Cat primary visual cortex has been used as an immunogen to produce monoclonal antibodies that detect subpopulations of neurons. When tested by immunofluorescence on tissue sections of areas 17 and 18, 2 of these antibodies, VC1.1 and VC5.1, outlined a rare subpopulation of neurons located mainly in layer 4 but also in layers 5 and 6. Double-labeling immunofluorescence experiments in area 17 revealed that all VC1.1-reactive cells were also VC5.1-reactive and 83% of VC5.1-reactive cells were VC1.1-reactive, suggesting that the antibodies were reacting with the same subpopulation of cells. Both antibodies labeled similar or identical subpopulations of cells in other areas of the cat CNS, including the superior colliculus, parts of hippocampus, cerebellar deep nuclei, and rostral spinal cord. Neither antibody labeled cell bodies in the lateral geniculate nucleus. In the retina, VC1.1 labeled cell bodies and processes of some horizontal and amacrine cells, whereas VC5.1 labeled only ganglion cell axons. In the cerebellar cortex, the most prominent labeling of VC1.1 was of Purkinje cells, whereas that of VC5.1 was of Lugaro cells. Immunoblotting analyses of cat cortical homogenates demonstrated that VC1.1 recognized a major polypeptide band of Mr 95,000-105,000 and additional bands of Mr 145,000 and Mr 170,000. VC5.1 recognized bands of Mr 97,000 and Mr 150,000. Subcellular fractionation and extraction studies showed that the VC1.1 antigens were integral membrane proteins preferentially located in a synaptosomal plasma membrane fraction. The VC5.1 antigens were preferentially located in a soluble cytoplasmic or extracellular fraction. The results indicate that antibodies VC1.1 and VC5.1 recognize unique epitopes in the cat CNS and define a previously unrecognized subpopulation of cells in cat visual cortex.

The developmental mechanism that contributes to the highly organized axonal connections within the cerebral cortex is not well understood. This is partly due to the lack of molecular markers specifically expressed in corticocortical associative neurons during the period of circuit formation. We have shown previously that latexin, a carboxypeptidase A inhibitor, is expressed in intrahemispheric corticocortical neurons from the second postnatal week in the rat (Arimatsu et al. [1999] Cereb. Cortex 9:569-576). In the present study, we first demonstrate in the adult rat that the orphan nuclear receptor Nurr1 is coexpressed in latexin-expressing neurons located in layer V, sublayer VIa, and the white matter of the lateral sector of the neocortex, and also in latexin-negative early born neurons in sublayer VIb of the entire neocortex. Virtually all Nurr1-expressing neurons exhibit immunoreactivity for phosphate-activated glutaminase but not for gamma-aminobutyric acid, suggesting that they are glutamatergic-excitatory neurons. By combining Nurr1 immunohistochemistry and 5-bromo-2'-deoxyuridine-birthdating, we then show that Nurr1 is expressed in (early born) subplate neurons and (later born) presumptive latexin-expressing neurons from embryonic day 18 onward. Finally, by combination of Nurr1 immunohistochemistry and retrograde tracing, we show that Nurr1-expressing neurons, including those in sublayer VIb, contribute predominantly to long-range intrahemispheric corticocortical projections. These results raise the possibility that Nurr1 plays a role in the establishment and maintenance of normal corticocortical circuitry and function.

The timing of neocortical regional specification was examined using a monoclonal antibody, designated PC3

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