The proposed research will deal with three aspects of descending control of somatosensory input and it involves several advanced neuroanatomical techniques. In one project we will pursue a research on the organization of descending cortical input to the cuneate nucleus of cats by intraaxonal staining with horseradish peroxidase. Light- and electro microscopical study of individual cortical fibers and their terminals from different functional areas will be carried out to obtain evidence relevant to the mechanisms by which the cerebral cortex modulates information processing by cuneate neurons. Another part of the proposed research is aimed at verifying the hypothesis that inerneurons in the ventrobasal complex of cats are GABA-ergic and to explore, by immunocytochemical and autoradiographic techniques, synaptic relationships within the somatosensory thalamic relay. A third project will employ the selective retrograde transport of amino acids to investigate whether cortical neurons to the ventrobasal complex of cats may use glutamate or aspartate as neurotransmitters and to verify the assumption that selective retrograde transport is a reliable experimental approach for the identification of the neurotransmitter used by a given pathway.