A sparse projection from the suprachiasmatic nucleus to the sleep active ventrolateral preoptic area in the rat

Physiological links of circadian clock and biological clock of aging

Circadian rhythms orchestrate biochemical and physiological processes in living organisms to respond the day/night cycle. In mammals, nearly all cells hold self-sustained circadian clocks meanwhile couple the intrinsic rhythms to systemic changes in a hierarchical manner. The suprachiasmatic nucleus (SCN) of the hypothalamus functions as the master pacemaker to initiate daily synchronization according to the photoperiod, in turn determines the phase of peripheral cellular clocks through a variety of signaling relays, including endocrine rhythms and metabolic cycles. With aging, circadian desynchrony occurs at the expense of peripheral metabolic pathologies and central neurodegenerative disorders with sleep symptoms, and genetic ablation of circadian genes in model organisms resembled the aging-related features. Notably, a number of studies have linked longevity nutrient sensing pathways in modulating circadian clocks. Therapeutic strategies that bridge the nutrient sensing pathways and circadian clock might be rational designs to defy aging.

The dynamics of GABA signaling: Revelations from the circadian pacemaker in the suprachiasmatic nucleus

Virtually every neuron within the suprachiasmatic nucleus (SCN) communicates via GABAergic signaling. The extracellular levels of GABA within the SCN are determined by a complex interaction of synthesis and transport, as well as synaptic and non-synaptic release. The response to GABA is mediated by GABA_A receptors that respond to both phasic and tonic GABA release and that can produce excitatory as well as inhibitory cellular responses. GABA also influences circadian control through the exclusively inhibitory effects of GABA_B receptors. Both GABA and neuropeptide signaling occur within the SCN, although the functional consequences of the interactions of these signals are not well understood. This review considers the role of GABA in the circadian pacemaker, in the mechanisms responsible for the generation of circadian rhythms, in the ability of non-photic stimuli to reset the phase of the pacemaker, and in the ability of the day-night cycle to entrain the pacemaker.

Circadian Rhythms, Sleep, and Disorders of Aging

Sleep-wake cycles are known to be disrupted in people with neurodegenerative disorders. These findings are now supported by data from animal models for some of these disorders, raising the question of whether the disrupted sleep/circadian regulation contributes to the loss of neural function. As circadian rhythms and sleep consolidation also break down with normal aging, changes in these may be part of what makes aging a risk factor for disorders like Alzheimer's disease (AD). Mechanisms underlying the connection between circadian/sleep dysregulation and neurodegeneration remain unclear, but several recent studies provide interesting possibilities. While mechanistic analysis is under way, it is worth considering treatment of circadian/sleep disruption as a means to alleviate symptoms of neurodegenerative disorders.

Promotion of Wakefulness and Energy Expenditure by Orexin-A in the Ventrolateral Preoptic Area

STUDY OBJECTIVES

The ventrolateral preoptic area (VLPO) and the orexin/hypocretin neuronal system are key regulators of sleep onset, transitions between vigilance states, and energy homeostasis. Reciprocal projections exist between the VLPO and orexin/hypocretin neurons. Although the importance of the VLPO to sleep regulation is clear, it is unknown whether VLPO neurons are involved in energy balance. The purpose of these studies was to determine if the VLPO is a site of action for orexin-A, and which orexin receptor subtype(s) would mediate these effects of orexin-A. We hypothesized that orexin-A in the VLPO modulates behaviors (sleep and wakefulness, feeding, spontaneous physical activity [SPA]) to increase energy expenditure.

DESIGN AND MEASUREMENTS

Sleep, wakefulness, SPA, feeding, and energy expenditure were determined after orexin-A microinjection in the VLPO of male Sprague-Dawley rats with unilateral cannulae targeting the VLPO. We also tested whether pretreatment with a dual orexin receptor antagonist (DORA, TCS-1102) or an OX2R antagonist (JNJ-10397049) blocked the effects of orexin-A on the sleep/wake cycle or SPA, respectively.

RESULTS

Orexin-A injected into the VLPO significantly increased wakefulness, SPA, and energy expenditure (SPA-induced and total) and reduced NREM sleep and REM sleep with no effect on food intake. Pretreatment with DORA blocked the increase in wakefulness and the reduction in NREM sleep elicited by orexin-A, and the OX2R antagonist reduced SPA stimulated by orexin-A.

CONCLUSIONS

These data show the ventrolateral preoptic area is a site of action for orexin-A, which may promote negative energy balance by modulating sleep/wakefulness and stimulating spontaneous physical activity and energy expenditure.

A Path to Sleep Is through the Eye

Sleep is expressed as a circadian rhythm and the two phenomena exist in a poorly understood relationship. Light affects each, simultaneously influencing rhythm phase and rapidly inducing sleep.

Light has long been known to modulate sleep, but recent discoveries support its use as an effective nocturnal stimulus for eliciting sleep in certain rodents. “Photosomnolence” is mediated by classical and ganglion cell photoreceptors and occurs despite the ongoing high levels of locomotion at the time of stimulus onset. Brief photic stimuli trigger rapid locomotor suppression, sleep, and a large drop in core body temperature (Tc; Phase 1), followed by a relatively fixed duration interval of sleep (Phase 2) and recovery (Phase 3) to pre-sleep activity levels. Additional light can lengthen Phase 2. Potential retinal pathways through which the sleep system might be light-activated are described and the potential roles of orexin (hypocretin) and melanin-concentrating hormone are discussed. The visual input route is a practical avenue to follow in pursuit of the neural circuitry and mechanisms governing sleep and arousal in small nocturnal mammals and the organizational principles may be similar in diurnal humans. Photosomnolence studies are likely to be particularly advantageous because the timing of sleep is largely under experimenter control. Sleep can now be effectively studied using uncomplicated, nonintrusive methods with behavior evaluation software tools; surgery for EEG electrode placement is avoidable. The research protocol for light-induced sleep is easily implemented and useful for assessing the effects of experimental manipulations on the sleep induction pathway. Moreover, the experimental designs and associated results benefit from a substantial amount of existing neuroanatomical and pharmacological literature that provides a solid framework guiding the conduct and interpretation of future investigations.

Circadian rhythms have broad implications for understanding brain and behavior

Circadian rhythms are generated by an endogenously organized timing system that drives daily rhythms in behavior, physiology and metabolism. In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus is the locus of a master circadian clock. The SCN is synchronized to environmental changes in the light:dark cycle by direct, monosynaptic innervation via the retino-hypothalamic tract. In turn, the SCN coordinates the rhythmic activities of innumerable subordinate clocks in virtually all bodily tissues and organs. The core molecular clockwork is composed of a transcriptional/post-translational feedback loop in which clock genes and their protein products periodically suppress their own transcription. This primary loop connects to downstream output genes by additional, interlocked transcriptional feedback loops to create tissue-specific 'circadian transcriptomes'. Signals from peripheral tissues inform the SCN of the internal state of the organism and the brain's master clock is modified accordingly. A consequence of this hierarchical, multilevel feedback system is that there are ubiquitous effects of circadian timing on genetic and metabolic responses throughout the body. This overview examines landmark studies in the history of the study of circadian timing system, and highlights our current understanding of the operation of circadian clocks with a focus on topics of interest to the neuroscience community.

Sex differences in circadian timing systems: implications for disease

Virtually every eukaryotic cell has an endogenous circadian clock and a biological sex. These cell-based clocks have been conceptualized as oscillators whose phase can be reset by internal signals such as hormones, and external cues such as light. The present review highlights the inter-relationship between circadian clocks and sex differences. In mammals, the suprachiasmatic nucleus (SCN) serves as a master clock synchronizing the phase of clocks throughout the body. Gonadal steroid receptors are expressed in almost every site that receives direct SCN input. Here we review sex differences in the circadian timing system in the hypothalamic-pituitary-gonadal axis (HPG), the hypothalamic-adrenal-pituitary (HPA) axis, and sleep-arousal systems. We also point to ways in which disruption of circadian rhythms within these systems differs in the sexes and is associated with dysfunction and disease. Understanding sex differentiated circadian timing systems can lead to improved treatment strategies for these conditions.

Modeling Interindividual Differences in Spontaneous Internal Desynchrony Patterns

A physiologically based mathematical model of a putative sleep-wake regulatory network is used to investigate the transition from typical human sleep patterns to spontaneous internal desynchrony behavior observed under temporal isolation conditions. The model sleep-wake regulatory network describes the neurotransmitter-mediated interactions among brainstem and hypothalamic neuronal populations that participate in the transitions between wake, rapid eye movement (REM) sleep, and non-REM (NREM) sleep. Physiologically based interactions among these sleep-wake centers and the suprachiasmatic nucleus (SCN), whose activity is driven by an established circadian oscillator model, mediate circadian modulation of sleep-wake behavior. When the sleep-wake and circadian rhythms are synchronized, the model simulates stereotypically normal human sleep-wake behavior within the limits of individual variation, including typical NREM-REM cycling across the night. When effects of temporal isolation are simulated by increasing the period of the sleep-wake cycle, the model replicates spontaneous internal desynchrony with the appropriate dependence of multiple features of REM sleep on circadian phase. In temporal isolation experiments, subjects have exhibited different desynchronized sleep-wake behaviors. Our model can generate similar ranges of desynchronized behaviors by variations in the period of the sleep-wake cycle and the strength of interactions between the SCN and the sleep-wake centers. Analysis of the model suggests that similar mechanisms underlie several different desynchronized behaviors and that the phenomenon of phase trapping may be dependent on SCN modulation of REM sleep-promoting centers. These results provide predictions for physiologically plausible mechanisms underlying interindividual variations in sleep-wake behavior observed during temporal isolation experiments.

Neuroanatomy of the extended circadian rhythm system

The suprachiasmatic nucleus (SCN), site of the primary clock in the circadian rhythm system, has three major afferent connections. The most important consists of a retinohypothalamic projection through which photic information, received by classical rod/cone photoreceptors and intrinsically photoreceptive retinal ganglion cells, gains access to the clock. This information influences phase and period of circadian rhythms. The two other robust afferent projections are the median raphe serotonergic pathway and the geniculohypothalamic (GHT), NPY-containing pathway from the thalamic intergeniculate leaflet (IGL). Beyond this simple framework, the number of anatomical routes that could theoretically be involved in rhythm regulation is enormous, with the SCN projecting to 15 regions and being directly innervated by about 35. If multisynaptic afferents to the SCN are included, the number expands to approximately brain 85 areas providing input to the SCN. The IGL, a known contributor to circadian rhythm regulation, has a still greater level of complexity. This nucleus connects abundantly throughout the brain (to approximately 100 regions) by pathways that are largely bilateral and reciprocal. Few of these sites have been evaluated for their contributions to circadian rhythm regulation, although most have a theoretical possibility of doing so via the GHT. The anatomy of IGL connections suggests that one of its functions may be regulation of eye movements during sleep. Together, neural circuits of the SCN and IGL are complex and interconnected. As yet, few have been tested with respect to their involvement in rhythm regulation.

Disease and Degeneration of Aging Neural Systems that Integrate Sleep Drive and Circadian Oscillations

Sleep/wake and circadian rest-activity rhythms become irregular with age. Typical outcomes include fragmented sleep during the night, advanced sleep phase syndrome and increased daytime sleepiness. These changes lead to a reduction in the quality of life due to cognitive impairments and emotional stress. More importantly, severely disrupted sleep and circadian rhythms have been associated with an increase in disease susceptibility. Additionally, many of the same brain areas affected by neurodegenerative diseases include the sleep and wake promoting systems. Any advances in our knowledge of these sleep/wake and circadian networks are necessary to target neural areas or connections for therapy. This review will discuss research that uses molecular, behavioral, genetic and anatomical methods to further our understanding of the interaction of these systems.

Circadian regulation of sleep-wake behaviour in nocturnal rats requires multiple signals from suprachiasmatic nucleus

The dynamics of sleep and wake are strongly linked to the circadian clock. Many models have accurately predicted behaviour resulting from dynamic interactions between these two systems without specifying physiological substrates for these interactions. By contrast, recent experimental work has identified much of the relevant physiology for circadian and sleep-wake regulation, but interaction dynamics are difficult to study experimentally. To bridge these approaches, we developed a neuronal population model for the dynamic, bidirectional, neurotransmitter-mediated interactions of the sleep-wake and circadian regulatory systems in nocturnal rats. This model proposes that the central circadian pacemaker, located within the suprachiasmatic nucleus (SCN) of the hypothalamus, promotes sleep through single neurotransmitter-mediated signalling to sleep-wake regulatory populations. Feedback projections from these populations to the SCN alter SCN firing patterns and fine-tune this modulation. Although this model reproduced circadian variation in sleep-wake dynamics in nocturnal rats, it failed to describe the sleep-wake dynamics observed in SCN-lesioned rats. We thus propose two alternative, physiologically based models in which neurotransmitter- and neuropeptide-mediated signalling from the SCN to sleep-wake populations introduces mechanisms to account for the behaviour of both the intact and SCN-lesioned rat. These models generate testable predictions and offer a new framework for modelling sleep-wake and circadian interactions.

Interactions between cognition and circadian rhythms: attentional demands modify circadian entrainment

Animals and humans are able to predict and synchronize their daily activity to signals present in their environments. Environmental cues are most often associated with signaling the beginning or the end of a daily activity cycle, but they can also be used to time the presentation or availability of scarce resources. If the signal occurs consistently, animals can begin to anticipate its arrival and ultimately become entrained to its presence. While many stimuli can produce anticipation for a daily event, these events rarely lead to changes in activity patterns during the rest of the circadian cycle. Here the authors demonstrate that performance of a task requiring sustained attention not only produces entrainment, but produces a robust modification in the animals' activity throughout the entire circadian cycle. In particular, normally nocturnal rats, when trained during the light phase (ZT 4) adopted a significant and reversible diurnal activity pattern. Of importance, control experiments demonstrated that this entrainment could not be attributed to the noncognitive components of task performance, such as handling, water deprivation, access to water used as a reward, or animal activity associated with operant training. These findings additionally indicate that levels of cognitive performance are modulated by the circadian cycle and that such activity can act as a highly effective entrainment signal. These results form the basis for future research on the role of neuronal systems mediating interactions between cognitive activity and circadian rhythms.

Functional neuroanatomy of sleep and circadian rhythms

The daily sleep-wake cycle is perhaps the most dramatic overt manifestation of the circadian timing system, and this is especially true for the monophasic sleep-wake cycle of humans. Considerable recent progress has been made in elucidating the neurobiological mechanisms underlying sleep and arousal, and more generally, of circadian rhythmicity in behavioral and physiological systems. This paper broadly reviews these mechanisms from a functional neuroanatomical and neurochemical perspective, highlighting both historical and recent advances. In particular, I focus on the neural pathways underlying reciprocal interactions between the sleep-regulatory and circadian timing systems, and the functional implications of these interactions. While these two regulatory systems have often been considered in isolation, sleep-wake and circadian regulation are closely intertwined processes controlled by extensively integrated neurobiological mechanisms.

A Network of (Autonomic) Clock Outputs

The circadian clock in the suprachiasmatic nuclei (SCN) is composed of thousands of oscillator neurons, each dependent on the cell-autonomous action of a defined set of circadian clock genes. A major question is still how these individual oscillators are organized into a biological clock that produces a coherent output capable of timing all the different daily changes in behavior and physiology. We investigated which anatomical connections and neurotransmitters are used by the biological clock to control the daily release pattern of a number of hormones. The picture that emerged shows projections contacting target neurons in the medial hypothalamus surrounding the SCN. The activity of these pre-autonomic and neuro-endocrine target neurons is controlled by differentially timed waves of vasopressin, GABA, and glutamate release from SCN terminals, among other factors. Together our data indicate that, with regard to the timing of their main release period within the LD cycle, at least four subpopulations of SCN neurons should be discernible. The different subgroups do not necessarily follow the phenotypic differences among SCN neurons. Thus, different subgroups can be found within neuron populations containing the same neurotransmitter. Remarkably, a similar distinction of four differentially timed subpopulations of SCN neurons was recently also discovered in experiments determining the temporal patterns of rhythmicity in individual SCN neurons by way of the electrophysiology or clock gene expression. Moreover, the specialization of the SCN may go as far as a single body structure, i.e., the SCN seems to contain neurons that specifically target the liver, pineal gland, and adrenal gland.

A Network of (Autonomic) Clock Outputs

The circadian clock in the suprachiasmatic nuclei (SCN) is composed of thousands of oscillator neurons, each of which is dependent on the cell-autonomous action of a defined set of circadian clock genes. A major question is still how these individual oscillators are organized into a biological clock producing a coherent output that is able to time all the different daily changes in behavior and physiology. We investigated which anatomical connections and neurotransmitters are used by the biological clock to control the daily release pattern of a number of hormones. The picture that emerged shows projections contacting target neurons in the medial hypothalamus surrounding the SCN. The activity of these pre-autonomic and neuro-endocrine target neurons is controlled by differentially timed waves of, among others, vasopressin, GABA, and glutamate release from SCN terminals. Together our data indicate that, with regard to the timing of their main release period within the light-dark (LD) cycle, at least 4 subpopulations of SCN neurons should be discerned. The different subgroups do not necessarily follow the phenotypic differences among SCN neurons. Thus, different subgroups can be found within neuron populations containing the same neurotransmitter. Remarkably, a similar distinction of 4 differentially timed subpopulations of SCN neurons was recently also discovered in experiments determining the temporal patterns of rhythmicity in individual SCN neurons by way of the electrophysiology or clock gene expression. Moreover, the specialization of the SCN may go as far as a single body structure; i.e., the SCN seems to contain neurons that specifically target the liver, pineal, and adrenal.

Suprachiasmatic modulation of noradrenaline release in the ventrolateral preoptic nucleus

As the major brain circadian pacemaker, the suprachiasmatic nucleus (SCN) is known to influence the timing of sleep and waking. We thus investigated here the effect of SCN stimulation on neurons of the ventrolateral preoptic nucleus (VLPO) thought to be involved in promoting sleep. Using an acute in vitro preparation of the rat anterior hypothalamus/preoptic area, we found that whereas single-pulse stimulations of the SCN evoked standard fast ionotropic IPSPs and EPSPs, train stimulations unexpectedly evoked a long-lasting inhibition (LLI). Such LLIs could also be evoked in VLPO neurons by pressure application of NMDA within the SCN, indicating the specific activation of SCN neurons. This LLI was shown to result from the presynaptic facilitation of noradrenaline release, because it was suppressed in presence of yohimbine, a selective antagonist of alpha2-adrenoreceptors. The LLI depended on the opening of a potassium conductance, because it was annulled at E(K) and could be reversed below E(K). These results show that the SCN can provide an LLI of the sleep-promoting VLPO neurons that could play a role in the circadian organization of the sleep-waking cycle.

Behavioral neuroendocrinology in nontraditional species of mammals: things the 'knockout' mouse CAN'T tell us

The exploration of many of the fundamental features of mammalian behavioral neuroendocrinology has benefited greatly throughout the short history of the discipline from the study of highly inbred, genetically characterized rodents and several other "traditional" exemplars. More recently, the impact of genomic variation in the determination of complex neuroendocrine and behavioral systems has advanced through the use of single and multiple gene knockouts or knockins. In our essay, we argue that the study of nontraditional mammals is an essential approach that complements these methodologies by taking advantage of allelic variation produced by natural selection. Current and future research will continue to exploit these systems to great advantage and will bring new techniques developed in more traditional laboratory animals to bear on problems that can only be addressed with nontraditional species. We highlight our points by discussing advances in our understanding of neuroendocrine and behavioral systems in phenomena of widely differing time scales. These examples include neuroendocrine variation in the regulation of reproduction across seasons in Peromyscus, variation in parental care by biparental male rodents and primates within a single infant rearing attempt, and circadian variation in the regulation of the substrates underlying mating in diurnal vs. nocturnal rodents. Our essay reveals both important divergences in neuroendocrine systems in our nontraditional model species, and important commonalities in these systems.

Indirect projections from the suprachiasmatic nucleus to major arousal-promoting cell groups in rat: implications for the circadian control of behavioural state

The circadian clock housed in the suprachiasmatic nucleus (SCN) controls various circadian rhythms including daily sleep-wake cycles. Using dual tract-tracing, we recently showed that the medial preoptic area (MPA), subparaventricular zone (SPVZ) and dorsomedial hypothalamic nucleus (DMH) are well positioned to relay SCN output to two key sleep-promoting nuclei, namely, the ventrolateral and median preoptic nuclei. The present study examined the possibility that these three nuclei may link the SCN with wake-regulatory neuronal groups. Biotinylated dextran-amine with or without cholera toxin B subunit was injected into selected main targets of SCN efferents; the retrograde labeling in the SCN was previously analyzed. Here, anterograde labeling was analyzed in immunohistochemically identified cholinergic, orexin/hypocretin-containing and aminergic cell groups. Tracer injections into the MPA, SPVZ and DMH resulted in moderate to dense anterograde labeling of varicose fibers in the orexin field and the tuberomammillary nucleus. The locus coeruleus, particularly the dendritic field, contained moderate anterograde labeling from the MPA and DMH. The ventral tegmental area, dorsal raphe nucleus, and laterodorsal tegmental nucleus all showed moderate anterograde labeling from the DMH. The substantia innominata showed moderate anterograde labeling from the MPA. These results suggest that the MPA, SPVZ and DMH are possible relay nuclei for indirect SCN projections not only to sleep-promoting preoptic nuclei as previously shown, but also to wake-regulatory cell groups throughout the brain. In the absence of major direct SCN projections to most of these sleep/wake-regulatory regions, indirect neuronal pathways probably play an important role in the circadian control of sleep-wake cycles and other physiological functions.

Octodon degus: a diurnal, social, and long-lived rodent

Octodon degus is a moderate-sized, precocious, but slowly maturing, hystricomorph rodent from central Chile. We have used this species to study a variety of questions about circadian rhythms in a diurnal mammal that readily adapts to most laboratory settings. In collaboration with others, we have found that a number of fundamental features of circadian function differ in this diurnal rodent compared with nocturnal rodents, specifically rats or hamsters. We have also discovered that many aspects of the circadian system are sexually dimorphic in this species. However, the sexual dimorphisms develop in the presence of pubertal hormones, and the sex differences do not appear until after gonadal puberty is complete. The developmental timing of the sex differences is much later than in the previously studied altricial, rapidly developing rat, mouse, or hamster. This developmental timing of circadian function is reminiscent of that reported for adolescent humans. In addition, we have developed a model that demonstrates how nonphotic stimuli, specifically conspecific odors, can interact with the circadian system to hasten recovery from a phase-shift of the light:dark cycle (jet lag). Interestingly, the production of the odor-based social signal and sensitivity to it are modulated by adult gonadal hormones. Data from degu circadian studies have led us to conclude that treatment of some circadian disorders in humans will likely need to be both age and gender specific. Degus will continue to be valuable research animals for resolving other questions regarding reproduction, diabetes, and cataract development.

Organization of suprachiasmatic nucleus projections in Syrian hamsters (Mesocricetus auratus): an anterograde and retrograde analysis

Circadian rhythms in physiology and behavior are controlled by pacemaker cells located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The mammalian SCN can be classified into two subdivisions (core and shell) based on the organization of neuroactive substances, inputs, and outputs. Recent studies in our laboratory indicate that these subdivisions are associated with functional specialization in Syrian hamsters. The core region, marked by calbindin-D(28K) (CalB)-containing cells, expresses light-induced, but not rhythmic, clock genes. In the shell compartment, marked by vasopressinergic cells and fibers, clock gene expression is rhythmic. Given these findings, an important question is how photic and rhythmic information are integrated and communicated from each of these regions to effector areas. The present study used localized, intra-SCN iontophoretic injections of the anterograde tracer biotinylated dextran amine (BDA) to investigate intra-SCN connectivity and the neural pathways by which information is communicated from SCN subregions to targets. Intra-SCN connections project from the core to the shell compartment of the SCN, but not from the shell to the CalB region of the SCN. Retrograde tracing experiments were performed using cholera toxin-beta (CTB) to determine more specifically whether SCN efferents originated in the core or shell using neurochemical markers for the rhythmic (vasopressin) and light-induced (CalB) SCN subregions. The combined results from anterograde and retrograde experiments suggest that all SCN targets receive information from both the light-induced and rhythmic regions of the SCN (albeit to varying degrees) and indicate that light and rhythmic information may be integrated both within the SCN and at target effector areas.

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.

A broad role for melanopsin in nonvisual photoreception

The rod and cone photoreceptors that mediate visual phototransduction in mammals are not required for light-induced circadian entrainment, negative masking of locomotor activity, suppression of pineal melatonin, or the pupillary light reflex. The photopigment melanopsin has recently been identified in intrinsically photosensitive retinal ganglion cells (RGCs) that project to the suprachiasmatic nucleus (SCN), intergeniculate leaflet (IGL), and olivary pretectal nucleus, suggesting that melanopsin might influence a variety of irradiance-driven responses. We have found novel projections from RGCs that express melanopsin mRNA to the ventral subparaventricular zone (vSPZ), a region involved in circadian regulation and negative masking, and the sleep-active ventrolateral preoptic nucleus (VLPO) and determined the subsets of melanopsin-expressing RGCs that project to the SCN, the pretectal area (PTA), and the IGL division of the lateral geniculate nucleus (LGN). Melanopsin was expressed in the majority of RGCs that project to the SCN, vSPZ, and VLPO and in a subpopulation of RGCs that innervate the PTA and the IGL but not in RGCs projecting to the dorsal LGN or superior colliculus. Two-thirds of RGCs containing melanopsin transcript projected to each of the SCN and contralateral PTA, and one-fifth projected to the ipsilateral IGL. Double-retrograde tracing from the SCN and PTA demonstrated a subpopulation of RGCs projecting to both sites, most of which contained melanopsin mRNA. Our results suggest that melanopsin expression defines a subset of RGCs that play a broad role in the regulation of nonvisual photoreception, providing collateralized projections that contribute to circadian entrainment, negative masking, the regulation of sleep-wake states, and the pupillary light reflex.

Representations of motivational drives in mesial cortex, medial thalamus, hypothalamus and midbrain

We propose that neural representations of motivational drives, including sexual desire, hunger, thirst, fear, power-dominance, the motivational aspect of pain, the need for sleep, and nurturance, are represented in four areas in the brain. These are located in the medial hypothalamic/preoptic area, the periaqueductal gray matter (PAG) in the midbrain/pons, the midline and intralaminar thalamic nuclei, and in the anterior part of the mesial cortex, including the medial prefrontal and anterior cingulate areas. We attempt to determine the locations of each of these representations within the hypothalamus/preoptic area, periaqueductal gray and cortex, based on the available literature on activation of brain structures by stimuli that evoke these forms of motivation, on the effects of electrical and chemical stimulation and lesions of candidate structures, and on hodological data. We discuss the hierarchical organization of the representations for a given drive, outputs from these representations to premotor structures in the medulla, caudate-putamen, and cortex, and their contributions to involuntary, learned-sequential (operant) and voluntary behaviors.

Indirect projections from the suprachiasmatic nucleus to the ventrolateral preoptic nucleus: a dual tract-tracing study in rat

The suprachiasmatic nucleus (SCN) contains a master clock for most circadian rhythms in mammals, including daily sleep-wake cycles. The ventrolateral preoptic nucleus (VLPO) plays a key role in sleep generation and, as such, might be an important target of the SCN circadian signal. However, direct SCN projections to the VLPO are limited, suggesting that most of the SCN output to the VLPO might be conveyed indirectly. We examined this possibility by microinjecting selected known major targets of SCN efferents with biotinylated dextran-amine and/or cholera toxin B subunit, followed by analyses of retrograde labelling in the SCN and anterograde labelling in the VLPO. Retrograde labelling results confirmed that the medial preoptic area, subparaventricular zone, dorsomedial hypothalamic nucleus and posterior hypothalamic area all received projections from the SCN; these projections arose predominantly from the shell, as opposed to the core, of the SCN. Anterograde labelling results indicated that these same nuclei also projected to the VLPO, mainly its medial and ventral aspects. Comparison of the results of injections of similar sizes across different target groups indicated that the rostral part of the medial preoptic area and the caudal part of the dorsomedial hypothalamic nucleus were particularly noteworthy for the abundance of both SCN source neurons and efferent fibres and terminals in the VLPO. These results suggest that the SCN might provide indirect input to the VLPO via the medial preoptic area and the dorsomedial hypothalamic nucleus, and that these indirect neuronal pathways might play a major role in circadian control of sleep-wake cycles.

Afferents to the ventrolateral preoptic nucleus

Sleep is influenced by diverse factors such as circadian time, affective states, ambient temperature, pain, etc., but pathways mediating these influences are unknown. To identify pathways that may influence sleep, we examined afferents to the ventrolateral preoptic nucleus (VLPO), an area critically implicated in promoting sleep. Injections of the retrograde tracer cholera toxin B subunit (CTB) into the VLPO produced modest numbers of CTB-labeled monoaminergic neurons in the tuberomammillary nucleus, raphe nuclei, and ventrolateral medulla, as well as a few neurons in the locus coeruleus. Immunohistochemistry for monoaminergic markers showed dense innervation of the VLPO by histaminergic, noradrenergic, and serotonergic fibers. Along with previous findings, these results suggest that the VLPO and monoaminergic nuclei may be reciprocally connected. Retrograde and anterograde tracing showed moderate or heavy inputs to the VLPO from hypothalamic regions including the median preoptic nucleus, lateral hypothalamic area, and dorsomedial hypothalamic nucleus (DMH), autonomic regions including the infralimbic cortex and parabrachial nucleus, and limbic regions including the lateral septal nucleus and ventral subiculum. Light to moderate inputs arose from orexin and melanin concentrating hormone neurons, but cholinergic or dopaminergic inputs were extremely sparse. Suprachiasmatic nucleus (SCN) projections to the VLPO were sparse, but the heavy input to the VLPO from the DMH, which receives direct and indirect SCN inputs, could provide an alternate pathway regulating the circadian timing of sleep. These robust pathways suggest candidate mechanisms by which sleep may be influenced by brain systems regulating arousal, autonomic, limbic, and circadian functions.

Electrophysiological analysis of suprachiasmatic nucleus projections to the ventrolateral preoptic area in the rat

The circadian pacemaker housed in the suprachiasmatic nucleus (SCN) synchronizes daily sleep-wake cycles, presumably by modulating the sleep-wake regulatory system, including ventrolateral preoptic area (VLPO) neurons. We used whole-cell patch-clamp recording to study the projections from the SCN to the VLPO in horizontal slices of rat hypothalamus. Single-pulse stimulation of the SCN region elicited postsynaptic currents (PSCs) in 20 of 66 neurons (30%) recorded within the VLPO region as verified by intracellular biocytin labelling. At a holding potential of -60 mV, the evoked PSCs had an amplitude of 17.6 +/- 3.2 pA (SEM) and a latency of 6.3 +/- 0.5 ms (n = 10). There was a trend for simple excitatory postsynaptic currents (EPSCs) to be evoked in the VLPO cluster, simple inhibitory postsynaptic currents (IPSCs) in the extended VLPO, and a combination of EPSCs and IPSCs in both regions. IPSCs were blocked reversibly by bicuculline (10 microm, n = 11). In both the presence and absence of bicuculline, EPSCs had fast and slow components that were blocked by 6,7-dinitroquinoxaline-2,3-dione (DNQX; 10 microm; n = 7), and (+/-)3-(2-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP; 10 microm, n = 6), respectively. Reversal potentials for the evoked IPSCs and EPSCs were consistent with mediation via GABAA and ionotropic glutamate receptors, respectively. These results suggest that the SCN region provides both inhibitory and excitatory inputs to single VLPO neurons, which are mediated, respectively, by GABAA receptors and by both non-NMDA and NMDA glutamate receptors. These projections may play important roles in conveying circadian input to systems in the preoptic area that regulate sleep and waking.

How does the circadian clock send timing information to the brain?

This paper discusses circadian output in terms of the signaling mechanisms used by circadian pacemaker neurons. In mammals, the suprachiasmatic nucleus houses a clock controlling several rhythmic events. This nucleus contains one or more pacemaker circuits, and exhibits diversity in transmitter content and in axonal projections. In Drosophila, a comparable circadian clock is located among period -expressing neurons, a sub-set of which (called LN-vs) express the neuropeptide PDF. Genetic experiments indicate LN-vs are the primary pacemakers neurons controlling daily locomotion and that PDF is the principal circadian transmitter. Further definition of pacemaker properties in several model systems will provide a useful basis with which to describe circadian output mechanisms.

Orexin A-like immunoreactivity in the hypothalamus and thalamus of the Syrian hamster (Mesocricetus auratus) and Siberian hamster (Phodopus sungorus), with special reference to circadian structures

The orexins are recently discovered neuropeptides that reportedly play a role in energy homeostasis, in addition to various other physiological processes. The synthesis of orexin A undergoes diurnal variation in certain areas of the brain, while the mutation of the orexin receptor 2 gene has been implicated in canine narcolepsy. Since the circadian pacemaker in the suprachiasmatic nucleus modulates the sleep/wake cycle, there is a putative role for orexins in the mammalian circadian system. In this study, immunohistochemical techniques were used to determine the distribution of orexin A in the structures of the hypothalamus and thalamus of Syrian and Siberian hamsters. In both species, the pattern of immunoreactivity was similar. Cells immunoreactive for orexin A were noted in the lateral hypothalamic area. Immunoreactive varicose orexin A fibres were found throughout the hypothalamus. The suprachiasmatic nucleus possessed little or no immunoreactive orexin A fibres in its core, but had fibres at its periphery. The thalamus of both species contained comparatively few immunoreactive fibres, which were mainly localised around the midline. The thalamic intergeniculate leaflet contained a plexus of immunoreactive orexin A fibres throughout its rostro-caudal extent. Three areas of the brainstem, the dorsal and median raphe nuclei and the locus coeruleus, were also investigated owing to their relevance to the circadian system and all were found to contain immunoreactive orexin A fibres. The presence of orexin A-immunoreactive fibres in the neural architecture of the mammalian circadian system suggests an important role for orexin A in circadian timekeeping processes.

Fos rhythms in the hypothalamus of Rattus and Arvicanthis that exhibit nocturnal and diurnal patterns of rhythmicity

This study compared patterns of Fos expression within the suprachiasmatic nucleus (SCN), the region immediately dorsal to the SCN (the lower subparaventricular zone, LSPV), and the supraoptic nucleus (SON) of grass rats (Arvicanthis niloticus) and lab rats (Rattus norvegicus). Among grass rats we also compared individuals exhibiting nocturnal and diurnal patterns of wheel running. In the SCN of both groups of grass rats, as well as laboratory rats, Fos was elevated during the light compared to the dark portions of the day, and was expressed in 7-12% of cells containing vasoactive intestinal polypeptide (VIP). Fos was higher in the LSPV during the night compared to the day in both forms of grass rats but not in laboratory rats. In the SON, Fos rose from day to night in the diurnal grass rats and in laboratory rats, but not in nocturnal grass rats. These patterns are consistent with the hypothesis that VIP cells in the SCN function similarly in nocturnal and diurnal rodents, but that the SON and the region dorsal to the SCN are associated with intra and interspecific differences in rhythmicity, respectively.

Suprachiasmatic nucleus projections to the paraventricular thalamic nucleus in nocturnal rats (Rattus norvegicus) and diurnal nile grass rats (Arviacanthis niloticus)

The circadian pacemaker of the suprachiasmatic nucleus (SCN) is likely to control the timing of the sleep-wake cycle in mammals by modulating the daily activity patterns of brain regions important in sleep and wakefulness. One such brain region is the paraventricular nucleus of the thalamus (PVT). In both nocturnal rats and the diurnal rodent Arvicanthis niltoicus (Nile grass rat), expression of Fos (the product of the immediate-early gene c-fos) in the PVT increases at times of day when the animals are most active. To compare the projections of the SCN to the PVT in these two species, the retrograde tracer cholera toxin (beta subunit; CTbeta) was microinjected into the PVT and the SCN was examined to identify labeled neurons. Further, the PVT-projecting SCN cells containing either arginine vasopressin (AVP) or gastrin releasing peptide (GRP) were also compared between species. In both nocturnal rats and diurnal Nile grass rats, the SCN sends a substantial projection to the PVT. In both species, many PVT-projecting SCN neurons contain AVP, and few contain GRP. Other work has shown that some AVP-containing neurons of the SCN function differently in rats and Nile grass rats. Projections from functionally distinct SCN neurons to the PVT may contribute to the difference in the temporal distribution of sleep and wakefulness seen between these two species.

Rhythms in Fos expression in brain areas related to the sleep-wake cycle in the diurnal Arvicanthis niloticus

Most mammals show daily rhythms in sleep and wakefulness controlled by the primary circadian pacemaker, the suprachiasmatic nucleus (SCN). Regardless of whether a species is diurnal or nocturnal, neural activity in the SCN and expression of the immediate-early gene product Fos increases during the light phase of the cycle. This study investigated daily patterns of Fos expression in brain areas outside the SCN in the diurnal rodent Arvicanthis niloticus. We specifically focused on regions related to sleep and arousal in animals kept on a 12:12-h light-dark cycle and killed at 1 and 5 h after both lights-on and lights-off. The ventrolateral preoptic area (VLPO), which contained cells immunopositive for galanin, showed a rhythm in Fos expression with a peak at zeitgeber time (ZT) 17 (with lights-on at ZT 0). Fos expression in the paraventricular thalamic nucleus (PVT) increased during the morning (ZT 1) but not the evening activity peak of these animals. No rhythm in Fos expression was found in the centromedial thalamic nucleus (CMT), but Fos expression in the CMT and PVT was positively correlated. A rhythm in Fos expression in the ventral tuberomammillary nucleus (VTM) was 180 degrees out of phase with the rhythm in the VLPO. Furthermore, Fos production in histamine-immunoreactive neurons of the VTM cells increased at the light-dark transitions when A. niloticus show peaks of activity. The difference in the timing of the sleep-wake cycle in diurnal and nocturnal mammals may be due to changes in the daily pattern of activity in brain regions important in sleep and wakefulness such as the VLPO and the VTM.