In humans 9, 10 and rodents 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, several studies in LD cycles have demonstrated diurnal modulation of learning/memory performance on the Morris water maze task 11, 12, the novel location recognition task 13, novel object recognition task 14, 15, 16 and fear-related tasks 17, 18, 19, 20. In non-mammals, such as the zebrafish 5, fruit fly 6, cockroach 7 and Aplysia 8, circadian fluctuations in learning and/or memory have been observed under constant dark (DD) conditions, while their peaking times in the performance diverge among the organisms and/or the experimental paradigms. The circadian clocks regulate a variety of neural functions, including cognitive performance. In the mouse hippocampus, expression levels of clock genes show circadian variations, supporting the presence of the peripheral clock regulated by the SCN 3, 4. These clocks are regulated by neuronal and hormonal signals from the SCN 1, 2 and are thought to play key roles for physiological functions of the peripheral tissues. On the other hand, peripheral clocks exist in most peripheral tissues, including extra-SCN brain regions. In mammals, the master circadian clock located in the hypothalamic suprachiasmatic nucleus (SCN) is synchronized to the LD cycle and governs the behavioural rhythms. Physiological and behavioural rhythms with circadian periodicities are generated by cell autonomous circadian clocks that are synchronized to daily environmental changes such as the light–dark (LD) cycle.
0 Comments
Leave a Reply. |