Influence of hypercapnia on the synthesis of neuropeptides and their receptors in murine brain

Relationships of orexigenic and anorexigenic hormones with body fat distribution in patients with obstructive sleep apnea syndrome

PURPOSE

We aimed to examine the relationships of disease activity and risk factors with serum levels of orexigenic and anorexigenic hormones in patients with obstructive sleep apnea syndrome (OSAS).

METHODS

Fasting blood samples were taken for hormonal analysis of all participants, abdominal/neck bioimpedance measurements were recorded, and polysomnography (PSG) analyses were performed. According to the apnea-hypopnea index (AHI), 34 patients with newly diagnosed OSAS and 34 participants without OSAS were compared.

RESULTS

The median body mass index (BMI) measured in the OSAS group was 30.39 kg/m² and AHI was 18.95 and these values were 25.40 kg/m² and 1.55 in the control group. There was a higher level of visceral adiposity and neuropeptide Y (NPY) in the moderate-to-severe OSAS group compared to the mild OSAS and control groups, and in the mild OSAS group compared to the control group (p = 0.001, p < 0.001). A positive correlation between the level of NPY and AHI and BMI (p < 0.001, p = 0.011), and a negative correlation between NPY levels and oxygen saturation (p = 0.001) was found. Oxygen saturation and desaturation rates were correlated with body fat percentage, body fat mass, abdominal adiposity, visceral adiposity, resting metabolic rate, and NPY levels.

CONCLUSIONS

The visceral adiposity ratio and increase in NPY levels are important parameters that increase the severity of OSAS. Considering the negative effects of NPY on vascular endothelium, measurement of basal NPY level before PSG in patients with OSAS is considered a parameter related to disease severity.

Adaptation of Respiratory-Related Brain Regions to Long-Term Hypercapnia: Focus on Neuropeptides in the RTN

Long-term hypercapnia is associated with respiratory conditions including obstructive sleep apnea, chronic obstructive pulmonary disease and obesity hypoventilation syndrome. Animal studies have demonstrated an initial (within hours) increase in ventilatory drive followed by a decrease in this response over the long-term (days–weeks) in response hypercapnia. Little is known about whether changes in the central respiratory chemoreflex are involved. Here we investigated whether central respiratory chemoreceptor neurons of the retrotrapezoid nucleus (RTN), which project to the respiratory pattern generator within the ventral respiratory column (VRC) have a role in the mechanism of neuroplasticity associated with long-term hypercapnia. Adult male C57BL/6 mice (n = 5/group) were used. Our aims were (1) to determine if galanin, neuromedin B and gastrin-releasing peptide gene expression is altered in the RTN after long-term hypercapnia. This was achieved using qPCR to measure mRNA expression changes of neuropeptides in the RTN after short-term hypercapnia (6 or 8 h, 5 or 8% CO₂) or long-term hypercapnia exposure (10 day, 5 or 8% CO₂), (2) in the mouse brainstem, to determine the distribution of preprogalanin in chemoreceptors, and the co-occurrence of the galanin receptor 1 (GalR1:Gi-coupled receptor) with inhibitory GlyT2 ventral respiratory column neurons using in situ hybridization (ISH) to better characterize galaninergic RTN-VRC circuitry, (3) to investigate whether long-term hypercapnia causes changes to recruitment (detected by cFos immunohistochemistry) of respiratory related neural populations including the RTN neurons and their galaninergic subset, in vivo. Collectively, we found that hypercapnia decreases neuropeptide expression in the RTN in the short-term and has the opposite effect over the long-term. Following long term hypercapnia, the number of RTN galanin neurons remains unchanged, and their responsiveness to acute chemoreflex is sustained; in contrast, we identified multiple respiratory related sites that exhibit blunted chemoreflex activation. GalR1 was distributed in 11% of preBötC and 30% of BötC glycinergic neurons. Our working hypothesis is that during long-term hypercapnia, galanin co-release from RTN neurons may counterbalance glutamatergic inputs to respiratory centers to downscale energetically wasteful hyperventilation, thereby having a role in neuroplasticity by contributing to a decrease in ventilation, through the inhibitory effects of galanin.

Combined hypoxia and hypercapnia, but not hypoxia alone, suppresses neurotransmission from orexin to hypothalamic paraventricular spinally-projecting neurons in weanling rats

Both orexin neurons in the lateral hypothalamus and spinally-projecting pre-sympathetic neurons (PSNs) in the paraventricular nucleus of the hypothalamus (PVN) play an important role in the regulation of cardiovascular function under normal conditions and during cardiovascular challenges such as hypoxia and/or hypercapnia. We have previously established, using selective optogenetic excitation of orexin neurons and pathways, there is a heterogeneous neurotransmission from orexin neurons to PSNs in the PVN. This study was undertaken to test whether this pathway is altered by acute exposure to hypoxia alone and/or combined hypoxia and hypercapnia (H/H). To test this hypothesis, we selectively expressed channelrhodopsin-2 (ChR2) in orexin neurons in the lateral hypothalamus and photoactivated ChR2-expressing fibers to evoke postsynaptic currents in spinally-projecting PSNs in an in vitro slice preparation in rats. In accordance with previously published data, two subpopulations of spinally-projecting PSNs were established, including those with glutamatergic or GABAergic inputs from orexin neurons. Hypoxia alone did not alter the peak amplitude of either glutamatergic or GABAergic neurotransmission, however, H/H significantly inhibited both glutamatergic and GABAergic neurotransmission from orexin neurons to SPNs. In conclusion, H/H may modulate cardiovascular function by affecting heterogeneous pathways from orexin neurons to spinally-projecting PSNs in the PVN.

Hypoxia and hypercapnia inhibit hypothalamic orexin neurons in rats

Evidence of impaired function of orexin neurons has been found in individuals with cardiorespiratory disorders, such as obstructive sleep apnea (OSA) and sudden infant death syndrome (SIDS), but the mechanisms responsible are unknown. Individuals with OSA and SIDS experience repetitive breathing cessations and/or rebreathing of expired air, resulting in hypoxia/hypercapnia (H/H). In this study, we examined the responses of fluorescently identified rat orexin neurons in the lateral hypothalamus to acute H/H to test if and how these neurons alter their activity and function during this challenge. Experiments were conducted in an in vitro slice preparation using voltage-clamp and current-clamp configurations. H/H (10 min) induced hyperpolarization, accompanied by rapid depression, and finally, cessation of firing activity in orexin neurons. Hypoxia alone had similar but less potent effects. H/H did not alter the frequency of inhibitory glycinergic postsynaptic currents. The frequency of GABAergic currents was diminished but only at 8-10 min of H/H. In contrast, the frequency of excitatory glutamatergic postsynaptic events was diminished as early as 2-4 min of H/H. In the presence of glutamatergic receptor blockers, the inhibitory effects of H/H on the firing activity and membrane potential of orexin neurons persisted but to a lesser extent. In conclusion, both direct alteration of postsynaptic membrane properties and diminished glutamatergic neurotransmission likely contribute to the inhibition of orexin neurons by H/H. These mechanisms could be responsible for the decreased function of orexin in individuals at risk for OSA and SIDS.

Sleep-Disordered Breathing in 2 Pediatric Patients on Peritoneal Dialysis

Sleep-disordered breathing (SDB) is prevalent in children with chronic kidney disease (CKD), and has the potential to worsen vascular and neuro-cognitive health and quality of life. We present 2 children with CKD who experience central sleep apnea and nocturnal hypoventilation and discuss the possible underlying mechanisms in relation to CKD and dialysis.

Sleep-disordered breathing in children with chronic kidney disease

BACKGROUND

The aim of our study was to ascertain the prevalence and type of sleep-disordered breathing (SDB) in paediatric patients with severe chronic kidney disease (CKD) based on the results of polysomnograms (PSGs).

METHODS

Overnight PSGs were conducted on children with CKD stages 3-5 (dialysis dependent). Data were collected on patient demographics from the medical records. Study participants and/or their caregivers completed the paediatric modification of the Epworth Sleepiness Scale Score, the Pediatric Sleep Questionnaire (PSQ) and the Pediatric Quality of Life Inventory at the time of the PSG.

RESULTS

Nineteen children were included in the study, of whom seven were on dialysis. The median (interquartile range) age at the time of the PSG was 13.5 (5.4-16.5) years, and eight (42%) of the children were male. There was a 37% (n = 7) prevalence of SDB in this cohort based on the PSG results. Central sleep apnea and obstructive sleep apnea were found in three children each. The PSQ scores did not correlate with the obstructive apnea-hypopnea index.

CONCLUSIONS

There was a high prevalence of SDB in this cohort of children with CKD. The PSG and validated sleep questionnaires yielded discordant results, reinforcing the limitations of diagnosing SDB in the CKD population based solely on sleep questionnaires.

Cumulative effects of repetitive intermittent hypercapnic hypoxia on orexin in the developing piglet hypothalamus

Orexin neuropeptides (OxA and OxB) and their receptors (OX1R and OX2R) are involved in maintenance of sleep and wakefulness, and are regulated by various environmental stimuli. We studied piglets, in the early neonatal period, exposed to 48-min of intermittent hypercapnic hypoxia (IHH; 7% O2/8% CO2) alternating with air. Three groups of 13-14 day-old piglets with IHH exposure of 1-day (1D-IHH) (n=7), 2-days (2D-IHH) (n=7) and 4-days (4D-IHH) (n=8) were compared to controls (exposed only to air, n=8). Immunoreactivity of OxA and OxB was studied in the piglet hypothalamic regions of the dorsomedial hypothalamus (DMH), perifornical area (PeF) and lateral hypothalamic area (LH). Results showed that after 1D- and 2D-IHH, total OxA and OxB expression decreased by 20% (p ≤ 0.005) and 40% (p<0.001), respectively. After 4D-IHH, the decrease in OxA and OxB was 50% (p<0.001). These findings indicate that a chronic IHH exposure induces greater changes in orexin neuropeptide expression than an acute 1-day exposure in the hypothalamus. This may be causally related to the dysregulation of sleep.

Decreased orexin (hypocretin) immunoreactivity in the hypothalamus and pontine nuclei in sudden infant death syndrome

Infants at risk of sudden infant death syndrome (SIDS) have been shown to have dysfunctional sleep and poor arousal thresholds. In animal studies, both these attributes have been linked to impaired signalling of the neuropeptide orexin. This study examined the immunoreactivity of orexin (OxA and OxB) in the tuberal hypothalamus (n = 27) and the pons (n = 15) of infants (1-10 months) who died from SIDS compared to age-matched non-SIDS infants. The percentage of orexin immunoreactive neurons and the total number of neurons were quantified in the dorsomedial, perifornical and lateral hypothalamus at three levels of the tuberal hypothalamus. In the pons, the area of orexin immunoreactive fibres were quantified in the locus coeruleus (LC), dorsal raphe (DR), laterodorsal tegmental (LDT), medial parabrachial, dorsal tegmental (DTg) and pontine nuclei (Pn) using automated methods. OxA and OxB were co-expressed in all hypothalamic and pontine nuclei examined. In SIDS infants, orexin immunoreactivity was decreased by up to 21 % within each of the three levels of the hypothalamus compared to non-SIDS (p ≤ 0.050). In the pons, a 40-50 % decrease in OxA occurred in the all pontine nuclei, while a similar decrease in OxB immunoreactivity was observed in the LC, LDT, DTg and Pn (p ≤ 0.025). No correlations were found between the decreased orexin immunoreactivity and previously identified risk factors for SIDS, including prone sleeping position and cigarette smoke exposure. This finding of reduced orexin immunoreactivity in SIDS infants may be associated with sleep dysfunction and impaired arousal.

Expression of mRNA in the frontal cortex and hypothalamus in a rat model of acute carbon dioxide poisoning

Acute carbon dioxide (CO2) poisoning causes no specific features that are revealed upon autopsy, and the pathophysiological mechanism of this syndrome is unclear. To address this issue, in the present study, we exposed rats to CO2 concentrations ranging from 10% to 60% and determined the effects on mRNA expression. According to the results of Gene Ontology (GO) and cluster analyses of microarrays data, we selected the following genes for further analysis: alkylglycerone phosphate synthase (Agps), hypocretin (Hcrt), tyrosine hydroxylase (Th), heat shock protein beta 2 (Hspb2), and opioid receptor delta 1 (Oprd1) expressed in the frontal cortex and renin (Ren), pancreatic polypeptide (Ppy), corticotropin releasing hormone receptor 2 (Crhr2), carbonic anhydrase 1 (Car1), and hypocretin receptor 1 (Hcrtr1) expressed in the hypothalamus. We found significant differences between the expression levels of Agps and Hspb2 mRNAs in the frontal cortex and that of Ppy, Crhr2 mRNAs in the hypothalamus in the presence of high concentrations of CO2. Further investigation of these genes may clarify the pathophysiology of acute CO2 poisoning and facilitate the development of novel forensic tests that can diagnose the cause of death.

Chronic intermittent hypoxia alters neurotransmission from lateral paragigantocellular nucleus to parasympathetic cardiac neurons in the brain stem

Patients with sleep-related disorders, including obstructive sleep apnea (OSA), have an increased risk of cardiovascular diseases. OSA events are more severe in rapid eye movement (REM) sleep. REM sleep further increases the risk of adverse cardiovascular events by diminishing cardioprotective parasympathetic activity. The mechanisms underlying REM sleep-related reduction in parasympathetic activity likely include activation of inhibitory input to cardiac vagal neurons (CVNs) in the brain stem originating from the lateral paragigantocellular nucleus (LPGi), a nucleus that plays a role in REM sleep control. This study tests the hypothesis that chronic intermittent hypoxia and hypercapnia (CIHH), an animal model of OSA, inhibits CVNs because of exaggeration of the GABAergic pathway from the LPGi to CVNs. GABAergic neurotransmission to CVNs evoked by electrical stimulation of the LPGi was examined with whole cell patch-clamp recordings in an in vitro brain slice preparation in rats exposed to CIHH and control rats. GABAergic synaptic events were enhanced after 4-wk CIHH in both male and female rats, to a greater degree in males. Acute hypoxia and hypercapnia (H/H) reversibly diminished the LPGi-evoked GABAergic neurotransmission to CVNs. However, GABAergic synaptic events were enhanced after acute H/H in CIHH male animals. Orexin-A elicited a reversible inhibition of LPGi-evoked GABAergic currents in control animals but evoked no significant changes in CIHH male rats. In conclusion, exaggerated inhibitory neurotransmission from the LPGi to CVNs in CIHH animals would reduce cardioprotective parasympathetic activity and enhance the risk of adverse cardiovascular events.

Effects of changes in energy homeostasis and exposure of noxious insults on the expression of orexin (hypocretin) and its receptors in the brain

This review summarizes data regarding the brain expression of the orexin (hypocretin) system including: prepro-orexin (PPO), orexin A (OxA), orexin B (OxB) and the two orexin receptors 1 and 2 (OxR1, OxR2). Clinical data is limited to OxA and OxB in cerebral spinal fluid and serum/plasma, thus necessitating the development of animal models to undertake mechanistic studies. We focus on changes in animal models that were either exposed to a regime of altered sleep, metabolic energy homeostasis, exposed to drugs and noxious insults. Many more expressional studies are available for PPO, OxA and OxB levels, compared to studies of the receptors. Interestingly, the direction and pattern of change for PPO, OxA and OxB is inconsistent amongst studies, whereas for the receptors, there tends to be increased expression for both OxR1 and OxR2 after alterations in energy homeostasis, and an increased expression after noxious insults or exposure to some drugs. The clinical implications of these results from animal models are discussed in light of the findings from human studies, and future research directions are suggested to fill knowledge gaps with regard to the orexin system, particularly during early brain development.

Orexin receptors in the developing piglet hypothalamus, and effects of nicotine and intermittent hypercapnic hypoxia exposures

Orexin and its receptors (OxR1 and OxR2) play a significant role in arousal and sleep regulation. Using developing piglets, we aimed to determine the effects of nicotine and Intermittent Hypercapnic Hypoxia (IHH), alone or in combination, on orexin receptor expression in the hypothalamus. Four piglet groups were studied: control (n=14), nicotine (n=14), IHH (n=10) and nic+IHH (n=14). Applying immunohistochemistry for OxR1 and OxR2 expression, eight nuclei/areas of the hypothalamus: dorsal medial nucleus (DMN), arcuate nucleus (ARC), perifornical area (PFA), paraventricular nucleus (PVN), lateral hypothalamic area (LHA), ventral medial nucleus (VMN), supraoptic nucleus, retrochiasmatic part (SONr) and tuberal mammillary nucleus (TMN), were studied. Compared to controls, OxR1 and OxR2 were increased due to exposures, however this was region dependent. Nicotine increased OxR1 in the DMN (P<0.001) and SONr (P=0.036), and OxR2 in the DMN (P<0.001), VMN (P=0.014) and the TMN (P=0.026). IHH increased OxR1 in the DMN, PVN, VMN and SONr (P<0.01 for all), and OxR2 in DMN (P<0.001), PFA (P=0.001), PVN (P=0.004), VMN (P=0.041) and the TMN (P<0.001). The nic+IHH exposure increased OxR1 expression in all nuclei (TMN excluded) however, the changes were not significantly different from IHH alone. For OxR2, the increased expression after nic+IHH was significant compared to IHH in the DMN, ARC and SONr. These results show that nicotine increases orexin receptor expression in a region dependent manner. IHH induced increases were specific to arousal and stress related regions and nic+IHH results suggest that for OxR1, nicotine has no additive effect whereas for OxR2 it does, and is region dependent.