Neuronal subsets in the geniculate ganglion

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Neurons in the geniculate ganglion provide sensory innervation to: 1) taste buds on the anterior region of the tongue, via the chorda tympani nerve; 2) taste buds on the incisive papilla and the soft palate, via the greater superficial petrosal nerve; and 3) the skin near the ear via a cutaneous nerve. Preliminary studies in our laboratory showed that three high affinity neurotrophin receptor tyrosine kinases (trkA, trkB and trkC) are found in neurons of the geniculate ganglion. The ligands for these receptors are a family of secreted neurotrophic factors: trkA binds Nerve Growth Factor, trkB binds Brain-Derived Neurotrophic Factor and Neurotrophin-4, and trk C binds Neurotrophin-3. In pilot studies on identified single neurons of the rat geniculate ganglion we have found that they can be divided into subsets according to the trk genes they express. Moreover, we have evidence suggesting that trk expression by rat geniculate ganglion neurons changes during development. This research will test two hypotheses: 1) The division of geniculate ganglion neurons into sub-populations based on differences in trk gene expression is related to one or more of the following: a) the projection of 3 main nerve branches from the ganglion, b) the taste and other modalities associated with neurons in the geniculate ganglion, or c) possible differences in central projections of the neurons. We shall use tracer molecules and electrophysiology to identify specific neurons, and subject these to RNA amplification and PCR methodology to look for functional correlates of the differences in trk expression. Hypothesis 2: The changes in trk gene expression in geniculate ganglion neurons reflect major developmental events, such as axon outgrowth, innervation of the peripheral target tissue, naturally occurring cell death, or innervation of the central target. We shall use in vitro and in vivo methods to relate changes in trk expression during development to other major developmental events.