The fungiform papilla as a multi-sensory end-organ

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Project Summary/Parent grant. Our perception of food in the mouth is a multi-modal sensory experience involving taste, olfaction, and somatosensation. All of these stimuli combine to contribute to the recognition and palatability of food. The first step leading to the perception of ‘mouth feel” in the oral cavity is activation of peripheral somatosensory neurons. Multiple classes of these trigeminal somatosensory neurons innervate the oral cavity and can be defined by the types of stimuli to which they respond. For example, low threshold mechanoreceptors (LTMRs) carry information concerning touch or mechanical stimuli. In skin, at least 7 clear subtypes of these neurons have been defined by using gene expression to relate the anatomical features of these neurons with their functions. We do not know how many specialized neuron subtypes mediate touch sensation in the tongue or their specialized anatomical and functional properties. Understanding the sensations evoked by a food stimulus requires us to determine the unique anatomical and functional properties of each LTMR subtype in the tongue. Equally important is how each LTMR’s activities are integrated with other oral sensory neuron types (i.e. taste and temperature). Toward these long-term goals we propose to use two genetic identifiers to examine trigeminal neuron subtypes innervating the fungiform (taste) papillae. Our Aims specifically test the following hypotheses: 1) Taste papillae are innervated by a Aβ LTMRs with a small receptive field that express the neurotrophin receptor, TrkC, and that they neurons can further be divided into rapidly adapting and slowly-adapting types based on the presence or absence of Parvalbumin. 2) These Aβ LTMRs project to the to a region of the brainstem immediately adjacent to the primary taste terminal field where they activate postsynaptic neurons that are responsive to touch. 3) These Aβ LTMRs are capable of modulating responses to taste stimuli at the first gustatory synaptic relay. Successful completion of these experiments establishes the fungiform papilla as a mechanosensory end organ and provides the genetic tools to explore the contribution of a specific subtype of LTMR in the integration of sensations produced by the multi- modal food stimulus.