Development of Circadian Rhythmicity and Light Responsiveness in the Rat Suprachiasmatic Nucleus: A Study Using the 2-deoxy[1-14C]glucose Method

The suprachiasmatic nucleus (SCN) of the hypothalamus is thought to play a critical role in circadian rhythm generation and entrainment to the light/dark cycle. In adult rats, the SCN shows a circadian rhythm in metabolic activity level as indicated by 2-deoxy[1-14C]glucose uptake. In the present study, the development of this rhythm was investigated. No diurnal difference in uptake was evident in fetal rats 1-2 days before birth. A significant diurnal difference in SCN 2-deoxy-glucose uptake was present on postnatal day 1, even in rats kept in constant darkness. By day 1, exposure to light at night increased the SCN metabolic levels. According to previous studies, on day 1 the SCN is poorly developed and contains few synapses. At this time the retinohypothalamic tract has not yet developed. We found progressive functional maturation of the SCN through day 21, when the rhythm and light responsiveness resembled those of adult rats.