Desert toads discriminate salt taste with chemosensory function of the ventral skin

Toads obtain water by absorption across their skin. When dehydrated, desert toads exhibit stereotyped hydration behavior in which they press their ven tral skin onto a moist surface. However, dehydrated toads avoid surfaces mo istened with hyperosmotic NaCl and KCl solutions (Hoff KvS, Hillyard SD. 19 93. J. Exp. Biol. 183:347-351). We have studied neural mechanisms for this avoidance with physiologic, behavioral, and morphologic approaches. Spinal nerves innervating the ventral skin could be stimulated by exposure to a hy perosmotic NaCl solution applied to the outer surface of the skin. This neu ral response occurred with much longer latency than to mechanical stimulati on and could be reduced by amiloride, a blocker for Na+ channels known to b e responsible for epithelial ion transport and salt taste transduction. In behavioral experiments, avoidance of a NaCl solution was also reduced by ad ding amiloride to the solution, suggesting involvement of amiloride-sensiti ve Na+ channels for detecting the hyperosmotic salt solution. Neural tracin g with fluorescent dye revealed spinal nerve endings and connections to put ative receptor cells, both located in the deeper layer of the epidermis. Ei ther of these or both may be associated with the transduction of Na+ flowin g into the skin. The ability of toads to detect hyperosmotic salt solutions in their environment reveals a previously unknown chemosensory function fo r spinal nerves in anuran amphibians. (C) 1999 Wiley-Liss, Inc.