The latency to awake from induced-obstructive sleep apnea is reduced in rats with chronic epilepsy.

OSA is known to increase the risk for SUDEP in persons with epilepsy, but the relationship between these two factors is not clear. Also, there is no study showing the acute responses to obstructive apnea in a chronic epilepsy model. Therefore, this study aimed to characterize cardiorespiratory responses to obstructive apnea and chemoreceptor stimulation in rats. In addition, we analyzed respiratory centers in the brain stem by immunohistochemistry. Epilepsy was induced with pilocarpine. About 30-60 days after the first spontaneous seizure, tracheal and thoracic balloons, and electrodes for recording the electroencephalogram, electromyogram, and electrocardiogram were implanted. Intermittent apneas were made by inflation of the tracheal balloon during wakefulness, NREM sleep, and REM sleep. During apnea, respiratory effort increased, and heart rate fell, especially with apneas made during wakefulness, both in control rats and rats with epilepsy. Latency to awake from apnea was longer with apneas made during REM than NREM, but rats with epilepsy awoke more rapidly than controls with apneas made during REM sleep. Rats with epilepsy also had less REM sleep. Cardiorespiratory responses to stimulation of carotid chemoreceptors with cyanide were similar in rats with epilepsy and controls. Immunohistochemical analysis of Phox2b, tryptophan hydroxylase, and NK1 in brain stem nuclei involved in breathing and sleep (retrotrapezoid nucleus, pre-Bötzinger complex, Bötzinger complex, and caudal raphe nuclei) revealed no differences between control rats and rats with epilepsy. In conclusion, our study showed that rats with epilepsy had a decrease in the latency to awaken from apneas during REM sleep, which may be related to neuroplasticity in some other brain regions related to respiratory control, awakening mechanisms, and autonomic modulation.

Role of Peripheral Chemoreceptors on Enhanced Central Chemoreflex Drive in Nonischemic Heart Failure.

Heart failure (HF) is a prevalent disease in elderly population. Potentiation of the ventilatory chemoreflex drive plays a pivotal role in disease progression, at least in part, through their contribution to the generation/maintenance of breathing disorders. Peripheral and central chemoreflexes are mainly regulated by carotid body (CB) and the retrotrapezoid nuclei (RTN), respectively. Recent evidence showed an enhanced central chemoreflex drive in rats with nonischemic HF along with breathing disorders. Importantly, increase activity from RTN chemoreceptors contribute to the potentiation of central chemoreflex response to hypercapnia. The precise mechanism driving RTN potentiation in HF is still elusive. Since interdependency of RTN and CB chemoreceptors has been described, we hypothesized that CB afferent activity is required to increase RTN chemosensitivity in the setting of HF. Accordingly, we studied central/peripheral chemoreflex drive and breathing disorders in HF rats with and without functional CBs (CB denervation). We found that CB afferent activity was required to increase central chemoreflex drive in HF. Indeed, CB denervation restored normal central chemoreflex drive and reduced the incidence of apneas by twofold. Our results support the notion that CB afferent activity plays an important role in central chemoreflex potentiation in rats with HF.

Cholinergic neurons in the pedunculopontine tegmental nucleus modulate breathing in rats by direct projections to the retrotrapezoid nucleus.

KEY POINTS: Cholinergic projections from the pedunculopontine tegmental nucleus (PPTg) to the retrotrapezoid nucleus (RTN) are considered to be important for sleep-wake state-dependent control of breathing. The RTN also receives cholinergic input from the postinspiratory complex. Stimulation of the PPTg increases respiratory output under control conditions but not when muscarinic receptors in the RTN are blocked. The data obtained in the present study support the possibility that arousal-dependent modulation of breathing involves recruitment of cholinergic projections from the PPTg to the RTN. ABSTRACT: The pedunculopontine tegmental nucleus (PPTg) in the mesopontine region has important physiological functions, including breathing control. The PPTg contains a variety of cell types, including cholinergic neurons that project to the rostral aspect of the ventrolateral medulla. In addition, cholinergic signalling in the retrotrapezoid nucleus (RTN), a region that contains neurons that regulate breathing in response to changes in CO2 /H+ , has been shown to activate chemosensitive neurons and increase inspiratory activity. The present study aimed to identify the source of cholinergic input to the RTN and determine whether cholinergic signalling in this region influences baseline breathing or the ventilatory response to CO2 in conscious male Wistar rats. Retrograde tracer Fluoro-Gold injected into the RTN labelled a subset of cholinergic PPTg neurons that presumably project directly to the chemosensitive region of the RTN. In unrestrained awake rats, unilateral injection of the glutamate (10 mm/100 nL) in the PPTg decreased tidal volume (VT ) but otherwise increased respiratory rate (fR ) and net respiratory output as indicated by an increase in ventilation (VE ). All respiratory responses elicited by PPTg stimulation were blunted by prior injection of methyl-atropine (5 mm/50-75 nL) into the RTN. These results show that stimulation of the PPTg can increase respiratory activity in part by cholinergic activation of chemosensitive elements of the RTN. Based on previous evidence that cholinergic PPTg projections may simultaneously activate expiratory output from the pFRG, we speculate that cholinergic signalling at the level of RTN region could also be involved in breathing regulation.

Inhibition of the hypercapnic ventilatory response by adenosine in the retrotrapezoid nucleus in awake rats.

The brain regulates breathing in response to changes in tissue CO2/H+ via a process called central chemoreception. Neurons and astrocytes in the retrotrapezoid nucleus (RTN) function as respiratory chemoreceptors. The role of astrocytes in this process appears to involve CO2/H+-dependent release of ATP to enhance activity of chemosensitive RTN neurons. Considering that in most brain regions extracellular ATP is rapidly broken down to adenosine by ectonucleotidase activity and since adenosine is a potent neuromodulator, we wondered whether adenosine signaling contributes to RTN chemoreceptor function. To explore this possibility, we pharmacologically manipulated activity of adenosine receptors in the RTN under control conditions and during inhalation of 7-10% CO2 (hypercapnia). In urethane-anesthetized or unrestrained conscious rats, bilateral injections of adenosine into the RTN blunted the hypercapnia ventilatory response. The inhibitory effect of adenosine on breathing was blunted by prior RTN injection of a broad spectrum adenosine receptor blocker (8-PT) or a selective A1-receptor blocker (DPCPX). Although RTN injections of 8PT, DPCPX or the ectonucleotidase inhibitor ARL67156 did not affected baseline breathing in either anesthetized or awake rats. We did find that RTN application of DPCPX or ARL67156 potentiated the respiratory frequency response to CO2, suggesting a portion of ATP released in the RTN during high CO2/H+ is converted to adenosine and serves to limit chemoreceptor function. These results identify adenosine as a novel purinergic regulator of RTN chemoreceptor function during hypercapnia.

Acute exercise-induced activation of Phox2b-expressing neurons of the retrotrapezoid nucleus in rats may involve the hypothalamus.

The rat retrotrapezoid nucleus (RTN) contains neurons that have a well-defined phenotype characterized by the presence of vesicular glutamate transporter 2 (VGLUT2) mRNA and a paired-like homeobox 2b (Phox2b)-immunoreactive (ir) nucleus and the absence of tyrosine hydroxylase (TH). These neurons are important to chemoreception. In the present study, we tested the hypothesis that the chemically-coded RTN neurons (ccRTN) (Phox2b(+)/TH(-)) are activated during an acute episode of running exercise. Since most RTN neurons are excited by the activation of perifornical and lateral hypothalamus (PeF/LH), a region that regulates breathing during exercise, we also tested the hypothesis that PeF/LH projections to RTN neurons contribute to their activation during acute exercise. In adult male Wistar rats that underwent an acute episode of treadmill exercise, there was a significant increase in c-Fos immunoreactive (c-Fos-ir) in PeF/LH neurons and RTN neurons that were Phox2b(+)TH(-) (p<0.05) compared to rats that did not exercise. Also the retrograde tracer Fluoro-Gold that was injected into RTN was detected in c-Fos-ir PeF/LH (p<0.05). In summary, the ccRTN neurons (Phox2b(+)TH(-)) are excited by running exercise. Thus, ccRTN neurons may contribute to both the chemical drive to breath and the feed-forward control of breathing associated with exercise.