Towards the sensory nature of the carotid body: hering, de castro and heymansdagger

Pathophysiology of Chronic Hypercapnic Respiratory Failure

Chronic hypercapnic respiratory failure occurs in several conditions associated with hypoventilation. The mechanisms underlying the development of chronic hypercapnia include a combination of processes that increase metabolic CO2 production, reduce minute ventilation (V'e), or increase dead space fraction (Vd/Vt). Fundamental to the pathophysiology is a mismatch between increased load and a reduction in the capacity of the respiratory pump to compensate. Though neural respiratory drive may be decreased in a subset of central hypoventilation disorders, it is more commonly increased in attempting to maintain the load-capacity homeostatic balance.

Three-Dimensional Ultrastructure of Flower-Spray Nerve Endings in the Rat Carotid Sinus

The flower-spray nerve endings are afferent nerve terminals in the carotid sinus that arise from carotid sinus nerve of glossopharyngeal nerve. However, the three-dimensional ultrastructural characteristics of flower-spray nerve endings and spatial relationships between the terminal parts and other cellular elements have not been fully understood. To elucidate their detailed relationship, backscattered electron imaging of serial sections was performed with a scanning electron microscope to produce a three-dimensional reconstruction of the flower-spray endings. The terminal parts of flower-spray endings were distributed horizontally approximately 5 µm outside the external elastic membrane in the tunica adventitia of the internal carotid artery. The three-dimensional reconstruction showed that the terminal parts of flower-spray endings were flat with irregular contours and were partially covered by the thin cytoplasmic processes of Schwann cells. The complex consisting of the nerve terminals and associated Schwann cells was surrounded by a multilayered basement membrane. The terminal parts of the endings were also surrounded by fibroblasts with elastic fibers and collagen fibrils. Secretory vesicles without an electron-dense core were observed in the terminal parts of the endings. The accumulation of vesicles just below the axonal membrane was observed in terminal parts not covered by Schwann cell cytoplasmic processes on both the luminal and basal sides. Swollen mitochondria, concentric membranous structures, and glycogen granule-like electron-dense materials were often noted in some of the terminal parts of the endings and the parent axon. Collectively, the present results suggest that flower-spray endings are baroreceptors because their morphology was similar to other mechanoreceptors. Furthermore, flower-spray endings may be affected by glutamate secreted in an autocrine manner.

[The search for the "node of life": breathing regulation]

The article provides a historical overview of developments in the understanding of respiratory rhythm and its control mechanisms over the last two centuries. In the 19th century, a structure in the medulla oblongata was first described as the "node of life". In 1743, Taube discovered the carotid body, and in 1927 the Spaniard de Castro described its morphology and innervation. It was only with the work of father and son Heymans that the physiological and pharmacological significance of the carotid and aortic body was recognized. Today we understand that the generation and control of respiration are mediated by a complex neuronal network in the brainstem. Chemo-, mechano- and proprioreceptos convey information from blood, airways and muscles to the control centre. The respiratory centre integrates the afferent input from the receptors, the autonomic nervous system, the cardiovascular system, and voluntary input from the cerebral cortex to modulate the degree of respiratory activation of motoneurons and respiratory muscles.

Novel approaches: targeting sympathetic outflow in the carotid sinus

Uncontrolled hypertension drives the global burden of increased cardiovascular disease (CVD) morbidity and mortality. Although high blood pressure (BP) is treatable and preventable, only half of the patients with hypertension undergoing treatment have their BP controlled. The failure of polypharmacy to attain adequate BP control may be due to a lack of physiological response, however, medication non-adherence and clinician inertia to increase treatment intensity are critical factors associated with poor hypertension management. The long-time medication titration, lifelong drug therapy, and often multi-drug treatment strategy are frustrating when the BP goal is not achieved, leading to increased CVD risk and a substantial burden on the healthcare system. Growing evidence indicates that neurohumoral activation is critical in initiating and maintaining elevated BP and its adverse consequences. Over the past decades, device-based therapies targeting the mechanisms underlying hypertension pathophysiology have been extensively studied. Among these, robust clinical experience for hypertension management exists for renal denervation (RDN) and baroreflex activation therapy (BAT), carotid body denervation (CBD), central arteriovenous anastomosis, and to a lesser extent, deep brain stimulation. Future studies are warranted to define the role of device-based approaches as an alternative or adjunctive treatment option to treat hypertension.

The carotid body: A novel key player in neuroimmune interactions

The idea that the nervous system communicates with the immune system to regulate physiological and pathological processes is not new. However, there is still much to learn about how these interactions occur under different conditions. The carotid body (CB) is a sensory organ located in the neck, classically known as the primary sensor of the oxygen (O2) levels in the organism of mammals. When the partial pressure of O2 in the arterial blood falls, the CB alerts the brain which coordinates cardiorespiratory responses to ensure adequate O2 supply to all tissues and organs in the body. A growing body of evidence, however, has demonstrated that the CB is much more than an O2 sensor. Actually, the CB is a multimodal sensor with the extraordinary ability to detect a wide diversity of circulating molecules in the arterial blood, including inflammatory mediators. In this review, we introduce the literature supporting the role of the CB as a critical component of neuroimmune interactions. Based on ours and other studies, we propose a novel neuroimmune pathway in which the CB acts as a sensor of circulating inflammatory mediators and, in conditions of systemic inflammation, recruits a sympathetic-mediated counteracting mechanism that appears to be a protective response.

Vagus nerve afferent stimulation: Projection into the brain, reflexive physiological, perceptual, and behavioral responses, and clinical relevance

The afferent vagus nerves project to diverse neural networks within the brainstem and forebrain, based on neuroanatomical, neurophysiological, and functional (fMRI) brain imaging evidence. In response to afferent vagal stimulation, multiple homeostatic visceral reflexes are elicited. Physiological stimuli and both invasive and non-invasive electrical stimulation that activate the afferent vagus elicit perceptual and behavioral responses that are of physiological and clinical significance. In the present review, we address these multiple roles of the afferent vagus under normal and pathological conditions, based on both animal and human evidence.

Immunohistochemical Characteristics of the Human Carotid Body in the Antenatal and Postnatal Periods of Development

The evolutionary and ontogenetic development of the carotid body is still understudied. Research aimed at studying the comparative morphology of the organ at different periods in the individual development of various animal species should play a crucial role in understanding the physiology of the carotid body. However, despite more than two centuries of study, the human carotid body remains poorly understood. There are many knowledge gaps in particular related to the antenatal development of this structure. The aim of our work is to study the morphological and immunohistochemical characteristics of the human carotid body in the antenatal and postnatal periods of development. We investigated the human carotid bodies from 1 embryo, 20 fetuses and 13 adults of different ages using samples obtained at autopsy. Immunohistochemistry revealed expression of βIII-tubulin and tyrosine hydroxylase in the type I cells and nerve fibers at all periods of ontogenesis; synaptophysin and PGP9.5 in the type I cells in some of the antenatal cases and all of the postnatal cases; 200 kDa neurofilaments in nerve fibers in some of the antenatal cases and all of the postnatal cases; and GFAP and S100 in the type II cells and Schwann cells in some of the antenatal cases and all of the postnatal cases. A high level of tyrosine hydroxylase in the type I cells was a distinctive feature of the antenatal carotid bodies. On the contrary, in the type I cells of adults, the expression of tyrosine hydroxylase was significantly lower. Our data suggest that the human carotid body may perform an endocrine function in the antenatal period, while in the postnatal period of development, it loses this function and becomes a chemosensory organ.

Carotid body chemoreceptors: physiology, pathology, and implications for health and disease

The carotid body (CB) is the main peripheral chemoreceptor for arterial respiratory gases O2 and CO2 and pH, eliciting reflex ventilatory, cardiovascular, and humoral responses to maintain homeostasis. This review examines the fundamental biology underlying CB chemoreceptor function, its contribution to integrated physiological responses, and its role in maintaining health and potentiating disease. Emphasis is placed on 1) transduction mechanisms in chemoreceptor (type I) cells, highlighting the role played by the hypoxic inhibition of O2-dependent K+ channels and mitochondrial oxidative metabolism, and their modification by intracellular molecules and other ion channels; 2) synaptic mechanisms linking type I cells and petrosal nerve terminals, focusing on the role played by the main proposed transmitters and modulatory gases, and the participation of glial cells in regulation of the chemosensory process; 3) integrated reflex responses to CB activation, emphasizing that the responses differ dramatically depending on the nature of the physiological, pathological, or environmental challenges, and the interactions of the chemoreceptor reflex with other reflexes in optimizing oxygen delivery to the tissues; and 4) the contribution of enhanced CB chemosensory discharge to autonomic and cardiorespiratory pathophysiology in obstructive sleep apnea, congestive heart failure, resistant hypertension, and metabolic diseases and how modulation of enhanced CB reactivity in disease conditions may attenuate pathophysiology.

Women Neuroscientist Disciples of Pío del Río-Hortega: the Cajal School Spreads in Europe and South America

Pio del Rio-Hortega was not only the discoverer of the microglia and oligodendroglia but also possibly the most prolific mentor of all Santiago Ramon y Cajal’s disciples (Nobel awardee in Physiology or Medicine 1906 and considered as the father of modern Neuroscience). Among Río-Hortega’s mentees, three exceptional women are frequently forgotten, chronologically: Pio’s niece Asunción Amo del Río who worked with Río-Hortega at Madrid, Paris, and Oxford; the distinguished British neuropathologist Dorothy Russell who also worked with Don Pío at Oxford; and Amanda Pellegrino de Iraldi, the last mentee in his career. Our present work analyzes the figures of these three women who were in contact and collaborated with Don Pío del Río-Hortega, describing the influences received and the impact on their careers and the History of Neuroscience. The present work completes the contribution of women neuroscientists who worked with Cajal and his main disciples of the Spanish Neurological School both in Spain (previous work) and in other countries (present work).

The inevitability of ATP as a transmitter in the carotid body

Atmospheric oxygen concentrations rose markedly at several points in evolutionary history. Each of these increases was followed by an evolutionary leap in organismal complexity, and thus the cellular adaptions we see today have been shaped by the levels of oxygen within our atmosphere. In eukaryotic cells, oxygen is essential for the production of adenosine 5'-triphosphate (ATP) which is the 'Universal Energy Currency' of life. Aerobic organisms survived by evolving precise mechanisms for converting oxygen within the environment into energy. Higher mammals developed specialised organs for detecting and responding to changes in oxygen content to maintain gaseous homeostasis for survival. Hypoxia is sensed by the carotid bodies, the primary chemoreceptor organs which utilise multiple neurotransmitters one of which is ATP to evoke compensatory reflexes. Yet, a paradox is presented in oxygen sensing cells of the carotid body when during periods of low oxygen, ATP is seemingly released in abundance to transmit this signal although the synthesis of ATP is theoretically halted because of its dependence on oxygen. We propose potential mechanisms to maintain ATP production in hypoxia and summarise recent data revealing elevated sensitivity of purinergic signalling within the carotid body during conditions of sympathetic overactivity and hypertension. We propose the carotid body is hypoxic in numerous chronic cardiovascular and respiratory diseases and highlight the therapeutic potential for modulating purinergic transmission.

Tentonin 3 as a baroreceptor mechanosensor: not a stretch

Mechanical stretch of baroreceptors in the wall of the aortic arch and carotid sinus initiates autonomic reflexes to change heart rate and blood pressure for cardiovascular homeostasis. In this issue of the JCI, Lu et al. show that tentonin 3 (TTN3), a recently identified stretch-sensitive ion channel, was present at the vagus afferent nerve endings innervating the aortic arch to function as a baroreceptor. This study expands the molecular profiles of baroreceptors and provides new insights into molecular mechanisms underlying the regulation of cardiovascular functions through baroreceptor function.

Historical setting and neuropathology of lathyrism: Insights from the neglected 1944 report by Oliveras de la Riva

Lathyrism is a central motor system disorder recognized since antiquity resulting from prolonged dietary dependence on the grasspea (Lathyrus sativus). The neuropathology underlying the characteristic spastic paraparesis of lathyrism is sketchy. Described here is a landmark but little-known Spanish-language neuropathological study of two patients with lathyrism of recent onset. Due to erroneous interpretations of Filimonov's influential work in 1926, it was assumed that spastic paraparesis of lathyrism was explained by destruction of Betz's pyramidal cells in the motor cortex. Contrary to present understanding, Betz cells and anterior horn cells were preserved, and pathological findings dominated by myelin loss were largely limited to pyramidal tracts in the lumbar cord. Thickening of the adventitia of capillaries and arterioles, together with proliferation of perivascular astrocytes, was found along the length of the spinal cord. Oliveras de la Riva proposed that the segmental spinal pathology arose because distal regions of elongate pyramidal tract axons are distant from their trophic center in the motor cortex, a view not far from the current distal axonopathy concept of lathyrism. In addition, we review the historical circumstances of Filimonov's work in Russia, a summary of the epidemic of lathyrism in Spain following its Civil War (1936-1939), and some historical aspects of the Cajal Institute in Madrid, where Oliveras de la Riva's work was carried out under the supervision of Fernando de Castro, one of Cajal's favorite students.

The Women Neuroscientists in the Cajal School

At the beginning of the 20th century, in view of the growing international recognition of Santiago Ramón y Cajal, the Spanish authorities took some important steps to support Cajal's scientific work. This recognition peaked in 1906, when Camillo Golgi and Santiago Ramón y Cajal shared the Nobel Prize in Physiology or Medicine. The Spanish government provided Cajal a state-of-the-art laboratory in Madrid to allow him to continue with his research and they funded salaries to pay his first tenured collaborators, the number of which increased further after the creation of the Junta para Ampliación de Estudios (JAE). The JAE was an organism set up to help promising researchers develop their careers in different ways, thereby contributing to the development of science in Spain. Although largely forgotten or relatively unknown, there has been a recent revival in the recognition of the school that developed around Cajal, collectively referred to as the Spanish Neurological School (or colloquially, as the Cajal School or School of Madrid). Almost all Cajal's collaborators were men, although a limited number of female scientists spent part of their careers at the heart of the Cajal School. Here we discuss these women and their work in the laboratory in Madrid. We have tracked the careers of Laura Forster (from Australia/United Kingdom), Manuela Serra, María Soledad Ruiz-Capillas and María Luisa Herreros (all Spanish), through their scientific publications, both in the journal founded by Cajal and elsewhere, and from other documentary sources. To complete the picture, we also outline the careers of other secondary figures that contributed to the production and running of Cajal's laboratory in Madrid. We show here that the dawn of Spanish neuroscience included a number of contributions from female researchers who to date, have received little recognition.

Fernando de Castro: Cajal's Man on the Peripheral Nervous System

Santiago Ramón y Cajal developed his initial scientific career working alone. After the publication of his opus magna ("Textura del sistema nervioso del hombre y los vertebrados") and the general recognition of the scientific environments that crystallized with the concession of the International Moscow Prize (1900), the Spanish Government decided to officially support Cajal with a laboratory and the first salaries to pay collaborators. Is then when the Spanish Neurological School births: in 1902, Francisco Tello is the first one to be incorporated. With new additions, Cajal's work is complimented in new aspects, including Neuropathologies. Fernando de Castro is one of his youngest direct disciples, one of the closest and more beloved. Fernando de Castro worked from 1916 in Cajal's lab, until the death of El Maestro. He was specially committed by Cajal to unravel different aspects of the structure of the peripheral ganglia: sensitive and vegetative. Afterward, Fernando de Castro described by first time the nature of arterial chemoreceptors in the carotid body. While trying to confirm his anatomical description with physiological demonstrations, and accumulating delays because of scientific decision and the sociopolitical circumstances in Spain, Corneille Heymans was awarded with the Nobel Prize in Physiology or Medicine 1938 for his contributions to the knowledge of cardiorespiratory reflexes. The Karolinska Institutet forgot Heinrich Hering and Fernando de Castro in their decision. Undoubtedly, Fernando de Castro was the most important disciple of Cajal working in the different structures of the peripheral nervous system, and this work is now reviewed here. Anat Rec, 303:1206-1214, 2020. © 2019 American Association for Anatomy.

Cajal and the Spanish Neurological School: Neuroscience Would Have Been a Different Story Without Them

Santiago Ramón y Cajal was still young when he came across the reazione nera, discovered by the Italian Camillo Golgi. Cajal became absolutely entranced by the fine structure of the nervous system this technique revealed, which led him to embark on one of the last truly epic endeavors in Modern History: the characterization of nervous cells, and of their organization to form the brain. Cajal remained in Spain throughout his scientific career, working for years alone. With international recognition, Cajal began recruiting brilliant students as collaborators. A handful of his pupils also made decisive discoveries that served to lay the foundations of modern Neuroscience. Cajal's brother Pedro, Tello, Domingo Sánchez, Achúcarro, Lafora, Río-Hortega, de Castro and Lorente de Nó worked side by side with El Maestro. While Cajal himself pronounced some of the basic rules that have helped us to understand the nervous system (the neuron theory, the law of dynamic polarization of the neuron), as well as providing innumerable details about the histological organization of the different neural structures, it was Pío del Río-Hortega who identified two of the four main cell types in the CNS (oligodendrocytes and microglia), and Fernando de Castro who described the innervation of the blood vessels and identified the first chemoreceptors in the carotid body. Together, this group of scientists is known as the Spanish Neurological School, and if they had not existed, the History of Neuroscience would surely have been quite a different story; and proof that Cajal was a truly exceptional scientist but he was not an exception for Spanish Science.

Distribution and morphology of baroreceptors in the rat carotid sinus as revealed by immunohistochemistry for P2X3 purinoceptors

The morphological characteristics of baroreceptors in the rat carotid sinus were reevaluated by whole-mount preparations with immunohistochemistry for P2X3 purinoceptors using confocal scanning laser microscopy. Immunoreactive nerve endings for P2X3 were distributed in the internal carotid artery proximal to the carotid bifurcation, particularly in the region opposite the carotid body. Some pre-terminal axons in nerve endings were ensheathed by myelin sheaths immunoreactive for myelin basic protein. Pre-terminal axons ramified into several branches that extended two-dimensionally in every direction. The axon terminals of P2X3-immunoreactive nerve endings were flat and leaf-like in shape, and extended hederiform- or knob-like protrusions in the adventitial layer. Some axons and axon terminals with P2X3 immunoreactivity were also immunoreactive for P2X2, and axon terminals were closely surrounded by terminal Schwann cells with S100 or S100B immunoreactivity. These results revealed the detailed morphology of P2X3-immunoreactive nerve endings and suggested that these endings respond to a mechanical deformation of the carotid sinus wall with their flat leaf-like terminals.

Oxygen Sensing by the Carotid Body: Past and Present

It is now well established that carotid bodies are sensory organs for monitoring arterial blood oxygen levels and trigger reflexes that are critical for maintaining homeostasis during hypoxemia. This review article provides a brief account of the early studies leading to the discovery of the carotid body as a sensory receptor and addresses current views of O₂ sensing mechanism(s) in the carotid body and their physiological importance.

Sympathetic regulation of blood pressure in normotension and hypertension: when sex matters

NEW FINDINGS

What is the topic of this review? Hypertension is a major problem in Western society. Risk of hypertension increases with age, especially in women, who have lower risk compared with men until menopause. This review outlines the sex differences in the sympathetic control of blood pressure and how these mechanisms change with age. What advances does it highlight? It has recently been recognized that men and women regulate blood pressure by different physiological mechanisms. This is important for both the understanding and the clinical management of individual patients with hypertension. This review summarizes recent advances in understanding how the regulation of blood pressure in hypertension by the sympathetic nervous system differs between men and women. The sympathetic nervous system has a central role in the regulation of arterial blood pressure (BP) and in the development of hypertension in humans. Recent evidence points to differences between the sexes in the integrative mechanisms by which BP is controlled, suggesting that the development of hypertension may follow distinct pathways in women compared with men. An important aspect of sympathetic control of BP is its substantial interindividual variability. In healthy young men, the variability in sympathetic nerve activity (SNA) is balanced by variability in cardiac output and vascular adrenergic responses, such that BP remains similar, and normal, across a severalfold range of resting SNA values. In young women, variability in resting SNA is similar to that seen in men, but the 'balancing' mechanisms are strikingly different; women exhibit greater β-adrenergic vasodilatation compared with men, which minimizes the pressor effects of a given level of SNA. Ageing is associated with increased SNA and a loss of the balancing factors seen in younger people, leading to an increased risk of hypertension in older people. Loss of oestrogen with menopause in women appears to be linked mechanistically with the decrease in β-adrenergic vasodilatation and the increased risk of hypertension in older women. Other important factors contributing to hypertension via sympathetic mechanisms are obesity and arterial stiffening, both of which increase with ageing. We conclude with a discussion of important areas in which more work is needed to understand and manage appropriately the sex-specific mechanisms in the development and maintenance of hypertension.

The Cajal School in the Peripheral Nervous System: The Transcendent Contributions of Fernando de Castro on the Microscopic Structure of Sensory and Autonomic Motor Ganglia

The fine structure of the autonomic nervous system was largely unknown at the beginning of the second decade of the 20th century. Although relatively anatomists and histologists had studied the subject, even the assays by the great Russian histologist Alexander Dogiel and the Spanish Nobel Prize laureate, Santiago Ramón y Cajal, were incomplete. In a time which witnessed fundamental discoveries by Langley, Loewi and Dale on the physiology of the autonomic nervous system, both reputed researchers entrusted one of their outstanding disciples to the challenge to further investigate autonomic structures: the Russian B.I. Lawrentjew and the Spanish Fernando de Castro developed new technical approaches with spectacular results. In the mid of the 1920's, both young neuroscientists were worldwide recognized as the top experts in the field. In the present work we describe the main discoveries by Fernando de Castro in those years regarding the structure of sympathetic and sensory ganglia, the organization of the synaptic contacts in these ganglia, and the nature of their innervation, later materialized in their respective chapters, personally invited by the editor, in Wilder Penfield's famous textbook on Neurology and the Nervous System. Most of these discoveries remain fully alive today.

Evaluating the importance of the carotid chemoreceptors in controlling breathing during exercise in man

Only the carotid chemoreceptors stimulate breathing during hypoxia in Man. They are also ideally located to warn if the brain's oxygen supply falls, or if hypercapnia occurs. Since their discovery ~80 years ago stimulation, ablation, and recording experiments still leave 3 substantial difficulties in establishing how important the carotid chemoreceptors are in controlling breathing during exercise in Man: (i) they are in the wrong location to measure metabolic rate (but are ideally located to measure any mismatch), (ii) they receive no known signal during exercise linking them with metabolic rate and no overt mismatch signals occur and (iii) their denervation in Man fails to prevent breathing matching metabolic rate in exercise. New research is needed to enable recording from carotid chemoreceptors in Man to establish whether there is any factor that rises with metabolic rate and greatly increases carotid chemoreceptor activity during exercise. Available evidence so far in Man indicates that carotid chemoreceptors are either one of two mechanisms that explain breathing matching metabolic rate or have no importance. We still lack key experimental evidence to distinguish between these two possibilities.

The Combinatorial Nature of Osmosensing in Fishes

Organisms exposed to altered salinity must be able to perceive osmolality change because metabolism has evolved to function optimally at specific intracellular ionic strength and composition. Such osmosensing comprises a complex physiological process involving many elements at organismal and cellular levels of organization. Input from numerous osmosensors is integrated to encode magnitude, direction, and ionic basis of osmolality change. This combinatorial nature of osmosensing is discussed with emphasis on fishes.

Peripheral chemoreceptors: function and plasticity of the carotid body

The discovery of the sensory nature of the carotid body dates back to the beginning of the 20th century. Following these seminal discoveries, research into carotid body mechanisms moved forward progressively through the 20th century, with many descriptions of the ultrastructure of the organ and stimulus-response measurements at the level of the whole organ. The later part of 20th century witnessed the first descriptions of the cellular responses and electrophysiology of isolated and cultured type I and type II cells, and there now exist a number of testable hypotheses of chemotransduction. The goal of this article is to provide a comprehensive review of current concepts on sensory transduction and transmission of the hypoxic stimulus at the carotid body with an emphasis on integrating cellular mechanisms with the whole organ responses and highlighting the gaps or discrepancies in our knowledge. It is increasingly evident that in addition to hypoxia, the carotid body responds to a wide variety of blood-borne stimuli, including reduced glucose and immune-related cytokines and we therefore also consider the evidence for a polymodal function of the carotid body and its implications. It is clear that the sensory function of the carotid body exhibits considerable plasticity in response to the chronic perturbations in environmental O2 that is associated with many physiological and pathological conditions. The mechanisms and consequences of carotid body plasticity in health and disease are discussed in the final sections of this article.