3. Visceral Noxious Stimulus Suppresses c-Fos Expression in the Nucleus of the Solitary Tract Produced by a Sweet Solution

Using the c-Fos immunohistochemical method, we examined whether the inhibition of brainstem neurons in the nucleus tractus solitarius(NTS)during ingestion with colorectal distention(CRD)is due to orosensory afferent neuronal activation in response to intraoral infusions of sweet stimuli. Rats were deprived of water and food overnight. Twelve male SD rats were used to establish a visceral pain model. A 5-cm-long flexible latex balloon was inserted intraanally into each rat, and colorectal distensions were carried out by inflating the balloon with air. A pressure of 80mmHg was applied for 12min. One minute after the start of CRD fluid stimulation, 5.0ml of distilled water or sucrose(0.5M)was injected with a infusion pump at a speed of 0.5ml/min. Animals were randomly divided into five groups:(A) sham operated, (B) inserted balloon with distilled water, (C) inserted and distended balloon, (D) inserted balloon with sucrose, and (E) inserted and distended balloon with sucrose. There was no c-Fos-like immunoreactivity(c-FLI)in group A;little c-FLI in groups B and C; and high levels of c-FLI in groups D and E, with a significant difference between these two groups. These results suggest that sweet responses of NTS neurons are inhibited by CRD.