Effects of neonatal hypoxia on the medulla-spinal cord descending neurons

Effect of neonatal hypoxia on the development of intraspinal serotonergic fibers in relation to spinal motoneurons

Serotonin (5-hydroxytryptamine; 5-HT)-containing neurons trophically affect target neurons and modulate central nervous system neuronal activity. We studied effects of neonatal hypoxia on postnatal development of intraspinal 5-HT fibers in spinal motoneuron pools. Postnatal day (PND) 0 Sprague-Dawley rats received a hypoxic load and survivors were used for histological analyzes on PNDs 1, 7, and 14. Spinal motoneurons were labeled using choleratoxin B subunit as a retrograde neurotracer, and 5-HT fibers were detected immunohistochemically. On PND 1, 5-HT fibers were present in the lateral portion of the ventral horn at the cervical level, but were sparsely distributed at the lumbar level. On PND 14, cervical and lumbar level distributions were nearly identical. The 5-HT fibers and varicosities in close apposition to motoneurons increased from PNDs 1-14, however, the close apposition of cervical motoneurons was significantly different from lumbar motoneurons only on PND 1. Density of 5-HT fibers in control and hypoxic rats was not different on PND 1, while those in hypoxic rats were significantly reduced on PND 14. Close appositions of lumbar motoneurons were reduced more than cervical MNs after neonatal hypoxia. Neurodevelopmental deficit after neonatal hypoxia with a rostro-caudal gradient is associated with significant changes in the 5-HT system.

Postnatal development of brainstem serotonin-containing neurons projecting to lumbar spinal cord in rats

We quantified postnatal changes in brainstem serotonin (5-hydroxytryptamine, 5-HT)-containing neurons projecting to lumbar spinal cord. The medulla-spinal cord descending neurons were identified by a retrograde neurotracer, choleratoxin B subunit (CTb), and 5-HT neurons were stained by immunohistochemistry. Double-labeled neurons were assumed to be 5-HT neurons projecting to the lumbar spinal cord, and were quantitatively analyzed in each raphe nucleus in the medulla. The following results were obtained: (1) At PND 3, numerous CTb-labeled neurons (CTLN) were already present in the raphe pallidus (B1), while few CTLN were seen in raphe obscurus (B2) and raphe magnus (B3). CTLN then rapidly increased in number and were separately distributed after PND 7 in B3 and after PND 14 in B2. (2) At PND 3, numerous 5-HT-containing neurons were already present in B1-B3, with 23.4% and 14.0% of them labeled with CTb in B1 and B2, respectively, while there were few double-labeled neurons in B3. From PND 3 to 28, although the proportion of double-labeled to 5-HT neurons remained unchanged in B1 and B2, that in B3 rapidly increased from 5.8% at PND 7 to 28.8% at PND 14. Previous studies have shown that the 5-HT neurons in B3 send fibers mainly to the dorsal horn, while those in B1 and B2 send fibers mainly to the ventral horn at all spinal cord levels. Taken together, the present findings suggest that the brainstem 5-HT systems influence the ventral horn of the spinal cord, where spinal motoneurons exist earlier than in the dorsal horn. The functional significance of these early 5-HT systems in motor development and/or disabilities is discussed.