Effects of enucleation on the direct reciprocal corticocortical connections between primary visual and somatosensory cortices of the mouse

Multisensory convergence is present in the cerebral cortex even at the initial stages of processing in the primary and low-level sensory cortices. Direct connections between primary sensory cortices are a particular feature of rodent cortical connectivity. Our previous studies have shown asymmetric projections between the primary visual and somatosensory cortices in mice. Binocular enucleation produces a relative reduction of the projections from the somatosensory cortex to the visual cortex in mice. The purpose of this study is to compare the direct reciprocal cross-modal connections between the primary visual (V1) and somatosensory (S1) cortices in intact and enucleated C57Bl/6 mice, and to determine quantitative differences in the proportion and laminar distribution of neurons and terminals in these projections. CTB labeled neurons were used to estimate the relative importance of projections between V1 and S1, and their laminar distribution used to classify them as feedback, feedforward or lateral projections. The size of axonal swellings was measured and frequency distribution determined for each cortical layer. Axon diameters were also sampled in these connections. Injections in V1 resulted in a reduced proportion of labeled cells in S1 of enucleated mice. There was a relative decrease of the projection from the somatosensory cortex to the visual cortex in enucleated mice due to a greater relative reduction of supragranular layers neurons. Enucleation otherwise had no effect on the connectivity of the somatosensory cortex with other motor and somatosensory cortices. In the projection from the visual to the somatosensory cortex in enucleated mice, the size of axonal swellings was reduced in all layers. Conversely, in the projection from the somatosensory cortex to the visual cortex, some larger swellings appeared in the supragranular layers. This shows pathway and laminar specific changes in cortical circuitry following loss of sensory afferent activity that results in changes in the relationship between visual and somatosensory cortices in the enucleated mice.