Distribution and synaptology of glossopharyngeal afferent nerve terminals in the nucleus of the solitary tract of the hamster

Abstract

The distribution and synaptology of the afferent fibers of the glossopharyngeal nerve (IXN) in the hamster were studied by using horseradish peroxidase (HRP) histochemistry visualized with light and electron microscopy. Crystals of HRP were applied to the trunk of IXN in the vicinity of the petrosal ganglion. The densest IXN afferent label was distributed within the nucleus of the solitary tract (nst), just caudal to but overlapping with the area of termination of the facial nerve. Labeled IXN fibers extended rostrally to the principal trigeminal nucleus and caudally to the cervical spinal cord. There was significant labeling within the spinal trigeminal complex; the area postrema and the medullary reticular formation contained some labeled fibers. Ultrastructurally, 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 endings were compared to comparable endings in control material and to published descriptions of VIIth nerve afferent terminals in the hamster nst. The synaptic relations of IXN afferent endings were predominantly with dendritic spines and shafts. The majority (86.6%) of IXN afferent endings were with dendritic processes that were not in apparent contact with other, unlabeled processes. Only 13.4% of IXN synaptic relationships were with dendritic processes that were also contacted by unlabeled vesicle-containing processes. This is in contrast to 31.2% of facial nerve afferent endings in the nst which make synaptic contact with such processes. There were more direct synaptic contacts between facial endings and unlabeled vesicle-containing processes (26.1%) than between IXN endings and unlabeled vesicle-containing processes (1.3%). Thus, unlike the glomerular-like endings of the gustatory fibers of the VIIth nerve, less complex relations appeared to characterize IXN synapses in the nst. These differences were related to the differential physiology of gustatory fibers in the VIIth nerve and IXN.

Synaptic relationships between the chorda tympani and tyrosine hydroxylase-immunoreactive dendritic processes in the gustatory zone of the nucleus of the solitary tract in the hamster

“…In the hamster, individual neurons in the gustatory NST respond to salty, sour (acidic), sweet-tasting, and occasionally bitter-tasting compounds (Smith et al, 1983). This broad responsiveness could be established by the convergence of inputs from other regions of the oral cavity that terminate on a single second-order dendrite (Brining and Smith, 1996;Travers et al, 1986). The anatomical substrate for such convergence could be the glomerular-like synaptic contacts of the type identified in the present study.…”

“…The incidences of these three synaptic profiles of the CT differ from those of the lingual branch of IX, which innervates the taste buds in the foliate and vallate papillae. In the hamster gustatory NST, about 87% of the terminal boutons of the lingual branch of IX are in 1:1 contact with second-order dendrites (Brining and Smith, 1996). Multiple contacts on single dendrites, including glomerular-like synaptic profiles, are uncommon.…”

Mammals

“…In cats and primates, the nucleus of the solitary tract extends rostral to the level of entrance of the facial nerve as a group of neurons embedded in a fibrous capsule within the dorsal part of the pars oralis of the spinal trigeminal nucleus and the principal sensory nucleus of the trigeminal nerve ( Fig. A rostral extension of the nucleus of the solitary tract and ascending components of the facial, glossopharyngeal and vagus nerves appear to be absent in rodents (see, however, Brining and Smith 1996). In humans, this rostral extension of the nucleus of the solitary tract was referred to as the nucleus ovalis by Olszewski and Baxter (1954).…”

Afferent connections of the parabrachial nucleus in C57BL/6J mice

“…In these model organisms, afferent fibers of the VIIth, IXth, and Xth cranial nerves convey gustatory information to the rostral nucleus of the solitary tract (NST) (Brining and Smith, 1996; Contreras et al, 1982; Hamilton and Norgren, 1984; Hayakawa et al, 2001; Torvik, 1956; Whitehead and Frank, 1983; Whitehead, 1986). From the NST, ascending fibers project to the pontine parabrachial nucleus (PbN) (Halsell et al, 1996; Herbert et al, 1990; Norgren, 1978; Norgren and Leonard, 1971; Travers, 1988; Whitehead, 1990).…”