| Based on electrophysiological studies of the rat gustatory system, the glossopharyngeal nerve (GL) contains a fiber type that is differentially responsive to quinine on one hand and acids and salts on the other. In contrast, the chorda tympani (CT) contains fibers that respond well to quinine, acids, and salts. It was predicted that GL transection should therefore disrupt behavioral discriminations between quinine and either acids or salts. In the context of other behavioral findings on the role of narrowly-tuned sodium-responsive fibers in behavioral discriminations between sodium and nonsodium salts, this result would imply that narrowly-tuned gustatory fibers are critical in behavioral discrimination. In Experiment 1, water-restricted rats were trained to press one of two levers if a sampled taste stimulus were quinine (0.068-0.75 mM), and the opposite lever if the sampled stimulus were citric acid (3-120 mM). Transection of the CT (CTX), combined CT and greater superficial petrosal (GSP) transection, and combined GL and CT transection (GLX+CTX) significantly impaired discrimination performance. GL transection alone (GLX) had no effect. In Experiment 2, an identical behavioral paradigm examined discrimination between quinine (0.1-1 mM) and potassium chloride (KCl, 0.1-1.0 M). As in Experiment 1, CTX and GLX+CTX significantly impaired performance, whereas GLX had no effect. Although these findings were contrary to prediction, they nonetheless compare favorably with other behavioral studies finding little effect of GLX on behavioral responses to quinine. These results suggest that narrowly-tuned fibers may not be critical in gustatory discriminations for all stimuli. On the other hand, it may be that narrowly-tuned quinine units exist in the CT, but for methodological reasons, remain undiscovered. |
