Synaptic organization of the interstitial subdivision of the nucleus tractus solitarii and of its laryngeal afferents in the rat

Abstract

The nucleus tractus solitarii, the first central relay for gustatory and a variety of visceral afferents, is also an integrative center for numerous functions. Its interstitial subdivision is involved in swallowing and respiratory reflexes. The ultrastructural characteristics of this subdivision and of its laryngeal afferents were investigated in adult rat by a serial-section study and by application of wheat germ agglutinin-horseradish peroxidase conjugate to the peripheral afferent fibers. The interstitial subnucleus contained scattered small neuronal cell bodies with such ultrastructural features as a large nucleus with deep indentations and an organelle-poor cytoplasm. On the basis of their size and vesicular content, the axon terminals were classified into three categories. Group I and group II terminals were small or large, respectively, and contained mainly small, round, and clear synaptic vesicles. Group III terminals were also small but contained small, pleomorphic, and clear vesicles. Axodendritic synapses were the most numerous. They were either asymmetrical, comprised of group I and II terminals, or symmetrical, comprised of group III terminals. More than 50% were part of complex synaptic arrangements in the form of rosettes or glomeruli. Axosomatic contacts involved both group I and group III terminals and were always symmetrical. A high frequency of axoaxonic synapses was found. They were symmetrical, comprised of group III terminals on group I or II terminals. Different types of symmetrical synaptic contacts made by dendrites were also found. This study indicates also that the ipsilateral interstitial subdivision constitutes the preferential site of termination for superior laryngeal afferents. The labeled axon terminals belonged exclusively to groups I and II and were involved in both axodendritic and axoaxonic synapses. Some of the axodendritic synapses were part of rosettes or glomeruli. All these synaptic arrangements may be considered a morphological substrate for important processing of afferent information in the nucleus tractus solitarii. They may account for some of the integrative functions of the interstitial subnucleus such as physiological processes triggered from the superior laryngeal nerve.

Silent Synapses in Developing Rat Nucleus Tractus Solitarii Have AMPA Receptors

“…More specifically related to synaptic formation, spillover may recruit extrasynaptic NMDARs or activate distant NMDAR clusters floating in the dendritic membrane (Friedman et al, 2000;Washbourne et al, 2002). For the adult NTS, vagal afferent terminals are generally large, and most synapses are part of synaptic glomeruli (Mrini and Jean, 1995;Hayakawa et al, 2003). This peculiar organization may favor crosstalk between active synapses during development and contribute to synaptic processing as well as autonomic circuit formation.…”

Central nervous system control of the laryngeal muscles in humans

“…The laryngeal adductor response involves afferent fibers contained in the iSLN with the cell bodies in the nodose ganglion terminating in the nucleus tractus solitarius (Sessle, 1973;Mrini and Jean, 1995;Gestreau et al, 1997;Ambalavanar et al, 2004). On the other hand, the afferents involved in cough are located in the tracheal bifurcation and are also thought to have their cell bodies in the nodose ganglion although in the guinea pig these are contained in the jugular ganglion (McAlexander et al, 1999).…”

Glutamate receptor subunits in the nucleus of the tractus solitarius and other regions of the medulla oblongata in the cat

“…The most numerous synapses in the interstitial subnucleus of the NTS are axodendritic synapses (Glaum and Miller, 1995;Mrini and Jean, 1995), and the terminals of vagal afferents make symmetric and asymmetric contacts with the dendrites in the interstitial subnucleus of the NTS (Chazal et al, 1991). There is some evidence that these terminals are glutamatergic (see discussion in Mrini and Jean, 1995), but immunocytochemical studies at the ultrastructural level are necessary to clarify this point. It generally is assumed that the vagal afferents form synapses on dendrites of the most common neuron type, i.e., the small to medium, round to ovoid cells, but this has not been proven.…”