Behavioral comparison of sucrose and l-2-amino-4-phosphonobutyrate (l-AP4) tastes in rats: Does l-AP4 have a sweet taste?

Abstract

Even though it is generally thought that umami stimuli such as monosodium glutamate (MSG) and sweet stimuli such as sucrose are detected by different taste receptors, these stimuli appear to share taste qualities when amiloride (a sodium channel blocker) is present to reduce the sodium taste. Single fiber recording studies of the facial and glossopharyngeal nerves have shown that encoding of l-2-amino-4-phosphonobutyrate (l-AP4), a potent mGluR4 agonist that elicits a taste quite similar to MSG, may occur in the same fibers that also encode sweet stimuli. This suggests that l-AP4 and sweet substances may activate common receptors or afferent signaling mechanisms. We report results of behavioral experiments that test this hypothesis. In the first study, rats conditioned to avoid sucrose or l-AP4 generalized the aversion to the opposite substance, indicating that both substances elicited similar tastes. However, two taste discrimination experiments showed that rats easily discriminated between sucrose and l-AP4 over a wide range of concentrations, even when the cue function of sodium associated with l-AP4 was reduced by amiloride and neutralized by adding equimolar concentrations of NaCl to sucrose. These data suggest that even though l-AP4 and sucrose elicit similar taste qualities, one or both substances also elicit other taste qualities not shared by the opposite substance. They also suggest that the taste-mGluR4 receptor and the signal pathway activated by l-AP4 are not the same as those activated by sucrose. These data, when combined with fiber recording data, suggest that there is convergence of l-AP4 and sucrose signals at some point early in the gustatory pathway.