Masatoshi Mizutani

Role of 5‐HT3 receptors in peristaltic reflex elicited by stroking the mucosa in the canine jejunum.

1. The role played by the 5‐HT3 receptor, a serotonin subtype receptor, in peristaltic reflexes was studied in dogs first given ketamine, then anaesthetized with urethane (1.0 g kg‐1, I.V.) and alpha‐chloralose (100 mg kg‐1, I.V.). The jejunal loop was partitioned into two segments with respect to blood supply. Drugs were infused intra‐arterially into each segment. 2. Stroking of the mucosa of the aboral and oral segments elicited an ascending contraction and a descending relaxation, respectively. 3. The ascending contraction was concentration‐dependently inhibited by treatment of the aboral segment with the 5‐HT3 receptor antagonists ICS 205‐930 and ondansetron (1.4 pmol min‐1 to 14 nmol min‐1 for both). The maximal inhibition was 49.5 and 69.3%, respectively. The response was not affected by treatment of the oral segment with these drugs. The descending relaxation was inhibited by 51.4 and 60.8%, respectively, by treatment of the oral segment with ICS 205‐930 and ondansetron (1.4 nmol min‐1 for both). 4. The ascending contraction was markedly inhibited by treatment of either segment with hexamethonium (140 nmol min‐1). The response was abolished by treating both segments with hexamethonium and by treating the oral segment with atropine (14 nmol min‐1). 5. These results suggest firstly that, in the canine jejunum, enteric neurons with 5‐HT3 receptors play a role as sensory neurons or interneurons in the ascending excitatory and the descending inhibitory pathways of the peristaltic reflex elicited by stroking the mucosa, and secondly, that the ascending limb is composed of cholinergic interneurons and motoneurons.

Histochemical study of the lumbar colonic nerve supply to the internal anal sphincter and its physiological role in dogs

The sympathetic innervation of the internal and sphincter (IAS) and its physiological role in maintaining sphincter tone were histochemically and mechanically studied in dogs anesthetized with pentobarbital sodium. Numerous catecholamine-fluorescent nerve fibers with varicosities were identified in the IAS of normal dogs. Such fibers were markedly reduced at one week and one month after resection of the hypogastric nerves (HGNs) or the lumbar colonic nerve (LCN), and disappeared after combined HGN and LCN resection. IAS tone decreased to 37.2% of baseline at 1 h after LCN resection and to 69.9% after HGN resection. It returned to the preoperative level at one week and one month after resection. The restored IAS tone was decreased again by acute transection of the previously intact HGNs or LCN. Combined LCN and HGN resection also caused a marked reduction of IAS tone (36.9%) at 1 h after the procedure, and was then restored to the preoperative level with time. The restored tone was not decreased by phentolamine administration. These findings confirmed that both the LCN and the HGN innervate the IAS and play a physiological role in the development of resting tone. The restoration of IAS tone after denervation may be due to intrinsic myogenic properties of the sphincter.

Indomethacin-induced lesion modifies contractile activity in rat small intestines.

BACKGROUND Indomethacin induces intestinal lesions. The change in contractility of segments with lesions was studied. METHODS Motility was recorded in segments with lesions isolated from the rat small intestine 24 h after a subcutaneous injection of indomethacin (20 mg/kg). RESULTS Concentration-contraction curves for acetylcholine shifted leftward independently of the degree of severity of lesions, but the curves for carbachol and 5-hydroxytryptamine did not. Contractions produced by intramural nerve stimulation were enhanced in segments with no visible damage but decreased with progression of lesions. Neostigmine augmented them in normal rats but not in indomethacin-treated rats. The peristaltic activity was enhanced in segments with no visible lesions. CONCLUSIONS The results suggest that treatment of the rat with indomethacin enhances contractility of the small intestine owing to diminution of acetylcholinesterase activity independently of the degree of lesions and reduces it owing to decreased responsiveness of enteric neurons with the progression of lesions.

Capsaicin‐sensitive afferents activate a sympathetic intestinointestinal inhibitory reflex in dogs.

1. In urethane‐anaesthetized dogs, an intra‐arterial infusion of capsaicin (0.7‐14 nmol min‐1) into a separated jejunal segment inhibited a vagally evoked cholinergic contraction of the other non‐infused segments. The mechanism of this reflex was investigated. 2. The inhibition by capsaicin was abolished after bilateral splanchnic nerve section or cervical spinal cord transection (C5 or C6), but was unaffected by bilateral vagotomy. Decerebration partially reduced the inhibition. 3. The inhibition by capsaicin was abolished by pre‐treatment with phentolamine or yohimbine, but was unaffected by prazosin or propranolol. 4. Sympathetic efferent discharge of the mesenteric nerve increased with capsaicin application, during which time vagally evoked contractions were inhibited. 5. Single‐unit discharges of the major splanchnic and mesenteric afferents increased with capsaicin infusion to the loop which was innervated by the units. 6. Together the results implied that capsaicin stimulated canine intestinal primary afferents, resulting in the sympathetic intestinointestinal inhibitory reflex supraspinally. The inhibition of vagally evoked contractions may be due to a presynaptic inhibition via alpha 2‐adrenoceptors, which are activated by the reflex.

Involvement of substance P neurons in contractions of canine small intestine produced by mesenteric nerve stimulation

Pathways for contractions of in vivo canine small intestine produced by mesenteric nerve stimulation (MNS) were studied. In intact and chronically sympathectomized dogs, contractions of jejunal and ileal segments were largely reduced by intra-arterial infusion of capsaicin (10-100 microM, 0.07 ml/min), substance P (SP) antagonist, (D-Pro4, D-Trp7.9) SP (4-11) (100 microM, 0.14 ml/min), hexamethonium (100-1000 microM, 0.07 ml/min) or atropine (100 microM, 0.07 ml/min). In chronically vagotomized dogs, capsaicin, SP-antagonist or atropine significantly reduced MNS-induced contractions, but hexamethonium did not. In dogs in which the coeliac and superior mesenteric ganglia had previously been removed, MNS caused no response although intra-arterial injection of 1,1-dimethyl-4-phenylpiperazinium iodide (DMPP, 0.1 mumol) caused marked contractions. It may therefore be suggested that extrinsic SP neurons probably originating in spinal ganglia and intrinsic SP neurons receiving input from vagal preganglionic cholinergic neurons are involved in the excitatory pathways to MNS-induced contractions and that activation of these neurons excites myenteric cholinergic neurons, thereby causing contractions of the small intestine.

Antidromic activation of vagal and sympathetic afferents does not produce intestinal contractions in dogs

The question has been asked whether vagal and sympathetic afferents activated antidromically play a role as motor nerves on the in vivo small intestine in dogs anesthetized with urethane. The vagus nerve of one side was cut above the nodose ganglion and the efferent fibers allowed to degenerate. Peripheral stimulation (5-50 Hz, 0.5-3 ms, 5-25 V) of an intact cervical vagus, being able to excite both efferent and afferent fibers, caused large contractions in the jejunum and stomach, whereas stimulation of the contralateral cut cervical vagus could not produce any response in the jejunum but small contractions in the stomach. Peripheral stimulation of the cut cervical vagus did not produce bradycardia and hypotension. Single- and multi-unit discharges to distension of the jejunal segments could be recorded from the peripheral cut end of the cut cervical vagus. Immunohistochemically, there were many substance P-containing cells in both nodose ganglia. Antidromic stimulation of the dorsal roots (T7-T10) did not induce any response in the jejunum but contractions in the stomach. The results may confirm that vagal and sympathetic afferents have no antidromic motor function at least in the in vivo canine small intestine.