Glossopharyngeal nerve transection impairs unconditioned avoidance of diverse bitter stimuli in rats

There is growing evidence of heterogeneity among responses to bitter stimuli at the peripheral, central and behavioral levels. For instance, the glossopharyngeal (GL) nerve and neurons receiving its projections are more responsive to bitter stimuli than the chorda tympani (CT) nerve, and this is particularly true for some bitter stimuli like PROP & cycloheximide that stimulate the GL to a far greater extent. Given this information, we hypothesized that cutting the GL would have a greater effect on behavioral avoidance of cycloheximide and PROP than quinine and denatonium, which also stimulate the CT, albeit to a lesser degree than salts and acids. Forty male SD rats were divided into four surgery groups: bilateral GL transection (GLX), chorda tympani transection (CTX), SHAM surgery, and combined transection (CTX + GLX). Postsurgical avoidance functions were generated for the four bitter stimuli using a brief-access test. GLX significantly compromised avoidance compared to both CTX and SHAM groups for all stimuli (p < .02), while CTX and SHAM groups did not differ. Contrary to our hypothesis, GLX had a greater effect on quinine than cycloheximide (mean shift of 1.02 vs. 0.27 log10 units). Moreover, combined CTX + GLX transection shifted the concentration-response function further than GLX alone for every stimulus except cycloheximide (ps < .03), suggesting that the GSP nerve is capable of maintaining avoidance of this stimulus to a large degree. This hypothesis is supported by reports of cycloheximide-responsive cells with GSP-innervated receptive fields in the NST and PBN.