Uncovering the neural control of laryngeal activity and subglottic pressure in anaesthetized rats: insights from mesencephalic regions

To assess the possible interactions between the dorsolateral periaqueductal gray matter (dlPAG) and the different domains of the nucleus ambiguus (nA), we have examined the pattern of double-staining c-Fos/FoxP2 protein immunoreactivity (c-Fos-ir/FoxP2-ir) and tyrosine hydroxylase (TH) throughout the rostrocaudal extent of nA in spontaneously breathing anaesthetised male Sprague-Dawley rats during dlPAG electrical stimulation. Activation of the dlPAG elicited a selective increase in c-Fos-ir with an ipsilateral predominance in the somatas of the loose (p < 0.05) and compact formation (p < 0.01) within the nA and confirmed the expression of FoxP2 bilaterally in all the domains within the nA. A second group of experiments was made to examine the importance of the dlPAG in modulating the laryngeal response evoked after electrical or chemical (glutamate) dlPAG stimulations. Both electrical and chemical stimulations evoked a significant decrease in laryngeal resistance (subglottal pressure) (p < 0.001) accompanied with an increase in respiratory rate together with a pressor and tachycardic response. The results of our study contribute to new data on the role of the mesencephalic neuronal circuits in the control mechanisms of subglottic pressure and laryngeal activity.

Influence of Brainstem's Area A5 on Sympathetic Outflow and Cardiorespiratory Dynamics

Area A5 is a noradrenergic cell group in the brain stem characterised by its important role in triggering sympathetic activity, exerting a profound influence on the sympathetic outflow, which is instrumental in the modulation of cardiovascular functions, stress responses and various other physiological processes that are crucial for adaptation and survival mechanisms. Understanding the role of area A5, therefore, not only provides insights into the basic functioning of the sympathetic nervous system but also sheds light on the neuronal basis of a number of autonomic responses. In this review, we look deeper into the specifics of area A5, exploring its anatomical connections, its neurochemical properties and the mechanisms by which it influences sympathetic nervous system activity and cardiorespiratory regulation and, thus, contributes to the overall dynamics of the autonomic function in regulating body homeostasis.

Central Autonomic Mechanisms Involved in the Control of Laryngeal Activity and Vocalization

In humans, speech is a complex process that requires the coordinated involvement of various components of the phonatory system, which are monitored by the central nervous system. The larynx in particular plays a crucial role, as it enables the vocal folds to meet and converts the exhaled air from our lungs into audible sounds. Voice production requires precise and sustained exhalation, which generates an air pressure/flow that creates the pressure in the glottis required for voice production. Voluntary vocal production begins in the laryngeal motor cortex (LMC), a structure found in all mammals, although the specific location in the cortex varies in humans. The LMC interfaces with various structures of the central autonomic network associated with cardiorespiratory regulation to allow the perfect coordination between breathing and vocalization. The main subcortical structure involved in this relationship is the mesencephalic periaqueductal grey matter (PAG). The PAG is the perfect link to the autonomic pontomedullary structures such as the parabrachial complex (PBc), the Kölliker-Fuse nucleus (KF), the nucleus tractus solitarius (NTS), and the nucleus retroambiguus (nRA), which modulate cardiovascular autonomic function activity in the vasomotor centers and respiratory activity at the level of the generators of the laryngeal-respiratory motor patterns that are essential for vocalization. These cores of autonomic structures are not only involved in the generation and modulation of cardiorespiratory responses to various stressors but also help to shape the cardiorespiratory motor patterns that are important for vocal production. Clinical studies show increased activity in the central circuits responsible for vocalization in certain speech disorders, such as spasmodic dysphonia because of laryngeal dystonia.

Impact of the glutamatergic neurotransmission within the A5 region on the cardiorespiratory response evoked from the midbrain dlPAG

Stimulation of the dorsolateral periaqueductal grey matter (dlPAG) in rats evokes an active defensive behaviour together with a cardiorespiratory response characterised by tachypnoea, tachycardia and hypertension. The dlPAG neurons involved in these responses are excitatory, presumably glutamatergic, due to the presence of vesicular glutamate transporter VGLUT2 within their axon terminals. Previously, our group described a functional interaction between dlPAG and the pontine A5 region. Accordingly, in the present work, in order to characterize the role of glutamate within this interaction, experiments were carried out in spontaneously breathing anaesthetized rats (sodium pentobarbitone 60 mg/kg i.p., suplemented with 20 mg/kg i.p.). The cardiorespiratory response evoked by electrical stimulation of the dlPAG (1 ms pulses, 20-50 μA, given at 100 Hz, during 5 s) was analysed before and after the microinjection, within the A5 region, of either kynurenic acid (non-specific glutamate receptor antagonist; 5-10 nmol), DAP-5 (NMDA antagonist; 1 pmol), CNQX (non-NMDA antagonist; 1 pmol) or MCPG (metabotropic antagonist; 0,1 nmol). Kynurenic acid decreased the intensity of both the tachypnoea (p < 0,001) and tachycardia (p < 0,001) induced by dl-PAG stimulation. Blockade of no-NMDA receptors reduced the increase of respiratory frequency, heart rate and pressor response to dl-PAG stimulation (p < 0,01, p < 0,001, p < 0,05 respectively). Blockade of either NMDA or metabotropic receptors reduced the dlPAG-evoked tachycardia and pressor response (p < 0,01; p < 0,05 respectively). These results suggest a neuromodulatory role for A5 region via glutamate neurotransmission of the dlPAG-evoked cardiorespiratory response, confirming the role of the ventrolateral pons in the neuronal circuits involved in respiratory and heart rate control.

Central control of cardiac activity as assessed by intra-cerebral recordings and stimulations

Some of the most important integrative control centers for the autonomic nervous system are located in the brainstem and the hypothalamus. However, growing recent neuroimaging evidence support that a set of cortical regions, named the central autonomic network (CAN), is involved in autonomic control and seems to play a major role in continuous autonomic cardiac adjustments to high-level emotional, cognitive or sensorimotor cortical activities. Intracranial explorations during stereo-electroencephalography (SEEG) offer a unique opportunity to address the question of the brain regions involved in heart-brain interaction, by studying: (i) direct cardiac effects produced by the electrical stimulation of specific brain areas; (ii) epileptic seizures inducing cardiac modifications; (iii) cortical regions involved in cardiac interoception and source of cardiac evoked potentials. In this review, we detail the available data assessing cardiac central autonomic regulation using SEEG, address the strengths and also the limitations of this technique in this context, and discuss perspectives. The main cortical regions that emerge from SEEG studies as being involved in cardiac autonomic control are the insula and regions belonging to the limbic system: the amygdala, the hippocampus, and the anterior and mid-cingulate. Although many questions remain, SEEG studies have already demonstrated afferent and efferent interactions between the CAN and the heart. Future studies in SEEG should integrate these afferent and efferent dimensions as well as their interaction with other cortical networks to better understand the functional heart-brain interaction.

Sex-Specific Effects of Stress on Respiratory Control: Plasticity, Adaptation, and Dysfunction

As our understanding of respiratory control evolves, we appreciate how the basic neurobiological principles of plasticity discovered in other systems shape the development and function of the respiratory control system. While breathing is a robust homeostatic function, there is growing evidence that stress disrupts respiratory control in ways that predispose to disease. Neonatal stress (in the form of maternal separation) affects "classical" respiratory control structures such as the peripheral O₂ sensors (carotid bodies) and the medulla (e.g., nucleus of the solitary tract). Furthermore, early life stress disrupts the paraventricular nucleus of the hypothalamus (PVH), a structure that has emerged as a primary determinant of the intensity of the ventilatory response to hypoxia. Although underestimated, the PVH's influence on respiratory function is a logical extension of the hypothalamic control of metabolic demand and supply. In this article, we review the functional and anatomical links between the stress neuroendocrine axis and the medullary network regulating breathing. We then present the persistent and sex-specific effects of neonatal stress on respiratory control in adult rats. The similarities between the respiratory phenotype of stressed rats and clinical manifestations of respiratory control disorders such as sleep-disordered breathing and panic attacks are remarkable. These observations are in line with the scientific consensus that the origins of adult disease are often found among developmental and biological disruptions occurring during early life. These observations bring a different perspective on the structural hierarchy of respiratory homeostasis and point to new directions in our understanding of the etiology of respiratory control disorders. © 2021 American Physiological Society. Compr Physiol 11:1-38, 2021.

Pontine A5 region modulation of the cardiorespiratory response evoked from the midbrain dorsolateral periaqueductal grey

Connections between the midbrain dorsolateral periaqueductal grey (dlPAG) and the pontine A5 region have been shown. The stimulation of both regions evokes similar cardiovascular responses: tachycardia and hypertension. Accordingly, we have studied the interactions between dlPAG and A5 region in spontaneously breathing anesthetized rats. dlPAG was electrically stimulated (20-30 μA 1-ms pulses were given for 5 s at 100 Hz). Changes in the evoked cardiorespiratoy response were analysed before and after ipsilateral microinjections of muscimol (GABAergic agonist, 50 nl, 0.25 nmol, 5 s) within the A5 region. Electrical stimulation of the dlPAG produces, in the rat, a response characterized by tachypnoea (p < 0.001), hypertension (p < 0.001) and tachycardia (p < 0.001). The increase in respiratory rate was due to a decrease in expiratory time (p < 0.01). Pharmacological inhibition of the A5 region with muscimol produced a marked reduction of the tachycardia (p < 0.001) and the tachypnoea (p < 0.01) evoked from the dlPAG. Finally, to assess functional interactions between A5 and dlPAG, extracellular activity of putative A5 neurones were recorded during dlPAG electrical stimulation. Forty A5 cells were recorded, 16 of which were affected by dlPAG stimulation (40%). 4 cells showed activation, 5 cells excitation and 7 cells decreased spontaneous activity to dlPAG stimulation (p < 0.001). These results confirm a link between the A5 region and dlPAG. The potential role of these connections in the modulation of dlPAG evoked cardiorespiratory responses and their possible clinical implications are discussed.

Monosynaptic Input Mapping of Diencephalic Projections to the Cerebrospinal Fluid-Contacting Nucleus in the Rat

Objective: To investigate the projections the cerebrospinal fluid-contacting (CSF-contacting) nucleus receives from the diencephalon and to speculate on the functional significance of these connections.

Methods: The retrograde tracer cholera toxin B subunit (CB) was injected into the CSF-contacting nucleus in SD rats according to the experimental formula of the stereotaxic coordinates. Animals were perfused 7–10 days after the injection, and the diencephalon was sliced at 40 μm with a freezing microtome. CB-immunofluorescence was performed on all diencephalic sections. The features of CB-positive neuron distribution in the diencephalon were observed with a fluorescence microscope.

Results: The retrograde labeled CB-positive neurons were found in the epithalamus, subthalamus, and hypothalamus. Three functional diencephalic areas including 43 sub-regions revealed projections to the CSF-contacting nucleus. The CB-positive neurons were distributed in different density ranges: sparse, moderate, and dense.

Conclusion: Based on the connectivity patterns of the CSF-contacting nucleus that receives anatomical inputs from the diencephalon, we preliminarily assume that the CSF-contacting nucleus participates in homeostasis regulation, visceral activity, stress, emotion, pain and addiction, and sleeping and arousal. The present study firstly illustrates the broad projections of the CSF-contacting nucleus from the diencephalon, which implies the complicated functions of the nucleus especially for the unique roles of coordination in neural and body fluids regulations.

Integration of the Proprioceptive and Central Inspiratory Inhibitory Afferent Inputs by Pontine Noradrenergic A5 Neurons in Rats

Inhibitory afferent inputs to pontine A5 noradrenergic neurons (A5 NN) are not known, except partial baroreceptor input. In spontaneously breathing pentobarbital-anesthetized rats, we registered 35 A5 NN that were activated by hypoxia (100% N₂, 10 sec) by more than 5 times in comparison with the background. Cooling of retrotrapezoid nucleus (15°C, 6 sec) completely blocked the motor inspiratory output and A5 NN discharge frequency increased (23/23) by more than 7 times in comparison with the background values. The beginning of A5 NN activation coincided with cessation of inspiratory activity. Short-term passive stretching of the shin muscles (1 sec, 100 g) caused BP drop and complete inhibition of A5 NN (12/12) activated by hypoxia. Inhibitory afferent inputs from proprioceptors and central inspiratory neurons that can limit A5 NN activity were demonstrated.

Glutamate receptors of the A5 region modulate cardiovascular responses evoked from the dorsomedial hypothalamic nucleus and perifornical area

To assess the possible function of glutamate in the interaction between the dorsomedial hypothalamic nucleus-perifornical area (DMH-PeF) and the A5 pontine region (A5), cardiovascular and respiratory changes were studied in response to electrical stimulation of the DMH-PeF (1 ms pulses, 30-50 μA given at 100 Hz for 5 s) before and after the microinjection of kynurenic acid (non-specific glutamate receptor antagonist; 50 nl, 5 nmol), MK-801 (NMDA receptor antagonist; 50 nl, 50 nmol), CNQX (non-NMDA receptor antagonist; 50 nl, 50 nmol) or MCPG (metabotropic glutamate receptor antagonist; 50 nl, 5 nmol) within the A5 region. DMH-PeF electrical stimulation elicited a pressor (p < 0.001) and tachycardic response (p < 0.001) which was accompanied by an inspiratory facilitation characterised by an increase in respiratory rate (p < 0.001) due to a decrease in expiratory time (p < 0.01). Kynurenic acid within the A5 region decreased the tachycardia (p < 0.001) and the intensity of the blood pressure response (p < 0.001) to DMH-PeF stimulation. After the microinjection of MK-801 and CNQX into the A5 region, the magnitude of the tachycardia and the pressor response were decreased (p < 0.05 and p < 0.01; p < 0.001 and p < 0.05, respectively). After MCPG microinjection into the A5 region, a decrease in the tachycardia (p < 0.001) with no changes in the pressor response was observed during DMH-PeF stimulation. The respiratory response elicited by DMH-PeF stimulation was not changed after the microinjection of kynurenic acid, MK-801, CNQX or MCPG within the A5 region. These results suggest that A5 region glutamate receptors play a role in the cardiovascular response elicited from the DMH-PeF. The possible mechanisms involved in these interactions are discussed.

Role of A5 noradrenergic neurons in the chemoreflex control of respiratory and sympathetic activities in unanesthetized conditions

The A5 area at the ventrolateral pons contains noradrenergic neurons connected with other medullary areas involved in the cardiorespiratory control. Its contribution to the cardiorespiratory regulation was previously evidenced in anesthetized conditions. In the present study, we investigated the involvement of the A5 noradrenergic neurons to the basal and chemoreflex control of the sympathetic and respiratory activities in unanesthetized conditions. A5 noradrenergic neurons were lesioned using microinjections of anti-dopamine β-hydroxylase saporin (anti-DβH-SAP). After 7-8days, we evaluated the arterial pressure levels, heart rate and minute ventilation in freely moving adult rats (280-350g) as well as recorded from thoracic sympathetic (tSN) and phrenic nerves (PN) using the arterially perfused in situ preparation of juvenile rats (80-90g). Baseline cardiovascular, sympathetic and respiratory parameters were similar between control (n=7-8) and A5-lesioned rats (n=5-6) in both experimental preparations. In adult rats, lesions of A5 noradrenergic neurons did not modify the reflex cardiorespiratory adjustments to hypoxia (7% O₂) and hypercapnia (7% CO₂). In the in situ preparations, the sympatho-excitation, but not the PN reflex response, elicited by either the stimulation of peripheral chemoreceptors (ΔtSN: 110±12% vs 58±8%, P<0.01) or hypercapnia (ΔtSN: 9.5±1.4% vs 3.9±1.7%, P<0.05) was attenuated in A5-lesioned rats compared to controls. Our data demonstrated that A5 noradrenergic neurons are part of the circuitry recruited for the processing of sympathetic response to hypoxia and hypercapnia in unanesthetized conditions.