In vitro responses of caudal hypothalamic neurons to hypoxia and hypercapnia

Brainstem and hypothalamic areas activated by tissue hypoxia: Fos-like immunoreactivity induced by carbon monoxide inhalation in the rat

Acute ambient hypoxia interacts with the ventilatory and cardiocirculatory control systems, via the concomitant activation of arterial chemoreceptors and tissue oxygen-sensing mechanisms. Whether these latter mechanisms may trigger a specific pathway had not yet been elucidated. We addressed this issue, mapping Fos expression in adult conscious rats subjected to tissue hypoxia elicited by carbon monoxide inhalation, under conditions of minimal activation of arterial chemoreceptors. Brief stimuli have been delivered (1% carbon monoxide inhaled during 5, 10 or 20 min) to produce steady tissue hypoxia. Compared to normoxia, even the briefest stimuli led to marked neuronal activation within areas involved in ventilatory and cardiocirculatory control. In the brainstem, stimulated rats exhibited enhanced Fos expression in the nucleus of the solitary tract, the area postrema, the dorsal motor nucleus of the vagus nerve, the ventrolateral medulla, the parapyramidal group, the nucleus raphe pallidus, the lateral paragigantocellular nucleus, the locus coeruleus, the dorsal raphe nucleus, the lateral parabrachial area, and the ventrolateral central gray. In the hypothalamus, activated neurons were identified at the ventral border and in the supramamillary, posterior, and dorsomedial nuclei. Fos expression appeared with increasing the severity of tissue hypoxia in the retrotrapezoid nucleus, the ventral tegmental area and the arcuate and paraventricular hypothalamic nuclei. The present data support the idea that inputs related to tissue hypoxia might play a crucial role in patterning the physiological response to hypoxia.

Chemosensitivity of serotonergic neurons in the rostral ventral medulla

The medullary raphé contains two subtypes of chemosensitive neuron: one that is stimulated by acidosis and another that is inhibited. Both types of neuron are putative chemoreceptors, proposed to act in opposite ways to modulate respiratory output and other pH sensitive brain functions. In this review, we will discuss the cellular properties of these chemosensitive raphé neurons when studied in vitro using brain slices and primary dissociated cell culture. Quantification of chemosensitivity of raphé neurons indicates that they are highly sensitive to small changes in extracellular pH (pH(o)) between 7.2 and 7.6. Stimulation by acidosis occurs only in the specific phenotypic subset of neurons within the raphé that are serotonergic. These serotonergic neurons also have other properties consistent with a specialized role in chemoreception. Homologous serotonergic neurons are present within the ventrolateral medulla (VLM), and may have contributed to localization of respiratory chemoreception to that region. Chemosensitivity of raphé neurons increases in the postnatal period in rats, in parallel with development of respiratory chemoreception in vivo. An abnormality of serotonergic neurons of the ventral medulla has been identified in victims of sudden infant death syndrome (SIDS). The cellular properties of serotonergic raphé neurons suggest that they play a role in the CNS response to hypercapnia, and that they may contribute to interactions between the sleep/wake cycle and respiratory control.

Monoaminergic neurons, chemosensation and arousal

In recent years, immense progress has been made in understanding central chemosensitivity at the cellular and functional levels. Combining molecular biological techniques (early gene expression as an index of cell activation) with neurotransmitter immunohistochemistry, new information has been generated related to neurochemical coding in chemosensory cells. We found that CO(2) exposure leads to activation of discrete cell groups along the neuraxis, including subsets of cells belonging to monoaminergic cells, noradrenaline-, serotonin-, and histamine-containing neurons. In part, they may play a modulatory role in the respiratory response to hypercapnia that could be related to their behavioral state control function. Activation of monoaminergic neurons by an increase in CO(2)/H(+) could facilitate respiratory related motor discharge, particularly activity of upper airway dilating muscles. In addition, these neurons coordinate sympathetic and parasympathetic tone to visceral organs, and participate in adjustments of blood flow with the level of motor activity. Any deficit in CO(2) chemosensitivity of a network composed of inter-related monoaminergic nuclei might lead to disfacilitation of motor outputs and to failure of neuroendocrine and homeostatic responses to life-threatening challenges (e.g. asphyxia) during sleep.

Microinjection of acetazolamide into the fastigial nucleus augments respiratory output in the rat

The rostral fastigial nucleus (FNr) of the cerebellum facilitates the respiratory response to hypercapnia. We hypothesized that some FNr sites are chemosensitive to focal tissue acidosis and contribute, at least partially, to respiratory modulation. Minute ventilation (VE) was recorded in 21 anesthetized and spontaneously breathing rats. Acetazolamide (AZ; 50 microM) was microinjected unilaterally into the FNr while an isocapnic condition was maintained throughout the experiment. AZ (1 or 20 nl) injection into the FNr significantly elevated VE (46.0 +/- 6.7%; P < 0.05), primarily via an increase in tidal volume (31.7 +/- 3.8%; P < 0.05), with little effect on arterial blood pressure. This augmented ventilatory response was initiated at 6.3 +/- 0.8 min and reached the peak at 19.7 +/- 4.1 min after AZ administration. The same dose of AZ delivered into the interposed and lateral cerebellar nuclei, or vehicle injection into the FNr, failed to elicit detectable cardiorespiratory responses. To determine whether the ventilatory response to AZ injection into the FNr resulted from an increase in respiratory central drive, the minute phrenic nerve activity (MPN) was recorded in seven paralyzed and ventilated rats. Similar to VE, MPN was increased by 38.9 +/- 8.9% (P < 0.05) after AZ administration. Our results suggest that elevation of CO2/H+ within the FNr facilitates respiratory output, supporting the presence of ventilatory chemoreception in rat FNr.

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.

Development of hypoxia-induced Fos expression in rat caudal hypothalamic neurons

The caudal hypothalamus is an important CNS site controlling cardiorespiratory integration during systemic hypoxia. Previous findings from this laboratory have identified caudal hypothalamic neurons of anesthetized rats that are stimulated during hypoxia. In addition, patch-clamp recordings in an in vitro brain slice preparation have revealed that there is an age-dependent response to hypoxia in caudal hypothalamic neurons. The present study utilized the expression of the transcription factor Fos as an indicator of neuronal depolarization to determine the hypoxic response of caudal hypothalamic neurons throughout postnatal development in conscious rats. Sprague-Dawley rats, aged three to 56 days, were placed in a normobaric chamber circulated with either 10% oxygen or room air for 3h. Following the hypoxic/normoxic exposure period, tissues from the caudal hypothalamus, periaqueductal gray, rostral ventrolateral medulla and nucleus tractus solitarius were processed immunocytochemically for the presence of the Fos protein. There was a significant increase in the density of neurons expressing Fos in the caudal hypothalamus of hypoxic compared to normoxic adult rats that was maintained in the absence of peripheral chemoreceptors. In contrast, no increase in the density of Fos-expressing caudal hypothalamic neurons was observed during hypoxia in rats less than 12 days old. Increases in Fos expression were also observed in an age-dependent manner in the periaqueductal gray, rostral ventrolateral medulla and nucleus tractus solitarius. These results show an increase in Fos expression in caudal hypothalamic neurons during hypoxia in conscious rats throughout development, supporting the earlier in vitro reports suggesting that these neurons are stimulated by hypoxia.

Biophysical characterization of rat caudal hypothalamic neurons: calcium channel contribution to excitability

Neurons in the caudal hypothalamus (CH) are responsible for the modulation of various processes including respiratory and cardiovascular output. Previous results from this and other laboratories have demonstrated in vivo that these neurons have firing rhythms matched to the respiratory and cardiovascular cycles. The goal of the present study was to characterize the biophysical properties of neurons in the CH with particular emphasis in those properties responsible for rhythmic firing behavior. Whole cell, patch-clamped CH neurons displayed a resting membrane potential of -58.0 +/- 1.1 mV and an input resistance of 319.3 +/- 16.6 MOmega when recorded in current-clamp mode in an in vitro brain slice preparation. A large proportion of these neurons displayed postinhibitory rebound (PIR) that was dependent on the duration and magnitude of hyperpolarizing current as well as the resting membrane potential of the cell. Furthermore these neurons discharged tonically in response to a depolarizing current pulse at a depolarized resting membrane potential (more positive than -65 mV) but switched to a rapid burst of firing to the same stimulus when the resting membrane potential was lowered. The PIR observed in these neurons was calcium dependent as demonstrated by the ability to block its amplitude by perfusion of Ca(2+)-free bath solution or by application of Ni(2+) (0.3-0.5 mM) or nifedipine (10 microM). These properties suggest that low-voltage-activated (LVA) calcium current is involved in the PIR and bursting firing of these CH neurons. In addition, high-voltage-activated calcium responses were detected after blockade of outward potassium current or in Ba(2+)-replacement solution. In addition, almost all of the CH neurons studied showed spike frequency adaptation that was decreased following Ca(2+) removal, indicating the involvement of Ca(2+)-dependent K(+) current (I(K,Ca)) in these cells. In conclusion, CH neurons have at least two different types of calcium currents that contribute to their excitability; the dominant current is the LVA or T-type. This LVA current appears to play a significant role in the bursting characteristics that may underlie the rhythmic firing of CH neurons.

Hypoxic augmentation of fast-inactivating and persistent sodium currents in rat caudal hypothalamic neurons

Previous work from this laboratory has indicated that TTX-sensitive sodium channels are involved in the hypoxia-induced inward current response of caudal hypothalamic neurons. Since this inward current underlies the depolarization and increased firing frequency observed in these cells during hypoxia, the present study utilized more detailed biophysical methods to specifically determine which sodium currents are responsible for this hypoxic activation. Caudal hypothalamic neurons from approximately 3-wk-old Sprague-Dawley rats were acutely dissociated and patch-clamped in the voltage-clamp mode to obtain recordings from fast-inactivating and persistent (noninactivating) whole cell sodium currents. Using computer-generated activation and inactivation voltage protocols, rapidly inactivating sodium currents were analyzed during normal conditions and during a brief (3-6 min) period of severe hypoxia. In addition, voltage-ramp and extended-voltage-activation protocols were used to analyze persistent sodium currents during normal conditions and during hypoxia. A polarographic oxygen electrode determined that the level of oxygen in this preparation quickly dropped to 10 Torr within 2 min of initiation of hypoxia and stabilized at <0.5 Torr within 4 min. During hypoxia, the peak fast-inactivating sodium current was significantly increased throughout the entire activation range, and both the activation and inactivation values (V(1/2)) were negatively shifted. Furthermore both the voltage-ramp and extended-activation protocols demonstrated a significant increase in the persistent sodium current during hypoxia when compared with normoxia. These results demonstrate that both rapidly inactivating and persistent sodium currents are significantly enhanced by a brief hypoxic stimulus. Furthermore the hypoxic-induced increase in these currents most likely is the primary mechanism for the depolarization and increased firing frequency observed in caudal hypothalamic neurons during hypoxia. Since these neurons are important in modulating cardiorespiratory activity, the oxygen responsiveness of these sodium currents may play a significant role in the centrally mediated cardiorespiratory response to hypoxia.

Hypoxic excitation in neurons cultured from the rostral ventrolateral medulla of the neonatal rat

Neurons within cardiorespiratory regions of the rostral ventrolateral medulla (RVLM) have been shown to be excited by local hypoxia. To determine the electrophysiological properties of these excitatory responses to hypoxia, we developed a primary dissociated cell culture system to examine the intrinsic response of RVLM neurons to hypoxia. Neonatal rat neurons plated on medullary astrocyte monolayers were studied using the whole cell perforated patch-clamp technique. Sodium cyanide (NaCN, 0.5-10 mM) was used, and membrane potential (V(m)), firing frequency, and input resistance were examined. In 11 of 19 neurons, NaCN produced a V(m) depolarization, an increase in firing frequency, and a decrease in input resistance, suggesting the opening of a cation channel. The hypoxic depolarization had a linear dose response and was dependent on baseline V(m), with a greater response at more hyperpolarized V(m). In 8 of 19 neurons, NaCN produced a V(m) hyperpolarization, decrease in firing frequency, and variable changes in input resistance. The V(m) hyperpolarization exhibited an all-or-none dose response and was independent of baseline V(m). These differential responses to NaCN were retained after synaptic blockade with low Ca(2+)-high Mg(2+) or TTX. Thus hypoxic excitation 1) is maintained in cell culture, 2) is an intrinsic response, and 3) is likely due to the increase in a cation current. These hypoxia-excited neurons are likely candidates to function as central oxygen sensors.

Ventilatory responses to specific CNS hypoxia in sleeping dogs

Our study was concerned with the effect of brain hypoxia on cardiorespiratory control in the sleeping dog. Eleven unanesthetized dogs were studied; seven were prepared for vascular isolation and extracorporeal perfusion of the carotid body to assess the effects of systemic [and, therefore, central nervous system (CNS)] hypoxia (arterial PO(2) = 52, 45, and 38 Torr) in the presence of a normocapnic, normoxic, and normohydric carotid body during non-rapid eye movement sleep. A lack of ventilatory response to systemic boluses of sodium cyanide during carotid body perfusion demonstrated isolation of the perfused carotid body and lack of other significant peripheral chemosensitivity. Four additional dogs were carotid body denervated and exposed to whole body hypoxia for comparison. In the sleeping dog with an intact and perfused carotid body exposed to specific CNS hypoxia, we found the following. 1) CNS hypoxia for 5-25 min resulted in modest but significant hyperventilation and hypocapnia (minute ventilation increased 29 +/- 7% at arterial PO(2) = 38 Torr); carotid body-denervated dogs showed no ventilatory response to hypoxia. 2) The hyperventilation was caused by increased breathing frequency. 3) The hyperventilatory response developed rapidly (<30 s). 4) Most dogs maintained hyperventilation for up to 25 min of hypoxic exposure. 5) There were no significant changes in blood pressure or heart rate. We conclude that specific CNS hypoxia, in the presence of an intact carotid body maintained normoxic and normocapnic, does not depress and usually stimulates breathing during non-rapid eye movement sleep. The rapidity of the response suggests a chemoreflex meditated by hypoxia-sensitive respiratory-related neurons in the CNS.

Chemosensitivity of non-respiratory rat CNS neurons in tissue culture

Neurons from many brainstem nuclei involved in respiratory control increase their firing rate in response to acidosis in vitro, suggesting that they are central chemoreceptors. This property has been considered to be either unique to neurons involved in respiratory control, or at least very unusual for non-respiratory neurons. However, recordings of intrinsic pH responses of neurons have not been made from enough non-respiratory regions of the CNS to be certain this assumption is true. Here, we have quantified changes in firing rate of neurons cultured from the hippocampus (n=43), neocortex (n=33), and cerebellum (n=29) in response to changes in CO(2) between 3% and 9% (pH approximately 7.6-7.2) after blockade of glutamatergic and GABAergic transmission. The responses of neurons from these three regions were similar, with a subset of neurons (12% of the total 105) inhibited by acidosis, decreasing their firing rate to a mean of 70% of control in response to a decrease in pH of 0.2. Some neurons (5% of total) were stimulated by acidosis, with an increase in firing rate to a mean of 175% of control in response to a decrease in pH of 0.2. We previously quantified chemosensitivity of neurons from the medullary raphe using the same methods [W. Wang, J.H. Pizzonia, G.B. Richerson, Chemosensitivity of rat medullary raphe neurones in primary tissue culture, J. Physiol., 511 (1998) 433-450]. Compared to these non-respiratory neurons, more raphe neurons were stimulated by acidosis (22%), and the average response was greater (to 300% of control) in response to the same stimulus. Thus, over a physiologically relevant pH range, stimulation by acidosis occurs in a significant percentage of neurons not involved in respiratory chemoreception. However, the degree of chemosensitivity of these neurons was less than medullary raphe neurons under the same conditions. Chemosensitivity is not an all-or-none neuronal property, and the degree of chemosensitivity may be relevant to the role neurons play in sensing pH in vivo.

Brainstem and hypothalamic areas involved in respiratory chemoreflexes: a Fos study in adult rats

The adaptation to hypoxia and hypercapnia requires the activation of several anatomical structures along the neuraxis. In this study, using Fos immunoreactivity, we sought to map neuronal populations involved in chemoreflex networks activated during the responses to moderate hypoxia (O(2) 11%), and hypercapnia (CO(2) 5%) in the brainstem and the hypothalamus of the rat. In the medulla, hypoxia elicited marked and significant staining in the nucleus of the solitary tract (NTS), and in parapyramidal neurons located near the ventral surface, whereas hypercapnia evoked significantly c-fos only near the ventral surface in paraolivar neurons. In contrast, within pontine and suprapontine structures, both hypoxia and hypercapnia evoked similarly Fos immunoreactivity in the lateral parabrachialis area, the central grey, the caudal hypothalamus (dorsomedial and posterior hypothalamic nuclei), and in a ventro-lateral hypothalamic area, extending from the rostral limit of the mammillary nuclei to the retrochiasmatic area. More rostrally, labelling was observed in the paraventricular nucleus of the hypothalamus in response to hypercapnia, and in the supraoptic nucleus in response to hypoxia. These results support the hypothesis that chemoreflexes pathways are not only restricted to medulla and pons but also involved mesencephalic and hypothalamic regions. The parabrachialis area and the central grey may be key relays between caudal and ventral hypothalamic neurons, and medullary neurons involved in the response to hypoxia and hypercapnia.

Effect of hypoxia on respiratory activity in the foetus

1. Breathing movements stop soon after the induction of hypoxia in foetal animals, a response attributed to the active inhibition of the respiratory centres. Separate studies using punctate lesions have identified lateral pontine and thalamic sites in foetal sheep from which respiratory inhibition may arise, but whether either of these loci are actually oxygen sensitive or whether they receive input from other regions responsive to hypoxia, has not been established. 2. FOS immunocytochemistry was used to identify neuronal pools activated by hypoxia in the brain of late-gestation foetal and newborn sheep. FOS-positive cells were found in the pons in regions corresponding to the medial parabrachial and Kolliker-Fuse nuclei and were shown to be catecholaminergic. Neurons in this pontine region were also activated by the piperizine drug almitrine, which, like hypoxia, inhibits respiratory output in the foetus but is a respiratory stimulant in the newborn and adult. Because these pontine neurons do not express FOS protein after challenge with hypoxia or almitrine in the newborn lamb, we argue that they are crucial to the hypoxic inhibition of respiratory activity in foetal life. 3. The role of the placenta in determining these foetal responses and how this may change at birth is discussed.

CO2, brainstem chemoreceptors and breathing

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

In vitro responses of neurons in the periaqueductal gray to hypoxia and hypercapnia

Hypoxia-sensitive neurons in the caudal hypothalamus (CH) have been shown to project to the periaqueductal gray (PAG) which, in turn, sends descending projections to an area of the ventrolateral medulla (VLM) containing neurons inherently excited by hypoxia. The purpose of this study was to determine if neurons in the PAG are excited by hypoxia or hypercapnia in an in vitro environment. Extracellular responses to hypoxia and hypercapnia of neurons located throughout the PAG were recorded in a rat brain slice (400-500 microm thick) preparation. Hypoxic (10% O(2)/5% CO(2)/85% N(2)) and hypercapnic (7% CO(2)/93% O(2)) stimuli were delivered to the tissue through gas bubbled into the brain slice chamber. A majority (39 of 53) of the neurons tested responded to hypoxia. Of these neurons, 92% responded to hypoxia with an increase in firing rate. Neurons in the dorsolateral/lateral regions increased firing rates to a greater extent than neurons located in ventrolateral regions. All neurons tested (n=6) also responded to hypoxia after perfusion of the tissue with a low Ca(2+)/high Mg(2+) solution to block classic synaptic transmission. Only a small proportion (7/33) of neurons tested responded to hypercapnia. These findings indicate that neurons in the periaqueductal gray region of the brain have an inherent responsiveness to hypoxia and, thus, may contribute to the overall coordination of cardiorespiratory responses to systemic hypoxia.

The distribution of FOS-immunoreactive neurons in the brainstem, midbrain and diencephalon of fetal sheep in response to acute hypoxia in mid and late gestation

FOS immunohistochemistry was used to map the distribution of neuronal pathways activated by hypoxia in fetal sheep. Conscious pregnant sheep were exposed to hypoxia (7-9% O2, 1-2% CO2, balance N2) for 2 h at either 100-105 days (n=5) or 130-133 days gestation (n=5); term is approximately 147 days. The hypoxia caused cessation of breathing movements at both fetal ages, and increased FOS staining in the medulla (area postrema, dorsal motor nucleus of vagus, nucleus solitary tract, ventrolateral medulla); pons (locus coeruleus and subcoeruleus, lateral and medial parabrachial nuclei); midbrain (habenula, periaqueductal grey, substantia nigra, areas ventrolateral to Red Nucleus); and hypothalamus (anterior and lateral hypothalamic areas, paraventricular and supraoptic nuclei). The results were essentially the same at both gestational ages, except that hypoxia increased FOS-staining in the habenula only in the older fetuses. The presence of FOS protein in pontomedullary cardiorespiratory nuclei at 100-105 days gestation indicates that the peripheral chemoreceptors respond to hypoxia at this early age, and in the subcoeruleus and medial parabrachial regions of the pons is consistent with lesion studies suggesting these areas mediate the inhibition of fetal breathing in response to hypoxia. FOS staining in the ventrolateral periaqueductal grey and habenula was unexpected, and suggests that pathways normally involved in response to noxious stimuli, or which are part of the hypothalamic 'defense' response are activated by hypoxia in the fetus. Some FOS-labelling could arise secondarily as a consequence of the cardiovascular and endocrine responses to hypoxia.

Effects of the respiratory stimulant almitrine on breathing and FOS expression in the brain of fetal and newborn sheep

Almitrine is a piperazine derivative known to stimulate breathing in the adult but cause apnea in fetal sheep. In fetal sheep (127-133 d gestation; term = 147 d) we confirmed this finding, but found that almitrine (4 mg/kg, i.v. or intra-arterial) had a biphasic effect, briefly stimulating and then suppressing breathing movements for at least 3 h. In 2- to 3-d-old (n = 4) and 7- to 14-d-old (n = 4) lambs almitrine increased both tidal volume and breath frequency, increased arterial partial pressure of oxygen and pH, and decreased partial pressure of carbon dioxide. The changes of tidal volume, partial pressure of oxygen and partial pressure of carbon dioxide were less in the 2- to 3-d-old compared with the 7- to 14-d-old lambs. The distribution of the nuclear phosphoprotein FOS, a marker of neuronal activation was examined in fetal and newborn brains. FOS protein was increased in cardiorespiratory areas of the medulla and pons, in the periaqueductal region of the midbrain, and in the supraoptic and paraventricular regions of the hypothalamus. In the pons, FOS protein was increased in the medial parabrachial and subcoeruleus nuclei in the fetuses but not in the 2- to 3- or 7- to 14-d-old lambs. These observations are similar to those reported for hypoxia, and consistent with the hypothesis that both almitrine and hypoxia inhibit fetal breathing movements by an action on a select group of pontine neurons. Whether these neurons respond directly to these stimuli or receive input from the other centers is yet to be elucidated. The mechanisms that change the almitrine (and hypoxia) response from inhibition to excitation at birth have not been identified, but may be important in preventing apnea in the newborn.

Developmental aspects and mechanisms of rat caudal hypothalamic neuronal responses to hypoxia

Previous reports from this laboratory have shown that a high percentage of neurons in the caudal hypothalamus are stimulated by hypoxia both in vivo and in vitro. This stimulation is in the form of an increase in firing frequency and significant membrane depolarization. The goal of the present study was to determine if this hypoxia-induced excitation is influenced by development. In addition, we sought to determine the mechanism by which hypoxia stimulates caudal hypothalamic neurons. Caudal hypothalamic neurons from neonatal (4-16 days) or juvenile (20-40 days) rats were patch-clamped, and the whole cell voltage and current responses to moderate (10% O2) or severe (0% O2) hypoxia were recorded in the brain slice preparation. Analysis of tissue oxygen levels demonstrated no significant difference in the levels of tissue oxygen in brain slices between the different age groups. A significantly larger input resistance, time constant and half-time to spike height was observed for neonatal neurons compared with juvenile neurons. Both moderate and severe hypoxia elicited a net inward current in a significantly larger percentage of caudal hypothalamic neurons from rats aged 20-40 days (juvenile) as compared with rats aged 4-16 days (neonatal). In contrast, there was no difference in the magnitude of the inward current response to moderate or severe hypoxia between the two age groups. Those cells that were stimulated by hypoxia demonstrated a significant decrease in input resistance during hypoxic stimulation that was not observed in those cells unaffected by hypoxia. A subset of neurons were tested independent of age for the ability to maintain the inward current response to hypoxia during synaptic blockade (11.4 mM Mg2+/0. 2 mM Ca2+). Most of the neurons tested (88.9%) maintained a hypoxic excitation during synaptic blockade, and this inward current response was unaffected by addition of 2 mM cobalt chloride to the bathing medium. In contrast, perfusion with the Na+ channel blocker, tetrodotoxin (1-2 microM) or Na+ replacement with N-methyl-D-glucamine (NMDG) significantly reduced the inward current response to hypoxia. Furthermore, the input resistance decrease observed during hypoxia was attenuated significantly during perfusion with NMDG. These results indicate the excitation elicited by hypoxia in hypothalamic neurons is age dependent. In addition, the inward current response of caudal hypothalamic neurons is not dependent on synaptic input but results from a sodium-dependent conductance.

Development of chemosensitivity of rat medullary raphe neurons

In many neonatal mammals, including humans and rats, there is a developmental increase in the ventilatory response to elevated pCO2. This maturation of central respiratory chemoreception may result from maturation of intrinsic chemosensitivity of brainstem neurons. We have examined age-related changes in chemosensitivity of neurons from the rat medullary raphe, a putative site for central chemoreception, using perforated patch-clamp recordings in vitro. In brain slices from rats younger than 12 days old, firing rate increased in 3% of neurons and decreased in 17% of neurons in response to respiratory acidosis (n = 36). In contrast, in slices from rats 12 days and older, firing rate increased in 18% of neurons and decreased in 15% of neurons in response to the same stimulus (n = 40). A tissue culture preparation of medullary raphe neurons was used to examine changes in chemosensitivity with age from three to 74 days in vitro. In cultured neurons younger than 12 days in vitro, firing rate increased in 4% of neurons and decreased in 44% of neurons in response to respiratory acidosis (n = 54). In contrast, in neurons 12 days in vitro and older, firing rate increased in 30% of neurons and decreased in 24% of neurons in response to respiratory acidosis (n = 105). In both types of chemosensitive neuron ("stimulated" and "inhibited"), the magnitudes of the changes in firing rate were greater in older neurons than in young neurons. These results indicate that the incidence and the degree of chemosensitivity of medullary raphe neurons increase with age in brain slices and in culture. This age-related increase in cellular chemosensitivity may underlie the development of respiratory chemoreception in vivo. Delays in this maturation process may contribute to developmental abnormalities of breathing, such as sudden infant death syndrome.

Suprapontine control of respiration

Despite focus on brainstem areas in central respiratory control, regions rostral to the medulla and pons are now recognized as being important in modulating respiratory outflow during various physiological states. The focus of this review is to highlight the role that suprapontine areas of the mammalian brain play in ventilatory control mechanisms. New imaging techniques have become invaluable in confirming and broadening our understanding of the manner in which the cerebral cortex of humans contributes to respiratory control during volitional breathing. In the diencephalon, the integration of respiratory output in relation to changes in homeostasis occurs in the caudal hypothalamic region of mammals. Most importantly, neurons in this region are strongly sensitive to perturbations in oxygen tension which modulates their level of excitation. In addition, the caudal hypothalamus is a major site for 'central command', or the parallel activation of locomotion and respiration. Furthermore, midbrain regions such as the periaqueductal gray and mesencephalic locomotor region function in similar fashion as the caudal hypothalamus with regard to locomotion and more especially the defense reaction. Together these suprapontine regions exert a strong modulation upon the basic respiratory drive generated in the brainstem.

Chemosensitivity of rat medullary raphe neurones in primary tissue culture

1. The medullary raphe, within the ventromedial medulla (VMM), contains putative central respiratory chemoreceptors. To study the mechanisms of chemosensitivity in the raphe, rat VMM neurones were maintained in primary dissociated tissue culture, and studied using perforated patch-clamp recordings. Baseline electrophysiological properties were similar to raphe neurones in brain slices and in vivo. 2. Neurones were exposed to changes in CO2 from 5% to 3 or 9% while maintaining a constant [NaHCO3]. Fifty-one per cent of neurones (n = 210) did not change their firing rate by more than 20% in response to hypercapnic acidosis. However, 22% of neurones responded to 9% CO2 with an increase in firing rate ('stimulated'), and 27% of neurones responded with a decrease in firing rate ('inhibited'). 3. Chemosensitivity has often been considered an all-or-none property. Instead, a method was developed to quantify the degree of chemosensitivity. Stimulated neurones had a mean increase in firing rate to 298 +/- 215% of control when pH decreased from 7.40 to 7.19. Inhibited neurones had a mean increase in firing rate to 232 +/- 265% of control when pH increased from 7. 38 to 7.57. 4. Neurones were also exposed to isocapnic acidosis. All CO2-stimulated neurones tested (n = 15) were also stimulated by isocapnic acidosis, and all CO2-inhibited neurones tested (n = 19) were inhibited by isocapnic acidosis. Neurones with no response to hypercapnic acidosis also had no response to isocapnic acidosis (n = 12). Thus, the effects of CO2 on these neurones were mediated in part via changes in pH. 5. In stimulated neurones, acidosis induced a small increase in the after-hyperpolarization level of 1.38 +/- 1. 15 mV per -0.2 pH units, which was dependent on the level of tonic depolarizing current injection. In voltage clamp mode at a holding potential near resting potential, there were small and inconsistent changes in whole-cell conductance and holding current in both stimulated and inhibited neurones. These results suggest that pH modulates a conductance in stimulated neurones that is activated during repetitive firing, with a reversal potential close to resting potential. 6. The two subtypes of chemosensitive VMM neurones could be distinguished by characteristics other than their response to acidosis. Stimulated neurones had a large multipolar soma, whereas inhibited neurones had a small fusiform soma. Stimulated neurones were more likely than inhibited neurones to fire with the highly regular pattern typical of serotonergic raphe neurones in vivo. 7. Within the medullary raphe, chemosensitivity is a specialization of two distinct neuronal phenotypes. The response of these neurones to physiologically relevant changes in pH is of the magnitude that suggests that this chemosensitivity plays a functional role. Elucidating their mechanisms in vitro may help to define the cellular mechanisms of central chemoreception in vivo.

Expression of c-fos in the rat brainstem after exposure to hypoxia and to normoxic and hyperoxic hypercapnia

In this study, Fos immunohistochemistry was used to map brainstem neuronal pathways activated during hypercapnia and hypoxia. Conscious rats were exposed to six different gas mixtures: (a) air; (b) 8% CO2 in air; (c) 10% CO2 in air; (d) 15% CO2 in air; (e) 15% CO2 + 60% O2, balance N2; (f) 9% O2, balance N2. Double-staining was performed to show the presence of tyrosine hydroxylase. Hypercapnia, in a dose-dependent way caused Fos expression in the following areas: caudal nucleus tractus solitarius (NTS), with few labeled A2 noradrenergic neurons; noradrenergic A1 cells and noncatecholaminergic neurons in the caudal ventrolateral medulla; raphe magnus and gigantocellular nucleus pars alpha (GiA); many noncatecholaminergic (and relatively few C1) neurons in the lateral paragigantocellular nucleus (PGCl), and in the retrotrapezoid nucleus (RTN); locus coeruleus (LC), external lateral parabrachial and Kölliker-Fuse nuclei, and A5 noradrenergic neurons at pontine level; and in caudal mesencephalon, the ventrolateral column of the periaqueductal gray (vlPAG). In most of these nuclei, hypoxia also induced Fos expression, albeit generally less than after hypercapnia. However, hypoxia did not cause labeling in RTN, juxtafacial PGCl, GiA, LC, or vlPAG. After normoxic hypercapnia, more labeled cells were present in NTS and PGCl than after hyperoxic hypercapnia. Part of the observed labeling could be caused by stress- or cardiovascular-related sequelae of hypoxia and hypercapnia. Possible implications for the neural control of breathing are also discussed, particularly with regard to the finding that several nuclei, not belonging to the classical brainstem respiratory centres, contained labeled cells.

Oxygen-sensing neurons in the caudal hypothalamus and their role in cardiorespiratory control

Work from this laboratory has shown that the caudal hypothalamus modulates the cardiorespiratory responses to hypoxia. The purpose of this review is to describe the modulation of respiratory output by the caudal hypothalamus during hypoxia and how neurons in this area respond to hypoxia. The diaphragmatic activity response to hypoxia was significantly attenuated following microinjection of either cobalt chloride or kynurenic acid into the caudal hypothalamus of rats. In addition, caudal hypothalamic neurons in anesthetized rats and cats responded to hypoxia with an increased firing frequency. This response was maintained in the absence of input from the vagus and carotid sinus nerves in the cat. When recorded extracellularly or by whole-cell patch clamp in vitro, these neurons responded to hypoxia with an increase in firing frequency, membrane potential and inward current. These results suggest that the caudal hypothalamus exerts excitatory influence on respiration during hypoxia, that may originate from the ability of these neurons to sense changes in oxygen levels.

Carbon dioxide regulates the tonic activity of locus coeruleus neurons by modulating a proton- and polyamine-sensitive inward rectifier potassium current

The electrophysiological effects of CO2 on locus coeruleus noradrenergic neurons were investigated in rat brain slices. Under control conditions, when slices were perfused with artificial cerebrospinal fluid containing 24 mM NaHCO3/5% CO2 (pH approximately 7.34, 33 degrees C) and exposed to 5% CO2/95% O2 arriving through an interface chamber, locus coeruleus neurons discharged spontaneously at approximately 1 Hz. Extracellular recordings showed that lowering CO2 that arrived through the chamber below 5% resulted in reductions in firing rate, often with a complete cessation of activity when exogenous CO2 was removed completely. Intracellular recordings revealed that lowering CO2 produced an outward current with an increase in slope conductance and a reversal potential near the potassium equilibrium potential; doubling the concentration of external potassium shifted the reversal potential of the current activated by CO2 removal by approximately +20 mV. Raising CO2 above 5% induced an increase in firing rate, an inward current, a decreased slope conductance at potentials near resting membrane voltage, and an increased slope conductance at more negative potentials. These effects of CO2 were mimicked by other manipulations that are known to affect intracellular pH. For example, NH4Cl, which acutely induces intracellular alkalinization, caused a marked reduction in firing rate, an outward current and an increased slope conductance that reversed near the potassium equilibrium potential. Bath-applied barium blocked the effects induced by removal of CO2 or addition of NH4Cl. The polyamine spermine (tetrahydrochloride) applied via intracellular micropipettes blocked the outward current induced by removal of CO2 or addition of NH4Cl. Spermine (free base) or an equivalent concentration of putrescine failed to alter the CO2 (0%)- or NH4Cl-induced effects. We conclude that CO2 maintains the tonic activity of locus coeruleus neurons by decreasing intracellular pH which, in turn, closes inward rectifier potassium channels, an effect that may be mediated by a protonated polyamine. According to this model, when there is alkalinization of locus coeruleus cells through removal of CO2 or addition of NH4Cl, endogenous spermine or a similar polyamine becomes partially deprotonated, releasing the channel block and allowing the cell to hyperpolarize. The possible implications of these results for the physiological effects of CO2 in the locus coeruleus are discussed.

Identification of brainstem neurons responding to hypoxia in fetal and newborn sheep

Hypoxia causes a reversible decrease in the level of respiratory, oculomotor and postural muscle activity in fetal sheep, an effect not seen in newborn lambs. We have used Fos immunohistochemistry to identify neurons which are activated by hypoxia and which may mediate this motor inhibition in the fetus. Pregnant sheep of either 117 or 138 days gestation were made hypoxic by allowing them to breathe 8-9% O2 for 2 h. Compared to age-matched control fetuses, hypoxia caused a significant increase in Fos-immunoreactivity in several medullary nuclei including the nucleus tractus solitarius, lateral reticular nucleus and the rostral ventrolateral medulla and also in the lateral parabrachial nucleus, locus coeruleus and subcoeruleus region in the pons. Hypoxia in newborn lambs, 7-18 days old, resulted in Fos staining in the same medullary and pontine nuclei with the exception of the subcoeruleus region which was devoid of Fos-immunoreactivity. In newborn lambs in which the carotid sinus nerves had been sectioned bilaterally, Fos-immunoreactivity was increased in the nucleus tractus solitarius in the medulla and in the locus coeruleus, lateral parabrachial and Kölliker-Fuse nuclei in the pons when compared to intact control newborn lambs. When carotid sinus nerve denervated-lambs were subjected to hypoxia the pattern of Fos-ir was similar to the pattern seen in the denervated control lambs but in addition staining was present in the subcoeruleus. These results suggest that a specific set of pontine neurons are activated by low oxygen levels in the fetus but not in the newborn lamb in the presence of an intact innervation from the carotid sinus. We hypothesise that: (a) in the fetus hypoxia activates neurons in the region of the subcoeruleus and this causes cessation of breathing movements and muscle atonia; and (b) that after birth stimulation of the carotid chemoreceptors by hypoxia normally inhibits activation of these subcoeruleus neurons.

Sudden Infant Death Syndrome: The Role of Central Chemosensitivity

Clinical and basic research on Sudden Infant Death Syndrome (SIDS) has focused on sleep-disordered cardiorespiratory control during a critical period of brainstem maturation. Recently, some SIDS cases have been reported to have abnormalities of the arcuate nucleus of the medulla. The human arcuate nucleus is thought to be homologous to the medullary raphe in rats and cats, a widely projecting serotonergic system that is functionally linked to both respiration and sleep. Neurons of the medullary raphe are now known to be highly sensitive to respiratory acidosis in vitro and are candidates for central chemoreceptors. The relevance of changes in the arcuate nucleus to the mechanisms of death in SIDS remains controversial. However, based on this new data, a specific hypothesis is proposed here. In combination with Immaturity of respiratory control mechanisms, dysfunction of arcuate neurons may lead to a fatal exaggeration of a normal physiologic inhibition of central chemoreception during sleep. The major elements of this working hypothesis are testable in animal experiments and clinical studies.

Plasticity of tyrosine hydroxylase gene expression in the rat nucleus tractus solitarius after ventilatory acclimatization to hypoxia

The aim of this study was to define the influence of long-term hypoxia on gene expression of tyrosine hydroxylase (TH) in the rat nucleus tractus solitarius (NTS). Animals were exposed to normobaric hypoxia (10% O2 in nitrogen) for 2 weeks. At this time, the hypoxia-induced hyperventilation reached a plateau, indicating ventilatory acclimatization. In horizontal brainstem sections, hypoxia-induced changes in TH protein and TH mRNA were assessed by immunocytochemistry and in-situ hybridization, respectively. Long-term hypoxia increased TH mRNA levels seen as both an increase in the number of grains per cell and an extension of the labeled area. The highest degree of labeling was found selectively located in caudal NTS. Hypoxia also enhanced TH immunoreactivity in the caudal NTS but this labeling extended more rostrally than that of TH mRNA. The data suggest that there is an hypoxia-induced plasticity of gene expression at the gene level in the NTS, which is associated with ventilatory acclimatization. The hypoxia model described in this study may serve as a framework for future regulatory studies.

In vitro responses of VLM neurons to hypoxia after normobaric hypoxic acclimatization

Hypoxic acclimatization involves an initial rapid ventilatory response followed by a more gradual increase in ventilation over a period of 24 to 48 h in both humans and rats. In addition, the acute ventilatory response to hypoxia is accentuated following hypoxic acclimatization. The purpose of the present investigation was to determine if hypoxic acclimatization augments the acute hypoxic response of neurons in the ventrolateral medulla (VLM). Brain slices (400 microns) containing the ventrolateral medulla were prepared from Sprague-Dawley rats acclimatized to hypoxia (10% O2) for 4-5 days (n = 4) and 9-10 days (n = 4) and from rats maintained in a normoxic environment (n = 4). Extracellular recordings demonstrated that there were no significant differences in the basal pattern or discharge rate of VLM neurons from animals exposed to short (10.8 +/- 0.9 Hz, n = 51), or long (10.1 +/- 1.1 Hz, n = 59) periods of hypoxia compared to control neurons (10.8 +/- 1.1 Hz, n = 52). The proportion of neurons stimulated (approximately 70%), inhibited (approximately 20%) and unaffected (approximately 10%) by an acute bout of hypoxia (10% O2) was also similar among groups. However, acute hypoxia elicited a greater increase in discharge frequency in neurons from rats exposed to the short period of hypoxia compared to the responses from neurons in the control and longer acclimatization groups. Thus, the responsivity of VLM neurons during the early stages of hypoxic acclimatization is altered in a manner consistent with the respiratory responses associated with acclimatization.

Effect of sinus denervation and vagotomy on c-fos expression in the nucleus tractus solitarius after exposure to CO2

Exposure to hypercapnia and electrical stimulation of the carotid sinus nerve (CSN) has been shown to induce c-fos expression in several brain stem regions including the nucleus tractus solitarius (NTS). To test whether the labeled neurons were activated directly by hypercapnia or secondarily via the carotid bodies (sinus nerve), adult rats were exposed to either air or 14-16% CO2 for 1 h. Experiments were done on eight groups: (1) exposure to air, (2) exposure to CO2, (3) chronic CSN denervation/CO2, (4) chronic unilateral CSN denervation/CO2, (5) chronic sham CSN denervation/CO2, (6) anesthetized/CO2, (7) anesthetized and acute vagotomy/CO2, and (8) premedicated with morphine, 10 mg s.c., 20 min before exposure to CO2. After exposure to CO2 or air the rats were anesthetized, perfused with 4% paraformaldehyde and the brains processed for immunohistochemical staining for c-fos protein using the PAP (i.e. peroxidase anti-peroxidase) technique. Labeled neurons in the area of the NTS in every second 50- "mu"m section were counted and their position plotted using a microscope and camera lucida attachment. Rats exposed to CO2 had a significantly greater number of labeled neurons in the NTS than those exposed to air. Other interventions, such as CSN denervation, surgery, anesthesia, vagotomy or injection of morphine did not significantly affect the level of c-fos expression in rats exposed to hypercapnia, indicative of central stimulation rather than secondary peripheral input. These responsive neurons may be part of a widespread central chemoreceptive complex.

Ventrolateral medullary neurons show age-dependent depolarizations to hypoxia in vitro

Central mechanisms are likely responsible for the larger respiratory activation in response to hypoxia in the adult compared to the neonatal animal. One possible site for this effect is in the ventrolateral medulla, an area known to be involved in the cardiorespiratory responses to hypoxia. Neurons in this area are stimulated by hypoxia both in vivo and in vitro. The purpose of the present study was to determine if changes in the magnitude of this excitatory response occur during early postnatal development. Whole-cell patch recordings were made from neurons in the ventrolateral medulla (VLM) in a 400-micrograms brain slice preparation. The basal properties and responses to a brief (90 s) hypoxic stimulus (5% CO2/95% N2) were compared between neurons from neonatal (P < 16) and juvenile (P16-38) rats. An excitation consisting of a depolarization, increase in spike frequency and decrease in input resistance was observed during hypoxia in eighty-three percent of juvenile but in only 58% of the neonatal VLM neurons. Moreover, the magnitude of this response was greater in the juvenile (8.2 +/- 1.3 mV) than in the neonatal (4.8 +/- 0.5 mV) neurons. A second type of depolarizing response, consisting of a more pronounced depolarization interrupted by a brief hyperpolarization that returned to a depolarized state and not associated with an increased discharge frequency, occurred in only 3% of the neurons from the juvenile animals compared to 18% of those from neonatal rats. The remaining proportion of the VLM neurons studied were hyperpolarized or were unaffected by hypoxia. Measurements of tissue pO2 indicate that none of the above differences are due to variations in the hypoxic stimulus between neonatal and adult slices. The results of this study suggest that the hypoxic-induced depolarizations observed in VLM neurons change during development. These developmental changes may contribute to the changes that occur in cardiorespiratory responses to acute systemic hypoxia during early development.

Rostral ventral medullary surface activity during hypercapnic challenges in awake and anesthetized goats

Regions within the rostral ventral medullary surface (RVMS) play an important role in cardiorespiratory responses to CO2 during anesthesia. Activity within a RVMS area, in which local cooling elicited marked ventilatory and blood pressure reductions, was measured as 660 nm scattered light changes in 5 goats following 5% CO2 challenges during waking and anesthetic states. During wakefulness, hypercapnia elicited a substantial, short latency transient (1-1.5 min) activity increase, followed by a sustained decrease. Stimulus cessation elicited a large and rapid off-transient activity increase which persisted for approximately 20 min. In contrast, during halothane anesthesia, the initial activation was absent, and the later activity decline and off-response were much reduced. We conclude that biphasic RVMS activity responses emerge to CO2 stimulation, and are state-dependent.

Hypoxia sensitive neurons in the caudal hypothalamus project to the periaqueductal gray

Previous studies have demonstrated that the caudal hypothalamus modulates the respiratory responses to hypoxia and hypercapnia. In addition, many of the neurons in this area have a basal discharge related to the cardiac and/or respiratory cycles and are stimulated by hypoxia or hypercapnia. The purpose of the present study was to determine if these hypothalamic neurons project to a known cardiorespiratory area, the periaqueductal gray in the rat. In a first set of experiments, rhodamine-tagged microspheres were injected into the periaqueductal gray (PAG) to determine the areas of the caudal hypothalamus that project to the PAG. These studies revealed that the caudal hypothalamus sends strong ipsilateral and weak contralateral projections to the PAG. In a second set of experiments, single unit recordings were made from neurons in the caudal hypothalamus; the basal discharge of these neurons were examined with signal averaging techniques. Each neuron (n = 79) was tested for a response to inhalation of a hypoxic (10% O2) and a hypercapnic (5% CO2) gas. Antidromic activation techniques were then used to determine if neurons in the caudal hypothalamus send projections to or through the PAG. Nineteen percent (n = 15) of the hypothalamic neurons studied could be activated from the PAG; approximately 53% (n = 8) of these were excited by hypoxia and 27% (n = 4) by hypercapnia. Most of these neurons tested (42 of 64 neurons) had a basal discharge related temporally to the cardiac and/or respiratory cycles. These findings suggest that a caudal hypothalamic to periaqueductal gray projection is involved in the integrated response to hypoxia.

Decerebration does not alter hypoxic sympathoexcitatory responses in rats

In anesthetized, paralyzed and ventilated rats, hypoxia, produced by intratracheal administration of 100% N2 for 20 s, increases sympathetic nerve activity and produces cardiovascular responses. Acute midcollicular decerebration has no effect on these responses in chemo-innervated or chemo-denervated animals. Suprapontine neural structures are, therefore, not required for the rapid sympathetic and cardiovascular responses to acute hypoxia. The results support the view that sympathoexcitatory responses to acute hypoxia depend entirely on the functions of reticulospinal sympathoexcitatory vasomotor neurons of the rostral ventrolateral medulla (RVL).

In vitro effects of GABA and hypoxia on posterior hypothalamic neurons from spontaneously hypertensive and Wistar-Kyoto rats

Recent studies from this laboratory have shown that neurons in this hypothalamic region are stimulated by hypoxia in vivo and in vitro. In addition, GABAergic activity is depressed in the posterior hypothalamus of the spontaneously hypertensive rat compared to the normotensive rat. The major purposes of the present study were to: a) evaluate if posterior hypothalamic neurons respond differently to GABA in the hypertensive rat compared to the normotensive rat; and b) examine the possibility that hypothalamic neurons from spontaneously hypertensive rats respond differently to hypoxia than those from normotensive rats. In addition, the effects of GABA on hypoxia-sensitive neurons was recorded. Extracellular single unit recordings of hypothalamic neurons were performed in a rat brain slice preparation. Neuronal responses to hypoxia (10% O2/5% CO2/85% N2) and to GABA were recorded from slices taken from both Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats. Administration of three different concentrations of GABA evoked a dose-related decrease in discharge rate in similar percentages of neurons from both SHR and WKY rats. The magnitude of the depression elicited by GABA did not differ significantly between the neurons from SHR and WKY rats. Hypoxia increased the firing rate of 75% and 69% of the SHR and WKY neurons, respectively; no differences (p > 0.05) were noted in the magnitude of the response or in the percentage of neurons responding to hypoxia between the two strains of rats. The discharge rate of most of these neurons fell to below control level following removal of the hypoxic stimulus. A significant percentage of SHR (75%) and WKY (75%) neurons that were stimulated by hypoxia were inhibited by exogenously applied GABA. These results indicate that a) an altered sensitivity of hypothalamic neurons to GABA does not contribute to hypertension in the SHR and b) the depressed respiratory response to hypoxia in the SHR is not due to a decreased responsiveness of hypothalamic neurons to hypoxia.

A review of the control of breathing during exercise

During the past 100 years many experimental investigations have been carried out in an attempt to determine the control mechanisms responsible for generating the respiratory responses observed during incremental and constant-load exercise tests. As a result of these investigations a number of different and contradictory control mechanisms have been proposed to be the sole mediators of exercise hyperpnea. However, it is now becoming evident that none of the proposed mechanisms are solely responsible for eliciting the exercise respiratory response. The present-day challenge appears to be one of synthesizing the proposed mechanisms, in order to determine the role that each mechanism has in controlling ventilation during exercise. This review, which has been divided into three primary sections, has been designed to meet this challenge. The aim of the first section is to describe the changes in respiration that occur during constant-load and incremental exercise. The second section briefly introduces the reader to traditional and contemporary control mechanisms that might be responsible for eliciting at least a portion of the exercise ventilatory response during these types of exercise. The third section describes how the traditional and contemporary control mechanisms may interact in a complex fashion to produce the changes in breathing associated with constant-load exercise, and incorporates recent experimental evidence from our laboratory.

Modulation of the respiratory responses to hypoxia and hypercapnia by synaptic input onto caudal hypothalamic neurons

Prior results from this laboratory have demonstrated that the respiratory response to hypercapnia is enhanced by microinjection of GABA antagonists or GABA synthesis inhibitors into the caudal hypothalamus of both cats and rats. However, no evidence was found for modulation of the respiratory response to hypoxia by a hypothalamic GABAergic mechanism. The purpose of the present study was to determine if synaptic input other than GABAergic onto caudal hypothalamic neurons affects the respiratory responses to hypoxia. The respiratory (diaphragmatic EMG) responses to hypoxia (10% O2) and hypercapnia (5% CO2) were recorded in anesthetized rats before and after bilateral microinjection of a blocker of synaptic transmission (CoCl2, 100 mM) or an excitatory amino acid receptor antagonist (kynurenic acid, 50 mM) into the caudal hypothalamus. Both hypoxia and hypercapnia elicited increases in tidal diaphragmatic activity and respiratory frequency prior to the microinjections. The respiratory response to hypercapnia was increased (+10.5%) after CoCl2 microinjections, which is consistent with prior results obtained with blockade of GABAergic input. Kynurenic acid did not alter the respiratory response to hypercapnia. A new finding was that the respiratory response to hypoxia was diminished after both CoCl2 (-13.0%) and kynurenic acid (-25.0%) microinjections. The results of this study support our prior findings that neurons in the caudal hypothalamus modulate the respiratory response to hypercapnia. In addition, our findings suggest that an excitatory input acting through excitatory amino acid receptors in the caudal hypothalamus modulates the respiratory responses to hypoxia.

Ventilatory and metabolic responses to cold and CO-induced hypoxia in awake rats

Experiments were carried out in awake rats to compare the effects of ambient and CO-induced hypoxia on thermoregulation and ventilatory control. Measurements of metabolic rate (VO2), ventilation (V), shivering (EMG) and colonic temperature (Tc) were made at fixed ambient temperature (Ta) of 25, 15 and 5 degrees C. Animals were exposed to ambient hypoxia (FIO2 of 21, 17, 14, 12 and 10%) or to CO hypoxia (FICO of 0.03% in air). The results show that: (1) Both ambient and CO-induced hypoxia provoked decreases in VO2 and Tc which were more marked at low Ta values; non-shivering thermogenesis was depressed with both types of hypoxia, whereas shivering was depressed only with ambient hypoxia; (2) Ventilatory response to ambient hypoxia was blunted at low Ta values and CO-induced hypoxia did not affect ventilation. It is concluded that: (1) hypoxia affects markedly the control of Tc by altering thermogenesis: inhibition of non-shivering thermogenesis seems to result from a decrease in CaO2 whereas inhibition of shivering seems to result from a decrease in PaO2; (2) during hypoxia, ventilation is controlled by the opposite stimulation from chemoreceptors and inhibition from hypometabolism. However, as revealed by CO-induced hypoxia, another stimulatory factor may also interact with the control of breathing.

In vivo and in vitro responses of neurons in the ventrolateral medulla to hypoxia

Neurons in the ventrolateral medulla (VLM) are known to be involved in several cardiorespiratory reflexes and to provide tonic drive to sympathetic preganglionic neurons. Recent studies have suggested that VLM neurons modulate the respiratory responses to hypoxia and to hypercapnia. The purpose of the present study was to determine with electrophysiological techniques if the discharge of these neurons is altered by hypoxia and/or by hypercapnia both in vivo and in vitro. Extracellular single-unit activity of VLM neurons (n = 39) was recorded during inhalation of a hypoxic gas (10% O2) and during inhalation of a hypercapnic gas (5% CO2) in anesthetized, spontaneously breathing rats (n = 16). Hypoxia elicited an increase in the discharge frequency in 64% of the VLM neurons studied; hypercapnia stimulated 42% of the neurons. Fifty-two percent of the neurons were stimulated by both hypoxia and hypercapnia. Signal averaging revealed that 76% of the hypoxia-stimulated neurons had a resting discharge related to the cardiac and/or respiratory cycle. Similar percentages of VLM neurons (35/54) were stimulated by hypoxia in a second group of animals (n = 14) that were studied after sinoaortic denervation. A rat brain slice preparation was then used to determine if hypoxia exerts a direct effect upon neurons in the VLM. Perfusing a hypoxic gas over the surface of medullary slices evoked an increase in the discharge frequency in the majority (39/49) of VLM neurons studied; responses were graded in relation to the magnitude of the hypoxic stimulus. Similar responses to hypoxia were observed in VLM neurons studied during perfusion with a synaptic blockade medium. Retrograde labeling of VLM neurons with rhodamine tagged microspheres injected into the thoracic intermediolateral cell column demonstrated that the hypoxia sensitive neurons were located in a region of the VLM that projects to the thoracic spinal cord. These results demonstrate that neurons in the ventrolateral medulla are excited by a direct effect of hypoxia; these neurons may play a critical role in the cardiorespiratory responses to hypoxia.

Effect of medullary lesions, vagotomy and carotid sinus denervation on fetal breathing

Chronically prepared fetal sheep were subjected to bilateral surface lesions of the Area "S" on the ventrolateral medulla and/or to peripheral chemoreceptor denervation by section of the vagus, sinus or both nerves. Sino-aortic denervation or Area "S" lesions reduced the incidence of fetal breathing (FB) for several days. Area "S" lesions also disrupted the pattern of FB; diaphragmatic EMG activity initially was mostly tonic and then of very high frequency, up to 7 Hz. Incidence and pattern of FB generally recovered by 7 days, but mean Ti was reduced in Area "S" lesioned fetuses (0.14 +/- 0.01 sec) compared to nonlesioned fetuses (0.19 +/- 0.01 sec) (P < 0.0001). Respiratory sensitivity to CO2 was variable but not different between control, denervated, and Area "S" lesioned groups. Eight of eight fetuses with Area "S" lesions were unable to initiate breathing at birth, but three sham operated fetuses were born normally. These data suggest that the classical peripheral and central chemoreceptors have a negligible influence on the control of FB, and that breathing activity in the fetus is mediated by a different mechanism than during postnatal life.