c-Fos Expression in NST 441 According to human study, salt stimulation produces two different tastes depending on its concentration. Human subject reported sweet and salty taste for 0.01 M NaCl stimulation and salty only for higher than 0.03 M NaCl stimulation [20]. Recently, Chen at al. [5] reported interest- ing result that temporal coding of the NST neurons depends on NaCl concentration. The electrical activity of single neu- ron was higher for 0.6 M NaCl than 0.1 M NaCl and also the responses were dose-dependently increased. Their re- sult may not directly be related to FLI expression level, since FLI expression study focused on the number of cells that expressed c-Fos. Therefore, there was no systematic approach to discriminate salt taste perception related to salt concentration. For systematic approach for FLI expression in the NST, there were several studies previously. Travers [10] reported that the FLI expression following 0.3 M NaCl stimulation in the rat NST. The NST sections from five different levels based on the area postrema were collected and were divided to six subregions as described by King et al. [19]. The FLI expressions in five different anterior-posterior levels, of the overall NST and in six subregions, were not significantly different from DW stimulation. Meanwhile, Stratford and Finger [18] reported different result using mice as ex- perimental animals. Nine NST sections from different ante- rior-posterior levels were collected and their FLI expre- ssions in the overall NST were compared. There was sig- nificant difference in FLI expressions between 0.15 M NaCl and DW stimulation in the NST. In their study, the NST was divided into six subregions as Travers [10] did. But unfortunately, they did not compare the difference of FLI expressions between NaCl and DW stimulation, since their main issue was to study the role of purinergic receptors in taste sensation. They only compared FLI expressions be- tween wild type and P2s2-dblKO mice. We also collected six NST sections from different anterior-posterior levels. In the present study, FLI expressions were not significantly increased in low salt concentration (0.3 and 0.6 M NaCl), than DW stimulation. This result is consistent with Travers [10] (0.3 M NaCl) and Stratford and Finger [18] (0.15 M Fig. 5. Comparison of FLI cell num- bers in the each subregions in the right NST. (A) dorsal lateral, (B) dorsal middle, (C) dorsal medial, (D) ventral lateral, (E) ventral middle, (F) ventral medial. The animal numbers are marked in parenthesis; Control (n=6), 0.1 M (n=6), 0.3 M (n=5), 0.6 M (n=6) and 1.0 M (n=4) NaCl concentration. Error bar is SEM. *p＜0.05. NaCl). Meanwhile, there was the higher expression of FLI in the intermediate NST levels with high salt (1.0 M NaCl) taste stimulation than DW stimulation. In subregions of the NST, the dorsal-medial and dorsal-middle regions in the intermediated NST showed higher FLI than DW stimula- tion. Therefore, our results may provide a morphological evidence to salt taste perception related with dose-depen- dence. There was similarity in the collecting and positioning NST sections among Travers [10], Stratford and Finger [18], and our study, but the naming of each levels were somewhat different. It is not easy to position each coronal section in serial section of the NST along the rostral-caudal axis among animals. Therefore, we focused on the position where the NST begin to meet the 4V because we thought it is the only clear section to calibrate each section. This section was named i1-NST in Stratford and Finger [18] and ours. The posterior region to i1 area was named as inter- mediate in Travers [10], and Stratford and Finger [18], and ours. The intermediate NST was further divided into i1, i2, i3 and i4 areas depending on their anterior-posterior level. These regional divisions will allow comparison of the FLI expression in each regional level. It should be note that rostral NST in Travers [10], and Stratford and Finger [18] were different from ours. The r1 and r2 areas in this study covers 400μm distance from i1 area, so the rostral NST that appear with cochlear nucleus was not included in the FLI analysis. This rostral NST were included in serial sec- tion and also its FLI expression was analyzed. But they were very lower than r1-NST and we exclude them in FLI comparison. It is interesting to determine where FLI was more prom- inent in the NST for specific tastant, because the area would imply the taste signal pathway of that tastant in the NST. Therefore, the division of the NST into six sub- regions is reasonable to test the information processing pathway of specific tastant simulation. But there were few studies that considered this issue. Quinine stimulation in- duced dorsal-medially distributed FLI in the NST [10,19]. Sucrose stimulation induced FLI in dorsal-middle sub-