The rate and pattern of neuronal discharge in the suprachiasmatic nucleus (SCN) during developmental stages were studied and compared with those of the ventromedial (VMH) and the lateral hypothalamus (LHA) using rat hypothalamic slices. The firing rate of the SCN neurons was low on the 7th and 11th days; however, it dramatically increased by the 14th day to reach the adult rate, while firing rates of VMH and LHA neurons increased gradually with age. The discharge rate of neurons in the ventrolateral part of the SCN (VL-SCN) was higher than that of the dorsomedial SCN (DM-SCN) neurons in 14-, 21- and 70-100-day-old rats. Activity of the DM-SCN neurons on day 21 were unaffected by bilateral enucleation on the third day, while activity in the VL-SCN decreased; that of both parts was significantly decreased by a constant light schedule.
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