The effect of large unilateral cortical lesions on rubrospinal tract sprouting in newborn rats

Many previous reports have demonstrated the development of aberrant neural connections in response to neonatal brain lesions. This investigation was undertaken to study possible alterations, particularly axonal sprouting, in rodent rubrospinal projections after neonatal destruction of the corticospinal tract through frontal cortical ablation. The neonatal ablations were made by aspiration in 1 to 2-day-old rats under hypothermic anesthesia. At three to six months after neonatal surgery, the rubrospinal tracts were ablated bilaterally in these small animals as well as in controls, by stereotaxically transecting the ventral tegmental decussation. Animals were killed two to six days after adult surgery, and rubrospinal projections were demonstrated using the Fink-Heimer degeneration stain. No differences in the pattern of rubrospinal projections were observed between animals with neonatal cortical lesions and controls. In all animals rubrospinal projections were located primarily in Rexed's lamina VI with a slight distribution into lamina V and the dorsal portion of lamina VII. Various hypotheses explaining the lack of rubrospinal sprouting after neonatal cortical lesions are presented, along with possible experiments to test these hypotheses.