Descending supraspinal pathways in amphibians: III. Development of descending projections to the spinal cord in Xenopus laevis with emphasis on the catecholaminergic inputs.

In developmental stages of the clawed toad, Xenopus laevis, we describe the ontogeny of descending supraspinal connections, catecholaminergic projections in particular, by means of retrograde tracing techniques with dextran amines. Already at embryonic stages (stage 40), spinal projections from the reticular formation, raphe nuclei, Mauthner neurons, vestibular nuclei, the locus coeruleus, the interstitial nucleus of the medial longitudinal fasciculus, the posterior tubercle, and the periventricular nucleus of the zona incerta are well developed. At the beginning of the premetamorphic period (stage 46), spinal projections arise from the suprachiasmatic nucleus, the torus semicircularis, the pretectal region, and the ventral telencephalon. After stage 48, tectospinal and cerebellospinal projections develop, with spinal projections from the preoptic area following at stage 51. Rubrospinal projections are present at stage 50. During the prometamorphic period, spinal projections arise in the nucleus of the solitary tract, the lateral line nucleus, and the mesencephalic trigeminal nucleus. With in vitro double-labeling methods, based on retrograde tracing of dextran amines in combination with tyrosine hydroxylase (TH) immunohistochemistry, we show that at stage 40/41, catecholaminergic (CA) neurons in the posterior tubercle are the first to project to the spinal cord. Subsequently, at stage 43, new projections arise in the periventricular nucleus of the zona incerta and the locus coeruleus. The last CA projection to the spinal cord originates from neurons in the nucleus of the solitary tract at the beginning of prometamorphosis (stage 53). Our data show a temporal, rostrocaudal sequence in the development of the CA cell groups projecting to the spinal cord. Moreover, the early appearance of CA fibers, preterminals and terminal-like structures in dorsal, intermediate, and ventral zones of the embryonic spinal cord, suggests an important role for catecholamines during development in nociception, autonomic functions, and motor control at the spinal level.