Development of catecholaminergic nerves in the spinal cord of the rat

The development of the noradrenergic and dopaminergic innervations in the spinal cord of rat was studied using fluorescence histochemical and neurochemical methods. From fetal day (FD) 16 to neonatal day (ND) 26, the cord increased in weight by 4-6 mg/day, except for the period between ND 14 and 20, when the increase was 13 mg/day. Norepinephrine was first detectable in the whole cord at ND 18, and then increased rapidly thereafter, peaking at ND 14, then declining at the end of neonatal life to the values found in the young adult spinal cord. The innervation in the intermediolateral cell column of the thoracic cord appeared to be more extensive at ND 14 than in the adult, raising the possibility of the selective destruction of a part of this noradrenergic innervation during later development. The nerve terminals in the ventral horn were first visualized clearly at birth, with a pattern similar to that of the adult. When the fetal locus coeruleus is transplanted into the transected spinal cord of the adult rat, it induces an extensive proliferation of the cut rostral axons in the ventral horn specifically. It is proposed that the transplanted fetal locus coeruleus produces a neurotrophic substance which stimulates the proliferation of the cut rostral axons derived from the locus coeruleus. Dopamine was first detectable in the cord at ND 20. Unlike noradrenergic nerves, dopaminergic nerves developed slowly throughout neonatal life. The adult innervation presumably develops slowly between ND 26 and young adulthood. In the fetus and very young neonate, DA was most concentrated in the thoracic region. Dopamine metabolism in the cord during neonatal life was a fraction of that found in the adult. It is concluded that the spinal dopaminergic and noradrenergic innervations develop with quite different time sequences. The rapid peaking of the noradrenergic innervation of ND 14 may play a significant role in the overall development and functional maturation of the cord.