THE SUPERIMPOSED EFFECTS OF CHRONIC PHRENICOTOMY AND CERVICAL SPINAL-CORD HEMISECTION ON SYNAPTIC CYTOARCHITECTURE IN THE RAT PHRENIC NUCLEUS

The present study was carried out to determine the effects of a combin ed peripheral phrenicotomy and rostral spinal cord hemisection on the synaptic architecture in the ipsilateraI rat phrenic nucleas. Young ad ult female Sprague-Dawley rats were divided into a hemisection-alone a nd two hemisection-plus-phrenicotomy (HPP) groups. In all animals, DiI , a fluorescent carbocyanine dye was injected into the left hemidiaphr agm to retrogradely label the ipsilateral phrenic motoneurons. ill the HPP groups, left intrathoracic phrenicotomies were carried out at 2 a nd 4 weeks prior to sacrificing, Hemisection-alone animals were not su bjected to phrenicotomy. In all animals, a left C2 spinal cord hemisec tion was performed 24 h prior to death. Quantitative morphometric anal ysis of the phrenic nucleus showed that the number of synapses contact ing phrenic profiles is significantly less in the HPP (2 week) group a s compared to the hemisection-alone group, but this number returns to a level not significantly different from the hemisection-alone value i n the HPP (4 week) group. The results suggest that the transient chang e in the number of synapses might contribute 60 the differential expre ssion of the crossed phrenic phenomenon documented in another group of animals subjected 60 the same surgical procedures. Furthermore, the d ifferent stages of glial reaction induced by phrenicotomy/spinal cord hemisection might underlie the change in synaptic number. (C) 1997 Aca demic Press.