Suprachiasmatic Nucleus Circadian Rhythms Research Articles

Neuropeptidergic input from the lateral hypothalamus to the suprachiasmatic nucleus alters the circadian period in mice.

In mammals, the central circadian clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus, which transmits circadian information to other brain regions and regulates the timing of sleep and wakefulness. Neurons in the lateral hypothalamus (LH), particularly those producing melanin-concentrating hormone (MCH)- and orexin are key regulators of sleep and wakefulness. Although the SCN receives non-photic input from other brain regions, the mechanisms of functional input from the LH to the SCN remain poorly understood. Here, we show that orexin and MCH peptides influence the circadian period within the SCN of both sexes. When these neurons are ablated, the circadian behavioral rhythms are lengthened under constant darkness. Using anterograde and retrograde tracing, we found that orexin and MCH neurons project to the SCN. Furthermore, the application of these peptides to cultured SCN slices shortened circadian rhythms and reduced intracellular cAMP levels. Additionally, pharmacological reduction of intracellular cAMP levels similarly shortened the circadian period in SCN slices. These findings suggest that orexin and MCH peptides from the LH contribute to the modulation of the circadian period in the SCN.Significance statement In mammals, the central circadian clock is located in the suprachiasmatic nucleus (SCN) of the hypothalamus, where it regulates circadian rhythms, including sleep and wakefulness. The SCN receives both neuronal and humoral input signals from external brain regions, which can modify circadian rhythms within the SCN. While several brain regions that project to the SCN have been anatomically identified, the specific regions, neuronal cell types, and neurotransmitters that influence SCN circadian rhythms remain largely uncharacterized. This study identifies two neuronal populations within the lateral hypothalamus that project to the SCN and modulate the circadian period.

Network rewiring and plasticity promotes synchronization of suprachiasmatic nucleus neurons.

In mammals, circadian rhythms throughout the body are orchestrated by the master clock in the hypothalamic suprachiasmatic nucleus (SCN), where SCN neurons are coupled with neurotransmitters to generate a uniform circadian rhythm. How the SCN circadian rhythm is so robust and flexible is, however, unclear. In this paper, we propose a temporal SCN network model and investigate the effects of dynamical rewiring and flexible coupling due to synaptic plasticity on the synchronization of the neural network in SCN. In networks consisting of simple Poincaré oscillators and complex circadian clocks, we found that dynamical rewiring and coupling plasticity enhance the synchronization in inhomogeneous networks. We verified the effect of enhanced synchronization in different architectures of random, scale-free, and small-world networks. A simple mean-field analysis for synchronization in plastic networks is proposed. Intuitively, the synchronization is greatly enhanced because both the random rewiring and coupling plasticity in the heterogeneous network have effectively increased the coupling strength in the whole network. Our results suggest that a proper network model for the master SCN circadian rhythm needs to take into account the effects of dynamical changes in topology and plasticity in neuron interactions that could help the brain to generate a robust circadian rhythm for the whole body.

Pituitary Adenylate Cyclase-Activating Peptide (PACAP)-Glutamate Co-transmission Drives Circadian Phase-Advancing Responses to Intrinsically Photosensitive Retinal Ganglion Cell Projections by Suprachiasmatic Nucleus

Results from a variety of sources indicate a role for pituitary adenylate cyclase-activating polypeptide (PACAP) in light/glutamate-induced phase resetting of the circadian clock mediated by the retinohypothalamic tract (RHT). Attempts to block or remove PACAP’s contribution to clock-resetting have generated phenotypes that differ in their responses to light or glutamate. For example, previous studies of circadian behaviors found that period-maintenance and early-night phase delays are intact in PACAP-null mice, yet there is a consistent deficit in behavioral phase-resetting to light stimulation in the late night. Here we report rodent stimulus–response characteristics of PACAP release from the RHT, and map these to responses of the suprachiasmatic nucleus (SCN) in intact and PACAP-deficient mouse hypothalamus with regard to phase-resetting. SCN of PACAP-null mice exhibit normal circadian rhythms in neuronal activity, but are “blind” to glutamate stimulating phase-advance responses in late night, although not in early night, consistent with previously reported selective lack of late-night light behavioral responsiveness of these mice. Induction of CREB phosphorylation, a hallmark of the light/glutamate response of the SCN, also is absent in SCN-containing ex vivo slices from PACAP-deficient mouse hypothalamus. PACAP replacement to the SCN of PACAP-null mice restored wild-type phase-shifting of firing-rate patterns in response to glutamate applied to the SCN in late night. Likewise, ex vivo SCN of wild-type mice post-orbital enucleation are unresponsive to glutamate unless PACAP also is restored. Furthermore, we demonstrate that the period of efficacy of PACAP at SCN nerve terminals corresponds to waxing of PACAP mRNA expression in ipRGCs during the night, and waning during the day. These results validate the use of PACAP-deficient mice in defining the role and specificity of PACAP as a co-transmitter with glutamate in ipRGC-RHT projections to SCN in phase advancing the SCN circadian rhythm in late night.

Genetic Disruption of Circadian Rhythms in the Suprachiasmatic Nucleus Causes Helplessness, Behavioral Despair, and Anxiety-like Behavior in Mice

BackgroundMajor depressive disorder is associated with disturbed circadian rhythms. To investigate the causal relationship between mood disorders and circadian clock disruption, previous studies in animal models have employed light/dark manipulations, global mutations of clock genes, or brain area lesions. However, light can impact mood by noncircadian mechanisms; clock genes have pleiotropic, clock-independent functions; and brain lesions not only disrupt cellular circadian rhythms but also destroy cells and eliminate important neuronal connections, including light reception pathways. Thus, a definitive causal role for functioning circadian clocks in mood regulation has not been established. MethodsWe stereotactically injected viral vectors encoding short hairpin RNA to knock down expression of the essential clock gene Bmal1 into the brain’s master circadian pacemaker, the suprachiasmatic nucleus (SCN). ResultsIn these SCN-specific Bmal1-knockdown (SCN-Bmal1-KD) mice, circadian rhythms were greatly attenuated in the SCN, while the mice were maintained in a standard light/dark cycle, SCN neurons remained intact, and neuronal connections were undisturbed, including photic inputs. In the learned helplessness paradigm, the SCN-Bmal1-KD mice were slower to escape, even before exposure to inescapable stress. They also spent more time immobile in the tail suspension test and less time in the lighted section of a light/dark box. The SCN-Bmal1-KD mice also showed greater weight gain, an abnormal circadian pattern of corticosterone, and an attenuated increase of corticosterone in response to stress. ConclusionsDisrupting SCN circadian rhythms is sufficient to cause helplessness, behavioral despair, and anxiety-like behavior in mice, establishing SCN-Bmal1-KD mice as a new animal model of depression.

Photoperiodic and circadian bifurcation theories of depression and mania.

Seasonal effects on mood have been observed throughout much of human history. Seasonal changes in animals and plants are largely mediated through the changing photoperiod (i.e., the photophase or duration of daylight). We review that in mammals, daylight specifically regulates SCN (suprachiasmatic nucleus) circadian organization and its control of melatonin secretion. The timing of melatonin secretion interacts with gene transcription in the pituitary pars tuberalis to modulate production of TSH (thyrotropin), hypothalamic T3 (triiodothyronine), and tuberalin peptides which modulate pituitary production of regulatory gonadotropins and other hormones. Pituitary hormones largely mediate seasonal physiologic and behavioral variations. As a result of long winter nights or inadequate illumination, we propose that delayed morning offset of nocturnal melatonin secretion, suppressing pars tuberalis function, could be the main cause for winter depression and even cause depressions at other times of year. Irregularities of circadian sleep timing and thyroid homeostasis contribute to depression. Bright light and sleep restriction are antidepressant and conversely, sometimes trigger mania. We propose that internal desynchronization or bifurcation of SCN circadian rhythms may underlie rapid-cycling manic-depressive disorders and perhaps most mania. Much further research will be needed to add substance to these theories.

Participation of morphofunctional zones in aging processes

There are morphofunctional zones in organism tissues where proliferation and differentiation processes occur. Daughter cells are differentiated in the electric field excited by 12 mother and daughter cell pairs, produced during cambial cell division. With aging, the number of cambial cells is reduced to seven, which is close to threshold level (six cells) at which the differentiation of daughter cells is absent. The depression of number of cambial cells with aging is connected with the work of another morphofunctional zone, i.e., the hypothalamus, which is the major center of vegetative regulation and initially has very high RhoA activity, which is established during embryogenesis. Estrogens, which influence the hypothalamus and activate Src kinase in its nuclei and reduce the level of RhoA activity, including in the suprachiasmatic nucleus, are responsible for many of an organism’s biorhythms. As a result, hyperestrogenemia and, therefore, connective tissue develop first. Then, hypoestrogenemia takes place, which leads to a sharp drop in the proliferative activity of cells, causing a decrease in the number of cambial cells and the possibility of a malignant tumor development. Along with this, there are deep lesions of hormone regulation that lead to some lethal diseases. Thus, an increase in RhoA in the hypothalamus and especially in suprachiasmatic nucleus circadian rhythm can counteract the Src kinase intensifying and prevent the processes connected with this.

Cryptochromes are critical for the development of coherent circadian rhythms in the mouse suprachiasmatic nucleus

Cryptochrome (Cry) 1 and Cry2 are regarded as critical components for circadian rhythm generation in mammals. Nevertheless, cultured suprachiasmatic nucleus (SCN) of neonatal Cry double deficient (Cry1(-/-)/Cry2(-/-)) mice exhibit circadian rhythms that damp out in several cycles. Here, by combining bioluminescence imaging of Per1-luc and PER2::LUC with multielectrode recording, we show developmental changes in SCN circadian rhythms in Cry1(-/-)/Cry2(-/-) mice. At the tissue level, circadian rhythms are found in neonatal but not in adult SCN, whereas at the cellular level, rhythms are detected in both SCN. Cellular circadian rhythms are synchronized in neonates, but not in adults, indicating a loss of rhythm synchrony in the course of development. Synchronized circadian rhythms in adult Cry1(-/-)/Cry2(-/-) SCN are restored by coculture of neonatal, but not of juvenile, SCN. These findings indicate that CRY1 and CRY2 are necessary for the development of intercellular networks that subserve coherent rhythm expression in adult SCN.

Advanced Light-Entrained Activity Onsets and Restored Free-Running Suprachiasmatic Nucleus Circadian Rhythms in Per2/Dec Mutant Mice

Many behavioral and physiological processes display diurnal (24-h) rhythms controlled by an internal timekeeping system—the circadian clock. In mammals, a circadian pacemaker is located in the suprachiasmatic nucleus (SCN) of the hypothalamus and synchronizes peripheral oscillators found in most other tissues with the external light-dark (LD) cycle. At the molecular level, circadian clocks are regulated by transcriptional translational feedback loops (TTLs) involving a set of clock genes. The mammalian core TTL includes the transcriptional modulators PER (1–3) and CRY (1/2) that inhibit their own expression by interaction with CLOCK/NPAS2 and BMAL1 (ARNTL). The basic helix-loop-helix transcription factors DEC1 (BHLHE40) and DEC2 (BHLHE41) can interact with this core TTL, forming an accessory feedback mechanism. The authors measured circadian locomotor behavior and clock gene expression in the SCN of Per2/Dec double- and triple-mutant mice to analyze the functional interaction of PER2 and DEC feedback on circadian pacemaker function in the SCN. The data suggest a synergistic interaction of Per2 and Dec1/2 in activity entrainment to a standard LD cycle, correlating with a cumulative deficiency in negative-masking capacities in Per2/Dec double- and triple-mutant mice and suggesting an involvement of Per2-Dec1/2 interactivity in activity-onset regulation and masking under LD, but not under constant conditions. In contrast, under constant darkness (DD) conditions, a deletion of either Dec1 or Dec2 partially rescued the Per2 mutant short-period/arrhythmicity phenotype, accompanied by a restoration of time-of-day effects on clock gene expression in the SCN. Together, these results show an interaction of Per2 and Dec1/2 feedback processes in the SCN with differential modes of interactivity under entrained and free-run conditions. (Author correspondence: henrik.oster@mpibpc.mpg.de)

Dissociation between Circadian Per1 and Neuronal and Behavioral Rhythms Following a Shifted Environmental Cycle

The suprachiasmatic nucleus (SCN) of the anterior hypothalamus contains a major circadian pacemaker that imposes or entrains rhythmicity on other structures by generating a circadian pattern in electrical activity [1, 2]. The identification of “clock genes” within the SCN [3–6] and the ability to dynamically measure their rhythmicity by using transgenic animals open up new opportunities to study the relationship between molecular rhythmicity and other well-documented rhythms within the SCN. We investigated SCN circadian rhythms in Per1-luc bioluminescence, electrical activity in vitro and in vivo, as well as the behavioral activity of rats exposed to a 6-hr advance in the light-dark cycle followed by constant darkness. The data indicate large and persisting phase advances in Per1-luc bioluminescence rhythmicity, transient phase advances in SCN electrical activity in vitro, and an absence of phase advances in SCN behavioral or electrical activity measured in vivo. Surprisingly, the in vitro phase-advanced electrical rhythm returns to the phase measured in vivo when the SCN remains in situ. Our study indicates that hierarchical levels of organization within the circadian timing system influence SCN output and suggests a strong and unforeseen role of extra-SCN areas in regulating pacemaker function.

Pre- and postsynaptic actions of serotonin on rat suprachiasmatic nucleus neurons

Serotoninergic transmission is implicated in the photic and non-photic regulation of circadian rhythms. 5-HT (1–100 μM), carboxamidotryptamine (5-CT 0.1–10 μM) and (+)-8-hydroxy-dipropylaminotetraline (8-OH-DPAT, 1–30 μM) dose-dependently activated an outward current (5–100 pA) in 30% of neurons voltage-clamped at −60 mV in the suprachiasmatic nucleus (SCN) in vitro slice. EC 50 values were 7.0 μM for 5-HT and 0.2 μM for 5-CT. Serotonin-induced outward current was associated with an increase in input conductance, and the current was blocked by Ba 2+ (1 mM). The amplitude of the current was enhanced by depolarization, reduced by hyperpolarization, and reversed its polarity during a hyperpolarization beyond the potassium equilibrium potential. Mean amplitudes of the 5-HT outward current changed with time of the subjective circadian day. The value near CT2 (23.8 pA) was about 4 times greater than that around CT14 (6.7 pA). Cells that responded with an outward current showed four types of morphology: monopolar, simple bipolar, curly bipolar and radial shaped; they were localized in all parts of the SCN. The EPSC evoked by retino-hypothalamic-tract (RHT) stimulation was inhibited 26% but the inward current induced by exogenously applied glutamate or NMDA was not affected by serotonin agonists. Focal stimulation-induced and spontaneous IPSC but not the exogenous GABA-induced outward current were inhibited by 5-HT agonists in a subpopulation of cells. In conclusion, 5-HT regulates SCN neurons by both pre- and post-synaptic inhibitory mechanisms; the latter may play a key role in modulating SCN circadian rhythm by activation of 5-HT receptors and opening of a potassium channel.

Cholinergic regulation of the suprachiasmatic nucleus circadian rhythm via a muscarinic mechanism at night

In mammals, the suprachiasmatic nucleus (SCN) is responsible for the generation of most circadian rhythms and for their entrainment to environmental cues. Carbachol, an agonist of acetylcholine (ACh), has been shown to shift the phase of circadian rhythms in rodents when injected intracerebroventricularly. However, the site and receptor type mediating this action have been unknown. In the present experiments, we used the hypothalamic brain-slice technique to study the regulation of the SCN circadian rhythm of neuronal firing rate by cholinergic agonists and to identify the receptor subtypes involved. We found that the phase of the oscillation in SCN neuronal activity was reset by a 5 min treatment with a carbachol microdrop (1 microliter, 100 microM), but only when applied during the subjective night, with the largest phase shift (+ 6 hr) elicited during the middle of the subjective night. This effect also was produced by ACh and two muscarinic receptor (mAChR) agonists, muscarine and McN-A-343 (M1-selective), but not by nicotine. Furthermore, the effect of carbachol was blocked by the mAChR antagonist atropine (0.1 microM), not by two nicotinic antagonists, dihydro-beta-erythroidine (10 microM) and d-tubocurarine (10 microM). The M1-selective mAChR antagonist pirenzepine completely blocked the carbachol effect at 1 microM, whereas an M3-selective antagonist, 4,2-(4,4'-diacetoxydiphenylmethyl)pyridine, partially blocked the effect at the same concentration. These results demonstrate that carbachol acts directly on the SCN to reset the phase of its firing rhythm during the subjective night via an M1-like mAChR.