0066 Glycinergic Postsynaptic Inhibition is Responsible for the Suppression of Hypoglossal Motoneuron Activity During Naturally-Occurring REM Sleep

Introduction

The present study was undertaken to explore the role of glycinergic postsynaptic inhibition and monoaminergic disfacilitation (a withdrawal of excitatory noradrenergic and serotonergic inputs) in the control of hypoglossal motoneuron activity during REM sleep. Accordingly, glycinergic, noradrenergic and serotonergic antagonists were microinjected into the hypoglossal nucleus, and their effects on the hypoglossal nerve activity during REM sleep were examined in chronically-instrumented, unanesthetized cats.

Methods

Adults cats were prepared for monitoring behavioral states of sleep and wakefulness, and for extracellular recordings from hypoglossal nerve. Strychnine (a glycinergic antagonist) and a mixture of prazosin (a noradrenergic antagonist) and methysergide (a serotonergic antagonist) were microinjected, separately, into the hypoglossal nucleus during naturally-occurring states of sleep and wakefulness.

Results

During REM sleep, compared to non-REM sleep, the hypoglossal nerve activity decreased by 17.4±1.5% (n=17) in the control recordings (prior to the injection of strychnine). Following the microinjection of strychnine, there was only a mean decrease of 7.2±1.2% (n=12) in the nerve activity during REM sleep versus NREM sleep. The strychnine effect was statistically significant compared to control (p<0.001; unpaired t-test), which indicates that strychnine blocks REM sleep-related suppression of hypoglossal nerve activity. In contrast, the microinjection of prazosin and methysergide did not significantly reduce the hypoglossal nerve activity during REM sleep (control: 15.9±2.3, n=9 vs. prazosin+methysergide: 12.6±1.4%, n=10, p=0.229, unpaired t-test).

Conclusion

The present results demonstrate that the microapplication of strychnine, but not prazosin and methysergide, into the hypoglossal nucleus significantly reduces the suppression of the hypoglossal nerve activity during naturally-occurring REM sleep. We therefore suggest that glycinergic postsynaptic inhibition is primarily responsible for the suppression of hypoglossal motoneuron activity during REM sleep.

Support

5R01NS094062

0067 An Anatomic Substrate for GABAergic Processes to Suppress Active Sleep and Promote Wakefulness in the Nucleus Pontis Oralis

Introduction

Our previous electrophysiologic data have provided compelling evidence that GABAergic processes in the nucleus pontis oralis (NPO) play a critical role in the generation and maintenance of wakefulness as well as active (REM) sleep (AS). We therefore hypothesized that one of the neuronal mechanisms of GABA actions in the NPO to promote wakefulness and suppress AS is due to a direct GABAergic inhibition of NPO neurons that generate AS (AS-generator neurons). However, the anatomical substrate for this inhibition is undetermined. Accordingly, the present study was undertaken to examine whether there is any direct interaction between GABAergic neurons and glutamatergic AS-generator neurons in the NPO.

Methods

Adult cats were deeply anesthetized and perfused transcardially. The brainstem containing the NPO was removed, postfixed and cut into 15 μm coronal sections with a Reichert-Jung cryostat. The sections were incubated with a mixture of a rabbit polyclonal antibodies against glutamine and GABA following the procedure of double fluorescence immunohistochemistry.

Results

There was a large number of neuronal somata labeled by anti-glutamine antibody and terminals labeled by anti-GABA antibody in the NPO. These glutamine-positive neurons were medium to large, multipolar cells (> 20 μm), which resemble glutamatergic, AS-generator neurons that have been previously identified in the NPO. Specifically, majority of glutamatergic neuronal somata were closely apposed by multiple GABAergic terminals, indicating that AS-generator neurons in the NPO receive direct GABAergic inputs.

Conclusion

The present results demonstrate that a direct connection exists between glutamatergic AS-generator neurons and GABAergic processes in the NPO. These data provide the anatomical evidence which supports our hypothesis that the pontine GABAergic control of wakefulness and active sleep is partially mediated via GABAergic processes project to NPO AS-generator neurons that suppress the activity of these cells.

Support

NS092383

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.

Neocortical 40 Hz oscillations during carbachol-induced rapid eye movement sleep and cataplexy

Higher cognitive functions require the integration and coordination of large populations of neurons in cortical and subcortical regions. Oscillations in the gamma band (30-45 Hz) of the electroencephalogram (EEG) have been involved in these cognitive functions. In previous studies, we analysed the extent of functional connectivity between cortical areas employing the 'mean squared coherence' analysis of the EEG gamma band. We demonstrated that gamma coherence is maximal during alert wakefulness and is almost absent during rapid eye movement (REM) sleep. The nucleus pontis oralis (NPO) is critical for REM sleep generation. The NPO is considered to exert executive control over the initiation and maintenance of REM sleep. In the cat, depending on the previous state of the animal, a single microinjection of carbachol (a cholinergic agonist) into the NPO can produce either REM sleep [REM sleep induced by carbachol (REMc)] or a waking state with muscle atonia, i.e. cataplexy [cataplexy induced by carbachol (CA)]. In the present study, in cats that were implanted with electrodes in different cortical areas to record polysomnographic activity, we compared the degree of gamma (30-45 Hz) coherence during REMc, CA and naturally-occurring behavioural states. Gamma coherence was maximal during CA and alert wakefulness. In contrast, gamma coherence was almost absent during REMc as in naturally-occurring REM sleep. We conclude that, in spite of the presence of somatic muscle paralysis, there are remarkable differences in cortical activity between REMc and CA, which confirm that EEG gamma (≈40 Hz) coherence is a trait that differentiates wakefulness from REM sleep.

Melanin-Concentrating Hormone (MCH): Role in REM Sleep and Depression

The melanin-concentrating hormone (MCH) is a peptidergic neuromodulator synthesized by neurons of the lateral sector of the posterior hypothalamus and zona incerta. MCHergic neurons project throughout the central nervous system, including areas such as the dorsal (DR) and median (MR) raphe nuclei, which are involved in the control of sleep and mood. Major Depression (MD) is a prevalent psychiatric disease diagnosed on the basis of symptomatic criteria such as sadness or melancholia, guilt, irritability, and anhedonia. A short REM sleep latency (i.e., the interval between sleep onset and the first REM sleep period), as well as an increase in the duration of REM sleep and the density of rapid-eye movements during this state, are considered important biological markers of depression. The fact that the greatest firing rate of MCHergic neurons occurs during REM sleep and that optogenetic stimulation of these neurons induces sleep, tends to indicate that MCH plays a critical role in the generation and maintenance of sleep, especially REM sleep. In addition, the acute microinjection of MCH into the DR promotes REM sleep, while immunoneutralization of this peptide within the DR decreases the time spent in this state. Moreover, microinjections of MCH into either the DR or MR promote a depressive-like behavior. In the DR, this effect is prevented by the systemic administration of antidepressant drugs (either fluoxetine or nortriptyline) and blocked by the intra-DR microinjection of a specific MCH receptor antagonist. Using electrophysiological and microdialysis techniques we demonstrated also that MCH decreases the activity of serotonergic DR neurons. Therefore, there are substantive experimental data suggesting that the MCHergic system plays a role in the control of REM sleep and, in addition, in the pathophysiology of depression. Consequently, in the present report, we summarize and evaluate the current data and hypotheses related to the role of MCH in REM sleep and MD.

Interactions between hypocretinergic and GABAergic systems in the control of activity of neurons in the cat pontine reticular formation

Anatomical studies have demonstrated that hypocretinergic and GABAergic neurons innervate cells in the nucleus pontis oralis (NPO), a nucleus responsible for the generation of active (rapid eye movement (REM)) sleep (AS) and wakefulness (W). Behavioral and electrophysiological studies have shown that hypocretinergic and GABAergic processes in the NPO are involved in the generation of AS as well as W. An increase in hypocretin in the NPO is associated with both AS and W, whereas GABA levels in the NPO are elevated during W. We therefore examined the manner in which GABA modulates NPO neuronal responses to hypocretin. We hypothesized that interactions between the hypocretinergic and GABAergic systems in the NPO play an important role in determining the occurrence of AS or W. To determine the veracity of this hypothesis, we examined the effects of the juxtacellular application of hypocretin-1 and GABA on the activity of NPO neurons, which were recorded intracellularly, in chloralose-anesthetized cats. The juxtacellular application of hypocretin-1 significantly increased the mean amplitude of spontaneous EPSPs and the frequency of discharge of NPO neurons; in contrast, the juxtacellular microinjection of GABA produced the opposite effects, i.e., there was a significant reduction in the mean amplitude of spontaneous EPSPs and a decrease in the discharge of these cells. When hypocretin-1 and GABA were applied simultaneously, the inhibitory effect of GABA on the activity of NPO neurons was reduced or completely blocked. In addition, hypocretin-1 also blocked GABAergic inhibition of EPSPs evoked by stimulation of the laterodorsal tegmental nucleus. These data indicate that hypocretin and GABA function within the context of a neuronal gate that controls the activity of AS-on neurons. Therefore, we suggest that the occurrence of either AS or W depends upon interactions between hypocretinergic and GABAergic processes as well as inputs from other sites that project to AS-on neurons in the NPO.

Postsynaptic inhibition of hypoglossal motoneurons produces atonia of the genioglossal muscle during rapid eye movement sleep

STUDY OBJECTIVES

Hypoglossal motoneurons were recorded intracellularly to determine whether postsynaptic inhibition or disfacilitation was responsible for atonia of the lingual muscles during rapid eye movement (REM) sleep.

DESIGN

Intracellular records were obtained of the action potentials and subthreshold membrane potential activity of antidromically identified hypoglossal motoneurons in cats during wakefulness, nonrapid eye movement (NREM) sleep, and REM sleep. A cuff electrode was placed around the hypoglossal nerve to antidromically activate hypoglossal motoneurons. The state-dependent changes in membrane potential, spontaneous discharge, postsynaptic potentials, and rheobase of hypoglossal motoneurons were determined.

ANALYSES AND RESULTS

During quiet wakefulness and NREM sleep, hypoglossal motoneurons exhibited spontaneous repetitive discharge. In the transition from NREM sleep to REM sleep, repetitive discharge ceased and the membrane potential began to hyperpolarize; maximal hyperpolarization (10.5 mV) persisted throughout REM sleep. During REM sleep there was a significant increase in rheobase, which was accompanied by barrages of large-amplitude inhibitory postsynaptic potentials (IPSPs), which were reversed following the intracellular injection of chloride ions. The latter result indicates that they were mediated by glycine; IPSPs were not present during wakefulness or NREM sleep.

CONCLUSIONS

We conclude that hypoglossal motoneurons are postsynaptically inhibited during naturally occurring REM sleep; no evidence of disfacilitation was observed. The data also indicate that glycine receptor-mediated postsynaptic inhibition of hypoglossal motoneurons is crucial in promoting atonia of the lingual muscles during REM sleep.

Coherent neocortical gamma oscillations decrease during REM sleep in the rat

Higher cognitive functions require the integration and coordination of large populations of neurons in cortical and subcortical regions. Oscillations in the high frequency band (30-100 Hz) of the electroencephalogram (EEG), that have been postulated to be a product of this interaction, are involved in the binding of spatially separated but temporally correlated neural events, which results in a unified perceptual experience. The extent of this functional connectivity can be examined by means of the mathematical algorithm called "coherence", which is correlated with the "strength" of functional interactions between cortical areas. As a continuation of previous studies in the cat [6,7], the present study was conducted to analyze EEG coherence in the gamma band of the rat during wakefulness (W), non-REM (NREM) sleep and REM sleep. Rats were implanted with electrodes in different cortical areas to record EEG activity, and the magnitude squared coherence values within the gamma frequency band of EEG (30-48 and 52-100 Hz) were determined. Coherence between all cortical regions in the low and high gamma frequency bands was greater during W compared with sleep. Remarkably, EEG coherence in the low and high gamma bands was smallest during REM sleep. We conclude that high frequency interactions between cortical areas are radically different during sleep and wakefulness in the rat. Since this feature is conserved in other mammals, including humans, we suggest that the uncoupling of gamma frequency activity during REM sleep is a defining trait of REM sleep in mammals.

Inter-hemispheric coherence of neocortical gamma oscillations during sleep and wakefulness

Oscillations in the gamma frequency band (mainly ≈40 Hz) of the electroencephalogram (EEG) have been involved in the binding of spatially separated but temporally correlated neural events that result in a unified perceptual experience. The extent of these interactions can be examined by means of a mathematical algorithm called "coherence", which reflects the "strength" of functional interactions between cortical areas. As a continuation of a previous study of our group, the present study was conducted to analyze the inter-hemispheric coherence of the EEG gamma frequency band in the cat during alert wakefulness (AW), quiet wakefulness (QW), non-REM (NREM) sleep and REM sleep. Cats were implanted with electrodes in the frontal, parietal and occipital cortices to monitor EEG activity. The degree of coherence in the low (30-45 Hz) and high (60-100 Hz) gamma frequency bands from pairs of EEG recordings was analyzed. A large increase in coherence between all inter-hemispheric cortical regions in the low gamma bands during AW was present compared to the other behavioral states. Furthermore, both low and high gamma coherence between inter-hemispheric heterotopic cortices (different cortical areas of both hemispheres) decreased during REM sleep; this is a pattern that we previously reported between the cortical areas of the same hemisphere (intrahemispheric coherence). In the high gamma band, coherence during REM sleep also decreased compared to the other behavioral states. In contrast, between most of the inter-hemispheric homotopic cortical areas (equivalent or mirror areas of both hemispheres), low gamma coherence was similar during NREM compared to REM sleep. We conclude that in spite of subtle differences between homotopic and heterotopic inter-hemispheric cortices, functional interactions at high frequency decrease during REM sleep.

Control of hypoglossal motoneurones during naturally occurring sleep and wakefulness in the intact, unanaesthetized cat: a field potential study

The present electrophysiological study was designed to determine the discharge threshold of hypoglossal motoneurones during naturally occurring states of sleep and wakefulness in the intact, unanaesthetized cat. The antidromic field potential, which reflects the net level of membrane excitability of motoneurones and therefore their discharge threshold, was recorded in the hypoglossal nucleus following stimulation of the hypoglossal nerve. The amplitude of the antidromic field potential was larger during wakefulness and non-rapid eye movement (NREM) sleep compared with REM sleep. There was no significant difference in the amplitude of the field potential when wakefulness was compared with NREM sleep (P = 0.103, df = 3, t = 2.324). However, there was a 46% reduction in amplitude during REM sleep compared with NREM sleep (P < 0.001, df = 10, t = 6.421) or wakefulness (P < 0.01, df = 4, t = -4.598). These findings indicate that whereas the excitability of motoneurones that comprise the hypoglossal motor pool is relatively constant during wakefulness and NREM sleep, their excitability is significantly reduced during REM sleep. This state-dependent pattern of control of hypoglossal motoneurones during REM sleep is similar to that reported for motoneurones in other motor nuclei at all levels of the neuraxis. The decrease in the evoked response of hypoglossal motoneurones, which reflects a significant increase in the discharge threshold of individual motoneurones, results in atonia of the lingual and related muscles during REM sleep.

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.

A unified survival theory of the functioning of the hypocretinergic system

This article advances the theory that the hypocretinergic (orexinergic) system initiates, coordinates, and maintains survival behaviors and survival-related processes (i.e., the Unified Survival Theory of the Functioning of the Hypocretinergic System or "Unified Hypocretinergic Survival Theory"). A priori presumptive support for the Unified Hypocretinergic Survival Theory emanates from the fact that neurons that contain hypocretin are located in the key executive central nervous system (CNS) site, the lateral hypothalamus, that for decades has been well-documented to govern core survival behaviors such as fight, flight, and food consumption. In addition, the hypocretinergic system exhibits the requisite morphological and electrophysiological capabilities to control survival behaviors and related processes. Complementary behavioral data demonstrate that all facets of "survival" are coordinated by the hypocretinergic system and that hypocretinergic directives are not promulgated except during survival behaviors. Importantly, it has been shown that survival behaviors are selectively impacted when the hypocretinergic system is impaired or rendered nonfunctional, whereas other behaviors are relatively unaffected. The Unified Hypocretinergic Survival Theory resolves the disparate, perplexing, and often paradoxical-appearing results of previous studies; it also provides a foundation for future hypothesis-driven basic science and clinical explorations of the hypocretinergic system.

Coherent neocortical 40-Hz oscillations are not present during REM sleep

During cognitive processes there are extensive interactions between various regions of the cerebral cortex. Oscillations in the gamma frequency band (≈40 Hz) of the electroencephalogram (EEG) are involved in the binding of spatially separated but temporally correlated neural events, which results in a unified perceptual experience. The extent of these interactions can be examined by means of a mathematical algorithm called 'coherence', which reflects the 'strength' of functional interactions between cortical areas. The present study was conducted to analyse EEG coherence in the gamma frequency band of the cat during alert wakefulness (AW), quiet wakefulness (QW), non-rapid eye movement (NREM) sleep and rapid eye movement (REM) sleep. Cats were implanted with electrodes in the frontal, parietal and occipital cortices to monitor EEG activity. Coherence values within the gamma frequency (30-100 Hz) from pairs of EEG recordings were analysed. A large increase in coherence occurred between all cortical regions in the 30-45 Hz frequency band during AW compared with the other behavioral states. As the animal transitioned from AW to QW and from QW to NREM sleep, coherence decreased to a moderate level. Remarkably, there was practically no EEG coherence in the entire gamma band spectrum (30-100 Hz) during REM sleep. We conclude that functional interactions between cortical areas are radically different during sleep compared with wakefulness. The virtual absence of gamma frequency coherence during REM sleep may underlie the unique cognitive processing that occurs during dreams, which is principally a REM sleep-related phenomenon.

Hypocretinergic and non-hypocretinergic projections from the hypothalamus to the REM sleep executive area of the pons

Within the postero-lateral hypothalamus neurons that utilize hypocretin or melanin-concentrating hormone (MCH) as neuromodulators are co-distributed. These neurons have been involved in the control of behavioral states, and a deficit in the hypocretinergic system is the pathogenic basis of narcolepsy with cataplexy. In this report, utilizing immunohistochemistry and retrograde tracing techniques, we examined the hypocretinergic innervation of the nucleus pontis oralis (NPO), which is the executive site that is responsible for the generation of REM sleep in the cat. The retrograde tracer cholera toxin subunit b (CTb) was administered in pontine regions where carbachol microinjections induced REM sleep. Utilizing immunohistochemical techniques, we found that approximately 1% of hypocretinergic neurons in the tuberal area of the hypothalamus project to the NPO. In addition, approximately 6% of all CTb+ neurons in this region were hypocretinergic. The hypocretinergic innervation of the NPO was also compared with the innervation of the same site by MCH-containing neurons. More than three times as many MCHergic neurons were found to project to the NPO compared with hypocretinergic cells; both neuronal types exhibited bilateral projections. We also identified a group of non-hypocretinergic non-MCHergic neuronal group of neurons that were intermingled with both hypocretinergic and MCHergic neurons that also projected to this same brainstem region. These neurons were grater in number that either hypocretin or MCH-containing neurons; their soma size was also smaller and their projections were mainly ipsilateral. The present anatomical data suggest that hypocretinergic, MCHergic and an unidentified companion group of neurons of the postero-lateral hypothalamus participate in the regulation of the neuronal activity of NPO neurons, and therefore, are likely to participate in the control of wakefulness and REM sleep.

Quantitative analysis of the excitability of hypoglossal motoneurons during natural sleep in the rat

We describe a novel approach to assess the excitability of hypoglossal motoneurons in rats during naturally occurring states of sleep and wakefulness. Adult rats were surgically prepared with permanently placed electrodes to record the EEG, EOG and neck EMG. A stimulating/recording miniature tripolar cuff electrode was implanted around the intact hypoglossal nerve and a head-restraining device was bonded to the calvarium. After a period of adaptation to head-restraint, the animals did not exhibit any sign of discomfort and readily transitioned between the states of wakefulness, NREM and REM sleep. There was no spontaneous respiratory or tonic activity present in the hypoglossal nerve during sleep or wakefulness. Hypoglossal motoneurons were activated by electrical stimulation of the hypoglossal nerve (antidromically) or by microstimulation directly applied to the hypoglossal nucleus. Microstimulation of hypoglossal motoneurons evoked compound action potentials in the ipsilateral hypoglossal nerve. The magnitude of their integrals tended to be higher during wakefulness (112.6% ± 15; standard deviation) and were strongly depressed during REM sleep (24.7% ± 3.4), compared to the integral magnitude during NREM sleep. Lidocaine, which was delivered using pressure microinjection to the microstimulation site, verified that the responses evoked in hypoglossal nerve can be affected pharmacologically. We conclude that this animal model can be utilized to study the neurotransmitter mechanisms that control the excitability of hypoglossal motoneurons during naturally occurring states of sleep and wakefulness.

Apnea produces excitotoxic hippocampal synapses and neuronal apoptosis

Obstructive sleep apnea (OSA) results in the degeneration of neurons in the hippocampus that eventuates in neurocognitive deficits. We were therefore interested in determining the effects of apnea on monosynaptic excitatory processes in a hippocampal pathway (cornu ammonis 3-cornu ammonis 1, CA3-CA1) that has been shown to mediate the processing of cognitive information. In addition, to substantiate an anatomical basis for the cognitive dysfunction that occurs in OSA patients, we examined the effects of apnea with respect to neurodegenerative changes (apoptosis) in the same hippocampal pathway. In order to determine the effects of apnea, an automated system for the generation and analysis of single and recurrent periods of apnea was developed. Utilizing this system, the field excitatory postsynaptic potential (fEPSP) generated by pyramidal neurons in the CA1 region of the hippocampus was monitored in α-chloralose anesthetized rats following stimulation of glutamatergic afferents in the CA3 region. A stimulus-response (input-output) curve for CA3-CA1 synaptic activity was determined. In addition, a paired-pulse paradigm was employed to evaluate, electrophysiologically, the presynaptic release of glutamate. Changes in the synaptic efficacy were assessed following single episodes of apnea induced by ventilatory arrest (60 to 80 s duration, mean=72 s; mean oxygen desaturation was 53% of normoxia level). Apnea resulted in a significant potentiation of the amplitude (mean=126%) and slope (mean=117%) of the baseline CA1 fEPSP. This increase in the fEPSP was accompanied by a significant decrease in the amplitude (71%) and slope (81%) of normalized paired-pulse facilitation (PPF) ratios. Since the potentiation of the fEPSP is inversely proportional to changes in PPF ratio, the potentiated fEPSP accompanied by the reduced PPF reveals that apnea produces an abnormal increase in the preterminal release of glutamate that results in the over-activation (and calcium overloading) of hippocampal CA1 neurons. Thus, we conclude that individual episodes of apnea result in the development of excitotoxic processes in the hippocampal CA3-CA1 pathway that is critically involved in the processing of cognitive information. Morphologically, the deleterious effect of recurrent apnea was substantiated by the finding of apoptosis in CA1 neurons of apneic (but not normoxic) animals.

The role of mesopontine NGF in sleep and wakefulness

The microinjection of nerve growth factor (NGF) into the cat pontine tegmentum rapidly induces rapid eye movement (REM) sleep. To determine if NGF is involved in naturally-occurring REM sleep, we examined whether it is present in mesopontine cholinergic structures that promote the initiation of REM sleep, and whether the blockade of NGF production in these structures suppresses REM sleep. We found that cholinergic neurons in the cat dorso-lateral mesopontine tegmentum exhibited NGF-like immunoreactivity. In addition, the microinjection of an oligodeoxyribonucleotide (OD) directed against cat NGF mRNA into this region resulted in a reduction in the time spent in REM sleep in conjunction with an increase in the time spent in wakefulness. Sleep and wakefulness returned to baseline conditions 2 to 5 days after antisense OD administration. The preceding antisense OD-induced effects occurred in conjunction with the suppression of NGF-like immunoreactivity within the site of antisense OD injection. These data support the hypothesis that NGF is involved in the modulation of naturally-occurring sleep and wakefulness.

Excitatory projections from the amygdala to neurons in the nucleus pontis oralis in the rat: an intracellular study

There is a consensus that active (REM) sleep (AS) is controlled by cholinergic projections from the laterodorsal and pedunculopontine tegmental nuclei (LDT/PPT) to neurons in the nucleus pontis oralis (NPO) that generate AS (i.e. AS-Generator neurons). The present study was designed to provide evidence that other projections to the NPO, such as those from the amygdala, are also capable of inducing AS. Accordingly, the responses of neurons, recorded intracellularly in the NPO, were examined following stimulation of the ipsilateral central nucleus of the amygdala (CNA) in urethane-anesthetized rats. Single pulse stimulation in the CNA produced an early, fast depolarizing potential (EPSP) in neurons within the NPO. The mean latency to the onset of these excitatory postsynaptic potentials (EPSPs) was 3.6±0.2 ms. A late, small-amplitude inhibitory synaptic potential (IPSP) was present following EPSPs in a portion of the NPO neurons. Following stimulation of the CNA with a train of 8-10 pulses, NPO neurons exhibited a sustained depolarization (5-10 mV) of their resting membrane potential. When single subthreshold intracellular depolarizing current pulses were delivered to NPO neurons, CNA-induced EPSPs were sufficient to promote the discharge of these cells. Stimulation of the CNA with a short train of stimuli induced potent temporal facilitation of EPSPs in NPO neurons. Two forms of synaptic plasticity were revealed by the patterns of response of NPO neurons following stimulation of the CNA: paired-pulse facilitation (PPF) and post-tetanic potentiation (PTP). Six of recorded NPO neurons were identified morphologically with neurobiotin. They were medium to large, multipolar cells with diameters >20 μM, which resemble AS-on cells in the NPO. The present results demonstrate that amygdalar projections are capable of exerting a powerful excitatory postsynaptic drive that activates NPO neurons. Therefore, we suggest that the amygdala is capable of inducing AS via direct projections to AS-Generator neurons in the NPO.

Hypocretinergic neurons are activated in conjunction with goal-oriented survival-related motor behaviors

Hypocretinergic neurons are located in the area of the lateral hypothalamus which is responsible for mediating goal-directed, survival-related behaviors. Consequently, we hypothesize that the hypocretinergic system functions to promote these behaviors including those patterns of somatomotor activation upon which they are based. Further, we hypothesize that the hypocretinergic system is not involved with repetitive motor activities unless they occur in conjunction with the goal-oriented behaviors that are governed by the lateral hypothalamus. In order to determine the veracity of these hypotheses, we examined Fos immunoreactivity (as a marker of neuronal activity) in hypocretinergic neurons in the cat during: a) Exploratory Motor Activity; b) Locomotion without Reward; c) Locomotion with Reward; and d) Wakefulness without Motor Activity. Significantly greater numbers of hypocretinergic neurons expressed c-fos when the animals were exploring an unknown environment during Exploratory Motor Activity compared with all other paradigms. In addition, a larger number of Hcrt+Fos+neurons were activated during Locomotion with Reward than during Wakefulness without Motor Activity. Finally, very few hypocretinergic neurons were activated during Locomotion without Reward and Wakefulness without Motor Activity, wherein there was an absence of goal-directed activities. We conclude that the hypocretinergic system does not promote wakefulness per se or motor activity per se but is responsible for mediating specific goal-oriented behaviors that take place during wakefulness. Accordingly, we suggest that the hypocretinergic system is responsible for controlling the somatomotor system and coordinating its activity with other systems in order to produce successful goal-oriented survival-related behaviors that are controlled by the lateral hypothalamus.

A restricted parabrachial pontine region is active during non-rapid eye movement sleep

The principal site that generates both rapid eye movement (REM) sleep and wakefulness is located in the mesopontine reticular formation, whereas non-rapid eye movement (NREM) sleep is primarily dependent upon the functioning of neurons that are located in the preoptic region of the hypothalamus. In the present study, we were interested in determining whether the occurrence of NREM might also depend on the activity of mesopontine structures, as has been shown for wakefulness and REM sleep. Adult cats were maintained in one of the following states: quiet wakefulness (QW), alert wakefulness (AW), NREM, or REM sleep induced by microinjections of carbachol into the nucleus pontis oralis (REM-carbachol). Subsequently, they were euthanized and single-labeling immunohistochemical studies were undertaken to determine state-dependent patterns of neuronal activity in the brainstem based upon the expression of the protein Fos. In addition, double-labeling immunohistochemical studies were carried out to detect neurons that expressed Fos as well as choline acetyltransferase, tyrosine hydroxylase, or GABA. During NREM, only a few Fos-immunoreactive cells were present in different regions of the brainstem; however, a discrete cluster of Fos+ neurons was observed in the caudolateral parabrachial region (CLPB). The number of Fos+ neurons in the CLPB during NREM was significantly greater (67.9±10.9, P<0.0001) compared with QW (8.0±6.7), AW (5.2±4.2), or REM-carbachol (8.0±4.7). In addition, there was a positive correlation (R=0.93) between the time the animals spent in NREM and the number of Fos+ neurons in the CLPB. Fos-immunoreactive neurons in the CLPB were neither cholinergic nor catecholaminergic; however, about 50% of these neurons were GABAergic. We conclude that a group of GABAergic and unidentified neurons in the CLPB are active during NREM and likely involved in the control of this behavioral state. These data open new avenues for the study of NREM, as well as for the explorations of interactions between these neurons that are activated during NREM and cells of the adjacent pontine tegmentum that are involved in the generation of REM sleep.

The injection of hypocretin-1 into the nucleus pontis oralis induces either active sleep or wakefulness depending on the behavioral state when it is administered

STUDY OBJECTIVES

We previously reported that the microinjection of hypocretin (orexin) into the nucleus pontis oralis (NPO) induces a behavioral state that is comparable to naturally occurring active (rapid eye movement) sleep. However, other laboratories have found that wakefulness occurs following injections of hypocretin into the NPO. The present study tested the hypothesis that the discrepancy in behavioral state responses to hypocretin injections is due to the fact that hypocretin was not administered during the same states of sleep or wakefulness.

DESIGN

Adult cats were implanted with electrodes to record sleep and waking states. Hypocretin-1 (0.25 microL, 500microM) was microinjected into the NPO while the animals were awake or in quiet (non-rapid eye movement) sleep.

MEASUREMENTS AND RESULTS

When hyprocretin-1 was microinjected into the NPO during quiet sleep, active sleep occurred with a short latency. In addition, there was a significant increase in the time spent in active sleep and in the number of episodes of this state. On the other hand, the injection of hyprocretin-1 during wakefulness resulted not only in a significant increase in wakefulness, but also in a decrease in the percentage and frequency of episodes of active sleep.

CONCLUSIONS

The present data demonstrate that the behavioral state of the animal dictates whether active sleep or wakefulness is induced following the injection of hypocretin. Therefore, we suggest that hypocretin-1 enhances ongoing states of wakefulness and their accompanying patterns of physiologic activity and that hypocretin-1 is also capable of promoting active sleep and the changes in various processes that occur during this state.

Projection neurons from the central nucleus of the amygdala to the nucleus pontis oralis

The present retrograde labeling study was designed to determine the presence and pattern of projections from individual subdivisions of the central nucleus of the amygdala (CNA) to the nucleus pontis oralis (NPO), which is a critical brainstem site involved in the generation and maintenance of active (REM) sleep. Projections from the CNA were labeled with the retrograde tracer cholera toxin B-subunit (CTB), which was injected, unilaterally, via microiontophoresis, into the NPO. Sections of the amygdala were immunostained in order to identify CTB-labeled CNA neurons and CNA neurons that contained CTB plus the vesicular glutamate transporter 2 (VGLUT2), which is a marker for glutamatergic neurons. Histological analyses revealed that retrogradely labeled neurons that project to the NPO were localized, ipsilaterally, within the medial, lateral, and capsular subdivisions of the CNA. In addition, a substantial proportion (24%) of all retrogradely labeled CNA neurons also exhibited VGLUT2 immunoreactivity. The present study demonstrates that glutamatergic neurons, which are present within various subdivisions of the CNA, project directly to the NPO. These data lend credence to the hypothesis that NPO neurons that are involved in the control of active sleep are activated by glutamatergic projections from the amygdala.

Nitrergic ventro-medial medullary neurons activated during cholinergically induced active (rapid eye movement) sleep in the cat

The rostral ventro-medial medullary reticular formation is a complex structure that is involved with a variety of motor functions. It contains glycinergic neurons that are activated during active (rapid eye movement (REM)) sleep (AS); these neurons appear to be responsible for the postsynaptic inhibition of motoneurons that occurs during this state. We have reported that neurons in this same region contain nitric oxide (NO) synthase and that they innervate brainstem motor pools. In the present study we examined the c-fos expression of these neurons after carbachol-induced active sleep (C-AS). Three control and four experimental cats were employed to identify c-fos expressing nitrergic neurons using immunocytochemical techniques to detect the Fos protein together with neuronal nitric oxide synthase (nNOS) or nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase activity. The classical neurotransmitter content of the nitrergic cells in this region was examined through the combination of immunocytochemical techniques for the detection of glutamate, glycine, choline acetyltransferase (ChAT), tyrosine hydroxilase (TH) or GABA together with nNOS. During C-AS, there was a 1074% increase in the number of nitrergic neurons that expressed c-fos. These neurons did not contain glycine, ChAT, TH or GABA, but a subpopulation (15%) of them displayed glutamate-like immunoreactivity. Therefore, some of these neurons contain both an excitatory neurotransmitter (glutamate) and an excitatory neuromodulator (NO); the neurotransmitter content of the rest of them remains to be determined. Because some of the nitrergic neurons innervate brainstem motoneurons it is possible that they participate in the generation of tonic and excitatory phasic motor events that occur during AS. We also suggest that these nitrergic neurons may be involved in autonomic regulation during this state. In addition, because NO has trophic effects on target neurons, the present findings represent the first, albeit indirect, evidence for a possible trophic function of this nature during AS.

Prevention of apnea-induced apoptosis in NREM- and REM-generating nuclei of adult guinea pigs

The present study was designed to investigate the effects of recurrent periods of apnea/hypoxia on the morphology of neurons in sites that control NREM and REM sleep. In addition, we determined whether the administration of a GABA agonist, eszopiclone, was capable of preventing the degenerative, i.e., apoptotic, sequelae of hypoxia in these sleep-promoting neurons. Adult guinea pigs were divided into control (normoxic) and hypoxic groups; a separate group of hypoxic animals was administered eszopiclone. Recurrent periods of hypoxia and normoxia lasted for a duration of 3h. Subsequently, the brains were sectioned, and areas in the CNS that control NREM sleep as well as REM sleep were examined after staining with an antibody raised against single-stranded DNA, which labels apoptotic neurons. In the group of control (normoxic) animals, apoptotic neurons were not observed in CNS regions that control NREM or REM sleep. In hypoxic animals, a large number of apoptotic neurons were found in the preceding regions. In the hypoxic animals that were administered eszopiclone, there were almost no apoptotic neurons in the brain regions that control NREM or REM sleep. These results demonstrate that recurrent periods of apnea induce extensive apoptosis in CNS nuclei that control NREM and REM sleep and that eszopiclone is capable of preventing neuronal degeneration in these sites. We suggest that the degeneration of neurons in sites that control the states of sleep is responsible for those sleep disturbances that arise as a consequence of hypoxia in individuals with sleep-related breathing disorders.

Apnea produces neuronal degeneration in the pons and medulla of guinea pigs

Obstructive sleep apnea and other sleep-related breathing disorders result in recurrent periods of oxygen deprivation (hypoxia), hypercapnia and an increase in the cellular production of reactive oxygen species (oxidative stress-related injury). Individuals with these disorders suffer from a variety of cellular abnormalities that result in cardiopulmonary dysfunctions, disturbances in sleep and other pathologies. In the present experiment, using an animal model of sleep apnea, we determined that the degeneration of neurons and glia, due to apoptosis, occurs in specific regions of the pons and medulla. Adult guinea pigs, which were divided into control (normoxic) and experimental (hypoxic) groups, were anesthetized with alpha-chloralose and immobilized with Flaxedil. Apnea (hypoxia) was induced by ventilatory arrest in order to desaturate the oxyhemoglobin to 75% SpO(2). A sequence of apnea, followed by ventilation with recovery to >95% SpO(2), was repeated for a period of 3h. At the end of the period of recurrent apnea, the animals were perfused and brain sections were immunostained with a mouse monoclonal antibody raised against single-stranded DNA (ssDNA). Apoptotic neurons and glia, which were not found in the control group of animals, were present in brainstem regions in hypoxic group of animals; these regions involved in the control of respiration (e.g., the parafacial respiratory group and the ventral respiratory group), cardiovascular functions (e.g., the nucleus ambiguus, the nucleus tractus solitarius and the dorsal motor nucleus of the vagus) as well as REM sleep (the nucleus pontis oralis) and wakefulness (e.g., the dorsal raphe and locus ceruleus). We suggest apoptotic neurons and glia in critical areas of the pons and medulla results in many of the comorbidities experienced by patients with sleep-disordered breathing pathologies.

Serotoninergic control of glycinergic inhibitory postsynaptic currents in rat hypoglossal motoneurons

This report presents the results of a study of the frequency potentiation of inhibitory postsynaptic currents (IPSCs) in hypoglossal motoneurons and its modulation by serotonin. A release-site model of synaptic plasticity was used to characterize the frequency-related potentiation of evoked IPSCs. Data were obtained to determine if the frequency potentiation of IPSCs occurs as a consequence of a low baseline quantal content of evoked IPSCs using whole cell patch-clamp recordings from hypoglossal motoneurons in the neonatal rat brainstem slice preparation. In these motoneurons, EPSCs and GABAergic IPSCs were blocked by the application of CNQX, AP-5 and bicuculline. Glycinergic IPSCs were evoked by threshold stimulation of inhibitory neurons in the nucleus of Roller, which is located ventro-lateral to the hypoglossal nucleus. IPSC responses to trains of stimuli were recorded in control solutions and in solutions containing serotonin, which is known to reduce IPSPs in this preparation. The amplitude of non-potentiated IPSCs was reduced and their frequency potentiation was enhanced when serotonin was added to the bath. These data were examined using a release-site model of synaptic plasticity in which facilitation is attributed to a time-dependent increase in the probability of transmitter release; depression is attributed to a time-dependent decrease in the number of sites available for release. Using this model, the effect of serotonin on frequency potentiation was explained by a combination of a reduction in the baseline probability of transmitter release and an increase in the time constant of decay of the increase in probability of release that follows a stimulus.

Eszopiclone prevents excitotoxicity and neurodegeneration in the hippocampus induced by experimental apnea

STUDY OBJECTIVE

This study was designed to determine the effects of eszopiclone on apnea-induced excitotoxic synaptic processes and apoptosis in the hippocampus.

DESIGN

Recurrent periods of apnea, which consisted of a sequence of apnea (75% SpO2), followed by ventilation with recovery to normoxia (> 95% SpO2), were induced for a period of three hours in anesthetized guinea pigs. The CA3 Schaffer collateral pathway in the hippocampus was stimulated and the field excitatory postsynaptic potential (fEPSP) response was recorded in CA1. Animals in the experimental group received an intravenous injection of eszopiclone (3 mg/kg) 10 min prior to the initiation of the periods of recurrent apnea, and once every 60 min thereafter; control animals received comparable injections of vehicle. At the end of the 3-h period of recurrent apnea, the animals were perfused, and hippocampal sections were immunostained in order to determine the presence of apoptosis, i.e., programmed cell death. ANALYSES AND RESULTS: Apnea resulted in a persistent increase in synaptic responsiveness of CA1 neurons as determined by analyses of the fEPSP. Eszopiclone antagonized the apnea-induced increase in the fEPSP. Morphological analyses revealed significant apoptosis of CA1 neurons in control animals; however, there was no significant apoptosis in eszopiclone-treated animals.

CONCLUSIONS

Eszopiclone was determined to suppress the apnea-induced hyperexcitability of hippocampal CA1 neurons, thereby reducing/eliminating neurotoxicity. These data lend credence to our hypothesis that eszopiclone, exclusive of its hypnotic actions, has the capacity to function as a potent neuroprotective agent.

State-dependent control of lumbar motoneurons by the hypocretinergic system

Neurons in the lateral hypothalamus (LH) that synthesize hypocretins (Hcrt-1 and Hcrt-2) are active during wakefulness and excite lumbar motoneurons. Because hypocretinergic cells also discharge during phasic periods of rapid eye movement (REM) sleep, we sought to examine their action on the activity of motoneurons during this state. Accordingly, cat lumbar motoneurons were intracellularly recorded, under alpha-chloralose anesthesia, prior to (control) and during the carbachol-induced REM sleep-like atonia (REMc). During control conditions, LH stimulation induced excitatory postsynaptic potentials (composite EPSP) in motoneurons. In contrast, during REMc, identical LH stimulation induced inhibitory PSPs in motoneurons. We then tested the effects of LH stimulation on motoneuron responses following the stimulation of the nucleus reticularis gigantocellularis (NRGc) which is part of a brainstem-spinal cord system that controls motoneuron excitability in a state-dependent manner. LH stimulation facilitated NRGc stimulation-induced composite EPSP during control conditions whereas it enhanced NRGc stimulation-induced IPSPs during REMc. These intriguing data indicate that the LH exerts a state-dependent control of motor activity. As a first step to understand these results, we examined whether hypocretinergic synaptic mechanisms in the spinal cord were state dependent. We found that the juxtacellular application of Hcrt-1 induced motoneuron excitation during control conditions whereas motoneuron inhibition was enhanced during REMc. These data indicate that the hypocretinergic system acts on motoneurons in a state-dependent manner via spinal synaptic mechanisms. Thus, deficits in Hcrt-1 may cause the coexistence of incongruous motor signs in cataplectic patients, such as motor suppression during wakefulness and movement disorders during REM sleep.

The impact of age on the hypnotic effects of eszopiclone and zolpidem in the guinea pig

RATIONALE

Eszopiclone and zolpidem are hypnotics that differentially affect sleep and waking states in adult animals. Therefore, it was of interest to compare their effects on the states of sleep and wakefulness in aged animals.

OBJECTIVES

Our objective was to determine the responses to eszopiclone and zolpidem vis-à-vis sleep and waking states in aged guinea pigs and to compare them with the effects of these hypnotics in adult animals.

METHODS

Aged guinea pigs were prepared to monitor sleep and waking states and to perform a frequency analysis of the EEG. Eszopiclone and zolpidem were administered intraperitoneally (1, 3, and 10 mg/kg).

RESULTS

Eszopiclone produced a more rapid and greater increase in NREM sleep as well as longer duration episodes of NREM sleep compared with zolpidem. There was also a significant increase in the latency to REM sleep with eszopiclone, but not with zolpidem. EEG power during NREM sleep increased in the delta band and decreased in the theta band following eszopiclone administration, whereas zolpidem had no effect on any of the frequency bands analyzed.

CONCLUSIONS

In aged as well as adult guinea pigs, eszopiclone is a more effective hypnotic insofar as it produces a shorter latency to NREM sleep, a greater amount of NREM sleep and EEG delta waves. Differences in the effects produced by eszopiclone and zolpidem as a function of the aging process likely reflect the fact that they bind to different subunits of the GABA(A) receptors, which are differentially reactive to the aging process.

MCHergic projections to the nucleus pontis oralis participate in the control of active (REM) sleep

Neurons that utilize melanin-concentrating hormone (MCH) as a neuromodulator are located in the lateral hypothalamus and incerto-hypothalamic area and project diffusely throughout the central nervous system, including areas that participate in the generation and maintenance of sleep and wakefulness. Recent studies have shown that hypothalamic MCHergic neurons are active during active sleep (AS), and that intraventricular microinjections of MCH induce AS sleep; however, there are no data available regarding the manner in which MCHergic neurons participate in the control of this behavioral state. Utilizing immunohistochemical and retrograde tracing techniques, we examined, in the cat, projections from MCHergic neurons to the nucleus pontis oralis (NPO), which is considered to be the executive area that is responsible for the generation and maintenance of AS. In addition, we explored the effects on sleep and waking states produced by the microinjection of MCH into the NPO. We first determined that MCHergic fibers and terminals are present in the NPO. We also found that when a retrograde tracer (cholera toxin subunit B) was placed in the NPO MCHergic neurons of the hypothalamus were labeled. When MCH was microinjected into the NPO, there was a significant increase in the amount of AS (19.8+/-1.4% versus 11.9+/-0.2%, P<0.05) and a significant decrease in the latency to AS (10.4+/-4.2 versus 26.6+/-2.3 min, P<0.05). The preceding anatomical and functional data support our hypothesis that the MCHergic system participates in the regulation of AS by modulating neuronal activity in the NPO.

Confirmation of the consensus that glycinergic postsynaptic inhibition is responsible for the atonia of REM sleep

An overwhelmingly coherent, integrated body of data developed by independent laboratories, over many decades, using intracellular recording in conjunction with the juxtacellular microiontophoretic ejection of neurotransmitters and antagonists, demonstrates conclusively that postsynaptic inhibition, mediated by glycine, is the critical and sufficient process that completely accounts for the suppression of motoneuron discharge during the tonic and phasic periods of REM sleep. These studies, many of which were conducted in intact, naturally sleeping, adult animals, eliminate potential interpretive complications that arise using reduced, in vitro slice or even intact in vivo preparations; they also provide for levels of resolutions that are not possible with microdialysis. On the other hand, when infusing a cocktail of substances for two to four hours into the trigeminal motor pool and adjacent regions, it is to be expected that uninterpretable and nonphysiological results would be obtained, especially when thousands of receptors on thousands of cells that are exclusively responsible for promoting waking-related functions of trigeminal motoneurons are activated. Because receptors in such a large region were indiscriminately activated by substances that Brooks and Peever dialyzed, it is clearly impossible to conclude that any change in EMG activity was due only to the activation of receptors on alpha motoneurons that are involved in state-dependent processes. In addition, because the results that Brooks and Peever obtained cannot be attributed to any specific class of receptors, synaptic process, or cell type, it is not possible to compare their findings with data obtained from intracellular studies. The preceding notwithstanding, the technical execution of their experiments was of an extremely high quality. Given this obvious strength of Brooks and Peever, it is unfortunate that they did not utilize a technique that would have allowed them to obtain meaningful data, such as intracellular recording. In point of fact, the generation of a preparation in which it is possible to record intracellularly and eject substances juxtacellularly during naturally occurring states of sleep and wakefulness was developed, over a period of two years, specifically to avoid the problems that are inherent in the microdialysis technique that Brooks and Peever employed. In conclusion, during wakefulness, numerous receptors on a great many neuronal elements in and in the vicinity of the trigeminal motor nucleus are normally activated in highly regulated sequences depending upon the specific behavior that is being performed, such as vocalization, biting, chewing, swallowing, etc. On the other hand, during REM sleep, only receptors on alpha motoneurons in the trigeminal motor nucleus, which are involved in state-dependent control processes, are excited. These latter receptors have been identified as glycinergic and have been shown to be activated, monosynaptically, by projections from the region of the nucleus reticularis gigantocellularis. Therefore, there is no justification for Brooks and Peever to claim that an unknown "biochemical substrate" is responsible for atonia during REM sleep, nor do they provide any data or reason not to continue to believe in the veracity of their initial statement, reflecting the consensus that "glycinergic inhibition of somatic motoneurons is responsible for loss of postural muscle tone in REM sleep".

Recurrent apnea induces neuronal apoptosis in the guinea pig forebrain

Obstructive sleep apnea (OSA) and sleep-disordered breathing (SDB) can result in impaired cognition and mental acuity, and the generation of mood disorders, including depression. However, the mechanisms of neuronal damage for these complications have not been elucidated. Accordingly, using immunohistochemical technique with monoclonal antibody against single-stranded DNA, we examined the morphological effects of chronic recurrent apnea on neurons in the hippocampus and related forebrain sites in guinea pigs. Our results show that a large number of neurons labeled by anti-ssDNA antibody were present in the cingulate, insular and frontal cortices, the hippocampus and the amygdala in conjunction with periods of recurrent apnea. However, no labeling was observed in comparable regions of the brain in control guinea pigs. In the cortices of experimental animals, labeled neurons were detected mainly in the superficial layers (II-III) in the frontal, insular and cingulate cortex. In the hippocampus, most labeled neurons were located in the CA1 region, in which most of stained neurons were observed in strata pyramidal, while only a few positive neurons were located in the strata radiatum and the strata oriens. In addition, a large number of labeled neurons were also detected in the central nucleus of amygdala in the guinea pigs underwent recurrent periods of apnea. The present data indicate that recurrent apnea results in cell death in the hippocampus and related forebrain regions via mechanisms of apoptosis, which may represent the basis for the clinical complications of obstructive sleep apnea and sleep-disordered breathing.

Effects of eszopiclone and zolpidem on sleep and waking states in the adult guinea pig

STUDY OBJECTIVE

The present study was designed to compare and contrast the effects of eszopiclone and zolpidem on the states of sleep and wakefulness in chronically instrumented, unanesthetized adult guinea pigs.

DESIGN

Adult guinea pigs were implanted with electrodes to record sleep and waking states and to perform a frequency analysis of the EEG. Eszopiclone (1 and 3 mg/kg) and zolpidem (1 and 3 mg/kg) were administered intraperitoneally.

MEASUREMENTS AND RESULTS

The administration of eszopiclone (1 and 3 mg/kg) resulted in a significant dose-dependent increase in NREM sleep. Zolpidem produced a significant increase in NREM sleep, but only at a dose of 3 mg/kg. The following changes in NREM and REM sleep, as well as in the power spectra, were all significant when the effects of 1 and 3 mg/kg of eszopiclone were compared with responses induced with 1 and 3 mg/kg of zolpidem, respectively: The increase in NREM sleep produced by eszopiclone was greater than that following the administration of zolpidem. The mean latency to NREM sleep following the administration of eszopiclone was significantly shorter than zolpidem. Eszopiclone significantly increased the latency to REM sleep. The mean duration of episodes of NREM sleep was increased by eszopiclone, but not by zolpidem. The EEG power increased in the delta band and decreased in the theta band during NREM sleep following the administration of eszopiclone. No significant changes occurred in any of the frequency bands analyzed following zolpidem administration.

CONCLUSIONS

The differences in the effects of eszopiclone and zolpidem on sleep and waking states and the power spectra of the EEG likely reflect the fact that eszopiclone and zolpidem bind to different subunits of the GABAA receptor complex.

Apnea promotes glutamate-induced excitotoxicity in hippocampal neurons

Patients with obstructive sleep apnea (OSA) exhibit hippocampal damage and cognitive deficits. To determine the effect of apnea on the synaptic transmission in the hippocampus, we performed electrophysiological studies in an in vivo guinea pig model of OSA. Specifically, we determined the cornu ammonis region 1 (CA1) field excitatory postsynaptic potential (fEPSP) response to cornu ammonis region 3 (CA3) stimulation and examined the presynaptic mechanisms underlying the changes in the fEPSP. Single episodes of apnea resulted in a maximal potentiation of the fEPSPs at 1 to 3 min after the termination of each episode of apnea. The mean amplitude and slope of the post-apneic fEPSP was significantly larger compared with the pre-apneic control. These changes were accompanied by a significant decrease in the paired-pulse facilitation ratio during the post-apneic period compared with the pre-apneic control. The N-methyl-D-aspartate (NMDA) glutamate receptor antagonist MK-801, when applied locally to the CA1 recording site by pressure ejection, blocked the apnea-induced potentiation of the fEPSP. In the experimental animals that were subjected to extended periods of recurrent apnea, CA1 neurons exhibited positive immunoreactivity for fragmented DNA strands, which indicates apoptotic cell death. The present results demonstrate that apnea-induced potentiation of the hippocampal CA1 fEPSP is mediated by an NMDA receptor mechanism. We therefore conclude that recurrent apnea produces abnormally high levels of glutamate that results in the apoptosis of CA1 neurons. We hypothesize that this damage is reflected by the cognitive deficits that are commonly observed in patients with breathing disorders such as OSA.

GABAergic processes in the mesencephalic tegmentum modulate the occurrence of active (rapid eye movement) sleep in guinea pigs

The ventrolateral subdivision of the periaqueductal gray (vlPAG) and the adjacent dorsal mesencephalic reticular formation (dMRF) are involved in the modulation of active (rapid eye movement) sleep (AS). In order to determine the effects on AS of the suppression of neuronal activity in these regions, muscimol, a GABA receptor A (GABA(A)) receptor agonist, and bicuculline, a GABA(A) receptor antagonist, were microinjected bilaterally in guinea pigs and the states of sleep and wakefulness were examined. The main effect of muscimol was an increase in AS; this increase occurred in conjunction with a reduction in the time spent in wakefulness. The powerful effect of muscimol was striking especially when considering the small amount of naturally-occurring AS that is present in this species. Additional observable effects that were induced by muscimol were: 1) long lasting episodes of hypotonia/atonia during wakefulness and quiet sleep that included a lack of extensor tone in the hind limbs, and 2) frequently occurring cortical spindles, similar to those observed during naturally-occurring quiet sleep (sleep spindles), that were present during wakefulness. Conversely, bilateral microinjections of bicuculline induced a prolonged state of wakefulness and blocked the effect of subsequent injections of muscimol. These data suggest that endogenous GABA acts on GABA(A) receptors within the vlPAG and dMRF to promote AS in the guinea pig.

MCH-containing neurons in the hypothalamus of the cat: searching for a role in the control of sleep and wakefulness

Neurons that utilize melanin-concentrating hormone (MCH) and others that employ hypocretin as neurotransmitter are located in the hypothalamus and project diffusely throughout the CNS, including areas that participate in the generation and maintenance of the states of sleep and wakefulness. In the present report, immunohistochemical methods were employed to examine the distribution of MCHergic and hypocretinergic neurons. In order to test the hypothesis that the MCHergic system is capable of influencing specific behavioral states, we studied Fos immunoreactivity in MCH-containing neurons during (1) quiet wakefulness, (2) active wakefulness with motor activity, (3) active wakefulness without motor activity, (4) quiet sleep and (5) active sleep induced by carbachol (AS-carbachol). We determined that MCHergic neuronal somata in the cat are intermingled with hypocretinergic neurons in the dorsal and lateral hypothalamus, principally in the tuberal and tuberomammillary regions; however, hypocretinergic neurons extended more in the anterior-posterior axis than MCHergic neurons. Axosomatic and axodendritic contacts were common between these neurons. In contrast to hypocretinergic neurons, which are known to be active during motor activity and AS-carbachol, Fos immunoreactivity was not observed in MCH-containing neurons in conjunction with any of the preceding behavioral conditions. Non-MCHergic, non-hypocretinergic neurons that expressed c-fos during active wakefulness with motor activity were intermingled with MCH and hypocretin-containing neurons, suggesting that these neurons are related to some aspect of motor function. Further studies are required to elucidate the functional sequela of the interactions between MCHergic and hypocretinergic neurons and the phenotype of the other neurons that were active during motor activity.

Brainstem glycinergic neurons and their activation during active (rapid eye movement) sleep in the cat

It is well established that, during rapid eye movement (REM) sleep, somatic motoneurons are subjected to a barrage of inhibitory synaptic potentials that are mediated by glycine. However, the source of this inhibition, which is crucial for the maintenance and preservation of REM sleep, has not been identified. Consequently, the present study was undertaken to determine in cats the location of the glycinergic neurons, that are activated during active sleep, and are responsible for the postsynaptic inhibition of motoneurons that occurs during this state. For this purpose, a pharmacologically-induced state of active sleep (AS-carbachol) was employed. Antibodies against glycine-conjugated proteins were used to identify glycinergic neurons and immunocytochemical techniques to label the Fos protein were employed to identify activated neurons. Two distinct populations of glycinergic neurons that expressed c-fos were distinguished. One population was situated within the nucleus reticularis gigantocellularis (NRGc) and nucleus magnocellularis (Mc) in the rostro-ventral medulla; this group of neurons extended caudally to the ventral portion of the nucleus paramedianus reticularis (nPR). Forty percent of the glycinergic neurons in the NRGc and Mc and 25% in the nPR expressed c-fos during AS-carbachol. A second population was located in the caudal medulla adjacent to the nucleus ambiguus (nAmb), wherein 40% of the glycinergic cells expressed c-fos during AS-carbachol. Neither population of glycinergic cells expressed c-fos during quiet wakefulness or quiet (non-rapid eye movement) sleep. We suggest that the population of glycinergic neurons in the NRGc, Mc, and nPR participates in the inhibition of somatic brainstem motoneurons during active sleep. These neurons may also be responsible for the inhibition of sensory and other processes during this state. It is likely that the group of glycinergic neurons adjacent to the nucleus ambiguus (nAmb) is responsible for the active sleep-selective inhibition of motoneurons that innervate the muscles of the larynx and pharynx.