DISTRIBUTION AND SYNAPTOLOGY OF GLOSSOPHARYNGEAL AFFERENT NERVE-TERMINALS IN THE NUCLEUS OF THE SOLITARY TRACT OF THE HAMSTER

The distribution and synaptology of the afferent fibers of the glossop haryngeal nerve (IXN) in the hamster were studied by using horseradish peroxidase (HRP) histochemistry visualized with light and electron mi croscopy. Crystals of HRP were applied to the trunk of IXN in the vici nity of the petrosal ganglion. The densest IXN afferent label was dist ributed within the nucleus of the solitary tract (nst), just caudal to but overlapping with the area of termination of the facial nerve. Lab eled IXN fibers extended rostrally to the principal trigeminal nucleus and caudally to the cervical spinal cord. There was significant label ing within the spinal trigeminal complex; the area postrema and the me dullary reticular formation contained some labeled fibers. Ultrastruct urally, the synaptic arrangements of anterogradely labeled IXN fibers were examined in the nst. Quantitative measures were taken of the area , maximum diameter, perimeter, and vesicles of labeled endings and the length of their synaptic junctions with dendritic processes. These en dings were compared to comparable endings in control material and to p ublished descriptions of VIIth nerve afferent terminals in the hamster nst. The synaptic relations of IXN afferent endings were predominantl y with dendritic spines and shafts. The majority (86.6%) of IXN affere nt endings were with dendritic processes that were not in apparent con tact with other, unlabeled processes. Only 13.4% of IXN synaptic relat ionships were with dendritic processes that were also contacted by unl abeled vesicle-containing processes. This is in contrast to 31.2% of f acial nerve afferent endings in the nst which make synaptic contact wi th such processes. There were more direct synaptic contacts between fa cial endings and unlabeled vesicle-containing processes (26.1%) than b etween IXN endings and unlabeled vesicle-containing processes (1.3%). Thus, unlike the glomerular-like endings of the gustatory fibers of th e VIIth nerve, less complex relations appeared to characterize IXN syn apses in the nst. These differences were related to the differential p hysiology of gustatory fibers in the VIIth nerve and IXN. (C) 1996 Wil ey-Liss, Inc.