Firing properties of two types of nucleus raphe dorsalis neurons during the sleep-waking cycle and their responses to sensory stimuli

Selective activation of serotoninergic dorsal raphe neurons facilitates sleep through anxiolysis

A role for the brain's serotoninergic (5HT) system in the regulation of sleep and wakefulness has been long suggested. Yet, previous studies employing pharmacological, lesion and genetically driven approaches have produced inconsistent findings, leaving 5HT's role in sleep-wake regulation incompletely understood. Here we sought to define the specific contribution of 5HT neurons within the dorsal raphe nucleus (DRN5HT) to sleep and arousal control. To do this, we employed a chemogenetic strategy to selectively and acutely activate DRN5HT neurons and monitored sleep-wake using electroencephalogram recordings. We additionally assessed indices of anxiety using the open field and elevated plus maze behavioral tests and employed telemetric-based recordings to test effects of acute DRN5HT activation on body temperature and locomotor activity. Our findings indicate that the DRN5HT cell population may not modulate sleep-wake per se, but rather that its activation has apparent anxiolytic properties, suggesting the more nuanced view that DRN5HT neurons are sleep permissive under circumstances that produce anxiety or stress.

Dorsal raphe nucleus projecting retinal ganglion cells: Why Y cells?

Retinal ganglion Y (alpha) cells are found in retinas ranging from frogs to mice to primates. The highly conserved nature of the large, fast conducting retinal Y cell is a testament to its fundamental task, although precisely what this task is remained ill-defined. The recent discovery that Y-alpha retinal ganglion cells send axon collaterals to the serotonergic dorsal raphe nucleus (DRN) in addition to the lateral geniculate nucleus (LGN), medial interlaminar nucleus (MIN), pretectum and the superior colliculus (SC) has offered new insights into the important survival tasks performed by these cells with highly branched axons. We propose that in addition to its role in visual perception, the Y-alpha retinal ganglion cell provides concurrent signals via axon collaterals to the DRN, the major source of serotonergic afferents to the forebrain, to dramatically inhibit 5-HT activity during orientation or alerting/escape responses, which dis-facilitates ongoing tonic motor activity while dis-inhibiting sensory information processing throughout the visual system. The new data provide a fresh view of these evolutionarily old retinal ganglion cells.

Functional connectivity of the dorsal and median raphe nuclei at rest

Serotonin (5-HT) is a neurotransmitter critically involved in a broad range of brain functions and implicated in the pathophysiology of neuropsychiatric illnesses including major depression, anxiety and sleep disorders. Despite being widely distributed throughout the brain, there is limited knowledge on the contribution of 5-HT to intrinsic brain activity. The dorsal raphe (DR) and median raphe (MR) nuclei are the source of most serotonergic neurons projecting throughout the brain and thus provide a compelling target for a seed-based probe of resting-state activity related to 5-HT. Here we implemented a novel multimodal neuroimaging approach for investigating resting-state functional connectivity (FC) between DR and MR and cortical, subcortical and cerebellar target areas. Using [(11)C]DASB positron emission tomography (PET) images of the brain serotonin transporter (5-HTT) combined with structural MRI from 49 healthy volunteers, we delineated DR and MR and performed a seed-based resting-state FC analysis. The DR and MR seeds produced largely similar FC maps: significant positive FC with brain regions involved in cognitive and emotion processing including anterior cingulate, amygdala, insula, hippocampus, thalamus, basal ganglia and cerebellum. Significant negative FC was observed within pre- and postcentral gyri for the DR but not for the MR seed. We observed a significant association between DR and MR FC and regional 5-HTT binding. Our results provide evidence for a resting-state network related to DR and MR and comprising regions receiving serotonergic innervation and centrally involved in 5-HT related behaviors including emotion, cognition and reward processing. These findings provide a novel advance in estimating resting-state FC related to 5-HT signaling, which can benefit our understanding of its role in behavior and neuropsychiatric illnesses.

Stress, serotonin, and hippocampal neurogenesis in relation to depression and antidepressant effects

Chronic stressful life events are risk factors for developing major depression, the pathophysiology of which is strongly linked to impairments in serotonin (5-HT) neurotransmission. Exposure to chronic unpredictable stress (CUS) has been found to induce depressive-like behaviours, including passive behavioural coping and anhedonia in animal models, along with many other affective, cognitive, and behavioural symptoms. The heterogeneity of these symptoms represents the plurality of corticolimbic structures involved in mood regulation that are adversely affected in the disorder. Chronic stress has also been shown to negatively regulate adult hippocampal neurogenesis, a phenomenon that is involved in antidepressant effects and regulates subsequent stress responses. Although there exists an enormous body of data on stress-induced alterations of 5-HT activity, there has not been extensive exploration of 5-HT adaptations occurring presynaptically or at the level of the raphe nuclei after exposure to CUS. Similarly, although hippocampal neurogenesis is known to be negatively regulated by stress and positively regulated by antidepressant treatment, the role of neurogenesis in mediating affective behaviour in the context of stress remains an active area of investigation. The goal of this review is to link the serotonergic and neurogenic hypotheses of depression and antidepressant effects in the context of stress. Specifically, chronic stress significantly attenuates 5-HT neurotransmission and 5-HT1A autoreceptor sensitivity, and this effect could represent an endophenotypic hallmark for mood disorders. In addition, by decreasing neurogenesis, CUS decreases hippocampal inhibition of the hypothalamic-pituitary-adrenal (HPA) axis, exacerbating stress axis overactivity. Similarly, we discuss the possibility that adult hippocampal neurogenesis mediates antidepressant effects via the ventral (in rodents; anterior in humans) hippocampus' influence on the HPA axis, and mechanisms by which antidepressants may reverse chronic stress-induced 5-HT and neurogenic changes. Although data are as yet equivocal, antidepressant modulation of 5-HT neurotransmission may well serve as one of the factors that could drive neurogenesis-dependent antidepressant effects through these stress regulation-related mechanisms.

Optogenetic drive of neocortical pyramidal neurons generates fMRI signals that are correlated with spiking activity

Local fluctuations in the blood oxygenation level-dependent (BOLD) signal serve as the basis of functional magnetic resonance imaging (fMRI). Understanding the correlation between distinct aspects of neural activity and the BOLD response is fundamental to the interpretation of this widely used mapping signal. Analysis of this question requires the ability to precisely manipulate the activity of defined neurons. To achieve such control, we combined optogenetic drive of neocortical neurons with high-resolution (9.4 T) rodent fMRI and detailed analysis of neurophysiological data. Light-driven activation of pyramidal neurons resulted in a positive BOLD response at the stimulated site. To help differentiate the neurophysiological correlate(s) of the BOLD response, we employed light trains of the same average frequency, but with periodic and Poisson distributed pulse times. These different types of pulse trains generated dissociable patterns of single-unit, multi-unit and local field potential (LFP) activity, and of BOLD signals. The BOLD activity exhibited the strongest correlation to spiking activity with increasing rates of stimulation, and, to a first approximation, was linear with pulse delivery rate, while LFP activity showed a weaker correlation. These data provide an example of a strong correlation between spike rate and the BOLD response. This article is part of a Special Issue entitled Optogenetics (7th BRES).

A new theoretical approach to the functional meaning of sleep and dreaming in humans based on the maintenance of 'predictive psychic homeostasis'

Different theories have been put forward during the last decade to explain the functional meaning of sleep and dreaming in humans. In the present paper, a new theory is presented which, while taking advantage of these earlier theories, introduces the following new and original aspects:   • Circadian rhythms relevant to various organs of the body affect the reciprocal interactions which operate to maintain constancy of the internal milieu and thereby also affect the sleep/wakefulness cycle. Particular attention is given to the constancy of natraemia and osmolarity and to the permissive role that the evolution of renal function has had for the evolution of the central nervous system and its integrative actions. • The resetting of neuro-endocrine controls at the onset of wakefulness leads to the acquisition of new information and its integration within previously stored memories. This point is dealt with in relation to Moore-Ede's proposal for the existence of a 'predictive homeostasis'. • The concept of 'psychic homeostasis' is introduced and is considered as one of the most important states since it is aimed at the well-being, or eudemonia, of the human psyche. Sleep and dreaming in humans are discussed as important functions for the maintenance of a newly proposed composite state: that of 'predictive psychic homeostasis'. On the basis of these assumptions, and in accordance with the available neurobiological data, the present paper puts forward the novel hypothesis that sleep and dreaming play important functions in humans by compensating for psychic allostatic overloads. Hence, both consolatory dreams and disturbing nightmares can be part of the vis medicatrix naturae, the natural healing power, in this case, the state of eudemonia.

Spike-timing relationship of neurochemically-identified dorsal raphe neurons during cortical slow oscillations

The firing activity of dorsal raphe neurons is related to arousal state. However, it is unclear how this firing activity is precisely related to cortical activity, in particular oscillations occurring during sleep rhythms. Here we conducted single-cell extracellular recordings and juxtacellular labelling while monitoring electrocorticogram (ECoG) activity in urethane anaesthetised rats, to relate activity in neurochemically identified groups of neurons to cortical slow-wave activity (SWA). We observed that electrophysiological heterogeneity in dorsal raphe neurons revealed different neurochemical groups of DRN neurons and was mirrored by significant differences in the phase and strength of coupling to the cortical slow oscillations. Spike firing relationship of clock-like neurons, identified as 5-HT (5-hydroxytryptamine) or serotonin neurons, was higher during the inactive component of the oscillations. In contrast, half of the identified bursting 5-HT neurons did not exhibit strong cortical entrainment; those that did fired most during the inactive component of the SWA. Two groups of putatively non-5-HT neurons (irregular slow-firing and fast-firing) exhibited significant coherence and fired most during the active component of the SWA. These findings indicate that within the DRN electrophysiologically and neurochemically discrete neuronal groups exhibit distinct relations to cortical activity.

Sleep and Emotional Memory Processing

Cognitive neuroscience continues to build meaningful connections between affective behavior and human brain function. Within the biological sciences, a similar renaissance has taken place, focusing on the role of sleep in various neurocognitive processes, and most recently, the interaction between sleep and emotional regulation. In this article, we survey an array of diverse findings across basic and clinical research domains, resulting in a convergent view of sleep-dependent emotional brain processing. Based on the unique neurobiology of sleep, we outline a model describing the overnight modulation of affective neural systems and the (re)processing of recent emotional experiences, both of which appear to redress the appropriate next-day reactivity of limbic and associated autonomic networks. Furthermore, a REM sleep hypothesis of emotional-memory processing is proposed, the implications of which may provide brain-based insights into the association between sleep abnormalities and the initiation and maintenance of mood disturbances.

Overnight therapy? The role of sleep in emotional brain processing

Cognitive neuroscience continues to build meaningful connections between affective behavior and human brain function. Within the biological sciences, a similar renaissance has taken place, focusing on the role of sleep in various neurocognitive processes and, most recently, on the interaction between sleep and emotional regulation. This review surveys an array of diverse findings across basic and clinical research domains, resulting in a convergent view of sleep-dependent emotional brain processing. On the basis of the unique neurobiology of sleep, the authors outline a model describing the overnight modulation of affective neural systems and the (re)processing of recent emotional experiences, both of which appear to redress the appropriate next-day reactivity of limbic and associated autonomic networks. Furthermore, a rapid eye movement (REM) sleep hypothesis of emotional-memory processing is proposed, the implications of which may provide brain-based insights into the association between sleep abnormalities and the initiation and maintenance of mood disturbances.

Expression of TRPM8 in the distal cerebrospinal fluid-contacting neurons in the brain mesencephalon of rats

Background

It has been shown that distal cerebrospinal fluid-contacting neurons (dCSF-CNs) exist near the ventral midline of the midbrain aqueduct and also in the grey matter of the inferior third ventricle and the fourth ventricle floor in the superior segment of the pons. The dCSF-CNs communicate between the cerebrospinal fluid (CSF) and the brain parenchyma and may participate in the transduction and regulation of pain signals. The cold sensation receptor channel, TRPM8 is involved in analgesia for neuropathic pain, but whether the TRPM8 receptor exists on dCSF-CNs remains unknown. However, there is preliminary evidence that TRPM8 is expressed in dCSF-CNs and may participate in the transmission and regulation of sensory information between brain parenchyma and cerebrospinal fluid (CSF) in rats.

Methods

Retrograde tracing of the cholera toxin subunit B labeled with horseradish peroxidase (CB-HRP) injected into the lateral ventricle was used to identify dCSF-CNs. A double-labeled immunofluorescent technique and laser scanning confocal microscopy were used to identify the expression of TRPM8 in dCSF-CNs. Software Image-Pro Plus was used to count the number of neurons in three sections where CB-HRP positive neurons were located in the mesencephalon of six rats.

Results

The cell bodies of CB-HRP-positive dCSF-CNs were found in the brain parenchyma near the midline of the ventral Aq, also in the grey of the 3V, and the 4V floor in the superior segment of the pons. In the mesencephalon their processes extended into the CSF. TRPM8 labeled neurons were also found in the same area as were CB-HRP/TRPM8 double-labeled neurons. CB-HRP/TRPM8 double-labeled neurons were found in 42.9 ± 2.3% of neurons labeled by TRPM8, and all CB-HRP-labeled neurons were also labeled with TPRM8.

Conclusion

This study has demonstrated that the cold sensation receptor channel, TRPM8, is localised within the dCSF-CNs of the mesencephalon. TRPM8 acts as receptor of dCSF-CNs for sensation transmission and pain regulation.

Projections from the vestibular nuclei and nucleus prepositus hypoglossi to dorsal raphe nucleus in rats

The serotonergic system regulates processing in components of the vestibular nuclear complex, including the medial vestibular nucleus (MVe) and nucleus prepositus hypoglossi (PH). Recent studies using anterograde and retrograde tracers have shown that vestibular nuclei are targeted by regionally selective projections from the serotonergic dorsal raphe nucleus. The objective of the present investigation was to determine whether the DRN is targeted by projections from the vestibular nuclear complex in rats, using the anterograde tracer biotinylated dextran amine (BDA). After injection of BDA into PH or the caudal parvicellular division of MVe, labeled fibers and terminals were observed in the ventromedial and lateral subdivisions of DRN. These findings indicate that projections from the vestibular nuclei and PH are organized to modulate processing within specific functional domains of the DRN.

Selective anterograde tracing of nonserotonergic projections from dorsal raphe nucleus to the basal forebrain and extended amygdala

The dorsal raphe nucleus (DRN) contains both serotonergic and nonserotonergic projection neurons. Retrograde tracing studies have demonstrated that components of the basal forebrain and extended amygdala are targeted heavily by input from nonserotonergic DRN neurons. The object of this investigation was to examine the terminal distribution of nonserotonergic DRN projections in the basal forebrain and extended amygdala, using a technique that allows selective anterograde tracing of nonserotonergic DRN projections. To trace nonserotonergic DRN projections, animals were pretreated with nomifensine, desipramine and the serotonergic neurotoxin 5,7-dihydroxytryptamine (5,7-DHT), 7 days prior to placing an iontophoretic injection of biotinylated dextran amine (BDA) into the DRN. In animals treated with 5,7-DHT, numerous nonserotonergic BDA-labeled fibers ascended to the basal forebrain in the medial forebrain bundle system. Some of these labeled fibers crossed through the lateral hypothalamus, bed nucleus of the stria terminalis, and substantial innominata. These fibers entered the amygdala through the ansa peduncularis and ramified within the central and basolateral amygdaloid nuclei. Other fibers entered the diagonal band of Broca and formed a dense plexus of labeled fibers in the dorsal half of the intermediate portion of the lateral septal nucleus and the septohippocampal nucleus. These findings demonstrate that the basal forebrain and extended amygdala receive a dense projection from nonserotonergic DRN neurons. Given that the basal forebrain plays a critical role in processes such as motivation, affect, and behavioral control, these findings support the hypothesis that nonserotonergic DRN projections may exert substantial modulatory control over emotional and motivational functions.

The serotonergic inhibition of slowly bursting cells in the intergeniculate leaflet of the rat

Electrophysiological studies combined with local neurotoxic lesions were conducted on anaesthetized rats in order to determine whether the dorsal raphe nucleus (DRN) inhibits the intergeniculate leaflet (IGL) of the lateral geniculate nucleus by means of innervation by serotonin-containing fibres. In the control animals, electrical stimulation of the DRN induced the long-latency and long-lasting inhibition of the neuronal firing of the IGL cells that are characterized by rhythmic, slow-bursting activity in light conditions. The electrical destruction of the DRN resulted in an increase in the firing rate of the recorded IGL cells, whilst at the same time not affecting the rhythmic, bursting pattern of the activity. In the second group of animals, local neurotoxic lesion of serotonergic fibres was performed by injection of the toxin 5,7-dihydroxytryptamine into the IGL. After 10 days of postoperative recovery, electrophysiological experiments were performed on the toxin-treated rats. In these animals, electrical stimulation as well as electrical lesion of the DRN did not induce any change in the firing of the slowly bursting cells in the 5,7-dihydroxytryptamine-injected IGL. The results obtained provide evidence that inhibition of the IGL slowly bursting cells, by innervation from the dorsal raphe, is mediated by the release of serotonin. Furthermore, the observed serotonergic inhibition of the light-dependent activity of slowly bursting cells can contribute to the neuronal mechanism gating the information that flows through this nucleus to the vestibular, visuomotor, circadian and sleep/arousal systems, with which the IGL is strongly interconnected.

Different serotonin receptor agonists have distinct effects on sound-evoked responses in inferior colliculus

The neuromodulator serotonin has a complex set of effects on the auditory responses of neurons within the inferior colliculus (IC), a midbrain auditory nucleus that integrates a wide range of inputs from auditory and nonauditory sources. To determine whether activation of different types of serotonin receptors is a source of the variability in serotonergic effects, four selective agonists of serotonin receptors in the serotonin (5-HT) 1 and 5-HT2 families were iontophoretically applied to IC neurons, which were monitored for changes in their responses to auditory stimuli. Different agonists had different effects on neural responses. The 5-HT1A agonist had mixed facilitatory and depressive effects, whereas 5-HT1B and 5-HT2C agonists were both largely facilitatory. Different agonists changed threshold and frequency tuning in ways that reflected their effects on spike count. When pairs of agonists were applied sequentially to the same neurons, selective agonists sometimes affected neurons in ways that were similar to serotonin, but not to other selective agonists tested. Different agonists also differentially affected groups of neurons classified by the shapes of their frequency-tuning curves, with serotonin and the 5-HT1 receptors affecting proportionally more non-V-type neurons relative to the other agonists tested. In all, evidence suggests that the diversity of serotonin receptor subtypes in the IC is likely to account for at least some of the variability of the effects of serotonin and that receptor subtypes fulfill specialized roles in auditory processing.

Electrophysiological diversity of the dorsal raphe cells across the sleep-wake cycle of the rat

Through their widespread projections to the entire brain, dorsal raphe cells participate in many physiological functions and are associated with neuropsychiatric disorders. In previous studies, the width of action potentials was used as a criterion to identify putative serotonergic neurons, and to demonstrate that cells with broad spikes were more active in wakefulness, slowed down their activity in slow wave sleep and became virtually silent during paradoxical sleep. However, recent studies reported that about half of these presumed serotonergic cells were not immunoreactive for tyrosine hydroxylase. Here, we re-examine the electrophysiological properties of dorsal raphe cells across the sleep-wake cycle in rats by the extracellular recording of a large sample of single units (n = 770). We identified two major types of cells, which differ in spike waveform: a first population characterized by broad, mostly positive spikes, and a second one displaying symmetrical positive-negative spikes with a large distribution of spike durations (0.6-3.2 ms). Although we found classical broad-spike cells that were more active in wakefulness, we also found that about one-third of these cells increased or did not change their firing rate during sleep compared with wakefulness. Moreover, 62% of the latter cells were active in paradoxical sleep when most of raphe cells were silent. Such a diversity in the neuronal firing behaviour is important in the light of the recent controversy regarding the neurochemical identity of dorsal raphe cells exhibiting broad spikes. Our results also suggest that the dorsal raphe contains subpopulations of neurons with reciprocal activity across the sleep-wake cycle.

Photostimulation alters c-Fos expression in the dorsal raphe nucleus

Retinal afferents to the dorsal raphe nucleus (DRN) have been described in a number of species, including Mongolian gerbils, but functional correlates of this optic pathway are unknown at present. To determine whether temporally modulated photostimulation can affect c-Fos expression in the gerbil DRN, quantitative analysis of c-Fos-immunoreactive (c-Fos-ir) neurons was conducted following 60-min exposure to pulsed (2 Hz) photostimulation at selected times over the 12:12 h light/dark cycle. For comparison, c-Fos expression was also analyzed in the subnuclei of the lateral geniculate complex and in the suprachiasmatic nucleus (SCN). In the DRN, a substantial reduction was observed in the number of c-Fos immunoreactive (c-Fos-ir) neurons during the light period and early dark period in photostimulated vs. control animals. Similar results were obtained in the intergeniculate leaflet (IGL) and ventral lateral geniculate (VLG). However, no significant changes were observed in the number of c-Fos-ir neurons in the dorsal lateral geniculate nucleus or suprachiasmatic nucleus (SCN) following photostimulation, except for an increase in the middle of the dark period. These findings indicate that photic stimulation can lead to a suppression or down-regulation of c-Fos expression in the DRN that is probably mediated via the direct retinal pathway to the DRN in this species. The similarity between c-Fos expression profiles in the DRN and IGL/VGL suggest that efferent projections from the DRN may modulate c-Fos expression to visual stimulation in these subnuclei of the lateral geniculate complex.

Diurnal variation in serotonin immunoreactivity in the dorsal raphe nucleus

Serotonin (5-HT) immunostained sections were analyzed using integrated optical density (IOD) measures obtained throughout the dorsal raphe nucleus (DRN) in Mongolian gerbils at selected times during a 12:12 h light:dark cycle. Substantial diurnal variation occurred in 5-HT neuronal staining density, with lowest and highest IOD values occurring at the light/dark and dark/light transitions, respectively. The injection of pargyline and tryptophan increased 5-HT immunostaining comparable to the highest level observed in control animals. Transitions between light and dark periods appear to be major environmental events that influence 5-HT levels in the DRN.

Sleep-dependent learning and memory consolidation

While the functions of sleep remain largely unknown, one of the most exciting and contentious hypotheses is that sleep contributes importantly to memory. A large number of studies offer a substantive body of evidence supporting this role of sleep in what is becoming known as sleep-dependent memory processing. This review will provide evidence of sleep-dependent memory consolidation and sleep-dependent brain plasticity and is divided into five sections: (1) an overview of sleep stages, memory categories, and the distinct stages of memory development; (2) a review of the specific relationships between sleep and memory, both in humans and animals; (3) a survey of evidence describing sleep-dependent brain plasticity, including human brain imaging studies as well as animal studies of cellular neurophysiology and molecular biology. We close (4) with a consideration of unanswered questions as well as existing arguments against the role of sleep in learning and memory and (5) a concluding summary.

Sensorimotor-related discharge of simultaneously recorded, single neurons in the dorsal raphe nucleus of the awake, unrestrained rat

Multi-channel, multi-neuron recording procedures were used to monitor simultaneously the spike train activity of single neurons (n=7-16 cells/animal) in the dorsal raphe (DR) nucleus of the awake, freely moving rat. Putative serotonergic and non-serotonergic neurons were distinguished from one another on the basis of established criteria, i.e. waveform shape and duration, firing pattern and firing frequency. As a group, presumed serotonergic neurons exhibited low tonic discharge rates, depressed firing after serotonin (5HT)-1a agonist administration, and, except for the transition from sleep to waking, a general insensitivity to specific sensory or motor events. By contrast, non-serotonergic cells in midline and lateral wing sub-regions of the nucleus displayed responses to a variety of sensorimotor events including locomotion, grooming, head movement, chewing, auditory stimuli, and whisker movement (both passive and active). However, within this latter group, the sensorimotor response repertoire of individual cells was not uniform. Likewise, non-5HT cells with diverse response profiles were identified in both medial and lateral sub-regions of the nucleus. Cells categorized as non-serotonergic also had varied responses to 5HT1a agonist administration. These results emphasize the diverse input/output relationships of individual DR neurons and underscore the need for a more comprehensive analysis of such properties under waking conditions in order to obtain a better understanding of the role of the DR nucleus in brain function.

Activity of dorsal raphe cells across the sleep-waking cycle and during cataplexy in narcoleptic dogs

Cataplexy, a symptom associated with narcolepsy, represents a unique dissociation of behavioural states. During cataplectic attacks, awareness of the environment is maintained, as in waking, but muscle tone is lost, as in REM sleep. We have previously reported that, in the narcoleptic dog, noradrenergic cells of the locus coeruleus cease discharge during cataplexy. In the current study, we report on the activity of serotonergic cells of the dorsal raphe nucleus. The discharge patterns of serotonergic dorsal raphe cells across sleep-waking states did not differ from those of dorsal raphe and locus coeruleus cells recorded in normal rats, cats and monkeys, with tonic discharge in waking, reduced activity in non-REM sleep and cessation of activity in REM sleep. However, in contrast with locus coeruleus cells, dorsal raphe REM sleep-off neurones did not cease discharge during cataplexy. Instead, discharge continued at a level significantly higher than that seen in REM sleep and comparable to that seen in non-REM sleep. We also identified several cells in the dorsal raphe whose pattern of activity was the opposite of that of the presumed serotonergic cells. These cells were maximally active in REM sleep and minimally active in waking and increased activity during cataplexy. The difference between noradrenergic and serotonergic cell discharge profiles in cataplexy suggests different roles for these cell groups in the normal regulation of environmental awareness and muscle tone and in the pathophysiology of narcolepsy.

Is state-dependent alternation of slow dynamics in central single neurons during sleep present in the rat ventroposterior thalamic nucleus?

Based upon our previous results in cats, we hypothesized that neurons in the central processor systems of the brain generally exhibit state-dependent dynamics alternation of slow fluctuations in spontaneous activity during sleep. To test the validity of this hypothesis across species, we recorded single neuronal activity during sleep from the ventroposterior (VP) thalamic nucleus in unanesthetized, head-restrained rats. Spectral analysis was performed on successive spike-counts of neuronal activity recorded during three stages of the sleep-wakefulness cycle: wakefulness (W, n=6), slow-wave sleep (SWS, n=20), and paradoxical sleep (PS, n=32). We found that firing of VP neurons displayed white-noise-like dynamics over the range of 0.04-1.0 Hz during SWS and 1/f-noise-like dynamics over the same range during PS. We also demonstrated for the first time that the slow dynamics of neuronal activity during quiet wakefulness (but not drowsiness) are white-noise-like. These results suggest that our hypothesis is true across species. During W and SWS, the brain may be considered as under global inhibition. Conversely, PS may represent a state of global disinhibition in the brain, where neuronal activity exhibits 1/f-noise-like dynamics. Fluctuations observed in living organisms may be involved in essential processes in generation and function of sleep states.

The distributions and signaling directions of the cerebrospinal fluid contacting neurons in the parenchyma of a rat brain

Many studies have been made on the distributions of CSF contacting neurons (CSF-CNs) in the parenchyma of the brain with horseradish peroxidase (HRP) or autoradiographics. A significant amount of data has shown that both HRP and autoradiographical substances could pass freely through the spaces of ependyma into the parenchyma of the brain. It is therefore possible that the results were not exact. We found that CB-HRP was a dependable tracer to CSF-CNs and studied the distributions and the signaling directions of cerebrospinal fluid contacting neurons (CSF-CNs) in the parenchyma of the brain with the cholera toxin subunit B with horseradish peroxidase (CB-HRP) tracing combined with transmission electron microscopy. The results were as follows: (1) CSF contacting tanycytes existed not only in the wall of the third ventricle (3V), but also in the walls of the lateral ventricle (LV), the fourth ventricle (4V) and the central canal (CC) of the spinal cord. (2) Some CSF contacting glia cells were observed in the lateral septal nucleus (LS). (3)The distal CSF-CNs in the parenchyma were found in LS, the anterodorsal thalamic nucleus (AD), the supramammillary nucleus (SuM), the dorsal raphe nucleus (DR), the floor of 4V and the lateral superior olive (LSO), but they were mainly found in DR and divided into groups A and B. (4) Axon terminals labeled by CB-HRP were found in the cavity of the brain ventricle. (5) The synaptic relationships between the neurons were labeled by CB-HRP in DR and no-labeled by CB-HRP in the parenchyma. Both synapses Gray I and II were found. It was significant that the presynaptic elements were formed by the neurons no-labeled CB-HRP and the postsynaptic elements labeled CB-HRP. Our results suggested firstly that the signaling directions of CSF-CNs in DR were only from the parenchyma to CSF.

Retinal ganglion cells projecting to the dorsal raphe and lateral geniculate complex in Mongolian gerbils

Injections of rhodamine-B into the dorsal raphe nucleus (DRN) and Fluoro-Gold into the lateral geniculate nucleus (LGN) revealed double-labeled retinal ganglion cells (DL RGCs) projecting to both nuclei. The soma-size distribution of DL RGCs was compared with three other distributions: DRN-projecting RGCs, LGN-projecting RGCs, and a large sample of RGCs labeled via the optic nerve with DiI. DL RGC soma diameters fell primarily within the mid-to-upper size range of all three distributions. DL RGCs may provide information to both nuclei concerning comparable aspects of light and visual stimulation via collateralized axons.

Cognitive flexibility across the sleep-wake cycle: REM-sleep enhancement of anagram problem solving

Flexible or 'fluid' cognitive processes are regarded as fundamental to problem solving and creative ability, requiring a specific neurophysiological milieu. REM-sleep dreaming is associated with creative processes and abstract reasoning with increased strength of weak associations in cognitive networks. REM sleep is also mediated by a distinctive neurophysiological profile, different to that of wake and NREM sleep. This study compared the performance of 16 subjects on a test of cognitive flexibility using anagram word puzzles following REM and NREM awakenings across the night, and waking performances during the day. REM awakenings provided a significant 32% advantage in the number of anagrams solved compared with NREM awakenings and was equal to that of wake time trials. Correlations of individual performance profiles suggest that REM sleep may offer a different mode of problem solving compared with wake and NREM. When early and late REM and NREM awakening data were separated, a dissociation was evident, with NREM task performance becoming more REM-like later in the night, while REM performance remained constant. These data suggest that the neurophysiology of REM sleep represents a brain state more amenable to flexible cognitive processing than NREM and different from that in wake, and may offer insights into the neurocognitive properties of REM-sleep dreaming.

Modulation of noradrenergic and serotonergic transmission by noxious stimuli and intrathecal morphine differs in the dorsal raphe nucleus of anesthetized rat: in vivo voltammetric studies

We examined the effects of cutaneous noxious heat as well as the intrathecal administration of morphine on the oxidation current of peaks 1 and 2 in the dorsal raphe nucleus (DRN) of anesthetized rats. Differential normal pulse voltammetry with carbon fiber electrodes identified distinct oxidation currents at +120 mV (peak 1: catechol signals) and +280 mV (peak 2: 5-hydroxyindole signals). The catechol signal was significantly increased by 22.9 +/- 4.2% after applying cutaneous noxious heat at 52 degrees C. The 5-hydroxyindole signal was decreased by 39.8 +/- 4.3 and by 25.2 +/- 4.7% after stimulation with cutaneous noxious heat at 52 and 45 degrees C, respectively. A low dose of morphine (2.5 microg) potentiated the increase in the catechol signal and the decrease in the 5-hydroxyindole signal induced by noxious heat, and a high dose (10.0 microg) attenuated both. The effects of morphine at low (2.5 microg) and high doses (10.0 microg) were antagonized by naloxone (0.5 mg/kg, i.p.). These results indicate that noxious heat stimulation increased the catechol signal and decreased the 5-hydroxyindole signal in the DRN. The intrathecal administration of morphine affects the noxious stimulation-induced activity of noradrenergic and serotonergic neurotransmission in the DRN.

Differentiation of presumed serotonergic dorsal raphe neurons in relation to behavior and wake-sleep states

Using extracellular single unit recording, either alone or in combination with microdialysis application of drugs, we examined the characteristics of presumed serotonergic dorsal raphe neurons during wake-sleep states in the freely moving cat. Recordings were made from a total of 272 neurons in the dorsal raphe nucleus. Of these, 240 (88%) were classified as serotonergic on the basis of their typical long-duration action potential, slow discharge activity, and reduced spontaneous discharge rate during paradoxical sleep compared to during slow-wave sleep. An inhibitory response to serotonergic agonists and a slow conduction velocity were seen in all neurons of this type tested or identified by stimulation of the main ascending serotonergic pathway. These presumed serotonergic dorsal raphe neurons could be subdivided into two typical previously identified groups (types I-A and I-B) and four atypical new groups (types I-C, II-A, II-B, and II-C) according to differences in firing patterns during wake-sleep states. The typical neurons were evenly distributed in the dorsal raphe nucleus and their activity was related to the level of behavioral arousal, since they discharged regularly at a high rate during waking and at progressively slower rates during slow-wave sleep, and ceased firing either during slow-wave sleep with ponto-geniculo-occipital waves and paradoxical sleep (type I-A) or only during paradoxical sleep (type I-B). In contrast, the atypical subgroups were unevenly distributed in the dorsal raphe nucleus and exhibited firing patterns distinct from those of the typical neurons, such as sustained tonic activity during paradoxical sleep (types I-C and II-C) or showing their highest rate of tonic discharge during slow-wave sleep, with suppression of discharge during both waking and paradoxical sleep (type II-B). From these data we suggest that presumed serotonergic dorsal raphe neurons play different roles in behavioral state control and that there is functional topographic organization in the dorsal raphe nucleus.

Increase in antidromic excitability in presumed serotonergic dorsal raphe neurons during paradoxical sleep in the cat

Putative serotonergic dorsal raphe (DRN) neurons display a dramatic state-related change in behaviour, discharging regularly at a high rate during waking and at progressively slower rates during slow-wave sleep (SWS) and ceasing firing during paradoxical sleep (PS). Using the antidromic latency technique and extracellular recording, we have examined the change in neuronal excitability of presumed serotonergic DRN neurons during the wake-sleep cycle in freely moving cats. We found that, under normal conditions, suprathreshold stimulation of the main ascending serotonergic pathway resulted in a marked decrease in both the magnitude and variability of antidromic latency during PS, while subthreshold stimulation led to a marked increase in antidromic responsiveness during PS compared with during other behavioural states. The antidromic latency shift resulted from a change in the delay between the initial segment (IS) and soma-dendritic (SD) spikes, the antidromic latency being inversely related to the interval between the stimulus and the preceding spontaneous action potential. A marked decrease in the magnitude and variability of antidromic latency was also seen following suppression of the spontaneous discharge of DRN neurons by application of 5-HT autoreceptor agonists or muscimol, a potent GABA agonist. A marked IS-SD delay or blockage of SD spikes was, however, seen in association with the PS occurring during recovery from 5-HT autoreceptor agonist or during muscimol application. The present findings are discussed in the light of previous in vitro intracellular recording data and our recent findings of the disfacilitation mechanisms responsible for the cessation of discharge of DRN neurons during PS.

Retinal projection to the dorsal raphe nucleus in the Chilean degus (Octodon degus)

A substantial projection from the retina to the dorsal raphe nucleus (DRN) has been demonstrated in the Chilean degus, a diurnal/crepuscular hystricomorph rodent. Following intraocular injection of cholera toxin subunit B (CTB), immunocytochemically labeled CTB-positive axons and terminals were observed in all major retinorecipient nuclei as well as in the DRN and periaqueductal gray (PAG) of the mesencephalon. Two streams of optic axons to the DRN were observed: one descending from the optic tract at the level of the pretectum and anterior superior colliculus, the other emerging as a small fascicle at the anterior pole of the inferior colliculus and descending bilaterally through the PAG. Contralateral retinal afferents in the DRN appeared to terminate primarily in the dorsomedial and lateral subdivisions of the DRN, and a less extensive ipsilateral component also was observed. Axonal arborizations were characterized by short branches and multiple varicosities, both in the DRN and in the PAG. The extent and density of DRN retinal afferents were not as extensive as previously observed in Mongolian gerbils using identical techniques, but the retinal-DRN projection is considerably larger in degus than in rats. The functional significance of the retinal-DRN pathway remains to be determined, although a variety of evidence indicates that light may directly affect the activity of neurons and serotonin levels in the DRN.

GABAergic neurons of the cat dorsal raphe nucleus express c-fos during carbachol-induced active sleep

Serotonergic neurons of the dorsal raphe nucleus (DRN) cease firing during active sleep (AS, also called rapid-eye-movement sleep). This cessation of electrical activity is believed to play a 'permissive' role in the generation of AS. In the present study we explored the possibility that GABAergic cells in the DRN are involved in the suppression of serotonergic activity during AS. Accordingly, we examined whether immunocytochemically identified GABAergic neurons in the DRN were activated, as indicated by their expression of c-fos, during carbachol-induced AS (AS-carbachol). Three chronically-prepared cats were euthanized after prolonged episodes of AS that was induced by microinjections of carbachol into the nucleus pontis oralis. Another four cats (controls) were maintained 2 h in quiet wakefulness before being euthanized. Thereafter, immunocytochemical studies were performed on brainstem sections utilizing antibodies against Fos, GABA and serotonin. When compared with identically prepared tissue from awake cats, the number of Fos+ neurons was larger in the DRN during AS-carbachol (35.9+/-5.6 vs. 13.9+/-4.4, P<0.05). Furthermore, a larger number of GABA+ Fos+ neurons were observed during AS-carbachol than during wakefulness (24.8+/-3.3 vs. 4.0+/-1.0, P<0.001). These GABA+ Fos+ neurons were distributed asymmetrically with a larger number located ipsilaterally to the site of injection. There was no significant difference between control and experimental animals in the number of non-GABAergic neurons that expressed c-fos in the DRN. We therefore suggest that activated GABAergic neurons of the DRN are responsible for the inhibition of serotonergic neurons that occurs during natural AS.

Discharge modulation of rat dorsal raphe neurons during sleep and waking: effects of preoptic/basal forebrain warming

In cats, putative serotonergic neurons (PSNs) recorded from the dorsal raphe nucleus (DRN) across the sleep-wake cycle exhibit the so-called rapid eye movement sleep-off (REM-off) discharge pattern. Since, the sleep-wake discharge patterns of DRN neurons in behaving rats is poorly known, the present study examined this neuronal populations. The PSNs recorded in this study exhibited: (1) progressive decrease in discharge rate from waking to NREM to REM sleep; (2) long action potential duration, and (3) reduction of discharge rate after systemic administration of a selective 5-HT(1A) agonist, (+/-)-8-hydroxy-2-(di-n-propylamino) tetralin hydrobromide (8-OH-DPAT). Evidence supports the hypothesis that NREM sleep is modulated by thermoregulatory mechanisms localized in the preoptic area and adjacent basal forebrain (POA/BF). We previously reported that POA/BF warming suppresses the discharge of wake-promoting neurons in the posterior hypothalamus and the basal forebrain. Since the DRN is one component of the brainstem arousal system and receives projections from POA/BF, we examined the effects of local POA/BF warming by 1.5-2.0 degrees C during waking on the discharge of DRN neurons. POA/BF warming reduced the discharge in 14 of 19 PSNs and in 12 of 17 other wake-related neurons in the DRN. DRN neuronal discharge reduction occurred without accompanying EEG frequency or behavioral changes. These results suggest that PSNs recorded in DRN in unrestrained and unanesthetized rats exhibit a "wake-active REM-off" discharge pattern and further support the hypothesis that the POA/BF warm-sensitive hypnogenic system induces sleep by a coordinated inhibition of multiple arousal systems including that modulated by the DRN.

Role and origin of the GABAergic innervation of dorsal raphe serotonergic neurons

Extracellular electrophysiological recordings in freely moving cats have shown that serotonergic neurons from the dorsal raphe nucleus (DRN) fire tonically during wakefulness, decrease their activity during slow wave sleep (SWS), and are nearly quiescent during paradoxical sleep (PS). The mechanisms at the origin of the modulation of activity of these neurons are still unknown. Here, we show in the unanesthetized rat that the iontophoretic application of the GABA(A) antagonist bicuculline on dorsal raphe serotonergic neurons induces a tonic discharge during SWS and PS and an increase of discharge rate during quiet waking. These data strongly suggest that an increase of a GABAergic inhibitory tone present during wakefulness is responsible for the decrease of activity of the dorsal raphe serotonergic cells during slow wave and paradoxical sleep. In addition, by combining retrograde tracing with cholera toxin B subunit and glutamic acid decarboxylase immunohistochemistry, we demonstrate that the GABAergic innervation of the dorsal raphe nucleus arises from multiple distant sources and not only from interneurons as classically accepted. Among these afferents, GABAergic neurons located in the lateral preoptic area and the pontine ventral periaqueductal gray including the DRN itself could be responsible for the reduction of activity of the serotonergic neurons of the dorsal raphe nucleus during slow wave and paradoxical sleep, respectively.

Topographic organization of serotonergic dorsal raphe neurons projecting to the superior colliculus in the Mongolian gerbil (Meriones unguiculatus)

Recent evidence suggests that the dorsal raphe nucleus (DRN) of the brainstem is a collection of neuronal clusters having different neurochemical characteristics and efferent projection patterns. To gain further insight into the neuroanatomic organization of the DRN, neuronal populations projecting to the superior colliculus (SC) were mapped in a highly visual rodent, the Mongolian gerbil (Meriones unguiculatus). Retrograde tracers Fluoro-Gold (FG) or cholera toxin subunit-B (CTB) were injected into the superficial layers of the SC, and serotonin (5-hydroxytryptamine, 5-HT) -positive cells were identified by using immunocytochemistry in the FG-injected animals. Based on its projections to the SC, the DRN was divided into five rostrocaudal levels. In the rostral and middle levels of the DRN, virtually all FG-filled cells occurred in the lateral DRN, and 36-55% of 5-HT-immunoreactive (5-HT-ir) cells were also double-labeled with FG. Caudally, FG-filled cells occurred in the lateral, ventromedial, and interfascicular DRN; and 44, 12, and 31% of 5-HT-ir cells, respectively, were also FG-filled. The dorsomedial DRN contained only a small proportion of FG-filled cells at its most caudal level and was completely devoid of FG-filled cells more rostrally. The CTB-injected animals showed a similar distribution of retrogradely labeled cells in the DRN. Topographically, the dorsal tegmental nucleus and the laterodorsal tegmental nucleus appeared to be closely associated with 5-HT-ir cells in the caudal DRN. These results suggest that the lateral DRN and the ventromedial/interfascicular DRN may be anatomically, morphologically, and neurochemically unique subdivisions of the gerbil DRN.

The effects of median raphé electrical stimulation on serotonin release in the dorsal hippocampal formation of prenatally protein malnourished rats

Our previous work had shown an enhanced inhibition in the hippocampal formation of prenatally protein malnourished rats. We have also found a diminishment in 5-hydroxytryptamine (5-HT) fibers in the hippocampal formation of malnourished rats as well as increased levels of 5-HT in the brain. The purpose of the present study was to determine 5-HT release in the dorsal hippocampal formation following electrical stimulation of the median raphé nucleus (MRN) in unanesthetized prenatally malnourished rats. Stimulation of this nucleus at 20 Hz in malnourished rats resulted in a significantly diminished release of 5-HT compared to well-nourished rats. The latter group showed a lesser, though still significant, decrease in 5-HT release following raphé stimulation. Basal release of 5-HT prior to stimulation was significantly higher in malnourished rats as compared to well-nourished controls. This may be the result of a decreased density of 5-HT neurons leading to a diminished control of release. Stimulation of the MRN in behaving malnourished animals may markedly affect the recurrent negative feedback collaterals onto somatodendritic 5-HT(1A) and 5-HT(1D) autoreceptors thus enhancing the inhibitory effects of stimulation of the median raphé on 5-HT release. Studies are underway to examine the sensitivity of both the somatodendritic and terminal 5-HT autoreceptors in malnourished animals, in order to understand possible mechanisms for our findings.

Neural connections of the pontine reticular formation, which connects reciprocally with the nucleus prepositus hypoglossi in the rat

The pontine reticular formation connected with the nucleus prepositus hypoglossi was studied in the rat using anterograde and retrograde tracer techniques. The area reciprocally connected with the nucleus prepositus hypoglossi was evident in the pontine reticular formation of the rat. The region had intensive reciprocal connections with the ipsilateral subthalamic region, the contralateral pontine reticular formation and the nucleus prepositus hypoglossi. Furthermore, it was confirmed that the region received cholinergic projections mainly from the pedunculopontine tegmental nucleus and the laterodorsal tegmental nucleus, and aminergic projections from the dopaminergic cell groups A13 and A11, noradrenergic cell groups A7, A6 and A5, and the serotoninergic B9 cell group. This region in the rat was considered to be the preoculomotor structure in the function of horizontal gaze corresponding to the paramedian pontine reticular formation in other animals.

In vivo microdialysis measures of extracellular serotonin in the rat hippocampus during sleep-wakefulness

We investigated extracellular 5-hydroxytryptamine (5-HT) levels in rat hippocampus during different stages of the sleep-waking cycle using in vivo microdialysis. The extracellular 5-HT level was highest in active waking (AW) and, when compared to AW, 5-HT level was progressively lower in quiet waking (QW; 78%), quiet sleep (QS; 50%) and REM (which we termed active sleep (AS); 40%). Functional implications of AS related-decreased 5-HT in the hippocampus are discussed.

Differential c-Fos expression in cholinergic, monoaminergic, and GABAergic cell groups of the pontomesencephalic tegmentum after paradoxical sleep deprivation and recovery

Multiple lines of evidence indicate that neurons within the pontomesencephalic tegmentum are critically involved in the generation of paradoxical sleep (PS). From single-unit recording studies, evidence suggests that unidentified but "possibly" cholinergic tegmental neurons discharge at higher rates during PS than during slow wave sleep or even waking and would thus play an active role, whereas "presumed" monoaminergic neurons cease firing during PS and would thus play a permissive role in PS generation. In the present study performed on rats, c-Fos immunostaining was used as a reflection of neuronal activity and combined with immunostaining for choline acetyltransferase (ChAT), serotonin (Ser), tyrosine hydroxylase (TH), or glutamic acid decarboxylase (GAD) for immunohistochemical identification of active neurons during PS recovery ( approximately 28% of recording time) as compared with PS deprivation (0%) and PS control (approximately 15%) conditions. With PS recovery, there was a significant increase in ChAT+/c-Fos+ cells, a significant decrease in Ser+/c-Fos+ and TH+/c-Fos+ cells, and a significant increase in GAD+/c-Fos+ cells. Across conditions, the percent PS was correlated positively with tegmental cholinergic c-Fos+ cells, negatively with raphe serotonergic and locus coeruleus noradrenergic c-Fos+ cells, and positively with codistributed and neighboring GABAergic c-Fos+ cells. These results support the hypothesis that cholinergic neurons are active, whereas monoaminergic neurons are inactive during PS. They moreover indicate that GABAergic neurons are active during PS and could thus be responsible for inhibiting neighboring monoaminergic neurons that may be essential in the generation of PS.

Global and serial neurons form A hierarchically arranged interface proposed to underlie memory and cognition

It is hypothesized that the cholinergic and monoaminergic neurons of the brain from a global network. What is meant by a global network is that these neurons operate as a unified whole, generating widespread patterns of activity in concert with particular electroencephalographic states, moods and cognitive gestalts. Apart from cholinergic and monoaminergic global systems, most other mammalian neurons relay sensory information about the external and internal milieu to serially ordered loci. These "serial" neurons are neurochemically distinct from global neurons and commonly use small molecule amino acid neurotransmitters such as glutamate or aspartate. Viewing the circuitry of the mammalian brain within the global-serial dichotomy leads to a number of novel interpretations and predictions. Global systems seem to be capable of transforming incoming sensory data into cognitive-related activity patterns. A comparative examination of global and serial systems anatomy, development and physiology reveals how global systems might turn sensation into mentation. An important step in this process is the permanent encoding of memory. Global neurons are particularly plastic, as are the neurons receiving global inputs. Global afferents appear to be capable of reorganizing synapses on recipient serial cells, thus leading to enhanced responding to a signal, in a particular context and state of arousal.

Behaviour of raphe cells projecting to the dorsomedial medulla during carbachol-induced atonia in the cat

1. The activity of most brainstem serotonergic cells is suppressed during sleep, particularly the rapid eye movement (REM) phase. Thus, they may play a major role in state-dependent changes in CNS functioning. Our main goal was to search for medullary raphe cells having axonal branches in the region of the hypoglossal (XII) motor nucleus and assess their behaviour during the atonia produced by microinjections of a cholinergic agonist, carbachol, into the dorsal pontine tegmentum. In chronic animals, such microinjections evoke a desynchronized sleep-like state similar to natural REM sleep; in decerebrate animals, they produce eye movements and a motor suppression similar to the postural atonia of REM sleep. 2. In decerebrate, paralysed, vagotomized and artificially ventilated cats, we recorded extracellularly from medullary raphe cells antidromically activated from the XII nucleus region. Forty-five cells recorded in the raphe obscurus and pallidus nuclei were antidromically activated with latencies characteristic of non-myelinated fibres (4.4-42.0 ms). For thirty-three of the forty-five cells, we found one or more axonal branches within or just below the XII nucleus. The remaining twelve cells, in addition to the XII nucleus, had axonal ramifications in the medial nucleus of the solitary tract (NTS) and/or the dorsal motor nucleus of the vagus (DMV). 3. A subset of fourteen spontaneously active cells with identified axonal projections were held long enough to be recorded during the carbachol-induced atonia, and eight of these also during the subsequent recovery and a systemic administration of the serotonergic 1A receptor agonist (+/-)8-hydroxy-2-(di-N-propylamino)tetrealin hydrobromide (8-OH-DPAT). All but one were suppressed during the atonia in parallel to the suppression of XII, phrenic and postural nerve activities (firing rate, 1.3 +/- 0.7 Hz before and 0.1 +/- 0.2 Hz after carbachol (means +/- S.D.)). Following the recovery from the atonia, the firing rates of the eight cells increased to the pre-carbachol level (1.6 +/- 1.0 Hz). Subsequently, all were silenced by 8-OH-DPAT. 4. These cells fulfil most physiological criteria for serotonergic cells and have the potential to modulate, in a state-dependent manner, activities in the motor XII nucleus, visceral sensory NTS, and DMV. The decrements in serotonergic neuronal activity that occur during the carbachol-induced atonia suggest that a similar withdrawal of serotonergic input may occur during REM sleep and contribute to the characteristic reductions in upper airway motor tone.

Single-unit recordings at dorsal raphe nucleus in the awake-anesthetized rat: spontaneous activity and responses to cutaneous innocuous and noxious stimulations

In this study, we recorded the single-unit activity of the dorsal raphe nucleus (DRN) in rats tested first awake and, a few days later, anesthetized with sodium pentobarbital and recorded again. This was achieved by means of a small chronically implanted device supporting a 25 micron platinum-iridium wire as the recording electrode. In both the awake and anesthetized conditions, and in agreement with most of the studies performed at the DRN level, we found that a vast majority of the units, displaying small amplitude and long-duration action potentials, possessed a low level of spontaneous activity (0.2-4 Hz). Among these units, found in greater number under pentobarbital, it was possible to establish that this activity was regular or irregular, in accordance with the literature reports. However, as opposed to these studies, we determined that the 'regularity' was relative, only noticeable in more or less prolonged phases of activity. In particular, we never recorded the so-called 'clock-like' activity, largely reported as an unambiguous criterion for selecting the serotoninergic neurons. In both the awake and anesthetized conditions, the responses of the DRN neurons to peripheral mechanical innocuous and noxious stimulations were observed in only one-half of the units recorded and were weak in comparison to other results that we obtained at the nucleus raphe magnus level in previous studies. When present, these responses were excitation or inhibition, occurring during or after the stimulus application. These results question the direct involvement of the DRN in acute nociception.

The dorsal raphe: an important nucleus in pain modulation

The dorsal raphe nucleus (DRN) is an important nucleus in pain modulation. It has abundant 5-HT neurons and many other neurotransmitter and/or neuromodulator containing neurons. Its vast fiber connections to other parts of the central nervous system provide a morphological basis for its pain modulating function. Its descending projections, via the nucleus raphe magnus or directly, modulate the responses caused by noxious stimulation of the spinal dorsal horn neurons. In ascending projections, it directly modulates the responses of pain sensitive neurons in the thalamus. It can also be involved in analgesia effects induced by the arcuate nucleus of the hypothalamus. Neurophysiologic and neuropharmacologic results suggest that 5-HT neurons and ENKergic neurons in the DRN are pain inhibitory, and GABA neurons are the opposite. The studies of the intrinsic synapses between ENKergic neurons, GABAergic neurons, and 5-HT neurons within the DRN throw light on their relations in pain modulation functions, and further explain their functions in pain mediation.

Effects of locus coeruleus lesions on vigilance and attentive behaviour in cat

Previous data have suggested that in the cat, expectancy behaviour (waiting for a target to appear) and associated electrocortical, focal, synchronized activity ('mu' rhythms) are modulated by a noradrenergic system possibly originating from the locus coeruleus (LC). To test the latter hypothesis, we have examined the behavioural and ECoG changes induced after bilateral LC lesions. Our results demonstrated that destruction of the anterior 3/4th of the LC (A6 noradrenergic cell group) resulted in a considerable increase of mu rhythms and expectancy behaviour, without episodes of drowsiness that normally occur. Destruction of the posterior fourth of LC (A4 noradrenergic group) only increased the duration of slow sleep. Extending the A6 lesion to include the dorsal ascending noradrenergic bundle also increased the expectancy behaviour and mu rhythms. Finally, when the nucleus subcoeruleus was also involved, the duration of slow sleep and the frequency of paradoxical sleep episodes increased. These findings indicate that the LC exerts an inhibitory effect on structures involved in the induction and persistence of expectancy behaviour with accompanying mu rhythms.

Effects induced by the electrical stimulation of the nucleus raphe dorsalis upon hypothalamic release of 5-hydroxyindole compounds and sleep parameters in the rat

Electrical stimulation (30 min; pulse 0.5 ms, 150 microA, 20 Hz) of the antero-dorsal part of the nucleus raphe dorsalis (n.RD) induces in the basal hypothalamus (n. arcuate and surrounding areas) a large axonal release of 5-hydroxyindole compounds (5-OHLes; about +300%) measured by means of voltammetric technique. During such a release, evidence for a direct detection of serotonin (5-HT) is reported. The above stimulation induces also marked aversive behaviors together with prolonged polygraphic arousal. Three hours later, a significant paradoxical sleep (PS) rebound occurs. Both the 5-OHLes release and the PS rebound observed after n.RD stimulation are suppressed by a p-chlorophenylalanine (PCPA) pretreatment of the animals. The 5-HT-hypnogenic factor dependence of the PS rebound observed after n.RD stimulation is discussed.

Schizophrenia: a disease of interhemispheric processes at forebrain and brainstem levels?

The evidence is convincing that each human cerebral hemisphere is capable of human mental activity. This being so, every normal human thought and action demands either a consensus between the two hemispheres, or a dominance of one over the other, in any event integrated into a unity of conscious mentation. How this is achieved remains wholly mysterious, but anatomical and behavioral data suggest that the two hemispheres, and their respective bilateral, anatomical-functional components, maintain a dynamic equilibrium through neural competition. While the forebrain commissures must contribute substantially to this competitive process, it is emphasized in this review that the serotonergic raphé nuclei of pons and mesencephalon are also participants in interhemispheric events. Each side of the raphé projects heavily to both sides of the forebrain, and each is in receipt of bilateral input from the forebrain and the habenulo-interpeduncular system. A multifarious loop thus exists between the two hemispheres, comprised of both forebrain commissural and brainstem paths. There are many reasons for believing that perturbation of this loop, by a variety of pathogenic agents or processes, probably including severe mental stress in susceptible individuals, underlies the extraordinarily diverse symptomatology of schizophrenia. Abnormality of features reflecting interhemispheric processes is common in schizophrenic patients; and the 'first rank' symptoms of delusions or hallucinations are prototypical of what might be expected were the two hemispheres unable to integrate their potentially independent thoughts. Furthermore, additional evidence suggests that the disorder lies within, or is focused primarily through, the raphé serotonergic system, that plays such a fundamental role in consciousness, in dreaming, in response to psychotomimetic drugs, and probably in movement, and even the trophic state of the neocortex. This system is also well situated to control the dopaminergic neurons of the ventral tegmental area, thus relating to the prominence of dopaminergic features in schizophrenia; and the lipofuscin loading and intimate relation with blood vessels and ependyma may make neurons of the raphé uniquely vulnerable to deleterious agents.

Slow fluctuations of single unit activities of hippocampal and thalamic neurons in cats. II. Role of serotonin on the stability of neuronal activities

A series of experiments was carried out both in the hippocampal pyramidal and thalamic ventrobasal neurons to investigate the effect of serotonin level in the brain on slow fluctuations of neuronal discharges. Single neuronal activities were recorded in the following two pharmacologically treated states: (1) a 5-hydroxytryptamine depleted state by p-chlorophenylalanine administration (PCPA phase) and (2) a 5-methoxy-N,N-dimethyltryptamine administered state under the PCPA pretreatment (5-MeODMT phase). The slow fluctuations of neuronal activities in the frequency range of 0.02-1.0 Hz in both nuclei were prominent during the PCPA phase and were similar to those during the paradoxical sleep. In contrast, slow fluctuations were suppressed during the 5-MeODMT phase and neuronal activities during this phase were similar to those during slow wave sleep (SWS). The results show that serotonin in the brain definitely plays a role in stabilizing single neuronal activities.

Slow fluctuations of single unit activities of hippocampal and thalamic neurons in cats. I. Relation to natural sleep and alert states

Spontaneous unit discharges during the natural sleep-wakefulness cycle in two different neuronal groups, the hippocampal pyramidal cells and thalamic ventrobasal neurons, have been analyzed. The results show that both neurons fire with white-noise-like fluctuations during the slow-wave sleep, and with slow fluctuations with power spectral densities inversely proportional to the frequency in the frequency range of 0.02-1.0 Hz, during the paradoxical sleep. This confirms that the characteristics of fluctuations in neuronal activities of the mesencephalic reticular formation observed in our previous study are more general phenomena in the cat's brain. Partly similar behavior of spectral densities is also observed during the alert state. These observations are quantitatively confirmed by the statistical time series analysis of the spike density processes of spontaneous activities.

Hippocampal Noradrenaline and Serotonin Release over 24 Hours as Measured by the Dialysis Technique in Freely Moving Rats: Correlation to Behavioural Activity State, Effect of Handling and Tail-Pinch

Hippocampal extracellular levels of noradrenaline (NA), 5-hydroxytryptamine (5-HT), and 5-hydroxyindoleacetic acid (5-HIAA) were monitored with the microdialysis technique in freely moving rats. In one experiment 30 min samples were collected during 24 h of continuous perfusion, and the monoamine output was compared to the behavioural activity state, as arbitrarily classified in three categories: sleep/rest, drowsiness and full alertness associated with complex behaviours. In the individual animal the hippocampal NA and 5-HT output showed pronounced fluctuations during the 24 h period, but the 30 min sampling times did not allow for a clear-cut correlation to behavioural activity state. However, the mean NA and 5-HT output for all animals during the dark period of the day was 43 and 38% higher, respectively, than during the light period, and the average NA and 5-HT levels in samples collected during periods of high behavioural activity was 34 and 45% higher, respectively, than during periods of rest or sleep. In contrast, there were no detectable changes in extracellular 5-HIAA. The selective serotonin uptake blocker indalpine, added to the perfusion fluid at 1 microM, increased the extracellular 5-HT levels 6-fold, with a similar correlation to behavioural activity state as without indalpine. In a second experiment the effect of handling and tail-pinch was studied in 15 min sample fractions. Gentle handling of the animals during the sampling period increased the hippocampal NA and 5-HT output by 32 and 72%, respectively, and a similar increase (63 and 48%) was obtained by application of tail-pinch. Maximum NA output was reached during the handling or tail-pinch period, whereas maximal 5-HT levels were detected in the subsequent 15 min sample fraction. No changes in extracellular 5-HIAA was observed. It is concluded (1) that intracerebral microdialysis provides a useful method for the study of extracellular NA and 5-HT in the hippocampal formation of conscious rats during active behaviour; (2) that there are substantial fluctuations in hippocampal NA and 5-HT output in freely moving rats which correlate with the light - dark cycle as well as with the activity state of the animals; (3) that the spontaneous variations in 5-HT output are maintained during reuptake blockade; and (4) that behavioural activation through gentle handling or tail-pinch elicits NA and 5-HT release. The present data support a role of the forebrain NA and 5-HT systems in behavioural state control and highlights the necessity of experimental designs in which the spontaneous fluctuations in transmitter release are controlled for in studies of, for example, drug effects on NA and 5-HT release in conscious animals.