- Email: [email protected]
The effect of time of day on apnoea index in the sleeping rat
Respiratory Physiology & Neurobiology 154 (2006) 351–355
The effect of time of day on apnoea index in the sleeping rat Richard Stephenson a,∗ , Richard L. Horner b a
Departments of Zoology, University of Toronto, Ramsay Wright Building, 25 Harbord Street, Toronto, Ont., Canada M5S 3G5 b Departments of Medicine and Physiology, University of Toronto, Toronto, Ont., Canada Accepted 9 February 2006
Abstract This study tested the hypothesis that apnoea index would be greater during daytime sleep than nighttime sleep in the rat. Electroencephalogram and electromyogram were monitored via biotelemetry implant and respiration was measured using whole body plethysmography in six male rats in two separate 34 h recording sessions per animal. Apnoeas were classified as “spontaneous” or “post-sigh”. Daily average spontaneous apnoea index was 35 times greater (p < 0.0001) during rapid eye movement (REM) sleep than in non-REM (NREM) sleep. In contrast, daily average post-sigh apnoea index was not significantly greater in REM sleep than in non-REM (NREM) sleep (p = 0.39). There was a greater post-sigh apnoea index during daytime REM than during nighttime REM (p = 0.043) but REM-related spontaneous apnoea index was unaffected by time of day. There was no day to night difference in spontaneous apnoea index or post-sigh apnoea index during NREM sleep. Respiratory variability (coefficient of variation for breath duration and tidal volume) was not affected by time of day in REM or NREM sleep. We conclude that the circadian timing system has no effect on apnoea index during NREM sleep in the rat, but it may influence the propensity for post-sigh apnoea during REM sleep. © 2006 Elsevier B.V. All rights reserved. Keywords: Circadian rhythms; Rat; Respiratory variability; Sleep apnoea
1. Introduction The circadian timing system has been shown to modulate respiration and the respiratory control system across the day–night cycle in human beings (Spengler et al., 2000; Stephenson et al., 2000). Furthermore, rats have been found to exhibit significant diurnal rhythms ∗ Corresponding author. Tel.: +1 416 978 3491; fax: +1 416 978 8532. E-mail address: [email protected] (R. Stephenson).
1569-9048/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.resp.2006.02.007
in respiration, metabolic rate and body temperature (Peever and Stephenson, 1997; Seifert and Mortola, 2002a). The circadian rhythms in these variables are synchronized, rising to a peak in the night in the nocturnal (night active) rat and in the daytime in the diurnal (day active) human. In rats the circadian rhythms and sleep–wake states have additive effects on respiration (Stephenson et al., 2001), but it is presently unknown if this is also true in humans. The rat has emerged in recent years as an important animal model for the study of neural mechanisms underlying sleep and breathing, and this species has been observed to exhibit intermit-
352
R. Stephenson, R.L. Horner / Respiratory Physiology & Neurobiology 154 (2006) 351–355
tent apnoea during sleep (Mendelson et al., 1988; Sato et al., 1990; Christon et al., 1996). Sleep apnoea syndrome is considered to be a major public health burden (Phillipson, 1993). Research has established that repetitive apnoeas are sleep-state dependent, implicating sleep-related neural control mechanisms in the manifestation of the disorder. It has recently been suggested that the circadian timing system may also play a role via circadian modulation of chemoreflex control (Stephenson, 2003), which may cause day to night differences in the propensity for respiratory instability and apnoea at sleep onset in humans. Specifically, it was predicted that circadian modulation would increase the propensity for apnoea during stages I and II of non-rapid eye movement sleep (NREM) at night and decrease it during NREM in the daytime. The plausibility of the hypothesis was supported by a theoretical analysis based on a mathematical model of the human respiratory control system (Stephenson, 2004) but an experimental test of the hypothesis has not yet been reported. The key mechanism underlying this putative circadian effect is a significant day to night difference in the human chemoreflex threshold, which has been found to be increased at night (Stephenson et al., 2000). The potential role of such a mechanism in other species has not yet been assessed, either theoretically or experimentally. Unfortunately, the chemoreflex parameters (threshold and chemosensitivity) have not yet been accurately quantified in awake and sleeping animals, precluding the application of theoretical modeling to this question for non-human species. A coarse experimental analysis of day to night differences in overall responsiveness to respiratory gases has been attempted in awake rats. Peever and Stephenson (1997) found that the metabolic rate specific hyperventilatory response (V˙ /V˙ CO2 ) to gaseous hypercapnia was independent of circadian time, but that the contributions of specific mechanisms involved in the response (respiratory frequency, tidal volume and metabolic rate) did differ between night and day. In contrast, Seifert and Mortola (2002b) concluded that there was no effect of circadian time on chemoreflex responses to hypercapnia or hypoxia in this species. Thus, experimental data in the rat are inconsistent raising uncertainty about the possibility that the circadian system may influence sleep apnoea in this species. The present analysis was conducted to test the hypothesis directly, and thereby
to evaluate the suitability of the rat as an animal model of circadian modulation of sleep apnoea. Since rats are primarily nocturnal we predicted that respiratory variability would be greater and sleep apnoea would be more frequent during sleep in the light phase of the light–dark cycle.
2. Materials and methods This paper represents a supplementary analysis of data that were acquired during a previously published study and details of the methods can be found elsewhere (Stephenson et al., 2001). Briefly, six adult male Sprague–Dawley rats (359 ± 19 g) were maintained on a 12:12 h light–dark cycle and provided with food and water ad libitum throughout the study. Experiments conformed to the guidelines set out by the Canadian Council on Animal Care and were approved by the animal care committee of the University of Toronto. Recordings began after a week-long recovery from surgical implantation of a biotelemetry device for recording electroencephalogram (EEG), neck electromyogram (EMG) and abdominal temperature (Tb). Rats were placed in a whole body plethysmograph for two separate 34 h recording sessions in which data were acquired during the final 24 h. The plethysmograph operated in intermittent flow mode so that respiration was recorded for intervals of one to several minutes during periods of rapid eye movement (REM) and non-REM (NREM) sleep. In the present analysis sleep–wake state was scored by visual interpretation of EEG and EMG records using standard criteria as described in more detail in Stephenson et al. (2001). Apnoea was defined as an expiratory pause lasting greater than 1 s, which is longer than one missed breath. Apnoeas were divided into two categories; post-sigh apnoeas, in which the apnoeic interval occurred within 5 s following a “sigh” or augmented breath (breath with tidal volume (VT ) more than 50% greater than those of preceding breaths), and spontaneous apnoeas in which the apnoea was not preceded by an augmented breath. The 5 s criterion was chosen because it was found to include all apnoeas occurring in close association with a sigh. Occasionally, sighs were observed without apnoea and these were followed by a brief
R. Stephenson, R.L. Horner / Respiratory Physiology & Neurobiology 154 (2006) 351–355
period of hypoventilation that was completed within 5 s. Indeed, all except four post-sigh apnoea’s occurred within three breaths of a sigh and reanalysis using a segregation limit of 3 s did not change the overall conclusions. Due to the low frequency of apnoea during sleep in rats (Mendelson et al., 1988; Sato et al., 1990; Christon et al., 1996), we pooled the data from each rat into two time bins; light versus dark. Apnoea index was taken as the number of apnoeas per hour of recording time during either NREM or REM sleep for each of postsigh apnoea (post-sigh apnoea index) and spontaneous apnoea (spontaneous apnoea index). A coarse estimate of overall respiratory variability was obtained by calculating the coefficient of variation (100 × standard deviation/mean (%)) for breath duration (Ttot ) and tidal volume (VT ). The original experiment was not intended to evaluate the present question and plethysmograph data included intermittent baseline correction making them unsuitable for a more sophisticated analysis of respiratory variability using time series analysis. Data were analyzed on a rat by rat basis, pooling the data from the two recording sessions for each rat. Data are presented as a grand mean (±S.E.) for n = 6 animals. Statistical comparisons used Student’s
353
t-test for paired data, with significance inferred when p < 0.05.
3. Results The overall apnoea index (24 h average for postsigh apnoea and spontaneous apnoea combined) was eight-fold greater in REM sleep than in NREM sleep (31.2 ± 3.6 versus 4.0 ± 0.3 apnoeas h−1 , respectively, p = 0.0007). This was due to a state-related difference in spontaneous apnoea frequency (24.6 ± 0.7 versus 0.7 ± 0.2 apnoeas h−1 in REM versus NREM respectively, p < 0.0001), with no significant difference between sleep states in the frequency of postsigh apnoea (6.6 ± 3.2 versus 3.4 ± 0.4, p = 0.39). During NREM the post-sigh apnoea index was 4.9 times greater than the spontaneous apnoea index, whereas during REM the converse was observed; spontaneous apnoea index was 3.7 times greater than post-sigh apnoea index. The day–night comparisons for apnoea index are presented in Fig. 1. There were no significant differences between daytime and nighttime NREM sleep with respect to both post-sigh apnoea index and spon-
Fig. 1. Apnoea index (events h−1 ) during NREM sleep (a and b) and REM sleep (c and d) recorded in the light phase (day, when rats are mainly asleep) and dark phase (night, when rats are mainly active) of the 12:12 light–dark cycle. Data are shown for each of six adult male rats (connected dots) together with the group mean ± S.E. Apnoeas were classified as spontaneous apnoeas (a and c) and post-sigh apnoeas (b and d). * Significantly different from day value (p < 0.05).
354
R. Stephenson, R.L. Horner / Respiratory Physiology & Neurobiology 154 (2006) 351–355
taneous apnoea index (Fig. 1a and b). There was a trend for a lower spontaneous apnoea index during REM at night (Fig. 1c) but this was not statistically significant (p = 0.214). However post-sigh apnoea index was significantly lower (p = 0.043) during REM sleep at night than during the day (Fig. 1d). For both NREM sleep and REM sleep there were no day–night differences in overall respiratory variability as measured by coefficient of variation for Ttot and VT . Comparing the 24 h average values, coefficient of variation (%) was significantly greater in REM than in NREM for both of these respiratory variables (Ttot , 28.1 ± 2.1 versus 11.3 ± 1.4; VT , 26.8 ± 1.3 versus 15.8 ± 1.0, REM versus NREM, respectively; both p < 0.002).
4. Discussion This study does not support the hypothesis that there is a significant effect of time of day on the propensity for apnoea during NREM sleep in rats. It remains to be determined whether this negative result can be extrapolated to human subjects. We conclude that the rat is not a suitable animal model for the specific investigation of circadian influences on chemoreflex-related sleep apnoea because it is during NREM that the chemoreflexes are predicted to contribute to apnoea (Stephenson, 2003, 2004). The hypothesis that the circadian timing system may contribute to the propensity for sleep apnoea (Stephenson, 2003, 2004) was based on the observation that healthy human subjects exhibit significant circadian rhythms in chemoreflex control characteristics (Spengler et al., 2000; Stephenson et al., 2000), which have a major influence on respiratory stability during NREM sleep (Smith et al., 2003). However the evidence that awake adult rats exhibit day–night differences in respiratory responsiveness to inhaled CO2 is contradictory (Peever and Stephenson, 1997; Seifert and Mortola, 2002b), and it is not yet known whether a circadian rhythm in chemoreflex control persists during sleep in either species. However, such circadian effects, if present, had no apparent effect on respiratory variability or spontaneous apnoea index during NREM sleep in rats. The incidence of apnoea (both spontaneous apnoea and post-sigh apnoea) was low during NREM sleep
in rats (Fig. 1a and b), confirming previous reports (Mendelson et al., 1988; Sato et al., 1990; Christon et al., 1996). In fact, the present data may overestimate NREM apnoea index because in almost every case of post-sigh apnoea (but not spontaneous apnoea), there was EEG evidence of a very brief change of state (usually brief NREM to REM transitions or occasionally very brief awakenings) associated with the sigh and apnoea. In contrast to NREM sleep, the apnoea index during REM sleep was relatively high, again confirming previous reports (Mendelson et al., 1988; Sato et al., 1990; Christon et al., 1996). In addition, there was evidence that the circadian system may play a role in REM-related apnoea (Fig. 1c and d), a possibility that has hitherto been overlooked. During REM, average spontaneous apnoea index was 68% greater during the day (when rats express most of their REM sleep) than at night. However this was not statistically significant because two of six animals exhibited a slight inverse day–night difference (Fig. 1c). Average postsigh apnoea index during REM was twice as high during the day than at night, a difference that was statistically significant. However it is worth noting that there were differences between animals in that four of six rats failed to exhibit post-sigh apnoea at all during nocturnal REM sleep whereas the remaining two showed no effect of time of day (Fig. 1d). These data therefore suggest a need for further experimental studies in the rat into the mechanisms (including circadian influences on mechanoreflex control of breathing) underlying apnoea during REM sleep. A day–night comparison of respiratory variability and apnoea–hypopnoea index in healthy human subjects and sleep apnoea patients is needed to assess whether there are qualitative species differences between rats and humans with regard to circadian influences on propensity for apnoea during NREM sleep.
Acknowledgements The technical assistance of K.S. Liao, H. Hamrahi and B. Chan is gratefully acknowledged. This study was supported by operating and equipment grants from the Natural Sciences and Engineering Research Council of Canada, Medical Research Council of Canada and the Ontario Thoracic Society.
R. Stephenson, R.L. Horner / Respiratory Physiology & Neurobiology 154 (2006) 351–355
References Christon, J., Carley, D.W., Monti, D., Radulovacki, M., 1996. Effects of inspired gas on sleep-related apnea in the rat. J. Appl. Physiol. 80, 2102–2107. Mendelson, W.B., Martin, J.V., Perlis, M., Giesen, H., Wagner, R., Rapoport, S.I., 1988. Periodic cessation of respiratory effort during sleep in adult rats. Physiol. Behav. 43, 229–234. Peever, J.H., Stephenson, R., 1997. Day–night differences in the respiratory response to hypercapnia in awake adult rats. Respir. Physiol. 109, 241–248. Phillipson, E.A., 1993. Sleep apnea—a major public health problem. New Engl. J. Med. 328, 1271–1273. Sato, T., Saito, H., Seto, K., Takatsuji, H., 1990. Sleep apneas and cardiac arrhythmias in freely moving rats. Am. J. Physiol. 259, R282–R287. Seifert, E.L., Mortola, J.P., 2002a. The circadian pattern of breathing in conscious adult rats. Respir. Physiol. 129, 297–305. Seifert, E.L., Mortola, J.P., 2002b. Circadian pattern of ventilation during acute and chronic hypercapnia in conscious adult rats.
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Am. J. Physiol. Regulat. Integr. Comp. Physiol. 282, R244– R251. Smith, C.A., Nakayama, H., Dempsey, J.A., 2003. The essential role of carotid body chemoreceptors in sleep apnea. Can. J. Physiol. Pharmacol. 81, 774–779. Spengler, C.M., Czeisler, C.A., Shea, S.A., 2000. An endogenous circadian rhythm of respiratory control in humans. J. Physiol. 526, 683–694. Stephenson, R., 2003. Do circadian rhythms in respiratory control contribute to sleep-related breathing disorders? Sleep Med. Rev. 7, 475–490. Stephenson, R., 2004. A theoretical study of the effect of circadian rhythms on sleep-induced periodic breathing and apnoea. Respir. Physiol. Neurobiol. 139, 303–319. Stephenson, R., Liao, K.S., Hamrahi, H., Horner, R.L., 2001. Circadian rhythms and sleep have additive effects on respiration in the rat. J. Physiol. 536, 225–235. Stephenson, R., Mohan, R.M., Duffin, J., Jarsky, T.M., 2000. Circadian rhythms in the chemoreflex control of breathing. Am. J. Physiol. Regulat. Integr. Comp. Physiol. 278, R282–R286.
The effect of time of day on apnoea index in the sleeping rat Richard Stephenson a,∗ , Richard L. Horner b a
Departments of Zoology, University of Toronto, Ramsay Wright Building, 25 Harbord Street, Toronto, Ont., Canada M5S 3G5 b Departments of Medicine and Physiology, University of Toronto, Toronto, Ont., Canada Accepted 9 February 2006
Abstract This study tested the hypothesis that apnoea index would be greater during daytime sleep than nighttime sleep in the rat. Electroencephalogram and electromyogram were monitored via biotelemetry implant and respiration was measured using whole body plethysmography in six male rats in two separate 34 h recording sessions per animal. Apnoeas were classified as “spontaneous” or “post-sigh”. Daily average spontaneous apnoea index was 35 times greater (p < 0.0001) during rapid eye movement (REM) sleep than in non-REM (NREM) sleep. In contrast, daily average post-sigh apnoea index was not significantly greater in REM sleep than in non-REM (NREM) sleep (p = 0.39). There was a greater post-sigh apnoea index during daytime REM than during nighttime REM (p = 0.043) but REM-related spontaneous apnoea index was unaffected by time of day. There was no day to night difference in spontaneous apnoea index or post-sigh apnoea index during NREM sleep. Respiratory variability (coefficient of variation for breath duration and tidal volume) was not affected by time of day in REM or NREM sleep. We conclude that the circadian timing system has no effect on apnoea index during NREM sleep in the rat, but it may influence the propensity for post-sigh apnoea during REM sleep. © 2006 Elsevier B.V. All rights reserved. Keywords: Circadian rhythms; Rat; Respiratory variability; Sleep apnoea
1. Introduction The circadian timing system has been shown to modulate respiration and the respiratory control system across the day–night cycle in human beings (Spengler et al., 2000; Stephenson et al., 2000). Furthermore, rats have been found to exhibit significant diurnal rhythms ∗ Corresponding author. Tel.: +1 416 978 3491; fax: +1 416 978 8532. E-mail address: [email protected] (R. Stephenson).
1569-9048/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.resp.2006.02.007
in respiration, metabolic rate and body temperature (Peever and Stephenson, 1997; Seifert and Mortola, 2002a). The circadian rhythms in these variables are synchronized, rising to a peak in the night in the nocturnal (night active) rat and in the daytime in the diurnal (day active) human. In rats the circadian rhythms and sleep–wake states have additive effects on respiration (Stephenson et al., 2001), but it is presently unknown if this is also true in humans. The rat has emerged in recent years as an important animal model for the study of neural mechanisms underlying sleep and breathing, and this species has been observed to exhibit intermit-
352
R. Stephenson, R.L. Horner / Respiratory Physiology & Neurobiology 154 (2006) 351–355
tent apnoea during sleep (Mendelson et al., 1988; Sato et al., 1990; Christon et al., 1996). Sleep apnoea syndrome is considered to be a major public health burden (Phillipson, 1993). Research has established that repetitive apnoeas are sleep-state dependent, implicating sleep-related neural control mechanisms in the manifestation of the disorder. It has recently been suggested that the circadian timing system may also play a role via circadian modulation of chemoreflex control (Stephenson, 2003), which may cause day to night differences in the propensity for respiratory instability and apnoea at sleep onset in humans. Specifically, it was predicted that circadian modulation would increase the propensity for apnoea during stages I and II of non-rapid eye movement sleep (NREM) at night and decrease it during NREM in the daytime. The plausibility of the hypothesis was supported by a theoretical analysis based on a mathematical model of the human respiratory control system (Stephenson, 2004) but an experimental test of the hypothesis has not yet been reported. The key mechanism underlying this putative circadian effect is a significant day to night difference in the human chemoreflex threshold, which has been found to be increased at night (Stephenson et al., 2000). The potential role of such a mechanism in other species has not yet been assessed, either theoretically or experimentally. Unfortunately, the chemoreflex parameters (threshold and chemosensitivity) have not yet been accurately quantified in awake and sleeping animals, precluding the application of theoretical modeling to this question for non-human species. A coarse experimental analysis of day to night differences in overall responsiveness to respiratory gases has been attempted in awake rats. Peever and Stephenson (1997) found that the metabolic rate specific hyperventilatory response (V˙ /V˙ CO2 ) to gaseous hypercapnia was independent of circadian time, but that the contributions of specific mechanisms involved in the response (respiratory frequency, tidal volume and metabolic rate) did differ between night and day. In contrast, Seifert and Mortola (2002b) concluded that there was no effect of circadian time on chemoreflex responses to hypercapnia or hypoxia in this species. Thus, experimental data in the rat are inconsistent raising uncertainty about the possibility that the circadian system may influence sleep apnoea in this species. The present analysis was conducted to test the hypothesis directly, and thereby
to evaluate the suitability of the rat as an animal model of circadian modulation of sleep apnoea. Since rats are primarily nocturnal we predicted that respiratory variability would be greater and sleep apnoea would be more frequent during sleep in the light phase of the light–dark cycle.
2. Materials and methods This paper represents a supplementary analysis of data that were acquired during a previously published study and details of the methods can be found elsewhere (Stephenson et al., 2001). Briefly, six adult male Sprague–Dawley rats (359 ± 19 g) were maintained on a 12:12 h light–dark cycle and provided with food and water ad libitum throughout the study. Experiments conformed to the guidelines set out by the Canadian Council on Animal Care and were approved by the animal care committee of the University of Toronto. Recordings began after a week-long recovery from surgical implantation of a biotelemetry device for recording electroencephalogram (EEG), neck electromyogram (EMG) and abdominal temperature (Tb). Rats were placed in a whole body plethysmograph for two separate 34 h recording sessions in which data were acquired during the final 24 h. The plethysmograph operated in intermittent flow mode so that respiration was recorded for intervals of one to several minutes during periods of rapid eye movement (REM) and non-REM (NREM) sleep. In the present analysis sleep–wake state was scored by visual interpretation of EEG and EMG records using standard criteria as described in more detail in Stephenson et al. (2001). Apnoea was defined as an expiratory pause lasting greater than 1 s, which is longer than one missed breath. Apnoeas were divided into two categories; post-sigh apnoeas, in which the apnoeic interval occurred within 5 s following a “sigh” or augmented breath (breath with tidal volume (VT ) more than 50% greater than those of preceding breaths), and spontaneous apnoeas in which the apnoea was not preceded by an augmented breath. The 5 s criterion was chosen because it was found to include all apnoeas occurring in close association with a sigh. Occasionally, sighs were observed without apnoea and these were followed by a brief
R. Stephenson, R.L. Horner / Respiratory Physiology & Neurobiology 154 (2006) 351–355
period of hypoventilation that was completed within 5 s. Indeed, all except four post-sigh apnoea’s occurred within three breaths of a sigh and reanalysis using a segregation limit of 3 s did not change the overall conclusions. Due to the low frequency of apnoea during sleep in rats (Mendelson et al., 1988; Sato et al., 1990; Christon et al., 1996), we pooled the data from each rat into two time bins; light versus dark. Apnoea index was taken as the number of apnoeas per hour of recording time during either NREM or REM sleep for each of postsigh apnoea (post-sigh apnoea index) and spontaneous apnoea (spontaneous apnoea index). A coarse estimate of overall respiratory variability was obtained by calculating the coefficient of variation (100 × standard deviation/mean (%)) for breath duration (Ttot ) and tidal volume (VT ). The original experiment was not intended to evaluate the present question and plethysmograph data included intermittent baseline correction making them unsuitable for a more sophisticated analysis of respiratory variability using time series analysis. Data were analyzed on a rat by rat basis, pooling the data from the two recording sessions for each rat. Data are presented as a grand mean (±S.E.) for n = 6 animals. Statistical comparisons used Student’s
353
t-test for paired data, with significance inferred when p < 0.05.
3. Results The overall apnoea index (24 h average for postsigh apnoea and spontaneous apnoea combined) was eight-fold greater in REM sleep than in NREM sleep (31.2 ± 3.6 versus 4.0 ± 0.3 apnoeas h−1 , respectively, p = 0.0007). This was due to a state-related difference in spontaneous apnoea frequency (24.6 ± 0.7 versus 0.7 ± 0.2 apnoeas h−1 in REM versus NREM respectively, p < 0.0001), with no significant difference between sleep states in the frequency of postsigh apnoea (6.6 ± 3.2 versus 3.4 ± 0.4, p = 0.39). During NREM the post-sigh apnoea index was 4.9 times greater than the spontaneous apnoea index, whereas during REM the converse was observed; spontaneous apnoea index was 3.7 times greater than post-sigh apnoea index. The day–night comparisons for apnoea index are presented in Fig. 1. There were no significant differences between daytime and nighttime NREM sleep with respect to both post-sigh apnoea index and spon-
Fig. 1. Apnoea index (events h−1 ) during NREM sleep (a and b) and REM sleep (c and d) recorded in the light phase (day, when rats are mainly asleep) and dark phase (night, when rats are mainly active) of the 12:12 light–dark cycle. Data are shown for each of six adult male rats (connected dots) together with the group mean ± S.E. Apnoeas were classified as spontaneous apnoeas (a and c) and post-sigh apnoeas (b and d). * Significantly different from day value (p < 0.05).
354
R. Stephenson, R.L. Horner / Respiratory Physiology & Neurobiology 154 (2006) 351–355
taneous apnoea index (Fig. 1a and b). There was a trend for a lower spontaneous apnoea index during REM at night (Fig. 1c) but this was not statistically significant (p = 0.214). However post-sigh apnoea index was significantly lower (p = 0.043) during REM sleep at night than during the day (Fig. 1d). For both NREM sleep and REM sleep there were no day–night differences in overall respiratory variability as measured by coefficient of variation for Ttot and VT . Comparing the 24 h average values, coefficient of variation (%) was significantly greater in REM than in NREM for both of these respiratory variables (Ttot , 28.1 ± 2.1 versus 11.3 ± 1.4; VT , 26.8 ± 1.3 versus 15.8 ± 1.0, REM versus NREM, respectively; both p < 0.002).
4. Discussion This study does not support the hypothesis that there is a significant effect of time of day on the propensity for apnoea during NREM sleep in rats. It remains to be determined whether this negative result can be extrapolated to human subjects. We conclude that the rat is not a suitable animal model for the specific investigation of circadian influences on chemoreflex-related sleep apnoea because it is during NREM that the chemoreflexes are predicted to contribute to apnoea (Stephenson, 2003, 2004). The hypothesis that the circadian timing system may contribute to the propensity for sleep apnoea (Stephenson, 2003, 2004) was based on the observation that healthy human subjects exhibit significant circadian rhythms in chemoreflex control characteristics (Spengler et al., 2000; Stephenson et al., 2000), which have a major influence on respiratory stability during NREM sleep (Smith et al., 2003). However the evidence that awake adult rats exhibit day–night differences in respiratory responsiveness to inhaled CO2 is contradictory (Peever and Stephenson, 1997; Seifert and Mortola, 2002b), and it is not yet known whether a circadian rhythm in chemoreflex control persists during sleep in either species. However, such circadian effects, if present, had no apparent effect on respiratory variability or spontaneous apnoea index during NREM sleep in rats. The incidence of apnoea (both spontaneous apnoea and post-sigh apnoea) was low during NREM sleep
in rats (Fig. 1a and b), confirming previous reports (Mendelson et al., 1988; Sato et al., 1990; Christon et al., 1996). In fact, the present data may overestimate NREM apnoea index because in almost every case of post-sigh apnoea (but not spontaneous apnoea), there was EEG evidence of a very brief change of state (usually brief NREM to REM transitions or occasionally very brief awakenings) associated with the sigh and apnoea. In contrast to NREM sleep, the apnoea index during REM sleep was relatively high, again confirming previous reports (Mendelson et al., 1988; Sato et al., 1990; Christon et al., 1996). In addition, there was evidence that the circadian system may play a role in REM-related apnoea (Fig. 1c and d), a possibility that has hitherto been overlooked. During REM, average spontaneous apnoea index was 68% greater during the day (when rats express most of their REM sleep) than at night. However this was not statistically significant because two of six animals exhibited a slight inverse day–night difference (Fig. 1c). Average postsigh apnoea index during REM was twice as high during the day than at night, a difference that was statistically significant. However it is worth noting that there were differences between animals in that four of six rats failed to exhibit post-sigh apnoea at all during nocturnal REM sleep whereas the remaining two showed no effect of time of day (Fig. 1d). These data therefore suggest a need for further experimental studies in the rat into the mechanisms (including circadian influences on mechanoreflex control of breathing) underlying apnoea during REM sleep. A day–night comparison of respiratory variability and apnoea–hypopnoea index in healthy human subjects and sleep apnoea patients is needed to assess whether there are qualitative species differences between rats and humans with regard to circadian influences on propensity for apnoea during NREM sleep.
Acknowledgements The technical assistance of K.S. Liao, H. Hamrahi and B. Chan is gratefully acknowledged. This study was supported by operating and equipment grants from the Natural Sciences and Engineering Research Council of Canada, Medical Research Council of Canada and the Ontario Thoracic Society.
R. Stephenson, R.L. Horner / Respiratory Physiology & Neurobiology 154 (2006) 351–355
References Christon, J., Carley, D.W., Monti, D., Radulovacki, M., 1996. Effects of inspired gas on sleep-related apnea in the rat. J. Appl. Physiol. 80, 2102–2107. Mendelson, W.B., Martin, J.V., Perlis, M., Giesen, H., Wagner, R., Rapoport, S.I., 1988. Periodic cessation of respiratory effort during sleep in adult rats. Physiol. Behav. 43, 229–234. Peever, J.H., Stephenson, R., 1997. Day–night differences in the respiratory response to hypercapnia in awake adult rats. Respir. Physiol. 109, 241–248. Phillipson, E.A., 1993. Sleep apnea—a major public health problem. New Engl. J. Med. 328, 1271–1273. Sato, T., Saito, H., Seto, K., Takatsuji, H., 1990. Sleep apneas and cardiac arrhythmias in freely moving rats. Am. J. Physiol. 259, R282–R287. Seifert, E.L., Mortola, J.P., 2002a. The circadian pattern of breathing in conscious adult rats. Respir. Physiol. 129, 297–305. Seifert, E.L., Mortola, J.P., 2002b. Circadian pattern of ventilation during acute and chronic hypercapnia in conscious adult rats.
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