Synaptophysin immunoreactivity in spinal white matter of young adult rats

Patterns of synaptophysin immunoreactivity were examined in the ventral and lateral funiculi of rat lumbosacral spinal cords. In normal young adults, dendrites from neurons in the spinal gray matter extended into the ventral and lateral white matter as finger‐like projections, immunopositive for synaptophysin. These projections appeared to diminish in size as they extended peripherally and, in general, did not reach the surface of the spinal cord, so that the outer one‐third to one‐fourth of the funiculi contained little or no immunoreactivity. The spinal cords of some of the animals studied were X‐irradiated on the third postnatal day. When examined 6 weeks to 5 months later, the pattern of synaptophysin immunoreactivity was found to be markedly altered in these animals. In general, the synaptophysin immunoreactivity in the white matter was less organized than in the non‐irradiated rat. As a result, the finger‐like projections, particularly into the lateral funiculi, were not as distinct, and the immunoreactivity appeared to be more diffusely distributed in the white matter. Further, the immunoreactivity was present throughout the thickness of the white matter in the irradiated animals and subpial concentrations were evident, especially along the lateral aspect of the spinal cord. Ultrastructural evaluation of the synaptic profiles revealed no differences between irradiated and non‐irradiated animals. The synapses occurred on both the shafts of the dendrites and on the spines. In general, both dendrites and axon terminals were covered by astrocyte processes except at synaptic sites, and the synaptic complexes were surrounded by astrocyte processes. Although the mechanisms underlying the altered pattern of synaptophysin immunoreactivity are not yet understood, they may be related to radiation‐induced effects on the glial populations previously reported by the investigators and/or to radiation‐induced alterations in reorganization or maturation of dendritic trees.