Effects of Competition on the Gustatory Nuclear of the Solitary Tract

Competitive interactions play a major role in the development of sensory systems. Here, for the first time, I show that competition plays a role in not only the development of, but also in the maintenance of terminal field organization within the rat's gustatory system. The terminal fields of the chorda tympani, greater superficial petrosal, and glossopharyngeal nerves all terminate in the rostral nucleus of the solitary tract (NTS), in discrete yet overlapping fields. Regardless of the age when the greater superficial petrosal and glossopharyngeal nerves are sectioned (P15, P25 or adulthood), the chorda tympani nerve terminal field expanded to a volume four times larger than expected from controls, into territory normally occupied by the greater superficial petrosal and glossopharyngeal nerve terminal fields. The expanded chorda tympani nerve terminal field was equal in volume and distribution to the terminal field of immature animals (P15). Despite the remarkable reorganization of the chorda tympani nerve terminal field, and despite the lack of sensory input from the two sectioned nerves, taste-related behavioral responses to NaCl, quinine hydrochloride, and sucrose were unaffected. Examination of all three terminal fields after regeneration of the sectioned nerves revealed that if the sectioned nerves successfully regenerated, the chorda tympani nerve terminal field volume was reduced to that found in controls. Successful regeneration of the glossopharyngeal nerve was dependent on age of section, with sectioning occurring at younger ages leading to less regeneration. Regeneration of the greater superficial petrosal nerve was also dependent on age of nerve section and took longer than glossopharyngeal nerve regeneration. In rats that received nerve cuts at adulthood, I propose that the regenerated iii glossopharyngeal nerve terminal field re-exerts competitive influence over the expanded chorda tympani nerve terminal field, and is responsible for the decrease in density in the dorsal portion of the NTS. All together, the work presented here shows a prominent role for competition in the development and life-long maintenance of terminal field organization with the rodent central gustatory system.

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