Date on Master's Thesis/Doctoral Dissertation

12-2021

Document Type

Doctoral Dissertation

Degree Name

Ph. D.

Department

Anatomical Sciences and Neurobiology

Degree Program

Anatomical Sciences and Neurobiology, PhD

Committee Chair

Krimm, Robin

Committee Co-Chair (if applicable)

Samuelsen, Chad

Committee Member

Samuelsen, Chad

Committee Member

Borghuis, Bart

Committee Member

McGee, Aaron

Committee Member

Stirling, David

Committee Member

Scott, Patrick

Author's Keywords

Taste; neuron; anatomy; taste bud; neuron type

Abstract

Taste neurons are functionally and molecularly diverse, but their morphological diversity was, until recently, completely unexplored. Taste neurons were considered relay cells, communicating information from taste-transducing cells to the brain without variation in morphology. Instead, individual taste neurons are tremendously morphologically variable. To determine how differences in branching relate to the number and types of taste-transducing cells providing neuronal input, I combined sparse cell genetic labeling with a whole-mount immunohistochemistry and analysis workflow. I found that the maximum number of taste-transducing cells capable of providing convergent input onto individual gustatory neurons varied with a range of 1-22 taste-transducing cells. Consistently, simple taste neurons contact a few taste-transducing cells of the same type (either sour or sweet/bitter/umami), whereas branched neurons contact many taste-transducing cells of both types. These results suggest differential convergence within the peripheral taste system in the type(s) of taste-transducing cells providing input and the number of taste-transducing cells of the same type. Taste arbors (the portion of the neuron within the taste bud) also vary in morphology and the number and types of taste-transducing cells contacted. I reconstructed 151 taste arbors from the full taste neuron population to analyze differences in size, complexity, symmetry and number and type to taste-transducing cells contacted. I determined that these features are not determined by the size or cellular composition of the taste bud. Taste arbors exist on a continuum of complexity, not in discrete categories of stereotyped endings. Large, asymmetrical arbors contacted more taste-transducing cells of both types, whereas smaller, symmetrical arbors contacted 1-2 taste-transducing cells. Differences in arbor complexity are consistent with regulation by plasticity rather than neuron type. To test this idea, I examined the arbors and ganglion neurons of the first molecularly and functionally described taste neuron type defined by the expression preproenkephalin (Penk). The arbors of Penk neurons do not represent a discrete subset of the arbor morphologies present in the full population. However, more symmetrical arbors from Penk+ neurons contact two taste-transducing cells of different types than the full population. All Penk+ ganglion neurons contact cells of both types while consistently contacting more Car4+ cells than PLCβ2+ cells. Together, these results indicate that plasticity dictates arbor morphology, whereas neuron type influences the number and type of taste-transducing cells providing input.

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