The reticulovestibular projection in the rabbit: an experimental study with the retrograde horseradish peroxidase method.

The reticulovestibular projections of the brainstem in the rabbit were studied by the retrograde transport of horseradish peroxidase (HRP). After selective iontophoretic injections of the tracer into various subdivisions of the vestibular nuclear complex (VNC), labeled neurons were found in defined regions of the reticular formation (RF) of the caudal pons and the rostral medulla. The results indicate that all four vestibular nuclei receive projection from RF. This projection is bilateral with a contralateral predominance. The major projection originates from dorsal and dorsolateral regions of the caudal pontine reticular nucleus (RPc) and the gigantocellular reticular nucleus (RGc) at the transitional level between them. A modest projection originates from pars alpha of the caudal pontine reticular nucleus (RPc alpha), the parvocellular reticular nucleus (Rpc) and pars alpha of the parvocellular nucleus (Rpc alpha), mostly from their ventral regions. A small projection arises from pars alpha of the gigantocellular reticular nucleus (RGc alpha), as well as from the ventral reticular subnucleus (Rv) and cell group a in the caudal aspect of the medulla. No clear-cut topical relationship was noted between the location of neurons in RF and projection site in VNC. The superior vestibular nucleus (SV) and the medial vestibular nucleus (MV) receive projections exclusively from RPc and RGc, whereas the lateral reticular nucleus (LV) and the inferior vestibular nucleus (IV) receive additional projections from the remaining RF nuclei. The termination areas of reticular fibers within SV and IV seem to be diffuse but in MV and LV there is a clear preponderance to the regions located ventrally. The present study has established cells of origin for the reticulovestibular projections from the pontomedullary RF to individual VNC nuclei in the rabbit.