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Department of Biological Sciences, Fordham University, Bronx, New York 10458
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ABSTRACT |
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 TOP
 ABSTRACT
 INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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We hypothesized that nitric
oxide (NO) may play a role in homeostatic sleep regulation. To test
this hypothesis, we studied the sleep deprivation (SD)-induced
homeostatic sleep responses after intraperitoneal administration of an
NO synthase inhibitor, N
-nitro-L-arginine methyl
ester (L-NAME, a cumulative dose of 100 mg/kg). Amounts and
intensity of sleep were increased in response to 8 h of SD in control
rats (n = 8). Sleep amounts remained above baseline for 16 h
after SD followed by a negative rebound. Rapid eye movement sleep
(REMS) and non-REMS (NREMS) intensities were elevated for 16 and 4 h,
respectively. L-NAME treatment (n = 8) suppressed
the rebound increases in NREMS amount and intensity. REMS rebound was
attenuated by L-NAME in the first dark period after SD;
however, a second rebound appeared in the subsequent dark period. REMS
intensity did not increase after SD in L-NAME-injected rats. The finding that the NO synthase inhibitor suppressed rebound increases in NREMS suggests that NO may play a role as a signaling molecule in homeostatic regulation of NREMS.
rapid eye movement sleep; fast Fourier analysis; N
-nitro-L-arginine methyl ester; electroencephalography; slow-wave activity
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INTRODUCTION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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SLEEP IS UNDER THE INFLUENCE of the circadian and
homeostatic regulatory processes (7). The circadian process is a
clocklike mechanism that is mainly controlled by the suprachiasmatic
nucleus (SCN) and is independent of prior wakefulness. The homeostatic component of non-rapid eye movement sleep (NREMS) regulation is a
mechanism that elicits compensatory increases in NREMS after sleep
loss. The existence of homeostatic NREMS-promoting mechanisms is
evident from sleep deprivation (SD) studies. SD causes compensatory increases in time spent in NREMS and NREMS intensity, as indicated by
electroencephalographic (EEG) 
-wave activity in several species, including rats (8). The homeostatic process is independent of the
circadian rhythm. In contrast to the circadian modulation of sleep, the
specific brain structure where homeostatic sleep-regulating mechanisms
reside has not been identified. Homeostatic increases in sleep can be
due to increased activity of a putative neuronal network(s) or the
accumulation of an endogenous substance during wakefulness. There are
mathematical models that describe homeostatic sleep regulation in
quantitative terms, simulate homeostatic sleep responses, and predict
changes induced by varying the duration of wakefulness (1).
A growing body of evidence suggests that nitric oxide (NO) may play a role in the circadian and homeostatic processes of sleep regulation. The circadian clock is entrained to the environmental light-dark cycle via the retinohypothalamic pathway (reviewed in Ref. 30). Glutamate is the primary neurotransmitter mediating the transmission of photic signals from the retinal afferents to the SCN (12). NO synthase (NOS) inhibitors block the behavioral circadian responses to photic stimuli in hamsters (12) and autonomic responses in rats (3). Also, NOS inhibitors block the glutamate-induced phase shifts of SCN neuron activity in vitro (35). These results suggest that the transduction of photic stimuli in the SCN and the entrainment of the biological clock to photic stimuli require the formation of NO.
Indirect evidence supports the hypothesis that NO is an endogenous sleep-promoting substance involved in homeostatic sleep regulation. First, intracerebroventricular injection of NO donors, such as 3-morpholinosydnonimine and S-nitroso-N-acetylpenicillamine, mimics the effects of prolonged wakefulness, i.e., increases time spent in NREMS as well as NREMS intensity in rats (21). Injection of S-nitroso-N-acetylpenicillamine in cats (11) or the NO precursor L-arginine in rats (18) into the pedunculopontine region causes increases in NREMS and REMS. Conversely, systemic (14, 15, 19), intracerebroventricular (19), or intrapedunculopontine (18, 20) injection of NOS inhibitor suppresses sleep in rats. Similarly, systemic and intracerebroventricular injection of NOS inhibitor suppresses sleep in rabbits (22), and intrapedunculopontine injection of NOS inhibitor suppresses sleep in cats (11). The activity of NOS shows diurnal variation. NOS activity increases during the behaviorally active period of the day in the hypothalamus, brain stem, cerebellum, and hippocampus of rats (4). Direct measurement of NO release from the cerebral cortex (9) or thalamus (36) of rats showed that production of NO is higher during wakefulness than during NREMS.
One approach to study the involvement of NO in homeostatic sleep
regulation is to test the effects of inhibiting NOS on homeostatic sleep responses. In the present experiments we studied the homeostatic sleep responses to SD in control rats and rats injected with
N-nitro-L-arginine methyl ester
(L-NAME), an inhibitor of NOS. The results indicate that
administration of NOS inhibitor decreased NREMS amounts and intensity
after SD, suggesting that NO plays a key role in homeostatic sleep regulation.
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METHODS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Animals
Male Sprague-Dawley rats (290-440 g) were used. With use of combined ketamine (87 mg/kg) and xylazine (13 mg/kg) anesthesia, animals were implanted with cortical EEG electrodes, nuchal electromyographic (EMG) electrodes, and a brain thermistor. The EEG electrodes were placed over the frontal, parietal, and occipital cortices, and the thermistor was placed on the dura over the parietal cortex. After a 1-wk recovery period, the animals were placed into sound-attenuated individual sleep-recording cages for habituation to the experimental conditions. During this adaptive period, the animals were connected to the recording cables and injected daily with isotonic NaCl solution intraperitoneally at light onset. Animals were kept on a 12:12-h light-dark cycle (light onset at 0600) and an ambient temperature of 26 ± 1°C for
2 wk before surgeries, during the recovery period, and throughout the experimental procedure. Water and
food were provided ad libitum during this time.
Recordings
EEG, EMG, and brain temperature (Tbr) were recorded by computer. EMG activity served the sole purpose of aiding in determining the vigilance states of the animals and was not further quantified. EEG was filtered below 0.1 Hz and above 40 Hz. The amplified signals were digitized at the frequency of 128 Hz for EEG and EMG and 2 Hz for Tbr. Single Tbr samples were saved on the hard disk in 10-s intervals. Average Tbr was calculated in 1-h time blocks. The vigilance states were determined offline in 10-s epochs. EEG, EMG, and Tbr were displayed in the computer in 10-s epochs and also simultaneously in a more condensed form in 12-min epochs. Wakefulness, NREMS, and REMS were distinguished as described in detail previously (23). Time spent in NREMS and REMS was calculated in 2-h time blocks. Online fast Fourier analysis of the EEG was also performed in 10-s intervals on 2-s segments of the EEG in 0.5-Hz bands of the 0.5- to 16-Hz frequency range. The EEG power density values were summed in four frequency bands for each 10-s epoch; (0.5-4
Hz)-, 
(4.5-8 Hz)-, 
(8.5-12 Hz)-, and 
(12.5-30 Hz)-activities were calculated. The spectral data were
paired with the vigilance states, and EEG power was computed in each of
the four bands separately for each vigilance state. Hourly average
-, -, -, and -activities (µV2) were then
calculated for the NREMS, REMS, and wakefulness epochs. On the baseline
day, average power densities were computed across the entire 23 h for
each rat to obtain a reference value for each animal. Power densities
in 2-h blocks on the baseline day and the test days were then expressed
as a percentage of that reference value. Group averages were also
calculated for the entire light and dark periods (experiment
III) or the last 4 h of the first light period and the subsequent
entire dark and light periods (experiments I and II).
Experimental Protocol
Experiment I: effect of SD on sleep, EEG power density, and Tbr. On the control day, sleep was recorded for 24 h starting at 0600. On the next day, all the animals were sleep deprived by gentle handling for 8 h, starting at light onset. Food and water were provided ad libitum during the SD. Animals received isotonic NaCl (2 ml/kg ip) at 0700 and 1100. They were returned to their home cages at 1400, and sleep was recorded for 48 h.
Experiment II: effect of L-NAME on SD-induced changes in sleep, EEG power density, and Tbr. The experimental design was similar to experiment I, except during SD, the animals received L-NAME (50 mg/kg ip; Sigma Chemical, St. Louis, MO) at 0700 and 1100. L-NAME was dissolved in isotonic NaCl at a concentration of 25 mg/ml. Recordings for experiments I and II were performed on the same animals (n = 8). On a given test day, one-half of the animals received saline, and the others received L-NAME. One week later, the treatments were reversed and the recordings were repeated.
Experiment III: effect of L-NAME on spontaneous sleep, EEG power density, and Tbr. On the control day, the rats (n = 8) received isotonic NaCl (2 ml/kg ip) at 0700 and 1100. Sleep was recorded for 23 h starting at 0600. On the test day, all the rats received L-NAME (50 mg/kg ip) at 0700 and 1100. Sleep was recorded continuously for 56 h from 0600.
Statistical Analysis
Statistical comparisons were made between the baseline days and the test days. In experiments I and II, ANOVA was done separately on five segments of the recordings: the first 4 h after the end of SD (i.e., the last 4 h of the light period), hours 5-16 after the end of SD (the 1st dark period), hours 17-28 (the 2nd light period), hours 29-40 (the 2nd dark period), and hours 41-48 (the 1st 8 h of the 3rd light period, i.e., the last 8 h of the recording). In experiment III the first segment of the recordings analyzed by ANOVA was the entire light period of the first experimental day. Two-way ANOVA for repeated measures were performed on 2-h time blocks for sleep and 1-h averages for Tbr. EEG spectra values were analyzed by two-way ANOVA on 2-h averages. When ANOVA indicated significant treatment effects, paired t-tests were performed a posteriori.| |
RESULTS |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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Experiment I: Effect of SD on Sleep, EEG Power Density, and Tbr in Control Rats
NREMS and REMS amounts and NREMS intensity were increased after SD in saline-treated rats (Fig. 1, Table 1). Time spent in NREMS was 196.0 ± 29.9 min (~58%) above baseline in the 16-h period after SD (hours 9-24). In the subsequent light period, hours 25-36, there was a negative rebound in NREMS, then NREMS returned to baseline levels by the beginning of the second dark period. In the last 8 h of the recordings, there was again a small, but statistically significant, decrease in the amount of NREMS.
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REMS did not change significantly in the 2 h immediately after SD (Fig. 1). Subsequently, time spent in REMS increased for 14 h by 77.3 ± 9.7 min (~165%). From the beginning of the second light period (hour 25), REMS remained at baseline levels. Amount of REMS was increased by 73.2 ± 10.9 min across the 48-h recording period.
Time spent in wakefulness decreased by 47% in hours 9-24 and increased in the second light period, corresponding to the changes in sleep. Amount of wakefulness returned to baseline levels from the second dark period (Fig. 1).
SD induced significant changes in the EEG -, -, and -frequency
ranges in NREMS (Table 1). The -activity during NREMS, a measure of
NREMS intensity, increased by 116% in the first 4 h (Figs. 1 and
2) and then exhibited a negative rebound
that lasted from hours 19 to 26 (Fig. 1). The - and
-activities were also increased in the first 4-h period, and there
was a tendency toward increased - and -activities for the
remainder of the recording period in NREMS. In fact, -activity was
significantly elevated during the second dark period (Fig. 2). SD
induced an increase in EEG -activity during REMS (Figs. 1 and 2). It
increased by 27.2% in the first 4 h and by 20.1% during the
subsequent dark period. These increases gradually diminished throughout
the rest of the recording period. SD did not affect the -activity
during REMS, but it increased - and -activities during the first
and the first and second dark periods, respectively. Changes in EEG power spectra during wakefulness followed a pattern similar to that
seen in NREMS (Fig. 2).
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ANOVA for repeated measures indicated significant changes in Tbr during two light periods (hours 25-36 and 49-56) after SD (Table 1). Post hoc analysis, however, did not reveal any time point when the differences between the control and SD day were statistically significant.
Experiment II: Effect of SD on Sleep, EEG Power Density, and Tbr in L-NAME-Treated Rats
The pattern of NREMS responses to SD in L-NAME-treated animals was similar to that in saline-treated rats but was shorter in duration and lower in amplitude (Fig. 3, Table 1). The initial positive NREMS rebound lasted only for 6 h. Time spent in NREMS was increased in hours 9-24 by 154.8 ± 15.6 min (40% above baseline), a significantly smaller change than in experiment I. These increases were followed by a negative rebound in the subsequent light period, and NREMS returned to baseline values thereafter.
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Time spent in REMS showed biphasic increases after SD in L-NAME-treated rats (Fig. 3). Values were below baseline in the first 2-h time block and then exhibited a significant positive rebound in hours 13-24. This increase was 45.9 ± 16.5 min, 107% above baseline, a significantly attenuated response compared with that in saline-treated animals after SD. REMS amount returned to baseline levels in the following light phase. In the second dark period, there was a second positive rebound; time spent in REMS increased again by 35.4 ± 12.2 min (78% above baseline), a response that was absent in saline-treated rats. For the rest of the recording period, REMS returned to baseline. Time spent in REMS for the entire 48-h recording period in L-NAME-treated rats was increased by 77.2 ± 41.2 min.
Wakefulness remained below baseline for the first 16 h after SD, then there was a tendency toward a positive rebound lasting for 10 h (Fig. 2). In the second dark period, coinciding with the second increase in REMS, the amount of wakefulness was again below baseline.
EEG -activity during NREMS was increased only in the first 2-h time
block after SD, by 56% above baseline (Fig. 3). This is a smaller
response than that seen in saline-treated rats, which showed an
increase of 198% during the first 2 h. When EEG power spectra were
averaged across the first 4 h, there was no significant change in NREMS
-activity, in contrast to the significant response in saline-treated
animals (Fig. 2). From hour 13, -activity in NREMS was
significantly below baseline until the end of the recording. Similar
tendencies were observed for -activity during NREMS. The - and
-frequencies showed no significant changes (Fig. 2). During REMS,
there was a gradual increase in EEG -activity, which reached
statistical significance in the last dark period (Fig. 2).
L-NAME-treated rats showed suppressed -activity in REMS
during the second light period after SD. REMS -activities were
increased during the second and -activities during the first and
second dark periods. There were no significant changes in EEG power
spectra during wakefulness (Fig. 2).
Sleep-deprived L-NAME-treated rats showed a statistically significant decrease in Tbr during the first dark phase after SD and an increase in Tbr in the following light period (Fig. 3).
Experiment III: Effect of L-NAME on Sleep, EEG Power Density, and Tbr in Rats Not Subjected to SD
L-NAME treatment alone decreased REMS and NREMS during the light periods of the day, thus resulting in increased wakefulness at these times (Fig. 4, Table 2). NREMS amounts were significantly suppressed during the second light period, hours 25-36, by 63.2 ± 18.4 min (~17% below baseline).
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Time spent in REMS was more affected by L-NAME treatment than NREMS (Fig. 4). REMS responses were significantly suppressed in all light periods, i.e., hours 1-12, 25-36, and 49-56, by 26.9 ± 6.7 min (~38%), 25.4 ± 8.8 min (~37%), and 23.6 ± 5.1 min (~43%), respectively. During the dark periods, the amount of REMS remained at baseline levels.
Time spent in wakefulness was significantly increased in all the light periods and remained at baseline levels during the dark periods (Fig. 4). In the first 12-h period, wakefulness increased by 51.3 ± 17.2 min (~21%) compared with the baseline day. In the second light period, hours 25-36, the amount of wakefulness increased by 88.6 ± 16.2 min (~38%) and in the last 8-h period of the recording period, hours 49-56, by 73.2 ± 16.7 min (~57%).
EEG -activity during NREMS in L-NAME-treated non-SD rats
was significantly below baseline levels for the entire recording period
(Figs. 2 and 4, Table 2). NREMS -activity was suppressed in
hours 13-36. There was also a tendency toward decreased
- and -frequencies during NREMS. During REMS, EEG -, -, and
-activities were not affected by L-NAME treatment. REMS
-activity decreased gradually, reaching statistical significance in
the second light period. EEG -activity in wakefulness was
significantly suppressed in L-NAME-treated rats in
hours 13-36. In wakefulness, - and -frequencies were
not affected by L-NAME. -Activity in wakefulness was
significantly suppressed during the second light period.
In the first 12 h, Tbr was decreased in L-NAME-treated non-SD rats. There was still a strong tendency toward suppressed Tbr in the first dark period. Thereafter, Tbr returned to baseline levels (Fig. 4).
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DISCUSSION |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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The present study confirms our previous findings that systemic
administration of the NOS inhibitor L-NAME suppresses
spontaneous NREMS, REMS, and EEG -activity during NREMS and lowers
Tbr in rats (19). To study the role of NO and NOS in the
brain, it is important that L-NAME be administered at doses
high enough to suppress brain NOS activities. We did not measure brain
NOS activity in the present study, but previous experiments support our
assumption that NOS activities in the brain were significantly suppressed after L-NAME administration. In the present
experiments we injected L-NAME (50 mg/kg ip) twice, at 4-h
intervals. Intraperitoneal bolus injection of 50 mg/kg
L-NAME causes a ~30% decrease in the NOS activity of the
hypothalamus and ~60% suppression in the brain stem 330 min after
the treatments in rats (5).
The sleep-suppressive and EEG effects of L-NAME were
relatively long lasting. This is consistent with our previous
observations and the finding that L-NAME is an irreversible
inhibitor of NOS (13). The effects of L-NAME were
restricted to the lower-frequency, - and -, range of the EEG. The
most robust EEG effects occurred during NREMS; - and -activities
of the EEG were suppressed for 36-48 h after L-NAME
injection. Also, there were decreases in slow-wave activities during
wakefulness. Another NOS inhibitor, 3-bromo-7-nitroindazole, also
suppresses EEG power in wakefulness and NREMS in rats (14). However,
unlike 3-bromo-7-nitroindazole, which suppresses EEG in a wide
(0.5-20 Hz) frequency range, the effect of L-NAME was
restricted to the - and -bands.
The amount of sleep under normal, baseline conditions is determined by
the interaction of three major factors. First, the duration of prior
wakefulness, which is the homeostatic factor (7). Prolonged periods of
wakefulness are followed by lengthened sleep periods and increased
sleep intensity. Second, a circadian factor also contributes to sleep
regulation (7). In nocturnal animals, sleep threshold is lower during
the day and higher during the night; therefore, sleep occurs
predominantly during the light phase of the day. Finally, several
environmental factors, such as ambient temperature, also influence
sleep amount. Various neurotransmitters (17), hormones (17), and
cytokines, such as interleukin-1 and tumor necrosis factor- (24),
may be involved in some or all aspects of sleep regulation. We
hypothesized that NO plays a role in the homeostatic regulation of
sleep. The function of homeostatic sleep-regulatory mechanisms can be
studied using SD. Our results confirm the well-known findings that
total SD elicits rebound increases in the amounts of NREMS and REMS.
The magnitude of these increases depends on the duration of the SD, the
circadian phase when the SD was performed, and other factors. Under our experimental conditions, NREMS rebound started immediately after the
cessation of SD, whereas compensatory REMS increases did not become
evident until the second 2-h time block after SD.
It has been proposed that the -activity of the EEG during NREMS is
an indicator of NREMS intensity (7) and -activity in REMS is a
measure of REMS intensity (8) in several species, including rats and
humans. Our results confirm that -activity in NREMS shows a biphasic
response to SD (16). After an initial increase for 4 h, NREMS was
suppressed in hours 12-18 after SD. In contrast to NREMS
-activities, -activity in REMS exhibited a monophasic increase
after SD. These findings indirectly support the hypothesis that -
and -activities may be considered a measure for NREMS and REMS
intensities, respectively. It is of particular interest that the
increases in -activities in REMS preceded the increases in time
spent in REMS after SD, indicating that REMS -activity (i.e., REMS
intensity) is more sensitive to SD than REMS amounts. Similarly,
increased sleep intensity without concomitant increase in sleep
duration has been reported for NREMS after SD in rats (8). Our results
also confirm that in wakefulness, as well as in NREMS, there is an
increase in EEG power over nearly the entire frequency range after SD
(8). Tbr did not change significantly after SD, although
there was a slight tendency toward decreased Tbr in
hours 4-6 after SD, which is in agreement with previously
reported observations in rats (16).
Our major finding is that L-NAME shortened and decreased the amplitude of NREMS responses to SD. The rebound increases in NREMS amounts and intensities were suppressed by L-NAME. This suggests that the homeostatic increase in NREMS requires an intact NO-generating capacity of the brain. Homeostatic sleep-promoting mechanisms are not only active after SD but also play a role in determining the amount of spontaneous sleep under normal conditions. That is, the amount of sleep during the rest period is, in part, set by the amount of wakefulness in the preceding behaviorally active period. L-NAME suppressed spontaneous sleep predominantly in the light, which is the rest period for rats. This is likely due to the inhibitory effect of L-NAME on the homeostatic component of NREMS regulation. In the dark period, when the activity of homeostatic sleep-promoting mechanisms is low, L-NAME did not affect NREMS.
SD induced increases in the amount of REMS and EEG -activity during
REMS. In contrast to NREMS, the total rebound increase in REMS after SD
was not affected by L-NAME. This suggests that NO is not
likely to play a key role in the homeostatic regulation of REMS
amounts. The time course of the REMS rebound, however, was greatly
affected by L-NAME. Only a partial REMS rebound took place
during the first dark period after SD, and it was followed by a second
wave of rebound in the second dark phase after SD. The presence of the
second REMS rebound indicates that homeostatic REMS pressure is still
high 28 h after the end of SD in L-NAME-injected rats. The
finding that L-NAME did not affect the total compensatory REMS increase after SD suggests that it is not the homeostatic signaling for REMS that L-NAME suppresses. In addition to
homeostatic and circadian signaling, normal sleep requires the intact
function of the neuronal mechanisms that generate and maintain sleep.
It is possible that L-NAME interferes with the
REMS-generating mechanisms, and this is why REMS responses during the
first night after SD were decreased. Cholinergic and cholinoceptive
neurons, essential for triggering and maintaining REMS, reside in the
lateral pontine region (reviewed in Refs. 6 and 31). In rats, NOS is
colocalized with ACh in the pons (34), and inhibition of NOS suppresses pontine ACh release in cats (25). This raises the possibility that
L-NAME may attenuate the activity of pontine
REMS-generating mechanisms immediately after SD. Because REMS pressure
is not completely discharged in the first dark period, a second REMS rebound takes place in the subsequent dark phase. The lack of rebound
increases during the light period is likely due to the fact that,
during the light period, REMS-promoting mechanisms are already fully
activated, and further increases in REMS are difficult to achieve. It
has been suggested that EEG -activity during REMS indicates the
intensity of REMS in rats (8). L-NAME did not affect REMS
-activities under normal conditions but completely prevented the
SD-induced increases in REMS -activity. This suggests that although
NO is unlikely to have a central role in the homeostatic regulation of
the REMS amounts, it may be involved in the induction of increased REMS
intensity after SD.
Our present study has its limitations. First, it does not address the question of which cells or neural circuits mediate the effects of L-NAME. It is likely that L-NAME acts on cellular elements in the brain that contain NOS. Many brain structures implicated in sleep regulation also contain relatively high concentrations of NOS. For example, neurons in the basal forebrain, various nuclei of the hypothalamus (e.g., the preoptic area, paraventricular nucleus, and lateral hypothalamic area), and the laterodorsal and pedunculopontine tegmental nuclei show strong staining for NOS-like immunoreactivity (33). Microinjections of L-NAME into these structures are needed to localize the effects of L-NAME in the brain. It is possible that at least some of the effects of L-NAME are mediated by the pedunculopontine tegmental nucleus, since microinjection of L-NAME into this structure suppresses spontaneous sleep in rats (18, 20) and cats (11). Second, in the present study we measured the effects of an NOS inhibitor on homeostatic sleep responses. Other approaches, such as measuring NOS activities or NO formation after SD, are needed to complement our understanding of the role of NO in homeostatic sleep regulation. Third, we injected L-NAME systemically. After systemic injections, L-NAME exerts its effects on central and peripheral targets. It has been speculated that central and peripheral NO may have opposite roles in regulating sleep (10).
A substance involved in homeostatic NREMS-promoting mechanisms should fulfill several criteria. For example, the injection of the substance should increase NREMS, whereas decreasing the endogenous levels of the substance should suppress spontaneous sleep as well as homeostatic NREMS responses to SD. The production of the substance is expected to increase during wakefulness and decrease during sleep. NO seems to fulfill these criteria. Injection of NO donors stimulates NREMS (11, 21), and inhibition of NOS suppresses spontaneous NREMS (11, 14, 15, 19, 22) and REMS (11, 14, 15, 19, 22, 26). NO production in the brain is higher during wakefulness than during NREMS (4, 9, 36). The present study indicates that inhibition of NOS also suppresses NREMS rebound after SD. These findings are consistent with the hypothesis that NO may play a role in the homeostatic regulation of NREMS. In addition to its role in homeostatic sleep regulation, NO may have a function in regulatory mechanisms mediating the effects of circadian (12, 35) and environmental influences on sleep.
Perspectives
It is hypothesized that a complex network of endogenous sleep-promoting and -inhibiting substances, also called "sleep factors," may play a role in sleep regulation (reviewed in Ref. 24). Sleep factors may mediate the circadian and homeostatic signaling of sleep, or they may be involved in executive mechanisms that generate and maintain sleep. Our results support the notion that NO may be a signaling molecule involved in the homeostatic control of NREMS regulation. There are other sleep factors, e.g., interleukin-1 (29), tumor
necrosis factor- (32), and growth hormone-releasing hormone (GHRH)
(28), the inhibition of which also attenuates homeostatic sleep
responses after SD. Cytokines (27) and GHRH (2) stimulate NO release in
the brain. Future experiments are needed to clarify the interactions
among homeostatic sleep factors. NO may serve as a common mediator of
effects of cytokines and GHRH in sleep homeostasis.
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ACKNOWLEDGEMENTS |
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This work was supported by National Institute of Neurological Disorders and Stroke Grant NS-30514.
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FOOTNOTES |
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The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked "advertisement" in accordance with 18 U.S.C. §1734 solely to indicate this fact.
Address for reprint requests and other correspondence: L. Kapás, Dept. of Biological Sciences, Fordham University, 441 E. Fordham Rd., Bronx, NY 10458 (E-mail: kapas{at}fordham.edu).
Received 26 January 1999; accepted in final form 22 October 1999.
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REFERENCES |
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TOP
ABSTRACT
INTRODUCTION
METHODS
RESULTS
DISCUSSION
REFERENCES
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, Control day; 
, SD day.
Amounts of non-rapid eye movement sleep (NREMS), rapid eye movement
sleep (REMS), wakefulness, EEG 
