Biological Rhythms - Heat Stress and Animal Productivity - page 133

Biological Rhythms

Contents
1 Introduction .................................................... 138
2 Circadian Timekeeping Mechanisms ........... 140
2.1 Biological Clock ..................................... 140
3 Daily Rhythms ................................................ 141
4 Annual Rhythms ............................................ 141
5

13 Circadian Rhythm of Cardiovascular

Parameters ...................................................... 158

14 Conclusions ..................................................... 158

References ............................................................... 159

Abstract
A biological rhythm is a cyclical change in the
biological or chemical function of body. The
biological rhythms are endogenously con-
trolled by self-contained circadian clocks.
The daily alternation of light and dark is the
main regulatory factor of the pineal hormone
melatonin. Alterations in long-term lighting
conditions during the year result in metabolic
and behavioural changes in most living
beings. The suprachiasmatic nuclei (SCN) in
the hypothalamus are regarded as the anatomi-
cal loci of the circadian pacemaker. The most
important synchronising trigger of circadian
rhythm is environmental light/dark (LD) cycle.
The circadian pacemaker in the mammalian
SCN consists of a double complex of circa-
dian genes (Per1/Cry1 and Per2/Cry2), which
is able to maintain the endogenous rhythm.
Melatonin has the ability to entrain biological
rhythms and has important effects on biological
function like reproduction of many mammals
and livestock. The daily rhythmicity of mela-
tonin is considered to be a very reliable phase
marker of the endogenous timing system.
The results on the patterns of cortisol levels in

Functions of Sleep .......................................... 142

5.1 Evolution of Sleep................................... 142

6 Pineal Gland and Melatonin ......................... 144

6.1 Synthesis and Metabolism
of Melatonin............................................ 145
6.2 Melatonin Receptors ............................... 146
6.3 Daily Rhythm of Melatonin .................... 146
6.4 Annual Rhythm of Melatonin ................. 148
7 Cortisol ............................................................ 148
7.1 Daily Rhythm of Cortisol........................ 149
7.2 Annual Rhythm of Cortisol..................... 151
7.3 Altered Circadian HPA
Activity and Rhythmicity........................ 151
8 Leptin .............................................................. 152
8.1 Daily Rhythm of Leptin .......................... 153
8.2 Annual Rhythm of Leptin ....................... 153
9 Lipid Metabolism ........................................... 154

9.1 Lipid Metabolism in Ruminants ............. 154

9.2 Daily Rhythms of Free Fatty

Acids (FFA) and Glycerol....................... 154

9.3 Seasonal Variation of Lipid

Metabolism ............................................. 155

10 Feeding-Related Circadian Rhythms ........... 155

11 Circadian Clocks as Mediators

of the Homeorhetic Response ........................ 156

12 Oxidative Stress .............................................. 157