Horseradish peroxidase tracing of dorsal root ganglion afferents within fetal mouse spinal cord explants chronically exposed to tetrodotoxin.

Afferent projection patterns within organotypic explants of fetal mouse spinal cord-dorsal root ganglia (DRG) were mapped out histologically using an HRP-staining method. The role of spontaneous bioelectric activity in the development of cord-DRG connections was studied using tetrodotoxin (TTX) to eliminate action potential discharges in the experimental cultures. Cultures grown in TTX-supplemented medium had a significantly lower proportion of DRG afferents within the dorsal half of the cord explant than did untreated cultures. In addition, afferent fiber entrances were made predominantly on the dorsolateral aspect in control cord explants, in contrast with the more diffuse entrance pattern displayed by the experimental group. Comparison of those cultures in both groups where the fiber entrances were chiefly dorsal revealed a greater tendency in the TTX group for DRG afferents to grow ventrally after penetrating the cord. Thus, it appears that bioelectric activity of the target neurons may be required by DRG afferents for the development of selective innervation patterns.