Sublingual Caruncle Taste Buds Drive Taste Perception and Salivary Reflexes

Taste buds (TBs) are the fundamental units of taste perception and are distributed throughout the oral cavity, including the tongue, soft palate, epiglottis, pharynx, and retromolar pad. Despite their widespread presence, a complete and systematic understanding of TB distribution and functional organization remains incomplete. In this study, we focus on a small population of TBs located in the mandibular sublingual caruncle of mice, adjacent to the openings of the Wharton's ducts, a site previously noted in anatomical and physiological studies that has not been comprehensively characterized. TBs of the sublingual caruncle exhibit canonical morphology and cellular composition, comprising type I (NTPDase2 +), type II (Trpm5 +), and type III (Car4 +) cells, as confirmed by transgenic reporter lines and immunofluorescence staining. Molecular analysis revealed the expression of key taste transduction components, including Gnat3, Trpm5, and Plcb2, as well as 26 of 35 bitter taste receptors. Immunofluorescence confirmed TB innervation by βIII-tubulin-positive nerve fibers, including P2X3-positive sensory afferents, Synapsin-1-positive presynaptic terminals, and CGRP-positive peptidergic endings, indicating integration into the peripheral gustatory circuitry. Retrograde neuronal tracing (Dil/DiO) further demonstrated that nerve fibers associated with sublingual caruncle TBs project specifically to the geniculate but not the petrosal ganglion, supporting their attribution to the facial gustatory pathway. Functionally, localized sucrose stimulation of the sublingual caruncle elicited cFos activation in the nucleus of the solitary tract, indicating central connectivity. Furthermore, tastants representing all 5 basic taste qualities (sweet, sour, bitter, umami, and salty) elicited measurable saliva secretion, with bitter-, sweet-, and umami-induced salivation reduced in transgenic mouse models with impaired gustatory function. Together, these findings establish the sublingual caruncle as a functional gustatory site involved in taste-evoked salivary reflexes, expanding current understanding of taste system organization and its coordination with salivary function.