Functional recovery of taste sensitivity to sodium chloride depends on regeneration of the chorda tympani nerve after transection in the rat.

Abstract

Chorda tympani nerve (CT) transection (CTX) raises sodium chloride (NaCl) taste detection threshold, but the effect of CT regeneration on NaCl threshold is unknown. This experiment examined whether CT regeneration supports normal NaCl threshold in the rat. Thresholds were measured with a 2-lever operant procedure. Thresholds increased more than 1 order of magnitude after CTX regardless of recovery period length. Postsurgical thresholds in rats with regenerated CTs did not differ from presurgical values. Stimulus adulteration with amiloride raised thresholds in rats with intact or regenerated CTs by about 1 order of magnitude but did not raise thresholds beyond postsurgical levels in rats with transected CTs. Thus, the regenerated CT supports normal NaCl threshold, which is raised by amiloride. Because thresholds remained elevated 62 days after CTX when regeneration was prevented, compensatory processes alone cannot support normal NaCl threshold.

Nerve regeneration-induced recovery of quinine avoidance after complete gustatory deafferentation of the tongue

“…Hamsters that have successfully acquired a conditioned taste aversion to NaCl subsequently fail to avoid the salt solution after CT transection but can reacquire the aversion on regeneration of the nerve (3). In rats, CT transection produces an increase in NaCl detection threshold of Ͼ1 order of magnitude and impairs the ability to discriminate NaCl from KCl, with sensitivity and salt discriminability returning to presurgical levels once the CT has regenerated (15,16). Moreover, in contrast to CT-transected rats, animals with regenerated CTs are just as sensitive to the performance-disrupting effects of amiloride treatment on NaCl detectability as intact rats (16).…”

“…In rats, CT transection produces an increase in NaCl detection threshold of Ͼ1 order of magnitude and impairs the ability to discriminate NaCl from KCl, with sensitivity and salt discriminability returning to presurgical levels once the CT has regenerated (15,16). Moreover, in contrast to CT-transected rats, animals with regenerated CTs are just as sensitive to the performance-disrupting effects of amiloride treatment on NaCl detectability as intact rats (16). These findings suggest that regeneration of the CT leads to normal recovery of salt taste in rodents.…”

“…The amount of reinnervation that takes place depends on several factors, including the species, how and where the injury is made, and whether surgical anastomosis is performed (see 5,8,19,20,23,24,34,42,43). In rodents, the CT and GL can be completely transected and, without any further intervention, find their way back to their native receptor fields, unless a large section of the nerve is removed or neurotoxins are specifically applied (2,7,13,15,16,21,34,36,40). Although significant regeneration occurs, only about three-fourths of the normal complement of taste buds appear to return, and the volume of the regenerated taste buds is reduced by Ͼ40% (13, 16, 25, 34, 36).…”

Learning-based recovery from perceptual impairment in salt discrimination after permanently altered peripheral gustatory input

“…Interestingly, the performance of Sham-Int rats was similar to that of Sham animals, despite testing with concentrations that were 1.0 log 10 unit lower (P Ͼ 0.05). This group was included as a control for a likely attenuation in stimulus intensity that occurs after CTx (28,46,58). We lowered the stimulus concentrations by a log unit during postsurgical testing in a subset of rats subjected to sham surgery to test whether the initial drop in performance following nerve transection could have been simply due to a weakened stimulus.…”

“…Behavioral studies using rodent models have shown that the chorda tympani (CT) nerve, a branch of the seventh cranial nerve that provides the sole innervation of taste receptor cells in the anterior tongue, is necessary for normal taste-guided behavior related to salts. Both sodium detection and salt discrimination are severely affected by CT transection (CTx) (18,27,28,45,54,58,60). The behavioral findings are not surprising, given that the CT is highly responsive to salts (2,37,38,40,41) and incorporates a sodium-specific signal via a subpopulation of fibers thought to be the basis for NaCl vs. KCl discriminability, called N-fibers or sodium specialists (8,15,30,37).…”