The contribution of central mechanisms rostral to the pons in high altitude ventilatory acclimatization

The Amyloid-β Pathway in Alzheimer's Disease

Breakthroughs in molecular medicine have positioned the amyloid-β (Aβ) pathway at the center of Alzheimer's disease (AD) pathophysiology. While the detailed molecular mechanisms of the pathway and the spatial-temporal dynamics leading to synaptic failure, neurodegeneration, and clinical onset are still under intense investigation, the established biochemical alterations of the Aβ cycle remain the core biological hallmark of AD and are promising targets for the development of disease-modifying therapies. Here, we systematically review and update the vast state-of-the-art literature of Aβ science with evidence from basic research studies to human genetic and multi-modal biomarker investigations, which supports a crucial role of Aβ pathway dyshomeostasis in AD pathophysiological dynamics. We discuss the evidence highlighting a differentiated interaction of distinct Aβ species with other AD-related biological mechanisms, such as tau-mediated, neuroimmune and inflammatory changes, as well as a neurochemical imbalance. Through the lens of the latest development of multimodal in vivo biomarkers of AD, this cross-disciplinary review examines the compelling hypothesis- and data-driven rationale for Aβ-targeting therapeutic strategies in development for the early treatment of AD.

Oxygen sensing: applications in humans

Our concepts of oxygen sensing have been transformed over the years. We now appreciate that oxygen sensing is not a unique property limited to “chemoreceptors” but is a common property of tissues and that responses to changes in oxygen levels are not static but can change over time. Respiratory responses initiated at the carotid body are modified by the excitatory and depressant effects of hypoxia at the brain and on the pathways connecting the carotid body to the brain. Equally important is that we are beginning to use our understanding of the cellular and molecular pathways triggered by hypoxia and hyperoxia to identify therapeutic targets to treat diseases such as cancer. We also have a better understanding of the complexities of the human respiratory responses to hypoxia; however, major deficiencies remain in our ability to alter or even measure human ventilatory responses to oxygen deficiency.

Adaptation in the respiratory control system

Exposure to hypoxia, whether for short or prolonged periods or for repeated episodes, produces alterations in the ventilatory responses. This review presents evidence that these adaptations are likely to be mediated by adaptations in the respiratory chemoreflexes, particularly the peripheral chemoreflex, and proposes models of respiratory control explaining the observed changes in ventilation. After a brief introduction to the respiratory control system, a graphical model is developed that illustrates the operation of the system in the steady state, which will be used later. Next, the adaptations in ventilatory responses to hypoxia that have been observed are described, and methods of measuring the alterations in the chemoreflexes that might account for them are discussed. Finally, experimental data supporting the view that changes in the activity of the peripheral chemoreflex can account for the ventilatory adaptations to hypoxia are presented and incorporated into models of chemoreflex behaviour during exposures to hypoxia of various durations.

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.

Human hypoxic ventilatory response with blood dopamine content under intermittent hypoxic training

Adaptation to intermittent hypoxia can enhance a hypoxic ventilatory response (HVR) in healthy humans. Naturally occurring oscillations in blood dopamine (DA) level may modulate these responses. We have measured ventilatory response to hypoxia relative to blood DA concentration and its precursor DOPA before and after a 2-week course of intermittent hypoxic training (IHT). Eighteen healthy male subjects (mean 22.8 ± 2.1 years old) participated in the study. HVRs to isocapnic, progressive, hypoxic rebreathing were recorded and analyzed using piecewise linear approximation. Rebreathing lasted for 5-6 min until inspired O2 reached 8 to 7%. IHT consisted of three identical daily rebreathing sessions separated by 5-min breaks for 14 consecutive days. Before and after the 2-week course of IHT, blood was sampled from the antecubital vein to measure DA and DOPA content. The investigation associated pretraining high blood DA and DOPA values with low HVR (r = -0.66 and -0.75, respectively), elevated tidal volume (r = 0.58 and 0.37) and vital capacity (r = 0.69 and 0.58), and reduced respiratory frequency (r = -0.89 and -0.82). IHT produced no significant change in ventilatory responses to mild hypoxic challenge (PetO2 from 110 to 70-80 mmHg; 1 mmHg = 133.3 Pa) but elicited a 96% increase in ventilatory response to severe hypoxia (from 70-80 to 45 mmHg). Changes in HVRs were not accompanied by statistically significant shifts in blood DA content (24% change), although a twofold increase in DOPA concentration was observed. Individual subject's changes in DA and DOPA content were not correlated with HVR changes when these two parameters were evaluated in relation to the IHT. We hypothesize that DA flowing to the carotid body through the blood may provoke DA autoreceptor-mediated inhibition of endogenous DA synthesis-release, as shown in our baseline data.Key words: hypoxic ventilatory response, dopamine, intermittent hypoxia.

Hyperoxic ventilatory responses of high altitude acclimatized cats

We have examined the effect of steady-state hyperoxia on the ventilation of sea level (SL) cats and cats acclimatized to simulated high altitude (HA) at 5500 m for three weeks. Three groups of cats were studied. In group I, the ventilatory responses to 10%, 21% and 100% O2 were studied at SL, and after acclimatization to HA, the ventilatory responses to 10% and 100% O2 were measured. In group II the ventilatory responses and femoral artery and superior sagittal sinus blood gases were measured in two sets of cats, one at SL and one at HA, during exposure to the gases outlined in group I. In group III, we examined the effect of chronic vagotomy on the ventilatory responses to the gas mixtures outlined in group I. Breathing 100% O2 at SL had no significant effect on ventilation, tidal volume, respiratory frequency, or cerebral blood flow (inferred from the cerebral veno-arterial CO2 difference). Ventilation was constant in the HA acclimatized cats while breathing 10% and 100% O2, but the ventilatory pattern changed dramatically during hyperoxia: respiratory frequency increased and tidal volume fell. Breathing 100% O2 was associated with changes in CBF, and venous PCO2 that might be expected to stimulate ventilation, but the change in ventilatory pattern suggests to us that hyperoxic disinhibition of central respiratory processes (which were modified by HA acclimatization) is the mechanism whereby ventilation is sustained during hyperoxia at HA. After vagotomy at HA, ventilation remained constant while breathing 100% O2, but the changes in respiratory pattern were no longer apparent. Therefore, vagal afferents seems to have a role in determining the pattern, but not necessarily the absolute level, of ventilation during hyperoxia. Cats vagotomized at SL prior to HA exposure did not show any evidence of HA ventilatory acclimatization; thus, the vagi may also play a heretofore unrecognized role in the process of acclimatization.

Carotid body hypercapnia does not elicit ventilatory acclimatization in goats

The carotid body (CB) perfusion model utilizes surgical vascular ligations to allow isolated blood supply to a single in situ CB in awake goats. The contralateral CB was excised. By use of an extracorporeal pump-oxygenator system the blood gas composition perfusing the CB can be controlled independently from that of the systemic arterial system including the brain. Using this model we compared the responses of systemically normoxic goats to CB hypercapnia and CB hypoxia. In 6 goats CB stimulation with hypercapnic-normoxic blood (mean PcbCO2 = 78 Torr, mean PcbO2 congruent to 100 Torr) produced acute hyperventilation (mean decrease in PaCO2 of 5.2 Torr, P less than 0.05) which remained constant over the 4-h perfusion period. Lack of a progressively increasing hyperventilation indicates that ventilatory acclimatization did not occur with hypercapnic CB perfusion. Hypoxic-normocapnic CB stimulation (mean PcbO2 = 40 Torr, mean PcbCO2 = 39 Torr) produced an acute mean decrease in PaCO2 of 5.5 Torr (P less than 0.05) in 6 additional goats. In contrast to CB hypercapnia, the acute hyperventilation induced by CB hypoxia was followed by a progressive time-dependent additional mean decrease in PaCO2 of 5.6 Torr (P less than 0.05) over a 4-h period (ventilatory acclimatization). These data are compatible with the concept of separate receptor mechanisms for hypercapnia and hypoxia in the CB and suggest that the early phase of ventilatory acclimatization to hypoxia in goats may result from a time-dependent increase in CB afferent output.

Ventilatory responses and blood gases in susceptible and resistant rats to high altitude

On exposure to a stimulated altitude of 5500 m (18 000 ft), the Hilltop (H) strain of Sprague-Dawley rats develops signs of chronic mountain sickness (CMS) (severe polycythemia, severe pulmonary hypertension and right ventricular hypertrophy) associated with a high mortality rate. In contrast, the Madison (M) strain of Sprague-Dawley rats remains healthy with less severe cardiopulmonary and hematological responses. We tested the hypothesis that hypoventilation in the H rats relative to the M rats, leading to greater alveolar hypoxia or hypoxemia, could account for the different hematological and cardiopulmonary responses between the two strains. Ventilatory responses and blood gases were compared under normoxia and acute and chronic hypoxia in fully awake and unrestrained animals of the two strains. There were no differences in VE, Pao2, PaCO2, pHa, P-vO2, PvCO2 and pH-v under either acute or chronic hypoxia between the two strains of rats. It is concluded that relative hypoventilation does not contribute to altitude susceptibility in H rats.