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rNST Circuits.


Bradley RM.


In: Bradley RM, editor.


The Role of the Nucleus of the Solitary Tract in Gustatory Processing. Boca Raton (FL): CRC Press; 2007. Chapter 7.
Frontiers in Neuroscience.


In previous chapters, the current knowledge of the neurobiology of the mammalian brainstem gustatory relay nucleus has been detailed. Information presented shows that all chemosensory information derived from stimulating taste receptors, no matter where they are located, has to pass through the rostral nucleus of the solitary tract (rNST), and by the 1960s, the basic brainstem projection pattern of the afferent gustatory nerves had been established in outline form (summarized in Chapter 1, Figure 1.1). Details of the development of the connections are only now being studied and found to consist of complex overlapping terminal fields that suggest highly convergent input to the second-order neurons (Figure 6.2). Further anatomical pathway tracing mainly in rodents has established the projection patterns from the rNST to both rostral brain areas and brainstem sites (Figure 7.1). As described in Chapter 4, the brainstem connections from rNST are the secretomotor output to the salivary glands and motor output to various muscles involved in oral reflexes and facial expression. The rostral projection divides at the parabrachial nucleus, with one pathway passing through the thalamus to the cortex, whose function is believed to be involved in the sensory discriminative aspect of taste perception, whereas the other, limbic pathway is believed to be involved in the hedonic component of taste perception and its control of feeding. In addition, descending connections from forebrain areas to the rNST have also been described and to some extent investigated by examining the effects of electrical stimulation of these areas on the response characteristics of rNST neurons (Chapter 5). Extracellular recordings were initially used to functionally establish that the rNST was in fact the brainstem taste relay, and then this technique was used by numerous later investigations to probe the role of the relay nucleus in taste processing. It is only in recent years that intracellular recordings have been used to characterize the biophysical and synaptic properties of rNST neurons. Despite extensive investigations by many laboratories, the rNST essentially remains a “black box,” and there is little information on what the rNST neurons do or how they interact as a neural circuit to process gustatory information. Nevertheless, investigators have made conclusions based on extracellular recordings from unidentified neurons in a limited part of the rNST on how the nucleus processes taste information. These experimental approaches have used similar techniques: Extracellular recordings are made from a sample of rNST neurons, and then a variety of ever more complex analyses applied to the data set. Often, only one concentration of a stimulus is used, and some feature of the resultant neural discharge is analyzed. It is apparent, therefore, though never stated, that several underlying assumptions are made in all these experiments that, if challenged, could undermine the conclusions that are drawn. It is my intent in this chapter to examine these assumptions more closely and suggest other ways to approach the study of the role of the rNST in processing taste stimulus-initiated neural activity.

Copyright © 2007, Taylor & Francis Group, LLC

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