Morphological and Electrophysiological Studies on the Formation of the Neural Tract and the Function in the Taste of Frog Chimerae.

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1. Of various agar-soaked gustatory stimuli, Xenopus (X. borealis and X. laevis) accepted L-Pro, L-Trp, and L-Val, while rejected L-Arg, quinine, and HCl. In general, X. borealis was more active and had lower thresholds for these gustatory stimuli than X. laevis. Feeding behavior of chimerae, which were produced by joining a half of an embryo of one species with that of another species, revealed similar behavior to X. laevis, regardless of the location of a half of an embryo of X. borealis.2. Taste buds calculated by staining with Ponceau S were about 300 on the palate, and about 200 on the floor of the mouth, and densely distributed on the inner lip and around the internal nostril. Both the number and location of the taste bud were similar among the two species and chimera. It was confirmed by staining with quinacrine that taste cells originated from an embryo of X. borealis are located ipsilaterally in chimerae. The extent of their contralateral location, however, were left unclear. Morphological tracing of glossopharyngeal nerves with HRP techniques showed that central gustatory neurons are distributed ipsilaterally in the medulla oblongata.3. Major differences in gustatory responses of the glossopharyngeal nerves to 60 gustatory stimuli between the two species were as follow: (1) L-Phe caused significant differences in responses over all concentrations tested, whereas L- and D-Trp, and L- and D-Tyr produced prominent differences in responses within a given concentration range. (2) differences in responses to quinine and strychnine were demonstrated throughout a wide concentration range. (3) among various nucleotides and nucleosides only adenosine produced fairly large responses in X. borealis, but very small responses in X. laevis. It was demonstrated using these substances as key stimuli that glossopharyngeal nerves of chimerae cause species-specific responsiveness during young adult, but that they tend to produce the responsiveness of X. laevis with aging.