Abstract
The pigmented rat is an increasingly important model in visual neuroscience research, yet the lamination of retinal projections in the dLGN has not been examined in sufficient detail. From previous studies it was known that most of the rat dLGN receives monocular input from the contralateral eye, with a small island receiving predominantly ipsilateral projections. Here we revisit the question using cholera toxin B subunit, a tracer that efficiently fills retinal terminals after intra-ocular injection. We imaged retinal termini throughout the dLGN at 0.5 μm resolution and traced areas of ipsilateral and contralateral terminals to obtain a high resolution 3D reconstruction of the projection pattern. Retinal termini in the dLGN are well segregated by eye of origin, as expected. We find, however, that the ipsilateral projections form multiple discrete projection zones in three dimensions, not the single island previously described. It remains to be determined whether these subdomains represent distinct functional sublaminae, as is the case in other mammals.
Highlights
In mammals, the retinal ganglion cells (RGC) that contribute to cortical vision send their projections to the dorsal lateral geniculate nucleus (LGN) of the thalamus
The projections of distinct ganglion cell types further segregate into discrete dLGN laminae, each containing a retinotopic map of visual space (Bishop et al, 1962; Laties and Sprague, 1966; Garey and Powell, 1968; Kinston et al, 1969; Sanderson, 1971a,b; Jones, 2007, for review)
The data and analysis presented here confirm the basic findings of an earlier preliminary report (Discenza et al, 2008): retinal projections to the dLGN of the rat are well segregated by eye of origin, and the ipsilateral projections form multiple spatially separated subdomains
Summary
The retinal ganglion cells (RGC) that contribute to cortical vision send their projections to the dorsal lateral geniculate nucleus (LGN) of the thalamus (dLGN). The dLGN of the macaque monkey (Macaca mulatta) contains six layers, each receiving inputs from a different subset of RGC (parasol or midget; ON or OFF; ipsilateral or contralateral) (Malpeli and Baker, 1975; Schiller and Malpeli, 1978; Connolly and Van Essen, 1984; Shapley and Perry, 1986; Szmajda et al, 2006; Murray et al, 2008). In the cat (Felis catus), six layers have been distinguished in the dLGN, receiving inputs from different subsets of RGC types (X, Y, or W) and segregated by eye of origin (Guillery, 1970; Sherman and Spear, 1982; Shapley and Perry, 1986). In the California ground squirrel (Spermophilus beecheyi), a diurnal rodent, the retinal projections form three layers with alternating eye of origin in the dLGN; six sublaminae have been distinguished (Roe et al, 1989)
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