Salt taste responses in the frog glossopharyngeal nerve: different receptor sites for Mg2+ and Na+.

The glossopharyngeal nerve of the frog responds to MgCl2 and NaCl applied to the tongue. To learn whether or not Mg2+ and Na+ react with different receptor sites, a proteolytic enzyme was topically applied to the tongue dorsum. Responses were recorded from the frog glossopharyngeal nerve during stimulation of the tongue with 100 mM MgCl2 and 500 mM NaCl. The magnitude of the MgCl2 response was selectively reduced after application of 0.1% pronase E to the dorsal tongue surface. The selective suppression of the MgCl2 response by the pronase treatment indicates that the receptor site responsible for the MgCl2 response differs from that for the NaCl response. The effect of pronase on the MgCl2 response was due to the proteolytic action. This suggests that the receptor site responsible for the MgCl2 response is composed of a protein or interacts secondarily with a protein affected by pronase treatment. It is suggested that there are multiple receptor sites for cations in salt taste reception of the frog.