Acute intermittent hypoxia in rat in vivo elicits a robust increase in tonic sympathetic nerve activity that is independent of respiratory drive

Acute intermittent hypoxia elicits sympathetic neuroplasticity independent of peripheral chemoreflex activation and spinal cord tissue hypoxia in a rodent model of high-thoracic spinal cord injury

The loss of medullary control of spinal circuits controlling the heart and blood vessels is a unifying mechanism linking both hemodynamic instability and the risk for cardiovascular diseases (CVD) following spinal cord injury (SCI). As such, new avenues to regulate sympathetic activity are essential to mitigate CVD in this population. Acute intermittent hypoxia (AIH) induces a type of neuroplasticity known as long-term facilitation (LTF), a persistent increase in nerve activity post-AIH in spinal motor circuits. Whether LTF occurs within the sympathetic circuit following SCI is largely unknown. We aimed to test whether AIH elicits sympathetic LTF (i.e., sLTF) and attenuates hypoactivity in sub-lesional splanchnic sympathetic circuits in a male rat model of SCI. In 3 experimental series, we tested whether 1) high-thoracic contusion SCI induces hypoactivity in splanchnic sympathetic nerve activity, 2) AIH elicits sLTF following SCI, and 3) sLTF requires carotid chemoreflex activation or spinal cord tissue hypoxia. Our results indicate that a single-session of AIH therapy (10 × 1 min of FiO2 = 0.1, interspersed with 2 min of FiO2 = 1.0) delivered at 2 weeks following SCI attenuates SCI-induced sympathetic hypoactivity by eliciting sLTF 90 min post-treatment that is independent of peripheral chemoreflex activation and/or spinal cord hypoxia. These findings advance our mechanistic understanding of AIH in the field and yield new insights into factors underpinning AIH-induced sLTF following SCI in a rat model. Our findings also set the stage for the chronic application of AIH to alleviate secondary complications resulting from sympathetic hypoactivity following SCI.

The left ventricle increases contractility in response to baroreceptor unloading, which is sympathetically mediated in the anesthetized rat

Contemporary discussion of the baroreflex includes the efferent vascular-sympathetic and cardiovagal arms. Since sympathetic postganglionic neurons also innervate the left ventricle (LV), it is often assumed that the LV produces a sympathetically mediated increase in contractility during baroreceptor unloading, but this has not been characterized using a load-independent index of contractility. We aimed to determine 1) whether LV contractility increases in response to baroreceptor unloading and 2) whether such increases are mediated via the sympathetic or parasympathetic arm of the autonomic nervous system. Ten male Wistar rats were anesthetized (urethane) and instrumented with arterial and LV pressure-volume catheters to measure mean arterial pressure (MAP) and load-independent LV contractility [maximal rate of increase in pressure adjusted to end-diastolic volume (PAdP/dtmax)], respectively. Rats were placed in a servo-controlled lower-body negative pressure (LBNP) chamber to reduce MAP by 10% for 60 s to mechanically unload baroreceptors under control conditions. LBNP was repeated in each animal following infusions of cardiac autonomic blockers using esmolol (sympathetic), atropine (parasympathetic), and esmolol + atropine. Under control conditions, PAdP/dtmax increased during baroreceptor unloading (26 ± 6 vs. 31 ± 9 mmHg·s-1·μL-1, P = 0.031). During esmolol, there was no increase in LV contractility during baroreceptor unloading (11 ± 2 vs. 12 ± 2, P = 0.125); however, during atropine, there was an increase in LV contractility during baroreceptor unloading (26 ± 6 vs. 31 ± 9, P = 0.019). During combined esmolol and atropine, there was a small increase in contractility versus control (13 ± 3 vs. 15 ± 4, P = 0.046). Our results demonstrate that, in anesthetized rats, LV contractility increases in response to baroreceptor unloading, which is largely sympathetically mediated.NEW & NOTEWORTHY This study empirically demonstrates a sympathetically mediated increase in LV contractility in response to baroreceptor unloading using a load-independent index of cardiac contractility in the anesthetized rat.

Advancements in the neurocirculatory reflex response to hypoxia

Hypoxia is a pivotal factor in the pathophysiology of various clinical conditions, including obstructive sleep apnea, which has a strong association with cardiovascular diseases like hypertension, posing significant health risks. Although the precise mechanisms linking hypoxemia-associated clinical conditions with hypertension remains incompletely understood, compelling evidence suggests that hypoxia induces plasticity of the neurocirculatory control system. Despite variations in experimental designs and the severity, frequency, and duration of hypoxia exposure, evidence from animal and human models consistently demonstrates the robust effects of hypoxemia in triggering reflex-mediated sympathetic activation. Both acute and chronic hypoxia alters neurocirculatory regulation and, in some circumstances, leads to sympathetic outflow and elevated blood pressures that persist beyond the hypoxic stimulus. Dysregulation of autonomic control could lead to adverse cardiovascular outcomes and increase the risk of developing hypertension.

The Effects of Volatile Anesthetics on Renal Sympathetic and Phrenic Nerve Activity during Acute Intermittent Hypoxia in Rats

Coordinated activation of sympathetic and respiratory nervous systems is crucial in responses to noxious stimuli such as intermittent hypoxia. Acute intermittent hypoxia (AIH) is a valuable model for studying obstructive sleep apnea (OSA) pathophysiology, and stimulation of breathing during AIH is known to elicit long-term changes in respiratory and sympathetic functions. The aim of this study was to record the renal sympathetic nerve activity (RSNA) and phrenic nerve activity (PNA) during the AIH protocol in rats exposed to monoanesthesia with sevoflurane or isoflurane. Adult male Sprague-Dawley rats (n = 24; weight: 280-360 g) were selected and randomly divided into three groups: two experimental groups (sevoflurane group, n = 6; isoflurane group, n = 6) and a control group (urethane group, n = 12). The AIH protocol was identical in all studied groups and consisted in delivering five 3 min-long hypoxic episodes (fraction of inspired oxygen, FiO2 = 0.09), separated by 3 min recovery intervals at FiO2 = 0.5. Volatile anesthetics, isoflurane and sevoflurane, blunted the RSNA response to AIH in comparison to urethane anesthesia. Additionally, the PNA response to acute intermittent hypoxia was preserved, indicating that the respiratory system might be more robust than the sympathetic system response during exposure to acute intermittent hypoxia.

Paraventricular nucleus projections to the nucleus tractus solitarii are essential for full expression of hypoxia-induced peripheral chemoreflex responses

The hypothalamic paraventricular nucleus (PVN) is essential to peripheral chemoreflex neurocircuitry, but the specific efferent pathways utilized are not well defined. The PVN sends dense projections to the nucleus tractus solitarii (nTS), which exhibits neuronal activation following a hypoxic challenge. We hypothesized that nTS-projecting PVN (PVN-nTS) neurons contribute to hypoxia-induced nTS neuronal activation and cardiorespiratory responses. To selectively target PVN-nTS neurons, rats underwent bilateral nTS nanoinjection of retrogradely transported adeno-associated virus (AAV) driving Cre recombinase expression. We then nanoinjected into PVN AAVs driving Cre-dependent expression of Gq or Gi designer receptors exclusively activated by designer drugs (DREADDs) to test the degree that selective activation or inhibition, respectively, of the PVN-nTS pathway affects the hypoxic ventilatory response (HVR) of conscious rats. We used immunohistochemistry for Fos and extracellular recordings to examine how DREADD activation influences PVN-nTS neuronal activation by hypoxia. Pathway activation enhanced the HVR at moderate hypoxic intensities and increased PVN and nTS Fos immunoreactivity in normoxia and hypoxia. In contrast, PVN-nTS inhibition reduced both the HVR and PVN and nTS neuronal activation following hypoxia. To further confirm selective pathway effects on central cardiorespiratory output, rats underwent hypoxia before and after bilateral nTS nanoinjections of C21 to activate or inhibit PVN-nTS terminals. PVN terminal activation within the nTS enhanced tachycardic, sympathetic and phrenic (PhrNA) nerve activity responses to hypoxia whereas inhibition attenuated hypoxia-induced increases in nTS neuronal action potential discharge and PhrNA. The results demonstrate the PVN-nTS pathway enhances nTS neuronal activation and is necessary for full cardiorespiratory responses to hypoxia. KEY POINTS: The hypothalamic paraventricular nucleus (PVN) contributes to peripheral chemoreflex cardiorespiratory responses, but specific PVN efferent pathways are not known. The nucleus tractus solitarii (nTS) is the first integration site of the peripheral chemoreflex, and the nTS receives dense projections from the PVN. Selective GqDREADD activation of the PVN-nTS pathway was shown to enhance ventilatory responses to hypoxia and activation (Fos immunoreactivity (IR)) of nTS neurons in conscious rats, augmenting the sympathetic and phrenic nerve activity (SSNA and PhrNA) responses to hypoxia in anaesthetized rats. Selective GiDREADD inhibition of PVN-nTS neurons attenuates ventilatory responses, nTS neuronal Fos-IR, action potential discharge and PhrNA responses to hypoxia. These results demonstrate that a projection from the PVN to the nTS is critical for full chemoreflex responses to hypoxia.

Nucleus tractus solitarii is required for the development and maintenance of phrenic and sympathetic long-term facilitation after acute intermittent hypoxia

Exposure to acute intermittent hypoxia (AIH) induces prolonged increases (long term facilitation, LTF) in phrenic and sympathetic nerve activity (PhrNA, SNA) under basal conditions, and enhanced respiratory and sympathetic responses to hypoxia. The mechanisms and neurocircuitry involved are not fully defined. We tested the hypothesis that the nucleus tractus solitarii (nTS) is vital to augmentation of hypoxic responses and the initiation and maintenance of elevated phrenic (p) and splanchnic sympathetic (s) LTF following AIH. nTS neuronal activity was inhibited by nanoinjection of the GABA_A receptor agonist muscimol before AIH exposure or after development of AIH-induced LTF. AIH but not sustained hypoxia induced pLTF and sLTF with maintained respiratory modulation of SSNA. nTS muscimol before AIH increased baseline SSNA with minor effects on PhrNA. nTS inhibition also markedly blunted hypoxic PhrNA and SSNA responses, and prevented altered sympathorespiratory coupling during hypoxia. Inhibiting nTS neuronal activity before AIH exposure also prevented the development of pLTF during AIH and the elevated SSNA after muscimol did not increase further during or following AIH exposure. Furthermore, nTS neuronal inhibition after the development of AIH-induced LTF substantially reversed but did not eliminate the facilitation of PhrNA. Together these findings demonstrate that mechanisms within the nTS are critical for initiation of pLTF during AIH. Moreover, ongoing nTS neuronal activity is required for full expression of sustained elevations in PhrNA following exposure to AIH although other regions likely also are important. Together, the data indicate that AIH-induced alterations within the nTS contribute to both the development and maintenance of pLTF.

The Effects of Therapeutic Intermittent Hypoxia Implementation on Complete Blood Count Parameters: An Experimental Animal Model

Objective: Intermittent hypoxia (IH) implementation is a method performed by intermittently decreasing oxygen concentration in inhaled air at specific rate. This method varies between studies in terms of its application. This study aims to examine the changes in Complete Blood Count (CBC) parameters caused by IH implementation at therapeutic dose ranges with a single model. Methods: Ten Sprague Dawley type adult male rats were divided into two groups. In the study group, FiO2 level of inhaled air, was reduced to 10% in hypoxic cycle. 5 minutes normoxia-hypoxia cycle was used in each 30 minutes experiment period for study group. Control group remained in normoxic air for 30 minutes. 1 cc of blood was taken from mandibular vein from all rats at the end of 6th day. CBC analyzes were performed and differences between two groups were investigated. Results: Significant differences were detected in some CBC parameters between the two groups. It was determined that significant increase in MONO (p

Leptin: A Potential Link Between Obstructive Sleep Apnea and Obesity

Chronic intermittent hypoxia (CIH), a pathophysiological manifestation of obstructive sleep apnea (OSA), is strongly correlated with obesity, as patients with the disease experience weight gain while exhibiting elevated plasma levels of leptin. This study was done to determine whether a relationship may exist between CIH and obesity, and body energy balance and leptin signaling during CIH. Sprague-Dawley rats were exposed to 96 days of CIH or normoxic control conditions, and were assessed for measures of body weight, food and water intake, and food conversion efficiency. At the completion of the study leptin sensitivity, locomotor activity, fat pad mass and plasma leptin levels were determined within each group. Additionally, the hypothalamic arcuate nucleus (ARC) was isolated and assessed for changes in the expression of proteins associated with leptin receptor signaling. CIH animals were found to have reduced locomotor activity and food conversion efficiency. Additionally, the CIH group had increased food and water intake over the study period and had a higher body weight compared to normoxic controls at the end of the study. Basal plasma concentrations of leptin were significantly elevated in CIH exposed animals. To test whether a resistance to leptin may have occurred in the CIH animals due to the elevated plasma levels of leptin, an acute exogenous (ip) leptin (0.04 mg/kg carrier-free recombinant rat leptin) injection was administered to the normoxic and CIH exposed animals. Leptin injections into the normoxic controls reduced their food intake, whereas CIH animals did not alter their food intake compared to vehicle injected CIH animals. Within ARC, CIH animals had reduced protein expression of the short form of the obese (leptin) receptor (isoform OBR₁₀₀) and showed a trend toward an elevated protein expression of the long form of obese (leptin) receptor (OBRb). In addition, pro-opiomelanocortin (POMC) protein expression was reduced, but increased expression of the phosphorylated extracellular-signal-regulated kinase 1/2 (pERK1/2) and of the suppressor of cytokine signaling 3 (SOCS3) proteins was observed in the CIH group, with little change in phosphorylated signal transducer and activator of transcription 3 (pSTAT3). Taken together, these data suggest that long-term exposure to CIH, as seen in obstructive sleep apnea, may contribute to a state of leptin resistance promoting an increase in body weight.

Pentobarbital Anesthesia Suppresses the Glucose Response to Acute Intermittent Hypoxia in Rat

A key feature of sleep disordered breathing syndromes, such as obstructive sleep apnea is intermittent hypoxia. Intermittent hypoxia is well accepted to drive the sympathoexcitation that is frequently associated with hypertension and diabetes, with measurable effects after just 1 h. The aim of this study was to directly measure the glucose response to 1 h of acute intermittent hypoxia in pentobarbital anesthetized rats, compared to conscious rats. However, we found that while a glucose response is measurable in conscious rats exposed to intermittent hypoxia, it is suppressed in anesthetized rats. Intermittent hypoxia for 1, 2, or 8 h increased blood glucose by 0.7 ± 0.1 mmol/L in conscious rats but had no effect in anesthetized rats (-0.1 ± 0.2 mmol/L). These results were independent of the frequency of the hypoxia challenges, fasting state, vagotomy, or paralytic agents. A supraphysiological challenge of 3 min of hypoxia was able to induce a glycemic response indicating that the reflex response is not abolished under pentobarbital anesthesia. We conclude that pentobarbital anesthesia is unsuitable for investigations into glycemic response pathways in response to intermittent hypoxia in rats.

Central AT1 receptor signaling by circulating angiotensin II is permissive to acute intermittent hypoxia-induced sympathetic neuroplasticity

Acute intermittent hypoxia (AIH) triggers sympathetic long-term facilitation (sLTF), a progressive increase in sympathetic nerve activity (SNA) linked to central AT1 receptor (AT1R) activation by circulating angiotensin II (ANG II). Here, we investigated AIH activation of the peripheral renin-angiotensin system (RAS) and the extent to which the magnitude of RAS activation predicts the magnitude of AIH-induced sLTF. In anesthetized male Sprague-Dawley rats, plasma renin activity (PRA) increased in a linear fashion in response to 5 (P = 0.0342) and 10 (P < 0.0001) cycles of AIH, with PRA remaining at the 10th cycle level 1 h later, a period over which SNA progressively increased. On average, SNA ramping began at the AIH cycle 4.6 ± 0.9 (n = 12) and was similar in magnitude 1 h later whether AIH consisted of 5 or 10 cycles (n = 6/group). Necessity of central AT1R in post-AIH sLTF was affirmed by intracerebroventricular (icv) losartan (40 nmol, 2 µL; n = 5), which strongly attenuated both splanchnic (P = 0.0469) and renal (P = 0.0018) sLTF compared with vehicle [artificial cerebrospinal fluid (aCSF), 2 µL; n = 5]. Bilateral nephrectomy largely prevented sLTF, affirming the necessity of peripheral RAS activation. Sufficiency of central ANG II signaling was assessed in nephrectomized rats. Whereas ICV ANG II (0.5 ng/0.5 µL, 30 min) in nephrectomized rats exposed to sham AIH (n = 4) failed to cause SNA ramping, it rescued sLTF in nephrectomized rats exposed to five cycles of AIH [splanchnic SNA (SSNA), P = 0.0227; renal SNA (RSNA), P = 0.0390; n = 5]. Findings indicate that AIH causes progressive peripheral RAS activation, which stimulates an apparent threshold level of central AT1R signaling that plays a permissive role in triggering sLTF.NEW & NOTEWORTHY Acute intermittent hypoxia (AIH) triggers sympathetic long-term facilitation (sLTF) that relies on peripheral renin-angiotensin system (RAS) activation. Here, increasing AIH cycles from 5 to 10 proportionally increased RAS activity, but not the magnitude of post-AIH sLTF. Brain angiotensin II (ANG II) receptor blockade and nephrectomy each largely prevented sLTF, whereas central ANG II rescued it following nephrectomy. Peripheral RAS activation by AIH induces time-dependent neuroplasticity at an apparent central ANG II signaling threshold, triggering a stereotyped sLTF response.

PACAP-PAC1 Receptor Activation Is Necessary for the Sympathetic Response to Acute Intermittent Hypoxia

Repetitive hypoxia is a key feature of obstructive sleep apnoea (OSA), a condition characterized by intermittent airways obstruction. Patients with OSA present with persistent increases in sympathetic activity and commonly develop hypertension. The objectives of this study were to determine if the persistent increases in sympathetic nerve activity, known to be induced by acute intermittent hypoxia (AIH), are mediated through activation of the pituitary adenylate cyclase activating polypeptide (PACAP) signaling system. Here, we show that the excitatory neuropeptide PACAP, acting in the spinal cord, is important for generating the sympathetic response seen following AIH. Using PACAP receptor knockout mice, and pharmacological agents in Sprague Dawley rats, we measured blood pressure, heart rate, pH, PaCO₂, and splanchnic sympathetic nerve activity, under anaesthesia, to demonstrate that the sympathetic response to AIH is mediated via the PAC1 receptor, in a cAMP-dependent manner. We also report that both intermittent microinjection of glutamate into the rostroventrolateral medulla (RVLM) and intermittent infusion of a sub-threshold dose of PACAP into the subarachnoid space can mimic the sympathetic response to AIH. All the sympathetic responses are independent of blood pressure, pH or PaCO₂ changes. Our results show that in AIH, PACAP signaling in the spinal cord helps drive persistent increases in sympathetic nerve activity. This mechanism may be a precursor to the development of hypertension in conditions of chronic intermittent hypoxia, such as OSA.

Centrally acting adrenomedullin in the long-term potentiation of sympathetic vasoconstrictor activity induced by intermittent hypoxia in rats

NEW FINDINGS

What is the central question of this study? Adrenomedullin in the rostral ventrolateral medulla (RVLM) increases sympathetic activity; given that adrenomedullin is released during hypoxia, what are the effects of its agonism and antagonism in the RVLM after chronic intermitent hypoxia (CIH) exposure? What is the main finding and its importance? CIH exposure sensitizes adrenomedullin-dependent mechanisms in the RVLM, supporting its role as a sympathoexcitatory neuromodulator. A novel mechanism was identified for the generation of sympathetic overdrive and hypertension associated with hypoxia, providing potential guidance on new therapeutic approaches for controlling sympathetic hyperactivity in diseases such as sleep apnoea and neurogenic hypertension.

ABSTRACT

Adrenomedullin in the rostral ventrolateral medulla (RVLM) has been shown to increase sympathetic activity whereas the antagonism of its receptors inhibited this autonomic activity lowering blood pressure in conditions of hypertension. Given that hypoxia is a stimulant for releasing adrenomedullin, we hypothesized that the presence of this peptide in the RVLM associated with chronic intermittent hypoxia (CIH) would cause sympathetic overdrive. Juvenile male rats (50-55 g) submitted to CIH (6% oxygen every 9 min, 8 h day^-1 for 10 days) were studied in an arterially perfused in situ preparation where sympathetic activity was recorded. In control rats (n = 6), exogenously applied adrenomedullin in the RVLM raised baseline sympathetic activity when combined with episodic activation of peripheral chemoreceptors (KCN 0.05%, 5 times every 5 min). This sympathoexcitatory response was markedly amplified in rats previously exposed to CIH (n = 6). The antagonism of adrenomedullin receptors in the RVLM caused a significant reduction in sympathetic activity in the CIH group (n = 7), but not in controls (n = 8). The transient reflex-evoked sympathoexcitatory response to peripheral chemoreceptor stimulation was not affected by either adrenomedullin or adrenomedullin receptor antagonism in the RVLM of control and CIH rats. Our findings indicate that CIH sensitizes the sympathoexcitatory networks within the RVLM to adrenomedullin, supporting its role as an excitatory neuromodulator when intermittent hypoxia is present. These data reveal novel state-dependent mechanistic insights into the generation of sympathetic overdrive and provide potential guidance on possible unique approaches for controlling sympathetic discharge in diseases such as sleep apnoea and neurogenic hypertension.

Circulatory control of phrenic motor plasticity

Acute intermittent hypoxia (AIH) elicits distinct mechanisms of phrenic motor plasticity initiated by brainstem neural network activation versus local (spinal) tissue hypoxia. With moderate AIH (mAIH), hypoxemia activates the carotid body chemoreceptors and (subsequently) brainstem neural networks associated with the peripheral chemoreflex, including medullary raphe serotonergic neurons. Serotonin release and receptor activation in the phrenic motor nucleus then elicits phrenic long-term facilitation (pLTF). This mechanism is independent of tissue hypoxia, since electrical carotid sinus nerve stimulation elicits similar serotonin-dependent pLTF. In striking contrast, severe AIH (sAIH) evokes a spinal adenosine-dependent, serotonin-independent mechanism of pLTF. Spinal tissue hypoxia per se is the likely cause of sAIH-induced pLTF, since local tissue hypoxia elicits extracellular adenosine accumulation. Thus, any physiological condition exacerbating spinal tissue hypoxia is expected to shift the balance towards adenosinergic pLTF. However, since these mechanisms compete for dominance due to mutual cross-talk inhibition, the transition from serotonin to adenosine dominant pLTF is rather abrupt. Any factor that compromises spinal cord circulation will limit oxygen availability in spinal cord tissue, favoring a shift in the balance towards adenosinergic mechanisms. Such shifts may arise experimentally from treatments such as carotid denervation, or spontaneous hypotension or anemia. Many neurological disorders, such as spinal cord injury or stroke compromise local circulatory control, potentially modulating tissue oxygen, adenosine levels and, thus, phrenic motor plasticity. In this brief review, we discuss the concept that local (spinal) circulatory control and/or oxygen delivery regulates the relative contributions of distinct pathways to phrenic motor plasticity.

Activation of the hypothalamic paraventricular nucleus by acute intermittent hypoxia: Implications for sympathetic long-term facilitation neuroplasticity

Exposure to acute intermittent hypoxia (AIH) induces a progressive increase of sympathetic nerve activity (SNA) that reflects a form of neuroplasticity known as sympathetic long-term facilitation (sLTF). Our recent findings indicate that activity of neurons in the hypothalamic paraventricular nucleus (PVN) contributes to AIH-induced sLTF, but neither the intra-PVN distribution nor the neurochemical identity of AIH responsive neurons has been determined. Here, awake rats were exposed to 10 cycles of AIH and c-Fos immunohistochemistry was performed to identify transcriptionally activated neurons in rostral, middle and caudal planes of the PVN. Effects of graded intensities of AIH were investigated in separate groups of rats (n = 6/group) in which inspired oxygen (O₂) was reduced every 6 min from 21% to nadirs of 10%, 8% or 6%. All intensities of AIH failed to increase c-Fos counts in the caudally located lateral parvocellular region of the PVN. c-Fos counts increased in the dorsal parvocellular and central magnocellular regions, but significance was achieved only with AIH to 6% O₂ (P < 0.002). By contrast, graded intensities of AIH induced graded c-Fos activation in the stress-related medial parvocellular (MP) region. Focusing on AIH exposure to 8% O₂, experiments next investigated the stress-regulatory neuropeptide content of AIH-activated MP neurons. Tissue sections immunostained for corticotropin-releasing hormone (CRH) or arginine vasopressin (AVP) revealed a significantly greater number of neurons stained for CRH than AVP (P < 0.0001), though AIH induced expression of c-Fos in a similar fraction (~14%) of each neurochemical class. Amongst AIH-activated MP neurons, ~30% stained for CRH while only ~2% stained for AVP. Most AIH-activated CRH neurons (~82%) were distributed in the rostral one-half of the PVN. Results indicate that AIH recruits CRH, but not AVP, neurons in rostral to middle levels of the MP region of PVN, and raise the possibility that these CRH neurons may be a substrate for AIH-induced sLTF neuroplasticity.

Life Under Hypoxia Lowers Blood Glucose Independently of Effects on Appetite and Body Weight in Mice

Blood glucose and the prevalence of diabetes are lower in mountain than lowland dwellers, which could among other factors be due to reduced oxygen availability. To investigate metabolic adaptations to life under hypoxia, male mice on high fat diet (HFD) were continuously maintained at 10% O2. At variance to preceding studies, the protocol was designed to dissect direct metabolic effects from such mediated indirectly via hypoxia-induced reductions in appetite and weight gain. This was achieved by two separate control groups on normal air, one with free access to HFD, and one fed restrictedly in order to obtain a weight curve matching that of hypoxia-exposed mice. Comparable body weight in restrictedly fed and hypoxic mice was achieved by similar reductions in calorie intake (-22%) and was associated with parallel effects on body composition as well as on circulating insulin, leptin, FGF-21, and adiponectin. Whereas the effects of hypoxia on the above parameters could thus be attributed entirely to blunted weight gain, hypoxia improved glucose homeostasis in part independently of body weight (fasted blood glucose, mmol/l: freely fed control, 10.2 ± 0.7; weight-matched control, 8.0 ± 0.3; hypoxia, 6.8 ± 0.2; p < 0.007 each; AUC in the glucose tolerance test, mol/l*min: freely fed control, 2.54 ± 0.15; weight-matched control, 1.86 ± 0.08; hypoxia, 1.67 ± 0.05; p < 0.05 each). Although counterintuitive to lowering of glycemia, insulin sensitivity appeared to be impaired in animals adapted to hypoxia: In the insulin tolerance test, hypoxia-treated mice started off with lower glycaemia than their weight-matched controls (initial blood glucose, mmol/l: freely fed control, 11.5 ± 0.7; weight-matched control, 9.4 ± 0.3; hypoxia, 8.1 ± 0.2; p < 0.02 each), but showed a weaker response to insulin (final blood glucose, mmol/l: freely fed control, 7.0 ± 0.3; weight-matched control, 4.5 ± 0.2; hypoxia, 5.5 ± 0.3; p < 0.01 each). Furthermore, hypoxia weight-independently reduced hepatic steatosis as normalized to total body fat, suggesting a shift in the relative distribution of triglycerides from liver to fat (mg/g liver triglycerides per g total fat mass: freely fed control, 10.3 ± 0.6; weight-matched control, 5.6 ± 0.3; hypoxia, 4.0 ± 0.2; p < 0.0004 each). The results show that exposure of HFD-fed mice to continuous hypoxia leads to a unique metabolic phenotype characterized by improved glucose homeostasis along with evidence for impaired rather than enhanced insulin sensitivity.

Glia and central cardiorespiratory pathology

Respiration and blood pressure are primarily controlled by somatic and autonomic motor neurones, respectively. Central cardiorespiratory control is critical in moment-to-moment survival, but it also has a role in the development and maintenance of chronic pathological conditions such as hypertension. The glial cells of the brain are non-neuronal cells with metabolic, immune, and developmental functions. Recent evidence shows that glia play an active role in supporting and regulating the neuronal circuitry which drives the cardiorespiratory system. Here we will review the activities of two key types of glial cell, microglia and astrocytes, in assisting normal central cardiorespiratory control and in pathology.

Acute intermittent hypoxia with concurrent hypercapnia evokes P2X and TRPV1 receptor-dependent sensory long-term facilitation in naïve carotid bodies

KEY POINTS

Activity-dependent plasticity can be induced in carotid body (CB) chemosensory afferents without chronic intermittent hypoxia (CIH) preconditioning by acute intermittent hypoxia coincident with bouts of hypercapnia (AIH-Hc). Several properties of this acute plasticity are shared with CIH-dependent sensory long-term facilitation (LTF) in that induction is dependent on 5-HT, angiotensin II, protein kinase C and reactive oxygen species. Several properties differ from CIH-dependent sensory LTF; H2 O2 appears to play no part in induction, whereas maintenance requires purinergic P2X2/3 receptor activation and is dependent on transient receptor potential vanilloid type 1 (TRPV1) receptor sensitization. Because P2X2/3 and TRPV1 receptors are located in carotid sinus nerve (CSN) terminals but not presynaptic glomus cells, a primary site of the acute AIH-Hc induced sensory LTF appears to be postsynaptic. Our results obtained in vivo suggest a role for TRPV1-dependent CB activity in acute sympathetic LTF. We propose that P2X-TRPV1-receptor-dependent sensory LTF may constitute an important early mechanism linking sleep apnoea with hypertension and/or cardiovascular disease.

ABSTRACT

Apnoeas constitute an acute existential threat to neonates and adults. In large part, this threat is detected by the carotid bodies, which are the primary peripheral chemoreceptors, and is combatted by arousal and acute cardiorespiratory responses, including increased sympathetic output. Similar responses occur with repeated apnoeas but they continue beyond the last apnoea and can persist for hours [i.e. ventilatory and sympathetic long-term facilitation (LTF)]. These long-term effects may be adaptive during acute episodic apnoea, although they may prolong hypertension causing chronic cardiovascular impairment. We report a novel mechanism of acute carotid body (CB) plasticity (sensory LTF) induced by repeated apnoea-like stimuli [i.e. acute intermittent hypoxia coincident with bouts of hypercapnia (AIH-Hc)]. This plasticity did not require chronic intermittent hypoxia preconditioning, was dependent on P2X receptors and protein kinase C, and involved heat-sensitive transient receptor potential vanilloid type 1 (TRPV1) receptors. Reactive oxygen species (O2 ·¯) were involved in initiating plasticity only; no evidence was found for H2 O2 involvement. Angiotensin II and 5-HT receptor antagonists, losartan and ketanserin, severely reduced CB responses to individual hypoxic-hypercapnic challenges and prevented the induction of sensory LTF but, if applied after AIH-Hc, failed to reduce plasticity-associated activity. Conversely, TRPV1 receptor antagonism had no effect on responses to individual hypoxic-hypercapnic challenges but reduced plasticity-associated activity by ∼50%. Further, TRPV1 receptor antagonism in vivo reduced sympathetic LTF caused by AIH-Hc, although only if the CBs were functional. These data demonstrate a new mechanism of CB plasticity and suggest P2X-TRPV1-dependent sensory LTF as a novel target for pharmacological intervention in some forms of neurogenic hypertension associated with recurrent apnoeas.

Sympathoexcitation following intermittent hypoxia in rat is mediated by circulating angiotensin II acting at the carotid body and subfornical organ

KEY POINTS

In anaesthetized rats, acute intermittent hypoxia increases sympathetic nerve activity, sympathetic peripheral chemoreflex sensitivity and central sympathetic-respiratory coupling. Renin-angiotensin system inhibition prevents the sympathetic effects of intermittent hypoxia, with intermittent injections of angiotensin II into the systemic circulation replicating these effects. Bilateral carotid body denervation reduces the sympathetic effects of acute intermittent hypoxia and eliminates the increases in chemoreflex sensitivity and sympathetic-respiratory coupling. Pharmacological inhibition of the subfornical organ also reduces the sympathetic effects of acute intermittent hypoxia, although it has no effect on the increases in chemoreflex sensitivity and central sympathetic-respiratory coupling. Combining both interventions eliminates the sympathetic effects of both intermittent hypoxia and angiotensin II.

ABSTRACT

Circulating angiotensin II (Ang II) is vital for arterial pressure elevation following intermittent hypoxia in rats, although its importance in the induction of sympathetic changes is unclear. We tested the contribution of the renin-angiotensin system to the effects of acute intermittent hypoxia (AIH) in anaesthetized and ventilated rats. There was a 33.7 ± 2.9% increase in sympathetic nerve activity (SNA), while sympathetic chemoreflex sensitivity and central sympathetic-respiratory coupling increased by one-fold following AIH. The sympathetic effects of AIH were prevented by blocking angiotensin type 1 receptors with systemic losartan. Intermittent systemic injections of Ang II (Int.Ang II) elicited similar sympathetic responses to AIH. To identify the neural pathways responsible for the effects of AIH and Int.Ang II, we performed bilateral carotid body denervation, which reduced the increase in SNA by 56% and 45%, respectively. Conversely, pharmacological inhibition of the subfornical organ (SFO), an established target of circulating Ang II, reduced the increase in SNA following AIH and Int.Ang II by 65% and 59%, respectively, although it did not prevent the sensitization of the sympathetic peripheral chemoreflex, nor the increase in central sympathetic-respiratory coupling. Combined carotid body denervation and inhibition of the SFO eliminated the enhancement of SNA following AIH and Int.Ang II. Repeated systemic injections of phenylephrine caused an elevation in SNA similar to AIH, and this effect was prevented by a renin inhibitor, aliskiren. Our findings show that the sympathetic effects of AIH are the result of RAS-mediated activations of the carotid bodies and the SFO.

Acute intermittent hypoxia enhances corticospinal synaptic plasticity in humans

Acute intermittent hypoxia (AIH) enhances voluntary motor output in humans with central nervous system damage. The neural mechanisms contributing to these beneficial effects are unknown. We examined corticospinal function by evaluating motor evoked potentials (MEPs) elicited by cortical and subcortical stimulation of corticospinal axons and the activity in intracortical circuits in a finger muscle before and after 30 min of AIH or sham AIH. We found that the amplitude of cortically and subcortically elicited MEPs increased for 75 min after AIH but not sham AIH while intracortical activity remained unchanged. To examine further these subcortical effects, we assessed spike-timing dependent plasticity (STDP) targeting spinal synapses and the excitability of spinal motoneurons. Notably, AIH increased STDP outcomes while spinal motoneuron excitability remained unchanged. Our results provide the first evidence that AIH changes corticospinal function in humans, likely by altering corticospinal-motoneuronal synaptic transmission. AIH may represent a novel noninvasive approach for inducing spinal plasticity in humans.

Hypothalamic PVN contributes to acute intermittent hypoxia-induced sympathetic but not phrenic long-term facilitation

Blackburn MB, Andrade MA, Toney GM. Hypothalamic PVN contributes to acute intermittent hypoxia-induced sympathetic but not phrenic long-term facilitation. J Appl Physiol 124: 1233-1243, 2018. First published December 19, 2017; doi: 10.1152/japplphysiol.00743.2017 .- Acute intermittent hypoxia (AIH) repetitively activates the arterial chemoreflex and triggers a progressive increase of sympathetic nerve activity (SNA) and phrenic nerve activity (PNA) referred to as sympathetic and phrenic long-term facilitation (S-LTF and P-LTF), respectively. Neurons of the hypothalamic paraventricular nucleus (PVN) participate in the arterial chemoreflex, but their contribution to AIH-induced LTF is unknown. To determine this, anesthetized rats were vagotomized and exposed to 10 cycles of AIH, each consisting of ventilation for 3 min with 100% O₂ followed by 3 min with 15% O₂. Before AIH, rats received bilateral PVN injections of artificial cerebrospinal fluid (aCSF; vehicle) or the GABA-A receptor agonist muscimol (100 pmol in 50 nl) to inhibit neuronal activity. Thirty minutes after completing the AIH protocol, during which rats were continuously ventilated with 100% O₂, S-LTF and P-LTF were quantified from recordings of integrated splanchnic SNA and PNA, respectively. PVN muscimol attenuated increases of SNA during hypoxic episodes occurring in later cycles (6-10) of AIH ( P < 0.03) and attenuated post-AIH S-LTF ( P < 0.001). Muscimol, however, did not consistently affect peak PNA responses during hypoxic episodes and did not alter AIH-induced P-LTF. These findings indicate that PVN neuronal activity contributes to sympathetic responses during AIH and to subsequent generation of S-LTF. NEW & NOTEWORTHY Neural circuits mediating acute intermittent hypoxia (AIH)-induced sympathetic and phrenic long-term facilitation (LTF) have not been fully elucidated. We found that paraventricular nucleus (PVN) inhibition attenuated sympathetic activation during episodes of AIH and reduced post-AIH sympathetic LTF. Neither phrenic burst patterning nor the magnitude of AIH-induced phrenic LTF was affected. Findings indicate that PVN neurons contribute to AIH-induced sympathetic LTF. Defining mechanisms of sympathetic LTF could improve strategies to reduce sympathetic activity in cardiovascular and metabolic diseases.

Burst patterning of hypothalamic paraventricular nucleus-driven sympathetic nerve activity in ANG II-salt hypertension

ANG II-salt hypertension selectively increases splanchnic sympathetic nerve activity (sSNA), but the extent to which this reflects increased respiratory versus cardiac rhythmic bursting is unknown. Here, integrated sSNA was elevated in ANG II-infused rats fed a high-salt (2% NaCl) diet (ANG II-HSD) compared with vehicle-infused rats fed a normal-salt (0.4% NaCl) diet (Veh-NSD; P < 0.01). Increased sSNA was not accompanied by increased inspiratory or expiratory bursting, consistent with no group difference in central inspiratory drive. Consistent with preserved inhibitory baroreflex entrainment of elevated sSNA in ANG II-HSD rats, the time integral ( P < 0.05) and amplitude ( P < 0.01) of cardiac rhythmic sSNA were increased. Consistent with activity of hypothalamic paraventricular nucleus (PVN) neurons supporting basal SNA in ANG II-salt hypertension, inhibition of PVN with the GABA-A receptor agonist muscimol reduced mean arterial pressure (MAP) and integrated sSNA only in the ANG II-HSD group ( P < 0.001). PVN inhibition had no effect on respiratory rhythmic sSNA bursting in either group but reduced cardiac rhythmic sSNA in ANG II-HSD rats only ( P < 0.01). The latter likely reflected reduced inhibitory baroreflex entrainment subsequent to the fall of MAP. Of note is that MAP as well as integrated and rhythmic burst patterns of sSNA were similar in vehicle-infused rats whether they were fed a normal or high-salt diet. Findings indicate that PVN neurons support elevated sSNA in ANG II-HSD rats by driving a tonic component of activity without altering respiratory or cardiac rhythmic bursting. Because sSNA was unchanged in Veh-HSD rats, activation of PVN-driven tonic sSNA appears to require central actions of ANG II. NEW & NOTEWORTHY ANG II-salt hypertension is strongly neurogenic and depends on hypothalamic paraventricular nucleus (PVN)-driven splanchnic sympathetic nerve activity (sSNA). Here, respiratory and cardiac bursts of sSNA were preserved in ANG II-salt rats and unaltered by PVN inhibition, suggesting that PVN neurons drive a tonic component of sSNA rather than modulating dominant patterns of burst discharge.

Generation of active expiration by serotoninergic mechanisms of the ventral medulla of rats

Abdominal expiratory activity is absent at rest and is evoked during metabolic challenges, such as hypercapnia and hypoxia, or after the exposure to intermittent hypoxia (IH). The mechanisms engaged during this process are not completely understood. In this study, we hypothesized that serotonin (5-HT), acting in the retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG), is able to generate active expiration. In anesthetized (urethane, ip), tracheostomized, spontaneously-breathing adult male Holtzman rats we microinjected a serotoninergic agonist and antagonist bilaterally in the RTN/pFRG and recorded diaphragm and abdominal muscle activities. We found that episodic (3 times, 5 min apart), but not single microinjections of 5-HT (1 mM) in the RTN/pFRG elicited an enduring (>30 min) increase in abdominal activity. This response was amplified in vagotomized rats and blocked by previous 5-HT receptor antagonism with ketanserin (10 µM). Episodic 5-HT microinjections in the RTN/pFRG also potentiated the inspiratory and expiratory reflex responses to hypercapnia. The antagonism of 5-HT receptors in the RTN/pFRG also prevented the long-term facilitation (>30 min) of abdominal activity in response to acute IH exposure (10 × 6-7% O for 45 s every 5 min). Our findings indicate the activation of serotoninergic mechanisms in the RTN/pFRG is sufficient to increase abdominal expiratory activity at resting conditions and required for the emergence of active expiration after IH in anesthetized animals.

Intrathecal Intermittent Orexin-A Causes Sympathetic Long-Term Facilitation and Sensitizes the Peripheral Chemoreceptor Response to Hypoxia in Rats

Intermittent hypoxia causes a persistent increase in sympathetic nerve activity (SNA), which progresses to hypertension in conditions such as obstructive sleep apnea. Orexins (A and B) are hypothalamic neurotransmitters with arousal-promoting and sympathoexcitatory effects. We investigated whether the sustained elevation of SNA, termed sympathetic long-term facilitation, after acute intermittent hypoxia (AIH) is caused by endogenous orexin acting on spinal sympathetic preganglionic neurons. The role of orexin in the increased SNA response to AIH was investigated in urethane-anesthetized, vagotomized, and artificially ventilated Sprague-Dawley rats (n = 58). A spinally infused subthreshold dose of orexin-A (intermittent; 0.1 nmol × 10) produced long-term enhancement in SNA (41.4% ± 6.9%) from baseline. This phenomenon was not produced by the same dose of orexin-A administered as a bolus intrathecal infusion (1 nmol; 7.3% ± 2.3%). The dual orexin receptor blocker, Almorexant, attenuated the effect of sympathetic long-term facilitation generated by intermittent orexin-A (20.7% ± 4.5% for Almorexant at 30 mg∙kg(-1) and 18.5% ± 1.2% for 75 mg∙kg(-1)), but not in AIH. The peripheral chemoreflex sympathoexcitatory response to hypoxia was greatly enhanced by intermittent orexin-A and AIH. In both cases, the sympathetic chemoreflex sensitization was reduced by Almorexant. Taken together, spinally acting orexin-A is mechanistically sufficient to evoke sympathetic long-term facilitation. However, AIH-induced sympathetic long-term facilitation appears to rely on mechanisms that are independent of orexin neurotransmission. Our findings further reveal that the activation of spinal orexin receptors is critical to enhance peripheral chemoreceptor responses to hypoxia after AIH.

Long-term facilitation of expiratory and sympathetic activities following acute intermittent hypoxia in rats

AIM

Acute intermittent hypoxia (AIH) promotes persistent increases in ventilation and sympathetic activity, referred as long-term facilitation (LTF). Augmented inspiratory activity is suggested as a major component of respiratory LTF. In this study, we hypothesized that AIH also elicits a sustained increase in expiratory motor activity. We also investigated whether the expiratory LTF contributes to the development of sympathetic LTF after AIH.

METHODS

Rats were exposed to AIH (10 × 6-7% O2 for 45 s, every 5 min), and the cardiorespiratory parameters were evaluated during 60 min using in vivo and in situ approaches.

RESULTS

In unanesthetized conditions (n = 9), AIH elicited a modest but sustained increase in baseline mean arterial pressure (MAP, 104 ± 2 vs. 111 ± 3 mmHg, P < 0.05) associated with enhanced sympathetic and respiratory-related variabilities. In the in situ preparations (n = 9), AIH evoked LTF in phrenic (33 ± 12%), thoracic sympathetic (75 ± 25%) and abdominal nerve activities (69 ± 14%). The sympathetic overactivity after AIH was phase-locked with the emergence of bursts in abdominal activity during the late-expiratory phase. In anesthetized vagus-intact animals, AIH increased baseline MAP (113 ± 3 vs. 122 ± 2 mmHg, P < 0.05) and abdominal muscle activity (535 ± 94%), which were eliminated after pharmacological inhibition of the retrotrapezoid nucleus/parafacial respiratory group (RTN/pFRG).

CONCLUSION

These findings indicate that increased expiratory activity is also an important component of AIH-elicited respiratory LTF. Moreover, the development of sympathetic LTF after AIH is linked to the emergence of active expiratory pattern and depends on the integrity of the neurones of the RTN/pFRG.

Repetitive acute intermittent hypoxia increases growth/neurotrophic factor expression in non-respiratory motor neurons

Repetitive acute intermittent hypoxia (rAIH) increases growth/trophic factor expression in respiratory motor neurons, thereby eliciting spinal respiratory motor plasticity and/or neuroprotection. Here we demonstrate that rAIH effects are not unique to respiratory motor neurons, but are also expressed in non-respiratory, spinal alpha motor neurons and upper motor neurons of the motor cortex. In specific, we used immunohistochemistry and immunofluorescence to assess growth/trophic factor protein expression in spinal sections from rats exposed to AIH three times per week for 10weeks (3×wAIH). 3×wAIH increased brain-derived neurotrophic factor (BDNF), its high-affinity receptor, tropomyosin receptor kinase B (TrkB), and phosphorylated TrkB (pTrkB) immunoreactivity in putative alpha motor neurons of spinal cervical 7 (C7) and lumbar 3 (L3) segments, as well as in upper motor neurons of the primary motor cortex (M1). 3×wAIH also increased immunoreactivity of vascular endothelial growth factor A (VEGFA), the high-affinity VEGFA receptor (VEGFR-2) and an important VEGF gene regulator, hypoxia-inducible factor-1α (HIF-1α). Thus, rAIH effects on growth/trophic factors are characteristic of non-respiratory as well as respiratory motor neurons. rAIH may be a useful tool in the treatment of disorders causing paralysis, such as spinal injury and motor neuron disease, as a pretreatment to enhance motor neuron survival during disease, or as preconditioning for cell-transplant therapies.

Neurophysiological assessment of sympathetic cardiovascular activity after loss of postganglionic neurons in the anesthetized rat

The goal of this study was to determine the degree of sympathetic postganglionic neuronal loss required to impair cardiovascular-related sympathetic activity. To produce neuronal loss separate groups of rats were treated daily with guanethidine for either 5days or 11days, followed by a recovery period. Sympathetic activity was measured by renal sympathetic nerve activity (RSNA). Stereology of thoracic (T13) ganglia was performed to determine neuronal loss. Despite loss of more than two thirds of neurons in T13 ganglia in both treated groups no effect on resting blood pressure (BP) or heart rate (HR) was detected. Basal RSNA in rats treated for 5days (0.61±0.10μV∗s) and 11days (0.37±0.08μV∗s) was significantly less than vehicle-treated rats (0.99±0.13μV∗s, p<0.05). Increases in RSNA by baroreceptor unloading were significantly lower in 5-day (1.09±0.19μV∗s) and 11-day treated rats (0.59±0.11μV∗s) compared with vehicle-treated rats (1.82±0.19μV∗s, p<0.05). Increases in RSNA to chemoreceptor stimulation were significantly lower in 5-day treated rats (1.54±0.25μV∗s) compared with vehicle-treated rats (2.69±0.23μV∗s, p<0.05). Increases in RSNA in 11-day treated rats were significantly lower (0.75±0.15μV∗s, p<0.05) compared with both vehicle-treated and 5-day treated rats. A positive correlation of neurons to sympathetic responsiveness but not basal activity was detected. These data suggest that diminished capacity for reflex sympathetic responsiveness rather than basal activity alone must be assessed for complete detection of neurophysiological cardiovascular impairment.

Intermittent hypoxia-induced cardiorespiratory long-term facilitation: A new role for microglia

Intermittent hypoxia induces plasticity in neural networks controlling breathing and cardiovascular function. Studies demonstrate that mechanisms causing cardiorespiratory plasticity rely on intracellular signalling pathways that are activated by specific neurotransmitters. Peptides such as serotonin, PACAP and orexin are well-known for their physiological significance in regulating the cardiorespiratory system. Their receptor counterparts are present in cardiorespiratory centres of the brainstem medulla and spinal cord. Microglial cells are also important players in inducing plasticity. The phenotype and function of microglial cells can change based on the physiological state of the central nervous system. Here, we propose that in the autonomic nuclei of the ventral brainstem the relationship between neurotransmitters and neurokines, neurons and microglia determines the overall neural function of the central cardiorespiratory system.

Age protects from harmful effects produced by chronic intermittent hypoxia

Obstructive sleep apnoea (OSA) affects an estimated 3–7% of the adult population, the frequency doubling at ages >60–65 years. As it evolves, OSA becomes frequently associated with cardiovascular, metabolic and neuropsychiatric pathologies defining OSA syndrome (OSAS). Exposing experimental animals to chronic intermittent hypoxia (CIH) can be used as a model of the recurrent hypoxic and O2 desaturation patterns observed in OSA patients. CIH is an important OSA event triggering associated pathologies; CIH induces carotid body (CB)-driven exaggerated sympathetic tone and overproduction of reactive oxygen species, related to the pathogenic mechanisms of associated pathologies observed in OSAS. Aiming to discover why OSAS is clinically less conspicuous in aged patients, the present study compares CIH effects in young (3–4 months) and aged (22–24 months) rats. To define potential distinctive patterns of these pathogenic mechanisms, mean arterial blood pressure as the final CIH outcome was measured. In young rats, CIH augmented CB sensory responses to hypoxia, decreased hypoxic ventilation and augmented sympathetic activity (plasma catecholamine levels and renal artery content and synthesis rate). An increased brainstem integration of CB sensory input as a trigger of sympathetic activity is suggested. CIH also caused an oxidative status decreasing aconitase/fumarase ratio and superoxide dismutase activity. In aged animals, CIH minimally affected CB responses, ventilation and sympathetic-related parameters leaving redox status unaltered. In young animals, CIH caused hypertension and in aged animals, whose baseline blood pressure was augmented, CIH did not augment it further. Plausible mechanisms of the differences and potential significance of these findings for the diagnosis and therapy of OSAS are discussed.

Mild Chronic Intermittent Hypoxia in Wistar Rats Evokes Significant Cardiovascular Pathophysiology but No Overt Changes in Carotid Body-Mediated Respiratory Responses

Models of chronic intermittent hypoxia (CIH), the main feature of obstructive sleep apnoea (OSA), have demonstrated dysregulation of the cardiovascular and respiratory systems resulting in hypertension, cardiac hypertrophy and alterations in the hypoxic ventilatory response (HVR) due to changes in sympathetic and respiratory control by the carotid body. In the UK, treatment of OSA is only offered to patients with an apnoea-hypopnoea index (AHI) >15, we investigated whether mild CIH produced significant pathophysiological changes, which might inform treatment guidelines.Rats were exposed to CIH (6 h(-1), 8 h day(-1), 5 % O(2) nadir) for 2 weeks and then arterial blood pressure (ABP), heart rate (HR) and ventilation were recorded in these and normoxic control rats (N) under Alfaxan anaesthesia, at baseline and in response to Dejours test, graded hypoxia and hypercapnia. Hearts were analysed post-mortem.CIH induced significant increases in baseline ABP (142 ± 5 vs 122 ± 2 mmHg), HR (448 ± 9 vs 412 ± 5 bpm) and cardiac mass (3.5 ± 0.1 vs 2.7 ± 0.1 g kg body mass(-1)) as a result of a selective left ventricular hypertrophy (1.6 ± 0.1 vs 1.3 ± 0.08 g kg body mass(-1); FCSA 464 ± 32 μm(2) vs 314 ± 9 μm(2)). There was no significant difference between N and CIH in baseline respiration or the response to Dejours test, graded hypoxia and hypercapnia.These results demonstrate that mild CIH can induce the significant cardiovascular changes associated with OSA without overt changes in respiratory function. Given evidence that CIH changes carotid body sensory activity, a possible explanation for these results is that there is differential integration of chemoreceptor input with respiratory and cardiac sympathetic outputs.

Transient Intermittent Hypoxia Exposure Disrupts Neonatal Bone Strength

A brief intermittent hypoxia (IH, ambient O2 levels alternating between room air and 12% O2) for 1 h immediately after birth resulted in pancreatic islet dysfunction associated with zinc deficiency as previously reported. We hypothesized that IH exposure modulates zinc homeostasis in bone as well, which leads to increased bone fragility. To test this hypothesis, we used neonatal rats and human osteoblasts (HObs). To examine IH influences on osteoblasts devoid of neural influences, we quantified amounts of alkaline phosphatase and mineralization in IH-treated HObs. Bones harvested from IH-treated animals showed significantly reduced hardness and elasticity. The IH group also showed discretely decreased levels of alkaline phosphatase and mineralization amounts. The IH group showed a decreased expression of ZIP8 or Zrt and Irt-like protein 8 (a zinc uptake transporter), Runx2 (or Runt-related transcription factor 2, a master protein in bone formation), Collagen-1 (a major protein comprising the extracellular matrix of the bone), osteocalcin, and zinc content. When zinc was eliminated from the media containing HObs using a zinc chelate and added later with zinc sulfate, Runx2, ZIP8, and osteocalcin expression decreased first, and recovered with zinc supplementation. Adenovirus-mediated ZIP8 over-expression in osteoblasts increased mineralization significantly as well. We conclude that IH impairs zinc homeostasis in bones and osteoblasts, and that such disturbances decrease bone strength, which can be recovered by zinc supplementation.

Regulation of the renal sympathetic nerves in heart failure

Heart failure (HF) is a serious debilitating condition with poor survival rates and an increasing level of prevalence. HF is associated with an increase in renal norepinephrine (NE) spillover, which is an independent predictor of mortality in HF patients. The excessive sympatho-excitation that is a hallmark of HF has long-term effects that contribute to disease progression. An increase in directly recorded renal sympathetic nerve activity (RSNA) has also been recorded in animal models of HF. This review will focus on the mechanisms controlling sympathetic nerve activity (SNA) to the kidney during normal conditions and alterations in these mechanisms during HF. In particular the roles of afferent reflexes and central mechanisms will be discussed.

Leptin dependent changes in the expression of tropomyosin receptor kinase B protein in nucleus of the solitary tract to acute intermittent hypoxia

To investigate the possibility that leptin exerts an effect in NTS by inducing changes in the expression of pre- and/or post-synaptic proteins, experiments were done in Sprague-Dawley wild-type rats (WT) rats and leptin-deficient rats (Lep(Δ151/Δ151); KILO rat) exposed to 8h of continuous intermittent hypoxia (IH) or normoxia. Protein was extracted from the caudal medial NTS and analyzed by western blot for the expression of brain-derived neurotrophic factor (BDNF), tropomyosin receptor kinase B (TrkB), synaptophysin, synaptopodin and growth-associated protein-43 (GAP-43). In WT rats, BDNF and GAP 43 protein expression levels were not altered after IH or normoxia, although there was a trend towards an increase in BDNF expression. On the other hand, after IH, protein expression of both isoforms of the BDNF receptor TrkB (gp95 and gp145) was higher. Furthermore, synaptophysin protein expression was lower compared to normoxic WT rats. In the KILO rat, no changes were observed in the protein expression of BDNF, TrkB, or GAP 43 after IH when compared to KILO normoxic controls. However, synaptophysin was lower in the IH exposed KILO rat compared to normoxic controls, as found in the WT rat. Expression of synaptopodin was not detected in NTS in either IH or normoxic animals of all groups. These results suggest that leptin released during IH may contribute to neurotrophic changes occurring within NTS and that these changes may be associated with altered chemoreceptor reflex function.

Chemoreflexes, sleep apnea, and sympathetic dysregulation

Obstructive sleep apnea (OSA) and hypertension are closely linked conditions. Disordered breathing events in OSA are characterized by increasing efforts against an occluded airway while asleep, resulting in a marked sympathetic response. This is predominantly due to hypoxemia activating the chemoreflexes, resulting in reflex increases in sympathetic neural outflow. In addition, apnea - and the consequent lack of inhibition of the sympathetic system that occurs with lung inflation during normal breathing - potentiates central sympathetic outflow. Sympathetic activation persists into the daytime, and is thought to contribute to hypertension and other adverse cardiovascular outcomes. This review discusses chemoreflex physiology and sympathetic modulation during normal sleep, as well as the sympathetic dysregulation seen in OSA, its extension into wakefulness, and changes after treatment. Evidence supporting the role of the peripheral chemoreflex in the sympathetic dysregulation seen in OSA, including in the context of comorbid obesity, metabolic syndrome, and systemic hypertension, is reviewed. Finally, alterations in cardiovascular variability and other potential mechanisms that may play a role in the autonomic imbalance in OSA are also discussed.

Acute intermittent optogenetic stimulation of nucleus tractus solitarius neurons induces sympathetic long-term facilitation

Acute intermittent hypoxia (AIH) induces sympathetic and phrenic long-term facilitation (LTF), defined as a sustained increase in nerve discharge. We investigated the effects of AIH and acute intermittent optogenetic (AIO) stimulation of neurons labeled with AAV-CaMKIIa, hChR2(H134R), and mCherry in the nucleus of the solitary tract (NTS) of anesthetized, vagotomized, and mechanically ventilated rats. We measured renal sympathetic nerve activity (RSNA), phrenic nerve activity (PNA), power spectral density, and coherence, and we made cross-correlation measurements to determine how AIO stimulation and AIH affected synchronization between PNA and RSNA. Sixty minutes after AIH produced by ventilation with 10% oxygen in balanced nitrogen, RSNA and PNA amplitude increased by 80% and by 130%, respectively (P < 0.01). Sixty minutes after AIO stimulation, RSNA and PNA amplitude increased by 60% and 100%, respectively, (P < 0.01). These results suggest that acute intermittent stimulation of NTS neurons can induce renal sympathetic and phrenic LTF in the absence of hypoxia or chemoreceptor afferent activation. We also found that while acute intermittent optogenetic and hypoxic stimulations increased respiration-related RSNA modulation (P < 0.01), they did not increase synchronization between central respiratory drive and RSNA. We conclude that mechanisms that induce LTF originate within the caudal NTS and extend to other interconnecting neuronal elements of the central nervous cardiorespiratory network.

Respiratory modulation of sympathetic activity is attenuated in adult rats conditioned with chronic hypobaric hypoxia

Respiratory modulation of sympathetic nerve activity (SNA) depends on numerous factors including prior experience. In our studies, exposing naïve adult, male Sprague-Dawley rats to acute intermittent hypoxia (AIH) enhanced respiratory-modulation of splanchnic SNA (sSNA); whereas conditioning them to chronic hypobaric hypoxia (CHH) attenuated modulation. Further, AIH can evoke increased SNA in the absence phrenic long-term facilitation. We hypothesized that AIH would restore respiratory modulation of SNA in CHH rats. In anesthetized, CHH-conditioned (0.5 atm, 2 wks) rats (n=16), we recorded phrenic and sSNA before during and after AIH (8% O2 for 45s every 5min for 1h). At baseline, sSNA was not modulated with respiration. The sSNA was not recruited during a single brief exposure of hypoxia nor after 10 repetitive exposures. Further, the sSNA chemoresponse was not restored 1h after completing AIH. Thus, CHH-conditioning blocked the short-term plasticity expressed in sympatho-respiratory efferent activities and this was associated with reduced respiratory modulation of sympathetic activity and with attenuation of the sympatho-respiratory chemoresponse.

Increased cardio-respiratory coupling evoked by slow deep breathing can persist in normal humans

Slow deep breathing (SDB) has a therapeutic effect on autonomic tone. Our previous studies suggested that coupling of the cardiovascular to the respiratory system mediates plasticity expressed in sympathetic nerve activity. We hypothesized that SDB evokes short-term plasticity of cardiorespiratory coupling (CRC). We analyzed respiratory frequency (fR), heart rate and its variability (HR&HRV), the power spectral density (PSD) of blood pressure (BP) and the ventilatory pattern before, during, and after a 20-min epoch of SDB. During SDB, CRC and the relative PSD of BP at fR increased; mean arterial pressure decreased; but HR varied; increasing (n = 3), or decreasing (n = 2) or remaining the same (n = 5). After SDB, short-term plasticity was not apparent for the group but for individuals differences existed between baseline and recovery periods. We conclude that a repeated practice, like pranayama, may strengthen CRC and evoke short-term plasticity effectively in a subset of individuals.

Vagal afferent control of abdominal expiratory activity in response to hypoxia and hypercapnia in rats

In the present study, we tested the hypothesis that vagal afferent information modulates the pattern of expiratory response to hypercapnia and hypoxia. Simultaneous recordings of airflow, diaphragmatic (DIA) and oblique abdominal muscle (ABD) activities were performed in anesthetized (urethane, 1.2g/kg), tracheostomized, spontaneously breathing male Wistar rats (290-320g, n=12). The animals were exposed to hypercapnia (7 and 10% CO2 for 5min) and hypoxia (7% O2 for 1min) before and after bilateral vagotomy. We verified that the percentage increase in DIA burst amplitude elicited by hypercapnia and hypoxia episodes was similar between intact and vagotomized rats (P>0.05). In contrast, hypercapnia and hypoxia promoted a marked increase in ABD activity in vagotomized, but not in intact rats (P<0.01). These amplified expiratory motor changes after vagotomy were associated with enhanced expiratory airflow (P<0.01) and augmented tidal volume responses (P<0.01). Our data indicates that, in anesthetized conditions, the removal of peripheral afferent inputs facilitates the processing of active expiration in response to hypercapnia and hypoxia in rats.

Blood pressure is maintained during dehydration by hypothalamic paraventricular nucleus-driven tonic sympathetic nerve activity

Resting sympathetic nerve activity (SNA) consists primarily of respiratory and cardiac rhythmic bursts of action potentials. During homeostatic challenges such as dehydration, the hypothalamic paraventricular nucleus (PVN) is activated and drives SNA in support of arterial pressure (AP). Given that PVN neurones project to brainstem cardio-respiratory regions that generate bursting patterns of SNA, we sought to determine the contribution of PVN to support of rhythmic bursting of SNA during dehydration and to elucidate which bursts dominantly contribute to maintenance of AP. Euhydrated (EH) and dehydrated (DH) (48 h water deprived) rats were anaesthetized, bilaterally vagotomized and underwent acute PVN inhibition by bilateral injection of the GABA-A receptor agonist muscimol (0.1 nmol in 50 nl). Consistent with previous studies, muscimol had no effect in EH rats (n = 6), but reduced mean AP (MAP; P < 0.001) and integrated splanchnic SNA (sSNA; P < 0.001) in DH rats (n = 6). Arterial pulse pressure was unaffected in both groups. Muscimol reduced burst frequency of phrenic nerve activity (P < 0.05) equally in both groups without affecting the burst amplitude-duration integral (i.e. area under the curve). PVN inhibition did not affect the amplitude of the inspiratory peak, expiratory trough or expiratory peak of sSNA in either group, but reduced cardiac rhythmic sSNA in DH rats only (P < 0.001). The latter was largely reversed by inflating an aortic cuff to restore MAP (n = 5), suggesting that the muscimol-induced reduction of cardiac rhythmic sSNA in DH rats was an indirect effect of reducing MAP and thus arterial baroreceptor input. We conclude that MAP is largely maintained in anaesthetized DH rats by a PVN-driven component of sSNA that is neither respiratory nor cardiac rhythmic.

Tonic arterial chemoreceptor activity contributes to cardiac sympathetic activation in mild ovine heart failure

Heart failure (HF) is associated with a large increase in cardiac sympathetic nerve activity (CSNA), which has detrimental effects on the heart and promotes arrhythmias and sudden death. There is increasing evidence that arterial chemoreceptor activation plays an important role in stimulating renal sympathetic nerve activity (RSNA) and muscle sympathetic nerve activity in HF. Given that sympathetic nerve activity to individual organs is differentially controlled, we investigated whether tonic arterial chemoreceptor activation contributes to the increased CSNA in HF. We recorded CSNA and RSNA in conscious normal sheep and in sheep with mild HF induced by rapid ventricular pacing (ejection fraction <40%). Tonic arterial chemoreceptor function was evaluated by supplementing room air with 100% intranasal oxygen (2-3 l min(-1)) for 20 min, thereby deactivating chemoreceptors. The effects of hyperoxia on resting levels and baroreflex control of heart rate, CSNA and RSNA were determined. In HF, chemoreceptor deactivation induced by hyperoxia significantly reduced CSNA [90 ± 2 versus 75 ± 5 bursts (100 heart beats)(-1), P < 0.05, n = 10; room air versus hyperoxia] and heart rate (96 ± 4 versus 85 ± 4 beats min(-1), P < 0.001, n = 12). There was no change in RSNA burst incidence [93 ± 4 versus 92 ± 4 bursts (100 heart beats)(-1), n = 7], although due to the bradycardia the RSNA burst frequency was decreased (90 ± 8 versus 77 ± 7 bursts min(-1), P < 0.001). In normal sheep, chemoreceptor deactivation reduced heart rate without a significant effect on CSNA or RSNA. In summary, deactivation of peripheral chemoreceptors during HF reduced the elevated levels of CSNA, indicating that tonic arterial chemoreceptor activation plays a critical role in stimulating the elevated CSNA in HF.

Cardiorespiratory coupling: common rhythms in cardiac, sympathetic, and respiratory activities

Cardiorespiratory coupling is an encompassing term describing more than the well-recognized influences of respiration on heart rate and blood pressure. Our data indicate that cardiorespiratory coupling reflects a reciprocal interaction between autonomic and respiratory control systems, and the cardiovascular system modulates the ventilatory pattern as well. For example, cardioventilatory coupling refers to the influence of heart beats and arterial pulse pressure on respiration and is the tendency for the next inspiration to start at a preferred latency after the last heart beat in expiration. Multiple complementary, well-described mechanisms mediate respiration's influence on cardiovascular function, whereas mechanisms mediating the cardiovascular system's influence on respiration may only be through the baroreceptors but are just being identified. Our review will describe a differential effect of conditioning rats with either chronic intermittent or sustained hypoxia on sympathetic nerve activity but also on ventilatory pattern variability. Both intermittent and sustained hypoxia increase sympathetic nerve activity after 2 weeks but affect sympatho-respiratory coupling differentially. Intermittent hypoxia enhances sympatho-respiratory coupling, which is associated with low variability in the ventilatory pattern. In contrast, after constant hypobaric hypoxia, 1-to-1 coupling between bursts of sympathetic and phrenic nerve activity is replaced by 2-to-3 coupling. This change in coupling pattern is associated with increased variability of the ventilatory pattern. After baro-denervating hypobaric hypoxic-conditioned rats, splanchnic sympathetic nerve activity becomes tonic (distinct bursts are absent) with decreases during phrenic nerve bursts and ventilatory pattern becomes regular. Thus, conditioning rats to either intermittent or sustained hypoxia accentuates the reciprocal nature of cardiorespiratory coupling. Finally, identifying a compelling physiologic purpose for cardiorespiratory coupling is the biggest barrier for recognizing its significance. Cardiorespiratory coupling has only a small effect on the efficiency of gas exchange; rather, we propose that cardiorespiratory control system may act as weakly coupled oscillator to maintain rhythms within a bounded variability.

Mechanism of sympathetic activation and blood pressure elevation in humans and animals following acute intermittent hypoxia

Sleep apnea is associated with repeated episodes of hypoxemia, causing marked increase in sympathetic nerve activity and blood pressure. Considerable evidence suggests that intermittent hypoxia (IH) resulting from apnea is the primary stimulus for sympathetic overactivity in sleep apnea patients. Several IH protocols have been developed either in animals or in humans to investigate mechanisms underlying the altered autonomic regulation of the circulation. Most of these protocols involve several days (10-40 days) of IH exposure, that is, chronic intermittent hypoxia (CIH). Recent data suggest that a single session of IH exposure, that is, acute intermittent hypoxia (AIH), is already capable of increasing tonic sympathetic nerve output (sympathetic long-term facilitation, LTF) and altering chemo- and baroreflexes with or without elevation of blood pressure. This indicates that IH alters the autonomic neurocirculatory at a very early time point, although the mechanisms underlying this neuroplasticity have not been explored in detail. The purpose of this chapter is to briefly review the effects of AIH on sympathetic LTF and alteration of autonomic reflexes in comparison with the studies from CIH studies. We will also discuss the potential central and peripheral mechanism underlying sympathetic LTF.

Glucose homoeostasis in rats exposed to acute intermittent hypoxia

AIM

Chronic exposure to intermittent hypoxia commonly induces the activation of sympathetic tonus and the disruption of glucose homoeostasis. However, the effects of exposure to acute intermittent hypoxia (AIH) on glucose homoeostasis are not yet fully elucidated. Herein, we evaluated parameters related to glucose metabolism in rats exposed to AIH.

METHODS

Male adult rats were submitted to 10 episodes of hypoxia (6% O2 , for 45 s) interspersed with 5-min intervals of normoxia (21%), while the control (CTL) group was kept in normoxia.

RESULTS

Acute intermittent hypoxia rats presented higher fasting glycaemia, normal insulinaemia, increased lactataemia and similar serum lipid levels, compared to controls (n = 10, P < 0.05). Additionally, AIH rats exhibited increased glucose tolerance (GT) (n = 10, P < 0.05) and augmented insulin sensitivity (IS) (n = 10, P < 0.05). The p-Akt/Akt protein ratio was increased in the muscle, but not in the liver and adipose tissue of AIH rats (n = 6, P < 0.05). The elevated glycaemia in AIH rats was associated with a reduction in the hepatic glycogen content (n = 10, P < 0.05). Moreover, the AIH-induced increase in blood glucose concentration, as well as reduced hepatic glycogen content, was prevented by prior systemic administration of the β-adrenergic antagonist (P < 0.05). The effects of AIH on glycaemia and Akt phosphorylation were transient and not observed after 60 min.

CONCLUSIONS

We suggest that AIH induces an increase in blood glucose concentration as a result of hepatic glycogenolysis recruitment through sympathetic activation. The augmentation of GT and IS might be attributed, at least in part, to increased β-adrenergic sympathetic stimulation and Akt protein activation in skeletal muscles, leading to a higher glucose availability and utilization.

Baroreflex and chemoreflex controls of sympathetic activity following intermittent hypoxia

There is a large amount of evidence linking obstructive sleep apnea (OSA), and the associated intermittent hypoxia that accompanies it, with the development of hypertension. For example, cross-sectional studies demonstrate that the prevalence of hypertension increases with the severity of OSA (Bixler et al., 2000; Grote et al., 2001) and an initial determination of OSA is associated with a three-fold increase for future hypertension (Peppard et al., 2000). Interestingly, bouts of intermittent hypoxia have also been shown to affect sympathetic output associated with the baroreflex and chemoreflex, important mechanisms in the regulation of arterial blood pressure. As such, the possibility exists that changes in the baroreflex and chemoreflex may contribute to the development of chronic hypertension observed in OSA patients. The aim of the current article is to briefly review the response of the baroreflex and chemoreflex to intermittent hypoxic exposure and to evaluate evidence for the hypothesis that modification of these autonomic reflexes may, at least in part, support the comorbidity observed between chronic hypertension and OSA.