The Circadian Physiology: Implications in Livestock Health

Abstract

Circadian rhythms exist in almost all types of cells in mammals. Thousands of genes exhibit approximately 24 h oscillations in their expression levels, making the circadian clock a crucial regulator of their normal functioning. In this regard, environmental factors to which internal physiological processes are synchronized (e.g., nutrition, feeding/eating patterns, timing and light exposure), become critical to optimize animal physiology, both by managing energy use and by realigning the incompatible processes. Once the circadian clock is disrupted, animals will face the increased risks of diseases, especially metabolic phenotypes. However, little is known about the molecular components of these clocks in domestic species and by which they respond to external stimuli. Here we review evidence for rhythmic control of livestock production and summarize the associated physiological functions, and the molecular mechanisms of the circadian regulation in pig, sheep and cattle. Identification of environmental and physiological inputs that affect circadian gene expressions will help development of novel targets and the corresponding approaches to optimize production efficiency in farm animals.

Keywords: circadian physiology; clock gene; lipid metabolism; livestock; melatonin.

Figure 1

The circadian clock actions on physiology and health of livestock. Circadian rhythm plays a critical role in livestock’s growth performance, reproduction, metabolism, and the quality of their products. The core mechanism, the transcription–translation–feedback-loop (TTFL) drives the diurnal oscillations to maintain circadian activities. In which CLOCK and BMAL1 directly control the transcription of PER and CRY genes. Whilst PER and CRY proteins can repress their modulator, CLOCK and BMAL1-stimulated transcription occurs in a negative feedback loop, allowing the cycle to begin anew when PER and CRY actions are turned-over. Importantly, REV-ERBs and RORs dominate the BMAL1 gene expression, are the primary players of the interlocked loop. In particular, hormones reach a peak level in the morning. Synthesis and release of melatonin is stimulated in the dark at night, while it is suppressed by light during the day. Loss of function of the circadian genes results in behavioral arrhythmicity, the disruption of the autoregulatory loop, and short period-length phenotypes.