Developmental characteristics in the daily rhythm of norepinephrine concentration within rabbit brainstem regions.

To examine the development of daily variations in norepinephrine levels, norepinephrine concentrations were measured within five distinct brainstem regions in 3-day-old, 21-day-old, and adult rabbits at 6-h intervals throughout the day. Norepinephrine was measured by radioenzymatic assay, and norepinephrine concentration was expressed relative to wet tissue weight. The data suggest that daily variations for norepinephrine concentrations are established by the third day of life. In the brainstem as a whole, there was an early nocturnal peak (2130 hours) for 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 hours) peak. These gross daily variations constitute the sum of distinct region-specific patterns in the development of daily variations in norepinephrine concentration. Norepinephrine is involved in cardiorespiratory regulation and in the regulation of sleep/wake cycles. The observed developmental patterns may relate to the maturation and integration of these physiologic processes.