Most living organisms have a circadian timing system adapted to optimize the daily rhythm of exposure to the environment. This circadian system modulates several behavioral and physiological processes, including the response to natural and drug rewards. Food is the most potent natural reward across species. Food-seeking is known to be mediated by dopaminergic and serotonergic transmission in cortico-limbic pathways. In the present work, we show evidence of a circadian modulation of motivation for food reward in young (4-months old) and aged (over 1.5 years old) C57BL/6 mice. Motivation was assayed through the progressive ratio (PR) schedule. Mice under a 12:12 light/dark (LD) cycle exhibited a diurnal rhythm in motivation, becoming more motivated during the night, coincident with their active phase. This rhythm was also evident under constant dark conditions, indicating the endogenous nature of this modulation. However, circadian arrhythmicity induced by chronic exposure to constant light conditions impaired the performance in the task causing low motivation levels. Furthermore, the day/night difference in motivation was also evident even without caloric restriction when using a palatable reward. All these results were found to be unaffected by aging. Taken together, our results indicate that motivation for food reward is regulated in a circadian manner, independent of the nutritional status and the nature of the reward, and that this rhythmic modulation is not affected by aging. These results may contribute to improve treatment related to psychiatric disorders or drugs of abuse, taking into account potential mechanisms of circadian modulation of motivational states.

中文翻译：

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小鼠的昼夜节律调节。

大多数活生物体都有一个昼夜节律系统，可以优化暴露在环境中的日常节奏。这种昼夜节律系统可调节多种行为和生理过程，包括对自然和药物奖励的反应。食物是所有物种中最有效的自然奖励。已知食物的摄取是通过皮质-边缘途径中的多巴胺能和5-羟色胺能的传递介导的。在目前的工作中，我们显示了年轻（4个月大）和年龄较大（1.5岁以上）C57BL / 6小鼠的食物奖励动机的昼夜节律调节。通过累进比率（PR）时间表来分析动机。在12:12的明/暗（LD）周期下的小鼠表现出动力的昼夜节律，在夜间变得更加活跃，与它们的活跃期相吻合。在恒定的黑暗条件下，这种节奏也很明显，表明这种调节的内源性。但是，长期暴露于恒定光照条件下引起的昼夜节律性心律失常会损害任务的执行能力，从而导致较低的动机水平。此外，当使用可口的奖励时，即使没有热量限制，白天/晚上的动机差异也很明显。发现所有这些结果均不受老化的影响。两者合计，我们的结果表明，食物奖励的动机以昼夜节律的方式进行调节，与营养状况和奖励的性质无关，并且这种节律性调节不受衰老的影响。这些结果可能有助于改善与精神疾病或滥用药物有关的治疗，