Department of Psychology, Simon Fraser University, Burnaby, BC, V5A1S6, Canada.
Sci Rep. 2024 Sep 16;14(1):21566. doi: 10.1038/s41598-024-72548-z.
The suprachiasmatic nucleus (SCN) contains a population of cell-autonomous circadian oscillators essential for entrainment to daily light-dark (LD) cycles. Synchrony among SCN oscillators is modified by photoperiod and determines functional properties of SCN clock cycling, including its amplitude, phase angle of entrainment, and free running periodicity (τ). For many species, encoding of daylength in SCN output is critical for seasonal regulation of metabolism and reproduction. C57BL/6 mice do not show seasonality in these functions, yet do show photoperiodic modulation of SCN clock output. The significance of this for brain systems and functions downstream from the SCN in these species is largely unexplored. C57BL/6 mice housed in a long-day photoperiod have been reported to perform better on tests of object, spatial and fear memory compared to mice in a standard 12 h photoperiod. We previously reported that encoding of photoperiod in SCN output, evident in τ in constant dark (DD), can be blocked by limiting food access to a 4 h mealtime in the light period. To determine whether this might also block the effect of long days on memory, mice entrained to 18 h:6 h (L18) or 6 h:18 h (L6) LD cycles were tested for 24 h object memory (novel object preference, NOP) and spatial working memory (Y-maze spontaneous alternation, SA), at 4 times of day, first with food available ad libitum and then during weeks 5-8 of daytime restricted feeding. Photoperiod modified τ as expected, but did not affect performance on the NOP and SA tests, either before or during restricted feeding. NOP performance did improve in the restricted feeding condition in both photoperiods, eliminating a weak time of day effect evident with food available ad-libitum. These results highlight benefits of restricted feeding on cognitive function, and suggest a dose-response relationship between photoperiod and memory, with no benefits at daylengths up to 18 h.
视交叉上核（SCN）包含一群自主的生物钟振荡器，对于与每日光暗（LD）周期的同步至关重要。SCN 振荡器之间的同步性受光周期的影响，并决定了 SCN 时钟周期的功能特性，包括其幅度、同步的相位角和自由运行的周期性（τ）。对于许多物种来说，SCN 输出中对日照长度的编码对于代谢和繁殖的季节性调节至关重要。C57BL/6 小鼠在这些功能上没有表现出季节性，但确实表现出 SCN 时钟输出的光周期调制。对于这些物种的 SCN 下游的大脑系统和功能，这一现象的意义在很大程度上尚未得到探索。据报道，在长日照光周期下饲养的 C57BL/6 小鼠在物体、空间和恐惧记忆测试中的表现优于在标准 12 小时光周期下饲养的小鼠。我们之前报道过，在持续黑暗（DD）中，SCN 输出中的光周期编码可以通过将光照期的 4 小时进餐时间限制来阻断，这在 τ 中是明显的。为了确定这是否也会阻断长日照对记忆的影响，我们将小鼠分别适应 18 小时：6 小时（L18）或 6 小时：18 小时（L6）LD 周期，并在 4 个时间点进行 24 小时物体记忆（新物体偏好，NOP）和空间工作记忆（Y 迷宫自发交替，SA）测试，首先是在自由进食的情况下，然后是在白天限时进食的第 5-8 周。光周期如预期的那样改变了 τ，但无论是在限时进食之前还是期间，都没有影响 NOP 和 SA 测试的表现。在两种光周期下，在限时进食条件下，NOP 的表现都有所提高，消除了在自由进食时明显的时间效应。这些结果强调了限时进食对认知功能的益处，并表明光周期与记忆之间存在剂量反应关系，在长达 18 小时的光照长度下没有益处。
