MODULATION OF RAT CHORDA TYMPANI NERVE ACTIVITY BY LINGUAL NERVE-STIMULATION

1. A subpopulation of lingual nerve (LN) fibers surround and/or termin ate in taste buds in fungiform papillae. One possible function of thes e fibers is to modulate chorda tympani fiber (CT) or taste responses. To test this hypothesis, the rat LN was stimulated electrically at var ious voltages (to 20 V), and single- and multiunit CT responses to wat er-0.1 M NaCl cycles were recorded before, during, and after LN stimul ation. 2. When a thermally controlled water-0.1 M NaCl stimulus cycle was applied onto the tongue's surface, the surface temperature remaine d constant, independent of the stimulation voltage. In the absence of a liquid stimulus, the tongue's surface temperature increased similar to 4 degrees C upon LN stimulation for voltages greater than or equal to 5 V. This tem perature increase, caused by vasodilation by way of t he axon reflex flare mechanism, was taken as evidence that LN stimulat ion induces peptide release. 3. Comparison of CT activity before LN st imulation with the activity either during or after stimulation reveale d statistically significant changes in CT activity. During LN stimulat ion the CT activity decreased. After LN stimulation, the variability i n amount of CT activity increased. 4. In rats treated postnatally with subcutaneous injections of capsaicin to reduce or eliminate polymodal nociceptors, LN stimulation did not produce increases in the tongue' s surface temperature or changes in CT activity. 5. Changes in CT acti vity could be detected seconds after LN stimulation, suggesting that t he intragemmal and/or perigemmal LN fibers modulate CT activity. 6. Th e physiological implications of this study suggest that CT responses t o salt can be modulated by endogenous compounds (probably peptides), e ating foods that activate LN responses (e.g., foods that are very acid ic or contain capsaicin) may modulate taste responses, and peri- and i ntragemmal fibers should be considered an integral part of the taste r eceptor system.