Figure 2.

Effects of exogenous 5-HT, endogenous 5-HT potentiated by fluoxetine, 2-me-5-HT, and RS 67506 on the threshold pressure for initiation of propulsive contractions. Histograms showing the mean (±s.e.m.) thresholds in cm H₂O for initiation of peristalsis in control, with the drug present or with drug and antagonist present. 5-HT (30 nM), fluoxetine (1 nM), 2-me-5-HT (1 mM), and RS 67506 (1 μM) each reversibly reduced the threshold for initiation of propulsive motor patterns in the ileum when applied to the lumen. The reduction in threshold was significant in each case [(A) 5-HT, P < 0.01; (B) fluoxetine, P < 0.01, 2-me-5-HT, P < 0.0001, RS 67506, P < 0.01]. The histograms in the left column show the effects of the 5-HT₄ receptor antagonist SB 207266 (10 nM) in the lumenal perfusate on the changes in threshold for initiation of propulsive motor patterns in the ileum induced by 5-HT (A), fluoxetine (B), 2-me-5-HT (C), or RS 67506 (D). The right column shows the effects of the 5-HT₃ receptor antagonist granisetron (1 μM) on the changes induced by the same compounds. SB 207266 blocked the facilitation caused by luminal 5-HT, fluoxetine, and RS 67506 [panels A(I),B(I),D(I)] but did not abolish the effects of 2-me-5-HT [panel C(I)]. In contrast, granisetron blocked the facilitation of peristalsis produced by all of the agonists tested [A(II),B(II),C(II),D(II)]. ^*indicates significantly different from control with P < 0.05 in all cases.