Trigeminal nerve responses in the rat elicited by chemical stimulation of the tongue.

Epithelial and neural mechanisms underlying trigeminal chemoreception were investigated by recording lingual nerve responses to chemical stimulation of the tongue. The chloride salts, NaCl, KCl, NH4Cl, and CaCl2, each elicited distinctly different, integrated whole-nerve responses (thresholds, 0.5-2.0 M). Incubation of the tongue with lanthanum (2.5 mM), which reduces the permeability of epithelial tight junctions, reversibly attenuated these salts responses. It did not affect neural responses to mechanical and thermal stimuli. Incubation with other established transport inhibitors--amiloride, tetra-ethyl ammonium or tetrodotoxin, had no effect on the salt responses. Acetic and hydrochloric acids (thresholds, 0.1-1.0 M), and MgCl2 and BaCl2 (greater than or equal to 0.5 M), also elicited distinctive responses. Other salts, MgSO4, Na isethionate and LaCl3 (greater than or equal to 1.0 M), and also ethanol (4 M), capsaicin (100 mM), nicotine (29.6 mM) and dextrose (0.5-2.5 M), did not elicit responses. These results indicate that ions of selected salts can diffuse through the tight junctions of the lingual epithelium to activate the trigeminal nerve, and suggest that both the cation and the anion may be important in determining if the nerves are activated and the wave-form of the responses.