Chronic upper airway obstruction induces abnormal sleep/wake dynamics in juvenile rats

Abstract

Objectives: Conventional scoring of sleep provides little information about the process of transitioning between vigilance-states. We used the state space technique to explore whether rats with chronic upper airway obstruction (UAO) have abnormal sleep/wake states, faster movements between states, or abnormal transitions between states.

Design: The tracheae of 22-day-old Sprague-Dawley rats were surgically narrowed to increase upper airway resistance with no evidence for frank obstructed apneas or hypopneas; 24-h electroencephalography of sleep/wake recordings of UAO and sham-control animals was analyzed using state space technique. This non-categorical approach allows quantitative and unbiased examination of vigilance-states and state transitions. Measurements were performed 2 weeks post-surgery at baseline and following administration of ritanserin (5-HT2 receptor antagonist) the next day to stimulate sleep.

Measurements and results: UAO rats spent less time in deep (delta-rich) slow wave sleep (SWS) and near transition zones between states. State transitions from light SWS to wake and vice versa and microarousals were more frequent and rapid in UAO rats, indicating that obstructed animals have more regions where vigilance-states are unstable. Ritanserin consolidated sleep in both groups by decreasing the number of microarousals and trajectories between wake and light SWS, and increasing deep SWS in UAO.

Conclusions: State space technique enables visualization of vigilance-state transitions and velocities that were not evident by traditional scoring methods. This analysis provides new quantitative assessment of abnormal vigilance-state dynamics in UAO in the absence of frank obstructed apneas or hypopneas.

Figure 1. Spontaneous sleep in control and obstructive rats.

Hourly values of wake (W), slow wave sleep (SWS), and paradoxical sleep (PS) are shown. Controls (n = 9) - open circles, Obstructive (n = 10) rats -filled circles. Black horizontal bars represent the light-off (active) period on a 12∶12-h cycle lights on at 09∶00. Upper airway obstruction (UAO) group had significantly more wake and less SWS and PS than controls during light period. During dark period obstructive group had significantly less PS than controls. #Indicates statistically significant (p<0.01) difference between the groups, ANOVA-2. Values are mean±SEM.

Figure 2. Spectral ratios of EEG activity define a 2-dimensional state space with distinct clusters.

(A) control and (B) UAO; spectral ratios of EEG activity define a 2D state space with distinct clusters. Each plot shows 24 hours of EEG activity, and each point represents 1 second of EEG activity. (C) and (D) show 2D state space after application of a Hanning window (20 seconds) of one control and one UAO animal, respectively. Panels A–D: Blue–Wake; Green–slow wave sleep (SWS); Black–paradoxical sleep (PS). Panels E and F: light SWS (LSWS)–green; deep SWS (DSWS)–red.

Figure 3. Point densities averaged across all control (A) and UAO (B) animals.

Warm colors indicate regions where the average density is high and cool colors indicate low density. (C), (D)–average state space densities for control and UAO rats projected into ratio 1. This projection yields two peaks; left peak is associated with wake and the right peak is associated with SWS sleep.

Figure 4. Difference plot showing average density pattern of UAO rats subtracted from control rats (A).

B–projection of this graph into ratio 1. The color scale highlights differences between the groups: warm colors indicate regions where the average density is higher in controls and cool colors indicate higher density in UAO group.

Figure 5. UAO group has faster movements in all regions of the 2D state space plot compared to controls.

Plots represent four different control (A) and UAO (B) animals. The velocity of spectral change was calculated as the distance between two consecutive data points and so temporal resolution was second-by-second. Values represent average velocities originating at that site. Warm colors show faster velocity (transition regions) indicating unstable states, and cool colors low velocities at stable states. UAO animals have faster movements.

Figure 6. Box plot shows the median velocity of control and UAO groups following calculation of the sum of the velocities in each animal.

*p<0.05.

Figure 7. Effect of ritanserin on microarousal during the first 6 hours of lights on.

UAO group has significantly more microarousals. Treatment with ritanserin reduces the number of microarousals. **p<0.001– comparing baseline vehicle to ritanserin. #p<0.05– comparing controls to UAO group.

Figure 8. Effect of ritanserin on spontaneous sleep during first six hours of light-on period.

On day one animals were given vehicle (4% methyl alcohol in saline) and on day two animals were treated with ritanserin (2 mg/kg) at lights on. W–wake, SWS–slow-wave sleep, PS–paradoxical sleep. **p<0.001– comparing vehicle to ritanserin study; + p<0.001 comparing vehicle study of control rats to obstructive rats. Values are mean (SEM).

Figure 9. Mean difference averaged densities during first 6 hours lights on in control (A) and UAO (B) groups.

Differences were calculated by subtraction of baseline vehicle study from the ritanserin (2 mg/kg) study. Warm colors indicate regions where the average density of the ritanserin study is higher relative to vehicle study and cool colors indicate higher density of vehicle study. In both groups ritanserin increased delta rich deep SWS and decreased light SWS. Projection of ratio 1 (C) control and (D) UAO group.

Figure 10. Effects of ritanserin on differences in number of trajectories.

Difference in number of trajectories was calculated by subtraction of ritanserin results from baseline vehicle measurements. In both groups the number of trajectories significantly decreases between wake and light SWS (LSWS) and vice versa after ritanserin (Rit) administration. Ritanserin significantly increased the number of trajectories of LSWS to deep SWS (DSWS) in UAO group. *p<0.05 comparing ritanserin results to baseline vehicle measurements in the same animals. #p<0.05 comparing change in trajectories between control and UAO groups.