Rhythms, Sleep and Homeostasis

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Abstract

Mammals display 24 hour oscillations in physiology and behaviour, ranging from blood pressure and body temperature rhythms to eating and drinking patterns. These rhythms are a product of an internal timing system that has evolved to enable organisms to align themselves with the 24 h nature of the external environment. In mammals these 24 h patterns, circadian rhythms, are primarily generated by the suprachiasmatic nuclei (SCN), a bilateral structure in the ventral hypothalamus. The SCN is synchronised by the external light:dark cycle as well as from non-photic stimuli such as periodic arousal, exercise and feeding. However, emerging evidence is demonstrating the importance of systemic and local oscillators separate to the SCN in regulating circadian physiology and behaviour. Sleep and energy balance involve interacting processes and neural substrates. When rodents can only access food in their inactive phase, they demonstrate disrupted sleep-wake behaviour patterns in order to robustly anticipate misaligned food. This process is driven by a food entrainable oscillator outside of SCN control, however, little is known about what drives this anticipatory activity and how it effects sleep homeostasis. Here, I demonstrate that during daytime restricted feeding, sleep homeostasis is generally maintained but sleep drive is reduced. This hints at an evolutionary adaption ensuring animals are more likely to wake when food is only available out of sync with typical behaviour patterns. Homeostatic behaviours involved in fluid and energy balance display circadian rhythmicity, yet little is known how this occurs. Through my studies, I discovered novel oscillators in the forebrain and hindbrain in the sensory circumventricular organs and the nucleus of the solitary tract; areas well-known for regulating fluid and energy balance. I demonstrated the properties and rhythm maintenance mechanisms of these extra-SCN oscillators in fluid balance areas. Furthermore, my results provide hints at the functional characteristics of the hindbrain oscillators in daily regulation of responses to metabolic signals and temporal changes in permeability to circulating factors. Collectively, these results reveal that systemic and local oscillators outside of the SCN are also important in regulating homeostatic systems and behaviours that display temporal variation.

Keyword(s)

Circadian; Energy balance; Extra-scn oscillators; Fluid balance; Food entrainment; Homeostasis; Metabolism; Sleep

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