Sweet taste transduction begins with the binding of sweet tasting compounds to membrane receptors on taste bus cells, followed by activation of G proteins and change in the levels of 2nd messengers. The 2nd messengers block a K+ channel to depolarize taste cells, increase intracellular Ca2+, and eventually, activate gustatory afferent neurons. Although much has been learned recently about the molecular identities of sweet taste receptors, comparatively little is know about the downstream events linking the receptors to the effector ion channels. Our goal for the proposed experiments is to elucidate the molecular cascade forming this link. Previous studies on sweet taste transduction have examined lingual taste buds, which are relatively insensitive to sweet in rodents. Our proposal focuses instead on that buds of the soft palate, which, in rodents is most sweet-sensitive taste field. Cyclic AMP cGMP and IP3 have been implicated as 2nd messengers in sweet transduction, but no single study has examined all three messengers in any one field, nor have the roles of the G proteins and effector enzymes involved in these modulation been assessed. We Present evidence that sweet transduction in the soft palate is mediated in part by the G protein, Galpha-gustducin. We will utilize transgenic mice expressing green fluorescent protein (GFP) in gustducin-lineage cells to select taste cells for electrophysiological recordings. The morphological marker is available in both wild type (Gus-GFP) and gustducin knockout ( gus-null-GFP) mice. Recording form GFP-labeled and unlabeled palatal taste cells and biochemical measurements of 2nd messengers will define the roles of alpha-gustducin in sweet transduction and will reveal whether gustducin-independent mechanisms also exist for sweet transduction. Using a combination of molecular, biochemical, and electrophysiological techniques, we will (1) identiy the G proteins activated by sucrose and synthetic sweeterners. (2) defne the isoforms of adenyly cylcase present in sweet-sensitive taste cells, and (3) determine the mechanisms by which second messengers modulate the sweet-sensitive conductance. Results from these studies will provide important information about downstream signaling mechanisms in sweet taste transduction.
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