[The hypothalamic suprachiasmatic nucleus and pineal gland in the circadian rhythmic organization of mammals].

The hypothalamic suprachiasmatic nucleus (SCN) is a most important circadian pacemaker, which controls physiological and behaviour rhythm in mammals. SCN owns a intrinsic rhythm itself, and is entrained by photoperiodic signal and some endogenous chemical substances. Melatonin (MEL) is secreted by pineal gland, which is regulated by SCN. MEL triggers the second and third message systems, and regulates SCN circadian activity through high affinity MEL receptor within SCN. This regulation is time-sensitive.