Coherency of circadian rhythms in the SCN is governed by the interplay of two coupling factors

Abstract

Circadian clocks are autonomous oscillators driving daily rhythms in physiology and behavior. In mammals, a network of coupled neurons in the suprachiasmatic nucleus (SCN) is entrained to environmental light-dark cycles and orchestrates the timing of peripheral organs. In each neuron, transcriptional feedbacks generate noisy oscillations. Coupling mediated by neuropeptides such as VIP and AVP lends precision and robustness to circadian rhythms. The detailed coupling mechanisms between SCN neurons are debated. We analyze organotypic SCN slices from neonatal and adult mice in wild-type and multiple knockout conditions. Different degrees of rhythmicity are quantified by pixel-level analysis of bioluminescence data. We use empirical orthogonal functions (EOFs) to characterize spatio-temporal patterns. Simulations of coupled stochastic single cell oscillators can reproduce the diversity of observed patterns. Our combination of data analysis and modeling provides deeper insight into the enormous complexity of the data: (1) Neonatal slices are typically stronger oscillators than adult slices pointing to developmental changes of coupling. (2) Wild-type slices are completely synchronized and exhibit specific spatio-temporal patterns of phases. (3) Some slices of Cry double knockouts obey impaired synchrony that can lead to co–existing rhythms (“splitting”). (4) The loss of VIP-coupling leads to desynchronized rhythms with few residual local clusters. Additional information was extracted from co–culturing slices with rhythmic neonatal wild-type SCNs. These co–culturing experiments were simulated using external forcing terms representing VIP and AVP signaling. The rescue of rhythmicity via co–culturing lead to surprising results, since a cocktail of AVP-antagonists improved synchrony. Our modeling suggests that these counter-intuitive observations are pointing to an antagonistic action of VIP and AVP coupling. Our systematic theoretical and experimental study shows that dual coupling mechanisms can explain the astonishing complexity of spatio-temporal patterns in SCN slices.

Different Roles for VIP Neurons in the Neonatal and Adult Suprachiasmatic Nucleus

“…Individual SCN neurons express cell autonomous oscillations driven through a negative transcription-translation feedback loop involving the core clock genes, which include Bmal1, Clock, Period 1 and 2, and Cryptochrome 1 and 2 (Takahashi, 2017; Webb et al, 2009; Welsh et al, 1995). Intercellular neurotransmission is necessary to couple single-cell oscillators and generate high-amplitude, precise, circuit-wide rhythms capable of driving behavioral circadian rhythms (Herzog et al, 2004; Tokuda et al, 2018; Welsh et al, 1995; Yamaguchi et al, 2003).…”

Targeted modification of the Per2 clock gene alters circadian function in mPer2luciferase (mPer2Luc) mice

“…This suggests that a robust rhythmic signal is produced in the shell that regulates rhythmicity in the core. In the model the shell → core coupling parameter represents average synaptic input to the core, the values being represented currently by inhibitory and excitatory GABA signaling, but can be adjusted readily to differentiate other potential signals such as arginine vasopressin [ 37 , 38 ].…”

Arginine Vasopressin-Containing Neurons of the Suprachiasmatic Nucleus Project to CSF

“…Specifically, paracrine signaling with a role for AVP, has been demonstrated in culture preparations of the SCN [ Honma, 2020 (see Fig. 5 ), Maywood et al, 2011 ; Tokuda et al, 2018 ]. Furthermore, SCN grafts encapsulated in a polymer material can restore circadian activity rhythms through volume transmission ( Silver et al, 1996 ).…”

“…Specifically, paracrine signaling with a role for AVP, has been demonstrated in culture preparations of the SCN (Honma, 2020 [see Fig. 5], Maywood et al, 2011, Tokuda et al, 2018.…”