Development of daily variations in methionine enkephalin within rabbit brainstem regions

To examine the development of daily variations in methionine enkephalin, we measured methionine enkephalin 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 h. Methionine enkephalin was measured by radioimmunoassay and methionine enkephalin concentration was expressed relative to wet tissue weight. In addition to defining the presence of a daily variation in methionine enkephalin concentration in the whole brainstem, we were interested in identifying brainstem region specific differences in this daily variation. Our data suggest that daily variations for methionine enkephalin are established by three days of life. Analysis of gross brainstem daily variation data suggests a nocturnal peak in methionine enkephalin concentration for 3-day-old animals in contrast to a diurnal peak for 21-day-old animals. Adult animals, showed a biphasic pattern. These gross daily variations reflect the net sum of distinct region specific patterns in methionine enkephalin concentration. Analysis by region reflects a region specific ontogeny in the development of daily variations for methionine enkephalin. Our data also suggest a caudal-rostral progression in the establishment of daily rhythms. Methionine enkephalin is involved in nociception and cardiorespiratory regulation. The observed developmental patterns may relate to the maturation and integration of these physiologic processes.