Intrinsic, nondeterministic circadian rhythm generation in identified mammalian neurons

Self Cite

100

In the mammalian suprachiasmatic nucleus (SCN), noisy cellular oscillators communicate within a neuronal network to generate precise system-wide circadian rhythms. Although the intracellular genetic oscillator and intercellular biochemical coupling mechanisms have been examined previously, the network topology driving synchronization of the SCN has not been elucidated. This network has been particularly challenging to probe, due to its oscillatory components and slow coupling timescale. In this work, we investigated the SCN network at a single-cell resolution through a chemically induced desynchronization. We then inferred functional connections in the SCN by applying the maximal information coefficient statistic to bioluminescence reporter data from individual neurons while they resynchronized their circadian cycling. Our results demonstrate that the functional network of circadian cells associated with resynchronization has small-world characteristics, with a node degree distribution that is exponential. We show that hubs of this small-world network are preferentially located in the central SCN, with sparsely connected shells surrounding these cores. Finally, we used two computational models of circadian neurons to validate our predictions of network structure.

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Functional network inference of the suprachiasmatic nucleus

Abel

Meeker

Granados‐Fuentes

et al. 2016

Self Cite

100

“…Clearly, more research needs to be done to define the nature of this food-entrained network. Recently it has been shown that the oscillatory property of the SCN is possibly a network property rather than the property of single neurons (43). We may therefore need to view in a similar manner the FEO as a neuronal network: its oscillatory capacity is neither derived nor dependent (as we have shown here) on one structure, but rather the interaction between the participating structures shapes, different aspects of FAA.…”

Section: Faa Is Lost After Lesion Of the Dmh But Reappears After Amentioning

confidence: 94%

Interaction between hypothalamic dorsomedial nucleus and the suprachiasmatic nucleus determines intensity of food anticipatory behavior

Acosta-Galvan

Vliet

et al. 2011

162

131

Food anticipatory behavior (FAA) is induced by limiting access to food for a few hours daily. Animals anticipate this scheduled meal event even without the suprachiasmatic nucleus (SCN), the biological clock. Consequently, a food-entrained oscillator has been proposed to be responsible for meal time estimation. Recent studies suggested the dorsomedial hypothalamus (DMH) as the site for this food-entrained oscillator, which has led to considerable controversy in the literature. Herein we demonstrate by means of c-Fos immunohistochemistry that the neuronal activity of the suprachiasmatic nucleus (SCN), which signals the rest phase in nocturnal animals, is reduced when animals anticipate the scheduled food and, simultaneously, neuronal activity within the DMH increases. Using retrograde tracing and confocal analysis, we show that inhibition of SCN neuronal activity is the consequence of activation of GABA-containing neurons in the DMH that project to the SCN. Next, we show that DMH lesions result in a loss or diminution of FAA, simultaneous with increased activity in the SCN. A subsequent lesion of the SCN restored FAA. We conclude that in intact animals, FAA may only occur when the DMH inhibits the activity of the SCN, thus permitting locomotor activity. As a result, FAA originates from a neuronal network comprising an interaction between the DMH and SCN. Moreover, this study shows that the DMH-SCN interaction may serve as an intrahypothalamic system to gate activity instead of rest overriding circadian predetermined temporal patterns. P hysiology and behavior of all mammals is organized in an alternating pattern of rest and activity cycles, whereby the endogenous and light-induced daily neuronal activity of the suprachiasmatic nucleus (SCN) signals rest in nocturnal rodents and activity in diurnal primates, such as humans (1, 2). Restricting food access to a short and predictable episode during the rest phase changes this behavioral pattern, such that an animal becomes active and for up to several hours anticipates the upcoming feeding event. This food anticipatory activity (FAA) is even exhibited without the known circadian oscillator, the SCN (3), and thus may rely on a different circadian pacemaker.In search of the location of this so-called "food entrained oscillator" (FEO), two recent studies have claimed that its position is within the dorsomedial nucleus of the hypothalamus (DMH) (4, 5). The designation of the DMH as master clock for food entrainment is, however, controversial because some groups reported unimpaired FAA despite large lesions of the DMH (6, 7); others have shown that lesions of the DMH disturb and diminish the intensity of FAA (6). The possible participation of other brain structures in FAA is evident from studies that demonstrate modulation of neuronal activity and induction of clock-gene rhythmicity in hypothalamic and limbic structures by feeding schedules (8-11). Thus, it has been suggested that FAA depends on a multioscillatory system comprised of a complex and redundant neuronal netwo...

“…Isolated SCN neurons are less reliable oscillators (11) and "neuronal network properties are integral to normal function of the SCN" (ref. 9, p. 551).…”

mentioning

confidence: 99%

A diversity of paracrine signals sustains molecular circadian cycling in suprachiasmatic nucleus circuits

Maywood

Chesham²,

O’Brien³

et al. 2011

264

311

The suprachiasmatic nucleus (SCN) is the principal circadian pacemaker of mammals, coordinating daily rhythms of behavior and metabolism. Circadian timekeeping in SCN neurons revolves around transcriptional/posttranslational feedback loops, in which Period (Per) and Cryptochrome (Cry) genes are negatively regulated by their protein products. Recent studies have revealed, however, that these "core loops" also rely upon cytosolic and circuit-level properties for sustained oscillation. To characterize interneuronal signals responsible for robust pacemaking in SCN cells and circuits, we have developed a unique coculture technique using wild-type (WT) "graft" SCN to drive pacemaking (reported by PER2::LUCIFER-ASE bioluminescence) in "host" SCN deficient either in elements of neuropeptidergic signaling or in elements of the core feedback loop. We demonstrate that paracrine signaling is sufficient to restore cellular synchrony and amplitude of pacemaking in SCN circuits lacking vasoactive intestinal peptide (VIP). By using grafts with mutant circadian periods we show that pacemaking in the host SCN is specified by the genotype of the graft, confirming graft-derived factors as determinants of the host rhythm. By combining pharmacological with genetic manipulations, we show that a hierarchy of neuropeptidergic signals underpins this paracrine regulation, with a preeminent role for VIP augmented by contributions from arginine vasopressin (AVP) and gastrin-releasing peptide (GRP). Finally, we show that interneuronal signaling is sufficiently powerful to maintain circadian pacemaking in arrhythmic Cry-null SCN, deficient in essential elements of the transcriptional negative feedback loops. Thus, a hierarchy of paracrine neuropeptidergic signals determines cell-and circuit-level circadian pacemaking in the SCN.T he circadian pacemaker of the suprachiasmatic nucleus (SCN) directs, on a daily basis, the most fundamental processes of physiology and metabolism, sleep and wakefulness (1). The established molecular model of the SCN oscillator involves interlocked transcriptional/posttranslational negative feedback loops, centered on rhythmic expression of the transcriptional inhibitors Period (Per) and Cryptochrome (Cry), driven by the positive activators Clock and Bmal1. The period of the daily oscillation is determined by the transcriptional activity of Clock (1) and by the stability of Per and Cry proteins (2, 3). More recently the model of SCN neurons as autonomous transcriptional/translational feedback oscillators has been reappraised. Rhythmic cytosolic signaling pathways, particularly cAMP and Ca 2+ , not only are driven by the core feedback loops, but also support them in a mutually dependent fashion (4, 5). Furthermore, within SCN circuits, loss of signaling by the neuropeptide vasoactive intestinal peptide (VIP) through its VPAC2 receptor (a positive regulator of cAMP) both desynchronizes and damps cellular transcriptional pacemaking (6, 7). Conversely, intercellular coupling maintains robustness within SCN cells deficie...