Changes in EEG associated with sleep-awake behavior in young adult versus aged adult fischer-344 rats

Physiology & Behavior, Vol. 44, pp. 38%392. Copyright©PergamonPress plc, 1988. Printedin the U.S.A.

0031-9384/88$3.00 + .00

Changes in EEG Associated With Sleep-Awake Behavior in Young Adult Versus Aged Adult Fischer-344 Rats YOSHIHIRO TANP AND TAKAFUMI ISHIHARA

Laboratory o f Experimental Pharmacology, Suntory Institute for Biomedical Research, Osaka 618 Japan R e c e i v e d 30 N o v e m b e r 1987 TANI, Y. AND T. ISHIHARA. Changes in EI£G associated with sleep-awake behavior in young adult versus aged adult Fischer-344 rats. PHYSIOL BEHAV 44(3) 38%392, 1988.--Age-related spontaneous cortical and hippocampal EEG changes associated with level of arousal in rats were investigated. The EEG of rats with chronically implanted electrodes were recorded using bipolar leads and simultaneously analyzed with a computer to obtain the power spectra. In the awake stage, EEGs in the aged rats consisted predominantly of slow waves when compared with those in the young rats. The cortical EEG activity of the aged rats exhibited specific irregular burst waves which consisted of two spectral peaks at 8-9 Hz and 15-16 Hz during the drowsy period. The relative powers of the cortical and hippocampal waves in the aged rats differed from those of the young rats during the slow-wave sleep stage. These findings suggest that the slowing of the EEG in the aged rats during the awake stage may be related to decreased brain activity associated with aging, and that the irregular burst waves in the cortical EEG in the aged rats appear to correlate with the changes observed in age-related human sleep patterns. Aging

EEG

Power spectra analysis

Cortical irregular burst wave

IN recent years, many reports dealing with pharmacological evaluation of antiamnesic or nootropic drugs (3, 4, 15, 17) have appeared. Several animal models with various abnormalities in brain function (4-6, 9, 10), as well as aged rats, have been used in these studies. Schuurman et al. (14) reported that because they are inferior to young rats with respect to learning ability, aged rats might serve as a useful model for studying age-related changes in brain function. Electroencephalogram (EEG) is in fact the only direct method which reflects brain function (2). Recent development of computer techniques have enabled us to quantitatively measure the changes in brain EEG activity. However, in animal experiments, it is necessary to relate the level of arousal to their spontaneous movement in order to obtain reliable information. The present study, therefore, was designed to investigate age-related qualitative and quantitative EEG changes in the frontal cortex and dorsal hippocampus in close association with the awake and sleep stages in aged and young rats.

were 11 to 12 weeks old (young group, weighing 200-250 g, n=7), and 24 to 25 months old (aged group, 350-450 g, n=7) at the beginning of the experiments. All animals were housed in an individual home cage maintained under standardized conditions of room temperature (23_+2°C) and a light-dark cycle (lights on between 07:00-19:00) throughout the experiments. Food and water were supplied ad lib.

Surgical Procedures The animals were anesthetized with sodium pentobarbital (40 mg/kg given intraperitoneally) and placed on a stereotaxic instrument. For EEG recordings, a bipolar electrode (screw, diameter= 1.0 mm) made of stainless steel was attached to the surface of the frontal cortex (FC) and a bipolar stainless steel electrode, insulated except for the 0.5 mm from the tip, was inserted into the dorsal hippocampus (HPC, 3.3 mm posterior from the bregma, 1.5 mm lateral to the midline and 3.5 mm horizontal below the skull surface) according to the brain atlas by Paxinos and Watson (11).

METHOD

EEG Recordings and Power Spectral Analysis

Animals

Two weeks after the surgical operations, EEG recordings were started under a freely moving condition. At the time of recording, a conscious rat was transferred to the test chamber

All experiments were carried out using male Fischer-344 rats purchased from Charles River Japan, Inc. The animals

~Requests for reprints should be addressed to Y. Tani, Laboratory of Experimental Pharmacology, Suntory Institute for Biomedical Research, l-l-1 Wakayamadai, Shimamoto-cho, Mishima-gun, Osaka 618, Japan.

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FIG. 1. EEG activity from the frontal cortex and dorsal hippocampus during awake, drowsy and slow-wave sleep in young and aged rats. The vertical scales indicate 100 p.V. and a horizontal line represents 5 sec. FC: frontal cortex, HPC: dorsal hippocampus.

(25 x 32 x 30 cm) from the home cage and allowed to acclimate to the situation for 10 min. A spontaneous EEG was recorded for 10 min at each stage. Stages were judged according to behavioral changes and the EEG pattern itself. Recording was made on an 8-channel polygraph (Nihon Kohden, RM-6000) at a paper speed of 5 mrrdsec, a time constant of 0.1 sec, and whose output was connected to a data analyzer (Nihon Kohden, ATAC-450, Tokyo, Japan). Fast Fourier Transformations (FFT) were performed with a data analyzer (Nihon Kohden, 4ch EEG analysis Prog., Tokyo, Japan) to obtain spectral power densities at frequencies ranging from 2.0 to 29,6 Hz. Data reported in this paper are divided into the following six frequency bands: delta (2.0-3.6), theta(4.0-7.6), alpha-l(8.0--9.6), alpha-2(10,0-12.8), beta-1(13,2--19.6) and beta-2(20.0-29.6 Hz), EEG samples were stored on disc (Nippon Data General, Tokyo, Japan).

Histology After completion of the experiments, the animals were anesthetized with ether and the brains were perfused through the heart with 10% Formalin. Brain sections (50 ttm thick) were made and stained with cresyl violet. The sites of the electrodes were verified histologically. If the electrodes were not located in the proper position, the results from the rats were discarded.

Statistics The results were statistically analyzed using the twotailed Student's t-test. RESULTS

The cortical (FC) and hippocampal (HPC) EEG activities,

the power densities and the spectral activities from the aged and young rats during the awake, drowsy and slow-wave sleep stages are presented in Figs. l, 2 and 3, respectively. In the awake stage, the FC EEG activity in the aged rats was almost the same as that in the young rats. in other words, a low amplitude and a high frequency (Fig. 1). However, power analysis revealed Lhat the relative powers of the alpha-1 waves (8.0-9.6 Hz) and alpha-2 waves (10.0--12.8 Hz) in the aged rats were significantly lower than those in the young rats (Fig. 3A). The HPC EEG power density in the aged rats was dominated by slow waves (Fig. 2C), and the relative power of the delta waves (2.0-3.6 Hz) was significantly higher (o<0.05) and that of atpha-1 waves significantly lower (p <0.05) than those in the young rats (Fig. 3B). In the sleep stage, the EEG activity in the aged rats was found to be quite different from that in the young rats. The young rats, upon failing asleep, exhibited desynchronization of hippocampal theta waves and an increase in the amplitude of cortical slow waves (Fig. 1). However. during the drowsy period, the FC EEG activity in the aged rats showed a few normal waves intermingled with clusters of irregular burst waves (Fig. 1). The burst activity was associated with two power density peaks at 8-9 Hz and 15-16 H z (Fig. 2D). All 7 aged rats exhibited this characteristic E E G pattern-during the drowsy period which appeared approximately I hr after the beginning of EEG recordings and lasted for 1 to 2 hr thereafter. The FC EEG activity in the young rats never exhibited this type of burst activity, even during the drowsy period. The relative HPC theta (4.0-7.6 Hz) activity in the aged rats during slow-wave sleep was s ~ l y (p <0,05) higher and alpha-2 activity significantly 09 <0.01) lower than those in the young rats (Fig. 3B). In FC of aged rats, the relative power of the delta waves during slow-wave sleep was significantly lower (p
391

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FIG. 2. Representative EEG power spectra from frontal cortex and dorsal hippocampus of young and aged rats. (A) during the awake stage in a young rat, (B) during the slow-wave sleep stage in a young rat, (C) during the awake stage in an aged rat, (D) during the drowsy stage in an aged rat.

a

B

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60

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DELTA 2.0-36

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THETA 4,0-76

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PERCENTOFTOTALSPECTRALACTIVITY

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FIG. 3. EEG power spectra analyses during awake and slow-wave sleep in young and aged rats. DELTA(2.0-3.6), THETA(4.0-7.6), ALP-l(8.0-9.6), ALP-2(10.0-12.8), BET-l(13.2-19.6), BET2(20.0-29.6 Hz). (A) frontal cortex, (B) dorsal hippocampus. *p<0.05, **p<0.01; significantly different from young rats (two-tailed Student's t-test). significantly higher (p<0.05) than those in the young rats (Fig. 3A). DISCUSSION

In the present study, we have evaluated spontaneous

EEG activity during awake and sleep stages in the frontal cortex (FC) and dorsal hippocampus (HPC) in qualitative and quantitative ways in aged and young rats. We confirmed the observations in aged rats by Poschel et al. (12), and in humans by Wang and Busse (16), that the low frequency

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TAN1 AND ISHIHARA

waves dominate spontaneous EEG activity in aged rats. This is in contrast to the young rats in which the high frequency waves were dominant in the awake stage. It was also found that the FC EEG activities in the aged rats exhibited characteristic irregular burst activities which consisted of two spectral peaks at 8--9 Hz and 15-16 Hz during the drowsy period, and that during slow-wave sleep the relative power densities of FC and HPC in the aged rats differed from those in the young rats. There are many reports dealing with age-related functional (1,14) and biochemical (7,8) deterioration in the central nervous system. Thus, the slowing of EEG rhythm during the awake stage observed in the aged rats in the present study may reflect global deterioration of brain function with aging. In the young rats, desynchronization of the HPC theta waves and amplification of the FC slow waves, which are the qualitative changes in spontaneous EEG activity, developed

upon falling asleep. However, the characteristic changes in alpha and beta waves observed in the aged rats were never seen in the FC E E G during the drowsy and slow-wave sleep period in the young rats. In clinical studies, it has been reported that the human sleep pattern changes with aging; i.e.. the amount of daily deep non-REM and REM sleep diminish in old age (13). The functional significance of the irregular burst waves observed during the drowsy period in the cortical EEG of the aged rats remains unclear. While further research is required to resolve the sleep pattern changes with aging, there is a possibility that this kind of EEG activity may reflect the cerebral arousal pattern to be observed during aging. ACKNOWLEDGEMENTS We express our gratitude to Drs. T. Noguchi (Director) and Y. Kas6 (Executive advisor) for their support and encouragement throughout this study. We are also indebted to Miss N. Hirose for typing the manuscript.

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