NEURAL RESPONSES OF THERMAL-SENSITIVE LINGUAL FIBERS TO BRIEF MENTHOLSTIMULATION

The addition of the coolant menthol to several oral and facial product s is to increase their attractiveness and commercial value. Little is, however, known about the physiological basis of menthol's sensory eff ects. We studied the electrophysiological responses of 45 thermal-sens itive lingual fibers to anterior tongue stimulation (10 s) with mentho l in male Sprague-Dawley rats. Menthol responses were unlike the respo nses to cold water. Cold water (6-degrees-C, 15-degrees-C) elicited an immediate sustained increase impulse frequencies of thermal-sensitive fibers adapted to room temperature water (22-24-degrees-C). Inhibitor y off-response followed cold water stimulation. Depending on the conce ntration and time of measurement, menthol stimulation either excited, inhibited. or had no effect on impulse frequencies of thermal-sensitiv e fibers. Strong menthol (0.64 mM, 1.28 mM) unequivocally excited ther mal-sensitive fibers with a response latency of 4-6 s. In most cases a fter menthol stimulation. the impulse frequencies returned to baseline ; there were no off-responses. Weak menthol (0.0128 mM, 0.064 mM, 0.12 8 mM) inhibited impulse frequencies of 14 thermal-sensitive fibers and excited impulse frequencies of 6 fibers primarily during the first 2 s of stimulation. Menthol responses were also unlike responses to stim ulation with taste solutions. Most taste solutions (30 and 100 mM NaCl , 0.3 and 1 mM quinine-HCl, 0.3 mM citric acid) significantly inhibite d impulse frequencies but only during the first 2-5 s of stimulation. The effect of NaCl was biphasic with the initial inhibitory phase foll owed by an excitatory phase during the second 5 s of stimulation. An e xcitatory off-response followed quinine stimulation. While considered principally a coolant. menthol elicits a unique pattern of responses f rom trigeminal and taste nerve endings quite unlike those of conventio nal thermal and taste stimuli.