Temporal and spatial distribution of immunoreactive PER1 and PER2 proteins in the suprachiasmatic nucleus and peri-suprachiasmatic region of the diurnal grass rat (Arvicanthis niloticus)

Action potential firing rhythms in the suprachiasmatic nucleus of the diurnal grass rat, Arvicanthis niloticus

In mammals, daily physiological activities are regulated by a central circadian pacemaker located in the hypothalamic suprachiasmatic nucleus (SCN). Recently, an increasing number of studies have used diurnal grass rats to analyze neuronal mechanisms regulating diurnal behavior. However, spontaneous action potential firing rhythms in SCN neurons have not been demonstrated clearly in diurnal grass rats. Therefore, the present study examined extracellular single-unit recordings from SCN neurons in acute hypothalamic slices of Arvicanthis niloticus (Nile grass rats). The results of this study found that circadian firing rhythms with the highest frequency occurred at dusk (6.4 Hz at zeitgeber time (ZT)10-12), while the secondary peak occurred at dawn (5.6 Hz at ZT0-2), and the lowest frequency took place in the middle of the night (3.6 Hz at ZT14-16). Locomotor activity recordings from a separate group of animals demonstrated that the Nile grass rats of the laboratory colony used in this study displayed diurnal behaviors that coincided with large crepuscular peaks under 12:12 h light-dark cycles and bimodal rhythms under constant dim red light. Thus, a positive correlation between SCN firing frequencies and locomotor activity levels was observed in the Nile grass rats. Previously, behavioral coupling of action potential firings in SCN neurons has been suggested by in vivo recordings while the present study demonstrates that the sustenance of bimodal firing rhythms in grass rat SCN neurons can last at least one day in vitro.

Arginine Vasopressin-Containing Neurons of the Suprachiasmatic Nucleus Project to CSF

While it is well established that there are robust circadian rhythms of arginine vasopressin (AVP) in the cerebrospinal fluid (CSF), the route whereby the peptide reaches the CSF is not clear. A, AVP neurons constitute the largest fraction of the SCN neuronal population. Here, we show that processes of AVP-expressing SCN neurons cross the epithelium of the 3rd ventricular wall to reach the CSF (black arrows). Additionally, we report rostro-caudal differences in AVP neuron size and demonstrate that the localization of cells expressing the clock protein PER2 extend beyond the AVP population, thereby indicating that the size of this nucleus is somewhat larger than previously understood. B, Following lateral ventricle (LV) injection of cholera toxin β subunit (CTβ ; magenta) the retrograde tracer is seen in AVP neurons of the SCN, supporting the anatomical evidence that AVP neuronal processes directly contact the CSF.

Arginine vasopressin (AVP) expressing neurons form the major population in the brain’s circadian clock located in the hypothalamic suprachiasmatic nucleus (SCN). They participate in inter-neuronal coupling and provide an output signal for synchronizing daily rhythms. AVP is present at high concentrations in the cerebrospinal fluid (CSF) and fluctuates on a circadian timescale. While it is assumed that rhythms in CSF AVP are of SCN origin, a route of communication between these compartments has not been delineated. Using immunochemistry (ICC) and cell filling techniques, we determine the morphology and location of AVP neurons in mouse and delineate their axonal and dendritic processes. Cholera toxin β subunit (CTβ) tracer injected into the lateral ventricle tests whether AVP neurons communicate with CSF. Most importantly, the results indicate that AVP neurons lie in close proximity to the third ventricle, and their processes cross the ventricular wall into the CSF. We also report that contrary to widely held assumptions, AVP neurons do not fully delineate the SCN borders as PER2 expression extends beyond the AVP region. Also, AVP neurons form a rostral prong originating in the SCN medial-most and ventral-most aspect. AVP is lacking in the mid-dorsal shell but does occur at the base of the SCN just above the optic tract. Finally, neurons of the rostral SCN are smaller than those lying caudally. These findings extend our understanding of AVP signaling potential, demonstrate the heterogeneity of AVP neurons, and highlight limits in using this peptide to delineate the mouse SCN.

Biological Timing and Neurodevelopmental Disorders: A Role for Circadian Dysfunction in Autism Spectrum Disorders

Autism spectrum disorders (ASDs) are a spectrum of neurodevelopmental disorders characterized by impaired social interaction and communication, as well as stereotyped and repetitive behaviors. ASDs affect nearly 2% of the United States child population and the worldwide prevalence has dramatically increased in recent years. The etiology is not clear but ASD is thought to be caused by a combination of intrinsic and extrinsic factors. Circadian rhythms are the ∼24 h rhythms driven by the endogenous biological clock, and they are found in a variety of physiological processes. Growing evidence from basic and clinical studies suggest that the dysfunction of the circadian timing system may be associated with ASD and its pathogenesis. Here we review the findings that link circadian dysfunctions to ASD in both experimental and clinical studies. We first introduce the organization of the circadian system and ASD. Next, we review physiological indicators of circadian rhythms that are found disrupted in ASD individuals, including sleep-wake cycles, melatonin, cortisol, and serotonin. Finally, we review evidence in epidemiology, human genetics, and biochemistry that indicates underlying associations between circadian regulation and the pathogenesis of ASD. In conclusion, we propose that understanding the functional importance of the circadian clock in normal and aberrant neurodevelopmental processes may provide a novel perspective to tackle ASD, and clinical treatments for ASD individuals should comprise an integrative approach considering the dynamics of daily rhythms in physical, mental, and social processes.

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.

Time management in a co-housed social rodent species (Arvicanthis niloticus)

Sociality has beneficial effects on fitness, and timing the activities of animals may be critical. Social cues could influence daily rhythmic activities via direct effects on the circadian clock or on processes that bypass it (masking), but these possibilities remain incompletely addressed. We investigated the effects of social cues on the circadian body temperature (Tb) rhythms in pairs of co-housed and isolated grass rats, Arvicanthis niloticus (a social species), in constant darkness (DD). Cohabitation did not induce synchronization of circadian Tb rhythms. However, socio-sexual history did affect circadian properties: accelerating the clock in sexually experienced males and females in DD and advancing rhythm phase in the females in a light-dark cycle. To address whether synchronization occurs at an ultradian scale, we analyzed Tb and activity rhythms in pairs of co-housed sisters or couples in DD. Regardless of pair type, co-housing doubled the percentage of time individuals were simultaneously active without increasing individual activity levels, suggesting that activity bouts were synchronized by redistribution over 24 h. Together, our laboratory findings show that social cues affect individual "time allocation" budgets via mechanisms at multiple levels of biological organization. We speculate that in natural settings these effects could be adaptive, especially for group-living animals.

Clock Gene Expression in the Suprachiasmatic Nucleus of Hibernating Arctic Ground Squirrels

Most organisms have a circadian system, entrained to daily light-dark cycles, that regulates 24-h rhythms of physiology and behavior. It is unclear, however, how circadian systems function in animals that exhibit seasonal metabolic suppression, particularly when this coincides with the long-term absence of a day-night cycle. The arctic ground squirrel, Urocytellus parryii, is a medium-sized, semi-fossorial rodent that appears above-ground daily during its short active season in spring and summer before re-entering a constantly dark burrow for 6 to 9 months of hibernation. This hibernation consists of multiple week-long torpor bouts interrupted by short (< 20 h) arousal intervals when metabolism and body temperature (Tb) return to normal levels. Here, we used immunohistochemistry to measure the expression of daily or circadian rhythms of the protein products of 3 circadian clock genes, PER1, PER2, BMAL1, and the neural activity marker c-FOS in the suprachiasmatic nucleus (SCN) of arctic ground squirrels before, during, and after the first torpor bout of hibernation. Before torpor, while under 12:12-h light:dark conditions, animals showed significant daily rhythms in their Tb, as well as in protein expression levels of PER1 and PER2, but not BMAL1. Upon entering first torpor (Tb < 30°C), animals were moved into constant darkness. When sampled at 6-h intervals-beginning 24 h after the last light out, with Tb 3°C to 4°C-there were no circadian oscillations in PER1, PER2, or c-FOS expression. Sampling across 24 h during the first spontaneous arousal interval, c-FOS expression was elevated only when Tb reached 20°C and PER1 and PER2 expression did not show any Tb- or time-dependent changes. These results suggest that the central circadian clock might have stopped functioning during hibernation in this species, and the timing of arousal from torpor in arctic ground squirrels is unlikely to be controlled by the circadian clock within the SCN.

Plastic oscillators and fixed rhythms: changes in the phase of clock-gene rhythms in the PVN are not reflected in the phase of the melatonin rhythm of grass rats

The same clock-genes, including Period (PER) 1 and 2, that show rhythmic expression in the suprachiasmatic nucleus (SCN) are also rhythmically expressed in other brain regions that serve as extra-SCN oscillators. Outside the hypothalamus, the phase of these extra-SCN oscillators appears to be reversed when diurnal and nocturnal mammals are compared. Based on mRNA data, PER1 protein is expected to peak in the late night in the paraventricular nucleus of the hypothalamus (PVN) of nocturnal laboratory rats, but comparable data are not available for a diurnal species. Here we use the diurnal grass rat (Arvicanthis niloticus) to describe rhythms of PER1 and 2 proteins in the PVN of animals that either show the species-typical day-active (DA) profile, or that adopt a night-active (NA) profile when given access to running wheels. For DA animals housed with or without wheels, significant rhythms of PER1 or PER2 protein expression featured peaks in the late morning; NA animals showed patterns similar to those expected from nocturnal laboratory rats. Since the PVN is part of the circuit that controls pineal rhythms, we also measured circulating levels of melatonin during the day and night in DA animals with and without wheels and in NA wheel runners. All three groups showed elevated levels of melatonin at night, with higher levels during both the day and night being associated with the levels of activity displayed by each group. The differential phase of rhythms in the clock-gene protein in the PVN of diurnal and nocturnal animals presents a possible mechanism for explaining species differences in the phase of autonomic rhythms controlled, in part, by the PVN. The present study suggests that the phase of the oscillator of the PVN does not determine that of the melatonin rhythm in diurnal and nocturnal species or in diurnal and nocturnal chronotypes within a species.

The circadian gene Clock oscillates in the suprachiasmatic nuclei of the diurnal rodent Barbary striped grass mouse, Lemniscomys barbarus: a general feature of diurnality?

A major challenge in the field of circadian rhythms is to understand the neural mechanisms controlling the oppositely phased temporal organization of physiology and behaviour between night- and day-active animals. Most identified components of the master clock in the suprachiasmatic nuclei (SCN), called circadian genes, display similar oscillations according to the time of day, independent of the temporal niche. This has led to the predominant view that the switch between night- and day-active animals occurs downstream of the master clock, likely also involving differential feedback of behavioral cues onto the SCN. The Barbary striped grass mouse, Lemniscomys barbarus is known as a day-active Muridae. Here we show that this rodent, when housed in constant darkness, displays a temporal rhythmicity of metabolism matching its diurnal behaviour (i.e., high levels of plasma leptin and hepatic glycogen during subjective midday and dusk, respectively). Regarding clockwork in their SCN, these mice show peaks in the mRNA profiles of the circadian gene Period1 (Per1) and the clock-controlled gene Vasopressin (Avp), which occur during the middle and late subjective day, respectively, in accordance with many observations in both diurnal and nocturnal species. Strikingly, expression of the circadian gene Clock in the SCN of the Barbary striped grass mouse was not constitutive as in nocturnal rodents, but it was rhythmic. As this is also the case for the other diurnal species investigated in the literature (sheep, marmoset, and quail), a hypothesis is that the transcriptional control of Clock within the SCN participates in the mechanisms underlying diurnality and nocturnality.

Efferent projections of the suprachiasmatic nucleus based on the distribution of vasoactive intestinal peptide (VIP) and arginine vasopressin (AVP) immunoreactive fibers in the hypothalamus of Sapajus apella

The suprachiasmatic nucleus (SCN), which is considered to be the master circadian clock in mammals, establishes biological rhythms of approximately 24 h that several organs exhibit. One aspect relevant to the study of the neurofunctional features of biological rhythmicity is the identification of communication pathways between the SCN and other brain areas. As a result, SCN efferent projections have been investigated in several species, including rodents and a few primates. The fibers originating from the two main intrinsic fiber subpopulations, one producing vasoactive intestinal peptide (VIP) and the other producing arginine vasopressin (AVP), exhibit morphological traits that distinguish them from fibers that originate from other brain areas. This distinction provides a parameter to study SCN efferent projections. In this study, we mapped VIP (VIP-ir) and AVP (AVP-ir) immunoreactive (ir) fibers and endings in the hypothalamus of the primate Sapajus apella via immunohistochemical and morphologic study. Regarding the fiber distribution pattern, AVP-ir and VIP-ir fibers were identified in regions of the tuberal hypothalamic area, retrochiasmatic area, lateral hypothalamic area, and anterior hypothalamic area. VIP-ir and AVP-ir fibers coexisted in several hypothalamic areas; however, AVP-ir fibers were predominant over VIP-ir fibers in the posterior hypothalamus and medial periventricular area. This distribution pattern and the receiving hypothalamic areas of the VIP-ir and AVP-ir fibers, which shared similar morphological features with those found in SCN, were similar to the patterns observed in diurnal and nocturnal animals. This finding supports the conservative nature of this feature among different species. Morphometric analysis of SCN intrinsic neurons indicated homogeneity in the size of VIP-ir neurons in the SCN ventral portion and heterogeneity in the size of two subpopulations of AVP-ir neurons in the SCN dorsal portion. The distribution of fibers and morphometric features of these neuronal populations are described and compared with those of other species in the present study.

Responses of brain and behavior to changing day-length in the diurnal grass rat (Arvicanthis niloticus)

Seasonal affective disorder (SAD) is a major depressive disorder that recurs in the fall and winter when day-length gets short. It is well accepted that day-length is encoded by the principal circadian clock located in the suprachiasmatic nucleus (SCN), but very little is known about day-length encoding in diurnal mammals. The present study utilized the grass rat, Arvicanthis niloticus, to investigate how the circadian system responds to photoperiodic changes in a diurnal mammal that shows day-length-dependent mood changes. The animals were initially housed in equatorial day-length (12h, EP) followed by either long (16h, LP) or short (8h, SP) photoperiods. The LP animals showed an expansion of the peak phase of the PER1 and PER2 rhythm in the SCN as well as an extended behavioral active phase. In contrast, the SP animals did not show any compression of their active phase nor a change in the peak duration of PER1 or PER2 expression, compared to those in EP. The results suggest that the circadian system in the diurnal grass rats is less responsive when day-length gets short compared to when it gets longer. The depression-like behaviors were assessed using sweet solution preference (SSP) and forced swimming test (FST). Animals in the SP group showed decreased SSP and increased immobility time in FST as compared to the EP group, suggesting a depressive phenotype. The present study serves as the first step toward exploring the role that the circadian system plays in SAD using a diurnal rodent model.

Human blood metabolite timetable indicates internal body time

A convenient way to estimate internal body time (BT) is essential for chronotherapy and time-restricted feeding, both of which use body-time information to maximize potency and minimize toxicity during drug administration and feeding, respectively. Previously, we proposed a molecular timetable based on circadian-oscillating substances in multiple mouse organs or blood to estimate internal body time from samples taken at only a few time points. Here we applied this molecular-timetable concept to estimate and evaluate internal body time in humans. We constructed a 1.5-d reference timetable of oscillating metabolites in human blood samples with 2-h sampling frequency while simultaneously controlling for the confounding effects of activity level, light, temperature, sleep, and food intake. By using this metabolite timetable as a reference, we accurately determined internal body time within 3 h from just two anti-phase blood samples. Our minimally invasive, molecular-timetable method with human blood enables highly optimized and personalized medicine.

Orexinergic signaling mediates light-induced neuronal activation in the dorsal raphe nucleus

Seasonal affective disorder (SAD), a major depressive disorder recurring in the fall and winter, is caused by the reduction of light in the environment, and its depressive symptoms can be alleviated by bright light therapy. Both circadian and monoaminergic systems have been implicated in the etiology of SAD. However, the underlying neural pathways through which light regulates mood are not well understood. The present study utilized a diurnal rodent model, Arvicanthis niloticus, to explore the neural pathways mediating the effects of light on brain regions involved in mood regulation. Animals kept in constant darkness received light exposure in early subjective day, the time when light therapy is usually applied. The time course of neural activity following light exposure was assessed using Fos protein as a marker in the following brain regions/cells: the suprachiasmatic nucleus (SCN), orexin neurons in the perifornical-lateral hypothalamic area (PF-LHA) and the dorsal raphe nucleus (DRN). A light-induced increase in Fos expression was observed in orexin neurons and the DRN, but not in the SCN. As the DRN is densely innervated by orexinergic inputs, the involvement of orexinergic signaling in mediating the effects of light on the DRN was tested in the second experiment. The animals were injected with the selective orexin receptor type 1 (OXR1) antagonist SB-334867 prior to the light exposure. The treatment of SB-334867 significantly inhibited the Fos induction in the DRN. The results collectively point to the role of orexin neurons in mediating the effects of light on the mood-regulating monoaminergic areas, suggesting an orexinergic pathway that underlies light-dependent mood fluctuation and the beneficial effects of light therapy.

Antibodies for assessing circadian clock proteins in the rodent suprachiasmatic nucleus

Research on the mechanisms underlying circadian rhythmicity and the response of brain and body clocks to environmental and physiological challenges requires assessing levels of circadian clock proteins. Too often, however, it is difficult to acquire antibodies that specifically and reliably label these proteins. Many of these antibodies also lack appropriate validation. The goal of this project was to generate and characterize antibodies against several circadian clock proteins. We examined mice and hamsters at peak and trough times of clock protein expression in the suprachiasmatic nucleus (SCN). In addition, we confirmed specificity by testing the antibodies on mice with targeted disruption of the relevant genes. Our results identify antibodies against PER1, PER2, BMAL1 and CLOCK that are useful for assessing circadian clock proteins in the SCN by immunocytochemistry.

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.

Development, maturation, and necessity of transcription factors in the mouse suprachiasmatic nucleus

The suprachiasmatic nucleus (SCN) of the hypothalamus is the master mammalian circadian clock. The SCN is highly specialized because it is responsible for generating a near 24 h rhythm, integrating external cues, and translating the rhythm throughout the body. Currently, our understanding of the developmental origin and genetic program involved in the proper specification and maturation of the SCN is limited. Herein, we provide a detailed analysis of transcription factor (TF) and developmental-gene expression in the SCN from neurogenesis to adulthood in mice (Mus musculus). TF expression within the postmitotic SCN was not static but rather showed specific temporal and spatial changes during prenatal and postnatal development. In addition, we found both global and regional patterns of TF expression extending into the adult. We found that the SCN is derived from a distinct region of the neuroepithelium expressing a combination of developmental genes: Six3, Six6, Fzd5, and transient Rx, allowing us to pinpoint the origin of this region within the broader developing telencephalon/diencephalon. We tested the necessity of two TFs in SCN development, RORα and Six3, which were expressed during SCN development, persisted into adulthood, and showed diurnal rhythmicity. Loss of RORα function had no effect on SCN peptide expression or localization. In marked contrast, the conditional deletion of Six3 from early neural progenitors completely eliminated the formation of the SCN. Our results provide the first description of the involvement of TFs in the specification and maturation of a neural population necessary for circadian behavior.

Global depletion of dopamine using intracerebroventricular 6-hydroxydopamine injection disrupts normal circadian wheel-running patterns and PERIOD2 expression in the rat forebrain

Normal circadian rhythms of behavior are disrupted in disorders involving the dopamine (DA) system, such as Parkinson's disease. We have reported previously using unilateral injections of the catecholamine toxin, 6-hydroxydopamine (6-OHDA), into the medial forebrain bundle that DA signaling regulates daily expression of the clock protein, PERIOD2 (PER2), in the dorsal striatum of the rat. In the present study, we made widespread lesions of DA fibers using large injections of 6-OHDA into the third ventricle to determine the involvement of DA in normal daily rhythms of wheel-running activity and PER2 patterns in the suprachiasmatic nucleus (SCN) and several regions of the limbic forebrain. Rats injected with 6-OHDA and housed in constant darkness were less active in the wheel and showed a disorganized pattern of activity in which wheel running was not confined to a specific phase over 24 h. The 6-OHDA injection had no effect on the daily PER2 pattern in the SCN, but blunted the normal rise in PER2 in the dorsal striatum. 6-OHDA also blunted PER2 expression in the periventricular nucleus of the hypothalamus, a region in which a daily PER2 pattern has not been previously reported in male rats, and in the oval nucleus of the bed nucleus of the stria terminalis, but not in the central nucleus of the amygdala. These results indicate that DA plays a prominent role in regulating circadian activity at both behavioral and molecular levels.

Phase preference for the display of activity is associated with the phase of extra-suprachiasmatic nucleus oscillators within and between species

Many features of the suprachiasmatic nucleus (SCN) are the same in diurnal and nocturnal animals, suggesting that differences in phase preference are determined by mechanisms downstream from the SCN. Here, we examined this hypothesis by characterizing rhythmic expression of Period 1 (PER1) and Period 2 (PER2) in several extra-SCN areas in the brains of a diurnal murid rodent, Arvicanthis niloticus (grass rats). In the shell of the nucleus accumbens, dorsal striatum, piriform cortex, and CA1 of the hippocampus, both PER1 and PER2 were rhythmic, with peak expression occurring at ZT10. PER1 in the dentate gyrus also peaked at ZT10, but PER2 was arrhythmic in this region. In general, these patterns are 180 degrees out of phase with those reported for nocturnal species. In a second study, we examined inter-individual differences in the multioscillator system of grass rats. Here, we housed grass rats in cages with running wheels, under which conditions some individuals spontaneously adopt a day active (DA) and others a night active (NA) phase preference. In the majority of the extra-SCN regions sampled, the patterns of PER1 and PER2 expression of NA grass rats resembled those of nocturnal species, while those of DA grass rats were similar to the ones seen in grass without access to running wheels. In contrast, the rhythmic expression of both PER proteins was identical in the SCN and ventral subparaventricular zone (vSPZ) of DA and NA animals. Differences in the phase of oscillators downstream from the SCN, and perhaps the vSPZ, appear to determine the phase preference of particular species, as well as that of members of a diurnal species that show voluntary phase reversals. The latter observation has important implications for the understanding of health problems associated with human shift work.

Projections of the suprachiasmatic nucleus and ventral subparaventricular zone in the Nile grass rat (Arvicanthis niloticus)

The phases of many circadian rhythms differ between diurnal and nocturnal species. However, rhythms within the hypothalamic suprachiasmatic nucleus (SCN), which contains the central circadian pacemaker, are very similar, suggesting that the mechanisms underlying phase preference lie downstream of the SCN. Rhythms in Fos expression in the ventral subparaventricular zone (vSPVZ), a major target of the SCN, differ substantially between diurnal Nile grass rats and nocturnal lab rats, raising the possibility that the vSPVZ modulates the effects of SCN signals at its targets. To understand better how and where the SCN and vSPVZ communicate circadian signals within the grass rat brain, we mapped their projections using the anterograde tracer biotinylated dextran amine (BDA). Adult female grass rats received unilateral BDA injections directed at the SCN or vSPVZ and their brains were perfusion-fixed several days later. Immunohistochemistry revealed that the distribution patterns of SCN and vSPVZ efferents were very similar. Labeled fibers originating in each region were heavily concentrated in the medial preoptic area, paraventricular thalamic nucleus, the subparaventricular zone, and the hypothalamic paraventricular and dorsomedial nuclei. BDA-labeled fibers from the SCN and vSPVZ formed appositions with orexin neurons and gonadotropin-releasing hormone neurons, two cell populations whose rhythms in Fos expression track temporally reversed patterns of locomotor and reproductive behavior, respectively, in diurnal and nocturnal rodents. These data demonstrate that projections of the SCN and vSPVZ are highly conserved in diurnal and nocturnal rodents, and the vSPVZ projections may enable it to modulate the responsiveness of target cells to signals from the SCN.

PER2 rhythms in the amygdala and bed nucleus of the stria terminalis of the diurnal grass rat (Arvicanthis niloticus)

The suprachiasmatic nucleus (SCN) of the hypothalamus is the central pacemaker that controls circadian rhythms in mammals. In diurnal grass rats (Arvicanthis niloticus), many functional aspects of the SCN are similar to those of nocturnal rodents, making it likely that the difference in the circadian system of diurnal and nocturnal animals lies downstream from the SCN. Rhythms in clock genes expression occur in several brain regions outside the SCN that may function as extra-SCN oscillators. In male grass rats PER1 is expressed in the oval nucleus of the bed nucleus of the stria terminalis (BNST-ov) and in the central and basolateral amygdala (CEA and BLA, respectively); several features of PER1 expression in these regions of the grass rat brain differ substantially from those of nocturnal species. Here we describe PER2 rhythms in the same three brain regions of the grass rat. In the BNST-ov and CEA PER2 expression peaked early in the light period Zeitgeber time (ZT) 2 and was low during the early night, which is the reverse of the pattern of nocturnal rodents. In the BLA, PER2 expression was relatively low for most of the 24-h cycle, but showed an acute elevation late in the light period (ZT10). This pattern is also different from that of nocturnal rodents that show elevated PER2 expression in the mid to late night and into the early day. These results are consistent with the hypothesis that diurnal behavior is associated with a phase change between the SCN and extra-SCN oscillators.

Daily rhythms and sex differences in vasoactive intestinal polypeptide, VIPR2 receptor and arginine vasopressin mRNA in the suprachiasmatic nucleus of a diurnal rodent, Arvicanthis niloticus

Diurnal and nocturnal animals differ with respect to the time of day at which the ovulatory surge in luteinizing hormone occurs. In some species this is regulated by the suprachiasmatic nucleus (SCN), the primary circadian clock, via cells that contain vasoactive intestinal polypeptide (VIP) and vasopressin (AVP). Here, we evaluated the hypothesis that chronotype differences in the timing of the luteinizing hormone surge are associated with rhythms in expression of the genes that encode these neuropeptides. Diurnal grass rats (Arvicanthis niloticus) were housed in a 12/12-h light-dark cycle and killed at one of six times of day (Zeitgeber time 1, 5, 9, 13, 17, 21; ZT 0 = lights-on). In-situ hybridization was used to compare levels of vip, avp and VIP receptor mRNA (vipr2) in the SCN of intact females, ovariectomized females, ovariectomized females given estradiol and intact males. We found a sex difference in vip rhythms with a peak occurring at ZT 13 in males and ZT 5 in intact females. In all groups avp mRNA rhythms peaked during the day, from ZT 5 to ZT 9, and had a trough in the dark at ZT 21. There was a modest rhythm and sex difference in the pattern of vipr2. Most importantly, the patterns of each of these SCN rhythms relative to the light-dark cycle resembled those seen in nocturnal rodents. Chronotype differences in timing of neuroendocrine events associated with ovulation are thus likely to be generated downstream of the SCN.

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.

Daily rhythms in PER1 within and beyond the suprachiasmatic nucleus of female grass rats (Arvicanthis niloticus)

Although circadian rhythms of males and females are different in a variety of ways in many species, their mechanisms have been primarily studied in males. Furthermore, rhythms are dramatically different in diurnal and nocturnal animals but have been studied predominantly in nocturnal ones. In the present study, we examined rhythms in one element of the circadian oscillator, the PER1 protein, in a variety of cell populations in brains of diurnal female grass rats. Every 4 h five adult female grass rats kept on a 12-h light/dark (LD) cycle were perfused and their brains were processed for immunohistochemical detection of PER1. Numbers of PER1-labeled cells were rhythmic not only within the suprachiasmatic nucleus (SCN), the locus of the primary circadian clock in mammals, but also in the peri-suprachiasmatic region, the oval nucleus of the bed nucleus of the stria terminalis, the central amygdala, and the nucleus accumbens. In addition, rhythms were detected within populations of neuroendocrine cells that contain tyrosine hydroxylase. The phase of the rhythm within the SCN was advanced compared with that seen previously in male grass rats. Rhythms beyond the SCN were varied and different from those seen in most nocturnal species, suggesting that signals originating in the SCN are modified by its direct and/or indirect targets in different ways in nocturnal and diurnal species.

Retinal pathways influence temporal niche

In mammals, light input from the retina entrains central circadian oscillators located in the suprachiasmatic nuclei (SCN). The phase of circadian activity rhythms with respect to the external light:dark cycle is reversed in diurnal and nocturnal species, although the phase of SCN rhythms relative to the light cycle remains unchanged. Neural mechanisms downstream from the SCN are therefore believed to determine diurnality or nocturnality. Here, we report a switch from nocturnal to diurnal entrainment of circadian activity rhythms in double-knockout mice lacking the inner-retinal photopigment melanopsin (OPN4) and RPE65, a key protein used in retinal chromophore recycling. These mice retained only a small amount of rod function. The change in entrainment phase of Rpe65(-/-);Opn4(-/-) mice was accompanied by a reversal of the rhythm of clock gene expression in the SCN and a reversal in acute masking effects of both light and darkness on activity, suggesting that the nocturnal to diurnal switch is due to a change in the neural response to light upstream from the SCN. A switch from nocturnal to diurnal activity rhythms was also found in wild-type mice transferred from standard intensity light:dark cycles to light:dark cycles in which the intensity of the light phase was reduced to scotopic levels. These results reveal a novel mechanism by which changes in retinal input can mediate acute temporal-niche switching.

Rhythms in expression of PER1 protein in the amygdala and bed nucleus of the stria terminalis of the diurnal grass rat (Arvicanthis niloticus)

In the diurnal rodent Arvicanthis niloticus (grass rats) the pattern of expression of the clock genes and their proteins in the suprachiasmatic nucleus (SCN) is very similar to that seen in nocturnal rodents. Rhythms in clock gene expression have been also documented in several forebrain regions outside the SCN in nocturnal Ratus norvegicus (lab rats). To investigate the neural basis for differences in the circadian systems of diurnal and nocturnal mammals, we examined PER1 expression in the oval nucleus of the bed nucleus of the stria terminalis (BNST-OV), and in the basolateral (BLA) and the central (CEA) amygdala of male grass rats kept in a 12:12 light/dark cycle. In the BNST-OV, peak levels of PER1 expression were seen early in the light phase of the cycle, 12h out of phase with what has been reported for nocturnal lab rats. In the BLA the pattern of PER1 expression featured sustained high levels during the day and low levels at night. PER1 expression in the CEA was also at its highest early in the light phase, but the effect of sampling time was not statistically significant (p<0.06). The results are consistent with the hypothesis that differences between nocturnal and diurnal species are due to differences in neural systems downstream from the SCN.

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.

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.

Cholinergic projections to the suprachiasmatic nucleus and lower subparaventricular zone of diurnal and nocturnal rodents

In nocturnal species cholinergic agonists alter circadian rhythm phase when injected intraventricularly or directly into the suprachiasmatic nucleus (SCN), but the phase shifts obtained differ depending upon the site being injected. Cholinergic projections reach the SCN of nocturnal laboratory rats, however, nothing is known about these projections in diurnal rodents. The first objective of this study was to evaluate the hypothesis that cholinergic projections to the SCN are only present in nocturnal species. The second objective was to evaluate the hypothesis that the lower part of the subparaventricular zone (LSPV) is a candidate for being a site that mediates the phase shifts observed when cholinergic agonists are injected intraventricularly. These hypotheses were tested in the diurnal unstriped Nile grass rat (Arvicanthis niloticus) and the nocturnal laboratory rat. Additionally, we evaluated if the light-dark (LD) cycle had an effect on the expression of the vesicular acetylcholine transporter (VAChT) in the SCN, LSPV, and in two control areas. Animals were kept in a 12:12 LD cycle and perfused at six time points. VAChT immunoreactivity was observed in the SCN, LSPV, and in the control areas of both species. The SCN and LSPV showed a differential distribution and density of cholinergic projections between the two species, but similar temporal patterns of VAChT expression were found across species. These results provide evidence for a differential cholinergic stimulation of the SCN between grass rats and laboratory rats that may reflect a rewiring of neural projections brought about by the adoption of a diurnal activity profile.

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.