A Chronic Sleep Fragmentation Model using Vibrating Orbital Rotor to Induce Cognitive Deficit and Anxiety-Like Behavior in Young Wild-Type Mice

Summary

Presented here is a protocol for chronic sleep fragmentation (CSF) model achieved by an electrically controlled orbital rotor, which could induce confirmed cognitive deficit and anxiety-like behavior in young wild-type mice. This model can be applied to explore the pathogenesis of chronic sleep disturbance and related disorders.

Abstract

Sleep disturbance is generally common in populations as a chronic disease or a complained event. Chronic sleep disturbance is proposed to be closely linked to the pathogenesis of diseases, especially neurodegenerative diseases. We recently found that 2 months of sleep fragmentation initiated Alzheimer’s disease (AD)-like behavioral and pathological changes in young wild-type mice. Herein, we present a standardized protocol to achieve chronic sleep fragmentation (CSF). Briefly, CSF was induced by an orbital rotor vibrating at 110 rpm and operating with a repetitive cycle of 10 s-on, 110 s-off, during light-ON phase (8:00 AM–8:00 PM) continuously for up to 2 months. Impairments of spatial learning and memory, anxiety-like but not depression-like behavior in mice as consequences of CSF modeling, were evaluated with Morris water maze (MWM), Novel object recognition (NOR), Open field test (OFT) and Forced swimming test (FST). In comparison with other sleep manipulations, this protocol minimizes the handling labors and maximizes the modeling efficiency. It produces stable phenotypes in young wild-type mice and can be potentially generated for a variety of research purposes.

Introduction

Sleep disturbance is increasingly common both in patients with sleep-disturbing conditions and healthy people with sleep-disturbing events. It has been observed that patients with neurodegenerative diseases, chronic pain, emotional stress, respiratory system diseases, urinary system diseases, etc., usually complain about unpleasant sleep experiences^1,2,3,4,5. Obstructive sleep apnea (OSA), periodic limb movements in sleep (PLMS), sleep maintenance insomnia among other sleep disorders are the most common causes, which induce sleep fragmentation^6,7. In developed countries, OSA has over 5% to 9% prevalence in adult population and 2% in child population^8,9,10. Meanwhile, there is an increasing proportion of the healthy population experiencing sleep disturbance due to the overuse of smart phones, irregular sleep habits, annoying noises, and work duties, such as night shifts for caregivers. Sleep is acknowledged to be important for brain waste clearance^11,12, memory consolidation^13,14, metabolic balance^15,16, among many other physiological processes. Yet, it still remains largely unknown whether long-term sleep disturbance gives rise to irreversible pathogenesis alterations in healthy human beings, and whether it is the etiology or a contributing factor of developing central nervous system diseases, such as neurodegenerative diseases in a couple of years down the road. Our goal is to report an experimental model that generates stable and evident cognitive deficit and anxiety-like behavior in young wild-type mice after a 2-month sleep fragmentation treatment. This model would be applied for answering the scientific questions listed above.

Sleep disturbance is listed as a potential risk factor for developing Alzheimer’s disease (AD) or dementia. Kang et al. first found and described the exacerbation of AD pathology by 6 h acute sleep deprivation¹⁷. Thereafter, many other studies reported that sleep deprivation or fragmentation could aggravate pathogenesis in transgenic AD mice models^18,19,20. However, very few researchers have studied the consequence of sleep disturbance in young wild type mice; that is, whether sleep disturbance gives rise to AD-like behavior or pathological changes in young wild-type mice. In our recent publication, we reported that 2 months of sleep fragmentation induced evident spatial memory deficit and anxiety-like behavior, as well as increased intracellular Amyloid-β (Aβ) accumulation both in cortex and hippocampus in 2–3 month-old wild-type mice²¹. We also observed altered expression levels of endosome-autophagosome-lysosome pathway markers and microglia activation, which was similar to the pathological changes reported in APP/PS1 mice^21,22.

This presented sleep fragmentation (SF) protocol was validated by Sinton et al.²³ and modified by Li et al.²⁴. In brief, an orbital rotor vibrating at 110 rpm interrupts sleep for 10 s every 2 min during light-ON phase (8:00 AM–8:00 PM). Sleep structure alteration in this model was previously characterized with electrophysiological sleep recordings and reported by Li et al.²⁴, indicating a significant increase in the wake time and decrease in rapid eye movement (REM) sleep during the light-ON phase, with the total sleep and wake times (in 24 hour) unaffected after more than 4 weeks’ modeling²⁴. Currently, total sleep or partial sleep deprivation are the most commonly used sleep manipulation models. Total sleep deprivation is usually performed by sustained gentle handling or exposing the animal to novel objects, alternatively by continuously rotating a bar or a running treadmill^{25,26,27,28,29}. Due to ethical reasons, total sleep deprivation is usually shorter than 24 h. The most commonly applied partial sleep deprivation model is the water platform method, which primarily ablating REM sleep^30,31,32. Other approaches using either a treadmill or a bar that sweeps along the bottom of the cage, could induce sleep fragmentation when set on at fixed intervals^{33,34,35,36,37,38}. It is noteworthy that SF interrupts sleep and intermittently causes arousals across all sleep stages²⁴. One of the prominent advantages of this CSF model applying orbital rotor is that it can be performed continuously for months automatically controlled by machines, which avoids frequent processing labor daily except for regular monitoring. Furthermore, the apparatus would allow to simultaneously model multiple cages of mice under uniformed interventions. During entire modeling sessions, mice are housed in their home cages with usual bedding and nesting materials, while some other methods require exposure to diversified environments and inevitable stress.

Sleep fragmentation was previously characterized by the sleep manipulation method, which mimics frequent arousals during the sleep phase and substantial sleep rebound during the wake phase. In some literatures, CSF was regarded as the animal model for OSA^39,40. In this study, the rationale of the chosen frequency of arousal to be 30 times per hour is based on the observation of arousal indices in patients with moderate-to-severe sleep apnea. It was observed that 4 weeks’ sleep fragmentation significantly increased hypercapnic arousal latency and the tactile arousal threshold, which could at least last 2 weeks after recovery²⁴. This phenotype was explained by revealing c-fos activation reduction in noradrenergic, orexinergic, histaminergic, and cholinergic wake-active neurons in response to hypercapnia, as well as reduced catecholaminergic and orexinergic projections into the cingulate cortex²⁴. However, it is necessary to note that the most important feature in OSA is hypoxia caused by airway obstruction, which results in sleep disruption^41,42. Sleep disturbance and repetitive hypoxia reciprocally interact with each other in OSA pathogenesis. Therefore, sleep fragmentation alone might not be able to fully demonstrate all key features of OSA in mice.

Herein, we present a standardized protocol to model chronic sleep fragmentation in young wild-type mice. Cognitive deficit and anxiety-like as well as depression-like behaviors after CSF treatment were evaluated by Morris water maze, Novel object recognition, Open field test, and Forced swimming test. It is important to note that this model should be taken as a whole that generates phenotypes of dysregulated sleep pattern, cognitive deficit, and anxiety-like behavior. The current model could potentially be applied, but not be limited, to the following purposes: 1) Further investigating the functional or molecular pathogenesis mechanisms induced by chronic sleep disturbance in young mice without genetic predisposition, 2) Identifying the direct pathway leading to neurodegeneration initiated by sleep disturbance, 3) Exploring the therapeutics for improving phenotypes induced by chronic sleep disturbance, 4) Studying the intrinsic protective/compensatory mechanisms in wild-type mice upon chronic sleep disturbance, 5) To be applied for studying sleep-wake regulation and state-transition mechanisms.

Protocol

This protocol was approved by the Institutional Animal Care and Use Committee of Tongji Hospital, Tongji Medical College, Huazhong University of Science and Technology.

1. Mice screening and preparation for the experiment

1. Select wild-type adult (8–10 weeks old) male mice with weight of 20–28 g for the whole experiment.
   NOTE: Wild-type C57BL/6 mice are obtained from the Hubei Research Center for Laboratory Animals, Hubei, China.
2. Randomly assign all mice to the CSF and the control group. House 3–5 mice in each cage to avoid social isolation stress. The number of mice housed in the control cages is matched with that housed in the paired CSF cages.
   NOTE: Mice in the same group cages are pooled to perform follow-up behavioral experiments.
3. Locate the control cages in the same room with the CSF cages, to keep the surrounding environment and labor effects identical.
4. Number and mark the mice in each group on their ears using an ear tag for monitoring purposes.
5. Maintain ambient temperature and humidity between 21–23 °C and 35%–60 %.
6. Maintain the ambient environment in 12-hour light-dark cycle (8:00 AM–8:00 PM light-ON, 8:00 PM–8:00 AM light-OFF), to avoid biased effect on normal sleep rhythm in mice.
7. Minimize the noise and interference while the researcher is present in the modeling room.
8. Provide mice with sufficient food and water. Use long nozzles with ball valve tips on water bottles, to prevent water leakage upon the platform movements. Fasten the water bottle on top of the cage with a spring to avoid the dislocation of the bottle during rotor running.

2. Preparation and setting of the orbital rotor

1. Prepare an electrically controlled orbital rotor with enlarged platform (67 cm x 110 cm), on which 10 cages can be placed at most.
2. Set the orbital rotor on during light-ON phase (8:00 AM–8:00 PM) controlled by a program timer, which is the time when mice exhibit the majority of their daily sleep.
3. Set the orbital rotor with a speed of 110 rpm and a repetitive cycle of 10 s-on, 110 s-off controlled with a solid-state timer.
   NOTE: The load capacity of the platform is 50 kg. The fixed amplitude of the rotor horizon vibrating is 2.5 cm.
4. Fasten the CSF cages on top of the rotor platform by thick springs to prevent dislocation of cages upon platform rotations.

3. Chronic sleep fragmentation modeling and monitoring

1. Place cages of the CSF and the control mice into the modeling room for one week prior to experiments, to let mice adapt to the ambient environment.
2. At the beginning of the modeling, ensure that all mice have free access to food and water during orbital rotations.
3. At the beginning of the modeling, observe at least for 1 h to ensure the orbital rotor operating in gear.
4. During the period of modeling, check that the orbital rotor is operating properly and mice conditions every 2 days to ensure mice have enough food and water. Change beddings of cages weekly.
5. During the period of modeling, weigh the mice weekly at 8:00 AM when changing the bedding. Remove the mice with significant weight loss from the modeling, and also from the experimental groups.
   NOTE: Significant weight loss is defined as weighing less than 20 g lasting for 2 weeks.
6. During the entire modeling sessions, remove the aggressor, if any, from the cage and, also from the experimental groups.
7. After the termination of modeling, continue to maintain and feed the mice in the original room.

4. Morris water maze (MWM) test

1. Preparation for the test
   1. Prepare the apparatus of a circular tank filled with warm water (20–23 °C).
   2. Suspend four signs with different shapes and colors on the curtain surrounding the tank in four quadrant directions as the distant vision reference. Make the water to appear opaque by the addition of powdered milk.
   3. Locate a platform in the middle of the southwest quadrant.
2. The training test
   1. Subject mice to four consecutive trials between 8:00 AM and 12:00 AM each day over a 5-day training period.
   2. Release each mouse into the water facing the sidewall at one of four quadrants in four trials. In each trial, allow the mouse to swim for 60 s to find the platform. If the mouse is unable to arrive at the platform within 60 s, guide it to the platform and to remain there for 15 s.
   3. Use a video tracking system to automatically record the escape latency of mice to find the hidden platform.
3. The probe test
   1. Conduct the probe test on the sixth day after 5 training days.
   2. Remove the platform. Release each mouse from the northeast quadrant and allow it to swim for 60 s
   3. Use a video tracking system to automatically record the track data of mice.

5. Novel object recognition (NOR) test

1. The familiar phase
   1. Place mice in a tank (length 30 cm, width 28 cm, height 35 cm) in sequence, which contains two copies of objects (A1 and A2). Allow the mice to explore freely (10 min per trial).
   2. Use a video tracking system to automatically record the track data of mice.
2. The test phase
   1. Conduct the test trial after a 1 h delay of the familiar phase. Replace one of the original objects by a novel object (“novel”) in the tank keeping the other one unchanged. Return the mice to the tank and allow it to explore for 5 min per trial.
   2. Use a video tracking system to automatically record the time spent in exploration of each object by each mouse.
      NOTE: The exploration of the object is determined by licking, sniffing, chewing, or moving vibrissae while orienting the nose toward and less than 1 cm from the object. The Discrimination Index (DI) is calculated with the equation (TN − TF)/(TN + TF), where TN = time spent exploring the “novel” object and TF = time spent exploring the “familiar” object.

6. Open field test (OFT)

1. Prepare the apparatus of a tank (30 cm x 28 cm x 35 cm).
2. During the test, place each mouse into the center of the tank and allow it to explore freely for 5 min. Clean the tank with 75% ethanol after each trial to avoid the leftover effects of the previous mouse.
3. Use a video tracking system to automatically record the track data of mice.

7. Forced swimming test (FST)

1. Prepare the apparatus of an open cylindrical vessel, which contains water (20–23 °C) that is 15 cm deep.
2. During the test, place each mouse into the cylinder and allow it to remain there for 6 min.
3. Use a video track system to automatically record the immobility time during the last 4 min of the test by each mouse.
   NOTE: The mouse is determined to be immobile when it stops struggling and floats in the water, making only movements which are necessary to keep its head above water.

8. Data Analysis

1. Analyze data using statistical analysis software (e.g., GraphPad Prism 6.0).
2. Express all data as the mean ± SEM.
3. Compare the escape latency in MWM test between two groups using two-way ANOVA with repeated measures followed by Bonferroni posttests. Other comparisons between the CSF and the control groups are determined by unpaired t tests.
4. Consider differences significant if P < 0.05 in all tests.

Results

All the representative results and figures were reproduced from our recent publication²¹. The reuse of the figures was permitted by the original journal.

The entire experimental design is illustrated in the order of time, which indicates the timing of CSF modeling, behavioral tests of MWM, NOR, OFT, and FST (Figure 1A). We obtained weights of mice every week from the CSF and the control groups, to monitor their general conditions during the...

Discussion

Critical steps in the current protocol include setting up sleep fragmentation machines with the optimized parameters according to the study purpose and maintaining the mice in comfortable and quiet living environment throughout the entire modeling sessions. It is also crucial to decide the proper timing to interrupt or stop sleep fragmentation and arrange behavioral tests for those mice. Like other sleep manipulation models, it is important to perform the protocol in a dedicated room with controlled light cycles and void...

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Any-maze behavior tracking system	Stoelting,Inc,USA	-	A video-tracking system which was used to record the behavior track of mice.
C57BL/6J mice	Hubei Research Center for Laboratory Animals, Hubei, China.	-	healthy male C57BL/6J mice aged 10-12 weeks were purchased from Hubei Research Center for Laboratory Animals
Graphpad Prism 6.0 Software	Graphpad Software,Inc.USA	-	Graphpad Prism 6.0 software was used to draw statistical graphs.
Morris water maze system	Shanghai XinRuan Information Technology Co.,Ltd,China	XR-XM101	The system was used to perform Morris water maze test
Orbial rotor	Shanghai ShiPing Laboratory Equipment Co.,Ltd,China	SPH-331	The orbital rotor was used to establish the chronic sleep fragmentation model
Solid state timer	OMRON Corporation, Kyoto, Japan	H3CR-F8-300	The solid state time was used to control the frequency and time of the rotor running
Wooden Lusterless Tank	-	-	length 30 cm, width 28 cm, height 35 cm The tank was used to perform open field test and novel object recognition test