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Background: Pelvic surgery may damage extrinsic nerves, resulting in colonic dysmotility and constipation. Adaptation restores motility after extrinsic denervation. We showed that intrinsic 5-HT₃ and 5-HT₄ receptors are upregulated to compensate for the loss of extrinsic 5-HT₃ receptors after parasympathetic denervation in rats (J Surg Res 2011, 171:510-516). However, the specific mechanism of action of these receptors remains unclear. We studied the role of 5-HT₃ and 5-HT₄ receptors in colonic transit and peristalsis in guinea pigs in vivo and in vitro.

Methods: For in vivo colonic transit study, ⁵¹Cr was infused into the proximal colon after saline, ondansetron (a 5-HT₃ receptor antagonist; 1 mg/kg), or GR 125487 (a 5-HT₄ receptor antagonist; 1 mg/kg) injection. Three hours later, geometric center (GC) of the ⁵¹Cr distribution in the entire colon was calculated. For in vitro studies, distal colonic segments were laid flat in an organ bath with Krebs-Henseleit buffer. Oral ends of segments were connected to an infusion syringe, and anal ends to a pressure transducer. Pressure changes in response to luminal infusion (0.2 ml) were recorded in the presence of ondansetron (3x10^-6 M) or GR 125487 (3x10^-6 M). In another setting, oral and anal ends were opened and the peristaltic reflex in response to pellet insertion or luminal balloon inflation was studied.

Results: Colonic transit was impaired by ondansetron (GC=4.5±0.3, n=6, p<0.01) and GR 125487 (GC=5.3±0.3, n=7, p<0.01) compared to controls (GC=6.8±0.3, n=10). Ondansetron reduced intraluminal pressure increase by 40±9% (n=4, p<0.01), whereas GR 125487 increased it by 76±28% (n=7, p<0.01) (Fig. 1). Pellet transit time was 46±9 sec (n=5) in controls, which was completely abolished by ondansetron (n=4) and prolonged by GR 125487 to 137±41 sec (n=8, p<0.05). In response to balloon distention, contractions observed at the anal side were smaller than those at the oral side (n=8). Ondansetron reduced the magnitude of oral and anal contractions in response to balloon distention. In contrast, GR 125487 reduced oral contractions and increased anal contractions. As a result, the ratio of anal/oral contractions was increased to 1.4±0.2 by GR 125487, compared to controls (0.6±0.1; n=9, p<0.01). Similarly, L-NAME (a nitric oxide inhibitor, 10^-4 M) significantly increased anal contractions (Fig. 2).

Conclusion: Ondansetron impairs colonic transit by lowering the magnitude of peristaltic contractions. GR 125487 impairs colonic transit by generating potent contractions on the anal side. Because L-NAME has a similar effect with GR 125487, it is suggested that 5-HT₄ receptors stimulate nitric oxide release distally. In contrast, 5-HT₃ receptors stimulate excitatory neurotransmission proximally. Our study offers new insight into the function of 5-HT₃ and 5-HT₄ receptors in regulating colonic peristalsis.

Figure 1. Luminal infusion-induced pressure increase in the presence of ondansetron and GR 125487 of the guinea pig distal colon. Ondansetron reduced, while GR 125487 increased motor responses to luminal infusion (**p<0.01, n=4-7).

Figure 2. Magnitude of oral and anal contractions in response to balloon distention of the guinea pig distal colon. Ondansetron significantly reduced both anal and oral contractions. In contrast, GR 125487 or L-NAME significantly decreased the magnitude of oral contraction and increased anal contractions, compared to controls (*p<0.05, **p<0.01, n=4-7).