Extracts of Thladiantha grosvenorii fruits, Stevia rebaudiana leaves, and Abrus precatorius leaves were investigated using Mongolian gerbil electrophysiological and conditioned taste aversion procedures, which were designed to respond to sucrose. A close correlation was observed between extracts of these sweet plants known to contain sweet principles and those extracts indicated as being sweet by a combination of these gerbil bioassays. The methods employed seem to be suitable for use in aiding the purification of highly sweet compounds of plant origin.

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The gustatory responses from the chorda tympani nerve of the Mongolian gerbil, Meriones unguiculatus, were tested with saccharin and some closely related compounds. Only two compounds, saccharin and 6-chlorosaccharin, stimulated the gerbil's taste receptors to any degree and in the same order in which they are sweet to man, saccharin > 6-chlorosaccharin.Those compounds which are tasteless to man did not stimulate and included N-methyl saccharin, phthalimide, and Osulfobenzoic acid. These nonstimulating derivatives did not inhibit the saccharin electrophysiological response. The saccharin taste response pH optimum is from 5 to 8. At pH 7 and 8, saccharin is fully ionized and is binding to the receptor site by an anionic mechanism. Kinetic plots of the taste response at pH 7 indicate that saccharin is forming 1:l complexes with it. In addition, mixtures of saccharin and sucrose stimulate in such a manner as to suggest that they are interacting at different receptor sites.In a previous study (Jakinovich, 1981), data were accumulated on the gerbil's taste response to a group of artificial sweeteners, including saccharin. We tested them both behaviorally and electrophysiologically, anticipating uniform responses because sweetness in humans is unified under Shallenberger and Acree's (1967) "AH,B" theory. This proved not to be the case. Of the 21 sweeteners tested electrophysiologically, only 14 stimulated the gerbil's taste nerve, and of those 14, 6 did not taste like sucrose according to behavioral experiments.The purpose of the present investigation was to focus on the gerbil's taste response to saccharin. This sweetener is important not only because it stimulates the gerbil's taste nerve and resembles the taste of sucrose behaviorally (Jakinovich, 1981) but because, in mammals, it is the most uniformly preferred artificial sweetener (Jakinovich, 1980), indicating a common sweet taste receptor mechanism. With regard to this mechanism, there is the intriguing possibility that saccharin and sugars are stimulating the taste receptors through separate receptor sites (Zawalich, 1973 saccharin were interacting at the same or at different receptor sites. Finally, a pH experiment was conducted because saccharin's amide group (the "AH" group of Shallenberger and Acree (1967) ) will ionize depending upon the pH and the results could shed some light on the binding mechanism.The results of these experiments suggest that the gerbil's taste specificity to saccharin is the same as man's inasmuch as the taste responses were similar. The saccharin derivatives which taste sweet to man also stimulated the gerbil's taste receptors, and the tasteless ones did not stimulate. However, it would seem that the taste stimulation mechanism is not consistent with the "AH,B" theory. We have found that the saccharin taste response has a pH optimum range from 5 to 8. At pH 7 and 8, saccharin is fully ionized, lacks the AH group, and therefore, cannot be forming hydrogen bonds with the receptor site. Moreover, contrary to the theory, mixtur...

The sugar methyl 4,6-dichloro-4,6-dideoxy-alpha-D-galactopyranoside (DiCl-gal) is a new type of inhibitor of the gerbil's electrophysiological taste response to sucrose or saccharin. Saturated solutions of this compound alone barely stimulate the gerbil's taste nerve. But, when mixed with sucrose or saccharin, DiCl-gal suppresses the gerbil's taste response to these two sweeteners. In contrast, when mixed with sodium chloride or hydrochloric acid, DiCl-gal does not affect the taste responses to these compounds. However, unlike other inhibitors of sweet taste, the DiCl-gal taste suppression is short-lived and occurs only when the inhibitor is combined with the sweetener.

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We have discovered that the gerbil's chorda tympani nerve response to sucrose is suppressed by p-nitrophenyl alpha-D-glucopyranoside (PNP-Glu) and chloramphenicol (CAP). Mixture experiments of PNP-Glu and CAP with sodium chloride, potassium chloride, hydrochloric acid, and sucrose gave rise to the following observations: Neither PNP-Glu nor CAP alone stimulates the gerbil's taste nerve; while the sucrose response is suppressed by these inhibitors, taste responses produced by sodium chloride, potassium chloride, and hydrochloric acid are unaffected by the presence of PNP-Glu or CAP; the antagonisms of PNP-Glu and CAP were surmounted by a high concentration of sucrose; CAP is a more potent antagonist (IC50 = 0.0013 M) than PNP-Glu (IC50 = 0.022 M), and both are more potent than methyl 4,6-dichloro-4,6-dideoxy-alpha-D-galactopyranoside (IC50 = 0.048 M); and sucrose antagonism occurs only when PNP-Glu and CAP are mixed with sucrose. It is short-lived and ceases when the mixtures are rinsed from the gerbil's tongue. Structure-activity studies provided the following information: The alpha anomer of PNP-Glu is a more potent inhibitor than its beta anomer; among the PNP-Glu derivatives tested (p-aminophenyl, p-nitrophenyl, and phenyl) only p-nitrophenyl inhibited; among the nitrophenyl galactosides, the para derivative was more potent than the ortho or meta; and p-nitrophenyl alpha-D-mannopyranoside and p-nitrophenyl alpha-D-galactoside are slightly more potent than PNP-Glu. On the basis of concentration experiments, we believe that the inhibitory mechanisms of PNP-Glu and CAP are different, which suggests the existence of at least 2 sucrose receptor sites.

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