Oral thermal modulation of taste processing in the mouse brain

Combinations of taste, somatosensory, and olfactory inputs contribute to the multimodal perception of flavor, which critically guides ingestive choices and can impact overall health. However the neural mechanisms underlying this perception are not fully understood. Human psychophysical studies showed that unimodal warm and cool thermal somatosensory stimuli applied to the tongue evoked sweet and salty taste perceptions, respectively. Additionally, warming sucrose solutions enhances their perceived sweetness. These data suggest that there is overlap in the neural mechanisms underlying taste and oral somesthesis. The rostral nucleus tractus solitarii (rNTS) is the first central nucleus in the taste pathway and shows co-sensitivity to oral thermal stimuli. However data on this topic are scarce. The purpose of these studies was to investigate the extent to which temperature modulates taste processing in the rNTS. To investigate the impact of temperature on prototypical taste processing in the rNTS we electrophysiologically recorded oral sensory responses (spikes) from 39 rNTS neurons in anesthetized C3HeB/FeJ mice during oral application of temperature-varied tastants. We found that some rNTS taste responses were heavily influenced by temperature. To understand if temperature was impacting rNTS representations of taste quality or taste intensity we recorded 35 rNTS neurons in anesthetized C57BL/6J mice during application of temperature-and concentration-varied stimuli. The major findings of these studies are as follows. Rostral NTS neural responses to prototypical taste stimuli are heavily modulated by changes in stimulus temperature, particularly oral warmth. Furthermore, changes in stimulus temperature appear to modulate the neural code for taste intensity. Together these results suggest that integration of temperature and taste inputs can occur much earlier in the flavor neuraxis than previously thought, and that temperature may be a parameter of the neural code for taste intensity akin to concentration.