The mechanisms underlying the generation of the colonic migrating motor complex in both wild-type and nNOS knockout mice

Colonic migrating motor complexes (CMMCs) propel fecal contents and are altered in diseased states, including slow-transit constipation. However, the mechanisms underlying the CMMCs are controversial because it has been proposed that disinhibition (turning off of inhibitory neurotransmission) or excitatory nerve activity generate the CMMC. Therefore, our aims were to reexamine the mechanisms underlying the CMMC in the colon of wild-type and neuronal nitric oxide synthase (nNOS)−/− mice. CMMCs were recorded from the isolated murine large bowel using intracellular recordings of electrical activity from circular muscle (CM) combined with tension recording. Spontaneous CMMCs occurred in both wild-type (frequency: 0.3 cycles/min) and nNOS−/− mice (frequency: 0.4 cycles/min). CMMCs consisted of a hyperpolarization, followed by fast oscillations (slow waves) with action potentials superimposed on a slow depolarization (wild-type: 14.0 ± 0.6 mV; nNOS−/−: 11.2 ± 1.5 mV). Both atropine (1 μM) and MEN 10,376 [neurokinin 2 (NK2) antagonist; 0.5 μM] added successively reduced the slow depolarization and the number of action potentials but did not abolish the fast oscillations. The further addition of RP 67580 (NK1 antagonist; 0.5 μM) blocked the fast oscillations and the CMMC. Importantly, none of the antagonists affected the resting membrane potential, suggesting that ongoing tonic inhibition of the CM was maintained. Fecal pellet propulsion, which was blocked by the NK2 or the NK1 antagonist, was slower down the longer, more constricted nNOS−/− mouse colon (wild-type: 47.9 ± 2.4 mm; nNOS−/−: 57.8 ± 1.4 mm). These observations suggest that excitatory neurotransmission enhances pacemaker activity during the CMMC. Therefore, the CMMC is likely generated by a synergistic interaction between neural and interstitial cells of Cajal networks

Critical role of 5-HT1A, 5-HT3, and 5-HT7 receptor subtypes in the initiation, generation, and propagation of the murine colonic migrating motor complex

The colonic migrating motor complex (CMMC) is necessary for fecal pellet propulsion in the murine colon. We have previously shown that 5-hydroxytryptamine (5-HT) released from enterochromaffin cells activates 5-HT3 receptors on the mucosal processes of myenteric Dogiel type II neurons to initiate the events underlying the CMMC. Our aims were to further investigate the roles of 5-HT1A, 5-HT3, and 5-HT7 receptor subtypes in generating and propagating the CMMC using intracellular microelectrodes or tension recordings from the circular muscle (CM) in preparations with and without the mucosa. Spontaneous CMMCs were recorded from the CM in isolated murine colons but not in preparations without the mucosa. In mucosaless preparations, ondansetron (3 μM; 5-HT3 antagonist) plus hexamethonium (100 μM) completely blocked spontaneous inhibitory junction potentials, depolarized the CM. Ondansetron blocked the preceding hyperpolarization associated with a CMMC. Spontaneous CMMCs and CMMCs evoked by spritzing 5-HT (10 and 100 μM) or nerve stimulation in preparations without the mucosa were blocked by SB 258719 or SB 269970 (1–5 μM; 5-HT7 antagonists). Both NAN-190 and (S)-WAY100135 (1–5 μM; 5-HT1A antagonists) blocked spontaneous CMMCs and neurally evoked CMMCs in preparations without the mucosa. Both NAN-190 and (S)-WAY100135 caused an atropine-sensitive depolarization of the CM. The precursor of 5-HT, 5-hydroxytryptophan (5-HTP) (10 μM), and 5-carboxamidotryptamine (5-CT) (5 μM; 5-HT1/5/7 agonist) increased the frequency of spontaneous CMMCs. 5-HTP and 5-CT also induced CMMCs in preparations with and without the mucosa, which were blocked by SB 258719. 5-HT1A, 5-HT3, and 5-HT7 receptors, most likely on Dogiel Type II/AH neurons, are important in initiating, generating, and propagating the CMMC. Tonic inhibition of the CM appears to be driven by ongoing activity in descending serotonergic interneurons; by activating 5-HT7 receptors on AH neurons these interneurons also contribute to the generation of the CMMC