Calbindin and Fos within the suprachiasmatic nucleus and the adjacent hypothalamus of Arvicanthis niloticus and Rattus norvegicus

Clock Gene Expression in the Eye Exhibits Circadian Oscillation and Light Responsiveness but is Not Necessary for Nocturnal Locomotor Activity of Japanese Loach, Misgurnus anguillicaudatus

There are few model fish that are both edible and suitable for use in the laboratory. The Japanese loach (Misgurnus anguillicaudatus) is a traditional food in Japan, but is highly neglected despite its great nutritional value. To understand its circadian system and photic input pathway for synchronization of physiological activities to environmental light-dark cycles, we measured locomotor activity under light-dark and constant dark (DD) conditions. Locomotor activity was found to be higher in the nighttime than daytime, and its rhythmicity was weakened under DD conditions. The nocturnal activity of the Japanese loach is mainly controlled by environmental light, rather than the circadian clock. We explored the circadian regulation and light-responsiveness of clock gene expression in the eyes of loaches. The daily expression profiles of its mRNA revealed that most of the examined Cry and Per genes were likely regulated by internal circadian and/or environmental light signals. Among the Opsin genes transcribed in the eye, we detected the retinal photopigment porphyropsin at the protein level, which was lower than in mice. This property of loach eyes prompted us to analyze the locomotor activities of eye-enucleated fish. As a result, they still showed nocturnal circadian activity. Thus, it is likely that extraocular photoreceptive tissue(s) also contribute to the photic input pathway, although loach eyes are a circadian photosensitive tissue. This suggests that the loach mainly uses not its vision but other stimuli, such as mechanical or chemical stimuli, detected by barbels, to coordinate its nocturnal behavior.

Sleep Deprivation and Caffeine Treatment Potentiate Photic Resetting of the Master Circadian Clock in a Diurnal Rodent

Circadian rhythms in nocturnal and diurnal mammals are primarily synchronized to local time by the light/dark cycle. However, nonphotic factors, such as behavioral arousal and metabolic cues, can also phase shift the master clock in the suprachiasmatic nuclei (SCNs) and/or reduce the synchronizing effects of light in nocturnal rodents. In diurnal rodents, the role of arousal or insufficient sleep in these functions is still poorly understood. In the present study, diurnal Sudanian grass rats, Arvicanthis ansorgei, were aroused at night by sleep deprivation (gentle handling) or caffeine treatment that both prevented sleep. Phase shifts of locomotor activity were analyzed in grass rats transferred from a light/dark cycle to constant darkness and aroused in early night or late night. Early night, but not late night, sleep deprivation induced a significant phase shift. Caffeine on its own induced no phase shifts. Both sleep deprivation and caffeine treatment potentiated light-induced phase delays and phase advances in response to a 30 min light pulse, respectively. Sleep deprivation in early night, but not late night, potentiated light-induced c-Fos expression in the ventral SCN. Caffeine treatment in midnight triggered c-Fos expression in dorsal SCN. Both sleep deprivation and caffeine treatment potentiated light-induced c-Fos expression in calbindin-containing cells of the ventral SCN in early and late night. These findings indicate that, in contrast to nocturnal rodents, behavioral arousal induced either by sleep deprivation or caffeine during the sleeping period potentiates light resetting of the master circadian clock in diurnal rodents, and activation of calbindin-containing suprachiasmatic cells may be involved in this effect.SIGNIFICANCE STATEMENT Arousing stimuli have the ability to regulate circadian rhythms in mammals. Behavioral arousal in the sleeping period phase shifts the master clock in the suprachiasmatic nuclei and/or slows down the photic entrainment in nocturnal animals. How these stimuli act in diurnal species remains to be established. Our study in a diurnal rodent, the Grass rat, indicates that sleep deprivation in the early rest period induces phase delays of circadian locomotor activity rhythm. Contrary to nocturnal rodents, both sleep deprivation and caffeine-induced arousal potentiate the photic entrainment in a diurnal rodent. Such enhanced light-induced circadian responses could be relevant for developing chronotherapeutic strategies.

Nocturnal Activity in a Diurnal Rodent (Arvicanthis Niloticus): The Importance of Masking

It is known that day-active Nile grass rats, Arvicanthis niloticus, increase the amount of activity in the night relative to that in the day when provided with running wheels. This was confirmed in the present study. Animals without a wheel displayed 69.0% of their general activity in the L phase of a 12:12 h light-dark cycle; animals provided with wheels had only 48.6% of their wheel revolutions in the light. The contribution of direct (masking) responses to light to the increased nocturnality of animals with wheels was examined in two experiments. In experiment 1, masking was tested by exposing the animals to repeated cycles of 30 min of entraining light and 30 min of a different, usually dimmer light, during the L phase of a 12:12 h light-dark cycle. For animals with wheels, there was more running during the 30-min pulses of dim light or darkness than during the 30-min periods of entraining light. In contrast, for animals without wheels, there was more general activity during the 30-min periods of entraining light than during the 30-min pulses of dim light or darkness. In experiment 2, the animals were first exposed to a 12:12 h light-dark cycle and then put on a 1:10:1:12 h LDLD skeleton photoperiod. Animals with wheels increased their running during the subjective day of the skeleton photoperiod compared to that in the actual day of the 12:12 h light-dark cycle. Animals without wheels showed similar levels of general activity during the subjective day of the skeleton photoperiod and the actual day of the 12:12 h cycle. These experiments demonstrate that when Nile rats have running wheels, their increased nocturnal activity is associated with an increased suppression of locomotion in direct response to light. It is possible that changes in masking responses to light may be an essential and integral component of switching between diurnal and nocturnal activity profiles.

Calcium Dynamics and Circadian Rhythms in Suprachiasmatic Nucleus Neurons

The hypothalamic suprachiasmatic nucleus (SCN) has a pivotal role in the mammalian circadian clock. SCN neurons generate circadian rhythms in action potential firing frequencies and neurotransmitter release, and the core oscillation is thought to be driven by “clock gene” transcription-translation feedback loops. Cytosolic Ca2+mobilization followed by stimulation of various receptors has been shown to reset the gene transcription cycles in SCN neurons, whereas contribution of steady-state cytosolic Ca2+levels to the rhythm generation is unclear. Recently, circadian rhythms in cytosolic Ca2+levels have been demonstrated in cultured SCN neurons. The circadian Ca2+rhythms are driven by the release of Ca2+from ryanodine-sensitive internal stores and resistant to the blockade of action potentials. These results raise the possibility that gene translation/transcription loops may interact with autonomous Ca2+oscillations in the production of circadian rhythms in SCN neurons.

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.

Targeted mutation of the calbindin D 28k gene selectively alters nonvisual photosensitivity

Light intensity is an important determinant of diverse physiological and behavioral responses within the non-image-forming visual system. Thresholds differ among various photic responses, namely control of circadian rhythms, vigilance state, activity level and pupil constriction, but the mechanisms that regulate photosensitivity are not known. Calbindin D(28k) (CalB) is a calcium-binding protein associated with light processing in the mammalian circadian clock. Loss-of-function studies indicate that CalB-deficient mice (CalB(-/-)) have deficits in their ability to entrain to light-dark cycles. To explore the role of CalB in modulating photosensitivity, thresholds for three behaviors mediated by the non-image-forming visual system (entrainment, masking and pupillary light reflex; PLR) were compared in CalB(-/-) and wildtype mice, and the localization of CalB protein in these circuits was examined in adult and juvenile mice. The results reveal a divergence in how CalB affects thresholds to photic cues among these responses. Entrainment and masking were 40- to 60-fold less sensitive in CalB(-/-) than in wildtype mice. On the other hand, the PLR in CalB(-/-) mice was 80- to 200-fold more sensitive. Though CalB is expressed in the retina and in brain circuits regulating entrainment we found no CalB expression in any component of the PLR pathway, namely the olivary pretectal nucleus, Edinger-Westphal nucleus and ciliary ganglion. The behavioral and anatomical data together suggest that, in normal animals, the retinal response to light is blunted in the presence of CalB, but responsiveness of the higher order processes that transduce afferent retinal input is enhanced.

Internal temporal order in the circadian system of a dual-phasing rodent, the Octodon degus

Daily rhythms in different biochemical and hematological variables have been widely described in either diurnal or nocturnal species, but so far no studies in the rhythms of these variables have been conducted in a dual-phasing species such as the degus. The Octodon degus is a rodent that has the ability to switch from diurnal to nocturnal activity under laboratory conditions in response to wheel-running availability. This species may help us discover whether a complete temporal order inversion occurs parallel to the inversion that has been observed in this rodent's activity pattern. The aim of the present study is to determine the phase relationships among 26 variables, including behavioral, physiological, biochemical, and hematological variables, during the day and at night, in diurnal and nocturnal degus chronotypes induced under controlled laboratory conditions through the availability of wheel running. A total of 39 male degus were individually housed under a 12:12 light-dark (LD) cycle, with free wheel-running access. Wheel-running activity (WRA) and body temperature (Tb) rhythms were recorded throughout the experiment. Melatonin, hematological, and biochemical variables were determined by means of blood samples obtained every 6 h (ZT1, ZT7, ZT13, and ZT19). In spite of great differences in WRA and Tb rhythms between nocturnal and diurnal degus, no such differences were observed in the temporal patterns of most of the biological variables analyzed for the two chronotypes. Variation was only found in plasma urea level and lymphocyte number. A slight delay in the phase of the melatonin rhythm was also observed. This study shows the internal temporal order of a dual-phasing mammal does not show a complete inversion in accordance with its activity and body temperature pattern; it would appear that the switching mechanism involved in the degu's nocturnalism is located downstream from the pacemaker.

Immunocytochemical evidence for different patterns in daily rhythms of VIP and AVP peptides in the suprachiasmatic nucleus of diurnal Funambulus palmarum

The suprachiasmatic nucleus (SCN) is the principal pacemaker that coordinates circadian rhythmicity in mammals. The studies on understanding the circadian system in diurnal rodents are limited. In this study, we have used the 3 striped South Indian Palm Squirrel (Funambulus palmarum). The locomotor activity showed a diurnal pattern of activity in LD 12:12, constant darkness (DD) and light (LL) conditions with circadian periods (τ) of 24.19 ± 0.1, 24.11 ± 0.03 and 24.92 ± 0.35 h respectively. Anatomical study of the brain revealed that this animal had short, thick and stout optic nerves with SCN elliptical in shape with a higher neuronal population as distinct from nocturnal rodents. Since the neuropeptides, vasoactive intestinal polypeptide (VIP) and arginine vasopressin (AVP) play important roles in photic entrainment and relay of information respectively in nocturnal rodents, we studied the distribution and daily rhythms of VIP-ir and AVP-ir in squirrel SCN. The VIP-ir and AVP-ir cells in the SCN showed a ventrolateral and dorsomedial distribution with daily rhythmicity in their levels. The peak time of VIP-ir rhythm was found ahead of AVP-ir. The VIP-ir levels were higher for longer duration than AVP-ir levels. The maximum and minimum VIP-ir levels were at ZT-6 and ZT-0 respectively and AVP-ir levels at ZT-12 and ZT-0 respectively. Thus, VIP and AVP maximum and minimum levels appeared 6 and 12h apart respectively in squirrel, though 12 and 8h apart in rat. These findings in the present report could be a step towards underpinning the mechanisms regulating diurnality.

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.

Intracellular calcium spikes in rat suprachiasmatic nucleus neurons induced by BAPTA-based calcium dyes

Background

Circadian rhythms in spontaneous action potential (AP) firing frequencies and in cytosolic free calcium concentrations have been reported for mammalian circadian pacemaker neurons located within the hypothalamic suprachiasmatic nucleus (SCN). Also reported is the existence of “Ca²⁺ spikes” (i.e., [Ca²⁺]_c transients having a bandwidth of 10∼100 seconds) in SCN neurons, but it is unclear if these SCN Ca²⁺ spikes are related to the slow circadian rhythms.

Methodology/Principal Findings

We addressed this issue based on a Ca²⁺ indicator dye (fluo-4) and a protein Ca²⁺ sensor (yellow cameleon). Using fluo-4 AM dye, we found spontaneous Ca²⁺ spikes in 18% of rat SCN cells in acute brain slices, but the Ca²⁺ spiking frequencies showed no day/night variation. We repeated the same experiments with rat (and mouse) SCN slice cultures that expressed yellow cameleon genes for a number of different circadian phases and, surprisingly, spontaneous Ca²⁺ spike was barely observed (<3%). When fluo-4 AM or BAPTA-AM was loaded in addition to the cameleon-expressing SCN cultures, however, the number of cells exhibiting Ca²⁺ spikes was increased to 13∼14%.

Conclusions/Significance

Despite our extensive set of experiments, no evidence of a circadian rhythm was found in the spontaneous Ca²⁺ spiking activity of SCN. Furthermore, our study strongly suggests that the spontaneous Ca²⁺ spiking activity is caused by the Ca²⁺ chelating effect of the BAPTA-based fluo-4 dye. Therefore, this induced activity seems irrelevant to the intrinsic circadian rhythm of [Ca²⁺]_c in SCN neurons. The problems with BAPTA based dyes are widely known and our study provides a clear case for concern, in particular, for SCN Ca²⁺ spikes. On the other hand, our study neither invalidates the use of these dyes as a whole, nor undermines the potential role of SCN Ca²⁺ spikes in the function of SCN.

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.

Calbindin D28K protein cells in a primate suprachiasmatic nucleus: localization, daily rhythm and age-related changes

In mammals, the suprachiasmatic nucleus (SCN) of the hypothalamus is the master circadian pacemaker. The SCN controls daily rhythms and synchronizes the organism to its environment and especially to photic signals. Photic signals via the retinohypothalamic tract reach the ventral part of the SCN, where the majority of calbindin-containing neurons are located. Calbindin cells seem important for the control of circadian rhythmicity. As ageing leads to marked changes in the expression of circadian rhythms, we investigated in the mouse lemur, a nocturnal primate, age-related changes in the oscillation of calbindin protein expression in SCN neurons. We used immunohistochemistry and quantitative analysis of calbindin expression in the SCN of adult and aged mouse lemurs. In this primate, a dense cluster of calbindin-positive neurons was found in the ventral part of the SCN. In adult animals, calbindin-positive SCN neurons did not exhibit daily rhythms in their number or intensity, but exhibited significant daily variations in the percentage of cells with a calbindin-positive nucleus, characterized by high values during the daytime and low values during the night. Immunoreactive intensity peaked in the middle of the daytime. Calbindin expression in the nuclei of calbindin cells in the SCN tends to be modified by ageing. The amplitude of daily variation in calbindin expression was damped, with a lower immunointensity during the daytime and a delayed decrease during the night. These changes may affect the ability of the SCN to transmit rhythmic information to other SCN cells and thereby modify the synchronization of the different cell populations in the SCN.

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.

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.

The circadian visual system, 2005

The primary mammalian circadian clock resides in the suprachiasmatic nucleus (SCN), a recipient of dense retinohypothalamic innervation. In its most basic form, the circadian rhythm system is part of the greater visual system. A secondary component of the circadian visual system is the retinorecipient intergeniculate leaflet (IGL) which has connections to many parts of the brain, including efferents converging on targets of the SCN. The IGL also provides a major input to the SCN, with a third major SCN afferent projection arriving from the median raphe nucleus. The last decade has seen a blossoming of research into the anatomy and function of the visual, geniculohypothalamic and midbrain serotonergic systems modulating circadian rhythmicity in a variety of species. There has also been a substantial and simultaneous elaboration of knowledge about the intrinsic structure of the SCN. Many of the developments have been driven by molecular biological investigation of the circadian clock and the molecular tools are enabling novel understanding of regional function within the SCN. The present discussion is an extension of the material covered by the 1994 review, "The Circadian Visual System."

Individual differences in rhythms of behavioral sleep and its neural substrates in Nile grass rats

Laboratory populations of grass rats (Arvicanthis niloticus) housed with a running wheel show considerable variation in patterns of locomotor activity. At the extremes are "day-active" (DA) animals with a monophasic distribution of running throughout the light phase and "night-active" (NA) animals exhibiting a biphasic pattern with an extended peak at the beginning of the dark phase and a brief peak shortly before lights-on. Here, the authors use this intraspecific variation to explore interactions between circadian and homeostatic influences on sleep and the effects of these interactions on the activity of brain regions involved in sleep regulation. Male animals were singly housed with running wheels in a 12:12 LD cycle, videotaped for 24 h, and perfused at ZT 4 or 16. Behavioral sleep was scored from the videotapes, and brains were processed for cFos immunoreactivity (cFos-ir). Sleep duration within the light and dark phases was higher in NA and DA animals, respectively, but these groups did not differ with respect to total sleep. In both groups, sleep bouts were shortest in the light phase and longest between ZT 20 and ZT 23. In the ventrolateral preoptic area (VLPO), cFos-ir was higher at ZT 16 than at ZT 4 in DA but not NA grass rats, and it was correlated with behavioral sleep at ZT 16 but not ZT 4. In OXA neurons, cFos-ir was high at ZT 4 in DA grass rats and at ZT 16 in NA grass rats, and it was correlated with behavioral sleep at both times. In the lower subparaventricular zone (LSPV), cFos-ir was higher at ZT 16 in both DA and NA animals, and it was unrelated to behavioral sleep. Thus, patterns of cFos-ir in the LSPV and OXA neurons were most tightly linked to time and sleep, respectively, whereas cFos-ir in the VLPO was influenced by an interaction between these 2 variables.

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)

The suprachiasmatic nucleus (SCN) of the hypothalamus contains the primary circadian pacemaker in both diurnal and nocturnal mammals. The lower subparaventricular zone (LSPV) immediately dorsal to the SCN may also play an important role in the regulation of circadian rhythms. The SCN contains a multitude of oscillator cells that generate circadian rhythms through transcriptional/translational feedback loops involving a set of clock genes including per1 and per2. Little is known about the temporal and spatial features of the proteins encoded by these genes in day-active mammals. The first objective of this study was to characterize the expression of PER1 and PER2 in the SCN of a diurnal rodent, the unstriped Nile grass rat (Arvicanthis niloticus). The second objective was to evaluate the hypothesis that a molecular clock could exist in the LSPV, where endogenous rhythms in Fos expression are seen in grass rats but not in laboratory rats. Animals were kept on a 12:12 light/dark cycle and perfused at 4-h intervals, and their brains were processed for immunohistochemical detection of PER1 and PER2. Both proteins were seen in the SCN where they peaked early in the dark phase, providing further evidence that the differences between diurnal and nocturnal patterns of behavior emerge from mechanisms lying downstream from the pacemaker within the SCN. Rhythmic expression of PER1 and PER2 was also seen in the LSPV providing support for the hypothesis that this region might participate in circadian time keeping in the diurnal grass rat. In addition, rhythms were seen lateral to the LSPV and the SCN. Results of this study are discussed in light of similarities and differences in the circadian time-keeping systems of day- and night-active animals.

Complex organization of mouse and rat suprachiasmatic nucleus

The suprachiasmatic nucleus, site of the dominant mammalian circadian clock, contains a variety of different neurons that tend to form groups within the nucleus. The present investigation used single and multiple label tract tracing and immunofluorescence methods to evaluate the relative locations of the neuron groups and to compare them with the distributions of the three major afferent projections, the retinohypothalamic tract, geniculohypothalamic tract and the serotonergic pathway from the median raphe nucleus. The suprachiasmatic nucleus has a complex order characterized by peptidergic cell groups (vasopressin, gastrin releasing peptide, vasoactive intestinal polypeptide, calbindin, calretinin, corticotrophin releasing factor and enkephalin) that, in most cases, substantially overlap. The retinohypothalamic tract projects bilaterally to virtually all the suprachiasmatic nucleus in both rat (predominantly contralateral) and mouse (symmetric) and its terminal field overlaps that for the geniculohypothalamic tract, but with distinctions visible according to density criteria; neither provides more than sparse innervation of the dorsomedial suprachiasmatic nucleus. In the mouse, the serotonergic terminal field is densest medially and ventrally, but is also distributed elsewhere with varying density. The serotonergic terminal plexus in the rat is densest centromedially and largely, but not completely, overlaps the complete distribution of retinal terminals with density much reduced in the lateral suprachiasmatic nucleus. The locations of vasopressin neurons, retinohypothalamic tract terminals and serotonergic (mouse, rat) or geniculohypothalamic tract (rat) provide evidence for three clear, but not exclusionary, sectors of the suprachiasmatic nucleus. The data, in conjunction with emerging knowledge concerning rhythmically dynamic changes in the size of regions of neuropeptide gene expression in suprachiasmatic nucleus cells, support the view that suprachiasmatic nucleus organization is more complex than a simple "core" and "shell" arrangement. While generalizations about suprachiasmatic nucleus organization can be made with respect to location of cell phenotypes or terminal fields, oversimplification may hinder, rather than facilitate, understanding of suprachiasmatic nucleus structure-function relationships.

Orexin fibers form appositions with Fos expressing neuropeptide-Y cells in the grass rat intergeniculate leaflet

Neuropeptide-Y (NPY) cells in the intergeniculate leaflet (IGL) are known to modulate effects of arousal on the mammalian circadian system. However, the route through which this information reaches the IGL has not been established. Here, we provide evidence that the orexins (hypocretins) are uniquely positioned as a potential source of activity state feedback to the IGL in the grass rat, Arvicanthis niloticus. Specifically, many NPY cells in the grass rat IGL exhibit orexin-A (OXA) fiber appositions. Furthermore, NPY cells contacted by OXA fibers are significantly more likely to express Fos during nocturnal wheel running than are NPY cells without such contacts (P < 0.001).

Targeted microlesions reveal novel organization of the hamster suprachiasmatic nucleus

The role of the suprachiasmatic nuclei (SCN) in generating circadian rhythms in physiology and behavior is well established. Recent evidence based on clock gene expression indicates that the rodent SCN are composed of at least two functional subdivisions. In Syrian hamsters (Mesocricetus auratus), cells in a subregion of the caudal SCN marked by calbindin-D(28K) (CalB) express light-induced, but not rhythmic, clock genes (Per1, Per2, and Per3). In the SCN region marked by vasopressinergic cells and fibers, clock gene expression is rhythmic. Importantly, lesions of the CalB subregion that spare a significant portion of the SCN abolish rhythms in locomotor behavior. One possibility is that the CalB subregion is required to maintain SCN function necessary to support all behavioral and physiological rhythms. Alternatively, this subregion may control circadian rhythms in locomotor behavior, whereas other circadian responses in physiology and behavior are sustained by different SCN compartments. The present study sought to distinguish between these possibilities by examining the role of the CalB subregion in a battery of rhythms within an individual animal. The results indicate that lesions of the CalB subregion of the SCN abolish circadian rhythms in behavior (locomotion, drinking, gnawing), physiology (body temperature, heart rate), and hormone secretion (melatonin, cortisol), even when other SCN compartments are spared. Together, these findings suggest a novel fundamental property of SCN organization, with a subset of cells being critical for the maintenance of SCN function manifest in circadian rhythms in physiology and behavior.

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.

Temporal organization of the 24-h corticosterone rhythm in the diurnal murid rodent Arvicanthis ansorgei Thomas 1910

Arvicanthis ansorgei is a diurnal murid rodent from sub-Saharan Africa. The present study reports on the temporal organization of one of the major hormonal rhythms, i.e. the adrenal steroid hormone corticosterone, in an attempt to characterize further the diurnal nature of this species. The data were obtained by means of two different physiological methods: blood sampling and intracerebral microdialysis. The results show a 12-h rhythm of corticosterone release with peak values close to the light-dark (ZT10) and dark-light transition (ZT22-24), which is clearly different from that in a nocturnal animal. Both corticosterone peaks are closely correlated with the occurrence of two major bouts of running wheel activity. As far as we are aware, this is the first demonstration of a hormonal rhythm with a clear crepuscular appearance (peak values around dusk and dawn). In conclusion, these data show that also in a rodent with a diurnal/crepuscular activity pattern, the tight association between the daily corticosterone peak and the onset of activity is maintained. In addition, intracerebral microdialysis is a suitable technique to measure hormonal rhythms when repeated blood sampling is not possible.

Developmental changes in calbindin-D28k and calretinin expression in the mouse suprachiasmatic nucleus

The hypothalamic suprachiasmatic nucleus, the primary circadian pacemaker in mammals, and the retinohypothalamic tract, the retinal afferent fibres to the suprachiasmatic nucleus, both mature during early postnatal life. The establishment of circadian rhythms is thought to depend on input from the retina, but the mechanism remains unknown. Here we examined developmental changes in the expression of the Ca2+-binding proteins calbindin-D28k and calretinin in the mouse hypothalamus. Robust calbindin-D28k immunoreactivity was observed in the dorsomedial suprachiasmatic nucleus and the supraoptic nucleus in neonatal mice (postnatal day 3). The calbindin-D28k immunoreactivity decreased significantly in the suprachiasmatic nucleus but not in the supraoptic nucleus during postnatal days 9-15, when retinohypothalamic tract projections to the suprachiasmatic nucleus are completed. Calretinin immunoreactivity was low in the neonatal suprachiasmatic nucleus and increased with development in the ventrolateral suprachiasmatic nucleus, in parallel with the developmental reduction of calbindin-D28k immunoreactivity observed in the dorsomedial suprachiasmatic nucleus. Developmentally stable calretinin immunoreactivity was also observed in retinohypothalamic tract fibres. Organotypic slice cultures of the suprachiasmatic nucleus were prepared from postnatal day 3 mice to examine the effect of the absence of retinohypothalamic tract inputs on developmental changes in calbindin-D28k and calretinin expression. After 12 days in vitro, the cultured suprachiasmatic nucleus slices exhibited dense calbindin-D28k immunoreactivity similar to neonatal mice, and calretinin immunoreactivity in the ventrolateral suprachiasmatic nucleus similar to young adult mice. These results demonstrate a developmental reduction in calbindin-D28k expression that paralleled retinohypothalamic tract formation and a developmental increase in calretinin expression that is independent of retinohypothalamic tract connections to suprachiasmatic nucleus neurons.

Beyond the suprachiasmatic nucleus

The suprachiasmatic nucleus is the master oscillator controlling circadian rhythms in mammals. Yet extensive temporal restructuring of behavior can occur without participation of the suprachiasmatic nucleus. This raises questions about current thinking about how to cope with jet lag and shift work.

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

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

Phase response curve and light-induced fos expression in the suprachiasmatic nucleus and adjacent hypothalamus of Arvicanthis niloticus

This article describes the phase response curve (PRC), the effect of light on Fos immunoreactivity (Fos-IR) in the suprachiasmatic nucleus (SCN), and the effect of SCN lesions on circadian rhythms in the murid rodent, Arvicanthis niloticus. In this species, all individuals are diurnal when housed without a running wheel, but running in a wheel induces a nocturnal pattern in some individuals. First, the authors characterized the PRC in animals with either the nocturnal or diurnal pattern. Both groups of animals were less affected by light during the middle of the subjective day than during the night and were phase delayed and phase advanced by pulses in the early and late subjective night, respectively. Second, the authors characterized the Fos response to light at circadian times 5, 14, or 22. Light induced an increase in Fos-IR within the SCN during the subjective night but not subjective day; this effect was especially pronounced in the ventral SCN, where retinal inputs are most concentrated, but was also evident in other regions. Both light and time influenced Fos-IR within the lower subparaventricular area. Third, SCN lesions caused animals to become arrhythmic when housed in a light-dark cycle as well as constant darkness. In summary, Arvicanthis appear to be very similar to nocturnal rodents with respect to their PRC, temporal patterns of light-induced Fos expression in the SCN, and the effects of SCN lesions on activity rhythms.

Patterns of wheel running are related to Fos expression in neuropeptide-Y-containing neurons in the intergeniculate leaflet of Arvicanthis niloticus

A variety of nonphotic influences on circadian rhythms have been documented in mammals. In hamsters, one such influence, running in a novel wheel, is mediated in part by the pathway extending from neuropeptide-Y (NPY)-containing cells within the intergeniculate leaflet (IGL) of the thalamus to the hypothalamic suprachiasmatic nucleus (SCN). Arvicanthis niloticus is a species in which all individuals are diurnal with respect to general activity and body temperature when they are housed without a running wheel, but access to a running wheel induces a subset of individuals to become nocturnal. In the first study, the authors evaluated the possibility that nocturnal and diurnal patterns of wheel running in Arvicanthis are correlated with differences in IGL function. Adult male Arvicanthis housed in a 12:12 light-dark (LD) cycle were monitored in wheels, classified as nocturnal or diurnal, and then perfused either 4 h after lights-on or 4 h after lights-off. Sections through the intergeniculate leaflet were processed for immunohistochemical labeling of Fos and NPY. The percentage of NPY cells that expressed Fos was significantly influenced by an interaction between time of day and phenotype such that it rose from night to day in diurnal animals, and from day to night in nocturnal animals. In the second experiment, the authors established that running in a wheel actually induces Fos in the IGL of Arvicanthis. Specifically, the proportion of NPY cells expressing Fos was increased by access to wheels in nocturnal animals at night and in diurnal animals during the day. In the third experiment, the authors established that lesions of the IGL eliminate NPY fibers within the SCN, suggesting that these IGL cells project to the SCN in this species as has been established in other rodents. Together, these data demonstrate a clear difference in NPY cell function in nocturnal and diurnal Arvicanthis that appears to be caused, at least in part, by the differences in their wheel-running patterns, and that NPY cells within the IGL project to the SCN in Arvicanthis.