Corticothalamic projections from the cortical barrel field to the somatosensory thalamus in rats: a single-fibre study using biocytin as an anterograde tracer.

This study investigated the pattern of axonal projections of single corticothalamic neurons from the cortical barrel field representing the vibrissae in the rat. Microiontophoretic injections of biocytin were performed in cortical layers V and VI to label small pools of corticothalamic cells and their intrathalamic axonal projections. After a survival period of 48 h, the animals were perfused and the tissue was processed for biocytin histochemistry. On the basis of the intrathalamic distribution of axonal fields and of the types of terminations found in the thalamus, four types of corticothalamic projections were identified. (i) Cells of the upper part of layer VI projected exclusively to the ventral posteromedial (VPm) nucleus, where they arborized in long rostrocaudally oriented bands or 'rods'. (ii) All cells of the lower part of layer VI projected to the medial part of the thalamic posterior group (Pom) but the vast majority of them also collateralized in VPm where they participated in the formation of rods. (iii) A minority of corticothalamic cells in the lower portion of layer VI, possibly located under the interbarrel spaces (septae), arborized exclusively in Pom. (iv) The corticothalamic projection of layer V cells originated from collaterals of corticofugal cells whose main axons ran caudally towards the brainstem. These collaterals arborized exclusively in Pom or in the central lateral nucleus. All corticothalamic cells from layer VI displayed the same type of axonal network, made of long branches decorated by terminal buttons emitted en passant at the tip of fine stalks. Corticothalamic fibres arising from layer V pyramids, however, remained smooth as they ran across the lateral thalamus and they generated in Pom one or two clusters of large boutons. All corticothalamic axons derived from layer VI cells, but not those derived from layer V cells, gave off collaterals as they traversed the thalamic reticular complex. These observations are discussed in the light of previous studies bearing on the topological organization and function of corticothalamic projections to VPm and Pom in rats. The possibility that a similar cellular specificity and a similar organizational plan may characterize corticothalamic relationships in other sensory systems is also considered.