Circadian Modulation of c-Fos Expression Occurs only in the SCN and not in Other Visual Projection Areas in the Rat

Brief photic stimuli at different circadian times induce differential expression of c-Fos in the suprachiasmatic nuclei (SCN). Whether circadian modulation of light-induced c-Fos expression occurs in other visual projection areas is not known. We addressed this question by estimating the immunohistochemical expression of c-Fos induced by 60 min light pulses at three different circadian times. The areas studied were the SCN, the ventral lateral geniculate nucleus, the intergeniculate leaflet, the ventral tegmental area, the superior colliculus and a non-visual control, the paraventricular thalamic nucleus (PVT). Light pulses induced an increase in the number of c-Fos immunoreactive cells in the SCN as a function of the circadian time. Remaining visual structures showed a light-induced increase in c-Fos expression but this was not dependent on the circadian time. The non-visual control area (PVT) did not respond to light pulses. Since no circadian modulation was found in the intergeniculate leaflet, which rec eives collateral projections from the same retinal ganglion cells that project to the SCN, nor in other primary visual projection areas, the present findings suggest that the circadian modulation of light-induced c-Fos expression in the SCN depends mainly on the functional properties of its intrinsic neurons.     

Neuropeptide Y microinjected into the suprachiasmatic region phase shifts circadian rhythms in constant darkness

The geniculohypothalamic tract (GHT) is a projection from the intergeniculate leaflet to the suprachiasmatic nucleus (SCN). The GHT exhibits neuropeptide Y (NPY) immunoreactivity and appears to communicate photic information to the SCN. Microinjection of NPY into the SCN has been found to phase shift circadian rhythms of hamsters housed in constant light in a manner similar to the phase shifts produced by pulses of darkness or triazolam injections. In the present study, NPY was injected into the SCN of Syrian hamsters housed in constant darkness and was found to produce phase shifts similar to those seen in hamsters housed in constant light. Microinjections were not followed by wheel running during the subjective day (the time when NPY microinjections are followed by significant phase advances). These data suggest that NPY produces phase shifts by some mechanism other than by inducing wheel running or by inhibiting the response of SCN neurons to light and supports a role for NPY in nonphotic shifting of the circadian clock.     

Response of the mouse circadian system to serotonin 1A/2/7 agonists in vivo: surprisingly little

Serotonin (5-HT) is thought to play a role in regulating nonphotic phase shifts and modulating photic phase shifts of the mammalian circadian system, but results with different species (rats vs. hamsters) and techniques (in vivo vs. in vitro; systemic vs. intracerebral drug delivery) have been discordant. Here we examined the effects of the 5-HT1A/7 agonist 8-OH-DPAT and the 5-HT1/2 agonist quipazine on the circadian system in mice, with some parallel experiments conducted with hamsters for comparative purposes. In mice, neither drug, delivered systemically at a range of circadian phases and doses, induced phase shifts significantly different from vehicle injections. In hamsters, quipazine intraperitoneally (i.p.) did not induce phase shifts, whereas 8-OH-DPAT induced phase shifts after i.p. but not intra-SCN injections. In mice, quipazine modestly increased c-Fos expression in the SCN (site of the circadian pacemaker) during the subjective day, whereas 8-OH-DPAT did not affect SCN c-Fos. In hamsters, both drugs suppressed SCN c-Fos in the subjective day. In both species, both drugs strongly induced c-Fos in the paraventricular nucleus (within-subject positive control). 8-OH-DPAT did not significantly attenuate light-induced phase shifts in mice but did in hamsters (between-species positive control). These results indicate that in the intact mouse in vivo, acute activation of 5-HT1A/2/7 receptors in the circadian system is not sufficient to reset the SCN pacemaker or to oppose phase-shifting effects of light. There appear to be significant species differences in the susceptibility of the circadian system to modulation by systemically delivered serotonergics.     

Rhythmicity of the cGMP-related signal transduction pathway in the mammalian circadian system

Entrainment of mammalian circadian rhythms requires the activation of specific signal transduction pathways in the suprachiasmatic nuclei (SCN). Pharmacological inhibition of kinases such as cGMP-dependent kinase (PKG) or Ca2+/calmodulin-dependent kinase, but not cAMP-dependent kinase, blocks the circadian responses to light in vivo. Here we show a diurnal and circadian rhythm of cGMP levels and PKG activity in the hamster SCN, with maximal values during the day or subjective day. This rhythm depends on phosphodiesterase but not on guanylyl cyclase activity. Five-minute light pulses increased cGMP levels at the end of the subjective night [circadian time 18 (CT18)], but not at CT13.5. Western blot analysis indicated that the PKG II isoform is the one present in the SCN. Inhibition of PKG or guanylyl cyclase in vivo significantly attenuated light-induced phase shifts at CT18 (after 5-min light pulses) but did not affect c-Fos expression in the SCN. These results suggest that cGMP and PKG are related to SCN responses to light and undergo diurnal and circadian changes.     

Signaling in the mammalian circadian clock: the NO/cGMP pathway

Mammalian circadian rhythms are generated by a hypothalamic suprachiasmatic nuclei (SCN) clock. Light pulses synchronize body rhythms by inducing phase delays during the early night and phase advances during the late night. Phosphorylation events are known to be involved in circadian phase shifting, both for delays and advances. Pharmacological inhibition of the cGMP-dependent kinase (cGK) or Ca2+/calmodulin-dependent kinase (CaMK), or of neuronal nitric oxide synthase (nNOS) blocks the circadian responses to light in vivo. Light pulses administered during the subjective night, but not during the day, induce rapid phosphorylation of both p-CAMKII and p-nNOS (specifically phosphorylated by CaMKII). CaMKII inhibitors block light-induced nNOS activity and phosphorylation, suggesting a direct pathway between both enzymes. Furthermore, SCN cGMP exhibits diurnal and circadian rhythms with maximal values during the day or subjective day. This variation of cGMP levels appears to be related to temporal changes in phosphodiesterase (PDE) activity and not to guanylyl cyclase (GC) activity. Light pulses increase SCN cGMP levels at circadian time (CT) 18 (when light causes phase advances of rhythms) but not at CT 14 (the time for light-induced phase delays). cGK II is expressed in the hamster SCN and also exhibits circadian changes in its levels, peaking during the day. Light pulses increase cGK activity at CT 18 but not at CT 14. In addition, cGK and GC inhibition by KT-5823 and ODQ significantly attenuated light-induced phase shifts at CT 18. This inhibition did not change c-Fos expression SCN but affected the expression of the clock gene per in the SCN. These results suggest a signal transduction pathway responsible for light-induced phase advances of the circadian clock which could be summarized as follows: Glu-Ca2+-CaMKII-nNOS-GC-cGMP-cGK-->-->clock genes. This pathway offers a signaling window that allows peering into the circadian clock machinery in order to decipher its temporal cogs and wheels.     

Extracellular nitric oxide signaling in the hamster biological clock

Nocturnal light pulses induce phase shifts in circadian rhythms and activate cFos expression in the suprachiasmatic nuclei (SCN). We have studied the role of nitric oxide (NO) in the intercellular communication within the dorsal and ventral portions of the SCN in Syrian hamsters. Administration of the NO scavenger 2-phenyl-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide blocked photic phase advances in a dose-dependent manner and inhibited light-induced cFos-ir, without affecting light-induced circadian phase delays. These results suggest that NO may act as an intercellular messenger in the SCN, mediating light-induced phase advances.     

Anterior paraventricular thalamus modulates light-induced phase shifts in circadian rhythmicity in rats

The reciprocal connections between the paraventricular thalamic nucleus (PVT) and the suprachiasmatic nuclei suggest that PVT may participate in the regulation of circadian rhythms. We studied in rats the effect of lesions of the anterior and midposterior regions of the PVT on phase shifts of drinking circadian rhythm induced by light pulses at circadian times 6, 12, and 23, as well as the phase shifts produced by electrical or glutamatergic stimulation of the anterior PVT at the same circadian times. Lesion of the anterior PVT abolishes the advances induced by light during late subjective night, whereas midposterior PVT lesions did not affect the phase shifts. Electrical stimulation or glutamate injections in the anterior PVT mimic the phase-shifting effects of light pulses. These results indicate the participation of the anterior PVT as a modulator of entrainment of circadian rhythms to light.     

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.     

Experience-independent development of the hamster circadian visual system

Experience-dependent functional plasticity is a hallmark of the primary visual system, but it is not known if analogous mechanisms govern development of the circadian visual system. Here we investigated molecular, anatomical, and behavioral consequences of complete monocular light deprivation during extended intervals of postnatal development in Syrian hamsters. Hamsters were raised in constant darkness and opaque contact lenses were applied shortly after eye opening and prior to the introduction of a light-dark cycle. In adulthood, previously-occluded eyes were challenged with visual stimuli. Whereas image-formation and motion-detection were markedly impaired by monocular occlusion, neither entrainment to a light-dark cycle, nor phase-resetting responses to shifts in the light-dark cycle were affected by prior monocular deprivation. Cholera toxin-b subunit fluorescent tract-tracing revealed that in monocularly-deprived hamsters the density of fibers projecting from the retina to the suprachiasmatic nucleus (SCN) was comparable regardless of whether such fibers originated from occluded or exposed eyes. In addition, long-term monocular deprivation did not attenuate light-induced c-Fos expression in the SCN. Thus, in contrast to the thalamocortical projections of the primary visual system, retinohypothalamic projections terminating in the SCN develop into normal adult patterns and mediate circadian responses to light largely independent of light experience during development. The data identify a categorical difference in the requirement for light input during postnatal development between circadian and non-circadian visual systems.     

Responses of the circadian system of rats to conditioned and unconditioned stimuli

The circadian systems of rodents respond to light pulses presented during the subjective night with phase shifts and altered cellular activity in the suprachiasmatic nuclei (SCN), including expression of immediate-early genes (IEGs) such as c-fos. A recent study showed that a nonphotic stimulus (an air disturbance generated by a fan) that does not normally induce the expression of c-fos-like immunoreactivity in the SCN of rats can be made to do so after being paired repeatedly with a light pulse in a Pavlovian conditioning paradigm. Furthermore, after conditioning (but not after noncontingent exposure to these stimuli), the fan also induced phase shifts in activity and body temperature rhythms comparable to those produced by light. The authors performed three experiments designed to replicate and extend these findings in rats. In experiment 1, rats were tested for conditioning effects of repeated pairings of a light pulse with a neutral air disturbance under a full photoperiod. In experiment 2, a modified conditioning paradigm was used in which a skeleton photoperiod served as both the entraining zeitgeber and the unconditioned stimulus. Animals in the paired and unpaired training conditions were exposed to both the light pulse and the air disturbance, but the air disturbance signaled the onset of light in the paired condition only. Phase shifts of wheel-running activity rhythms and gene expression in the SCN, intergeniculate leaflet, and paraventricular nucleus of the thalamus were assessed in animals following either of the training conditions or the control procedures. Experiment 3 assessed whether the air disturbance could entrain the circadian activity rhythms of rats with or without previous pairing with light in a classical conditioning paradigm. No evidence for classical conditioning, nor for unconditioned effects of the air disturbance on the circadian system, was found in these studies.     

A Noradrenergic Mechanism Influences the Circadian Timing System in Rats and Hamsters

Brainstem monoaminergic projections to the suprachiasmatic nucleus (SCN), and to the intergeniculate leaflet (IGL), appear to modulate both photic and non-photic effects on the circadian system. Recent work in this laboratory has concentrated on the role of noradrenaline in the regulation of circadian period and phase. Previously, this lab has shown that chronic administration of the alpha₂ adrenergic agonist, clonidine, to rats maintained in constant light (LL) shortens free-running circadian period and promotes dissociation of rhythmicity, while acute clonidine administration to hamsters produces phase shifts similar to those observed with photic stimuli. These results suggest an interaction between clonidine and photic input on circadian rhythmicity, and so the present study was designed to examine systematically the relationship between chronic clonidine administration and photic input in both rats and hamsters. In DD and low intensity LL, clonidine did not alter free-running circadian wheel-running rhythms of rats, but under moderate to high intensity LL, clonidine significantly reduced the period-lengthening effects of LL. Chronic clonidine administration also altered several aspects of circadian phase in hamsters; phase shifts in response to light pulses of varying intensity at CT 19 were reduced; steady-state entrainment phase under a 24-h light-dark cycle (LD 14:10)was delayed; and synchronization to a 23-h light-dark cycle (LD 13:10) was impaired. Clonidine appeared to have little effect on free-running period of hamsters, but a trend towards dissociation of rhythmicity under LL was observed. These effects may reflect an action of clonidine at the photic input pathways to the circadian system, or directly at the circadian pacemaker, since alpha 2 adrenoceptors have been localized both in the suprachiasmatic nucleus (SCN) and in several of its projection areas. As both clinical and experimental studies suggest that clonidine may have depressogenic properties, chronic administration of clonidine to rodents may provide an animal model of the alterations in circadian rhythmicity seen in human depression.     

A Noradrenergic Mechanism Influences the Circadian Timing System in Rats and Hamsters

Brainstem monoaminergic projections to the suprachiasmatic nucleus (SCN), and to the intergeniculate leaflet (IGL), appear to modulate both photic and non-photic effects on the circadian system. Recent work in this laboratory has concentrated on the role of noradrenaline in the regulation of circadian period and phase. Previously, this lab has shown that chronic administration of the alpha₂ adrenergic agonist, clonidine, to rats maintained in constant light (LL) shortens free-running circadian period and promotes dissociation of rhythmicity, while acute clonidine administration to hamsters produces phase shifts similar to those observed with photic stimuli. These results suggest an interaction between clonidine and photic input on circadian rhythmicity, and so the present study was designed to examine systematically the relationship between chronic clonidine administration and photic input in both rats and hamsters. In DD and low intensity LL, clonidine did not alter free-running circadian wheel-running rhythms of rats, but under moderate to high intensity LL, clonidine significantly reduced the period-lengthening effects of LL. Chronic clonidine administration also altered several aspects of circadian phase in hamsters; phase shifts in response to light pulses of varying intensity at CT 19 were reduced; steady-state entrainment phase under a 24-h light-dark cycle (LD 14:10)was delayed; and synchronization to a 23-h light-dark cycle (LD 13:10) was impaired. Clonidine appeared to have little effect on free-running period of hamsters, but a trend towards dissociation of rhythmicity under LL was observed. These effects may reflect an action of clonidine at the photic input pathways to the circadian system, or directly at the circadian pacemaker, since alpha 2 adrenoceptors have been localized both in the suprachiasmatic nucleus (SCN) and in several of its projection areas. As both clinical and experimental studies suggest that clonidine may have depressogenic properties, chronic administration of clonidine to rodents may provide an animal model of the alterations in circadian rhythmicity seen in human depression.     

Characterization of the Circadian System in a Brazilian Species of Monkey (Callithrix jacchus): Immunohistochemical Analysis and Retinal Projections

The hypothalamic suprachiasmatic nucleus (SCN) and the thalamic pregeniculate nucleus (PGN), which appears to include the intergeniculate leaflet (IGL), comprise circadian related centers in the primate brain. In this study, these centers were analysed in respect to their cytoarchitecture, retinal afferents and chemical of major cells and axon terminals with tract tracers and immunohistochemical techniques to define cytoarchitecture and connections, in the common marmoset. The SCN was shown to be a triangularly shaped cluster of compact cells just dorsal to the optic chiasm and lateral to the third ventricle. It is innervated in its ventral portion by terminals from the retina, and NPY-ergic fibers. Serotonergic and SP-staining processes are distributed throughout. VIP-neurons form a dorsolateral group of cells and CB-immunoreactive neurons fill much of the nucleus. The PGN was shown to be a wedge-shaped cluster of cells located dorsomedially to the dorsal lateral geniculate nucleus. It appears to comprise a ventral portion which receives a bilateral retinal projection and contains NPY-neurons, suggesting that this portion may correspond to IGL. The PGN also contains CB-neurons, PV-neurons and fibers, and SP- and 5-HT-fibers. These results in marmoset show that, beside a common plan revealed for most mammals, there are significant interspecific variations in the circadian timing system. Future studies are needed in order to elucidate the circadian organization in this primate species.     

Characterization of the Circadian System in a Brazilian Species of Monkey (Callithrix jacchus): Immunohistochemical Analysis and Retinal Projections

The hypothalamic suprachiasmatic nucleus (SCN) and the thalamic pregeniculate nucleus (PGN), which appears to include the intergeniculate leaflet (IGL), comprise circadian related centers in the primate brain. In this study, these centers were analysed in respect to their cytoarchitecture, retinal afferents and chemical of major cells and axon terminals with tract tracers and immunohistochemical techniques to define cytoarchitecture and connections, in the common marmoset. The SCN was shown to be a triangularly shaped cluster of compact cells just dorsal to the optic chiasm and lateral to the third ventricle. It is innervated in its ventral portion by terminals from the retina, and NPY-ergic fibers. Serotonergic and SP-staining processes are distributed throughout. VIP-neurons form a dorsolateral group of cells and CB-immunoreactive neurons fill much of the nucleus. The PGN was shown to be a wedge-shaped cluster of cells located dorsomedially to the dorsal lateral geniculate nucleus. It appears to comprise a ventral portion which receives a bilateral retinal projection and contains NPY-neurons, suggesting that this portion may correspond to IGL. The PGN also contains CB-neurons, PV-neurons and fibers, and SP- and 5-HT-fibers. These results in marmoset show that, beside a common plan revealed for most mammals, there are significant interspecific variations in the circadian timing system. Future studies are needed in order to elucidate the circadian organization in this primate species.     

Loss of dexras1 alters nonphotic circadian phase shifts and reveals a role for the intergeniculate leaflet (IGL) in gene-targeted mice

Loss of Dexras1 in gene-targeted mice impairs circadian entrainment to light cycles and produces complex changes to phase-dependent resetting responses (phase shifts) to light. The authors now describe greatly enhanced and phase-specific nonphotic responses induced by arousal in dexras1(-/-) mice. In constant conditions, mutant mice exhibited significant arousal-induced phase shifts throughout the subjective day. Unusual phase advances in the late subjective night were also produced when arousal has little effect in mice. Bilateral lesions of the intergeniculate leaflet (IGL) completely eliminated both the nonphotic as well as the light-induced phase shifts of circadian locomotor rhythms during the subjective day, but had no effect on nighttime phase shifts. The expression of FOS-like protein in the suprachiasmatic nucleus (SCN) was not affected by either photic or nonphotic stimulation in the subjective day in either genotype. Therefore, the loss of Dexras1 (1) enhances nonphotic phase shifts in a phase-dependent manner, and (2) demonstrates that the IGL in mice is a primary mediator of circadian phase-resetting responses to both photic and nonphotic events during the subjective day, but plays a different functional role in the subjective night. Furthermore, (3) the change in FOS level does not appear to be a critical step in the entrainment pathways for either light or arousal during the subjective day. The cumulative evidence suggests that Dexras1 regulates multiple photic and nonphotic signal-transduction pathways, thereby playing an essential role modulating species-specific characteristics of circadian entrainment.     

The intergeniculate leaflet partially mediates effects of light on circadian rhythms

Photic signals affect circadian activity rhythms by both phasic and tonic mechanisms that modulate pacemaker phase and period. In mammals, the effects of light on circadian activity are mediated by the retina, which communicates with the suprahiasmatic nucleus (SCN) by two different anatomical routes: the retino-hypothalamic tract (RHT), originating in the retina, and the geniculo-hypothalamic tract (GHT), arising from a retino-recipient nucleus, the intergeniculate leaflet (IGL). We assessed the roles of these two afferent systems in mediating phasic and tonic effects of light on circadian activity in IGL-lesioned animals. Destruction of the IGL significantly affected phase shifts produced by brief light pulses (phasic effect) and modified the change in period (tau) of the free-running activity rhythm produced by changing the level of constant light (LL) (tonic effect). Phase advances produced by brief light pulses were decreased in amplitude while phase delays were increased in IGL-lesioned animals as compared to controls. The free-running period in constant dark (tau DD) of IGL-lesioned animals was greater than tau DD of controls, and the lengthening of tau normally produced by LL was not observed or was greatly reduced in IGL-lesioned animals. Entrainment to light-dark cycles was unaffected by the lesions, as were other aspects of the circadian activity rhythm that normally change in response to LL (e.g., activity-rest ratio, total activity, splitting). Our data support the interpretation that the IGL plays a significant role in relaying information regarding illumination intensity to the SCN.     

In search of the pathways for light-induced pacemaker resetting in the suprachiasmatic nucleus

Within the suprachiasmatic nucleus (SCN) of the mammalian hypothalamus is a circadian pacemaker that functions as a clock. Its endogenous period is adjusted to the external 24-h light-dark cycle, primarily by light-induced phase shifts that reset the pacemaker's oscillation. Evidence using a wide variety of neurobiological and molecular genetic tools has elucidated key elements that comprise the visual input pathway for SCN photoentrainment in rodents. Important questions remain regarding the intracellular signals that reset the autoregulatory molecular loop within photoresponsive cells in the SCN's retino-recipient subdivision, as well as the intercellular coupling mechanisms that enable SCN tissue to generate phase shifts of overt behavioral and physiological circadian rhythms such as locomotion and SCN neuronal firing rate. Multiple neurotransmitters, protein kinases, and photoinducible genes add to system complexity, and we still do not fully understand how dawn and dusk light pulses ultimately produce bidirectional, advancing and delaying phase shifts for pacemaker entrainment.     

Effects of illumination and enucleation on substance-P-immunoreactive structures in subcortical visual centers of golden hamster and Wistar rat

The undecapeptide substance P is found in different entities of the visual system that control eye movement and synchronize endogenous rhythms with the light cycle (i.e., superior colliculus, suprachiasmatic nucleus, intergeniculate leaflet). Immunocytochemical methods were used to compare the reactivity to substance P in the brain of five groups of golden hamsters and two groups of Wistar rats: (1) untreated hamsters kept under 14L:10D and sacrificed at noon; (2) identically maintained animals sacrificed at midnight; (3) enucleated animals kept under control conditions; (4) hamsters kept under constant darkness; (5) hamsters kept under the same conditions as the controls, but intraventricularly injected with colchicine. The results obtained in golden hamsters of groups (1) and (3) were compared with findings in Wistar rats treated accordingly [groups (6) and (7)]. Substance P-immunoreactive perikarya were found in the suprachiasmatic nucleus and superior colliculus of hamsters and Wistar rats. Substance P-immunoreactive nerve fibers were abundant in the hypothalamic area ventral to the paraventricular nucleus, in the intergeniculate leaflet, in some thalamic nuclei, and in the superior colliculus. Immunoreactivity to substance P in the suprachiasmatic nucleus and intergeniculate leaflet did not vary among the experimental groups. However, a conspicuous decrease in reactivity to substance P was observed in the superficial layers of the superior colliculus of enucleated hamsters and rats, compared with all other groups. These results indicate that substance P immunoreactivity in the superior colliculus, but not that in the suprachiasmatic nucleus or intergeniculate leaflet, depends on the integrity of the retinal projection.     

c-Fos expression in the brains of behaviorally "split" hamsters in constant light: calling attention to a dorsolateral region of the suprachiasmatic nucleus and the medial division of the lateral habenula

"Splitting" of circadian activity rhythms in Syrian hamsters maintained in constant light appears to be the consequence of a reorganized SCN, with left and right halves oscillating in antiphase; in split hamsters, high mRNA levels characteristic of day and night are simultaneously expressed on opposite sides of the paired SCN. To visualize the splitting phenomenon at a cellular level, immunohistochemical c-Fos protein expression in the SCN and brains of split hamsters was analyzed. One side of the split SCN exhibited relatively high c-Fos levels, in a pattern resembling that seen in normal, unsplit hamsters during subjective day in constant darkness; the opposite side was labeled only within a central-dorsolateral area of the caudal SCN, in a region that likely coincides with a photo-responsive, glutamate receptor antagonist-insensitive, pERK-expressing cluster of cells previously identified by other laboratories. Outside the SCN, visual inspection revealed an obvious left-right asymmetry of c-Fos expression in the medial preoptic nucleus and subparaventricular zone of split hamsters killed during the inactive phase and in the medial division of the lateral habenula during the active phase (when the hamsters were running in their wheels). Roles for the dorsolateral SCN and the mediolateral habenula in circadian timekeeping are not yet understood.     

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

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