Patterned activation of hippocampal network (∼10 Hz) during in vitro sharp wave-ripples

Sharp waves (SPWs) are endogenous hippocampal network activity that occurs during certain behaviors and it is thought to be involved in the process of memory consolidation. Frequently, SPWs are generated in bursts or clusters of several consecutive events forming discrete episodes of activity, a hitherto unexplored feature of this prominent hippocampal network activity. In the present study, using rat ventral hippocampal slices, we show that clusters of SPWs consist of two to four consecutive events occurring at a frequency of ∼10 Hz (range, 7–14 Hz). Similarly to the first (primary) event in a cluster the following (secondary) SPWs correspond to inhibitory postsynaptic potentials in CA1 pyramidal cells. Furthermore, the initiation of secondary SPWs in the 23% of cells coincides with postinhibitory rebound excitation. Antagonists of NMDA receptors reversibly abolish secondary but not primary SPWs suggesting that their generation depend on the activation of NMDA receptors. Furthermore, the generation of clusters of SPWs is very sensitive to moderate pharmacological reduction or enhancement of the GABA (A) receptor-mediated transmission suggesting that precise levels of GABAergic transmission are required for the clustered generation of SPWs. In addition, enhancement of GABA (A) receptor-mediated transmission affects the timing of secondary SPWs initiation. Trains of high-frequency (100 Hz) or theta burst stimulation at the Schaffer collaterals that induce long-term potentiation of the evoked field response enhance the incidence of SPWs' clusters and the amplitude of the primary SPWs. We propose that sequential ∼10 Hz clustered activation of the local hippocampal circuit occurring under the dynamics of SPWs and depending on NMDA receptors and an accurate level of GABAergic synaptic transmission is an essential pattern of precisely controlled network activity involved in synaptic plasticity processes with potential implications in mnemonic functions.     

Hippocampal sharp wave-ripples linked to slow oscillations in rat slow-wave sleep

Slow oscillations originating in the prefrontal neocortex during slow-wave sleep (SWS) group neuronal network activity and thereby presumably support the consolidation of memories. Here, we investigated whether the grouping influence of slow oscillations extends to hippocampal sharp wave-ripple (SPW) activity thought to underlie memory replay processes during SWS. The prefrontal surface EEG and multiunit activity (MUA), along with hippocampal local field potentials (LFP) from CA1, were recorded in rats during sleep. Average spindle and ripple activity and event correlation histograms of SPWs were calculated, time-locked to half-waves of slow oscillations. Results confirm decreased prefrontal MUA and spindle activity during EEG slow oscillation negativity and increases in this activity during subsequent positivity. A remarkably close temporal link was revealed between slow oscillations and hippocampal activity, with ripple activity and SPWs being also distinctly decreased during negative half-waves and increased during slow oscillation positivity. Fine-grained analyses of temporal dynamics revealed for the slow oscillation a phase delay of approximately 90 ms with reference to up and down states of prefrontal MUA, and of only approximately 60 ms with reference to changes in SPWs, indicating that up and down states in prefrontal MUA precede corresponding changes in hippocampal SPWs by approximately 30 ms. Results support the notion that the depolarizing surface-positive phase of the slow oscillation and the associated up state of prefrontal excitation promotes hippocampal SPWs via efferent pathways. The preceding disfacilitation of hippocampal events temporally coupled to the negative slow oscillation half-wave appears to serve a synchronizing role in this neocorticohippocampal interplay.     

5-Hydroxytryptamine neurons and the sleep-wakefulness cycle. Effects of methergoline and zimelidine

Methergoline, a 5-hydroxytryptamine (5-HT) receptor blocking agent, produced a significant decrease in the number of slow wave sleep 2 (SWS 2) and paradoxical sleep (PS) episodes and an increase in the length of wakefulness (W) episodes. Zimelidine, a specific 5-HT uptake blocking agent, produced a significant reduction of the number of PS episodes and an increase in the number of spisodes spent in W and slow wave sleep 1 (SWS 1), the total time spent in SWS 1 being increased. The findings demonstrate that increases and decreases of 5-HT receptor activity will produce differential effects on SWS 2 events and on W mechanisms.     

Entry into hibernation in M. Flaviventris: Sleep and behavioral thermoregulation

Hibernators of the genus Marmota (wt=3–5 kg) differ from smaller hibernators (wt<1 kg) in thermoregulatory characteristics during entry into hibernation. They might be expected to differ also with respect to the distributions of activity, awake and sleep states during entry. Marmots implanted stereotaxically with electrodes to record electroencephalograms and brain temperature (T_Br) were monitored remotely by a polygraph as well as video transmission as they entered hibernation. During entry, awake (A), slow wave sleep (SWS), and paradoxical sleep (PS) states could be identified. Activity which included nest building, grooming, and shivering slowed entry and became progressively more stereotyped as T_Br decreased. All animals exhibited at least one PS bout between T_Br=33?32°C. SWS as percent of total sleep increased (80 to 92%) as T_Br decreased from 35?25°C. This increase represented a decrease in number and increase in duration of SWS episodes. The length of individual SWS and PS episodes of the marmot did not differ from those reported in Citellus. These data suggest that entry into hibernation is a more complex phenomenon than merely an extension of slow wave sleep. Similar changes in arousal state distribution occur in hibernators of different sizes.     

Sleep patterns in the pony with observations on partial perceptual deprivation

Sleep characteristics were investigated in ponies under normal stall conditions and during partial (visual and auditive) perceptual deprivation. Polygraphic recordings of slow waves sleep (SWS) and paradoxical sleep (PS) including changes in hippocampal activity are described. A notable feature was the modulation of rhythmic slow hippocampal activity during PS. Sleep patterns are described with a relatively short sleep cycle (mean: 13.5 min). An intermediary phase (IP), lasting about 2.0 min, frequently occurred (40%) between SWS and PS. Perceptual deprivation reduced the occurrence of IP but an increase in SWS was observed. After-effects showed an augmentation in both SWS and PS.     

NMDA receptor activation mediates hydrogen peroxide-induced pathophysiology in rat hippocampal slices

Endogenous reactive oxygen species (ROS) can act as modulators of neuronal activity, including synaptic transmission. Inherent in this process, however, is the potential for oxidative damage if the balance between ROS production and regulation becomes disrupted. Here we report that inhibition of synaptic transmission in rat hippocampal slices by H2O2 can be followed by electrical hyperexcitability when transmission returns during H2O2 washout. As in previous studies, H2O2 exposure (15 min) reversibly depressed the extracellular population spike (PS) evoked by Schaffer collateral stimulation. Recovery of PS amplitude, however, was typically accompanied by mild epileptiform activity. Inclusion of ascorbate (400 microM) during H2O2 washout prevented this pathophysiology. No protection was seen with isoascorbate, which is a poor substrate for the stereoselective ascorbate transporter and thus remains primarily extracellular. Epileptiform activity was also prevented by the N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonopentanoic acid (AP5) during H2O2 washout. Once hyperexcitability was induced, however, AP5 did not reverse it. When present during H2O2 exposure, AP5 did not alter PS depression by H2O2 but did inhibit the recovery of PS amplitude seen during pulse-train stimulation (10 Hz, 5 s) in H2O2. Inhibition of glutamate uptake by l-trans-2,4-pyrrolidine dicarboxylate (PDC; 50 microM) during H2O2 washout markedly enhanced epileptiform activity; coapplication of ascorbate with PDC prevented this. These data indicate that H2O2 exposure can cause activation of normally silent NMDA receptors, possibly via inhibition of redox-sensitive glutamate uptake. When synaptic transmission returns during H2O2 washout, enhanced NMDA receptor activity leads to ROS generation and consequent oxidative damage. These data reveal a pathological cycle that could contribute to progressive degeneration in neurological disorders that involve oxidative stress, including cerebral ischemia.     

Uridine as an active component of sleep-promoting substance: its effects on nocturnal sleep in rats

A 10-h intraventricular infusion of 10 pmol of uridine from 19.00 to 05.00 h resulted in significant increases in sleep in otherwise saline-infused male rats (n = 8) during the environmental dark period (20.00-08.00 h). Increments of slow wave sleep (SWS) and paradoxical sleep (PS) were 21.0% and 68.1%, respectively, of the baseline value. This was due to increases in the frequencies of both SWS and PS episodes but not to their durations. Similar increases occurred the first recovery night under saline infusion, but sleep amounts returned to the baseline levels the second night. Brain temperature was not affected by uridine administration. A small dose of uridine (1 pmol/10 h) exerted no effect (n = 6) while larger doses (100 and 1000 pmol/10 h, each n = 5) resulted in slight but insignificant increases in SWS and PS. The 1000-pmol uridine administration seemed to be non-physiological since it brought about irregularities in locomotor activity and sleep-waking rhythms. Thus, authentic uridine exhibited the same sleep-enhancing effects as a naturally occurring active component of sleep-promoting substance, which was recently identified with uridine.     

Temporal pattern of hippocampal high-frequency oscillations during sleep after stimulant-evoked waking

Hippocampal ripple oscillations (140-200 Hz) are believed to be critically involved in the consolidation of memory traces during slow-wave sleep (SWS). We investigated the temporal pattern of ripple occurrence in relation to sleep phases following different types of waking. Amphetamine, the atypical wakening drug modafinil or non-pharmacological sleep deprivation lead to an increased ripple occurrence ("rebound") during the subsequent SWS episode. Waking of the same duration evoked by amphetamine or sleep deprivation led to a ripple rebound of similar extent (approximately 200%). The mean intraripple frequency was also elevated by up to 20 Hz during SWS following all treatments. Ripple amplitude was significantly increased only in experiments with amphetamine. Ripple occurrence but not intraripple frequency clearly correlated with the antecedent waking duration independent of treatment. Recovery of ripple occurrence and frequency to the pretreatment level during SWS depended on SWS duration. At the end of the recovery period paradoxical sleep (PS) acted like waking, elevating ripple occurrence during subsequent SWS episodes. On the other hand, PS decreased ripple occurrence if recovery from the rebound was not yet complete. Thus occurrence and structure of ripple oscillations are regulated by the timing and duration of previous SWS, PS and waking episodes.     

Effects of fornix lesions on waking and sleep patterns in white rats.

Polygraphic 12-hr daytime observation of male albino rats revealed increased spontaneous motor activity by a coronal fornical transection but not by a frontal cortical lesion. Fornicotomy alone affected sleep patterns by reducing the total time spent in slow wave sleep (SWS) and paradoxical sleep (PS) and in frequency of occurrence of SWS and PS and by increasing mean duration in the PS phase and the inter PS-interval. However, fornicotomy failed to affect the ratio of the two sleep phases (PS/SWS) either in total time, frequency of occurrence or duration and it did not alter the rate of tiny muscle twitching in PS. Since hippocampal theta activity was completely eliminated by the fornix lesions, the previously hypothesized relationship between theta activity and behaviors such as voluntary movement and PS is in need of revision.     

State-dependent changes in high-frequency oscillations recorded in the rat nucleus accumbens

Among the local field potentials recorded in the rat nucleus accumbens (NAc) spontaneous high frequency oscillations (HFO) are typically represented by a small peak in the power spectra in the range of 140–180 Hz. These HFO are known to occur in the awake state, but their distribution over the sleep–wake cycle has not been investigated. To address this issue we firstly examined the power of HFO during periods of quiet waking, slow-wave sleep (SWS) and rapid eye movement (REM) sleep. Since general anesthesia resembles certain features of naturally occurring SWS we went on to examine the effect of pentobarbital, isoflurane or urethane anesthesia on spontaneous and ketamine-induced increases in HFO. With respect to waking, the power of spontaneous HFO decreased significantly during periods of SWS but did not differ during bouts of REM sleep. General anesthetics also reduced the power of spontaneous HFO recorded in the NAc and prevented the ketamine-induced increase. These findings suggest that behavioural states where the generation of mental activity is most intense are associated with the presence of HFO in the NAc. In line with this, states which lead to decreased mentation, such as naturally occurring SWS and general anesthesia are associated with reductions in the power of HFO. Our results also suggest that the awake state is necessary for NMDA antagonists to produce enhancement of HFO.     

EEG bands during wakefulness, slow-wave, and paradoxical sleep as a result of principal component analysis in the rat

Rat EEG has been empirically divided in bands that frequently do not correspond with EEG generators nor with the functional meaning of EEG rhythms. Power spectra from wakefulness (W), slow-wave sleep (SWS), and paradoxical sleep (PS) of Wistar rats were submitted to Principal Component Analyses (PCA) to investigate which frequencies are covariant. Three independent eigenvectors were identified for SWS: a band between 1-6, an intermediate band between 7-15, and a fast band between 16-32 Hz (90.74% of the variance); two independent eigenvectors were extracted for PS: slow frequencies between 1-6 covarying together with frequencies between 11-16 Hz, and activity between 6-10 covarying together with fast frequencies between 17-32 Hz (80.38% of the variance); four eigen-vectors were obtained for W: 3-7, 8-9, 10-21 and 21-32 Hz (81.47% of the variance). Vigilance states showed significant differences in AP from 1 to 22 Hz. PCA extracted broad bands different for each vigilance state, which included the most representative EEG activities characteristic of them. These results indicate that during SWS, slow oscillations include frequencies up to 6 Hz, and spindle oscillations frequencies down to 7 Hz. No alpha frequencies were identified as an independent band. Frequencies within theta and beta were gathered in the same eigenvector during PS and in different eigenvectors during W suggesting coordinated activation of hippocampal and cortical systems during PS. These bands are consistent with the underlying neurophysiological mechanisms of sleep and wakefulness and with firing frequencies of generators of rhythmic activity obtained in cellular studies in animals.     

Interacting sleep-modulatory effects of simultaneously administered delta-sleep-inducing peptide, muramyl dipeptide and uridine in unrestrained rats

A 10-h nocturnal intracerebroventricular infusion of 2 or 3 sleep substances such as 2.5 nmol delta-sleep-inducing peptide (DSIP), 2.0 nmol muramyl dipeptide (MDP) and 10 pmol uridine resulted in a significant, combination-dependent change in sleep-waking dynamics, which was quite different from the time-course sleep promotion induced by the single administration of each substance. The DSIP-MDP combination was characterized by a profound increase in SWS only at the middle infusion period. PS was little affected. The DSIP-uridine combination exerted a slight increase in SWS at the late infusion period with little change in PS parameters. The MDP-uridine combination caused a marked increase in both SWS and PS only at the early to middle infusion period. The DSIP-MDP-uridine combination induced an extremely large increase in SWS at the early to middle phase of the infusion period. PS significantly increased only at the early phase. These results may suggest that the sleep substances acted in tandem, either synergistically or antagonistically, on the sleep regulatory mechanism.     

Polygraphic and behavioral study of sleep in geese: existence of nuchal atonia during paradoxical sleep

In adult geese, chronic polygraphic recordings of EEG, EOG, EMG, ECG and respiratory rate completed with behavioral observations allowed the characterization of four states of vigilance: wakefulness (W), drowsiness (D), slow wave sleep (SWS) and paradoxical sleep (PS). The EEG, EOG, EMG general patterns observed during W, D, SWS and PS episodes with nuchal isotonia or hypotonia were similar to those reported in other birds. The characteristic brevity of avian PS was confirmed since this sleep state occupied only 2.8% of the nycthemere in geese. For the first time in an adult bird it was shown that numerous PS episodes were accompanied, as in mammals, by a total disappearance of nuchal EMG activity. These observations made in a bird species with a stable head support when sleeping, suggest that, as in mammals, inhibitory mechanisms leading to a PS related nuchal atonia do exist and that head falling is not the cause of PS episodes brevity in birds.     

Level of arousal during the small irregular activity state in the rat hippocampal EEG

The sleeping rat cycles between two well-characterized hippocampal physiological states, large irregular activity (LIA) during slow-wave sleep (SWS) and theta activity during rapid-eye-movement sleep (REM). A third, less well-characterized electroencephalographic (EEG) state, termed "small irregular activity" (SIA), has been reported to occur when an animal is startled out of sleep without moving and during active waking when it abruptly freezes. We recently found that the hippocampal population activity of a spontaneous sleep state whose EEG resembles SIA reflects the rat's current location in space, suggesting that it is also a state of heightened arousal. To test whether this spontaneous SIA state corresponds to the SIA state reported in the literature and to compare the level of arousal during SIA to the other well-characterized physiological states, we recorded unit activity from ensembles of hippocampal CA1 pyramidal cells, EEG from the hippocampus and the neocortex, and electromyography (EMG) from the dorsal neck musculature in rats presented with auditory stimuli while foraging for randomly scattered food pellets and while sleeping. Auditory stimuli presented during sleep reliably induced SIA episodes very similar to spontaneous SIA in hippocampal and neocortical EEG amplitudes and power spectra, EMG amplitude, and CA1 population activity. Both spontaneous and elicited SIA exhibited neocortical desynchronization, and both had EMG amplitude comparable to that of waking LIA. We conclude based on this and other evidence that spontaneous SIA and elicited SIA correspond to a single state and that the level of arousal in SIA is higher than in the well-characterized sleep states but lower than the active theta state.     

Blockade of NMDA receptors enhances spontaneous sharp waves in rat hippocampal slices

An in vitro model of sharp waves (SPWs) and ripples was used to investigate the involvement of NMDA receptors in SPW/ripple production. Intracellular recordings from CA3 pyramidal cells confirmed that SPWs are composed of primarily excitatory currents. Unexpectedly, NMDA receptor antagonists greatly increased the size of SPWs and ripples. This effect may have involved decreased calcium influx through NMDA receptors and a subsequent reduction in the activation of SK2 calcium-activated potassium channels. The results support the claim that activation of NMDA receptors can serve to dampen the excitation of SPWs.     

Offsetting of aberrations associated with seizure proneness in rat hippocampus area CA1 by theta pulse stimulation-induced activity pattern

Epileptiform activity induces long term aberrations in hippocampal network functions. This study was conducted in pentylenetetrazol (PTZ) -kindled rats to examine offsetting of aberrations associated with seizure proneness in hippocampus area CA1 by theta pulse stimulation (TPS: 5 Hz trains for 3 min) -induced activity pattern. In hippocampal slices from both control and kindled rats, the field excitatory postsynaptic potentials (fEPSP) and population spikes (PS) were simultaneously recorded through electrodes in the apical dendrites and stratum pyramidale, respectively. The following changes in kindled vs. control slices were observed. The fEPSP needed to be greater to produce the PS recorded in the cell body layer. The fEPSP was reduced by paired stimuli whereas the PS amplitude was increased. TPS selectively depressed the PS in a lasting fashion, and shifted the fEPSP slope and the PS amplitude relation toward what was observed in controls. Both the fEPSP and PS were increased by paired stimuli at 60 min after TPS application. The lasting depressive effect of TPS on the PS amplitude was converted into facilitation by adenosine A1 receptor antagonist 8-cyclopentyl-1, 3-dipropylxanthine (CPX). Potentiation of the PS amplitude by TPS in the presence of CPX was blocked by an N-methyl-d-aspartate receptor antagonist AP5. We hypothesize that the extracellular adenosine spillover, acting through adenosine A1 receptors, during TPS-induced activity pattern could trigger a homeostatic process for correcting network imbalances caused by epileptiform activity.     

Daytime sleep-wake cycle in the rat.

The daytime sleep-wake cycle of 20 male, albino rats was observed for a 8:00 a.m. to 8:00 p.m. period. EEG, EMG, gross body movement and eyelid position were continuously monitored throughout the 12 hr observation period. Based on percent of total time the results indicate that the rat's sleep-wake activities during the day are: awake, 24% (active, 17%; restings, 7%); SWS, 67% (light SWS, 13%; deep SWS, 54%); PS, 9%; and total sleep, 76%. The rats had a mean of 396 wake-ups from either SWS and PS per day. Visual observations revealed a sleep phenomenon not previously reported for the rat and termed “paradoxical awake”; an animal would generate typical light and deep SWS, EEG waves with either one or both eyes opened. The present results compared to previous findings revealed significant variability in estimates of the rat's sleep-wake activities which may be a function of illumination cycles, sleep observation times or duration of the adaptation periods.     

Sleep patterns in the Mongolian gerbil, Meriones unguiculatus

Sleep patterns were studied in Mongolian gerbils and normative values were derived from 48 hour recordings, during a 24-hr light-dark cycle (LD 12:12). Behavioral and electrographic observations confirmed the existence of well defined sleep states: slow-wave sleep (SWS) and paradoxical sleep (PS). During the light period, sleep occupied slightly more than half of the 12 hour period, 57.13 +/- 0.002% out of which 49.64 +/- 0.007% was occupied by SWS and 8.07 +/- 0.007% by PS. There were 23 +/- 0.01 episodes of PS with a mean duration of 2.32 +/- 0.01 min. During the dark period, sleep occupied slightly less than half of the recording time (51.75 +/- 0.01%). They spent 41.62 +/- 0.006% in SWS and 10.12 +/- 0.02% in PS. The number of PS episodes was 32 +/- 0 with a mean duration of 2.28 +/- 0.01 min. Sleep cycle duration was 7.80 +/- 3.76 min. The ratio day/night sleep was 1.17 +/- 0.002 min. We found that the gerbil in captivity, unlike most rodents that are nocturnal, is a crepuscular animal, being more active at the transitions between light and dark.     

Fluctuations in spontaneous discharge of hippocampal theta cells during sleep-waking states and PCPA-induced insomnia

1. The spontaneous activity of hippocampal theta cells in head-restrained cats was recorded during slow-wave sleep (SWS), paradoxical sleep (PS), and the attentive state of bird watching (BW). We also recorded theta cell activity during a state of insomnia with pontogeniculoocipital (PGO) waves, which was induced by the administration of p-chlorophenylalanine (PCPA), a selective inhibitor of serotonin synthesis. 2. The time-dependent structure of fluctuations in theta cell activity was evaluated by power spectral analysis and Markovian analysis. The coefficient of variation for these time series was used as a measure of the variability of theta cell activity, which indicates the relative amplitude of fluctuations. 3. During SWS, theta cell activity showed a larger variability and a flat spectrum, i.e., low Markovian properties. During PS, this activity exhibited smaller variability and high spectral density in a low-frequency band (0.01-1.0 Hz), i.e., high Markovian properties. During BW, variability, spectral and Markovian properties were intermediate. 4. The firing pattern of theta cells during PCPA-induced insomnia was similar to that during PS. However, after the administration of either a serotonin agonist, 5-Methoxy-N,N-dimethyltryptamine, or a choline antagonist, atropine sulfate, theta cell activity no longer exhibited PS-like fluctuations, revealing instead a firing pattern similar to that during SWS. 5. During PS and PCPA-induced insomnia, not only the unit activity of theta cells, but PGO activity and theta wave frequency exhibited slow fluctuations, i.e., the high spectral density in the low-frequency band (0.01-1.0 Hz). 6. Cross-correlation analyses were performed between the fluctuations in theta cell activity, theta wave frequency, and PGO activity. These fluctuations correlated with each other during both PS and PCPA-induced insomnia. Bursts of PGO waves especially contributed to these cross-correlations. 7. These results suggest, first, that the slow fluctuations of the theta cell activity during PS and PCPA-induced insomnia is the physiological expression of the removal of aminergic influences and, secondly, that they are dependent on cholinergic mechanisms, including PGO generators.     

Insomnia caused by administration of para-chlorophenylalanine: reversibility by peripheral or central injection of 5-hydroxytryptophan and serotonin

Parachlorophenylalanine (PCPA) produces a total insomnia with a permanent discharge of pontogeniculooccipital (PGO) activity. We studied the reversibility of this insomnia in physiological slow-wave sleep (SWS) and paradoxical sleep (PS) after 5-hydroxytryptophan (5HTP) and serotonin (5HT) administration. Whereas D-5HTP (5 mg/kg) had no effect, parenteral injection of L-5HTP (2.5 mg/kg) or DL-5HTP (5 mg/kg) immediately suppressed PGO activity, then gave rise to the return of SWS and PS with delays of 26 and 60 min, respectively. Intraventricular or intracisternal administration of 5HTP (250 to 1500 micrograms) or 5HT produced physiological sleep with variable delays. If chloramphenicol (which selectively suppresses PS in normal cat) is administered in a PCPA-pretreated cat, 5HTP still suppressed PGO activity and gave rise to a lower amount of SWS but did not restore PS. The results suggest that 5HTP is rapidly decarboxylated into 5HT in restoring the PGO gating effect. Thus, 5HT would seem to act as a classic neurotransmitter. The long latency for PS (and its suppression by chloramphenicol) suggests that 5HT would appear to be a neurohormone controlling another sleep-inducing factor.