A neural circuit for circadian regulation of arousal

Abnormal development of the locus coeruleus in Ear2(Nr2f6)-deficient mice impairs the functionality of the forebrain clock and affects nociception

The orphan nuclear receptor Ear2 (Nr2f6) is transiently expressed in the rostral part of the rhombic lip in which the locus coeruleus (LC) arises. LC development, regulated by a signaling cascade (Mash1 --> Phox2b --> Phox2a), is disrupted in Ear2-/- embryos as revealed by an approximately threefold reduction in the number of Phox2a- and Phox2b-expressing LC progenitor cells. Mash1 expression in the rhombic lip, however, is unaffected, placing Ear2 in between Mash1 and Phox2a/b. Dopamine-beta-hydroxylase and tyrosine hydroxylase staining demonstrate that >70% of LC neurons are absent in the adult with agenesis affecting primarily the dorsal division of the LC. Normally, this division projects noradrenergic efferents to the cortex that appear to be diminished in Ear2-/- since the cortical concentration of noradrenaline is four times lower in these mice. The rostral region of the cortex is known to contain a circadian pacemaker regulating adaptability to light- and restricted food-driven entrainment. In situ hybridization establishes that the circadian expression pattern of the clock gene Period1 is abolished in the Ear2-/- forebrain. Behavioral experiments reveal that Ear2 mutants have a delayed entrainment to shifted light-dark cycles and adapt less efficiently to daytime feeding schedules. We propose that neurons in the dorsal division of LC contribute to the regulation of the forebrain clock, at least in part, through targeted release of noradrenaline into the cortical area.

Melatonin enhances the hypoxic response of rat carotid body chemoreceptor

Melatonin attenuates carotid chemoreceptor response to hypercapnic acidosis and may contribute to the effect of circadian rhythms on the chemoreflex. The purpose of this study was to test the hypothesis that melatonin modulates rat carotid chemoreceptor response to hypoxia. To examine the effect of melatonin on the hypoxic response of the chemosensitive cells, cytosolic calcium ([Ca2+]i) was measured by spectrofluorometry in fura-2-loaded type-I (glomus) cells dissociated from rat carotid bodies. Melatonin (0.01-10 nm) did not change the resting Ca2+]i level of the glomus cells but it concentration-dependently increased peak Ca2+]i response to cyanide or deoxygenated buffer. An agonist of melatonin receptors, iodomelatonin also enhanced the Ca2+]i response to hypoxia. The melatonin-induced enhancement of the Ca2+]i response was abolished by pretreatment with nonselective mt1/MT2 antagonist, luzindole, and by MT2 antagonists, 4-phenyl-2-propionamidotetraline or DH97. These findings suggest that melatonin receptors in the glomus cells mediate the effect of melatonin on the chemoreceptor response to hypoxia. In addition, melatonin increased the carotid afferent response to hypoxia in unitary activities recorded from the sinus nerve in isolated carotid bodies superfused with bicarbonate-buffer saline. Furthermore, plethysmographic measurement of ventilatory activities in unanesthetized rats revealed that melatonin (1 mg/kg, i.p.) increased the ventilatory response to hypoxia. Hence, the circadian rhythm of melatonin in arterial blood can modulate the carotid chemoreceptor response to hypoxia. This modulation may be a physiological mechanism involved in the day-light differences in ventilatory activities.

Persistent inward currents in motoneuron dendrites: implications for motor output

The dendrites of motoneurons are not, as once thought, passive conduits for synaptic inputs. Instead they have voltage-dependent channels that provide the capacity to generate a very strong persistent inward current (PIC). The amplitude of the PIC is proportional to the level of neuromodulatory input from the brainstem, which is mediated primarily by the monoamines serotonin and norepinephrine. During normal motor behavior, monoaminergic drive is likely to be moderately strong and the dendritic PIC generates many of the characteristic features of motor unit firing patterns. Most of the PIC activates at or below recruitment threshold and thus motor unit firing patterns exhibit a linear increase just above recruitment. The dendritic PIC allows motor unit derecruitment to occur at a lower input level than recruitment, thus providing sustained tonic firing with little or no synaptic input, especially in low-threshold units. However the dendritic PIC can be readily deactivated by synaptic inhibition. The overall amplification due to the dendritic PIC and other effects of monoamines on motoneurons greatly increases the input-output gain of the motor pool. Thus the brainstem neuromodulatory input provides a mechanism by which the excitability of motoneurons can be varied for different motor behaviors. This control system is lost in spinal cord injury but PICs nonetheless recover near-normal amplitudes in the months following the initial injury. The relationship of these findings to the cause of the spasticity syndrome developing after spinal cord injury is discussed.

Pharmacological modulation of circadian rhythms: a new drug target in psychotherapeutics

Daily variation in an organism's physiology and behaviour is regulated by the synchrony that is achieved between the internal timing mechanisms - the circadian rhythms of the biological clock - and the prevailing environmental cues. Proper synchrony constitutes an adaptive response; improper or lost synchrony may well yield maladaptation and, in the case of humans, a psychiatric disorder. On a basic level, the circadian system is comprised of three parts: a central oscillator, its various neuronal inputs and its outputs. For all three of these parts, the dissemination of new information is moving at an unprecedented pace, and the number of molecular targets for the opportunistic pharmacologist is growing in step. Monoamines, neuropeptides, kinases - sorting through all these, much less developing one into a drug discovery programme, may be the biggest challenge. However, the potential benefits in targeting a basic flaw in a fundamental biological system may be enormous.

Central administration of transforming growth factor-alpha and neuregulin-1 suppress active behaviors and cause weight loss in hamsters

Transforming growth factor-alpha (TGF-alpha) is a candidate output signal of the hypothalamic circadian pacemaker. TGF-alpha is expressed in the suprachiasmatic nucleus (SCN) of rats, hamsters, and rhesus macaques [A. Kramer, F.C. Yang, P. Snodgrass, X. Li, T.E. Scammell, F.C. Davis and C.J. Weitz, Regulation of daily locomotor activity and sleep by hypothalamic EGF receptor signaling, Science, 294 (2001) 2511-5., X. Li, N. Sankrithi and F.C. Davis, Transforming growth factor-alpha is expressed in astrocytes of the suprachiasmatic nucleus in hamster: role of glial cells in circadian clocks, Neuroreport, 13 (2002) 2143-7., Y.J. Ma, M.E. Costa and S.R. Ojeda, Developmental expression of the genes encoding transforming growth factor alpha and its receptor in the hypothalamus of female rhesus macaques, Neuroendocrinology, 60 (1994) 346-59., Y.J. Ma, M.P. Junier, M.E. Costa and S.R. Ojeda, Transforming growth factor-alpha gene expression in the hypothalamus is developmentally regulated and linked to sexual maturation, Neuron, 9 (1992) 657-70.]. TGF-alpha reversibly inhibits wheel-running activity during long-term infusions into the third ventricle of hamsters (2 weeks, intracerebroventricular or ICV) [A. Kramer, F.C. Yang, P. Snodgrass, X. Li, T.E. Scammell, F.C. Davis and C.J. Weitz, Regulation of daily locomotor activity and sleep by hypothalamic EGF receptor signaling, Science, 294 (2001) 2511-5.], and this effect appears to be mediated by the epidermal growth factor receptor (EGFR or ErbB-1) [A. Kramer, F.C. Yang, P. Snodgrass, X. Li, T.E. Scammell, F.C. Davis and C.J. Weitz, Regulation of daily locomotor activity and sleep by hypothalamic EGF receptor signaling, Science, 294 (2001) 2511-5.]. Here, we demonstrate that this inhibitory effect is not restricted to wheel-running behavior or to mediation by the EGFR. Using direct observation, we found the effects of long-term TGF-alpha infusion (ICV, 12 microl/day, 3.3 microM) to be more general than previously reported. Other active behaviors such as grooming and feeding were reversibly inhibited and hamsters showed dramatic weight loss as a result of reduced feeding (34% of body weight over 19 days). TGF-alpha did not disrupt a non-behavioral rhythm, the rhythm in pineal melatonin. Wheel-running activity was also inhibited by another epidermal growth factor-like (EGF-like) peptide, neuregulin (NRG-1), that binds to different ErbB receptors. Like TGF-alpha, NRG-1 caused a significant weight loss. We also show that an acute injection of TGF-alpha inhibits activity (ICV, 5 microl, 3.3 microM over 2 min), with inhibition and recovery occurring over a few hours. Although the results are consistent with the proposed [A. Kramer, F.C. Yang, P. Snodgrass, X. Li, T.E. Scammell, F.C. Davis and C.J. Weitz, Regulation of daily locomotor activity and sleep by hypothalamic EGF receptor signaling, Science, 294 (2001) 2511-5.] role for EGF-like peptides in the daily regulation of activity, the actions of these peptides might also contribute to the behavioral etiology of diseases in which EGF-like peptides are expressed.

The hypothalamic integrator for circadian rhythms

Although the suprachiasmatic nucleus (SCN) is well established as providing a genetically based clock for timing circadian rhythms, the mechanisms by which the timing signal is translated into circadian rhythms of behavior and underlying physiology have only recently come to light. The bulk of the SCN outflow terminates in a column of tissue that arches upward and backward from the SCN, and which includes the subparaventricular zone (SPZ) and the dorsomedial nucleus of the hypothalamus. Neurons within the dorsal SPZ are necessary for organizing circadian rhythms of body temperature, whereas neurons in the ventral SPZ are needed for circadian rhythms of sleep and waking. Ventral SPZ neurons in turn relay to the dorsomedial nucleus, which is crucial for producing circadian rhythms of sleep and waking, locomotor activity, feeding and corticosteroid production. This multistage processor provides the animal with flexibility so that environmental cues, such as food availability, ambient temperature and social interactions, can be integrated with the clock signal to sculpt an adaptive pattern of rhythmic daily activities that maximize the chances of survival and reproduction.

Sleep-wake states in transgenic mouse models overexpressing the human beta-amyloid precursor protein

Studies testing the amyloid hypothesis and recent advances in mouse molecular genetic technologies have played a critical role in improving our understanding of Alzheimer's disease (AD). Mouse models of AD currently available show only some of the characteristic neuropathology in human AD. Studies have demonstrated, however, that these models are excellent tools for characterizing different aspects of the molecular pathology of AD and the neurobiological basis for the clinical heterogeneity in AD. The present discussion focuses on behavioral and physiological data obtained in transgenic (Tg) mice overexpressing the mutant human beta-amyloid precursor protein (hbetaAPP). This mouse model exhibits memory and neurophysiological deficits at ages preceding amyloid-beta-peptide (Abeta) plaque formation that worsened with age and Abeta plaque formation. In spite of these findings, very little emphasis has been placed on characterizing the neurobiological basis of the diverse neuropsychiatric symptoms that are also observed in AD, including sleep disturbances. Taking into consideration the relationship between memory processes and sleep, the use of animal models of AD as a preclinical bioassay has the potential to characterize the neural substrates mediating clinical manifestations of AD, such as sleep-wake states, and contribute to the development of treatments for early stages of AD.

Toward an integrative theory of sleep and dreaming

Non-rapid-eye-movement sleep (NREMS) is triggered by the accumulation of adenosine, as a result of the perceptual overload of the brain cortex. NREMS starts in the most burdened regions of the cortex first and then eventually, after the released adenosine has reached the ventrolateral pre-optic nucleus area of the hypothalamus, triggers the "general NREMS pattern". This is accompanied by the usual familiar changes in the thalamocortical system. When NREMS reaches the slow-wave sleep (SWS) phase, with its predominant delta activity, brain metabolism drops significantly with the brain temperature, and this is recognized by the alarm system in the pre-optic anterior hypothalamus and/or the other thermostat circuit in the brainstem as a life-threatening situation. This alarm system triggers a reaction similar to abortive or partial awakening called rapid-eye-movement sleep (REMS), which is aimed at restoring the optimal body-core temperature. As soon as this restoration is accomplished by the activation of the brainstem-to-cortex ascending pathways, NREMS may continue, as may the interchange of the two sleep phases during the entire sleep period. During both NREMS and REMS, the same essential pattern occurs in the cortex: the loops "used" during the previous waking period, now deprived of external input, replay their waking activity at a lower frequency, one which enables them to restore the membrane's potential (possibly by means of LTD). During REMS, however, the cholinergic flood originating in the LTD/PPT nuclei of the pons tegmentum, increases in the basal forebrain and, provoking theta activity in the medial septum is extended to the hippocampus, causing the circuits that are active at that particular moment in the cortex, to store the information they carry as memory. This is the explanation of both the memory improvement known to be related to REMS and of dreams. Both phenomena are clearly side effects of REMS.

Brain structures and receptors involved in alertness

Transitions between sleep and wakefulness are regulated by complex neurobiological mechanisms, which ultimately can be delineated as oscillations between two opponent processes--one promoting sleep and the other promoting wakefulness. The suprachiasmatic nuclei (SCN) provide temporal organization to the sleep-wake cycle through arousal mechanisms that oppose homeostatic drive or sleep. Assuming that individual cells in the SCN are competent circadian oscillators, it is important to understand how these cells communicate and remain synchronized with each other. Examination of the brain structures and receptors that are involved in alertness and the complex phenomena involved in regulation of the circadian sleep-wake cycle has provided evidence for an important role for the noradrenergic locus coeruleus (LC) system in the circadian regulation of alertness and performance. However, the broad interest in mechanisms underlying alertness is not solely to understand wakefulness but also to gain insight into how to maintain alertness and cognitive performance while awake. Few studies have attempted to link the role of a brain system in sleep-wake regulation with a role in cognitive performance during waking. We hypothesize that the dorsomedial hypothalamic nucleus (DMH) modulates the circadian rhythm of sleep and waking via projections to the LC. We propose a SCN-DMH-LC signalling pathway that may influence the activity of the LC and thereby a variety of central nervous system functions related to noradrenergic innervations, including alertness, vigilance, attention, learning and memory. The influence of sleep drive on the LC system may be important for our understanding of the deleterious effects of sleep loss on performance, and presents a logical target for developing new treatments to counteract impairments in alertness and performance due to poor quality sleep.

Electrical synapses coordinate activity in the suprachiasmatic nucleus

In the suprachiasmatic nucleus (SCN), the master circadian pacemaker, neurons show circadian variations in firing frequency. There is also considerable synchrony of spiking across SCN neurons on a scale of milliseconds, but the mechanisms are poorly understood. Using paired whole-cell recordings, we have found that many neurons in the rat SCN communicate via electrical synapses. Spontaneous spiking was often synchronized in pairs of electrically coupled neurons, and the degree of this synchrony could be predicted from the magnitude of coupling. In wild-type mice, as in rats, the SCN contained electrical synapses, but electrical synapses were absent in connexin36-knockout mice. The knockout mice also showed dampened circadian activity rhythms and a delayed onset of activity during transition to constant darkness. We suggest that electrical synapses in the SCN help to synchronize its spiking activity, and that such synchrony is necessary for normal circadian behavior.

Circadian firing-rate rhythms and light responses of rat habenular nucleus neurons in vivo and in vitro

The suprachiasmatic nuclei of the anterior hypothalamus serve as the principal pacemaker of the mammalian circadian system. Among its efferent targets are the habenular nucleus (Hb), especially the lateral Hb (LHb), which plays an important role in conveying input from the limbic forebrain to midbrain structures. We recorded extracellularly from single neurons in the LHb and medial Hb (MHb), both in vivo and using an in vitro slice preparation, to assess their responses to retinal illumination and the rhythmicity of their firing rates. Of cells recorded in the LHb, 42% were tonically activated or suppressed by retinal illumination, while significantly fewer cells recorded in the MHb responded to retinal illumination (19%). Of photically responsive cells, 68% in the LHb were activated and the remainder suppressed, while only 25% of those recorded in the MHb were activated. Cells in both the LHb and MHb showed higher baseline firing rates during the day than during the night in vivo, while photic responses were of significantly larger amplitude among LHb cells during the projected night than during the projected day. LHb cells recorded in vitro maintained their rhythmicity for two circadian cycles, but MHb cells did not show a rhythm in vitro. The habenula may play a role in linking circadian and motivational systems and may contribute to photic regulation of these systems, as well as to the rhythmicity of their function.

Behavioral state instability in orexin knock-out mice

Narcolepsy is caused by a lack of orexin (hypocretin), but the physiologic process that underlies the sleepiness of narcolepsy is unknown. Using orexin knock-out (KO) mice as a model of narcolepsy, we critically tested the three leading hypotheses: poor circadian control of sleep and wakefulness, inadequate activation of arousal regions, or abnormal sleep homeostasis. Compared with wild-type (WT) littermates, orexin KO mice had essentially normal amounts of sleep and wake, but wake and non-rapid eye movement (NREM) bouts were very brief, with many more transitions between all behavioral states. In constant darkness, orexin KO mice had normal amplitude and timing of sleep-wake rhythms, providing no evidence for disordered circadian control. When placed in a new, clean cage, both groups of mice remained awake for approximately 45 min, demonstrating that, even in the absence of orexin, fundamental arousal regions can be engaged to produce sustained wakefulness. After depriving mice of sleep for 2-8 hr, orexin KO mice recovered their NREM and rapid eye movement sleep deficits at comparable rates and to the same extent as WT mice, with similar increases in EEG delta power, suggesting that their homeostatic control of sleep is normal. These experiments demonstrate that the fragmented wakefulness of orexin deficiency is not a consequence of abnormal sleep homeostasis, poor circadian control, or defective fundamental arousal systems. Instead, the fragmented behavior of orexin KO mice may be best described as behavioral state instability, with apparently low thresholds to transition between states.

Mammalian circadian biology: elucidating genome-wide levels of temporal organization

During the past decade, the molecular mechanisms underlying the mammalian circadian clock have been defined. A core set of circadian clock genes common to most cells throughout the body code for proteins that feed back to regulate not only their own expression, but also that of clock output genes and pathways throughout the genome. The circadian system represents a complex multioscillatory temporal network in which an ensemble of coupled neurons comprising the principal circadian pacemaker in the suprachiasmatic nucleus of the hypothalamus is entrained to the daily light/dark cycle and subsequently transmits synchronizing signals to local circadian oscillators in peripheral tissues. Only recently has the importance of this system to the regulation of such fundamental biological processes as the cell cycle and metabolism become apparent. A convergence of data from microarray studies, quantitative trait locus analysis, and mutagenesis screens demonstrates the pervasiveness of circadian regulation in biological systems. The importance of maintaining the internal temporal homeostasis conferred by the circadian system is revealed by animal models in which mutations in genes coding for core components of the clock result in disease, including cancer and disturbances to the sleep/wake cycle.

Nonvisual Responses to Light Exposure in the Human Brain during the Circadian Night

The brain processes light information to visually represent the environment but also to detect changes in ambient light level. The latter information induces non-image-forming responses and exerts powerful effects on physiology such as synchronization of the circadian clock and suppression of melatonin. In rodents, irradiance information is transduced from a discrete subset of photosensitive retinal ganglion cells via the retinohypothalamic tract to various hypothalamic and brainstem regulatory structures including the hypothalamic suprachiasmatic nuclei, the master circadian pacemaker. In humans, light also acutely modulates alertness, but the cerebral correlates of this effect are unknown. We assessed regional cerebral blood flow in 13 subjects attending to auditory and visual stimuli in near darkness following light exposures (>8000 lux) of different durations (0.5, 17, 16.5, and 0 min) during the biological night. The bright broadband polychromatic light suppressed melatonin and enhanced alertness. Functional imaging revealed that a large-scale occipito-parietal attention network, including the right intraparietal sulcus, was more active in proportion to the duration of light exposures preceding the scans. Activity in the hypothalamus decreased in proportion to previous illumination. These findings have important implications for understanding the effects of light on human behavior.

The influence of spike rate and stimulus duration on noradrenergic neurons

We model spiking neurons in locus coeruleus (LC), a brain nucleus involved in modulating cognitive performance, and compare with recent experimental data. Extracellular recordings from LC of monkeys performing target detection and selective attention tasks show varying responses dependent on stimuli and performance accuracy. From membrane voltage and ion channel equations, we derive a phase oscillator model for LC neurons. Average spiking probabilities of a pool of cells over many trials are then computed via a probability density formulation. These show that: (1) Post-stimulus response is elevated in populations with lower spike rates; (2) Responses decay exponentially due to noise and variable pre-stimulus spike rates; and (3) Shorter stimuli preferentially cause depressed post-activation spiking. These results allow us to propose mechanisms for the different LC responses observed across behavioral and task conditions, and to make explicit the role of baseline firing rates and the duration of task-related inputs in determining LC response.

Why Trypanosomes Cause Sleeping Sickness

African trypanosomiasis or sleeping sickness is hallmarked by sleep and wakefulness disturbances. In contrast to other infections, there is no hypersomnia, but the sleep pattern is fragmented. This overview discusses that the causative agents, the parasites Trypanosoma brucei, target circumventricular organs in the brain, causing inflammatory responses in hypothalamic structures that may lead to dysfunctions in the circadian-timing and sleep-regulatory systems.

Effects of hypocretin2-saporin and antidopamine-beta-hydroxylase-saporin neurotoxic lesions of the dorsolateral pons on sleep and muscle tone

The hypocretin neurons have been implicated in regulating sleep-wake states as they are lost in patients with the sleep disorder narcolepsy. Hypocretin (HCRT) neurons are located only in the perifornical region of the posterior hypothalamus and heavily innervate pontine brainstem neurons, such as the locus coeruleus (LC), which have traditionally been implicated in promoting arousal. It is not known how the hypocretin innervation of the pons regulates sleep-wake states as pontine lesions have never been shown to increase sleep. It is likely that in previous studies specific neurons were not lesioned. Therefore, in this study, we applied saporin-based neurotoxins to the dorsolateral pons and monitored sleep in rats. Anti-dopamine-beta-hydroxylase-saporin killed the LC neurons but sleep was affected only during a two hour light-dark transition period. Application of hypocretin2-saporin killed fewer LC neurons relative to other adjacent neurons. This occurred because the LC neurons possess the hypocretin receptor 1 but the ligand hypocretin 2 binds to this receptor with less affinity relative to the hypocretin receptor 2. The hypocretin2-saporin lesioned rats compared to controls had increased sleep during the dark period and displayed increased limb movements during REM sleep. None of the lesioned rats had sleep onset REM sleep periods or cataplexy. We conclude that the hypocretin innervation to the pons functions to awaken the animal when the lights turn off (via its innervation of the LC), sustains arousal and represses sleep during the rest of the night (via a wider innervation of other pontine neurons), and modulates muscle tone.

Intergeniculate leaflet and ventral lateral geniculate nucleus afferent connections: An anatomical substrate for functional input from the vestibulo-visuomotor system

The intergeniculate leaflet (IGL) has widespread projections to the basal forebrain and visual midbrain, including the suprachiasmatic nucleus (SCN). Here we describe IGL-afferent connections with cells in the ventral midbrain and hindbrain. Cholera toxin B subunit (CTB) injected into the IGL retrogradely labels neurons in a set of brain nuclei most of which are known to influence visuomotor function. These include the retinorecipient medial, lateral and dorsal terminal nuclei, the nucleus of Darkschewitsch, the oculomotor central gray, the cuneiform, and the lateral dorsal, pedunculopontine, and subpeduncular pontine tegmental nuclei. Intraocular CTB labeled a retinal terminal field in the medial terminal nucleus that extends dorsally into the pararubral nucleus, a location also containing cells projecting to the IGL. Distinct clusters of IGL-afferent neurons are also located in the medial vestibular nucleus. Vestibular projections to the IGL were confirmed by using anterograde tracer injection into the medial vestibular nucleus. Other IGL-afferent neurons are evident in Barrington's nucleus, the dorsal raphe, locus coeruleus, and retrorubral nucleus. Injection of a retrograde, trans-synaptic, viral tracer into the SCN demonstrated transport to cells as far caudal as the vestibular system and, when combined with IGL injection of CTB, confirmed that some in the medial vestibular nucleus polysynaptically project to the SCN and monosynaptically to the IGL, as do cells in other brain regions. The results suggest that the IGL may be part of the circuitry governing visuomotor activity and further indicate that circadian rhythmicity might be influenced by head motion or visual stimuli that affect the vestibular system.

Circadian variation in base rate measures of cardiac autonomic activity

To investigate the role of the circadian pacemaker in autonomic modulation of base rate cardiac activity, 29 healthy subjects participated in a constant routine protocol. They were randomly divided into two groups in order to manipulate prior wakefulness. Group 1 started at 0900 hours immediately after a monitored sleep period, while group 2 started 12 h later. Measures of interbeat intervals (IBIs), respiratory sinus arrhythmia (RSA, an estimate of parasympathetic activity), pre-ejection period (PEP, an estimate of sympathetic activity), and core body temperature (CBT) were recorded continuously. Multilevel regression analyses (across-subjects) revealed significant 24- and/or 12-h sinusoidal circadian variation for CBT, IBI, and RSA, but not for PEP. Subject-specific 24+12 h sinusoidal fits demonstrated a convergence of phase distribution for IBI and RSA of group 1 similar to CBT, while PEP showed a relatively large (i.e. random) distribution of phase. In group 2, all cardiac measures showed large distributions of phase. Unexpected results in the cardiac measures were found in group 2, probably caused by group differences in prior activation. Also, effects of sleep deprivation were observed for IBI and RSA in group 2. Consequently, all cardiac measures revealed significant sinusoidal x group interactions, a result not shown in CBT. These findings were interpreted as an indication for circadian endogenous parasympathetic modulation of cardiac activity that is mainly confounded by prior wakefulness that extends 24 h, while the sympathetic modulation is relatively uncoupled from the endogenous circadian drive and mainly influenced by prior activation.

Chlorotoxin-mediated disinhibition of noradrenergic locus coeruleus neurons using a conditional transgenic approach

The noradrenergic locus coeruleus (LC) has been implicated in the promotion of arousal, in focused attention and learning, and in the regulation of the sleep/waking cycle. The complex biological functions of the central noradrenergic system have been investigated largely through electrophysiological recordings and neurotoxic lesions of LC neurons. Activation of LC neurons through electrical or chemical stimulation has also led to important insights, although these techniques have limited cellular specificity and short-term effects. Here, we describe a novel method aimed at stimulating the central noradrenergic system in a highly selective manner for prolonged periods of time. This was achieved through the conditional expression of a transgene for chlorotoxin (Cltx) in the LC of adult mice. Chlorotoxin is a component of scorpion venom that partially blocks small conductance chloride channels. In this manner, the influence of GABAergic and glycinergic inhibitory inputs on LC cells is greatly reduced, while their ability to respond to excitatory inputs is unaffected. We demonstrate that the unilateral induction of Cltx expression in the LC is associated with a concomitant ipsilateral increase in the expression of markers of noradrenergic activity in LC neurons. Moreover, LC disinhibition is associated with the ipsilateral induction of the immediate early gene NGFI-A in cortical and subcortical target areas. Unlike previous gain of function approaches, transgenic disinhibition of LC cells is highly selective and persists for at least several weeks. This method represents a powerful new tool to assess the long-term effects of LC activation and is potentially applicable to other neuronal systems.

Transgenic approach reveals expression of the VPAC2 receptor in phenotypically defined neurons in the mouse suprachiasmatic nucleus and in its efferent target sites

Circadian rhythms in mammals depend on the properties of cells in the suprachiasmatic nucleus (SCN). The retino-recipient core of the mouse SCN is characterized by vasoactive intestinal peptide (VIP) neurons. Expression within the SCN of VPAC2, a VIP receptor, is required for circadian rhythmicity. Using transgenic mice with beta-galactosidase as a marker for VPAC2, we have phenotyped VPAC2-expressing cells within the SCN and investigated expression of the VPAC2 marker at sites previously shown to receive VIP-containing SCN efferents. In situ hybridization and immunohistochemistry demonstrated identical distributions for VPAC2 mRNA and beta-galactosidase and coexpression of the two signals in the SCN. Double-label confocal immunofluorescence identified beta-galactosidase in 32% of the VIP and 31% of the calretinin neurons in the SCN core. Of the arginine-vasopressin neurons that characterize the SCN shell, 45% expressed beta-galactosidase. In contrast, this marker was not apparent in astrocytes within the SCN core or shell. Cell bodies containing beta-galactosidase were detected at sites reportedly receiving VIP-containing SCN efferents, including the subparaventricular zone and lateral septum and the anteroventral periventricular, preoptic suprachiasmatic, medial preoptic and paraventricular hypothalamic nuclei. The detection of a marker for VPAC2 expression in the SCN in almost one-third of the VIP and calretinin core neurons and nearly half of the arginine-vasopressin shell neurons and also in cell bodies at sites receiving VIP-immunoreactive projections from the SCN indicates that VPAC2 may contribute to autoregulation and/or coupling within the SCN core and to the control of the SCN shell and sites distal to this nucleus.

Electroencephalographic brain dynamics following manually responded visual targets

Scalp-recorded electroencephalographic (EEG) signals produced by partial synchronization of cortical field activity mix locally synchronous electrical activities of many cortical areas. Analysis of event-related EEG signals typically assumes that poststimulus potentials emerge out of a flat baseline. Signals associated with a particular type of cognitive event are then assessed by averaging data from each scalp channel across trials, producing averaged event-related potentials (ERPs). ERP averaging, however, filters out much of the information about cortical dynamics available in the unaveraged data trials. Here, we studied the dynamics of cortical electrical activity while subjects detected and manually responded to visual targets, viewing signals retained in ERP averages not as responses of an otherwise silent system but as resulting from event-related alterations in ongoing EEG processes. We applied infomax independent component analysis to parse the dynamics of the unaveraged 31-channel EEG signals into maximally independent processes, then clustered the resulting processes across subjects by similarities in their scalp maps and activity power spectra, identifying nine classes of EEG processes with distinct spatial distributions and event-related dynamics. Coupled two-cycle postmotor theta bursts followed button presses in frontal midline and somatomotor clusters, while the broad postmotor "P300" positivity summed distinct contributions from several classes of frontal, parietal, and occipital processes. The observed event-related changes in local field activities, within and between cortical areas, may serve to modulate the strength of spike-based communication between cortical areas to update attention, expectancy, memory, and motor preparation during and after target recognition and speeded responding.

Adrenergic signaling plays a critical role in the maintenance of waking and in the regulation of REM sleep

Many experiments have suggested that the adrenergic system is important for arousal and the regulation of sleep/wake states. Electrophysiological studies have found strong correlations between the firing of adrenergic neurons and arousal state. Lesions of adrenergic neurons have been reported to cause changes in sleep/wake regulation, although findings have been variable and sometimes transient. To more specifically address the role of adrenergic signaling in sleep/wake regulation, we performed electroencephalographic and electromyographic recordings in mice with a targeted disruption of the gene for dopamine beta-hydroxylase, the enzyme that converts dopamine to norepinephrine. These mice are unable to synthesize the endogenous adrenergic ligands norepinephrine and epinephrine. The mutant mice sleep approximately 2 h more each day. The decrease in waking is due to a considerable decrease in the duration of waking bouts in spite of an increase in the number of waking bouts and transitions from sleep to waking. In contrast, the amount of rapid-eye-movement (REM) sleep is only half that in control mice due to a decrease in the number and duration of REM sleep bouts. Delta power is selectively increased in the mutant mice, and there is much less variation in non-REM sleep delta power over 24 h. After 6 h of total sleep deprivation during the first half of the light period, there is no rebound recovery of sleep time in the mutant mice. These results provide genetic evidence that adrenergic signaling acts to maintain waking and is important for the regulation of REM sleep and possibly sleep homeostasis.

Cataplexy-active neurons in the hypothalamus: implications for the role of histamine in sleep and waking behavior

Noradrenergic, serotonergic, and histaminergic neurons are continuously active during waking, reduce discharge during NREM sleep, and cease discharge during REM sleep. Cataplexy, a symptom associated with narcolepsy, is a waking state in which muscle tone is lost, as it is in REM sleep, while environmental awareness continues, as in alert waking. In prior work, we reported that, during cataplexy, noradrenergic neurons cease discharge, and serotonergic neurons greatly reduce activity. We now report that, in contrast to these other monoaminergic "REM-off" cell groups, histamine neurons are active in cataplexy at a level similar to or greater than that in quiet waking. We hypothesize that the activity of histamine cells is linked to the maintenance of waking, in contrast to activity in noradrenergic and serotonergic neurons, which is more tightly coupled to the maintenance of muscle tone in waking and its loss in REM sleep and cataplexy.

Expression of VIP and/or PACAP receptor mRNA in peptide synthesizing cells within the suprachiasmatic nucleus of the rat and in its efferent target sites

The suprachiasmatic nucleus (SCN) contains the predominant circadian pacemaker in mammals. Considerable evidence indicates that VPAC(2) and PAC(1), receptors for vasoactive intestinal peptide (VIP) and pituitary adenylate cyclase-activating peptide (PACAP), play critical roles in maintaining and entraining circadian rhythms. Retinal projections to the rat SCN contain PACAP and terminate mostly in the ventral SCN, the site of VIP neurons. The incidence of VPAC(2) and PAC(1) mRNAs within distinct neuronal populations of the rat SCN has been determined using double-label in situ hybridization. VPAC(2) mRNA was detected in almost all arginine-vasopressin (AVP) neurons of the dorsomedial SCN and in 41% of the VIP neurons; somatostatin (SST) neurons, predominantly in dorsomedial and intermediate regions, showed a decreased incidence (23%). PAC(1) mRNA was present in nearly half of the VIP and SST neurons (45% and 40%, respectively) and in one-third of the AVP neurons (32%). Cells expressing VPAC(2) mRNA also were detected in diencephalic areas that receive VIP-immunoreactive SCN efferents, such as the peri-suprachiasmatic region, lateral subparaventricular zone, parvocellular hypothalamic paraventricular subdivisions, dorsomedial hypothalamic nucleus, and anterior thalamic paraventricular and paratenial nuclei. The extensive distribution of PAC(1) mRNA within the SCN suggests that actions of PACAP are not restricted to the predominantly retinorecipient region. The presence of VPAC(2) mRNA in nearly half the VIP neurons, in almost all the AVP neurons, and at sites receiving VIP-immunoreactive SCN efferents suggests that the SCN VIP neurons are coupled and/or autoregulated and also influence the AVP-containing dorsomedial SCN and distal sites via VPAC(2).

Numerous GABAergic afferents to locus ceruleus in the pericerulear dendritic zone: possible interneuronal pool

Most nuclei in the CNS are composed of principal neurons that project to other areas and interneurons that serve to integrate information among afferents. The noradrenergic brain nucleus locus ceruleus (LC) has appeared to be an exception to this general rule, because the LC is composed almost entirely of noradrenergic principal neurons. Here, we report that numerous small neurons in the peri-LC region become retrogradely labeled after focal injections of wheat germ agglutinin-apo (inactivated) horseradish peroxidase conjugated to colloidal gold, or pseudorabies virus (PRV), into the nuclear core of the rat LC. A substantial number of these neurons were routinely found within the dendritic field of the LC, in the area surrounding the compact cell-dense region classically defined as LC. Double labeling revealed that a large percentage of these cells stained for GABA. Ultrastructural analyses revealed axodendritic and axosomatic contacts between PRV-labeled afferents and LC neurons labeled with tyrosine hydroxylase immunohistochemistry. In addition, PRV-labeled neurons or axons were immunopositive for GABA in ultrastructural localizations. Analysis of the synaptology of immunopositive profiles demonstrated that these LC afferents in the peri-LC region receive several non-LC synaptic inputs. These results indicate that a population of small GABAergic neurons in the peri-LC dendritic zone may provide interneuronal integration for LC noradrenergic neurons.

Suprachiasmatic nucleus input to autonomic circuits identified by retrograde transsynaptic transport of pseudorabies virus from the eye

Intraocular injection of the Bartha strain of pseudorabies virus (PRV Bartha) results in transsynaptic infection of the hypothalamic suprachiasmatic nucleus (SCN), a retinorecipient circadian oscillator. PRV Bartha infection of a limited number of retinorecipient structures, including the SCN, was initially interpreted as the differential infection of a subpopulation of rat retinal ganglion cells, followed by replication and anterograde transport via the optic nerve. A recent report that used a recombinant strain of PRV Bartha (PRV152) expressing enhanced green fluorescent protein demonstrated that SCN infection actually results from retrograde transneuronal transport of the virus via the autonomic innervation of the eye in the golden hamster. In the present study using the rat, the pattern of infection after intravitreal inoculation with PRV152 was examined to determine if infection of the rat SCN is also restricted to retrograde transsynaptic transport. It was observed that infection in preganglionic autonomic nuclei (i.e., Edinger-Westphal nucleus, superior salivatory nucleus, and intermediolateral nucleus) precedes infection in the SCN. Sympathetic superior cervical ganglionectomy did not abolish label in the SCN after intraocular infection, nor did lesions of parasympathetic preganglionic neurons in the Edinger-Westphal nucleus. However, combined Edinger-Westphal nucleus ablation and superior cervical ganglionectomy eliminated infection of the SCN. This observation allowed a detailed examination of the SCN contribution to descending autonomic circuits afferent to the eye. The results indicate that in the rat, as in the hamster, SCN infection after intraocular PRV152 inoculation is by retrograde transsynaptic transport via autonomic pathways to the eye.

Effects of sustained gamma-hydroxybutyrate treatments on spontaneous and evoked firing activity of locus coeruleus norepinephrine neurons

BACKGROUND

Gamma-hydroxybutyrate is currently used to promote nighttime sleep in the treatment of narcolepsy; however, it is also a drug of abuse ("Liquid Ecstasy") associated with a withdrawal syndrome with anxiety features. Of interest, the activity of locus coeruleus neurons is a reflective index of these above mentioned behavioral states.

METHODS

Using in vivo extracellular unitary recordings, sustained administration of gamma-hydroxybutyrate (40 mg/kg/day via minipump implanted subcutaneously) on the spontaneous and sensory-evoked burst firing of locus coeruleus norepinephrine neurons was assessed in rats.

RESULTS

A 2-day and 10-day gamma-hydroxybutyrate administration decreased the spontaneous firing activity of locus coeruleus neurons by 52% and 54%, respectively, when compared with controls. A similar degree of attenuation on evoked burst firing of norepinephrine neurons also occurred in these rats (2-day gamma-hydroxybutyrate: 47% and 10-day gamma-hydroxybutyrate: 58%), when compared with controls. In contrast, rats treated with gamma-hydroxybutyrate for 10 days followed by removal of the minipump for 36 hours resulted in a 33% augmentation in spontaneous locus coeruleus activity as compared with controls. Furthermore, a robust 79% increase in burst firing in response to paw-pinch was exhibited in theses rats.

CONCLUSIONS

Chronic gamma-hydroxybutyrate treatment inhibits the spontaneous and sensory-evoked burst firing of locus coeruleus norepinephrine neurons, whereas these indices are enhanced during drug withdrawal. The alteration in norepinephrine activity during chronic gamma-hydroxybutyrate administration may contribute to the ability of this agent to induce sleep and regulate narcoleptic episodes. Enhanced norepinephrine activity during withdrawal may be related to symptoms of anxiety on rapid termination of this drug in abusers.

Sniffing in infant rats during sleep and wakefulness

Sniffing, a behavior that enhances detection and localization of odorants, is typically assumed to require behavioral arousal. In an effort to determine whether sniffing and arousal are dissociable, dimethyl disulfide (DMDS) was presented to 8-day-old rats while respiration and behavioral state were monitored. Pups sniffed in response to the highest concentrations of DMDS, exhibiting a lower olfactory threshold when awake. Surprisingly, sniffing occurred even while pups remained asleep. Sniffing was mediated by the olfactory system, as evidenced by the abolition of sniffing when the lateral olfactory tracts, were cut and the retention of rapid arousal in response to a trigeminal stimulant, acetic acid. Finally, sleeping pups presented with acetic acid awakened without sniffing. Thus, although olfactory threshold increases during sleep, sleeping does not preclude sniffing.

Hypothalamic contribution to sleep–wake cycle development

Infant mammals cycle rapidly between sleep and wakefulness and only gradually does a more consolidated sleep pattern develop. The neural substrates responsible for this consolidation are unknown. To establish a reliable measure of sleep-wake cyclicity in infant rats, nuchal muscle tone was measured in 2-, 5-, and 8-day-old rats, as were motor behaviors associated with sleep (i.e. myoclonic twitching) and wakefulness (e.g. kicking, stretching). Sleep-wake cycles of 2-day-old rats were characterized by short periods of muscle atonia followed by equally short periods of high tone. In 8-day-olds, sleep periods lengthened significantly and disproportionately in relation to awake periods. Next, locus coeruleus (LC) lesions in 8-day-olds resulted in rapid sleep-wake cycling similar to that exhibited by 2-day-olds; in addition, LC lesions had no effect on the duration of awake periods. Finally, transections caudal, but not rostral, to the anterior hypothalamus also reinstated rapid cycling in 8-day-olds, again without affecting the duration of awake periods. This last finding implicates neural structures within the anterior hypothalamus (e.g. ventrolateral preoptic area) in the modulation of sleep-wake cyclicity. The temporal coherence of atonia and myoclonic twitching was not disrupted by any of the manipulations. These results suggest the presence of a bistable mesopontine circuit governing rapid sleep-wake cycling that does not include the LC and that comes increasingly under hypothalamic control during the first postnatal week. This circuit may represent a basic building block with which other sleep components become integrated during ontogeny.

Role of locus coeruleus ?1-adrenoceptors in motor activity in rats

The question of whether or not the locus coeruleus (LC) participates in the control of motor activity has been controversial due to difficulties in demonstrating permanent motor deficits after neurotoxic lesions of this nucleus or of the dorsal noradrenergic bundle (DNB). In the present experiments it was shown in rats that acute local blockade (with terazosin) or stimulation (with phenylephrine) of LC alpha(1)-adrenoceptors respectively blocked or stimulated exploratory behavior in a novel cage and the home cage. Moreover, previous lesion of the DNB by i.p. DSP4 abolished the behavioral changes to local LC alpha(1)-receptor manipulation but did not affect motor activity in the novel or home cage by itself. These findings are consistent with the hypothesis that the intact LC does contribute to motor activity control, exerted in part by its alpha(1)-receptors; however, the permanent loss of this nucleus is compensated for by remaining CNS motor structures.

Critical role of dorsomedial hypothalamic nucleus in a wide range of behavioral circadian rhythms

The suprachiasmatic nucleus (SCN) contains the brain's circadian pacemaker, but mechanisms by which it controls circadian rhythms of sleep and related behaviors are poorly understood. Previous anatomic evidence has implicated the dorsomedial hypothalamic nucleus (DMH) in circadian control of sleep, but this hypothesis remains untested. We now show that excitotoxic lesions of the DMH reduce circadian rhythms of wakefulness, feeding, locomotor activity, and serum corticosteroid levels by 78-89% while also reducing their overall daily levels. We also show that the DMH receives both direct and indirect SCN inputs and sends a mainly GABAergic projection to the sleep-promoting ventrolateral preoptic nucleus, and a mainly glutamate-thyrotropin-releasing hormone projection to the wake-promoting lateral hypothalamic area, including orexin (hypocretin) neurons. Through these pathways, the DMH may influence a wide range of behavioral circadian rhythms.

The circadian and homeostatic modulation of sleep pressure during wakefulness differs between morning and evening chronotypes

The purpose of this study was to evaluate homeostatic and circadian sleep process in 'larks' and 'owls' under daily life conditions. Core body temperature, subjective sleepiness and waking electroencephalogram (EEG) theta-alpha activity (6.25-9 Hz) were assessed in 18 healthy men (nine morning and nine evening chronotypes, 21.4 +/- 1.9 years) during a 36-h constant routine that followed a week of a normal 'working' sleep-wake schedule (bedtime: 23.30 h, wake time: 07.30 h). The phase of the circadian rhythm of temperature and sleepiness occurred respectively, 1.5 h (P = 0.01) and 2 h (P = 0.009) later in evening- than in morning-type subjects. Only morning-type subjects showed a bimodal rhythm of sleep-wake propensity. The buildup of subjective sleepiness, as quantified by linear regression, was slower in evening than in morning types (P = 0.04). The time course of EEG theta-alpha activity of both chronotypes could be closely fitted by an exponential curve. The time constant of evening types was longer than that of morning types (P = 0.03), indicating a slower increase in sleep pressure during extended wakefulness. These results suggest that both the circadian signal and the kinetics of sleep pressure buildup differ between the two chronotypes even under prior naturalistic conditions mimicking the usual working day.

Indirect projections from the suprachiasmatic nucleus to the median preoptic nucleus in rat

We recently showed, using dual tract-tracing, that the suprachiasmatic nucleus (SCN), the site of the principal circadian clock in mammals, may have indirect projections to the sleep-promoting ventrolateral preoptic nucleus (VLPO) via relays in the medial preoptic area (MPA), dorsomedial hypothalamic nucleus (DMH), and, to a lesser extent, the subparaventricular zone (SPVZ). Here, we found that the injection of the rostral MPA, the periventricular nucleus/medial SPVZ, and the caudal DMH with a mixture of anterograde and retrograde tracers resulted in dense anterograde labeling in the median preoptic nucleus (MnPO), another key sleep-promoting nucleus in the preoptic region. The retrograde labeling in the SCN was evident as previously reported. The injections in either the MPA or the DMH produced similar densities of varicose fibers between the MnPO and the VLPO, while the injections in the SPVZ yielded a greater density of varicose fibers in the MnPO than in the VLPO. These results suggest that the MPA and DMH are potential relay nuclei to mediate SCN output to the MnPO, as well as to the VLPO, for the circadian control of sleep-wake states.

Comparative distribution of neuropeptide Y Y1 and Y5 receptors in the rat brain by using immunohistochemistry

Neuropeptide Y (NPY) Y1 and Y5 receptor subtypes mediate many of NPY's diverse actions in the central nervous system. The present studies use polyclonal antibodies directed against the Y1 and Y5 receptors to map and compare the relative distribution of these NPY receptor subtypes within the rat brain. Antibody specificity was assessed by using Western analysis, preadsorption of the antibody with peptide, and preimmune serum controls. Immunostaining for the Y1 and Y5 receptor subtypes was present throughout the rostral-caudal aspect of the brain with many regions expressing both subtypes: cerebral cortex, hippocampus, hypothalamus, thalamus, amygdala, and brainstem. Further studies using double-label immunocytochemistry indicate that Y1R immunoreactivity (-ir) and Y5R-ir are colocalized in the cerebral cortex and caudate putamen. Y1 receptor ir was evident in the central amygdala, whereas both Y1- and Y5-immunoreactive cells and fibers were present in the basolateral amygdala. Corresponding with the physiology of NPY in the hypothalamus, both Y1R- and Y5R-ir was present within the paraventricular (PVN), supraoptic, arcuate nuclei, and lateral hypothalamus. In the PVN, Y5R-ir and Y1R-ir were detected in cells and fibers of the parvo- and magnocellular divisions. Intense immunostaining for these receptors was observed within the locus coeruleus, A1-5 and C1-3 nuclei, subnuclei of the trigeminal nerve and nucleus tractus solitarius. These data provide a detailed and comparative mapping of Y1 and Y5 receptor subtypes within cell bodies and nerve fibers in the brain which, together with physiological and electrophysiological studies, provide a better understanding of NPY neural circuitries.

Genetic evidence for a role of CREB in sustained cortical arousal

The cyclic AMP-response element binding protein (CREB) is an activity-dependent transcription factor important for synaptic plasticity and memory storage. Levels of phosphorylated CREB within the cortex are higher in waking than in sleep, suggesting that CREB plays a role in sleep/wake regulation in mammals. We tested the hypothesis that CREB is critical for sleep/wake regulation by examining behavioral state parameters in mice lacking the alpha and Delta isoforms of CREB. Over 24 h, time spent awake was significantly decreased in CREB alphaDelta mutant mice by approximately 100 min, and time spent in nonrapid eye movement sleep (NREM) sleep was increased correspondingly. Wake and REM sleep periods were shorter in CREB alphaDelta mice, and CREB alphaDelta mice had decreased levels of -activity during wake and REM sleep, consistent with an impairment in the ability to maintain an activated electroencephalogram. These results suggest that the CREB protein contributes to the mechanisms by which wakefulness is maintained and demonstrate that specific genetic alterations in species as diverse as Drosophila and mice produce similar phenotypes in arousal and wakefulness.

The hypocretin/orexin system in health and disease

Using positional cloning in a canine model of narcolepsy and mouse gene knockouts, genes involved in the pathogenesis of narcolepsy in animals have been identified. Hypocretin/orexin ligand and hypocretin/orexin receptor genes are key to the pathogenesis of narcolepsy in animals. Mutations in hypocretin-related genes are rare in humans, but hypocretin-ligand deficiency is found in many cases. Hypocretins/orexins are novel hypothalamic neuropeptides involved in various hypothalamic mechanisms, such as energy homeostasis and neuroendocrine function. Hypocretin-deficient human narcolepsy appears to be a more complex condition than a simple sleep disorder, and it may serve as an important disease model for studying hypothalamic function in health and disease.

alpha 1d Adrenoceptor signaling is required for stimulus induced locomotor activity

alpha 1 Adrenergic receptors mediate a variety of physiological responses and have been well studied in the cardiovascular and peripheral nervous system. However, their role in the central nervous system remains ill defined because of the lack of highly specific ligands to the alpha1 receptor subtypes. Here, we have employed gene targeting to elucidate the role of alpha 1d receptors in vivo. In addition to disrupting function, the insertion of the lacZ gene into the alpha 1d receptor locus enabled the specific identification of cells expressing the alpha 1d gene. These cells are localized in the cortex, hippocampus, olfactory bulb, dorsal geniculate and ventral posterolateral nuclei of the thalamus. Behaviorally, the alpha 1d(-/-) mice show normal locomotor activity during the subjective day, or resting phase of their cycle. However, during subjective night, or active phase, wheel-running activity is significantly reduced in mutant mice. Furthermore, these mice show a reduction in exploratory rearing behavior in a novel cage environment. Lastly, alpha 1d(-/-) mice show reduced hyperlocomotion after acute amphetamine administration. Together, these data reveal the functional importance of alpha 1d adrenoceptors in mediating a variety of stimulus-induced changes in locomotor behaviors. While the sensitivity of noradrenergic neurons to environmental stimuli has been well documented, our data demonstrate that at least some of these post-synaptic responses are mediated by alpha 1d adrenergic receptors.

A molecular perspective of human circadian rhythm disorders

A large number of physiological variables display 24-h or circadian rhythms. Genes dedicated to the generation and regulation of physiological circadian rhythms have now been identified in several species, including humans. These clock genes are involved in transcriptional regulatory feedback loops. The mutation of these genes in animals leads to abnormal rhythms or even to arrhythmicity in constant conditions. In this view, and given the similarities between the circadian system of humans and rodents, it is expected that mutations of clock genes in humans may give rise to health problems, in particular sleep and mood disorders. Here we first review the present knowledge of molecular mechanisms underlying circadian rhythmicity, and we then revisit human circadian rhythm syndromes in light of the molecular data.

Circadian Dynamics of Cytosolic and Nuclear Ca2+ in Single Suprachiasmatic Nucleus Neurons

Intracellular free Ca(2+) regulates diverse cellular processes, including membrane potential, neurotransmitter release, and gene expression. To examine the cellular mechanisms underlying the generation of circadian rhythms, nucleus-targeted and untargeted cDNAs encoding a Ca(2+)-sensitive fluorescent protein (cameleon) were transfected into organotypic cultures of mouse suprachiasmatic nucleus (SCN), the primary circadian pacemaker. Circadian rhythms in cytosolic but not nuclear Ca(2+) concentration were observed in SCN neurons. The cytosolic Ca(2+) rhythm period matched the circadian multiple-unit-activity (MUA)-rhythm period monitored using a multiple-electrode array, with a mean advance in phase of 4 hr. Tetrodotoxin blocked MUA, but not Ca(2+) rhythms, while ryanodine damped both Ca(2+) and MUA rhythms. These results demonstrate cytosolic Ca(2+) rhythms regulated by the release of Ca(2+) from ryanodine-sensitive stores in SCN neurons.

Suprachiasmatic nucleus projection to the medial prefrontal cortex: a viral transneuronal tracing study

The viral transneuronal labeling method was used to examine whether the suprachiasmatic nucleus (SCN) is linked by multisynaptic connections to the medial prefrontal cortex of the rat. In separate experiments, pseudorabies virus (PRV) was injected into one of the three different cytoarchitectonic regions that comprise the medial prefrontal cortex: infralimbic (Brodmann area 25), prelimbic (Brodmann area 32), and cingulate (Brodmann area 24) cortical areas. After 4-days survival, extensive SCN transneuronal labeling was found following infralimbic cortex (ILC) injections, but almost none occurred when the PRV injections were centered in the prelimbic or cingulate areas. In the ILC cases, transneuronal labeling was localized mainly in the dorsomedial SCN, although a moderate number of labeled neurons were found in the ventrolateral SCN. About 13% of the infected neurons were vasopressin immunoreactive and 4% were vasoactive intestinal polypeptide-positive. Another set of experiments was performed in which the paraventricular thalamic nucleus (PVT) was destroyed 2 weeks prior to making PRV injections into the ILC. Almost no SCN transneuronal labeling occurred in these animals, suggesting that the SCN projection to the ILC is dependent on a relay in the PVT. We propose that the SCN sends timing signals, via its relay in the PVT, to the ILC. This pathway may modulate higher-level brain functions, such as attention, mood, or working memory. Assuming that a homologous circuit exists in humans, we speculate that neurochemical changes affecting this pathway may account for some of the symptoms associated with clinical depression and attention-deficit/hyperactivity disorder.

Separation of circadian and wake duration-dependent modulation of EEG activation during wakefulness

The separate contribution of circadian rhythmicity and elapsed time awake on electroencephalographic (EEG) activity during wakefulness was assessed. Seven men lived in an environmental scheduling facility for 4 weeks and completed fourteen 42.85-h 'days', each consisting of an extended (28.57-h) wake episode and a 14.28-h sleep opportunity. The circadian rhythm of plasma melatonin desynchronized from the 42.85-h day. This allowed quantification of the separate contribution of circadian phase and elapsed time awake to variation in EEG power spectra (1-32 Hz). EEG activity during standardized behavioral conditions was markedly affected by both circadian phase and elapsed time awake in an EEG frequency- and derivation-specific manner. The nadir of the circadian rhythm in alpha (8-12 Hz) activity in both fronto-central and occipito-parietal derivations occurred during the biological night, close to the crest of the melatonin rhythm. The nadir of the circadian rhythm of theta (4.5-8 Hz) and beta (20-32 Hz) activity in the fronto-central derivation was located close to the onset of melatonin secretion, i.e. during the wake maintenance zone. As time awake progressed, delta frequency (1-4.5 Hz) and beta (20-32 Hz) activity rose monotonically in frontal derivations. The interaction between the circadian and wake-dependent increase in frontal delta was such that the intrusion of delta was minimal when sustained wakefulness coincided with the biological day, but pronounced during the biological night. Our data imply that the circadian pacemaker facilitates frontal EEG activation during the wake maintenance zone, by generating an arousal signal that prevents the intrusion of low-frequency EEG components, the propensity for which increases progressively during wakefulness.

Anatomical and functional evidence for a stress-responsive, monoamine-accumulating area in the dorsomedial hypothalamus of adult rat brain

The dorsomedial hypothalamus (DMH) plays an important role in relaying information to neural pathways mediating neuroendocrine, autonomic, and behavioral responses to stress. Evidence suggests that the DMH is a structurally and functionally diverse integrative structure that contributes to both facilitation and inhibition of the hypothalamo-pituitary-adrenal axis, depending on the nature of the stimulus and the specific neural circuits involved. Previous studies have determined that stress or stress-related stimuli elevate tissue concentrations of serotonin (5-hydroxytryptamine; 5-HT), 5-hydroxyindoleacetic acid (5-HIAA), dopamine, and noradrenaline selectively within the DMH. In order to determine the specific region of the rat DMH involved, we used high-performance liquid chromatography with electrochemical detection to measure tissue concentrations of 5-HT, 5-HIAA, dopamine, and noradrenaline within five different subregions of the DMH in adult female Lewis and Fischer rats immediately or 4 h following a 30-min period of restraint stress. Compared to unrestrained control rats, restrained rats had elevated concentrations of 5-HT, 5-HIAA, dopamine, and noradrenaline immediately after a 30-min period of restraint and had elevated concentrations of 5-HT 4 h following the onset of a 30-min period of restraint stress. These effects were confined to a specific region that included medial portions of the dorsal hypothalamic area and dorsal ependymal, subependymal, and neuronal components of the periventricular nucleus. Furthermore, these effects were observed in Lewis rats, but not Fischer rats, two closely related rat strains with well-documented differences in neurochemical, neuroendocrine, autonomic, and behavioral responses to stress. These data provide support for the existence of a stress-responsive, amine-accumulating area in the DMH that may play an important role in the differential stress responsiveness of Lewis and Fischer rats.

The role of chronobiology in sleep disorders medicine

This review is concerned with circadian (approximately 24 h) aspects of chronobiology, and how they relate to sleep disorders medicine. We begin with an introduction to the key concepts and paradigms of circadian rhythms research in general, including a description of homeostatic and circadian determinants of sleep timing. This is followed by a brief history of chronobiology in relation to sleep disorders medicine. Both animal and human circadian rhythm studies are considered. We trace historical changes in the relative emphasis placed on social contacts, light, and melatonin in human circadian rhythms research. Special attention is given to free-running, forced desynchrony, and ultra-short sleep/wake cycle findings of particular relevance to sleep disorders medicine. The latter part of the review comprises a description of the circadian rhythm sleep disorders, highlighting insights derived from basic circadian rhythms research, including recent advances in molecular genetics. We conclude that the role of chronobiology in sleep disorders medicine is profound and pervasive, and that the two disciplines will move ever closer as a natural function of important new insights into sleep and sleep disorders provided by basic circadian rhythms research.