Post-trial sleep sequences including transition sleep are involved in avoidance learning of adult rats

High resolution computerized EEG analyses, and behavioral observations were used to identify slow wave sleep (SS), paradoxical sleep (PS) and transition sleep (TS) in adult male Wistar rats exposed to a session of two-way active avoidance training. Of the four sleep sequences that could be identified, two included TS (SS-->TS-->W and SS-->TS-->PS), while the other two did not (SS-->W and SS-->PS). Comparison of post-trial sleep variables between fast learning rats (FL, reaching criterion in the training session), slow learning rats (SL, reaching criterion in the retention session the following day), and non learning rats (NL, failing to reach criterion) indicated that the total amounts of SS, TS and PS of the SS-->TS-->PS sequence was markedly higher in FL rats than in SL rats. In addition, in comparison with the corresponding baseline period, the average duration and total amount of SS and TS episodes of the SS-->TS-->PS sequence increased in FL rats, while the number of SS-->TS-->W sequences decreased. On the other hand, the average duration of SS episodes increased in the SS-->TS-->W and SS-->W sequences of SL rats, and in the SS-->W and SS-->TS-->PS sequences of NL rats. Correlative analyses between number of avoidances and post-trial sleep variables demonstrated that avoidances were directly correlated with the duration of SS episodes of the SS-->TS-->PS sequence and with the duration of TS episodes of the SS-->TS-->W sequence, but inversely correlated with the number and amount of SS episodes of the SS-->W sequence and with the duration and amount of SS episodes of the SS-->PS sequence. On the whole, the data supported the view that TS-containing sleep sequences are involved in long-term storage of novel adaptive behavior, while sleep sequences lacking TS are involved in the maintenance of innate behavioral responses.     

Trains of sleep sequences are indices of learning capacity in rats

In previous work dealing with the identification of four sleep sequences (SS-->W, SS-->PS, SS-->TS-->W and SS-->TS-->PS) in the baseline session of adult male Wistar rats [Mandile P, Vescia S, Montagnese P, Romano F, Giuditta A. Characterization of transition sleep episodes in baseline EEG recordings of adults rats, Physiol Behav 1996;60:1435-1439], we have shown that those containing an intervening episode of transition sleep (TS) strongly correlate with the number of avoidances scored the following day [Vescia S, Mandile P, Montagnese P, Romano F, Cataldo G, Cotugno M, Giuditta A. Baseline transition sleep and associated sleep episodes are related to the learning ability of rats, Physiol Behav 1996;60:1513-152]. More recently, clusters of sleep sequences (trains) separated by waking intervals longer than 60 s have been identified in the baseline session of the same rats [Piscopo S, Mandile P, Montagnese P, Cotugno M, Giuditta A, Vescia S. Identification of trains of sleep sequences in adult rats, Behav Brain Res, this volume], and distinguished in homogeneous or mixed trains according to the presence of a single sleep sequence or more than one sequence. Mixed trains have been further separated into trains containing the SS-->TS-->W sequence (+TSW trains) and trains lacking it (-TSW trains). Analysis of the distribution of variables of baseline trains (and of their sleep sequences and components) among fast learning (FL), slow learning (SL), or non-learning (NL) rats, indicates that variables of +TSW trains prevail in FL rats, while variables of -TSW trains prevail in NL rats. In addition, variables of +TSW trains correlate with the number of avoidances of the training session, while variables of -TSW trains do not significantly correlate, or show inverse correlations. Interestingly, sleep sequences such as SS-->W or SS-->TS-->W show direct or inverse correlations with avoidances depending on whether they are included in +TSW trains or in -TSW trains. The data are interpreted to suggest that the outcome of brain operations performed during a sleep sequence may selectively condition the appearance of later sequences within a time interval shorter than a given threshold. An analogous mechanism may be responsible for the aggregation of sleep components in sleep sequences.     

Learning and sleep: the sequential hypothesis

During the last 30 years, paradoxical sleep (PS) has been generally considered as the only type of sleep involved in memory processing, mainly for the consistent increase of PS episodes in laboratory animals learning a relatively complex task, and for the retention deficits induced by post-training PS deprivation. The vicissitudes of this idea, examined in detail by several laboratories, have been critically presented in a number of review articles However, according to a more comprehensive unitary proposal (the sequential hypothesis), memory processing during sleep does require the initial participation of slow-wave sleep (SS) in addition to the subsequent involvement of PS. The evidence supporting this hypothesis, largely derived from experiments concerning rats trained for a two-way active avoidance task, is reviewed here in some detail. Recent studies of human sleep are in full agreement with this view. In the rat, the main effect of learning on post-training SS consists in the selective increment in the average duration of SS episodes initiating different types of sleep sequences. Notably, following training for a two-way active avoidance task, the occurrence of this effect in sleep sequences including transition sleep (TS), such as SS→TS→W and SS→TS→PS, appears related to the processing of memories of the novel avoidance response. Conversely, the occurrence of the same effect in sleep sequences lacking TS may reflect the processing of memories of innate responses (escapes and freezings). Memories of innate and novel responses are assumed to engage in a dynamic competitive interaction to attain control of waking behaviour. Interestingly, in baseline sleep, variables of SS→TS→W and SS→TS→PS sequences, such as the average duration of SS, TS, and PS episodes, have proved to be good indices of the capacity to learn, as shown by their strong correlations with the number of avoidances scored by rats the following day. Comparable correlations have not been displayed by variables of baseline SS→W and SS→PS sequences.     

Synaptosomal protein synthesis is selectively modulated by learning

Synaptosomes from rat brain have long been used to investigate the properties of synaptic protein synthesis. Comparable analyses have now been made in adult male rats trained for a two-way active avoidance task to examine the hypothesis of its direct participation in brain plastic events. Using Ficoll-purified synaptosomes from neocortex, hippocampus and cerebellum, our data indicate that the capacity of synaptosomal protein synthesis and the specific activity of newly synthesized proteins were not different in trained rats in comparison with home-caged control rats. On the other hand, the synthesis of two proteins of 66.5 kDa and 87.6 kDa separated by SDS-PAGE and analyzed by quantitative densitometry was selectively enhanced in trained rats. In addition, the synthesis of the 66.5 kDa protein, but not of the 87.6 kDa protein, correlated with avoidances and escapes and inversely correlated with freezings in the neocortex, while in the cerebellum it correlated with avoidances and escapes. The data demonstrate the participation of synaptic protein synthesis in plastic events of behaving rats, and the selective, region-specific modulation of the synthesis of a synaptic 66.5 kDa protein by the newly acquired avoidance response and by the reprogramming of innate neural circuits subserving escape and freezing responses.     

Training old rats selectively modulates synaptosomal protein synthesis

We have previously shown that the local synthesis of two synaptic proteins of 66.5‐kDa and 87.6‐kDa is selectively enhanced in male adult rats trained for a two‐way active avoidance task. We report here that a comparable but not identical response occurs in 2‐year‐old male rats trained for the same task. In the latter age group, the local synthesis of the 66.5‐kDa protein markedly increases in cerebral cortex, brainstem, and cerebellum, with a somewhat lower increment in synthesis of the 87.6‐kDa protein. On the other hand, the newly synthesized 87.6‐kDa protein correlates with avoidances and escapes and inversely correlates with freezings in cerebral cortex and brainstem, whereas the correlations of the newly synthesized 66.5‐kDa protein remain below significance. These correlative patterns are sharply at variance with those present in trained adult rats. Our data confirm that the local system of synaptic protein synthesis is selectively modulated by training and show that the synaptic response of old rats differs from that of adult rats as reflected in behavioral responses. © 2012 Wiley Periodicals, Inc.     

Functional topography of the human nonREM sleep electroencephalogram

The sleep EEG of healthy young men was recorded during baseline and recovery sleep after 40 h of waking. To analyse the EEG topography, power spectra were computed from 27 derivations. Mean power maps of the nonREM sleep EEG were calculated for 1-Hz bins between 1.0 and 24.75 Hz. Cluster analysis revealed a topographic segregation into distinct frequency bands which were similar for baseline and recovery sleep, and corresponded closely to the traditional frequency bands. Hallmarks of the power maps were the frontal predominance in the delta and alpha band, the occipital predominance in the theta band, and the sharply delineated vertex maximum in the sigma band. The effect of sleep deprivation on EEG topography was determined by calculating the recovery/baseline ratio of the power spectra. Prolonged waking induced an increase in power in the low-frequency range (1-10.75 Hz) which was largest over the frontal region, and a decrease in power in the sigma band (13-15.75 Hz) which was most pronounced over the vertex. The topographic pattern of the recovery/baseline power ratio was similar to the power ratio between the first and second half of the baseline night. These results indicate that changes in sleep propensity are reflected by specific regional differences in EEG power. The predominant increase of low-frequency power in frontal areas may be due to a high 'recovery need' of the frontal heteromodal association areas of the cortex.     

Improvement of shuttle-box avoidance with post-training intracranial self-stimulation, in rats: a parametric study

Rats were trained in a two-way active avoidance task followed immediately by a lateral hypothalamic intracranial self-stimulation (ICSS) treatment, during 5 consecutive days. The effects of the number of ICSS trains allowed (0, 500, 2500 or 4500) were studied upon acquisition and long-term retention (LTR, 10 and 30 days). The number of ICSS trains administered and the number of avoidances at the last acquisition session (5th) showed a positive lineal relation, that is, the more number of ICSS trains, the more number of avoidances. The level of learning achieved during the 5th session was maintained after the LTR periods in all experimental groups. It is concluded that the number of ICSS trains could be a critical parameter in the facilitatory effect of lateral hypothalamic ICSS upon learning, and it is suggested that the facilitatory effect of post-training lateral hypothalamic ICSS might be due to the activation of general activatory neural systems.     

The Structure of Sleep is Related to the Learning Ability of Rats

Using electroencephalographic methods, rats learning or not learning a two-way active avoidance task were found to differ significantly in the structure of sleep determined the day before training. The main differences concerned (i) synchronized sleep episodes followed by wakefulness, which were longer and fewer in learning rats; (ii) paradoxical sleep episodes, which were longer in learning rats. Significant correlations were present between the number and/or the average duration of synchronized sleep episodes followed by wakefulness or by paradoxical sleep and the number of avoidances or escapes scored in the training session. Power spectral analysis indicated that the relative output in the 6 – 7-Hz region was higher in learning rats, notably during short episodes of synchronized sleep followed by paradoxical sleep. As two-way active avoidance training induces comparable modifications in postacquisition sleep (Ambrosini et al., Physiol. Behav. , 51 , 217 – 226, 1992), the features of preacquisition sleep which prevail in learning rats might directly determine their capacity to learn. Alternatively, they might reflect the existence of a genetic determinant independently conditioning the ability to learn.     

Cortical oscillatory activity and the induction of plasticity in the human motor cortex

Repetitive transcranial magnetic stimulation paradigms such as continuous theta burst stimulation (cTBS) induce long-term potentiation- and long-term depression-like plasticity in the human motor cortex. However, responses to cTBS are highly variable and may depend on the activity of the cortex at the time of stimulation. We investigated whether power in different electroencephalogram (EEG) frequency bands predicted the response to subsequent cTBS, and conversely whether cTBS had after-effects on the EEG. cTBS may utilize similar mechanisms of plasticity to motor learning; thus, we conducted a parallel set of experiments to test whether ongoing electroencephalography could predict performance of a visuomotor training task, and whether training itself had effects on the EEG. Motor evoked potentials (MEPs) provided an index of cortical excitability pre- and post-intervention. The EEG was recorded over the motor cortex pre- and post-intervention, and power spectra were computed. cTBS reduced MEP amplitudes; however, baseline power in the delta, theta, alpha or beta frequencies did not predict responses to cTBS or learning of the visuomotor training task. cTBS had no effect on delta, theta, alpha or beta power. In contrast, there was an increase in alpha power following visuomotor training that was positively correlated with changes in MEP amplitude post-training. The results suggest that the EEG is not a useful state-marker for predicting responses to plasticity-inducing paradigms. The correlation between alpha power and changes in corticospinal excitability following visuomotor training requires further investigation, but may be related to disengagement of the somatosensory system important for motor memory consolidation.     

Sleep homeostasis in suprachiasmatic nuclei-lesioned rats: effects of sleep deprivation and triazolam administration

The electroencephalogram (EEG) and electromyogram of rats with lesions in the suprachiasmatic nuclei (SCNx) were recorded during two series of 24-h baseline, 6-h sleep deprivation (SD), and 24-h recovery. At recovery onset, rats were injected i.p. with vehicle (VEH) control solution or 0.4 mg/kg triazolam (TRZ) in a balanced crossover design. Consecutive 10-s epochs were scored for vigilance states and EEG power spectra were computed. Arousal states were uniformly distributed during 24-h baseline (wake 47% of recording time, non-rapid-eye movement sleep (nonREMS) 47%, REMS 7%), and EEG spectra (0-25 Hz) were devoid of significant trends. State-specific EEG power spectra profiles in SCNx rats were similar to those of intact animals reported previously. However, EEG delta power (0.5-3.5 Hz) of nonREMS was markedly lower in SCNx rats. Recovery from 6-h SD was characterised by a short-lasting reduction of REMS, and a long-lasting increase of nonREMS time at the cost of wakefulness. EEG delta power rebounded during the first 8 h in recovery, and fell below baseline level after 12 h in recovery. During 0-2 h TRZ recovery, rats spent more time in nonREMS with higher EEG slow wave activity as compared to the corresponding VEH recovery period. EEG slow wave activity fell below baseline levels 10 h after TRZ injection and termination of SD. We conclude that major features of homeostatic sleep EEG regulation are present in SCNx rats.(ABSTRACT TRUNCATED AT 250 WORDS)     

Angiotensin II protects cultured midbrain dopaminergic neurons against rotenone-induced cell death

In humans, EEG power in the theta frequency band (5-8 Hz) during quiet waking increases during sleep deprivation (SD), and predicts the subsequent homeostatic increase of sleep slow-wave activity (SWA; EEG power between 0.5 and 4.0 Hz). These findings indicate that theta power in waking is an EEG variable, which reflects the rise in sleep propensity. In rodents, a number of short sleep attempts, as well as SWA in the waking EEG increase in the course of SD, but neither variable predicts the subsequent homeostatic increase of EEG SWA during recovery sleep. To investigate whether there is an EEG marker for sleep propensity also in rodents, the EEG of the rat was recorded during 6 h SD in the first half of the light period (SDL, n = 7). During SDL, power of the waking EEG showed an increase in the delta (1.5-4 Hz) and low theta (5-6.5 Hz) band. Based on the neck muscle EMG, wakefulness was subdivided into active (high EMG activity) and quiet (low EMG activity) waking. During quiet waking, the theta peak occurred at 5.5 Hz, the frequency at which the increase of EEG power during SD was most pronounced. This increase was due to higher amplitude of theta waves, while wave incidence (frequency) was unchanged. Correlation analysis showed that the rise in EEG power in the 5-7 Hz band during SD predicted the subsequent enhancement of SWA in non-rapid eye movement sleep. The analysis of data of a further batch of rats which were sleep deprived for 6 h after dark onset (SDD, n = 7) revealed a significant increase in theta-wave amplitude during the SD and a tendency for a similar, positive correlation between the increase of theta power (5-7 Hz) and subsequent SWA. The results indicate that in rats, as in humans, a specific waking EEG frequency, i.e., theta power in quiet waking is a marker of sleep propensity.     

High frequency waking EEG: reflection of a slow ultradian rhythm in daytime arousal

The ultradian dynamics of the human waking EEG was studied using a short visual fixation task repeated every 10 min throughout the daytime. The EEG spectra obtained from the tasks were assessed for time effect and ultradian periodicity. Fronto-central EEG high frequency powers (22.5-44.5 Hz) decreased at the time of the midafternoon vigilance dip (14.00-17.00 h) along with slight concomitant increases in parietal alpha (7.5-13.5 Hz) and delta (1-3 Hz) powers. A slow ultradian rhythm with a 3-4 h periodicity strongly modulated EEG power in all frequency bands between 1 and 44.5 Hz. The high frequency waking EEG may well reflect the activity of a brain arousal process underlying maintenance of the waking state probably throughout the 24 h cycle.     

Sleep deprivation: effect on sleep stages and EEG power density in man

Sleep was analysed in 8 young adults subjects during two baseline nights and two recovery nights following 40.5 h sleep deprivation. Sleep stages were scored from the polygraph records according to conventional criteria. In addition, the EEG records of the entire nights were subjected to spectral analysis to compute the frequency distribution of the power density in the 0.25-25 Hz range for 0.5 Hz or 1.0 Hz bins. In the first recovery night, the power density in the delta band was significantly higher than baseline for total sleep time as well as for sleep stages 2, 3 and 4, 4 and REM. These changes were not restricted to the delta band, but extended to higher frequency bands. Minor, but significant, effects of sleep deprivation were seen in the power density distribution of the second recovery night. In the baseline nights, a progressive reduction of power density in the delta/theta range was present for successive non-REM-REM sleep cycles for total sleep time and stages 2, 3 and 4, and 4. The results show that effects of sleep deprivation as well as trends within the sleep periods are readily apparent from spectral analysis, but are inadequately reflected by conventional sleep scoring. When the power density values were integrated over the entire frequency range (0.75-25 Hz) for each non-REM-REM sleep cycle, an exponential decline from cycle 1 to cycle 3 was suggested. The present findings support the hypothesis that the EEG power density in the low frequency range is an indicator of a progressively declining process during sleep whose initial value is determined by the duration of prior waking.     

Changes in EEG power spectra and behavioral states in rats exposed to the acetylcholinesterase inhibitor chlorpyrifos and muscarinic agonist oxotremorine

Organophosphates (OPs) inhibit acetylcholinesterase (AChE) activity causing cholinergic stimulation in the central nervous system (CNS). Cholinergic systems are crucial in electroencephalogram (EEG) generation and regulation of behavior; however, little is known about how OP exposure affects the EEG and behavioral states. We recorded EEG, core temperature and motor activity before and after exposure to the OP pesticide chlorpyrifos (CHP) in adult female rats implanted with telemetric transmitters. The recording and reference electrodes were placed in the occipital and frontal bones, respectively. The animals received CHP, 25 mg/kg, p.o., or oxotremorine (OX), 0.2 mg/kg, s.c. CHP led to a significant increase in delta (0.1-3.5 Hz), slow theta (4-6.5 Hz), gamma 2 (35.5-50 Hz), reduction in fast theta (7-8.5 Hz), alpha/sigma (9-14 Hz), beta 1 (14.5-24 Hz), beta 2 (24.5-30 Hz) and gamma 1 (30.5-35 Hz) powers, slowing of peak frequencies in 1-9 Hz range, hypothermia and decrease in motor activity. The drop in 7-14 Hz was associated with cholinergic suppression of sleep spindles. Changes in behavioral state were characterized by dramatic diminution of sleep postures and exploring activity and prolongation of quiet waking. There was recovery in all bands in spite of continued inhibition of AChE activity [44,45] in rats exposed to CHP. OX-induced EEG and behavioral alterations were similar to CHP except there was no increase in delta and the onset and recovery were more rapid. We did not find a correlation between the EEG and core temperature alterations. Overall, changes in EEG (except in delta band) and behavior following CHP were attributable to muscarinic stimulation. Cortical arousal together with increased quiet waking and decreased sleep after CHP occurred independently from inhibition of motor activity and lowering of core temperature.     

Post-training intra-amygdala amphetamine injections given during acquisition of a stimulus-response (S-R) habit task enhance the expression of stimulus-reward learning: further evidence for incidental amygdala learning

The effect of post-training intra-amygdala amphetamine injections was examined on the acquisition and expression of a visual discrimination task. Rats were trained to enter four lit arms for food (stimulus-response) and avoid unlit arms on an eight-arm radial maze visual discrimination task. Post-training intra-amygdala amphetamine injections (10 microg) were given for 4 consecutive days during the mid-point of training (days 20-23). The number of lit arm entries was used as a measure of stimulus-response habit learning 24 h after each injection. Twenty-four hours after the last injection, a transfer test was run to assess the effect of the same post-training manipulation. This transfer test assessed the amount of time spent in the lit arms and was used as a measure of stimulus-reward learning. Compared to saline-injected rats, rats that received post-training amphetamine spent more time in lit as opposed to dark arms during the transfer test. This occurred in the absence of an increase in the number of correct arm entries during visual discrimination training. This suggests that post-training amphetamine strengthened a stimulus-reward association that did not immediately affect behavioral output. This association may reflect a mnemonic representation stored in an ensemble of amygdala neurons.     

EEG power changes are related to regional cerebral glucose metabolism in vascular dementia.

OBJECTIVE: In patients with vascular dementia (VD), the relationship between the EEG power within the 4 frequency bands and the regional metabolic disturbances was investigated. METHODS: Twenty-eight patients (age 69.0+/-6.54 years) with VD according to NINDS-AIREN criteria underwent quantitative EEG recording, according to the 10-20 system, and fluodeoxyglucose F18 positron emission tomography (PET) at resting condition within 24 h. EEG power FFT-analysis was performed for delta (2-3.5 Hz), theta (4-7.5 Hz), alpha (8-13 Hz) and beta (13.5-20 Hz) frequency bands. Regional EEG power bands were related to regional glucose metabolism in anatomically defined regions corresponding to locations of the 10-20 system. RESULTS: Correlation between slow frequency band power and glucose metabolism was found. A widespread inverse relationship of delta power to metabolism was found between various regions; additionally, delta power was negatively correlated to cerebral glucose metabolism in individual regions. Frontal theta power correlated especially with thalamic CMRglc. Alpha power correlated directly with metabolism in the occipital lobe. No significant relationships were found between beta power and metabolism. CONCLUSION: We conclude that EEG power in VD is linked to glucose metabolism, indicating specific regional dependencies.     

Analysis of the vigilance stages in the rat by fast Fourier transformation

The characteristics of the power density spectra of the frontal and occipital cortical EEG between 1 and 31 Hz as well as the total power of the neck muscle EMG during an 8-second epoch were used to differentiate 8 vigilance stages in normal rats. The following stages and the time spent within these stages during a 6-hour recording (9.00-15.00) were evaluated: waking with active EMG (about 127 min), waking (7 +/- 3 min), theta-waking (25.5 +/- 9 min), classical sleep I (dozing; 107 +/- 11 min), classical sleep II (41 +/- 6 min), paradoxical sleep (PS; 30 +/- 9 min), transitional phase (PS spindles; 9.5 +/- 4 min) and transitional dozing (13 +/- 3 min). Two additional stages were differentiated in rats suffering from petit mal: paroxysmally occurring spikes and waves and an especially marked transitional phase. The absolute power spectra of these vigilance stages (except active waking) were plotted as well as the statistically analyzed deviations from the mean power spectra of all epochs without active EMG. A comparison between the mean power spectra of a control recording and a recording after oral application of 10 mg/kg diazepam indicates that these differences are less marked than between the vigilance stages.     

Memory consolidation in children with specific language impairment: Delayed gains and susceptibility to interference in implicit sequence learning

In this study, the time course of the procedural learning of a visuomotor sequence skill was followed over a 24-hour and a 1-week time period in children with and without specific language impairment (SLI). Two aspects of memory consolidation in implicit sequence learning were examined: the evolution of post-training gains in sequence knowledge (Experiment 1) and the susceptibility to interference (Experiment 2). In the first experiment, 18 children with SLI and 17 control children matched for sex, age, and nonverbal intelligence completed a serial reaction-time (SRT) task and were tested 24 hours and 1 week after practicing. The two groups of children attained an equal level of sequence knowledge in the training session, but the children with SLI lacked the consolidation gains displayed by the control children in the two post-training sessions. Working with a new group of children, 17 with SLI and 17 control peers, Experiment 2 examined resistance to interference by introducing a second sequence 15 min after the first training session. Similar results were obtained for the performance of both groups in the training session. However, although the performance of the control group improved in the post-training sessions, the performance of the SLI group deteriorated significantly during the consolidation phase due to the interfering sequence. These findings suggest that the consolidation phase of sequence learning is impaired in children with SLI.     

Classical sleep stages and the spectral content of the EEG signal

Polygraphic sleep recordings during 12 nights in 5 healthy volunteers were classified manually into waking and the various sleep stages. The smoothed power spectra of EEG signal segments defined as waking or one of the sleep stages were calculated via segmentation of the EEG signal, using the autoregressive model, and time-dependent fuzzy clustering. The spectra were derived from the prediction coefficients of the segments. The relative power in the delta frequency band was found to increase monotonically with increasing depth of sleep, together with a parallel decrease in the alpha relative power. In most cases alpha relative power had a small peak during REM sleep, and on average the relative power in the sigma frequency band during REM sleep was smaller than the beta relative power. The power spectra from subjects with no waking alpha differed from those of subjects with abundant waking alpha mostly in the relative spectral content of stages 1 and REM. The significance of these findings is discussed in relation to future standardisation of automatic analysis of sleep recordings.