Hypoxic adaptation of the peptidergic innervation in the rat carotid body

Neurochemical plasticity of the carotid body in hypertension

The carotid body (CB), a main peripheral arterial chemoreceptor, has lately been implicated in the pathophysiology of various cardiovascular disorders. Emerging experimental evidence supports a causal relationship between CB dysfunction and augmented sympathetic outflow which is the common hallmark of human sympathetic-related diseases, including essential hypertension. To gain insight into the neurotransmitter profile of chemosensory cells in the hypertensive CB, we examined the expression and cellular localization of some classical neurotransmitters, neuropeptides, and gaseous signaling molecules as well as neurotrophic factors and their receptors in the CB of spontaneously hypertensive rats, a common animal model of hypertension. Our immunohistochemical experiments revealed an elevated catecholamine and serotonin content in the hypertensive CB compared to normotensive controls. GABA immunostaining was seen in some peripherally located glomus cells in the CB of SHR and it was significantly lower than in control animals. The density of substance P and vasoactive intestinal peptide-immunoreactive fibers was diminished whereas that of neuropeptide Y-immunostained nerve fibers was increased and that of calcitonin gene-related peptide-containing fibers remained almost unchanged in the hypertensive CB. We have further demonstrated that in the hypertensive state the production of nitric oxide is impaired and that the components of the neurotrophin signaling system display an abnormal enhanced expression. Our results provide immunohistochemical evidence that the altered transmitter phenotype of CB chemoreceptor cells and the elevated production of neurotrophic factors modulate the chemosensory processing in hypertensive animals which contributes to autonomic dysfunction and elicits sympathetic hyperactivity, consequently leading to elevated blood pressure.

Neurochemical Plasticity of the Carotid Body

A striking feature of the carotid body (CB) is its remarkable degree of plasticity in a variety of neurotransmitter/modulator systems in response to environmental stimuli, particularly following hypoxic exposure of animals and during ascent to high altitude. Current evidence suggests that acetylcholine and adenosine triphosphate are two major excitatory neurotransmitter candidates in the hypoxic CB, and they may also be involved as co-transmitters in hypoxic signaling. Conversely, dopamine, histamine and nitric oxide have recently been considered inhibitory transmitters/modulators of hypoxic chemosensitivity. It has also been revealed that interactions between excitatory and inhibitory messenger molecules occur during hypoxia. On the other hand, alterations in purinergic neurotransmitter mechanisms have been implicated in ventilatory acclimatization to hypoxia. Chronic hypoxia also induces profound changes in other neurochemical systems within the CB such as the catecholaminergic, peptidergic and nitrergic, which in turn may contribute to increased ventilatory and chemoreceptor responsiveness to hypoxia at high altitude. Taken together, current data suggest that complex interactions among transmitters markedly influence hypoxia-induced transmitter release from the CB. In addition, the expression of a wide variety of growth factors, proinflammatory cytokines and their receptors have been identified in CB parenchymal cells in response to hypoxia and their upregulated expression could mediate the local inflammation and functional alteration of the CB under hypoxic conditions.

Neurochemical Anatomy of the Mammalian Carotid Body

Carotid body (CB) glomus cells in most mammals, including humans, contain a broad diversity of classical neurotransmitters, neuropeptides and gaseous signaling molecules as well as their cognate receptors. Among them, acetylcholine, adenosine triphosphate and dopamine have been proposed to be the main excitatory transmitters in the mammalian CB, although subsequently dopamine has been considered an inhibitory neuromodulator in almost all mammalian species except the rabbit. In addition, co-existence of biogenic amines and neuropeptides has been reported in the glomus cells, thus suggesting that they store and release more than one transmitter in response to natural stimuli. Furthermore, certain metabolic and transmitter-degrading enzymes are involved in the chemotransduction and chemotransmission in various mammals. However, the presence of the corresponding biosynthetic enzyme for some transmitter candidates has not been confirmed, and neuroactive substances like serotonin, gamma-aminobutyric acid and adenosine, neuropeptides including opioids, substance P and endothelin, and gaseous molecules such as nitric oxide have been shown to modulate the chemosensory process through direct actions on glomus cells and/or by producing tonic effects on CB blood vessels. It is likely that the fine balance between excitatory and inhibitory transmitters and their complex interactions might play a more important than suggested role in CB plasticity.

Comparative morphological and molecular studies on the oxygen-chemoreceptive cells in the carotid body and fish gills

Oxygen-chemoreceptive cells play critical roles for the respiration control. This review summarizes the chemoreceptive cells in the carotid body and fish gills from a morphological and molecular perspective. The cells synthesize and secrete biogenic amines, neuropeptides, and neuroproteins and also express many signaling molecules and transcription factors. In mammals, birds, reptiles, and amphibians, the carotid body primordium is consistently formed in the wall of the third arch artery which gives rise to the common carotid artery and the basal portion of the internal carotid artery. Consequently, the carotid body is located in the carotid bifurcation region, except birds in which the organ is situated at the lateral side of the common carotid artery. The carotid body receives branches of the cranial nerves IX and/or X dependent on the location of the organ. The glomus cell progenitors in mammals and birds are derived from the neighboring ganglion, i.e., the superior cervical sympathetic ganglion and the nodose ganglion, respectively, and immigrate into the carotid body primordium, constituting a solid cell cluster. In other animal species, the glomus cells are dispersed singly or forming small cell groups in intervascular stroma of the carotid body. In fishes, the neuroepithelial cells, corresponding to the glomus cells, are distributed in the gill filaments and lamellae. All oxygen-chemoreceptive cells sensitively respond to acute or chronic hypoxia, exhibiting degranulation, hypertrophy, hyperplasia, and upregulated expression of many genes.

Mass Spectrometric Profiling of Neuropeptides in Callinectes sapidus during Hypoxia Stress

Oxygen (O₂) is a critical component of life; without proper O₂ levels, cells are unable to respire, meaning glucose cannot be utilized. Thus, hypoxia (low O₂ levels) is a well-documented stressor, especially in aquatic environments. Neuropeptides are a major class of regulators for stress-induced responses; however, their global expression changes during stress are not well characterized due to the natural complexity of the nervous system. Beyond being a neurological model organism, crustaceans are regularly exposed to hypoxia, making them a relevant system for this study. Several neuropeptide families, including orcokinins, RFamides, and allatostatin A-types, show dynamic dysregulation due to hypoxic stress. In particular, the brain showed the most dynamic changes with a survival mechanism "switching" (i.e., significant increase to decrease) of neuropeptide content between moderate and severe hypoxia (e.g., NFDEDRSGFA, FDAFTTGFGHS, NRNFLRFamide, and APSGFLGMRamide). Globally, neuropeptides in different tissues appeared to exhibit unique expression patterns at the various severities of hypoxia, including LSSSNSPSSTPL and NFDEIDRSSFGF. Overall, this study provides clear evidence for the benefits of globally analyzing biomolecules and that neuropeptides play a critical role in how crustaceans adapt due to hypoxic stress.

Vasoactive Intestinal Polypeptide in the Carotid Body-A History of Forty Years of Research. A Mini Review

Vasoactive intestinal polypeptide (VIP) consists of 28 amino acid residues and is widespread in many internal organs and systems. Its presence has also been found in the nervous structures supplying the carotid body not only in mammals but also in birds and amphibians. The number and distribution of VIP in the carotid body clearly depends on the animal species studied; however, among all the species, this neuropeptide is present in nerve fibers around blood vessels and between glomus cell clusters. It is also known that the number of nerves containing VIP located in the carotid body may change under various pathological and physiological factors. The knowledge concerning the functioning of VIP in the carotid body is relatively limited. It is known that VIP may impact the glomus type I cells, causing changes in their spontaneous discharge, but the main impact of VIP on the carotid body is probably connected with the vasodilatory effects of this peptide and its influence on blood flow and oxygen delivery. This review is a concise summary of forty years of research concerning the distribution of VIP in the carotid body.

The Carotid Sinus Nerve-Structure, Function, and Clinical Implications

Interest has been renewed in the anatomy and physiology of the carotid sinus nerve (CSN) and its targets (carotid sinus and carotid body, CB), due to recent proposals of surgical procedures for a series of common pathologies, such as carotid sinus syndrome, hypertension, heart failure, and insulin resistance. The CSN originates from the glossopharyngeal nerve soon after its appearance from the jugular foramen. It shows frequent communications with the sympathetic trunk (usually at the level of the superior cervical ganglion) and the vagal nerve (main trunk, pharyngeal branches, or superior laryngeal nerve). It courses on the anterior aspect of the internal carotid artery to reach the carotid sinus, CB, and/or intercarotid plexus. In the carotid sinus, type I (dynamic) carotid baroreceptors have larger myelinated A-fibers; type II (tonic) baroreceptors show smaller A- and unmyelinated C-fibers. In the CB, afferent fibers are mainly stimulated by acetylcholine and ATP, released by type I cells. The neurons are located in the petrosal ganglion, and centripetal fibers project on to the solitary tract nucleus: chemosensory inputs to the commissural subnucleus, and baroreceptor inputs to the commissural, medial, dorsomedial, and dorsolateral subnuclei. The baroreceptor component of the CSN elicits sympatho-inhibition and the chemoreceptor component stimulates sympatho-activation. Thus, in refractory hypertension and heart failure (characterized by increased sympathetic activity), baroreceptor electrical stimulation, and CB removal have been proposed. Instead, denervation of the carotid sinus has been proposed for the "carotid sinus syndrome." Anat Rec, 302:575-587, 2019. © 2018 Wiley Periodicals, Inc.

Short-term hypoxia transiently increases dopamine β-hydroxylase immunoreactivity in glomus cells of the rat carotid body

Under long-term hypoxia, noradrenaline (NA) content in the carotid body (CB) increases, suggesting that NA plays an important role in CB chemotransduction. However, it is unknown whether short-term hypoxia upregulates NA biosynthesis in CB. Therefore, we examined dopamine β-hydroxylase (DBH) expression in the CB of rats exposed to hypoxia (10% O(2)) for 0 to 24 hr with immunoblotting and immunohistochemistry. Using immunoblotting, the signal intensity for DBH appeared to be the most intense in rats exposed to hypoxia for 12 hr. Using immunohistochemistry, DBH immunoreactivity was observed in the cytoplasm of some glomus cells and varicosities in controls and rats exposed to hypoxia for 6 hr. In rats exposed to hypoxia for 12 hr, DBH immunoreactive intensities in DBH-positive glomus cells were significantly higher compared with controls (p<0.05). In the CB of rats exposed to hypoxia for 18 and 24 hr, DBH immunoreactive intensities in DBH-positive glomus cells were significantly lower than that of rats exposed to hypoxia for 12 hr (p<0.05). These results demonstrate that DBH immunoreactivity is transiently increased in glomus cells by short-term hypoxia, suggesting that NA biosynthesis is transiently facilitated in glomus cells at an early stage of hypoxia.

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.

Carotid body remodelling in l-NAME-induced hypertension in the rat

The carotid body (CB) is a chemoreceptor organ located at the bifurcation of the common carotid artery. It is made up of the carotid glomus, a structure containing type 1 cells surrounded by type 2 cells. The aim of this study was to evaluate the morphological changes of the CB and carotid glomus in the rat model of l-NAME-induced hypertension. Male Wistar rats were divided in two groups: control untreated rats (C) and rats receiving l-NAME 40 mg/kg/day (LN) for 6 weeks. At the end of the experiment, the systolic blood pressure was 63% higher in the LN group compared with the C group. Morphometric analysis showed that the area of the CB was 29% greater in the LN group compared with the C group. The density of nuclei in the CB was similar between groups, but it was 31% less in the carotid glomus of the LN group. Cells in the CB of the LN group displayed cytoplasmic vacuolation and expressed several biogenic amines. There were more elastic fibres, proteoglycans and collagen fibres in the LN group compared with the C group. Immunohistochemistry showed increased expression of nuclear factor kB, substance P, vascular endothelial growth factor and neuronal nitric oxide synthase in the LN group, while expression of the protein gene product 9.5 was decreased. l-NAME alters cell morphology and the expression of extracellular matrix molecules in the CB and carotid glomus in rats with l-NAME-induced hypertension.

Trophic factors in the carotid body

The aim of the present study is to provide a review of the expression and action of trophic factors in the carotid body. In glomic type I cells, the following factors have been identified: brain-derived neurotrophic factor, glial cell line-derived neurotrophic factor, artemin, ciliary neurotrophic factor, insulin-like growth factors-I and -II, basic fibroblast growth factor, epidermal growth factor, transforming growth factor-alpha and -beta1, interleukin-1beta and -6, tumour necrosis factor-alpha, vascular endothelial growth factor, and endothelin-1 (ET-1). Growth factor receptors in the above cells include p75LNGFR, TrkA, TrkB, RET, GDNF family receptors alpha1-3, gp130, IL-6Ralpha, EGFR, FGFR1, IL1-RI, TNF-RI, VEGFR-1 and -2, ETA and ETB receptors, and PDGFR-alpha. Differential local expression of growth factors and corresponding receptors plays a role in pre- and postnatal development of the carotid body. Their local actions contribute toward producing the morphologic and molecular changes associated with chronic hypoxia and/or hypertension, such as cellular hyperplasia, extracellular matrix expansion, changes in channel densities, and neurotransmitter patterns. Neurotrophic factor production is also considered to play a key role in the therapeutic effects of intracerebral carotid body grafts in Parkinson's disease. Future research should also focus on trophic actions on carotid body type I cells by peptide neuromodulators, which are known to be present in the carotid body and to show trophic effects on other cell populations, that is, angiotensin II, adrenomedullin, bombesin, calcitonin, calcitonin gene-related peptide, cholecystokinin, erythropoietin, galanin, opioids, pituitary adenylate cyclase-activating polypeptide, atrial natriuretic peptide, somatostatin, tachykinins, neuropeptide Y, neurotensin, and vasoactive intestinal peptide.

Control of breathing in anuran amphibians

The primary role of the respiratory system is to ensure adequate tissue oxygenation, eliminate carbon dioxide and help to regulate acid-base status. To maintain this homeostasis, amphibians possess an array of receptors located at peripheral and central chemoreceptive sites that sense respiration-related variables in both internal and external environments. As in mammals, input from these receptors is integrated at central rhythmogenic and pattern-forming elements in the medulla in a manner that meets the demands determined by the environment within the constraints of the behavior and breathing pattern of the animal. Also as in mammals, while outputs from areas in the midbrain may modulate respiration directly, they do not play a significant role in the production of the normal respiratory rhythm. However, despite these similarities, the breathing patterns of the two classes are different: mammals maintain homeostasis of arterial blood gases through rhythmic and continuous breathing, whereas amphibians display an intermittent pattern of aerial respiration. While the latter is also often rhythmic, it allows a degree of fluctuation in key respiratory variables that has led some to suggest that control is not as tight in these animals. In this review we will focus specifically on recent advances in studies of the control of ventilation in anuran amphibians. This is the group of amphibians that has attracted the most recent attention from respiratory physiologists.

Chemoreceptor activity is normal in mice lacking the NK1 receptor

Substance P has been proposed to be an important neurotransmitter in the carotid body with the neurokinin 1 (NK1) receptor, mediating excitation between the glomus cells and afferent nerve endings. In order to better understand the role of substance P, this study examined chemoreceptor afferent activity, in vitro, and tissue catecholamine levels and release in adult, wild-type mice and mice lacking the gene for the NK1 receptor (NK1-KO). Groups did not differ significantly in body weight, carotid body dopamine content or carotid body norepinephrine content. In wild-type mice, single unit activity increased from 0.59 +/- 0.14 Hz to 19.78 +/- 2.27 Hz during superfusion with strong hypoxia (PO2 approximately 25 Torr). Chemoreceptor activity in NK1-KO mice, increased from 0.71 +/- 0.23 to 21.50 +/- 3.62 Hz, and neither baseline or peak frequencies were significantly different from the wild-type group. Less severe hypoxia (PO2 approximately 45 torr), evoked peak activities of 12.50 +/- 1.88 and 10.64 +/- 3.72 Hz in wild-type and NK1-KO mice, which were also not significantly different. In response to severe hypoxia, free-tissue catecholamine increased to 4.92 +/- 0.85 microm in wild-type mice and 4.26 +/- 0.63 microm in NK1-KO mice, which were also not significantly different. It may therefore be concluded that loss of NK1 receptors has little effect on chemoreceptor function in the mouse, and thus they play, at best, a minor role in the hypoxic chemoreception process.

Chronic hypoxia-induced morphological and neurochemical changes in the carotid body

The carotid body (CB) plays an important role in the control of ventilation. Type I cells in CB are considered to be the chemoreceptive element which detects the levels of PO(2), PCO(2), and [H(+)] in the arterial blood. These cells originate from the neural crest and appear to retain some neuronal properties. They are excitable and produce a number of neurochemicals. Some of these neurochemicals, such as dopamine and norepinephrine, are considered to be primarily inhibitory to CB function and others, such as adenosine triphosphate, acetylcholine, and endothelin, are thought to be primarily excitatory. Chronic hypoxia (CH) induces profound morphological as well as neurochemical changes in the CB. CH enlarges the size of CB and causes hypertrophy and mitosis of type I cells. Also, CH changes the vascular structure of CB, including inducing marked vasodilation and the growth of new blood vessels. Moreover, CH upregulates certain neurochemical systems within the CB, e.g., tyrosine hydroxylase and dopaminergic activity in type I cells. There is also evidence that CH induces neurochemical changes within the innervation of the CB, e.g., nitric oxide synthase. During CH the sensitivity of the CB chemoreceptors to hypoxia is increased but the mechanisms by which the many CH-induced structural and neurochemical changes affect the sensitivity of CB to hypoxia remains to be established.

Innervation of the carotid body: Immunohistochemical, denervation, and retrograde tracing studies

This review presents information about multiple neurochemical substances in the carotid body. Nerve fibers around blood vessels and glomus cells within the chemoreceptive organ contain immunoreactivities (IR) for tyrosine hydroxylase (TH), calcitonin gene-related peptide (CGRP), substance P (SP), galanin (GAL), vasoactive intestinal polypeptide (VIP), neuropeptide Y (NPY), calretinin (CR), calbindin D-28k (CB), parvalbumin (PV), and nitric oxide synthase (NOS). Parasympathetic neurons scattered around the carotid body contain VIP, choline acetyltransferase, and vanilloid receptor 1-like receptor. In the mammalian carotid body, transection of the carotid sinus nerve (CSN) causes the absence or decrease of CGRP-, SP-, and NOS-immunoreactive (IR) nerve fibers, whereas all NPY-IR nerve fibers disappear after removal of the superior cervical ganglion. Most VIP-IR nerve fibers disappear but a few persist after sympathetic ganglionectomy. In addition, the CSN transection appears to cause the acquisition of GAL-IR in originally immunonegative glomus cells and nerve fibers within the rat carotid body. On the other hand, 4%, 25%, 17%, and less than 1% of petrosal neurons retrogradely labeled from the rat CSN contain TH-, CGRP-, SP-, and VIP-IR, respectively. In the chicken carotid body, many CGRP- and SP-IR nerve fibers disappear after vagus nerve transection or nodose ganglionectomy. GAL-, NPY-, and VIP-IR nerve fibers mostly disappear after removal of the 14th cervical ganglion of the sympathetic trunk. The origin and functional significance of the various neurochemical substances present in the carotid body is discussed.

Regulation of gene expression for neurotransmitters during adaptation to hypoxia in oxygen-sensitive neuroendocrine cells

Reduced oxygen tension (hypoxia) in the environment stimulates oxygen-sensitive cells in the carotid body (CB). Upon exposure to hypoxia, the CB immediately triggers a reflexive physiological response, thereby increasing respiration. Adaptation to hypoxia involves changes in the expression of various CB genes, whose products are involved in the transduction and modulation of the hypoxic signal to the central nervous system (CNS). Genes encoding neurotransmitter-synthesizing enzymes and receptors are particularly important in this regard. The cellular response to hypoxia correlates closely with the release and biosynthesis of catecholamines. The gene expression of tyrosine hydroxylase (TH), the rate-limiting enzyme for catecholamine biosynthesis, is regulated by hypoxia in the CB and in the oxygen-sensitive cultured PC12 cell line. Recently, genomic microarray studies have identified additional genes regulated by hypoxia. Patterns of gene expression vary, depending on the type of applied hypoxia, e.g., intermittent vs. chronic. Construction of a hypoxia-regulated, CB-specific, subtractive cDNA library will enable us to further characterize regulation of gene expression in the CB.

Carotid labyrinth of amphibians

The amphibian carotid labyrinth is a characteristic maze-like vascular expansion at the bifurcation of the common carotid artery into the internal and external carotid arteries. The carotid labyrinths of anurans are spherical and those of urodeles are oblong. In the intervascular stroma of both anuran and urodelan carotid labyrinths, the glomus cells (type I cells, chief cells) are distributed singly or in clusters between connective tissue cells and smooth muscle cells. In fluorescence histochemistry, the glomus cells emit intense fluorescence for biogenic monoamines. In fine structure, the glomus cells are characterized by a number of dense-cored vesicles in their cytoplasm. The glomus cells have long, thin cytoplasmic processes, some of which are closely associated with smooth muscle cells, endothelial cells, and pericytes. Afferent, efferent, and reciprocal synapses are found on the glomus cells. The morphogenesis of the carotid labyrinth starts in the larvae at the point where the carotid arch descends to the internal gills. Through the early stages of larval development, the slightly expanded region of the external carotid artery becomes closely connected with the carotid arch. By the end of the foot stage, the expanded region becomes globular, and at the final stage of metamorphosis the carotid labyrinth is close to its adult form. In fine structure, the glomus cells appear as early as the initial stage of larval development. At the middle stages of development, the number of dense-cored vesicles increases remarkably. Distinct afferent synapses are found in juveniles, although efferent synapses can be seen during metamorphosis. The carotid labyrinth is innervated by nerve fibers containing several kinds of regulatory neuropeptides. Double-immunolabeling in combination with a multiple dye filter system demonstrates the coexistence of two different neuropeptides. The amphibian carotid labyrinth has been electrophysiologically confirmed to have arterial chemo- and baroreceptor functions analogous to those of the mammalian carotid body and carotid sinus. The ultrastructural characteristics of the glomus cells during and after metamorphosis suggest that the glomus cells contribute to the chemoreception after metamorphosis. The three-dimensional fine structure of vascular corrosion casts suggests that the amphibian carotid labyrinth has the appropriate architecture for controlling vascular tone and the findings throughout metamorphosis reveal that the vascular regulatory function begins at an early stage of metamorphosis. In addition, immunohistochemical studies suggest that the vascular regulation in the carotid labyrinth is under peptidergic innervation. Thus, the multiple functions of the carotid labyrinth underline the importance of this relatively small organ for maintenance of homeostasis and appropriate blood supply to the cephalic region.

Carotid body and glomus cells distributed in the wall of the common carotid artery in the bird

In the bird the carotid body is located between the distal (nodose) ganglion of the vagus nerve and the recurrent laryngeal nerve at the beginning of the common carotid artery, that is, the organ is located at the cervicothoracic border. The chicken carotid body receives numerous branches from the vagus and the recurrent laryngeal nerves. In addition, dense networks of the peptidergic nerve fibers immunoreactive for substance P, calcitonin gene-related peptide (CGRP), vasoactive intestinal peptide (VIP), galanin, and neuropeptide Y (NPY) are distributed in and around the carotid body parenchyma. The substance P- and CGRP-immunoreactive fibers are derived from both the superior and inferior ganglia of the vagus nerve. The VIP-, galanin-, and NPY-immunoreactive fibers originate from the 14th cervical ganglion of the sympathetic trunk. The endocrine organs including the thyroid gland, parathyroid glands, carotid body, and ultimobranchial gland are situated as a continuous series along the common carotid artery. The organs are supplied with arteries arising as one trunk from the common carotid artery. Glomus cells are widely distributed not only in the carotid body but also in the wall of the common carotid artery and around the common trunk and its branches. The glomus cells of the chicken carotid body exhibit intense immunoreactivity for serotonin, tyrosine hydroxylase, and chromogranin A. The cells located in the wall of the common carotid artery further express NPY mRNA and peptide. In the chickens exposed to isocapnic hypoxia for 35 days, 3-4-fold increase of the carotid body volume is induced and the carotid body glomus cells show enhanced synthetic and secretory activities. On the other hand, the cells in the wall of the common carotid artery display little changes after the long-term hypoxia, having different functions from the carotid body. The carotid body rudiment is formed in the lateral wall of the third branchial artery. The neural cells immunoreactive for TuJ1, PGP 9.5, and HNK-1, which are continuous with the inferior vagal (nodose) ganglion, first surround and then invade both the carotid body rudiment and the other portions of the third branchial artery, becoming glomus cells.

Depression of hypoxic arousal response in adolescent mice following antenatal vasoactive intestinal polypeptide blockade

Late-gestation blockade of vasoactive intestinal polypeptide (VIP) activity in pregnant mice produces discrete morphological abnormalities in the somatosensory cortex of offspring. We investigated the functional implications of this lesion on the behavioural arousal response to moderate hypoxia. Pregnant mice received twice-daily injections of 200 microl saline (control), or saline + 50 microg VIP antagonist (anti-VIP) on embryonic days 17 and 18. Offspring were studied unrestrained at 6-7 weeks after birth, in a bias-flow whole-body plethysmograph during behavioural quiet sleep. Arousal was defined by movement (MVT) lasting > or =1 s. Hypoxic ventilatory (HVR) and arousal responses were measured during a 5 min exposure to 10 % O(2)-3 % CO(2) (hypoxia); peripheral chemoreflex drive was estimated by transient hyperoxia administered at rest and end-hypoxia (Dejours-type test). MVTs increased in all mice during hypoxia, but in anti-VIP mice: (a) MVT onset was delayed (174 +/- 90 vs. 108 +/- 59 s from the start of hypoxia, anti-VIP vs. control; P = 0.008); and (b) MVTs were less frequent, and total MVT time in hypoxia was less (8 +/- 7 vs. 15 +/- 9 %; P = 0.03). The HVR, and peripheral drive at rest and end-hypoxia were comparable in control and anti-VIP mice. In conclusion, a significant arousal deficit was evident in anti-VIP mice. This was not associated with obviously deranged peripheral or brainstem-mediated responses to hypoxia during sleep. This may signal a general deficit in the way hypoxic distress is monitored and processed, and arousal initiated and sustained in these mice.

Carotid body mechanisms in acclimatization to hypoxia

Most studies oriented toward examining mechanisms increasing carotid body (CB) sensitivity to hypoxia during ventilatory acclimatization (VAH) have focussed on the role of known neuromodulators of CB function. Two general categories of the neuromodulatory agents studied most extensively could be considered: those thought to be primarily inhibitory to CB function: dopamine, norepinephrine, nitric oxide and those thought to be primarily excitatory: substance P, endothelin. There is evidence that these putative inhibitory agents are up-regulated in the first weeks of chronic hypoxia and that substance P is down-regulated. All these changes would favor a decrease in CB sensitivity to hypoxia. There are data suggesting that CB endothelin activity is up-regulated in rats subjected to chronic hypoxia, a direction suggesting increased CB sensitivity to hypoxia. Dopamine may have an excitatory as well as an inhibitory role on the CB, but there is not yet evidence to indicate that an excitatory role for DA exists in chronic hypoxia. Ion channel studies of type I CB cells suggest increased excitability after prolonged hypoxia. The role of excitatory CB nicotinic receptors and putative serotonin type 3 receptors should be examined further for their potential role in VAH. It is suggested that a balance of excitatory and inhibitory modulation is responsible for increased CB sensitivity to hypoxia during VAH.