Axonal and somato-dendritic modalities of serotonin release: their involvement in sleep preparation, triggering and maintenance

Serotonergic system may be involved in alterations of sleep homeostasis in spontaneously hypertensive rats

Hypertension is associated with sleep disorders. Spontaneously hypertensive rats are derived from Wistar-Kyoto rats and widely used in research on hypertension. The present study investigated the propensity to sleep and electroencephalographic spectrum changes over 24 hr in spontaneously hypertensive rats, and proposed the involvement of the serotonergic system in these alterations. Time-course analysis showed that spontaneously hypertensive rats exhibit hyperarousal during the light phase but hypersomnia during the dark phase. Spontaneously hypertensive rats also exhibited less slight fluctuation in electroencephalographic delta power density over 24 hr as compared with Wistar-Kyoto rats, suggesting that the accumulation or elimination of sleep pressure was disrupted. Sleep deprivation disrupted the regulation of sleep homeostasis in spontaneously hypertensive rats, reflected by less sleep time and poor sleep quality during the recovery period. The density and activity of serotonergic neurons in the dorsal raphe nucleus were higher in spontaneously hypertensive rats compared with Wistar-Kyoto rats. Interestingly, we observed the absence of fluctuations in 5-hydroxytryptamine and 5-hydroxyindoleacetic acid across the sleep, wake, sleep deprivation and sleep recovery stages in spontaneously hypertensive rats, which were dramatically different from Wistar-Kyoto rats. These results indicate that the disruption of sleep-wake pattern and sleep homeostasis in spontaneously hypertensive rats might be related to abnormalities of the serotonergic system.

Role of nitric oxide and WNK-SPAK/OSR1-KCC2 signaling in daily changes in GABAergic inhibition in the rat dorsal raphe neurons

Serotonergic neurons in the dorsal raphe nucleus (DRN) act as wake-inducing neurons in the sleep-wake cycle and are controlled by gamma-aminobutyric acid (GABA) synaptic inputs. We investigated daily changes in GABAergic inhibition of the rat DRN neurons and the role of nitric oxide (NO) and cation-chloride co-transporters in the GABAergic action. Neuronal NO synthase (nNOS) was co-expressed in 74% of serotonergic DRN neurons and nNOS expression was higher during daytime (the sleep cycle) than that during nighttime (the wake cycle). GABAergic hyperpolarization of DRN neurons produced by GABAA receptor agonist muscimol was greater and the equilibrium potential of muscimol showed a hyperpolarizing shift during daytime compared to that during nighttime. Expression levels of potassium-chloride co-transporter 2 (KCC2) were higher during daytime than that during nighttime, whereas there were no changes in sodium-potassium-chloride co-transporter 1 (NKCC1) expression. With-no-lysine kinase (WNK) isoform 1 was more highly expressed during daytime than that during nighttime. Total Ste20-related proline alanine rich kinase (SPAK) and oxidative stress response kinase 1 (OSR1) were also higher during daytime than during nighttime, while there were no changes in phosphorylated SPAK and OSR1. Consistent with the findings during the sleep-wake cycle, ex vivo treatment of DRN slices with a NO donor sodium nitroprusside (SNP) increased the expression of KCC2, WNK1, WNK2, WNK3, SPAK, and OSR1, whilst decreasing phosphorylated SPAK. These results suggest that GABAergic synaptic inhibition of DRN serotonergic neurons shows daily changes during the sleep-wake cycle, which might be regulated by daily changes in nNOS-derived NO and WNK-SPAK/OSR1-KCC2 signaling.

REM Sleep Rebound as an Adaptive Response to Stressful Situations

Stress and sleep are related to each other in a bidirectional way. If on one hand poor or inadequate sleep exacerbates emotional, behavioral, and stress-related responses, on the other hand acute stress induces sleep rebound, most likely as a way to cope with the adverse stimuli. Chronic, as opposed to acute, stress impairs sleep and has been claimed to be one of the triggering factors of emotional-related sleep disorders, such as insomnia, depressive- and anxiety-disorders. These outcomes are dependent on individual psychobiological characteristics, conferring even more complexity to the stress-sleep relationship. Its neurobiology has only recently begun to be explored, through animal models, which are also valuable for the development of potential therapeutic agents and preventive actions. This review seeks to present data on the effects of stress on sleep and the different approaches used to study this relationship as well as possible neurobiological underpinnings and mechanisms involved. The results of numerous studies in humans and animals indicate that increased sleep, especially the rapid eye movement phase, following a stressful situation is an important adaptive behavior for recovery. However, this endogenous advantage appears to be impaired in human beings and rodent strains that exhibit high levels of anxiety and anxiety-like behavior.

Nitric oxide in the regulation of the sleep-wake states

Nitric oxide (NO) production involves four different NO-synthases (NOSs) that are either constitutive (neuronal, nNOS; endothelial, eNOS; mitochondrial, mNOS) or inducible (iNOS) in nature. Three main processes regulate NO/NOSs output, i.e., the L-arginine/arginase substrate-competing system, the L-citrulline/arginosuccinate-recycling system and the asymmetric dimethyl-/monomethyl-L-arginine-inhibiting system. In adult animals, nNOS exhibits a dense innervation intermingled with pontine sleep structures. It is well established that the NO/nNOS production makes a key contribution to daily homeostatic sleep (slow-wave sleep, SWS; rapid eye movement sleep, REM sleep). In the basal hypothalamus, the NO/nNOS production further contributes to the REM sleep rebound that takes place after a sleep deprivation (SD). This production may also contribute to the sleep rebound that is associated with an immobilization stress (IS). In adult animals, throughout the SD time-course, an additional NO/iNOS production takes place in neurons. Such production mediates a transitory SD-related SWS rebound. A transitory NO/iNOS production is also part of the immune system. Such a production contributes to the SWS increase that accompanies inflammatory events and is ensured by microglial cells and astrocytes. Finally, with aging, the iNOS expression becomes permanent and the corresponding NO/iNOS production is important to ensure an adequate maintenance of REM sleep and, to a lesser extent, SWS. Despite such maintenance, aged animals, however, are not able to elicit a sleep rebound to deal with the challenge of SD or IS. Sleep regulatory processes in adult animals thus become impaired with age. Reduced iNOS expression during aging may contribute to accelerated senescence, as observed in senescence-accelerated mice (SAMP-8 mice).

The involvement of noradrenaline in rapid eye movement sleep mentation

Noradrenaline, one of the main brain monoamines, has powerful central influences on forebrain neurobiological processes which support the mental activities occurring during the sleep-waking cycle. Noradrenergic neurons are activated during waking, decrease their firing rate during slow wave sleep, and become silent during rapid eye movement (REM) sleep. Although a low level of noradrenaline is still maintained during REM sleep because of diffuse extrasynaptic release without rapid withdrawal, the decrease observed during REM sleep contributes to the mentation disturbances that occur during dreaming, which principally resemble symptoms of schizophrenia but seemingly also of attention deficit hyperactivity disorder.

The development of the science of dreaming

Although the main peripheral features of dreaming were identified two millennia ago, the neurobiological study of the basic and higher integrated processes underlying rapid eye movement (REM) sleep only began about 70 years ago. Today, the combined contributions of the successive and complementary methods of electrophysiology, imaging, pharmacology, and neurochemistry have provided a good level of knowledge of the opposite but complementary activating and inhibitory processes which regulate waking mentation and which are disturbed during REM sleep, inducing a schizophrenic-like mental activity.

Brain structures and mechanisms involved in the control of cortical activation and wakefulness, with emphasis on the posterior hypothalamus and histaminergic neurons

Wakefulness is a functional brain state that allows the performance of several "high brain functions", such as diverse behavioural, cognitive and emotional activities. Present knowledge at the whole animal or cellular level suggests that the maintenance of the cerebral cortex in this highly complex state necessitates the convergent and divergent activity of an ascending network within a large reticular zone, extending from the medulla to the forebrain and involving four major subcortical structures (the thalamus, basal forebrain, posterior hypothalamus and brainstem monoaminergic nuclei), their integral interconnections and several neurotransmitters, such as glutamate, acetylcholine, histamine and noradrenaline. In this mini-review, the importance of the thalamus, basal forebrain and brainstem monoaminergic neurons in wake control is briefly summarized, before turning our attention to the posterior hypothalamus and histaminergic neurons, which have been far less studied. Classical and recent experimental data are summarized, supporting the hypothesis that (1) the posterior hypothalamus constitutes one of the brain ascending activating systems and plays an important role in waking; (2) this function is mediated, in part, by histaminergic neurons, which constitute one of the excitatory sources for cortical activation during waking; (3) the mechanisms of histaminergic arousal involve both the ascending and descending projections of histaminergic neurons and their interactions with diverse neuronal populations, such as neurons in the pre-optic area and cholinergic neurons; and (4) other widespread-projecting neurons in the posterior hypothalamus also contribute to the tonic cortical activation during wakefulness and/or paradoxical sleep.

Monoaminergic system lesions increase post-sigh respiratory pattern disturbance during sleep in rats

Monoamines are important regulators of behavioral state and respiratory pattern, and the impact of monoaminergic control during sleep is of particular interest for the stability of breathing regulation. The aim of this study was to test the effects of systemically induced chemical lesions to noradrenergic and serotonergic efferent systems, on the expression of sleep-wake states, pontine wave activity, and sleep-related respiratory pattern and its variability. In chronically instrumented male adult Sprague-Dawley rats we lesioned noradrenergic terminal axonal branches by a single intraperitoneal dose of DSP-4 (N-(2-chloroethyl)-N-ethyl-2-brombenzilamine; 50 mg/kg, i.p.), and serotonergic axonal terminals by two intraperitoneal doses, 24 h apart, of PCA (p-chloroamphetamine; 6 mg/kg, i.p.). In each animal, we recorded sleep, pontine waves (P-waves) and breathing at baseline, following sham injection, and every week for 5 weeks following injection of either systemic neurotoxin. Distinct responses were observed to the two lesions. DSP-4 lesions were associated with a trend toward increased NREM sleep (p < 0.06), decreased wakefulness (p < 0.05) and increased respiratory tidal volume during NREM (p = 0.0002) and REM (p = 0.0001) sleep with respect to baseline. None of these effects, however, were observed during the first 14 days after injection. No significant changes were observed in the frequency of apneas or sighs, nor in the coupling between these two, at any time after DSP-4 injection. Conversely, selective serotonergic lesion by PCA produced no change in the baseline respiratory frequency or tidal volume during sleep or wakefulness, nor was the expression of Wake, NREM or REM sleep affected. Instead, PCA injection resulted in a sustained increase in the frequency and duration of post-sigh apneas (PS) during NREM sleep (p = 0.002). This reflected increased coupling between sighs and apneas, because neither the frequency nor the amplitude of spontaneous sighs was altered by PCA.

Behavioral correlates of activity in identified hypocretin/orexin neurons

Micropipette recording with juxtacellular Neurobiotin ejection, linked micropipette-microwire recording, and antidromic and orthodromic activation from the ventral tegmental area and locus coeruleus were used to identify hypocretin (Hcrt) cells in anesthetized rats and develop criteria for identification of these cells in unanesthetized, unrestrained animals. We found that Hcrt cells have broad action potentials with elongated later positive deflections that distinguish them from adjacent antidromically identified cells. They are relatively inactive in quiet waking but are transiently activated during sensory stimulation. Hcrt cells are silent in slow wave sleep and tonic periods of REM sleep, with occasional burst discharge in phasic REM. Hcrt cells discharge in active waking and have moderate and approximately equal levels of activity during grooming and eating and maximal activity during exploratory behavior. Our findings suggest that these cells are activated during emotional and sensorimotor conditions similar to those that trigger cataplexy in narcoleptic animals.

Nitric oxide and sleep

Nitric oxide (NO) is a biological messenger synthesized by three main isoforms of NO synthase (NOS): neuronal (nNOS, constitutive calcium dependent), endothelial (eNOS, constitutive, calcium dependent) and inducible (iNOS, calcium independent). NOS is distributed in the brain either in circumscribed neuronal sets or in sparse interneurons. Within the laterodorsal tegmentum (LDT), pedunculopontine tegmentum and dorsal raphe nucleus, NOS-containing neurons overlap neurons grouped according to their contribution to sleep mechanisms. The main target for NO is the soluble guanylate cyclase that triggers an overproduction of cyclic guanosine monophosphate. NO in neurons of the pontine tegmentum facilitates sleep (particularly rapid-eye-movement sleep), and NO contained within the LDT intervenes in modulating the discharge of the neurons through an auto-inhibitory process involving the co-synthesized neurotransmitters. Moreover, NO synthesized within cholinergic neurons of the basal forebrain, while under control of the LDT, may modulate the spectral components of the EEG instead of the amounts of different sleep states. Finally, impairment of NO production (e.g. neurodegeneration, iNOS induction) has identifiable effects, including ageing, neuropathologies and parasitaemia.

Inhibition of NADH oxidation by chloramphenicol in the freely moving rat measured by picosecond time-resolved emission spectroscopy

Owing to the lack of methods capable to monitor the energetic processes taking place within small brain regions (i.e. nucleus raphe dorsalis, nRD), the neurotoxicity of various categories of substances, including antibiotics and psycho-active drugs, still remains difficult to evaluate. Using an in vivo picosecond optical spectroscopy imaging method, we report that chloramphenicol (CAP), besides its well-known ability to inhibit the mitochondria protein synthesis, also influences the NADH/NAD+ redox processes of the respiratory chain. At a 200-mg/kg dose, CAP indeed produces a marked increase in the fluorescent signal of the nRD which, according to clear evidence, is likely to be related to the NADH concentration. This effect also implies an efficient inhibition of complex I of the respiratory chain by CAP. It refers to the mechanism through which the adverse effects of the antibiotic may take place. It could explain why paradoxical sleep, a state needing aerobic energy to occur, is suppressed after CAP administration. The present approach constitutes the first attempt to determine by fluorescence methods the effects of substances on deep brain structures of the freely moving animal. It points out that in vivo ultrafast optical methods are innovative and adequate tools for combined neurochemical and behavioural approaches.

The effect of restraint stress on paradoxical sleep is influenced by the circadian cycle

It is well known that the physiological impact imposed by events or behaviors displayed during the waking period determines the way organisms sleep. Among the situations known to affect sleep both in its duration and quality, stress has been widely studied and it is now admitted that its effects on sleep architecture depend on several factors specific to the stressor or the individual itself. Although numerous reports have highlighted the prominent role of the circadian cycle in the physiological, endocrine and behavioral consequences of restraint stress, a possible circadian influence in the effects of stress on the sleep-wake cycle has never been studied. Thus the present study was designed to compare the effects on sleep of a 1 h-lasting restraint stress applied at light onset to those observed after the same stressor was applied at light offset. We report that in both conditions stress induced a marked paradoxical sleep increase, whereas wakefulness displayed a moderate decrease and slow wave sleep a moderate augmentation. Although the effects of stress at lights on were of similar magnitude than those of stress at lights off, important differences in the sleep rebound latencies were observed: whatever the time of day the stress was applied, its effects on sleep always occurred during the dark period. This result thus shows that restraint stress could be efficiently used to study the interaction between the circadian and homeostatic components of sleep regulation.

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.

Differential effects of acute cold and footshock on the sleep of rats

Several studies have shown that 1 h of immobilisation stress during the rat's active period results in rebound of paradoxical (PS) and slow wave sleep (SWS). Since the effects of stress on behaviour and physiological parameters vary according to the stimulus, the present study sought to examine the activation of the hypothalamic-pituitary-adrenal (HPA) axis and the sleep pattern of rats submitted to 1 h of footshock, immobilisation or cold, or 18 h of PS deprivation (PSD). Stress sessions began between 0900 and 0930 h. Immediately after the end of the stress session, or at the corresponding time for controls, animals were blood sampled for determination of ACTH and corticosterone (CORT) plasma levels. In Experiment 2, animals were implanted with electrodes for basal and post-stress polysomnographic recording (6 h long). The results showed that all stressors produced an activation of the HPA axis; however, footshock induced the largest ACTH levels, whereas cold resulted in the highest CORT levels. In regard to the sleep data, immobilisation and PSD led to a rebound of SWS (+16.87% and +9.37%, respectively) and PS (+42.45% and +55.25%, respectively). Immobilisation, however, induced an increased number of PS episodes, whereas PSD resulted in longer PS episodes. Cold stress produced an exclusive rebound of SWS (+14.23%) and footshock promoted sustained alertness during the animal's resting period (+47.18%). These results indicate that different stimuli altered the sleep pattern in a distinct manner; and these alterations might be related to the state of the HPA axis activation.

Serotonin and the sleep/wake cycle: special emphasis on microdialysis studies

Several areas in the brainstem and forebrain are important for the modulation and expression of the sleep/wake cycle. Even if the first observations of biochemical events in relation to sleep were made only 40 years ago, it is now well established that several neurotransmitters, neuropeptides, and neurohormones are involved in the modulation of the sleep/wake cycle. Serotonin has been known for many years to play a role in the modulation of sleep, however, it is still very controversial how and where serotonin may operate this modulation. Early studies suggested that serotonin is necessary to obtain and maintain behavioral sleep (permissive role on sleep). However, more recent microdialysis experiments provide evidence that the level of serotonin during W is higher in most cortical and subcortical areas receiving serotonergic projections. In this view the level of extracellular serotonin would be consistent with the pattern of discharge of the DRN serotonergic neurons which show the highest firing rate during W, followed by a decrease in slow wave sleep and by virtual electrical silence during REM sleep. This suggests that during waking serotonin may complement the action of noradrenaline and acetylcholine in promoting cortical responsiveness and participate to the inhibition of REM-sleep effector neurons in the brainstem (inhibitory role on REM sleep). The apparent inconsistency between an inhibitory and a facilitatory role played by serotonin on sleep has at least two possible explanations. On the one hand serotonergic modulation on the sleep/wake cycle takes place through a multitude of post-synaptic receptors which mediate different or even opposite responses; on the other hand the achievement of a behavioral state depends on the complex interaction between the serotonergic and other neurotransmitter systems. The main aim of this commentary is to review the role of brain serotonin in relation to the sleep/wake cycle. In particular we highlight the importance of microdialysis for on-line monitoring of the level of serotonin in different areas of the brain across the sleep/wake cycle.

Nitric oxide and sleep in the rat: a puzzling relationship

To date, only a few studies indicate that nitric oxide may play a role in the regulation of the sleep-wake cycle. However, data reported are controversial and the part played by nitric oxide in sleep-wake cycle regulation still remains uncertain. In the present report, we studied the effects on sleep amounts of two different nitric oxide synthase inhibitors: N-nitro-L-arginine methyl ester, a non-selective nitric oxide synthase inhibitor, and 7-nitro-indazole, a specific inhibitor of neuronal nitric oxide synthase. The above compounds were administered via two routes, i.e. intraperitoneally or locally in the dorsal raphe nucleus, a structure involved in sleep regulation. In order to evaluate their efficiency to inhibit nitric oxide synthesis in the rat brain, they were first administered intraperitoneally to a group of animals, and the cortical release of nitric oxide was determined by means of voltammetric measurements. N-Nitro-L-arginine methyl ester (100 mg/kg, i.p.) did not affect the cortical release of nitric oxide, whereas it increased both slow-wave sleep and paradoxical sleep durations. On the contrary, 7-nitro-indazole (40 mg/kg, i.p.) significantly decreased the cortical release of nitric oxide (-25%) and paradoxical sleep duration. Furthermore, following microinjection of either N-nitro-L-arginine methyl ester or 7-nitro-indazole at 100 ng/0.20 microl into the nitric oxidergic cell area of the dorsal raphe nucleus, decreases in paradoxical sleep duration were obtained (-32.8% and -25.3%, respectively). The results obtained support the existence of a duality in the sleep regulation modalities exerted by nitric oxide, i.e. a peripheral inhibiting influence and a central facilitating role for the nitric oxide-serotoninergic neurons of the dorsal raphe nucleus.

Key role of 5-HT1B receptors in the regulation of paradoxical sleep as evidenced in 5-HT1B knock-out mice

The involvement of 5-HT1B receptors in the regulation of vigilance states was assessed by investigating the spontaneous sleep-waking cycles and the effects of 5-HT receptor ligands on sleep in knock-out (5-HT1B-/-) mice that do not express this receptor type. Both 5-HT1B-/- and wild-type 129/Sv mice exhibited a clear-cut diurnal sleep-wakefulness rhythm, but knock-out animals were characterized by higher amounts of paradoxical sleep and lower amounts of slow-wave sleep during the light phase and by a lack of paradoxical sleep rebound after deprivation. In wild-type mice, the 5-HT1B agonists CP 94253 (1-10 mg/kg, i.p.) and RU 24969 (0.25-2.0 mg/kg, i.p.) induced a dose-dependent reduction of paradoxical sleep during the 2-6 hr after injection, whereas the 5-HT1B/1D antagonist GR 127935 (0.1-1.0 mg/kg, i.p.) enhanced paradoxical sleep. In addition, pretreatment with GR 127935, but not with the 5-HT1A antagonist WAY 100635, prevented the effects of both 5-HT1B agonists. In contrast, none of the 5-HT1B receptor ligands, at the same doses as those used in wild-type mice, had any effect on sleep in 5-HT1B-/- mutants. Finally, the 5-HT1A agonist 8-OH-DPAT (0.2-1.2 mg/kg, s.c.) induced in both strains a reduction in the amount of paradoxical sleep. Altogether, these data indicate that 5-HT1B receptors participate in the regulation of paradoxical sleep in the mouse.

Activity of ventromedial hypothalamic neurons suppressing heart rate is associated with paradoxical sleep in the rat

Cardiovascular change is one of the common features of paradoxical sleep. Our study offers evidence that one of the central areas regulating the circulation during sleep is the ventromedial nucleus of the hypothalamus (VMH). We found a group of neurons in this hypothalamic nucleus of rats whose electrical activity was exclusively increased during paradoxical sleep, and was associated with a reduction in heart rate. The onset of this neural activity usually followed that of paradoxical sleep. The incidence and duration of paradoxical sleep was increased by means of microinjection of carbachol, a cholinergic agonist, into the pontine reticular formation, and the neural activity of the VMH still appeared in synchrony with carbachol-induced paradoxical sleep. These results suggest that the cholinergic paradoxical sleep-inducing mechanism in the pons facilitate the excitability of these neurons. We have previously shown that these VMH neurons suppress blood pressure and heart rate via inhibition of the vasomotor neurons in the medulla oblongata. Taken together, our findings suggest that a group of neurons in the VMH suppresses the circulatory system during paradoxical sleep.

Effects of interferon-gamma, interleukin-1 beta, and tumor necrosis factor-alpha on the serotonin metabolism in the nucleus raphe dorsalis of the rat

The effects of the cytokines interferon (IFN)-gamma, interleukin (IL)-1, and tumor necrosis factor (TNF)-alpha on the serotoninergic transmission in the nucleus raphe dorsalis (NRD) were studied after peripheral and central application. The studies were performed in the freely moving rat using differential pulse voltammetry with multicarbon fibre electrodes to study the extracellular levels of the serotonin (5-HT) metabolite 5-hydroxyindoleacetic acid (5-HIAA). The extracellular 5-HIAA levels were not changed in the NRD after peripheral application of rat recombinant IFN-gamma, but elevated by the cytokines IL-1 beta and TNF-alpha. After intracerebroventricular (i.c.v.) application the cytokines IFN-gamma, IL-1 beta and TNF-alpha stimulated the serotoninergic transmission in the NRD. Our data suggest that the effect of peripherally elevated cytokine concentrations on the serotonin metabolism in the NRD of the rat is cytokine-dependent. In this respect the T-cell and NK-cell cytokine IFN-gamma acts clearly different when compared to the mainly macrophage-derived cytokines IL-1 beta and TNF-alpha, and plays a different role in the communication between immune and central nervous system.

Determination of NADH in the rat brain during sleep-wake states with an optic fibre sensor and time-resolved fluorescence procedures

The present paper reports a nanosecond time-resolved fluorescence derived from the cortex and the area of the periaqueductal gray including the nucleus raphe dorsalis (PAG-nRD) in unanaesthetized freely moving rats. The measurements were acquired through a single optic fibre transmitting a subnanosecond nitrogen laser pulse (337 nm, 15 Hz) and collecting the brain fluorescence occurring at 460 nm which might depend on mitochondrial NADH (reduced form of nicotinamide adenine dinucleotide). The fluorometric method was combined with polygraphic recordings, and this procedure allowed us to define, for the first time, variations of the 460 nm signal occurring throughout the sleep-wake cycle. In the PAG-nRD, the signal exhibited moderate heterogeneous variation in amplitude during slow-wave as compared to the waking state. Constant increases were observed during paradoxical sleep as compared to the waking state. For this state of sleep the magnitude of the variations depended on the optic fibre location. In the cortex and during either slow-wave sleep or paradoxical sleep, the signal presented moderate increases which were significant during paradoxical sleep. The magnitude of the redox variations observed either in the PAG-nRD or in the cortex might be ascribed to the oxidative energy balance which is related to sleep states.

Inter-individual differences in the effects of acute stress on the sleep-wakefulness cycle in the rat

It has been described that an acute immobilization stress (IS) modifies subsequent paradoxical sleep (PS). However, its effects are complex because some subjects remain unaffected. This discrepancy might result from constitutive inter-individual psychobiological differences. In order to test this hypothesis, an inter-individual analysis of sleep patterns and their modifications after 60 min IS has been performed. Even though global analysis showed a PS increase after IS, inter-individual analysis evidenced different PS reactivity; subjects which had the least PS during control recordings were those with the largest PS increase. Unlike global analysis, an inter-individual study evidenced different modifications of wakefulness and slow wave sleep according to individuals. Subjects presenting the highest amount of wakefulness in control conditions (the lowest amount of slow wave sleep) decreased their wakefulness amount, while subjects with the lowest amount of wakefulness increased it. Thus, individual characteristics of the sleep-wakefulness cycle should be considered while studying its modifications induced by different treatments.

Influence of a 1 h immobilization stress on sleep states and corticotropin-like intermediate lobe peptide (CLIP or ACTH18-39, Ph-ACTH18-39) brain contents in the rat

A 1 h immobilization stress (IS) was imposed to rats at the beginning of the dark period, i.e., when the animals start to be active. The IS was accompanied by an intense polygraphic waking and followed, over 12 h of the dark period, by a significant rebound of slow-wave sleep (SWS, +17%) and paradoxical sleep (PS, +57%). In order to estimate the IS-related changes in the endogenous concentrations of corticotropin-like intermediate lobe peptide (CLIP, ACTH18-39) and related compounds, a specific radioimmunoassay (RIA) was used. Assays performed in cerebral biopsies taken from arcuate (AN) and raphe dorsalis (nRD) nuclei led to the obtention of 2 main immunoreactive peaks, corresponding to CLIP and its phosphorylated form Ph-CLIP. Just after end of the IS and within the nRD. Ph-CLIP immunoreactivity increased by about 95%. Four hours later, i.e., when PS rebound was maximal, a 37% increase in Ph-CLIP immunoreactivity was measured in the AN. These observations have never been described before. In the blood, at the end of the restraint, CLIP/ACTH1-39 total immunoreactivity was increased by 330%. It returned to baseline level 4 h later. Blood concentration of corticosterone was also increased by 56% at the end of the IS and was close to baseline level 4 h later. Data reported here indicate that the IS first triggers an increase in Ph-CLIP within the nRD. Since the nRD contains sleep permissive components, this increase might be determinant for the SWS and PS rebound induction. The changes observed in the blood as regards CLIP/ACTH1-39 total immunoreactivity and corticosterone concentration testify to the efficacy of the IS and are part of the conventional picture accompanying such a situation. Finally, the increase in Ph-CLIP, occurring in the AN 4 h after the end of the restraint, might be part of the restorative processes necessary to compensate the stress overshoot.

The vigilance promoting drug modafinil decreases GABA release in the medial preoptic area and in the posterior hypothalamus of the awake rat: possible involvement of the serotonergic 5-HT3 receptor

The effect of modafinil on endogenous gamma-aminobutyric acid (GABA) release in the medial preoptic area (MPA) and posterior hypothalamus (PH) and the role of local 5-HT3 receptors in this effect was investigated in the awake rat using in vivo microdialysis. Modafinil (30-100 mg/kg i.p.) dose-dependently decreased GABA release from the MPA, while only the 100 mg/kg dose markedly reduced GABA release in the PH. The modafinil (100 mg/kg) induced inhibition of GABA release in the MPA and the PH was partially counteracted by the 5-HT3 receptor antagonist MDL72222 (1 microM) when perfused locally alone or together with the non-selective 5-HT receptor antagonist methysergide (1 microM). Thus, the reduction of GABA transmission induced by modafinil in the MPA and in the PH, at least in part, involves local 5-HT3 receptors. The GABA release inhibition by modafinil in the above areas may be relevant for its vigilance promoting action.

5-HT2 receptors are partially involved in the relationship between renin release and delta relative power

A strong relationship was previously described between the nocturnal oscillations of plasma renin activity (PRA) and the sleep cycles, with levels of PRA that increase during non rapid eye movement sleep and decrease during rapid eye movement sleep. This study was designed to determine whether ritanserin, a 5-hydroxytryptamine-2 (5-HT2) receptor antagonist known to increase slow wave sleep both in human and in animals and to decrease plasma renin activity response to serotonergic stimulation in the rat, would uncouple this relationship. Eight subjects underwent two randomized night studies after having received either placebo or 5 mg ritanserin administered in the morning. They were subjected to 8 hour polysomnography, including spectral analysis of the electroencephalogram and to continuous blood sampling. Blood was sampled from 2300 to 700h every 10 min and plasma renin activity (PRA) was measured by radioimmunoassay of angiotensin 1. The nocturnal profiles were analysed using the pulse detection program ULTRA. Ritanserin produced the expected increase in slow wave sleep (SWS) duration (132 +/- 10 min under ritanserin vs 72 +/- 9 min under placebo; p < 0.001) and a significant increase in delta relative power (69 +/- 2% under ritanserin vs 60 +/- 2% under placebo; p < 0.01). The mean overnight PRA levels had a tendency to decrease under ritanserin (1.66 +/- 0.34 ngAngl/ml per h under ritanserin vs 1.48 +/- 0.31 ngAngl/ml per h under placebo; p = 0.08). Individual PRA oscillations were preserved and remained strongly associated with delta power oscillations. PRA peak levels were similar in both experimental conditions, but the absolute amplitude of the oscillations was decreased under ritanserin (1.50 +/- 0.36 ngAngl/ml per h vs 1.04 +/- 0.14 ngAngl/ml per h; p < 0.05). These results demonstrate that ritanserin, at a dose that augments delta power, only weakly affects renin release, which suggests that 5-HT2 receptors are only partially involved in the processes coupling renin release and SWS and that other mechanisms probably control the sleep-associated variations in PRA.

Sleep permissive components within the dorsal raphe nucleus in the rat

Two peptides known for their hypnogenic properties, CLIP (corticotropin-like intermediate lobe peptide or ACTH 18-39) or VIP (vasoactive intestinal polypeptide), were injected locally into the nucleus raphe dorsalis (nRD) of rats pretreated with p-chlorophenylalanine (PCPA). During the dark period, the PCPA insomnia was primarily associated with a reduction in paradoxical sleep (PS), whereas both slow wave sleep (SWS) and PS were decreased during the light period. Immunohistochemistry of serotonin in PCPA-pretreated animals indicated a clear disappearance of 5-HT fibers in the basal hypothalamus and the nRD as compared to control animals. Local injections of CLIP or VIP in the nRD restored PS and SWS. The positive injection sites corresponded to the anatomical distribution of either CLIP or VIP fibers, i.e., the entire nRD for VIP and the antero-dorsal part of this nucleus for CLIP. The sleep effects obtained in PCPA-pretreated rats involve a non-5-HT sleep permissive component within the nRD upon which these injected peptides act.

Sleep regulation: interactions among cytokines and classical neurotransmitters

The role of classical neurotransmitters in sleep regulation is amply documented (Hobson and Steriade, 1986). In recent years evidence has been gathered that immunoactive molecules, infectious agents and their components, or cytokines play some part in sleep regulation (Krueger and Obál, 1994; Opp et al., 1992; Moldofsky, 1994). Different cytokines possess hypnogenic properties when injected centrally or systemically to different animal species and their role in physiological sleep regulation is currently under investigation. Little is known of how cytokines and classical neurotransmitters interact and of the relevance of this interaction in sleep induction and maintenance. The present paper (i) reviews data on this topic; (ii) proposes a unitary interpretation whenever possible; and (iii) raises questions that might be addressed by future studies.

The influence of ipsapirone, a 5-HT1A agonist, on sleep patterns of healthy subjects

Ipsapirone is a new pyrimidinylpiperazine ligand specific for 5-HT1A receptors, with potential therapeutic use in affective disorders. Because 5-HT is involved in the regulation of sleep, we investigated the effect of ipsapirone hydrochloride on sleep patterns in 18 normal, healthy subjects of both sexes. Compared to placebo, ipsapirone 5 mg administered by mouth three times daily for 14 days decreased rapid eye movement (REM) sleep duration and, by the tenth day of treatment, began to reduce slow wave sleep (SWS) duration. The decrease in REM sleep occurred in the first 3 h of sleep. The latency to REM sleep was increased from the first night following ipsapirone administration, remained increased throughout the 14 days of administration, and fell to equal latency on placebo immediately administration ended. Subjective assessments of sleep revealed no differences between ipsapirone and placebo. Our experiments confirm a role of 5-HT1A receptors in sleep. The effects of ipsapirone on the sleep patterns of patients with affective disorders still need to be determined.

c-Fos expression during wakefulness and sleep

We have recently demonstrated that c-fos expression is strongly induced by both spontaneous and forced wakefulness in many brain regions. c-Fos expression was considerably increased in regions involved in the regulation of arousal states, such as the locus coeruleus (noradrenergic neurons) and the medial preoptic area (non-GABAergic neurons). With c-fos antisense injection in the medial preoptic area, we demonstrated that c-fos expression in this region is causally involved in sleep regulation. c-Fos expression in other areas, such as the cerebral cortex and the hippocampus, may be related to the functional consequences of prolonged wakefulness and to the need of sleep. Further work should explore the mechanisms leading to changes in the expression of c-fos, and possibly of its target genes, during the sleep-wake cycle.

Evidence for a sleep-promoting influence of stress

In the present review the data supporting the existence at the central level of a stress-sleep relation are reported and discussed. An immobilization stress of 1 or 2 hour(s) is accompanied by a marked polygraphic waking and followed by a significant sleep rebound concerning mainly paradoxical sleep (PS). During the restraint, an important release of 5-hydroxyindoles [5-OHles, a good index of serotonin (5-HT) release] occurs in the basal hypothalamus (BH). This release, produced by the nerve endings originating from the nucleus raphe dorsalis (nRD), might secondarily influence the release and/or the synthesis of hypnogenic substances directly involved in the sleep rebound production. Corticotropin-like intermediate lobe peptide (CLIP, or ACTH18-39) is a peptide possessing hypnogenic properties and derived from proopiomelanocortin (POMC) whose perikarya are contained within the BH (arcuate nucleus). The POMC nerve endings impinge on the nucleus raphe dorsalis, a structure containing sleep permissive components upon which CLIP acts to trigger sleep. It remains to be defined how the activity of the neuronal loop described above is impaired under chronic stress conditions.

Sleep-associated variations in plasma renin activity and blood pressure in the rat

Plasma renin activity (PRA) was determined in plasma samples obtained approximately at 6-min intervals during consecutive non-rapid eye movement sleep (NREMS) and rapid eye movement sleep (REMS) periods in rats chronically implanted with EEG electrodes, a brain thermistor, and an intracardiac catheter. PRA was low in REMS and high in NREMS: this difference was statistically significant. The PRAs in wakefulness and NREMS were not different. Sleep-associated variations in systemic blood pressure (BP) were also recorded in a group of rats implanted with a chronic aortic catheter. During REMS, large oscillations superimposed on a tonic rise in BP were observed, and the end of REMS was followed by an abrupt fall in BP. This is the first demonstration of sleep-associated variations in PRA in a species other than man. The changes in BP in the rat during REMS confirm previous reports and, unlike those in many other species, are similar to those previously described in humans. The rat therefore provides a model for study of the mechanisms of the sleep-related variations in PRA and BP. The changes in PRA may reflect a regulatory response to variations in BP or may result from central mechanisms, e.g. the sleep-associated changes in serotonergic activity.

Behavioral state-related changes of extracellular serotonin concentration in the dorsal raphe nucleus: a microdialysis study in the freely moving cat

For direct measurement of the extracellular concentration of serotonin (5-HT) in the dorsal raphe nucleus (DRN) over the sleep-wake cycle we used the technique of in vivo microdialysis in six freely moving, naturally sleeping cats whose behavioral state was polygraphically determined. Perfusate samples from microdialysis probes histologically localized to the DRN showed the following significantly different levels of extracellular 5-HT, with the numbers in parentheses indicating successively the mean value in fmol/5 microliters perfusate sample, the % level relative to waking, and the sample n: waking (4.02, 100%, n = 38) > slow wave sleep (2.02, 50%, n = 30) > REM sleep (1.61, 38%, n = 17). These data, to our knowledge the first direct DRN 5-HT measurements during behavioral state changes, directly parallel the levels of serotonergic neuronal action potential activity and suggest that DRN extracellular 5-HT is determined by this action potential activity through synaptic release by recurrent axonal collaterals in the DRN.

Immunocytochemical study of the CLIP/ACTH-immunoreactive nerve fibres in the dorsal raphe nucleus of the rat

The injection of corticotrophin-like intermediate lobe peptide (CLIP) in the lateral ventricle or the dorsal raphe nucleus (DRN) of the rat is followed by a significant increase in the amount of paradoxical sleep. In the DRN, CLIP would act through a somatic and/or dendritic release of serotonin. To establish the anatomical basis of these effects, the nerve fibres immunoreactive for CLIP/ACTH were labelled and their fine anatomical relationships with the neuronal elements of the DRN were studied at the ultrastructural level. Half of the profiles of labelled varicosities were 'free' in the neuropile, the other half was in close contact with dendrites, either shafts or spines. It was interesting to note that few contacts with dendritic shafts were under the form of a synapse, whereas a large number of the contacts with spines were synaptic. The chemical nature of these dendritic targets remains to be determined but preliminary results indicate that they are partly serotoninergic.

Is the nucleus raphe dorsalis a target for the peptides possessing hypnogenic properties?

Several peptides exhibiting hypnogenic properties when administered i.p., i.v. or i.c.v. are now known. No data, however, are available concerning their targets in the brain. In the present work we hypothesize that the nucleus raphe dorsalis (nRD) may be one such target since it contains 2 sleep permissive components that must be influenced for sleep to occur. One of these components is serotoninergic in nature and gates the occurrence of ponto-geniculo-occipital (PGO) waves. The other, of unknown nature, influences tonic sleep phenomena. For hypnogenic peptides, a putative mechanism permitting the triggering and maintenance of sleep might consist of influencing both the above components. In the present work, 3 hypnogenic substances, CLIP (corticotropin-Like intermediate lobe peptide), VIP (vasoactive intestinal polypeptide) and DSIP (delta sleep inducing peptide), were injected into the nRD in order to determine whether these compounds still induce sleep by local administration. To verify that such local injections do not spread outside the nRD, radiolabelled CLIP and VIP were also injected. Autoradiograms obtained with either labeled CLIP or VIP indicate that these compounds, injected in a 0.2 microliter volume, do not spread outside the nRD. The sleep data obtained confirm that CLIP, at a dose of 10 ng, induces an increase in duration of paradoxical sleep (PS); this effect is observed only for injection sites located in the dorsolateral part of the nRD, an area where CLIP immunoreactive (IR) fibers are present. VIP, at a dose of 100 ng, also increases PS duration, whereas at 10 ng, only slow wave sleep duration is increased. In this case, the positive injection sites are scattered throughout the entire nRD as are the VIP-IR fibers. With DSIP, no sleep effect was found whatever the dose used or the site injected; in the same manner, no DSIP-IR fibers have been located in this structure. These data suggest that the nRD is a target for the expression of the hypnogenic properties of CLIP and VIP, but not for DSIP. The nature of the possible mechanisms permitting such expression are discussed.

The effects of selective activation of the 5-HT3 receptor with m-chlorophenylbiguanide on sleep and wakefulness in the rat

The effects of the 5-HT3 receptor agonist, m-chlorophenylbiguanide, were compared with those of the 5-HT3 receptor antagonist, MDL 72222, in rats implanted with electrodes for chronic sleep recordings. m-Chlorophenylbiguanide (12.5-50.0 micrograms) injected into the left lateral ventricle increased wakefulness and rapid eye movement (REM) sleep latency, whereas slow wave sleep, REM sleep and the number of REM periods were reduced. MDL 72222 (0.1-1.0 mg/kg, s.c.) induced a delayed and dose-dependent increase of slow wave sleep. Pretreatment with MDL 72222 (0.1-0.5 mg/kg) prevented the effects of m-chlorophenylbiguanide (50 micrograms) on wakefulness and sleep. It is suggested that the increase of wakefulness after 5-HT3 receptor activation could be related to the release of endogenous serotonin and dopamine.

Re-examination of the effects of raphe lesions on the sleep/wakefulness cycle states in cats

To study the specific effects of central superior raphe nucleus (CeSR) lesions on the different sleep/wakefulness cycle states of the cat, nine animals with implanted electrodes for EOG, EMG and EEG recordings were used. Seven cats received diathermocoagulation lesions that destroyed between 13 and 100 percent of the CeSR; the remaining two cats, which suffered lesions in the paramedial region of the oral pontine reticular nucleus (RPO), were used to determine the effects on sleep/wakefulness states caused by damage to adjacent CeSR structures and/or passage fibres. Three prelesion and five postlesion weekly 24 h recordings were obtained from each cat. Recordings were scored according to the polygraphic criteria for wakefulness (W), drowsiness (D), slow wave sleep (SWS) and paradoxical sleep (PS). Results indicated that insomnia is not produced exclusively by CeSR lesions, since adjacent paramedial RPO lesions also decrease both SWS and PS; however, increased W occurred after the former while increased D occurred after the latter. Correlation coefficient analyses showed that W is the only state that correlates significantly with the volume of CeSR destroyed. The following correlations between different states of the sleep/wakefulness cycle were, however, significant: W-D, W-SWS and SWS-PS. Disinhibition of W, therefore, and not sleep loss seems to be the primary effect of CeSR lesions. Thus, the CeSR nucleus appears to be involved in arousal mechanisms rather than in direct sleep promotion.