Taste Bud Connectome: Implications for Taste Information Processing.

Taste buds contain multiple cell types, 2 of which mediate transduction of specific taste qualities: Type III cells transduce sour while Type II cells transduce either sweet, or bitter or umami. In order to discern the degree of interaction between different cell types and specificity of connectivity with the afferent nerve fibers, we employed serial blockface scanning electron microscopy through 5 circumvallate mouse taste buds. Points of contact between Type II and III cells are rare, and lack morphologically identifiable synapses, suggesting that interaction between these cell types does not occur via synapses. Of the 127 nerve fibers that make synaptic contacts with taste cells in the sampling volume, about 70% (n=91) synapse with only one taste cell while 32 fibers synapse exclusively with multiple Type II cells or multiple Type III cells. Our data do not rule out multimodal fibers innervating Type II cells of separate taste qualities. Notably, 4 fibers (∼3%) synapse with both Type II and Type III cells, forming both mitochondrial and vesicular synapses on the different cell types. Since Type II and Type III cells transduce different taste qualities, these dual connected fibers are not consistent with a absolute labeled-line encoding system. Further, our data reveal considerable variation in both the number of synapses per cell/nerve pair and the number of innervating nerve fibers per taste cell, both of which likely have consequences for encoding taste quality and concentration. Finally, we identify a subset of Type II cells which may represent an immature stage.SIGNIFICANCE STATEMENTTaste buds, the sensory end organs for the sense of taste, contain multiple types of sensory cells, with each responding to one of the primary tastes: salt, sweet, sour, bitter and umami. In order to determine the degree of interaction between cell types and specificity of connectivity to afferent nerves, we employed serial blockface EM of mouse circumvallate taste buds. We find no synapses between cell types within the taste bud suggesting that any interactions are indirect. While the majority of nerve fibers connect to a single type of taste cell, 3.1% of the fibers branch to receive input from taste cells of different specificities. Thus taste cannot entirely be carried along nerve fibers dedicated to single taste qualities.