Peripheral chemosensitivity and central integration: neuroplasticity of catecholaminergic cells under hypoxia

Erythropoietin-Mediated Regulation of Central Respiratory Command

Erythropoietin (Epo) is a cytokine expressed throughout the body, including in the central nervous system where it can act as a breathing modulator in the central respiratory network. In vitro, Epo allows maintaining the activity of respiratory neurons during acute hypoxia, resulting in inhibition of the hypoxia-induced rhythm depression. In vivo, Epo action on the central respiratory command results in enhancement of the acute hypoxic ventilatory response, allowing a better oxygenation of the body by improvement of gases exchanges in the lungs. Importantly, this effect of Epo is age-dependent, being observed at adulthood and at both early and late postnatal ages, but not at middle postnatal ages, when an important setup of the central respiratory command occurs. Epo regulation of the central respiratory command involves at least two intracellular signaling pathways, PI3K-Akt and MEK-ERK pathways. However, the exact mechanism underlying the action of Epo on the central respiratory control remains to be deciphered, as well as the exact cell types and nuclei involved in this control. Epo-mediated effect on the central respiratory command is regulated by several factors, including hypoxia, sex hormones, and an endogen antagonist. Although more knowledge is needed before reaching the clinical trial step, Epo seems to be a promising therapeutic treatment, notably against newborn breathing disorders.

Key Brainstem Structures Activated during Hypoxic Exposure in One-day-old Mice Highlight Characteristics for Modeling Breathing Network in Premature Infants

We mapped and characterized changes in the activity of brainstem cell groups under hypoxia in one-day-old newborn mice, an animal model in which the central nervous system at birth is particularly immature. The classical biphasic respiratory response characterized by transient hyperventilation, followed by severe ventilation decline, was associated with increased c-FOS immunoreactivity in brainstem cell groups: the nucleus of the solitary tract, ventral reticular nucleus of the medulla, retrotrapezoid/parafacial region, parapyramidal group, raphe magnus nucleus, lateral, and medial parabrachial nucleus, and dorsal subcoeruleus nucleus. In contrast, the hypoglossal nucleus displayed decreased c-FOS immunoreactivity. There were fewer or no activated catecholaminergic cells activated in the medulla oblongata, whereas ~45% of the c-FOS-positive cells in the dorsal subcoeruleus were co-labeled. Approximately 30% of the c-FOS-positive cells in the parapyramidal group were serotoninergic, whereas only a small portion were labeled for serotonin in the raphe magnus nucleus. None of the c-FOS-positive cells in the retrotrapezoid/parafacial region were co-labeled for PHOX2B. Thus, the hypoxia-activated brainstem neuronal network of one-day-old mice is characterized by (i) the activation of catecholaminergic cells of the dorsal subcoeruleus nucleus, a structure implicated in the strong depressive pontine influence previously reported in the fetus but not in newborns, (ii) the weak activation of catecholaminergic cells of the ventral reticular nucleus of the medulla, an area involved in hypoxic hyperventilation, and (iii) the absence of PHOX2B-positive cells activated in the retrotrapezoid/parafacial region. Based on these results, one-day-old mice could highlight characteristics for modeling the breathing network of premature infants.

Arousal From Sleep and Sympathetic Excitation During Wakefulness

Obstructive apnea during sleep elevates the set point for efferent sympathetic outflow during wakefulness. Such resetting is attributed to hypoxia-induced upregulation of peripheral chemoreceptor and brain stem sympathetic function. Whether recurrent arousal from sleep also influences daytime muscle sympathetic nerve activity is unknown. We therefore tested, in a cohort of 48 primarily nonsleepy, middle-aged, male (30) and female (18) volunteers (age: 59±1 years, mean±SE), the hypothesis that the frequency of arousals from sleep (arousal index) would relate to daytime muscle sympathetic burst incidence, independently of the frequency of apnea or its severity. Polysomnography identified 24 as having either no or mild obstructive sleep apnea (apnea–hypopnea index <15 events/h) and 24 with moderate-to-severe obstructive sleep apnea (apnea–hypopnea index >15 events/h). Burst incidence correlated significantly with arousal index ( r =0.53; P <0.001), minimum oxygen saturation ( r =−0.43; P =0.002), apnea–hypopnea index ( r =0.41; P =0.004), age ( r =0.36; P =0.013), and body mass index ( r =0.33; P =0.022) but not with oxygen desaturation index ( r =0.28; P =0.056). Arousal index was the single strongest predictor of muscle sympathetic nerve activity burst incidence, present in all best subsets regression models. The model with the highest adjusted R 2 (0.456) incorporated arousal index, minimum oxygen saturation, age, body mass index, and oxygen desaturation index but not apnea–hypopnea index. An apnea- and hypoxia-independent effect of sleep fragmentation on sympathetic discharge during wakefulness could contribute to intersubject variability, age-related increases in muscle sympathetic nerve activity, associations between sleep deprivation and insulin resistance or insomnia and future cardiovascular events, and residual adrenergic risk with persistence of hypertension should therapy eliminate obstructive apneas but not arousals.

Consequences of peripheral chemoreflex inhibition with low-dose dopamine in humans

Low-dose dopamine inhibits peripheral chemoreceptors and attenuates the hypoxic ventilatory response (HVR) in humans. However, it is unknown: (1) whether it also modulates the haemodynamic reactions to acute hypoxia, (2) whether it also modulates cardiac baroreflex sensitivity (BRS) and (3) if there is any effect of dopamine withdrawal. We performed a double-blind, placebo-controlled study on 11 healthy male volunteers. At sea level over 2 days every subject was administered low-dose dopamine (2 μg kg(-1) min(-1)) or saline infusion, during which we assessed both ventilatory and haemodynamic responses to acute hypoxia. Separately, we evaluated effects of initiation and withdrawal of each infusion and BRS. The initiation of dopamine infusion did not affect minute ventilation (MV) or mean blood pressure (MAP), but increased both heart rate (HR) and cardiac output. Concomitantly, it decreased systemic vascular resistance. Dopamine blunted the ventilatory, MAP and HR reactions (hypertension, tachycardia) to acute hypoxia. Dopamine attenuated cardiac BRS to falling blood pressure. Dopamine withdrawal evoked an increase in MV. The magnitude of the increment in MV due to dopamine withdrawal correlated with the size of the HVR and depended on the duration of dopamine administration. The ventilatory reaction to dopamine withdrawal constitutes a novel index of peripheral chemoreceptor function.

Differential regulation of tyrosine hydroxylase by continuous and intermittent hypoxia

Although continuous hypoxia (CH) and intermittent hypoxia (IH) cause reduction in oxygen availability, organisms adapt to the effects of chronic CH whereas IH adversely impacts autonomic functions. Catecholamines are expressed both in the central and peripheral nervous systems and they play important roles in the regulation of cardio-respiratory functions during hypoxia. Tyrosine hydroxylase (TH) is the rate-limiting enzyme for catecholamine synthesis. Several studies have examined the effects of hypoxia on catecholamines by focusing on the regulation of TH. In this article, we present a brief overview of the impact of chronic CH and IH on TH expression, activity and the associated cellular mechanism(s).

The Ventilatory Response to Hypoxia in Mammals: Mechanisms, Measurement, and Analysis

The respiratory response to hypoxia in mammals develops from an inhibition of breathing movements in utero into a sustained increase in ventilation in the adult. This ventilatory response to hypoxia (HVR) in mammals is the subject of this review. The period immediately after birth contains a critical time window in which environmental factors can cause long-term changes in the structural and functional properties of the respiratory system, resulting in an altered HVR phenotype. Both neonatal chronic and chronic intermittent hypoxia, but also chronic hyperoxia, can induce such plastic changes, the nature of which depends on the time pattern and duration of the exposure (acute or chronic, episodic or not, etc.). At adult age, exposure to chronic hypoxic paradigms induces adjustments in the HVR that seem reversible when the respiratory system is fully matured. These changes are orchestrated by transcription factors of which hypoxia-inducible factor 1 has been identified as the master regulator. We discuss the mechanisms underlying the HVR and its adaptations to chronic changes in ambient oxygen concentration, with emphasis on the carotid bodies that contain oxygen sensors and initiate the response, and on the contribution of central neurotransmitters and brain stem regions. We also briefly summarize the techniques used in small animals and in humans to measure the HVR and discuss the specific difficulties encountered in its measurement and analysis.

Breathing at high altitude

Acclimatization to long-term hypoxia takes place at high altitude and allows gradual improvement of the ability to tolerate the hypoxic environment. An important component of this process is the hypoxic ventilatory acclimatization (HVA) that develops over several days. HVA reveals profound cellular and neurochemical re-organization occurring both in the peripheral chemoreceptors and in the central nervous system (in brainstem respiratory groups). These changes lead to an enhanced activity of peripheral chemoreceptor and re-inforce the central translation of peripheral inputs to efficient respiratory motor activity under the steady low O(2) pressure. We will review the cellular processes underlying these changes with a particular emphasis on changes of neurotransmitter function and ion channel properties in peripheral chemoreceptors, and present evidence that low O(2) level acts directly on brainstem nuclei to induce cellular changes contributing to maintain a high tonic respiratory drive under chronic hypoxia.

Do monoamine-synthesizing cells constitute a complex network of oxygen sensors?

Oxygen represents an essential molecule for organisms. Because of this, sophisticated systems of sensors have evolved to monitor oxygenation of tissues. We propose that monoamine-synthesizing cells represent an important part of this system. It is well known that the carotid body, which contains chromaffin cells, serves as a chemical sensor of blood oxygenation. Similarly, the activity of adrenal medullary chromaffin cells is increased during hypoxia. Moreover, neurons located in the central nervous system containing catecholamines, serotonin, and histamine are also sensitive to hypoxia. On the basis of this common sensitivity of monoamine-synthesizing cells to changes in oxygenation we propose the hypothesis that these cells constitute a widely distributed network of sensors that monitor oxygen levels. The role of monoamine-synthesizing cells in monitoring tissue oxygen supply during both physiological and pathological conditions is also discussed.

Noradrenergic modulation of respiratory motor output during tadpole development: Role of alpha-adrenoceptors

Noradrenaline (NA) is an important modulator of respiratory activity. Results from in vitro studies using immature rodents suggest that the effects exerted by NA change during development, but these investigations have been limited to neonatal stages. To address this issue, we used in vitro brainstem preparations of an ectotherm, Rana catesbeiana, at three developmental stages: pre-metamorphic tadpoles, metamorphic tadpoles and fully mature adult bullfrogs. We first compared the effects of NA bath application (0.02-10 micromol l(-1)) on brainstem preparations from both pre-metamorphic (Taylor-Köllros stages VII-XI) and metamorphic tadpoles (TK stages XVIII-XXIII) and adult frogs. The fictive lung ventilation frequency response to NA application was both dose- and stage-dependent. Although no net change was observed in the pre-metamorphic group, NA application decreased fictive lung burst frequency in preparations from more mature animals. These effects were attenuated by application of alpha-adrenoceptor antagonists. Conversely, NA application elicited dose- and stage-dependent increases in fictive buccal ventilation frequency. We then assessed the contribution of alpha-adrenoceptors towards these responses by applying specific agonists (alpha1: phenylephrine; alpha2: clonidine; concentration range from 10 to 200 micromol l(-1) for both). Of the two agonists used, only phenylephrine application consistently mimicked the lung burst frequency response observed during NA application in each stage group. However, both agonists decreased buccal burst frequency, thus suggesting that other (beta) adrenoceptor types mediate this response. We conclude that modulation of both buccal and lung-related motor outputs change during development. NA modulation affects both types of respiratory activities in a distinct fashion, owing to the different adrenoceptor type involved.

Chemoreceptor plasticity and respiratory acclimation in the zebrafishDanio rerio

The goals of this study were to assess the respiratory consequences of exposing adult zebrafish Danio rerio to chronic changes in water gas composition (hypoxia, hyperoxia or hypercapnia) and to determine if any ensuing effects could be related to morphological changes in branchial chemoreceptors. To accomplish these goals, we first modified and validated an established non-invasive technique for continuous monitoring of breathing frequency and relative breathing amplitude in adult fish. Under normal conditions 20% of zebrafish exhibited an episodic breathing pattern that was composed of breathing and non-breathing (pausing/apneic) periods. The pausing frequency was reduced by acute hypoxia (PwO2&lt;130 mmHg)and increased by acute hyperoxia (PwO2&gt;300 mmHg), but was unaltered by acute hypercapnia.

Fish were exposed for 28 days to hyperoxia (PwO2&gt;350 mmHg), or hypoxia (PwO2=30 mmHg) or hypercapnia(PwCO2=9 mmHg). Their responses to acute hypoxia or hypercapnia were then compared to the response of control fish kept for 28 days in normoxic and normocapnic water. In control fish, the ventilatory response to acute hypoxia consisted of an increase in breathing frequency while the response to acute hypercapnia was an increase in relative breathing amplitude. The stimulus promoting the hyperventilation during hypercapnia was increased PwCO2 rather than decreased pH. Exposure to prolonged hyperoxia decreased the capacity of fish to increase breathing frequency during hypoxia and prevented the usual increase in breathing amplitude during acute hypercapnia. In fish previously exposed to hyperoxia,episodic breathing continued during acute hypoxia until PwO2 had fallen below 70 mmHg. In fish chronically exposed to hypoxia, resting breathing frequency was significantly reduced (from 191±12 to 165±16 min–1); however, the ventilatory responses to hypoxia and hypercapnia were unaffected. Long-term exposure of fish to hypercapnic water did not markedly modify the breathing response to acute hypoxia and modestly blunted the response to hypercapnia.

To determine whether branchial chemoreceptors were being influenced by long-term acclimation, all four groups of fish were acutely exposed to increasing doses of the O2 chemoreceptor stimulant, sodium cyanide,dissolved in inspired water. Consistent with the blunting of the ventilatory response to hypoxia, the fish pre-exposed to hyperoxia also exhibited a blunted response to NaCN. Pre-exposure to hypoxia was without effect whereas prior exposure to hypercapnia increased the ventilatory responses to cyanide.

To assess the impact of acclimation to varying gas levels on branchial O2 chemoreceptors, the numbers of neuroepithelial cells (NECs) of the gill filament were quantified using confocal immunofluorescence microscopy. Consistent with the blunting of reflex ventilatory responses, fish exposed to chronic hyperoxia exhibited a significant decrease in the density of NECs from 36.8±2.8 to 22.7±2.3 filament–1.

Effect of chronic hypoxia on cholinergic chemotransmission in rat carotid body

Current views suggest that oxygen sensing in the carotid body occurs in chemosensory type I cells, which excite synaptically apposed chemoafferent nerve terminals in the carotid sinus nerve (CSN). Prolonged exposure in a low-oxygen environment [i.e., chronic hypoxia (CH)] elicits an elevated stimulus-evoked discharge in chemoreceptor CSN fibers (i.e., increased chemosensitivity). In the present study, we evaluated cholinergic chemotransmission in the rat carotid body in an effort to test the hypothesis that CH enhances ACh-mediated synaptic activity between type I cells and chemoafferent nerve terminals. Animals were exposed in a hypobaric chamber (barometric pressure = 380 Torr) for 9–22 days before evaluation of chemoreceptor activity using an in vitro carotid body/CSN preparation. Nerve activity evoked by ACh was significantly larger ( P < 0.01) after CH, suggesting increased expression of cholinergic receptors. Approximately 80% of the CSN impulse activity elicited by ACh (100- or 1,000-μg bolus) in both normal and CH preparations was blocked by the specific nicotinic receptor antagonist mecamylamine (100 μM). CSN activity elicited by acute hypoxia or hypercapnia in normal preparations was likewise blocked (≥80%) in the presence of 100 μM mecamylamine, but after CH the enhanced CSN activity elicited by acute hypoxia or hypercapnia was not reduced in the presence of 100 or 500 μM mecamylamine. A muscarinic receptor antagonist, atropine (10 μM), and a specific nicotinic receptor α7 subunit antagonist, methyllycaconatine (50 nM), blocked ∼50% of the hypoxia-evoked activity in normal preparations but were ineffective after CH. Prolonged exposure to hypoxia appears to dramatically alter chemotransmission in the carotid body, and may induce alternative neurotransmitter mechanisms and/or electrical coupling between type I cells and chemoafferent nerve terminals.

Identification of neurons responding to hypoxia in sudden infant death syndrome

The pathogenesis of sudden infant death syndrome (SIDS) is still not understood, although one of the most credited current hypotheses is the respiratory theory. Considerable evidence has been assembled suggesting that hypoxia in human infants produces an initial increase in ventilation, after which respiration is rapidly inhibited. We investigated the expression of the c-fos proto-oncogene, a marker of activated neurons, particularly by hypoxia, in the medulla oblongata nuclei involved in breathing after birth, with special reference to SIDS. We utilized c-fos protein immunohistochemistry on serial transverse sections of medulla oblongata from 22 SIDS victims. In 60% of the analyzed cases, we observed numerous positive c-fos neurons in the dorsal motor nucleus of the vagal nerve. In control cases, the immunohistochemical labeling was negative or very low. The c-fos protein was expressed in the rostral-intermediate portion of the dorsal motor vagal nucleus, where motoneurons with respiratory-related activity are located. The positive c-fos immunoreactivity observed in SIDS suggests that the neurons of the dorsal motor vagal nucleus involved in the regulation of breathing are able to yield an intense, immediate ventilatory response to hypoxia. Our results support the respiratory theory of SIDS.

Evidence for hypoxia-induced neuronal-to-chromaffin metaplasia in neuroblastoma

We present evidence that in neuroblastoma, a pediatric malignancy of embryonal sympathetic origin, hypoxia, underlies a phenotypic switch from a primitive neuronal to a chromaffin cell type. This conclusion is based on morphological and molecular data on 116 clinical tumors and is supported by data on the phenotypic effects of hypoxia on neuroblastoma cell lines when studied in monolayer culture and as tumor xenografts. In the clinical material, extensive chromaffin features were seen in regions of chronic tumor hypoxia. This was the exclusive form of intra-tumoral maturation of stroma-poor tumors and was also seen in stroma-rich tumors, either exclusively or in combination with ganglion-like cells. In neuroblastoma cell lines, hypoxia induced changes in gene expression associated with the chromaffin features observed in vivo. We therefore propose tumor hypoxia as a major cue determining phenotype in sympathetic tumors of neuroblastic origin. Because it appears to be reversible upon reoxygenation in monolayer culture, we suggest the term metaplasia for the phenomenon.

Neuroplasticity in respiratory motor control

Although recent evidence demonstrates considerable neuroplasticity in the respiratory control system, a comprehensive conceptual framework is lacking. Our goals in this review are to define plasticity (and related neural properties) as it pertains to respiratory control and to discuss potential sites, mechanisms, and known categories of respiratory plasticity. Respiratory plasticity is defined as a persistent change in the neural control system based on prior experience. Plasticity may involve structural and/or functional alterations (most commonly both) and can arise from multiple cellular/synaptic mechanisms at different sites in the respiratory control system. Respiratory neuroplasticity is critically dependent on the establishment of necessary preconditions, the stimulus paradigm, the balance between opposing modulatory systems, age, gender, and genetics. Respiratory plasticity can be induced by hypoxia, hypercapnia, exercise, injury, stress, and pharmacological interventions or conditioning and occurs during development as well as in adults. Developmental plasticity is induced by experiences (e.g., altered respiratory gases) during sensitive developmental periods, thereby altering mature respiratory control. The same experience later in life has little or no effect. In adults, neuromodulation plays a prominent role in several forms of respiratory plasticity. For example, serotonergic modulation is thought to initiate and/or maintain respiratory plasticity following intermittent hypoxia, repeated hypercapnic exercise, spinal sensory denervation, spinal cord injury, and at least some conditioned reflexes. Considerable work is necessary before we fully appreciate the biological significance of respiratory plasticity, its underlying cellular/molecular and network mechanisms, and the potential to harness respiratory plasticity as a therapeutic tool.

PDGF-beta receptor expression and ventilatory acclimatization to hypoxia in the rat

Activation of platelet-derived growth factor-beta (PDGF-beta) receptors in the nucleus of the solitary tract (nTS) modulates the late phase of the acute hypoxic ventilatory response (HVR) in the rat. We hypothesized that temporal changes in PDGF-beta receptor expression could underlie the ventilatory acclimatization to hypoxia (VAH). Normoxic ventilation was examined in adult Sprague-Dawley rats chronically exposed to 10% O(2), and at 0, 1, 2, 7, and 14 days, Northern and Western blots of the dorsocaudal brain stem were performed for assessment of PDGF-beta receptor expression. Although no significant changes in PDGF-beta receptor mRNA occurred over time, marked attenuation of PDGF-beta receptor protein became apparent after day 7 of hypoxic exposure. Such changes were significantly correlated with concomitant increases in normoxic ventilation, i.e., with VAH (r: -0.56, P < 0.005). In addition, long-term administration of PDGF-BB in the nTS via osmotic pumps loaded with either PDGF-BB (n = 8) or vehicle (Veh; n = 8) showed that although no significant changes in the magnitude of acute HVR occurred in Veh over time, the typical attenuation of HVR by PDGF-BB decreased over time. Furthermore, PDGF-BB microinjections did not attenuate HVR in acclimatized rats at 7 and 14 days of hypoxia (n = 10). We conclude that decreased expression of PDGF-beta receptors in the dorsocaudal brain stem correlates with the magnitude of VAH. We speculate that the decreased expression of PDGF-beta receptors is mediated via internalization and degradation of the receptor rather than by transcriptional regulation.

Central nervous system mechanisms of ventilatory acclimatization to hypoxia

Ventilatory acclimatization to hypoxia is the time-dependent increase in ventilation that occurs with chronic exposure to hypoxia. Despite decades of research, the physiological mechanisms that increase the hypoxic ventilatory response during chronic hypoxia are not well understood. This review focuses on adaptations within the central nervous system (CNS) that increase the hypoxic ventilatory response. Although an increase in CNS responsiveness had been proposed many years ago, only recently has strong experimental evidence been provided for an increase in the CNS gain in the rat, which has proved to be a good model of VAH in humans. Within the CNS, several neuroanatomical sites could be involved as well as changes in various neurotransmitters, neuromodulators or signalling mechanisms within any of those sites. Lastly, adaptations within the CNS could involve both direct effects of decreased P(O(2)) and indirect effects of increased afferent nerve activity due to chronic stimulation of the peripheral arterial chemoreceptors.

C-fos gene expression in rat brain around birth: effect of asphyxia and catecholamines

At birth, the mammalian nervous system must adapt rapidly to the new conditions it encounters in the extra-uterine environment. One aspect of this adaptation, known as arousal, is mediated by catecholamines, the levels of which in the brain increase rapidly after birth. The pattern of gene expression also changes. Shortly after birth, expression of the immediate early gene c-fos, known to reflect general neural activity, is up-regulated. Furthermore, asphyxia often occurs in connection with birth. In order to examine the effects of this phenomenon on the expression of c-fos, as well as on the rate of noradrenaline (NA) turnover, asphyxia was induced in rat pups delivered by caesarean section. Northern blot analysis and in situ hybridization revealed that the increase in expression of c-fos in certain areas of the brain was greatly enhanced by asphyxia of moderate duration; whereas more prolonged asphyxia lowered the level of c-fos mRNA. Asphyxia had a similar effect on the rate of NA turnover. Adrenergic receptor antagonists administered prior to birth attenuated the birth-related induction of c-fos mRNA. However, the potentiation of c-fos expression by asphyxia was not altered by these antagonists. Therefore, we propose that while catecholamines play an important role in the induction of c-fos in the brain at birth, the effects of asphyxia involve a different mechanism.

Plasticity in the phenotypic expression of catecholamines and vasoactive intestinal peptide in adult rat superior cervical and stellate ganglia after long-term hypoxia in vivo

Sympathetic ganglia in the adult rat contain various populations of nerve cells which demonstrate plasticity with respect to their transmitter phenotype. The plasticity of the neuronal cell bodies and of the small intensely fluorescent cells in the superior cervical and stellate ganglia in response to hypoxia in vivo (10% O2 for seven days) was assessed by studying the expression of catecholamines and vasoactive intestinal peptide. The levels of norepinephrine, dopamine, 3,4-dihydroxyphenylacetic acid and vasoactive intestinal peptide immunoreactivity were determined. In addition, the density of the immunohistochemical staining of cells for tyrosine hydroxylase and vasoactive intestinal peptide was evaluated. In the intact superior cervical ganglion, hypoxia increased the dopamine level as well as the density of small intensely fluorescent cells immunolabelled for tyrosine hydroxylase and vasoactive intestinal peptide. In the axotomized ganglion, hypoxia elicited a twofold rise in the level of the vasoactive intestinal peptide as well as enhancing the density of neuronal cell bodies immunostained for this peptide. Thus, the effect of hypoxia on the expression of vasoactive intestinal peptide expression in neurons was dependent on neural interactions. In the intact stellate ganglion, hypoxia alone induced a 1.5-fold increase in the density of neuronal cell bodies immunostained for vasoactive intestinal peptide. Thus, ganglia-specific factors appeared to play a role in determining changes in neuronal phenotype in response to hypoxia. The present study provides evidence for the involvement of dopamine and vasoactive intestinal peptide in ganglionic responses to long-term hypoxia as well as for differential responses by the two ganglionic cell populations, i.e. neuronal cell bodies and small intensely fluorescent cells. Changes in the expression of the vasoactive intestinal peptide during long-term hypoxia may be of energetic, trophic and/or synaptic significance. Hypoxia may be considered to be a vasoactive intestinal peptide-inducing factor in sympathetic ganglia.

Differential brainstem Fos-like immunoreactivity after laryngeal-induced coughing and its reduction by codeine

We used the expression of the immediate-early gene c-fos, a marker of neuronal activation, to localize brainstem neuronal populations functionally related to fictive cough (FC). In decerebrate, paralyzed, and ventilated cats, the level of Fos-like immunoreactivity (FLI) was examined in five groups of animals: (1) controls, sham-operated unstimulated animals; (2) coughing cats, including both animals in which FC was elicited by unilateral electrical stimulation of the superior laryngeal nerve (SLN) and (3) those in which FC was elicited by bilateral SLN stimulation; (4) stimulated-treated cats, in which bilateral SLN stimulation was applied after selective blockade of FC by codeine; and (5) codeine controls, sham-operated unstimulated cats subjected to administration of codeine. Fifteen brainstem structures were compared for numbers of labeled cells. Because codeine selectively blocks FC, brainstem nuclei activated specifically during FC were identified as regions showing increased FLI after FC and significant reductions in FLI after FC suppression by codeine in stimulated-treated cats. In coughing animals, we observed a selective immunoreactivity in the interstitial and ventrolateral subdivisions of the nucleus of the tractus solitarius, the medial part of the lateral tegmental field, the internal division of the lateral reticular nucleus, the nucleus retroambiguus, the para-ambigual region, the retrofacial nucleus, and the medial parabrachial nucleus. FLI in all these nuclei was significantly reduced in stimulated-treated cats. Our results are consistent with the involvement of neurons overlapping the main brainstem respiratory-related regions as well as the lateral tegmental field and the lateral reticular nucleus in the neural processing of laryngeal-induced FC.