Ultrastructural organization of the interstitial subnucleus of the nucleus of the tractus solitarius in the cat: Identification of vagal afferents

Abstract

This electron microscopic study, based on serial section analysis, describes the synaptic organization of the interstitial subnucleus of the nucleus of the solitary tract and identifies the terminals of the vagal primary afferents utilizing degeneration and HRP transport. The interstitial subnucleus contains sparsely scattered cell bodies, numerous dendrites and axon terminals, and bundles of unmyelinated and myelinated axons. The cell bodies which are small in diameter have an organelle poor cytoplasm and a large invaginated nucleus. Axon terminals can be classified into two main types according to their vesicular shape. The first type contains clear, round vesicles and can be further subdivided into two subgroups on the basis of their morphology and the size of their vesicles. In the first subgroup the terminals are small, contain a few mitochondria and their vesicles are densely packed with an homogeneous size. In the second subgroup the terminals which vary from small to large, contain many mitochondria and contain round vesicles which are heterogeneous in size. The second main terminal type consists of axon terminals containing pleomorphic vesicles which are associated with asymmetrical or symmetrical synaptic contacts on dendrites. Axo-axonic contacts are present in the interstitial subnucleus. In general, the presynaptic axon terminals contain pleomorphic vesicles and the postsynaptic elements contain round vesicles of varying size. In some dendrites, identified by the presence of ribosomes, groups of round and/or pleomorphic vesicles are found associated with synaptic contacts. These dendrites are presynaptic to conventional dendrites and postsynaptic to axon terminals. After removal of the nodose ganglion, degenerative alterations are seen only at the caudal and middle levels of the interstitial subnucleus. Degeneration occurs in a few myelinated axons and in axon terminals which usually contain a mixture of small and larger round, clear vesicles. After HRP injection into the vagus nerve, the HRP reaction product is visible in axon terminals filled with clear, round vesicles which are heterogeneous in size. The labelled axon terminals establish single or multiple synaptic contacts. This study demonstrates that terminals of vagal primary afferents consist principally of terminals of the second subgroup. The morphology of these terminals are compared to primary afferents in the brainstem and spinal cord.

Glycine-immunoreactive synaptic terminals in the nucleus tractus solitarii of the cat: Ultrastructure and relationship to GABA-immunoreactive terminals

“…Although many axonal profiles in the tractus solitarius did contain high levels of gold labeling, they probably originate from glycinergic projection neurones sited within the medulla or spinal cord. Neither do the observed effects of glycine seem likely to be through presynaptic inhibition of afferent terminals, since such contacts were not observed in the present study, besides which, axo-axonic synaptic contacts have only been rarely observed in most regions of the NTS (Chazal et al, 1991;Chen et al, 1992;Izzo et al, 1992;Maqbool et al, 1991;Saha et al, 1995a). As reported previously for the rat (Cassell et al, 1992), the vast majority of glycine-IR synapses in the NTS contacted small or medium-sized dendritic profiles; therefore, the effects of glycine are most likely brought about via postsynaptic inhibition of interneurones.…”

The ultrastructure of perisynaptic glia in the nucleus tractus solitarii of the adult rat: Comparison between single synapses and multisynaptic arrangements

“…Scale bars: 500 nm (electron micrographs), 400 nm (three-dimensional reconstructions). Chazal et al, 1991;Davis, 1998;Hayakawa et al, 2003;May et al, 2007;Whitehead, 1986). Accordingly, about half of our reconstructed synapses were directly apposed to at least one other synapse without intervening glia.…”

“…Studies performed at the cerebellar mossy fiber to granule cells synapse indicate that glutamate spillover in divergent glomeruli enhances the efficacy of fast synaptic transmission by decreasing fluctuations resulting from probabilistic transmitter release (DiGregorio et al, 2002;Sargent et al, 2005). In the NTS, sensory afferent terminals are frequently involved in divergent arrangements (Chazal et al, 1991;Davis, 1998;Hayakawa et al, 2003;May et al, 2007;Whitehead, 1986). One may postulate that glutamate spillover within these synaptic clusters helps to insure reliable transmission of sensory information.…”

“…Most NTS synapses are glutamatergic including those made by visceral primary afferents (Lachamp et al, 2006; see also Baude et al, 2009 for review). In addition to single synapses, the NTS contains numerous divergent and convergent multisynaptic arrangements (Brining and Smith, 1996;Chazal et al, 1991;Davis, 1998;Hayakawa et al, 2003;May et al, 2007;Whitehead, 1986). The present study was undertaken to quantitatively analyze and compare the ultrastructure of perisynaptic glia at single and complex NTS synapses.…”

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

“…Saha et al (1995) showed that most glutamatergic terminals in the medial, dorsal, gelatinosus, dorsolateral, and interstitial subnuclei of the NTS of the cat form on dendrites of different sizes. 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.…”