A di-synaptic projection from the superior colliculus to the head of the caudate nucleus via the centromedian-parafascicular complex in the cat: an anterograde and retrograde labeling study

Centromedian-parafascicular (CM-Pf) complex of the thalamus receives inputs from the superior colliculus (SC). The CM-Pf neurons, in turn, project to the neostriatum. In order to establish a circuitry that the efferent termin als of SC neurons make direct synaptic contacts with thalamostriatal neuron s in the CM-Pf complex, the anterograde tracer (biocytin) was injected into the unilateral SC and the retrograde tracer (wheat germ agglutinin conjuga ted to horseradish peroxidase, WGA-HRP) into the ipsilateral head of the ca udate nucleus of the cat. The anterogradely labeled SC fibers and their ter minals and retrogradely labeled CM-Pf neuronal elements were examined under light and electron microscope. At the light micrsocopic level, biocytin-la beled terminal-like varicosities were observed densely in the latero-dorsal part of the CM and the dorsal part of the Pf lateral to the fasciculus ret roflexus. These varicosities were often in dose proximity to HRP-labeled so mata and dendrites of CM-Pf neurons. Electron microscopic analysis revealed that the biocytin-labeled synaptic boutons contained mainly round synaptic vesicles and established asymmetrical synaptic contacts with retrogradely labeled thalamostriatal neuronal elements, including perikarya, and small a nd large dendrites. In addition, anterogradely labeled terminals made synap tic contacts with unlabeled somata, small and large dendrites, and spines a s well as profiles containing synaptic vesicles. These vesicle-containing p rofiles were considered to be pre-synaptic elements contacting on thalamost riatal neurons or the vesicle-containing dendrites of local circuit neurons . These results demonstrate that the neurons of the CM-Pf complex of the th alamus is involved in a di-synaptic tecto-striatal circuit. (C) 1998 Elsevi er Science Ireland Ltd. All rights reserved.