Abstract
Central projections of the mesencephalic trigeminal nucleus (Vmes) were investigated using anterograde transport of a combination of horseradish peroxidase and wheat germ agglutinin-conjugated horseradish peroxidase. Small, extracellular deposits of the axonal tracer were positioned in the mesencephalic nucleus or surrounding regions using electrophysiological techniques as guidance and iontophoretic injections. This technique proved useful in delineating the central processes of mesencephalic neurons; all central processes of mesencephalic neurons terminated ipsilaterally. The central branch of mesencephalic neurons were confined to the tract of Probst, while collaterals from this branch innervated several brainstem structures. Neurons in Vmes were found to innervate the trigeminal motor nucleus, the supratrigeminal nucleus, the intertrigeminal reticular region, the caudal portion of the dorsal nucleus of the principal sensory nucleus, the dorsal nuclei of trigeminal subnucleus oralis, and the parvicellular reticular region. Fine collaterals were found extending to but no terminal endings were found in the solitary system. No labeling was found below the approximate level of the obex. In order to confirm the termination of the tract of Probst, additional animals were injected with the tracer in regions of the lateral reticular formation above and below the obex. Retrogradely labeled Vmes cells were never found with injections below the obex. These findings indicate a distinct caudal limit to the tract of Probst in the rat and a consistent and selective projection to the subnucleus oralis and principalis of the trigeminal brainstem nuclei.
