Abstract

The circadian time-keeping system is the physiological system that allows predictive adaptation of individuals to the reproducible 24-h day/night alternations of our planet. This system generates the 24-h rhythms (circadian rhythms) present in hormones (melatonin, prolactin, gonadotropins, testosterone, cortisol, TSH, etc.), and in cardiovascular (blood pressure, heart rate), biophysical (temperature) and behavioral (activity/rest, sleep/wake) functions. Conceptually, the circadian time-keeping system is described as formed by a biological clock receiving inputs from the environment and sending commanding outputs to the peripheral clocks involved in the generation of overt circadian rhythms. In mammals, the master clock resides in the suprachiasmatic nucleus (SCN), located bilaterally over the optic chiasm, in the hypothalamus. The network of neurons forming the SCN oscillate with a period of 24-h. The molecular events determining their oscillatory function involve the expression of genes encoding for at least 4 types of proteins that interact in a transcription/transduction feedback loop. In the animal, signals from the retina, conveyed by a glutaminergic monosynaptic pathway, the retinohypothalamic tract, act upon some of these genes, shifting the phase of the oscillation of the neurons and entraining them to the light/dark cycle. The emerging concept is that neural/humoral output signals from the SCN impinge upon peripheral clocks located in other areas of the brain, pituitary, liver, kidney, adrenal, fibroblasts, and probably every cell of the body resulting in overt circadian rhythms in integrated physiological functions. In this chapter we review some of the molecular mechanisms involved in SCN circadian function, and its relationship with the physiology of reproduction.