Mechanisms of Ageing and Development 146 (2015) 12–22 Contents lists available at ScienceDirect Mechanisms of Ageing and Development jou rn al hom epage : www.elsevier.com/locat e/mechagedev Aging induced cortical drive alterations during sleep in rats Jelena Ciric a , Katarina Lazic a , Jelena Petrovic a , Aleksandar Kalauzi b , Jasna Saponjic a,∗ a University of Belgrade, Department of Neurobiology, Institute for Biological Research – Sinisa Stankovic, 11 060 Belgrade, Serbia b University of Belgrade, Department for Life Sciences, Institute for Multidisciplinary Research, 11 030 Belgrade, Serbia a r t i c l e i n f o Article history: Received 1 October 2014 Received in revised form 26 February 2015 Accepted 3 March 2015 Available online 12 March 2015 Keywords: Corticomuscular coherence Aging Sleep Cortical drive EEG EMG a b s t r a c t We followed the impact of healthy aging on cortical drive during sleep in rats by using the corticomuscular coherence (CMC). We employed the chronic electrodes implantation for sleep recording in adult, male Wistar rats, and followed the aging impact during sleep from 3 to 5.5 months age. We have analyzed the sleep/wake states architecture, and the sleep/wake state related EEG microstructure and CMCs. We evidenced the topographically distinct impact of aging on sleep/wake states architecture within the sensorimotor (SMCx) vs. motor cortex (MCx) from 4.5 to 5.5 months age. Healthy aging consistently altered only the SMCx sleep/wake states architecture, and increased the delta and beta CMCs through both cortical drives during Wake, but only through the MCx drive during REM. According to the delta and beta CMCs values, aging impact through the SMCx drive was opposite, but it was convergent through the MCx drive during Wake vs. REM, and there was a dual and inverse mode for the motor control during REM. © 2015 Elsevier Ireland Ltd. All rights reserved. 1. Introduction Decline of the motor functions with aging is characterized by increased movement variability, reduced coordination abilities, and slowing of movement speed (Krampe, 2002; Seidler et al., 2010). The musculo-skeletal alterations, changes of peripheral and central nerve conduction, decreased levels of neurotransmitters, and loss of gray and white matter have been accounted for the impaired motor functions in elderly (Kamp et al., 2013; Krampe, 2002). Although there is a lot of evidence for the slowing of brain and muscle dynamics during healthy aging, it is still unknown how the aging induced motor control decline is mediated by central processes. The corticomuscular coherence (CMC) is an established method for detecting early markers of healthy aging (Kamp et al., 2013), a neurophysiological marker of functional coupling between primary motor cortex and peripheral muscles, a method for quantifying the functional coupling between the motor cortex and contralat- eral peripheral muscles in frequency domain (Krause et al., 2014; Kristeva et al., 2007; Salenius and Hari, 2003), and the measure of ∗ Corresponding author at: University of Belgrade, Department of Neurobiology, Institute for Biological Research – Sinisa Stankovic, Despot Stefan Blvd. 142, 11060 Belgrade, Serbia. Tel.: +381 11 2078426; fax: +381 11 2761433. E-mail addresses: [email protected], [email protected] (J. Saponjic). pyramidal system integrity (Kristeva et al., 2007; Mima and Hallett, 1999). CMC plays a crucial role for sensorimotor integration, rep- resenting a key mechanism for appropriate motor control (Kamp et al., 2013; Kristeva et al., 2007). While CMC is prominent in the beta frequency band during weak to medium isometric muscle con- traction in healthy humans (Kristeva et al., 2007), the decreased beta band CMC has been related to reduced high frequency corti- cally generated drives to contralateral muscles, affecting ongoing motor control mechanisms, and contributing to the Parkinson dis- ease’s motor symptoms such as bradykinesia and rigidity (Krause et al., 2014; Salenius et al., 2002). Conversely, the increased beta band CMC, as a measure of increased cortical locomotor drive, is evidenced during sleep in patients with rapid eye movement (REM) behavioral disorder (Jung et al., 2012). It is well documented that the sleep/wake states architecture and corresponding electroencephalographic changes accompany the process of healthy aging in both humans and animals (Mendelson and Bergmann 1999a,b, 2000; Moyanova et al., 2002), but age-related motor control during sleep is less understood. Recent study in a rat model of the Parkinson disease’s cholinergic neuropathology evidenced two distinct REM states with regard to the total EMG power, the topographically distinct EEG microstruc- tures, and the SMCx and MCx locomotor drives to the dorsal nuchal musculature (Petrovic et al., 2014). In this rat model, the cortico- muscular control was altered dominantly through the SMCx drive, during both REM states, but more severely during healthy REM http://dx.doi.org/10.1016/j.mad.2015.03.002 0047-6374/© 2015 Elsevier Ireland Ltd. All rights reserved.
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Mechanisms of Ageing and Development 146 (2015) 12–22
Contents lists available at ScienceDirect
Mechanisms of Ageing and Development
jou rn al hom epage : www.elsev ier .com/ locat e/mechagedev
ging induced cortical drive alterations during sleep in rats
elena Ciric a, Katarina Lazic a, Jelena Petrovic a, Aleksandar Kalauzi b, Jasna Saponjic a,∗
University of Belgrade, Department of Neurobiology, Institute for Biological Research – Sinisa Stankovic, 11 060 Belgrade, SerbiaUniversity of Belgrade, Department for Life Sciences, Institute for Multidisciplinary Research, 11 030 Belgrade, Serbia
r t i c l e i n f o
rticle history:eceived 1 October 2014eceived in revised form 26 February 2015ccepted 3 March 2015vailable online 12 March 2015
eywords:orticomuscular coherence
a b s t r a c t
We followed the impact of healthy aging on cortical drive during sleep in rats by using the corticomuscularcoherence (CMC).
We employed the chronic electrodes implantation for sleep recording in adult, male Wistar rats, andfollowed the aging impact during sleep from 3 to 5.5 months age. We have analyzed the sleep/wake statesarchitecture, and the sleep/wake state related EEG microstructure and CMCs.
We evidenced the topographically distinct impact of aging on sleep/wake states architecture withinthe sensorimotor (SMCx) vs. motor cortex (MCx) from 4.5 to 5.5 months age. Healthy aging consistently
gingleeportical driveEGMG
altered only the SMCx sleep/wake states architecture, and increased the delta and beta CMCs throughboth cortical drives during Wake, but only through the MCx drive during REM. According to the delta andbeta CMCs values, aging impact through the SMCx drive was opposite, but it was convergent through theMCx drive during Wake vs. REM, and there was a dual and inverse mode for the motor control duringREM.
© 2015 Elsevier Ireland Ltd. All rights reserved.
. Introduction
Decline of the motor functions with aging is characterized byncreased movement variability, reduced coordination abilities,nd slowing of movement speed (Krampe, 2002; Seidler et al.,010). The musculo-skeletal alterations, changes of peripheral andentral nerve conduction, decreased levels of neurotransmitters,nd loss of gray and white matter have been accounted for thempaired motor functions in elderly (Kamp et al., 2013; Krampe,002). Although there is a lot of evidence for the slowing of brainnd muscle dynamics during healthy aging, it is still unknown howhe aging induced motor control decline is mediated by centralrocesses.
The corticomuscular coherence (CMC) is an established methodor detecting early markers of healthy aging (Kamp et al., 2013), aeurophysiological marker of functional coupling between primaryotor cortex and peripheral muscles, a method for quantifying
he functional coupling between the motor cortex and contralat-ral peripheral muscles in frequency domain (Krause et al., 2014;risteva et al., 2007; Salenius and Hari, 2003), and the measure of
∗ Corresponding author at: University of Belgrade, Department of Neurobiology,nstitute for Biological Research – Sinisa Stankovic, Despot Stefan Blvd. 142, 11060elgrade, Serbia. Tel.: +381 11 2078426; fax: +381 11 2761433.
E-mail addresses: [email protected], [email protected]. Saponjic).
ttp://dx.doi.org/10.1016/j.mad.2015.03.002047-6374/© 2015 Elsevier Ireland Ltd. All rights reserved.
pyramidal system integrity (Kristeva et al., 2007; Mima and Hallett,1999). CMC plays a crucial role for sensorimotor integration, rep-resenting a key mechanism for appropriate motor control (Kampet al., 2013; Kristeva et al., 2007). While CMC is prominent in thebeta frequency band during weak to medium isometric muscle con-traction in healthy humans (Kristeva et al., 2007), the decreasedbeta band CMC has been related to reduced high frequency corti-cally generated drives to contralateral muscles, affecting ongoingmotor control mechanisms, and contributing to the Parkinson dis-ease’s motor symptoms such as bradykinesia and rigidity (Krauseet al., 2014; Salenius et al., 2002). Conversely, the increased betaband CMC, as a measure of increased cortical locomotor drive, isevidenced during sleep in patients with rapid eye movement (REM)behavioral disorder (Jung et al., 2012).
It is well documented that the sleep/wake states architectureand corresponding electroencephalographic changes accompanythe process of healthy aging in both humans and animals(Mendelson and Bergmann 1999a,b, 2000; Moyanova et al., 2002),but age-related motor control during sleep is less understood.
Recent study in a rat model of the Parkinson disease’s cholinergicneuropathology evidenced two distinct REM states with regard tothe total EMG power, the topographically distinct EEG microstruc-tures, and the SMCx and MCx locomotor drives to the dorsal nuchal
musculature (Petrovic et al., 2014). In this rat model, the cortico-muscular control was altered dominantly through the SMCx drive,during both REM states, but more severely during healthy REM
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J. Ciric et al. / Mechanisms of Agei
REM with atonia), and it was expressed as a decreased beta bandMC (Petrovic et al., 2014). In addition, REM behavioral disorderery frequently go unnoticed in patients with neurodegenerativeiseases, and as a symptom precede the onset of motor and cog-itive disturbances by years or even decades (Boeve et al., 2007;imic et al., 2009; Whitwell et al., 2007). Therefore, the complemen-arity of locomotor drive alteration in humans and in the animal
odel of Parkinson disease’s cholinergic neuropathology (Petrovict al., 2014) highlighted the potential relevance of corticomuscularoherence use to investigate the motor control during sleep, partic-larly in healthy aging, and aging impaired by neurodegenerativeiseases (different dementias, Parkinson’s disease, Alzheimer’s dis-ase, etc.).
On the base of all above mentioned experimental evidence ourim, in this study, was to follow the impact of healthy aging onortical drive to dorsal nuchal muscle during sleep in rats by usinghe CMCs, as a possible measure for the onset of healthy aging-elated locomotor drive alterations, and for its further possible uses a valuable tool to detect the REM behavioral disorder onset.
. Material and methods
We performed the experiments in 42 adult, male Wistar controlats, chronically instrumented for sleep recording at the age of 2.5
onths. Prior to surgery and consistently throughout the experi-ental protocol, the animals were maintained on a 12-h light-dark
ycle, and were housed at 25 ◦C with free access to food and water.All animal procedures were in compliance with the EEC
irective (86/609/EEC) on the protection of animals used for exper-mental and other scientific purposes, and were approved by thethical Committee for the Use of Laboratory Animals of the Instituteor Biological Research “Sinisa Stankovic”, University of BelgradeApproval No. 2-21/10).
.1. Surgical procedure
The surgical procedures employed for the chronic electrodemplantation for sleep recording have been described previ-usly (Petrovic et al., 2014, 2013a,b; Saponjic et al., 2013, 2007),nd are outlined below. We implanted in 2.5 months old ratswo pairs of the epidural parietal stainless-steel screw elec-rodes for EEG cortical activity recording from the motor (MCx;/P: +1.0 mm from bregma; R/L: 2.0 mm from sagittal suture),nd the sensorimotor (SMCx; A/P: −3.0 mm from bregma; R/L:.0 mm from sagittal suture) cortex (Paxinos and Watson, 2005)nder ketamine/diazepam anesthesia (Zoletil 50, VIRBAC, France,0 mg/kg; i.p.). Bilateral electromyogram (EMG) wire electrodesere implanted into the dorsal nuchal musculature to assess
keletal muscle activity, and a stainless-steel screw electrode wasmplanted in the nasal bone. All the electrode leads were soldered to
miniature connector plug (39F1401, Newark Electronics, Schaum-urg, IL, USA), and the assembly was fixed to the screw electrodesnd skull using acrylic dental cement (Biocryl-RN, Galenika a.d.eograd, Serbia).
.2. Recording procedure
At the end of surgical procedure, the scalp wounds wereutured and the rats were allowed to recover 13 days beforeheir adaptation to the recording cable and plexiglass chamber30 cm × 30 cm × 30 cm) for one day. The EEG and EMG activitiesere carried from the connector plug on the rat head by cable,
assed through a sealed port on the recording box, and differen-ially recorded. Differential mode consisted of 6 inputs (left MCx,ight MCx, left SMCx, right SMCx, left EMG, right EMG), each with
(+) on the left and a (−) on the right side, and all with the same
Development 146 (2015) 12–22 13
ground (a screw electrode implanted in the nasal bone). The activ-ities were displayed on a computer monitor, and stored on diskfor further off-line analysis. After conventional amplification andfiltering (0.3–100 Hz band pass; A-M System Inc., Model 3600, Carl-borg, WA, USA), the analog data were digitized (sampling frequency256/s), and recorded for 6 h, during the normal inactive circadianphase for rats (from 9 a.m. to 3 p.m.), using DataWave SciWorksExperimenter Verson 8.0 (Datawave Technologies, Longmont, CO,USA). The sleep recording sessions were done in four experimentalgroups of different ages: 3 months old rats (n = 15); 3.5 months oldrats (n = 10); 4.5 months old rats (n = 10); and 5.5 months old rats(n = 7).
2.3. Data analysis
Analysis of the recorded signals was conducted using softwarewe developed using MATLAB 6.5. We applied Fourier analysis to thesignals acquired throughout each 6 h recording (2160 10 s Fourierepochs), and each 10 s epoch was differentiated as Wake, NREMor REM state for further analysis of the Wake, NREM and REMrelated EEG relative amplitudes of all the conventional frequencybands (ı = 0.3–4 Hz; � = 4.1–8 Hz; � = 10.1–15 Hz; ̌ = 15.1–30 Hz;� = 30.1–50 Hz).
First, we extracted all the 10 s Wake epochs from each 6 hrecording, based on the product of EEG sigma and theta frequencypower on the y-axis, and the total EMG power on the x-axis. Fur-ther, the differentiation of NREM and REM 10 s epochs was doneusing the total EMG power on the y-axis, and the EEG delta/thetapower ratio on the x-axis (Petrovic et al., 2013a). Differentiation ofthe Wake epochs from sleep epochs, and further differentiation ofthe NREM and REM epochs was achieved using the two clustersK means algorithm. We improved these differentiation results byusing the logarithmic values of quantities on both axes (Petrovicet al., 2013a,b; Saponjic et al., 2013).
To analyze the sleep/wake state related EEG amplitude changeswe calculated group probability density distributions of all theWake, NREM and REM conventional EEG frequency bands rela-tive amplitudes over 6 h, using the Probability Density Estimate(PDE) routine supplied with MATLAB 6.5. In order to eliminate anyinfluence from absolute signal amplitude variations on the record-ings, we computed the relative Fourier amplitudes (Petrovic et al.,2013a,b; Saponjic et al., 2013):
(RA)b =˙b
Amp
˙tot
Amp, b = {ı, �, �, ˇ, �}
For each sleep/wake state and each frequency band PDE analysiswas performed on the ensembles of relative amplitudes by poolingmeasured values (RA)b from all animals belonging to a specific agegroup (3 months old, 3.5 months old, 4.5 months old, 5.5 monthsold).
Additionally, we have analyzed the Wake, NREM and REM cor-ticomuscular coherences (CMCs) separately for each age group andfor all the conventional EEG frequency bands, using the SMCx orMCx EEG, and the EMG of the dorsal nuchal muscles (Petrovic et al.,2014). CMC values were calculated using the “cohere” routine ofthe MATLAB 6.5 Signal Processing Toolbox. It actually computes themagnitude squared coherence between signals x(EEG) and y(EMG)as
|Pxy(f )|2
Cxy(f ) =Pxx(f )Pyy(f )
where Pxy(f) stands for the cross spectrum of x and y, while Pxx(f)and Pyy(f) denote the power spectra of the two signals.
14 J. Ciric et al. / Mechanisms of Ageing and Development 146 (2015) 12–22
Fig. 1. Topography of the sleep/wake states architecture during healthy aging (3–5.5months old rats): mean duration of Wake/NREM/REM/6 h + SE. SMCx – sensorimotorcA
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ortex; MCx – motor cortex; asterisk–p ≤ 0.02.ging induced changes of the sleep/wake states architecture only within SMCx.
All Pxy, Pxx, and Pyy values were determined for every 10 s of each h recording, and for each frequency within the overall 0.3–50 Hzange, with 0.1 Hz resolution.
Namely, previously identified Wake/NREM/REM EEG and EMG0 s epochs were concatenated and pooled within each age group.hen, the CMC spectra were calculated for every 60 min of Wakend NREM, and for every 30 min of REM, using 10 s FFT epochs forhe MATLAB “cohere” routine, resulting in 0.1 Hz frequency reso-ution. Then, the CMC values within each conventional frequencyand (ı, �, �, ˇ, �) were averaged for each spectrum, and finallyheir means were calculated from the collection of all available CMCpectra, for each state and each age group.
For the statistical analysis of PDE/6 h and CMC/6 h we calculatedhe relative amplitude means for Wake and NREM per each 60 min,nd for REM per each 30 min (Petrovic et al., 2013a,b; Saponjic et al.,013).
We employed the Kruskal–Wallis ANOVA and Mann–Whitney two-tailed tests for the statistical analysis of all group means over
h: the group mean durations of Wake, NREM, and REM; the groupean number and group mean duration of Wake, NREM and REM
pisodes; the group means of Wake, NREM and REM EEG relativemplitudes for all frequency bands; and the group CMC means of
ake, NREM and REM. In all cases the differences were consideredtatistically significant for p ≤ 0.05.
. Results
.1. Topography of the sleep/wake states architecture duringealthy aging
We have evidenced the distinct sleep/wake states architectureithin the sensorimotor (SMCx) vs. motor cortex (MCx). Aging
nduced changes only the SMCx sleep/wake states architecture Tab
le
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rap
hse
nso
rim
ot
SMC
x
3
Mon
ths
3.5
Mon
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J. Ciric et al. / Mechanisms of Ageing and Development 146 (2015) 12–22 15
Fig. 2. Individual examples of the distinct SMCx vs. MCx hypnograms during 6 h of sleep (recorded from 9 a.m. to 3 p.m.) at the onset (3 months old rats), and at the end (5.5m .I Cx an2
(gfF
dtR(pduaaNTaTdlot
td
onths old rats) of aging follow-up period with their corresponding 90 min insertsn this rat the Wake/NREM/REM numbers of episodes/6 h were 188/188/106 for SM22/297/260 for MCx, at the end of aging follow-up period.
Fig. 1, Table 1). The individual examples of SMCx vs. MCx hypno-rams/6 h, at the onset and at the end of aging follow-up period,rom one rat representing the group data (Table 1), are depicted inig. 2.
Beside all consistently expressed topographic (SMCx vs. MCx)ifferences, aging altered only the SMCx sleep/wake states archi-ecture from 4.5 to 5.5 months (Fig. 1, Table 1): it increasedEM duration (X2 = 18.34; p = 10−4), and decreased NREM durationX2 = 10.37; p = 0.02), with no change of Wake duration (X2 = 3.04;
= 0.39). Conversely, there were no changes of Wake/NREM/REMurations within the MCx throughout the overall aging follow-p period. Since aging altered only the SMCx sleep/wake statesrchitecture, the typical examples of age-related SMCx hypnogramsnd their corresponding group age-related distributions of Wake,REM, and REM episodes durations are depicted (Fig. 3A and B,able 1). Although aging increased the mean number of NREMnd REM episodes of shorter duration within the SMCx (Fig. 3B,able 1; X2 ≥ 9.21; p ≤ 0.03), the overall mean duration of NREMecreased (Fig. 1; Table 1) due to much higher decrement of the
onger NREM episodes (Fig. 3B), whereas the overall mean durationf REM increased (Fig. 1; Table 1) due to much higher increment ofhe shorter REM episodes (Fig. 3B).
Although aging did not impact the MCx sleep/wake states archi-
ecture (Fig. 1; X2 ≥ 3.19; p ≥ 0.27), it decreased REM episodesurations at 5.5. months (Table 1).d 294/313/146 for MCx, at the onset, while there were 202/202/183 for SMCx and
3.2. Topography of the sleep/wake states related EEGmicrostructure during healthy aging
Healthy aging did not change the Wake EEG microstructure ofSMCx (X2 ≥ 1.41; p ≥ 0.09), and MCx (X2 ≥ 3.23; p ≥ 0.08), except theSMCx Wake sigma amplitude increased at the end of our follow-up period – at 5.5 months age (X2 = 10.41; p = 0.01). Since thisaugmented sigma amplitude during Wake was expressed only tran-siently, and only within the SMCx, at the end of follow-up periodin our study, we did not present the Wake EEG microstructuredata.
Also, healthy aging did not change the NREM EEG microstruc-ture within the SMCx (X2 ≥ 3.72; p ≥ 0.08; Fig. 4), but it transientlyaugmented delta amplitude (X2 = 13.01; p = 0.005), and attenuatedsigma and beta amplitudes (X2 ≥ 8.72; p ≤ 0.03) within the MCx(Fig. 4) from 3.5 to 4.5 months age, when they returned to controlvalues.
During REM (Fig. 5), aging only transiently augmented deltaamplitude (X2 = 28.07; p = 10−4), but attenuated theta, sigma andbeta amplitudes within the SMCx (X2 ≥ 12.88; p ≤ 0.005) of the4.5 months old rats (Fig. 5). Also, aging augmented REM thetaamplitude in the MCx (X2 = 14.35; p = 0.002) from 3 to 4.5 months,followed by attenuated sigma only at 4.5 months age (X2 = 15.37;
p = 0.002; Fig. 5). During REM, sigma amplitude increased in theMCx only at the end of aging follow up period (5.5 months old rats).
16 J. Ciric et al. / Mechanisms of Ageing and Development 146 (2015) 12–22
Fig. 3. Typical examples of the age-related SMCx hypnograms during 6 h of sleep with their corresponding 60 min inserts (A), and their corresponding group age-relatedd e distrtA long
3a
sumuwfca
i(bhasd
a
istributions of the Wake, NREM, and REM episodes durations (B). Missing lines in tho be identified as “not a number”.ging did not change the Wake episodes duration, but it significantly decreased the
.3. Topography of the sleep/wake states related cortical drivelterations during healthy aging
In our study we followed the specific topography of sleep/waketates related cortical muscle control by calculating the CMC val-es (as a measure of the propagated oscillation from the cortex touscle). The highest mean control (at the onset of aging follow-
p period) sensorimotor cortex-dorsal nuchal muscles CMC valuesere for Wake theta, NREM sigma, and REM theta and sigma
requency bands (Fig. 6), while the highest mean control motorortex-dorsal nuchal CMC values were for Wake theta, NREM delta,nd REM sigma (Fig. 7).
Whereas healthy aging through the MCx drive consistentlyncreased the delta and beta CMCs (Fig. 7) during Wake and REMX2 ≥ 16.68; p ≤ 0.001), the SMCx drive oppositely altered delta andeta CMCs during Wake vs. REM (Fig. 6). In addition, whereasealthy aging consistently decreased the sigma CMCs during NREMnd REM, through both cortical drives (X2 ≥ 15.76; p ≤ 0.001), theigma CMCs were more consistently increased through the SMCx
rive during Wake (Fig. 6, Fig. 7; X2 ≥ 20.37; p ≤ 0.00).Since the common onset of aging induced alteration of the Wakend REM delta and beta CMCs was at 3.5 months age, and lasted
ibutions depicts the zero group mean number of episodes that causes the logarithm
NREM and increased the short REM episodes.
up to 5.5 months, we additionally applied the linear correlationanalysis between the Wake/NREM/REM individual frequency EEGamplitudes and their corresponding CMCs to reveal the possiblefunctional coupling, but we only presented the highest statisticallysignificant results, or the highest r values (Fig. 8).
The most significant and negative correlation wasfor the REM delta CMCs augmentation in 4.5 monthsold rats and between MCx delta amplitude and deltaCMC (Fig. 8), although there was no an augmentation of theREM delta amplitude within the MCx (Fig. 5). During Wake theonly significant, but positive correlation was between SMCx deltaamplitude and delta CMC (Fig. 8).
At the end of aging follow-up period (5.5 months old rats) therewere only the significant positive linear correlations during REMbetween MCx beta amplitude and beta CMC, and during NREMbetween SMCx beta amplitude and beta CMC (Fig. 8).
According to the linear correlation analysis results (the sig-nificantly highest r values) aging increased delta CMCs throughthe SMCx delta drive during Wake, and through the MCx delta
drive during REM, while it increased beta CMCs through theSMCx drive during NREM and through the MCx drive duringREM.
J. Ciric et al. / Mechanisms of Ageing and Development 146 (2015) 12–22 17
F o 5.5 ma .
4
ad(nhao
tdaaaWodANCd
acMMaasdtc
ig. 4. Topography of the NREM EEG microstructure during healthy aging (from 3 tnd beta relative amplitudes/6 h. SMCx – sensorimotor cortex; MCx – motor cortex
. Discussion
We have shown that aging altered the sleep/wake statesrchitecture only within the SMCx: whereas the NREM durationecreased, REM duration increased from 4.5 to 5.5 months ageFig. 1) due to decreased number of the long NREM and increasedumber of the short REM episodes (Table 1, Fig. 3B). However,ealthy aging did not alter consistently throughout the overallging follow-up period the Wake/NREM/REM EEG microstructuresf both cortices.
We have shown for the first time that healthy aging induced theopographically and state-related distinct alteration of the corticalrives to the dorsal nuchal muscles during sleep in rats. Healthyging in rats induced the long-lasting SMCx and MCx drives alter-tions, mostly and consistently expressed through the delta, sigmand beta CMCs alterations from 4.5 to 5.5 months (Fig. 6, Fig. 7).
hereas the age-related SMCx delta and beta drives impacts wereppositely expressed (Fig. 6), the age-related MCx delta and betarives were convergently expressed (Fig. 7) during Wake and REM.lso, while aging consistently decreased the sigma CMCs duringREM and REM, through both cortical drives, conversely the sigmaMCs increased during Wake more consistently due to the SMCxrive.
It is very important to note here that all well documentedge-related changes in sleep/wake states architecture, and theirorresponding EEG changes (Feinberg et al., 2002; Kirov andoyanova 2002; Mendelson and Bergmann, 1999a,b, 2000;oyanova et al., 2002) considered 3–5 months old rats as young
dults, compared to middle-aged at 10–12 months and old ratst 21–24 months. In our study we followed the aging impact on
leep/wake states architecture, EEG microstructure, and corticalrives alterations during spontaneous sleep in young adult Wis-ar rats (3 months old rats), and up to 5.5 months age. We havehosen this aging follow-up period because we aimed this study toonths): the group probability density distributions of the NREM EEG delta, sigma
determine the onset of healthy age-related sleep disorders (partic-ularly cortical drive alterations), for our further studies of the onsetof sleep disorders in the rat models of neurodegenerative diseases.Also, in our study we consider as age-related changes only con-sistent changes, changes that lasted at least in two last follow-upperiods with increment of 1.5 month. Our results of the age-relatedchanges, during spontaneous sleep in young adults, are in accor-dance with the evidence for the age – related reduction of NREMsleep, increase of REM sleep, as well as a stable NREM delta power,but during recovery after sleep deprivation, and in the middle agedrats (Mendelson and Bergmann, 1999a,b, 2000).
Although sleep is classically considered as a global phenomenon,orchestrated by central specialized neuronal networks modulatingwhole-brain activity (Nobili et al., 2012), recently it has been pro-posed that sleep is local in nature, or a fundamental propriety ofsmall neuronal groups (Krueger et al., 2008). A lot of experimentalevidence in animals and humans suggest that: sleep and wakeful-ness might be simultaneously present in different cerebral regions;that the boundaries between these behavioral states are not strictlydefined; and that brain-sleep state may be spatially non-uniform,as well as that sleep and wakefulness may not be temporally dis-crete behavioral states (Nobili et al., 2012). In addition, recentstudy in rat evidenced that topography of the sleep/wake statesrelated EEG microstructure and transitions structure differentiatesthe functionally distinct cholinergic innervation disorders (Petrovicet al., 2013b). In our former study, we evidenced the emergence oftwo REM states with regard to total EMG power, topographicallydistinct EEG microstructures, and the locomotor drives to dorsalnuchal muscles in the rat model of cholinergic Parkinson’s diseaseneuropathology (Petrovic et al., 2014). In this rat model of neurode-
generative disease, we evidenced impaired beta CMCs during bothREM states (Petrovic et al., 2014), due to altered SMCx drive, butmore severely during healthy REM (REM with atonia) than duringpathological REM (REM without atonia).
18 J. Ciric et al. / Mechanisms of Ageing and Development 146 (2015) 12–22
F o 5.5
s cortex
caiaaFcfdRd(OttaiW
dat
ia
ig. 5. Topography of the REM EEG microstructure during healthy aging (from 3 tigma and beta relative amplitudes/6 h. SMCx – sensorimotor cortex; MCx – motor
Our present results have shown that healthy aging in rats isommonly and consistently expressed as the delta and beta CMCsugmentation during Wake and REM, although there was no agingnduced Wake and REM EEG amplitude changes. Namely, healthyging did not alter Wake EEG microstructure but it consistentlyugmented Wake delta CMCs through both cortical drives (Fig. 6,ig. 7), and Wake beta CMC only through the MCx drive (Fig. 7), indi-ating aging augmented propagation of the Wake delta oscillationrom both cortices, but Wake beta oscillation only from the MCx toorsal nuchal muscles. Moreover, healthy aging transiently alteredEM delta and beta amplitudes only within the SMCx, but REMelta and beta CMCs consistently decreased through the SMCx driveFig. 6), and consistently increased through the MCx drive (Fig. 7).ur results have shown aging augmented simultaneous propaga-
ion of the REM delta and beta oscillations from the MCx, but alsoheir simultaneous attenuation from the SMCx. In addition, whileging, through both cortical drives, decreased the sigma CMCs dur-ng NREM and REM, in contrast it increased sigma CMCs during
ake.These results indicate the dual and inverse motor control drives
uring REM in physiological controls, expressed through the deltand beta CMCs, that could be the homeostatic mechanism for main-
aining motor control during healthy aging.It was shown that CMC is prominent in beta frequency band dur-ng weak to medium isometric contraction (Kristeva et al., 2007),nd that in contrast to tremor, which has been related to increased
months): the group probability density distributions of the REM EEG delta, theta,.
oscillatory beta band synchronization (Krause et al., 2014), themovement slowing in Parkinson’s disease has been related to thedecreased beta band CMC (Salenius et al., 2002). On the other hand,the increased beta CMC is evidenced during REM sleep in the REMbehavioural disorder patients, without changes during Wake andNREM (Jung et al., 2012). Recent studies in the rat model of Parkin-son disease’s cholinergic neuropathology (Petrovic et al., 2013a,2014) evidenced the long-lasting simultaneous attenuation of theWake and NREM delta band amplitude, and an augmentation of thebeta band EEG amplitude, during all sleep/wake states within theSMCx. In this rat model of the severe cholinergic neuronal pathol-ogy, there was also an emergence of two REM sleep states, and onlythe altered SMCx drive decreased beta band CMC during REM sleep(Petrovic et al., 2014). Actually, the impaired cholinergic innerva-tion augmented beta amplitude within the SMCx, but attenuatedthe beta CMC during REM, and indicated the breakdown of thedescending propagation of the oscillations (Petrovic et al., 2014).
There is evidence in healthy humans that the voluntary toniccontractions of the limbs are driven by three main descendingactivities: a 10 Hz drive (associated with the physiological postu-ral tremor); a drive around 20 Hz (characterizes the weaker musclecontractions); and a 40 Hz drive (characterizes the very strong mus-
cle contractions). In contrast to the 10 Hz drive, whose origin isunclear, the 20 Hz and 40 Hz drives clearly originate in the pri-mary motor cortex (Brown, 2000; Salenius et al., 2002). Also, itwas shown that levodopa in Parkinsonian patients switches the pri-
J. Ciric et al. / Mechanisms of Ageing and Development 146 (2015) 12–22 19
Fig. 6. Sleep/wake states related sensorimotor cortex (SMCx) drive alterations during healthy aging (from 3 to 5.5 months): the Wake/NREM/REM group mean coherences ing hf isk–p
A M delt
muam(
cthaii
pectra between the SMCx EEG and dorsal nuchal muscle EMG, and their correspondor each conventional EEG frequency band (delta, theta, sigma, beta, gamma). Asterging consistently increased Wake delta CMCs, and decreased NREM sigma and RE
ary motor cortex from a mode of inefficient recruitment of motornits, often associated with the synchronization of cortical outputt around 10 Hz, to one that involves improved motor unit recruit-ent and synchronous activity in the 15–30 Hz and 35–60 Hz band
Salenius et al., 2002).On the base of all above mentioned experimental evidence, we
an speculate that in our present study, the augmented beta CMCshrough the MCx drive during Wake and REM, may serve as aomeostatic mechanism for the compensation of simultaneously
ugmented delta CMCs and improvement of motor control dur-ng healthy aging. In addition, the inverse SMCx and MCx drivesmpacts, expressed through the delta and beta CMCs during REM,istograms of the individual group corticomuscular coherence (CMC) means/6 h + SE≤ 0.03.a, sigma, beta and gamma CMCs through the SMCx drive.
may serve for the maintaining of healthy REM sleep with atoniaduring healthy aging.
Our results are in accordance to the evidence that age-relatedincrease of CMC strength might represent a compensatory mech-anism, or a stronger cortical involvement to maintain isometriccontraction (Kamp et al., 2013; Kristeva et al., 2007), and also withthe evidence that the CMC presents an efferent phenomenon, oran oscillation that propagate from the cortical source to spinalmotoneurons via corticospinal tract (Ushiyama et al., 2010).
In our study the linear correlation analysis have shown theestablished significant functional coupling for the MCx delta andbeta drives of the highest strength during REM (r ≥ 0.75; p ≤ 0.00),
20 J. Ciric et al. / Mechanisms of Ageing and Development 146 (2015) 12–22
Fig. 7. Sleep/wake states related motor cortex (MCx) drive alterations during healthy aging (from 3 to 5.5 months): the Wake/NREM/REM group mean coherence spectrab vidualfA EM si
aaSastw
pftc
etween the MCx EEG and dorsal nuchal muscle EMG, and their corresponding indirequency band (delta, theta, sigma, beta, gamma). Asterisk–p ≤ 0.04.ging consistently increased Wake and REM delta and beta, decreased NREM and R
nd for the SMCx delta drive during Wake (r = 0.58; p = 0.03). Welso have to consider that although the distinct alterations of theMCx and MCx drives during Wake and REM in our study are prob-bly due to perfectly functional sensory system in Wake vs. REM, ithould be noted that we recorded sleep only during normal inac-ive phase for rats, and our Wake state was rather awakening thanakefulness.
Although our study is in accordance with the evidence that CMCresents an efferent phenomenon, or an oscillation that propagate
rom the cortical source to spinal motoneurons via corticospinalract, the pharmacological studies suggest that CMC and corti-al oscillations represent separate phenomena, and strengthen the
group corticomuscular coherence (CMC) means/6 h + SE for each conventional EEG
gma, and increased REM gamma CMCs through the MCx drive.
hypothesis that CMC reflects a significant mechanism rather thana transfer of an essentially cortical phenomenon (Baker and Baker,2003; Riddle et al., 2004).
Our study evidenced for the first time that aging alteredsleep/wake states architecture, simultaneously followed by alteredmotor control, in 4.5–5.5 months old rats. Also, we have evidencedfor the first time that Wake and REM are the most sensitive stateson healthy aging induced cortical drive alterations, whereas theNREM states is stable. Healthy aging was state-dependently and
oppositely expressed in Wake vs. REM, and through the MCx vs.SMCx drive during sleep. Beside the common aging impact throughboth cortical drives, expressed as augmented Wake delta CMCs,
J. Ciric et al. / Mechanisms of Ageing and Development 146 (2015) 12–22 21
F EEG rm tional
b (5.5 m
aRaimaticih
miwp
A
e
R
B
ig. 8. Significant linear correlations between the Wake/NREM/REM delta or beta
eans (see Figs. 6, 7). The state-related topography of established significant funceta CMC alterations (4.5 months), until the end of healthy aging follow-up period
nd through the MCx drive, expressed as augmented Wake andEM beta CMCs, aging through SMCx drive attenuated REM deltand beta CMCs. We have evidenced for the first time the dual andnverse motor control during REM sleep: aging induced opposite
otor control from SMCx vs. MCx, expressed through the deltand beta CMCs alterations during REM sleep in rats, and could behe homeostatic mechanism for maintaining of motor control dur-ng REM in healthy aging. Augmented beta oscillation propagationould be the compensatory homeostatic mechanism for dampen-ng the augmented delta oscillation propagation and maintainingealthy REM (REM with atonia) during healthy aging.
Our present study opens a new perspective of using the animalodels and corticomuscular coherence, as the promising tools to
nvestigate mechanisms of motor control during healthy aging, asell as during pathological aging in neurodegenerative diseases,
articularly during sleep.
cknowledgement
This work was supported by Serbian Ministry of Education, Sci-nce and Technological Development Grant OI 173022.
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