Experimentally Induced Retinal Projections to the Ferret Auditory Thalamus: Development of Clustered Eye-Specific Patterns in a Novel Target

Abstract

We have examined the relative role of afferents and targets in pattern formation using a novel preparation, in which retinal projections in ferrets are induced to innervate the medial geniculate nucleus (MGN). We find that retinal projections to the MGN are arranged in scattered clusters. Clusters arising from the ipsilateral eye are frequently adjacent to, but spatially segregated from, clusters arising from the contralateral eye. Both clustering and eye-specific segregation in the MGN arise as a refinement of initially diffuse and overlapped projections. The shape, size, and orientation of retinal terminal clusters in the MGN closely match those of relay cell dendrites arrayed within fibrodendritic laminae in the MGN. We conclude that specific aspects of a projection system are regulated by afferents and others by targets. Clustering of retinal projections within the MGN and eye-specific segregation involve progressive remodeling of retinal axon arbors, over a time period that closely parallels pattern formation by retinal afferents within their normal target, the lateral geniculate nucleus (LGN). Thus, afferentdriven mechanisms are implicated in these events. However, the termination zones are aligned within the normal cellular organization of the MGN, which does not differentiate into eye-specific cell layers similar to the LGN. Thus, target-driven mechanisms are implicated in lamina formation and cellular differentiation.

Brainstem inputs to the ferret medial geniculate nucleus and the effect of early deafferentation on novel retinal projections to the auditory thalamus

“…8 and Table 1). As previously reported (Angelucci et al, 1997), retinal axons were consistently found to ''prefer'' the rostral MGN. Our estimates indicated that approximately 10% of the rostral half of the nucleus receives retinal afferents.…”

“…Consistent with this hypothesis, the ''deafferented'' MGN was found to receive some normal inputs from auditory and non-auditory brainstem nuclei spared by the neonatal lesions, as well as novel inputs from the retina and from other sensory nuclei, including somatosensory and visceral nuclei. Support for the hypothesis of competition between afferent axons in the MGN was previously demonstrated by Angelucci et al (1997) who reported that axons from the two eyes segregate into eye-specific clusters in the MGN of experimentally manipulated ferrets. It is conceivable that the undamaged axons innervating the MGN at the time of lesion have a competitive advantage over retinal and other more distantly located axons.…”

Cross-modal reorganization of callosal connectivity without altering thalamocortical projections

“…1). Under these conditions, retinal axons project to the contralateral medial geniculate nucleus (MGN) and to a lesser extent to the ipsilateral MGN (11,(23)(24)(25)(26).…”

Expression of regulatory genes during differentiation of thalamic nuclei in mouse and monkey

“…In the case of the dorsal thalamus, specific sets of nuclei receive inputs from defined afferent pathways and become bidirectionally connected with distinct functional areas of the neocortex (Jones, 1985). During these phases afferent activity and competition between innervating fibers play an important role in determining the definitive input-output relationships and the finer grained cytoarchitecture (e.g., Sretevan et al, 1988;Sur et al, 1988;Bhide and Frost, 1992;Angelucci et al, 1997;Penn et al, 1998;Hahm et al, 1999;Stellwagen and Shatz, 2002).…”