Transneuronal Retinal Projections to V1 Form “Minicolumns” in Layer 1: Comparison with Projections to Layer 4 in Normal and Visually Deprived Long Evans Rats

Lattice-like patterns in layer 1 (L1) of primary visual cortex (V1) of mice have been demonstrated following injections of tracers into the lateral geniculate nucleus (LGN) of the thalamus (Ji et al., 2015). To distinguish the ipsilateral and contralateral components of this projection, we made unilateral intravitreal injections of the transneuronal tracer WGA-HRP in Long Evans rats, a strain in which projections to L4 form ocular dominance columns (ODCs, Laing et al., 2015). We have shown that ODCs form by postnatal day 10 (P10), and that they are susceptible to monocular enucleation and monocular deprivation by eyelid suture during development (Olavarria et al., 2021). We now show that lattice-like patterns in L1 are also visible by P10, but unlike the normal contralateral projection to L4, which does not encroach into ipsilateral eye territory, the contralateral projections to layer 1 in P10 and adult normal rats are distributed throughout V1, including ipsilateral eye territories. Moreover, this pattern does not change in visually deprived rats, suggesting that L1 projections are not susceptible to visual deprivation as L4 projections are. Notably, contralateral projections to L4 in visually deprived rats do encroach into ipsilateral eye territory, resembling the projection pattern in L1. Together, these observations suggest that geniculate projections to L1 or L4 differ not only in the cues guiding their target selection, but also in cues determining their distribution within V1, and the way they respond to visual deprivation during development.