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.