Effect on breathing of surface ventrolateral medullary cooling in awake, anesthetized and asleep goats

Contributions of carotid bodies, retrotrapezoid nucleus neurons and preBötzinger complex astrocytes to the CO2 -sensitive drive for breathing

Current models of respiratory CO2 chemosensitivity are centred around the function of a specific population of neurons residing in the medullary retrotrapezoid nucleus (RTN). However, there is significant evidence suggesting that chemosensitive neurons exist in other brainstem areas, including the rhythm-generating region of the medulla oblongata - the preBötzinger complex (preBötC). There is also evidence that astrocytes, non-neuronal brain cells, contribute to central CO2 chemosensitivity. In this study, we reevaluated the relative contributions of the RTN neurons, the preBötC astrocytes, and the carotid body chemoreceptors in mediating the respiratory responses to CO2 in experimental animals (adult laboratory rats). To block astroglial signalling via exocytotic release of transmitters, preBötC astrocytes were targeted to express the tetanus toxin light chain (TeLC). Bilateral expression of TeLC in preBötC astrocytes was associated with ∼20% and ∼30% reduction of the respiratory response to CO2 in conscious and anaesthetized animals, respectively. Carotid body denervation reduced the CO2 respiratory response by ∼25%. Bilateral inhibition of RTN neurons transduced to express Gi-coupled designer receptors exclusively activated by designer drug (DREADDGi ) by application of clozapine-N-oxide reduced the CO2 response by ∼20% and ∼40% in conscious and anaesthetized rats, respectively. Combined blockade of astroglial signalling in the preBötC, inhibition of RTN neurons and carotid body denervation reduced the CO2 -induced respiratory response by ∼70%. These data further support the hypothesis that the CO2 -sensitive drive to breathe requires inputs from the peripheral chemoreceptors and several central chemoreceptor sites. At the preBötC level, astrocytes modulate the activity of the respiratory network in response to CO2 , either by relaying chemosensory information (i.e. they act as CO2  sensors) or by enhancing the preBötC network excitability to chemosensory inputs. KEY POINTS: This study reevaluated the roles played by the carotid bodies, neurons of the retrotrapezoid nucleus (RTN) and astrocytes of the preBötC in mediating the CO2 -sensitive drive to breathe. The data obtained show that disruption of preBötC astroglial signalling, blockade of inputs from the peripheral chemoreceptors or inhibition of RTN neurons similarly reduce the respiratory response to hypercapnia. These data provide further support for the hypothesis that the CO2 -sensitive drive to breathe is mediated by the inputs from the peripheral chemoreceptors and several central chemoreceptor sites.

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.

Redefining the components of central CO2 chemosensitivity--towards a better understanding of mechanism

Abstract

The field of CO₂ chemosensitivity has developed considerably in recent years. There has been a mounting number of competing nuclei proposed as chemosensitive along with an ever increasing list of potential chemosensory transducing molecules. Is it really possible that all of these areas and candidate molecules are involved in the detection of chemosensory stimuli? How do we discriminate rigorously between molecules that are chemosensory transducers at the head of a physiological reflexversusthose that just happen to display sensitivity to a chemosensory stimulus? Equally, how do we differentiate between nuclei that have a primary chemosensory function, versusthose that are relays in the pathway? We have approached these questions by proposing rigorous definitions for the different components of the chemosensory reflex, going from the salient molecules and ions, through the components of transduction to the identity of chemosensitive cells and chemosensitive nuclei. Our definitions include practical and rigorous experimental tests that can be used to establish the identity of these components. We begin by describing the need for central CO₂ chemosensitivity and the problems that the field has faced. By comparing chemosensory mechanisms to those in the visual system we suggest stricter definitions for the components of the chemosensory pathway. We then, considering these definitions, re-evaluate current knowledge of chemosensory transduction, and propose the ‘multiple salient signal hypothesis’ as a framework for understanding the multiplicity of transduction mechanisms and brain areas seemingly involved in chemosensitivity.

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.

Contributions of central and peripheral chemoreceptors to the ventilatory response to CO2/H+

The major objective of this review is to evaluate existing information and reach conclusions regarding whether there is interaction between P(CO(2))/H(+) stimulation of carotid (peripheral) and intracranial (central) chemoreceptors. Interaction is defined as a ventilatory response to simultaneous changes in the degree of Pco2/H(+) stimulation of both chemoreceptors that is greater (hyperadditive) or less (hypoadditive) than the sum of the responses when stimulation of each set of chemoreceptors is individually altered. Simple summation of the simultaneous changes in stimuli results in no interaction (i.e., additive interaction). Knowledge of the nature of central/peripheral interaction is crucial for determining the physiological significance of newer models of ventilatory control based on recent neuroanatomic observations of the circuitry of key elements of the ventilatory control system. In this review, we will propose that these two sets of receptors are not functionally separate but rather that they are dependent on one another such that the sensitivity of the medullary chemoreceptors is critically determined by input from the peripheral chemoreceptors and possibly other breathing-related reflex afferents as well. The short format of this minireview demands that we be somewhat selective in developing our ideas. We will briefly discuss the limitations of experiments used to study CO(2)/H(+) sensitivity and interaction to date, traditional views of the relative contributions of peripheral and central chemoreceptors to CO(2)/H(+) sensitivity, the evidence for and against different types of interaction, and the effect of tonic carotid chemoreceptor afferent activity on central control mechanisms.

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.

Central chemoreception 2005: A brief review

This brief review will place recent findings on specific neurons and receptors identified as putative central chemoreceptors, namely glutamatergic and serotonergic neurons and purogenic receptors, into the context of our working hypothesis that central chemoreception is a distributed property.

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.

Medullary serotonergic neurones and adjacent neurones that express neurokinin-1 receptors are both involved in chemoreception in vivo

Neurokinin-1 receptor (NK1R)-expressing neurones that are involved in chemoreception at the retrotrapezoid nucleus (Nattie & Li, 2002b) are also prominent at locations that contain medullary serotonergic neurones, which are chemosensitive in vitro. In medullary regions containing both types, we evaluated their role in central chemoreception by specific cell killing. We injected (2 x 100 nl) (a) substance P-saporin (SP-SAP; 1 microm) to kill NK1R-expressing neurones, (b) a novel conjugate of a monoclonal antibody to the serotonin transporter (SERT) and saporin (anti-SERT-SAP; 1 microm) to kill serotonergic neurones, or (c) SP-SAP and anti-SERT-SAP together to kill both types. Controls received IgG-SAP injections (1 microm). There was no double-labelling of NK1R-immunoreactive (ir) and tryptophan-hydroxylase (TPOH)-ir neurones. Cell (somatic profile) counts showed that NK1R-ir neurones in the SP-SAP group were reduced by 31%; TPOH-ir neurones in the anti-SERT-SAP group by 28%; and NK1R-ir and TPOH-ir neurones, respectively, in the combined lesion group by 55% and 31% (P < 0.001; two-way ANOVA; P < 0.05, Tukey's post hoc test). The treatments had no significant effect on sleep/wake time, body temperature, or oxygen consumption but all three reduced the ventilatory response to 7% inspired CO(2) in wakefulness and sleep by a similar amount. SP-SAP treatment decreased the averaged CO(2) responses (3, 7 and 14 days after lesions) in wakefulness and sleep by 21% and 16%, anti-SERT-SAP decreased the responses by 15% and 18%, and the combined treatment decreased the responses by 12% and 12% (P < 0.001; two-way ANOVA; P < 0.05, Tukey's post hoc test). We conclude that separate populations of serotonergic and adjacent NK1R-expressing neurones in the medulla are both involved in central chemoreception in vivo.

Ontogeny of central CO2 chemoreception: chemosensitivity in the ventral medulla of developing bullfrogs

Sites of central CO2 chemosensitivity were investigated in isolated brain stems from Rana catesbeiana tadpoles and frogs. Respiratory neurograms were made from cranial nerve (CN) 7 and spinal nerve 2. Superfusion of the brain stem with hypercapnic artificial cerebrospinal fluid elicited increased fictive lung ventilation. The effect of focal perfusion of hypercapnic artificial cerebrospinal fluid on discrete areas of the ventral medulla was assessed. Sites of chemosensitivity, which are active continuously throughout development, were identified adjacent to CN 5 and CN 10 on the ventral surface of the medulla. In early- and middle-stage tadpoles and frogs, unilateral stimulation within either site was sufficient to elicit the hypercapnic response, but simultaneous stimulation within both sites was required in late-stage tadpoles. The chemosensitive sites were individually disrupted by unilateral application of 1 mg/ml protease, and the sensitivity to bath application or focal perfusion of hypercapnia was reassessed. Protease lesions at CN 10 abolished the entire hypercapnic response, but lesions at CN 5 affected only the hypercapnic response originating from the CN 5 site. Neurons within the chemosensitive sites were also destroyed by unilateral application of 1 mM kainic acid, and the sensitivity to bath or focal application of hypercapnia was reassessed. Kainic acid lesions within either site abolished the hypercapnic response. Using a vital dye, we determined that kainic acid destroyed neurons by only within 100 microm of the ventral medullary surface. Thus, regardless of developmental stage, neurons necessary for CO2 sensitivity are located in the ventral medulla adjacent to CN 5 and 10.

Do neurotoxic lesions in rostral medullary nuclei induce/accentuate hypoventilation during NREM sleep?

Experimentally induced neuronal dysfunction in respiratory regions of the rostral medulla decrease breathing more in anesthetized mammals than in awake mammals. Sleep is similar to anesthesia in that excitatory inputs to respiratory neurons are reduced compared to the awake state; thus, we hypothesized that neurotoxic lesions in rostral medullary nuclei would, relative to wakefulness (WK), induce and/or accentuate hypoventilation during non-rapid eye movement (NREM) sleep. To test the hypothesis, goats were studied between 21:00 h and 03:00 h: (1) before and 30 days after chronically implanting microtubules bilaterally into the rostral medulla and, (2) 9-15 h and 2-17 days after unilateral injections of 100 nl to 1 microl, 50 mM ibotenic acid into the vestibular, gigantocellularis reticularis, or facial nuclei, or the retrotrapezoid nucleus/parapyramidal region. Arterial blood was repeatedly sampled in all studies during WK, and NREM and rapid eye movement (REM) sleep states. There was no significant (P>0.10) change in Pa(CO(2)) between WK and NREM sleep (and REM sleep when sufficient data were obtained) before or after implantation of microtubules and in studies after creating the neurotoxic lesions. Breathing frequency also did not significantly (P>0.10) differ between states in any of the studies. The data thus did not support the hypothesis. We speculate that in goats efficient compensatory mechanisms maintain Pa(CO(2)) homeostasis during normal sleep and the same and/or other mechanisms maintain homeostasis when excitatory drive is further reduced by lesions in rostral medullary nuclei.

Carotid body denervation effect on cytochrome oxidase activity in pre-Botzinger complex of developing rats

Previously, we found that the rat pre-Bötzinger complex (PBC) exhibited reduced cytochrome oxidase (CO) activity on postnatal days (P) 3-4 and especially on P12, with a concomitant decrease in glutamate and N-methyl-d-aspartate receptor subunit 1, and an increase in GABA, GABA(B), glycine receptor, and glutamate subunit 2. We hypothesized that the PBC would be more affected by carotid body denervation (CBD) during the two critical windows than at other times. Pairs of CBD and sham animals at each postnatal day from P2 to P14 and at P21 were operated on and survived for 3 days. Brain stems were processed for CO and neurokinin-1 receptor for the identification of PBC. Results indicate that CBD caused a significant loss in body weight in all animals and a reduction in PBC somal size when the surgery was between P2 and P7. CBD also induced a significant decrease in CO activity of the PBC in most animals and a distinct delay, as well as prolongation of the maturational process, especially when induced close to P3 and P11-P13.

Plasticity in the control of breathing following sensory denervation

The purpose of this manuscript is to review the results of studies on the recovery or plasticity following a denervation- or lesion-induced change in breathing. Carotid body denervation (CBD), lung denervation (LD), cervical (CDR) and thoracic (TDR) dorsal rhizotomy, dorsal spinal column lesions, and lesions at pontine, medullary, and spinal sites all chronically alter breathing. The plasticity after these is highly variable, ranging from near complete recovery of the peripheral chemoreflex in rats after CBD to minimal recovery of the Hering-Breuer inflation reflex in ponies after LD. The degree of plasticity varies among the different functions of each pathway, and plasticity varies with the age of the animal when the lesion was made. In addition, plasticity after some lesions varies between species, and plasticity is greater in the awake than in the anesthetized state. Reinnervation is not a common mechanism of plasticity. There is evidence supporting two mechanisms of plasticity. One is through upregulation of an alternate sensory pathway, such as serotonin-mediated aortic chemoreception after CBD. The second is through upregulation on the efferent limb of a reflex, such as serotonin-mediated increased responsiveness of phrenic motoneurons after CDR, TDR, and spinal cord injury. Accordingly, numerous components of the ventilatory control system exhibit plasticity after denervation or lesion-induced changes in breathing; this plasticity is uniform neither in magnitude nor in underlying mechanisms. A major need in future research is to determine whether "reorganization" within the central nervous system contributes to plasticity following lesion-induced changes in breathing.

Breathing: rhythmicity, plasticity, chemosensitivity

Breathing is a vital behavior that is particularly amenable to experimental investigation. We review recent progress on three problems of broad interest. (i) Where and how is respiratory rhythm generated? The preBötzinger Complex is a critical site, whereas pacemaker neurons may not be essential. The possibility that coupled oscillators are involved is considered. (ii) What are the mechanisms that underlie the plasticity necessary for adaptive changes in breathing? Serotonin-dependent long-term facilitation following intermittent hypoxia is an important example of such plasticity, and a model that can account for this adaptive behavior is discussed. (iii) Where and how are the regulated variables CO2 and pH sensed? These sensors are essential if breathing is to be appropriate for metabolism. Neurons with appropriate chemosensitivity are spread throughout the brainstem; their individual properties and collective role are just beginning to be understood.

Substance P-saporin lesion of neurons with NK1 receptors in one chemoreceptor site in rats decreases ventilation and chemosensitivity

All medullary central chemoreceptor sites contain neurokinin-1 receptor immunoreactivity (NK1R-ir). We ask if NK1R-ir neurons and processes are involved in chemoreception. At one site, the retrotrapezoid nucleus/parapyramidal region (RTN/Ppy), we injected a substance P-saporin conjugate (SP-SAP; 0.1 pmol in 100 nl) to kill NK1R-ir neurons specifically, or SAP alone as a control. We made measurements for 15 days after the injections in two groups of rats. In group 1, with unilateral injections made in the awake state via a pre-implanted guide cannula, we compared responses within rats using initial baseline data. In group 2, with bilateral injections made under anaesthesia at surgery, we compared responses between SP-SAP- and SAP-treated rats. SP-SAP treatment reduced the volume of the RTN/Ppy region that contained NK1R-ir neuronal somata and processes by 44 % (group 1) and by 47 and 40 % on each side, respectively (group 2). Ventilation (.V(E)) and tidal volume (V(T)) were decreased during air breathing in sleep and wakefulness (group 2; P < 0.001; two-way ANOVA) and P(a,CO2) was increased (group 2; P < 0.05; Student's t test). When rats breathed an air mixture containing 7 % CO(2) during sleep and wakefulness, .V(E) and V(T) were lower (groups 1 and 2; P < 0.001; ANOVA) and the Delta.V(E) in air containing 7 % CO(2) compared to air was decreased by 28-30 % (group 1) and 17-22 % (group 2). SP-SAP-treated rats also slept less during air breathing. We conclude that neurons with NK1R-ir somata or processes in the RTN/Ppy region are either chemosensitive or they modulate chemosensitivity. They also provide a tonic drive to breathe and may affect arousal.

Anatomical arrangement of hypercapnia-activated cells in the superficial ventral medulla of rats

The anatomical structure of central respiratory chemoreceptors in the superficial ventral medulla of rats was studied by using hypercapnia-induced c-fos labeling to identify cells directly stimulated by extracellular pH or PCO(2). The distribution of c-fos-positive cells was found to be predominantly perivascular to surface vessels. In the superficial ventral medullary midline, parapyramidal, and ventrolateral regions where c-fos-positive cells were concentrated, we found a common, characteristic, anatomical architecture. The medullary surface showed an indentation covered by a surface vessel, and the marginal glial layer was thickened. We classified c-fos-positive cells into two types. One (type I cell) was small, was located inside the marginal glial layer and close to the medullary surface, and surrounded fine vessels. The other (type II cell) was large and located dorsal to the marginal glial layer. c-fos Expression under synaptic blockade suggested that type I cells are intrinsically chemosensitive. The chemosensitivity of surface cells (possible type I cells) surrounding vessels was confirmed electrophysiologically in slice preparations. We suggest that this characteristic anatomical structure may be the central chemoreceptor.

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.

Effects of neural drives on breathing in the awake state in humans

We have developed a mathematical model of the regulation of ventilation that successfully simulates breathing in the awake as well as in sleeping states. In previous models, which were used to simulate Cheyne-Stokes breathing and respiration during sleep, the controller was only responsive to chemical stimuli, and allowed no ventilation at sub-normal carbon dioxide levels. The current model includes several new features. The chemical controller responds continuously to changes in P(CO(2)) with a lower sensitivity during hypocapnia than in the hypercapnic ranges. Hypoxia interacts multiplicatively with P(CO(2)) over the entire range of activity. The controller in the current model, besides the chemical drive, includes also a neural component. This neural drive increases and decreases as the level of alertness changes, and adds or subtracts from ventilation levels demanded by the chemical controller. The model also includes the effects of post-stimulus potentiation (PSP) and hypoxic ventilatory depression (HVD). While PSP eliminates apneas after a disturbance and also dampens the subsequent dynamics of the respiration, it is not a major factor in the damping of the response. Another finding is that HVD is destabilizing. The model is the first to reproduce results reported in conscious humans after hyperventilation and after acute and longer-term hypoxia. It also reproduces the effects of NREM sleep.

Central chemosensitivity, sleep, and wakefulness

Neurons in many regions of the lower brain are chemosensitive in vitro. Focal acidification of these same and other regions in vivo can stimulate breathing indicating the presence of chemoreception. Why are there so many sites for central chemoreception? This review evaluates data obtained from unanesthetized rats at three central chemoreceptor sites, the retrotrapezoid nucleus (RTN), the medullary raphé, and the nucleus tractus solitarius (NTS) and extends ideas concerning two hypotheses, which were recently formulated (Nattie, E., 2000. Respir. Physiol. 122, 223-235). (1) The high overall sensitivity of the respiratory control system in the unanesthetized state to small increases in arterial CO(2) relies on an additive or greater effect of these multiple chemoreceptor sites. (2) Chemoreceptor sites can vary in effectiveness dependent on the state of arousal. These ideas fit into a more speculative and general hypothesis that central chemoreceptors are organized in a hierarchical manner as proposed for temperature sensing and thermoregulation (Satinoff, E., 1978. Science 201, 16-22). The presence of a number of chemosensitive sites with varying thresholds, sensitivity, and arousal dependence provides finely tuned control and stability for breathing.

Central chemosensitivity and breathing asleep in unilateral medullary lesion patients: comparisons to animal data

The rostro-ventrolateral medulla (RVLM) is a site of chemosensitivity in animals; such site(s) have not been defined in humans. We studied the effect of unilateral focal lesions in the rostrolateral medulla (RLM) of man, on the ventilatory CO(2) sensitivity and during awake and sleep breathing. Nine patients with RLM lesions (RLM group), and six with lesions elsewhere (non-RLM group) were studied. The ventilatory CO(2) sensitivity was lower in the RLM compared with the non-RLM group (mean (S.D.), RLM, 1.4 (0.9), non-RLM 3.0 (0.6) L min(-1) mmHg(-1)). In both groups resting breathing was normal. During sleep all RLM patients had frequent arousals, four had significant sleep disordered breathing (SDB), only one non-RLM patient had SDB. Our findings in humans resemble those in animals with focal RVLM lesions. This review provides evidence that in humans there is an area of chemosensitivity in the RLM. We propose that in humans, dorsal displacement of the RVLM area of chemosensitivity in animals, arises from development of the olive plus the consequences of the evolution of the cerebellum/inferior peduncle.

Multiple rostral medullary nuclei can influence breathing in awake goats

The purpose of this study was to determine the effect on breathing of neuronal dysfunction in the retrotrapezoid (RTN), facial (FN), gigantocellularis reticularis (RGN), or vestibular (VN) nuclei of adult awake goats. Microtubules were chronically implanted to induce neuronal dysfunction by microinjection of an excitatory amino acid (EAA) receptor antagonist or a neurotoxin. The EAA receptor antagonist had minimal effect on eupneic breathing, but 8--10 days after injection of the neurotoxin, 7 of 10 goats hypoventilated (arterial PCO(2) increased 3.2 +/- 0.7 Torr). Overall there were no significant (P > 0.10) effects of the EAA receptor antagonist on CO(2) sensitivity. However, for all nuclei, > or =66% of the antagonist injections altered CO(2) sensitivity by more than the normal 12.7 +/- 1.6% day-to-day variation. These changes were not uniform, inasmuch as the antagonist increased (RTN, n = 2; FN, n = 7; RGN, n = 6; VN, n = 1) or decreased (RTN, n = 2; RGN, n = 3; VN, n = 2) CO(2) sensitivity. Ten days after injection of the neurotoxin into the FN (n = 3) or RGN (n = 5), CO(2) sensitivity was also reduced. Neuronal dysfunction also did not have a uniform effect on the exercise arterial PCO(2) response, and there was no correlation between effects on CO(2) sensitivity and the exercise hyperpnea. We conclude that there is a heterogeneous population of neurons in these rostral medullary nuclei (or adjacent tissue) that can affect breathing in the awake state, possibly through chemoreception or chemoreceptor-related mechanisms.

Relative importance of rostral ventrolateral medulla in sympathoinhibitory action of rilmenidine in conscious and anesthetized rabbits

The pressor region of the rostral ventrolateral medulla (RVLM) is a critical site in the sympathoinhibitory action of imidazoline receptor agonists as shown by studies in anesthetized animals. The aim of this study was to compare the importance of the RVLM in mediating the inhibitory action of rilmenidine on renal sympathetic nerve activity (RSNA) and arterial pressure in urethane-anesthetized rabbits (n = 11) and in conscious, chronically instrumented rabbits (n = 6). Bilateral microinjection of rilmenidine (4 nmol in 100 nl) into the RVLM caused a greater decrease in resting arterial pressure in anesthetized animals (-19 mm Hg) than in conscious animals (-8 mm Hg). By contrast, the decrease in resting RSNA evoked by rilmenidine was similar in conscious (-27%) and anesthetized (-36%) rabbits. Furthermore, rilmenidine microinjection into the RVLM was equally effective in inhibiting the RSNA baroreflex in both groups of animals. The upper plateau of the RSNA baroreflex decreased by 37% and 42%, and gain decreased by 41% and 44% after rilmenidine treatments in conscious and anesthetized rabbits, respectively. We conclude that the RVLM plays an equally important role in the inhibitory action of rilmenidine on RSNA in conscious and anesthetized rabbits either at rest or during baroreflex responses. A relatively moderate effect of rilmenidine on arterial pressure in conscious, chronically instrumented rabbits may relate to a lower level of sympathetic drive compared with anesthetized animals.

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.

Influence of rostral ventrolateral medulla on renal sympathetic baroreflex in conscious rabbits

Previous studies with anesthetized animals have shown that the pressor region of the rostral ventrolateral medulla (RVLM) is a critical site in vasomotor control. The aim of this study was to develop, in conscious rabbits, a technique for microinjecting into the RVLM and to determine the influence of this area on renal sympathetic nerve activity (RSNA) and arterial pressure (AP) using local injections of glutamate, rilmenidine, ANG II and sarile. Rabbits were implanted with guide cannulas for bilateral microinjections into the RVLM (n = 7) or into the intermediate ventrolateral medulla (IVLM, n = 6) and an electrode for measuring RSNA. After 7 days of recovery, injections of glutamate (10 and 20 nmol) into the RVLM increased RSNA by 81 and 88% and AP by 17 and 25 mmHg, respectively. Infusion of glutamate (2 nmol/min) into the RVLM increased AP by 15 mmHg and the RSNA baroreflex range by 38%. By contrast, injection of the imidazoline receptor agonist rilmenidine (4 nmol) into the RVLM decreased AP by 8 mmHg and the RSNA baroreflex range by 37%. Injections of rilmenidine into the IVLM did not alter AP or RSNA. Surprisingly, treatments with ANG II (4 pmol/min) or the ANG II receptor antagonist sarile (500 pmol) into the RVLM did not affect the resting or baroreflex parameters. Infusion of ANG II (4 pmol/min) into the fourth ventricle increased AP and facilitated the RSNA baroreflex. Our results show that agents administered via a novel microinjecting system for conscious rabbits can selectively modulate neuronal activity in circumscribed regions of the ventrolateral medulla. We conclude that the RVLM plays a key role in circulatory control in conscious rabbits. However, we find no evidence for the role of ANG II receptors in the RVLM in the moment-to-moment regulation of AP and RSNA.

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.

Important role of carotid chemoreceptor afferents in control of breathing of adult and neonatal mammals

This review provides a summary and prospective on the importance of carotid/peripheral chemoreceptors to the control of breathing during physiologic conditions. For several days after carotid body denervation (CBD), adult mammals hypoventilate (+10 mmHg increase in Pa(CO(2))) at rest and during exercise and CO(2) sensitivity is attenuated by about 60%. In addition, if the rostral ventrolateral medulla is cooled during NREM sleep after CBD, a sustained apnea is observed. Eventually, days or weeks after CBD, a peripheral ventilatory chemoreflex redevelops and there is a normalization of breathing (rest and exercise) and CO(2) sensitivity. The site (s) of the regained chemosensitivity has not been established. This plasticity/redundancy after CBD appears greater in neonates than in adult mammals. These data suggest the carotid and other peripheral chemoreceptors provide an important excitatory input to medullary respiratory neurons that is essential for breathing when wakeful stimuli and central chemoreceptors are absent.

Unilateral focal lesions in the rostrolateral medulla influence chemosensitivity and breathing measured during wakefulness, sleep, and exercise

OBJECTIVES

The rostrolateral medulla (RLM) has been identified in animals as an important site of chemosensitivity; in humans such site(s) have not been defined. The aim of this study was to investigate the physiological implications of unilateral lesions in the lower brainstem on the control of breathing.

METHODS

In 15 patients breathing was measured awake at rest, asleep, during exercise, and during CO(2) stimulation. The lesions were located clinically and by MRI; in nine patients they involved the RLM (RLM group), in six they were in the pons, cerebellum, or medial medulla (Non-RLM group). All RLM group patients, and three non-RLM group patients had ipsilateral Horner's syndrome.

RESULTS

Six of the RLM group had a ventilatory sensitivity to inhaled CO(2) (V/P(ET) CO(2)) below normal (group A: V/P(ET) CO(2), mean, 0.87; range 0.3-1.4 l. min(-1)/mm Hg). It was normal in all of the non-RLM group (group B: V/P(ET) CO(2), mean, 3.0; range, 2.6-3.9 min(-1)/mmHg). There was no significant difference in breathing between groups during relaxed wakefulness (V, group A: 7.44 (SD 2.5) l.min(-1); group B: 6.02 (SD 1.3) l.min(-1); P(ET) CO(2), group A: 41.0 (SD 4.2) mm g; group B: 38.3 (SD2.0) mm Hg) or during exercise (V/VO(2): group A: 21 (SD 6. 0) l.min(-1)/l.min(-1); group B: 24 (SD 7.3) l.min(-1)/l.min(-1)). During sleep, all group A had fragmented sleep compared with only one patient in group B (group A: arousals, range 13 to > 60 events/hour); moreover, in group A there was a high incidence of obstructive sleep apnoea associated with hypoxaemia.

CONCLUSION

Patients with unilateral RLM lesions require monitoring during sleep to diagnose any sleep apnoea. The finding that unilateral RLM lesions reduce ventilatory sensitivity to inhaled CO(2) is consistent with animal studies. The reduced chemosensitivity had a minimal effect on breathing awake at rest or during exercise.

Maturation of the mammalian respiratory system

In this review, the maturational changes occurring in the mammalian respiratory network from fetal to adult ages are analyzed. Most of the data presented were obtained on rodents using in vitro approaches. In gestational day 18 (E18) fetuses, this network functions but is not yet able to sustain a stable respiratory activity, and most of the neonatal modulatory processes are not yet efficient. Respiratory motoneurons undergo relatively little cell death, and even if not yet fully mature at E18, they are capable of firing sustained bursts of potentials. Endogenous serotonin exerts a potent facilitation on the network and appears to be necessary for the respiratory rhythm to be expressed. In E20 fetuses and neonates, the respiratory activity has become quite stable. Inhibitory processes are not yet necessary for respiratory rhythmogenesis, and the rostral ventrolateral medulla (RVLM) contains inspiratory bursting pacemaker neurons that seem to constitute the kernel of the network. The activity of the network depends on CO2 and pH levels, via cholinergic relays, as well as being modulated at both the RVLM and motoneuronal levels by endogenous serotonin, substance P, and catecholamine mechanisms. In adults, the inhibitory processes become more important, but the RVLM is still a crucial area. The neonatal modulatory processes are likely to continue during adulthood, but they are difficult to investigate in vivo. In conclusion, 1) serotonin, which greatly facilitates the activity of the respiratory network at all developmental ages, may at least partly define its maturation; 2) the RVLM bursting pacemaker neurons may be the kernel of the network from E20 to adulthood, but their existence and their role in vivo need to be further confirmed in both neonatal and adult mammals.

Important role of carotid afferents in control of breathing

The purpose of the present study was to determine the effect on breathing in the awake state of carotid body denervation (CBD) over 1-2 wk after denervation. Studies were completed on adult goats repeatedly before and 1) for 15 days after bilateral CBD (n = 8), 2) for 7 days after unilateral CBD (n = 5), and 3) for 15 days after sham CBD (n = 3). Absence of ventilatory stimulation when NaCN was injected directly into a common carotid artery confirmed CBD. There was a significant (P < 0.01) hypoventilation during the breathing of room air after unilateral and bilateral CBD. The maximum PaCO2 increase (8 Torr for unilateral and 11 Torr for bilateral) occurred approximately 4 days after CBD. This maximum was transient because by 7 (unilateral) to 15 (bilateral) days after CBD, PaCO2 was only 3-4 Torr above control. CO2 sensitivity was attenuated from control by 60% on day 4 after bilateral CBD and by 35% on day 4 after unilateral CBD. This attenuation was transient, because CO2 sensitivity returned to control temporally similar to the return of PaCO2 during the breathing of room air. During mild and moderate treadmill exercise 1-8 days after bilateral CBD, PaCO2 was unchanged from its elevated level at rest, but, 10-15 days after CBD, PaCO2 decreased slightly from rest during exercise. These data indicate that 1) carotid afferents are an important determinant of rest and exercise breathing and ventilatory CO2 sensitivity, and 2) apparent plasticity within the ventilatory control system eventually provides compensation for chronic loss of these afferents.