The development of catecholaminergic innervation in chick spinal cord.

This combined biochemical and histofluorescent study of the embryonic chick was designed to investigate the temporal and spatial development of noradrenergic pathways in the spinal cord. The sequence of catecholaminergic innervation was analyzed by measuring the specific uptake of [3H]norepinephrine, the levels of endogenous norepinephrine, and the distribution of catecholamine histofluorescence. Uptake studies showed early axons present in all levels of spinal cord by 10 days incubation with subsequent increases of uptake activity appearing in a rostrocaudal fashion. Endogenous norepinephrine values were low until day 14, at which time transmitter levels began to increase, approaching hatched values on incubation day 17. Morphologic studies demonstrated catecholaminergic terminals first in the intermediate gray matter and later concentrated in the ventral and dorsal horns. These observations were interpreted to indicate that: (1) noradrenergic axons are an early descending supraspinal pathway; (2) arborization of this system occurs in a rostral-caudal sequence; and (3) monoaminergic uptake mechanisms develop prior to and independent of neurotransmitter synthesis and storage. The development of this noradrenergic system parallels alteration in spinal cord physiology and refinements in the motility of the embryo.