PER1-like immunoreactivity in oxytocin cells of the hamster hypothalamo-neurohypophyseal system.
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Animals↗
Biomedical subjects
William J Schwartz
Publications and source records attributed to William J Schwartz.
Hamsters running on time: is the lateral habenula a part of the clock?
Previous anatomical and physiological studies have implicated the lateral habenula, and especially its medial division (LHbM), as a candidate component of the circadian timing system in rodents. We assayed lateral habenula rhythmicity in rodents using c-FOS immunohistochemistry and found a robust rhythm in immunoreactive cell counts in the LHbM, with higher counts during the dark phase of a light-dark (LD) cycle and during subjective night in constant darkness. We have also observed an obvious asymmetry of c-FOS expression in the LHbM of behaviorally "split" hamsters in constant light, but only during their active phase (when they were running in wheels). Locomotor activity rhythms appear to be regulated by the suprachiasmatic nucleus (SCN) via multiple output pathways, one of which might be diffusible while the other might be neural, involving the lateral habenula.
Animals↗
Minireview: timely ovulation: circadian regulation of the female hypothalamo-pituitary-gonadal axis.
The preovulatory surge in the secretion of LH is timed by a neuroendocrine integrative mechanism that involves ovarian estradiol levels and the endogenous circadian system. Studies in female rats and hamsters have established that the clock in the hypothalamic suprachiasmatic nucleus has a preeminent role in setting the LH surge, and anatomical, physiological, and pharmacological data are revealing the responsible connections between suprachiasmatic nucleus neurons and GnRH and estradiol-receptive areas. Recent investigations show that GnRH and pituitary cells express circadian clock genes that might play a role in the release and reception of the GnRH signal. Analysis of the circadian regulation of the LH surge may provide a model for understanding how multiple neural oscillators function within other neuroendocrine axes.
Animals↗
Photoinducible and rhythmic ICER-CREM immunoreactivity in the rat suprachiasmatic nucleus.
Several genes expressed in the suprachiasmatic nucleus (SCN) are induced by light and are candidate links in the photic entrainment pathway of the SCN's circadian clock. Since the cAMP response element binding protein (CREB) and CRE-mediated gene transcription in the SCN appears to be crucial for light-induced phase shifts of circadian rhythmicity, we analyzed the immunohistochemical expression of proteins encoded by the cAMP response element modulator (CREM) gene, including a repressor isoform (inducible cAMP early repressor [ICER]). ICER-CREM immunoreactivity was detected in cells of the ventrolateral subdivision of the rat SCN after light administration during the subjective night in constant darkness; but only late after light onset (at 240 min), following earlier successive peaks of phosphorylated CREB protein (by 5 min), c-fos mRNA (by 40 min), per 1 mRNA (by 55 min), and c-Fos protein (by 60 min). In constant darkness, there was a modest but significant endogenous rhythm of ICER-CREM immunoreactivity, with a two-fold difference between high levels at circadian time (CT) 10 and low levels at CT 22. Our data raise the possibility that ICER-CREM might be involved in downregulating the SCN expression of immediate-early and "clock" genes after their induction by phase-shifting light pulses.
Analysis of Variance↗
Myelination and long diffusion times alter diffusion-tensor-imaging contrast in myelin-deficient shiverer mice.
Diffusion tensor imaging (DTI) using variable diffusion times (t(diff)) was performed to investigate wild-type (wt) mice, myelin-deficient shiverer (shi) mutant mice and shi mice transplanted with wt neural precursor cells that differentiate and function as oligodendrocytes. At t(diff) = 30 ms, the diffusion anisotropy "volume ratio" (VR), diffusion perpendicular to the fibers (lambda( perpendicular)), and mean apparent diffusion coefficient ( ) of the corpus callosum of shi mice were significantly higher than those of wt mice by 12 +/- 2%, 13 +/- 2%, and 10 +/- 1%, respectively; fractional anisotropy (FA) and relative anisotropy (RA) were lower by 10 +/- 1% and 11 +/- 3%, respectively. Diffusion parallel to the fibers (lambda(//)) was not statistically different between shi and wt mice. Normalized T(2)-weighted signal intensities showed obvious differences (27 +/- 4%) between wt and shi mice in the corpus callosum but surprisingly did not detect transplant-derived myelination. In contrast, diffusion anisotropy maps detected transplant-derived myelination in the corpus callosum and its spatial distribution was consistent with the donor-derived myelination determined by immunohistochemical staining. Anisotropy indices (except lambda(//)) in the corpus callosum showed strong t(diff) dependence (30-280 ms), and the differences in lambda( perpendicular) and VR between wt and shi mice became significantly larger at longer t(diff), indicative of improved DTI sensitivity at long t(diff). In contrast, anisotropy indices in the hippocampus showed very weak t(diff) dependence and were not significantly different between wt and shi mice across different t(diff). This study provides insights into the biological signal sources and measurement parameters influencing DTI contrast, which could lead to developing more sensitive techniques for detection of demyelinating diseases.
Algorithms↗
The suprachiasmatic nucleus is a functionally heterogeneous timekeeping organ.
Ever since the locus of the brain clock in the suprachiasmatic nucleus (SCN) was first described, methods available have both enabled and encumbered our understanding of its nature at the level of the cell, the tissue, and the animal. A combination of in vitro and in vivo approaches has shown that the SCN is a complex heterogeneous neuronal network. The nucleus is composed of cells that are retinorecipient and reset by photic input; those that are reset by nonphotic inputs; slave oscillators that are rhythmic only in the presence of the retinohypothalamic tract; endogenously rhythmic cells, with diverse period, phase, and amplitude responses; and cells that do not oscillate, at least on some measures. Network aspects of SCN organization are currently being revealed, but mapping these properties onto cellular characteristics of electrical responses and patterns of gene expression are in early stages. While previous mathematical models focused on properties of uniform coupled oscillators, newer models of the SCN as a brain clock now incorporate oscillator and gated, nonoscillator elements.
Animals↗
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.
Animals↗
Sunrise and sunset in fly brains.
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Animals↗
Using Per gene expression to search for photoperiodic oscillators in the hamster suprachiasmatic nucleus.
The circadian pacemaker in the suprachiasmatic nucleus (SCN) is also believed to underlie photoperiodic (seasonal) timekeeping in mammals. This clock has been modeled as a complex pacemaker composed of two coupled circadian oscillators; variability in their mutual phase relationship could account for the ability to measure daylength, with putative morning and evening oscillators synchronized to dawn and dusk, respectively. Recently, several genes have been identified that are believed to be part of the clock's core oscillatory mechanism. Here, we investigate how such molecular oscillations are altered as a function of photoperiod by analyzing Period (Per1, Per2, and Per3) gene expression at the mRNA level using SCN tissue sections and in situ hybridization. Golden hamsters were entrained to complete 24-h light-dark (LD) cycles with either a long (16 h) or a short (8 h) photophase, or they were entrained to the long complete photoperiod and then allowed to free-run in constant darkness. The results show large photoperiod-dependent changes in the duration of high daytime SCN Per1 and Per2 mRNA levels and small changes in the phase difference between their rhythms.
Animals↗
Forced desynchronization of dual circadian oscillators within the rat suprachiasmatic nucleus.
The circadian clock in the suprachiasmatic nucleus of the hypothalamus (SCN) contains multiple autonomous single-cell circadian oscillators and their basic intracellular oscillatory mechanism is beginning to be identified. Less well understood is how individual SCN cells create an integrated tissue pacemaker that produces a coherent read-out to the rest of the organism. Intercellular coupling mechanisms must coordinate individual cellular periods to generate the averaged, genotype-specific circadian period of whole animals. To noninvasively dissociate this circadian oscillatory network in vivo, we (T.C. and A.D.-N.) have developed an experimental paradigm that exposes animals to exotic light-dark (LD) cycles with periods close to the limits of circadian entrainment. If individual oscillators with different periods are loosely coupled within the network, perhaps some of them would be synchronized to the external cycle while others remain unentrained. In fact, rats exposed to an artificially short 22 hr LD cycle express two stable circadian motor activity rhythms with different period lengths in individual animals. Our analysis of SCN gene expression under such conditions suggests that these two motor activity rhythms reflect the separate activities of two oscillators in the anatomically defined ventrolateral and dorsomedial SCN subdivisions. Our "forced desychronization" protocol has allowed the first stable separation of these two regional oscillators in vivo, correlating their activities to distinct behavioral outputs, and providing a powerful approach for understanding SCN tissue organization and signaling mechanisms in behaving animals.
Analysis of Variance↗
Real-time imaging reveals spatiotemporal dynamics of cellular circadian clocks.
Circadian clocks have been localized to discrete sites within the nervous systems of several organisms, and in mammals to the suprachiasmatic nucleus (SCN) in the anterior hypothalamus. The clock in the SCN is composed of multiple autonomous single-cell oscillators, and new studies now allow an unprecedented look at their oscillatory activities over repeated cycles in tissue slices in vitro. Initial data reinforce the importance of intercellular membrane events for constructing a functional and reliable tissue pacemaker.
Animals↗
Lateralization of circadian pacemaker output: Activation of left- and right-sided luteinizing hormone-releasing hormone neurons involves a neural rather than a humoral pathway.
Locomotor activity and luteinizing hormone (LH) secretion in golden hamsters share a common circadian pacemaker in the suprachiasmatic nucleus (SCN), but the rhythms do not seem to share a common output pathway from the SCN. Locomotion is believed to be driven by humoral factor(s), whereas LH secretion may depend on specific ipsilateral neural efferents from the SCN to LH releasing hormone (LHRH)-containing neurons in the preoptic area. In this paper we provide the first functional evidence for such efferents in neurologically intact hamsters by exploiting a phenomenon known as "splitting" in constant light, in which circa-12 hr (approximately 12 hr) locomotor activity bouts reflect an antiphase oscillation of the left and right sides of the bilaterally paired SCN. In ovariectomized, estrogen-treated (OVX + E2) female hamsters, splitting is also known to include circa-12 hr LH secretory surges. Here we show that behaviorally "split" OVX + E2 females exhibit a marked left-right asymmetry in immunoreactive c-Fos expression in both SCN and activated LHRH neurons, with the percentage of LHRH+/c-Fos+ double-labeled cells approximately fivefold higher on the side corresponding to the side of the SCN with higher c-Fos immunoreactivity. Our results suggest that splitting involves alternating left- and right-sided stimulation of LHRH neurons; under such circumstances, the functional activity of the neuroendocrine hypothalamus mirrors intrinsic side-to-side differences in SCN gene expression. The circadian regulation of reproductive activity depends on lateralized, point-to-point axonal projections rather than on diffusible factors.
Animals↗
Circadian rhythms: in the loop at last.
The basic molecular mechanisms underlying circadian oscillators follow the same general plan across the phylogenetic spectrum: oscillating feedback loops in which clock gene products negatively regulate their own expression. The circadian clocks of animals involve at least two interacting feedback loops. This Viewpoint compares and contrasts the circadian clocks of mammals and of Drosophila, emphasizing how different players are used to create the same basic script. Both the general script and the specific details of the murine and Drosophila circadian pathways are available at Science's Signal Transduction Knowledge Environment Connections Maps.
ARNTL Transcription Factors↗
Systemic administration of betamethasone delays endotoxin-induced preterm labor in the murine model.
OBJECTIVE: The purpose of this study was to determine whether the administration of betamethasone decreases the endotoxin-induced preterm parturition rate and inhibits the risk of cytokines in the murine model. STUDY DESIGN: Endotoxin was administered intraperitoneally at gestational day 15 (75% of gestation). In phase I, the duration of gestation was measured in 36 gravid C3H/HeOu mice that were equally divided into four treatment groups: control, endotoxin only, and two different dose regimens of betamethasone followed by endotoxin. In phase II, maternal serum and amniotic fluid concentrations of cytokines (interleukin-1alpha, tumor necrosis factor-alpha, and interleukin-6) were measured at 4 hours after endotoxin injection in 44 gravid mice divided equally in the four treatment groups. RESULTS: The group that was exposed only to endotoxin was delivered at a significantly earlier gestational age compared with the control group (16.2 +/- 0.4 days vs 19.6 +/- 0.2 days; P <.01). The two groups that were pretreated with betamethasone before the endotoxin were delivered at gestational ages similar to the control group. There was a marked increase of tumor necrosis factor-alpha and interleukin-6 levels in amniotic fluid of mice that were treated with endotoxin only compared with the control group (P <.001). No difference in cytokine levels was found in those mice that were premedicated with betamethasone compared with the control group. CONCLUSION: Antenatal administration of betamethasone to mice delayed preterm parturition that was induced by endotoxin. Elevations of amniotic fluid cytokine concentrations that were observed with endotoxin were not observed with pretreatment with betamethasone.
Amniotic Fluid↗
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.
Animals↗
A subpopulation of efferent neurons in the mouse suprachiasmatic nucleus is also light responsive.
The suprachiasmatic nucleus (SCN) is the site of a circadian clock with input (afferent) pathways for photic entrainment and output (efferent) pathways for expression of overt, measurable rhythms. To determine whether there are individual neurons in the mouse SCN that might be part of both pathways, we performed double-label immunohistochemistry for light-induced c-Fos and the retrograde tracer cholera toxin subunit B (CtB), 2 weeks after CtB was iontophoresed into the subparaventricular area (subPVA). A minority of neurons was found that were both efferent to the subPVA and responsive to light. This cellular subset may function as a direct channel through the SCN for photic inputs to influence neural outputs, and its existence highlights the topographical heterogeneity of SCN tissue.
Afferent Pathways↗
Transplanted clonal neural stem-like cells respond to remote photic stimulation following incorporation within the suprachiasmatic nucleus.
Multipotent neural stem-like cells (NSCs) obtained from one brain region and transplanted to another region appear to differentiate into neuronal and glial phenotypes indigenous to the implantation site. Whether these donor-derived cells are appropriately integrated remains unanswered. In order to test this possibility, we exploited the suprachiasmatic nucleus (SCN) of the hypothalamus, site of a known circadian clock, as a novel engraftment target. When a clone of NSCs initially derived from neonatal mouse cerebellum was transplanted into mouse embryos, the cells incorporated within the SCN over a narrow gestational window that corresponded to the conclusion of SCN neurogenesis. Immunocytochemical staining suggested that donor-derived cells in the SCN synthesized a peptide neurotransmitter (arginine vasopressin) characteristic of SCN neurons. Donor-derived SCN cells reacted to light pulses by expressing immunoreactive c-Fos protein in a pattern that is appropriate for native SCN cells. This region-specific and physiologically appropriate response to the natural stimulation of a remote sensory input implies that donor-derived and endogenous cells formed true SCN chimeras, suggesting that exogenous NSCs engrafted to ectopic locations can integrate in a meaningful fashion.
Animals↗