinergic PPTg neurons that presumably project directly to the chemosensitive region of the RTN. In unrestrained awake rats, unilateral injection of the glutamate (10 mm/100 nL) in the PPTg decreased tidal volume (V_T ) but otherwise increased respiratory rate (f_R ) and net respiratory output as indicated by an increase in ventilation (V_E ). All respiratory responses elicited by PPTg stimulation were blunted by prior injection of methyl-atropine (5 mm/50-75 nL) into the RTN. These results show that stimulation of the PPTg can increase respiratory activity in part by cholinergic activation of chemosensitive elements of the RTN. Based on previous evidence that cholinergic PPTg projections may simultaneously activate expiratory output from the pFRG, we speculate that cholinergic signalling at the level of RTN region could also be involved in breathing regulation.

PIEZOs mediate neuronal sensing of blood pressure and the baroreceptor reflex

Activation of stretch-sensitive baroreceptor neurons exerts acute control over heart rate and blood pressure. Although this homeostatic baroreflex has been described for more than 80 years, the molecular identity of baroreceptor mechanosensitivity remains unknown. We discovered that mechanically activated ion channels PIEZO1 and PIEZO2 are together required for baroreception. Genetic ablation of both Piezo1 and Piezo2 in the nodose and petrosal sensory ganglia of mice abolished drug-induced baroreflex and aortic depressor nerve activity. Awake, behaving animals that lack Piezos had labile hypertension and increased blood pressure variability, consistent with phenotypes in baroreceptor-denervated animals and humans with baroreflex failure. Optogenetic activation of Piezo2-positive sensory afferents was sufficient to initiate baroreflex in mice. These findings suggest that PIEZO1 and PIEZO2 are the long-sought baroreceptor mechanosensors critical for acute blood pressure control.

Next-Generation Tools to Study Autonomic Regulation In Vivo

The recent development of tools to decipher the intricacies of neural networks has improved our understanding of brain function. Optogenetics allows one to assess the direct outcome of activating a genetically-distinct population of neurons. Neurons are tagged with light-sensitive channels followed by photo-activation with an appropriate wavelength of light to functionally activate or silence them, resulting in quantifiable changes in the periphery. Capturing and manipulating activated neuron ensembles, is a recently-designed technique to permanently label activated neurons responsible for a physiological function and manipulate them. On the other hand, neurons can be transfected with genetically-encoded Ca²⁺ indicators to capture the interplay between them that modulates autonomic end-points or somatic behavior. These techniques work with millisecond temporal precision. In addition, neurons can be manipulated chronically to simulate physiological aberrations by transfecting designer G-protein-coupled receptors exclusively activated by designer drugs. In this review, we elaborate on the fundamental concepts and applications of these techniques in research.

Sleep disordered breathing in children disrupts the maturation of autonomic control of heart rate and its association with cerebral oxygenation

KEY POINTS

Sleep disordered breathing (SDB) affects 4-11% of children and is associated with adverse neurocognitive, behavioural and cardiovascular outcomes, including reduced autonomic control. The relationship between heart rate variability (HRV; a measure of autonomic control) and age found in non-snoring control children was absent during sleep in children with SDB. Age significantly predicted increasing cerebral oxygenation during wake in non-snoring control children, whereas during sleep, HRV significantly predicted decreasing cerebral oxygenation. Cerebral oxygenation was not associated with either age or HRV in children with SDB during both wake and sleep. SDB significantly disrupts the normal maturation of autonomic control and the positive association between autonomic control and cerebral oxygenation found in non-snoring children, and we speculate that the dampened autonomic control exhibited by children with SDB may have an attenuating effect on cerebral autoregulation via the moderating influence of HRV on cerebral blood flow.

ABSTRACT

The repetitive episodes of hypoxia that are features of sleep disordered breathing (SDB) in children are associated with alterations in autonomic control of heart rate in an age-dependent manner. We aimed to relate heart rate variability (HRV) parameters to age and measures of cerebral oxygenation in children (3-12 years old) with SDB and non-snoring controls. Children (SDB, n = 117; controls, n = 42; 3-12 years) underwent overnight polysomnography. Total (TP), low- (LF) and high-frequency (HF) power, tissue oxygenation index (TOI) and fractional tissue oxygen extraction (FTOE) were analysed during wake and sleep. Pearson's correlations determined the association between age and HRV parameters, and multiple linear regressions between HRV, age and cerebral oxygenation parameters. During wake, age had a positive association with LF power, reflecting increased parasympathetic and sympathetic activity with increasing age for both control and SDB groups. This association was also evident during sleep in controls, but was absent in children with SDB. In controls, during wake TOI had a positive, and FTOE a negative association with age. During sleep, TP, LF and HF power were significant, negative determinants of TOI and positive determinants of FTOE. These associations were not seen in children with SDB during wake or sleep. SDB disrupts the normal maturation of the autonomic control of heart rate and the association between HRV and cerebral oxygenation exhibited by non-snoring control children of primary school age. These results highlight the impact SDB has on cardiovascular control and the potential impact on adverse cardiovascular outcomes.

Firing properties of ventral medullary respiratory neurons in sino-aortic denervated rats

NEW FINDINGS

What is the central question of this study? After sino-aortic denervation (SAD), rats present normal levels of mean arterial pressure (MAP), high MAP variability and changes in breathing. However, mechanisms involved in SAD-induced respiratory changes and their impact on the modulation of sympathetic activity remain unclear. Herein, we characterized the firing frequency of medullary respiratory neurons after SAD. What is the main finding and its importance? Sino-aortic denervation-induced prolonged inspiration was associated with a reduced interburst frequency of pre-inspiratory/inspiratory neurons and an increased long-term variability of late inspiratory neurons, but no changes were observed in the ramp-inspiratory and post-inspiratory neurons. This imbalance in the respiratory network might contribute to the modulation of sympathetic activity after SAD.

ABSTRACT

In previous studies, we documented that after sino-aortic denervation (SAD) in rats there are significant changes in the breathing pattern, but no significant changes in sympathetic activity and mean arterial pressure compared with sham-operated rats. However, the neural mechanisms involved in the respiratory changes after SAD and the extent to which they might contribute to the observed normal sympathetic activity and mean arterial pressure remain unclear. Here, we hypothesized that after SAD, rats present with changes in the firing frequency of the ventral medullary inspiratory and post-inspiratory neurons. To test this hypothesis, male Wistar rats underwent SAD or sham surgery and 3 days later were surgically prepared for an in situ experiment. The duration of inspiration significantly increased in SAD rats. During inspiration, the total firing frequency of ramp-inspiratory, pre-inspiratory/inspiratory and late-inspiratory neurons was not different between groups. During post-inspiration, the total firing frequency of post-inspiratory neurons was also not different between groups. Furthermore, the data demonstrate a reduced interburst frequency of pre-inspiratory/inspiratory neurons and an increased long-term variability of late-inspiratory neurons in SAD compared with sham-operated rats. These findings indicate that the SAD-induced prolongation of inspiration was not accompanied by alterations in the total firing frequency of the ventral medullary respiratory neurons, but it was associated with changes in the long-term variability of late-inspiratory neurons. We suggest that the timing imbalance in the respiratory network in SAD rats might contribute to the modulation of presympathetic neurons after removal of baroreceptor afferents.

Glutamatergic neurons of the paraventricular nucleus are critical contributors to the development of neurogenic hypertension

KEY POINTS

Recurrent periods of over-excitation in the paraventricular nucleus (PVN) of the hypothalamus could contribute to chronic over-activation of this nucleus and thus enhanced sympathetic drive. Stimulation of the PVN glutamatergic population utilizing channelrhodopsin-2 leads to an immediate frequency-dependent increase in baseline blood pressure. Partial lesions of glutamatergic neurons of the PVN (39.3%) result in an attenuated rise in blood pressure following Deoxycorticosterone acetate (DOCA)-salt treatment and reduced index of sympathetic activity. These data suggest that stimulation of PVN glutamatergic neurons is sufficient to cause autonomic dysfunction and drive the increase in blood pressure during hypertension.

ABSTRACT

Neuro-cardiovascular dysregulation leads to increased sympathetic activity and neurogenic hypertension. The paraventricular nucleus (PVN) of the hypothalamus is a key hub for blood pressure (BP) control, producing or relaying the increased sympathetic tone in hypertension. We hypothesize that increased central sympathetic drive is caused by chronic over-excitation of glutamatergic PVN neurons. We tested how stimulation or lesioning of excitatory PVN neurons in conscious mice affects BP, baroreflex and sympathetic activity. Glutamatergic PVN neurons were unilaterally transduced with channelrhodopsin-2 using an adeno-associated virus (CamKII-ChR2-eYFP-AAV2) in wildtype mice (n = 7) to assess the impact of acute stimulation of excitatory PVN neurons selectively on resting BP in conscious mice. Stimulation of the PVN glutamatergic population resulted in an immediate frequency-dependent (2, 10 and 20 Hz) increase in BP from baseline by ∼9 mmHg at 20 Hz stimulation (P < 0.001). Additionally, in vGlut2-cre mice glutamatergic neurons of the PVN were bilaterally lesioned utilizing a cre-dependent caspase (AAV2-flex-taCASP3-TEVp). Resting BP and urinary noradrenaline (norepinephrine) levels were then recorded in conscious mice before and after DOCA-salt hypertension. Partial lesions of glutamatergic neurons of the PVN (39.3%, P < 0.05) resulted in an attenuated rise in BP following DOCA-salt treatment (P < 0.05 at 7 day time point, n = 8). Noradrenaline levels as an index of sympathetic activity between the lesion and wildtype groups showed a significant reduction after DOCA-salt treatment in the lesioned animals (P < 0.05). These experiments suggest that stimulation of PVN glutamatergic neurons is sufficient to cause autonomic dysfunction and drive the increase in BP.

Analysis of the neuronal network of the medullary respiratory center in transgenic rats expressing archaerhodopsin-3 in Phox2b-expressing cells

Preinspiratory (Pre-I) neurons in the parafacial respiratory group (pFRG) comprise one of the respiratory rhythm generators in the medulla of the neonatal rat. A subgroup of pFRG/Pre-I neurons expresses the transcription factor Phox2b. To further analyze detailed neuronal mechanisms of respiratory rhythm generation in the neonatal rat, we developed a transgenic (Tg) rat line in which Phox2b-positive cells expressed archaerhodopsin-3 (Arch). Brainstem-spinal cord preparations were isolated from 0-2-day-old Tg newborn rats and were superfused with artificial cerebrospinal fluid equilibrated with 95% O₂ and 5% CO₂, pH 7.4, at 25-26 °C. Inspiratory fourth cervical ventral root (C4) activity was monitored, and membrane potentials of neurons in the pFRG including Pre-I and inspiratory neurons were recorded. Phox2b-positive cells in the Tg rats were essentially positive for enhanced green fluorescent protein (EGFP) signals (reporter for Arch) in the pFRG. Continuous photo-stimulation of the rostral ventral medulla for up to 90 s by covering the pFRG with green laser light (532 nm) induced a decrease of respiratory rate measured at C4 accompanied by membrane hyperpolarization of Phox2b-positive pFRG/Pre-I neurons. In contrast, Phox2b-negative inspiratory neurons were not hyperpolarized during the photo-stimulation. Our findings showed that Phox2b-expressing pFRG/Pre-I neurons are involved in the maintenance of the basic respiratory rhythm in neonatal rat.

Brain metabolic sensing and metabolic signaling at the level of an astrocyte

Astrocytes support neuronal function by providing essential structural and nutritional support, neurotransmitter trafficking and recycling and may also contribute to brain information processing. In this article we review published results and report new data suggesting that astrocytes function as versatile metabolic sensors of central nervous system (CNS) milieu and play an important role in the maintenance of brain metabolic homeostasis. We discuss anatomical and functional features of astrocytes that allow them to detect and respond to changes in the brain parenchymal levels of metabolic substrates (oxygen and glucose), and metabolic waste products (carbon dioxide). We report data suggesting that astrocytes are also sensitive to circulating endocrine signals-hormones like ghrelin, glucagon-like peptide-1 and leptin, that have a major impact on the CNS mechanisms controlling food intake and energy balance. We discuss signaling mechanisms that mediate communication between astrocytes and neurons and consider how these mechanisms are recruited by astrocytes activated in response to various metabolic challenges. We review experimental data suggesting that astrocytes modulate the activities of the respiratory and autonomic neuronal networks that ensure adaptive changes in breathing and sympathetic drive in order to support the physiological and behavioral demands of the organism in ever-changing environmental conditions. Finally, we discuss evidence suggesting that altered astroglial function may contribute to the pathogenesis of disparate neurological, respiratory and cardiovascular disorders such as Rett syndrome and systemic arterial hypertension.

Anaphylactic hypotension causes renal and adrenal sympathoexcitaion and induces c-fos in the hypothalamus and medulla oblongata

NEW FINDINGS

What is the central question of this study? Whether anaphylaxis affects sympathetic outflows to the brown adipose tissue (BAT) and adrenal gland and whether anaphylaxis affects some brain areas in association with sympathetic regulation. What is the main finding and its importance? Sympathoexcitatory responses to anaphylaxis occurred regionally in the kidney and adrenal gland, but not in the thermogenesis-related BAT. Further, anaphylactic hypotension also caused increase in c-fos immunoreactivity in the hypothalamic and medullary areas. Moreover, catecholaminergic neurons of the brainstem cause adrenal sympathoexcitation in a baroreceptor-independent manner.

ABSTRACT

We previously reported that sympathetic nerve activity (SNA) to the kidney and the hindlimb increases during anaphylactic hypotension in anaesthetized rats. Based on this evidence, we examined effects of anaphylactic hypotension on SNA to the brown adipose tissue (BAT), and the adrenal gland and kidney in anaesthetized rats. We demonstrated that adrenal and renal SNA, but not BAT-SNA, were stimulated. In addition, the effects of anaphylaxis on neural activities of the hypothalamic and medullary nuclei, which are candidates for relaying efferent SNA to the peripheral organs, were investigated via immunohistochemical staining of c-fos. Anaphylaxis increased c-fos expression in the neurons of the paraventricular nucleus (PVN) of the hypothalamus and in those of the nucleus tractus solitarii (NTS) and rostral ventrolateral medulla (RVLM) of the medulla oblongata; c-fos was expressed in γ-aminobutyric acid (GABA)-ergic neurons of the NTS and in the catecholaminergic neurons of the RVLM. In addition, c-fos expression in the rostral NTS and mid NTS during anaphylaxis was reduced by sinoaortic baroreceptor denervation; however, increased c-fos expression in the caudal NTS and RVLM or adrenal sympathoexcitation were not affected by sinoaortic baroreceptor denervation. These results indicated that anaphylactic hypotension activates the hypothalamic PVN and the medullary NTS and RVLM independently of the baroreflex pathway. Further, it stimulated efferent SNA to the adrenal gland and kidney to restore blood pressure.

Brainstem catecholaminergic neurones and breathing control during postnatal development in male and female rats

KEY POINTS

The brainstem catecholaminergic (CA) modulation on ventilation changes with development. We determined the role of the brainstem CA system in ventilatory control under normocapnic and hypercapnic conditions during different phases of development [postnatal day (P)7-8, P14-15 and P20-21] in male and female Wistar rats. Brainstem CA neurones produce a tonic inhibitory drive that affects breathing frequency in P7-8 rats and provide an inhibitory drive during hypercapnic conditions in both males and females at P7-8 and P14-15. In pre-pubertal rats, brainstem CA neurones become excitatory for the CO₂ ventilatory response in males but remain inhibitory in females. Diseases such as sudden infant death syndrome, congenital central hypoventilation syndrome and Rett syndrome have been associated with abnormalities in the functioning of CA neurones; therefore, the results of the present study contribute to a better understanding of this system.

ABSTRACT

The respiratory network undergoes significant development during the postnatal phase, including the maturation of the catecholaminergic (CA) system. However, postnatal development of this network and its effect on the control of pulmonary ventilation ( V̇E ) is not fully understood. We investigated the involvement of brainstem CA neurones in respiratory control during postnatal development [postnatal day (P)7-8, P14-15 and P20-21], in male and female rats, through chemical injury with conjugated saporin anti-dopamine β-hydroxylase (DβH-SAP). Thus, DβH-SAP (420 ng μL^-1 ), saporin (SAP) or phosphate buffered solution (PBS) was injected into the fourth ventricle of neonatal Wistar rats of both sexes. V̇E and oxygen consumption were recorded 1 week after the injections in unanaesthetized neonatal and juvenile rats during room air and hypercapnia. The resting ventilation was higher in both male and female P7-8 lesioned rats by 33%, with a decrease in respiratory variability being observed in males. The hypercapnic ventilatory response (HCVR) was altered in male and female lesioned rats at all postnatal ages. At P7-8, the HCVR for males and females was increased by 37% and 30%, respectively. For both sexes at P14-15 rats, the increase in V̇E during hypercapnia was 37% higher for lesioned rats. A sex-specific difference in HCRV was observed at P20-21, with lesioned males showing a 33% decrease, and lesioned females showing an increase of 33%. We conclude that brainstem CA neurones exert a tonic inhibitory effect on V̇E in the early postnatal days of the life of a rat, increase variability in P7-8 males and modulate HCRV during the postnatal phase.

Possible Breathing Influences on the Control of Arterial Pressure After Sino-aortic Denervation in Rats

PURPOSE OF REVIEW

Surgical removal of the baroreceptor afferents [sino-aortic denervation (SAD)] leads to a lack of inhibitory feedback to sympathetic outflow, which in turn is expected to result in a large increase in mean arterial pressure (MAP). However, few days after surgery, the sympathetic nerve activity (SNA) and MAP of SAD rats return to a range similar to that observed in control rats. In this review, we present experimental evidence suggesting that breathing contributes to control of SNA and MAP following SAD.The purpose of this review was to discuss studies exploring SNA and MAP regulation in SAD rats, highlighting the possible role of breathing in the neural mechanisms of this modulation of SNA.

RECENT FINDINGS

Recent studies show that baroreceptor afferent stimulation or removal (SAD) results in changes in the respiratory pattern. Changes in the neural respiratory network and in the respiratory pattern must be considered among mechanisms involved in the modulation of the MAP after SAD.

The Role Of Parafacial Neurons In The Control Of Breathing During Exercise

Neuronal cell groups residing within the retrotrapezoid nucleus (RTN) and C1 area of the rostral ventrolateral medulla oblongata contribute to the maintenance of resting respiratory activity and arterial blood pressure, and play an important role in the development of cardiorespiratory responses to metabolic challenges (such as hypercapnia and hypoxia). In rats, acute silencing of neurons within the parafacial region which includes the RTN and the rostral aspect of the C1 circuit (pF_RTN/C1), transduced to express HM₄D (G_i-coupled) receptors, was found to dramatically reduce exercise capacity (by 60%), determined by an intensity controlled treadmill running test. In a model of simulated exercise (electrical stimulation of the sciatic or femoral nerve in urethane anaesthetised spontaneously breathing rats) silencing of the pF_RTN/C1 neurons had no effect on cardiovascular changes, but significantly reduced the respiratory response during steady state exercise. These results identify a neuronal cell group in the lower brainstem which is critically important for the development of the respiratory response to exercise and, determines exercise capacity.

Deciphering the Neural Control of Sympathetic Nerve Activity: Status Report and Directions for Future Research

Sympathetic nerve activity (SNA) contributes appreciably to the control of physiological function, such that pathological alterations in SNA can lead to a variety of diseases. The goal of this review is to discuss the characteristics of SNA, briefly review the methodology that has been used to assess SNA and its control, and to describe the essential role of neurophysiological studies in conscious animals to provide additional insights into the regulation of SNA. Studies in both humans and animals have shown that SNA is rhythmic or organized into bursts whose frequency varies depending on experimental conditions and the species. These rhythms are generated by brainstem neurons, and conveyed to sympathetic preganglionic neurons through several pathways, including those emanating from the rostral ventrolateral medulla. Although rhythmic SNA is present in decerebrate animals (indicating that neurons in the brainstem and spinal cord are adequate to generate this activity), there is considerable evidence that a variety of supratentorial structures including the insular and prefrontal cortices, amygdala, and hypothalamic subnuclei provide inputs to the brainstem regions that regulate SNA. It is also known that the characteristics of SNA are altered during stress and particular behaviors such as the defense response and exercise. While it is a certainty that supratentorial structures contribute to changes in SNA during these behaviors, the neural underpinnings of the responses are yet to be established. Understanding how SNA is modified during affective responses and particular behaviors will require neurophysiological studies in awake, behaving animals, including those that entail recording activity from neurons that generate SNA. Recent studies have shown that responses of neurons in the central nervous system to most sensory inputs are context-specific. Future neurophysiological studies in conscious animals should also ascertain whether this general rule also applies to sensory signals that modify SNA.

Role of ventral medullary catecholaminergic neurons for respiratory modulation of sympathetic outflow in rats

Sympathetic activity displays rhythmic oscillations generated by brainstem inspiratory and expiratory neurons. Amplification of these rhythmic respiratory-related oscillations is observed in rats under enhanced central respiratory drive or during development of neurogenic hypertension. Herein, we evaluated the involvement of ventral medullary sympatho-excitatory catecholaminergic C1 neurons, using inhibitory Drosophila allatostatin receptors, for the enhanced expiratory-related oscillations in sympathetic activity in rats submitted to chronic intermittent hypoxia (CIH) and following activation of both peripheral (hypoxia) and central chemoreceptors (hypercapnia). Pharmacogenetic inhibition of C1 neurons bilaterally resulted in reductions of their firing frequency and amplitude of inspiratory-related sympathetic activity in rats in normocapnia, hypercapnia or after CIH. In contrast, hypercapnia or hypoxia-induced enhanced expiratory-related sympathetic oscillations were unaffected by C1 neuronal inhibition. Inhibition of C1 neurons also resulted in a significant fall in arterial pressure and heart rate that was similar in magnitude between normotensive and CIH hypertensive rats, but basal arterial pressure in CIH rats remained higher compared to controls. C1 neurons play a key role in regulating inspiratory modulation of sympathetic activity and arterial pressure in both normotensive and CIH hypertensive rats, but they are not involved in the enhanced late-expiratory-related sympathetic activity triggered by activation of peripheral or central chemoreceptors.

Central Network Dynamics Regulating Visceral and Humoral Functions

The brain processes information from the periphery and regulates visceral and immune activity to maintain internal homeostasis, optimally respond to a dynamic external environment, and integrate these functions with ongoing behavior. In addition to its relevance for survival, this integration underlies pathology as evidenced by diseases exhibiting comorbid visceral and psychiatric symptoms. Advances in neuroanatomical mapping, genetically specific neuronal manipulation, and neural network recording are overcoming the challenges of dissecting complex circuits that underlie this integration and deciphering their function. Here we focus on reciprocal communication between the brain and urological, gastrointestinal, and immune systems. These studies are revealing how autonomic activity becomes integrated into behavior as part of a social strategy, how the brain regulates innate immunity in response to stress, and how drugs impact emotion and gastrointestinal function. These examples highlight the power of the functional organization of circuits at the interface of the brain and periphery.