The Role of the Nucleus of the Solitary Tract in Gustatory Processing PDF

Preface

Recent discoveries of G-protein coupled membrane receptors in taste cells have resulted in a reexamination of the mechanisms of taste transduction and how different taste qualities are encoded. In particular, sweet, bitter, and amino acid (umami) receptors have been identified and apparently localized to separate cells within the taste bud. Investigators now describe taste buds as having sweet cells, amino acid cells, and bitter cells. The existence of salt and sour cells is usually not mentioned. The assumption increasingly made is that quality coding in the taste system is hardwired with direct connections (so-called labeled-line) between taste cells that express a particular receptor type and the central nervous system area that is responsible for perceptual behavior, the concept being that there are separate taste pathways for preference and aversive behaviors. Many of the previous investigations of taste coding argued against this labeled-line explanation based on taste bud cell turnover and peripheral and central nervous system neurophysiological investigations. More to the point, the labeled-line hypothesis assumes that the relays in the central taste pathway have a minimal role in sensory processing, merely serving to connect the different ascending tracts of the gustatory pathway. However, much research has established that although specificity might exist to some extent at the taste periphery, it is gradually degraded at each relay nucleus in the ascending taste pathway.

The first relay in the taste pathway is the nucleus of the solitary tract, a gateway to central taste processing. However, the neurobiology of the solitary tract nucleus and the other central nervous system taste relays has received relatively little attention. Investigators who examined the anatomical and neurophysiological properties of the central taste relay nuclei have demonstrated that afferent taste information changes as it passes through each relay. For example, central neurons that respond to chemicals that taste sweet or bitter also respond to salts and acids, demonstrating that convergence of information occurs. Although early research seemed to indicate that the nucleus of the solitary tract is a relatively simple relay, more recent investigations using a variety of techniques have revealed that this nucleus is quite complex; besides being part of the ascending taste pathway, the nucleus also serves to connect to several brainstem sites responsible for a number of important taste-initiated reflex functions

A principal motivation for assembling this book was to bring together in one volume the expertise of a number of investigators who have studied the nucleus of the solitary tract and have contributed substantially to the current knowledge of the anatomy, physiology, and developmental biology of the solitary nucleus.

My aim was to provide the reader with information that has never before been gathered in one place to serve as a reference and hopefully motivate others to bring new approaches to advance knowledge about the central processing of gustatory information. As the reader will find, much has been accomplished, but the time is ripe for progress in revealing what the solitary nucleus does with the gustatory information delivered to its afferent portal.