Toshiaki Neya

Sympathetic activity in the recto-rectal reflex of the guinea pig.

In the guinea pig, defecation is controlled by the myenteric plexus, whose activity is modulated by the sacral spinal and supraspinal centers. The purpose of this study is to clarify the control of defecation reflex by sympathetic nerves. The propulsive contractions of the rectum produced by rectal distension (recto-rectal excitatory reflex response) were abolished after transection of the Th 13 and/or the L 4 segment. This response was reproduced again after removal of the lumbar segments (L1-4), division of the lumbar dorsal roots (L1-4), the lumbar splanchnic nerves or lumbar colonic nerves (LCN). The frequency of efferent discharges of LCN was increased slightly by rectal distension and remarkably increased after Th 13 and/or L 4 transection. Thus, there occurs during the recto-rectal reflex not only mucosal intrinsic reflex and sacral excitatory reflex via the pelvic nerves but also a lumbar inhibitory reflex via the colonic nerves, whose center may be located in the upper lumbar segments.But, the activity of the inhibitory center was depressed by the supraspinal center, so that an excitatory reflex is produced more dominantly than an inhibitory one in normal animals. All these extrinsic reflexes coordinate the activity of the myenteric plexus in defecation reflex.

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.

Pelvic afferent reflex control of rectal motility and lumbar colonic efferent discharge mediated by the pontine sympatho-inhibitory region in guinea pigs.

Rectal motility and the efferent discharge of lumbar colonic nerves (LCED) have previously been shown to be affected by reflex activity activated by rectal stimulation. The sensory limb of this reflex is represented by afferent fibers in pelvic nerves. The present study revealed that this reflex is modulated by supraspinal sympatho-inhibitory regions. Pelvic afferent stimulation led to rectal contraction through the withdrawal of a tonic inhibitory influence of lumbar colonic nerves. The supraspinal region responsible for this antagonism ofthe rectal-inhibitory colonic nerve activity was localized to the pons. Neither the intravenous administration of atropine nor that of guanethidine (and Eisai compound 865–123, another adrenergic neuron blocking agent) effected the ability of pelvic afferent stimulation to inhibit tonic discharge of lumbar colonic efferent nerves; nervertheless, both agents eliminated the mechanical response of the rectum to stimulation of pelvic afferents. These observations suggest that lumbar sympathetic nerves may tonically inhibit the release of acetylcholine from excitatory neurons in the rectal myenteric plexus. We conclude that descending fibers from the pons are activated as a result of pelvic afferent nerve stimulation. These descending pontine fibers in turn inhibit the firing of sympathetic lumbar colonic nerves. Removal of this tonic restraint leads to rectal contraction.

Role and localization of a region in the pons which has a descending inhibitory influence on sympathetically mediated inhibition of the recto-rectal reflex of guinea pigs

The present study revealed the site of origin and the possible function of a supraspinal descending-inhibitory influence over the lumbar sympathetic component of the recto-rectal reflex of guinea pigs. The recto-rectal reflex contraction was not changed by suprapontine transection. It completely disappeared after subpontine transection, but returned immediately after additional section of the colonic nerves, which contain the sympathetic inhibitory outflow to the rectum, i.e., subpontine transection with the lumbar colonic nerves transected did not suppress the recto-rectal reflex. These results indicate that a descending pathway which can inhibit the lumbar sympathetic component of the reflex may originate in the pons. On stimulation at sites within the pons of animals which had been spinalized at L4 we were able to evoke an increase of rectal motility and an inhibition of the lumbar colonic efferent discharges, thus producing a response which is comparable to the reflex response produced by afferent stimulation of the rectum.The sites from which this effect could be evoked were mainly located in a band running rostrocaudally through the lateral reticular formation of the rostral part of the pons, medial to the sensory nucleus of the trigeminal nerve.

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.

Myenteric 5-HT-containing neurones activate the descending cholinergic excitatory pathway to the circular muscle of guinea-pig ileum.

1 Participation of myenteric 5‐hydroxytryptamine (5‐HT)‐containing neurones in the ascending and descending pathways of the guinea‐pig isolated ileum was investigated in a new preparation. Transmural electrical stimulation of the longitudinal muscle‐myenteric plexus (LM‐MP) portion of the preparation caused ascending and descending contractions of circular or longitudinal muscle in the attached, intact segments situated orally or anally to the point of stimulation 2 All contractions to LM‐MP stimulation were abolished by tetrodotoxin (0.2 μm). The ascending and descending contractions of circular muscles were also abolished by atropine and inhibited to about 50% by hexamethonium. They were not affected by desensitization to substance P (SP) or by the SP antagonist, (D‐Pro2,D‐Trp7,9)‐substance P. The contractions of longitudinal muscles were inhibited by about 45% by hexamethonium and abolished by a combination of atropine with SP desensitization or the SP antagonist, (D‐Pro2,D‐Trp7,9)‐substance P. 3 Desensitization to 5‐HT, ICS 205–930 (1 μm) or cocaine (1 μm) reduced the descending contraction of circular muscle by 80–90%, without significantly affecting the ascending contraction. Methysergide (0.2 μm) failed to alter either contraction. 4 5‐HT desensitization, ICS 205–930 and cocaine only partially reduced the descending contraction of longitudinal muscle. A similar reduction of the ascending contraction (20–30%) was also observed. Methysergide had no effects on either contraction. 5 Contractions of either circular or longitudinal muscle produced by field stimulation of the intact segment were not significantly affected by any of the 5‐HT receptor antagonists tested. 6 The results imply that 5‐HT‐containing neurones, as interneurones, are involved mainly in the descending cholinergic excitatory pathway to the circular muscles.

Contractions of the guinea-pig ileum evoked by stimulation of the submucous plexus

Neural pathways from the submucous plexus to the longitudinal muscle of an adjacent segment of isolated guinea-pig ileum were studied. It was found that electrical field stimulation of a strip of submucosa-submucous plexus produced frequency-dependent longitudinal contractions of an intact segment of intestine lying oral to the point of stimulation. The responses were reduced to less than 10% of control by tetrodotoxin, atropine, morphine and chymotrypsin and by desensitization to substance P (SP). The responses were only inhibited by one-third by hexamethonium and were not affected by desensitization to 5-hydroxytryptamine. The effect of desensitization to SP was reversible, but the effect of chymotrypsin was irreversible. SP-induced desensitization and chymotrypsin did not inhibit the twitch response produced by field stimulation of the whole ileal segment. The same results were observed with preparations made from ileal segments that had been extrinsically denervated. The results suggest that intrinsic neurons with processes in the submucous plexus can excite cholinergic and SP-containing neurons in the myenteric plexus, thereby causing the longitudinal muscle to contract.