In vivo calcium imaging identifies functionally and molecularly distinct subsets of tongue-innervating mechanosensory neurons

Abstract

Mechanosensory neurons in the mouth provide essential information to guide feeding and speech. How classes of oral mechanoreceptors contribute to oral behaviors is not well understood; in particular, the functional properties of lingual mechanoreceptors remain elusive. Previous work identified putative mechanosensory endings in the tongue with novel morphologies; how these fit into current knowledge of mechanosensory neuron classification is not known. To identify functional classes of lingual mechanosensory neurons, we used in vivo calcium imaging of trigeminal ganglia. We first investigated calcium responses of tongue-innervating trigeminal neurons to thermal and mechanical stimulation (e.g., pressure, fluid flow, temperature changes). We found that around 17% of neurons responded to pressure, and that these pressure responders were significantly larger than neurons that only responded to temperature changes. To further investigate the cadre of functionally distinct mechanosensory neurons, we tested responses to brushing and sustained pressures and found that brush-sensitive neurons comprise the majority of tongue-innervating mechanosensory trigeminal neurons. Qualitatively, mechanosensory neurons responded to pressure with distinct kinetics, suggesting the presence of multiple classes of mechanoreceptors. To determine the number of classes, we developed an unbiased multi-layer hierarchical clustering approach to classify calcium response characteristics to pressure stimulation. This approach revealed that mechanosensory neurons displayed five distinct stimulus-response profiles to pressure. Classes include neuronal populations with sustained, transient, high-threshold, and negative responses to force as well as neurons that responded only to brushing. Analysis of cluster representation in transgenic animals with only subsets of labeled neurons reveals molecular markers of clusters and end organ structures. These studies are amongst the first to determine the functional properties of low-threshold mechanosensory neurons innervating the mouse tongue.