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

An open-source tool for automated analysis of breathing behaviors in common marmosets and rodents

The respiratory system maintains homeostatic levels of oxygen (O2) and carbon dioxide (CO2) in the body through rapid and efficient regulation of breathing frequency and depth (tidal volume). The commonly used methods of analyzing breathing data in behaving experimental animals are usually subjective, laborious, and time-consuming. To overcome these hurdles, we optimized an analysis toolkit for the unsupervised study of respiratory activities in animal subjects. Using this tool, we analyzed breathing behaviors of the common marmoset (Callithrix jacchus), a New World non-human primate model. Using whole-body plethysmography in room air as well as acute hypoxic (10% O2) and hypercapnic (6% CO2) conditions, we describe breathing behaviors in awake, freely behaving marmosets. Our data indicate that marmosets' exposure to acute hypoxia decreased metabolic rate and increased sigh rate. However, the hypoxic condition did not augment ventilation. Hypercapnia, on the other hand, increased both the frequency and depth (i.e., tidal volume) of breathing.

The Impact and Mechanism of a Novel Allosteric AMPA Receptor Modulator LCX001 on Protection Against Respiratory Depression in Rodents

Analgesics and sedative hypnotics in clinical use often give rise to significant side effects, particularly respiratory depression. For emergency use, specific antagonists are currently administered to counteract respiratory depression. However, antagonists are often short-lasting and eliminate drug generated analgesia. To resolve this issue, novel positive AMPA modulators, LCX001, was tested to alleviate respiratory depression triggered by different drugs. The acetic acid writhing and hot-plate test were conducted to evaluate analgesic effect of LCX001. Binding assay, whole-cell recording, live cell imaging, and Ca²⁺ imaging were used to clarify mechanism and impact of LCX001 on respiratory protection. Results showed that LCX001 effectively rescued and prevented opioid (fentanyl and TH-030418), propofol, and pentobarbital-induced respiratory depression by strengthening respiratory frequency and minute ventilation. The acetic acid writhing test and hot-plate test revealed potent anti-nociceptive efficacy of LCX001, in contrast to other typical ampakines that did not affect analgesia. Furthermore, LCX001 potentiated [³H]AMPA and L-glutamate binding affinity to AMPA receptors, and facilitated glutamate-evoked inward currents in HEK293 cells stably expressing GluA2(R). LCX001 had a typical positive modulatory impact on AMPAR-mediated function. Importantly, application of LCX001 generated a significant increase in GluA2(R) surface expression, and restrained opioid-induced abnormal intracellular Ca²⁺ load, which might participate in breathing modulation. Our study improves therapeutic interventions for the treatment of drug induced respiratory depression, and increases understanding of potential mechanism of AMPA receptor modulators.

Erythropoietin, a putative neurotransmitter during hypoxia, is produced in RVLM neurons and activates them in neonatal Wistar rats

Recent studies indicate that erythropoietin (EPO) is present in many areas of the brain and is active in the restoration of impaired neurons. In this study, we examined the presence of EPO and its role in bulbospinal neurons in the rostral ventrolateral medulla (RVLM). Hypoxia is often accompanied by a high blood pressure (BP). We hypothesized that EPO is produced in response to hypoxia in RVLM neurons and then activates them. To investigate whether RVLM neurons are sensitive to EPO, we examined the changes in the membrane potentials (MPs) of bulbospinal RVLM neurons using the whole cell patch-clamp technique during superfusion with EPO. A brainstem-spinal cord preparation was used for the experiments. EPO depolarized the RVLM neurons, and soluble erythropoietin receptor (SEPOR), an antagonist of EPO, hyperpolarized them. Furthermore, hypoxia-depolarized RVLM neurons were significantly hyperpolarized by SEPOR. In histological examinations, the EPO-depolarized RVLM neurons showed the presence of EPO receptor (EPOR). The RVLM neurons that possessed EPORs showed the presence of EPO and hypoxia-inducible factor (HIF)-2α. We also examined the levels of HIF-2α and EPO messenger RNA (mRNA) in the ventral sites of the medullas (containing RVLM areas) in response to hypoxia. The levels of HIF-2α and EPO mRNA in the hypoxia group were significantly greater than those in the control group. These results suggest that EPO is produced in response to hypoxia in RVLM neurons and causes a high BP via the stimulation of those neurons. EPO may be one of the neurotransmitters produced by RVLM neurons during hypoxia.

Intrinsic chemosensitivity of rostral ventrolateral medullary sympathetic premotor neurons in the in situ arterially perfused preparation of rats

Brainstem hypoperfusion is a major excitant of sympathetic activity triggering hypertension, but the exact mechanisms involved remain incompletely understood. A major source of excitatory drive to preganglionic sympathetic neurons originates from the ongoing activity of premotor neurons in the rostral ventrolateral medulla (RVLM sympathetic premotor neurons). The chemosensitivity profile of physiologically characterized RVLM sympathetic premotor neurons during hypoxia and hypercapnia remains unclear. We examined whether physiologically characterized RVLM sympathetic premotor neurons can sense brainstem ischaemia intrinsically. We addressed this issue in a unique in situ arterially perfused preparation before and after a complete blockade of fast excitatory and inhibitory synaptic transmission. During hypercapnic hypoxia, respiratory modulation of RVLM sympathetic premotor neurons was lost, but tonic firing of most RVLM sympathetic premotor neurons was elevated. After blockade of fast excitatory and inhibitory synaptic transmission, RVLM sympathetic premotor neurons continued to fire and exhibited an excitatory firing response to hypoxia but not hypercapnia. This study suggests that RVLM sympathetic premotor neurons can sustain high levels of neuronal discharge when oxygen is scarce. The intrinsic ability of RVLM sympathetic premotor neurons to maintain responsivity to brainstem hypoxia is an important mechanism ensuring adequate arterial pressure, essential for maintaining cerebral perfusion in the face of depressed ventilation and/or high cerebral vascular resistance.

Timing, sleep, and respiration in health and disease

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

Substance P differentially modulates firing rate of solitary complex (SC) neurons from control and chronic hypoxia-adapted adult rats

NK1 receptors, which bind substance P, are present in the majority of brainstem regions that contain CO2/H(+)-sensitive neurons that play a role in central chemosensitivity. However, the effect of substance P on the chemosensitive response of neurons from these regions has not been studied. Hypoxia increases substance P release from peripheral afferents that terminate in the caudal nucleus tractus solitarius (NTS). Here we studied the effect of substance P on the chemosensitive responses of solitary complex (SC: NTS and dorsal motor nucleus) neurons from control and chronic hypoxia-adapted (CHx) adult rats. We simultaneously measured intracellular pH and electrical responses to hypercapnic acidosis in SC neurons from control and CHx adult rats using the blind whole cell patch clamp technique and fluorescence imaging microscopy. Substance P significantly increased the basal firing rate in SC neurons from control and CHx rats, although the increase was smaller in CHx rats. However, substance P did not affect the chemosensitive response of SC neurons from either group of rats. In conclusion, we found that substance P plays a role in modulating the basal firing rate of SC neurons but the magnitude of the effect is smaller for SC neurons from CHx adult rats, implying that NK1 receptors may be down regulated in CHx adult rats. Substance P does not appear to play a role in modulating the firing rate response to hypercapnic acidosis of SC neurons from either control or CHx adult rats.

Sodium and calcium mechanisms of rhythmic bursting in excitatory neural networks of the pre-Bötzinger complex: a computational modelling study

The neural mechanisms generating rhythmic bursting activity in the mammalian brainstem, particularly in the pre-Bötzinger complex (pre-BötC), which is involved in respiratory rhythm generation, and in the spinal cord (e.g. locomotor rhythmic activity) that persist after blockade of synaptic inhibition remain poorly understood. Experimental studies in rodent medullary slices containing the pre-BötC identified two mechanisms that could potentially contribute to the generation of rhythmic bursting: one based on the persistent Na(+) current (I(NaP)), and the other involving the voltage-gated Ca(2+) current (I(Ca)) and the Ca(2+) -activated nonspecific cation current (I(CAN)), activated by intracellular Ca(2+) accumulated from extracellular and intracellular sources. However, the involvement and relative roles of these mechanisms in rhythmic bursting are still under debate. In this theoretical/modelling study, we investigated Na(+)-dependent and Ca(2+)-dependent bursting generated in single cells and heterogeneous populations of synaptically interconnected excitatory neurons with I(NaP) and I(Ca) randomly distributed within populations. We analysed the possible roles of network connections, ionotropic and metabotropic synaptic mechanisms, intracellular Ca(2+) release, and the Na(+)/K(+) pump in rhythmic bursting generated under different conditions. We show that a heterogeneous population of excitatory neurons can operate in different oscillatory regimes with bursting dependent on I(NaP) and/or I(CAN), or independent of both. We demonstrate that the operating bursting mechanism may depend on neuronal excitation, synaptic interactions within the network, and the relative expression of particular ionic currents. The existence of multiple oscillatory regimes and their state dependence demonstrated in our models may explain different rhythmic activities observed in the pre-BötC and other brainstem/spinal cord circuits under different experimental conditions.

Modulation of bulbospinal rostral ventral lateral medulla neurons by hypoxia/hypercapnia but not medullary respiratory activity

Although sympathetic vasomotor discharge has respiratory modulation, the site(s) responsible for this cardiorespiratory interaction is unknown. One likely source for this coupling is the rostral ventral lateral medulla (RVLM), where presympathetic neurons originate in close apposition to respiratory neurons. The current study tested the hypothesis that RVLM bulbospinal neurons are modulated by medullary respiratory network activity using whole-cell patch-clamp electrophysiological recordings of RVLM neurons while simultaneously recording fictive respiratory bursting activity from the hypoglossal rootlet. Additionally, we examined whether challenges to cardiorespiratory function, mainly hypoxia/hypercapnia, alter the activity of bulbospinal neurons and, secondarily, whether changes in synaptic input mediate these responses. Surprisingly, our results indicate that inspiratory-related activity did not modulate glutamatergic, γ-aminobutyric acid-ergic, or glycinergic synaptic events or spontaneous action potential firing in these RVLM neurons. However, hypoxia/hypercapnia reversibly decreased the frequency of γ-aminobutyric acid and glycine inhibitory postsynaptic currents. Glycinergic inhibitory postsynaptic current frequency was depressed from the fifth through the 10th minute, whereas the depression of γ-aminobutyric acid-ergic events became significant only at the 10th minute of hypoxia/hypercapnia. On the basis of spontaneous firing activity, there were 2 populations of RVLM bulbospinal neurons. The firing frequency of low-discharging RVLM neurons was facilitated by hypoxia/hypercapnia, and this increase depended on reduced inhibitory neurotransmission. The firing frequency in RVLM neurons with high-discharge rates was inhibited, independent of synaptic input, by hypoxia/hypercapnia. This article demonstrates that sympathetic-respiratory coupling is not active in the neonatal brain stem slice, and reductions in inhibitory neurotransmission to low spontaneously active bulbospinal RVLM neurons are responsible for hypoxia/hypercapnia-elicited increases in activity.

Heme oxygenase-1 and chronic hypoxia

A myriad of changes are necessary to adapt to chronic hypoxemia. Key among these changes increases in arterial oxygen carrying capacity, ventilation and sympathetic activity. This requires the induction of several gene products many of which are regulated by the activity of HIF-1α, including HO-1. Induction of HO-1 during chronic hypoxia is necessary for the continued breakdown of heme for the enhanced production of hemoglobin and the increased respiratory and sympathetic responses. Several human HO-1 polymorphisms have been identified that can affect the expression or activity of HO-1. Associations between these polymorphisms and the prevalence of hypertension have recently been assessed in specific populations. There are major gaps in our understanding of the mechanisms of how HO-1 mediates changes in the activity of the hypoxia-sensitive chemosensors and whether HO-1 polymorphisms are an important factor in the integrated response to chronic hypoxia. Understanding how HO-1 mediates cardiorespiratory responses could provide important insights into clinical syndromes such as obstructive sleep apnea.

Contribution of BK(Ca) channels of neurons in rostral ventrolateral medulla to CO-mediated central regulation of respiratory rhythm in medullary slices of neonatal rats

We recently described that carbon monoxide (CO) participated in the regulation of rhythmic respiration in medullary slices. The present study was undertaken to further assess whether the large-conductance calcium-activated potassium channels (BK(Ca) channels) are involved in the CO-mediated central regulation of respiratory rhythm in medullary slices. The rhythmic discharge of hypoglossal rootlets of medullary slices of neonatal rats was recorded. We observed that blocking BK(Ca) channels could partially abolish the effects of CO on the rhythmic bursts of hypoglossal rootlets. With whole-cell patch-clamp recording technique, we further observed that CO could reversibly augment potassium current density of the neurons in the rostral ventrolateral medulla. The CO-induced increase in potassium current was entirely blocked by the pretreatment of slices with BK(Ca) channels blocker; whereas blockade of CO generation with zinc protoporphyrin-IX produced an opposite response. Altogether, these data indicate that BK(Ca) channels of the neurons in neonatal rostral ventrolateral medulla could be activated by CO and involved in CO-mediated central regulation of respiratory rhythm in medullary slices.

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

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

Heme oxygenase-1-dependent central cardiorespiratory adaptations to chronic hypoxia in mice

Adaptations to chronic hypoxia (CH) could reflect cellular changes within the cardiorespiratory regions of the rostral ventrolateral medulla (RVLM), the C1 region, and the pre-Bötzinger complex (pre-BötC). Previous studies have shown that the hypoxic chemosensitivity of these regions are heme oxygenase (HO) dependent and that CH induces HO-1. To determine the time course of HO-1 induction within these regions and explore its relevance to the respiratory and sympathetic responses during CH, the expression of HO-1 mRNA and protein in the RVLM and measures of respiration, sigh frequency, and sympathetic activity (spectral analysis of heart rate) were examined during 10 days of CH. Respiratory and sympathetic responses to acute hypoxia were obtained in chronically instrumented awake wild-type (WT) and HO-1 null mice. After 4 days of CH, there was a significant induction of HO-1 within the C1 region and pre-BötC. WT mice acclimated to CH by increasing peak diaphragm EMG after 10 days of CH but had no change in the respiratory response to acute hypoxia. There were no significant differences between WT and HO-1 null mice. In WT mice, hypoxic sigh frequency and hypoxic sensitivity of sympathetic activity initially declined before returning toward baseline after 5 days of CH, correlating with the induction of HO-1. In contrast, HO-1 null mice had a persistent decline in hypoxic sigh frequency and hypoxic sensitivity of sympathetic activity. We conclude that induction of HO-1 in these RVLM cardiorespiratory regions may be important for the hypoxic sensitivity of sighs and sympathetic activity during CH.

Acute hyperoxia increases lipid peroxidation and induces plasma membrane blebbing in human U87 glioblastoma cells

Atomic force microscopy (AFM), malondialdehyde (MDA) assays, and amperometric measurements of extracellular hydrogen peroxide (H(2)O(2)) were used to test the hypothesis that graded hyperoxia induces measurable nanoscopic changes in membrane ultrastructure and membrane lipid peroxidation (MLP) in cultured U87 human glioma cells. U87 cells were exposed to 0.20 atmospheres absolute (ATA) O(2), normobaric hyperoxia (0.95 ATA O(2)) or hyperbaric hyperoxia (HBO(2), 3.25 ATA O(2)) for 60 min. H(2)O(2) (0.2 or 2 mM; 60 min) was used as a positive control for MLP. Cells were fixed with 2% glutaraldehyde immediately after treatment and scanned with AFM in air or fluid. Surface topography revealed ultrastructural changes such as membrane blebbing in cells treated with hyperoxia and H(2)O(2). Average membrane roughness (R(a)) of individual cells from each group (n=35 to 45 cells/group) was quantified to assess ultrastructural changes from oxidative stress. The R(a) of the plasma membrane was 34+/-3, 57+/-3 and 63+/-5 nm in 0.20 ATA O(2), 0.95 ATA O(2) and HBO(2), respectively. R(a) was 56+/-7 and 138+/-14 nm in 0.2 and 2 mM H(2)O(2). Similarly, levels of MDA were significantly elevated in cultures treated with hyperoxia and H(2)O(2) and correlated with O(2)-induced membrane blebbing (r(2)=0.93). Coapplication of antioxidant, Trolox-C (150 microM), significantly reduced membrane R(a) and MDA levels during hyperoxia. Hyperoxia-induced H(2)O(2) production increased 189%+/-5% (0.95 ATA O(2)) and 236%+/-5% (4 ATA O(2)) above control (0.20 ATA O(2)). We conclude that MLP and membrane blebbing increase with increasing O(2) concentration. We hypothesize that membrane blebbing is an ultrastructural correlate of MLP resulting from hyperoxia. Furthermore, AFM is a powerful technique for resolving nanoscopic changes in the plasma membrane that result from oxidative damage.

Heme oxygenase is necessary for the excitatory response of cultured neonatal rat rostral ventrolateral medulla neurons to hypoxia

Heme oxygenase has been linked to the oxygen-sensing function of the carotid body, pulmonary vasculature, cerebral vasculature, and airway smooth muscle. We have shown previously that the cardiorespiratory regions of the rostral ventrolateral medulla are excited by local hypoxia and that heme oxygenase-2 (HO-2) is expressed in the hypoxia-chemosensitive regions of the rostral ventrolateral medulla (RVLM), the respiratory pre-Bötzinger complex, and C1 sympathoexcitatory region. To determine whether heme oxygenase is necessary for the hypoxic-excitation of dissociated RVLM neurons (P1) cultured on confluent medullary astrocytes (P5), we examined their electrophysiological responses to hypoxia (NaCN and low Po(2)) using the whole-cell perforated patch clamp technique before and after blocking heme oxygenase with tin protoporphyrin-IX (SnPP-IX). Following the electrophysiological recording, immunocytochemistry was performed on the recorded neuron to correlate the electrophysiological response to hypoxia with the expression of HO-2. We found that the responses to NaCN and hypoxia were similar. RVLM neurons responded to NaCN and low Po(2) with either depolarization or hyperpolarization and SnPP-IX blocked the depolarization response of hypoxia-excited neurons to both NaCN and low Po(2) but had no effect on the hyperpolarization response of hypoxia-depressed neurons. Consistent with this observation, HO-2 expression was present only in the hypoxia-excited neurons. We conclude that RVLM neurons are excited by hypoxia via a heme oxygenase-dependent mechanism.

Role of chemoreceptors in mediating dyspnea

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

Transient carbon monoxide inhibits the ventilatory responses to hypoxia through peripheral mechanisms in the rat

Hypoxia inhibits K+ channels of chemoreceptors of the carotid body (CB), which is reversed by transient carbon monoxide (CO), suggesting an inhibitory effect of CO on hypoxic stimulation of carotid chemoreceptors. Therefore, we hypothesized that the ventilatory responses to hypoxic stimulation of the CB might be depressed in intact rats by transient inhalation of CO. Anesthetized, spontaneously breathing rats were exposed to room air, and 1 min of 11% O2 (HYP) and CO (0.25-2%) alone and in combination (HYP+CO). We found that transient CO did not affect baseline cardiorespiratory variables, but significantly attenuated hypoxic ventilatory augmentation, predominantly via reduction of tidal volume. To distinguish whether this CO modulation occurs at the CB or within the central nervous system, the cardiorespiratory responses to electrical stimulation of the fastigial nucleus (FN), a cerebellar nucleus known excitatory to respiration, were compared before and during transient CO. Our results showed that the FN-mediated cardiorespiratory responses were not significantly changed by transient CO exposure. To evaluate the effect of CO accumulation, we also compared baseline cardiorespiratory responses to 5 min of 1% and 2% CO, respectively. Interestingly, only the latter produced a biphasic ventilatory response (initial increase followed by decrease) associated with hypotension. We conclude that eupneic breathing in anesthetized rat was not affected by transient CO, but was altered by prolonged exposure to higher levels of CO. Moreover, transient CO depresses hypoxic ventilatory responses mainly through peripherally inhibiting hypoxic stimulation of carotid chemoreceptors.

Diencephalic and mesencephalic influences on ponto-medullary respiratory control in normoxic and hypoxic conditions: an in vitro study on central nervous system preparations from newborn rat

We investigated the effects of the diencephalon and mesencephalon on the central respiratory drive originating from ponto-medullary regions in normoxic and hypoxic conditions, using central nervous system preparations from newborn rats. We used two approaches: 1) electrophysiological analysis of respiratory frequency and the amplitude of inspiratory C4 activity and 2) immunohistochemical detection of Fos protein, an activity-dependent neuronal marker. We found that, in normoxic conditions, the mesencephalon moderated respiratory frequency, probably by means of an inhibitory effect on ventral medullary respiratory neurons. Diencephalic inputs restored respiratory frequency. Moreover, O(2)-sensing areas in the diencephalon (caudal lateral and posterior hypothalamic areas) and mesencephalon (ventrolateral and dorsolateral periaqueductal gray) seem to increase the amplitude of respiratory bursts during adaptation of the central respiratory drive to hypoxia. In contrast, decrease in respiratory frequency during hypoxia is thought to be mediated by a cluster of ventral hypothalamic neurons.

Hypoxic excitability changes and sodium currents in hippocampus CA1 neurons

1. The objective of the present study was to distinguish if inhibition of neuronal activity by hypoxia is related to a block of voltage-gated Na+ channels. 2. The effect of chemical hypoxia induced by cyanide (0.5 mM, 10 min perfusion) was studied with patch-clamp technique in visualized intact CA1 pyramidal neurons in rat brain slices. Action potentials were elicited in whole cell current-clamp recordings and the threshold was estimated by current pulses of 50-ms duration and incremental amplitudes (n = 31). The effect of cyanide on the Na+ current and conductance was studied in voltage clamp recordings from cell-attached patches (n = 13). 3. Cyanide perfusion during 10 min increased the threshold for excitation by 73 +/- 79 pA (p = 0.001), which differed from the effect in control cells (11 +/- 41 pA, ns). The change in current threshold was correlated to a change in membrane potential (r = -0.88, p < 0.0001). Cyanide had no significant effect on the peak amplitude, duration, or rate of rise of the action potential. 4. Cyanide perfusion did not change the Na+ current size, but caused a small decrease in ENa (-17 +/- 22 mV, ns) and a slight increase in Na+ conductance (+14 +/- 26%, ns), which differed (p = 0.045) from controls (-19 +/- 23 %, ns). 5. In conclusion, chemical hypoxia does not cause a decrease in Na+ conductance. The decreased excitability during hypoxia can be explained by an increase in the current threshold, which is correlated with the effect on the membrane potential.

Oxygen-sensing neurons in the central nervous system

This mini-review summarizes the present knowledge regarding central oxygen-chemosensitive sites with special emphasis on their function in regulating changes in cardiovascular and respiratory responses. These oxygen-chemosensitive sites are distributed throughout the brain stem from the thalamus to the medulla and may form an oxygen-chemosensitive network. The ultimate effect on respiratory or sympathetic activity presumably depends on the specific neural projections from each of these brain stem oxygen-sensitive regions as well as on the developmental age of the animal. Little is known regarding the cellular mechanisms involved in the chemotransduction process of the central oxygen sensors. The limited information available suggests some conservation of mechanisms used by other oxygen-sensing systems, e.g., carotid body glomus cells and pulmonary vascular smooth muscle cells. However, major gaps exist in our understanding of the specific ion channels and oxygen sensors required for transducing central hypoxia by these central oxygen-sensitive neurons. Adaptation of these central oxygen-sensitive neurons during chronic or intermittent hypoxia likely contributes to responses in both physiological conditions (ascent to high altitude, hypoxic conditioning) and clinical conditions (heart failure, chronic obstructive pulmonary disease, obstructive sleep apnea syndrome, hypoventilation syndromes). This review underscores the lack of knowledge about central oxygen chemosensors and highlights real opportunities for future research.

Hypoxic response in newborn rat is attenuated by neurokinin-1 receptor blockade

Substance P (SP) is considered to be involved in the regulation of respiration, in particular when respiratory demands are increased, such as during hypoxic stress. In the present study we have investigated the effects of intracerebroventricular pre-treatment with the selective NK-1 receptor antagonist RP67580 on the respiratory response to hypoxia in 5-day-old rat pups. Basal respiration was not altered by RP67580. When subjected to hypoxia (10% O(2)), rat pups pre-treated with RP67580 were unable to sustain the increased respiratory frequency at 10 min. In situ hybridisation demonstrated increased expression of c-fos mRNA in several brainstem areas following hypoxia. This activation was blocked by the antagonist in the retrotrapezoid nucleus and the rostral ventrolateral medulla, areas known to be involved in the hypoxic ventilatory response. This study corroborates a role of endogenously released SP, mediated via NK-1 receptors, in the sustained response to hypoxia in 5-day-old rat pups and suggests that neurons in the rostral ventrolateral medulla are important in this function. It also represents a further example that neuropeptides are released under stressful conditions.

Endogenous rhythm generation in the pre-Bötzinger complex and ionic currents: modelling and in vitro studies

The pre-Bötzinger complex is a small region in the mammalian brainstem involved in generation of the respiratory rhythm. As shown in vitro, this region, under certain conditions, can generate endogenous rhythmic bursting activity. Our investigation focused on the conditions that may induce this bursting behaviour. A computational model of a population of pacemaker neurons in the pre-Bötzinger complex was developed and analysed. Each neuron was modelled in the Hodgkin-Huxley style and included persistent sodium and delayed-rectifier potassium currents. We found that the firing behaviour of the model strongly depended on the expression of these currents. Specifically, bursting in the model could be induced by a suppression of delayed-rectifier potassium current (either directly or via an increase in extracellular potassium concentration, [K+]o) or by an augmentation of persistent sodium current. To test our modelling predictions, we recorded endogenous population activity of the pre-Bötzinger complex and activity of the hypoglossal (XII) nerve from in vitro transverse brainstem slices (700 micro m) of neonatal rats (P0-P4). Rhythmic activity was absent at 3 mm[K+]o but could be triggered by either the elevation of [K+]o to 5-7 mm or application of potassium current blockers (4-AP, 50-200 micro m, or TEA, 2 or 4 mm), or by blocking aerobic metabolism with NaCN (2 mm). This rhythmic activity could be abolished by the persistent sodium current blocker riluzole (25 or 50 micro m). These findings are discussed in the context of the role of endogenous bursting activity in the respiratory rhythm generation in vivo vs. in vitro and during normal breathing in vivo vs. gasping.

A novel O2-sensing mechanism in rat glossopharyngeal neurones mediated by a halothane-inhibitable background K+ conductance

Modulation of K+ channels by hypoxia is a common O2-sensing mechanism in specialised cells. More recently, acid-sensitive TASK-like background K+ channels, which play a key role in setting the resting membrane potential, have been implicated in O2-sensing in certain cell types. Here, we report a novel O2 sensitivity mediated by a weakly pH-sensitive background K+ conductance in nitric oxide synthase (NOS)-positive neurones of the glossopharyngeal nerve (GPN). This conductance was insensitive to 30 mM TEA, 5 mM 4-aminopyridine (4-AP) and 200 microM Cd2+, but was reversibly inhibited by hypoxia (O2 tension (PO2) = 15 mmHg), 2-5 mM halothane, 10 mM barium and 1 mM quinidine. Notably, the presence of halothane occluded the inhibitory effect of hypoxia. Under current clamp, these agents depolarised GPN neurones. In contrast, arachidonic acid (5-10 microM) caused membrane hyperpolarisation and potentiation of the background K+ current. This pharmacological profile suggests the O2-sensitive conductance in GPN neurones is mediated by a class of background K+ channels different from the TASK family; it appears more closely related to the THIK (tandem pore domain halothane-inhibited K+) subfamily, or may represent a new member of the background K+ family. Since GPN neurones are thought to provide NO-mediated efferent inhibition of the carotid body (CB), these channels may contribute to the regulation of breathing during hypoxia via negative feedback control of CB function, as well as to the inhibitory effect of volatile anaesthetics (e.g. halothane) on respiration.

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.

Expression of heme oxygenase in the oxygen-sensing regions of the rostral ventrolateral medulla

Recently, unique regions in the rostral ventrolateral medulla (RVLM) have been found to be oxygen sensitive. However, the mechanism of sensing oxygen in these RVLM regions is unknown. Because heme oxygenase (HO) has been shown to be involved in the hypoxic responses of the carotid body and pulmonary artery, the aim of this study was to determine whether HO is present in the RVLM and whether expression of HO is altered by chronic hypoxia. Adult rats were exposed to hypoxia (10% O(2)) or normoxia (21% O(2)) for 10 days, and the mRNA for HO-1 and HO-2 was examined in the RVLM by using RT-PCR. Expression of HO-2 mRNA was seen in the RVLM of both control and hypoxic samples, whereas expression of HO-1 mRNA was only seen in the RVLM of hypoxic samples. HO-2 was immunocytochemically localized in brain sections (40 microm) to the C1 region and pre-Bötzinger complex of the RVLM. Together, these results indicate that HO-2 is present in the RVLM under control conditions and that HO-1 is induced in the RVLM during chronic hypoxia, consistent with a potential role for HO in the oxygen-sensing function of these cardiorespiratory RVLM regions.

Invited review: Physiological and pathophysiological responses to intermittent hypoxia

This mini-review summarizes the physiological adaptations to and pathophysiological consequences of intermittent hypoxia with special emphasis given to the pathophysiology associated with obstructive sleep apnea. Intermittent hypoxia is an effective stimulus for evoking the respiratory, cardiovascular, and metabolic adaptations normally associated with continuous chronic hypoxia. These adaptations are thought by some to be beneficial in that they may provide protection against disease as well as improve exercise performance in athletes. The long-term consequences of chronic intermittent hypoxia may have detrimental effects, including hypertension, cerebral and coronary vascular problems, developmental and neurocognitive deficits, and neurodegeneration due to the cumulative effects of persistent bouts of hypoxia. Emphasis is placed on reviewing the available data on intermittent hypoxia, making extensions from applicable information from acute and chronic hypoxia studies, and pointing out major gaps in information linking the genomic and cellular responses to intermittent hypoxia with physiological or pathophysiological responses.