Developmental changes in the noradrenergic innervations of spinal motoneurons in neonatal rats

Developmental changes in the noradrenergic innervations of spinal motoneurons in both the cervical and lumbar cords were studied in neonatal rats. The labeling of motoneurons was done using choleratoxin B subunit as a retrograde neurotracer. The noradrenergic fibers were detected by immunohistochemistry for tyrosine hydroxylase. At postnatal day 1, tyrosine hydroxylase immunoreactive fibers were evident in the entire ventral horn, including the triceps brachii motoneuron pools at the cervical level. In contrast, they were observed only in that portion of the ventral horn medial to the quadriceps femoris motoneuron pools at the lumbar level. Subsequently, tyrosine hydroxylase immunoreactive fibers increased at both levels, and they were distributed in most of the gray matter at postnatal day 14. At this age, the distribution pattern of tyrosine hydroxylase immunoreactive fibers in the lumbar level was almost identical to that of the cervical level. The number of closely apposed tyrosine hydroxylase immunoreactive varicosities on motoneurons (close appositions) increased continuously from postnatal day 1 to 14 at both the cervical and lumbar levels. At postnatal day 1, triceps brachii motoneurons had more close appositions than quadriceps femoris motoneurons in number and, after postnatal day 7, there was no difference in the number of close appositions between triceps brachii motoneurons and quadriceps femoris motoneurons. Based on these results, we discuss the significance of monoaminergic influences on the postnatal development of spinal motoneurons and of motor behavior with a rostrocaudal gradient.