DEVELOPMENTAL CHARACTERISTICS IN THE DAILY RHYTHM OF NOREPINEPHRINE CONCENTRATION WITHIN RABBIT BRAIN-STEM REGIONS

To examine the development of daily variations in norepinephrine level s, norepinephrine concentrations were measured within five distinct br ainstem regions in 3-day-old, 21-day-old, and adult rabbits at 6-h int ervals throughout the day. Norepinephrine was measured by radioenzymat ic assay, and norepinephrine concentration was expressed relative to w et tissue weight. The data suggest that daily variations for norepinep hrine concentrations are established by the third day of life. In the brainstem as a whole, there was an early nocturnal peak (2130 hours) f or 3-day-old animals in contrast to a late nocturnal peak (0330 hours) for 21-day-old animals. Adult animals showed a late diurnal (1530 hou rs) peak. These gross daily variations constitute the sum of distinct region-specific patterns in the development of daily variations in nor epinephrine concentration. Norepinephrine is involved in cardiorespira tory regulation and in the regulation of sleep/wake cycles. The observ ed developmental patterns may relate to the maturation and integration of these physiologic processes.