Taste buds comprise 50-100 elongate taste cells which detect sapid molecules in the oral cavity. The apical pole contains the taste receptors, while the basolateral membranes contact sensory nerve fibers that transmit taste information to the brain. Taste buds contain three types of fusiform cells distinguishable by morphological, molecular and functional criteria. Of these, Type II cells are the best understood. They contain the taste receptors and downstream signaling effectors for bitter, sweet and umami taste stimuli and when activated, release ATP to activate purinergic receptors on sensory nerves. The Type I cells serve a glial like support function, expressing enzymes for uptake and degradation of transmitters, including a specific ectoATPase for degrading the ATP released from the Type II cells. The Type III cells are the most enigmatic. They are required for sour taste and some aspects of salty taste, but the receptor mechanisms remain elusive. Further, Type III cells are the only cells in the taste bud to form discrete synapses with afferent fibers, but neither the transmitter released, nor its cognate receptor on the sensory nerve fibers has been identified molecularly. We have found that a specific population of geniculate ganglion neurons, those that express the 5-HT3a receptor, selectively innervate the Type III taste cells of the anterior tongue. Using mice expressing GFP from the 5-HT3a promoter, we have shown that although 5-HT selectively stimulates the GFP-expressing ganglion neurons, 5-HT is not required for transmission of sour or salty taste to afferent fibers, suggesting other transmitter(s) and receptors are required. Although 5-HT is not crucial to neurotransmission in this system, the expression of 5-HT3a by those ganglion cells innervating Type III taste cells allows us to distinguish this population from other gustatory ganglion neurons.
In Aim 1, we will utilize the Fluidigm C1, a new technology recently acquired by our Genomics and Microarray Core to generate and compare the individual transcriptomes of GFP-labeled (innervating Type III taste cells) and unlabeled geniculate ganglion neurons (which innervate Type II taste cells). This comparison will identify candidate neurotransmitter receptors expressed preferentially by the ganglion cells innervating sour- and salty-responsive taste cells.
In Aim 2, we will use the bioinformatics information obtained from Aim 1 to test candidate transmitters on GFP- labeled ganglion cells in vitro to test functionality of the candidate receptor. Then we will utilize chord tympani nerve responses and corresponding antagonists to test functionality in vivo and the necessity for these receptors in transmission of taste information. Together the results will allow for the development of a new approach for studying synaptic transmission in taste buds as well as provide an important database for researchers studying proteins involved in signal recognition between taste cells and nerve fibers.
The function of the gustatory system is to discriminate nutritious substances, such as salts, sugars, and amino acids from harmful substances, such as bitter alkaloids and acids. Understanding the molecular events underlying detection and transmission of taste information may lead to the development of pharmaceutical agents that can modulate food intake, a major factor in controlling obesity. The proposed studies will develop new tools to understand how sour- and salty-responsive taste cells communicate with afferent nerve fibers.