NUCLEUS-TRACTUS-SOLITARII - NEUROANATOMY, NEUROCHEMISTRY AND FUNCTION

The nucleus tractus solitarii (NTS) has long been considered as the first central relay for gustatory and visceral afferent informations only. However, data obtained during the past ten years, with neuroanatomical, biochemical and electrophysiological techniques, clearly demonstrate that the NTS is a structure with a high degree of complexity, which plays, at the medullary level, a key role in several integrative processes. The NTS, located in the dorsomedial medulla, is a structure of small size containing a limited number of neurons scattered in a more or less dense fibrillar plexus. The distribution and the organization of both the cells and the fibrillar network are not homogeneous within the nucleus and the NTS has been divided cytoarchitectonically into various subnuclei, which are partly correlated with the areas of projection of peripheral afferent endings. At the ultrastructural level, the NTS shows several complex synaptic arrangements in form of glomeruli. These arrangements provide morphological substrates for complex mechanisms of intercellular communication within the NTS. The NTS is not only the site of vagal and glossopharyngeal afferent projections, it receives also endings from facial and trijeminal nerves as well as from some renal afferents. Gustatory and somatic afferents from the oropharyngeal region project with a crude somatotopy within the rostral part of the NTS and visceral afferents from cardiovascular, digestive, respiratory and renal systems terminate viscerotopically within its caudal part. Moreover the NTS is extensively connected with several central structures. It projects directly to multiple brain regions by means of short connections to bulbo-ponto-mesencephalic structures (parabrachial nucleus, motor nuclei of several cranial nerves, ventro-lateral reticular formation, raphe nuclei...) and long connections to the spinal cord and diencephalic and telencephalic structures, in particular the hypothalamus and some limbic structures. The NTS is also the recipient of several central afferent inputs. It is worth to note that most of the structures that receive a direct projection from the NTS project back to the nucleus. Direct projections from the cerebral cortex to the NTS have also been identified. These extensive connections indicate that the NTS is a key structure for autonomic and neuroendocrine functions as well as for integration of somatic and autonomic responses in certain behaviors. The NTS contains a great diversity of neuroactive substances. Indeed, most of the substances identified within the central nervous system have also been detected in the NTS and may act, at this level, as classical transmitters and/or neuromodulators. About forty neuroactive substances have been detected in the NTS, in particular excitatory and inhibitory amino acids, amines (acetylcholine, catecholamines, serotonin...) and several neuropeptides (hypothalamic and pituitary peptides (vasopressin, oxytocin, TRH...), opioid peptides, "gastro-intestinal" peptides (CCK, VIP, neuropeptide Y...), substance P, neurotensin, angiotensin II...). It is worth to note that within the medulla oblongata the NTS is the richest in neuropeptides and biogenic amines. The chemical transmitters have been identified either within the cell bodies of NTS neurons and/or in fibers and axon terminals. Thus they may be of intrinsic origin or originate from the peripheral and/or central afferent connections to the NTS. Several examples of co-localization of transmitters have been shown within NTS neural elements, as well as interactions between different neuroactive substances. It is also worth noting that transmitters can act, within the NTS, through synaptic as well as non-synaptic mechanisms. These data provide evidence for complex mechanisms of chemical transmission within the NTS. Being extensively connected and containing an impressive diversity of neuroactive substances, it is clear that the NTS does not play a simple role of primary afferent relay. Indeed, the NTS is involved in several integrative processes. It is mainly concerned with basic functions such as autonomic (cardiovascular regulation, respiratory rythmogenesis, motor and secretory digestive processes...) and endocrine functions (control of neurohormones and anterior pituitary secretions, control of blood glucose level...). Moreover, the NTS is involved in more complex activities such as certain emotional processes and the specific behaviors of hunger and thirst. The NTS intervenes also in the control of pain mechanisms and the regulation of the level of consciousness. It is worth to note that the NTS is not only a relay station for these activities, it is involved in the processing of the neural activity by means of a treatment both within local neural networks and through complex endogenous properties of its own neurons. It is puzzling that a structure of such a restricted size is involved in most mechanisms of major importance for the maintenance of life. However, it is clear that, at the medullary level, the NTS is a key structure for the control of homeostasis which requires coordination of autonomic, neuroendocrine and behavioral responses and also a major site for the integration of autonomic and skeletal motor functions.