CORTICOTHALAMIC PROJECTIONS FROM THE CORTICAL BARREL FIELD TO THE SOMATOSENSORY THALAMUS IN RATS - A SINGLE-FIBER STUDY USING BIOCYTIN AS AN ANTEROGRADE TRACER

This study investigated the pattern of axonal projections of single co rticothalamic neurons from the cortical barrel field representing the vibrissae in the rat. Microiontophoretic injections of biocytin were p erformed in cortical layers V and VI to label small pools of corticoth alamic cells and their intrathalamic axonal projections. After a survi val period of 48 h, the animals were perfused and the tissue was proce ssed for biocytin histochemistry. On the basis of the intrathalamic di stribution of axonal fields and of the types of terminations found in the thalamus, four types of corticothalamic projections were identifie d. (i) Cells of the upper part of layer VI projected exclusively to th e ventral posteromedial (VPm) nucleus, where they arborized in long ro strocaudally oriented bands or 'rods'. (ii) All cells of the lower par t of layer VI projected to the medial part of the thalamic posterior g roup (Pom) but the vast majority of them also collateralized in VPm wh ere they participated in the formation of rods. (iii) A minority of co rticothalamic 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 c ollaterals of corticofugal cells whose main axons ran caudally towards the brainstem, These collaterals arborized exclusively in Pom or in t he central lateral nucleus. All corticothalamic cells from layer VI di splayed the same type of axonal network, made of long branches decorat ed by terminal buttons emitted en passant at the tip of fine stalks. C orticothalamic 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 de rived from layer VI cells, but not those derived from layer V cells, g ave off collaterals as they traversed the thalamic reticular complex. These observations are discussed in the light of previous studies bear ing on the topological organization and function of corticothalamic pr ojections to VPm and Pom in rats. The possibility that a similar cellu lar specificity and a similar organizational plan may characterize cor ticothalamic relationships in other sensory systems is also considered .