Organization of the projections from barrel cortex to thalamus in mice studied with Phaseolus vulgaris-leucoagglutinin and HRP.

In order to elucidate the geometric organization of projections from the barrel cortex to the thalamus, iontophoretic injections of the anterograde tracer Phaseolus vulgaris-leucoagglutinin were made. The injections were confined to one barrel column (i.e. barrel in layer IV + cortical tissue above and below it). Axonal terminations could be demonstrated in three thalamic nuclei: reticularis (RT), ventrobasalis (VB) and posterior (PO). Anterograde terminal labelling was obtained in RT + VB; in PO only; or in RT + VB + PO. The terminals labelled in PO were much larger than those in RT and VB. The termination areas in RT, VB and PO were shaped like rods which have a rostro-caudal orientation. These cortico-thalamic projections are discretely and topographically organized. The clearest such arrangement was found in VB. Here, projections from the A row of barrels in BF terminate dorsally, whereas those from the C row end ventrally. Barrel A1 projects to the lateral part of VB, whereas A4, to more medial parts; other rows are arranged similarly. These results were compared with the distribution of thalamo-cortical projection neurons that were labelled after iontophoretic HRP injections in individual barrels. We concluded that the corticothalamic projections originating from one barrel column contact an are of barreloids in VB.