A Modest Petrosal: Elucidating Function and Molecular Identity in the Understudied Peripheral Taste Ganglion

In addition to identifying sweet, sour, bitter, salty, and umami, the sense of taste is involved in other cognitive and reflexive processes. Higher order taste processing leads to flavor, association, hedonic value, and top-down modulation of taste perception while brainstem reflexive circuits mediate orofacial reflexes, salivation, and other reflexes related to digestion preparation. Two anatomically distinct peripheral taste pathways from the anterior and posterior tongue provide parallel streams of taste information that feed into these different taste-processing circuits. Mounting evidence indicates that the anterior pathway is more important for taste identification while the posterior pathway is associated with reflexive responses. In order to better define the cells and circuits mediating these different functions of taste signaling, I set out to characterize the activity and molecular identity of the gustatory neurons of the petrosal ganglion, the primary sensory neurons of the posterior taste pathway, which have been understudied to date. Here, I use in vivo calcium imaging to characterize the responses of petrosal neurons to taste stimuli, compare and contrast these to the responses of geniculate ganglion neurons of the anterior taste pathway, and explore the physiological relevance for some of these differences. Finally, I use molecular profiling, histology, and in vivo calcium imaging to identify a universal gustatory neuron marker. Together, these studies provide new insight into the encoding of taste information by petrosal neurons, the importance of the posterior tongue for taste-evoked salivation, and the identification of Gabra1 as a general marker for peripheral gustatory neurons. This paves the way for future studies to further tease apart the contributions of petrosal neurons to important, but often overlooked taste functions such as salivation, digestion preparation, and orofacial reflex.