442 Y Kwak, et al regions [17]. Dorsal-middle subregion was sensitive to mon- osodium glutamate (umami taste) [18]. Salt stimulation in- duced more FLI expressions in dorsal-medial and dor- sal-middle subregions of the intermediate NST in the pres- ent study. These results imply the topographical dis- tribution of FLI for various tastants were intermingled around the dorsal-medial region. Recently, Corson and Hill [21] reported an anatomical evidence for the CT tract termination in the NST. The CT tract only was remained after other tracts in the tongue were dissected and was stained with fluorescent ante- rograde nerve labeling using biotinylated dextran amine. The terminals in the NST were clustered mostly in dor- sal-medial regions of the rostral NST. This clustering of ter- minal could explain the high FLI expressions in dorsal-me- dial region for most taste stimuli. In our result, the dor- sal-medial and dorsal-middle subregions showed signifi- cantly higher FLI expressions in i1, i2 and/or i3 areas. Meanwhile, there is inconsistency between CT terminal and FLI expression in the NST along anterior-posterior level. The CT terminals were clustered mostly in the rostral NST but the FLI expression was higher in the intermediate NST than rostral NST in Stratford and Finger [18] and our result. Although some part of CT terminals in the NST maybe specific for salt tastant, this discrepancy could not be explained fully. There is a possibility that the salt sol- ution was flowed or diffused to the posterior part of the tongue during stimulation, even though we tried to apply salt solution to the anterior 2/3 of the tongue. There are two interesting findings in the present study. The first is the position of maximum FLI expression region. The maximum FLI expression in the NST was more inter- mediately oriented than control (DW). This tendency was general in the overall NST and each subregions. This result may suggest the difference of taste signaling pathway in the NST between DW and salt taste. The other is the con- centration-dependent increase of FLI. Even though it was not statistically significant, there were strong trends. The last finding needs further research to verify the different expression pattern of NaCl stimulation from control. There was one unexpected result that FLI expression in the left and right subregion of the NST showed different expression pattern. In the left NST (Fig. 4), the medial and middle subregion of the NST showed higher FLI expression than lateral subregion. In the right NST (Fig. 5), however, the lateral subregion showed higher FLI expression than other subregions. Why this discrepancy is happened even though the overall FLI expression of the NST in the Fig. 3 showed similar pattern? Currently, we do not have the answer for this discrepancy but one possible explanation is the direc- tion of tastant stimulation. The tastant was applied toward the left side of the tongue and body was left side up, so this positioning may causes different FLI between left and right. c-Fos immunohistochemistry has been used as a valuable method to verify gustatory signaling in the central nerves system. But there were technical issues related with FLI expression. FLI may depend on anesthesia condition and stimulation modality. Takayam et al. [22] reported ure- thane anesthesia evoked more FLI cells than fentan- yl/midazolam. Rosen et al. [23] suggested water as an in- dependent taste modality because the NST cells electrically responded to water treatment. Nevertheless, c-Fos expre- ssion was used in many research fields including the learn- ing effect of food depletion [15,24] or aversion [14,16,25]. Furthermore, many researchers used FLI to figure out the location of taste pathway in the NST [9,17,26]. In our re- sult, water treatment evoked FLI expressions but the re- sponse was not significantly higher than high salt concen- tration treatment in dorsal-middle and dorsal-medial sub- regions of the NST. Recently, the taste information processing was analyzed in the gustatory cortex using two photon calcium imaging [27], intrinsic optical imaging [28] and they revealed spatial coding of tastant stimuli, different tastant occupy different region in the gustatory cortex. Direct electrical recordings from the NST were also conducted and revealed the differ- ent temporal coding for tastants and their concentration [5]. The c-Fos immunohistochemical study also shows the spa- tial coding of tastant. Therefore, combination of different approaches would extend our knowledge for gustatory sys- tem, especially in the NST. In conclusion, difference in the FLI expression between 1.0 M NaCl and DW was demonstrated and the dorsal-mid- dle and dorsal-medial subregions showed higher FLI ex- pression than other subregions. The results provide mor- phological evidence for perceptional and discrimination of salt taste signaling in the NST. ACKNOW LEDGEMENTS This study was supported by Korea Food Research Insti- tute (E0111501). REFERENCES 1. Paxinos G. The Rat Nervous System. 3rd ed. San Diego: Elsevier Academic Press; 2004. 2. Hamilton RB, Norgren R. Central projections of gustatory nerves in the rat. J Comp Neurol. 1984;222:560-577. 3. Morton GJ, Cummings DE, Baskin DG, Barsh GS, Schwartz MW. Central nervous system control of food intake and body weight. Nature. 2006;443:289-295. 4. Herrera DG, Robertson HA. Activation of c-fos in the brain. Prog Neurobiol. 1996;50:83-107. 5. Chen JY, Victor JD, Di Lorenzo PM. Temporal coding of intensity of NaCl and HCl in the nucleus of the solitary tract of the rat. 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