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Morphological and neurochemical analysis of sensory afferents of the vagus nerve

Sykes, Rachel Mary; (1995) Morphological and neurochemical analysis of sensory afferents of the vagus nerve. Doctoral thesis (Ph.D), UCL (University College London). Green open access

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Abstract

A detailed morphological analysis of sensory afferents throughout the different subnuclei of the rostrocaudal extent of the rat nucleus tractus solitarius (nTS) is described. Vagal afferents were labelled using neuroanatomical tracing techniques and examined at the electron microscope level. Labelled boutons were characterized as containing round, clear synaptic vesicles and occasionally dense core vesicles and forming asymmetric synaptic contacts with predominantly small to medium calibre dendrites. This anatomical organization was observed throughout the entire nTS. Only one possible vagal afferent was observed forming an axo-axonic contact suggesting modulation of vagal afferent input occurs by interactions on a common post-synaptic target. Further studies were undertaken to investigate the neurochemical content of vagal afferents. Glutamate, has been detected in a variety of afferent projections throughout the CNS but evidence for this amino acid in vagal afferents is equivocal. Initially the distribution of glutamate-immunoreactive structures in the nTS was examined using post-embedding immunocytochemical techniques. Immunoreactive perikarya were sparse at the more caudal and rostral extremes of the nTS but were more numerous at intermediate levels where distinct groups of glutamate-containing neurones were observed in the tractus solitarius and medial subnucleus. The nTS was densely innervated with glutamate-immunoreactive boutons which had a homogeneous distribution throughout its rostrocaudal extent. Ultrastructural analysis revealed that these immunolabelled boutons had an identical morphological profile to vagal afferents. When anterograde tracing and immunocytochemistry were combined in the same tissue immunolabelled boutons further identified as vagal afferents were detected, establishing that the sensory neurones of the nodose ganglia contribute to the glutamatergic innervation of the nTS. Substance P (SP), calcitonin gene-related peptide (CGRP) and 5-hydroxytryptamine (5-HT) had previously been demonstrated in nodose ganglia perikarya but there was no direct evidence for their presence in central vagal afferent terminations. A technique was developed that allowed the simultaneous detection of both the anterogradely labelled vagal afferents and putative neurotransmitter at the light microscope level using antibodies labelled with different fluorochromes. Problems encountered with identifying double-labelled structures with a standard fluorescent microscope led to the development of a method of analysis that ultilized a laser scanning confocal microscope and image capture system. Direct evidence for the presence of 5- HT, SP and CGRP immunoreactivity in the central terminals of sensory vagal afferents was obtained. Very few SP-immunoreactive vagal afferents were observed suggesting that SP may play a greater role in the periphery as opposed to the CNS. Vagal afferents containing 5-HT were detected in regions that receive mainly gastrointestinal tract afferents whilst vagal afferents immunoreactive for CGRP were concentrated in areas receiving a general visceral innervation. The differential distribution of vagal afferents immunoreactive for these neurochemicals suggests that the chemical expression of nodose ganglia perikarya may be related to their target tissues or the functional class of the afferent fibres.

Type: Thesis (Doctoral)
Qualification: Ph.D
Title: Morphological and neurochemical analysis of sensory afferents of the vagus nerve
Open access status: An open access version is available from UCL Discovery
Language: English
Additional information: Thesis digitised by ProQuest.
Keywords: Biological sciences; Sensory afferents
URI: https://discovery.ucl.ac.uk/id/eprint/10103241
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