Sodium appetite in the sham-drinking rat after chorda tympani nerve transection

Sodium depletion in the neurologically intact rat (Intact) produces a prompt and robust intake of NaCl. After chorda tympani nerve transection (CTX), there is a longer latency and a reduced intake of NaCl. The CTX rat depends on remaining gustatory and postingestive information to direct NaCl intake. In the present study, the effect of the removal of the postingestive signals of ingested NaCl (by means of a chronic gastric fistula) on the NaCl intakes and licking patterns of Intact and CTX rats was studied. When the gastric fistula was open (Sham), ingested NaCl did not pass beyond the stomach, thus negative postingestive stimulation was absent. After overnight sodium depletion, when postingestive stimulation was present (i.e., gastric fistula closed; Real), the CTX group drank significantly less 0.3 M NaCl than the Intact group over the 2-h test [11.7 +/- 1.6 (CTX) vs. 15.3 +/- 2.8 (Intact) ml]. In contrast, when postingestive signals were absent (i.e., Sham) the Intact group ingested 52.5 +/- 4.4 ml, whereas the CTX group had ingested only 12.4 +/- 3.1 ml of 0.3 M NaCl. Lickometer data analysis revealed that even during the first minute of the test the CTX/Real group generated significantly fewer licks than any of the other groups. Thus, although the CTX group was sensitive to inhibitory postingestive signals in the early portion of the appetite test, the absence of these signals did not release the robust and sustained intake of NaCl characteristic of the Intact group. These results suggest that information provided by the chorda tympani nerve is critically important to the strong motivational properties of NaCl after sodium depletion.