Experimentally induced visual projections to the auditory thalamus in ferrets: evidence for a W cell pathway.

We have previously reported that following specific neonatal brain lesions in ferrets, a retinal projection is induced into the auditory thalamus (Sur et al., Science 242:1437, '88). In these "rewired" ferrets, a novel visual pathway is established through auditory thalamus [the medial geniculate nucleus (MGN)] and primary auditory cortex (A1); cells in both MGN and A1 are visually responsive and exhibit properties similar to those of visual cells in the normal visual pathway. In this paper, we use three approaches--physiological, anatomical, and developmental--to examine which of the retinal ganglion cells project to the MGN in these rewired ferrets. We find that: 1) physiological response properties of postsynaptic visual cells in the MGN are W-like; 2) retinal ganglion cells back-filled from the MGN are small and similar to soma sizes of subsets of the normal retinal W cell population; and 3) subpopulations of these small cells can be preferentially rerouted to the MGN in response to different surgical manipulations at birth, consistent with differential W cell projection patterns in normal animals. These data suggest that retinal W cells come to project to the MGN in rewired animals. These findings not only provide a basis on which to interpret functional properties of this novel visual pathway, but also provide important information about the developmental capabilities of specific retinal ganglion cell classes and the regulation of their projections by target structures in the brain during development.