Altered spatial patterns of functional thalamocortical connections in the barrel cortex after neonatal infraorbital nerve cut revealed by optical recording.

In rodents, the somatosensory cortex has a cell aggregation cluster termed the barrel, reflecting a whisker vibrissa, and this barrel formation is disrupted by infraorbital nerve cut at birth. In the present study, we prepared thalamocortical slice preparations from rats that received infraorbital nerve cut either at birth or at postnatal day (P) 7 and those from normal rats, recorded the optical response reflecting neural excitation in the somatosensory cortex with a voltage-sensitive dye (RH482) and compared the optical responses from lesioned rats with those from normal rats. In normal rats at P10, the optical response elicited electrically by thalamic stimulation propagated to the cortex, and then several patchy clusters appeared in layer IV. The size and location of these patchy responses precisely matched either barrels identified by cytochrome oxidase staining or terminal arbors of thalamocortial axons stained with biotinylated dextran amine. In contrast, at P10 in P0-lesioned rats, clusters having a wider horizontal width but smaller amplitude than those seen in normal rats appeared in layer IV. Correspondingly, neither cytochrome oxidase staining nor biotinylated dextran amine labeling of thalamocortical axons showed any barrel-like clusters or glomerular axon terminals. Likewise, at P5-P6, the tangential width of clusters in layer IV were larger than that in normal rats. At P10 in P7-lesioned rats, small cluster-matched barrels were seen in the optical response as well as in normal rats. These results suggest that P0 infraorbital nerve cut interrupted segregation of functional synapses into the barrels and retarded the maturation of thalamocortical transmission.

Laminar specificity of extrinsic cortical connections studied in coculture preparations.

The formation of specific neural connections in the cerebral cortex was studied using organotypic coculture preparations composed of subcortical and cortical regions. Morphological and electrophysiological analysis indicated that several cortical efferent and afferent connections, such as the corticothalamic, thalamocortical, corticocortical, and corticotectal connections, were established in the cocultures with essentially the same laminar specificity as that found in the adult cerebral cortex, but without specificity of sensory modality. This suggests the existence of a cell-cell recognition system between cortical or subcortical neurons and their final targets. This interaction produces lamina-specific connections, but is probably insufficient for the formation of the modality-specific connections.