Rat neurophysiological taste responses to salt solutions

Abstract

1. Neurophysiological studies on the chemoresponsive tongue units of the rat geniculate ganglion readily detected and further characterized the two major functional neural groups designated as ‘acid’ and ‘salt’ units by other investigators in chorda tympani recordings. Units belonging to either of these groups were initially identified on the basis of their responses to a series of chemical solutions (a Test Series) developed to distinguish unit groups in the geniculate ganglion of the cat, dog and goat. 2. The nature of active ionic species effective in stimulating the units of the two different groups was further elaborated by studying their responses to a variety of salt solutions. From these studies, it was concluded that the optimum molecular species for the acid units is an acid, where an acid is defined as a Br0nsted acid or proton donating molecule. A variety of molecular species may be active in solution, including some nitrogen compounds. Responses to salt solutions by acid units are determined in part by the action of the cation functioning as a Brønsted acid (NH ₄ ), or in promoting proton donor molecules in water (mainly H ₃ O ⁺ ). The salt units, on the other hand, are almost exclusively responsive to solutions containing Na ⁺ and Li ⁺ . All other solutions were virtually inactive. It was concluded that under a wide variety of environmental conditions, Na+ would be the exclusive stimulus for the salt units. Na ⁺ and Li ⁺ were found to be highly stimulating when accompanied by a wide variety of solute anions, although those containing the halogens (Cl ⁻ , F ⁻ , I ⁻ , Br ⁻ ) were among the most stimulatory compounds tested. 3. The establishment that the stimulus is a proton donor molecule for the acid units and the Na ⁺ and Li ⁺ for the salt units has profound theoretical implications for the taste receptor. Biophysical models of taste receptors are reviewed with respect to this improved understanding of the stimuli for the two types of units. It is suggested, after consideration of certain basic biochemical aspects of the situation, that the acid receptor (possibly an imidazole group) forms, as part of a protein, a proton conducting circuit for driving an intracellular energy process devoted to proton sensing transmission. It is further suggested, in light of the extreme specificity shown to Na ⁺ , that receptor attachment is of the multidentate ligand variety and is probably linked to Na ⁺ , K ⁺ transport across the cell membrane, as proposed by DeSimone et al. (1981).