Effects of acute and chronic chorda tympani denervation on taste responses in the nucleus of the solitary tract

The central effects of peripheral deafferentation have been well described in visual, somatosensory and auditory systems. However little is known of central plasticity in the gustatory system. A limited number of anatomical, neurophysiological, behavioral and psychophysical studies suggest that changes do occur in the central taste system after peripheral deafferentation. The purpose of this project was to determine whether physiological changes occur in the first order central taste relay, the nucleus of the solitary tract (NST) after acute or chronic chorda tympani (CT) denervation. Ten percent of multi- and single-unit sites in NST increased their response to taste stimulation by an average of 33% during acute CT denervation (anesthesia). In addition, we noted a decrease in the mean spontaneous rate of NST cells during anesthesia. This effect was robust for cells that responded to taste mixture stimulation of the anterior tongue and hard palate. The CT innervates taste buds located on the anterior tongue and the greater superficial petrosal nerve innervates taste buds on the hard palate. Since the taste response elicited by palatal stimulation was affected little by anesthesia, but the spontaneous rate greatly decreased, in essence, there was an increase in the signal-to-noise ratio for these cells. It appears that taste compensation after CT anesthesia may be partly due to disinhibition and an increase in the signal-to-noise ratio of a subset of NST cells. Chronic CT denervation (transection) resulted in increased posterior tongueelicited taste responses of NST cells. Also, there was a trend for increased palatal-elicited taste responses of NST cells. However, the orotopic organization of taste and tactile responses in the NST was unaltered after denervation. These results suggest that intact nerves increase their functional input in their normal terminal fields in the NST. Based on these studies, the gustatory system at the level of the NST has the potential for neural plasticity to compensate for lost input, but appears more resistant to the large-scale changes that have been found in other sensory systems. More importantly, these results provide us with more information concerning the normal processing of taste input in the central nervous system.