Descending brainstem projections of the pedunculopontine tegmental nucleus in the rat

Descending brainstem projections from the pedunculopontine tegmental nucleus (PPN) were studied in the rat by use of the anterograde tracer Phaseolus vulgaris-leucoagglutinin (PHA-L) and the retrograde tracer lectin-conjugated horseradish peroxidase (HRP-WGA). Results of these experiments demonstrated prominent bilateral projections to the pontomedullary reticular nuclei, but direct connections to the motor and sensory nuclei of the cranial nerves could not be ascertained. The PPN fibers terminated mainly in the pontine reticular nuclei oralis and caudalis and in ventromedial portions (pars alpha and pars ventralis) of the gigantocellular reticular nucleus. A smaller number of labeled fibers distributed to more dorsal regions of the gigantocellular nucleus, lateral para-gigantocellular, ventral reticular nucleus of the medulla and lateral reticular nucleus. Although a significant number of PHA-L labeled fibers was seen in two cases in the contralateral medial portion of the facial nucleus, and all cases exhibited a sparse predominantly ipsilateral projection to the lateral facial motor neurons, the retrograde tracing experiments have revealed that these facial afferents originated in the nuclei surrounding the PPN. The results are discussed in the context of PPN involvement in motor functions. It is suggested that the PPN may participate in a complex network involved in the orienting reflex.