Ultrastructure of Rat Rostral Nucleus of the Solitary Tract Terminals in the Parabrachial Nucleus and Medullary Reticular Formation

Abstract

Neurons in the rostral nucleus of the solitary tract (rNST) receive taste information from the tongue and relay it mainly to the parabrachial nucleus (PBN) and the medullary reticular formation (RF) through two functionally different neural circuits. To help understand how the information from the rNST neurons is transmitted within these brainstem relay nuclei in the taste pathway, we examined the terminals of the rNST neurons in the PBN and RF by use of anterograde horseradish peroxidase (HRP) labeling, postembedding immunogold staining for glutamate, serial section electron microscopy, and quantitative analysis. Most of the anterogradely labeled, glutamate-immunopositive axon terminals made a synaptic contact with only a single postsynaptic element in PBN and RF, suggesting that the sensory information from rNST neurons, at the individual terminal level, is not passed to multiple target cells. Labeled terminals were usually presynaptic to distal dendritic shafts in both target nuclei. However, the frequency of labeled terminals that contacted dendritic spines was significantly higher in the PBN than in the RF, and the frequency of labeled terminals that contacted somata or proximal dendrites was significantly higher in the RF than in the PBN. Labeled terminals receiving axoaxonic synapses, which are a morphological substrate for presynaptic modulation frequently found in primary sensory afferents, were not observed. These findings suggest that the sensory information from rNST neurons is processed in a relatively simple manner in both PBN and RF, but in a distinctly different manner in the PBN as opposed to the RF.

Synaptic connectivity of the TRPV1-positive trigeminal afferents in the rat lateral parabrachial nucleus

“…In addition, number of dendritic spines per TRPV1+ bouton was much fewer in the LPBN than Vc. This difference between LPBN and Vc is analogous to the difference between the frequency of synapses of the tooth pulp afferents and the rostral nucleus of solitary tract afferents with dendritic spines in their respective functionally different target nuclei (Bae et al, 2003;Park et al, 2022). It is consistent with the idea that under pathological conditions, the change in synaptic strength and postsynaptic neuron excitability through spine plasticity can be less pronounced in the LPBN than in the Vc, thus affecting the emotional aspect of pain less than the pain perception per se.…”