c-Fos Expression in the Nucleus of the Solitary Tract in Response to Salt Stimulation in Rats

Abstract

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.

Keywords: Chorda tympani nerve; Nucleus of the solitary tract; Salt; c-Fos.

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 4^th 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 500 µm in (B).

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: 250 µm.

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.

Fig. 4

Comparison of FLI cell numbers in the subregions of the left 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=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.

Fig. 5

Comparison of FLI cell numbers 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.