Serotonin hyperpolarizes cholinergic low-threshold burst neurons in the rat laterodorsal tegmental nucleus in vitro

Sleep-Wake Neurobiology

Sleep and wakefulness are complex, tightly regulated behaviors that occur in virtually all animals. With recent exciting developments in neuroscience methodologies such as optogenetics, chemogenetics, and cell-specific calcium imaging technology, researchers can advance our understanding of how discrete neuronal groups precisely modulate states of sleep and wakefulness. In this chapter, we provide an overview of key neurotransmitter systems, neurons, and circuits that regulate states of sleep and wakefulness. We also describe long-standing models for the regulation of sleep/wake and non-rapid eye movement/rapid eye movement cycling. We contrast previous knowledge derived from classic approaches such as brain stimulation, lesions, cFos expression, and single-unit recordings, with emerging data using the newest technologies. Our understanding of neural circuits underlying the regulation of sleep and wakefulness is rapidly evolving, and this knowledge is critical for our field to elucidate the enigmatic function(s) of sleep.

A study on sleep apnea in patients with abnormal sewda type of depression

Objective: To preliminarily investigate the clinical features and PSG in abnormal sewda-type depressive insomnia.

Methods: A total of 127 abnormal sewda-type depressive insomnia patients were evaluated with overnight PSG, and 32 normal participants were compared.

Results: Patients with abnormal sewda-type depressive insomnia were compared with the control group; the sleep symptoms showed a long incubation period of sleep, low sleep maintenance rate, low sleep efficiency and poor sleep quality as well as daytime dysfunction. At process and continuity of sleep: Total sleep time, sleep efficiency, sleep maintenance rate in abnormal sewda-type depressive insomnia group were shorter than the control group. Wake after sleep onset, and sleep latency were longer than the control group. At sleep structure: N1 ratio and N2 ratio in depressive insomnia group were longer than the control group, N3 ratio and REM sleep ratio shorter than the control group. At REM index: REM latency, REM cycles, and REM sleep time were shorter than the control group.

Conclusion: Insomnia symptoms in abnormal sewda-type depression comorbid insomnia patients were similar to the ordinary insomnia patients. The PSG characteristics had significant changes in sleep process, sleep structure and REM indicators. The severity of the abnormal sewda-type depression was closely related to REM indicators. Change of REM sleep characteristics may be the specificity, and these could be taken as reference in diagnosis and identification of abnormal sewda-type depressive insomnia.

Serotonin Expression in the Song Circuitry of Adult Male Zebra Finches

Serotonin is an important neurotransmitter of the brain, but its role in song control remains to be fully demonstrated. Using male zebra finches (Taeniopygia guttata) that have song learning and production capabilities, we analysed the serotonin expression levels in the song nuclei and adjacent areas (peri-song nuclei) using immunohistochemistry. Key song nuclei were identified using combinations of Hoechst, choline acetyltransferase, and a neurofilament (NN18) marker in reference to the ZEBrA atlas. Mean serotonin expression was highest in interfacial nucleus (Nif) and lower in the other song nuclei in the following order (in order of highest first): interfacial nucleus (Nif) > Area X > dorsomedial part of the intercollicular nucelus (DM) > robust nucleus of the archistriatum (RA) > lateral magnocellular nucleus of the anterior neostriatum (LMAN) > ventral respiratory group (VRG) > dorsolateral nucleus of the medial thalamus (DLM) > the nucleus HVC (proper name) > tracheosyringeal motor nucleus (nXIIts). However, the mean serotonin expression (in order of highest first) in the peri-song nuclei regions was: peri-DM > peri-nXIIts > supra-peri-HVC > peri-RA > peri-DLM > peri-Area X > infra-peri-HVC > peri-VRG > peri-LMAN > peri-Nif. Interestingly, serotoninergic fibers immunostained for serotonin or the serotonin transporter can be found as a basket-like peri-neuronal structure surrounding cholinergic cell bodies, and appear to form contacts onto dopaminergic neurones. In summary, serotonin fibers are present at discrete song nuclei, and peri-song nuclei regions, which suggest serotonin may have a direct and/or modulatory role in song control.

Arousal and sleep circuits

The principal neurons of the arousal and sleep circuits are comprised by glutamate and GABA neurons, which are distributed within the reticular core of the brain and, through local and distant projections and interactions, regulate cortical activity and behavior across wake-sleep states. These are in turn modulated by the neuromodulatory systems that are comprised by acetylcholine, noradrenaline, dopamine, serotonin, histamine, orexin (hypocretin), and melanin-concentrating hormone (MCH) neurons. Glutamate and GABA neurons are heterogeneous in their profiles of discharge, forming distinct functional cell types by selective or maximal discharge during (1) waking and paradoxical (REM) sleep, (2) during slow wave sleep, (3) during waking, or (4) during paradoxical (REM) sleep. The neuromodulatory systems are each homogeneous in their profile of discharge, the majority discharging maximally during waking and paradoxical sleep or during waking. Only MCH neurons discharge maximally during sleep. They each exert their modulatory influence upon other neurons through excitatory and inhibitory receptors thus effecting a concerted differential change in the functionally different cell groups. Both arousal and sleep circuit neurons are homeostatically regulated as a function of their activity in part through changes in receptors. The major pharmacological agents used for the treatment of wake and sleep disorders act upon GABA and neuromodulatory transmission.

Circuit mechanisms and computational models of REM sleep

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REM sleep was discovered in the 1950s.

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Many hypothalamic and brainstem areas have been found to contribute to REM sleep.

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An up-to-date picture of REM-sleep-regulating circuits is reviewed.

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A brief overview of computational models for REM sleep regulation is provided.

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Outstanding issues for future studies are discussed.

Rapid eye movement (REM) sleep or paradoxical sleep is an elusive behavioral state. Since its discovery in the 1950s, our knowledge of the neuroanatomy, neurotransmitters and neuropeptides underlying REM sleep regulation has continually evolved in parallel with the development of novel technologies. Although the pons was initially discovered to be responsible for REM sleep, it has since been revealed that many components in the hypothalamus, midbrain, pons, and medulla also contribute to REM sleep. In this review, we first provide an up-to-date overview of REM sleep-regulating circuits in the brainstem and hypothalamus by summarizing experimental evidence from neuroanatomical, neurophysiological and gain- and loss-of-function studies. Second, because quantitative approaches are essential for understanding the complexity of REM sleep-regulating circuits and because mathematical models have provided valuable insights into the dynamics underlying REM sleep genesis and maintenance, we summarize computational studies of the sleep-wake cycle, with an emphasis on REM sleep regulation. Finally, we discuss outstanding issues for future studies.

The GABAergic Gudden's dorsal tegmental nucleus: A new relay for serotonergic regulation of sleep-wake behavior in the mouse

Serotonin (5-HT) neurons are involved in wake promotion and exert a strong inhibitory influence on rapid eye movement (REM) sleep. Such effects have been ascribed, at least in part to the action of 5-HT at post-synaptic 5-HT_1A receptors (5-HT_1AR) in the brainstem, a major wake/REM sleep regulatory center. However, the neuroanatomical substrate through which 5-HT_1AR influence sleep remains elusive. We therefore investigated whether a brainstem structure containing a high density of 5-HT_1AR mRNA, the GABAergic Gudden's dorsal tegmental nucleus (DTg), may contribute to 5-HT-mediated regulatory mechanisms of sleep-wake stages. We first found that bilateral lesions of the DTg promote wake at the expense of sleep. In addition, using local microinjections into the DTg in freely moving mice, we showed that local activation of 5-HT_1AR by the prototypical agonist 8-OH-DPAT enhances wake and reduces deeply REM sleep duration. The specific involvement of 5-HT_1AR in the latter effects was further demonstrated by ex vivo extracellular recordings showing that the selective 5-HT_1AR antagonist WAY 100635 prevented DTg neuron inhibition by 8-OH-DPAT. We next found that GABAergic neurons of the ventral DTg exclusively targets glutamatergic neurons of the lateral mammillary nucleus (LM) in the posterior hypothalamus by means of anterograde and retrograde tracing techniques using cre driver mouse lines and a modified rabies virus. Altogether, our findings strongly support the idea that 5-HT-driven enhancement of wake results from 5-HT_1AR-mediated inhibition of DTg GABAergic neurons that would in turn disinhibit glutamatergic neurons in the mammillary bodies. We therefore propose a Raphe→DTg→LM pathway as a novel regulatory circuit underlying 5-HT modulation of arousal.

Rapid eye movement sleep behavior disorder and the link to alpha-synucleinopathies

Rapid eye movement (REM) sleep behavior disorder (RBD) involves REM sleep without atonia in conjunction with a recurrent nocturnal dream enactment behavior, with vocalizations such as shouting and screaming, and motor behaviors such as punching and kicking. Secondary RBD is well described in association with neurological disorders including Parkinson's disease (PD), multiple system atrophy (MSA), and other conditions involving brainstem structures such as tumors. However, RBD alone is now considered to be a potential harbinger of later development of neurodegenerative disorders, in particular PD, MSA, dementia with Lewy bodies (DLB), and pure autonomic failure. These conditions are linked by their underpinning pathology of alpha-synuclein protein aggregation. In RBD, it is therefore important to recognize the potential risk for later development of an alpha-synucleinopathy, and to investigate for other potential causes such as medications. Other signs and symptoms have been described in RBD, such as orthostatic hypotension, or depression. While it is important to recognize these features to improve patient management, they may ultimately provide clinical clues that will lead to risk stratification for phenoconversion. A critical need is to improve our ability to counsel patients, particularly with regard to prognosis. The ability to identify who, of those with RBD, is at high risk for later neurodegenerative disorders will be paramount, and would in addition advance our understanding of the prodromal stages of the alpha-synucleinopathies. Moreover, recognition of at-risk individuals for neurodegenerative disorders may ultimately provide a platform for the testing of possible neuroprotective agents for these neurodegenerative disorders.

Neural Circuitry of Wakefulness and Sleep

Sleep remains one of the most mysterious yet ubiquitous animal behaviors. We review current perspectives on the neural systems that regulate sleep/wake states in mammals and the circadian mechanisms that control their timing. We also outline key models for the regulation of rapid eye movement (REM) sleep and non-REM sleep, how mutual inhibition between specific pathways gives rise to these distinct states, and how dysfunction in these circuits can give rise to sleep disorders.

The anatomical, cellular and synaptic basis of motor atonia during rapid eye movement sleep

Rapid eye movement (REM) sleep is a recurring part of the sleep-wake cycle characterized by fast, desynchronized rhythms in the electroencephalogram (EEG), hippocampal theta activity, rapid eye movements, autonomic activation and loss of postural muscle tone (atonia). The brain circuitry governing REM sleep is located in the pontine and medullary brainstem and includes ascending and descending projections that regulate the EEG and motor components of REM sleep. The descending signal for postural muscle atonia during REM sleep is thought to originate from glutamatergic neurons of the sublaterodorsal nucleus (SLD), which in turn activate glycinergic pre-motor neurons in the spinal cord and/or ventromedial medulla to inhibit motor neurons. Despite work over the past two decades on many neurotransmitter systems that regulate the SLD, gaps remain in our knowledge of the synaptic basis by which SLD REM neurons are regulated and in turn produce REM sleep atonia. Elucidating the anatomical, cellular and synaptic basis of REM sleep atonia control is a critical step for treating many sleep-related disorders including obstructive sleep apnoea (apnea), REM sleep behaviour disorder (RBD) and narcolepsy with cataplexy.

Is Autism Partly a Consolidation Disorder?

Computational modeling has been useful for understanding processes of encoding and consolidation in cortical structures. In particular, this work suggests a role of neuromodulators in setting dynamics for consolidation processes during different stages of waking and sleep. Because autistic individuals show symptoms of a cognitive nature coupled with a high prevalence of comorbid conditions such as epileptiform discharge during sleep and sleep disorders, it is possible that autism could involve a breakdown in consolidation processes, which are essential to build effective cognitive representations of the environment on the basis of individual experiences. In this article, theories of consolidation during different stages of waking and sleep and the role of different neuromodulators in these consolidation processes are reviewed in conjunction with different features of autism, which may be understood in the context of these theories.

Optogenetic activation of cholinergic neurons in the PPT or LDT induces REM sleep

Rapid eye movement (REM) sleep is an important component of the natural sleep/wake cycle, yet the mechanisms that regulate REM sleep remain incompletely understood. Cholinergic neurons in the mesopontine tegmentum have been implicated in REM sleep regulation, but lesions of this area have had varying effects on REM sleep. Therefore, this study aimed to clarify the role of cholinergic neurons in the pedunculopontine tegmentum (PPT) and laterodorsal tegmentum (LDT) in REM sleep generation. Selective optogenetic activation of cholinergic neurons in the PPT or LDT during non-REM (NREM) sleep increased the number of REM sleep episodes and did not change REM sleep episode duration. Activation of cholinergic neurons in the PPT or LDT during NREM sleep was sufficient to induce REM sleep.

Neurophysiological evidence for the presence of cannabinoid CB1 receptors in the laterodorsal tegmental nucleus

Marijuana, which acts within the endocannabinoid (eCB) system as an agonist of the cannabinoid type 1 receptor (CB1R), exhibits addictive properties and has powerful actions on the state of arousal of an organism. The laterodorsal tegmental nucleus (LDT), as a component of the reticular activating system, is involved in cortical activation and is important in the development of drug addiction-associated behaviours. Therefore, eCBs might exert behavioural effects by actions on the LDT; however, it is unknown whether eCBs have actions on neurons in this nucleus. Accordingly, whole-cell voltage- and current-clamp recordings were conducted from mouse brain slices, and responses of LDT neurons to the CB1R agonist WIN-2 were monitored. Our results showed that WIN-2 decreased the frequency of spontaneous and miniature inhibitory postsynaptic currents (sIPSCs and mIPSCs). Ongoing activity of endogenous eCBs was confirmed as AM251, a potent CB1R antagonist, elicited sIPSCs. WIN-2 reduced the firing frequency of LDT neurons. In addition, our RT-PCR studies confirmed the presence of CB1R transcript in the LDT. Taken together, we conclude that CB1Rs are functionally active in the LDT, and their activation changes the firing frequency and synaptic activity of neurons in this nucleus. Therefore, endogenous eCB transmission could play a role in processes involving the LDT, such as cortical activation and motivated behaviours and, further, behavioural actions of marijuana are probably mediated, in part, via cellular actions within the LDT induced by this addictive and behavioural state-altering drug.

The cholinergic agonist carbachol increases the frequency of spontaneous GABAergic synaptic currents in dorsal raphe serotonergic neurons in the mouse

Dorsal raphe nucleus (DRN) serotonin (5-HT) neurons play an important role in feeding, mood control and stress responses. One important feature of their activity across the sleep-wake cycle is their reduced firing during rapid-eye-movement (REM) sleep which stands in stark contrast to the wake/REM-on discharge pattern of brainstem cholinergic neurons. A prominent model of REM sleep control posits a reciprocal interaction between these cell groups. 5-HT inhibits cholinergic neurons, and activation of nicotinic receptors can excite DRN 5-HT neurons but the cholinergic effect on inhibitory inputs is incompletely understood. Here, in vitro, in DRN brain slices prepared from GAD67-GFP knock-in mice, a brief (3 min) bath application of carbachol (50 μM) increased the frequency of spontaneous inhibitory postsynaptic currents (sIPSCs) in GFP-negative, putative 5-HT neurons but did not affect miniature (tetrodotoxin-insensitive) IPSCs. Carbachol had no direct postsynaptic effect. Thus, carbachol likely increases the activity of local GABAergic neurons which synapse on 5-HT neurons. Removal of dorsal regions of the slice including the ventrolateral periaqueductal gray (vlPAG) region where GABAergic neurons projecting to the DRN have been identified, abolished the effect of carbachol on sIPSCs whereas the removal of ventral regions containing the oral region of the pontine reticular nucleus (PnO) did not. In addition, carbachol directly excited GFP-positive, GABAergic vlPAG neurons. Antagonism of both muscarinic and nicotinic receptors completely abolished the effects of carbachol. We suggest cholinergic neurons inhibit DRN 5-HT neurons when acetylcholine levels are lower i.e. during quiet wakefulness and the beginning of REM sleep periods, in part via excitation of muscarinic and nicotinic receptors located on local vlPAG and DRN GABAergic neurons. Higher firing rates or burst firing of cholinergic neurons associated with attentive wakefulness or phasic REM sleep periods leads to excitation of 5-HT neurons via the activation of nicotinic receptors located postsynaptically and presynaptically on excitatory afferents.

Off the beaten path: drug addiction and the pontine laterodorsal tegmentum

Drug addiction is a multileveled behavior controlled by interactions among many diverse neuronal groups involving several neurotransmitter systems. The involvement of brainstem-sourced, cholinergic neurotransmission in the development of addiction and in the persistent physiological processes that drive this maladaptive behavior has not been widely investigated. The major cholinergic input to neurons in the midbrain which are instrumental in assessment of reward and assignment of salience to stimuli, including drugs of abuse, sources from acetylcholine- (ACh-) containing pontine neurons of the laterodorsal tegmentum (LDT). Excitatory LDT input, likely cholinergic, is critical in allowing behaviorally relevant neuronal firing patterns within midbrain reward circuitry. Via this control, the LDT is positioned to be importantly involved in development of compulsive, addictive patterns of behavior. The goal of this review is to present the anatomical, physiological, and behavioral evidence suggesting a role of the LDT in the neurobiology underlying addiction to drugs of abuse. Although focus is directed on the evidence supporting a vital participation of the cholinergic neurons of the LDT, data indicating a contribution of noncholinergic LDT neurons to processes underlying addiction are also reviewed. While sparse, available information of actions of drugs of abuse on LDT cells and the output of these neurons as well as their influence on addiction-related behavior are also presented. Taken together, data from studies presented in this review strongly support the position that the LDT is a major player in the neurobiology of drug addiction. Accordingly, the LDT may serve as a future treatment target for efficacious pharmaceutical combat of drug addiction.

Why are seizures rare in rapid eye movement sleep? Review of the frequency of seizures in different sleep stages

Since the formal characterization of sleep stages, there have been reports that seizures may preferentially occur in certain phases of sleep. Through ascending cholinergic connections from the brainstem, rapid eye movement (REM) sleep is physiologically characterized by low voltage fast activity on the electroencephalogram, REMs, and muscle atonia. Multiple independent studies confirm that, in REM sleep, there is a strikingly low proportion of seizures (~1% or less). We review a total of 42 distinct conventional and intracranial studies in the literature which comprised a net of 1458 patients. Indexed to duration, we found that REM sleep was the most protective stage of sleep against focal seizures, generalized seizures, focal interictal discharges, and two particular epilepsy syndromes. REM sleep had an additional protective effect compared to wakefulness with an average 7.83 times fewer focal seizures, 3.25 times fewer generalized seizures, and 1.11 times fewer focal interictal discharges. In further studies REM sleep has also demonstrated utility in localizing epileptogenic foci with potential translation into postsurgical seizure freedom. Based on emerging connectivity data in sleep, we hypothesize that the influence of REM sleep on seizures is due to a desynchronized EEG pattern which reflects important connectivity differences unique to this sleep stage.

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.

Pharmacological evidence of functional inhibitory metabotrophic glutamate receptors on mouse arousal-related cholinergic laterodorsal tegmental neurons

Cholinergic neurons of the pontine laterodorsal tegmentum (LDT) are importantly involved in neurobiological mechanisms governing states of arousal such as sleep and wakefulness as well as other appetitive behaviors, such as drug-seeking. Accordingly, mechanisms controlling their excitability are important to elucidate if we are to understand how these LDT neurons generate arousal states. Glutamate mediates the vast majority of excitatory synaptic transmission in the vertebrate CNS and while presence of glutamate input in the LDT has been shown and ionotropic responses to glutamate have been reported in the LDT, characterization of metabotropic responses is lacking. Therefore, electrophysiological responses and changes in levels of intracellular Ca(2+) in mouse cholinergic LDT neurons following application of specific mGluR agonists and antagonists were examined. Unexpectedly, both the mGluR(5)specific agonist, CHPG, and the group II mGluR (mGlu(2/3)) agonist, LY379268 (LY), induced a TTX-insensitive outward current/hyperpolarization. Both outward currents were significantly reduced by the mGluR antagonist MCPG and the CHPG-induced current was blocked by the specific mGluR(5) antagonist MTEP. Concurrent Ca(2+)imaging revealed that while CHPG actions did include release of Ca(2+) from CPA/thapsigargin-sensitive intracellular stores, actions of LY did not. Both CHPG- and LY-induced outward currents were mediated by a TEA-sensitive potassium conductance. The large-conductance, Ca(2+)-dependent potassium (BK) channel blocker, iberiotoxin, attenuated CHPG actions. Consistent with actions on the BK conductance, CHPG enhanced the amplitude of the fast component of the after hyperpolarizing potential, concurrent with a reduction in the firing rate. We conclude that stimulation of mGluR(5) and group II (mGluR(2/3)) elicits postsynaptically-mediated outward currents/hyperpolarizations in cholinergic LDT neurons. Effects of glutamatergic input would be, thus, expected not only to be excitation via stimulation of ionotropic glutamate receptors and mGluR(1), but also inhibition via actions at mGluR(5) and mGluR(2/3) on these neurons. As these two processes counteract each other, these surprising findings necessitate revision of predictions regarding the net level of excitation generated by glutamate input to cholinergic LDT cells and, by extension, the functional outcome of glutamate transmission on processes which these neurons regulate. This article is part of a Special Issue entitled 'Metabotropic Glutamate Receptors'.

Interleukin-1 inhibits putative cholinergic neurons in vitro and REM sleep when microinjected into the rat laterodorsal tegmental nucleus

STUDY OBJECTIVES

REM sleep is suppressed during infection, an effect mimicked by the administration of cytokines such as interleukin-1 (IL-1). In spite of this observation, brain sites and neurochemical systems mediating IL-1-induced suppression of REM sleep have not been identified. Cholinergic neurons in the brainstem laterodorsal tegmental nucleus (LDT) are part of the neuronal circuitry responsible for REM sleep generation. Since IL-1 inhibits acetylcholine synthesis and release, the aim of this study was to test the two different, but related hypotheses. We hypothesized that IL-1 inhibits LDT cholinergic neurons, and that, as a result of this inhibition, IL-1 suppresses REM sleep.

DESIGN, MEASUREMENT, AND RESULTS

To test these hypotheses, the electrophysiological activity of putative cholinergic LDT neurons was recorded in a rat brainstem slice preparation. Interleukin-1 significantly inhibited the firing rate of 76% of recorded putative cholinergic LDT neurons and reduced the amplitude of glutamatergic evoked potentials in 60% of recorded neurons. When IL-1 (1 ng) was microinjected into the LDT of freely behaving rats, REM sleep was reduced by about 50% (from 12.7% +/- 1.5% of recording time [after vehicle] to 6.1% +/- 1.4% following IL-1 administration) during post-injection hours 3-4.

CONCLUSIONS

Results of this study support the hypothesis that IL-1 can suppress REM sleep by acting at the level of the LDT nucleus. Furthermore this effect may result from the inhibition of evoked glutamatergic responses and of spontaneous firing of putative cholinergic LDT neurons.

Altered sleep architecture and higher incidence of subsyndromal depression in low endogenous melatonin secretors

Melatonin secretion is synchronized to the sleep/wake cycle and has been suggested to have somnogenic properties. Sleep/wake cycle disruption and alterations in the secretary pattern of melatonin is present in various psychiatric disorders. The objective of this study was to investigate the sleep architecture and the presence of depression in individuals with low endogenous melatonin levels. The study included 16 participants (mean age 30.3 +/- 14.9 years). The first night of testing included psychiatric evaluation followed by melatonin secretion profile evaluation by Dim Light Melatonin Onset test and then standard montage polysomnographic testing. On the second night, only polysomnographic testing was carried out with an imposed sleep period of 8 h. Low endogenous melatonin secretors (LEMS) showed no discernible peaks in melatonin secretion compared to normal secretors (controls). LEMS demonstrated significant alterations in rapid eye movement sleep but not in non-rapid eye movement sleep along with poor sleep initiation and quality compared to controls. 55.6% of the low melatonin secretors group presented with subsyndromal depression. Melatonin has significant bearing on sleep architecture and a lack of melatonin may desynchronize endogenous rhythms allowing subsyndromal depression to manifest.

The structure of the dorsal raphe nucleus and its relevance to the regulation of sleep and wakefulness

Serotonergic (5-HT) cells in the rat dorsal raphe nucleus (DRN) appear in topographically organized groups. Based on cellular morphology, expression of other neurotransmitters, afferent and efferent connections and functional properties, 5-HT neurons of the DRN have been grouped into six cell clusters. The subdivisions comprise the rostral, ventral, dorsal, lateral, caudal and interfascicular parts of the DRN. In addition to 5-HT cells, neurons containing γ-aminobutyric acid (GABA), glutamate, dopamine, nitric oxide and the neuropeptides corticotropin-releasing factor, substance P, galanin, cholecystokinin, neurotensin, somatostatin, vasoactive intestinal peptide, neuropeptide Y, thyrotropin-releasing hormone, growth hormone, leu-enkephalin, met-enkephalin and gastrin have been characterized in the DRN. Moreover, numerous brain areas have neurons that project to the DRN and express monoamines (norepinephrine, histamine), amino acids (GABA, glutamate), acetylcholine or neuropeptides (orexin, melanin-concentrating hormone, corticotropin-releasing factor and substance P) that directly or indirectly, through local circuits, regulate the activity of 5-HT cells. The 5-HT cells predominate along the midline of the rostral, dorsal and ventral subdivisions of the DRN and outnumber the non-5-HT cells occurring in the raphe nucleus. The GABAergic and glutamatergic neurons are clustered mainly in the lateral and dorsal subdivisions of the DRN, respectively. The 5-HT(1A) receptor is located on the soma and the dendrites of 5-HT neurons and at postsynaptic sites (outside the DRN). It is expressed, in addition, by non-5-HT cells of the DRN. The 5-HT(1B) receptor is located at presynaptic and postsynaptic sites (outside the boundaries of the DRN). It has been described also in the ventromedial DRN where it is expressed by non-5-HT cells. The 5-HT(2A) and 5-HT(2C) receptors are located within postsynaptic structures. At the level of the DRN the 5-HT(2A) and 5-HT(2C) receptor-containing cells are predominantly GABAergic interneurons and projection neurons. Within the boundaries of the DRN the 5-HT(3) receptor is expressed by, among others, glutamatergic interneurons. 5-HT(7) receptors in the DRN are not localized to serotonergic neurons but, at least in part, to GABAergic cells and terminals. The complex structure of the DRN may have important implications for neural mechanisms underlying 5-HT modulation of wakefulness and REM sleep.

Equivalent effects of acute tryptophan depletion on REM sleep in ecstasy users and controls

INTRODUCTION

This study sought to test the association between 3,4-methylenedioxymethamphetamine use, serotonergic function and sleep.

MATERIALS AND METHODS

Ambulatory polysomnography was used to measure three nights sleep in 12 ecstasy users and 12 controls after screening (no intervention), a tryptophan-free amino acid mixture (acute tryptophan depletion (ATD)) and a tryptophan-supplemented control mixture.

RESULTS

ATD significantly decreased rapid eye movement (REM) sleep onset latency, increased the amount of REM sleep and increased the amount of stage 2 sleep in the first 3 h of sleep. There was no difference between ecstasy users' and controls' sleep on the screening night or after ATD.

DISCUSSION

These findings imply that the ecstasy users had not suffered significant serotonergic damage as indexed by sleep.

Antagonism of serotonergic 5-HT2A/2C receptors: mutual improvement of sleep, cognition and mood?

Serotonin [5-hydroxytryptamine (5-HT)] and 5-HT receptors are involved in sleep and in waking functions such as cognition and mood. Animal and human studies support a particular role for the 5-HT(2A) receptor in sleep, which has led to renewed interest in this receptor subtype as a target for the development of novel pharmacological agents to treat insomnia. Focusing primarily on findings in healthy human volunteers, a review of the available data suggests that antagonistic interaction with 5-HT(2A) receptors (and possibly also 5-HT(2C) receptors) prolongs the duration of slow wave sleep and enhances low-frequency (< 7 Hz) activity in the sleep electroencephalogram (EEG), a widely accepted marker of sleep intensity. Despite certain differences, the changes in sleep and the sleep EEG appear to be remarkably similar to those of physiologically more intense sleep after sleep deprivation. It is currently unclear whether these changes in sleep are associated with improved vigilance, cognition and mood during wakefulness. While drug-induced interaction with sleep must be interpreted cautiously, too few studies are available to provide a clear answer to this question. Moreover, functional relationships between sleep and waking functions may differ between healthy controls and patients with sleep disorders. A multimodal approach investigating subjective and objective aspects of sleep and wakefulness provides a promising research avenue for shedding light on the complex relationships among 5-HT(2A/2C) receptor-mediated effects on sleep, the sleep EEG, cognition and mood in health and various diseases associated with disturbed sleep and waking functions.

Serotoninergic dorsal raphe neurons possess functional postsynaptic nicotinic acetylcholine receptors

Very few neurons in the telencephalon have been shown to express functional postsynaptic nicotinic acetylcholine receptors (nAChRs), among them, the noradrenergic and dopaminergic neurons. However, there is no evidence for postsynaptic nAChRs on serotonergic neurons. In this study, we asked if functional nAChRs are present in serotonergic (5-HT) and nonserotonergic (non-5-HT) neurons of the dorsal raphe nucleus (DRN). In rat midbrain slices, field stimulation at the tegmental pedunculopontine (PPT) nucleus evoked postsynaptic currents (eEPSCs) with different components in DRN neurons. After blocking the glutamatergic and GABAergic components, the remaining eEPSCs were blocked by mecamylamine and reduced by either the selective alpha7 nAChR antagonist methyllycaconitine (MLA) or the selective alpha4beta2 nAChR antagonist dihydro-beta-eritroidine (DHbetaE). Simultaneous addition of MLA and DHbetaE blocked all eEPSCs. Integrity of the PPT-DRN pathway was assessed by both anterograde biocytin tracing and antidromic stimulation from the DRN. Inward currents evoked by the direct application of acetylcholine (ACh), in the presence of atropine and tetrodotoxin, consisted of two kinetically different currents: one was blocked by MLA and the other by DHbetaE; in both 5-HT and non-5-HT DR neurons. Analysis of spontaneous (sEPSCs) and evoked (eEPSCs) synaptic events led to the conclusion that nAChRs were located at the postsynaptic membrane. The possible implications of these newly described nAChRs in various physiological processes and behavioral events, such as the wake-sleep cycle, are discussed.

Dual orexin actions on dorsal raphe and laterodorsal tegmentum neurons: noisy cation current activation and selective enhancement of Ca2+ transients mediated by L-type calcium channels

The hypocretin/orexins (Hcrt/Orxs) are hypothalamic neuropeptides that regulate stress, addiction, feeding, and arousal behaviors. They depolarize many types of central neurons and can increase [Ca2+]i in some, including those of the dorsal raphe (DR) and laterodorsal tegmental (LDT) nuclei-two structures likely to contribute to the behavioral actions of Hcrt/Orx. In this study, we used simultaneous whole cell and Ca2+-imaging methods in mouse brain slices to compare the Hcrt/Orx-activated current in DR and LDT neurons and to determine whether it contributes to the Ca2+ influx evoked by Hcrt/Orx. We found Hcrt/Orx activates a similar noisy cation current that reversed near 0 mV in both cell types. Contrary to our expectation, this current did not contribute to the somatic Ca2+ influx evoked by Hcrt/Orx. In contrast, Hcrt/Orx enhanced the Ca2+ transients produced by voltage steps (-60 to -30 mV) by approximately 30% even in neurons lacking an inward current. This effect was abolished by nifedipine, augmented by Bay-K and abolished by bisindolylmaleimide I. Thus Hcrt/Orx has two independent actions: activation of noisy cation channels that generate depolarization and activation of a protein kinase C (PKC)-dependent enhancement of Ca2+ transients mediated by L-type Ca2+ channels. Immunocytochemistry verified that both these actions occurred in serotonergic and cholinergic neurons, indicating that Hcrt/Orx can function as a neuromodulator in these key neurons of the reticular activating system. Because regulation of Ca2+ transients mediated by L-channels is often linked to the control of transcriptional signaling, our findings imply that Hcrt/Orxs may also function in the regulation of long-term homeostatic or trophic processes.

Increased extracellular 5-HT but no change in sleep after perfusion of a 5-HT1A antagonist into the dorsal raphe nucleus of rats

AIM

The 5-HT(1A) receptor antagonist 4-Iodo-N-[2-[4-(methoxyphenyl)-1-piperazinyl]ethyl]-N-2-pyridinyl-benzamide hydrochloride (p-MPPI) (10 microM) was perfused into the dorsal raphe nucleus (DRN) to study simultaneously the effects of the drug on the DRN and frontal cortex extracellular serotonin (5-hydroxytryptamine, 5-HT) levels and concurring behavioural states.

METHODS

Waking, slow wave sleep and rapid eye movement sleep were determined by polygraphic recordings during microdialysis perfusion and extracellular sample collection. The samples were analysed by microbore high-performance liquid chromatography coupled with electrochemical detection for analysis of 5-HT.

RESULTS

p-MPPI perfusion into the DRN (n = 6) produced a sixfold 5-HT increase in the DRN during all behavioural states. The increased 5-HT level was most likely related to the blockage of 5-HT(1A) receptors in the DRN by p-MPPI. No significant effect was seen on sleep.

CONCLUSION

Despite the dramatic increase in DRN extracellular 5-HT produced by p-MPPI, only a transient and nonsignificant effect on sleep was recorded. It is suggested that the usual coupling between 5-HT level and behavioural state may be lost when an excessive serotonergic output is pharmacologically achieved.

The developmental decrease in REM sleep: the role of transmitters and electrical coupling

STUDY OBJECTIVES

This mini-review considers certain factors related to the developmental decrease in rapid eye movement (REM) sleep, which occurs in favor of additional waking time, and its relationship to developmental factors that may influence its potential role in brain development.

DESIGN

Specifically, we discuss some of the theories proposed for the occurrence of REM sleep and agree with the classic notion that REM sleep is, at the least, a mechanism that may play a role in the maturation of thalamocortical pathways. The developmental decrease in REM sleep occurs gradually from birth until close to puberty in the human, and in other mammals it is brief and coincides with eye and ear opening and the beginning of massive exogenous activation. Therefore, the purported role for REM sleep may change to involve a number of other functions with age.

MEASUREMENTS AND RESULTS

We describe recent findings showing that morphologic and physiologic properties as well as cholinergic, gamma amino-butyric acid, kainic acid, n-methyl-d-aspartic acid, noradrenergic, and serotonergic synaptic inputs to mesopontine cholinergic neurons, as well as the degree of electrical coupling between mostly noncholinergic mesopontine neurons and levels of the neuronal gap-junction protein connexin 36, change dramatically during this critical period in development. A novel mechanism for sleep-wake control based on well-known transmitter interactions, as well as electrical coupling, is described.

CONCLUSION

We hypothesize that a dysregulation of this process could result in life-long disturbances in arousal and REM sleep drive, leading to hypervigilance or hypovigilance such as that observed in a number of disorders that have a mostly postpubertal age of onset.

Sleep disturbances and suicide risk: A review of the literature

A growing body of research indicates that sleep disturbances are associated with suicidal ideation and behaviors. This article (1) provides a critical review of the extant literature on sleep and suicidality and (2) addresses shared underlying neurobiological factors, biological and social zeitgebers, treatment implications, and future directions for research. Findings indicate that suicidal ideation and behaviors are closely associated with sleep complaints, and in some cases, this association exists above and beyond depression. Several cross-sectional investigations indicate a unique association between nightmares and suicidal ideation, whereas the relationship between insomnia and suicidality requires further study. Underlying neurobiological factors may, in part, account for the relationship between sleep and suicide. Serotonergic neurotransmission appears to play a critical role in both sleep and suicide. Finally, it remains unclear whether or not sleep-oriented interventions may reduce risk for suicidal behaviors. Unlike other suicide risk factors, sleep complaints may be particularly amenable to treatment. As a warning sign, disturbances in sleep may thus be especially useful to research and may serve as an important clinical target for future suicide intervention efforts.

Neurobiology of REM and NREM sleep

This paper presents an overview of the current knowledge of the neurophysiology and cellular pharmacology of sleep mechanisms. It is written from the perspective that recent years have seen a remarkable development of knowledge about sleep mechanisms, due to the capability of current cellular neurophysiological, pharmacological and molecular techniques to provide focused, detailed, and replicable studies that have enriched and informed the knowledge of sleep phenomenology and pathology derived from electroencephalographic (EEG) analysis. This chapter has a cellular and neurophysiological/neuropharmacological focus, with an emphasis on rapid eye movement (REM) sleep mechanisms and non-REM (NREM) sleep phenomena attributable to adenosine. The survey of neuronal and neurotransmitter-related brainstem mechanisms of REM includes monoamines, acetylcholine, the reticular formation, a new emphasis on GABAergic mechanisms and a discussion of the role of orexin/hypcretin in diurnal consolidation of REM sleep. The focus of the NREM sleep discussion is on the basal forebrain and adenosine as a mediator of homeostatic control. Control is through basal forebrain extracellular adenosine accumulation during wakefulness and inhibition of wakefulness-active neurons. Over longer periods of sleep loss, there is a second mechanism of homeostatic control through transcriptional modification. Adenosine acting at the A1 receptor produces an up-regulation of A1 receptors, which increases inhibition for a given level of adenosine, effectively increasing the gain of the sleep homeostat. This second mechanism likely occurs in widespread cortical areas as well as in the basal forebrain. Finally, the results of a new series of experimental paradigms in rodents to measure the neurocognitive effects of sleep loss and sleep interruption (modeling sleep apnea) provide animal model data congruent with those in humans.

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.

How best to consider the structure and function of the pedunculopontine tegmental nucleus: Evidence from animal studies

This review presents the hypothesis that the best way to consider the pedunculopontine tegmental nucleus is by analogy with the substantia nigra. The substantia nigra contains two main compartments: the pars compacta and the pars reticulata. The former contains dopamine neurons that project widely within the basal ganglia while the latter is in receipt of corticostriatal output. Similarly, the PPTg contains the Ch5 acetylcholine containing neurons that project to the thalamus and corticostriatal systems (notably the pars compacta of substantia nigra and the subthalamic nucleus) while the non-cholinergic neurons of the pedunculopontine are in receipt of corticostriatal output. Assessment of the location, composition and connections of the pedunculopontine tegmental nucleus is made to support the hypothesis that it has structural similarities with substantia nigra. Assessment of the motor, sensory and cognitive functions of the pedunculopontine is also made, suggesting functional similarities exist also. Having a clear model of pedunculopontine structure and function is a matter of some importance. It is clearly involved in Parkinson's disease and could potentially be a target for therapeutic intervention. If this is to be realized it will be best to have as clear an understanding as possible of pedunculopontine structure and function in order to maximize positive benefits.

Electrophysiological characterization of neurons in the dorsolateral pontine rapid-eye-movement sleep induction zone of the rat: Intrinsic membrane properties and responses to carbachol and orexins

Pharmacological, lesion and single-unit recording techniques in several animal species have identified a region of the pontine reticular formation (subcoeruleus, SubC) just ventral to the locus coeruleus as critically involved in the generation of rapid-eye-movement (REM) sleep. However, the intrinsic membrane properties and responses of SubC neurons to neurotransmitters important in REM sleep control, such as acetylcholine and orexins/hypocretins, have not previously been examined in any animal species and thus were targeted in this study. We obtained whole-cell patch-clamp recordings from visually identified SubC neurons in rat brain slices in vitro. Two groups of large neurons (mean diameter 30 and 27 mum) were tentatively identified as cholinergic (rostral SubC) and noradrenergic (caudal SubC) neurons. SubC reticular neurons (non-cholinergic, non-noradrenergic) showed a medium-sized depolarizing sag during hyperpolarizing current pulses and often had a rebound depolarization (low-threshold spike, LTS). During depolarizing current pulses they exhibited little adaptation and fired maximally at 30-90 Hz. Those SubC reticular neurons excited by carbachol (n=27) fired spontaneously at 6 Hz, often exhibited a moderately sized LTS, and varied widely in size (17-42 mum). Carbachol-inhibited SubC reticular neurons were medium-sized (15-25 mum) and constituted two groups. The larger group (n=22) was silent at rest and possessed a prominent LTS and associated one to four action potentials. The second, smaller group (n=8) had a delayed return to baseline at the offset of hyperpolarizing pulses. Orexins excited both carbachol excited and carbachol inhibited SubC reticular neurons. SubC reticular neurons had intrinsic membrane properties and responses to carbachol similar to those described for other reticular neurons but a larger number of carbachol inhibited neurons were found (>50%), the majority of which demonstrated a prominent LTS and may correspond to pontine-geniculate-occipital burst neurons. Some or all carbachol-excited neurons are presumably REM-on neurons.

Transmitter modulation of spike-evoked calcium transients in arousal related neurons: muscarinic inhibition of SNX-482-sensitive calcium influx

Nitric oxide synthase (NOS)-containing cholinergic neurons in the laterodorsal tegmentum (LDT) influence behavioral and motivational states through their projections to the thalamus, ventral tegmental area and a brainstem 'rapid eye movement (REM)-induction' site. Action potential-evoked intracellular calcium transients dampen excitability and stimulate NO production in these neurons. In this study, we investigated the action of several arousal-related neurotransmitters and the role of specific calcium channels in these LDT Ca(2+)-transients by simultaneous whole-cell recording and calcium imaging in mouse (P14-P30) brain slices. Carbachol, noradrenaline and adenosine inhibited spike-evoked Ca(2+)-transients, while histamine, t-ACPD, a metabotropic glutamate receptor agonist, and orexin-A did not. Carbachol inhibition was blocked by atropine, was insensitive to blockade of G-protein-coupled inward rectifier (GIRK) channels and was not inhibited by nifedipine, omega-conotoxin GVIA or omega-agatoxin IVA, which block L-, N- and P/Q-type calcium channels, respectively. In contrast, SNX-482 (100 nm), a selective antagonist of R-type calcium channels containing the alpha1E (Cav2.3) subunit, attenuated carbachol inhibition of the somatic spike-evoked calcium transient. To our knowledge, this is the first demonstration of muscarinic inhibition of native SNX-482-sensitive R-channels. Our findings indicate that muscarinic modulation of these channels plays an important role in the feedback control of cholinergic LDT neurons and that inhibition of spike-evoked Ca(2+)-transients is a common action of neurotransmitters that also activate GIRK channels in these neurons. Because spike-evoked calcium influx dampens excitability, our findings suggest that these 'inhibitory' transmitters could boost firing rate and enhance responsiveness to excitatory inputs during states of high firing, such as waking and REM sleep.

Pedunculopontine neurons are involved in network changes in the kindling model of temporal lobe epilepsy

It is well known that epileptogenesis is associated with widespread neuronal network changes in brain regions adjacent to the seizure focus but also in remote structures including basal ganglia. Besides the superior colliculus, the pedunculopontine tegmental nucleus (PPN) is one of three main target regions of basal ganglia output activity and is reciprocally connected with the substantia nigra pars reticulata (SNr), which is critically involved in seizure propagation and manipulation. We here tested the hypothesis if, apart from the traditional view that the superior colliculus mediates seizure-gating mechanisms of the SNr, the PPN is involved in kindling-induced network changes. Rats were electrically kindled via the ipsilateral basolateral amygdala. In vivo extracellular single unit recordings of right PPN neurons were performed in kindled rats 1 day after a generalized seizure in order to examine kindling-associated rather than seizure-associated activity changes. The main findings of the study were (1) a seizure-outlasting drastically reduced firing rate of PPN neurons and (2) an increase in burst and irregular firing pattern in kindled rats compared with sham-kindled and naïve controls. These changes are likely to be caused by an altered inhibitory input from the SNr. Furthermore, kindling caused (3) the oscillation frequency of PPN neurons to shift towards lower frequencies. The kindling-induced activity changes were found to be anatomically restricted to the PPN, indicating that network changes follow distinct anatomical routes. We demonstrated that the PPN is strongly affected by the functional reorganization of neurocircuitry associated with kindling. The underlying mechanisms are discussed. The findings are relevant for a better understanding of kindling-associated network changes and might provide new targets for therapeutic manipulations in epilepsies.

Neuroimaging and sleep medicine

In sleep medicine, patients with sleep disorders are evaluated and treated. The primary assessment tool of the field has traditionally been polysomnography. While polysomnography has been helpful in the evaluation of some sleep disorders, such as sleep apnea syndrome and periodic limb movement disorder, it has been less helpful in others, such as the insomnias, or sleep disorders secondary to mental disorders. These disorders are presumed to stem from some alteration in brain function that disrupts sleep. The development of functional neuroimaging methods provides a means to understand brain function in patients with sleep disorders in a manner not accessible to polysomnography. This paper summarizes functional neuroimaging findings during healthy sleep, then, reviews available studies in sleep disorders patients, and studies addressing the pharmacology of sleep and sleep disorders. Areas in which functional neuroimaging methods may be helpful in sleep medicine, and in which future development is advised, include: (1) clarification of pathophysiology; (2) aid in differential diagnosis; (3) assessment of treatment response; (4) guiding new drug development; and (5) monitoring treatment response.

Calcium influx through presynaptic 5-HT3 receptors facilitates GABA release in the hippocampus: in vitro slice and synaptosome studies

Serotonin 5-hydroxytryptamine type 3 receptors (5HT3R) are Ca2+-permeant, non-selective cation channels that have been localized to presynaptic terminals and demonstrated to modulate neurotransmitter release. In the present study the effect of 5-HT on GABA release in the hippocampus was characterized using both electrophysiological and biochemical techniques. 5-HT elicited a burst-like, 6- to 10-fold increase in the frequency of GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) measured with whole-cell voltage-clamp recordings of CA1 neurons in hippocampal slices. When tetrodotoxin was used to block action potential propagation, the 5-HT-induced burst of IPSCs was still observed. Stimulation of hippocampal synaptosomes with 5-HT resulted in a significant increase in the amount of [3H]GABA released by hyperosmotic saline. In both preparations, the 5-HT effect was shown to be mediated by 5HT3Rs, as it was mimicked by the selective 5HT3R agonist m-chlorophenyl biguanide and blocked by the selective 5HT3R antagonist 3-tropanylindole-3-carboxylate hydrochloride. The 5HT3R-mediated increase in GABA release was blocked by 100 microM cadmium or by omitting Ca2+ in external solutions, indicating the Ca2+-dependence of the effect. The high voltage-activated Ca2+ channel blockers omega-conotoxin GVIA and omega-conotoxin MVIIC and 10 microM cadmium had no significant effect on the 5-HT3R-mediated enhancement of GABA release, indicating that Ca2+ influx through the 5-HT3R facilitates GABA release. Taken together, these data provide direct evidence that Ca2+ entry via presynaptic 5HT3Rs facilitates the release of GABA from hippocampal interneurons.

Pontine cholinergic mechanisms and their impact on respiratory regulation

Activation of pontomedullary cholinergic neurons may directly and indirectly cause depression of respiratory motoneuronal activity, activation of respiratory premotor neurons and acceleration of the respiratory rate during REM sleep, as well as activation of breathing during active wakefulness. These effects may be mediated by distinct subpopulations of cholinergic neurons. The relative inactivity of cholinergic neurons during slow-wave sleep also may contribute to the depressant effects of this state on breathing. Cholinergic muscarinic and nicotinic receptors are expressed in central respiratory neurons and motoneurons, thus allowing cholinergic neurons to act on the respiratory system directly. Additional effects of cholinergic activation are mediated indirectly by noradrenergic, serotonergic and other neurons of the reticular formation. Excitatory and suppressant respiratory effects with features of natural states of REM sleep or active wakefulness can be elicited in urethane-anesthetized rats by pontine microinjections of the cholinergic agonist, carbachol. Carbachol models help elucidate the neural basis of respiratory disorders associated with central cholinergic activation.

The human raphe nuclei and the serotonergic system

The raphe nuclei are distributed near the midline of the brainstem along its entire rostro-caudal extension. The serotonergic neurons are their main neuronal components, although a proportion of them lie in subdivisions of the lateral reticular formation. They develop from mesopontine and medullary primordia, and the resulting grouping into rostral and caudal clusters is maintained into adulthood, and is reflected in the connectivity. Thus, the mesencephalon and rostral pons, neurons within the rostral raphe complex (caudal linear, dorsal raphe, and median raphe nuclei) project primarily to the forebrain. By contrast, in the caudal pons and medulla oblongata, neurons within the caudal raphe complex (raphe magnus, raphe obscurus, raphe pallidus nuclei and parts of the adjacent lateral reticular formation) project to the brainstem nuclei and to the spinal cord. The median raphe and dorsal raphe nuclei provide parallel and overlapping projections to many forebrain structures with axon fibers exhibiting distinct structural and functional characteristics. The caudal group of the serotonergic system projects to the brainstem, and, by three parallel projections, to the dorsal, intermediate and ventral columns in the spinal cord. The serotonergic axons arborize over large areas comprising functionally diverse targets. Some projections form classical chemical synapses while many do not, thus contributing to the so-called paracrine or volume transmission. The serotonergic projections participate in the regulation of different functional (motor, somatosensory, limbic) systems; and have been associated with a wide range of neuropsychiatric and neurological disorders. Finally, recent experimental data support the role of serotonin in modulating brain development, such that a dysfunction in serotonergic transmission during early life could lead to long lasting structural and functional alterations.

A serotonergic (5-HT2) receptor mechanism in the laterodorsal tegmental nucleus participates in regulating the pattern of rapid-eye-movement sleep occurrence in the rat

Serotonin [5-hydroxytryptamine (5-HT)] plays an inhibitory role in rapid-eye-movement (REM) sleep although the exact mechanism(s) and site(s) of action are not known. It is commonly assumed that 5-HT exerts its influence on REM sleep via input from the dorsal raphe nucleus (DRN) directly onto cholinergic neurons involved in the generation of REM sleep. 5-HT(2) receptor sites have been found on cholinergic neurons in the laterodorsal tegmental nucleus (LDT) and pedunculopontine tegmental nucleus (PPT). We locally microinjected the 5-HT(2) agonist DOI ((+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl) and the 5-HT(2) antagonist, ketanserin, in LDT in rats to determine whether these receptor sites are involved in the regulation of behavioral states. DOI and ketanserin primarily affected REM sleep, by significantly decreasing or increasing, respectively, the number, but not the duration, of REM sleep episodes. DOI specifically decreased the occurrence of clusters of REM sleep episodes appearing at intervals less than or equal to 3 min (sequential episodes) without affecting single episodes separated by more than 3 min. An opposite effect of ketanserin on REM sleep clusters, although not statistically significant, was observed.

Acetylcholine systems and rhythmic activities during the waking–sleep cycle

The two processes of activation in thalamocortical systems exerted by mesopontine cholinergic neurons are (a) a direct depolarization associated with increased input resistance of thalamic relay neurons, which is antagonized by muscarinic blockers, and (b) a disinhibition of the same neurons via hyperpolarization of inhibitory thalamic reticular neurons. Low-frequency (< 15 Hz) oscillations during slow-wave sleep, characterized by rhythmic and prolonged hyperpolarizations, are suppressed by brainstem cholinergic neurons and nucleus basalis cholinergic and GABAergic neurons projecting to thalamic reticular neurons. Fast rhythms (20-60 Hz) appear during the sustained depolarization of thalamic and neocortical neurons during brain-active states that are accompanied by increased release of acetylcholine (ACh) in the thalamus and cerebral cortex. Such fast rhythms also occur during the depolarizing phases of the slow oscillation (0.5-1 Hz) in non-REM sleep. Intracellular recordings of neocortical neurons during natural states of waking and sleep demonstrate stable and increased input resistance of corticocortical and corticothalamic neurons during the sustained depolarization in wakefulness, compared to the depolarizing phase of the slow oscillation in non-REM sleep. Despite the highly increased synaptic inputs along different afferent systems that open many conductances of cortical neurons during wakefulness, the increased input resistance is attributed to the effect of acetylcholine on cortical neurons.