Brain stem generation of the hippocampal EEG

Coupling of hippocampal theta and ripples with pontogeniculooccipital waves

The hippocampus has a major role in encoding and consolidating long-term memories, and undergoes plastic changes during sleep¹. These changes require precise homeostatic control by subcortical neuromodulatory structures². The underlying mechanisms of this phenomenon, however, remain unknown. Here, using multi-structure recordings in macaque monkeys, we show that the brainstem transiently modulates hippocampal network events through phasic pontine waves known as pontogeniculooccipital waves (PGO waves). Two physiologically distinct types of PGO wave appear to occur sequentially, selectively influencing high-frequency ripples and low-frequency theta events, respectively. The two types of PGO wave are associated with opposite hippocampal spike-field coupling, prompting periods of high neural synchrony of neural populations during periods of ripple and theta instances. The coupling between PGO waves and ripples, classically associated with distinct sleep stages, supports the notion that a global coordination mechanism of hippocampal sleep dynamics by cholinergic pontine transients may promote systems and synaptic memory consolidation as well as synaptic homeostasis.

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.

Cell discharge correlates of posterior hypothalamic theta rhythm. Recipe for success in recording stable field potential

The theta rhythm discovered in the posterior hypothalamus area (PHa) differs from theta observed in the hippocampal formation. In comparison to hippocampal spontaneous theta, the theta recorded in the PHa is rarely registered, has lower amplitude, often disappears, and sometimes returns after a few minutes. These features indicate that spontaneous theta recorded in the PHa is not an appropriate experimental model to search for the correlation between PHa cell discharges and local field potential. In this paper we present standard experimental conditions necessary to record theta-related cells in the PHa in anesthetized rats. Three pharmacological agents were used in the experiments to induce PHa theta rhythm in urethanized rats: carbachol (CCH), carbenoxolone and kainic acid, which are potent enough to induce well-synchronized PHa theta. However, CCH was found to be the best pharmacological tool to induce PHa theta oscillations, due to its longest duration of action and lack of preliminary epileptogenic effects. It seems that CCH-induced theta can be the most suitable pharmacological model for experiments with the use of protocol of long-lasting recordings of PHa theta-related cell discharges.

Theta Rhythm of the Hippocampus: Subcortical Control and Functional Significance

The theta rhythm is the largest extracellular synchronous signal that can be recorded from the mammalian brain and has been strongly implicated in mnemonic processes of the hippocampus. We describe (a) ascending brain stem–forebrain systems involved in controlling theta and nontheta (desynchronization) states of the hippocampal electroencephalogram; (b) theta rhythmically discharging cells in several structures of Papez's circuit and their possible functional significance, specifically with respect to head direction cells in this same circuit; and (c) the role of nucleus reuniens of the thalamus as a major interface between the medial prefrontal cortex and hippocampus and as a prominent source of afferent limbic information to the hippocampus. We suggest that the hippocampus receives two main types of input: theta rhythm from ascending brain stem– diencephaloseptal systems and information bearing mainly from thalamocortical/cortical systems. The temporal convergence of activity of these two systems results in the encoding of information in the hippocampus, primarily reaching it from the entorhinal cortex and nucleus reuniens.

Supramammillary serotonin reduction alters place learning and concomitant hippocampal, septal, and supramammillar theta activity in a Morris water maze

Hippocampal theta activity is related to spatial information processing, and high-frequency theta activity, in particular, has been linked to efficient spatial memory performance. Theta activity is regulated by the synchronizing ascending system (SAS), which includes mesencephalic and diencephalic relays. The supramamillary nucleus (SUMn) is located between the reticularis pontis oralis and the medial septum (MS), in close relation with the posterior hypothalamic nucleus (PHn), all of which are part of this ascending system. It has been proposed that the SUMn plays a role in the modulation of hippocampal theta-frequency; this could occur through direct connections between the SUMn and the hippocampus or through the influence of the SUMn on the MS. Serotonergic raphe neurons prominently innervate the hippocampus and several components of the SAS, including the SUMn. Serotonin desynchronizes hippocampal theta activity, and it has been proposed that serotonin may regulate learning through the modulation of hippocampal synchrony. In agreement with this hypothesis, serotonin depletion in the SUMn/PHn results in deficient spatial learning and alterations in CA1 theta activity-related learning in a Morris water maze. Because it has been reported that SUMn inactivation with lidocaine impairs the consolidation of reference memory, we asked whether changes in hippocampal theta activity related to learning would occur through serotonin depletion in the SUMn, together with deficiencies in memory. We infused 5,7-DHT bilaterally into the SUMn in rats and evaluated place learning in the standard Morris water maze task. Hippocampal (CA1 and dentate gyrus), septal and SUMn EEG were recorded during training of the test. The EEG power in each region and the coherence between the different regions were evaluated. Serotonin depletion in the SUMn induced deficient spatial learning and altered the expression of hippocampal high-frequency theta activity. These results provide evidence in support of a role for serotonin as a modulator of hippocampal learning, acting through changes in the synchronicity evoked in several relays of the SAS.

Brainstem system of hippocampal theta induction: The role of the ventral tegmental area

This article summarizes the results of studies concerning the influence of the ventral tegmental area (VTA) on the hippocampal theta rhythm. Temporary VTA inactivation resulted in transient loss of the hippocampal theta. Permanent destruction of the VTA caused a long-lasting depression of the power of the theta and it also had some influence on the frequency of the rhythm. Activation of glutamate (GLU) receptors or decrease of GABAergic tonus in the VTA led to enhancement of dopamine release and increased hippocampal theta power. High time and frequency cross-correlation was detected for the theta band between the VTA and hippocampus during paradoxical sleep and active waking. Thus, the VTA may belong to the broad network involved in theta rhythm regulation. This article also presents a model of brainstem-VTA-hippocampal interactions in the induction of the hippocampal theta rhythm. The projections from the VTA which enhance theta rhythm are incorporated into the main theta generation pathway, in which the septum acts as the central node. The neuronal activity that may be responsible for the ability of the VTA to regulate theta probably derives from the structures associated with rapid eye movement (sleep) (REM) sleep or with sensorimotor activity (i.e., mainly from the pedunculopontine and laterodorsal tegmental nuclei and also from the raphe).

Mesopontine median raphe regulates hippocampal ripple oscillation and memory consolidation

Sharp-wave associated field-oscillations (~200 Hz) of the hippocampus, referred to as “ripples”, are believed to be important for consolidation of explicit memory. Little is known about how ripples are regulated by other brain regions. Here we show that the median raphe region (MnR) plays a key role in regulating hippocampal ripple activity and memory consolidation. We performed in vivo simultaneous recording in the MnR and hippocampus, and found that when a group of MnR neurons were active, ripples were absent. Consistently, optogenetic stimulation of MnR neurons suppressed ripple activity, while inhibition of these neurons increased ripple activity. Importantly, using a fear conditioning procedure, we provided evidence that photostimulation of MnR neurons interfered with memory consolidation. Our results demonstrate a critical role of the MnR in regulating ripples and memory consolidation.

The frequency of hippocampal theta rhythm is modulated on a circadian period and is entrained by food availability

The hippocampal formation plays a critical role in the generation of episodic memory. While the encoding of the spatial and contextual components of memory have been extensively studied, how the hippocampus encodes temporal information, especially at long time intervals, is less well understood. The activity of place cells in hippocampus has previously been shown to be modulated at a circadian time-scale, entrained by a behavioral stimulus, but not entrained by light. The experimental procedures used in the previous study of this phenomenon, however, necessarily conflated two alternative entraining stimuli, the exposure to the recording environment and the availability of food, making it impossible to distinguish between these possibilities. Here we demonstrate that the frequency of theta-band hippocampal EEG varies with a circadian period in freely moving animals and that this periodicity mirrors changes in the firing rate of hippocampal neurons. Theta activity serves, therefore, as a proxy of circadian-modulated hippocampal neuronal activity. We then demonstrate that the frequency of hippocampal theta driven by stimulation of the reticular formation also varies with a circadian period. Because this effect can be observed without having to feed the animal to encourage movement we were able to identify what stimulus entrains the circadian oscillation. We show that with reticular-activated recordings started at various times of the day the frequency of theta varies quasi-sinusoidally with a 25 h period and phase-aligned when referenced to the animal’s regular feeding time, but not the recording start time. Furthermore, we show that theta frequency consistently varied with a circadian period when the data obtained from repeated recordings started at various times of the day were referenced to the start of food availability in the recording chamber. This pattern did not occur when data were referenced to the start of the recording session or to the actual time of day when this was not also related to feeding time. This double dissociation demonstrates that hippocampal theta is modulated with a circadian timescale, and that this modulation is strongly entrained by food. One interpretation of this finding is that the hippocampus is responsive to a food entrainable oscillator (FEO) that might modulate foraging behavior over circadian periods.

Septal projections to nucleus incertus in the rat: bidirectional pathways for modulation of hippocampal function

Projections from the nucleus incertus (NI) to the septum have been implicated in the modulation of hippocampal theta rhythm. In this study we describe a previously uncharacterized projection from the septum to the NI, which may provide feedback modulation of the ascending circuitry. Fluorogold injections into the NI resulted in retrograde labeling in the septum that was concentrated in the horizontal diagonal band and areas of the posterior septum including the septofimbrial and triangular septal nuclei. Double-immunofluorescent staining indicated that the majority of NI-projecting septal neurons were calretinin-positive and some were parvalbumin-, calbindin-, or glutamic acid decarboxylase (GAD)-67-positive. Choline acetyltransferase-positive neurons were Fluorogold-negative. Injection of anterograde tracers into medial septum, or triangular septal and septofimbrial nuclei, revealed fibers descending to the supramammillary nucleus, median raphe, and the NI. These anterogradely labeled varicosities displayed synaptophysin immunoreactivity, indicating septal inputs form synapses on NI neurons. Anterograde tracer also colocalized with GAD-67-positive puncta in labeled fibers, which in some cases made close synaptic contact with GAD-67-labeled NI neurons. These data provide evidence for the existence of an inhibitory descending projection from medial and posterior septum to the NI that provides a "feedback loop" to modulate the comparatively more dense ascending NI projections to medial septum and hippocampus. Neural processes and associated behaviors activated or modulated by changes in hippocampal theta rhythm may depend on reciprocal connections between ascending and descending pathways rather than on unidirectional regulation via the medial septum.

NMDA-glutamatergic activation of the ventral tegmental area induces hippocampal theta rhythm in anesthetized rats

Glutamate afferents reaching the ventral tegmental area (VTA) affect dopamine (DA) cells in this structure probably mainly via NMDA receptors. VTA appears to be one of the structures involved in regulation of hippocampal theta rhythm, and this work aimed at assessing the role of glutamatergic activation of the VTA in the theta regulation. Male Wistar rats (n=17) were divided into groups, each receiving intra-VTA microinjection (0.5 μl) of either solvent (water), glutamatergic NMDA agonist (0.2 μg) or antagonist (MK-801, 3.0 μg). Changes in local field potential were assessed on the basis of peak power (Pmax) and corresponding peak frequency (Fmax) for the delta (0.5-3 Hz) and theta (3-6 Hz) bands. NMDA microinjection evoked long-lasting hippocampal theta. The rhythm appeared with a latency of ca. 12 min post-injection and lasted for over 30 min; Pmax in this band was significantly increased for 50 min, while simultaneously Pmax in the delta band remained lower than in control conditions. Theta Fmax and delta Fmax were increased in almost entire post-injection period (by 0.3-0.5 Hz and 0.3-0.7 Hz, respectively). MK-801 depressed the sensory-evoked theta: tail pinch could not induce theta for 30 min after the injection; Pmax significantly decreased in the theta band and at the same time it increased in the delta band. Theta Fmax decreased 10 and 20 min post injection (by 0.4-0.5 Hz) and delta Fmax decreased in almost entire post injection period (by 0.3-0.7 Hz). NMDA injection generates theta rhythm probably through stimulation of dopaminergic activity within the VTA.

A review of gamma oscillations in healthy subjects and in cognitive impairment

This review describes a wide range of functional correlates of gamma oscillations in whole-brain work, in neuroethology, sensory-cognitive dynamics, emotion, and cognitive impairment. This survey opens a new window towards understanding the brain's gamma activity. Gamma responses are selectively distributed in the whole brain, and do not reflect only a unique, specific function of the nervous system. Sensory responses from cortex, thalamus, hippocampus, and reticular formations in animal and human brains, and also cognitive responses, were described by several authors. According to reviewed results, it becomes obvious that cognitive disorders, and medication-which influence the transmitter release-change entirely the understanding of the big picture in cognitive processes. Gamma activity is evoked or induced by different sensory stimuli or cognitive tasks. Thus, it is argued that gamma-band synchronization is an elementary and fundamental process in whole-brain operation. In conclusion, reasoning and suggestions for understanding gamma activity are highlighted.

Distribution and targets of the relaxin-3 innervation of the septal area in the rat

Neural tracing studies have revealed that the rat medial and lateral septum are targeted by ascending projections from the nucleus incertus, a population of tegmental GABA neurons. These neurons express the relaxin-family peptide, relaxin-3, and pharmacological modulation of relaxin-3 receptors in medial septum alters hippocampal theta rhythm and spatial memory. In an effort to better understand the basis of these interactions, we have characterized the distribution of relaxin-3 fibers/terminals in relation to different septal neuron populations identified using established protein markers. Dense relaxin-3 fiber plexuses were observed in regions of medial septum containing hippocampal-projecting choline acetyltransferase (ChAT)-, neuronal nitric oxide synthase (nNOS)-, and parvalbumin (PV)-positive neurons. In lateral septum (LS), relaxin-3 fibers were concentrated in the ventrolateral nucleus of rostral LS and the ventral nucleus of caudal LS, with sparse labeling in the dorsolateral and medial nuclei of rostral LS, dorsal nucleus of caudal LS, and ventral portion nuclei. Relaxin-3 fibers were also observed in the septofimbrial and triangular septal nuclei. In the medial septum, we observed relaxin-3-immunoreactive contacts with ChAT-, PV-, and glutamate decarboxylase-67-positive neurons that projected to hippocampus, and contacts between relaxin-3 terminals and calbindin- and calretinin-positive neurons. Relaxin-3 colocalized with synaptophysin in nerve terminals in all septal areas, and ultrastructural analysis revealed these terminals were symmetrical and contacted spines, somata, dendritic shafts, and occasionally other axonal terminals. These data predict that this GABA/peptidergic projection modulates septohippocampal activity and hippocampal theta rhythm related to exploratory navigation, defensive and ingestive behaviors, and responses to neurogenic stressors.

The use of the EEG in measuring therapeutic drug action: focus on depression and antidepressants

A major issue in proof of concept studies and early clinical trials of novel therapeutic agents is that the active drugs can often have a relatively small additional effect compared with placebo. This is especially the case in psychiatry when we usually have no direct method of measuring the pathology underlying the disorder being studied but, rather, have to rely on the subjective assessment of psychiatric symptoms. The use of the electroencephalogram (EEG) offers two potential major means of addressing this problem. First it is able to provide direct data relating to neural activity that may be abnormal in certain disorders. As such there are opportunities for utilizing the EEG in a variety of ways as an objective outcome measure. Second there is growing evidence that in certain circumstances the EEG can be used to predict which patients are likely to respond to treatment, thus potentially increasing the power of studies by decreasing non-response rates and increasing mean changes in outcome measure. Both of these uses of the EEG are illustrated in reference to the study of mood disorders and in particular depression and its treatment with antidepressants.

Corticosterone microinjected into nucleus pontis oralis increases tonic immobility in rats

Tonic immobility (TI) is also known as "immobility response", "immobility reflex", "animal hypnosis", etc. It is an innate antipredatory behavior characterized by an absence of movement, varying degrees of muscular activity, and a relative unresponsiveness to external stimuli. Experimentally, TI is commonly produced by manually forcing an animal into an inverted position and restraining it in that position until the animal becomes immobile. Part of the neural mechanism(s) of TI involves the medullo-pontine reticular formation, with influence from other components of the brain, notably the limbic system. It has been observed that TI is more prolonged in stressed animals, and systemic injection of corticosterone (CORT) also potentiates this behavior. At present, the anatomical brain regions involved in the CORT modulation of TI are unknown. Thus, our study was made to determine if some pontine areas could be targets for the modulation of TI by CORT. A unilateral nucleus pontis oralis (PnO) microinjection of 1 μL of CORT (0.05 μg/1 μL) in rats resulted in clear behavioral responses. The animals had an increased duration of TI caused by clamping the neck (in this induction, besides of body inversion and restraint, there is also clamping the neck), with an enhancement in open-field motor activity, which were prevented by pretreatment injection into PnO with 1 μL of the mineralocorticoid-receptor antagonist spironolactone (0.5 μg/1 μL) or 1 μL of the glucocorticoid-receptor antagonist mifepristone (0.5 μg/1 μL). In contrast, these behavioral changes were not seen when CORT (0.05 μg/1 μL) was microinjected into medial lemniscus area or paramedian raphe. Our data support the idea that, in stressful situations, glucocorticoids released from adrenals of the prey reach the PnO to produce a hyper arousal state, which in turn can prolong the duration of TI.

Hippocampal θ dysfunction after lateral fluid percussion injury

Chronic memory deficits are a major cause of morbidity following traumatic brain injury (TBI). In the rat, the hippocampal theta rhythm is a well-studied correlate of memory function. This study sought to investigate disturbances in hippocampal theta rhythm following lateral fluid percussion injury in the rat. A total of 13 control rats and 12 TBI rats were used. Electrodes were implanted in bilateral hippocampi and an electroencephalogram (EEG) was recorded while the rats explored a new environment, and also while navigating a modified version of the Barnes maze. Theta power and peak theta frequency were significantly attenuated in the injured animals. Further, injured rats were less likely to develop a spatial strategy for Barnes maze navigation compared to control rats. In conclusion, rats sustaining lateral fluid percussion injury demonstrated deficits in hippocampal theta activity. These deficits may contribute to the underlying memory problems seen in chronic TBI.

Comparative analysis of the neurophysiological profile of group II metabotropic glutamate receptor activators and diazepam: effects on hippocampal and cortical EEG patterns in rats

Selective activation of the Group II metabotropic glutamate receptors 2/3 (mGlu2/3) by either full agonists or positive allosteric modulators (PAMs) show anxiolytic activity. In the present study the anxiolytic profile of mGlu2/3 receptor agonists LY-354740 and LY-404039 and the mGlu2 receptor PAM 1-methyl-2-((cis-3-methyl-4-(4-trifluoromethyl-2-methoxy)-phenyl)piperidin-1-yl)-1H-imidazo[4,5-b]pyridine (MTFIP) were evaluated using neurophysiology-based assays. Activation of mGlu2/3 receptors by these compounds, as well as the positive control diazepam, significantly decreased the frequency of hippocampal theta oscillation elicited by stimulation of the brainstem nucleus pontis oralis (nPO), a characteristic action of anxiolytic compounds. Since the nPO is a critical region involved in regulation of rapid eye movement sleep, mGlu2/3 receptor activators were also tested on sleep parameters, as well as on cortical and hippocampal encephalography (EEG) activity. Both mGlu2/3 agonists and the mGlu2 PAM significantly prolonged REM sleep latency and reduced total REM sleep duration while during the active awake state all compounds lowered hippocampal peak theta frequency. However, diazepam and mGlu2/3 agonists/PAM elicited opposite changes in cortical EEG delta and beta bands. Delta power significantly increased after any of the mGlu2/3 compounds but decreased after diazepam. In the beta band, mGlu2/3 receptor agonists dose-dependently decreased beta power in contrast to the well-known beta activation by diazepam. These effects lasted 3-4h and could not be explained by modest, transient changes (<1h) in waking and slow wave sleep. The current observations support the role of mGlu2/3 receptor activators as potential anxiolytic compounds, but indicate a distinct action on cortical EEG activity which is different from the effects of GABA(A) PAMs. This article is part of a Special Issue entitled 'Anxiety and Depression'.

Modulation of NMDA receptor function by inhibition of D-amino acid oxidase in rodent brain

Observations that N-Methyl-D-Aspartate (NMDA) antagonists produce symptoms in humans that are similar to those seen in schizophrenia have led to the current hypothesis that schizophrenia might result from NMDA receptor hypofunction. Inhibition of D-amino acid oxidase (DAAO), the enzyme responsible for degradation of D-serine, should lead to increased levels of this co-agonist at the NMDA receptor, and thereby provide a therapeutic approach to schizophrenia. We have profiled some of the preclinical biochemical, electrophysiological, and behavioral consequences of administering potent and selective inhibitors of DAAO to rodents to begin to test this hypothesis. Inhibition of DAAO activity resulted in a significant dose and time dependent increase in D-serine only in the cerebellum, although a time delay was observed between peak plasma or brain drug concentration and cerebellum D-serine response. Pharmacokinetic/pharmacodynamic (PK/PD) modeling employing a mechanism-based indirect response model was used to characterize the correlation between free brain drug concentration and D-serine accumulation. DAAO inhibitors had little or no activity in rodent models considered predictive for antipsychotic activity. The inhibitors did, however, affect cortical activity in the Mescaline-Induced Scratching model, produced a modest but significant increase in NMDA receptor-mediated synaptic currents in primary neuronal cultures from rat hippocampus, and resulted in a significant increase in evoked hippocampal theta rhythm, an in vivo electrophysiological model of hippocampal activity. These findings demonstrate that although DAAO inhibition did not cause a measurable increase in D-serine in forebrain, it did affect hippocampal and cortical activity, possibly through augmentation of NMDA receptor-mediated currents.

Distinct patterns of neuronal inputs and outputs of the juxtaparaventricular and suprafornical regions of the lateral hypothalamic area in the male rat

We have analyzed at high resolution the neuroanatomical connections of the juxtaparaventricular region of the lateral hypothalamic area (LHAjp); as a control and in comparison to this, we also performed a preliminary analysis of a nearby LHA region that is dorsal to the fornix, namely the LHA suprafornical region (LHAs). The connections of these LHA regions were revealed with a coinjection tract-tracing technique involving a retrograde (cholera toxin B subunit) and anterograde (Phaseolus vulgaris leucoagglutinin) tracer. The LHAjp and LHAs together connect with almost every major division of the cerebrum and cerebrospinal trunk, but their connection profiles are markedly different and distinct. In simple terms, the connections of the LHAjp indicate a possible primary role in the modulation of defensive behavior; for the LHAs, a role in the modulation of ingestive behavior is suggested. However, the relation of the LHAjp and LHAs to potential modulation of these behaviors, as indicated by their neuroanatomical connections, appears to be highly integrative as it includes each of the major functional divisions of the nervous system that together determine behavior, i.e., cognitive, state, sensory, and motor. Furthermore, although a primary role is indicated for each region with respect to a particular mode of behavior, intermode modulation of behavior is also indicated. In summary, the extrinsic connections of the LHAjp and LHAs (so far as we have described them) suggest that these regions have a profoundly integrative role in which they may participate in the orchestrated modulation of elaborate behavioral repertoires.

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.

Cognitive neuroscience of sleep

Mechanism is at the heart of understanding, and this chapter addresses underlying brain mechanisms and pathways of cognition and the impact of sleep on these processes, especially those serving learning and memory. This chapter reviews the current understanding of the relationship between sleep/waking states and cognition from the perspective afforded by basic neurophysiological investigations. The extensive overlap between sleep mechanisms and the neurophysiology of learning and memory processes provide a foundation for theories of a functional link between the sleep and learning systems. Each of the sleep states, with its attendant alterations in neurophysiology, is associated with facilitation of important functional learning and memory processes. For rapid eye movement (REM) sleep, salient features such as PGO waves, theta synchrony, increased acetylcholine, reduced levels of monoamines and, within the neuron, increased transcription of plasticity-related genes, cumulatively allow for freely occurring bidirectional plasticity, long-term potentiation (LTP) and its reversal, depotentiation. Thus, REM sleep provides a novel neural environment in which the synaptic remodelling essential to learning and cognition can occur, at least within the hippocampal complex. During non-REM sleep Stage 2 spindles, the cessation and subsequent strong bursting of noradrenergic cells and coincident reactivation of hippocampal and cortical targets would also increase synaptic plasticity, allowing targeted bidirectional plasticity in the neocortex as well. In delta non-REM sleep, orderly neuronal reactivation events in phase with slow wave delta activity, together with high protein synthesis levels, would facilitate the events that convert early LTP to long-lasting LTP. Conversely, delta sleep does not activate immediate early genes associated with de novo LTP. This non-REM sleep-unique genetic environment combined with low acetylcholine levels may serve to reduce the strength of cortical circuits that activate in the ~50% of delta-coincident reactivation events that do not appear in their waking firing sequence. The chapter reviews the results of manipulation studies, typically total sleep or REM sleep deprivation, that serve to underscore the functional significance of the phenomenological associations. Finally, the implications of sleep neurophysiology for learning and memory will be considered from a larger perspective in which the association of specific sleep states with both potentiation or depotentiation is integrated into mechanistic models of cognition.

Anxiolytic profile of pregabalin on elicited hippocampal theta oscillation

Previous published work with the novel anticonvulsant, analgesic and anti-anxiety medication, pregabalin (Lyrica), has shown that it has anxiolytic-like actions in several animal behavioral models. However, pregabalin is structurally and pharmacologically different from other classes of known anxiolytic drugs, and the mechanisms that alter brain activity to produce anxiolytic-like actions are not well understood. In an effort to determine more about the cellular mechanisms of pregabalin, we studied its effects on hippocampal theta activity of urethane-anesthetized rats that was elicited by electrical stimulation of the nucleus pontis oralis (nPO) in the brainstem. We found that systemic administration of pregabalin significantly reduced the frequency of stimulation-induced hippocampal theta activity similarly to the effects of diazepam. In addition, pregabalin (but not diazepam) significantly altered the stimulus intensity/frequency relationship, and increased slow delta oscillation (<3.0Hz) in spontaneous hippocampal EEG in a dose-dependent manner. Our findings suggest that pregabalin may alter evoked theta frequency activity in the hippocampus by reducing neurotransmitter-mediated activation of either the septal nucleus or the hippocampus, and that its actions are unlikely to be mediated by direct activation of GABA neurotransmitter systems. These observations provide further insight to the action of pregabalin, and support the utilization of stimulation-induced theta model in discovery of novel anxiolytic drugs.

A review of brain oscillations in cognitive disorders and the role of neurotransmitters

The analysis of the functional correlates of "brain oscillations" has become an important branch of neuroscience. Although research on the functional correlates of brain oscillation has progressed to a high level, studies on cognitive disorders are rare and mainly limited to schizophrenia patients. The present review includes the results of the changes in brain oscillations in patients with Alzheimer's, schizophrenia, bipolar disorders, mild cognitive impairment, attention-deficit hyperactivity disorder (ADHD), alcoholism and those with genetic disorders. Furthermore, the effects of pharmaca and the influence of neurotransmitters in patients with cognitive disorders are also reviewed. Following the review, a short synopsis is given related to the analysis of brain oscillations.

Cyclic and sleep-like spontaneous alternations of brain state under urethane anaesthesia

Background

Although the induction of behavioural unconsciousness during sleep and general anaesthesia has been shown to involve overlapping brain mechanisms, sleep involves cyclic fluctuations between different brain states known as active (paradoxical or rapid eye movement: REM) and quiet (slow-wave or non-REM: nREM) stages whereas commonly used general anaesthetics induce a unitary slow-wave brain state.

Methodology/Principal Findings

Long-duration, multi-site forebrain field recordings were performed in urethane-anaesthetized rats. A spontaneous and rhythmic alternation of brain state between activated and deactivated electroencephalographic (EEG) patterns was observed. Individual states and their transitions resembled the REM/nREM cycle of natural sleep in their EEG components, evolution, and time frame (∼11 minute period). Other physiological variables such as muscular tone, respiration rate, and cardiac frequency also covaried with forebrain state in a manner identical to sleep. The brain mechanisms of state alternations under urethane also closely overlapped those of natural sleep in their sensitivity to cholinergic pharmacological agents and dependence upon activity in the basal forebrain nuclei that are the major source of forebrain acetylcholine. Lastly, stimulation of brainstem regions thought to pace state alternations in sleep transiently disrupted state alternations under urethane.

Conclusions/Significance

Our results suggest that urethane promotes a condition of behavioural unconsciousness that closely mimics the full spectrum of natural sleep. The use of urethane anaesthesia as a model system will facilitate mechanistic studies into sleep-like brain states and their alternations. In addition, it could also be exploited as a tool for the discovery of new molecular targets that are designed to promote sleep without compromising state alternations.

Repeated cortisol administration attenuates the EEG response to buspirone in healthy volunteers: evidence for desensitization of the 5-HT1A autoreceptor

It has previously been postulated that the therapeutic effect of antidepressants, particularly selective serotonin re-uptake inhibitors (SSRIs), is mediated by a down-regulation of somatodendritic (presynaptic) 5-HT(1A) autoreceptors with chronic treatment. Animal studies have revealed that repeated administration of corticosteroids similarly down-regulate this receptor. However, it has previously been difficult to explore if this receptor is similarly modulated in man in vivo. The objective of this study was to explore the effect of repeated administration of cortisol to healthy volunteers utilising a novel putative index of somatodendritic 5-HT(1A) autoreceptor function. This method involves the administration of the 5-HT(1A) agonist buspirone and observing the subsequent negative shift in the frequency spectrum of the electroencephalogram (EEG). Healthy male volunteers were treated with cortisol 20 mg, or placebo, orally twice daily for 7 days in a double-blind random-order crossover study. After each treatment period volunteers were administered buspirone 30 mg orally prior to EEG recordings. Following a week's treatment with placebo, buspirone led to a negative shift in the EEG frequency spectrum as previously reported. However, following treatment with cortisol, the effect of buspirone was significantly attenuated. This is consistent with corticosteroids having a similar effect on somatodendritic 5-HT(1A) autoreceptors in man as seen in rodents.

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.

Activation of alpha7 acetylcholine receptors augments stimulation-induced hippocampal theta oscillation

In the septohippocampal formation alpha7 nicotinic receptors (alpha7 nAChRs) are predominantly expressed by neurons well positioned to modulate hippocampal theta oscillation, such as GABAergic interneurons in the hippocampus, and by both GABAergic and cholinergic septal neurons. In the present experiments, we evaluated the efficacy of the recently developed selective alpha7 nAChR agonist PNU-282987 on hippocampal theta oscillation in anaesthetized rats. This compound shows high affinity for the rat alpha7 nAChRs (Ki = 26 nM) but a negligible activity at other nAChRs. Systemic administration of PNU-282987 significantly enhanced the power (by 40%) of hippocampal theta oscillation induced by electrical stimulation of the brainstem reticular formation. In contrast, the amnesic and muscarinic receptor antagonist scopolamine significantly decreased the power (by 68%) of the stimulation-induced theta oscillation. Given the connection between hippocampal theta oscillation and cognitive processes, it is proposed that precognitive actions of alpha7 nAChR agonists could be mediated, at least in part, by modulation of hippocampal oscillatory activity.

Hippocampal theta rhythm: a tag for short-term memory

The theta rhythm is the largest extracellular synchronous signal that can be recorded from the mammalian brain, and has been strongly implicated in mnemonic functions of the hippocampus. We advance the proposal that the theta rhythm represents a "tag" for short-term memory processing in the hippocampus. We propose that the hippocampus receives two main types of input, theta from ascending brainstem-diencephalo-septal systems and "information bearing" mainly from thalamocortical and cortical systems. The temporal convergence of activity of these two systems results in the encoding of information in the hippocampus, primarily reaching it via cortical routes. By analogy to processes associated with long-term potentiation (LTP), we suggest that theta represents a strong depolarizing influence on NMDA receptor-containing cells of the hippocampus. The temporal coupling of a theta-induced depolarization and the release of glutamate to these cells from intra- and extrahippocampal sources activates them. This, in turn, initiates processes leading to a (short-term) strengthening of connections between presynaptic ("information bearing") and postsynaptic neurons of the hippocampus. Theta is selectively present in the rat during active exploratory movements. During exploration, a rat continually gathers and updates information about its environment. If this information is temporally coupled to theta (as with the case of locomotion), it becomes temporarily stored in the hippocampus by mechanisms similar to the early phase of LTP (E-LTP). If the exploratory behavior of the rat goes unreinforced, these relatively short-lasting traces (1-3 h) gradually weaken and eventually fade-to be reupdated. On the other hand, if the explorations of the rat lead to rewards (or punishments), additional modulatory inputs to the hippocampus become activated and convert the short-term, theta-dependent memory, into long-term stores.

Narcolepsy and Disorders of Excessive Somnolence

Recent studies provide valid criteria that help differentiate idiopathic narcolepsy from other disorders of excessive daytime somnolence [3]. Research to date suggests that idiopathic narcolepsy might properly be considered a disorder of excessive sleepiness with dysfunctional REM-sleep mechanisms, clinically evidenced as cataplexy and electrophysiologically recognized as SOREMPs. Given these criteria, a diagnosis can generally be made using a combination of history, PSG, and MSLT. Traditionally, the medical treatment of idiopathic narcolepsy has centered on a two-drug regimen (stimulants for sleepiness and TCAs for cataplexy and auxiliary symptoms). Some newer medications are proving efficacious for sleepiness with minimal adverse effects, whereas others may provide a single-drug regimen that simultaneously addresses sleepiness and cataplexy [18]. New research has allowed some experts to hypothesize that idiopathic narcolepsy may be the result of a genetic predisposition to autoimmune disease [176]. It is possible that aberrant genetic coding of elements in the hypocretin/orexin systems allows a sensitivity to inducible and possibly virally mediated changes, which leave cells in the lateral hypothalamus susceptible to autoimmune attack [96]. As such, genetic screening of high-risk individuals might eventually rationalize the prophylactic use of immunosuppressants in some instances. In the future, for atypical cases(poorly responsive to therapy), genetic, CSF, and brain imaging studies, and possibly even neuronal transplantation may prove beneficial in the assessment and treatment of idiopathic narcolepsy.

The supramammillary area: its organization, functions and relationship to the hippocampus

The supramammillary area of the hypothalamus, although small in size, can have profound modulatory effects on the hippocampal formation and related temporal cortex. It can control hippocampal plasticity and also has recently been shown to contain cells that determine the frequency of hippocampal rhythmical slow activity (theta rhythm). We review here its organization and anatomical connections providing an atlas and a new nomenclature. We then review its functions particularly in relation to its links with the hippocampus. Much of its control of behaviour and its differential activation by specific classes of stimuli is consistent with a tight relationship with the hippocampus. However, its ascending connections involve not only caudal areas of the cortex with close links to the hippocampus but also reciprocal connections with more rostral areas such as the infralimbic and anterior cingulate cortices. These latter areas appear to be the most rostral part of a network that, via the medial septum, hippocampus and lateral septum, is topographically mapped into the hypothalamus. The supramammillary area is thus diffusely connected with areas that control emotion and cognition and receives more topographically specific return information from areas that control cognition while also receiving ascending information from brain stem areas involved in emotion. We suggest that it is a key part of a network that recursively transforms information to achieve integration of cognitive and emotional aspects of goal-directed behaviour.

Functional connectivity between the red nucleus and the hippocampus supports the role of hippocampal formation in sensorimotor integration

Experiments were carried out in urethane-anesthetized rats to evaluate the hypothesis that the red nucleus has functional connections with the hippocampal formation. Depth profiles of electrical stimulation in experiment 1 confirmed that stimulation administered to the red nucleus elicited theta field activity in the hippocampal formation with a linear relationship between stimulus intensity and theta frequency. Experiment 2 showed that microinfusion of local anesthetic procaine hydrochloride into the medial septum resulted in a reversible blockade of theta field activity elicited by electrical stimulation of the red nucleus. In experiment 3, the discharge activity of red nucleus cells was recorded during the field conditions of hippocampal synchrony (theta) and hippocampal asynchrony [large amplitude irregular activity (LIA)]. Analysis revealed that 26/46 (56%) of red nucleus cells were theta-related, whereas the remaining 20 (44%) were nonrelated. The majority of theta-related cells were classified as tonic theta-on. A brief increase above the basal discharge rate of tonic theta-on red nucleus cells during LIA predicted the transition from LIA to theta with 400- to 500-ms latency. Furthermore, higher frequency transitional discharges predicted higher theta frequencies, whereas higher discharge rates during theta predicted shifts to higher theta frequencies. The results supported the conclusion that the red nucleus, traditionally associated with motor functions, is functionally connected with the neural circuitry involved in the generation of theta band oscillation and synchrony in the hippocampal formation, in agreement with the predictions of the sensorimotor integration model of hippocampal function.

The effect of posterior hypothalamic injection of cholinergic agents on hippocampal formation theta in freely moving cat

The effect of intra-hypothalamic micro-injection of muscarinic (atropine sulphate, pirenzepine and gallamine) and nicotinic (hexamethonium) antagonists on spontaneous, sensory and electrically-induced hippocampal formation (HPC) theta EEG activity was investigated in the freely behaving cat. Administration of hexamethonium and gallamine failed to elicit a detectable effect on HPC theta activity. However, the injection of atropine sulphate and pirenzepine abolished the theta rhythm recorded from HPC. This effect was reversible. A substantial difference in the recovery time course between frequency versus amplitude (microV) and power (microV2) of hippocampal theta was observed. While theta frequency exhibited a rapid reappearance with a shallow slope, the power and amplitude showed a gradual recovery with a steeper slope. The injection of carbachol into posterior hypothalamus (PH) produced almost a continuous HPC theta with increased power. These results demonstrate that cholinergic (M1) receptors localised in the posterior area of the hypothalamus are engaged in mechanisms responsible for generating hippocampal theta oscillations in the freely behaving cat. The contribution of posterior hypothalamic region to HPC theta frequency and amplitude is discussed.

Multiple hypothalamic sites control the frequency of hippocampal theta rhythm

Stimulation of a neural pathway originating in the brainstem reticular formation, with synapses in the medial hypothalamus, activates the hippocampal theta rhythm. The frequency of reticular-elicited theta is determined in the medial supramammillary nucleus (mSuM) completely in anaesthetised rats, but only partially when the animal is awake. We tested other medial hypothalamic sites for their capacity to control theta frequency in awake rats. Blockade of sodium channels (1 microl fast infusion of the local anaesthetic procaine, experiment 1) or increased inhibition by GABA (Chlordiazepoxide [CDP], experiment 2) was found to reduce or increase the frequency of reticular-elicited theta, depending on the precise site of injection, in the region of the dorsomedial hypothalamic nucleus (DMH) and the posterior hypothalamic nucleus (PH). A band of null sites for CDP was located in the region of the ventral border of PH and dorsal border of mSuM. Using 0.5 and 1 microl CDP, and slow infusions (experiment 3), it was found that effective PH sites were also separate from mSuM in the rostrocaudal direction. In experiment 4, the DMH/PH region was mapped with unilateral and bilateral slow infusions of 0.5 microl CDP. CDP significantly reduced frequency in medial (periventricular) and dorsal PH, but not DMH. Bilateral injections appeared to generally sum the usual effects of unilateral injection, producing effects of intermediate size. However, the absolute frequency change in any given site, or with any pair of sites, did not exceed 1 Hz, which is similar to what is seen with single injections in mSuM. Overall, it appears that at, any one time, theta frequency may be determined by a complex interplay between distinct but interacting modulatory regions in the medial hypothalamus.

Firing activity and postsynaptic properties of morphologically identified neurons of ventral oral pontine reticular nucleus

The ventral part of the oral pontine reticular nucleus (vRPO) is an important region for the generation and maintenance of REM sleep. Firing activity and synaptic response properties of morphologically identified vRPO neurons have been investigated in urethane-anaesthetized cats. Extracellular recordings were performed through recording micropipettes and neurons were extracellularly stained with biocytin. Two types of neurons were identified under spontaneous conditions: type I neurons (77%) are characterized by non-rhythmic firing; type II neurons (23%) display single spikes firing rhythmically at between 7 and 22 Hz. Type I neurons displayed ellipsoid somata with thick dendritic trunks and axons that arose from either the soma or the initial dendritic segment; these axons could not be clearly followed. Type II neurons showed polygonal somata with radial dendrites; their axons branched at a small distance from the soma. Electrical stimulation of the contralateral vRPO elicited responses in both neuron types (57% and 31%, respectively); this effect was blocked by the non-NMDA glutamatergic receptor antagonist CNQX. Electrical stimulation of the PpT evoked orthodromic responses in type I neurons (41%) and inhibited the firing rate of all type II neurons for 50-100 ms. Both effects were blocked by the muscarinic receptor antagonist atropine. The cholinergic agonist, carbachol, increased the firing rate in most type I neurons and inhibited most type II neurons in these animals. The results demonstrated that the activity of vRPO neurons is modulated through the postsynaptic activation exerted by extrinsic afferents on cholinergic and glutamatergic receptors.

Behavioral arrest: in search of the neural control system

Scientists have spent hundreds of years trying to understand how the brain controls movement. Why has there been so little interest in knowing how the brain STOPS movement? This review calls attention to behavioral phenomena in which an animal or human undergoes temporary total-body arrest of movement, that is, behavioral arrest (BA). These states can be actively induced by visual stimuli, by body and limb manipulations, and by drugs. Historically, these states have been considered as unrelated, and their literature does not cross-connect. What is known about the causal mechanisms is scant, limited mostly to implication of the brainstem in manipulation-induced BA and dopaminergic blockade in the striatum in the case of drug-induced BA. The possibility has not been experimentally tested that all of these states share with each other not only an active global immobility in which awkward postures are maintained, but also underlying neural mechanisms. This review identifies key brainstem, diencephalic, and basal forebrain areas that seem to be involved in causing BA. We review the evidence that suggest a possible role in BA for the following brain structures: entopeduncular nucleus, medullary and pontine reticular zones, parabrachial region, pedunculopontine nucleus and nearby areas, substantia nigra, subthalamic nucleus, ventromedial thalamic nucleus, and zona incerta. Such areas may operate as a BA control system. Confirmation of which brain areas operate collectively in BA would require testing of several kinds of BA in the same animals with the same kinds of experimental tests. Areas and mechanisms might be elucidated through a strategic combination of the following research approaches: imaging (fMRI, c-fos), lesions (of areas, of afferent and efferent pathways), chemical microstimulation, and electrical recording (of multiple units and field potentials, with an emphasis on testing coherence among areas). We suggest the working hypothesis that BA is created and sustained by coherent, perhaps oscillatory, activity among a group of basal forebrain and brainstem areas that collectively disrupt the normal spinal and supraspinal sequencing controls of reciprocal actions on the extensors and flexors that otherwise produce movement.