Exploring the role of locomotor sensitization in the circadian food entrainment pathway

Food entrainment is the internal mechanism whereby the phase and period of circadian clock genes comes under the control of daily scheduled food availability. Food entrainment allows the body to efficiently realign the internal timing of behavioral and physiological functions such that they anticipate food intake. Food entrainment can occur with or without caloric restriction, as seen with daily schedules of restricted feeding (RF) or restricted treat (RT) that restrict food or treat intake to a single feeding time. However, the extent of clock gene control is more pronounced with caloric restriction, highlighting the role of energy balance in regulating clock genes. Recent studies have implicated dopamine (DA) to be involved in food entrainment and caloric restriction is known to affect dopaminergic pathways to enhance locomotor activity. Since food entrainment results in the development of a distinct behavioral component, called food anticipatory activity (FAA), we examined the role of locomotor sensitization (LS) in food entrainment by 1) observing whether amphetamine (AMPH) sensitization results in enhanced locomotor output of FAA and 2) measuring LS of circadian and non-circadian feeding paradigms to an acute injection of AMPH (AMPH cross-sensitization). Unexpectedly, AMPH sensitization did not show enhancement of FAA. On the contrary, LS did develop with sufficient exposure to RF. LS was present after 2 weeks of RF, but not after 1, 3 or 7 days into RF. When food was returned and rats regain their original body weight at 10–15 days post-RF, LS remained present. LS did not develop to RT, nor to feedings of a non-circadian schedule, e.g. variable restricted feeding (VRF) or variable RT (VRT). Further, when RF was timed to the dark period, LS was observed only when tested at night; RF timed to the light period resulted in LS that was present during day and night. Taken together our results show that LS develops with food entrainment to RF, an effect that is dependent on the chronicity and circadian phase of RF but independent of body weight. Given that LS involves reorganization of DA-regulated motor circuitry, our work provides indirect support for the role of DA in the food entrainment pathway of RF. The findings also suggest differences in neuronal pathways involved in LS from AMPH sensitization and LS from RF.

食物节律同步（food entrainment）是指昼夜节律时钟基因的时相与周期受每日定时食物供给调控的内在生物学机制。该机制可使机体高效重调行为与生理功能的内在时序，使其提前预判食物摄入时刻。食物节律同步可在有或无热量限制的条件下发生，典型案例为将食物或零食摄入限制在单次饲喂时段的每日限制性饲喂（RF）或限制性零食饲喂（RT）方案。不过，在热量限制条件下，昼夜节律时钟基因的调控效应更为显著，凸显了能量平衡在调控昼夜节律时钟基因中的作用。已有研究表明多巴胺（DA）参与食物节律同步过程，且热量限制可调控多巴胺能通路以增强运动活性。由于食物节律同步会催生一类独特的行为特征——食物预期活动（FAA），本研究通过两项实验探究运动敏化（LS）在食物节律同步中的作用：1）观察苯丙胺（AMPH）敏化是否会增强FAA的运动输出；2）检测昼夜节律与非昼夜节律饲喂范式下，机体对急性苯丙胺注射的运动敏化水平（即苯丙胺交叉敏化，AMPH cross-sensitization）。出乎预料的是，苯丙胺敏化并未增强FAA。与之相反，经过足够时长的限制性饲喂暴露后，运动敏化确实得以形成：在实施限制性饲喂2周后可检测到运动敏化，但在饲喂1、3或7天时则未出现。当撤除限制性饲喂并让大鼠在10~15天内恢复至初始体重后，运动敏化依然存在。运动敏化并未在限制性零食饲喂或非昼夜节律饲喂方案（例如可变限制性饲喂VRF或可变限制性零食饲喂VRT）中形成。此外，若将限制性饲喂安排在黑暗时段，仅在夜间测试时可检测到运动敏化；若将饲喂安排在光照时段，则无论昼夜测试均可检测到运动敏化。综上，本研究结果表明，运动敏化会随限制性饲喂诱导的食物节律同步而形成，该效应依赖于限制性饲喂的持续时长与时相，却与体重无关。鉴于运动敏化涉及多巴胺调控的运动环路重构，本研究间接支持了多巴胺在限制性饲喂诱导的食物节律同步通路中的作用。本研究结果还提示，苯丙胺敏化诱导的运动敏化与限制性饲喂诱导的运动敏化所涉及的神经元通路存在差异。