Corticocortical associative neurons expressing latexin: Specific cortical connectivity formed in vivo and in vitro

Latexin, a carboxypeptidase A inhibitor, is expressed in a subset of neuron s in the infragranular layers of the lateral cortex in the rat. We here sho w that latexin-expressing neurons exhibit ultrastructural features common t o cortical pyramidal neurons. We show in combined retrograde tracing and im munofluorescent experiments that latexin-expressing neurons contribute to s pecific cortico-cortical pathways. Thus, injections of the retrograde trace r fluorogold into either the primary somatosensory (SI) or the primary moto r (MI) cortical area labeled many latexin expressing neurons in the infragr anular layers of the secondary somatosensory (SII) and visceral sensory (Vi ) areas. In contrast, tracer injections involving the thalamus, striatum, o r contralateral SII and Vi exclusively labeled latexin-nonexpressing neuron s in both the SII and Vi. Finally, we show that the correct corticocortical projections can be formed in organotypic slice cultures in vitro from late xin-expressing neurons: when slices of developing SII were cocultured with those from the SI and the thalamus, latexin-immunoreactive neurons in the S II projected preferentially to their normal SI target. The specific connect ivity formed in vivo and in vitro by this molecularly distinct neuronal pop ulation reveals its characteristic manner of cortical organization and prov ides a unique model system to analyze mechanisms underlying the formation o f precise corticocortical pathways.