Chronic intermittent hypoxia modulates nNOS mRNA and protein expression in the rat hypothalamus

Pulmonary Hypertension: Pharmacological and Non-Pharmacological Therapies

Pulmonary hypertension (PH) is a severe and chronic disease characterized by increased pulmonary vascular resistance and remodeling, often precipitating right-sided heart dysfunction and death. Although the condition is progressive and incurable, current therapies for the disease focus on multiple different drugs and general supportive therapies to manage symptoms and prolong survival, ranging from medications more specific to pulmonary arterial hypertension (PAH) to exercise training. Moreover, there are multiple studies exploring novel experimental drugs and therapies including unique neurostimulation, to help better manage the disease. Here, we provide a narrative review focusing on current PH treatments that target multiple underlying biochemical mechanisms, including imbalances in vasoconstrictor-vasodilator and autonomic nervous system function, inflammation, and bone morphogenic protein (BMP) signaling. We also focus on the potential of novel therapies for managing PH, focusing on multiple types of neurostimulation including acupuncture. Lastly, we also touch upon the disease's different subgroups, clinical presentations and prognosis, diagnostics, demographics, and cost.

Severity of sleep apnea as a prognostic factor for mortality in patients with multiple system atrophy

BACKGROUND

We determined whether the severity of sleep apnea increases the risk of mortality in patients with multiple system atrophy (MSA) with and without stridor. MethodsThis retrospective study included patients who underwent polysomnography within one year after diagnosis of probable MSA. Stridor, sleep apnea, and arousal from sleep were determined using full-night polysomnography. Disease severity was measured using the Unified MSA Rating Scale (UMSARS). Survival data were collected and analyzed using Cox regression analysis.

RESULTS

Sixty-four patients with MSA were included. During a median follow-up of 34.5 months, 49 (76.6 %) patients died. Stridor was present in 56.3 % of patients. Patients with stridor had more severe sleep apnea and shorter sleep time than those without, but the hazard ratio (HR) for death did not differ between patients with and without stridor. Among patients without stridor, apnea-hypopnea index ≥30/h (HR, 6.850; 95 % confidence interval [CI], 1.983-23.664; p = 0.002) and a score of UMSARS I + II (HR, 1.080; 95 % CI, 1.040-1.121; p < 0.001) were independently associated with death. In contrast, among patients with stridor, frequent arousals from sleep (HR, 0.254; 95 % CI, 0.089-0.729; p = 0.011) were a significant factor associated with longer survival, while MSA-cerebellar type tended to be associated with poor survival (HR, 2.195; 95 % CI, 0.941-5.120; p = 0.069).

CONCLUSION

The severity of sleep apnea might be a significant predictor of shorter survival in MSA patients without stridor, whereas frequent arousals from sleep might be a significant predictor for longer survival in MSA patients with stridor.

Elevated Oxidative Stress and Inflammation in Hypothalamic Paraventricular Nucleus Are Associated With Sympathetic Excitation and Hypertension in Rats Exposed to Chronic Intermittent Hypoxia

Obstructive sleep apnea (OSA), characterized by recurrent collapse of the upper airway during sleep leading to chronic intermittent hypoxia (CIH), is an independent risk factor for hypertension. Sympathetic excitation has been shown to play a major role in the pathogenesis of OSA-associated hypertension. Accumulating evidence indicates that oxidative stress and inflammation in the hypothalamic paraventricular nucleus (PVN), a critical cardiovascular and autonomic center, mediate sympathetic excitation in many cardiovascular diseases. Here we tested the hypothesis that CIH elevates oxidative stress and inflammation in the PVN, which might be associated with sympathetic excitation and increased blood pressure in a rat model of CIH that mimics the oxygen profile in patients with OSA. Sprague-Dawley rats were pretreated with intracerebroventricular (ICV) infusion of vehicle or superoxide scavenger tempol, and then exposed to control or CIH for 7 days. Compared with control+vehicle rats, CIH+vehicle rats exhibited increased blood pressure, and increased sympathetic drive as indicated by the blood pressure response to ganglionic blockade and plasma norepinephrine levels. Pretreatment with ICV tempol prevented CIH-induced increases in blood pressure and sympathetic drive. Molecular studies revealed that expression of NAD(P)H oxidase subunits, production of reactive oxygen species, expression of proinflammatory cytokines and neuronal excitation in the PVN were elevated in CIH+vehicle rats, compared with control+vehicle rats, but were normalized or reduced in CIH rat pretreated with ICV tempol. Notably, CIH+vehicle rats also had increased systemic oxidative stress and inflammation, which were not altered by ICV tempol. The results suggest that CIH induces elevated oxidative stress and inflammation in the PVN, which lead to PVN neuronal excitation and are associated with sympathetic excitation and increased blood pressure. Central oxidative stress and inflammation may be novel targets for the prevention and treatment of hypertension in patients with OSA.

Sleep biology updates: Hemodynamic and autonomic control in sleep disorders

Sleep disorders like obstructive sleep apnea syndrome, periodic limb movements in sleep syndrome, insomnia and narcolepsy-cataplexy are all associated with an increased risk of cardiovascular diseases. These disorders share an impaired autonomic nervous system regulation that leads to increased cardiovascular sympathetic tone. This increased cardiovascular sympathetic tone is, in turn, likely to play a major role in the increased risk of cardiovascular disease. Different stimuli, such as intermittent hypoxia, sleep fragmentation, decrease in sleep duration, increased respiratory effort, and transient hypercapnia may all initiate the pathophysiological cascade leading to sympathetic overactivity and some or all of these are encountered in these different sleep disorders. In this manuscript, we outline the different pathways leading to sympathetic over-activity in different sleep conditions. This augmented sympathetic tone is likely to play an important role in the development of cardiovascular disease in patients with sleep disorders, and it is further hypothesized to that sympathoexcitation contributes to the metabolic dysregulation associated with these sleep disorders.

Hypothalamic paraventricular nucleus neuronal nitric oxide synthase activity is a major determinant of renal sympathetic discharge in conscious Wistar rats

NEW FINDINGS

What is the central question of this study? Does chronic reduction of neuronally generated nitric oxide in the hypothalamic paraventricular nucleus affect the set-point regulation of blood pressure and sympathetic activity destined to the kidneys? What is the main finding and its importance? Within the hypothalamic paraventricular nucleus, nitric oxide generated by neuronal nitric oxide synthase plays a major constitutive role in suppressing long term the levels of both ongoing renal sympathetic activity and arterial pressure in conscious Wistar rats. This finding unequivocally demonstrates a mechanism by which the diencephalon exerts a tonic influence on sympathetic discharge to the kidney and may provide the basis for both blood volume and osmolality homeostasis.

ABSTRACT

The paraventricular nucleus (PVN) of the hypothalamus plays a crucial role in cardiovascular and neuroendocrine regulation. Application of nitric oxide donors to the PVN stimulates GABAergic transmission, and may suppress sympathetic nerve activity (SNA) to lower arterial pressure. However, the role of endogenous nitric oxide within the PVN in regulating renal SNA chronically remains to be established in conscious animals. To address this, we used our previously established lentiviral vectors to knock down neuronal nitric oxide synthase (nNOS) selectively in the PVN of conscious Wistar rats. Blood pressure and renal SNA were monitored simultaneously and continuously for 21 days (n = 14) using radio-telemetry. Renal SNA was normalized to maximal evoked discharge and expressed as a percentage change from baseline. The PVN was microinjected bilaterally with a neurone-specific tetracycline-controllable lentiviral vector, expressing a short hairpin miRNA30 interference system targeting nNOS (n = 7) or expressing a mis-sense as control (n = 7). Recordings continued for a further 18 days. The vectors also expressed green fluorescent protein, and successful expression in the PVN and nNOS knockdown were confirmed histologically post hoc. Knockdown of nNOS expression in the PVN resulted in a sustained increase in blood pressure (from 95 ± 2 to 104 ± 3 mmHg, P < 0.05), with robust concurrent sustained activation of renal SNA (>70%, P < 0.05). The study reveals a major role for nNOS-derived nitric oxide within the PVN in chronic set-point regulation of cardiovascular autonomic activity in the conscious, normotensive rat.

Umbilical Cord Blood NOS1 as a Potential Biomarker of Neonatal Encephalopathy

BACKGROUND

There are no definitive markers to aid in diagnosis of neonatal encephalopathy (NE). The purpose of our study was (1) to identify and evaluate the utility of neuronal nitric oxide synthase (NOS1) in umbilical cord blood as a NE biomarker and (2) to identify the source of NOS1 in umbilical cord blood.

METHODS

This was a nested case-control study of neonates >35 weeks of gestation. ELISA for NOS1 in umbilical cord blood was performed. Sources of NOS1 in umbilical cord were investigated by immunohistochemistry, western blot, ELISA, and quantitative PCR. Furthermore, umbilical cords of full-term neonates were subjected to 1% hypoxia ex vivo.

RESULTS

NOS1 was present in umbilical cord blood and increased in NE cases compared with controls. NOS1 was expressed in endothelial cells of the umbilical cord vein, but not in artery or blood cells. In ex vivo experiments, hypoxia was associated with increased levels of NOS1 in venous endothelial cells of the umbilical cord as well as in ex vivo culture medium.

CONCLUSION

This is the first study to investigate an early marker of NE. NOS1 is elevated with hypoxia, and further studies are needed to investigate it as a valuable tool for early diagnosis of neonatal brain injury.

Neural Control of Blood Pressure in Chronic Intermittent Hypoxia

Sleep apnea (SA) is increasing in prevalence and is commonly comorbid with hypertension. Chronic intermittent hypoxia is used to model the arterial hypoxemia seen in SA, and through this paradigm, the mechanisms that underlie SA-induced hypertension are becoming clear. Cyclic hypoxic exposure during sleep chronically stimulates the carotid chemoreflexes, inducing sensory long-term facilitation, and drives sympathetic outflow from the hindbrain. The elevated sympathetic tone drives hypertension and renal sympathetic activity to the kidneys resulting in increased plasma renin activity and eventually angiotensin II (Ang II) peripherally. Upon waking, when respiration is normalized, the sympathetic activity does not diminish. This is partially because of adaptations leading to overactivation of the hindbrain regions controlling sympathetic outflow such as the nucleus tractus solitarius (NTS), and rostral ventrolateral medulla (RVLM). The sustained sympathetic activity is also due to enhanced synaptic signaling from the forebrain through the paraventricular nucleus (PVN). During the waking hours, when the chemoreceptors are not exposed to hypoxia, the forebrain circumventricular organs (CVOs) are stimulated by peripherally circulating Ang II from the elevated plasma renin activity. The CVOs and median preoptic nucleus chronically activate the PVN due to the Ang II signaling. All together, this leads to elevated nocturnal mean arterial pressure (MAP) as a response to hypoxemia, as well as inappropriately elevated diurnal MAP in response to maladaptations.

Intermittent hypoxia and diet-induced obesity: effects on oxidative status, sympathetic tone, plasma glucose and insulin levels, and arterial pressure

Obstructive sleep apnea (OSA) consists of sleep-related repetitive obstructions of upper airways that generate episodes of recurrent or intermittent hypoxia (IH). OSA commonly generates cardiovascular and metabolic pathologies defining the obstructive sleep apnea syndrome (OSAS). Literature usually links OSA-associated pathologies to IH episodes that would cause an oxidative status and a carotid body-mediated sympathetic hyperactivity. Because cardiovascular and metabolic pathologies in obese patients and those with OSAS are analogous, we used models (24-wk-old Wistar rats) of IH (applied from weeks 22 to 24) and diet-induced obesity (O; animals fed a high-fat diet from weeks 12 to 24) to define the effect of each individual maneuver and their combination on the oxidative status and sympathetic tone of animals, and to quantify cardiovascular and metabolic parameters and their deviation from normality. We found that IH and O cause an oxidative status (increased lipid peroxides and diminished activities of superoxide dismutases), an inflammatory status (augmented C-reactive protein and nuclear factor kappa-B activation), and sympathetic hyperactivity (augmented plasma and renal artery catecholamine levels and synthesis rate); combined treatments worsened those alterations. IH and O augmented liver lipid content and plasma cholesterol, triglycerides, leptin, glycemia, insulin levels, and HOMA index, and caused hypertension; most of these parameters were aggravated when IH and O were combined. IH diminished ventilatory response to hypoxia, and hypercapnia and O created a restrictive ventilatory pattern; a combination of treatments led to restrictive hypoventilation. Data demonstrate that IH and O cause comparable metabolic and cardiovascular pathologies via misregulation of the redox status and sympathetic hyperactivity.

Enhanced expression of hypothalamic nitric oxide synthase in rats developmentally exposed to organophosphates

Nitric oxide synthase (NOS) is highly expressed in the hypothalamus, and nitric oxide (NO) specifically contributes to the regulation of neuronal activity within distinct hypothalamic regions. We studied the long-lasting effects of developmental exposure to low doses of organophosphate chlorpyrifos (CPF) and diazinon (DZN) on the expression of NOS in the hypothalamic subnuclei that subserve neuroendocrine, autonomic and cognitive functions. A daily dose of 1 mg/kg of either CPF or DZN was administered to developing rats during gestational days 15-18 or postnatal days (PND) 1-4. Brain sections from PND 60 rats were processed using NADPH-diaphorase (NADPH-d) and neuronal NOS (nNOS) immunohistochemistry. The number of labeled neurons and the optical density (OD) were assessed in the supraoptic (SON), paraventricular (PVN), medial septum, vertical limb, and horizontal limb of the diagonal band. Developmental exposure to organophosphates increased the number of labeled neurons and OD in different subnuclei in the hypothalamus without gender selectivity. The effect on OD was more pronounced and was significant for more cases. Prenatal exposure to CPF and DZN significantly increased the OD in all regions studied with the exception of PVN. Neonatal exposure to DZN also consistently increased OD in all studied subnuclei. For rats that treated with CPF during early postnatal period, this effect was statistically significant only for the SON and PVN. These findings suggest that overexpression of NOS in the hypothalamus may contribute to the mechanisms inducing or compensating for endocrine, autonomic and cognitive abnormalities after developmental exposure to organophosphates.

Nitric oxide and obstructive sleep apnea

Obstructive sleep apnea is a common disease, affecting 16% of the working age population. Although sleep apnea has a well-established connection to daytime sleepiness presumably mediated through repetitive sleep disruption, some other consequences are less well understood. Clinical, epidemiological, and physiological investigations have demonstrated a connection between sleep apnea and daytime hypertension. The elevation of arterial pressure is evident during waking, when patients are not hypoxic, and is mediated by sustained sympathoexcitation and by altered peripheral vascular reactivity. This review summarizes data suggesting that both the sympathoexcitation and the altered vascular reactivity are, at least in part, a consequence of reduced expression of nitric oxide synthase, in neural tissue and in endothelium. Reduced nitric oxide generation in central and peripheral sites of sympathoregulation and in endothelium together may, in part, explain the elevations in waking pressures observed in sleep apnea patients.

An adenovirus traffic update: from receptor engagement to the nuclear pore

Adenoviruses have a bipolar nature: they are ubiquitous pathogens that occasionally cause life-threatening diseases or they can be engineered into powerful gene transfer vectors. The goal of this article is to summarize the most recent advances in adenovirus receptor engagement, internalization, endosomal maturation, endosomal escape and trafficking to the nuclear pore. A better understanding of this initial part of the adenovirus lifecycle may identify new mechanistic-based treatments for adenovirus-induced diseases and help in the engineering of more efficient vectors.

Hypoxia. 3. Hypoxia and neurotransmitter synthesis

Central and peripheral neurons as well as neuroendocrine cells express a variety of neurotransmitters/modulators that play critical roles in regulation of physiological systems. The synthesis of several neurotransmitters/modulators is regulated by O(2)-requiring rate-limiting enzymes. Consequently, hypoxia resulting from perturbations in O(2) homeostasis can affect neuronal functions by altering neurotransmitter synthesis. Two broad categories of hypoxia are frequently encountered: continuous hypoxia (CH) and intermittent hypoxia (IH). CH is often seen during high altitude sojourns, whereas IH is experienced in sleep-disordered breathing with recurrent apneas (i.e., brief, repetitive cessations of breathing). This article presents what is currently known on the effects of both forms of hypoxia on neurotransmitter levels and neurotransmitter synthesizing enzymes in the central and peripheral nervous systems.

Tryptophan hydroxylase: a target for neuroendocrine disruption

Tryptophan hydroxylase (TPH), the rate-limiting enzyme in serotonin (5-HT) synthesis, performs an essential role in the maintenance of serotonergic functions in the central nervous system (CNS), including regulation of the neuroendocrine system controlling reproduction. The results of recent studies in a teleost model of neuroendocrine disruption, Atlantic croaker, indicated that hypothalamic TPH is a major site of interference of hypothalamic-pituitary-gonadal function by environmental stressors. The effects of exposure to two different types of environmental stressors, low dissolved oxygen (hypoxia) and a polychlorinated biphenyl mixture (Aroclor 1254), on the stimulatory brain serotonergic system controlling reproductive neuroendocrine function in Atlantic croaker are reviewed. Exposure to both stressors produced decreases in TPH activity, which were accompanied by a fall in hypothalamic 5-HT and gonadotropin-releasing hormone (GnRH I) content in the preoptic-anterior hypothalamic area and were associated with reduction in luteinizing hormone (LH) secretion and gonadal development. Pharmacological restoration of hypothalamic 5-HT levels after exposure to both stressors also restored neuroendocrine and reproductive functions, indicating that the serotonergic system is an important site for hypoxia- and Aroclor 1254-induced inhibition of reproductive neuroendocrine functions. The mechanisms underlying downregulation of TPH activity by these stressors remain unclear but may involve alterations in hypothalamic antioxidant status. In support of this hypothesis, treatment with an antioxidant, vitamin E, was found to reverse the inhibitory effects of Aroclor 1254 on TPH activity. The results suggest that TPH is a major target for neuroendocrine disruption by diverse environmental stressors.

Chronic intermittent hypoxia induces NMDA receptor-dependent plasticity and suppresses nitric oxide signaling in the mouse hypothalamic paraventricular nucleus

Chronic intermittent hypoxia (CIH) is a concomitant of sleep apnea that produces a slowly developing chemosensory-dependent blood pressure elevation ascribed in part to NMDA receptor-dependent plasticity and reduced nitric oxide (NO) signaling in the carotid body. The hypothalamic paraventricular nucleus (PVN) is responsive to hypoxic stress and also contains neurons that express NMDA receptors and neuronal nitric oxide synthase (nNOS). We tested the hypothesis that extended (35 d) CIH results in a decrease in the surface/synaptic availability of the essential NMDA NR1 subunit in nNOS-containing neurons and NMDA-induced NO production in the PVN of mice. As compared with controls, the 35 d CIH-exposed mice showed a significant increase in blood pressure and an increased density of NR1 immunogold particles located in the cytoplasm of nNOS-containing dendrites. Neither of these between-group differences was seen after 14 d, even though there was already a reduction in the NR1 plasmalemmal density at this time point. Patch-clamp recording of PVN neurons in slices showed a significant reduction in NMDA currents after either 14 or 35 d exposure to CIH compared with sham controls. In contrast, NO production, as measured by the NO-sensitive fluorescent dye 4-amino-5-methylamino-2',7'-difluorofluorescein, was suppressed only in the 35 d CIH group. We conclude that CIH produces a reduction in the surface/synaptic targeting of NR1 in nNOS neurons and decreases NMDA receptor-mediated currents in the PVN before the emergence of hypertension, the development of which may be enabled by suppression of NO signaling in this brain region.

Modulation of cardiorespiratory function mediated by the paraventricular nucleus

The hypothalamic paraventricular nucleus (PVN) coordinates autonomic and neuroendocrine systems to maintain homeostasis and to respond to stress. Neuroanatomic and neurophysiologic experiments have provided insight into the mechanisms by which the PVN acts. The PVN projects directly to the spinal cord and brainstem and, specifically, to sites that control cardio-respiratory function: the intermediolateral cell columns and phrenic motor nuclei in the spinal cord and rostral ventrolateral medulla (RVLM) and the rostral nuclei in the ventral respiratory column (rVRC) in the brainstem. Activation of the PVN increases ventilation (both tidal volume and frequency) and blood pressure (both heart rate and sympathetic nerve activity). Excitatory and inhibitory neurotransmitters including glutamate and GABA converge in the PVN to influence its neuronal activity. However, a tonic GABAergic input to the PVN directly modulates excitatory outflow from the PVN. Further, even within the PVN, microinjection of GABA(A) receptor blockers increases glutamate release suggesting an indirect mechanism by which GABA control contributes to PVN functions. PVN activity alters blood pressure and ventilation during various stresses, such as maternal separation, chronic intermittent hypoxia (CIH), dehydration and hemorrhage. Among the several PVN neurotransmitters and neurohormones, vasopressin and oxytocin modulate ventilation and blood pressure. Here, we review our data indicating that increases in vasopressin and vasopressin type 1A (V(1A)) receptor signalling in the RVLM and rVRC are mechanisms increasing blood pressure and ventilation after exposure to CIH. That blockade of V(1A) receptors in the medulla normalizes baseline blood pressure as well as blunts PVN-evoked blood pressure and ventilatory responses in CIH-conditioned animals indicate the role of vasopressin in cardiorespiratory control. In summary, morphological and functional studies have found that the PVN integrates sensory input and projects to the sympathetic and respiratory control systems with descending projections to the medulla and spinal cord.

Cyclic intermittent hypoxia enhances renal sympathetic response to ICV ET-1 in conscious rats

To test the hypothesis that central changes in sympathoregulation might contribute to sympathoexcitation after cyclic intermittent hypoxia (CIH) we exposed male Sprague-Dawley rats to CIH or to room air sham (Sham) for 8h/d for 3 weeks. After completion of the exposure we assessed heart rate, mean arterial pressure and renal sympathetic nerve activity in conscious animals before and after intracerebroventricular (i.c.v.) administration of endothelin-1 (ET-1, 3 pmol). CIH-exposed animals had a significantly greater sympathetic response to ET-1 than did Sham-exposed animals (CIH 137.8+/-15.6% of baseline; Sham 112.2+/-10.0% of baseline; CIH vs. Sham, P=0.0373). This enhanced sympathetic response to i.c.v. ET-1 was associated with greater expression of endothelin receptor A (ETA) protein in the subfornical organs of CIH-exposed relative to Sham-exposed rats. We conclude that 3-week CIH exposure enhances central ET-1 receptor expression and the sympathetic response to i.c.v. ET-1 suggesting central endothelin may contribute to the sympathetic and hemodynamic response to cyclic intermittent hypoxia.

Control of systemic and pulmonary blood pressure by nitric oxide formed through neuronal nitric oxide synthase

Nitric oxide formed by neuronal nitric oxide synthase (nNOS) in the brain, autonomic inhibitory (nitrergic) nerves, and heart plays important roles in the control of blood pressure. Activation of nitrergic nerves innervating the systemic vasculature elicits vasodilatation, decreases peripheral resistance, and lowers blood pressure. Impairment of nitrergic nerve function, as well as endothelial dysfunction, results in systemic and pulmonary hypertension and decreased regional blood flow. Blockade of nNOS activity in the brain, particularly the medulla and hypothalamus, causes systemic hypertension. Under hypertensive states, such as those in spontaneously hypertensive and Dahl salt-sensitive rats, the expression of the nNOS gene in the brain is increased; this appears to counteract the activated sympathetic function in the vasomotor center. The present article summarizes information concerning the modulation of systemic and pulmonary hypertension through nNOS-derived nitric oxide produced in the brain and periphery.

Increased vasopressin transmission from the paraventricular nucleus to the rostral medulla augments cardiorespiratory outflow in chronic intermittent hypoxia-conditioned rats

A co-morbidity of sleep apnoea is hypertension associated with elevated sympathetic nerve activity (SNA) which may result from conditioning to chronic intermittent hypoxia (CIH). Our hypothesis is that SNA depends on input to the rostral ventrolateral medulla (RVLM) from neurons in the paraventricular nucleus (PVN) that release arginine vasopressin (AVP) and specifically, that increased SNA evoked by CIH depends on this excitatory input. In two sets of neuroanatomical experiments, we determined if AVP neurons project from the PVN to the RVLM and if arginine vasopressin (V(1A)) receptor expression increases in the RVLM after CIH conditioning (8 h per day for 10 days). In the first set, cholera toxin beta subunit (CT-beta) was microinjected into the RVLM to retrogradely label the PVN neurons. Immunohistochemical staining demonstrated that 14.6% of CT-beta-labelled PVN neurons were double-labelled with AVP. In the second set, sections of the medulla were immunolabelled for V(1A) receptors, and the V(1A) receptor-expressing cell count was significantly greater in the RVLM (P < 0.01) and in the neighbouring rostral ventral respiratory column (rVRC) from CIH- than from room air (RA)-conditioned rats. In a series of physiological experiments, we determined if blocking V(1A) receptors in the medulla would normalize blood pressure in CIH-conditioned animals and attenuate its response to disinhibition of PVN. Blood pressure (BP), heart rate (HR), diaphragm (D(EMG)) and genioglossus muscle (GG(EMG)) activity were recorded in anaesthetized, ventilated and vagotomized rats. The PVN was disinhibited by microinjecting a GABA(A) receptor antagonist, bicuculline (BIC, 0.1 nmol), before and after blocking V(1A) receptors within the RVLM and rVRC with SR49059 (0.2 nmol). In RA-conditioned rats, disinhibition of the PVN increased BP, HR, minute D(EMG) and GG(EMG) activity and these increases were attenuated after blocking V(1A) receptors. In CIH-conditioned rats, a significantly greater dose of blocker (0.4 nmol) was required to blunt these physiological responses (P < 0.05). Further, this dose normalized the baseline BP. In summary, AVP released by a subset of PVN neurons modulates cardiorespiratory output via V(1A) receptors in the RVLM and rVRC, and increased SNA in CIH-conditioned animals depends on up-regulation of V(1A) receptors in the RVLM.

Sympathetic response to chemostimulation in conscious rats exposed to chronic intermittent hypoxia

Exposure to cyclic intermittent hypoxia (CIH) is associated with elevated arterial pressure and sustained sympathoexcitation, but the causes of the augmented sympathetic activity remain poorly understood. We recorded arterial pressure, heart rate, and renal sympathetic nerve (RSN) activity in conscious rats previously exposed to either CIH or Sham for 3 weeks during acute exposure to hypoxia (15% and 10% O(2)) or hypercapnia (7% CO(2)). Hemodynamic responses to both hypercapnia and hypoxia were similar between CIH-exposed and Sham-exposed rats, although the pattern of response was different for hypoxia (tachycardia with no change in arterial pressure) and hypercapnia (bradycardia and increased arterial pressure). RSN responses as a percent of the baseline were, however, significantly greater in CIH-exposed animals (CIH-exposed: 15% O(2) - 123.4+/-0.06%; 10% O(2) - 136.7+/-0.12%; 7% CO(2) - 138.3+/-0.18%; Sham-exposed: 15% O(2) - 106.6+/-0.03%; 10% O(2) - 107.6+/-0.01%; 7% CO(2) - 103.0+/-0.14% P<0.01 for all conditions). These data indicate that in conscious rats exposure to CIH enhances sympathetic responses to both hypoxia and hypercapnia.

A new model of chronic intermittent hypoxia in humans: effect on ventilation, sleep, and blood pressure

Obstructive sleep apnea is characterized by repetitive nocturnal upper airway obstructions that are associated with sleep disruption and cyclic intermittent hypoxia (CIH) The cyclic oscillations in O(2) saturation are thought to contribute to cardiovascular and other morbidity, but animal and patient studies of the pathogenic link between CIH and these diseases have been complicated by species differences and by the effects of confounding factors such as obesity, hypertension, and impaired glucose metabolism. To minimize these limitations, we set up a model of nocturnal CIH in healthy humans. We delivered O(2) for 15 s every 2 min during sleep while subjects breathed 13% O(2) in a hypoxic tent to create 30 cycles/h of cyclic desaturation-reoxygenation [saturation of peripheral O(2) (Sp(O(2))) range: 95-85%]. We exposed subjects overnight for 8-9 h/day for 2 wk (10 subjects) and 4 wk (8 subjects). CIH exposure induced respiratory disturbances (central apnea hypopnea index: 3.0 +/- 1.9 to 31.1 +/- 9.6 events/h of sleep at 2 wk). Exposure to CIH for 14 days induced an increase in slopes of hypoxic and hypercapnic ventilatory responses (1.5 +/- 0.6 to 3.1 +/- 1.2 l.min(-1).% drop in Sp(O(2)) and 2.2 +/- 1.0 to 3.3 +/- 0.9 l.min(-1).mmHg CO(2)(-1), respectively), consistent with hypoxic acclimatization. Waking normoxic arterial pressure increased significantly at 2 wk at systolic (114 +/- 2 to 122 +/- 2 mmHg) and for diastolic at 4 wk (71 +/- 1.3 to 74 +/- 1.7 mmHg). We propose this model as a new technique to study the cardiovascular and metabolic consequences of CIH in human volunteers.

Sympathetic neural mechanisms in human hypertension

Compared with substantial clinical research on the renin-angiotensin-aldosterone system (RAAS), much less is known about the importance of the sympathetic nervous system as a therapeutic target to slow the initiation and progression of human hypertension. Using microelectrode recordings of sympathetic activity and radiotracer measurements of regional norepinephrine spillover in hypertensive patients, recent research has advanced several provocative findings with novel--but still largely potential--therapeutic implications for clinical hypertension. These include a stronger scientific rationale for using 1) combined alpha/beta-blockers in the early phases of primary hypertension and obesity-related hypertension; 2) RAAS blockers as central sympatholytics in hypertension associated with chronic kidney disease; and 3) a higher dialysis dose--either nocturnal or short daily hemodialysis--to reduce uremic stimulation of a blood pressure--raising reflex arising in the failing kidneys. New outcomes trials are needed if we are to translate this largely theoretical body of research into clinical practice.

Morbidity due to obstructive sleep apnea: insights from animal models

PURPOSE OF REVIEW

Obstructive sleep apnea (OSA) is a prevalent disorder with clinically well known mid-term and long-term consequences. It is difficult, however, to investigate the mechanisms causing morbidity in OSA from human studies, owing to confounding factors in patients. Animal research is useful to analyze the various injurious stimuli--intermittent hypoxia/hypercapnia, mechanical stress and sleep disruption--that potentially cause OSA morbidity. This review is focused on the most recent advances in our understanding of the consequences of OSA, achieved as a result of animal models.

RECENT FINDINGS

Animal research has improved our knowledge of various aspects of the cardiovascular consequences of OSA: myocardial damage, left ventricular dysfunction, vasoconstriction, hypertension and atherosclerosis. The systemic and metabolic consequences of OSA--inflammation, insulin resistance, alterations in lipid metabolism and hepatic morbidity--have also been investigated with animal models. Our understanding of the mechanisms involved in the neurocognitive consequences of OSA--neuronal and brain alterations and cognitive dysfunctions--has also been improved through animal research. Moreover, animal models have recently been used to investigate the mechanisms of upper airway inflammation and dysfunction.

SUMMARY

The simple experimental models used to investigate OSA morbidity are useful for investigating isolated mechanisms. However, more complex and realistic models incorporating the various injurious challenges characterizing OSA are required to more precisely translate the results of animal research to patients and to design potentially preventive and therapeutic strategies.

Obstructive sleep apnea and hypertension: mechanisms, evaluation, and management

Obstructive sleep apnea (OSA) is a recognized cause of secondary hypertension. OSA episodes produce surges in systolic and diastolic pressure that keep mean blood pressure levels elevated at night. In many patients, blood pressure remains elevated during the daytime, when breathing is normal. Contributors to this diurnal pattern of hypertension include sympathetic nervous system overactivity and alterations in vascular function and structure caused by oxidant stress and inflammation. Treatment of OSA with nasal continuous positive airway pressure (CPAP) abolishes apneas, thereby preventing intermittent arterial pressure surges and restoring the nocturnal "dipping" pattern. CPAP treatment also has modest beneficial effects on daytime blood pressure. Because even small decreases in arterial pressure can contribute to reducing cardiovascular risk, screening for OSA is an essential element of evaluating patients with hypertension.

Erectile dysfunction in a murine model of sleep apnea

RATIONALE

Erectile dysfunction (ED) is frequent in obstructive sleep apnea syndrome (OSAS). Chronic intermittent hypoxia (CIH), one of the hallmarks of OSAS, could mediate ED.

OBJECTIVES

To determine whether intermittent hypoxia during sleep affects erectile dysfunction in mice.

METHODS

Three groups of C57BL/6 mice were exposed to CIH for 5 or 24 weeks. Sexual function was evaluated by in vivo telemetry of corpus spongiosum pressure. Spontaneous erections, sexual activity during mating, and noncontact tests were assessed after 5 weeks of CIH and after treatment with tadalafil. Plasma testosterone was measured after 8 and 24 weeks of CIH, and the expression of nitric oxide synthase (NOS) isoforms was examined in penile tissue.

MEASUREMENTS AND MAIN RESULTS

Noncontact, spontaneous, and contact sexual activity in the mice was suppressed after CIH. Spontaneous erection counts decreased after the first week of CIH by 55% (P < 0.001) and remained unchanged thereafter. Recovery of erectile activity during normoxia for 6 weeks was incomplete. Compared with control mice, latencies for mounts and intromissions increased by 60- and 40-fold, respectively (P < 0.001), and the sexual activity index decreased sixfold. Tadalafil treatment significantly attenuated these effects. Immunoblot analyses of NOS proteins in the erectile tissue showed decreased expression of endothelial NOS after CIH (P < 0.01), with no changes in plasma testosterone levels after 8 and 24 weeks of CIH.

CONCLUSIONS

CIH during sleep is associated with decreased libido in mice. The decreased expression of endothelial NOS protein in erectile tissue and the favorable response to tadalafil suggest that altered nitric oxide mechanisms underlie CIH-mediated ED. No changes in testosterone emerge after intermittent hypoxia.