Labeling knife cuts: a new method for revealing the functional anatomy of the CNS demonstrated on the noradrenergic dorsal bundle.

To assess the efficacy of using an HRP-coated wire knife to label cells of origin of cut axons, labeling cuts were made both through and parallel to the dorsal noradrenergic bundle. The damage to the dorsal bundle was evaluated independently by using fluorescence microscopy to detect the buildup of norepinephrine in cut axons. The noradrenergic axons in the dorsal bundle arise from cells in the locus coeruleus. When cuts severed the entire dorsal bundle, as could readily be seen by the dense fluorescent axons perpendicular to the center region of the cut on its caudal side, HRP-labeled cells in the locus coeruleus were so numerous that it was difficult to distinguish individual cells. Cuts parallel to the dorsal bundle that damaged only a few of its most lateral fibers resulted in a corresponding paucity of labeled cells in the locus coeruleus, although the numerous undamaged axons directly adjacent must have been exposed to HRP along the length of the cut. These results indicate that this technique may be used to identify the cells of origin of fibers damaged by knife cuts; and that axons passing near or even directly adjacent to the cut do not take up and transport label unless cut. Since survival times of several days can follow the labeling cut, both functional and anatomical effects of the knife cut may be assessed. The use of this technique in areas where cut pathways are not easily identified is recommended.