SCN, the master clock?

In 1960, Pittendrigh proposed that transients following Zeitgeber perturbations may be explained by a coupled two-oscillator model: a pacemaker receiving input signals and another reacting to pacemaker signals [87]. However, it took almost 30 more years until the suprachiasmatic nucleus (SCN) was accepted as master pacemaker (for review see [88]). Initial lesion experiments identified the SCN as two bilaterally paired clusters of ~20.000 densely packed neurons located superior to the optic chiasm. This region was shown to be required for hormone, activity and feeding rhythms in rats [24], [27], [28]. Elegant explanation and transplantation experiments, demonstrating autonomous rhythmicity [29]–[32] and pace-making function [33], consolidated the role of the SCN as master clock and sole driver of all body rhythms. However, in 2004, three independent groups were demonstrated that peripheral circadian oscillators display autonomous and self-sustained circadian rhythmicity ex vivo and in vitro [35], [47], [48], [89]. Almost 10 years later, the independence of peripheral tissue oscillations of rhythmic SCN derived (and environmental) signals was demonstrated in vivo [36], [90]. These findings shifted the role of the SCN from a master pacemaker to an

orchestrater of other body clocks. Nevertheless, unlike any other body clock, the SCN is indispensable for photic entrainment and transmission of light-dark signals to downstream tissue oscillators.

Each neuronal cluster of the SCN is divided into a core region and a shell region. The core region is closely located to the optic chiasm and receives direct input from the retinohypothalamic tract (RHT) [80]. The shell region receives input from the hypothalamus, limbic areas, as well as the SCN core region [80]. External time, in form of photic signals, is perceived by ocular opsin photoreceptors and, via the melanopsin expression retinal ipRGCs, transmitted to the SCN [91]. The RHT originates from the retina and forms synapses with SCN neurons, where the neurotransmitters pituitary adenylate cyclase-activating polypeptide (PACAP) and glutamate are released to transform electrical into biochemical signals [92]. Activation of their respective receptors (GluR and PAC1) induces kinase signaling pathways resulting in the rapid induction of so-called immediate early genes (e.g. c-fos, fos-B, c-myc, c-jun, jun-B), including components of the core clock machinery (Per1/2) [93], [94]. Ionotropic GluRs function as voltage-gated ion channels, metabotropic GluRs and PAC1 as G-protein coupled receptors (GPCR). Thus, several downstream signaling cascades may be activated by glutamate and PACAP. The most accepted pathways include voltage-gated calcium (Ca²⁺) channel and Ga GPCR signaling [95]. Both pathways result in the downstream elevation of cyclic AMP (cAMP) levels and the cAMP dependent activation of kinases, e.g. protein kinase A (PKA) or calmodulin-dependent protein kinase (CAMK), which phosphorylate cAMP response element binding proteins (CREB). CREBs belong to a family of transcription factors that, upon phosphorylation, induce target gene expression by binding to cAMP response elements (CRE) [93], [95].

Additionally, Ca²⁺ and Ras activation dependent MAP kinase (MAPK) pathways have been described to converge on the transcriptional induction of CRE and serum response elements (SRE), another enhancer element of immediate early genes [93].

Ultimately, light induced activation of clock gene transcription in the SCN results in time-of-day dependent phase responses, thereby entraining the SCN to environmental Zeitgeber cycles [96] (for details see 1.3). Besides the RHT, other afferent projections to the SCN, e.g. from the thalamus or the arousal centers, have been proposed as pathways of non-photic entrainment but not many details are known so far.

Regarding its efferent projections, shell and core region of the SCN differ in their neuronal connectivity, gene and neuropeptide expression profiles, as well as their response to external light information. Thus, these regions constitute functionally distinct compartments within the SCN [80], [97]–[99]. Predominant neuronal populations of shell and core region are arginine vasopressin (AVP) and vasoactive intestinal polypeptide (VIP) neurons, respectively [100]. Gamma-aminobutyric acid (GABA) or glutamate expressing neurons are common for both regions [101]. Despite their different molecular makeup, shell and core oscillators synchronize with each other. Intercellular coupling between neuronal oscillators is achieved via exchange of secreted neurotransmitters, e.g. AVP, VIP, GABA, gastrin-releasing peptide (GRP), or via gap junctions [102] (for details see 1.4). It has been shown that SCN core and shell innervate the same target structures of surrounding brain regions, which then project to other neuronal or endocrine tissues that pass on SCN derived time information to the rest of the body [80], [103], [104]. “SCN splitting” experiments have demonstrated that exposure to non 24 hour light-dark conditions results in desynchronization and anti-phasic oscillations of distinct SCN regions, as well as in aberrant rest-activity and hormonal cycles [105]–[113]. Moreover, in 2015 Evans et al. showed that even though the SCN shell can maintain phase relationships of peripheral tissue clocks by itself, synchronization of SCN regions is important for high amplitude rhythmicity within the SCN, as well as in non-SCN tissues [103].

These findings suggest that the SCN acts as orchestrator of peripheral tissue oscillations and enhances rhythmicity of autonomous peripheral clocks. Thus, ultimately the SCN may not be a master pacemaker in a strict sense. It is not required to drive circadian oscillations of peripheral tissue clocks. But it appears to be required for the establishment of stable phase relationships among body clocks, high-amplitude rhythms of peripheral oscillators, as well as for the entrainment to the light-dark cycle.

SCN dependent synchronization of the periphery can be achieved by various pathways, including direct neuronal or hormonal innervation of target tissues, indirect behavioral control (regulation of rest-activity or feeding-fasting cycles), or core body temperature variations [10] (for details see 1.3). Therefore, the SCN is still accepted as superior unit of mammalian timekeeping, even though the peripheral oscillators exhibit cell-autonomous and self-sustained rhythmicity.