Aging differentially affects the re-entrainment response of central and peripheral circadian oscillators

Abstract

Aging produces a decline in the amplitude and precision of 24 h behavioral, endocrine, and metabolic rhythms, which are regulated in mammals by a central circadian pacemaker within the suprachiasmatic nucleus (SCN) and local oscillators in peripheral tissues. Disruption of the circadian system, as experienced during transmeridian travel, can lead to adverse health consequences, particularly in the elderly. To test the hypothesis that age-related changes in the response to simulated jet lag will reflect altered circadian function, we examined re-entrainment of central and peripheral oscillators from young and old PER2::luciferase mice. As in previous studies, locomotor activity rhythms in older mice required more days to re-entrain following a shift than younger mice. At the tissue level, effects of age on baseline entrainment were evident, with older mice displaying earlier phases for the majority of peripheral oscillators studied and later phases for cells within most SCN subregions. Following a 6 h advance of the light:dark cycle, old mice displayed slower rates of re-entrainment for peripheral tissues but a larger, more rapid SCN response compared to younger mice. Thus, aging alters the circadian timing system in a manner that differentially affects the re-entrainment responses of central and peripheral circadian clocks. This pattern of results suggests that a major consequence of aging is a decrease in pacemaker amplitude, which would slow re-entrainment of peripheral oscillators and reduce SCN resistance to external perturbation.

Figure 1.

Effects of aging on locomotor activity rhythms under baseline conditions and during simulated jet lag. A, Representative double-plotted actograms illustrating wheel-running activity of a young and old mouse that re-entrained in 4 and 8 days, respectively. White and black bars above and below each actogram depict lighting conditions in place before and after the 6 h advance, respectively. Gray shading in each actogram indicates the presence of darkness. B, Measures of baseline activity rhythms and rate of re-entrainment in young (n = 9) and old (n = 7) mice. Old mice displayed more variability in the time of activity onset and a longer latency to re-entrain following the 6 h LD cycle advance, but they did not differ in phase angle of entrainment or activity duration. Student's t test, *p < 0.05.

Figure 2.

Effects of age on PER2::LUC rhythms of central and peripheral oscillators under baseline conditions (Day0). A, Representative time series of PER2::LUC rhythms displayed by SCN and peripheral oscillators from young and old mice. Time series are detrended and corrected for ZT start time, with the start of each recording corresponding to ZT12 on the day of culture. B, Peak time of PER2::LUC rhythms for esophagus (ESO), lung (LNG), spleen (SPLN), thymus (THY), and the SCN field rhythm in young and old mice during baseline. Dashed lines connect comparable structures in young and old mice to highlight the effects of age. Relative to young mice, old mice displayed a significantly earlier peak time for lung and thymus and a trend for an earlier peak time for the esophagus. The baseline phase of entrainment of the SCN field rhythm and spleen did not differ by age. C, Average phase maps illustrate the spatial location of each SCN subregion analyzed: dorsal SCN (d, dSCN), lateral SCN (l, lSCN), medial SCN (m, mSCN), and ventral SCN (v, vSCN). D, Representative time series of PER2::LUC rhythms displayed by SCN neurons from young and old mice, plotted as in A. E, Peak time of PER2::LUC rhythms displayed by cell-like ROIs within SCN subregions from young and old mice during baseline, plotted as in B. Number of cultures per group: ESO-Young = 12, ESO-Old = 8; LNG-Young = 13, LNG-Old = 8; SCN-Young = 4, SCN-Old = 4; SPLN-Young = 12, SPLN-Old = 8; THY-Young = 14, THY-Old = 8. Results of post hoc LSM Contrasts are illustrated adjacent to each symbol within the legend: ***p < 0.0005, **p < 0.001, p < 0.01, # p < 0.05.

Figure 3.

Re-entrainment of PER2::LUC rhythms for the SCN field rhythm, esophagus (ESO), lung (LNG), spleen (SPLN), and thymus (THY) from young mice (black symbols) and old mice (gray symbols). For each age group, peak time of each tissue is normalized to that displayed on Day0 (set to 0). Dashed vertical line illustrates target phase after the 6 h LD cycle advance. Number of cultures per group: ESO-Young = 12 (Day0), 16 (Day1), 6 (Day3), 16 (Day5), 8 (Day8), 7 (Day14); ESO-Old = 8, 10, 6, 11, 6, 4; LNG-Young = 13, 18, 6, 17, 9, 12; LNG-Old = 8, 10, 7, 12, 7, 8; SCN-Young = 4, 2, 2, 4, 3, 3; SCN-Old = 4, 4, 3, 4, 3, 4; SPLN-Young = 12, 16, 6, 17, 9, 12; SPLN-Old = 8, 10, 6, 3, 6, 7; THY-Young = 14, 16, 5, 16, 9, 10; THY-Old = 8, 10, 6, 12, 7, 7. LSM contrasts, *p < 0.01.

Figure 4.

Re-entrainment of the SCN from young mice (black symbols) and old mice (gray symbols). A, Average phase maps illustrating age-related changes in SCN spatiotemporal organization during re-entrainment. B, Re-entrainment PER2::LUC rhythms of cell-like ROIs pooled across all subregions (“All”) and within each individual subregion. For each age group, peak time is normalized to that displayed on Day0 (set to 0). C, Standard deviation of PER2::LUC peak time during re-entrainment for cell-like ROIs pooled across all subregions (“All”) and within each individual subregion. LSM contrasts, *p < 0.005. ^#p = 0.012.

Figure 5.

Re-entrainment responses of peripheral tissues during the first (filled symbols) and eighth (open symbols) 6 h LD cycle advance. Number of cultures per group: esophagus (ESO)-Young = 12 (Shift1-Day0), 16 (Shift1-Day1), 16 (Shift1-Day5), 4 (Shift8-Day0), 7 (Shift8-Day1), 4 (Shift8-Day5); ESO-Old = 8, 10, 11, 5, 6, 5; lung (LNG)-Young = 13, 18, 17, 4, 6, 4; LNG-Old = 8, 10, 12, 6, 7, 5; spleen (SPLN)-Young = 12, 16, 17, 2, 4, 2; SPLN-Old = 8, 10, 13, 6, 6, 5; thymus (THY)-Young = 14, 16, 16, 4, 6, 3; THY-Old = 8, 10, 12, 6, 7, 5. LSM contrasts, *p < 0.01.

Figure 6.

Aging differentially affects rate of re-entrainment for central and peripheral oscillators. To assess rate of re-entrainment, the normalized phase shift displayed on Day5 was divided by the number of days after the 6 h LD cycle advance. A, Rate of re-entrainment for peripheral tissues after the first shift of the LD cycle. B, Rate of re-entrainment for peripheral tissues after the eighth shift of the LD cycle. C, Rate of re-entrainment for the SCN field rhythm, cell-like ROIs pooled across all SCN subregions (“All”), and within each individual SCN subregion after the first shift of the LD cycle. Student's t test, ***p < 0.0001, **p < 0.005, *p < 0.05. ESO, Esophagus; LNG, lung; SPLN, spleen; THY, thymus.