Photic responses of suprachiasmatic area neurons in diurnal degus (Octodon degus) and nocturnal rats (Rattus norvegicus)

A subclass of evening cells promotes the switch from arousal to sleep at dusk

Animals exhibit rhythmic patterns of behavior that are shaped by an internal circadian clock and the external environment. Although light intensity varies across the day, there are particularly robust differences at twilight (dawn/dusk). These periods are also associated with major changes in behavioral states, such as the transition from arousal to sleep. However, the neural mechanisms by which time and environmental conditions promote these behavioral transitions are poorly defined. Here, we show that the E1 subclass of Drosophila evening clock neurons promotes the transition from arousal to sleep at dusk. We first demonstrate that the cell-autonomous clocks of E2 neurons primarily drive and adjust the phase of evening anticipation, the canonical behavior associated with "evening" clock neurons. We next show that conditionally silencing E1 neurons causes a significant delay in sleep onset after dusk. However, rather than simply promoting sleep, activating E1 neurons produces time- and light-dependent effects on behavior. Activation of E1 neurons has no effect early in the day but then triggers arousal before dusk and induces sleep after dusk. Strikingly, these activation-induced phenotypes depend on the presence of light during the day. Despite their influence on behavior around dusk, in vivo voltage imaging of E1 neurons reveals that their spiking rate and pattern do not significantly change throughout the day. Moreover, E1-specific clock ablation has no effect on arousal or sleep. Thus, we suggest that, rather than specifying "evening" time, E1 neurons act, in concert with other rhythmic neurons, to promote behavioral transitions at dusk.

A subclass of evening cells promotes the switch from arousal to sleep at dusk

Animals exhibit rhythmic patterns of behavior that are shaped by an internal circadian clock and the external environment. While light intensity varies across the day, there are particularly robust differences at twilight (dawn/dusk). These periods are also associated with major changes in behavioral states, such as the transition from arousal to sleep. However, the neural mechanisms by which time and environmental conditions promote these behavioral transitions are poorly defined. Here, we show that the E1 subclass of Drosophila evening clock neurons promotes the transition from arousal to sleep at dusk. We first demonstrate that the cell-autonomous clocks of E2 neurons alone are required to drive and adjust the phase of evening anticipation, the canonical behavior associated with "evening" clock neurons. We next show that conditionally silencing E1 neurons causes a significant delay in sleep onset after dusk. However, rather than simply promoting sleep, activating E1 neurons produces time- and light- dependent effects on behavior. Activation of E1 neurons has no effect early in the day, but then triggers arousal before dusk and induces sleep after dusk. Strikingly, these phenotypes critically depend on the presence of light during the day. Despite their influence on behavior around dusk, in vivo voltage imaging of E1 neurons reveals that their spiking rate does not vary between dawn and dusk. Moreover, E1-specific clock ablation has no effect on arousal or sleep. Thus, we suggest that, rather than specifying "evening" time, E1 neurons act, in concert with other rhythmic neurons, to promote behavioral transitions at dusk.

Inhibitory responses to retinohypothalamic tract stimulation in the circadian clock of the diurnal rodent Rhabdomys pumilio

In both diurnal and nocturnal mammals, the timing of activity is regulated by the central circadian clock of the suprachiasmatic nucleus (SCN). The SCN is synchronized to the external light cycle via the retinohypothalamic tract (RHT). To investigate potential differences in light processing between nocturnal mice and the diurnal rodent Rhabdomys pumilio, we mimicked retinal input by stimulation of the RHT ex vivo. Using Ca²⁺ imaging, we observed excitations as well as inhibitions of SCN neurons in response to electrical RHT stimulation. In mice, the vast majority of responses were excitatory (85%), whereas in Rhabdomys, the proportion of excitatory and inhibitory responses was similar (51% excitatory, 49% inhibitory). Glutamate blockers AP5 and CNQX blocked the excitatory responses to RHT stimulation but did not abolish the inhibitory responses in mice or Rhabdomys, indicating that the inhibitions were monosynaptically transmitted via the RHT. Simultaneous application of glutamate blockers with the GABA_A antagonist gabazine blocked all inhibitory responses in mice, but not in Rhabdomys. Collectively, our results indicate that in Rhabdomys, considerably more inhibitory responses to light are present and that these responses are driven directly by the RHT. We propose that this increased proportion of inhibitory input could reflect a difference in the entrainment mechanism employed by diurnal rodents.

The photobiology of the human circadian clock

SignificanceThe function of our biological clock is dependent on environmental light. Rodent studies have shown that there are multiple colors that affect the clock, but indirect measures in humans suggest blue light is key. We performed functional MRI studies in human subjects with unprecedented spatial resolution to investigate color sensitivity of our clock. Here, we show that narrowband blue, green, and orange light were all effective in changing neuronal activity of the clock. While the clock of nocturnal rodents is excited by light, the human clock responds with a decrease in neuronal activity as indicated by a negative BOLD response. The sensitivity of the clock to multiple colors should be integrated in light therapy aimed to strengthen our 24-h rhythms.

Bright daytime light enhances circadian amplitude in a diurnal mammal

Mammalian circadian rhythms are orchestrated by a master pacemaker in the hypothalamic suprachiasmatic nuclei (SCN), which receives information about the 24 h light-dark cycle from the retina. The accepted function of this light signal is to reset circadian phase in order to ensure appropriate synchronization with the celestial day. Here, we ask whether light also impacts another key property of the circadian oscillation, its amplitude. To this end, we measured circadian rhythms in behavioral activity, body temperature, and SCN electrophysiological activity in the diurnal murid rodent Rhabdomys pumilio following stable entrainment to 12:12 light-dark cycles at four different daytime intensities (ranging from 18 to 1,900 lx melanopic equivalent daylight illuminance). R. pumilio showed strongly diurnal activity and body temperature rhythms in all conditions, but measures of rhythm robustness were positively correlated with daytime irradiance under both entrainment and subsequent free run. Whole-cell and extracellular recordings of electrophysiological activity in ex vivo SCN revealed substantial differences in electrophysiological activity between dim and bright light conditions. At lower daytime irradiance, daytime peaks in SCN spontaneous firing rate and membrane depolarization were substantially depressed, leading to an overall marked reduction in the amplitude of circadian rhythms in spontaneous activity. Our data reveal a previously unappreciated impact of daytime light intensity on SCN physiology and the amplitude of circadian rhythms and highlight the potential importance of daytime light exposure for circadian health.

Widespread Doublecortin Expression in the Cerebral Cortex of the Octodon degus

It has been demonstrated that in adulthood rodents show newly born neurons in the subgranular layer (SGL) of the dentate gyrus (DG), and in the subventricular zone (SVZ). The neurons generated in the SVZ migrate through the rostral migratory stream (RMS) to the olfactory bulb. One of the markers of newly generated neurons is doublecortin (DCX). The degu similarly shows significant numbers of DCX-labeled neurons in the SGL, SVZ, and RMS. Further, most of the nuclei of these DCX-expressing neurons are also labeled by proliferating nuclear antigen (PCNA) and Ki67. Finally, whereas in rats and mice DCX-labeled neurons are predominantly present in the SGL and SVZ, with only a few DCX neurons present in piriform cortex, the degu also shows significant numbers of DCX expressing neurons in areas outside of SVZ, DG, and PC. Many areas of neocortex in degu demonstrate DCX-labeled neurons in layer II, and most of these neurons are found in the limbic cortices. The DCX-labeled cells do not stain with NeuN, indicating they are immature neurons.

Blue Light Deprivation Produces Depression-Like Responses in Mongolian Gerbils

BACKGROUND

Depression is a leading cause of disability worldwide and is a major contributor to the overall global burden of disease, but its etiology is poorly understood. It has been reported that a disrupted biological rhythm, in terms of a shortened light duration and total darkness, can cause depression-like behaviors in animals. Blue light was reported to have an inhibitory effect on melatonin, which is considered an important clock rhythm biomarker. In the present study, we investigated the effects of blue light deprivation on depressive-like behaviors in gerbils and explored the underlying mechanisms.

METHODS

Gerbils were housed under white light with a filter to block the blue light or without a filter. The behaviors of the gerbils were observed. The biological rhythm, 5-HT, hypothalamic-pituitary-adrenal (HPA) axis and melanopsin pathway were analyzed.

RESULTS

We found that blue light deprivation (BLD) induced depression-like behavior in gerbils. Melatonin lost its rhythm, and corticosterone (CORT) levels decreased in the morning in the BLD group. Lower corticotropin-releasing hormone (CRH) in the hypothalamus and lower adrenocorticotropin hormone (ACTH)/CORT in serum were observed after BLD. Furthermore, 5-HT in the serum and brain were decreased after BLD. Additionally, BLD affected the blue light sensitivity protein melanopsin and its pathway, with downregulation of the proteins melanopsin, PKCα, and c-Fos and the mRNA levels of c-fos and trpc3 and upregulation of the protein p-PKCα.

CONCLUSIONS

Our findings indicated that BLD might produce depression-like behaviors in gerbils. Melatonin arrhythmicity, HPA axis abnormalities, 5-HT decreases and melanopsin pathway changes might be associated with the depression behavioral phenotype in gerbils.

Circadian and photic modulation of daily rhythms in diurnal mammals

The temporal niche that an animal occupies includes a coordinated suite of behavioral and physiological processes that set diurnal and nocturnal animals apart. The daily rhythms of the two chronotypes are regulated by both the circadian system and direct responses to light, a process called masking. Here we review the literature on circadian regulations and masking responses in diurnal mammals, focusing on our work using the diurnal Nile grass rat (Arvicanthis niloticus) and comparing our findings with those derived from other diurnal and nocturnal models. There are certainly similarities between the circadian systems of diurnal and nocturnal mammals, especially in the phase and functioning of the principal circadian oscillator within the hypothalamic suprachiasmatic nucleus (SCN). However, the downstream pathways, direct or indirect from the SCN, lead to drastic differences in the phase of extra-SCN oscillators, with most showing a complete reversal from the phase seen in nocturnal species. This reversal, however, is not universal and in some cases the phases of extra-SCN oscillators are only a few hours apart between diurnal and nocturnal species. The behavioral masking responses in general are opposite between diurnal and nocturnal species, and are matched by differential responses to light and dark in several retinorecipient sites in their brain. The available anatomical and functional data suggest that diurnal brains are not simply a phase-reversed version of nocturnal ones, and work with diurnal models contribute significantly to a better understanding of the circadian and photic modulation of daily rhythms in our own diurnal species.

From clock to functional pacemaker

In mammals, the central pacemaker that coordinates 24‐hr rhythms is located in the suprachiasmatic nucleus (SCN). Individual neurons of the SCN have a molecular basis for rhythm generation and hence, they function as cell autonomous oscillators. Communication and synchronization among these neurons are crucial for obtaining a coherent rhythm at the population level, that can serve as a pace making signal for brain and body. Hence, the ability of single SCN neurons to produce circadian rhythms is equally important as the ability of these neurons to synchronize one another, to obtain a bona fide pacemaker at the SCN tissue level. In this chapter we will discuss the mechanisms underlying synchronization, and plasticity herein, which allows adaptation to changes in day length. Furthermore, we will discuss deterioration in synchronization among SCN neurons in aging, and gain in synchronization by voluntary physical activity or exercise.

Mathematical modeling of circadian rhythms

Circadian rhythms are endogenous ~24-hr oscillations usually entrained to daily environmental cycles of light/dark. Many biological processes and physiological functions including mammalian body temperature, the cell cycle, sleep/wake cycles, neurobehavioral performance, and a wide range of diseases including metabolic, cardiovascular, and psychiatric disorders are impacted by these rhythms. Circadian clocks are present within individual cells and at tissue and organismal levels as emergent properties from the interaction of cellular oscillators. Mathematical models of circadian rhythms have been proposed to provide a better understanding of and to predict aspects of this complex physiological system. These models can be used to: (a) manipulate the system in silico with specificity that cannot be easily achieved using in vivo and in vitro experimental methods and at lower cost, (b) resolve apparently contradictory empirical results, (c) generate hypotheses, (d) design new experiments, and (e) to design interventions for altering circadian rhythms. Mathematical models differ in structure, the underlying assumptions, the number of parameters and variables, and constraints on variables. Models representing circadian rhythms at different physiologic scales and in different species are reviewed to promote understanding of these models and facilitate their use. This article is categorized under: Physiology > Mammalian Physiology in Health and Disease Models of Systems Properties and Processes > Organ, Tissue, and Physiological Models.

Dependence of the entrainment on the ratio of amplitudes between two subgroups in the suprachiasmatic nucleus

Organisms can be synchronized not only to the natural 24-h light-dark cycle but also to artificial non-24-h cycles. Interestingly, when the period of the cycle is far from 24 h, organisms may show complicated behavioral patterns. For example, exposed to a 22-h light-dark cycle, in behavioral activity of rats, a phenomenon called "dissociation" emerges, i.e., one periodic component shows a 22-h period and the other shows a period close to the endogenous period of the animal (around 24 h). It has been found that these two components are regulated by two subgroups of the suprachiasmatic nucleus (SCN), respectively, with the ventrolateral part regulating the 22-h component and the dorsomedial part regulating the other component. In the present study, based on a mathematical model, we will examine how the ratio of amplitudes between these two subgroups affects the entrainment of the SCN to the external 22-h light-dark cycle. Our results show that the dissociation happens when the ratio is smaller than 1 and the maximal entrainment (synchronization) ability of the SCN to the external cycle is obtained when the ratio is larger than 1. Our finding sheds light on the dissociation between the subgroups and suggests that the heterogeneity in the amplitudes alter the entrainment ability of the SCN.

Neuronal oscillations on an ultra-slow timescale: daily rhythms in electrical activity and gene expression in the mammalian master circadian clockwork

Neuronal oscillations of the brain, such as those observed in the cortices and hippocampi of behaving animals and humans, span across wide frequency bands, from slow delta waves (0.1 Hz) to ultra-fast ripples (600 Hz). Here, we focus on ultra-slow neuronal oscillators in the hypothalamic suprachiasmatic nuclei (SCN), the master daily clock that operates on interlocking transcription-translation feedback loops to produce circadian rhythms in clock gene expression with a period of near 24 h (< 0.001 Hz). This intracellular molecular clock interacts with the cell's membrane through poorly understood mechanisms to drive the daily pattern in the electrical excitability of SCN neurons, exhibiting an up-state during the day and a down-state at night. In turn, the membrane activity feeds back to regulate the oscillatory activity of clock gene programs. In this review, we emphasise the circadian processes that drive daily electrical oscillations in SCN neurons, and highlight how mathematical modelling contributes to our increasing understanding of circadian rhythm generation, synchronisation and communication within this hypothalamic region and across other brain circuits.

Using light to tell the time of day: sensory coding in the mammalian circadian visual network

Circadian clocks are a near-ubiquitous feature of biology, allowing organisms to optimise their physiology to make the most efficient use of resources and adjust behaviour to maximise survival over the solar day. To fulfil this role, circadian clocks require information about time in the external world. This is most reliably obtained by measuring the pronounced changes in illumination associated with the earth's rotation. In mammals, these changes are exclusively detected in the retina and are relayed by direct and indirect neural pathways to the master circadian clock in the hypothalamic suprachiasmatic nuclei. Recent work reveals a surprising level of complexity in this sensory control of the circadian system, including the participation of multiple photoreceptive pathways conveying distinct aspects of visual and/or time-of-day information. In this Review, I summarise these important recent advances, present hypotheses as to the functions and neural origins of these sensory signals, highlight key challenges for future research and discuss the implications of our current knowledge for animals and humans in the modern world.

Light color importance for circadian entrainment in a diurnal (Octodon degus) and a nocturnal (Rattus norvegicus) rodent

The central circadian pacemaker (Suprachiasmatic Nuclei, SCN) maintains the phase relationship with the external world thanks to the light/dark cycle. Light intensity, spectra, and timing are important for SCN synchronisation. Exposure to blue-light at night leads to circadian misalignment that could be avoided by using less circadian-disruptive wavelengths. This study tests the capacity of a diurnal Octodon degus and nocturnal Rattus norvegicus to synchronise to different nocturnal lights. Animals were subjected to combined red-green-blue lights (RGB) during the day and to: darkness; red light (R); combined red-green LED (RG) lights; and combined red-green-violet LED (RGV) lights during the night. Activity rhythms free-ran in rats under a RGB:RG cycle and became arrhythmic under RGB:RGV. Degus remained synchronised, despite the fact that day and night-time lighting systems differed only in spectra, but not in intensity. For degus SCN c-Fos activation by light was stronger with RGB-light than with RGV. This could be relevant for developing lighting that reduces the disruptive effects of nocturnal light in humans, without compromising chromaticity.

Collective timekeeping among cells of the master circadian clock

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus is the master circadian clock that coordinates daily rhythms in behavior and physiology in mammals. Like other hypothalamic nuclei, the SCN displays an impressive array of distinct cell types characterized by differences in neurotransmitter and neuropeptide expression. Individual SCN neurons and glia are able to display self-sustained circadian rhythms in cellular function that are regulated at the molecular level by a 24h transcriptional-translational feedback loop. Remarkably, SCN cells are able to harmonize with one another to sustain coherent rhythms at the tissue level. Mechanisms of cellular communication in the SCN network are not completely understood, but recent progress has provided insight into the functional roles of several SCN signaling factors. This review discusses SCN organization, how intercellular communication is critical for maintaining network function, and the signaling mechanisms that play a role in this process. Despite recent progress, our understanding of SCN circuitry and coupling is far from complete. Further work is needed to map SCN circuitry fully and define the signaling mechanisms that allow for collective timekeeping in the SCN network.

Mammalian Diurnality: Some Facts and Gaps

A major factor contributing to the evolution of mammals was their ability to be active during the night, a niche previously underused by terrestrial vertebrates. Diurnality subsequently reemerged multiple times in a variety of independent lineages. This paper reviews some recent data on circadian mechanisms in diurnal mammals and considers general themes that appear to be emerging from this work. Careful examination of behavioral studies suggests that although subtle differences may exist, the fundamental functions of the circadian system are the same, as seems to be the case with respect to the molecular mechanisms of the clock. This suggests that responses to signals originating in the clock must be different, either within the SCN or at its targets or downstream from them. Some features of the SCN vary from species to species, but none of these has been clearly associated with diurnality. The region immediately dorsal to the SCN, which receives substantial input from it, exhibits dramatically different rhythms in nocturnal lab rats and diurnal grass rats. This raises the possibility that it functions as a relay that transforms the signal emitted by the SCN and transmits different patterns to downstream targets in nocturnal and diurnal animals. Other direct targets of the SCN include neurons containing orexin and those containing gonadotropin-releasing hormone, and both of these populations of cells exhibit patterns of rhythmicity that are inverted in at least one diurnal compared to one nocturnal species. The patterns that emerge from the data on diurnality are discussed in terms of the implications they have for the evolution and neural substrates of a day-active way of life.

Analysis of the Prokineticin 2 System in a Diurnal Rodent, the Unstriped Nile Grass Rat (Arvicanthis niloticus)

Prokineticin 2 (PK2) is a putative output molecule from the SCN. PK2 RNA levels are rhythmic in the mouse SCN, with high levels during the day, and PK2 administration suppresses nocturnal locomotor activity in rats. The authors examined the PK2 system in a diurnal rodent, Arvicanthis niloticus, to determine whether PK2 or PK2 receptors differ between diurnal and nocturnal species. The major transcript variant of A. niloticus PK2 ( AnPK2) encodes a 26-residue signal peptide followed by the presumed mature peptide of 81 residues. Within the grass rat signal sequence, polymorphic sequences and amino acid substitutions were observed relative to mouse and laboratory rats, but the hydrophobic core and cleavage site of the signal sequence were preserved. The mature PK2 peptide is identical among A. niloticus, rat, and mouse. AnPK2 mRNA is rhythmically expressed in the SCN, with peak RNAlevels occurring in the morning, preceding peaks of Per1 and Per2 as in mouse SCN. Analysis of prokineticin receptor 2 (PKR2) sequences revealed polymorphisms among the grass rats studied. PKR2 mRNAwas expressed in the SCN and paraventricular nuclei of the thalamus and hypothalamus. While further analysis is necessary, there is no clear evidence indicating that a difference in the PK2 ligand/receptor system accounts for diurnality in this rodent species. These data contribute to a growing body of evidence suggesting that the key to diurnality lies downstream of the SCN in A. niloticus.

The rat suprachiasmatic nucleus: the master clock ticks at 30 Hz

Key points

Light‐responsive neurones in the rat suprachiasmatic nucleus discharge with a harmonic distribution of interspike intervals, whereas unresponsive neurones seldom do.

This harmonic patterning has a fundamental frequency of close to 30 Hz, and is the same in light‐on cells as in light‐off cells, and is unaffected by exposure to light.

Light‐on cells are more active than light‐off cells in both subjective day and subjective night, and both light‐on cells and light‐off cells respond more strongly to changes in light intensity during the subjective night than during the subjective day.

Paired recordings indicate that the discharge of adjacent light‐responsive cells is very tightly synchronized.

The gap junction inhibitor carbenoxolone increases the spontaneous activity of suprachiasmatic nucleus neurones but does not block the harmonic discharge patterning.

Abstract

The suprachiasmatic nucleus (SCN) of the hypothalamus has an essential role in orchestrating circadian rhythms of behaviour and physiology. In the present study, we recorded from single SCN neurons in urethane‐anaesthetized rats, categorized them by the statistical features of their electrical activity and by their responses to light, and examined how activity in the light phase differs from activity in the dark phase. We classified cells as light‐on cells or light‐off cells according to how their firing rate changed in acute response to light, or as non‐responsive cells. In both sets of light‐responsive neurons, responses to light were stronger at subjective night than in subjective day. Neuronal firing patterns were analysed by constructing hazard functions from interspike interval data. For most light‐responsive cells, the hazard functions showed a multimodal distribution, with a harmonic sequence of modes, indicating that spike activity was driven by an oscillatory input with a fundamental frequency of close to 30 Hz; this harmonic pattern was rarely seen in non‐responsive SCN cells. The frequency of the rhythm was the same in light‐on cells as in light‐off cells, was the same in subjective day as at subjective night, and was unaffected by exposure to light. Paired recordings indicated that the discharge of adjacent light‐responsive neurons was very tightly synchronized, consistent with electrical coupling.

Light‐responsive neurones in the rat suprachiasmatic nucleus discharge with a harmonic distribution of interspike intervals, whereas unresponsive neurones seldom do.

This harmonic patterning has a fundamental frequency of close to 30 Hz, and is the same in light‐on cells as in light‐off cells, and is unaffected by exposure to light.

Light‐on cells are more active than light‐off cells in both subjective day and subjective night, and both light‐on cells and light‐off cells respond more strongly to changes in light intensity during the subjective night than during the subjective day.

Paired recordings indicate that the discharge of adjacent light‐responsive cells is very tightly synchronized.

The gap junction inhibitor carbenoxolone increases the spontaneous activity of suprachiasmatic nucleus neurones but does not block the harmonic discharge patterning.

In synch but not in step: Circadian clock circuits regulating plasticity in daily rhythms

The suprachiasmatic nucleus (SCN) is a network of neural oscillators that program daily rhythms in mammalian behavior and physiology. Over the last decade much has been learned about how SCN clock neurons coordinate together in time and space to form a cohesive population. Despite this insight, much remains unknown about how SCN neurons communicate with one another to produce emergent properties of the network. Here we review the current understanding of communication among SCN clock cells and highlight a collection of formal assays where changes in SCN interactions provide for plasticity in the waveform of circadian rhythms in behavior. Future studies that pair analytical behavioral assays with modern neuroscience techniques have the potential to provide deeper insight into SCN circuit mechanisms.

Synchronization of Biological Clock Neurons by Light and Peripheral Feedback Systems Promotes Circadian Rhythms and Health

In mammals, the suprachiasmatic nucleus (SCN) functions as a circadian clock that drives 24-h rhythms in both physiology and behavior. The SCN is a multicellular oscillator in which individual neurons function as cell-autonomous oscillators. The production of a coherent output rhythm is dependent upon mutual synchronization among single cells and requires both synaptic communication and gap junctions. Changes in phase-synchronization between individual cells have consequences on the amplitude of the SCN’s electrical activity rhythm, and these changes play a major role in the ability to adapt to seasonal changes. Both aging and sleep deprivation negatively affect the circadian amplitude of the SCN, whereas behavioral activity (i.e., exercise) has a positive effect on amplitude. Given that the amplitude of the SCN’s electrical activity rhythm is essential for achieving robust rhythmicity in physiology and behavior, the mechanisms that underlie neuronal synchronization warrant further study. A growing body of evidence suggests that the functional integrity of the SCN contributes to health, well-being, cognitive performance, and alertness; in contrast, deterioration of the 24-h rhythm is a risk factor for neurodegenerative disease, cancer, depression, and sleep disorders.

The suprachiasmatic nuclei as a seasonal clock

In mammals, the suprachiasmatic nucleus (SCN) contains a central clock that synchronizes daily (i.e., 24-h) rhythms in physiology and behavior. SCN neurons are cell-autonomous oscillators that act synchronously to produce a coherent circadian rhythm. In addition, the SCN helps regulate seasonal rhythmicity. Photic information is perceived by the SCN and transmitted to the pineal gland, where it regulates melatonin production. Within the SCN, adaptations to changing photoperiod are reflected in changes in neurotransmitters and clock gene expression, resulting in waveform changes in rhythmic electrical activity, a major output of the SCN. Efferent pathways regulate the seasonal timing of breeding and hibernation. In humans, seasonal physiology and behavioral rhythms are also present, and the human SCN has seasonally rhythmic neurotransmitter levels and morphology. In summary, the SCN perceives and encodes changes in day length and drives seasonal changes in downstream pathways and structures in order to adapt to the changing seasons.

Eye-specific visual processing in the mouse suprachiasmatic nuclei

Internal circadian clocks are important regulators of mammalian biology, acting to coordinate physiology and behaviour in line with daily changes in the environment. At present, synchronisation of the circadian system to the solar cycle is believed to rely on a quantitative assessment of total ambient illumination, provided by a bilateral projection from the retina to the suprachiasmatic nuclei (SCN). It is currently unclear, however, whether this photic integration occurs at the level of individual cells or within the SCN network. Here we use extracellular multielectrode recordings from the SCN of anaesthetised mice to show that most SCN neurons receive visual input from just one eye. While we find that binocular inputs to a subset of cells are important for rapid responses to changes in illumination, we find no evidence indicating that individual SCN cells are capable of reporting the average light intensity across the whole visual field. As a result of these local irradiance coding properties, our data establish that photic integration is primarily mediated at the level of the SCN network and suggest that accurate assessments of global light levels would be impaired by non-uniform illumination of either eye.

The proportion of light-responsive neurons determines the limit cycle properties of the suprachiasmatic nucleus

In mammals, the central clock in the suprachiasmatic nucleus (SCN) controls physiological and behavioral circadian rhythms and is entrained to the external light-dark cycle. The ability of the SCN to entrain can be measured by exposing the animal to a light-dark cycle with a duration that deviates from 24 h (T-cycles); a wider entrainment range reflects a higher ability to entrain. The neurons of the SCN are either light responsive or light unresponsive and are mutually synchronized. The coupling and synchronization between individual SCN neurons and between groups of neurons within the SCN influence the SCN's ability to entrain. Some studies suggest that enhanced coupling decreases the entrainment range, whereas others suggest that enhanced coupling increases the entrainment range. The latter results are surprising, as they are not consistent with the prevalent assumption that the SCN is a limit cycle oscillator that has larger phase shifts when the amplitude is smaller. Here, we used the Poincaré and Goodwin models to test entrainment properties using various proportions of neurons that are responsive to an external stimulus. If all neurons receive external input, the SCN shows limit cycle behavior in all conditions. If all neurons do not receive light input, we found that the entrainment range of the SCN was positively related to coupling strength when coupling was weak. When coupling strength was stronger and above a critical value, the entrainment range was negatively correlated with coupling strength. The results obtained from our simulations were confirmed by analytical studies. Thus, the limit cycle behavior of the SCN appears to be critically dependent on the coupling strength among the neurons and the proportion of neurons that respond to the entraining stimulus.

Retinal projections and neurochemical characterization of the pregeniculate nucleus of the common marmoset (Callithrix jacchus)

In mammals, the suprachiasmatic nucleus (SCN) and the intergeniculate leaflet (IGL) are the main components of the circadian timing system. The SCN is the site of the endogenous biological clock that generates rhythms and synchronizes them to environmental cues. The IGL is a key structure that modulates SCN activity and is responsible for the transmission of non-photic information to the SCN, thus participating in the integration between photic and non-photic stimuli. Both the SCN and IGL receive projections of retinal ganglion cells and the IGL is connected to the SCN through the geniculohypothalamic tract. Little is known about these structures in the primate brain and the pregeniculate nucleus (PGN) has been suggested to be the primate equivalent of the rodent IGL. The aim of this study was to characterize the PGN of a primate, the common marmoset (Callithrix jacchus), and to analyze its retinal afferents. Here, the marmoset PGN was found to be organized into three subsectors based on neuronal size, pattern of retinal projections, and the distribution of neuropeptide Y-, GAD-, serotonin-, enkephalin- and substance P-labeled terminals. This pattern indicates that the marmoset PGN is equivalent to the IGL. This detailed description contributes to the understanding of the circadian timing system in this primate species considering the importance of the IGL within the context of circadian regulation.

Physiology of circadian entrainment

Mammalian circadian rhythms are controlled by endogenous biological oscillators, including a master clock located in the hypothalamic suprachiasmatic nuclei (SCN). Since the period of this oscillation is of approximately 24 h, to keep synchrony with the environment, circadian rhythms need to be entrained daily by means of Zeitgeber ("time giver") signals, such as the light-dark cycle. Recent advances in the neurophysiology and molecular biology of circadian rhythmicity allow a better understanding of synchronization. In this review we cover several aspects of the mechanisms for photic entrainment of mammalian circadian rhythms, including retinal sensitivity to light by means of novel photopigments as well as circadian variations in the retina that contribute to the regulation of retinal physiology. Downstream from the retina, we examine retinohypothalamic communication through neurotransmitter (glutamate, aspartate, pituitary adenylate cyclase-activating polypeptide) interaction with SCN receptors and the resulting signal transduction pathways in suprachiasmatic neurons, as well as putative neuron-glia interactions. Finally, we describe and analyze clock gene expression and its importance in entrainment mechanisms, as well as circadian disorders or retinal diseases related to entrainment deficits, including experimental and clinical treatments.

The substructure of the suprachiasmatic nucleus: Similarities between nocturnal and diurnal spiny mice

Evolutionary transitions between nocturnal and diurnal patterns of adaptation to the day-night cycle must have involved fundamental changes in the neural mechanisms that coordinate the daily patterning of activity, but little is known about how these mechanisms differ. One reason is that information on these systems in very closely related diurnal and nocturnal species is lacking. In this study, we characterize the suprachiasmatic nucleus (SCN), the primary brain structure involved in the generation and coordination of circadian rhythms, in two members of the genus Acomys with very different activity patterns, Acomys russatus (the golden spiny mouse, diurnal) and Acomys cahirinus (the common spiny mouse, nocturnal). Immunohistochemical techniques were used to label cell bodies containing vasoactive intestinal polypeptide (VIP), vasopressin (VP), gastrin-releasing peptide (GRP) and calbindin (CalB) in the SCN, as well as two sets of inputs to it, those containing serotonin (5-HT) and neuropeptide Y (NPY), respectively. All were present in the SCN of both species and no differences between them were seen. On the basis of neuronal phenotype, the SCN was organized into three basic regions that contained VIP-immunoreactive (-ir), CalB-ir and VP-ir cells, in the ventral, middle and dorsal SCN, respectively. In the rostral SCN, GRP-ir cells were in both the VIP and the CalB cell regions, and in the caudal area they were distributed across a portion of each of the other three regions. Fibers containing NPY-ir and serotonin (5-HT)-ir were most concentrated in the areas containing VIP-ir and CalB-ir cells, respectively. The details of the spatial relationships among the labeled cells and fibers seen here are discussed in relation to different approaches investigators have taken to characterize the SCN more generally.

The response of Per1 to light in the suprachiasmatic nucleus of the diurnal degu (Octodon degus)

Several studies suggest that the circadian systems of diurnal mammals respond differently to daytime light than those of nocturnal mammals. We hypothesized that the photosensitive "clock" gene Per1 would respond to light exposure during subjective day in the suprachiasmatic nucleus of the diurnal rodent, Octodon degus. Tissue was collected 1.5-2 h after a 30 min light pulse presented at five timepoints across the 24 h day and compared to controls maintained under conditions of constant darkness. Per1 mRNA was quantified using in situ hybridization. Results showed that the rhythmicity and photic responsiveness of Per1 in the degu resembles that of nocturnal animals.

Phase Response Curve to Melatonin in a Putatively Diurnal Rodent,Octodon degus

The degu (Octodon degus) is a diurnal rodent, although phase inversions to nocturnal behavior have been reported under specific laboratory conditions. The reliability of this animal as a diurnal model of sleep therefore requires further characterization of intrinsic circadian pacemaker properties. A phase response curve to light has been reported in the degu, and is consistent with other diurnal animals. This study reports a phase response curve to melatonin in the degu, which is distinct in orientation from the light curve.

Compartmentalized expression of light-induced clock genes in the suprachiasmatic nucleus of the diurnal grass rat (Arvicanthis niloticus)

Photic responses of the circadian system are mediated through light-induced clock gene expression in the suprachiasmatic nucleus (SCN). In nocturnal rodents, depending on the timing of light exposure, Per1 and Per2 gene expression shows distinct compartmentalized patterns that correspond to the behavioral responses. Whether the gene- and region-specific induction patterns are unique to nocturnal animals, or are also present in diurnal species is unknown. We explored this question by examining the light-induced Per1 and Per2 gene expression in functionally distinct SCN subregions, using diurnal grass rats Arvicanthis niloticus. Light exposure during nighttime induced Per1 and Per2 expression in the SCN, showing unique spatiotemporal profiles depending on the phase of the light exposure. After a phase delaying light pulse (LP) in the early night, strong Per1 induction was observed in the retinorecipient core region of the SCN, while strong Per2 induction was observed throughout the entire SCN. After a phase advancing LP in the late night, Per1 was first induced in the core and then extended into the whole SCN, accompanied by a weak Per2 induction. This compartmentalized expression pattern is very similar to that observed in nocturnal rodents, suggesting that the same molecular and intercellular pathways underlying acute photic responses are present in both diurnal and nocturnal species. However, after an LP in early subjective day, which induces phase advances in diurnal grass rats, but not in nocturnal rodents, we did not observe any Per1 or Per2 induction in the SCN. This result suggests that in spite of remarkable similarities in the SCN of diurnal and nocturnal rodents, unique mechanisms are involved in mediating the phase shifts of diurnal animals during the subjective day.

Period gene expression in the diurnal degu (Octodon degus) differs from the nocturnal laboratory rat (Rattus norvegicus)

Recent data suggest that both nocturnal and diurnal mammals generate circadian rhythms using similarly phased feedback loops involving Period genes in the suprachiasmatic nuclei (SCN) of the hypothalamus. These molecular oscillations also exist in the brain outside of the SCN, but the relationship between SCN and extra-SCN oscillations is unclear. We hypothesized that a comparison of "diurnal" and "nocturnal" central nervous system Per rhythms would uncover differences in the underlying circadian mechanisms between these two chronotypes. Therefore, this study compared the 24-h oscillatory patterns of Per1 and Per2 mRNA in the SCN and putative striatum and cortex of Octodon degus (degu), a diurnal hystricognath rodent, with those of the nocturnal laboratory rat, Rattus norvegicus. The brains of adult male degus and rats were collected at 2-h intervals across 24 h in entrained light-dark and constant darkness conditions, and sections were analyzed via in situ hybridization. In the SCN, degu Per1 and Per2 hybridization signal exhibited 24-h oscillatory patterns similar in phasing to those seen in other rodents, with peaks occurring during the light period and troughs during the dark period. However, Per1 remained elevated for five fewer hours in the degu than in the rat, and Per2 remained elevated for two fewer hours in the degu. In brain areas outside of the SCN, the phase of Per2 hybridization signal rhythms in the degu were 180 degrees out of phase with those found in the rat, and Per1 hybridization signal lacked significant rhythmicity. These results suggest that, while certain basic components of the transcriptional-translational feedback loop generating circadian rhythms are similar in diurnal and nocturnal mammals, there are variations that may reflect adaptations to circadian niche.

Light deprivation damages monoamine neurons and produces a depressive behavioral phenotype in rats

Light is an important environmental factor for regulation of mood. There is a high frequency of seasonal affective disorder in high latitudes where light exposure is limited, and bright light therapy is a successful antidepressant treatment. We recently showed that rats kept for 6 weeks in constant darkness (DD) have anatomical and behavioral features similar to depressed patients, including dysregulation of circadian sleep-waking rhythms and impairment of the noradrenergic (NA)-locus coeruleus (LC) system. Here, we analyzed the cell viability of neural systems related to the pathophysiology of depression after DD, including NA-LC, serotoninergic-raphe nuclei and dopaminergic-ventral tegmental area neurons, and evaluated the depressive behavioral profile of light-deprived rats. We found increased apoptosis in the three aminergic systems analyzed when compared with animals maintained for 6 weeks in 12:12 light-dark conditions. The most apoptosis was observed in NA-LC neurons, associated with a significant decrease in the number of cortical NA boutons. Behaviorally, DD induced a depression-like condition as measured by increased immobility in a forced swim test (FST). DD did not appear to be stressful (no effect on adrenal or body weights) but may have sensitized responses to subsequent stressors (increased fecal number during the FST). We also found that the antidepressant desipramine decreases these neural and behavioral effects of light deprivation. These findings indicate that DD induces neural damage in monoamine brain systems and this damage is associated with a depressive behavioral phenotype. Our results suggest a mechanism whereby prolonged limited light intensity could negatively impact mood.

Minireview: Entrainment of the suprachiasmatic clockwork in diurnal and nocturnal mammals

Daily rhythmicity, including timing of wakefulness and hormone secretion, is mainly controlled by a master clock located in the suprachiasmatic nucleus (SCN) of the hypothalamus. The SCN clockwork involves various clock genes, with specific temporal patterns of expression that are similar in nocturnal and diurnal species (e.g. the clock gene Per1 in the SCN peaks at midday in both categories). Timing of sensitivity to light is roughly similar, during nighttime, in diurnal and nocturnal species. Molecular mechanisms of photic resetting are also comparable in both species categories. By contrast, in animals housed in constant light, exposure to darkness can reset the SCN clock, mostly during the resting period, i.e. at opposite circadian times between diurnal and nocturnal species. Nonphotic stimuli, such as scheduled voluntary exercise, food shortage, exogenous melatonin, or serotonergic receptor activation, are also capable of shifting the master clock and/or modulating photic synchronization. Comparison between day- and night-active species allows classifications of nonphotic cues in two, arousal-independent and arousal-dependent, families of factors. Arousal-independent factors, such as melatonin (always secreted during nighttime, independently of daily activity pattern) or gamma-aminobutyric acid (GABA), have shifting effects at the same circadian times in both nocturnal and diurnal rodents. By contrast, arousal-dependent factors, such as serotonin (its cerebral levels follow activity pattern), induce phase shifts only during resting and have opposite modulating effects on photic resetting between diurnal and nocturnal species. Contrary to light and arousal-independent nonphotic cues, arousal-dependent nonphotic stimuli provide synchronizing feedback signals to the SCN clock in circadian antiphase between nocturnal and diurnal animals.

Organization of cell and tissue circadian pacemakers: a comparison among species

In most animal species, a circadian timing system has evolved as a strategy to cope with 24-hour rhythms in the environment. Circadian pacemakers are essential elements of the timing system and have been identified in anatomically discrete locations in animals ranging from insects to mammals. Rhythm generation occurs in single pacemaker neurons and is based on the interacting negative and positive molecular feedback loops. Rhythmicity in behavior and physiology is regulated by neuronal networks in which synchronization or coupling is required to produce coherent output signals. Coupling occurs among individual clock cells within an oscillating tissue, among functionally distinct subregions within the pacemaker, and between central pacemakers and the periphery. Recent evidence indicates that peripheral tissues can influence central pacemakers and contain autonomous circadian oscillators that contribute to the regulation of overt rhythmicity. The data discussed in this review describe coupling and synchronization mechanisms at the cell and tissue levels. By comparing the pacemaker systems of several multicellular animal species (Drosophila, cockroaches, crickets, snails, zebrafish and mammals), we will explore general organizational principles by which the circadian system regulates a 24-hour rhythmicity.

Electrophysiology of the suprachiasmatic circadian clock

In mammals, an internal timekeeping mechanism located in the suprachiasmatic nuclei (SCN) orchestrates a diverse array of neuroendocrine and physiological parameters to anticipate the cyclical environmental fluctuations that occur every solar day. Electrophysiological recording techniques have proved invaluable in shaping our understanding of how this endogenous clock becomes synchronized to salient environmental cues and appropriately coordinates the timing of a multitude of physiological rhythms in other areas of the brain and body. In this review we discuss the pioneering studies that have shaped our understanding of how this biological pacemaker functions, from input to output. Further, we highlight insights from new studies indicating that, more than just reflecting its oscillatory output, electrical activity within individual clock cells is a vital part of SCN clockwork itself.

A comparative analysis of the distribution of immunoreactive orexin A and B in the brains of nocturnal and diurnal rodents

Background

The orexins (hypocretins) are a family of peptides found primarily in neurons in the lateral hypothalamus. Although the orexinergic system is generally thought to be the same across species, the orexins are involved in behaviors which show considerable interspecific variability. There are few direct cross-species comparisons of the distributions of cells and fibers containing these peptides. Here, we addressed the possibility that there might be important species differences by systematically examining and directly comparing the distribution of orexinergic neurons and fibers within the forebrains of species with very different patterns of sleep-wake behavior.

Methods

We compared the distribution of orexin-immunoreactive cell bodies and fibers in two nocturnal species (the lab rat, Rattus norvegicus and the golden hamster, Mesocricetus auratus) and two diurnal species (the Nile grass rat, Arvicanthis niloticus and the degu, Octodon degus). For each species, tissue from the olfactory bulbs through the brainstem was processed for immunoreactivity for orexin A and orexin B (hypocretin-1 and -2). The distribution of orexin-positive cells was noted for each species. Orexin fiber distribution and density was recorded and analyzed using a principal components factor analysis to aid in evaluating potential species differences.

Results

Orexin-positive cells were observed in the lateral hypothalamic area of each species, though there were differences with respect to distribution within this region. In addition, cells positive for orexin A but not orexin B were observed in the paraventricular nucleus of the lab rat and grass rat, and in the supraoptic nucleus of the lab rat, grass rat and hamster. Although the overall distributions of orexin A and B fibers were similar in the four species, some striking differences were noted, especially in the lateral mammillary nucleus, ventromedial hypothalamic nucleus and flocculus.

Conclusion

The orexin cell and fiber distributions observed in this study were largely consistent with those described in previous studies. However, the present study shows significant species differences in the distribution of orexin cell bodies and in the density of orexin-IR fibers in some regions. Finally, we note previously undescribed populations of orexin-positive neurons outside the lateral hypothalamus in three of the four species examined.

Changes in neuropeptide Y immunoreactivity and transcript levels in circadian system structures of the diurnal rodent, the thirteen-lined ground squirrel

The intergeniculate leaflet (IGL) and its neuropeptide Y (NPY) projection to the main circadian clock, the suprachiasmatic nucleus (SCN), have been the focus of extensive research conducted, for the most part, on nocturnal rodent species. However, a variety of anatomical and physiological differences between the circadian system of diurnal and nocturnal species have been reported. These differences led us to question whether the role of NPY in the circadian system of the diurnal ground squirrel differs from that in nocturnal rodents. We used semi-quantitative immunohistochemistry to analyze NPY content in SCN terminals of squirrels sacrificed at specific times of the day and compared the data to previous published results from the rat. Additionally, IGL NPY mRNA was quantified using real-time PCR to determine if varying NPY immunoreactivity (-ir) levels could be the result of changes in peptide transcription. Our results demonstrate that NPY-ir levels in the ground squirrel SCN peak during the middle of the night unlike what is observed in the rat. Cell counts of NPY-ir neurons in the IGL revealed a pattern of variation 6 h out of phase compared to what was observed in the SCN. NPY mRNA levels showed only one sharp increase in the middle of the night, coinciding with increases in NPY-ir levels observed in the SCN. Differences in the pattern of fluctuation of NPY in the SCN between the rat and squirrel suggest that this peptide may serve distinct roles in the circadian system of diurnal and nocturnal species. Our data provide the first evidence of the relationship between transcript and peptide levels in the circadian system of a diurnal species.

Light and GABAAreceptor activation alterPeriodmRNA levels in the SCN of diurnal Nile grass rats

We examined Period (Per) mRNA rhythms in the suprachiasmatic nucleus (SCN) of a diurnal rodent and assessed how phase-shifting stimuli acutely affect SCN Per mRNA using semiquantitative in situ hybridization. First, Per1 and Per2 varied rhythmically in the SCN over the course of one circadian cycle in constant darkness: Per1 mRNA was highest in the early to mid-subjective day, while Per2 mRNA levels peaked in the late subjective day. Second, acute light exposure in the early subjective night significantly increased both Per1 and Per2 mRNA. Third, Per2 but not Per1 levels decreased 1 and 2 h after injection of the gamma-aminobutyric acid (GABA)(A) receptor agonist muscimol into the SCN during the subjective day. Fourth, muscimol also reduced the light-induced Per2 in the early subjective night, but Per1 induction by light was not significantly affected. Consistent with previous studies, these data demonstrate that diurnal and nocturnal animals show very similar daily patterns of Per mRNA and light-induced Per increases in the SCN. As with light, muscimol alters circadian phase, and daytime phase alterations induced by muscimol are associated with significant decreases in Per2 mRNA. In diurnal animals, muscimol-induced decreases in Per are associated with phase delays rather than advances. The direction of the daytime phase shift may be determined by the relative suppression of Per1 vs. Per2 in SCN cells. As in nocturnal animals, changes in Per1 and Per2 mRNA by photic and non-photic stimuli appear to be associated with circadian phase alteration.

Circadian regulation of sleep in mammals: Role of the suprachiasmatic nucleus

Despite significant progress in elucidating the molecular basis for circadian oscillations, the neural mechanisms by which the circadian clock organizes daily rhythms of behavioral state in mammals remain poorly understood. The objective of this review is to critically evaluate a conceptual model that views sleep expression as the outcome of opponent processes-a circadian clock-dependent alerting process that opposes sleep during the daily wake period, and a homeostatic process by which sleep drive builds during waking and is dissipated during sleep after circadian alerting declines. This model is based primarily on the evidence that in a diurnal primate, the squirrel monkey (Saimiri sciureus), ablation of the master circadian clock (the suprachiasmatic nucleus; SCN) induces a significant expansion of total daily sleep duration and a reduction in sleep latency in the dark. According to this model, the circadian clock actively promotes wake but only passively gates sleep; thus, loss of circadian clock alerting by SCN ablation impairs the ability to sustain wakefulness and causes sleep to expand. For comparison, two additional conceptual models are described, one in which the circadian clock actively promotes sleep but not wake, and a third in which the circadian clock actively promotes both sleep and wake, at different circadian phases. Sleep in intact and SCN-damaged rodents and humans is first reviewed, to determine how well the data fit these conceptual models. Neuroanatomical and neurophysiological studies are then reviewed, to examine the evidence for direct and indirect interactions between the SCN circadian clock and sleep-wake circuits. Finally, sleep in SCN-ablated squirrel monkeys is re-examined, to consider its compatibility with alternative models of circadian regulation of sleep. In aggregate, the behavioral and neurobiological evidence suggests that in rodents and humans, the circadian clock actively promotes both wake and sleep, at different phases of the circadian cycle. The hypersomnia of SCN-ablated squirrel monkeys is unique in magnitude, but is not incompatible with a role for the SCN pacemaker in actively promoting sleep.

Diurnal mice (Mus musculus) and other examples of temporal niche switching

Examples are presented of nocturnal animals becoming diurnal or vice versa as a result of mutations, genetic manipulations, or brain lesions. Understanding these cases could give insight into mechanisms employed when switches of temporal niche occur as part of the life cycle, or in response to circumstances such as availability of food. A two-process account of niche switching is advocated, involving both a change in clock-controlled outputs and a change in the direct response to light (i.e. masking). An emerging theme from this review is the suggestion that retinal inputs have a greater role in switching than suspected previously.

The suprachiasmatic nucleus is a functionally heterogeneous timekeeping organ

Ever since the locus of the brain clock in the suprachiasmatic nucleus (SCN) was first described, methods available have both enabled and encumbered our understanding of its nature at the level of the cell, the tissue, and the animal. A combination of in vitro and in vivo approaches has shown that the SCN is a complex heterogeneous neuronal network. The nucleus is composed of cells that are retinorecipient and reset by photic input; those that are reset by nonphotic inputs; slave oscillators that are rhythmic only in the presence of the retinohypothalamic tract; endogenously rhythmic cells, with diverse period, phase, and amplitude responses; and cells that do not oscillate, at least on some measures. Network aspects of SCN organization are currently being revealed, but mapping these properties onto cellular characteristics of electrical responses and patterns of gene expression are in early stages. While previous mathematical models focused on properties of uniform coupled oscillators, newer models of the SCN as a brain clock now incorporate oscillator and gated, nonoscillator elements.

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.

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.

Differences in the suprachiasmatic nucleus and lower subparaventricular zone of diurnal and nocturnal rodents

Diurnal and nocturnal species are profoundly different in terms of the temporal organization of daily rhythms in physiology and behavior. The neural bases for these divergent patterns are at present unknown. Here we examine functional differences in the suprachiasmatic nucleus (SCN) and one of its primary targets in a diurnal rodent, the unstriped Nile grass rat (Arvicanthis niloticus) and in a nocturnal one, the laboratory rat (Rattus norvegicus). Grass rats and laboratory rats were housed in a 12:12 light:dark cycle, and killed at six time points. cFos-immunoreactive rhythms in the SCN of grass rats and laboratory rats were similar to those reported previously, with peaks early in the light phase and troughs in the dark phase. However, cFos-immunoreactivity in the lower subparaventricular zone (LSPV) of grass rats rose sharply 5 h into the dark phase, and remained high through the first hour after light onset, whereas in laboratory rats it peaked 1 h after light onset and was low at all other sampling times. Daily cFos rhythms in both the SCN and the LSPV persisted in grass rats, but not in laboratory rats, after extended periods in constant darkness. In grass rats, the endogenous cFos rhythm in the LSPV, but not the SCN, was present both in calbindin-positive and in calbindin-negative cells. Cells that expressed cFos at night in the region of the LSPV in grass rats were clearly outside of the boundaries of the SCN as delineated by Nissl stain and immunoreactivity for vasopressin and vasoactive intestinal peptide. The LSPV of the grass rat, a region that receives substantial input from the SCN, displays a daily rhythm in cFos expression that differs from that of laboratory rats with respect to its rising phase, the duration of the peak and its dependence on a light/dark cycle. These characteristics may reflect the existence of mechanisms in the LSPV that enable it to modulate efferent SCN signals differently in diurnal and nocturnal species.