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

Abstract

Toads obtain water by absorption across their skin. When dehydrated, desert toads exhibit stereotyped hydration behavior in which they press their ventral skin onto a moist surface. However, dehydrated toads avoid surfaces moistened with hyperosmotic NaCl and KCl solutions (Hoff KvS, Hillyard SD. 1993. 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 hyperosmotic NaCl solution applied to the outer surface of the skin. This neural response occurred with much longer latency than to mechanical stimulation and could be reduced by amiloride, a blocker for Na+ channels known to be responsible for epithelial ion transport and salt taste transduction. In behavioral experiments, avoidance of a NaCl solution was also reduced by adding amiloride to the solution, suggesting involvement of amiloride‐sensitive Na+ channels for detecting the hyperosmotic salt solution. Neural tracing with fluorescent dye revealed spinal nerve endings and connections to putative receptor cells, both located in the deeper layer of the epidermis. Either of these or both may be associated with the transduction of Na+ flowing into the skin. The ability of toads to detect hyperosmotic salt solutions in their environment reveals a previously unknown chemosensory function for spinal nerves in anuran amphibians. J. Comp. Neurol. 408:125–136, 1999. © 1999 Wiley‐Liss, Inc.

The spinal nerves innervate putative chemosensory cells in the ventral skin of desert toads, Bufo alvarius

“…The temperature was maintained between 21°C and 23°C, and the toads were fed crickets twice each week. A group of animals were first used to study chemosensitivity of the ventral skin (Nagai et al 1999a), and some of them were further sacrificed for the present histological study. (5) spinal nerve (small arrows) and sixth (6) spinal nerve (large arrow).…”

“…The procedure basically followed Sandell and Masland (1988) and our previous method (Nagai et al 1999a). Until illumination for photoconversion, the use of fluorescent microscopy was minimized to avoid fading of the fluorescence.…”

“…We have extended the study of WR by using another species of desert toad, namely Colorado River toads (Bufo alvarius; Nagai et al 1999a). We have combined physiological and anatomical approaches to elucidate neural mechanisms for the escape behavior of the toads.…”

“…Photoconversion of fluorescent dye in the presence of 3, 3'-diaminobenzidine tetrahydrochloride (DAB) allows electron-microscopic observations to be made (Maranto 1982). However, because of the variability inherent in this technique, cytological features of the cells were only studied to a limited extent in our previous study (Nagai et al 1999a). In the present study, with an improved technique for photoconversion, we have studied DiI-labeled cells in the epidermis of the toads, their distribution patterns in the epidermis, and their cytological features.…”

Cardiovascular and behavioural changes during water absorption in toads, Bufo alvarius and Bufo marinus

“…The resulting standard weights were recorded and the toads placed overnight in a dry terrarium to obtain a level of dehydration (12-20% relative to the standard weight) that is sufficient to consistently stimulate the toads to initiate water absorption behaviour (Maleek et al, 1999;Nagai et al, 1999). Following dehydration, BCF and water uptake read from the pipette were measured for 40·min with the reservoir containing either deionised water (DI) or 50·mmol·l -1 NaCl (NaCl).…”