Ultrastructural evidence for remodelling in a central noradrenergic pathway following electrolytic lesioning

Electrolytic lesions were carried out in the medial hypothalamus of adult rats to study remodelling responses in a central noradrenergic pathway, the medial forebrain bundle. Four days, two weeks, 4 weeks and 8 weeks post-lesion, the animals were perfused and processed for correlated fluorescence microscopic (FM) and electron microscopic (EM) study. FM evaluation 4 days post-lesion showed that, compared with control preparations, catecholaminergic fibers became thick, distorted and intensely fluorescent. With increasing survival times the caliber of these fibers became finer and fluorescence intensity was gradually diminished. Some of the small blood vessels in the vicinity of the lesion acquired an intense perivascular fluorescence. Electron microscopic examination of the lesion site 4 days post-lesion disclosed many degenerating axons and increased extracellular space. No increased extracellular space was discerned by 8 weeks post-lesion. After all survival periods greatly enlarged axonal profiles were seen, and these resembled 'growth cones' described in earlier tissue culture, developmental and peripheral nervous system studies.