An ultrastructural analysis of afferent terminals to the anterior pretectal nucleus in the cat.

The synaptic organization of three kinds of afferent projections in the feline anterior pretectal nucleus (PTA) was analyzed by a combination of the degeneration and retrograde transport of horseradish peroxidase (HRP) techniques, along with that of the degeneration and anterograde transport of HRP techniques. Retrograde labeling of PTA neurons was performed by injections of HRP in the dorsal accessory olivary nucleus (DAO). Three kinds of afferent sources of the PTA--the cerebral motor cortex, the anterior interpositus nucleus of the cerebellum, and the gracile nucleus--were subjected to electrolysis, suction, or injection of kainic acid or HRP for identification of axon terminals of each system. Axon terminals of these different afferent sources identified by degeneration or anterograde HRP transport techniques showed similar morphological features: They were relatively large (1-6.5 microns in diameter), contained round or ovoid synaptic vesicles, and made asymmetrical synaptic contacts. When the degeneration study was combined with the retrograde HRP transport technique, some degenerating terminals from the motor cortex, anterior interpositus, or gracile nuclei were found to synapse directly with HRP-labeled dendrites or somata of the PTA neurons projecting to the DAO. Each combination of the degeneration and anterograde HRP transport techniques revealed the fact that neither degenerating nor HRP-labeled terminals were found to synapse with the same neuronal structure. These observations indicate that the PTA neurons relay afferent inputs from three different sources directly to the DAO, and that there is a possibility of parallel processing rather than convergence of three different afferent systems via the PTA to the DAO.