An artificial sweetener stimulates the sweet taste in insect: Dual effectsof glycyrrhizin in Phormia regina

Glycyrrhizin, found in the root of licorice (Glycyrrhizia glabra), has been used extensively as a non-sugar sweetener for humans and also as a medicin e. As far as we know, the present work is the first report describing that a non-sugar sweetener for humans induces a sweet taste in insects. In behav ioural experiments, we found that glycyrrhizin induced the feeding response , including full proboscis extension in the blowfly, Phormia regina. Glycyr rhizin also induced impulses of the sugar receptor cell in the labeller che mosensillum, which is highly specialized for the tastes of sugars and nucle otides. The optimum concentration of glycyrrhizin was 3.0 mM, which is much lower than that of sucrose. It has been established that multiple receptor sites, the pyranose receptor site (P site) and the furanose receptor site (F site), are present in the sugar receptor cell of the blowfly and the fle shfly. The inhibitors specific to the P site, starch and PCMB (p-chloromerc uribenzoate), partially inhibited glycyrrhizin induced responses but not le van (an inhibitor to the F site), indicating that the P sire on the sugar r eceptor cell is involved in the glycyrrhizin action but not the F site. Whe n 30 s stimulation with 3.0 mM glycyrrhizin was repeated with an interval o f 3-10 min, the impulse frequency to the second stimulus was higher than th at to the first one and doubled within 6 min. The first stimulus lasting lo nger than 10 s potentiated the impulse generation and reduced the adaptatio n rate during the second stimulus. These results suggest that, in addition to the action via the P site, an additional mechanism, possibly in the sign al transduction cascade of the sugar receptor cell, may be involved in the action of glycyrrhizin.