TONGUE ADAPTATION TEMPERATURE INFLUENCES LINGUAL NERVE RESPONSES TO THERMAL AND MENTHOL STIMULATION

Menthol, a tangible substance present in many orally administered prod ucts, can produce a powerful influence on the perceived intensity of s ubsequent thermal stimulation in humans as well as the response magnit udes of thermally sensitive neurons in rats. However, there are no pri or studies examining the influence of adaptation temperature on percei ved intensity and/or response magnitudes of thermally sensitive neuron s to subsequent menthol stimulation. We identified 32 thermally sensit ive neurons that increased their discharge rate to a gradual temperatu re decrement beginning from 35 degrees C and dropping to 10 degrees C at 1 degrees C/s. Based on their response threshold, time-to-peak, and range of sensitivity, these thermally sensitive lingual neurons were found to be divisible into two distinct groups. Group 1 neurons (n = 2 1) responded best to the upper cool range (34-15 degrees C) of the tem perature decrement, whereas Group 2 neurons (n = 11) responded to the lower cold range (32-10 degrees C) of the temperature decrement. Our G roup 1 and Group 2 neurons may be analogous to low threshold and high threshold cold receptors identified previously in primates. We also ex amined the responses of lingual neurons to 0%, 25%, 50% and 75% diluti on's of a stock menthol concentration (1.28 mM) at 25 and 35 degrees C adaptation temperatures. Menthol responses across all concentrations were far larger after adaptation to 35 degrees C compared to 25 degree s C. Furthermore, only during 35 degrees C adaptation did responses to menthol stimulation persist during the ensuing 20 s after menthol off -set and water on-set. Although an effective coolant, the degree of pe rceived cooling due to menthol applied to lingual mucosa depends on th e temperature of the vehicle and the surface to which it is applied.