Focal central chemoreceptor sensitivity in the RTN studied with a CO2 diffusion pipette in vivo

Dorsal Raphe Serotonin Neurons Mediate CO2-Induced Arousal from Sleep

Arousal from sleep in response to CO₂ is a critical protective phenomenon. Dysregulation of CO₂-induced arousal contributes to morbidity and mortality from prevalent diseases, such as obstructive sleep apnea and sudden infant death syndrome. Despite the critical nature of this protective reflex, the precise mechanism for CO₂-induced arousal is unknown. Because CO₂ is a major regulator of breathing, prevailing theories suggest that activation of respiratory chemo- and mechano-sensors is required for CO₂-induced arousal. However, populations of neurons that are not involved in the regulation of breathing are also chemosensitive. Among these are serotonin (5-HT) neurons in the dorsal raphe nucleus (DRN) that comprise a component of the ascending arousal system. We hypothesized that direct stimulation of these neurons with CO₂ could cause arousal from sleep independently of enhancing breathing. Dialysis of CO₂-rich acidified solution into DRN, but not medullary raphe responsible for modulating breathing, caused arousal from sleep. Arousal was lost in mice with a genetic absence of 5-HT neurons, and with acute pharmacological or optogenetic inactivation of DRN 5-HT neurons. Here we demonstrate that CO₂ can cause arousal from sleep directly, without requiring enhancement of breathing, and that chemosensitive 5-HT neurons in the DRN critically mediate this arousal. Better understanding mechanisms underlying this protective reflex may lead to interventions to reduce disease-associated morbidity and mortality.SIGNIFICANCE STATEMENT Although CO₂-induced arousal is critical to a number of diseases, the specific mechanism is not well understood. We previously demonstrated that serotonin (5-HT) neurons are important for CO₂-induced arousal, as mice without 5-HT neurons do not arouse to CO₂ Many have interpreted this to mean that medullary 5-HT neurons that regulate breathing are important in this arousal mechanism. Here we found that direct application of CO₂-rich aCSF to the dorsal raphe nucleus, but not the medullary raphe, causes arousal from sleep, and that this arousal was lost with genetic ablation or acute inhibition of 5-HT neurons. We propose that 5-HT neurons in the dorsal raphe nucleus can be activated directly by CO₂ to cause arousal independently of respiratory activation.

The nervous system of airways and its remodeling in inflammatory lung diseases

Inflammatory lung diseases are associated with bronchospasm, cough, dyspnea and airway hyperreactivity. The majority of these symptoms cannot be primarily explained by immune cell infiltration. Evidence has been provided that vagal efferent and afferent neurons play a pivotal role in this regard. Their functions can be altered by inflammatory mediators that induce long-lasting changes in vagal nerve activity and gene expression in both peripheral and central neurons, providing new targets for treatment of pulmonary inflammatory diseases.

Optogenetic stimulation of adrenergic C1 neurons causes sleep state-dependent cardiorespiratory stimulation and arousal with sighs in rats

RATIONALE

The rostral ventrolateral medulla (RVLM) contains central respiratory chemoreceptors (retrotrapezoid nucleus, RTN) and the sympathoexcitatory, hypoxia-responsive C1 neurons. Simultaneous optogenetic stimulation of these neurons produces vigorous cardiorespiratory stimulation, sighing, and arousal from non-REM sleep.

OBJECTIVES

To identify the effects that result from selectively stimulating C1 cells.

METHODS

A Cre-dependent vector expressing channelrhodopsin 2 (ChR2) fused with enhanced yellow fluorescent protein or mCherry was injected into the RVLM of tyrosine hydroxylase (TH)-Cre rats. The response of ChR2-transduced neurons to light was examined in anesthetized rats. ChR2-transduced C1 neurons were photoactivated in conscious rats while EEG, neck muscle EMG, blood pressure (BP), and breathing were recorded.

MEASUREMENTS AND MAIN RESULTS

Most ChR2-expressing neurons (95%) contained C1 neuron markers and innervated the spinal cord. RTN neurons were not transduced. While the rats were under anesthesia, the C1 cells were faithfully activated by each light pulse up to 40 Hz. During quiet resting and non-REM sleep, C1 cell stimulation (20 s, 2-20 Hz) increased BP and respiratory frequency and produced sighs and arousal from non-REM sleep. Arousal was frequency-dependent (85% probability at 20 Hz). Stimulation during REM sleep increased BP, but had no effect on EEG or breathing. C1 cell-mediated breathing stimulation was occluded by hypoxia (12% FIO2), but was unchanged by 6% FiCO2.

CONCLUSIONS

C1 cell stimulation reproduces most effects of acute hypoxia, specifically cardiorespiratory stimulation, sighs, and arousal. C1 cell activation likely contributes to the sleep disruption and adverse autonomic consequences of sleep apnea. During hypoxia (awake) or REM sleep, C1 cell stimulation increases BP but no longer stimulates breathing.

Regulation of breathing and autonomic outflows by chemoreceptors

Lung ventilation fluctuates widely with behavior but arterial PCO2 remains stable. Under normal conditions, the chemoreflexes contribute to PaCO2 stability by producing small corrective cardiorespiratory adjustments mediated by lower brainstem circuits. Carotid body (CB) information reaches the respiratory pattern generator (RPG) via nucleus solitarius (NTS) glutamatergic neurons which also target rostral ventrolateral medulla (RVLM) presympathetic neurons thereby raising sympathetic nerve activity (SNA). Chemoreceptors also regulate presympathetic neurons and cardiovagal preganglionic neurons indirectly via inputs from the RPG. Secondary effects of chemoreceptors on the autonomic outflows result from changes in lung stretch afferent and baroreceptor activity. Central respiratory chemosensitivity is caused by direct effects of acid on neurons and indirect effects of CO2 via astrocytes. Central respiratory chemoreceptors are not definitively identified but the retrotrapezoid nucleus (RTN) is a particularly strong candidate. The absence of RTN likely causes severe central apneas in congenital central hypoventilation syndrome. Like other stressors, intense chemosensory stimuli produce arousal and activate circuits that are wake- or attention-promoting. Such pathways (e.g., locus coeruleus, raphe, and orexin system) modulate the chemoreflexes in a state-dependent manner and their activation by strong chemosensory stimuli intensifies these reflexes. In essential hypertension, obstructive sleep apnea and congestive heart failure, chronically elevated CB afferent activity contributes to raising SNA but breathing is unchanged or becomes periodic (severe CHF). Extreme CNS hypoxia produces a stereotyped cardiorespiratory response (gasping, increased SNA). The effects of these various pathologies on brainstem cardiorespiratory networks are discussed, special consideration being given to the interactions between central and peripheral chemoreflexes.

Timing, sleep, and respiration in health and disease

Breathing is perhaps the physiological function that is most vital to human survival. Without breathing and adequate oxygenation of tissues, life ceases. As would be expected for such a vital function, breathing occurs automatically, without the requirement of conscious input. Breathing is subject to regulation by a variety of factors including circadian rhythms and vigilance state. Given the need for breathing to occur continuously with little tolerance for interruption, it is not surprising that breathing is subject to both circadian phase-dependent and vigilance-state-dependent regulation. Similarly, the information regarding respiratory state, including blood-gas concentrations, can affect circadian timing and sleep-wake state. The exact nature of the interactions between breathing, circadian phase, and vigilance state can vary depending upon the species studied and the methodologies employed. These interactions between breathing, circadian phase, and vigilance state may have important implications for a variety of human diseases, including sleep apnea, asthma, sudden unexpected death in epilepsy, and sudden infant death syndrome.

The respiratory chemoreception conundrum: light at the end of the tunnel?

Arterial PCO₂ is tightly regulated via changes in breathing. A rise in PCO₂ activates the carotid bodies and exerts additional effects on neurons located within the CNS, causing an increase in lung ventilation. Central respiratory chemoreception refers to the component of this homeostatic reflex that is triggered by activation of receptors located within the brain (central chemoreceptors). Throughout the body, CO₂ generally operates via the proxy of pH. Since countless proteins, ion channels and neurons display some degree of pH-sensitivity, the notion that central respiratory chemoreception could rely on a few specialized neurons seems a priori counter-intuitive. Yet, two types of neurons currently stand out as critically important for breathing regulation by CO₂: the retrotrapezoid nucleus (RTN) and the raphe. RTN neurons are glutamatergic, strongly activated by hypercapnia in vivo and by CO₂ or protons in slices. These neurons target selectively the pontomedullary regions implicated in generating the respiratory rhythm and pattern. Their response to CO₂ seems to involve both cell-autonomous and paracrine effects of CO₂, the latter presumably mediated by the surrounding glia. The specific connections that these excitatory neurons establish with the rest of the breathing network are likely to be the main explanation of their importance to respiratory chemoreception. Serotonergic neurons have a powerful stimulatory effect on breathing, they facilitate the chemoreflexes and a subset of them likely function as CO₂ sensors. Opto- and pharmacogenetic methods have played an important role in assessing the contribution of RTN and serotonergic neurons as well as glial cells to respiration. These particular experiments are emphasized here for thematic reasons although the current perception of the importance of the RTN and serotonergic cells to respiratory chemoreception also relies on many other types of evidence. A small portion of this evidence is presented as background. This article is part of a Special Issue entitled Optogenetics (7th BRES).

Ventilatory effects of substance P-saporin lesions in the nucleus tractus solitarii of chronically hypoxic rats

During ventilatory acclimatization to hypoxia (VAH), time-dependent increases in ventilation lower Pco(2) levels, and this persists on return to normoxia. We hypothesized that plasticity in the caudal nucleus tractus solitarii (NTS) contributes to VAH, as the NTS receives the first synapse from the carotid body chemoreceptor afferents and also contains CO(2)-sensitive neurons. We lesioned cells in the caudal NTS containing the neurokinin-1 receptor by microinjecting the neurotoxin saporin conjugated to substance P and measured ventilatory responses in awake, unrestrained rats 18 days later. Lesions did not affect hypoxic or hypercapnic ventilatory responses in normoxic control rats, in contrast to published reports for similar lesions in other central chemosensitive areas. Also, lesions did not affect the hypercapnic ventilatory response in chronically hypoxic rats (inspired Po(2) = 90 Torr for 7 days). These results suggest functional differences between central chemoreceptor sites. However, lesions significantly increased ventilation in normoxia or acute hypoxia in chronically hypoxic rats. Hence, chronic hypoxia increases an inhibitory effect of neurokinin-1 receptor neurons in the NTS on ventilatory drive, indicating that these neurons contribute to plasticity during chronic hypoxia, although such plasticity does not explain VAH.

TRPV1 channels in the nucleus of the solitary tract mediate thermal prolongation of the LCR in decerebrate piglets

Elevating body temperature or just the temperature of the dorsal medulla by approximately 2°C prolongs the laryngeal chemoreflex (LCR) in decerebrate neonatal piglets. We tested the hypothesis that transient receptor potential vanilloid 1 (TRPV1) receptors in the nucleus of the solitary tract (NTS) mediate thermal prolongation of the LCR. We studied the effect of a selective TRPV1 receptor antagonist on thermal prolongation of the LCR, and we tested the effect of a TRPV1 agonist on the duration of the LCR under normothermic conditions. We studied 37 decerebrate neonatal piglets between the ages of post-natal days 4 and 7. The TRPV1 receptor antagonist, 5'-iodoresiniferatoxin (65μM/L in 100nL), blocked thermal prolongation of the LCR when injected bilaterally into the region of the NTS. The TRPV1 agonist, resiniferatoxin (0.65-1.0mM/L in 100nL), prolonged the LCR after bilateral injection into the NTS even when the body temperature of each piglet was normal. The effect of the TRPV1 agonists could be blocked by treatment with the GABA(A) receptor antagonist, bicuculline, whether given intravenously (0.3mg/kg) or focally injected bilaterally into the NTS (10mM in 100nL). We conclude that TRPV1 receptors in the NTS mediate thermal prolongation of the LCR.

Retrotrapezoid nucleus and parafacial respiratory group

The rat retrotrapezoid nucleus (RTN) contains about 2000 Phox2b-expressing glutamatergic neurons (ccRTN neurons; 800 in mice) with a well-understood developmental lineage. ccRTN neuron development fails in mice carrying a Phox2b mutation commonly present in the congenital central hypoventilation syndrome. In adulthood, ccRTN neurons regulate the breathing rate and intensity, and may regulate active expiration along with other neighboring respiratory neurons. Prenatally, ccRTN neurons form an autonomous oscillator (embryonic parafacial group, e-pF) that activates and possibly paces inspiration. The pacemaker properties of the ccRTN neurons probably vanish after birth to be replaced by synaptic drives. The neonatal parafacial respiratory group (pfRG) may represent a transitional phase during which ccRTN neurons lose their group pacemaker properties. ccRTN neurons are activated by acidification via an intrinsic mechanism or via ATP released by glia. In summary, throughout life, ccRTN neurons seem to be a critical hub for the regulation of CO(2) via breathing.

TRP channels are involved in mediating hypercapnic Ca2+ responses in rat glia-rich medullary cultures independent of extracellular pH

The medulla contains central chemosensitive cells important for the maintenance of blood gas and pH homeostasis. To identify the intrinsic chemosensitive cells, we measured responses of intracellular Ca(2+) ([Ca(2+)](i)) and H(+) ([H(+)](i)), and membrane potential of rat primary-cultured medullary cells to 6-s exposure to acidosis. The cells showed transient [Ca(2+)](i) increases to extracellular pH 6.8, which was inhibited by the specific ASIC1a blocker (psalmotoxin-1), but did not respond to pH 7.1 in the HEPES-buffered solution. Isocapnic acidosis induced no changes in [Ca(2+)](i), whereas hypercapnic acidosis induced a remarkable Ca(2+) response and an increase in membrane potential in the HCO(3)(-)-buffered solution (pH 7.1). In glia-rich cultures, intracellular acidification preceded the hypercapnic acidosis-induced Ca(2+) response, and acetazolamide, a carbonic anhydrase inhibitor suppressed these responses. Transient receptor potential (TRP) channel broad-spectrum blockers Ni(2+) and ruthenium red, and a TRPV1- and TRPM8-specific blocker N-(4-tertiarybutylphenyl)-4-(3-chloropyridin-2-yl)-tetrahydropyrazine-1(2H)-carbox-amide attenuated the hypercapnic acidosis-induced Ca(2+) response. Subpopulations of cells that exhibited the hypercapnic acidosis-induced Ca(2+) response also responded to the application of capsaicin (TRPV1 agonist) and menthol (TRPM8 agonist). These results suggest that the TRP channel family partially mediates the fast hypercapnic acidosis-induced Ca(2+) response via changes in [H(+)](i) and is a candidate of central chemosensing proteins.

Photostimulation of Phox2b medullary neurons activates cardiorespiratory function in conscious rats

RATIONALE

Hypoventilation is typically treated with positive pressure ventilation or, in extreme cases, by phrenic nerve stimulation. This preclinical study explores whether direct stimulation of central chemoreceptors could be used as an alternative method to stimulate breathing.

OBJECTIVES

To determine whether activation of the retrotrapezoid nucleus (RTN), which is located in the rostral ventrolateral medulla (RVLM), stimulates breathing with appropriate selectivity.

METHODS

A lentivirus was used to induce expression of the photoactivatable cationic channel channelrhodopsin-2 (ChR2) by RVLM Phox2b-containing neurons, a population that consists of central chemoreceptors (the ccRTN neurons) and blood pressure (BP)-regulating neurons (the C1 cells). The transfected neurons were activated with pulses of laser light. Respiratory effects were measured by plethysmography or diaphragmatic EMG recording and cardiovascular effects by monitoring BP, renal sympathetic nerve discharge, and the baroreflex.

MEASUREMENTS AND MAIN RESULTS

The RVLM contained 600 to 900 ChR2-transfected neurons (63% C1, 37% ccRTN). RVLM photostimulation significantly increased breathing rate (+42%), tidal volume (21%), minute volume (68%), and peak expiratory flow (48%). Photostimulation increased diaphragm EMG amplitude (19%) and frequency (21%). Photostimulation increased BP (4 mmHg) and renal sympathetic nerve discharge (43%) while decreasing heart rate (15 bpm).

CONCLUSIONS

Photostimulation of ChR2-transfected RVLM Phox2b neurons produces a vigorous stimulation of breathing accompanied by a small sympathetically mediated increase in BP. These results demonstrate that breathing can be relatively selectively activated in resting unanesthetized mammals via optogenetic manipulation of RVLM neurons presumed to be central chemoreceptors. This methodology could perhaps be used in the future to enhance respiration in humans.

Medullary serotonin neurons and central CO2 chemoreception

Serotonergic (5-HT) neurons are putative central respiratory chemoreceptors, aiding in the brain's ability to detect arterial changes in PCO2 and implement appropriate ventilatory responses to maintain blood homeostasis. These neurons are in close proximity to large medullary arteries and are intrinsically chemosensitive in vitro, characteristics expected for chemoreceptors. 5-HT neurons of the medullary raphé are stimulated by hypercapnia in vivo, and their disruption results in a blunted hypercapnic ventilatory response. More recently, data collected from transgenic and knockout mice have provided further insight into the role of 5-HT in chemosensitivity. This review summarizes current evidence in support of the hypothesis that 5-HT neurons are central chemoreceptors, and addresses arguments made against this role. We also briefly explore the relationship between the medullary raphé and another chemoreceptive site, the retrotrapezoid nucleus, and discuss how they may interact during hypercapnia to produce a robust ventilatory response.

The central respiratory chemoreceptor: where is it located?-Invited article

We review previous reports on the localization of the central chemoreceptor focusing on our studies that used various experimental techniques including lesioning (brainstem transection and removal of pia mater), analyses of neuronal responses to CO(2) by electrophysiological and optical recording, mapping of CO(2)-excitable neurons by c-fos immunohistochemistry and local acidic stimulation. Among these experimental techniques, voltage imaging with calculation of cross correlation coefficients between the respiratory output activity and each pixel, i.e., correlation coefficient imaging technique, enabled us to effectively analyze imaging data without empirical signal processing. The reviewed studies have indicated that the most superficial layer of the rostral ventral medulla, i.e., the surface portions of the nucleus retrotrapezoideus/parafacial respiratory group, nucleus parapyramidal superficialis and nucleus raphe pallidus, is important in central chemoreception. We suggest that one of the major respiratory rhythm generators, i.e., the preBötzinger complex, is not chemosensitive in itself or rather inhibited by CO(2). Based on our detailed analysis of c-fos immunohistochemistry, we propose a cell-vessel architecture model for the central respiratory chemoreceptor. Primary chemoreceptor cells are mainly located beneath large surface vessels within the marginal glial layer of the ventral medulla, and surround fine penetrating vessels that branch from a large surface vessel. Respiratory neurons in the rostral portion of the ventral respiratory group could be intrinsically chemosensitive, but their role in chemoreception might be secondary. Definitive identification of chemosensitive sites and chemoreceptor cells needs further studies.

Control of breathing and volitional respiratory rhythm in humans

When breathing frequency (f) is imperceptibly increased during a volitionally paced respiratory rhythm imposed by an auditory signal, tidal volume (Vt) decreases such that minute ventilation (V̇e) rises according to f-induced dead-space ventilation changes ( 18 ). As a result, significant change in alveolar ventilation and Pco2 are prevented as f varies. The present study was performed to determine what regulatory properties are retained by the respiratory control system, wherein the spontaneous automatic rhythmic activity is replaced by a volitionally paced rhythm. Six volunteers were asked to trigger each breath cycle on hearing a brief auditory signal. The time interval between subsequent auditory signals was imperceptibly changed for 10–15 min, during 1) air breathing ( condition 1), 2) the addition of a 300 ml of instrumental dead space ( condition 2), 3) an increase in the inspired level of CO2 ( condition 3), and 4) light exercise ( condition 4). We found that as f was slowly increased the elaborated Vt decreased in accordance to the background level of CO2 and metabolic rate. Indeed, for any given breath duration, Vt was shifted upward in condition 2 vs. 1, whereas the slope of Vt changes according to the volitionally rhythm was much steeper in conditions 3 and 4 vs. 1. The resulting changes in V̇e offset any f-induced changes in dead-space ventilation in all conditions. We conclude that there is an inherent, fundamental mechanism that elaborates Vt based on f when imposed by the premotor cortex in humans. The chemoreflex and exercise drive to breath interacts with this cortically mediated rhythm maintaining alveolar rather than V̇e constant as f changes. The implications of our findings are discussed in the context of our current understanding of the central generation of breathing rhythm.

Role of chemoreceptors in mediating dyspnea

Dyspnea, or the uncomfortable awareness of respiratory distress, is a common symptom experienced by most people at some point during their lifetime. It is commonly encountered in individuals with pulmonary disease, such as chronic obstructive pulmonary disease (COPD), but can also be seen in healthy individuals after strenuous exercise, at altitude or in response to psychological stress. Dyspnea is a multifactorial sensation involving the brainstem, cortex, and limbic system, as well as mechanoreceptors, irritant receptors and chemoreceptors. Chemoreceptors appear to contribute to the sensation of dyspnea in two ways. They stimulate the respiratory control system in response to hypoxia and/or hypercapnia, and the resultant increase respiratory motor output can be consciously perceived as unpleasant. They also can induce the sensation of dyspnea through an as yet undetermined mechanism-potentially via direct ascending connections to the limbic system and cortex. The goal of this article is to briefly review how changes in blood gases reach conscious awareness and how chemoreceptors are involved in dyspnea.

Peripheral chemoreceptor inputs to retrotrapezoid nucleus (RTN) CO2-sensitive neurons in rats

The rat retrotrapezoid nucleus (RTN) contains pH-sensitive neurons that are putative central chemoreceptors. Here, we examined whether these neurons respond to peripheral chemoreceptor stimulation and whether the input is direct from the solitary tract nucleus (NTS) or indirect via the respiratory network. A dense neuronal projection from commissural NTS (commNTS) to RTN was revealed using the anterograde tracer biotinylated dextran amine (BDA). Within RTN, 51% of BDA-labelled axonal varicosities contained detectable levels of vesicular glutamate transporter-2 (VGLUT2) but only 5% contained glutamic acid decarboxylase-67 (GAD67). Awake rats were exposed to hypoxia (n = 6) or normoxia (n = 5) 1 week after injection of the retrograde tracer cholera toxin B (CTB) into RTN. Hypoxia-activated neurons were identified by the presence of Fos-immunoreactive nuclei. CommNTS neurons immunoreactive for both Fos and CTB were found only in hypoxia-treated rats. VGLUT2 mRNA was detected in 92 +/- 13% of these neurons whereas only 12 +/- 9% contained GAD67 mRNA. In urethane-chloralose-anaesthetized rats, bilateral inhibition of the RTN with muscimol eliminated the phrenic nerve discharge (PND) at rest, during hyperoxic hypercapnia (10% CO(2)), and during peripheral chemoreceptor stimulation (hypoxia and/or i.v. sodium cyanide, NaCN). RTN CO(2)-activated neurons were recorded extracellularly in anaesthetized intact or vagotomized rats. These neurons were strongly activated by hypoxia (10-15% O(2); 30 s) or by NaCN. Hypoxia and NaCN were ineffective in rats with carotid chemoreceptor denervation. Bilateral injection of muscimol into the ventral respiratory column 1.5 mm caudal to RTN eliminated PND and the respiratory modulation of RTN neurons. Muscimol did not change the threshold and sensitivity of RTN neurons to hyperoxic hypercapnia nor their activation by peripheral chemoreceptor stimulation. In conclusion, RTN neurons respond to brain P(CO(2)) presumably via their intrinsic chemosensitivity and to carotid chemoreceptor activation via a direct glutamatergic pathway from commNTS that bypasses the respiratory network. RTN neurons probably contribute a portion of the chemical drive to breathe.

Response of membrane potential and intracellular pH to hypercapnia in neurons and astrocytes from rat retrotrapezoid nucleus

We compared the response to hypercapnia (10%) in neurons and astrocytes among a distinct area of the retrotrapezoid nucleus (RTN), the mediocaudal RTN (mcRTN), and more intermediate and rostral RTN areas (irRTN) in medullary brain slices from neonatal rats. Hypercapnic acidosis (HA) caused pH(o) to decline from 7.45 to 7.15 and a maintained intracellular acidification of 0.15 +/- 0.02 pH unit in 90% of neurons from both areas (n = 16). HA excited 44% of mcRTN (7/16) and 38% of irRTN neurons (6/16), increasing firing rate by 167 +/- 75% (chemosensitivity index, CI, 256 +/- 72%) and 310 +/- 93% (CI 292 +/- 50%), respectively. These responses did not vary throughout neonatal development. We compared the responses of mcRTN neurons to HA (decreased pH(i) and pH(o)) and isohydric hypercapnia (IH; decreased pH(i) with constant pH(o)). Neurons excited by HA (firing rate increased 156 +/- 46%; n = 5) were similarly excited by IH (firing rate increased 167 +/- 38%; n = 5). In astrocytes from both RTN areas, HA caused a maintained intracellular acidification of 0.17 +/- 0.02 pH unit (n = 6) and a depolarization of 5 +/- 1 mV (n = 12). In summary, many neurons (42%) from the RTN are highly responsive (CI 248%) to HA; this may reflect both synaptically driven and intrinsic mechanisms of CO(2) sensitivity. Changes of pH(i) are more significant than changes of pH(o) in chemosensory signaling in RTN neurons. Finally, the lack of pH(i) regulation in response to HA suggests that astrocytes do not enhance extracellular acidification during hypercapnia in the RTN.

P2 receptors modulate respiratory rhythm but do not contribute to central CO2 sensitivity in vitro

Multiple brainstem sites are proposed to contribute to central respiratory chemosensitivity, however, the underlying molecular mechanisms remain unknown. P2X2 subunit-containing ATP receptors, which mediate pH-sensitive currents, appear to contribute to central chemosensitivity in vivo [J. Physiol. 523 (2000) 441]. However, recent data from P2X2 knockout mice [J. Neurosci. 23 (2003) 11315] indicate that they are not essential. To further explore the role of P2 receptors in central chemosensitivity, we examined the effects of P2 receptor agonists/antagonists on respiratory-related activity and CO2-sensitivity of rhythmically-active in vitro preparations from neonatal rat. Our main findings: (i) that putative chemosensitive regions of the ventrolateral medulla are immunoreactive for the P2X2 subunit; (ii) that ATP potentiates respiratory frequency in a dose-dependent, and PPADS-sensitive (P2 receptor antagonist), manner; and (iii) that the increase in burst frequency produced by increasing CO2 is unaffected by PPADS, indicate that ATP is a potent modulator of respiratory activity, but that P2 receptors do not contribute to central chemosensitivity in vitro.

Connexin36 distribution in putative CO2-chemosensitive brainstem regions in rat

Recent work from our laboratory has demonstrated that the gap junction proteins connexin26 (Cx26) and connexin32 (Cx32) are expressed in neurons in putative CO2-chemosensitive brainstem regions in both neonatal and adult rats. Whether the recently identified neuron-specific gap junction protein connexin36 (Cx36) is also present in these brainstem regions remains to be determined. Therefore, in the current experiments, immunoblot and immunohistochemical protocols were used to investigate the regional distribution and cellular localization of Cx36 in putative CO2-chemosensitive brainstem regions in both neonatal and adult rats. Immunoblot analyses revealed Cx36 expression in putative CO2-chemosensitive brainstem regions in each of the age groups examined, although both regional and developmental differences in the relative expression levels were detected. Immunohistochemical analyses confirmed Cx36 expression in neurons in each of the putative CO2-chemosensitive brainstem regions and revealed both somal and dendritic labeling patterns. These findings provide additional morphological evidence supporting the potential for gap junctional communication in these regions in both neonatal and adult rats. We propose that the gap junction protein Cx36 also contributes to the neuroanatomical substrate for gap junctional communication, which is hypothesized to play a role in central CO2 chemoreception.

Effects on breathing in awake and sleeping goats of focal acidosis in the medullary raphe

Our aim was to determine the effects of focal acidification in the raphe obscurus (RO) and raphe pallidus (RP) on ventilation and other physiological variables in both the awake and sleep states in adult goats. Through chronically implanted microtubules, 1) a focal acidosis was created by microdialysis of mock cerebrospinal fluid (mCSF), equilibrated with various levels of CO2, and 2) medullary extracellular fluid (ECF) pH was measured by using a custom-made pH electrode. Focal acidosis in the RO or RP, by dialyzing either 25 or 80% CO2 (mCSF pH approximately 6.8 or 6.3), increased (P < 0.05) inspiratory flow by 8 and 12%, respectively, while the animals were awake during the day, but not at night while they were awake or in non-rapid eye movement sleep. While the animals were awake during the day, there were also increases in heart rate and blood pressure (P < 0.05) but no significant change in metabolic rate or arterial Pco2. Dialysis with mCSF equilibrated with 25 or 80% CO2 reduced ECF pH by the same amount (25%) or three times more (80%) than when inspired CO2 was increased to 7%. During CO2 inhalation, the reduction in ECF pH was only 50% of the reduction in arterial pH. Finally, dialysis in vivo only decreased ECF pH by 19.1% of the change during dialysis in an in vitro system. We conclude that 1) the physiological responses to focal acidosis in the RO and RP are consistent with the existence of chemoreceptors in these nuclei, and 2) local pH buffering mechanisms act to minimize changes in brain pH during systemic induced acidosis and microdialysis focal acidosis and that these mechanisms could be as or more important to pH regulation than the small changes in inspiratory flow during a focal acidosis.

Highly H+-sensitive neurons in the caudal ventrolateral medulla of the rat

The ventral surface of the caudal ventrolateral medulla (cVLM) has been shown to generate intense respiratory responses after surface acid-base stimulation. With respect to their chemosensitive characteristics, cVLM neurons have been less studied than other rostral-most regions of the brainstem. The purpose of these experiments was to determine the bioelectric responses of cVLM neurons to acidic stimuli and to determine their chemosensitive properties. Using extracellular and microiontophoretic techniques, we recorded electrical activities from 117 neurons in an area close to the ventral surface of the cVLM in anaesthetised rats. All neurons were tested for their sensitivity to H+. The fluorescent probe BCECF was used to measure extracellular pH changes produced by the microiontophoretic injection of H+ in brainstem slices. This procedure provided an estimation of the local changes in pH produced by microiontophoretic H+ application in the anaesthetised rat. Neurons coupled to the respiratory cycle, R (n = 51), were not responsive to direct stimulation with H+. Sixty-six neurons that did respond to H+ stimulation were uncoupled from respiration, and identified as NR neurons. These neurons presented distinct ranges of H+ sensitivity. The neuronal sensitivity to H+ was mainly assessed by the slope of the stimulus-response curve, where the steeper the slope, the higher the H+ sensitivity. On this basis, NR neurons were classed as being either weakly or highly sensitive to H+. NR neurons with a high H+ sensitivity (n = 12) showed an average value of 34.17 +/- 7.44 spikes s-1 (100 nC)-1 (mean +/- S.D.) for maximal slope and an EC50 of 126.76 +/- 33 nC. Suprathreshold H+ stimulation of highly sensitive NR neurons elicited bursting pattern responses coupled to the respiratory cycle. The bursting responses, which were synchronised with the inspiratory phase and the early expiratory phase of the respiratory cycle, lasted for several seconds before returning to the steady state firing pattern characteristic of the pre-stimulus condition. These NR neurons, which possess the capacity to detect distinct H+ concentrations in the extracellular microenvironment, are excellent candidates to serve in a chemoreceptor capacity in the caudal medulla.

Muscimol inhibition of medullary raphé neurons decreases the CO2 response and alters sleep in newborn piglets

Medullary raphé neurons are chemosensitive in vitro (Wang et al., J. Physiol. Lond. 511 (1998)), are involved in the ventilatory response to CO(2) in vivo (Dreshaj et al., Respir. Physiol. 111 (1998); Nattie and Li, J. Appl. Physiol. 90 (2001)), and are abnormal in many Sudden Infant Death Syndrome (SIDS) victims (Panigrahy et al., J. Neuropathol. Exp. Neurol. 59 (2000)). In this study we determine whether the ventilatory response to CO(2) is altered when medullary raphé neuronal function is focally and reversibly inhibited in chronically instrumented newborn piglets. Ventilation was measured by whole body plethysmography in room air and in 5% CO(2) before and during microdialysis of muscimol, a gamma-amino butyric acid (GABA(A)) receptor agonist, into the medullary raphé. Muscimol (10 mM in the dialysate), had no effect on eupneic ventilation, but reduced significantly the CO(2) response by 17% during wakefulness. Sleep cycling was also disrupted, as characterized by a significant increase in the percentage of time spent awake and a significant decrease in the percentage of time spent in NREM sleep. Disturbances of medullary raphé function can alter central chemoreception and normal sleep architecture, which may contribute to the pathogenesis of SIDS.

CO2 dialysis in one chemoreceptor site, the RTN: stimulus intensity and sensitivity in the awake rat

We stimulate single central chemoreceptor sites in the unanesthetized rat by focal microdialysis of artificial cerebrospinal fluid (aCSF) equilibrated with 25% CO(2). Here, in the retrotrapezoid nucleus (RTN) we measured the focal stimulus intensity with a pH electrode adjacent to the dialysis probe. During 25% CO(2) dialysis, RTN pH decreased by 0.069 (0.013, SEM) pH units (N=5), 44% of the change observed during 7% CO(2) breathing, -0.157 (0.019) pH units (N=4). During 7% CO(2) breathing, Pa(CO(2)) increased by 15 Torr (N=5). We calculate the deltaPa(CO(2)) that would produce a deltapH at the RTN approximately like that observed during 25% CO(2) dialysis as 44% of 15 Torr, or 6.6 Torr deltaPa(CO(2)). Using ventilatory response data from our lab, we estimate overall chemoreceptor sensitivity as 13% deltaVE/Torr deltaPa(CO(2)) and RTN sensitivity as 3% deltaVE/Torr deltaPa(CO(2)). The RTN provides 23% of the overall response. This may be an underestimate. During RTN stimulation Pa(CO(2)) decreases by 4.9 (0.7) Torr (N=5), which may inhibit other chemoreceptor sites. Multiple chemoreceptor sites may interact to provide high sensitivity in systemic hypercapnia and stability during heterogeneous stimulation and inhibition.

Gap junctions in CO(2)-chemoreception and respiratory control

Recent evidence indicates that gap junctions play a more prominent role in normal functioning of the mammalian central nervous system (CNS) than was once believed. Accumulating evidence from both neonatal and adult rodents indicates that gap junctions participate in multiple aspects of respiratory control, including central CO(2) chemoreception, respiratory rhythmogenesis, and respiratory motoneuron output. This review provides an overview of gap junction neurobiology in the mammalian CNS and presents the anatomical and electrophysiological evidence for gap junctions in CO(2) chemoreception and respiratory control.

Ventral medulla pHi measured in vivo by 31P NMR is not regulated during hypercapnia in anesthetized rat

Chemoreceptors in the ventral medulla contribute to the respiratory response to hypercapnia. Do they 'sense' intracellular pH (pHi)? We measured pHi in the ventral medulla or cortex (control) using 31P-NMR obtained via a novel 3 x 5 mm2 surface coil in anesthetized rats breathing air or 7% CO2. During air breathing over 240 min, pHi decreased slightly from 7.13 +/- 0.02 to 7.05 +/- 0.02 (SEM; n = 5; 2 cortex, 3 ventral medulla). During 180 min of hypercapnia, cortical pHi (n = 4) decreased from 7.17 +/- 0.02 to 6.87 +/- 0.01 by 90 min and recovered by 150 min. Ventral medulla pHi showed no such regulation. It decreased from 7.11 +/- 0.02 to 6.88 +/- 0.02 at 90 min and recovered only after cessation of hypercapnia (n = 5), results consistent with pHi being the chemoreceptor stimulus. However, non-chemoreceptor neurons that contribute to our medullary NMR signal also do not appear to regulate pHi in vitro. Regional differences in pHi regulation between cortex and ventral medulla may be due to both chemosensitive and non-chemosensitive neurons.

Localization of connexin26 and connexin32 in putative CO(2)-chemosensitive brainstem regions in rat

Recent studies have suggested that cell-to-cell coupling, which occurs via gap junctions, may play a role in CO(2) chemoreception. Here, we used immunoblot and immunohistochemical analyses to investigate the presence, distribution, and cellular localization of the gap junction proteins connexin26 (Cx26) and connexin32 (Cx32) in putative CO(2)-chemosensitive brainstem regions in both neonatal and adult rats. Immunoblot analyses revealed that both Cx subtypes were expressed in putative CO(2)-chemosensitive brainstem regions; however, regional differences in expression were observed. Immunohistochemical experiments confirmed Cx expression in each of the putative CO(2)-chemosensitive brainstem regions, and further demonstrated that Cx26 and Cx32 were found in neurons and Cx26 was also found in astrocytes in these regions. Thus, our findings suggest the potential for gap junctional communication in these regions in both neonatal and adult rats. We propose that the gap junction proteins Cx26 and Cx32, at least in part, form the neuroanatomical substrate for this gap junctional communication, which is hypothesized to play a role in central CO(2) chemoreception.

CO2 dialysis in the medullary raphe of the rat increases ventilation in sleep

Central chemoreceptors are widespread within the brain stem. We hypothesize that function at different sites varies with arousal state. In unanesthetized rats, we produced focal acidification at single sites by means of microdialysis using artificial cerebrospinal fluid equilibrated with 25% CO2. Tissue acidosis, measured under anesthesia, is equivalent to that observed with 63 Torr end-tidal PCO2 and is limited to 600 microm. Focal acidification of the retrotrapezoid nucleus increased ventilation by 24% only in wakefulness via an increase in tidal volume (Li A, Randall M, and Nattie E. J Appl Physiol 87: 910-919, 1999). In this study of the medullary raphe, the effect of such focal acidification was in sleep (defined by electroencephalographic and electromyographic criteria): ventilation and frequency increased by 15-20% in non-rapid eye movement sleep, and frequency increased by 15% in rapid eye movement sleep. There was no effect in wakefulness. Chemoreception in the medullary raphe appears to be responsive in sleep. Central chemoreceptors at two different locations appear to vary in effectiveness with arousal state.

Ventilatory effects of impaired glial function in a brain stem chemoreceptor region in the conscious rat

Glia are thought to regulate ion homeostasis, including extracellular pH; however, their role in modulating central CO2 chemosensitivity is unclear. Using a push-pull cannula in chronically instrumented and conscious rats, we administered a glial toxin, fluorocitrate (FC; 1 mM) into the retrotrapezoid nucleus (RTN), a putative chemosensitive site, during normocapnia and hypercapnia. FC exposure significantly increased expired minute ventilation (VE) to a value 38% above the control level during normocapnia. During hypercapnia, FC also significantly increased both breathing frequency and expired VE. During FC administration, maximal ventilation was achieved at approximately 4% CO2, compared with 8-10% CO2 during control hypercapnic trials. RTN perfusion of control solutions had little effect on any ventilatory measures (VE, tidal volume, or breathing frequency) during normocapnic or hypercapnic conditions. We conclude that unilateral impairment of glial function in the RTN of the conscious rat results in stimulation of respiratory output.

Location of central respiratory chemoreceptors in the developing tadpole

The location of central respiratory chemoreceptors in amphibian larvae may change as the central chemoreceptive function shifts from driving gill to driving lung ventilation during metamorphosis. We examined this possibility in the in vitro brain stem of the pre- and postmetamorphic Rana catesbeiana tadpole by microinjecting hypercapnic artificial cerebrospinal fluid (aCSF) while recording fictive lung ventilation. The rostral and caudal brain stem were separately explored systematically using injections of 11 nl of aCSF equilibrated with 100% CO2 that transiently acidified a 500-microm region, producing a maximum reduction in pH of 0.23 +/- 0.06 at the site of injection. In postmetamorphic tadpoles, chemoreceptive sites were concentrated in the rostral compared with the caudal brain stem. No such segregation was observed in the premetamorphic tadpole. We conclude that, as in lung rhythmogenic function, respiratory chemosensitivity emerges rostrally in the amphibian brain stem during development.

Sites of respiratory rhythmogenesis during development in the tadpole

During ontogeny, amphibian larvae experience a dramatic alteration in the motor act of breathing as the premetamorphic gill breather develops into the postmetamorphic lung ventilator. We tested the hypothesis that the site of lung rhythmogenesis relocates during metamorphosis by recording fictive lung ventilation before and after transecting the in vitro brain stem of pre- and postmetamorphic Rana catesbeiana into four segments. In premetamorphic tadpoles, the two caudalmost brain stem segments combined proved to be the minimum brain stem configuration necessary and sufficient for lung burst generation. In the postmetamorphic counterpart, this function was supplied by the combination of the two rostralmost brain stem segments. In the postmetamorphic brain stem, a 500-microm segment lying just rostral to cranial nerve IX conveys rhythmogenic capability to neighboring rostral or caudal segments. We conclude that lung rhythmogenic capability translocates rostrally during development as the tadpole shifts from gill to lung ventilation.

Muscimol dialysis in the rostral ventral medulla reduced the CO(2) response in awake and sleeping piglets

Some victims of sudden infant death syndrome have arcuate nucleus abnormalities. The arcuate nucleus may be homologous with ventral medullary structures in the cat known to be involved in the control of breathing and the response to systemic hypercapnia. We refer to putative arcuate homologues in the piglet collectively as the rostral ventral medulla (RVM). We inhibited the RVM in awake and sleeping, chronically instrumented piglets by microdialysis of the GABA(A) receptor agonist muscimol. Muscimol dialysis (10 and 40 mM) had no effect on eupnea but caused a significant reduction in the response to hypercapnia during both wakefulness (34.8 +/- 8.7 and 30.7 +/- 10.1%, respectively) and sleep (36.7 +/- 6.7 and 49.5 +/- 8.9%, respectively). The effect of muscimol on the CO(2) response was entirely via a reduction in tidal volume and appeared to be greater during non-rapid-eye-movement sleep. We conclude that the piglet RVM contains neurons of importance in the response to systemic CO(2) during both wakefulness and non-rapid-eye-movement sleep. We hypothesize that dysfunction of homologous regions in the human infant could lead to impaired ability to respond to hypercapnia, particularly during sleep, which could potentially be involved in the pathogenesis of sudden infant death syndrome.

Bicuculline dialysis in the retrotrapezoid nucleus (RTN) region stimulates breathing in the awake rat

Muscimol dialysis in the retrotrapezoid nucleus (RTN) region of awake rats reduces tidal volume during air breathing and decreases chemoreception (Nattie, Li, 2000. J. Appl. Physiol., 89, 153-162). Is there an endogenous GABAergic inhibition of the RTN as for medullary respiratory and pressor neurons? Bicuculline microdialysis (30 min; 1 mM) into the RTN region of awake rats reversibly increased tidal volume by 11-16% over the period from 10 to 60 min (P<0.01; six rats). Ventilation increased but this was significant (P<0.05) only at 5, 20, and 25 min as frequency tended to decrease during dialysis. The ventilatory response to 7% CO(2) was unaffected (six rats); dialysis of vehicle alone over 4 h had no effect (five rats). It was concluded that in the awake rat there is ongoing endogenous modulation of RTN effects on tidal volume by a GABAergic process of unknown origin. The lack of effect on the response to systemic hypercapnia suggests that the RTN provides an ongoing endogenous drive to respiration by a process that is independent of its role in chemoreception.

Lesion or muscimol in the rostral ventral medulla reduces ventilatory output and the CO(2) response in decerebrate piglets

Developmental abnormalities have been described in the arcuate nucleus of sudden infant death syndrome (SIDS) victims. The arcuate nucleus has putative homologues in chemosensitive areas of the ventral medulla in animals. We refer to some of these areas collectively as the rostral ventral medulla (RVM). In the RVM of decerebrate piglets 2-15 days of age, we studied the effects of electrolytic lesions (n=7) or microdialysis of muscimol (n=15), a GABAA receptor agonist, on ventilatory output and the response to hypercapnia. Lesions caused a 66.7+/-17.3% reduction in eupneic phrenic minute activity (MA) and abolished the response to hypercapnia. Muscimol dialysis caused a 32.4+/-10.4% reduction in MA with a significant downward displacement of the response to hypercapnia with no significant effect on the slope. We conclude that the piglet RVM contains neurons of vital importance in the maintenance of normal breathing and the response to systemic CO(2). We hypothesize that dysfunction of homologous regions in the human infant could lead to impaired ability to respond to hypercapnia and could potentially be involved in the pathogenesis of SIDS.

Multiple sites for central chemoreception: their roles in response sensitivity and in sleep and wakefulness

Central chemoreceptors appear to be widely distributed in the brainstem. Why are there so many central chemoreceptor sites? This review focuses on two hypotheses. (1) The high sensitivity of the respiratory control system as a whole to small changes in systemic P(CO(2)) results from an additive, or greater, effect of the multiple central chemoreceptor sites. Each site provides a fraction of the total response and, importantly, provides tonic excitatory input in eucapnia as well. (2) Individual central chemoreceptor sites vary in effectiveness depending on the arousal or vigilance state of the animal. For example, some sites are more important in wakefulness; others in sleep. Proof for these hypotheses depends critically on obtaining accurate measures of stimulus intensity at each chemoreceptor site in vivo.

Muscimol dialysis in the retrotrapezoid nucleus region inhibits breathing in the awake rat

Under anesthesia, inactivation of the retrotrapezoid nucleus (RTN) region markedly inhibits breathing and chemoreception. In conscious rats, we dialyzed muscimol for 30 min to inhibit neurons of the RTN region reversibly. Dialysis of artificial cerebrospinal fluid had no effect. Muscimol (1 or 10 mM) significantly decreased tidal volume (VT) (by 16-17%) within 15 min. VT remained decreased for 50 min or more, with recovery by 90 min. Ventilation (VE) decreased significantly (by 15-20%) within 15 min and then returned to baseline within 40 min as a result of an increase in frequency. This, we suggest, is a compensatory physiological response to the reduced VT. Oxygen consumption was unchanged. In response to 7% CO(2) in the 1 mM group, absolute VE and change in VE were significantly reduced (by 19-22%). In the 10 mM group, the response to dialysis included a time-related increase in frequency and decrease in body temperature, which may reflect greater spread of muscimol. In the awake rat, the RTN region provides a portion of the tonic drive to breathe, as well as a portion of the response to hypercapnia.

Mammalian brainstem chemosensitive neurones: linking them to respiration in vitro

Neurones which are excited by CO2 or H+ are present in a number of brainstem structures in addition to the ventrolateral region of the medulla, the site at which the respiratory response to hypercapnia is traditionally believed to originate. In this review we examine recent work concerned with establishing the relationship between these chemosensitive neurones and respiration, the emphasis being placed on the use for this purpose of in vitro preparations of the mammalian brainstem.

CO2, brainstem chemoreceptors and breathing

The regulation of breathing relies upon chemical feedback concerning the levels of CO2 and O2. The carotid bodies, which detect O2, provide tonic excitation to brainstem respiratory neurons under normal conditions and dramatic excitation if O2 levels fall. Feedback for CO2 involves the carotid body and receptors in the brainstem, central chemoreceptors. Small increases in CO2 produce large increases in breathing. Decreases in CO2 below normal can, in sleep and anesthesia, decrease breathing, even to apnea. Central chemoreceptors, once thought localized to the surface of the ventral medulla, are likely distributed more widely with sites presently identified in the: (1) ventrolateral medulla; (2) nucleus of the solitary tract; (3) ventral respiratory group; (4) locus ceruleus; (5) caudal medullary raphé; and (6) fastigial nucleus of the cerebellum. Why so many chemoreceptor sites? Hypotheses, some with supporting data, include the following. Geographical specificity; all regions of the brainstem with respiratory neurons contain chemoreceptors. Stimulus intensity; some sites operate in the physiological range of CO2 values, others only with more extreme changes. Stimulus specificity; CO2 or pH may be sensed by multiple mechanisms. Temporal specificity; some sites respond more quickly to changes on blood or brain CO2 or pH. Syncytium; chemosensitive neurons may be connected via low resistance, gap junctions. Arousal state: sites may vary in effectiveness and importance dependent on state of arousal. Overall, as judged by experiments of nature, and in the laboratory, central chemoreceptors are critical for adequate breathing in sleep, but other aspects of the control system can maintain breathing in wakefulness.

CO(2) microdialysis in retrotrapezoid nucleus of the rat increases breathing in wakefulness but not in sleep

Central chemoreceptors are widespread within the brain stem. We suggest that their function at some sites may vary with the state of arousal. In this study, we tested the hypothesis that the function of chemoreceptors in the retrotrapezoid nucleus (RTN) varies with sleep and wakefulness. In unanesthetized rats, we produced focal acidification of the RTN by means of a microdialysis probe (tip containing the semipermeable membrane = 1-mm length, 240-microm diameter, and 45-nl volume). With the use of a dialysate equilibrated with 25% CO(2), the tissue pH change (measured in anesthetized animals) was 1) limited to within 550 microm of the probe and, 2) at the probe tip, was equivalent to that observed with end-tidal PCO(2) of 63 Torr. This focal acidification of the RTN increased ventilation significantly by 24% above baseline, on average, in 13 trials in seven rats only during wakefulness. The effect was entirely due to an increase in tidal volume. During sleep defined by behavioral criteria, ventilation was unaffected, on average, in 10 trials in seven rats. During sleep, the chemoreceptors in the RTN appear to be inactive, or, if active, the respiratory control system either is not responding or is responding with very low gain. Because ventilation is increased during sleep with all central chemoreceptor sites stimulated via systemic CO(2) application, other central chemoreceptor locations must have enhanced effectiveness.

CO(2)-induced expression of c-fos in the nucleus of the solitary tract and the area postrema of developing swine

This investigation was performed to determine whether hypercapnic exposure elicited expression of the c-fos protooncogene product, FOS, in nucleus of the solitary tract (NTS) and area postrema (AP) neurons of developing swine. Mean arterial blood pressure (MAP) and heart rate (HR) were also monitored to evaluate whether numbers of neurons containing FOS were related to changes of MAP and HR. In each experiment, two litter-matched piglets were prepared simultaneously, i.e., Saffan anesthesia, paralysis, and artificial ventilation (100% O(2)). One animal was exposed to hypercapnia (1 h of 10% CO(2), balance oxygen), while the other continued to breathe 100% O(2). Animals were studied at three different ages: 5-8 days, 13-15 days, and 26-34 days old. In the NTS, FOS expression was prominent in regions corresponding to the general visceral afferent subdivision; the AP showed no such topographic distribution. The number of NTS and AP neurons with FOS in hypercapnic-exposed animals was significantly greater than those of unexposed animals. However, an age-related increase of FOS was observed only for NTS neurons, with the greatest number observed in 13- to 15-day-old animals. Increases of MAP, not HR, were noted during the early part of hypercapnia in the 5- to 8-day-old group; older animals exhibited no change of MAP. Our findings demonstrated that prolonged hypercapnic stimulation elicited FOS expression in AP and NTS neurons of developing animals, and that such expression was non-uniform, depending upon the region studied.

Brain stem lesion size determined by DEAD red or conjugation of neurotoxin to fluorescent beads

Neurotoxin microinjected into the retrotrapezoid nucleus of anesthetized rats decreases phrenic activity and eliminates the response to CO2. In unanesthetized rats, such treatment has no effect on awake, resting breathing and decreases CO2 sensitivity by 40% (M. Akilesh, M. Kamper, A. Li, and E. E. Nattie. J. Appl. Physiol. 82: 469-479, 1997). One important factor in explaining these disparate results is the actual size of the anatomic lesion. In the present study, we injected ibotenic acid into the retrotrapezoid nucleus of anesthetized rats and evaluated lesion size by using two new approaches: 1) DEAD red, a fluorescent probe that enters impaired cells through leaky membranes and binds to nucleic acids, and 2) conjugation of toxin to fluorescent beads. With the use of DEAD red, the region containing labeled dying cells was 313 +/- 104 nl (n = 4), six times larger than the initial injected volume, and the physiological effects on phrenic amplitude, the CO2 response, and blood pressure began within minutes and were substantial. With conjugated toxin, in theory, neuronal damage would be limited to the region of detectable fluorescence (49 +/- 10 nl; n = 4). Effects on phrenic amplitude, CO2 sensitivity, and blood pressure were absent until approximately 2 h postinjection. Control experiments, with 2 h of in vitro incubation of the neurotoxin-microbead conjugate and injection of the supernatant after centrifugation, showed similar results that suggest release of conjugated neurotoxin. We conclude that DEAD red provides a useful means to monitor neuronal impairment in acute studies in vivo. Conjugation of neurotoxin to microbeads may be less reliable in this regard.

Ventilatory effects of glial dysfunction in a rat brain stem chemoreceptor region

Glia are thought to be important in brain extracellular fluid ion and pH regulation, but their role in brain stem sites that sense pH and stimulate breathing is unknown. Using a diffusion pipette, we administered the glial toxin, fluorocitrate (FC; 1 mM) into one such brain stem region, the retrotrapezoid nucleus (RTN) for 45-60 min. This dose and time period were chosen so that the effects of FC would be largely reversible. Within minutes, tissue pH decreased, and respiratory output increased. Both recovered almost completely after cessation of FC administration. The response to systemic CO2 stimulation was unaffected by FC treatment compared with that following control diffusion. Anatomic analysis showed, at the center of FC administration, some small (mean diameter = 5.1 micrometer) cells that stained for DEAD Red, a marker for altered cell membrane permeability, and some fragmented glia (glial fibrillary acidic protein immunohistochemistry). The average RTN tissue volume that contained such DEAD Red-positive cells was 271 nl, approximately 23% of the volume of one RTN region. Reversible disruption of glia in the RTN, a region known to contain central chemoreception, results in an acidic local pH and in stimulation of respiratory output.

Hans Loeschcke, Robert Mitchell and the medullary CO2 chemoreceptors: a brief historical review

In the late 1950s, stimulated by reports from Leusen in Belgium and Winterstein in Germany on ventilatory responses to spinal fluid acid, Hans Loeschcke from Göttingen, and Robert Mitchell of the University of California in San Francisco were independently seeking the site of respiratory chemosensitivity to CO2 which they presumed to be mediated by cerebro-spinal fluid hydrogen ions. In 1960 Loeschcke came to San Francisco to join Mitchell for 3 months of intensive hunting for the site of action. This essay describes the events surrounding the localization of ventral medullary superficial (VMS) chemosensitivity to topical acidification, and some of their subsequent and largely independent work on the location, nature and function of this structure. The discovery led to a vast literature on all aspects of the regulation of respiration.

Neurogenesis of patterns of automatic ventilatory activity

Normal respiration, termed eupnea, is characterized by periodic filling and emptying of the lungs. Eupnea can occur 'automatically' without conscious effort. Such automatic ventilation is controlled by the brainstem respiratory centers of pons and medulla. Following removal of the pons, eupnea is replaced by gasping, marked by brief but maximal inspiratory efforts. The mechanisms by which the respiratory rhythms are generated have been examined intensively. Evidence is discussed that ventilatory activity can be generated in multiple regions of pons and medulla. Eupnea and gasping represent fundamentally different ventilatory patterns. Only for gasping has a critical region for neurogenesis been identified, in the rostral medulla. Gasping may be generated by the discharge of 'pacemaker' neurons. In eupnea, this pacemaker activity is suppressed and incorporated into the pontile and medullary neuronal circuit responsible for the neurogenesis of eupnea. Evidence for ventilatory neurogenesis which has been obtained from a number of in vitro preparations is discussed. A much-used preparation is that of a 'superfused' brainstem of the neonatal rat. However, activities of this preparation differ greatly from those of eupnea, as recorded in vitro or in arterially perfused in vitro preparations. Activities of this 'superfused' preparation are identical with gasping and, hence, results must be reinterpreted accordingly. The possibility is present that mechanisms responsible for generating respiratory rhythms may differ from those responsible for shaping respiratory-modulated discharge patterns of cranial and spinal nerves. The importance of pontile mechanisms in the neurogenesis and control of eupnea is reemphasized.