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

Criteria for central respiratory chemoreceptors: experimental evidence supporting current candidate cell groups

An interoceptive homeostatic system monitors levels of CO2/H+ and provides a proportionate drive to respiratory control networks that adjust lung ventilation to maintain physiologically appropriate levels of CO2 and rapidly regulate tissue acid-base balance. It has long been suspected that the sensory cells responsible for the major CNS contribution to this so-called respiratory CO2/H+ chemoreception are located in the brainstem-but there is still substantial debate in the field as to which specific cells subserve the sensory function. Indeed, at the present time, several cell types have been championed as potential respiratory chemoreceptors, including neurons and astrocytes. In this review, we advance a set of criteria that are necessary and sufficient for definitive acceptance of any cell type as a respiratory chemoreceptor. We examine the extant evidence supporting consideration of the different putative chemoreceptor candidate cell types in the context of these criteria and also note for each where the criteria have not yet been fulfilled. By enumerating these specific criteria we hope to provide a useful heuristic that can be employed both to evaluate the various existing respiratory chemoreceptor candidates, and also to focus effort on specific experimental tests that can satisfy the remaining requirements for definitive acceptance.

Altered 5-HT2A/C receptor binding in the medulla oblongata in the sudden infant death syndrome (SIDS): Part I. Tissue-based evidence for serotonin receptor signaling abnormalities in cardiorespiratory- and arousal-related circuits

The sudden infant death syndrome (SIDS), the leading cause of postneonatal infant mortality in the United States, is typically associated with a sleep period. Previously, we showed evidence of serotonergic abnormalities in the medulla (e.g. altered serotonin (5-HT)1A receptor binding), in SIDS cases. In rodents, 5-HT2A/C receptor signaling contributes to arousal and autoresuscitation, protecting brain oxygen status during sleep. Nonetheless, the role of 5-HT2A/C receptors in the pathophysiology of SIDS is unclear. We hypothesize that in SIDS, 5-HT2A/C receptor binding is altered in medullary nuclei that are key for arousal and autoresuscitation. Here, we report altered 5-HT2A/C binding in several key medullary nuclei in SIDS cases (n = 58) compared to controls (n = 12). In some nuclei the reduced 5-HT2A/C and 5-HT1A binding overlapped, suggesting abnormal 5-HT receptor interactions. The data presented here (Part 1) suggest that a subset of SIDS is due in part to abnormal 5-HT2A/C and 5-HT1A signaling across multiple medullary nuclei vital for arousal and autoresuscitation. In Part II to follow, we highlight 8 medullary subnetworks with altered 5-HT receptor binding in SIDS. We propose the existence of an integrative brainstem network that fails to facilitate arousal and/or autoresuscitation in SIDS cases.

The retrotrapezoid nucleus and the neuromodulation of breathing

Breathing is regulated by a host of arousal and sleep-wake state-dependent neuromodulators to maintain respiratory homeostasis. Modulators such as acetylcholine, norepinephrine, histamine, serotonin (5-HT), adenosine triphosphate (ATP), substance P, somatostatin, bombesin, orexin, and leptin can serve complementary or off-setting functions depending on the target cell type and signaling mechanisms engaged. Abnormalities in any of these modulatory mechanisms can destabilize breathing, suggesting that modulatory mechanisms are not overly redundant but rather work in concert to maintain stable respiratory output. The present review focuses on the modulation of a specific cluster of neurons located in the ventral medullary surface, named retrotrapezoid nucleus, that are activated by changes in tissue CO₂/H⁺ and regulate several aspects of breathing, including inspiration and active expiration.

Lethal physiological effects of carbon dioxide exposure at high concentration in rats

The acute toxicity of high concentrations of carbon dioxide (CO₂) was investigated in anesthetized rats using physiological parameters. At an oxygen concentration of 21%, the survival time decreased in a concentration-dependent manner from ≥7.3 h at 20% CO₂ to 1.0 h at 50% CO₂. The animals were divided into groups that were exposed to 40% CO₂ and 21% O₂ balanced with nitrogen (CO₂ group), 40% CO₂ and 12.6% O₂ (CO₂-Hypoxia group), 0% CO₂ and 12.6% O₂ (Hypoxia group), and 0% CO₂ and 21% O₂ (Control group) for 3 h. In the CO₂ group, mean blood pressure (MBP) increased temporarily in the first 60 min followed by a gradual decrease, while breathing rate (BR) decreased immediately up to 3 h and the concentration of serum indicators reflecting organ damage increased. Most of these effects progressed in the CO₂-Hypoxia group. The Hypoxia group showed a contrasting response to the CO₂ groups in MBP and BR, and a slight partial increase in the serum indicators. Histological changes were not observed in any primary organs of any group, except for eosinophilic or necrosis of pyramidal cells in the hippocampal CA1 region of the CO₂ group. These results indicate that high concentrations of CO₂ inhalation are toxic, likely due to BR suppression, and that hypoxia produced under a high CO₂ environment, while showing little effect on its own, enhances the toxic effects of CO₂.

GABAergic neurons of the medullary raphe regulate active expiration during hypercapnia

The parafacial respiratory group (pFRG), located in the lateral aspect of the rostroventral lateral medulla, has been described as a conditional expiratory oscillator that emerges mainly in conditions of high metabolic challenges to increase breathing. The convergence of inhibitory and excitatory inputs to pFRG and the generation of active expiration may be more complex than previously thought. We hypothesized that the medullary raphe, a region that has long been described to be involved in breathing activity, is also responsible for the expiratory activity under hypercapnic condition. To test this hypothesis, we performed anatomical and physiological experiments in urethane-anesthetized adult male Wistar rats. Our data showed anatomical projections from serotonergic (5-HT-ergic) and GABAergic neurons of raphe magnus (RMg) and obscurus (ROb) to the pFRG region. Pharmacological inhibition of RMg or ROb with muscimol (60 pmol/30 nL) did not change the frequency or amplitude of diaphragm activity and did not generate active expiration. However, under hypercapnia (9-10% CO₂), the inhibition of RMg or ROb increased the amplitude of abdominal activity, without changing the increased amplitude of diaphragm activity. Depletion of serotonergic neurons with saporin anti-SERT injections into ROb and RMg did not increase the amplitude of abdominal activity during hypercapnia. These results show that the presumably GABAergic neurons within the RMg and ROb may be the inhibitory source to modulate the activity of pFRG during hypercapnia condition.NEW & NOTEWORTHY Medullary raphe has been involved in the inspiratory response to central chemoreflex; however, these reports have never addressed the role of raphe neurons on active expiration induced by hypercapnia. Here, we showed that a subset of GABA cells within the medullary raphe directly project to the parafacial respiratory region, modulating active expiration under high levels of CO₂.

Lethal avian influenza A (H5N1) virus replicates in pontomedullary chemosensitive neurons and depresses hypercapnic ventilatory response in mice

The highly pathogenic H5N1 (HK483) viral infection causes a depressed hypercapnic ventilatory response (dHCVR, 20%↓) at 2 days postinfection (dpi) and death at 7 dpi in mice, but the relevant mechanisms are not fully understood. Glomus cells in the carotid body and catecholaminergic neurons in locus coeruleus (LC), neurokinin 1 receptor (NK1R)-expressing neurons in the retrotrapezoid nucleus (RTN), and serotonergic neurons in the raphe are chemosensitive and responsible for HCVR. We asked whether the dHCVR became worse over the infection period with viral replication in these cells/neurons. Mice intranasally inoculated with saline or the HK483 virus were exposed to hypercapnia for 5 min at 0, 2, 4, or 6 dpi, followed by immunohistochemistry to determine the expression of nucleoprotein of H5N1 influenza A (NP) alone and coupled with 1) tyrosine hydroxylase (TH) in the carotid body and LC, 2) NK1R in the RTN, and 3) tryptophan hydroxylase (TPH) in the raphe. HK483 viral infection blunted HCVR by ∼20, 50, and 65% at 2, 4, and 6 dpi. The NP was observed in the pontomedullary respiratory-related nuclei (but not in the carotid body) at 4 and 6 dpi, especially in 20% of RTN NK1R, 35% of LC TH, and ∼10% raphe TPH neurons. The infection significantly reduced the local NK1R or TPH immunoreactivity and population of neurons expressing NK1R or TPH. We conclude that the HK483 virus infects the pontomedullary respiratory nuclei, particularly chemosensitive neurons in the RTN, LC, and raphe, contributing to the severe depression of HCVR and respiratory failure at 6 dpi. NEW & NOTEWORTHY The H5N1 virus infection is lethal due to respiratory failure, but the relevant mechanisms remain unclear. In this study, we demonstrated a gradual diminution of hypercapnic ventilatory response to a degree, leading to respiratory failure over a 6-day infection. Death was associated with viral replication in the pontomedullary respiratory-related nuclei, especially the central chemosensitive neurons. These results not only provide insight into the mechanisms of the lethality of H5N1 viral infection but also offer clues in the development of corresponding treatments to minimize and prevent respiratory failure.

The contribution of endogenous glutamatergic input in the ventral respiratory column to respiratory rhythm

Glutamate is the predominant excitatory neurotransmitter in the ventral respiratory column; however, the contribution of glutamatergic excitation in the individual subregions to respiratory rhythm generation has not been fully delineated. In an adult, in vivo, decerebrate rabbit model during conditions of mild hyperoxic hypercapnia we blocked glutamatergic excitation using the receptor antagonists 2,3-dihydroxy-6-nitro-7-sulfamoyl-benzo[f]quinoxaline-2,3-dione (NBQX) and d(-)-2-amino-5-phosphonopentanoic acid (AP5). Disfacilitation of the preBötzinger Complex caused a decrease in inspiratory and expiratory duration as well as peak phrenic amplitude and ultimately apnea. Disfacilitation of the Bötzinger Complex caused a decrease in inspiratory and expiratory duration; subsequent disfacilitation of the preBötzinger Complex resulted in complete loss of the respiratory pattern but maintained tonic inspiratory activity. We conclude that glutamatergic drive to the preBötzinger Complex is essential for respiratory rhythm generation. Glutamatergic drive to the Bötzinger Complex significantly affects inspiratory and expiratory phase duration. Bötzinger Complex neurons are responsible for maintaining the silent expiratory phase of the phrenic neurogram.

Effects on breathing of agonists to μ-opioid or GABAA receptors dialyzed into the ventral respiratory column of awake and sleeping goats

Pulmonary ventilation (V̇I) in awake and sleeping goats does not change when antagonists to several excitatory G protein-coupled receptors are dialyzed unilaterally into the ventral respiratory column (VRC). Concomitant changes in excitatory neuromodulators in the effluent mock cerebral spinal fluid (mCSF) suggest neuromodulatory compensation. Herein, we studied neuromodulatory compensation during dialysis of agonists to inhibitory G protein-coupled or ionotropic receptors into the VRC. Microtubules were implanted into the VRC of goats for dialysis of mCSF mixed with agonists to either μ-opioid (DAMGO) or GABAA (muscimol) receptors. We found: (1) V̇I decreased during unilateral but increased during bilateral dialysis of DAMGO, (2) dialyses of DAMGO destabilized breathing, (3) unilateral dialysis of muscimol increased V̇I, and (4) dialysis of DAMGO decreased GABA in the effluent mCSF. We conclude: (1) neuromodulatory compensation can occur during altered inhibitory neuromodulator receptor activity, and (2) the mechanism of compensation differs between G protein-coupled excitatory and inhibitory receptors and between G protein-coupled and inotropic inhibitory receptors.

The Onset of the Fetal Respiratory Rhythm: An Emergent Property Triggered by Chemosensory Drive?

The mechanisms responsible for the onset of respiratory activity during fetal life are unknown. The onset of respiratory rhythm may be a consequence of the genetic program of each of the constituents of the respiratory network, so they start to interact and generate respiratory cycles when reaching a certain degree of maturation. Alternatively, generation of cycles might require the contribution of recently formed sensory inputs that will trigger oscillatory activity in the nascent respiratory neural network. If this hypothesis is true, then sensory input to the respiratory generator must be already formed and become functional before the onset of fetal respiration. In this review, we evaluate the timing of the onset of the respiratory rhythm in comparison to the appearance of receptors, neurotransmitter machinery, and afferent projections provided by two central chemoreceptive nuclei, the raphe and locus coeruleus nuclei.

Role of Astrocytes in Central Respiratory Chemoreception

Astrocytes perform various homeostatic functions in the nervous system beyond that of a supportive or metabolic role for neurons. A growing body of evidence indicates that astrocytes are crucial for central respiratory chemoreception. This review presents a classical overview of respiratory central chemoreception and the new evidence for astrocytes as brainstem sensors in the respiratory response to hypercapnia. We review properties of astrocytes for chemosensory function and for modulation of the respiratory network. We propose that astrocytes not only mediate between CO₂/H⁺ levels and motor responses, but they also allow for two emergent functions: (1) Amplifying the responses of intrinsic chemosensitive neurons through feedforward signaling via gliotransmitters and; (2) Recruiting non-intrinsically chemosensitive cells thanks to volume spreading of signals (calcium waves and gliotransmitters) to regions distant from the CO₂/H⁺ sensitive domains. Thus, astrocytes may both increase the intensity of the neuron responses at the chemosensitive sites and recruit of a greater number of respiratory neurons to participate in the response to hypercapnia.

CO2-inhibited neurons in the medullary raphé are GABAergic

Previous studies have reported subsets of medullary raphé neurons that are either stimulated or inhibited by CO2/pH in vitro, in situ, and in vivo. We tested the hypothesis that medullary raphé CO2-inhibited neurons are GABAergic. Extracellular recordings in unanesthetized juvenile in situ rat preparations showed reversible hypercapnia-induced suppression of 19% (63/323) of medullary raphé neurons, and this suppression persisted after antagonism of NMDA, AMPA/kainate, and GABAA receptors. We stained a subset of CO2-inhibited cells and found that most (11/12) had glutamic acid decarboxylase 67 immunoreactivity (GAD67-ir). These data indicate that the majority of acidosis-inhibited medullary raphé neurons are GABAergic, and that their chemosensitivity is independent of major fast synaptic inputs. Thus, CO2-sensitive GABAergic neurons may play a role in central CO2/pH chemoreception.

Purinergic signalling contributes to chemoreception in the retrotrapezoid nucleus but not the nucleus of the solitary tract or medullary raphe

Several brain regions are thought to function as important sites of chemoreception including the nucleus of the solitary tract (NTS), medullary raphe and retrotrapezoid nucleus (RTN). In the RTN, mechanisms of chemoreception involve direct H(+)-mediated activation of chemosensitive neurons and indirect modulation of chemosensitive neurons by purinergic signalling. Evidence suggests that RTN astrocytes are the source of CO2-evoked ATP release. However, it is not clear whether purinergic signalling also influences CO2/H(+) responsiveness of other putative chemoreceptors. The goals of this study are to determine if CO2/H(+)-sensitive neurons in the NTS and medullary raphe respond to ATP, and whether purinergic signalling in these regions influences CO2 responsiveness in vitro and in vivo. In brain slices, cell-attached recordings of membrane potential show that CO2/H(+)-sensitive NTS neurons are activated by focal ATP application; however, purinergic P2-receptor blockade did not affect their CO2/H(+) responsiveness. CO2/H(+)-sensitive raphe neurons were unaffected by ATP or P2-receptor blockade. In vivo, ATP injection into the NTS increased cardiorespiratory activity; however, injection of a P2-receptor blocker into this region had no effect on baseline breathing or CO2/H(+) responsiveness. Injections of ATP or a P2-receptor blocker into the medullary raphe had no effect on cardiorespiratory activity or the chemoreflex. As a positive control we confirmed that ATP injection into the RTN increased breathing and blood pressure by a P2-receptor-dependent mechanism. These results suggest that purinergic signalling is a unique feature of RTN chemoreception.

Neural control of the upper airway: integrative physiological mechanisms and relevance for sleep disordered breathing

The various neural mechanisms affecting the control of the upper airway muscles are discussed in this review, with particular emphasis on structure-function relationships and integrative physiological motor-control processes. Particular foci of attention include the respiratory function of the upper airway muscles, and the various reflex mechanisms underlying their control, specifically the reflex responses to changes in airway pressure, reflexes from pulmonary receptors, chemoreceptor and baroreceptor reflexes, and postural effects on upper airway motor control. This article also addresses the determinants of upper airway collapsibility and the influence of neural drive to the upper airway muscles, and the influence of common drugs such as ethanol, sedative hypnotics, and opioids on upper airway motor control. In addition to an examination of these basic physiological mechanisms, consideration is given throughout this review as to how these mechanisms relate to integrative function in the intact normal upper airway in wakefulness and sleep, and how they may be involved in the pathogenesis of clinical problems such obstructive sleep apnea hypopnea.

Medullary serotonin neurons are CO2 sensitive in situ

Brainstem central chemoreceptors are critical to the hypercapnic ventilatory response, but their location and identity are poorly understood. When studied in vitro, serotonin-synthesizing (5-HT) neurons within the rat medullary raphé are intrinsically stimulated by CO2/acidosis. The contributions of these neurons to central chemosensitivity in vivo, however, are controversial. Lacking is documentation of CO2-sensitive 5-HT neurons in intact experimental preparations and understanding of their spatial and proportional distribution. Here we test the hypothesis that 5-HT neurons in the rat medullary raphé are sensitive to arterial hypercapnia. We use extracellular recording and hypercapnic challenge of spontaneously active medullary raphé neurons in the unanesthetized in situ perfused decerebrate brainstem preparation to assess chemosensitivity of individual cells. Juxtacellular labeling of a subset of recorded neurons and subsequent immunohistochemistry for the 5-HT-synthesizing enzyme tryptophan hydroxylase (TPH) identify or exclude this neurotransmitter phenotype in electrophysiologically characterized chemosensitive and insensitive cells. We show that the medullary raphé houses a heterogeneous population, including chemosensitive and insensitive 5-HT neurons. Of 124 recorded cells, 16 cells were juxtacellularly filled, visualized, and immunohistochemically identified as 5-HT synthesizing, based on TPH-immunoreactivity. Forty-four percent of 5-HT cells were CO2 stimulated (increased firing rate with hypercapnia), while 56% were unstimulated. Our results demonstrate that medullary raphé neurons are heterogeneous and clearly include a subset of 5-HT neurons that are excited by arterial hypercapnia. Together with data identifying intrinsically CO2-sensitive 5-HT neurons in vitro, these results support a role for such cells as central chemoreceptors in the intact system.

Atropine microdialysis within or near the pre-Botzinger Complex increases breathing frequency more during wakefulness than during NREM sleep

Normal activity of neurons within the medullary ventral respiratory column (VRC) in or near the pre-Bötzinger Complex (preBötC) is dependent on the balance of inhibitory and excitatory neuromodulators acting at their respective receptors. The role of cholinergic neuromodulation during awake and sleep states is unknown. Accordingly, our objective herein was to test the hypotheses that attenuation of cholinergic modulation of VRC/preBötC neurons in vivo with atropine would: 1) decrease breathing frequency more while awake than during non-rapid-eye-movement (NREM) sleep and 2) increase other excitatory neuromodulators. To test these hypotheses, we unilaterally dialyzed mock cerebrospinal fluid (mCSF) or 50 mM atropine in mCSF in or near the preBötC region of adult goats during the awake (n = 9) and NREM sleep (n = 7) states. Breathing was monitored, and effluent dialysate was collected for analysis of multiple neurochemicals. Compared with dialysis of mCSF alone, atropine increased (P < 0.05) breathing frequency while awake during the day [+10 breaths (br)/min] and at night (+9 br/min) and, to a lesser extent, during NREM sleep (+5 br/min). Atropine increased (P < 0.05) effluent concentrations of serotonin (5-HT), substance P (SP), and glycine during the day and at night. When atropine was dialyzed in one preBötC and mCSF in the contralateral preBötC, 5-HT and SP increased only at the site of atropine dialysis. We conclude: 1) attenuation of a single neuromodulator results in local changes in other neuromodulators that affect ventilatory control, 2) effects of perturbations of cholinergic neuromodulation on breathing are state-dependent, and 3) interpretation of perturbations in vivo requires consideration of direct and indirect effects.

Central chemoreceptors: locations and functions

Central chemoreception traditionally refers to a change in ventilation attributable to changes in CO2/H(+) detected within the brain. Interest in central chemoreception has grown substantially since the previous Handbook of Physiology published in 1986. Initially, central chemoreception was localized to areas on the ventral medullary surface, a hypothesis complemented by the recent identification of neurons with specific phenotypes near one of these areas as putative chemoreceptor cells. However, there is substantial evidence that many sites participate in central chemoreception some located at a distance from the ventral medulla. Functionally, central chemoreception, via the sensing of brain interstitial fluid H(+), serves to detect and integrate information on (i) alveolar ventilation (arterial PCO2), (ii) brain blood flow and metabolism, and (iii) acid-base balance, and, in response, can affect breathing, airway resistance, blood pressure (sympathetic tone), and arousal. In addition, central chemoreception provides a tonic "drive" (source of excitation) at the normal, baseline PCO2 level that maintains a degree of functional connectivity among brainstem respiratory neurons necessary to produce eupneic breathing. Central chemoreception responds to small variations in PCO2 to regulate normal gas exchange and to large changes in PCO2 to minimize acid-base changes. Central chemoreceptor sites vary in function with sex and with development. From an evolutionary perspective, central chemoreception grew out of the demands posed by air versus water breathing, homeothermy, sleep, optimization of the work of breathing with the "ideal" arterial PCO2, and the maintenance of the appropriate pH at 37°C for optimal protein structure and function.

Central chemoreceptors: locations and functions

Central chemoreception traditionally refers to a change in ventilation attributable to changes in CO2/H(+) detected within the brain. Interest in central chemoreception has grown substantially since the previous Handbook of Physiology published in 1986. Initially, central chemoreception was localized to areas on the ventral medullary surface, a hypothesis complemented by the recent identification of neurons with specific phenotypes near one of these areas as putative chemoreceptor cells. However, there is substantial evidence that many sites participate in central chemoreception some located at a distance from the ventral medulla. Functionally, central chemoreception, via the sensing of brain interstitial fluid H(+), serves to detect and integrate information on (i) alveolar ventilation (arterial PCO2), (ii) brain blood flow and metabolism, and (iii) acid-base balance, and, in response, can affect breathing, airway resistance, blood pressure (sympathetic tone), and arousal. In addition, central chemoreception provides a tonic "drive" (source of excitation) at the normal, baseline PCO2 level that maintains a degree of functional connectivity among brainstem respiratory neurons necessary to produce eupneic breathing. Central chemoreception responds to small variations in PCO2 to regulate normal gas exchange and to large changes in PCO2 to minimize acid-base changes. Central chemoreceptor sites vary in function with sex and with development. From an evolutionary perspective, central chemoreception grew out of the demands posed by air versus water breathing, homeothermy, sleep, optimization of the work of breathing with the "ideal" arterial PCO2, and the maintenance of the appropriate pH at 37°C for optimal protein structure and function.

Respiration and autonomic regulation and orexin

Orexin, a small neuropeptide released from neurons in the hypothalamus with widespread projections throughout the central nervous system, has broad biological roles including the modulation of breathing and autonomic function. That orexin activity is fundamentally dependent on sleep-wake state, and circadian cycle requires consideration of orexin function in physiological control systems in respect to these two state-related activity patterns. Both transgenic mouse studies and focal orexin receptor antagonism support a role for orexins in respiratory chemosensitivity to CO₂ predominantly in wakefulness, with further observations limiting this role to the dark period. In addition, orexin neurons participate in the regulation of sympathetic activity, including effects on blood pressure and thermoregulation. Orexin is also essential in physiological responses to stress. Orexin-mediated processes may operate at two levels: (1) in sleep-wake and circadian states and (2) in stress, for example, the defense or "fight-or-flight" response and panic-anxiety syndrome.

Medullary serotonin neurons and their roles in central respiratory chemoreception

Much progress has been made in our understanding of central chemoreception since the seminal experiments of Fencl, Loeschcke, Mitchell and others, including identification of new brainstem regions and specific neuron types that may serve as central "sensors" of CO(2)/pH. In this review, we discuss key attributes, or minimal requirements a neuron/cell must possess to be defined as a central respiratory chemoreceptor, and summarize how well each of the various candidates fulfill these minimal criteria-especially the presence of intrinsic chemosensitivity. We then discuss some of the in vitro and in vivo evidence in support of the conclusion that medullary serotonin (5-HT) neurons are central chemoreceptors. We also provide an additional hypothesis that chemosensitive medullary 5-HT neurons are poised to integrate multiple synaptic inputs from various other sources thought to influence ventilation. Finally, we discuss open questions and future studies that may aid in continuing our advances in understanding central chemoreception.

State-dependent central chemoreception: a role of orexin

Sites involved in central chemoreception (CCR) are widely distributed in the brain. One possible explanation for the existence of multiple central chemoreceptor sites is the vigilance state-dependent hypothesis, that some sites are of greater importance in wakefulness others in sleep. We briefly summarize the evidence for a distributed network of central chemoreceptor sites and a vigilance state-dependent differentiation among them. We then discuss the role of orexin in vigilance state-dependent CCR based on our recent studies using orexin knockout mice and focal microdialysis of an orexin receptor antagonist at the retrotrapezoid nucleus and medullary raphe in rats. Orexin affects CCR in a vigilance state-dependent manner that varies with circadian time. Orexin also contributes to emotional stress- and other state-dependent related regulation of ventilation, e.g., the defense response. Diversity in central chemoreception including orexin neurons and the synaptic control of respiratory and cardiovascular output neurons appears to be necessary for animals to adapt themselves to constantly changing situations and behavioral states.

Central respiratory chemoreception

By definition central respiratory chemoreceptors (CRCs) are cells that are sensitive to changes in brain PCO(2) or pH and contribute to the stimulation of breathing elicited by hypercapnia or metabolic acidosis. CO(2) most likely works by lowering pH. The pertinent proton receptors have not been identified and may be ion channels. CRCs are probably neurons but may also include acid-sensitive glia and vascular cells that communicate with neurons via paracrine mechanisms. Retrotrapezoid nucleus (RTN) neurons are the most completely characterized CRCs. Their high sensitivity to CO(2) in vivo presumably relies on their intrinsic acid sensitivity, excitatory inputs from the carotid bodies and brain regions such as raphe and hypothalamus, and facilitating influences from neighboring astrocytes. RTN neurons are necessary for the respiratory network to respond to CO(2) during the perinatal period and under anesthesia. In conscious adults, RTN neurons contribute to an unknown degree to the pH-dependent regulation of breathing rate, inspiratory, and expiratory activity. The abnormal prenatal development of RTN neurons probably contributes to the congenital central hypoventilation syndrome. Other CRCs presumably exist, but the supportive evidence is less complete. The proposed locations of these CRCs are the medullary raphe, the nucleus tractus solitarius, the ventrolateral medulla, the fastigial nucleus, and the hypothalamus. Several wake-promoting systems (serotonergic and catecholaminergic neurons, orexinergic neurons) are also putative CRCs. Their contribution to central respiratory chemoreception may be behavior dependent or vary according to the state of vigilance.

Central serotonin neurons are required for arousal to CO2

There is a long-standing controversy about the role of serotonin in sleep/wake control, with competing theories that it either promotes sleep or causes arousal. Here, we show that there is a marked increase in wakefulness when all serotonin neurons are genetically deleted in mice hemizygous for ePet1-Cre and homozygous for floxed Lmx1b (Lmx1b(f/f/p)). However, this only occurs at cool ambient temperatures and can be explained by a thermoregulatory defect that leads to an increase in motor activity to generate heat. Because some serotonin neurons are stimulated by CO(2), and serotonin activates thalamocortical networks, we hypothesized that serotonin neurons cause arousal in response to hypercapnia. We found that Lmx1b(f/f/p) mice completely lacked any arousal response to inhalation of 10% CO(2) (with 21% O(2) in balance N(2)) but had normal arousal responses to hypoxia, sound, and air puff. We propose that serotonin neurons mediate the potentially life-saving arousal response to hypercapnia. Impairment of this response may contribute to sudden unexpected death in epilepsy, sudden infant death syndrome, and sleep apnea.

The caudal solitary complex is a site of central CO(2) chemoreception and integration of multiple systems that regulate expired CO(2)

The solitary complex is comprised of the nucleus tractus solitarius (NTS, sensory) and dorsal motor nucleus of the vagus (DMV, motor), which functions as an integrative center for neural control of multiple systems including the respiratory, cardiovascular and gastroesophageal systems. The caudal NTS-DMV is one of the several sites of central CO(2) chemoreception in the brain stem. CO(2) chemosensitive neurons are fully responsive to CO(2) at birth and their responsiveness seems to depend on pH-sensitive K(+) channels. In addition, chemosensitive neurons are highly sensitive to conditions such as hypoxia (e.g., neural plasticity) and hyperoxia (e.g., stimulation), suggesting they employ redox and nitrosative signaling mechanisms. Here we review the cellular and systems physiological evidence supporting our hypothesis that the caudal NTS-DMV is a site for integration of respiratory, cardiovascular and gastroesophageal systems that work together to eliminate CO(2) during acute and chronic respiratory acidosis to restore pH homeostasis.

Central chemoreception in wakefulness and sleep: evidence for a distributed network and a role for orexin

This minireview examines data showing the locations of central chemoreceptor sites as identified by the presence of ventilatory responses to focal, mild acidification produced in unanesthetized animals in vivo, how the site-specific responses vary by arousal state, and what the emerging role of orexin might be in this state-dependent central chemoreceptor system. We comment on the organization of this distributed central chemoreceptor system and suggest that interactions among sites are synergistic and not additive, which is an important aspect of its normal function.

High CO2/H+ dialysis in the caudal ventrolateral medulla (Loeschcke's area) increases ventilation in wakefulness

Central chemoreception, the detection of CO(2)/H(+) within the brain and the resultant effect on ventilation, was initially localized at two areas on the ventrolateral medulla, one rostral (rVLM-Mitchell's) the other caudal (cVLM-Loeschcke's), by surface application of acidic solutions in anesthetized animals. Focal dialysis of a high CO(2)/H(+) artificial cerebrospinal fluid (aCSF) that produced a milder local pH change in unanesthetized rats (like that with a approximately 6.6mm Hg increase in arterial P(CO2)) delineated putative chemoreceptor regions for the rVLM at the retrotrapezoid nucleus and the rostral medullary raphe that function predominantly in wakefulness and sleep, respectively. Here we ask if chemoreception in the cVLM can be detected by mild focal stimulation and if it functions in a state dependent manner. At responsive sites just beneath Loeschcke's area, ventilation was increased by, on average, 17% (P<0.01) only in wakefulness. These data support our hypothesis that central chemoreception is a distributed property with some sites functioning in a state dependent manner.

The role of medullary serotonin (5-HT) neurons in respiratory control: contributions to eupneic ventilation, CO2 chemoreception, and thermoregulation

The functional roles of the medullary raphé, and specifically 5-HT neurons, are not well understood. It has previously been stated that the role of 5-HT has been so difficult to understand, because "it is implicated in virtually everything, but responsible for nothing"(Cowen PJ. Foreword. In: Serotonin and Sleep: Molecular, Functional and Clinical Aspects, edited by Monti JM, Prandi-Perumal SR, Jacobs BL, Nutt DJ. Switzerland: Birkhauser, 2008). Are 5-HT neurons important, and can we assign a general, or even specific, function to them given their diffuse projections? Recent data obtained from transgenic animals and other model systems indicate that the 5-HT system is not expendable, particularly during postnatal development, and likely plays specific roles in vital functions such as respiratory and thermoregulatory control. We recently provided a detailed and updated review of one specific function of 5-HT neurons, as central respiratory chemoreceptors contributing to the brain's ability to detect changes in pH/CO2 and stimulate adjustments to ventilation accordingly (9). Here, we turn our focus to recent data demonstrating that 5-HT neurons provide an essential excitatory drive to the respiratory network. We then further discuss their role in the CO2 chemoreflex, as well as other homeostatic functions that are closely related to ventilatory control. Last, we provide additional hypotheses/concepts that are worthy of further study, and how 5-HT neurons may be involved in human disease.

Differences between three inbred rat strains in number of K+ channel-immunoreactive neurons in the medullary raphé nucleus

Ventilatory sensitivity to hypercapnia is greater in Dahl salt-sensitive (SS) rats than in Fawn Hooded hypertensive (FHH) and Brown Norway (BN) inbred rats. Since pH-sensitive potassium ion (K(+)) channels are postulated to contribute to the sensing and signaling of changes in CO(2)-H(+) in chemosensitive neurons, we tested the hypothesis that there are more pH-sensitive K(+) channel-immunoreactive (ir) neurons within the medullary raphé nuclei of the highly chemosensitive SS rats than in the other two strains. Medullary tissues from male and female BN, FHH, and SS rats were stained with cresyl violet or with antibodies targeting TASK-1, K(v)1.4, and Kir2.3 channels. K(+) channel-ir neurons were quantified and compared with the total neurons in the region. The total number of neurons in the medullary raphé 1) was greater in male FHH than the other male rats, 2) did not differ among the female rats, and 3) did not differ between sexes. The average number of K(+) channel-ir neurons per section was 30-60 neurons higher in the male SS than in the other rat strains. In contrast, for the females, the number of K(+) channel-ir neurons was greatest in the BN. We also found significant differences in the number of K(+) channel-ir neurons between sexes in SS (males > females) and BN (females > males) rats, but not the FHH strain. Our findings support the hypothesis for males but not for females, suggesting that both genetic background and sex are determinants of K(+) channel immunoreactivity of medullary raphé neurons, and that the expression of pH-sensitive K(+) channels in the medullary raphé does not correlate with the ventilatory sensitivity to hypercapnia.

Micro-opioid receptor agonist injections into the presumed pre-Botzinger complex and the surrounding region of awake goats do not alter eupneic breathing

Opioids are clinically important in the alleviation of pain. An undesirable side effect of opioids is depression of breathing. Data from isolated preparations suggest this effect is due to attenuation of discharge activity of neurons in the pre-Bötzinger complex (preBötzC), a medullary area with respiratory rhythmogenic properties. The purpose of this study was to examine how [d-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO), a mu-opioid receptor agonist, affected breathing after injection into the presumed preBötzC of the adult awake goat. We hypothesized that DAMGO would cause breathing to decrease and become irregular when injected into the presumed preBötzC and the surrounding region of the conscious animal. We further hypothesized that ventilatory sensitivity to CO(2) and hypoxia would be blunted after the injection of DAMGO. Microtubules were bilaterally implanted into the presumed preBötzC of 10 adult female goats. After recovery from the surgery, DAMGO (0.5-10 mul, 1 nM-10 muM) was injected into the presumed preBötzC during the awake state. DAMGO had no effect on pulmonary ventilation [inspiratory minute ventilation (Vi)], respiratory rhythm and pattern, the activation pattern of inspiratory and expiratory muscles, or arterial blood gases during eupneic breathing conditions (P > 0.10). However, DAMGO attenuated (P < 0.05) the evoked increase in breathing frequency when inspired CO(2) was increased, and DAMGO attenuated the Vi response to reduction of inspired O(2) to 10.8% (P < 0.05). We conclude that our data do not provide support for the concept that in awake mammals opioid depression of breathing is due to a directed action of opioids on preBötzC neurons.

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.

Severe spontaneous bradycardia associated with respiratory disruptions in rat pups with fewer brain stem 5-HT neurons

The medullary 5-HT system has potent effects on heart rate and breathing in adults. We asked whether this system mitigates the respiratory instability and bradycardias frequently occurring during the neonatal period. 5,7-Dihydroxytryptamine (5,7-DHT) or vehicle was administered to rat pups at postnatal day 2 (P2), and we then compared the magnitude of bradycardias occurring with disruptions to eupnea in treated and vehicle control littermates at P5-6 and P10-12. We then used a novel method that would allow accurate assessment of the ventilatory and heart rate responses to near square-wave challenges of hypoxia (10% O2), hypercapnia (5 and 8% CO2 in normoxia and hyperoxia), and asphyxia (8% CO2-10% O2), and to the induction of the Hering-Breuer inflation reflex (HBR), a potent, apnea-inducing reflex in newborns. The number of 5-HT-positive neurons was reduced approximately 80% by drug treatment. At both ages, lesioned animals had considerably larger bradycardias during brief apnea; at P5-6, average and severe events were approximately 50% and 70% greater, respectively, in lesioned animals (P = 0.002), whereas at P10-12, events were approximately 23% and 50% greater (P = 0.018). However, lesioning had no effect on the HR responses to sudden gas challenge or the HBR. At P5-6, lesioned animals had reduced breathing frequency and ventilation (Ve), but normal Ve relative to metabolic rate (Ve/Vo2). At P10-12, lesioned animals had a more unstable breathing pattern (P = 0.04) and an enhanced Ve response to moderate hypercapnia (P = 0.007). Within the first two postnatal weeks, the medullary 5-HT system plays an important role in cardiorespiratory control, mitigating spontaneous bradycardia, stabilizing the breathing pattern, and dampening the hypercapnic Ve response.

Focal acidosis in the pre-Botzinger complex area of awake goats induces a mild tachypnea

There are widespread chemosensitive areas in the brain with varying effects on breathing. In the awake goat, microdialyzing (MD) 50% CO(2) at multiple sites within the medullary raphe increases pulmonary ventilation (Vi), blood pressure, heart rate, and metabolic rate (Vo(2)) (11), while MD in the rostral and caudal cerebellar fastigial nucleus has a stimulating and depressant effect, respectively, on these variables (17). In the anesthetized cat, the pre-Bötzinger complex (preBötzC), a hypothesized respiratory rhythm generator, increases phrenic nerve activity after an acetazolamide-induced acidosis (31, 32). To gain insight into the effects of focal acidosis (FA) within the preBötzC during physiological conditions, we tested the hypothesis that FA in the preBötzC during wakefulness would stimulate breathing, by increasing respiratory frequency (f). Microtubules were bilaterally implanted into the preBötzC of 10 goats. Unilateral MD of mock cerebral spinal fluid equilibrated with 6.4% CO(2) did not affect Vi, tidal volume (Vt), or f. Unilateral MD of 25 and 50% CO(2) significantly increased Vi and f by 10% (P < 0.05, n = 10, 17 trials), but Vt was unaffected. Bilateral MD of 6.4, 25, or 50% CO(2) did not significantly affect Vi, Vt, or f (P > 0.05, n = 6, 6 trials). MD of 80% CO(2) caused a 180% increase in f and severe disruptions in airflow (n = 2). MD of any level of CO(2) did not result in any significant changes in mean arterial blood pressure, heart rate, or Vo(2). Thus the data suggest that the preBötzC area is chemosensitive, but the responses to FA at this site are unique compared with other chemosensitive sites.

Differential contribution of storage pools to the extracellular amount of accumbal dopamine in high and low responders to novelty: effects of reserpine

The present study examined the effects of reserpine on the extracellular concentration of accumbal dopamine in high responders (HR) and low responders (LR) to novelty rats. Reserpine reduced the baseline concentration of extracellular accumbal dopamine more in HR than in LR, indicating that the dopamine release is more dependent on reserpine-sensitive storage vesicles in non-challenged HR than in non-challenged LR. In addition, reserpine reduced the novelty-induced increase of the extracellular concentration of accumbal dopamine in LR, but not in HR, indicating that the dopamine release in response to novelty depends on reserpine-sensitive storage vesicles only in LR, not in HR. Our data clearly demonstrate that HR and LR differ in the characteristics of those monoaminergic storage vesicles that mediate accumbal dopamine release.

Ventilatory effects of muscimol microdialysis into the rostral medullary raphé region of conscious rats

We hypothesized that inhibition of the rostral medullary raphe region (MRR), a putative central chemoreceptor location, with the GABA(A) receptor agonist muscimol would decrease ventilatory responses to hypercapnia and hypoxia in conscious rats, and that its known effect at this site on body temperature might alter its effect upon these ventilatory responses. At ambient temperatures of 24.5-26.5 degrees C (Cool), microdialysis of 1mM muscimol into the MRR significantly decreased body temperature by approximately 0.5 degrees C, increased the ventilatory response to 7% CO(2) and decreased the response to 10% O(2). At ambient temperatures of 29.5-30.5 degrees C (Warm), 1 mM muscimol microdialysis no longer decreased body temperature and increased the ventilatory response to hypercapnia and to hypoxia. Muscimol did not significantly affect the VE/VO2 ratio at either temperature. Muscimol significantly increased the hypercapnic ventilatory responses in Cool and Warm conditions and the hypoxic response in Warm conditions, which indicates the presence of an inhibitory effect of rostral MRR neurons sensitive to muscimol. In the Cool condition the ventilatory response to hypoxia is inhibited but appropriately so for the lower VO2 .

Ventilatory response to hypercapnia and hypoxia after extensive lesion of medullary serotonergic neurons in newborn conscious piglets

Acute inhibition of serotonergic (5-HT) neurons in the medullary raphé (MR) using a 5-HT(1A) receptor agonist had an age-dependent impact on the "CO(2) response" of piglets (33). Our present study explored the effect of chronic 5-HT neuron lesions in the MR and extra-raphé on the ventilatory response to hypercapnia and hypoxia in piglets, with possible implications on the role of 5-HT in the sudden infant death syndrome. We established four experimental groups. Group 1 (n = 11) did not undergo any treatment. Groups 2, 3, and 4 were injected with either vehicle or the neurotoxin 5,7-dihydroxytryptamine in the cisterna magna during the first week of life (group 2, n = 9; group 4, n = 11) or second week of life (group 3, n = 10). Ventilation was recorded in response to 5% CO(2) (all groups) and 12% O(2) (group 2) during wakefulness and sleep up to postnatal day 25. Surprisingly, the piglets did not reveal changes in their CO(2) sensitivity during early postnatal development. Overall, considerable lesions of 5-HT neurons (up to 65% decrease) in the MR and extra-raphé had no impact on the CO(2) response, regardless of injection time. Postlesion raphé plasticity could explain why we observed no effect. 5,7-Dihydroxytryptamine-treated males, however, did present a lower CO(2) response during sleep. Hypoxia significantly altered the frequency during sleep in lesioned piglets. Further studies are necessary to elucidate the role of plasticity, sex, and 5-HT abnormalities in sudden infant death syndrome.

Inhibition of serotonergic medullary raphe obscurus neurons suppresses genioglossus and diaphragm activities in anesthetized but not conscious rats

Although exogenous serotonin at the hypoglossal motor nucleus (HMN) activates the genioglossus muscle, endogenous serotonin plays a minimal role in modulating genioglossus activity in awake and sleeping rats (Sood S, Morrison JL, Liu H, and Horner RL. Am J Respir Crit Care Med 172: 1338–1347, 2005). This result therefore implies that medullary raphe neurons also play a minimal role in the normal physiological control of the HMN, but this has not yet been established because raphe neurons release other excitatory neurotransmitters onto respiratory motoneurons in addition to serotonin. This study tests the hypothesis that inhibition of medullary raphe serotonergic neurons with 8-hydroxy-2-(di- n-propylamino)tetralin (8-OH-DPAT) suppresses genioglossus and diaphragm activities in awake and sleeping rats. Ten rats were implanted with electrodes to record sleep-wake states and genioglossus and diaphragm activities. Microdialysis probes were also implanted into the nucleus raphe obscurus (NRO). Experiments in 10 anesthetized and vagotomized rats were also performed using the same methodology. In anesthetized rats, microdialysis perfusion of 0.1 mM 8-OH-DPAT into the NRO decreased genioglossus activity by 60.7 ± 9.0% and diaphragm activity by 13.3 ± 3.4%. Diaphragm responses to 7.5% CO2 were also significantly reduced by 8-OH-DPAT. However, despite the robust effects observed in anesthetized and vagotomized rats, there was no effect of 0.1 mM 8-OH-DPAT on genioglossus or diaphragm activities in conscious rats awake or asleep. The results support the concept that endogenously active serotonergic medullary raphe neurons play a minimal role in modulating respiratory motor activity across natural sleep-wake states in freely behaving rodents. This result has implications for pharmacological strategies aiming to manipulate raphe neurons and endogenous serotonin in obstructive sleep apnea.

Stress-induced cardiac stimulation and fever: Common hypothalamic origins and brainstem mechanisms

Our past results provide considerable evidence that activation of neurons somewhere in the region of the dorsomedial hypothalamus (DMH) plays a key role in the generation of many of the effects typically seen in "emotional" stress in rats, including activation of the hypothalamic-pituitary-adrenal (HPA) axis, the neuroendocrine hallmark of the generalized response to stress, and sympathetically mediated tachycardia. More recently, we demonstrated that (1) the tachycardia resulting either from chemical stimulation of the DMH or from experimental stress is markedly attenuated by microinjection of the GABAA receptor agonist muscimol, a neuronal inhibitor, into the medullary raphe pallidus (RP); and (2) the specific subregion of the DMH mediating stimulation-induced tachycardia corresponds to the dorsal hypothalamic area (DHA), a site where neurons projecting to the RP are densely concentrated. Thus, the pathway from neurons in the DHA to sympathetic premotor neurons in the RP may constitute a key relay mediating the increases in heart rate seen in emotional stress--a role that had never been proposed previously for either of these regions. Instead, sympathetic premotor neurons were known to exist in the RP but had been most closely associated with sympathetic thermoregulatory mechanisms, including activation of brown fat, the principal means for nonshivering thermogenesis in rats, and cutaneous vasoconstriction in the tail, an important method of conserving body heat in this species. These sympathetic effects serve to maintain body temperature in a cold environment or to increase it in fever--and are typically accompanied by tachycardia. Interestingly, we and others have now shown that (1) disinhibition of neurons in the DMH also increases body temperature, at least in part through activation of brown fat, (2) microinjection of the neuronal inhibitor muscimol into the DMH reduces experimental fever and the associated tachycardia in rats. We hypothesize that activation of neurons in the DMH mediates both the increased body temperature and cardiac stimulation produced in rats by experimental "emotional" stress and fever, and that these effects are mediated in large part through direct projections to sympathetic premotor neurons in the RP. Thus, this pathway may constitute a common effector circuit upon which a variety of forebrain inputs converge in response to diverse environmental challenges.

CO2/H+ chemoreceptors in the cerebellar fastigial nucleus do not uniformly affect breathing of awake goats

Our objective in this study was to test the hypothesis that focal acidosis (FA) in the cerebellar fastigial nucleus (CFN) of awake goats arising from global brain acidosis induced by increasing inspired CO2 will increase breathing. FA was created by reverse microdialysis of mock cerebral spinal fluid, equilibrated with 6.4, 25, 50, or 80% CO2 through chronically implanted microtubules (cannula). Dialysis with 6.4% CO2 had no significant effects on any physiological parameters. However, microdialysis at higher levels of CO2 increased pulmonary ventilation (V(I)) in one group of studies and decreased V(I) in a second group and the difference between the groups was significant (t = 9.16, P < 0.001). In one group of studies (n = 8), FA with 50 and 80% CO2 significantly increased (P < 0.05) Vi by 16 and 12%, respectively, and significantly increased (P < 0.05) heart rate by 13 and 9%, respectively. In contrast, in another group of studies (n = 6), FA with 25 and 50% CO2 significantly decreased (P < 0.05) Vi by 7 and 10%, respectively. In this group oxygen consumption was decreased during dialysis with 80% CO2. On the basis of histology, we estimate that the increased and decreased responses were associated with FA primarily in the rCFN and cCFN, respectively. We conclude that there are CO2/H+-sensitive neurons in the CFN that do not uniformly affect breathing. In addition, the significant changes in heart rate and oxygen consumption during FA indicate that the CFN can also influence non-respiratory-related control systems.

Neurochemical development of brain stem nuclei involved in the control of respiration

The first two postnatal weeks are the most dynamic in the development of brain stem respiratory nuclei in the rat, the primary model for this review. Several neurochemicals (glutamate, glycine receptors, choline acetyltransferase, serotonin, norepinephrine, and thyrotropin-releasing hormone) increase expression with age, while others (GABA, serotonin receptor 1A, substance P, neurokinin 1 receptor, and somatostatin) decrease their expression. Surprisingly, a dramatic shift occurs at postnatal day (P) 12 in the rat. Excitatory neurotransmitter glutamate and its NMDA receptors fall precipitously, whereas inhibitory neurotransmitter GABA, GABA(B), and glycine receptors rise sharply. A concomitant drop in cytochrome oxidase activity occurs in respiratory neurons. Several receptor types undergo subunit switches during development. Notably, GABA(A) receptors switch prevalence from alpha3- to an alpha1-dominant form at P12 in the pre-Bötzinger complex of the rat. The transient dominance of inhibitory over excitatory neurotransmission around P12 may render the respiratory system sensitive to failure when stressed. Relating these neurochemical changes to physiological responses in animals and to sudden infant death syndrome in humans will be a challenge for future research.

Re: Retrotrapezoid nucleus: a litmus test for the identification of central chemoreceptors

Central chemoreception is the mechanism by which arterial blood P(CO2) is detected by the CNS to regulate breathing. Two main theories have been proposed to account for the phenomenon. The distributed chemosensitivity theory argues that pH sensitivity is a widespread attribute of brainstem neurones and that central chemoreception results from the cumulative effects of pH on countless neurones. The specialized chemoreceptor theory envisions the existence of small and specialized populations of CNS cells (chemoreceptors) that are unique in their ability to detect very small pH fluctuations and, via specific connections, regulate a respiratory network that is itself unresponsive to pH. The recently identified CO2-sensitive neurones of the retrotrapezoid nucleus (RTN) seem to possess most of the attributes that one would expect of such chemoreceptors. In this review we also suggest that many fewer medullary neurones are intrinsically responsive to CO2 in vivo than might have been anticipated from prior experimentation in vitro. The properties of RTN neurones provide renewed support for the specialized chemoreceptor theory of central chemoreception, proposed in the early 1960s. However, many uncertainties remain, especially as regards the molecular mechanisms of chemoreception, the type of cell that actually detects pH in vivo (neurone, glia or others) and the number and location of bona fide central chemoreceptors.

Medullary serotonergic neurones modulate the ventilatory response to hypercapnia, but not hypoxia in conscious rats

Serotonergic neurones in the mammalian medullary raphe region (MRR) have been implicated in central chemoreception and the modulation of the ventilatory response to hypercapnia, and may also be involved in the ventilatory response to hypoxia. In this study, we ask whether ventilatory responses across arousal states are affected when the 5-hydroxytryptamine 1A receptor (5-HT1A) agonist (R)-(+)-8-hydroxy-2(di-n-propylamino)tetralin (DPAT) is microdialysed into the MRR of the unanaesthetized adult rat. Microdialysis of 1, 10 and 30 mM DPAT into the MRR significantly decreased absolute ventilation values(VE) during 7% CO2 breathing by 21%, 19% and 30%, respectively, in wakefulness compared to artificial cerebrospinal fluid (aCSF) microdialysis, due to decreases in tidal volume (VT) and not in frequency (f), similar to what occurred during non-rapid eye movement (NREM) sleep. The concentration-dependence of the hypercapnic ventilatory effect might be due to differences in tissue distribution of DPAT. DPAT (30 mM) changed room air breathing pattern by increasing f and decreasing VT. As evidenced by a sham control group, repeated experimentation and microdialysis of aCSF alone had no effect on the ventilatory response to 7% CO2 during wakefulness or sleep. Unlike during hypercapnia, microdialysis of 30 mM DPAT into the MRR did not change the ventilatory response to 10% O2. Additionally, 10 and 30 mM DPAT MRR microdialysis decreased body temperature, and 30 mM DPAT increased the percentage of experimental time in wakefulness. We conclude that serotonergic activity in the MRR plays a role in the ventilatory response to hypercapnia, but not to hypoxia, and that MRR 5-HT1A receptors are also involved in thermoregulation and arousal.

Carotid body denervation alters ventilatory responses to ibotenic acid injections or focal acidosis in the medullary raphe

Our aim was to determine the effects of carotid body denervation (CBD) on the ventilatory responses to focal acidosis and ibotenic acid (IA) injections into the medullary raphe area of awake, adult goats. Multiple microtubules were chronically implanted into the midline raphe area nuclei either before or after CBD. For up to 15 days after bilateral CBD, arterial PCO2 (PaCO2) (13.3 +/- 1.9 Torr) was increased (P < 0.001), and CO2 sensitivity (-53.0 +/- 6.4%) was decreased (P <0.001). Thereafter, resting PaCO2 and CO2 sensitivity returned (P <0.01) toward control, but PaCO2 remained elevated (4.8 +/- 1.9 Torr) and CO2 sensitivity reduced (-24.7 +/- 6.0%) > or =40 days after CBD. Focal acidosis (FA) at multiple medullary raphe area sites 23-44 days post-CBD with 50 or 80% CO(2) increased inspiratory flow (Vi), tidal volume (Vt), metabolic rate (VO2), and heart rate (HR) (P <0.05). The effects of FA with 50% CO2 after CBD did not differ from intact goats. However, CBD attenuated (P <0.05) the increase in Vi, Vt, and HR with 80% CO2, but it had no effect on the increase in VO2. Rostral but not caudal raphe area IA injections increased Vi, BP, and HR (P < 0.05), and these responses were accentuated (P <0.001) after CBD. CO2 sensitivity was attenuated (-20%; P <0.05) <7 days after IA injection, but thereafter it returned to prelesion values in CBD goats. We conclude the following: 1) the attenuated response to FA after CBD provides further evidence that the carotid bodies provide a tonic facilitory input into respiratory control centers, 2) the plasticity after CBD is not due to increased raphe chemoreceptor sensitivity, and 3) the "error-sensing" function of the carotid body blunts the effect of strong stimulation of the raphe.

Chronic fluoxetine microdialysis into the medullary raphe nuclei of the rat, but not systemic administration, increases the ventilatory response to CO2

In conscious rats, focal CO2 stimulation of the medullary raphe increases ventilation, whereas interference with serotonergic function here decreases the ventilatory response to systemic hypercapnia. We sought to determine whether repeated administration of a selective serotonin reuptake inhibitor in this region would increase the ventilatory response to hypercapnia in unanesthetized rats. In rats instrumented with electroencephalogram-electromyogram electrodes, 250 or 500 μM fluoxetine or artificial cerebrospinal fluid (aCSF) was microdialyzed into the medullary raphe for 30 min daily over 15 days. To compare focal and systemic treatment, two additional groups of rats received 10 mg·kg−1·day−1 fluoxetine or vehicle systemically. Ventilation was measured in normocapnia and in 7% CO2 before treatment ( day 0), acutely ( days 1 or 3), on day 7, and on day 15. There was no change in normocapnic ventilation in any treatment group. Rats that received 250 μM fluoxetine microdialysis showed a significant 13% increase in ventilation in wakefulness during hypercapnia on day 7, due to an increase in tidal volume. In rats microdialyzed with 500 μM fluoxetine, there were 16 and 32% increases in minute ventilation during hypercapnia in wakefulness and sleep on day 7, and 20 and 28% increases on day 15, respectively, again due to increased tidal volume. There was no change in the ventilatory response to CO2 in rats microdialyzed with aCSF or in systemically treated rats. Chronic fluoxetine treatment in the medullary raphe increases the ventilatory response to hypercapnia in an unanesthetized rat model, an effect that may be due to facilitation of chemosensitive serotonergic neurons.

Effects on breathing of focal acidosis at multiple medullary raphe sites in awake goats

To gain insight into why there are chemoreceptors at widespread sites in the brain, mircrotubules were chronically implanted at two or three sites in the medullary raphe nuclei of adult goats (n = 7). After >2 wk, microdialysis (MD) probes were inserted into the microtubules to create focal acidosis (FA) in the awake state using mock cerebral spinal fluid (mCSF) equilibrated with 6.4% (pH = 7.3), 50% (pH = 6.5), or 80% CO(2) (pH = 6.3), where MD with 50 and 80% CO(2) reduces tissue pH by 0.1 and 0.18 pH unit, respectively. There were no changes in all measured variables with MD with 6.4% at single or multiple raphe sites (P > 0.05). During FA at single raphe sites, only 80% CO(2) elicited physiological changes as inspiratory flow was 16.9% above (P < 0.05) control. However, FA with 50 and 80% CO(2) at multiple sites increased (P < 0.05) inspiratory flow by 18.4 and 30.1%, respectively, where 80% CO(2) also increased (P < 0.05) tidal volume, heart rate, CO(2) production, and O(2) consumption. FA with 80% CO(2) at multiple raphe sites also led to hyperventilation (-2 mmHg), indicating that FA had effects on breathing independent of an increased metabolic rate. We believe these findings suggest that the large ventilatory response to a global respiratory brain acidosis reflects the cumulative effect of stimulation at widespread chemoreceptor sites rather than a large stimulation at a single site. Additionally, focal acidification of raphe chemoreceptors appears to activate an established thermogenic response needed to offset the increased heat loss associated with the CO(2) hyperpnea.

Transient attenuation of CO2 sensitivity after neurotoxic lesions in the medullary raphe area of awake goats

The major objective of this study was to gain insight into whether under physiological conditions medullary raphe area neurons influence breathing through CO(2)/H(+) chemoreceptors and/or through a postulated, nonchemoreceptor modulatory influence. Microtubules were chronically implanted into the raphe of adult goats (n = 13), and breathing at rest (awake and asleep), breathing during exercise, as well as CO(2) sensitivity were assessed repeatedly before and after sequential injections of the neurotoxins saporin conjugated to substance P [SP-SAP; neurokinin-1 receptor (NK1R) specific] and ibotenic acid (IA; nonspecific glutamate receptor excitotoxin). In all goats, microtubule implantation alone resulted in altered breathing periods, manifested as central or obstructive apneas, and fractionated breathing. The frequency and characteristics of the altered breathing periods were not subsequently affected by injections of the neurotoxins (P > 0.05). Three to seven days after SP-SAP or subsequent IA injection, CO(2) sensitivity was reduced (P < 0.05) by 23.8 and 26.8%, respectively, but CO(2) sensitivity returned to preinjection control values >7 days postinjection. However, there was no hypoventilation at rest (awake, non-rapid eye movement sleep, or rapid eye movement sleep) or during exercise after these injections (P > 0.05). The neurotoxin injections resulted in neuronal death greater than three times that with microtubule implantation alone and reduced (P < 0.05) both tryptophan hydroxylase-expressing (36%) and NK1R-expressing (35%) neurons at the site of injection. We conclude that both NK1R- and glutamate receptor-expressing neurons in the medullary raphe nuclei influence CO(2) sensitivity apparently through CO(2)/H-expressing chemoreception, but the altered breathing periods appear unrelated to CO(2) chemoreception and thus are likely due to non-chemoreceptor-related neuromodulation of ventilatory control mechanisms.