While the function of taste neurons (that connect taste receptor cells to the brain) has been well-studied, almost nothing is known about individual taste neuron morphology or peripheral connectivity. Specifically, it is unclear how many taste receptor cells provide input to a single neuron or even if those receptor cells are all the same type. This lack of knowledge contributes to our relatively poor understanding of peripheral taste coding mechanisms. This project will use a combination of sparse cell genetic labeling and serial blockface scanning electron microscopy (SBF-SEM) to 1) examine the number of taste receptor cells likely contacted by chorda tympani neurons of differing complexity 2) determine if individual neurons contact receptor cells of the same type or more than one type. I will accomplish these goals via three specific Aims.
In Aim 1, I will determine if there are any structural motifs present at synapses or non-synaptic structural connections using SBF-SEM, which can also be identified in 3-D confocal images of sparsely labeled nerve fibers and taste receptor cells. The goal is to permit better interpretation of the anatomical relationships between receptor cells and nerve fibers at the light level.
In Aim 2, I will reconstruct separate fibers entering the taste bud using SBF-SEM, which will determine the rules of connectivity of simply vs. sparsely branched fibers within the fungiform taste bud. These data will be compared with the contacts identified, using image analysis software at the light level between individual fibers entering the taste bud.
In Aim 3, I will analyze the morphology of whole chorda tympani nerve fibers, innervating anywhere from 1 to 7 taste buds, and relate their branching complexity to the number and types of peripheral contacts made with taste receptor cells. Together, these experiments will test the hypothesis that heavily branched neurons contact many more taste receptor cells of multiple types, compared to simply branched neurons which will contact only a few receptor cells of a single type. This project will be the first to examine the peripheral connectivity of individual chorda tympani neurons, and may reveal an anatomical substrate for the physiological finding that some neurons are narrowly tuned (respond to one taste stimulus) whereas others are broadly tuned (respond to multiple taste stimuli). Determining the peripheral connectivity of individual neurons will likely inform future experiments designed to look at taste coding. These baseline data for the normal connectivity of the peripheral taste system can be used to compare the potential disruption of connectivity that may result from disease states or manipulations of molecular factors regulating the formation of these connections.
Taste disorders impact quality of life, yet little is known about the underlying defects and mechanisms that result in these perceptual deficits. Diseases and drugs (i.e. chemotherapies) that disrupt taste function could have their effects by disturbing taste receptor cell neuron peripheral connectivity. This project will develop a baseline connectivity map of peripheral nerve fibers with taste receptor cells; both the map and novel methodological approach can be used to dissect the role of connectivity in taste disorders.