Alteraciones del ciclo circadiano en las enfermedades psiquiátricas: papel sincronizador de la melatonina en el ciclo sue？o-vigilia y la polaridad neuronal

circadian rhythms are oscillations of physiological functions. the period of their oscillation is about 24 h, and can be synchronized by environmental periodic signals as night-day cycle. the endogenous periodical changes depend on various structural elements of the circadian system which consists of the effectors, the secondary oscillators, the synchronizers and the circadian pacemaker. in mammalian species, the physiological function better understood respect their oscillation pattern are the synthesis and release of several hormones (i.e. cortisol and melatonin), the body temperature, the sleep-awake cycle, the locomotive activity, cell proliferation, neuronal activity among other rhythms. the suprachiasmatic nucleus is the main circadian pacemarker in mammals; its oscillation keeps the circadian system synchronized particularly with respect to the environment photo period. when light reaches the pigment melanopsin in ganglionar neurons in the retina, the photoperiod signal is sent to suprachiasmatic nucleus, and its postsinaptic neurons distributes the temporal signal to pheripheral oscillators by nervous or humoral pathways. among the oscillators, the pineal gland is a peripheral one modulated by suprachiasmatic nucleus. at night, the indolamine melatonin is synthesized and released from pinealocytes, and reaches other peripheral oscillators. melatonin interacts with membrane receptors on suprachiasmatic nucleus pacemarker neurons, reinforcing the signal of the photoperiod. in mammals, exogenous melatonin synchronizes several circadian rhythms including locomotive activity and melatonin release. when this indolamine is applied directly into the suprachiasmatic nucleus, it produces a phase advance of the endogenous melatonin peak and increases the amplitude of the oscillation. in humans, melatonin effect on the circadian system is evident because it changes the circadian rhythms phase in subjects with advanced sleep-phase syndrome, night workers or blind people.