The circadian rhythms are characterized by the manifestation in a cycle of 24 hours, such as the physiological rhythms of body temperature, hormones secretation, eating behavior, sleep-wake cycles, activity and relaxation, and performance rhythms in memory, sensory and motor perceptions. The hypothalamic's suprachiasmatic nucleus is considered the endogenous circadian oscillator responsible for the generation of these oscillations. The ability of SCN to generate circadian oscillations lies in the autonomous expression of so-called clock genes in their cells. The neurotransmitter dopamine, involved in the potential implications of Parkinson's disease, is one of the neurotransmitters that may influence the expression of some clock genes. The dopamine production can be found in the neural centers as the retrorubral field, substantia nigra pars compacta and ventral tegmental area. The substantia nigra pars compacta takes part in several circuits inside the nervous system, among them the motor circuit (nigrostriatal pathway), oculomotor, dorsolateral prefrontal (cognitive functions), orbitofrontal lateral (cognitive) and limbic (emotional and visceral functions). Although the role of dopamine in motor and non-motor functions may be well known, the role of the circadian system in the neural population of substantia nigra pars compacta has not been fully elucidated. Our hypothesis is that light-dark cycle could influence the dopamine-producers neurons of the substantia nigra, which probably would influence its functions as motor coordination, emotional and behavioral modulation. In this work, it is intended to explore neurochemical and neuroanatomic aspects of substantia nigra pars compacta of the diurnal primate Sapajus apella in different times of the day, as well as discuss potential implications from the investigation of patterns and temporal differences in Per2 clock protein expression in this area by the immunohistochemistry technique. The presence of clock genes in areas classically unrelated to circadian control, as well as the possibility of changes in their own expression in physiological conditions, expands the traditional forms of discuss the substantia nigra pars compacta functions and leads us to reconsider the physiological, behavioral and psychopathological importance of these zone-especific rhythms in different brain areas.
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