A6 Noradrenergic Cell Group Modulates the Hypoxic Ventilatory Response

Deficiency of Biogenic Amines Modulates the Activity of Hypoglossal Nerve in the Reserpine Model of Parkinson's Disease

The underlying cause of respiratory impairments appearing in Parkinson's disease (PD) is still far from being elucidated. To better understand the pathogenesis of respiratory disorders appearing in PD, we studied hypoglossal (HG) and phrenic (PHR) motoneuron dysfunction in a rat model evoked with reserpine administration. After reserpine, a decrease in the baseline amplitude and minute HG activity was noted, and no depressive phase of the hypoxic ventilatory response was observed. The pre-inspiratory time of HG activity along with the ratio of pre-inspiratory time to total respiratory cycle time and the ratio of pre-inspiratory to inspiratory amplitude were significantly reduced during normoxia, hypoxia, and recovery compared to sham rats. We suggest that the massive depletion of not only dopamine, but above all noradrenaline and serotonin in the brainstem observed in our study, has an impact on the pre-inspiratory activity of the HG. The shortening of the pre-inspiratory activity of the HG in the reserpine model may indicate a serious problem with maintaining the correct diameter of the upper airways in the preparation phase for inspiratory effort and explain the development of obstructive sleep apnea in some PD patients. Therapies involving the supplementation of amine depletion other than dopamine should be considered.

Respiratory pattern and phrenic and hypoglossal nerve activity during normoxia and hypoxia in 6-OHDA-induced bilateral model of Parkinson's disease

Respiratory disturbances present in Parkinson's disease (PD) are not well understood. Thus, studies in animal models aimed to link brain dopamine (DA) deficits with respiratory impairment are needed. Adult Wistar rats were lesioned with injection of 6-hydroxydopamine (6-OHDA) into the third cerebral ventricle. Two weeks after hypoxic test was performed in whole-body plethysmography chamber, phrenic (PHR) and hypoglossal (HG) nerve activities were recorded in normoxic and hypoxic conditions in anesthetized, vagotomized, paralyzed and mechanically ventilated rats. The effects of activation and blockade of dopaminergic carotid body receptors were investigated during normoxia in anesthetized spontaneously breathing rats. 6-OHDA injection affected resting respiratory pattern in awake animals: an increase in tidal volume and a decrease in respiratory rate had no effect on minute ventilation. Hypoxia magnified the amplitude and minute activity of the PHR and HG nerve of 6-OHDA rats. The ratio of pre-inspiratory to inspiratory HG burst amplitude was reduced in normoxic breathing. Yet, the ratio of pre-inspiratory time to total time of the respiratory cycle was increased during normoxia. 6-OHDA lesion had no impact on DA and domperidone effects on the respiratory pattern, which indicate that peripheral DA receptors are not affected in this model. Analysis of monoamines confirmed substantial striatal depletion of dopamine, serotonin and noradrenaline (NA) and reduction of NA content in the brainstem. In bilateral 6-OHDA model changes in activity of both nerves: HG (linked with increased apnea episodes) and PHR are present. Demonstrated respiratory effects could be related to specific depletion of DA and NA.

Sex differences in breathing

Breathing is a vital behavior that ensures both the adequate supply of oxygen and the elimination of CO₂, and it is influenced by many factors. Despite that most of the studies in respiratory physiology rely heavily on male subjects, there is much evidence to suggest that sex is an important factor in the respiratory control system, including the susceptibility for some diseases. These different respiratory responses in males and females may be related to the actions of sex hormones, especially in adulthood. These hormones affect neuromodulatory systems that influence the central medullary rhythm/pontine pattern generator and integrator, sensory inputs to the integrator and motor output to the respiratory muscles. In this article, we will first review the sex dependence on the prevalence of some respiratory-related diseases. Then, we will discuss the role of sex and gonadal hormones in respiratory control under resting conditions and during respiratory challenges, such as hypoxia and hypercapnia, and whether hormonal fluctuations during the estrous/menstrual cycle affect breathing control. We will then discuss the role of the locus coeruleus, a sexually dimorphic CO₂/pH-chemosensitive nucleus, on breathing regulation in males and females. Next, we will highlight the studies that exist regarding sex differences in respiratory control during development. Finally, the few existing studies regarding the influence of sex on breathing control in non-mammalian vertebrates will be discussed.

Pharmacological, but not genetic, alteration of neural Epo modifies the CO2/H+ central chemosensitivity in postnatal mice

Cerebral erythropoietin (Epo) plays a crucial role for respiratory control in newborn rodents. We showed previously that soluble Epo receptor (sEpoR: an Epo antagonist) reduces basal ventilation and hypoxic hyperventilation at postnatal day 10 (P10) and in adult mice. However, at these ages (P10 and adulthood), Epo had no effect on central chemosensitivity. Nevertheless, it is known that the sensitivity to CO₂/H⁺ during the mammalian respiratory network maturation process is age-dependent. Accordingly, in this study we wanted to test the hypothesis that cerebral Epo is involved in the breathing stimulation induced by the activation of central CO₂/H⁺ chemoreceptors at earlier postnatal ages. To this end, en bloc brainstem-spinal cord preparations were obtained from P4 mice and the fictive breathing response to CO₂-induced acidosis or metabolic acidosis was analyzed. This age (P4) was chosen because previous research from our laboratory showed that Epo altered (in a dose- and time-dependent manner) the fictive ventilation elicited in brainstem-spinal cord preparations. Moreover, as it was observed that peripheral chemoreceptors determined the respiratory sensitivity of central chemoreceptors to CO₂, the use of this technique restricts our observations to central modulation. Our results did not show differences between preparations from control and transgenic animals (Tg21: overexpressing cerebral Epo; Epo-TAg^h: cerebral Epo deficient mice). However, when Tg21 brainstem preparations were incubated for 1h with sEpoR, or with inhibitors of ERK/Akt (thus blocking the activation of the Epo molecular pathway), the fictive breathing response to CO₂-induced acidosis was blunted. Our data suggest that variation of the Epo/sEpoR ratio is central to breathing modulation during CO₂ challenges, and calls attention to clinical perspectives based on the use of Epo drugs at birth in hypoventilation cases.

Erythropoietin in the Locus coeruleus attenuates the ventilatory response to CO2 in rats

The Locus coeruleus (LC) is a pontine area that contributes to the CO₂/pH chemosensitivity. LC cells express erythropoietin (Epo) receptors (EpoR), and Epo in the brainstem is a potent normoxic and hypoxic respiratory stimulant. However, a recent study showed that the intra-cisternal injection (ICI) of Epo antagonist does not alter the hypercapnic ventilatory response in mice. As ICI leads to a widespread dispersal of the product throughout the brainstem, in this work we evaluated the specific impact of Epo in the LC-mediated ventilatory response to CO₂ (by whole body plethysmography) in juvenile male Wistar rats. Normocapnic and hypercapnic ventilation were evaluated before and after unilateral microinjection of Epo (1ng/100nL) into the LC. To evaluate the long-term effect of Epo, the HcVR was re-evaluated 24h later. Our results show that Epo attenuates the hypercapnic ventilation. We conclude that Epo in the LC tunes the hypercapnia-induced hyperpnea.

Erythropoietin and its antagonist regulate hypoxic fictive breathing in newborn mice

Clinical use of erythropoietin in adult and newborn patients has revealed its involvement in neuroprotection, neurogenesis, and angiogenesis. More recently, we showed in adult mouse, that brain erythropoietin interacts with the major brainstem centers associated with respiration to enhance the ventilatory response to acute and chronic conditions of physiological hypoxia (e.g., as occurring at high altitude). However, whether brain erythropoietin is involved in breathing regulation in newborns remains unknown. In this study, en bloc brainstem-spinal cord preparations were obtained from mice at postnatal day 4. After various periods (30, 60, or 90 min) of incubation with 0, 25, or 250 U of erythropoietin, preparations were superfused with artificial cerebrospinal fluid bubbled with normoxic or hypoxic gas mixtures. The electrophysiological fictive breathing produced by axons at the C4 ventral root was next recorded. Our results show that erythropoietin attenuates the hypoxia-mediated decrease of the central respiratory activity and improves post-hypoxic recovery. Additional analysis revealed that the soluble erythropoietin receptor (the endogenous erythropoietin antagonist) dramatically decreases neural hypoxic respiratory activity, confirming the specific erythropoietin effect on respiratory drive. These results imply that erythropoietin exerts main modulation and maintenance of respiratory motor output during hypoxic and post-hypoxic challenges in 4-days old mice.

Erythropoietin modulates the neural control of hypoxic ventilation

Numerous factors involved in general homeostasis are able to modulate ventilation. Classically, this comprises several kind of molecules, including neurotransmitters and steroids that are necessary for fine tuning ventilation under different conditions such as sleep, exercise, and acclimatization to high altitude. Recently, however, we have found that erythropoietin (Epo), the main regulator of red blood cell production, influences both central (brainstem) and peripheral (carotid bodies) respiratory centers when the organism is exposed to hypoxic conditions. Here, we summarize the effect of Epo on the respiratory control in mammals and highlight the potential implication of Epo in the ventilatory acclimatization to high altitude, as well as in the several respiratory sickness and syndromes occurring at low and high altitude.

Brain monoaminergic neurons and ventilatory control in vertebrates

Monoamines (noradrenaline (NA), adrenaline (AD), dopamine (DA) and serotonin (5-HT) are key neurotransmitters that are implicated in multiple physiological and pathological brain mechanisms, including control of respiration. The monoaminergic system is known to be widely distributed in the animal kingdom, which indicates a considerable degree of phylogenetic conservation of this system amongst vertebrates. Substantial progress has been made in uncovering the participation of the brain monoamines in the breathing regulation of mammals, since they are involved in the maturation of the respiratory network as well as in the modulation of its intrinsic and synaptic properties. On the other hand, for the non-mammalian vertebrates, most of the knowledge of central monoaminergic modulation in respiratory control, which is actually very little, has emerged from studies using anuran amphibians. This article reviews the available data on the role of brain monoaminergic systems in the control of ventilation in terrestrial vertebrates. Emphasis is given to the comparative aspects of the brain noradrenergic, adrenergic, dopaminergic and serotonergic neuronal groups in breathing regulation, after first briefly considering the distribution of monoaminergic neurons in the vertebrate brain.

Sex-dependent regulation of hypoxic ventilation in mice and humans is mediated by erythropoietin

Acclimatization to hypoxic exposure relies on an elevated ventilation and erythropoietic activity. We recently proposed that erythropoietin (Epo) links both responses: apart from red blood cell production, cerebral and plasma Epo interact with the central and peripheral respiratory centers. Knowing that women cope better than men with reduced oxygen supply (as observed at high altitude), we analyzed the hypoxic ventilatory response in Epo-overexpressing transgenic male and female mice with high Epo levels in brain and plasma (Tg6) or in wild-type animals injected with recombinant human Epo (rhEpo). Exposure to moderate and severe hypoxia as well as to hyperoxia and injection of domperidone, a potent peripheral ventilatory stimulant, revealed that the presence of transgenic or rhEpo extensively increased the hypoxic ventilatory response in female mice compared with their corresponding male siblings. Alterations of catecholamines in the brain stem's respiratory centers were also sex dependent. In a proof-of-concept study, human volunteers were intravenously injected with 5,000 units rhEpo and subsequently exposed to 10% oxygen. Compared with men, the hypoxic ventilatory response was significantly increased in women. We conclude that Epo exerts a sex-dependent impact on hypoxic ventilation improving the response in female mice and in women that most probably involves sexual hormones. Our data provides an explanation as to why women are less susceptible to hypoxia-associated syndromes than men.

Ventilatory responses to acute and chronic hypoxia are altered in female but not male Paskin-deficient mice

Proteins harboring a Per-Arnt-Sim (PAS) domain are versatile and allow archaea, bacteria, and plants to sense oxygen partial pressure, as well as light intensity and redox potential. A PAS domain associated with a histidine kinase domain is found in FixL, the oxygen sensor molecule of Rhizobium species. PASKIN is the mammalian homolog of FixL, but its function is far from being understood. Using whole body plethysmography, we evaluated the ventilatory response to acute and chronic hypoxia of homozygous deficient male and female PASKIN mice ( Paskin −/−). Although only slight ventilatory differences were found in males, female Paskin −/− mice increased ventilatory response to acute hypoxia. Unexpectedly, females had an impaired ability to reach ventilatory acclimatization in response to chronic hypoxia. Central control of ventilation occurs in the brain stem respiratory centers and is modulated by catecholamines via tyrosine hydroxylase (TH) activity. We observed that TH activity was altered in male and female Paskin −/− mice. Peripheral chemoreceptor effects on ventilation were evaluated by exposing animals to hyperoxia (Dejours test) and domperidone, a peripheral ventilatory stimulant drug directly affecting the carotid sinus nerve discharge. Male and female Paskin −/− had normal peripheral chemosensory (carotid bodies) responses. In summary, our observations suggest that PASKIN is involved in the central control of hypoxic ventilation, modulating ventilation in a gender-dependent manner.

Desferrioxamine enhances hypoxic ventilatory response and induces tyrosine hydroxylase gene expression in the rat brainstem in vivo

The iron chelator desferrioxamine (DFO) induces accumulation of the hypoxia-inducible factor (HIF-1), a transcription factor that up-regulates genes involved in adaptative responses to hypoxia. This property makes DFO a potential neuroprotector against hypoxic stress. We investigated in rats the effects of DFO on the ventilatory response to mild hypoxic tests and the expression of tyrosine hydroxylase (TH), a target gene of HIF-1. Two protocols were used, the first with repeated injections of 50 mg/kg DFO every 2 days during a 2-week period. This was aimed at define the time course of the ventilatory responses to a hypoxic test. In the second protocol, rats were given a single injection of 300 mg/kg DFO. Every day over 4 days, the hypoxic ventilatory response was recorded before the animal was sacrificed, and Western blot analysis of TH in the dorsal brainstem cardiorespiratory area was performed. DFO produced a delayed increase in the hypoxic ventilatory response, which appeared in the same time window as TH up-regulation (2-3 days after the bolus injection of DFO). This delay suggests a genic effect of the drug that improves the ventilatory response to hypoxia.

Erythropoietin regulates hypoxic ventilation in mice by interacting with brainstem and carotid bodies

Apart from its role in elevating red blood cell number, erythropoietin (Epo) exerts protective functions in brain, retina and heart upon ischaemic injury. However, the physiological non-erythroid functions of Epo remain unclear. Here we use a transgenic mouse line (Tg21) constitutively overexpressing human Epo in brain to investigate Epo's impact on ventilation upon hypoxic exposure. Tg21 mice showed improved ventilatory response to severe acute hypoxia and moreover improved ventilatory acclimatization to chronic hypoxic exposure. Furthermore, following bilateral transection of carotid sinus nerves that uncouples the brain from the carotid body, Tg21 mice adapted their ventilation to acute severe hypoxia while chemodenervated wild-type (WT) animals developed a life-threatening apnoea. These results imply that Epo in brain modulates ventilation. Additional analysis revealed that the Epo receptor (EpoR) is expressed in the main brainstem respiratory centres and suggested that Epo stimulates breathing control by alteration of catecholaminergic metabolism in brainstem. The modulation of hypoxic pattern of ventilation after i.v. injection of recombinant human Epo in WT mice and the dense EpoR immunosignal observed in carotid bodies showed that these chemoreceptors are sensitive to plasma levels of Epo. In summary, our results suggest that Epo controls ventilation at the central (brainstem) and peripheral (carotid body) levels. These novel findings are relevant to understanding better respiratory disorders including those occurring at high altitude.

The acute effects of insulin on the cardiorespiratory responses to hypoxia in streptozotocin-induced diabetic rats

AIMS

We examined whether or not streptozotocin (STZ)-induced diabetic rats, which have a lower heart rate (HR, beats min(-1)) than control rats, could maintain hypoxic ventilatory response.

METHODS

Twenty-six Wistar rats, which had been injected with STZ (60 mg kg(-1), EXP) or vehicle (0.1 m citrate buffer, CONT) intraperitoneally at 9 weeks of age, had their cardiorespiratory responses to normoxia and 12%O(2) examined after 5 weeks.

RESULTS

Compared with CONT rats, EXP rats had a higher blood glucose [24 +/- 3 vs. 5 +/- 1 (mean +/- SD) mmol L(-1)], a lower body weight (320 +/- 23 vs. 432 +/- 24 g), lower HR (303 +/- 49 vs. 380 +/- 44 in normoxia, and 343 +/- 56 vs. 443 +/- 60 in hypoxia) and a lower mean arterial blood pressure (MAP) (89 +/- 6 vs. 102 +/- 10 mmHg in hypoxia). In contrast, both groups had similar values in ventilation (V(E)), V(E)-metabolic rate (MR) ratio and arterial blood gases (ABGs). In EXP rats, with an acute insulin supplement (i.v., 0.75 U h(-1) for 1.5-2 h), not only blood glucose, but also HR, and MAP were normalized as those obtained in CONT rats, and in hypoxia further increased without affecting V(E)-MR ratio and ABGs. Such acute cardiorespiratory stimulating effects of insulin could not be obtained in non-diabetic rats (n = 7, 355 +/- 24 g), in which euglycaemia (mean 6.4 mmol L(-1)) was maintained during the measurements.

CONCLUSIONS

Our results suggest that, in STZ-induced diabetic rats: (1) ventilation is hardly suppressed by hyperglycaemia, (2) cardiorespiratory responses can be acutely stimulated by short insulin injection, and (3) the effects, including those through acute blood glucose normalization, are possibly specific for the diabetic impairments.