Monoaminergic system lesions increase post-sigh respiratory pattern disturbance during sleep in rats

Monoamines are important regulators of behavioral state and respiratory pattern, and the impact of monoaminergic control during sleep is of particular interest for the stability of breathing regulation. The aim of this study was to test the effects of systemically induced chemical lesions to noradrenergic and serotonergic efferent systems, on the expression of sleep-wake states, pontine wave activity, and sleep-related respiratory pattern and its variability. In chronically instrumented male adult Sprague-Dawley rats we lesioned noradrenergic terminal axonal branches by a single intraperitoneal dose of DSP-4 (N-(2-chloroethyl)-N-ethyl-2-brombenzilamine; 50 mg/kg, i.p.), and serotonergic axonal terminals by two intraperitoneal doses, 24 h apart, of PCA (p-chloroamphetamine; 6 mg/kg, i.p.). In each animal, we recorded sleep, pontine waves (P-waves) and breathing at baseline, following sham injection, and every week for 5 weeks following injection of either systemic neurotoxin. Distinct responses were observed to the two lesions. DSP-4 lesions were associated with a trend toward increased NREM sleep (p < 0.06), decreased wakefulness (p < 0.05) and increased respiratory tidal volume during NREM (p = 0.0002) and REM (p = 0.0001) sleep with respect to baseline. None of these effects, however, were observed during the first 14 days after injection. No significant changes were observed in the frequency of apneas or sighs, nor in the coupling between these two, at any time after DSP-4 injection. Conversely, selective serotonergic lesion by PCA produced no change in the baseline respiratory frequency or tidal volume during sleep or wakefulness, nor was the expression of Wake, NREM or REM sleep affected. Instead, PCA injection resulted in a sustained increase in the frequency and duration of post-sigh apneas (PS) during NREM sleep (p = 0.002). This reflected increased coupling between sighs and apneas, because neither the frequency nor the amplitude of spontaneous sighs was altered by PCA.

C-fiber activation exacerbates sleep-disordered breathing in rats

We reported previously that activation of vagal feedback by protoveratrines or serotonin exacerbates sleep apnea in rats, but each of these agents activates multiple afferent fiber types. To elucidate the specific impact of C-fiber activity on sleep apnea, the present study utilized capsaicin (CAP), which stimulates C-fibers via the CAP receptor. Nine adult Sprague-Dawley rats were instrumented for chronic polysomnography and recorded for 6 hours on four occasions. Prior to each recording, the animals received an intraperitoneal injection of either saline (control), or CAP 0.1, 1.0, or 10.0 mg/kg. Respiration was monitored by single-chamber plethysmography and apneas were scored as breaths longer than 2.5 seconds not preceded by a sigh. CAP increased apneas during non-rapid eye movement (NREM) sleep (p < 0.05 vs control) and reduced respiratory minute ventilation by about 15% in all behavioral states (waking, NREM, and REM sleep). We conclude that selective pharmacological activation of C-fibers produces a diathesis of sleep-disordered breathing specific to NREM sleep in rats.

Riluzole suppresses post-sigh, but not spontaneous apnoeas during sleep in rats

We conducted this experiment to determine the role of glutamate in the mechanism of sleep apnoeas by administering riluzole, a glutamate release inhibitor, to freely moving rats in which sleep-related apnoeas are physiological phenomena. Adult Sprague-Dawley rats were implanted with electrodes for electroencephalogram (EEG) and electromyogram (EMG) recording to monitor sleep and were placed inside a single-chamber plethysmograph to monitor respiration. Sleep and respiration were recorded for 6 h following intraperitoneal administration of 0.5, 5.0 and 10.0 mg kg(-1) riluzole. Riluzole dose-dependently suppressed post-sigh apnoeas during rapid eye movement (REM) sleep but had no effect on sleep-related spontaneous apnoeas. The drug (5.0 and 10.0 mg kg(-1)) also dose-dependently reduced wakefulness and increased sleep. It appears that glutamate, an excitatory neurotransmitter, plays a role in the genesis of the post-sigh apnoeas during REM sleep.

R-zacopride, a 5-HT3 antagonist/5-HT4 agonist, reduces sleep apneas in rats

The effects of R-zacopride, a benzamide with potent 5-HT3 receptor antagonist and 5-HT4 receptor agonist properties, on spontaneous apneas were studied in 10 Sprague-Dawley rats by monitoring respiration and sleep for 6 h. R-zacopride (0.5, 1.0 and 10.0 mg/kg) suppressed spontaneous central apneas during non-rapid-eye-movement (NREM) sleep by 50% (P=.05 for 0.5 mg/kg, P=.02 for 1.0 mg/kg and P=.001 for 10.0 mg/kg dose vs. control), and during rapid-eye-movement (REM) sleep by 80% by all doses tested (P<.0007) for at least 2 h after intraperitoneal injection. We conclude that R-zacopride, over a 20-fold dose range, significantly reduces central apnea expression during NREM and REM sleep in the rat. The efficacy of this compound to suppress central apneas most probably arises from its antagonist actions at 5-HT3 receptors or from its mixed agonist/antagonist profile at 5-HT4/5-HT3 receptors.

Sleep-disordered respiration in phenotypically normotensive, genetically hypertensive rats

Increased prevalence of sleep-related breathing disorders has been reported in patients with essential hypertension and we have described disordered breathing in spontaneously hypertensive rats, an animal model of genetic hypertension. The mechanisms coupling hypertension to respiratory dysfunction during sleep remain, however, largely unknown. To determine if sleep-related respiratory disorder reflects cardiovascular derangement or, alternatively, represents an independent phenotype in hypertensive rats, we polygraphically recorded groups (n = 10) of genetically hypertensive, genetically normotensive, and phenotypically normotensive rats carrying a genetic background for hypertension. Apnea index was elevated more than 15-fold during NREM sleep in both animal groups carrying hypertension-related genes (p < 0.0001 for each) versus normotensive Wistar Kyoto rats. During REM sleep, a genetic background for hypertension was associated with an increased apnea index of at least 500% versus normotensive Wistar Kyoto rats (p < 0.0001 for each comparison). Still, overall mean respiratory rate, minute ventilation, and sleep architecture were equivalent among all animal groups. As expected, blood pressure and heart period were similar in both normotensive groups but elevated in the hypertensive animals. Persistent sleep-related breathing disorder despite effective cardiovascular normalization in the phenotypically normotensive but genetically hypertensive rats suggests that disordered breathing represents a genetically determined phenotype in these animals that is not secondary to the cardiovascular derangements. The model system described here may provide a powerful tool for investigation of the determinants of sleep-related breathing disorder.

Mirtazapine, a mixed-profile serotonin agonist/antagonist, suppresses sleep apnea in the rat

Serotonin enhancing drugs, including L-tryptophan and, more recently, fluoxetine and paroxetine, have been tested as pharmacologic treatments for sleep apnea syndrome. Although some patients have demonstrated reduced apnea expression after treatment with these compounds, this improvement has been restricted to nonrapid eye movement (NREM) sleep, with some patients showing no improvement. This study reports the effects of mirtazapine, an antidepressant with 5-HT(1) agonist as well as 5-HT(2) and 5-HT(3) antagonist effects, on sleep and respiration in an established animal model of central apnea. We studied nine adult male Sprague-Dawley rats chronically instrumented for sleep staging. In random order on separate days, rats were recorded after intraperitoneal injection of: (1) saline, (2) 0.1 mg/kg +/- mirtazapine (labeled as Remeron), (3) 1 mg/kg mirtazapine, or (4) 5 mg/ kg mirtazapine. With respect to saline injections, mirtazapine at all three doses reduced apnea index during NREM sleep by more than 50% (p < 0.0001) and during REM sleep by 60% (p < 0.0001) for at least 6 h. In association with this apnea suppression normalized inspiratory minute ventilation increased during all wake/sleep states (p < 0.001 for each state). The duration of NREM sleep was unaffected by any dose of mirtazapine (p = 0.42), but NREM EEG delta power was increased by more than 30% at all doses (p = 0.04), indicating improved NREM sleep consolidation after mirtazapine injection. We conclude that mirtazapine, over a 50-fold dose range, significantly reduces central apnea expression during NREM and REM sleep in the rat. The efficacy of this compound to suppress apnea in all sleep stages most probably arises from its mixed agonist/antagonist profile at serotonin receptors. The implications of these findings for the management of sleep apnea syndrome must be verified by appropriate clinical trials.

Role of peripheral adenosine A(1) receptors in the regulation of sleep apneas in rats

The effects of administration of N(6)-p-sulfophenyladenosine (p-SPA), a peripheral adenosine A(1) receptor agonist, and 8-(p-sulfophenyl)theophylline (p-SPT), a peripheral adenosine A(1) receptor blocker, on spontaneous apneas were studied in 10 adult Sprague-Dawley rats by monitoring respiration, sleep, and blood pressure for 6 h. Intraperitoneal injection of p-SPA (1 mg/kg) to rats suppressed spontaneous central apneas during non-rapid eye movement sleep by 50% in comparison to control recordings (p = 0.03). This effect was blocked by pretreatment with an equimolar dose of p-SPT (0.67 mg/kg) indicating that p-SPA suppression of apneas was receptor mediated in the peripheral nervous system. Administration of p-SPA did not affect apnea expression in rapid eye movement sleep and had no effect on sleep or blood pressure at the dose tested. Administration of p-SPT (0.67, 6.7, and 30 mg/kg) to rats had no effect on apneas, sleep, or blood pressure. The lack of p-SPT effect on sleep apneas argues against a physiologic role for endogenous adenosine in the peripheral nervous system as a modulator of sleep apnea expression under baseline conditions.

Role of peripheral serotonin in the regulation of central sleep apneas in rats

STUDY OBJECTIVES

The aim of our study was to determine the effects of serotonin (5-HT), which does not penetrate the blood-brain barrier (BBB), and GR38032F, a 5-HT3 receptor antagonist that may cross the BBB, on spontaneous apneas in adult Sprague-Dawley rats.

MEASUREMENTS AND RESULTS

Rats were implanted with electrodes for EEG and electromyographic recording to monitor sleep, with a radiotelemetry transmitter for monitoring aortic BP and heart period (HP) and were placed inside a single chamber plethysmograph for monitoring respiration. Sleep, BP, HP, and respiration were monitored for 6 h following administration of drugs. Intraperitoneal injection of 5-HT (0.79 mg/kg) to rats increased spontaneous central apneas during rapid eye movement (REM) sleep by > 250% in comparison to control recording (p = 0.01). GR38032F (0.1 mg/kg), which produced no effect on apnea expression, completely blocked the 5-HT-induced increase in REM apneas. Administration of 5-HT did not affect apnea expression in non-REM sleep and had no effect on sleep or BP.

CONCLUSIONS

From these observations, we conclude that binding at 5-HT3 receptors in the peripheral nervous system promotes REM-related apnea genesis in rats. These findings further suggest that endogenous 5-HT, acting at least at peripheral 5-HT3 receptors, may play a baseline physiologic role in the expression of spontaneous central apneas in rats.

Chemo- and baroresponses differ in African-Americans and Caucasians in sleep

To determine sleep effects on baro- and ventilatory responses to transient chemo- and barostimulation in African-Americans and Caucasians, 26 nonobese normotensive young subjects (13 African-Americans and 13 Caucasians) were studied awake and in non-rapid-eye movement (NREM) and rapid-eye-movement sleep during induced transient hypoxemia (N2), hypertension (phenylephrine, PE), and concomitant hypoxemia and hypertension (N2 + PE). Arterial blood pressure was recorded by plethysmographic volume clamp, minute ventilation by pneumotachograph, and arterial O2 saturation by pulse oximeter. For all subjects, chronotropic baroresponse (Deltapulse interval/Deltasystolic blood pressure, where Delta is change) increased with NREM sleep (P = 0.007). Baroresponse slope was greater in Caucasians than in African-Americans (ANOVA, P = 0.02). Hypoxemic ventilatory response (Deltaminute ventilation/Deltaarterial O2 saturation) was greater in African-Americans than in Caucasians in NREM sleep (P = 0.01), as was hypoxemic attenuation of baroresponse (N2 + PE, P = 0.03). These data suggest sleep-related differences in arterial chemo- and baroreceptor responses in normal young African-Americans and Caucasians, which may have implications concerning development of systemic hypertension.

Serotonin 5-HT3-receptor antagonist GR 38032F suppresses sleep apneas in rats

The effects of administration of GR38032F, a 5-HT3 receptor antagonist, on spontaneous sleep apneas were studied in adult Sprague-Dawley rats by monitoring sleep, respiration and blood pressure for 6 hours. Intraperitoneal injection of GR38032F (1 mg/kg) suppressed spontaneous central apneas during non-rapid-eye-movement (NREM) and especially during rapid-eye-movement (REM) sleep. This effect was associated with increased respiratory drive but did not cause cardiovascular changes at the dose tested. The suppressive action of GR38032F on spontaneous sleep apneas is analogous to findings in anesthetized rats in which 5-HT and 2-methyl-5-HT provoked central apneas that were antagonized by GR38032 (Yoshioka et al, JPET 1992; 260:917-924). Our data implicate 5-HT3 receptor systems in determining sleep-related respiratory drive and apnea expression in rats, effects which are most probably mediated by vagal afferents.

Diazepam suppresses sleep apneas in rats

To test the respiratory effects of benzodiazepines in an established animal model of central apnea, we administered nonhypnotic and hypnotic doses of diazepam to nine adult male Sprague-Dawley rats chronically instrumented for sleep staging. In random order on separate days, rats were recorded following intraperitoneal injection of: (1) saline; (2) 0.05 mg/kg diazepam; or (3) 5 mg/kg diazepam. Normalized inspiratory minute ventilation increased significantly during wakefulness and non-rapid eye movement (non-REM) sleep following each dose of diazepam (p < 0.003 in each case) and following the highest dose during rapid eye movement (REM) sleep (p = 0.01). In accord with this respiratory stimulation, non-REM-related spontaneous and post-sigh apnea expression decreased following each dose of diazepam (p = 0.006 to 0.04), but REM-related apnea expression was unaffected despite significant respiratory stimulation. The durations of non-REM and REM sleep were unaffected by the low dose, but following 5 mg/kg of diazepam non-REM sleep was increased (p = 0.03) and REM sleep was decreased (p = 0.009). We conclude that both hypnotic and non-hypnotic doses of benzodiazepines may be associated with suppression of sleep-related central apnea. We further conclude that non-REM and REM-related apneas arise from at least partially distinct mechanisms.

Cardiopulmonary control in sleeping Sprague-Dawley rats treated with hydralazine

To test the hypothesis that hydralazine can suppress spontaneous sleep-related central apnea, respiratory pattern, blood pressure, and heart period were monitored in Sprague-Dawley rats. In random order and on separate days, rats were recorded after intraperitoneal injection of 1) saline or 2) 2 mg/kg hydralazine. Normalized minute ventilation (NVI) declined significantly with transitions from wake to non-rapid-eye-movement (NREM) sleep (-5.1%; P = 0.01) and rapid-eye-movement (REM) sleep (-4.2%; P = 0.022). Hydralazine stimulated respiration (NVI increased by 21%; P < 0.03) and eliminated the effect of state on NVI. Blood pressure decreased by 17% after hydralazine, and the correlation between fluctuations in mean blood pressure and NVI changed from strongly positive during control recordings to weakly negative after hydralazine (P < 0.0001 for each). Postsigh and spontaneous apneas were reduced during NREM and REM sleep after hydralazine (P < 0.05 for each). This suppression was strongly correlated with the reduction in blood pressure and with the degree of respiratory stimulation. We conclude that mild hydralazine-induced hypotension leads to respiratory stimulation and apnea suppression.

Respiratory and arousal responses to acoustic stimulation

STUDY OBJECTIVES

Although sleep-related obstructive apnea is most often associated with transient arousal, the impact of this arousal on respiratory control remains unclear. We tested the hypotheses that acoustic arousing stimulation can generate a significant respiratory response during sleep in healthy subjects and that the magnitude or timing of this response is affected by the presence of electrocortical arousal or inhaled carbon dioxide.

DESIGN

We employed binaural tone bursts (0.5-s duration, 4-KHz center frequency, 99-s interstimulus interval) to elicit repetitive transient arousals from sleep during nocturnal polysomnographic recordings beginning at 10 PM and ending at 6 AM.

PARTICIPANTS

Recordings were conducted in five healthy adult volunteers aged 24 to 37 years.

INTERVENTIONS

Inspired gas was alternated between room air and 3% to 7% CO2 (titrated to yield an approximate 50% increase in minute ventilation) at 1-h intervals.

MEASUREMENTS AND RESULTS

Each 30-s epoch was scored for sleep/wake stage according to standard criteria. Only results obtained during nonrapid eye movement sleep are presented herein. Tone-evoked arousals were detected by computer analysis as increased EEG frequency occurring within 3 s of acoustic stimulation. For each tone, respiratory parameters for each of three prestimulus and four poststimulus breaths were normalized to the overall mean of prestimulus breaths measured during room air breathing for each subject. Tone bursts elicited repetitive transient arousals with a mean duration of approximately 10 s from all stages of sleep. With respect to the three prestimulus breaths, acoustic stimulation was associated with increased tidal volume and decreased inspiratory duration for at least four breaths. These respiratory responses to acoustic stimulation were not significantly influenced by either presence of transient arousal from sleep or inspired gas.

CONCLUSIONS

We conclude that transient EEG arousal may be repeatedly evoked from nonrapid eye movement sleep by transient acoustic stimulation in normal sleepers. This sensory stimulation is associated with augmented ventilation, a response that is not significantly affected by inspired hypercapnia or the presence of generalized EEG arousal.

Adenosine A1 receptor agonist GR79236 suppresses apnea during all sleep stages in the rat

We tested the hypothesis that N-[(1S, trans)-2-hydroxycyclopentyl]adenosine (GR79236), a novel adenosine A1 receptor agonist, would suppress sleep-related apnea in the rat at doses not associated with hypotension or hypothermia. Nine adult Sprague-Dawley rats were instrumented for chronic recording of sleep by electroencephalographic and electromyographic monitoring. Respirations were measured by single chamber plethysmograph, and blood pressure and heart period were transduced by a telemetric implant. Each rat was polygraphically recorded for 6 hours on four occasions in random order, with recordings for an individual animal separated by at least 3 days. Fifteen minutes prior to each recording (0945 hours) each animal received a 1 ml/kg intraperitoneal bolus injection of one of four injectates: saline (control) or 0.03 mg/kg, 0.3 mg/kg, or 3 mg/kg of GR79236. The study was a repeated-measures balanced design such that each animal was recorded exactly once for each injectate. The rate of spontaneous apneas (pauses > 2.5 seconds) was significantly reduced during all sleep stages by all doses of GR79236. At the highest dose, apnea index was reduced by over 70% in both non-rapid eye movement (NREM) and rapid eye movement (REM) sleep. In contrast, GR79236 had no effect on sleep stage volumes or blood pressure at any dose tested. Heart rate and core temperature were reduced only at the highest dose (3 mg/kg). We conclude that the adenosine A1 receptor agonist GR79236 significantly suppresses apnea expression in all sleep stages at doses not associated with significant changes in sleep architecture, blood pressure, heart rate, or core temperature.

Protoveratrines A and B increase sleep apnea index in Sprague-Dawley rats

The action of protovertarines A and B, which stimulate carotid sinus baroreceptors and vagal sensory endings in the heart as well as pulmonary bed, were assessed on spontaneous and postsigh central sleep apneas in freely moving Sprague-Dawley rats. During the 6-h recording period, animals were simultaneously monitored for sleep by using electroencephalogram and electromyogram recordings, for respiration by single-chamber plethysmography, and for blood pressure and heart period by using radiotelemetry. After administration of 0.2, 0.5, or 1 mg/kg sc of protoveratrines, cardiopulmonary changes lasting at least 6 h were observed in all three behavioral states [heart period increased up to 23% in wakefulness, 21% in non-rapid-eye-movement (non-REM) sleep, and 20% in REM sleep; P < 0.005 for each]. At the same time, there was a substantial increase in the number of spontaneous (375% increase; P = 0.04) and postsigh (268% increase, P = 0.0002) apneas. Minute ventilation decreased by up to 24% in wakefulness, 25% in non-REM, and 35% in REM sleep (P < 0.05 for each). We conclude that pharmacological stimulation of baroreflexes promotes apnea expression in the sleeping rat.

Cardiopulmonary interactions following REM sleep deprivation in Sprague-Dawley rats

We characterized the effects of 48 h of rapid-eye-movement (REM) sleep deprivation on cardiovascular and respiratory variables and on sleep-related cardiopulmonary interactions in adult male Sprague-Dawley rats. Rats were instrumented for monitoring EEG, EMG, and aortic blood pressure. Respiratory rate and minute ventilation were measured by unrestrained single-chamber plethysmography. By using radiotelemetry to monitor blood pressure we clearly demonstrated progressive decreases in mean blood pressure with transitions from wakefulness to non-rapid-eye-movement and REM sleep which were unaffected by REM sleep deprivation. Mirror-image state-dependent increases in heart period suggest that baroreflexes were augmented during sleep with respect to wakefulness. REM sleep deprivation was also associated with lower blood pressure and longer heart period over all sleep/wake states, although this achieved statistical significance only during REM sleep and only during the first hour of recovery sleep. These cardiovascular changes coupled with the observed decreases in respiratory rate and minute ventilation suggest a further augmentation of baroreflexes following REM sleep deprivation.

Hypotension reduces sleep apneas in Zucker lean and Zucker obese rats

The effects of hypotension and obesity on spontaneous apnea (SA) and post-sigh sleep apnea (PSA) were studied in Zucker rats by monitoring blood pressure, respiration, and sleep state for 6 hours. Hypotension produced by intraperitoneal administration of hydralazine (2 mg/kg) was associated with reduced SA and PSA expression in nonrapid eye movement sleep in both lean and obese rats. In both animal groups, hypotension reduced rapid eye movement (REM) sleep by 50% but exerted no significant effect on REM-related expression of SA. Blood pressure lowering also correlated with increased respiratory rate and inspired minute ventilation during sleep, suggesting that the effects of hypotension on apnea expression may arrive via modulation of respiratory drive. These findings emphasize the interdependence of cardiorespiratory functions and may have implications regarding the mechanisms of central apnea in man.

Respiratory and electrocortical responses to acoustic stimulation

Although sleep-related obstructive apnea is most often associated with transient arousal, the impact of this arousal on respiratory control remains unclear. We employed binaural tone bursts (.5 second duration) to elicit repetitive transient arousals from sleep during polygraphic recordings in 5 adult volunteers. By this method, we elicited repetitive transient arousals with a mean duration of approximately 10 seconds from all stages of sleep. With respect to the 3 pre-stimulus breaths, acoustic stimulation was associated with increased tidal volume and decreased inspiratory duration for at least 4 breaths. These respiratory responses to acoustic stimulation were not significantly influenced by either presence of transient arousal from sleep or the sleep state from which arousal occurred. We conclude that transient electro-cortical state changes may be repeatedly evoked from all sleep stages by transient acoustic stimulation in normal sleepers. This sensory stimulation represents a significant respiratory stimulus even when generalized arousal from sleep does not occur.

Effects of inspired gas on sleep-related apnea in the rat

Central apneas have been reported to occur in the rat during all stages of sleep. Two types of apnea have been described: spontaneous and postsigh, which are immediately preceded by an augmented breath. We studied the effect of inspired gas on the number and type of apneas in nine adult male Sprague-Dawley rats that were surgically prepared with cortical electroencephalogram and nuchal electromyogram electrodes. In addition to the electroencephalogram and electromyogram, we recorded respiration by the barometric method by using a single-chamber plethysmograph. Each rat was recorded from 1000 until 1600 on 4 separate days by using different inspired gases: room air, 100% O2, 15% O2, and 5% CO2. We found that the sleep-related apnea index was significantly higher during 100% O2 compared with room air (P < 0.05) and was significantly lower during 15% O2 and 5% CO2 compared with room air (P < 0.05). Postsigh apneas occurred more frequently than did spontaneous apneas (P < 0.0001). The coupling between sighs and apneas was strengthened by hyperoxia and weakened by hypoxia and hypercapnia (P < 0.05 for each). We conclude that stimulation of chemoreceptors acts to oppose apnea in the rat.

Hydralazine reduces elevated sleep apnea index in spontaneously hypertensive (SHR) rats to equivalence with normotensive Wistar-Kyoto rats

The effects of lowering blood pressure (BP) by hydralazine (HY) (2 mg/kg) on spontaneous (SA) and post-sigh (PSA) sleep apneas have been studied in spontaneously hypertensive (SHR) rats by monitoring their respiration and sleep by the EEG for 6 hours. Normotensive Wistar-Kyoto (WKY) rats, from which the SHR rat strain was derived, were used as an appropriate control. The SHR rats had more SA (p < 0.02) and PSA (p < 0.0001) apneas/hour than WKY rats during nonrapid eye movement sleep and their mean BP was higher by 40 mm Hg (p < 0.0001) than WKY rats. Administration of HY to SHR rats equalized their BP with the arterial pressure of WKY rats and reduced the SA and the PSA apneas/hour to equivalence with WKY normotensive rats. These results demonstrate that even in the context of lifelong hypertension, acute normalization of BP significantly reduces sleep apneas in rats. They further suggest that improved management of BP may be clinical benefit to patients with apnea who have long-standing hypertension.

Sleep apnea in normal and REM sleep-deprived normotensive Wistar-Kyoto and spontaneously hypertensive (SHR) rats

The effects of hypertension and REM sleep deprivation on spontaneous and postsigh apneas have been studied in normotensive Wistar-Kyoto (WKY) and spontaneously hypertensive (SHR) rats by simultaneously monitoring their respiration and sleep by the EEG. The amount of REM and non-REM sleep in SHR rats was identical to WKY rats under control as well as REM sleep-deprived recording conditions. Hypertension was associated with an increase in postsigh apneas, as was evident by the increased postsigh apnea index in non-REM and total sleep in SHR rats when compared to normotensive WKY rats. In contrast, REM sleep deprivation suppressed the postsigh apnea expression both in non-REM and total sleep in SHR rats. The incidence of spontaneous apneas was increased by a combination of hypertension and REM sleep deprivation, as was shown in REM-deprived SHR rats, while each of these conditions alone had no effect on spontaneous apneas. These results suggest a role for hypertension in the postsigh apnea genesis and the existence of partially distinct mechanisms for the two types of apneas.

Effect of REM sleep deprivation on sleep apneas in rats

We studied the effects of 48 h of REM sleep deprivation on spontaneous and post-sigh central apneas in Sprague-Dawley rates by simultaneously monitoring sleep by the EEG and respiration for 6 h. During the recovery sleep following REM deprivation a decrease in post-sigh apneas occurred in total sleep. There was no change in spontaneous apneas. The results suggest the existence of partially distinct mechanisms for the two types of apneas.

p-SPA, a peripheral adenosine A1 analogue, reduces sleep apneas in rats

The actions of N6-p-sulfophenyladenosine (p-SPA), a novel peripherally selective adenosine A1 agonist, were assessed on spontaneous and postsigh central sleep apneas in freely moving, unanesthetized rats by simultaneously monitoring sleep and respiration. Intraperitoneal administration of 0.1, 0.3, and 1.0 mg/kg of the drug significantly decreased postsigh and spontaneous sleep apnea index (AI). This effect persisted throughout the 6-h recording period. Doses of 0.1 and 0.3 mg/kg did not affect sleep efficiency, whereas 1.0 mg/kg of p-SPA reduced it to 60% of baseline value.

Palatal muscle electromyogram activity in obstructive sleep apnea

Eight subjects (5 men, 3 women, ages 27 to 55) with obstructive sleep apnea syndrome (OSAS) were studied to quantify and compare electromyographic (EMG) activity of levator veli palatini (LVP) and palatoglossus (PG), two velopharyngeal muscles, and genioglossus (GG) during obstructive apnea cycles in non-rapid eye movement (NREM) sleep. EMG activity of three successive preapneic breaths, first and last apneic efforts, and three successive postapneic breaths was quantified for each muscle as peak phasic inspiratory EMG normalized as percent activity of the last preapneic breath. In all subjects, apnea onset coincided with simultaneous inspiratory EMG nadir of all three muscles (LVP = 63 +/- 40%, PG = 74 +/- 53%. GG = 83 +/- 48%. mean +/- SD activity of last preapneic breath). Apnea resolution did not occur until inspiratory EMG of all three muscles simultaneously reached maximal activity, at levels significantly greater than preapneic activity as well as activity of the last preapneic effort (LVP = 215 +/- 205%, PG = 227 +/- 240+, GG = 235 +/- 202%, mean +/- SD activity of last preapneic breath, p < 0.05, Fisher's partial least-squares difference [PLSD] test for each muscle). The presence or absence of electroencephalographic arousal at apnea resolution did not influence these patterns of EMG activity. Inspiratory recruitment of velopharyngeal as well as oropharyngeal muscles appears to be associated with upper airway patency during sleep in patients with OSAS.

Adenosine analogues modulate the incidence of sleep apneas in rats

The effects of adenosine A1 and A2 agonists on spontaneous central sleep apneas in rats have been examined by simultaneously monitoring sleep and respiration in freely moving unanesthetized animals. Intraperitoneal administration of 1.0 mg/kg of the A1 receptor agonist R(-)N6-L-(2-phenyl-isopropyl)adenosine (L-PIA) and 150 and 300 micrograms/kg of 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamido-adenosine hydrochloride (CGS 21680), a selective A2 agonist, decreased the apnea index (AI) during sleep. Within a sleep period, AI increased over time in both control and drug-treated animals. For both agonists, doses effective in reducing AI also significantly reduced sleep efficiency.

Effect of induced transient arousal on obstructive apnea duration

Six untreated male patients (age 19-55 yr) with obstructive sleep apnea underwent nocturnal polysomnography with acoustic stimulation to determine the effect of transient arousal on obstructive apneas during sleep. Binaural tone bursts (25-95 dB) were delivered in late expiration during the second obstructive apnea of a cycle consisting of four consecutive apneas. For the group, stimulated apneas were significantly shorter (P < 0.05, Fisher's protected least significant difference test) than were the unstimulated apneas when transient electrocortical arousal was elicited in both non-rapid-eye-movement (non-REM) sleep [mean 17 +/- 7 (SD) vs. 26 +/- 9, 23 +/- 10, and 26 +/- 12 s for 2nd vs. 1st, 3rd, and 4th apnea, respectively, of each cycle] and REM sleep (mean 19 +/- 10 vs. 35 +/- 15, 45 +/- 18, and 39 +/- 20 s). Without electrocortical arousal, the stimulated apnea was significantly shortened in non-REM (23 +/- 9 vs. 25 +/- 7, 24 +/- 8, and 26 +/- 8 s) but not in REM (32 +/- 16 vs. 37 +/- 12, 32 +/- 15, and 30 +/- 16 s). Tones delivered relatively early and late in the apnea were equally likely to be associated with resolution of the apnea. The nadir of arterial oxygen saturation of hemoglobin was inversely proportional to apnea length, with higher saturation nadirs associated with the stimulated apneas. These data indicate that transient arousal, induced by nonrespiratory stimulation, influences the resolution of obstructive apneas during sleep.

Acoustically induced cortical arousal increases phasic pharyngeal muscle and diaphragmatic EMG in NREM sleep

Six healthy subjects (3 males, 3 females) were studied to assess phasic inspiratory responses of upper airway (UA) and diaphragm muscles to electrocortical arousal independent of other potential respiratory stimulation. Transient electroencephalographic (EEG) arousal (abrupt EEG frequency shift > or = 3 s without awakening) was induced during supine stage 2 non-rapid-eye-movement (NREM) sleep with binaural tone bursts (0.5 s, 4 kHz, 25-95 dB). Electromyograms (EMG) of levator veli palatini (EMGlvp) and genioglossus (EMGgg) were obtained with intramuscular electrodes, and EMG of diaphragm (EMGdi) was obtained with esophageal electrodes. EMG signals were processed as moving time-averaged inspiratory activity over 100-ms windows. For each arousal, each of five consecutive postarousal breaths (R1-R5) was scored for peak inspiratory phasic EMG and normalized as percent averaged EMG of the three prearousal breaths for all muscles. After arousal, EMGlvp was increased for R1-R5 and EMGgg and EMGdi were increased for R1-R4. The increase in EMGlvp was greater than those of EMGgg and EMGdi for all response breaths. There was a significant increase in EMGlvp in all subjects, and EMGgg and EMGdi were significantly increased in three and two subjects, respectively. These data indicate that isolated transient electrocortical arousal is generally associated with phasic inspiratory recruitment of UA and diaphragm muscles in normal humans during NREM sleep; velopharyngeal muscle recruitment appears to be more consistent and of greater magnitude and duration than that of oropharyngeal muscle or diaphragm. We speculate that transient arousal from sleep may contribute to UA patency independent of chemical and mechanical respiratory stimuli.

Respiratory system stability and abnormal carbon dioxide homeostasis

We have tested the hypothesis that interactions among eight parameters of the respiratory and cardiovascular systems that determine the loop gain (LG) of the respiratory CO2 feedback control system might account for the degree of stability or instability of breathing patterns in healthy sleeping volunteers as well as in familial dysautonomia (FD) and congenital central hypoventilation syndrome (CCHS) patients. The predictability of cycle duration was tested as well. We measured the values of CO2 sensitivity, CO2 delivery capacity in the circulation, circulation delay, mean lung volume for CO2, and mixed venous PCO2 in 8 FD patients, 2 CCHS patients, and 19 healthy controls. The values of these parameters were used in a mathematical model to compute the LG of the respiratory control system during sleep for each epoch of respiration analyzed. The strength of the ventilatory oscillations (R) was quantified using power density spectra of the ventilation time series. All subjects were studied at inspiratory O2 concentrations (FIO2) of 0.21 and 0.15; CCHS patients and controls were also studied at 0.12 FIO2 to examine the effect of steady-state hypoxia on respiratory system stability. In 2 FD patients, LG was elevated at both levels of FIO2 and periodic breathing was observed; the values of R were elevated. Elevated mixed venous PCO2 and reduced CO2 delivery capacity were chiefly responsible for the abnormally high LG observed. In three healthy volunteers, high LG and unstable patterns were associated with high chemosensitivity. The CCHS patients, however, remained stable even at 0.12 FIO2 because LG remained equivalent to zero due to a lack of chemosensitivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Upper airway muscle activity and the thoracic volume dependence of upper airway resistance

Although a thoracic volume dependence of upper airway resistance and caliber is known to exist in seated subjects, the mechanisms mediating this phenomenon are unknown. To test the hypothesis that actively altered end-expiratory lung volume (EELV) affects upper airway resistance in the supine position and to explore the mechanisms of any EELV-induced resistance changes, we studied five normal males during wakefulness. Supraglottic upper airway resistance (Ruaw) was calculated at an inspiratory flow of 0.1 l/s. The genioglossal electromyogram was obtained with indwelling wire electrodes and processed as moving time average. End-tidal CO2 was monitored by infrared analyzer. Observations were made during four 20-breath voluntary maneuvers: two at high and two at low EELV in each subject. Each maneuver was preceded by a control period at functional residual capacity. At high lung volume the EELV was increased by 2.23 +/- 0.54 (SD) liters; Ruaw decreased to 67.8 +/- 35.1% of control, while tonic and phasic genioglossal activities declined to 79.0 +/- 23.1 and 72.4 +/- 29.8%, respectively. At low lung volume the EELV was decreased by 0.86 +/- 0.23 liters. Ruaw increased to 178.2 +/- 186.8%, while tonic and phasic genioglossal activities increased to 243.0 +/- 139.3 and 249.1 +/- 146.3%, respectively (P less than 0.0001 for all). The findings were not explained by CO2 perturbations or respiratory pattern. Multiple linear regression analysis indicated that the genioglossal responses blunted the EELV-induced changes in upper airway patency.(ABSTRACT TRUNCATED AT 250 WORDS)

Breath-by-breath respiratory timing and volume control during periodic breathing

We examined the control of respiratory pattern during non-rapid-eye-movement sleep-related periodic breathing (PB) in adults, with and without hypoxia. We analyzed 186 cycles of PB from 18 epochs occurring in eight subjects; the mean (+/- SD) cycle duration was 30.8 +/- 8.4 s. Significant oscillations occurred in inspired tidal volume (VT), inspiratory duration (TI), mean inspired flow, inspired minute ventilation, and expiratory duration (TE) (P less than 0.005). For each epoch of PB, moving cross-correlation (MCC) functions were employed to describe the time-dependent intervariable relationships between 1) TI vs. TE, 2) VT vs. TE, and 3) VT vs. breath duration (TT) as synchronization, a strong and consistent intervariable correlation; relative coordination (RC), a weaker interaction characterized by an unstable MCC function oscillating at a subharmonic of the PB frequency; or as independence, with no statistical evidence of interaction. Fourteen epochs showed RC between TI and TE, 11 and 12 of which also showed RC between VT and TE, and VT and TT, respectively. In 4 epochs negative synchronization was exhibited by all three variable pairs. In no case were the oscillations between any pair of variables independent. The modes of coupling between variables were not correlated to O2 saturation, end-tidal CO2 levels, or inspired O2 level. We conclude that during sleep-related PB a nonrandom but weak coupling usually exists between TI and TE, VT and TE, and VT and TT.(ABSTRACT TRUNCATED AT 250 WORDS)

Breath-by-breath interactions between inspiratory and expiratory duration in occlusive sleep apnea

We examined interactions between inspiratory duration (TI), expiratory duration (TE), and inspiratory (esophageal) pressure (Pes) generation in seven subjects with confirmed occlusive sleep apnea. Breath-by-breath values of TI, TE, and Pes were identified by digital computer during 21 260-s epochs of repetitive occlusive apnea during non-rapid-eye-movement sleep. The control theory of interacting nonlinear oscillators was used to categorize the interaction between TI and TE for each epoch as either 1) synchronization, the strongest possible interaction between biological oscillators; 2) relative entrainment, a moderate interaction between oscillators; or 3) relative coordination, a weak interaction. The latter two interactions were characterized by systemic oscillations in the moving cross-correlation between TI and TE. The relationship between TI and Pes was analyzed in a similar fashion. Significant oscillations were present in all three parameters (P less than 0.0001 for each). We observed significant negative correlations between TI and TE and between TI and Pes (P less than 0.001 for each) when all breaths for all epochs were pooled. In no epoch was there a significant positive correlation between TI and TE or Pes. All three interactions were observed between TI and TE: five epochs of synchronization, nine of relative entrainment, and seven of relative coordination. In contrast, 19 of 21 epochs exhibited synchronization between TI and Pes, with 2 epochs of relative entrainment. The relative frequency of TI vs. Pes synchronization was significantly greater than TI vs. TE synchronization (P less than 0.005).(ABSTRACT TRUNCATED AT 250 WORDS)

Upper airway and respiratory muscle responses to continuous negative airway pressure

To determine upper airway and respiratory muscle responses to nasal continuous negative airway pressure (CNAP), we quantitated the changes in diaphragmatic and genioglossal electromyographic activity, inspiratory duration, tidal volume, minute ventilation, and end-expiratory lung volume (EEL) during CNAP in six normal subjects during wakefulness and five during sleep. During wakefulness, CNAP resulted in immediate increases in electromyographic diaphragmatic and genioglossal muscle activity, and inspiratory duration, preserved or increased tidal volume and minute ventilation, and decreased EEL. During non-rapid-eye-movement and rapid-eye-movement sleep, CNAP was associated with no immediate muscle or timing responses, incomplete or complete upper airway occlusion, and decreased EEL. Progressive diaphragmatic and genioglossal responses were observed during non-rapid-eye-movement sleep in association with arterial O2 desaturation, but airway patency was not reestablished until further increases occurred with arousal. These results indicate that normal subjects, while awake, can fully compensate for CNAP by increasing respiratory and upper airway muscle activities but are unable to do so during sleep in the absence of arousal. This sleep-induced failure of load compensation predisposes the airways to collapse under conditions which threaten airway patency during sleep. The abrupt electromyogram responses seen during wakefulness and arousal are indicative of the importance of state effects, whereas the gradual increases seen during sleep probably reflect responses to changing blood gas composition.

A minimal mathematical model of human periodic breathing

Numerous mathematical models of periodic breathing (PB) currently exist. These models suggest mechanisms that may underlie many known causes of PB. However, each model that has been shown to simulate PB under reasonable conditions contains greater than 15 physiological parameters. Because some parameters exhibit a wide range of values in a population, such simulations cannot test a model's ability to account for the breathing patterns of individuals. Furthermore it is impractical to perform a direct experimental validation study that would require the estimation of each of 15 or more parameters for each subject. A minimal model of PB is presented that is suitable for direct validation. Analytic expressions are given that define the conditions for PB in terms of the following: 1) CO2 sensitivity, 2) Cardiac output, 3) Mixed venous CO2, 4) Circulation time, and 5) Mean lung volume for CO2. This model is shown to be consistent with previous models and experimental data regarding the degree of hypoxia or congestive heart failure required to produce PB. A quantitative measure of relative stability is defined as a metric of comparison to the human studies described in the accompanying paper (J. Appl. Physiol. 65: 1389-1399, 1988).

Relative stability of human respiration during progressive hypoxia

We have systematically studied the relationship between the relative stability (R) of respiration and the loop gain (LG) of the CO2 control system in 15 healthy awake adult males during progressive hypoxia. R was measured by the ventilatory oscillations after brief (less than 10 s) CO2 challenges. Control theory suggests that such oscillations are completely governed by LG. A significant positive correlation was found between R and LG (r = 0.74, P less than 0.01, n = 85). A minimal mathematical model of respiratory control was used to predict R as a function of LG. Serial correlation analysis (r = 0.09, P greater than 0.1) of the residuals indicated statistical agreement between predictions and observations. The mean residual (0.011) was not significantly different from zero (P greater than 0.1). Also, as the model predicted, sustained periodic breathing (PB) occurred whenever the estimated LG was greater than unity. The mean LG breathing room air was 0.51 and for the 13 epochs of PB was 1.17 (range 0.71-1.65). It is concluded that PB is a quantitative extension of the relative stability continuum and corresponds to unstable operation of the CO2 control system. Furthermore, relative stability can be quantitatively predicted for each subject by a minimal mathematical model.

Evaluation of autonomic dysfunction in familial dysautonomia by power spectral analysis

We examined the nature and extent of the autonomic control defect in patients with autonomic dysfunction using power spectral analysis of heart rate fluctuations. Heart rate variability and respiratory patterns were monitored and discrete blood pressure measurements were made during supine and standing positions in 10 ambulatory patients with familial dysautonomia and in controls. Postural hypotension without compensatory tachycardia was confirmed in the patients upon standing. The balance between sympathetic and parasympathetic activity was compared in both positions by quantifying the power of the low (0.04-0.095 Hz) and high (respiratory) frequency fluctuations in instantaneous heart rate. After changing from supine to standing position there was a small decrease in the low frequency power of heart rate fluctuations in the patients as opposed to a significant increase in controls. The mean power of fluctuations occurring at high frequency decreased only slightly in the patients compared to a marked decrease in the controls. We conclude that the fall in blood pressure, lack of appropriate heart rate modulation, and failure to increase low frequency heart rate power which occurred in the patients upon standing, are all due to lack of increased sympathetic output under the influence of gravity. The failure to decrease power in the respiratory frequency peak in the patients, suggests an abnormal retention of parasympathetic activity. This may be explained by parasympathetic compensation for the substantial sympathetic loss, or by a lack of appropriate inhibition of parasympathetic tone from baroreceptors or supraspinal structures.

Aging of modulation of heart rate

We postulated that measurements of autonomically mediated fluctuations in heart rate might provide a quantitative probe of biological aging. We used power spectrum analysis of instantaneous heart rate while 33 male subjects matched their breathing to a metronome at 15 breaths/min. Measurements were made in supine and standing position. Total power and its two major components, high- and low-frequency power, declined with age in both positions but at different rates. High-frequency power that represents parasympathetically mediated respiratory sinus arrhythmia declined linearly in supine position only in subjects 9-28 yr with a slope of -0.796, which was significantly different from zero at P = 0.0007. The absolute value of high-frequency power in standing position was approximately 60% of that in supine, a difference that was statistically significant (P = 0.01). Low-frequency power that represents beta-adrenergically mediated heart rate fluctuations, especially in standing position, declined linearly to 62 yr of age (P = 0.0001). Mean heart rate increased 17.2 beats/min, and diastolic blood pressure increased 8 mmHg in the entire group in the standing compared with supine position. There were no significant differences in these changes above and below 30 yr of age. We conclude that the influence of the two major mechanisms that modulate heart rate decline at significantly different rates with aging.