Altered taste responses in adult NST after neonatal chorda tympani denervation

Altered taste responses in adult NST after neonatal chorda tympani denervat ion. J. Neurophysiol. 82: 2565-2578, 1999. Anatomic and behavioral changes have been observed in the taste system after peripheral deafferentation, bu t their physiological consequences remain unknown. Interestingly, a recent behavioral study suggested that peripheral denervation could induce central plasticity. After neonatal chorda tympani (CT) transection, adult rats dem onstrated a marked preference for a normally avoided salt, NH4Cl. In the pr esent study, taste responses were recorded from the nucleus of the solitary tract (NST) in similarly CT-denervated rats to investigate a physiological basis for this behavioral phenomenon. We hypothesized that alterations in functional connectivity of remaining afferent nerves might underlie the beh avioral change. Specifically, if NST neurons formerly activated by sodium-s elective CT fibers were instead driven by more broadly tuned glossopharynge al (GL) afferents, neural coding of salt responses would be altered. Such a change should be accompanied by a shift in orotopic representation and inc reased NH4Cl responses. This hypothesis was not supported. After CT denerva tion, orotopy was unaltered, NH4Cl responsiveness declined, and no other ch anges occurred that could simply explain the behavioral effects. Indeed, th e most pronounced consequence of CT denervation was a 68% reduction in NaCl responses, supporting previous evidence for a critical role of this nerve in coding sodium salts. In addition, we found "reoganizational" changes sim ilar to, albeit smaller than, those observed in other sensory systems after deafferentation. There was a trend for increased responses elicited by sti mulation of receptor subpopulations innervated by the GL and greater superf icial petrosal nerves. In addition, the spontaneous rate of nasoincisor duc t-responsive cells increased significantly. This effect on spontaneous rate is opposite to that produced by CT anesthesia, suggesting that acute versu s chronic denervation may affect central taste neurons differently. In conc lusion, the taste system at the medullary level seems more resistant to lar ge-scale plasticity than other sensory systems, but nevertheless reacts to lost afferent input. Because the most robust plastic changes have been docu mented at cortical levels in other sensory pathways, the substrate for the behavioral effect of neonatal CT transection may be located more centrally in the gustatory system.