Circadian Rhythm, Hypothalamo-Pituitary Adrenal Axis, and Immunity: Physiological and Pathological Examples

Abstract

All living organisms; from single-celled microorganisms to humans, they have to adapt to changing environmental conditions to maintain their survival processes. Circadian rhythm is one of the most important mechanism that associated with this adaptation processes. There are biological clocks in the body, which are related to the circadian rhythm and have a hierarchical organization. The master circadian clock is located in the suprachiasmatic nucleus (SCN) of hypothalamus. SCN maintain body rhythms in synchronous with the light-dark cycle in the external environment. There are also peripheral oscillators that work in coordination with SCN. Neurological, endocrinological, and immunological functions in the body are under the influence of circadian and seasonal rhythms. Melatonin and cortisol (corticosterone in animals) are among the most important hormones that show circadian rhythm in the body. The body adapts to daily and seasonal changes with biological rhythms regulated by biological clocks. It is well known that the immune system is affected by the external environment. Changes in endocrine system, hypothalaomo-pituitary adrenal (HPA) axis, and immune system are marked, especially depending on the seasonal changes. Therefore, the immune system has close relationship with the circadian rhythm. Understanding relationship between physiological regulation of the circadian rhythm, HPA axis and immune activity is important for to keep our body in healthy conditions and struggle with the diseases as well. In current review, the interaction and relationship of genes and proteins related to the circadian rhythm with HPA axis and immune system parameters are discussed with both physiological and pathological examples.