Association of sleep parameters and memory in intact old rats and young rats with lesions in the nucleus basalis magnocellularis

Relations between sleep and memory were examined as a function of aging in rats. Sleep (24 hr), passive avoidance retention, and choline acetyltransferase (CAT) activity were assessed in 3 age-groups (6, 15, and 24 months old). Age-related alterations were evident in sleep, memory, and cortical and striatal CAT activity. Retention deficits in old rats were significantly correlated with several measures of paradoxical sleep. Similar analyses in 6- and 15-month-old rats with ibotenic acid-induced lesions of the nucleus basalis magnocellularis (NBM) showed several alterations in sleep, memory, and cortical CAT activity comparable to those seen in the old rats. One measure of paradoxical sleep, bout duration, correlated significantly with retention scores in rats with lesions. Thus, fragmented paradoxical sleep accompanies memory impairments in old rats and in young rats with NBM lesions.     

Individual differences in cognitive aging: implication of pregnenolone sulfate

In humans and animals, individual differences in aging of cognitive functions are classically reported. Some old individuals exhibit performances similar to those of young subjects while others are severely impaired. In senescent animals, we have previously demonstrated a significant correlation between the cognitive performance and the cerebral concentration of a neurosteroid, the pregnenolone sulfate (PREG-S).Neurotransmitter systems modulated by this neurosteroid were unknown until our recent report of an enhancement of acetylcholine (ACh) release in basolateral amygdala, cortex and hippocampus induced by intracerebroventricular (i.c.v.) or intracerebral administrations of PREG-S. Central ACh neurotransmission is known to be involved in the regulation of memory processes and is affected in normal aging and severely altered in human neurodegenerative pathologies like Alzheimer's disease.In the central nervous system, ACh neurotransmission is also involved in the modulation of sleep-wakefulness cycle, and particularly the paradoxical sleep (PS). Relationships between paradoxical sleep and memory are documented in the literature in old animals in which the spatial memory performance positively correlates with the basal amounts of paradoxical sleep. PREG-S infused at the level of ACh cell bodies (nucleus basalis magnocellularis, NBM, or pedunculopontine nucleus, PPT) increases paradoxical sleep in young animals.Finally, aging related cognitive dysfunctions, particularly those observed in Alzheimer's disease, have also been related to alterations of mechanisms underlying cerebral plasticity. Amongst these mechanisms, neurogenesis has been extensively studied recently. Our data demonstrate that PREG-S central infusions dramatically increase neurogenesis, this effect could be related to the negative modulator properties of this steroid at the GABA(A) receptor level.Taken together these data suggest that neurosteroids can influence cognitive processes, particularly in senescent subjects, through a modulation of ACh neurotransmission associated with paradoxical sleep modifications; furthermore, our recent data suggest a critical role for neurosteroids in the modulation of cerebral plasticity, mainly on hippocampal neurogenesis.     

The diurnal rhythm of adenosine levels in the basal forebrain of young and old rats

There are significant decrements in sleep with age. These include fragmentation of sleep, increased wake time, decrease in the length of sleep bouts, decrease in the amplitude of the diurnal rhythm of sleep, decrease in rapid eye movement sleep and a profound decrease in electroencephalogram Delta power (0.3-4 Hz). Old rats also have less sleep in response to 12 h-prolonged wakefulness (W) indicating a reduction in sleep drive with age. The mechanism contributing to the decline in sleep with aging is not known but cannot be attributed to loss of neurons implicated in sleep since the numbers of neurons in the ventral lateral preoptic area, a region implicated in generating sleep, is similar between young (3.5 months) and old (21.5 months) rats. One possibility for the reduced sleep drive with age is that sleep-wake active neurons may be stimulated less as a result of a decline in endogenous sleep factors. Here, we test this hypothesis by focusing on the purine, adenosine (AD), one such sleep factor that increases after prolonged W. In experiment 1, microdialysis measurements of AD in the basal forebrain at 1 h intervals reveal that old (21.5 months) rats have more extracellular levels of AD compared with young rats across the 24 h diurnal cycle. In experiment 2, old rats kept awake for 6 h (first half of lights-on period) accumulated more AD compared with young rats. If old rats have more AD then why do they sleep less? To investigate whether changes in sensitivity of the AD receptor contribute to the decline in sleep, experiments 3 and 4 determined that for the same concentration of AD or the AD receptor 1 agonist, cyclohexyladenosine, old rats have less sleep compared with young rats. We conclude that even though old rats have more AD, a reduction in the sensitivity of the AD receptor to the ligand does not transduce the AD signal at the same strength as in young rats and may be a contributing factor to the decline in sleep drive in the elderly.     

Age-related changes in sleep in the rat

Human sleep in old age is characterized by a number of changes, including reductions in sleep efficiency, amounts of visually scored slow-wave and REM sleep, and amplitude of the diurnal sleep/wake rhythm. In older rats, some, but not all, of these traits have been reported, including a decrease in the mean duration of sleep bouts, an increase in the number of sleep bouts, and a modest reduction of REM sleep. Studies of the diurnal rhythm of total sleep have had varied results. There are, however, virtually no data indicating at what point across the rat's lifetime the changes seen in old age begin to occur. In order to more fully characterize sleep in older rats, and to develop data on when they first appear, we have examined sleep in young adult (3 months), middle-aged (12 months), and older (24 months) rats during 24 hours under constant dim light. Analyses of variance revealed no age-related changes in total sleep, NREM or REM sleep, wake time after sleep onset, or three different measures of the amplitude of the sleep/wake circadian rhythm. There were, however, significant age-related reductions in high-voltage NREM sleep ("HS2"), the mean length of sleep bouts, and REM-onset duration. These were seen in the 1-year-old rats, indicating that the changes seen in the older animals were evident by midlife.     

Individual differences in aging: behavioral and neurobiological correlates

The goal of this experiment was to determine the correlations among different behavioral and neurobiological measures in aged rats. Aged Sprague-Dawley rats were given a battery of cognitive and sensorimotor tests, followed by electrophysiological assessment of sleep and biochemical measurements of various neurotransmitter systems. The behavioral tests included the following: Activity level in an open field; short-term and long-term memory of a spatial environment as assessed by habituation: spatial navigation, discrimination reversal, and cue learning in the Morris water pool; spatial memory in a T-maze motivated by escape from water; spatial memory and reversal on the Barnes circular platform task; passive avoidance; motor skills. Sleep was assessed by electrographic cortical records. The following neurotransmitter markers were examined: Choline acetyltransferase; the density of nicotinic, benzodiazepine and glutamine receptors in the cortex and caudate nucleus; endogenous levels of norepinephrine, dopamine, and serotonin in the cortex and hippocampus. The duration of bouts of paradoxical sleep was strongly correlated with several cognitive measures and selected serotonergic markers. This finding suggests that changes in sleep patterns and brain biochemistry contribute directly to deficits in learning and memory, or that the same neurobiological defect contributes to age-related impairments in sleep and in learning and memory.     

Glucocorticoids are not responsible for paradoxical sleep deprivation-induced memory impairments

STUDY OBJECTIVES: To evaluate whether paradoxical sleep deprivation-induced memory impairments are due to release of glucocorticoids, by means of corticosterone inhibition with metyrapone. DESIGN: The design was a 2 (Groups [control, paradoxical sleep-deprived]) x 2 (Treatments [vehicle, metyrapone]) study, performed in 2 experiments: Acute treatment (single injection given immediately after 96 hours of sleep deprivation) and chronic treatment (8 injections, twice per day, throughout the sleep-deprivation period). Animals were either paradoxical sleep-deprived or remained in their home cages for 96 hours before training in contextual fear conditioning and received intraperitoneal injections of a corticosterone synthesis inhibitor, metyrapone. Memory performance was tested 24 hours after training. SUBJECTS: Three-month old Wistar male rats. Measurements: Freezing behavior was considered as the conditioning index, and adrenocorticotropic hormone and corticosterone plasma levels were determined from trunk blood of animals sacrificed in different time points. Animals were weighed before and after the paradoxical sleep-deprivation period. RESULTS: Acute metyrapone treatment impaired memory in control animals and did not prevent paradoxical sleep deprivation-induced memory impairment. Likewise, in the chronic treatment, paradoxical sleep-deprived animals did not differ from control rats in their corticosterone or adrenocorticotropic hormone response to training, but still did not learn as well, and did not show any stress responses to the testing. Chronic metyrapone was, however, effective in preventing the weight loss typically observed in paradoxical sleep-deprived animals. CONCLUSIONS: Our results suggest that glucocorticoids do not mediate memory impairments but might be responsible for the weight loss induced by paradoxical sleep deprivation.     

Sleep wake profile and EEG spectral power in young or old senescence accelerated mice

Changes occurring with age in cortical EEG and sleep-wake states architecture were examined in senescence accelerated prone (SAMP8) or senescence resistant (SAMR1) mice (age: 2 and 12 months) under baseline conditions or after a 4 h sleep deprivation (SD). In baseline conditions, an increase in slow wave sleep (SWS) amount (21-24%) occurs at the expense of the wakefulness (W) in old SAMP8 and SAMR1 mice versus young animals. In these conditions, SWS latency is reduced (67-72%). Moreover, in SAMP8 and SAMR1 mice, aging deteriorates paradoxical sleep (PS) architecture with more pronounced changes in SAMP8 (amount: -63%; episode duration: -44%; latency: +286%; circadian component loss; and EEG theta (theta) peak frequency (TPF): -1 Hz). During the 4 h recovery subsequent to a 4 h sleep deprivation, old SAMP8 mice exhibit an enhanced sensitivity resulting in SWS (+62%) and PS (+120%) rebounds, a characteristic of this inbred strain. Results obtained are discussed in line with the age-related learning and memory impairments existing in SAMP8 animals. In particular, the reduced cognitive performances described in old SAMP8 might be linked to the TPF deterioration during PS.     

Selective deprivation of paradoxical sleep and consolidation of shuttle-box avoidance.

Investigated whether paradoxical sleep is implicated in the storage of information acquired during shuttle-box avoidance. Wistar rats were given 5 brief training sessions distributed over the light period of the diurnal cycle. During the intervals between sessions the animals were selectively deprived of paradoxical sleep by awakening them every time they showed this type of sleep. The onset of paradoxical sleep was identified when hippocampal theta rhythm occurred during behavioural sleep. Yoked control animals got the same treatment irrespective of their sleep-waking behaviour, whereas free sleep rats were allowed to sleep undisturbed. In spite of large differences in the amount of paradoxical sleep during the intersession intervals no differences in learning performances were found among the groups. A tendency toward more intertrial crossings was noted in the paradoxical sleep deprived group at the end of training. It is concluded that storage of information acquired during distributed shuttle-box avoidance is not dependent on the presence of paradoxical sleep immediately following learning. Some possibilities are considered that paradoxical sleep may still be involved in memory storage processes.     

Glucose and age-related changes in memory

Epinephrine, released from the adrenal medulla, enhances memory in young rats and mice and apparently does so, at least in part, by increasing blood glucose levels. Like epinephrine, administration of glucose enhances cognitive functions in humans and rodents, including reversing age-related impairments in learning and memory. Epinephrine responses to training are increased in aged rats but the subsequent increase in blood glucose levels is severely blunted. The absence of increases in blood glucose levels during training might contribute to age-related deficits in learning and memory. Also, extracellular glucose levels in the hippocampus are depleted during spontaneous alternation testing to a far greater extent in aged than in young rats. Importantly, systemic injections of glucose block the depletion in the hippocampus and also enhance performance on the alternation task. Thus, the extensive depletion of extracellular glucose during training in aged rats may be associated with age-related memory impairments, an effect that might be related to - or may exacerbate - the effects on learning and memory of an absence of the increases in blood glucose levels to training as seen in young rats. Together, these findings suggest that age-related changes in both peripheral and central glucose physiology contribute to age-related impairments in memory.     

The wake-promoting effects of hypocretin-1 are attenuated in old rats

Disruption of sleep is a frequent complaint among elderly humans and is also evident in aged laboratory rodents. The neurobiological bases of age-related sleep/wake disruption are unknown. Given the critical role of the hypocretins in sleep/wake regulation, we sought to determine whether the wake-promoting effect of hypocretin changes with age in Wistar rats, a strain in which age-related changes in both sleep and hypocretin signaling have been reported. Intracerebroventricular infusions of hypocretin-1 (10 and 30 μg) significantly increased wake time relative to vehicle in both young (3 mos) and old (25 mos) Wistar rats. However, the magnitude and duration of the wake-promoting effects were attenuated with age. An increase of parameters associated with homeostatic sleep recovery after sleep deprivation, including non-rapid eye movement (NR) sleep time, NR delta power, the ratio of NR to rapid eye movement (REM) sleep, and NR consolidation, occurred subsequent to Hcrt-induced waking in young but not old rats. ICV infusions of hypocretin-2 (10 and 30 μg) produced fewer effects in both young and old rats. These data demonstrate that activation of a major sleep/wake regulatory pathway is attenuated in old rats.     

Effect of paradoxical sleep deprivation and stress on passive avoidance behavior

Previous studies have shown that several types of stress can induce memory impairment. However, the memory effects of paradoxical sleep deprivation (PSD), a stressor in itself, are unclear. We therefore compared passive avoidance behavior of rats undergoing PSD and PSD stress yoked-control (PSC) using the "reversed flowerpot method." When rats were kept isolated on a PSC platform for 24 h immediately after criterion training, retention trials showed impaired aversive memory storage. When delayed for 24 h after criterion training, PSC stress did not disrupt retention performance. In rats subjected to PSD, either immediately or 24 h after criterion training, there was no disruption of aversive memory consolidation. These results suggest that, during stress, paradoxical sleep plays a role in erasing aversive memory traces, in line with the theory that we "dream in order to forget."     

Distinct sleep-wake stages in rats depend differentially on age

The objective was to study age-related changes in sleep-wake stages in the rat by using precise polyphysiograph criteria for stage identification. Cortical and hippocampal electroencephalogram, and ocular and myographic activities were recorded in young, middle-aged, and old male Wistar rats to define 6 stages: active and quiet wake (AW, QW); light and deep slow wave sleep (SWS: S1, S2); intermediate stage of sleep (IS); and paradoxical sleep (PS). The old rats displayed a decrease in S1, S2, and IS, accompanied by an enhancement of AW. No age changes were found for QW and PS. It is suggested that the consolidation of SWS is primarily disturbed in the old rats, which may lead to a facilitation of wake.     

Infusion of neurosteroids into the rat nucleus basalis affects paradoxical sleep in accordance with their memory modulating properties.

The neurosteroids pregnenolone sulfate and allopregnanolone affect memory processes in an opposite manner, pregnenolone sulfate acts as a potent memory-enhancer whereas allopregnanolone impairs memory performance. The mechanisms underlying these memory modulating properties have yet to be elucidated. We have previously reported that infusions of either neurosteroid into the nucleus basalis magnocellularis, one of the main forebrain cholinergic nuclei, differentially affect spatial memory in rats. The relationships between memory performance and paradoxical sleep are well documented, therefore we investigated whether neurosteroids infused into the nucleus basalis magnocellularis affected the sleep-wakefulness cycle in rats, measured by electroencephalographic recordings. Results show that pregnenolone sulfate (5 ng) increased by 12%, whereas allopregnanolone (2 ng) decreased by 24%, the duration of paradoxical sleep in the 24 h interval following injection compared to control recordings. Pregnenolone sulfate inhibits GABA(A) receptors whereas allopregnanolone stimulates them. Since cholinergic neurons of the nucleus basalis magnocellularis are GABA-modulated, it may be postulated that these neurosteroids modify paradoxical sleep by acting on the cholinergic transmission. This may account, at least in part, for the memory modulating properties of these compounds.     

Age-dependent changes in recovery sleep after 48 hours of sleep deprivation in rats

To characterize possible changes in homeostatic regulation of sleep with aging, we have examined sleep stages during recovery sleep after 48 h of sleep deprivation in young (3 months), middle aged (12 months), and old (24 months) rats. It was found that young and middle aged, in contrast to old rats, had large (21-24%) increases in total sleep time during recovery sleep; the old rats experienced a quantitatively small (8%) but significant rise in total sleep. NREM sleep increased significantly during the recovery period in young and middle aged, but not older rats. High voltage NREM sleep (HS2) declined by 30% during recovery in the young animals, but remained unchanged compared to baseline in the middle aged and old animals. The young and middle aged rats had increases in REM sleep during recovery compared to their baseline by 96% and 93%, respectively, which was significantly greater than a 65% increase during recovery in the old rats. Increases in total sleep and REM sleep during recovery were largely confined to the first 6 h in young and middle aged rats, but maxima for the old rats occurred in the second 6 h.     

Age-related changes in control of blood pressure and heart rate during sleep in the rat

STUDY OBJECTIVES: The aim of this study was to determine age-related changes in the control of mean arterial pressure (MAP) and heart rate (HR) during sleep, and its relationship to the baroreflex in aging. DESIGN: MAP, HR, body temperature (TP), spontaneous activity (ACT), and sleeping/waking duration were monitored for 24 hours in groups of young (10-12 wk old) and old (23-24 mo old) rats. SETTING: The sleep laboratory at the University of Tokushima. PARTICIPANTS: Subjects were 8 young (10-12 wk old) and 7 old (23-24 mo old) Wistar rats. INTERVENTIONS: Reflex control of HR was evaluated by examining various pressure responses to an intravenous bolus injection of phenylephrine and sodium nitroprusside. MEASUREMENTS AND RESULTS: MAP and TP were recorded by a radiotelemetry system. HR was detected from the AP signal. ACT was counted by a photo-sensor system. In the case of old rats, the sensitivity of baroreflex control of HR was significantly depressed, and the spontaneous increase of MAP and HR during REM sleep and the MAP drop at the end of REM sleep were significantly enhanced. The old rats showed no large deterioration of the circadian profiles of MAP, HR, TP, and the amount of sleep. CONCLUSIONS: The baroreflex dysfunction is considered to appear in an early stage of the aging process, and to affect the control of MAP and HR during sleep.     

Sleep deficits in rats after NMDA receptor blockade.

N-Methyl-D-aspartate (NMDA) receptor blockade disrupts a variety of functions associated with neural plasticity, including acquisition of learned responses and long-term potentiation. Deficits in memory are significantly correlated with deficits in measures of paradoxical sleep in several amnesic populations. The present experiment therefore assessed whether NPC 12626, a competitive NMDA receptor antagonist, also disrupts sleep. NPC 12626 (1, 10, 50, and 100 mg/kg) or saline was administered to Sprague-Dawley rats 30 min prior to 3-h daytime recording periods. Paradoxical sleep was selectively impaired at all but the highest dose, which prevented all sleep during the recording period. Some deficits in nonparadoxical sleep first appeared at the 10 mg/kg dose but did not became prominent until the 50 mg/kg dose. The results thus show that NPC 12626 impairs sleep states in rats and demonstrate that paradoxical sleep is particularly susceptible to the effects of NMDA receptor blockade. These findings, along with previous evidence that NMDA antagonists impair waking measures of arousal, provide evidence that all sleep-wake states are impaired by NMDA receptor blockade. More generally, the results suggest that some brain mechanisms underlying sleep and memory may share common elements.     

Changes in the sleep-wake cycle architecture and cortical nitric oxide release during ageing in the rat

Changes in sleep-wake states and nitric oxide release were examined in aged rats versus young-adult ones. Sleep-wake recordings and nitric oxide measurements were taken from animals chronically equipped with polygraphic and voltametric electrodes. Animals were examined in baseline conditions and in response to a 24-hour paradoxical sleep deprivation. In aged rats, basal amount of paradoxical sleep is decreased during the light phase versus young-adult animals. After paradoxical sleep deprivation, a paradoxical sleep rebound occurs with an amount and intensity that are less marked in aged animals than in young-adult rats. The amplitude of the circadian distribution for wakefulness, slow-wave sleep and paradoxical sleep amounts is reduced with age. Finally, delta-slow-wave sleep and theta-paradoxical sleep power spectra are attenuated either in baseline conditions or after paradoxical sleep deprivation in aged animals. It is also reported that cortical nitric oxide release exhibits a circadian rhythm with higher amplitude in aged rats than in young-adult ones. However, after paradoxical sleep deprivation, a limited overproduction of nitric oxide is obtained compared with young-adult ones. These results, evidencing the dynamics of the nitric oxide changes occurring in relation to the sleep-wake cycle, point out the homeostatic paradoxical sleep regulation as an age-dependent process in which the nitric oxide molecule is possibly involved.     

The effect of apomorphine on genital reflexes in male rats deprived of paradoxical sleep

Drugs that stimulate dopamine (DA) systems can stimulate sexual arousal in male rats and humans, and previous work has shown that cocaine enhances genital reflexes [penile erection (PE) and ejaculation (EJ)] in rats deprived of paradoxical sleep (PS). The present study sought to expand the latter finding by assessing the effects of DA receptor agonist apomorphine in sleep-deprived rats. Apomorphine in doses ranging from 10 to 240 microg/kg was administered intraperitoneally to rats that had been deprived of sleep for 4 days and to normal controls, and the incidence of PEs and EJs was measured for 60 min. Sleep deprivation alone induced PE and this effect was potentiated by apomorphine, with maximal effects occurring with the 120 microg/kg dose; results for this dose group differed from those of PSD groups treated with 0, 10, 20, 40, 80, and 240 microg/kg of apomorphine. Sleep deprivation alone also induced spontaneous EJ, but this response was not potentiated by apomorphine in the dose range tested. We suggest that the potentiating effects of apomorphine on PE are likely due to PSD-induced DA receptor supersensitivity.     

Deterioration of spatial and nonspatial reference and working memory in aged rats: protective effect of life-long calorie restriction.

Two different aspects of learning (spatial and nonspatial) and two different types of memory (reference and working) were simultaneously measured in populations of 3- (young), 11- (adult), and 25-month-old (aged) rats fed ad libitum either a standard (ST) or a hypocaloric (HY) diet. All groups, regardless of age or diet, showed ability in learning all four versions of the task. However, old ST rats were significantly slower and less efficient at learning than the young and adult ST rats. In contrast, senescent HY rats' cognitive abilities did not differ from those of their young and adult counterparts. The decline in reference and working memory in the aged ST rats was more pronounced in the spatial than the nonspatial version of the task. This study confirms and extends to more specific aspects of memory our earlier finding that age-related cognitive deterioration in rats was antagonized by life-long calorie restriction.     

Memory consolidation in senescence: effects of CO2, amphetamine and morphine

—Alterations in memory storage processes that occur in senescence were investigated by challenging young and old female “small Wistar” rats with posttraining administration of CO₂, ampethamine or morphine, and measuring retention performance. Neither duration of CO₂ immersion, nor the time of CO₂ immersion after training had a differential amnestic effect with age on retention of a one-trial, shock-motivated inhibitory avoidance task. This results indicate that the times during which memory is susceptible to disruption for old and young rats are similar. Challenge with drugs, however, did reveal age-related alterations in memory storage processes. Amphetamine attenuated CO₂-induced amnesia in young rats, but had no effect in old rats. This could not be attributed to a general decline in response to amphetamine in old rats because amphetamine increased open field activity of both young and old animals. Morphine also had a differential effect on memory with age: it caused amnesia in old rats trained in a one-trial hot plate escape task, while having no effect on retention performance of young rats. Thus, the modulatory influence of catecholamine and opioid systems on memory processes is probably altered in senescence.