The roles of specificity and competition in the formation of a laminated colliculogeniculate projection

In the present study, we examined the colliculogeniculate projection in normal adult tree shrews and in adults that were bilaterally enucleated at birth. We injected lectin-conjugated HRP into superficial superior colliculus and then mapped the pattern of anterogradely transported enzyme in the ipsilateral dLGN. In normal adult tree shrews, the results confirm that the colliculogeniculate projection is laminated and terminates predominantly in small-celled layers 3 and 6 and in the interlaminar space between layers 4 and 5 (Fitzpatrick et al., 1980); we report an additional sparse projection to layer 4. In bilaterally enucleated animals, the colliculogeniculate projection is unlaminated and tends to terminate in the lateral two-thirds of the dLGN even though synaptic sites are vacated throughout the nucleus. We suggest that this preference may be due to a specificity of the colliculogeniculate fibers for the lateral two-thirds of the dLGN, which, in normal adult tree shrews, contains cells with similar physiological characteristics. We further suggest that the normal lamination of the colliculogeniculate projection in the lateral two-thirds of the dLGN may be due to competition with retinogeniculate fibers so that colliculogeniculate fibers terminate predominantly in layers containing small, W-like cells.