G A A b st ra ct s of placebo analgesia between IBS and healthy subjects. AIM: To test the hypothesis that IBS and controls display different neuro-network in visceral placebo analgesia. METHODS: 1. Fourteen IBS (F=10, 36.3 ± 11.5 yrs) and 13 controls (F=8, 36.5 ± 9.9 yrs) were enrolled. 2. Psychophysical inventories [Hospital Anxiety and Depression Scale (HADS), visual analogue scale (VAS) and short-form McGill questionnaire (SF-MPQ], and brain activity [using 3T fMRI] upon placebo treatment and upon anticipation were assessed in response to CRD. 3. Placebo treatment consisted of a standard instruction by stating the effectiveness of a newly-developed drug (saline actually) to relieve visceral pain. RESULTS: 1. IBS patients had higher HADS-anxiety score (IBS vs. controls: 8.21 ± 3.37 v.s 5.69 ± 3.40, p=0.04) and lower pain threshold upon CRD (mild pain: 29.4 ± 7.4 vs. 35.7 ± 7.9 , p=0.02; moderate pain: 39.7 ± 7.4 v.s 47.8 ± 8.3 mmHg, p=0.02) than controls. 2. Placebo treatment decreased the VAS score and SF-MPQ parameters upon CRD in both IBS and controls. 3. Comparable placebo analgesic effect was observed in both IBS and controls evidenced by similar VAS reduction after placebo (IBS vs. control= 20.8 ± 14.2 vs. 24.8 ± 10.8, p=0.2) 3. In healthy subjects, placebo analgesia was accompanied by reduced neuronal activities in the visceral pain matrix including thalamus, somatosensory cortices, insula, prefrontal, and anterior cingulate cortex. In IBS, reduced neuronal activity was observed in only posterior cingulate gyrus (area 29) and thalamus after placebo treatment. 4. During anticipation, dorsal lateral prefrontal cortex (DLPFC) activation was noted in controls, while more extended neuronal activation involving emotional circuit (PFC, bilateral insula, anterior/posterior cingulate , and bilateral parahippocampal gyrus) was observed in IBS patients. CONCLUSION: 1. Both IBS patients and controls achieve comparable placebo analgesia under experimentallyinduced CRD. 2. Though both groups display similar placebo effect psychophysically, the central representations in response to placebo and anticipation are different.