Taste responses to electrolytes in the frog glossopharyngeal nerve: initial process of taste reception.

In taste reception, it has been proposed that changes in surface potential on the apical membrane of taste cells bring about activation of taste cells during chemical stimulation. To ascertain whether changes in the surface potential are involved in taste reception of electrolytes, unitary discharges were recorded from single water fibers of the frog glossopharyngeal nerve with a suction electrode. The surface potential is a function of both the charge density of the membrane surface and the ionic strength of the medium. Low concentrations of CaCl2 (less than 1 mM) were very effective stimuli. However, 0.01-1 mM LaCl3 and HCl (pH 3.0-4.5), which alter the surface potential in the positive direction, had no excitatory effect. Transition metal cations, such as Mn2+, Co2 and Ni2+, had an excitatory effect, but the responses to these cations appeared at relatively high concentrations (greater than 5 mM), in spite of the high affinity of the receptor membrane for these cations. The results suggest that the surface charge of the apical membrane is not associated with the excitation caused by electrolytes. MgCl2 (greater than 5 mM) and NaCl (greater than 100 mM) were also effective stimuli, whereas choline Cl (100-1000 mM) had no excitatory effect. An increase in the ionic strength was achieved by the addition of 100-300 mM choline Cl to stimulating solutions of MgCl2 or NaCl. The responses to Mg2+ and Na+ were not affected by the increase in the ionic strength. The results obtained here indicate that changes in the surface potential on the surface of the apical membrane are not involved in taste reception of electrolytes. Alteration of the surface potential of the membrane in the positive direction would bring about a reduction in the local concentration of cations in the vicinity of the membrane. Hence, the presence of divalent cations in the medium may affect the response to monovalent cations. However, addition of 100 mM MgCl2 to the stimulating solution of NaCl did not affect the concentration-response curve for NaCl. This result suggests that the surface charge density of the apical membrane is very low and hence the magnitude of the surface potential is very small. The results also suggest that Mg2+ and Na+ activate the taste cells by two separate, non-interacting processes. The present study suggests that, in the initial process of taste reception, only the binding of each separate cation to its appropriate receptor site (specific receptor site) leads to activation of the receptor.