Korean J Physiol Pharmacol Vol 15: 437−443, December, 2011 http://dx.doi.org/10.4196/kjpp.2011.15.6.437 c-Fos Expression in the N ucleus of the Solitary Tract in Response to Salt Stimulation in Rats Yongho Kwak1, Mee-Ra Rhyu5, Sun Joon Bai2,4, Young Hee Sa2, Min Jee Kwon1,3, Bae Hwan Lee1,3 Departments of 1Physiology, 2Anesthesiology and Pain Medicine, 3BK 21 Project for Medical Science, 4Anesthesia and Pain Research Institute, Yonsei University College of Medicine, Seoul 120-752, 5Functional Food Technology Research Group, Korea Food Research Institute, Sungnam 463-746, Korea Salt signals in tongue are relayed to the nucleus of the solitary tract (NST). This signaling is very important to determine whether to swallow salt-related nutrition or not and suggests some implications in discrimination of salt concentration. Salt concentration-dependent electrical responses in the chorda tympani and the NST were well reported. But salt concentration-dependency and spatial distribution of c-Fos in the NST were not well established. In the present study, NaCl signaling in the NST was studied in urethane-anesthetized rats. The c-Fos immunoreactivity in the six different NST areas along the rostral-caudal axis and six subregions in each of bilateral NST were compared between applications of distilled water and different concentrations of NaCl to the tongue of experimental animals. From this study, salt stimulation with high concentration (1.0 M NaCl) induced significantly higher c-Fos expression in intermediate NST and dorsal-medial and dorsal-middle subregions of the NST compared to distilled water stimulation. The result represents the specific spatial distribution of salt taste perception in the NST. Key Words: c-Fos, Chorda tympani nerve, Nucleus of the solitary tract, Salt INTRODUCTION The nucleus of the solitary tract (NST) is essential as the first central relay of the gustatory system in taste in- formation processing pathway. The taste signals from taste cells on the tongue are transported to the NST through the chorda tympani (CT) and glossopharyngeal nerves [1,2]. Furthermore, visceral taste information is conveyed to the NST from the gastrointestinal tract [3]. This taste signaling to the NST was well verified using c-Fos, an immediate ear- ly gene family [4], and electrical recordings [5] for various tastants. c-Fos-like immunoreactivity (FLI) increases in the NST by various chemicals including general tastants. Carstens et al. [6] reported that stimulation using irritant chemicals (nicotine, capsaicin, histamine, peperine, acetylcoline, and ethanol) increases FLI in the rat NST. Bitter chemicals such as QHCl, denatonium, propylthiouracil increase FLI in rat NST [7]. Yamamoto and Sawa [8] showed that the difference in FLI levels depends on chemicals. For example, Received December 3, 2011, Revised December 17, 2011, Accepted December 20, 2011 Corresponding to: Bae Hwan Lee, Department of Physiology, Yonsei University College of Medicine, 50, Yonsei-ro, Seodaemun-gu, Seoul 120-752, Korea. (Tel) 82-2-2228-1711, (Fax) 82-2-393-0203, (E-mail) bhlee@yuhs.ac CC This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/ by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited. 437 most remarkable induction of FLI was observed by stim- ulation with LiCl, lactose, and ethanol which have been of- ten used as unconditioned stimuli in conditioned taste aversion. Sucrose and maltose induce more FLI than glu- cose, fructose, and galactose [8]. Meanwhile, sweeteners such as saccharin, glycine, and alanine as well as other ba- sic taste stimuli such as NaCl, HCl, quinine, and umami substances induce relatively low level of FLI in rat NST [8]. The FLI responses to tastants showed topographic pat- terns in the NST. For example, sucrose stimulation acti- vated FLI which is evenly distributed along the medio- lateral axis in the rostral NST [9]. Quinine stimulation in- duced more FLI in medial subregion in the rat NST but the distributions of FLI are intermingled with sucrose-acti- vated FLI [9]. Along the rostral-caudal axis, FLI induced by quinine stimulation was reduced in rostral regions of the NST [10]. Salt is very important nutrition to keep the body meta- bolism. Specifically, sodium ion is the major cation of ex- tracellular fluids and essential to keep ions and water homeostasis. Therefore, there is naturally developed salt- sensing system in animals. The salt-sensitive receptors on the tongue are crucial to accept low concentration of sodium as salt appetite [11,12] or to deny high concentration as salt aversion [12,13]. Salt taste stimulation using 0.15 M NaCl was conducted for conditioned taste aversion studies [12,14]. Houpt et al. [15] used 0.3 M NaCl to study sodium ABBREVIATIONS: NST, nucleus of the solitary tract; CT, chorda tympani nerve; FLI, c-Fos-like immunoreactivity.