Cervicothalamic tract termination: a reexamination and comparison with the distribution of monoclonal antibody Cat-301 immunoreactivity in the cat.

The distribution of cervicothalamic tract (CTT) terminations, labeled with anterogradely transported tracers (WGA-HRP or biotinylated dextran amine) injected into the lateral cervical nucleus of cats, was compared with the distribution of immunoreactivity for a cell-surface antigen detected with the monoclonal antibody Cat-301. The most abundant CTT termination is present in the ventrobasal complex (VB), mainly in its lateral part (VPL) and only sparsely in its medial part (VPM). In the VPL, the CTT preferentially terminates in a Cat-301-sparse peripheral rim of the nucleus and in between its lateral and medial subdivisions (VPLI and VPLm). CTT terminations are sparse in the central Cat-301-dense parts of the VPL. In the ventral periphery of VB (VBvp), situated in between the VPL/VPM and the external medullary lamina, thin CTT fibers with spaced varicosities is seen among the large fibers of passage. The VBvp is essentially devoid of Cat-301 immunoreactivity. Scattered clusters of CTT termination are also seen caudal and dorsal to the VB in the medial division of the posterior complex (POm), which is virtually devoid of Cat-301 immunoreactivity. In the caudal thalamus, dense and focused CTT termination is present in the medial extension of the magnocellular medial geniculate nucleus (MGmc) but absent from its main lateral part. The termination in the MGmc is centered upon clusters of cells displaying dense Cat-301 immunoreactivity. The present study demonstrates previously unrecognized or unconfirmed CTT terminations in the VPM and in the VBvp, and confirm previously described projections to the VPL, POm and MGmc. The preferential termination of the CTT in the Cat-301-sparse peripheral region of the VPL demonstrates that the CTT is related to a chemically defined VPL compartment. In the light of previous data, this observation suggests that the CTT is related to one or more thalamocortical channels that are partly or completely separate from that (those) activated through the dorsal column-medial lemniscal pathway. The organization of the thalamocortical channel(s) activated through the CTT remains to be elucidated. In contrast to the termination in the VPL, CTT termination in the medial MGmc is focused to clusters of Cat-301 immunolabeled cells. The significance of this difference between CTT recipient cells in the VPL and in the MGmc is unclear.