Connection from the dorsal column nuclei to the superior colliculus in the rat: Topographical organization and somatotopic specific plasticity in response to neonatal enucleation

Abstract

Somatosensory stimuli from the body to deep and intermediate strata of the superior colliculus (SC) are relayed from the dorsal column nuclei (DCN), gracile (GrN) and cuneate (CuN). Electrophysiological studies have shown that the somatosensory representation in SC is arranged into a map-like pattern. However, there is a lack of studies confirming a morphological correlate of such an organization. On the other hand, after neonatal enucleation in rodents, somatosensory inputs ascend from their normal termination territory in intermediate and deep collicular strata to invade the more dorsally located visual strata. However, the origin of these reactive afferents has not been specified. By using anterograde (biotinylated dextran amine 10,000; BDA) and retrograde (Fluoro-Gold; FG) tracers, we studied separately the connection from GrN and CuN to the intact and neonatally deafferented SC. GrN-collicular afferents were found to terminate mainly within the periphery of the caudomedial SC quadrant, whereas CuN-collicular fibers innervated primarily the lateral part of the rostrolateral and caudolateral collicular quadrants, in a way consistent with previously described functional data. Retrograde tracing experiments using FG injected in SC confirmed this topographical arrangement. Injections of BDA in GrN or CuN of neonatally enucleated rats showed that reactive fibers reaching superficial strata are only those CuN-collicular fibers innervating the caudolateral SC quadrant, where the forelimb is represented. The present results provide an anatomical substrate for the known somatotopic organization of tactile representation in SC and further reinforce the previous proposal that the plastic reorganization of DCN-collicular afferents following neonatal enucleation constitutes a functional compensatory response.