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

To examine the development of daily rhythms in serotonin levels, we measured serotonin concentration within five distinct brainstem regions in 3-day-old, 21-day-old and adult rabbits at 09.30, 15.30, 21.30 and 03.30. Serotonin was measured by radioenzymatic assay in the superior and inferior colliculi, rostral pons, caudal pons and medulla, and serotonin concentration was expressed relative to wet tissue weight. In addition to defining the presence of a daily rhythm in serotonin concentration in whole brainstem, we were interested in identifying brainstem region-specific differences in this rhythm. Our data suggest that daily rhythms for serotonin are established by 3 days of life. Analysis of gross brainstem rhythm data suggests a nocturnal peak in serotonin concentration in 3-day-old and adult animals in contrast to a diurnal peak for 21-day-old animals. These gross rhythms reflect the net sum of distinct region-specific patterns in serotonin concentration. Analysis by region reflects a region-specific ontogeny in the development of daily rhythms in serotonin concentration. Serotonin is involved in the initiation of sleep. The observed developmental patterns may relate to the maturation and integration of sleep/wake states.