Direct synaptic connections between superior colliculus afferents and thalamo-insular projection neurons in the feline suprageniculate nucleus

The suprageniculate nucleus (Sg) of the feline thalamus, which subserves largely unimodal sensory and orientation behavior, receives input from the deep layers of the superior colliculus (SC), and projects to the suprasylvian cortical areas, such as the anterior ectosylvian visual area and the insular visual area (IVA), which contain visually responsive neurons. Through a double tract-tracing procedure involving the injection of wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP) into the IVA and the injection of kainic acid into the SC, this study sought to determine the nature of the synaptic relationship between the SC afferents and the thalamo-cortical projection neurons. WGA-HRP injections labeled numerous neurons in the Sg, while kainic acid injections destroyed many tectothalamic terminals in the Sg. The distributions of the WGA-HRP-labeled neurons and the degenerated axon terminals overlapped in the dorsal part of the Sg. Electron microscopic observations demonstrated that the degenerated axon terminals made synaptic contacts with the dendrites of the WGA-HRP-labeled neurons in this overlapping region of the Sg. These results provide the first anatomical evidence that the Sg may play a role in the key relay of visual information from the SC to the IVA, within an identified extrageniculo-cortical pathway.