Year of Publication

2007

Document Type

Dissertation

College

Agriculture

Department

Veterinary Science

First Advisor

Barry Fitzgerald

Abstract

The circadian system provides animals with a means to adapt internal physiology to the constantly changing environmental stimuli that exists on a rotating planet. Light information is translated into molecular timing mechanisms within individual pacemaker cells of the mammalian hypothalamic suprachiasmatic nucleus (SCN) via transcriptionaltranslational feedback loops. Humoral and neural outputs from this master clock result in circadian rhythms of physiology and behavior. The hierarchy of the circadian system involves SCN synchronization of cellular clocks within peripheral tissues so that differential transcriptional profiles in individual organs reflect their specific function. The first step to investigating equine circadian regulation was to identify and isolate the core components of the molecular clock in the horse. Successful isolation and sequencing of equine Bmal1, Per2, Cry1 and Clock cDNAs revealed high sequence homology with their human counterparts. Real Time RT-PCR assays were subsequently designed to quantitatively assess clock gene expression in equine peripheral tissues. Synchronization of equine fibroblasts revealed temporal profiles of clock gene expression identical to those of the SCN and peripheral tissues of other species. However, while clock gene expression varies over time in equine adipose tissue, there was no observable oscillation of clock gene transcripts in equine blood. Spurred by recent reports of immune-circadian interactions, this novel finding prompted an investigation of clock gene expression in equine blood during a systemic inflammatory response. The results demonstrated that acute inflammation upregulates Per2 and Bmal1 in equine blood. Subsequent experiments identified neutrophils as the source of this upregulation and highlighted exciting new immunecircadian interplay during an innate immune response. Finally, the effect of a 6-h phase advance of the light/dark cycle, mimicking an easterly transmeridian journey, on circadian melatonin and core body temperature rhythms was investigated. In contrast to the gradual adaptation observed in other species, these markers of equine circadian phase adapt immediately to a time zone transition. Combined, the results of these experiments highlight important interspecies differences in circadian regulation with practical implications regarding the potential impact of jet lag on equine athletes. Furthermore, the results underline the relevance of chronobiological investigation in a large mammalian species such as the horse.

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