The biological clock: the bodyguard of temporal homeostasis.
In order for any organism to function properly, it is crucial that it be table to control the timing of its biological functions. An internal biological clock, located, in mammals, in the suprachiasmatic nucleus of the hypothalamus (SCN), therefore carefully guards this temporal homeostasis by delivering its message of time throughout the body. In view of the large variety of body functions (behavioral, physiological, and endocrine) as well as the large variety in their preferred time of main activity along the light:dark cycle, it seems logical to envision different means of time distribution by the SCN. In the present review, we propose that even though it presents a unimodal circadian rhythm of general electrical and metabolic activity, the SCN seems to use several sorts of output connections that are active at different times along the light:dark cycle to control the rhythmic expression of different body functions. Although the SCN is suggested to use diffusion of synchronizing factors in the rhythmic control of behavioral functions, it also needs neuronal connections for the control of endocrine functions. The distribution of the time-of-day message to neuroendocrine systems is either directly onto endocrine neurons or via intermediate neurons located in specific SCN targets. In addition, the SCN uses its connections with the autonomic nervous system for spreading its time-of-day message, either by setting the sensitivity of endocrine glands (i.e., thyroid, adrenal, ovary) or by directly controlling an endocrine output (i.e., melatonin synthesis). Moreover, the SCN seems to use different neurotransmitters released at different times along the light:dark cycle for each of the different connection types presented. Clearly, the temporal homeostasis of endocrine functions results from a diverse set of biological clock outputs.