The Functional Consequences of Gustatory Nerve Regeneration as Assessed Behaviorally in a Rat Model

Abstract

When gustatory nerves are transected, the taste buds they normally innervate degenerate and specific deficits in taste-related behavior result depending on the nerves transected, the taste compounds used, and the nature of the behavioral test (see Spector, 2003). In rodents, the chorda tympani nerve (CT) which innervates the anterior tongue, and the glossopharyngeal nerve (GL) which innervates the posterior tongue, both have a great proclivity to regenerate after injury and reinnervate their native receptor fields. It is clear that there are some seemingly permanent anatomical consequences of CT regeneration including reductions in taste bud number and volume, decreases in the number of myelinated axons, decrease in the density of terminal projections, and a decline in the volume of the terminal field in the rostral nucleus of the solitary tract (NST) (Shuler et al., 2004; for review, see Spector, 2003). These regeneration induced changes in the anatomy of the system raise the issue of whether taste function would be altered in some way. Accordingly, my laboratory has been using behavioral procedures to examine whether functions that are disrupted by neurotomy recover upon regeneration of the nerve. The CT, which innervates ~13% of the total taste buds in the rat, is exceptionally responsive to NaCl. Although for many years the input of the CT was thought to be unnecessary to maintain sensibility to NaCl based on two-bottle preference tests, when more psychophysically rigorous tasks were applied to assess responsiveness to this salt, very severe and unequivocal behavioral deficits were revealed in rats that had the nerve transected. Thus, the ability of the regenerated CT to support taste-guided performance in tasks involving NaCl was a conceptually promising way to begin to assess the functional conse