The responses to choline ions induced by transition metal ions in single water fibers of the frog glossopharyngeal nerve.

In single water-sensitive fibers (water fibers) of the frog glossopharyngeal nerve, application of a solution of 500 mM choline C1 to the tongue elicited responses of varying magnitude. Some water fibers (plain choline-insensitive water fibers) barely responded to the solution, while some water fibers (plain choline-sensitive water fibers) exhibited a considerable response to this solution. NiCl2, which is barely effective in producing neural response at concentrations below 5 mM, induced the response of plain choline-insensitive water fibers to choline+ ions. It was confirmed, in a collision test, that the Ni(2+)-induced responses to choline+ ions were derived from water fibers. However, NiCl2 did not affect the magnitude of the response generated by choline+ ions in plain choline-sensitive water fibers. The concentration-response curve for choline C1 in the presence of 1 mM NiCl2 for plain choline-insensitive water fibers was similar to the curves obtained in the absence of NiCl2 for plain choline-sensitive water fibers. Other organic salts, such as tris(hydroxymethyl)aminomethane-HCl, triethanolamine-HCl and tetraethylammonium C1, elicited no response or only a very small response from water fibers, and NiCl2 did not affect these responses. It is suggested that there exists a choline receptor for the response to choline+ ions in the apical membrane of frog taste cells and that Ni2+ ions expose the sites of such choline receptors, which are deeply embedded in the receptor membrane, to the outside medium. The effect of Ni2+ ions results in an increase in the number of the choline receptor sites available for binding of choline+ ions. The rank order of effectiveness of transition metal ions in eliciting the appearance or enhancement of the response to choline C1 was Ni2+ > Co2+ > Mn2+. Mg2+ ions had no effect on the response to choline+ ions. A similar rank order was previously obtained in enhancement of the responses to Ca2+, Mg2+ and Na+ ions (Kitada, 1994a). It seems likely that the mechanism for enhancement or elicitation of the response to choline+ ions by the transition metal ions has features in common with that for enhancement of the responses to Ca2+, Mg2+ and Na+ ions.