Reduced Plasticity in Coupling Strength in the Aging SCN Clock as Revealed by Kuramoto Modeling.

The mammalian circadian clock is located in the suprachiasmatic nucleus (SCN) and consists of a network of coupled neurons, which are entrained to the environmental light-dark cycle. The phase coherence of the neurons is plastic and driven by the duration of daylight. With aging, the capacity to behaviorally adapt to seasonal changes in photoperiod reduces. The mechanisms underlying photoperiodic adaptation are largely unknown, but are important to unravel for the development of novel interventions to improve the quality of life of the elderly. We analyzed the phase coherence of single-cell PERIOD2::LUCIFERASE (PER2::LUC) expression rhythms in the SCN of young and old mice entrained to either long or short photoperiod. The phase coherence was used as input to a 2-community noisy Kuramoto model to estimate the coupling strength between and within neuronal subpopulations. The model revealed a correlation between coupling strength and photoperiod-induced changes in the phase relationship among neurons, suggesting a functional link. We found that the SCN of young mice adapts in coupling strength over a large range, with weak coupling in long photoperiod (LP) and strong coupling in short photoperiod (SP). In aged mice, we also found weak coupling in LP, but a reduced capacity to reach strong coupling in SP. The inability to respond with an increase in coupling strength suggests that manipulation of photoperiod is not a suitable strategy to enhance clock function with aging. We conclude that the inability of aged mice to reach strong coupling contributes to deficits in behavioral adaptation to seasonal changes in photoperiod.

Differential Phase Arrangement of Cellular Clocks along the Tonotopic Axis of the Mouse Cochlea Ex Vivo.

Topological distributions of individual cellular clocks have not been demonstrated in peripheral organs. The cochlea displays circadian patterns of core clock gene expression [1, 2]. PER2 protein is expressed in the hair cells and spiral ganglion neurons of the cochlea in the spiral ganglion neurons [1]. To investigate the topological organization of cellular oscillators in the cochlea, we recorded circadian rhythms from mouse cochlear explants using highly sensitive real-time tracking of PER2::LUC bioluminescence. Here, we show cell-autonomous and self-sustained oscillations originating from hair cells and spiral ganglion neurons. Multi-phased cellular clocks were arranged along the length of the cochlea with oscillations initiating at the apex (low-frequency region) and traveling toward the base (high-frequency region). Phase differences of 3 hr were found between cellular oscillators in the apical and middle regions and from isolated individual cochlear regions, indicating that cellular networks organize the rhythms along the tonotopic axis. This is the first demonstration of a spatiotemporal arrangement of circadian clocks at the cellular level in a peripheral organ. Cochlear rhythms were disrupted in the presence of either voltage-gated potassium channel blocker (TEA) or extracellular calcium chelator (BAPTA), demonstrating that multiple types of ion channels contribute to the maintenance of coherent rhythms. In contrast, preventing action potentials with tetrodotoxin (TTX) or interfering with cell-to-cell communication the broad-spectrum gap junction blocker (CBX [carbenoxolone]) had no influence on cochlear rhythms. These findings highlight a dynamic regulation and longitudinal distribution of cellular clocks in the cochlea.