Brainstem control of the events of REM sleep

Urethane anaesthesia exhibits neurophysiological correlates of unconsciousness and is distinct from sleep

Urethane is a general anaesthetic widely used in animal research. The state of urethane anaesthesia is unique because it alternates between macroscopically distinct electrographic states: a slow-wave state that resembles non-rapid eye movement (NREM) sleep and an activated state with features of both REM sleep and wakefulness. Although it is assumed that urethane produces unconsciousness, this has been questioned because of states of cortical activation during drug exposure. Furthermore, the similarities and differences between urethane anaesthesia and physiological sleep are still unclear. In this study, we recorded the electroencephalogram (EEG) and electromyogram in chronically prepared rats during natural sleep-wake states and during urethane anaesthesia. We subsequently analysed the power, coherence, directed connectivity and complexity of brain oscillations and found that EEG under urethane anaesthesia has clear signatures of unconsciousness, with similarities to other general anaesthetics. In addition, the EEG profile under urethane is different in comparison with natural sleep states. These results suggest that consciousness is disrupted during urethane. Furthermore, despite similarities that have led others to conclude that urethane is a model of sleep, the electrocortical traits of depressed and activated states during urethane anaesthesia differ from physiological sleep states.

A medullary hub for controlling REM sleep and pontine waves

Rapid-eye-movement (REM) sleep is a distinct behavioral state associated with vivid dreaming and memory processing. Phasic bursts of electrical activity, measurable as spike-like pontine (P)-waves, are a hallmark of REM sleep implicated in memory consolidation. However, the brainstem circuits regulating P-waves, and their interactions with circuits generating REM sleep, remain largely unknown. Here, we show that an excitatory population of dorsomedial medulla (dmM) neurons expressing corticotropin-releasing-hormone (CRH) regulates both REM sleep and P-waves in mice. Calcium imaging showed that dmM CRH neurons are selectively activated during REM sleep and recruited during P-waves, and opto- and chemogenetic experiments revealed that this population promotes REM sleep. Chemogenetic manipulation also induced prolonged changes in P-wave frequency, while brief optogenetic activation reliably triggered P-waves along with transiently accelerated theta oscillations in the electroencephalogram (EEG). Together, these findings anatomically and functionally delineate a common medullary hub for the regulation of both REM sleep and P-waves.

Sleep Disruption and Cancer: Chicken or the Egg?

Sleep is a nearly ubiquitous phenomenon across the phylogenetic tree, highlighting its essential role in ensuring fitness across evolutionary time. Consequently, chronic disruption of the duration, timing, or structure of sleep can cause widespread problems in multiple physiological systems, including those that regulate energy balance, immune function, and cognitive capacity, among others. Many, if not all these systems, become altered throughout the course of cancer initiation, growth, metastatic spread, treatment, and recurrence. Recent work has demonstrated how changes in sleep influence the development of chronic diseases, including cancer, in both humans and animal models. A common finding is that for some cancers (e.g., breast), chronic disruption of sleep/wake states prior to disease onset is associated with an increased risk for cancer development. Additionally, sleep disruption after cancer initiation is often associated with worse outcomes. Recently, evidence suggesting that cancer itself can affect neuronal circuits controlling sleep and wakefulness has accumulated. Patients with cancer often report difficulty falling asleep, difficulty staying asleep, and severe fatigue, during and even years after treatment. In addition to the psychological stress associated with cancer, cancer itself may alter sleep homeostasis through changes to host physiology and via currently undefined mechanisms. Moreover, cancer treatments (e.g., chemotherapy, radiation, hormonal, and surgical) may further worsen sleep problems through complex biological processes yet to be fully understood. This results in a “chicken or the egg” phenomenon, where it is unclear whether sleep disruption promotes cancer or cancer reciprocally disrupts sleep. This review will discuss existing evidence for both hypotheses and present a framework through which the interactions between sleep and cancer can be dissociated and causally investigated.

Hippocampal theta power pressure builds over non-REM sleep and dissipates within REM sleep episodes

The theta rhythm during waking has been associated with voluntary motor activity and learning processes involving the hippocampus. Theta also occurs continuously during rapid eye movement (REM) sleep where it likely serves memory consolidation. Theta amplitude builds across wakefulness and is the best indicator of the homeostatic need for non-REM (NREM) sleep. Although REM sleep is homeostatically regulated independently of NREM sleep, the drivers of REM sleep regulation are under debate. The dynamics of theta within REM sleep bouts have not been thoroughly explored. We equipped 20 male rats with sleep instrumentation and hippocampal electrodes to measure theta across normal sleep/waking periods over the first 4 h of the sleep phase on two consecutive days. We found that theta power decreased by a third, on average, within individual REM sleep bouts, but recovered between bouts. Thus, there was no general decline in theta power across the duration of the recording period or between days. The time constant of theta power decline within a REM sleep bout was the same whether the bout was short, midlength, or long, and did not predict the behavioral state immediately following the REM sleep bout. Interestingly, the more time spent in NREM sleep prior to REM sleep, the larger the decline in theta power during REM sleep, indicating that REM sleep theta may be homeostatically driven by NREM sleep just as NREM delta power is driven by the length of prior waking and by waking theta. Potential causes and implications for this phenomenon are discussed.

Partial homologies between sleep states in lizards, mammals, and birds suggest a complex evolution of sleep states in amniotes

It is crucial to determine whether rapid eye movement (REM) sleep and slow-wave sleep (SWS) (or non-REM sleep), identified in most mammals and birds, also exist in lizards, as they share a common ancestor with these groups. Recently, a study in the bearded dragon (P. vitticeps) reported states analogous to REM and SWS alternating in a surprisingly regular 80-s period, suggesting a common origin of the two sleep states across amniotes. We first confirmed these results in the bearded dragon with deep brain recordings and electro-oculogram (EOG) recordings. Then, to confirm a common origin and more finely characterize sleep in lizards, we developed a multiparametric approach in the tegu lizard, a species never recorded to date. We recorded EOG, electromyogram (EMG), heart rate, and local field potentials (LFPs) and included data on arousal thresholds, sleep deprivation, and pharmacological treatments with fluoxetine, a serotonin reuptake blocker that suppresses REM sleep in mammals. As in the bearded dragon, we demonstrate the existence of two sleep states in tegu lizards. However, no clear periodicity is apparent. The first sleep state (S1 sleep) showed high-amplitude isolated sharp waves, and the second sleep state (S2 sleep) displayed 15-Hz oscillations, isolated ocular movements, and a decrease in heart rate variability and muscle tone compared to S1. Fluoxetine treatment induced a significant decrease in S2 quantities and in the number of sharp waves in S1. Because S2 sleep is characterized by the presence of ocular movements and is inhibited by a serotonin reuptake inhibitor, as is REM sleep in birds and mammals, it might be analogous to this state. However, S2 displays a type of oscillation never previously reported and does not display a desynchronized electroencephalogram (EEG) as is observed in the bearded dragons, mammals, and birds. This suggests that the phenotype of sleep states and possibly their role can differ even between closely related species. Finally, our results suggest a common origin of two sleep states in amniotes. Yet, they also highlight a diversity of sleep phenotypes across lizards, demonstrating that the evolution of sleep states is more complex than previously thought.

Until recently, the general understanding about sleep was that only mammals and birds show two sleep states: slow-wave sleep and rapid eye movement (REM) sleep. Consequently, it was thought that these two states appeared independently in these warm-blooded animals. However, a recent paper reported the presence of these two states in the bearded dragon lizard (Pogona vitticeps), suggesting that these two states arose with the common ancestor of mammals, birds, and reptiles. We confirmed the presence of two sleep states in the bearded dragon and compared its sleep with that of another lizard, the Argentine tegu (Salvator merianae). Our results show that both lizard species have two sleep states with similarities to the two sleep states observed in mammals and birds. Additionally, our study of behavioral and physiological parameters as well as the brain activity associated with sleep in these lizards allowed us to also show important differences between these two species of lizards and between lizards, birds, and mammals. Our findings indicate that sleep in lizards is more complex than previously thought and raise further questions about the nature, function, and evolution of these two sleep states.

Distribution, physiology and pharmacology of relaxin-3/RXFP3 systems in brain

Relaxin-3 is a member of a superfamily of structurally-related peptides that includes relaxin and insulin-like peptide hormones. Soon after the discovery of the relaxin-3 gene, relaxin-3 was identified as an abundant neuropeptide in brain with a distinctive topographical distribution within a small number of GABAergic neuron populations that is well conserved across species. Relaxin-3 is thought to exert its biological actions through a single class-A GPCR - relaxin-family peptide receptor 3 (RXFP3). Class-A comprises GPCRs for relaxin-3 and insulin-like peptide-5 and other peptides such as orexin and the monoamine transmitters. The RXFP3 receptor is selectively activated by relaxin-3, whereas insulin-like peptide-5 is the cognate ligand for the related RXFP4 receptor. Anatomical and pharmacological evidence obtained over the last decade supports a function of relaxin-3/RXFP3 systems in modulating responses to stress, anxiety-related and motivated behaviours, circadian rhythms, and learning and memory. Electrophysiological studies have identified the ability of RXFP3 agonists to directly hyperpolarise thalamic neurons in vitro, but there are no reports of direct cell signalling effects in vivo. This article provides an overview of earlier studies and highlights more recent research that implicates relaxin-3/RXFP3 neural network signalling in the integration of arousal, motivation, emotion and related cognition, and that has begun to identify the associated neural substrates and mechanisms. Future research directions to better elucidate the connectivity and function of different relaxin-3 neuron populations and their RXFP3-positive target neurons in major experimental species and humans are also identified.

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

Variability in State-Dependent Plasticity of Intrinsic Properties during Cell-Autonomous Self-Regulation of Calcium Homeostasis in Hippocampal Model Neurons

How do neurons reconcile the maintenance of calcium homeostasis with perpetual switches in patterns of afferent activity? Here, we assessed state-dependent evolution of calcium homeostasis in a population of hippocampal pyramidal neuron models, through an adaptation of a recent study on stomatogastric ganglion neurons. Calcium homeostasis was set to emerge through cell-autonomous updates to 12 ionic conductances, responding to different types of synaptically driven afferent activity. We first assessed the impact of theta-frequency inputs on the evolution of ionic conductances toward maintenance of calcium homeostasis. Although calcium homeostasis emerged efficaciously across all models in the population, disparate changes in ionic conductances that mediated this emergence resulted in variable plasticity to several intrinsic properties, also manifesting as significant differences in firing responses across models. Assessing the sensitivity of this form of plasticity, we noted that intrinsic neuronal properties and the firing response were sensitive to the target calcium concentration and to the strength and frequency of afferent activity. Next, we studied the evolution of calcium homeostasis when afferent activity was switched, in different temporal sequences, between two behaviorally distinct types of activity: theta-frequency inputs and sharp-wave ripples riding on largely silent periods. We found that the conductance values, intrinsic properties, and firing response of neurons exhibited differential robustness to an intervening switch in the type of afferent activity. These results unveil critical dissociations between different forms of homeostasis, and call for a systematic evaluation of the impact of state-dependent switches in afferent activity on neuronal intrinsic properties during neural coding and homeostasis.

Ascending control of arousal and motivation: role of nucleus incertus and its peptide neuromodulators in behavioural responses to stress

Arousal is a process that involves the activation of ascending neural pathways originating in the rostral pons that project to the forebrain through the midbrain reticular formation to promote the activation of key cortical, thalamic, hypothalamic and limbic centres. Established modulators of arousal include the cholinergic, serotonergic, noradrenergic and dopaminergic networks originating in the pons and midbrain. Recent data indicate that a population of largely GABAergic projection neurones located in the nucleus incertus (NI) are also involved in arousal and motivational processes. The NI has prominent efferent connections with distinct hypothalamic, amygdalar and thalamic nuclei, in addition to dense projections to key brain regions associated with the generation and pacing of hippocampal activity. The NI receives strong inputs from the prefrontal cortex, lateral habenula and the interpeduncular and median raphe nuclei, suggesting it is highly integrated in circuits regulating higher cognitive behaviours (hippocampal theta rhythm) and emotion. Anatomical and functional studies have revealed that the NI is a rich source of multiple peptide neuromodulators, including relaxin-3, and may mediate extra-hypothalamic effects of the stress hormone corticotrophin-releasing factor, as well as other key modulators such as orexins and oxytocin. This review provides an overview of earlier studies and highlights more recent research that implicates this neural network in the integration of arousal and motivated behaviours and has begun to identify the associated mechanisms. Future research that should help to better clarify the connectivity and function of the NI in major experimental species and humans is also discussed.

Altered sigma-1 receptor expression in two animal models of cognitive impairment

PURPOSE

Sigma-1 receptors are involved in learning and memory processes. We assessed sigma-1 receptor expression and memory function in two animal models of cognitive impairment.

PROCEDURES

Male Wistar-Hannover rats were either lesioned by unilateral injection of N-methyl-D-aspartic acid in the nucleus basalis, or deprived of rapid eye movement sleep for 48 h, using the modified multiple platform method. Sigma-1 receptor expression was examined with the positron emission tomography radiotracer [(11)C]SA4503, immunohistochemistry, and Western blotting.

RESULTS

Cortical tracer uptake after 1 week was not significantly affected by lesioning. Immunohistochemistry revealed moderate increases of sigma-1 receptors at bregma level -2.8, in parietal cortex layer V of the lesioned hemisphere. Sleep deprivation lowered passive avoidance test scores and reduced [(11)C]SA4503 accumulation and sigma-1 receptor expression in pons.

CONCLUSIONS

Cholinergic lesioning causes an increase of sigma-1 receptor expression in a small cortical area which may be neuroprotective. Sleep deprivation decreases receptor expression in midbrain and pons.

Efferent connections of the parvalbumin-positive (PV1) nucleus in the lateral hypothalamus of rodents

A solitary cluster of parvalbumin-positive neurons--the PV1 nucleus--has been observed in the lateral hypothalamus of rodents. In the present study, we mapped the efferent connections of the PV1 nucleus using nonspecific antero- and retrograde tracers in rats, and chemoselective, Cre-dependent viral constructs in parvalbumin-Cre mice. In both species, the PV1 nucleus was found to project mainly to the periaqueductal grey matter (PAG), predominantly ipsilaterally. Indirectly in rats and directly in mice, a discrete, longitudinally oriented cylindrical column of terminal fields (PV1-CTF) was identified ventrolateral to the aqueduct on the edge of the PAG. The PV1-CTF is particularly dense in the rostral portion, which is located in the supraoculomotor nucleus (Su3). It is spatially interrupted over a short stretch at the level of the trochlear nucleus and abuts caudally on a second parvalbumin-positive (PV2) nucleus. The rostral and the caudal portions of the PV1-CTF consist of axonal endings, which stem from neurons scattered throughout the PV1 nucleus. Topographically, the longitudinal orientation of the PV1-CTF accords with that of the likewise longitudinally oriented functional modules of the PAG, but overlaps none of them. Minor terminal fields were identified in a crescentic column of the lateral PAG, as well as in the Edinger-Westphal, the lateral habenular, and the laterodorsal tegmental nuclei. So far, no obvious functions have been attributed to this small, circumscribed column ventrolateral to the aqueduct, the prime target of the PV1 nucleus.

REM Sleep Regulating Mechanisms in the Cholinergic Cell Compartment of the Brainstem

Rapid eye movement (REM) sleep is a highly evolved yet paradoxical behavioral state (highly activated brain in a paralyzed body) in mammalian species. Since the discovery of REM sleep and its physiological distinction from other sleep states¹, a vast number of studies in neurosciences have been dedicated toward understanding the mechanisms and functions of this behavioral state. Collectively, studies have shown that each of the physiological events that characterize the behavioral state of REM sleep is executed by distinct cell groups located in the brainstem. These cell groups are discrete components of a widely distributed network, rather than a single REM sleep center. The final activity within each of these executive cell groups is controlled by the ratio of cholinergic neurotransmission emanating from the pedunculopontine tegmentum (PPT) to aminergic neurotransmission emanating from the locus coeruleus (LC) and raphe nucleus (RN). In this review, we summarize the most recent findings on the cellular and molecular mechanisms in the PPT cholinergic cell compartment that underlie the regulation of REM sleep. This up-to-date review should allow clinicians and researchers to better understand the effects of drugs and neurologic disease on REM sleep.

REM sleep reorganizes hippocampal excitability

Sleep is composed of an alternating sequence of REM and non-REM episodes, but their respective roles are not known. We found that the overall firing rates of hippocampal CA1 neurons decreased across sleep concurrent with an increase in the recruitment of neuronal spiking to brief "ripple" episodes, resulting in a net increase in neural synchrony. Unexpectedly, within non-REM episodes, overall firing rates gradually increased together with a decrease in the recruitment of spiking to ripples. The rate increase within non-REM episodes was counteracted by a larger and more rapid decrease of discharge frequency within the interleaved REM episodes. Both the decrease in firing rates and the increase in synchrony during the course of sleep were correlated with the power of theta activity during REM episodes. These findings assign a prominent role of REM sleep in sleep-related neuronal plasticity.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

Why does rem sleep occur? A wake-up hypothesis

Brain activity differs in the various sleep stages and in conscious wakefulness. Awakening from sleep requires restoration of the complex nerve impulse patterns in neuronal network assemblies necessary to re-create and sustain conscious wakefulness. Herein I propose that the brain uses rapid eye movement (REM) to help wake itself up after it has had a sufficient amount of sleep. Evidence suggesting this hypothesis includes the facts that, (1) when first going to sleep, the brain plunges into Stage N3 (formerly called Stage IV), a deep abyss of sleep, and, as the night progresses, the sleep is punctuated by episodes of REM that become longer and more frequent toward morning, (2) conscious-like dreams are a reliable component of the REM state in which the dreamer is an active mental observer or agent in the dream, (3) the last awakening during a night's sleep usually occurs in a REM episode during or at the end of a dream, (4) both REM and awake consciousness seem to arise out of a similar brainstem ascending arousal system (5) N3 is a functionally perturbed state that eventually must be corrected so that embodied brain can direct adaptive behavior, and (6) cortico-fugal projections to brainstem arousal areas provide a way to trigger increased cortical activity in REM to progressively raise the sleeping brain to the threshold required for wakefulness. This paper shows how the hypothesis conforms to common experience and has substantial predictive and explanatory power regarding the phenomenology of sleep in terms of ontogeny, aging, phylogeny, abnormal/disease states, cognition, and behavioral physiology. That broad range of consistency is not matched by competing theories, which are summarized herein. Specific ways to test this wake-up hypothesis are suggested. Such research could lead to a better understanding of awake consciousness.

Cellular and chemical neuroscience of mammalian sleep

Extraordinary strides have been made toward understanding the complexities and regulatory mechanisms of sleep over the past two decades thanks to the help of rapidly evolving technologies. At its most basic level, mammalian sleep is a restorative process of the brain and body. Beyond its primary restorative purpose, sleep is essential for a number of vital functions. Our primary research interest is to understand the cellular and molecular mechanisms underlying the regulation of sleep and its cognitive functions. Here I will reflect on our own research contributions to 50 years of extraordinary advances in the neurobiology of slow-wave sleep (SWS) and rapid eye movement (REM) sleep regulation. I conclude this review by suggesting some potential future directions to further our understanding of the neurobiology of sleep.

Coupling changes in cortical and pontine sigma and theta frequency oscillations following monoaminergic lesions in rat

PURPOSE

Sigma and theta frequency electroencephalogram (EEG) oscillations exhibit substantial and well-recognized shifts with transitions across sleep and wake states. We aimed in this study to test the changes in coupling between these characteristic oscillations of non-rapid-eye-movement (NREM)/rapid-eye-movement (REM) sleep within and between cortical and pontine EEGs following monoaminergic lesion, by using the Pearson's product-moment correlation coefficients.

METHODS

Experiments were performed in 14 adult, male Sprague Dawley rats chronically instrumented for sleep recording. We lesioned the dorsal raphe nucleus axon terminals in four rats using PCA neurotoxin (p-chloroamphetamine; Sigma-Aldrich, MO) administered as two intraperitoneal (IP) injections (6 mg/kg) 24 h apart. Lesioning of locus coeruleus axon terminals was performed in five rats using DSP-4 neurotoxin (N-2-chloroethyl-N-ethyl-2-bromobenzilamine; Sigma-Aldrich, MO) in a single IP dose of 50 mg/kg.

RESULTS & CONCLUSIONS

Our previous study [Saponjic et al., Physiol Behav 90:1-10, 2007] demonstrated that these systemically induced monoaminergic lesions failed to produce significant changes in sleep/wake distribution from control conditions. The present study, by using spectral analysis and by examining the Pearson's correlation coefficients and their approximate probability density (APD) distribution profiles in control and lesion condition, demonstrates significant augmentation of the sigma/theta coupling strength, an inversion of cortical sigma/theta coupling direction and emergence of an additional sigma/theta coupling "mode" specific to the post-lesion state only within the cortex. By using the Pearson's correlation coefficients and their APD profiles, instead of classical sleep/wake distribution analysis, as a measure of direction and strength of sigma/theta coupling within and between cortex and pons, we were able to uncover the impact of a tonically decreased level of brain monoamines as altered strength and mode of coupling between sigma and theta oscillations. Specifically, a new mode of sigma/theta coupling emerged following lesion, which was specific to NREM sleep, suggests that loss of monoaminergic signaling interferes with NREM sleep consolidation. Our results also indicate an importance of monoamines in control of the sleep spindle and theta rhythm generators.

Evolving concepts of sleep cycle generation: From brain centers to neuronal populations

As neurophysiological investigations of sleep cycle control have provided an increasingly detailed picture of events at the cellular level, the concept that the sleep cycle is generated by the interaction of multiple, anatomically distributed sets of neurons has gradually replaced the hypothesis that sleep is generated by a single, highly localized neuronal oscillator.

Cell groups that discharge during rapid-eye-movement (REM) sleep (REM-on) and neurons that slow or cease firing during REM sleep (REM-off) have long been thought to comprise at least two neurochemically distinct populations. The fact that putatively cholinoceptive and/or cholinergic (REM-on) and putatively aminergic (REM-off) cell populations discharge reciprocally over the sleep cycle suggests a causal interdependence.

In some brain stem areas these cell groups are not anatomically segregated and may instead be neurochemically mixed (interpenetrated). This finding raises important theoretical and practical issues not anticipated in the original reciprocal-interaction model. The electrophysiological evidence concerning the REM-on and REM-off cell groups suggests a gradient of sleep-dependent membrane excitability changes that may be a function of the connectivity strength within an anatomically distributed neuronal network. The connectivity strength may be influenced by the degree of neurochemical interpenetration between the REM-on and REM-offcells. Recognition of these complexities forces us to revise the reciprocal-interaction model and to seek new methods to test its tenets.

Cholinergic microinjection experiments indicate that some populations of REM-on cells can execute specific portions of the REM sleep syndrome or block the generation of REM sleep. This observation suggests that the order of activation within the anatomically distributed generator populations may be critical in determining behavioral outcome. Support for the cholinergic tenets of the reciprocal-interaction model has been reinforced by observations from sleep-disorders medicine.

Specific predictions of the reciprocal-interaction model and suggestions for testing these predictions are enumerated for future experimental programs that aim to understand the cellular and molecular basis of the mammalian sleep cycle.

Modulation of hippocampal theta oscillations and spatial memory by relaxin-3 neurons of the nucleus incertus

Hippocampal theta rhythm is thought to underlie learning and memory, and it is well established that "pacemaker" neurons in medial septum (MS) modulate theta activity. Recent studies in the rat demonstrated that brainstem-generated theta rhythm occurs through a multisynaptic pathway via the nucleus incertus (NI), which is the primary source of the neuropeptide relaxin-3 (RLN3). Therefore, this study examined the possible contribution of RLN3 to MS activity, and associated hippocampal theta activity and spatial memory. In anesthetized and conscious rats, we identified the ability of intraseptal RLN3 signaling to modulate neuronal activity in the MS and hippocampus and promote hippocampal theta rhythm. Behavioral studies in a spontaneous alternation task indicated that endogenous RLN3 signaling within MS promoted spatial memory and exploratory activity significantly increased c-Fos immunoreactivity in RLN3-producing NI neurons. Anatomical studies demonstrated axons/terminals from NI/RLN3 neurons make close contact with septal GABAergic (and cholinergic) neurons, including those that project to the hippocampus. In summary, RLN3 neurons of the NI can modulate spatial memory and underlying hippocampal theta activity through axonal projections to pacemaker neurons of the MS. NI/RLN3 neurons are highly responsive to stress and express corticotropin-releasing factor type-1 receptors, suggesting that the effects observed could be an important component of memory processing associated with stress responses.

Identification of cholinergic and non-cholinergic neurons in the pons expressing phosphorylated cyclic adenosine monophosphate response element-binding protein as a function of rapid eye movement sleep

Recent studies have shown that in the pedunculopontine tegmental nucleus (PPT), increased neuronal activity and kainate receptor-mediated activation of intracellular protein kinase A (PKA) are important physiological and molecular steps for the generation of rapid eye movement (REM) sleep. In the present study performed on rats, phosphorylated cyclic AMP response element-binding protein (pCREB) immunostaining was used as a marker for increased intracellular PKA activation and as a reflection of increased neuronal activity. To identify whether activated cells were either cholinergic or noncholinergic, the PPT and laterodorsal tegmental nucleus (LDT) cells were immunostained for choline acetyltransferase (ChAT) in combination with pCREB or c-Fos. The results demonstrated that during high rapid eye movement sleep (HR, approximately 27%), significantly higher numbers of cells expressed pCREB and c-Fos in the PPT, of which 95% of pCREB-expressing cells were ChAT-positive. With HR, the numbers of pCREB-positive cells were also significantly higher in the medial pontine reticular formation (mPRF), pontine reticular nucleus oral (PnO), and dorsal subcoeruleus nucleus (SubCD) but very few in the locus coeruleus (LC) and dorsal raphe nucleus (DRN). Conversely, with low rapid eye movement sleep (LR, approximately 2%), the numbers of pCREB expressing cells were very few in the PPT, mPRF, PnO, and SubCD but significantly higher in the LC and DRN. The results of regression analyses revealed significant positive relationships between the total percentages of REM sleep and numbers of ChAT+/pCREB+ (Rsqr=0.98) cells in the PPT and pCREB+ cells in the mPRF (Rsqr=0.88), PnO (Rsqr=0.87), and SubCD (Rsqr=0.84); whereas significantly negative relationships were associated with the pCREB+ cells in the LC (Rsqr=0.70) and DRN (Rsqr=0.60). These results provide evidence supporting the hypothesis that during REM sleep, the PPT cholinergic neurons are active, whereas the LC and DRN neurons are inactive. More importantly, the regression analysis indicated that pCREB activation in approximately 98% of PPT cholinergic neurons, was caused by REM sleep. Moreover the results indicate that during REM sleep, PPT intracellular PKA activation and a transcriptional cascade involving pCREB occur exclusively in the cholinergic neurons.

Differential localization of carbachol- and bicuculline-sensitive pontine sites for eliciting REM sleep-like effects in anesthetized rats

Carbachol, a cholinergic agonist, and GABA(A) receptor antagonists injected into the pontine dorsomedial reticular formation can trigger rapid eye movement (REM) sleep-like state. Data suggest that GABAergic and cholinergic effects interact to produce this effect but the sites where this occurs have not been delineated. In urethane-anesthetized rats, in which carbachol effectively elicits REM sleep-like episodes (REMSLE), we tested the ability of 10 nL microinjections of carbachol (10 mm) and bicuculline (0.5 or 2 mm) to elicit REMSLE at 47 sites located within the dorsal pontine reticular formation at the levels -8.00 to -10.80 from bregma (B) (Paxinos and Watson, The Rat Brain in Stereotaxic Coordinates, Academic Press, San Diego, 1997). At rostral levels, most carbachol and some bicuculline injections elicited REMSLE with latencies that gradually decreased from 242 to 12 s for carbachol and from 908 to 38 s for bicuculline for more caudal injection sites. As the latencies decreased, the durations of bicuculline-elicited REMSLE increased from 104 s to over 38 min, and the effect was dose dependent, whereas the duration of carbachol-elicited REMSLE changed little (104-354 s). Plots of REMSLE latency versus the antero-posterior coordinates revealed that both drugs were maximally effective near B-8.80. At levels caudal to B-8.80, carbachol was effective at few sites, whereas bicuculline-elicited REMSLE to at least B-9.30 level. Thus, the bicuculline-sensitive sites extended further caudally than those for carbachol and antagonism of GABA(A) receptors both triggered REMSLE and controlled their duration, whereas carbachol effects on REMSLE duration were small or limited by its concurrent REMSLE-opposing actions.

Modulation of respiratory pattern and upper airway muscle activity by the pedunculopontine tegmentum: role of NMDA receptors

The pedunculopontine tegmental nucleus (PPT) is postulated to have important functions relevant to the regulation of rapid eye movement (REM) sleep and arousal, and various motor control systems including respiration. We have recently shown that pharmacologic activation of a neuronal subpopulation within the PPT, induced by micropipette injection of glutamate in nanoliter volumes, can produce respiratory rhythm disturbances and changes in genioglossus muscle activity in anesthetized rats. The aim of this study was to determine whether the respiratory pattern disturbance and increased genioglossus muscle tone induced by glutamate injection within the PPT are mediated by activation of N-methyl-D-aspartate (NMDA) receptors within the PPT. Experiments were performed in eight adult male spontaneously breathing Sprague-Dawley rats anesthetized using nembutal. Respiratory movements were monitored by piezoelectric strain gauge. Three-barrel glass pipettes were used to pressure inject glutamate (as a probe for respiratory modulating sites), ketamine (an NMDA channel blocker), and oil-red dye (to aid in histological verification of the injection sites) within the PPT. Electroencephalograms were recorded from the sensorimotor cortex, the hippocampus, and the pons, contralateral to the injection site. Electromyograms (EMGs) were recorded from the genioglossus muscle. The typical response to glutamate injection within the PPT respiratory-modulating region was immediate apnea followed by tachypnea and increased genioglossal tonic activity. The noncompetitive NMDA receptor channel-antagonist ketamine, injected at the same site and in the same volume as glutamate (5 nl), blocked respiratory dysrhythmia and genioglossal EMG responses to subsequent glutamate injections. For the first time, the present results suggest that respiratory rhythm and upper airway muscle tone are controlled by the activation of pedunculopontine tegmental nucleus NMDA receptors.

Neurobiological mechanisms for the regulation of mammalian sleep-wake behavior: reinterpretation of historical evidence and inclusion of contemporary cellular and molecular evidence

At its most basic level, the function of mammalian sleep can be described as a restorative process of the brain and body; recently, however, progressive research has revealed a host of vital functions to which sleep is essential. Although many excellent reviews on sleep behavior have been published, none have incorporated contemporary studies examining the molecular mechanisms that govern the various stages of sleep. Utilizing a holistic approach, this review is focused on the basic mechanisms involved in the transition from wakefulness, initiation of sleep and the subsequent generation of slow-wave sleep and rapid eye movement (REM) sleep. Additionally, using recent molecular studies and experimental evidence that provides a direct link to sleep as a behavior, we have developed a new model, the cellular-molecular-network model, explaining the mechanisms responsible for regulating REM sleep. By analyzing the fundamental neurobiological mechanisms responsible for the generation and maintenance of sleep-wake behavior in mammals, we intend to provide a broader understanding of our present knowledge in the field of sleep research.

The "ways" we look at dreams: evidence from unilateral spatial neglect (with an evolutionary account of dream bizarreness)

Despite decades of research, the question of whether the rapid eye movements (REMs) of paradoxical sleep (PS) are equivalent to waking saccades and whether their direction is congruent with visual spatial events in the dream scene is still very controversial. We gained an insight into these questions through the study of a right brain damaged patient suffering attentional neglect for the left side of space and drop of the optokinetic nystagmus (OKN) with alternating rightward slow/leftward fast phases evoked by rightward optic flow. During PS the patient had frequent Nystagmoid REMs with alternating leftward slow/rightward fast phases and reported dreams with visual events evoking corresponding OKN such as a train running leftward. By contrast, just as in waking OKN, Nystagmoid REMs with alternating rightward slow/leftward fast phases were virtually absent. REMs followed by staring eye position or by consecutive REMs were also observed: these showed no asymmetry comparable to that of Nystagmoid ones. The selective disappearance of Nystagmoid REMs in one horizontal direction proves, for the first time, that in humans different types of REMs exists and that these are driven by different premotor mechanisms. Concomitant drop of OKN and Nystagmoid REMs toward the same horizontal direction demonstrates that phylogenetically ancient oculomotor mechanisms, such as the OKN, are shared by waking and PS. On this evidence and converging findings from animal, neuropsychological and brain imaging studies, a new evolutionary account of dream bizarreness is proposed. Classification and labelling of the different types of REMs are also provided.

The role of tropomyosin-related kinase receptors in neurotrophin-induced rapid eye movement sleep in the cat

The microinjection of nerve growth factor and neurotrophin-3 into the rostro-dorsal pontine tegmentum of the cat evokes a state that is comparable to naturally-occurring rapid eye movement sleep. Using two experimental paradigms, we tested the hypothesis that neurotrophin high-affinity receptors (trkA and trkC, tropomyosin-related kinase A and C, respectively) mediate this effect. First, trk and fos immunohistochemistry were combined to determine whether tyrosine kinase receptor-containing neurons in the dorsal pontine tegmentum are active in cats that exhibit long-lasting periods of rapid eye movement sleep following the local microinjection of nerve growth factor. During approximately two hours of recording, nerve growth factor-treated cats spent 59.8% of the time in a rapid eye movement sleep-like state; vehicle-injected (control) animals remained in quiet wakefulness and non-rapid eye movement sleep. Whereas control and nerve growth factor-treated cats exhibited a similar mean number of trkA- and trkC-immunoreactive neurons in the dorsal pontine tegmentum, the number of trkA- and trkC-immunoreactive neurons that expressed Fos, i.e. double-labeled cells that are presumably activated, was significantly larger in cats that were injected with nerve growth factor. Axon terminals contained tyrosine kinase receptor immunoreactivity in this region; many were apposed to Fos-immunoreactive neurons. In addition, patterns of tyrosine kinase receptor and Fos immunoreactivity similar to those observed in nerve growth factor-injected cats were present, in conjunction with long-lasting rapid eye movement sleep, following the microinjection of carbachol into the dorsal pons. In a second series of studies, nerve growth factor or neurotrophin-3 was injected alone or after K-252a, a blocker of tyrosine kinase receptors, into the rostro-dorsal pontine tegmentum. Nerve growth factor or neurotrophin-3 alone produced, with a mean latency of 4 min, a rapid eye movement sleep-like state. However, neurotrophin injections preceded by K-252a were not effective in inducing rapid eye movement sleep. These results indicate that the activation of trkA and trkC receptors in neurons in the pontine tegmentum is responsible, at least in part, for the rapid eye movement sleep-inducing effect of nerve growth factor and neurotrophin-3. Furthermore, the data suggest that these neurotrophins are capable of acting both pre- and postsynaptically to activate pontine neurons that are involved in the generation of rapid eye movement sleep.

Microinjection of procaine and electrolytic lesion in the ventral tegmental area suppresses hippocampal theta rhythm in urethane-anesthetized rats

The midbrain ventral tegmental area (VTA), a key structure of the mesocorticolimbic system is anatomically connected with the hippocampal formation. In addition mesocortical dopamine was found to influence hippocampus-related memory and hippocampal synaptic plasticity, both being linked to the theta rhythm. Therefore, the aim of the present study was to evaluate the possible role of the VTA in the regulation of the hippocampal theta activity. The study was performed on urethane-anesthetized male Wistar rats in which theta rhythm was evoked by tail pinch. It was found that unilateral, temporal inactivation of the VTA by means of direct procaine injection resulted in bilateral suppression of the hippocampal theta which manifested as a loss of synchronization of hippocampal EEG and respective reduction of the power and also the frequency of the 3-6 Hz theta band. Depression of the power of the 3-6 Hz component of the EEG signal was also seen in spontaneous hippocampal EEG after procaine. The permanent destruction of the VTA by means of unilateral electrocoagulation evoked a long-lasting, mainly ipsilateral depression of the power of the theta with some influence on its frequency. Simultaneously, there was a substantial increase of the power in higher frequency bands indicating decrease of a synchrony of the hippocampal EEG activity. On the basis of these results indicating impairment of synchronization of the hippocampal activity the VTA may be considered as another part of the brainstem theta synchroning system.

Injection of glutamate into the pedunculopontine tegmental nuclei of anesthetized rat causes respiratory dysrhythmia and alters EEG and EMG power

The pedunculopontine tegmental nucleus (PPT) has been shown to have important functions relevant to the regulation of behavioral states and various motor control systems, including breathing control. Our previous work has shown that the activation of neurons within the PPT, a structure that is typically active during rapid eye movement (REM) sleep, can produce respiratory disturbances in freely moving and anesthetized rats. The aim of this study was to test the hypothesis that respiratory modulation by the PPT in anesthetized rats can be evoked in the absence of other signs of an REM-sleep-like state. We characterized electroencephalogram (EEG) and electromyogram (EMG) changes during respiratory disturbances induced by glutamatergic stimulation of the PPT in spontaneously breathing, adult male Sprague-Dawley rats anesthetized with a ketamine/xylazine combination or with nembutal. Respiratory movements were monitored by a piezoelectric strain gauge. Two-barrel glass pipettes were used to pressure inject glutamate, to probe for respiratory effective sites within the PPT, and to inject oil red dye at the end of the experiments for histological verification of the injection sites. The EEGs were recorded from the sensorimotor cortex, hippocampus, and from the pons contralateral from the injection site. The EMGs were recorded from the genioglossus muscle. The initial response to glutamate injection into the respiratory modulating region of the PPT was always a respiratory pattern disturbance. Subsequent activation of EMG and EEG often occurred in ketamine/xylazine-anesthetized rats, but REM-sleep-like patterns were not observed. Respiratory pattern and EMG power changes in nembutal-anesthetized rats were similar, but EEG activation was never observed. Thus, we conclude that respiratory suppression produced by the local activation of PPT neurons may not necessarily be accompanied by an REM-sleep-like cortical state in this anesthetized model.

Maternal cigarette-smoking during pregnancy disrupts rhythms in fetal heart rate

OBJECTIVE

To examine the effects of maternal cigarette smoking during pregnancy on the developing infant's autonomic regulation before the possible effects of parturition and neonatal withdrawal could alter outcome measures.

METHODS

Heart rate variability (HRV) was assessed for 10 min during late gestation for 21 cigarette-exposed (CE) and 22 nonexposed (NE) fetuses.

RESULTS

HRV was significantly lower in fetuses whose mothers smoked cigarettes during pregnancy. Spectrum analysis of that variability showed temporally organized rhythms at a frequency similar to rhythms previously found in fetal cyclic motility (approximately .3 cycles per min). Lower powered rhythms--associated with poorer development--at the first, second, and dominant rhythms, as well as lower overall power of the power spectrum, were found for CE fetuses. Pearson correlations showed significant negative correlations between the amount of maternal cigarette smoking during the first trimester of pregnancy and measures of fetal HRV and power of spectral peaks.

CONCLUSIONS

Results show that CE fetuses have lower HRV and disrupted temporal organization of autonomic regulation before effects of parturition, postnatal adaptation, and possible nicotine withdrawal contributes to differences in infant neurobehavioral function.

Memory consolidation in sleep; dream or reality

We discuss several lines of evidence refuting the hypothesis that procedural or declarative memories are processed/consolidated in sleep. One of the strongest arguments against a role for sleep in declarative memory involves the demonstration that the marked suppression or elimination of REM sleep in subjects on antidepressant drugs or with brainstem lesions produces no detrimental effects on cognition. Procedural memory, like declarative memory, undergoes a slow, time-dependent period of consolidation. A process has recently been described wherein performance on some procedural tasks improves with the mere passage of time and has been termed "enhancement." Some studies, but not others, have reported that the consolidation/enhancement of perceptual and motor skills is dependent on sleep. We suggest that consolidation or enhancement, initiated in waking with task acquisition, could in some instances extend to sleep, but sleep would serve no unique role in these processes. In sum, there is no compelling evidence to support a relationship between sleep and memory consolidation.

Activation of phasic pontine-wave generator prevents rapid eye movement sleep deprivation-induced learning impairment in the rat: a mechanism for sleep-dependent plasticity

Animal and human studies of sleep and learning have demonstrated that training on various tasks increases subsequent rapid eye movement (REM) sleep and phasic pontine-wave (P-wave) activity, followed by improvement in performance on the learned task. It is well documented that REM sleep deprivation after learning trials blocks the expected improvement in performance on subsequent retesting. Our aim was to test whether experimentally induced P-wave generator activation could eliminate the learning impairment produced by post-training REM sleep deprivation. Rats were trained on a two-way active avoidance-learning task. Immediately thereafter, two groups of those rats received a control vehicle (100 nl saline) microinjection and one group received a carbachol (50 ng in 100 nl saline) microinjection into the P-wave generator. The carbachol-injected group and one of the two control saline microinjected groups were selectively deprived of REM sleep during a 6 hr polygraphic recording session. All rats were then tested on the avoidance-learning task. The rats that received both the control saline injection and REM sleep deprivation showed learning deficits compared with the control saline-injected rats that were allowed to sleep normally. In contrast, the rats that received the carbachol microinjection and REM sleep deprivation demonstrated normal learning. These results demonstrate, for the first time, that carbachol-induced activation of the P-wave generator prevents the memory-impairing effects of post-training REM sleep deprivation. This evidence supports our hypothesis that the activation of the P-wave generator during REM sleep deprivation enhances a physiological process of memory, which occurs naturally during post-training REM sleep.

Influence of hypnogenic brain areas on wakefulness- and rapid-eye-movement sleep-related neurons in the brainstem of freely moving cats

Rapid-eye-movement (REM) sleep is normally preceded by non-REM sleep; however, every non-REM sleep episode is not followed by REM sleep. It has been proposed that, for the regulation of REM sleep, the brain areas modulating waking and non-REM sleep are likely to communicate with neurons promoting REM sleep. The former has been reported earlier, and in this study the latter has been investigated. Under surgical anaesthesia, cats were prepared for electrophysiological recording of sleep-wakefulness and electrical stimulation of caudal brainstem as well as preopticoanterior hypothalamic hypnogenic areas. Insulated microwires of 25-32 microm were used to record 52 single neuronal activities from the brainstem along with bipolar electroencephalogram, electromyogram, electrooculogram, and pontogeniculooccipital waves in freely moving, normally behaving cats. The neurons were classified into five groups based on changes in firing rates associated with different sleep-waking states compared with quiet wakefulness. Thereafter, the responses of these neurons to 1-Hz stimulation of the two non-REM sleep-promoting areas were studied. At the end of experiment, the stimulating and recording sites were histologically identified. It was observed that, among the affected neurons, the caudal brainstem non-REM sleep-promoting area excited more REM-on neurons, whereas the preopticoanterior hypothalamus hypnogenic area inhibited more awake-active neurons. Thus, the results suggest that, at the single neuronal level, the caudal brainstem non-REM sleep-modulating area, rather than the preopticoanterior hypothalamic hypnogenic area in the brain, plays a modulatory role in triggering REM sleep initiation at a certain depth of sleep.

Activation of the phasic pontine-wave generator enhances improvement of learning performance: a mechanism for sleep-dependent plasticity

The aim of this study was to test the hypothesis that supplementary activation of the phasic pontine wave (P-wave) generator during rapid eye movement (REM) sleep enhances consolidation and integration of memories, resulting in improved learning. To test this hypothesis, two groups of rats were trained on a two-way active avoidance learning task in the morning. Immediately after training, one group of rats received a carbachol microinjection into the P-wave generator and the other group was microinjected with control saline into the same target area. After training trials and microinjections, rats were allowed a 6-h period of undisturbed sleep in the polygraphic recording chamber. At the end of 6 h of undisturbed sleep-wake recordings, rats were retested in a session of avoidance learning trials. After learning trials, the total percentage of time spent in REM sleep was significantly increased in both saline (15.36%)- and carbachol (17.70%)-microinjected rats. After learning trials, REM sleep P-wave density was significantly greater throughout the 6-h period of recordings in carbachol treated rats than in the saline treated rats. In the retrial session, the improvement in learning task performance was 22.75% higher in the carbachol-microinjected rats than in the saline-microinjected rats. These findings show that the consolidation and integration of memories create a homeostatic demand for P-waves. In addition, these findings provide experimental evidence, for the first time, that activation of the P-wave generator may enhance consolidation and integration of memories, resulting in improved performance on a recently learned task.

Specific contributions of the basal forebrain corticopetal cholinergic system to electroencephalographic activity and sleep/waking behaviour

The present study examined the role of the basal forebrain corticopetal cholinergic projection in the regulation of cortical electroencephalographic activity across sleep/wake states in rats. Selective lesions of this projection were effected by local intraparenchymal infusions of the immunotoxin 192 IgG-saporin. Lesions spared the septo-hippocampal cholinergic system, as well as p75-receptor-bearing noncholinergic neurons in the suprachiasmatic nucleus. Relative to sham-lesioned control animals, rats with lesions of basal forebrain cholinergic neurons displayed a significant reduction in high frequency EEG activity, characterized especially by a reduction in gamma EEG power. Lesions did not significantly alter the overall proportion of sleeping and waking states as defined behaviourally, but the attenuation of high frequency EEG activity was apparent across all stages, including REM-like periods. Results are consistent with the view that the basal forebrain corticopetal cholinergic system exerts a general activational effect on the cortical mantle. Although this system may not be essential for sleep/wake stage-switching, it does impact on the cortical states associated with those stages.

The union of the state: myoclonic twitching is coupled with nuchal muscle atonia in infant rats

Active sleep (AS), as measured by the occurrence of myoclonic twitching (MT), is the most prevalent behavioral state in newborn rats. Historically, AS has been considered a developmental precursor of REM sleep, but recently this idea has been questioned. In the present study, the authors assess, in 2-, 5-, and 8-day-old rats, the relationship between MT and nuchal muscle atonia, a widely recognized component of REM sleep. At all ages, muscle atonia preceded MT and persisted until awake behaviors occurred. In addition, muscle tone decreased gradually during transitions from awake behavior to twitching. Thus, MT during infancy occurs against a backdrop of muscle atonia, a result that is consistent with the view that AS is a developmental precursor of REM sleep.

Microinjection of procaine into the pedunculopontine tegmental nucleus suppresses hippocampal theta rhythm in urethane-anesthetized rats

It was found that the cholinergic component of the pedunculopontine tegmental nucleus (PPN) is involved in the generation of theta rhythm in the hippocampus. However, it is still not known how important PPN is in the brainstem theta-generating system, where the nucleus reticularis pontis oralis is regarded as a primary generator. In the present experiment, performed on urethane-anesthetized rats, we studied the effect on the tail pinch-elicited hippocampal theta of unilateral inactivation of PPN by means of direct procaine microinjection. Procaine induced ipsilateral suppression of theta rhythm, manifested as desynchronization of hippocampal EEG, a shift of the fast Fourier transformation (FFT) power peak toward lower frequencies, and a reduction of FFT peak magnitude at theta band. Hippocampal field activity returned to normal (both its FFT peak frequency and magnitude) within 30 min after the injection. The results obtained indicate that PPN is critical for hippocampal theta generation but it may not be involved in encoding theta frequency.

Physiological effects of sustained blockade of excitatory synaptic transmission on spontaneously active developing neuronal networks--an inquiry into the reciprocal linkage between intrinsic biorhythms and neuroplasticity in early ontogeny

Spontaneous bioelectric activity (SBA) taking the form of extracellularly recorded spike trains (SBA) has been quantitatively analyzed in organotypic neonatal rat visual cortex explants at different ages in vitro, and the effects investigated of both short- and long-term pharmacological suppression of glutamatergic synaptic transmission. In the presence of APV, a selective NMDA receptor blocker, 1-2- (but not 3-)week-old cultures recovered their previous SBA levels in a matter of hours, although in imitation of the acute effect of the GABAergic inhibitor picrotoxin (PTX), bursts of action potentials were abnormally short and intense. Cultures treated either overnight or chronically for 1-3 weeks with APV, the AMPA/kainate receptor blocker DNQX, or a combination of the two were found to display very different abnormalities in their firing patterns. NMDA receptor blockade for 3 weeks produced the most severe deviations from control SBA, consisting of greatly prolonged and intensified burst firing with a strong tendency to be broken up into trains of shorter spike clusters. This pattern was most closely approximated by acute GABAergic disinhibition in cultures of the same age, but this latter treatment also differed in several respects from the chronic-APV effect. In 2-week-old explants, in contrast, it was the APV+DNQX treated group which showed the most exaggerated spike bursts. Functional maturation of neocortical networks, therefore, may specifically require NMDA receptor activation (not merely a high level of neuronal firing) which initially is driven by endogenous rather than afferent evoked bioelectric activity. Putative cellular mechanisms are discussed in the context of a thorough review of the extensive but scattered literature relating activity-dependent brain development to spontaneous neuronal firing patterns.

Reversible analgesia, atonia, and loss of consciousness on bilateral intracerebral microinjection of pentobarbital

Concussion, asphyxia, and systemically administered general anesthetics all induce reversible depression of the organism's response to noxious stimuli as one of the elements of loss of consciousness. This is so even for barbiturate anesthetics, which have only modest analgesic efficacy at subanesthetic doses. Little is known about the neural circuits involved in this form of antinociception, although for anesthetic agents, at least, it is usually presumed that the drugs act in widely distributed regions of the nervous system. We now report the discovery of a focal zone in the brainstem mesopontine tegmentum in rats at which microinjection of minute quantities of pentobarbital induces a transient, reversible anesthetic-like state with non-responsiveness to noxious stimuli, flaccid atonia, and absence of the righting reflex. The behavioral suppression is accompanied by slow-wave EEG and, presumably, loss of consciousness. This zone, which we refer to as the mesopontine tegmental anesthesia locus (MPTA), apparently contains a barbiturate-sensitive 'switch' for both cortical and spinal activity. The very existence of the MPTA locus has implications for an understanding of the neural circuits that control motor functions and pain sensation, and for the cerebral representation of consciousness.

Pharmacogenetics of psychoactive drugs

Response to psychoactive drugs is subject to genetic variation. Heritable differences in metabolism of antidepressants have been demonstrated and appear to be clinically relevant. More complex genetic variations in drug response are important in alcoholism and possibly other forms of substance abuse. Pharmacologic challenge studies in volunteer twins have shown familial variation in responses to the cholinergic agonist arecoline and to the monoaminergic agonist amphetamine. Arecoline responses also differentiate well-state bipolar patients from controls; this is consistent with a hypothesis of muscarinic supersensitivity in affective disorder. Genetically characterized animal models may be useful in further analyzing such neurochemical differences.

Avoidance task training potentiates phasic pontine-wave density in the rat: A mechanism for sleep-dependent plasticity

Behavioral studies of learning and memory in both humans and animals support a role for sleep in the consolidation and integration of memories. The present study explored possible physiological mechanisms of sleep-dependent behavioral plasticity by examining the relationship between learning and state-dependent phasic signs of rapid eye movement (REM) sleep. Cortical electroencephalogram, electromyogram, eye movement, hippocampal theta-wave, and pontine-wave (P-wave) measures were recorded simultaneously in freely moving rats after a session of conditioned avoidance learning or a control session. After learning trials, rats spent 25.5% more time in REM sleep and 180.6% more time in a transitional state between slow-wave sleep and REM sleep (tS-R) compared with that in control trials. Both REM sleep and tS-R behavioral states are characterized by the presence of P-waves. P-wave density was significantly greater in the first four episodes of REM sleep after the learning session compared with the control session. Furthermore, the P-wave density change between the first and third REM sleep episodes was proportional to the improvement of task performance between the initial training session and the post-sleep retest session. These findings show that the increase in P-wave density during the post-training REM sleep episodes is correlated with the effective consolidation and retention of avoidance task learning.

REM sleep behavior disorder

REM sleep is the stage associated with vivid dream mentation, desynchronous cortical EEG, and atonia of antigravitary muscles. REM sleep behavior disorder (RBD) is characterized by the intermittent loss of REM sleep atonia and by the appearance of elaborate motor activity associated with dream mentation. The animal model of REM sleep without atonia indicates that lesions to the perilocus coeruleus disrupt the excitatory connection to the nucleus reticularis magnocellularis in the descending medullary reticular formation and disable the hyperpolarization of the alpha spinal motoneurons. Extensive neurologic evaluations in humans suffering from both idiopathic and symptomatic forms have not identified specific lesions; however, findings in some patients suggest that diffuse lesions of the hemispheres, bilateral thalamic abnormalities, or primary brain-stem lesions may result in the RBD. Symptomatic RBD cases are associated with several neurologic disorders such as dementia, cerebrovascular diseases, multiple sclerosis, brain-stem neoplasm. RBD has been often documented to precede or to co-occur with neurodegenerative disorders, such as dementia, Parkinson's disease and multiple system atrophy. Most importantly, RBD is readily diagnosable and treatable. Patients and their bed partners usually report immediate improvement in sleep-related motor behavior with small doses of clonazepam.

Mesopontine contribution to the expression of active 'twitch' sleep in decerebrate week-old rats

Myoclonic twitching is a ubiquitous feature of infant behavior that has been used as an index of active sleep. Although the active sleep of infants differs in some ways from the REM sleep of adults, their marked similarities have led many to view them them as homologous behavioral states. Recently, however, this view has been challenged. One avenue for resolving this issue entails examination of the neural substrates of active sleep. If the neural substrates of active sleep were found to be similar to those of REM sleep, then this would support the view that the two states are homologous. Therefore, in the present study, decerebrations were performed in the pons and midbrain to determine whether the mesopontine region is important for the expression of active sleep in infants, just as it is for the expression of REM sleep in adults. It was found that, in comparison to controls, caudal pontine decerebrations reduced myoclonic twitching by 76%, rostral pontine decerebrations reduced twitching by 40%, and midbrain transections had no significant effect on twitching. Moreover, analysis of the temporal organization of twitching indicated that pontine decerebrations predominantly affected high-frequency twitching while leaving unaffected the low-frequency twitching that is thought to be contributed by local spinal circuits at this age. These results indicate that the mesopontine region plays a central role in the expression of active sleep in infant rats.

The neurophysiology of sleep and waking: intracerebral connections, functioning and ascending influences of the medulla oblongata

This paper focuses on the successive historical papers related to medulla oblongata (M.O.) intracerebral connections, its activities and ascending influences regulating sleep waking behavior. The M.O. certainly influences the quantitative and qualitative processes of waking. However, its neurophysiological properties are often concealed by those of the upper-situated brain stem structures. The M.O., particularly the solitary tract nucleus, is involved in sleep-inducing processes. This nucleus seem to act as a deactivating system of the above situated reticular formation, but it also impacts directly on the thalamocortical slow wave and spindle-inducing processes. The M.O. is significantly involved in paradoxical sleep mechanisms. Indeed, the mesopontine executive centers are unable to induce paradoxical sleep without the M.O. Moreover, stimulation of the solitary tract nucleus afferents can induce paradoxical sleep, and the M.O. metabolic functioning is specifically disturbed by paradoxical sleep deprivation. Finally. there seems to be a paradoxical sleep Zeitgeber. Our current knowledge shows that this lowest brain stem level is crucial for sleep waking mechanisms. It will undoubtedly be further highlighted by future electrophysiologial and neurochemical studies.