Changes in REM sleep occurrence due to rhythmical auditory stimulation in the rat

A Novel Insight of Effects of a 3-Hz Binaural Beat on Sleep Stages During Sleep

The dichotic presentation of two almost equivalent pure tones with slightly different frequencies leads to virtual beat perception by the brain. In this phenomenon, the so-called binaural beat has a frequency equaling the difference of the frequencies of the two pure tones. The binaural beat can entrain neural activities to synchronize with the beat frequency and induce behavioral states related to the neural activities. This study aimed to investigate the effect of a 3-Hz binaural beat on sleep stages, which is considered a behavioral state. Twenty-four participants were allocated to experimental and control groups. The experimental period was three consecutive nights consisting of an adaptation night, a baseline night, and an experimental night. Participants in both groups underwent the same procedures, but only the experimental group was exposed to the 3-Hz binaural beat on the experimental night. The stimulus was initiated when the first epoch of the N2 sleep stage was detected and stopped when the first epoch of the N3 sleep stage detected. For the control group, a silent sham stimulus was used. However, the participants were blinded to their stimulus group. The results showed that the N3 duration of the experimental group was longer than that of the control group, and the N2 duration of the experimental group was shorter than that of the control group. Moreover, the N3 latency of the experimental group was shorter.

Distribution of MCH-containing fibers in the feline brainstem: Relevance for REM sleep regulation

Neurons that utilize melanin-concentrating hormone (MCH) as a neuromodulator are localized in the postero-lateral hypothalamus and incerto-hypothalamic area. These neurons project diffusely throughout the central nervous system and have been implicated in critical physiological processes, such as sleep. Unlike rodents, in the order carnivora as well as in humans, MCH exerts its biological functions through two receptors: MCHR-1 and MCHR-2. Hence, the cat is an optimal animal to model MCHergic functions in humans. In the present study, we examined the distribution of MCH-positive fibers in the brainstem of the cat. MCHergic axons with distinctive varicosities and boutons were heterogeneously distributed, exhibiting different densities in distinct regions of the brainstem. High density of MCHergic fibers was found in the dorsal raphe nucleus, the laterodorsal tegmental nucleus, the periaqueductal gray, the pendunculopontine tegmental nucleus, the locus coeruleus and the prepositus hypoglossi. Because these areas are involved in the control of REM sleep, the present anatomical data support the role of this neuropeptidergic system in the control of this behavioral state.

Enhancement of sleep slow waves: underlying mechanisms and practical consequences

Even modest sleep restriction, especially the loss of sleep slow wave activity (SWA), is invariably associated with slower electroencephalogram (EEG) activity during wake, the occurrence of local sleep in an otherwise awake brain, and impaired performance due to cognitive and memory deficits. Recent studies not only confirm the beneficial role of sleep in memory consolidation, but also point to a specific role for sleep slow waves. Thus, the implementation of methods to enhance sleep slow waves without unwanted arousals or lightening of sleep could have significant practical implications. Here we first review the evidence that it is possible to enhance sleep slow waves in humans using transcranial direct-current stimulation (tDCS) and transcranial magnetic stimulation. Since these methods are currently impractical and their safety is questionable, especially for chronic long-term exposure, we then discuss novel data suggesting that it is possible to enhance slow waves using sensory stimuli. We consider the physiology of the K-complex (KC), a peripheral evoked slow wave, and show that, among different sensory modalities, acoustic stimulation is the most effective in increasing the magnitude of slow waves, likely through the activation of non-lemniscal ascending pathways to the thalamo-cortical system. In addition, we discuss how intensity and frequency of the acoustic stimuli, as well as exact timing and pattern of stimulation, affect sleep enhancement. Finally, we discuss automated algorithms that read the EEG and, in real-time, adjust the stimulation parameters in a closed-loop manner to obtain an increase in sleep slow waves and avoid undesirable arousals. In conclusion, while discussing the mechanisms that underlie the generation of sleep slow waves, we review the converging evidence showing that acoustic stimulation is safe and represents an ideal tool for slow wave sleep (SWS) enhancement.

The hypocretins (orexins) mediate the "phasic" components of REM sleep: A new hypothesis

In 1998, a group of phenotypically distinct neurons were discovered in the postero-lateral hypothalamus which contained the neuropeptides hypocretin 1 and hypocretin 2 (also called orexin A and orexin B), which are excitatory neuromodulators. Hypocretinergic neurons project throughout the central nervous system and have been involved in the generation and maintenance of wakefulness. The sleep disorder narcolepsy, characterized by hypersomnia and cataplexy, is produced by degeneration of these neurons. The hypocretinergic neurons are active during wakefulness in conjunction with the presence of motor activity that occurs during survival-related behaviors. These neurons decrease their firing rate during non-REM sleep; however there is still controversy upon the activity and role of these neurons during REM sleep. Hence, in the present report we conducted a critical review of the literature of the hypocretinergic system during REM sleep, and hypothesize a possible role of this system in the generation of REM sleep.

5-HT1A receptor-responsive pedunculopontine tegmental neurons suppress REM sleep and respiratory motor activity

Serotonin type 1A (5-HT(1A)) receptor-responsive neurons in the pedunculopontine tegmental nucleus (PPTn) become maximally active immediately before and during rapid eye movement (REM) sleep. A prevailing model of REM sleep generation indicates that activation of such neurons contributes significantly to the generation of REM sleep, and if correct then inactivation of such neurons ought to suppress REM sleep. We test this hypothesis using bilateral microperfusion of the 5-HT(1A) receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT, 10 μm) into the PPTn; this tool has been shown to selectively silence REM sleep-active PPTn neurons while the activity of wake/REM sleep-active PPTn neurons is unaffected. Contrary to the prevailing model, bilateral microperfusion of 8-OH-DPAT into the PPTn (n = 23 rats) significantly increased REM sleep both as a percentage of the total recording time and sleep time, compared with both within-animal vehicle controls and between-animal time-controls. This increased REM sleep resulted from an increased frequency of REM sleep bouts but not their duration, indicating an effect on mechanisms of REM sleep initiation but not maintenance. Furthermore, an increased proportion of the REM sleep bouts stemmed from periods of low REM sleep drive quantified electrographically. Targeted suppression of 5-HT(1A) receptor-responsive PPTn neurons also increased respiratory rate and respiratory-related genioglossus activity, and increased the frequency and amplitude of the sporadic genioglossus activations occurring during REM sleep. These data indicate that 5-HT(1A) receptor-responsive PPTn neurons normally function to restrain REM sleep by elevating the drive threshold for REM sleep induction, and restrain the expression of respiratory rate and motor activities.

Auditory input modulates sleep: an intra-cochlear-implanted human model

To properly demonstrate the effect of auditory input on sleep of intra-cochlear-implanted patients, the following approach was developed. Four implanted deaf patients were recorded during four nights: two nights with the implant OFF, with no auditory input, and two nights with the implant ON, that is, with normal auditory input, being only the common night sounds present, without any additional auditory stimuli delivered. The sleep patterns of another five deaf people were used as controls, exhibiting normal sleep organization. Moreover, the four experimental patients with intra-cochlear devices and the implant OFF also showed normal sleep patterns. On comparison of the night recordings with the implant ON and OFF, a new sleep organization was observed for the recordings with the implant ON, suggesting that brain plasticity may produce changes in the sleep stage percentages while maintaining the ultradian rhythm. During sleep with the implant ON, the analysis of the electroencephalographic delta, theta and alpha bands in the frequency domain, using the Fast Fourier Transform, revealed a diversity of changes in the power originated in the contralateral cortical temporal region. Different power shifts were observed, perhaps related to the exact position of the implant inside the cochlea and the scalp electrode location. In conclusion, this pilot study shows that the auditory input in humans can introduce changes in central nervous system activity leading to shifts in sleep characteristics, as previously demonstrated in guinea pigs. We are postulating that an intra-cochlear-implanted deaf patient may have a better recovery if the implant is maintained ON during the night, that is, during sleep.

Chronic exposure to an environmental noise permanently disturbs sleep in rats: Inter-individual vulnerability

Chronic exposure to an environmental noise (EN) induces sleep disturbances. However, discrepancies exist in the literature since many contradictory conclusions have been reported. These disagreements are largely due to inappropriate evaluation of sleep and also to uncontrolled and confounding factors such as sex, age and also inter-individual vulnerability. Based on a recently validated animal model, aims of the present study were (i) to determine the effects of a chronic exposure to EN on sleep and (ii) to evaluate the inter-individual vulnerability of sleep to EN. For this purpose, rats were exposed during 9 days to EN. Results show that a chronic exposure to EN restricts continually amounts of slow wave sleep (SWS) and paradoxical sleep (PS) and fragments these two sleep stages with no habituation effect. Results also evidence the existence of subpopulations of rats that are either resistant or vulnerable to these deleterious effects of EN on sleep and especially on SWS amounts, bouts number and bout duration. Furthermore, importance of SWS debt and daily decrease of SWS bout duration are correlated to each others and both correlate to the amplitude of the locomotor reactivity to novelty, a behavioral measure of reactivity to stress. This last result suggests that this psychobiological profile of subjects, known to induce profound differences in neural and endocrine systems, could be responsible for their SWS vulnerability under a chronic EN exposure.

Pattern of rapid-eye movement sleep episode occurrence after an immobilization stress in the rat

Rapid-eye movement sleep (REMS) in the rat occurs in the form of episodes separated by long (>3 min: isolated REMS episodes, IREMSEs) and short (<3 min: sequential REMS episodes, SREMSEs) intervals. The mechanism clustering SREMSEs is thought to be better suited to increase the amount of REMS when its drive is high. We tested this hypothesis in a new situation by transiently augmenting this drive with a moderate psychogenic stress. After a 90-min immobilization stress, REMS increased by 54% over the rest of the dark phase. This increase was associated with a modest increase in IREMSEs (+40%), but a massive increase in SREMSEs (+300%). Thus, the mechanism that clusters REMS episodes operates in response to a moderate psychogenic stress imposed in standard laboratory conditions and not preceded by sleep deprivation.

Inferior colliculus unitary activity in wakefulness, sleep and under barbiturates

The spontaneous unitary activity and the response to contralateral tone-burst were analyzed in the inferior colliculus (IC) of guinea pigs during the sleep-waking cycle and under the effects of pentobarbital anesthesia. Minor changes were observed in both spontaneous and evoked activity between wakefulness (W) and slow wave sleep (SWS). On the other hand, a consistent increase in the mean spontaneous firing rate and a significant decrement in the signal-to-noise ratio (S/N ratio) was observed during paradoxical sleep (PS). Pentobarbital anesthesia reduced the spontaneous and evoked firing rate, the duration of the excitatory response and increased the duration of the post-excitatory suppression. We conclude, that the processing of auditory information in the IC change markedly during PS. Because the IC is a compulsory station for almost all the ascending auditory pathways, the observed decrease in the S/N ratio may deeply affect the auditory perception during this behavioral state. Finally, the alteration of the neuronal activity induced by pentobarbital differs not only with the activity observed during W, but also with the activity observed during both SWS and PS.

REM sleep enhancement due to rhythmical auditory stimulation in the rat

From a physiological viewpoint, REM sleep (REMS) is a period during which homeostatic physiological regulations are impaired. In the rat, REMS occurs in two forms respectively characterized by episodes separated by long intervals (single REMS episodes) and by episodes which have short intervals and occur in sequences (REMS clusters). Since the partition of REMS in the form of either single or clustered episodes may reveal how the REMS drive and body homeostatic processes interact in the control of REMS occurrence, we have used this approach to clarify the effects of the rhythmical delivery of an auditory stimulus (1000 Hz, 63 or 88 dB, 50 ms, every 20 s), which has been previously observed by different authors to enhance REMS in the absence of a previous sleep deprivation. Stimuli were delivered to pairs of animals and triggered by the occurrence of REMS in one rat (REMS-selective stimulation), whilst the other animal received the same stimulus irrespectively of the stage of the wake-sleep cycle (REMS-unselective stimulation). The results showed that the REMS-selective stimulation did not change the overall amount of REMS, since an increase in the occurrence of REMS clusters was concomitant with a decrease in the occurrence of single REMS episodes. In contrast, under the REMS-unselective stimulation, the total amount of REMS was increased during the second day of stimulation through an increase in the duration of both types of REMS episodes. Since during the REMS-unselective stimulation 87% of the stimuli fell outside REMS (i.e., during the REMS interval), the results show that the occurrence of REMS is more consistently affected when the stimuli are delivered in a period during which homeostatic physiological regulations are fully operant.