A biological clock in the brain--the suprachiasmatic nucleus of the hypothalamus

Abstract

Rhythms of 24 hr periodicity underlie most aspects of daily life, although we are rarely aware of its presence. This circadian rhythmicity is a consequence of evolution of the living organism, which takes place on the earth for millions of years. Some of these rhythms are actually derived from an endogenous oscillating mechanism whose inherent period is not exactly 24 hrs. Therefore, it must be constantly adjusted to the solar cycle of day and night. It has been firmly established that the suprachiasmatic nucleus (SCN) of the hypothalamus is the highest center of these endogenous circadian rhythms. Lesions of the SCN eliminates most behavioral and physiological rhythms and actvities of the SCN exhibited a dramatic change between day and night. Furthermore, rhythms of SCN neurons persist under in vitro conditions. The cellular or molecular mechanism for these neurons to generate a 24 hr rhythm is unclear at present. The SCN contains many neuropeptides and some of them display circadian rhythms, while others do not. We found that this distinct behavior of peptides in the SCN was due to different regulation of MRNA or gene transcription. This observation indicate the possibility that genes are involved in the generation of circadian rhythms in the SCN.