separately. The positions of other sections were calculated using their sectioning interval from “0” position. We se- lected six NST areas along rostral-caudal axis with 200μm intervals: r1 and r2 to the anterior direction (r: rostral), and i1, i2, i3 and i4 to the posterior direction, here r1 is more rostral than r2, and i4 is more posterior to i1. Each NST had 8∼9 serial sections in 200μm distance and their FLI expressions were averaged to reduce variations of the number of FLI cells, because the FLI cell numbers were not consistent between two serial sections. The NST in ×100 magnified image was divided into six subregions as described by King et al. [19] in which the nucleus was divided into three equal parts such as medial, middle, and lateral, and two parts such as dorsal and ven- tral, as depicted in Fig. 1B. To detect FLI-positive cells, we used only NST area encircled with dotted line in Fig. 1B. The background intensity of the NST region was cap- tured and its mean and standard deviations (STD) were calculated. With these two values, the FLI-positive cells were selected if the cell’s FLI intensity was bigger than mean＋STD×4. This threshold value was chosen to match the FLI cell number with Travers [10] approximately. The brightness and contrast of all images were not adjusted to remove any subjective selection of FLI cells. Statistical analysis Data were analyzed using one-way ANOVA followed by Tukey’s post-hoc multiple comparisons. The level of stat- istical significance was set at p＜0.05.Analysis of images and statistics were done using MATLAB (R2009b, Natick, MA, USA). Fig. 1. FLI expression images and subregions of the NST. (A) Typical FLI coronal section along the rostral-caudal axis is compared. The arrow in the bottom indicates the direction of anterior-posterior axis. The middle image clearly shows that the left and right NST meet the 4th ventricles (4V). This section was designated as “0” to calculate other section’s position along rostral-caudal axis and named i1 area (middle). The other sections were show typical r1 (left) and i3 (right) areas. (B) This image shows how the right NST was divided to six subregions. The horizontal line a lateral-medial axis divides the NST to dorsal and ventral regions. The two vertical lines equally divide the NST to medial, middle, and lateral regions. Scale bar: 1 mm in (A) and 500μm in (B). c-Fos Expression in NST 439 RESULTS Salt taste-induced c-Fos expression in the NST A series of FLI expression images for typical r1, i1, and i3 area are shown in Fig. 2. There was strong tendency that the number of FLI cells is lower in the r1 and r1 area and higher in the i2 and i3 area for all stimulation conditions including DW and four NaCl solutions. The high salt stim- ulation with 1.0 M NaCl also induced very low FLI in the r1 area. Two moderate salt stimulations with 0.1 and 0.3 M NaCl induced respectively low FLI which was slightly lower than control through all rostral-caudal axis. Mean- while, FLI responses to 1.0 M NaCl stimulation were higher in the i1 and i3 area than the r1 area. This topographical FLI distribution shows strong tendency of higher FLI to- ward i3 area. Fig. 2. Typical left NST images for r1, i1 and i3 areas of the NST from control (CNT) and NaCl-treated (0.1, 0.3, 0.6, and 1.0 M) rats. The black dots in each NST are c-Fos expressed neurons. Scale bar: 250μm. Fig. 3. The number of FLI cells in the NST. (A) Left NST, (B) right NST. The position along the rostral-caudal axis is marked as r1, r2, i1, i2, i3, and i4. Here, i1 is position “0” section. The animal numbers are marked in parenthesis: Control (n=6), 0.1 M (n=5), 0.3 M (n=6), 0.6 M (n=6) and 1.0 M (n=4) NaCl. Error bars indicate SEM. *p＜0.05.