Heterotopic and homotopic callosal connections in rat visual cortex

Abstract

Heterotopic and homotopic callosal projections of rat visual cortex are evaluated. Callosal termination zones in visual cortex are identified with a degeneration technique following complete section of the corpus callosum. The zones which receive callosal afferents are the lateral one-third of area 17, an anteroposterior strip in dorsal and in ventral areas 18a, and 4 patches in area 18b. Following a large injection of lectin-bound horseradish peroxidase (WGA-HRP) into visual cortex, many retrogradely labeled neurons are found in the medial two-thirds of area 17 which does not receive callosal afferents, as well as in the lateral, callosal-recipient zone. These data suggest that hetereotopic callosal pathways exist in visual cortex. Injections of tritiated amino acids into restricted parts of visual cortex show the following heterotopic connections: lateral area 17 projects to dorsal area 18a; medial area 17 projects to lateral area 17 and dorsal area 18a; area 18a projects to lateral area 17 and anteromedial area 18b; area 18b projects to lateral area 17, dorsal area 18a, heterotopic sites in area 18b, and to area 29d. Heterotopic connections are generally less dense than homotopic ones. In addition, heterotopic projections terminate in the supragranular layers. This contrasts with the homotopic afferents of areas 17 and 18a which have additional strong projections to layer V. The distribution of label through the depth of the cortex in some of the callosal recipient zones has been quantified. Injections of WGA-HRP restricted to areas 17, 18a or 18b corroborate the presence of each of the heterotopic connections described above. Heterotopic afferents originate mostly from layer V neurons, whereas homotopic afferents arise from neurons primarily in layers II-V. Like the afferents, the numbers of callosal projection cells in heterotopic regions are substantially less than that in homotopic sites. Heterotopic callosal connections may be one factor responsible for binocular vision and also may provide the basis for large, non-oriented receptive fields of units in layer V of rodent visual cortex.