SYNAPTIC PLASTICITY OF 5-HYDROXYTRYPTAMINE-IMMUNOREACTIVE TERMINALS IN THE PHRENIC NUCLEUS FOLLOWING SPINAL-CORD INJURY - A QUANTITATIVE ELECTRON-MICROSCOPIC ANALYSIS

The present study was conducted to examine the plasticity of 5-hydroxy tryptamine (5-HT)-immunoreactive terminals in the rat phrenic nucleus following an ipsilateral C2 spinal cord hemisection and 30-day surviva l period. A retrograde horseradish peroxidase (HRP) labeling technique was used to identify the phrenic motoneurons at the electron microsco pic (EM) level. After employing a pre-embedding immunocytochemical tec hnique, the ultrastructural characteristics of 5-HT-immunoreactive ter minals were qualitatively and then quantitatively analyzed with a comp uterized morphometric system before and after injury in separate group s of rats. The results indicated that the majority of the 5-HT-labeled terminals formed axodendritic contacts, but some 5-HT-labeled termina ls made axosomatic contacts. 5-HT terminals were associated with eithe r asymmetrical or symmetrical synapses, and some displayed postsynapti c dense bodies. Approximately 2% of the 5-HT terminals had dense-core vesicles. Although the total number of labeled and unlabeled terminals in the phrenic nucleus was reduced after hemisection, the number of 5 -HT terminals in the hemisected group was greater than that of the con trol group. Moreover, the total number and length of asymmetrical and symmetrical synaptic active zones per 5-HT terminal were significantly greater after injury. Finally, the total number of 5-HT terminals wit h multiple synapses was significantly greater in the hemisected group as compared to controls. It is possible that 5-HT synaptic plasticity may be part of the morphological substrate for the unmasking of the la tent crossed phrenic pathway which mediates recovery of the ipsilatera l hemidiaphragm paralyzed by C2 spinal cord hemisection. (C) 1997 Wile y-Liss, Inc.