Kiyotada Naitou

Characterization of ghrelin-sensitive neurons in the lumbosacral defecation center in rats

Ghrelin is involved in the regulation of somatic growth, feeding behavior and energy homeostasis. Ghrelin stimulates neuropeptide Y (NPY) neurons and activates intracellular AMP‐activated protein kinase (AMPK) in the hypothalamus. These NPY neurons also express the leptin receptor and leptin inhibits ghrelin‐induced activation of NPY neurons. In the spinal cord, we have demonstrated colokinetic action of ghrelin. However, the precise characteristics of the ghrelin‐sensitive neurons remain to be clarified. The aim of this study was firstly to confirm that the action of ghrelin is mediated via a neurogenic pathway in the spinal cord, and secondly to characterize the ghrelin‐sensitive neurons by comparing with hypothalamic ghrelin‐sensitive neurons.

Functional roles of capsaicin-sensitive intrinsic neural circuit in the regulation of esophageal peristalsis in rats: in vivo studies using a novel method

A well-developed myenteric plexus exists in the esophagus composed of striated muscle layers, but its functional role in controlling peristaltic movements remains to be clarified. The purpose of this study was to clarify the role of a local neural reflex consisting of capsaicin-sensitive primary afferent neurons and intrinsic neurons in esophageal peristalsis. We firstly devised a method to measure peristaltic movement of esophagus in vivo in rats. Rats were anesthetized with urethane, and esophageal intraluminal pressure and propelled intraluminal liquid volume were recorded. In the experimental system, an intraluminal pressure stimulus evoked periodic changes in intraluminal pressure of the esophagus, which were consistently accompanied by intraluminal liquid propulsion. Bilateral vagotomy abolished changes in intraluminal pressure as well as liquid propulsion. These results indicate that the novel method is appropriate for inducing peristalsis in the esophagus composed of striated muscles. Then, by using the method, we examined functional roles of the local reflex in esophageal peristalsis. For that purpose, we used rats in which capsaicin-sensitive neurons had been destroyed. The esophagus of capsaicin-treated rats showed a multiphasic rise in intraluminal pressure, which may due to noncoordinated contractions of esophageal muscles, whereas a monophasic response was observed in the intact rat esophagus. In addition, destruction of capsaicin-sensitive neurons increased the propelled liquid volume and lowered the pressure threshold for initiating peristalsis. These results suggest that the local neural reflex consisting of capsaicin-sensitive neurons and intrinsic neurons contributes to coordination of peristalsis and suppresses mechanosensory function of vagal afferents in the esophagus.

Stimulation of dopamine D2‐like receptors in the lumbosacral defaecation centre causes propulsive colorectal contractions in rats

The pathophysiological roles of the CNS in bowel dysfunction in patients with irritable bowel syndrome and Parkinsons disease remain obscure. In the present study, we demonstrate that dopamine in the lumbosacral defaecation centre causes strong propulsive motility of the colorectum. The effect of dopamine is a result of activation of sacral parasympathetic preganglionic neurons via D2‐like dopamine receptors. Considering that dopamine is a neurotransmitter of descending pain inhibitory pathways, our results highlight the novel concept that descending pain inhibitory pathways control not only pain, but also the defaecation reflex. In addition, severe constipation in patients with Parkinsons disease can be explained by reduced parasympathetic outflow as a result of a loss of the effect of dopaminergic neurons.

Colokinetic effect of noradrenaline in the spinal defecation center: implication for motility disorders

Chronic abdominal pain in irritable bowel syndrome (IBS) usually appears in combination with disturbed bowel habits, but the etiological relationship between these symptoms remains unclear. Noradrenaline is a major neurotransmitter controlling pain sensation in the spinal cord. To test the hypothesis that the descending noradrenergic pathway from the brain stem moderates gut motility, we examined effects of intrathecal application of noradrenaline to the spinal defecation center on colorectal motility. Colorectal intraluminal pressure and expelled volume were recorded in vivo in anesthetized rats. Intrathecal application of noradrenaline into the L6-S1 spinal cord, where the lumbosacral defecation center is located, caused propulsive contractions of the colorectum. Inactivation of spinal neurons by tetrodotoxin blocked the effect of noradrenaline. Pharmacological experiments showed that the effect of noradrenaline is mediated primarily by alpha-1 adrenoceptors. The enhancement of colorectal motility by intrathecal noradrenaline was abolished by severing of the pelvic nerves. Our results demonstrate that noradrenaline acting on sacral parasympathetic preganglionic neurons through alpha-1 adrenoceptors causes propulsive motility of the colorectum in rats. Considering that visceral pain activates the descending inhibitory pathways including noradrenergic neurons, our results provide a rational explanation of the concurrent appearance of chronic abdominal pain and colonic motility disorders in IBS patients.

Exogenous serotonin regulates colorectal motility via the 5-HT2 and 5-HT3 receptors in the spinal cord of rats

We previously reported that intrathecal injection of noradrenaline or dopamine causes enhancement of colorectal motility. As these monoamines are neurotransmitters of descending pain inhibitory pathways in the spinal cord, we hypothesized that serotonin, which is one of the neurotransmitters involved in descending pain inhibition, also influences the lumbosacral defecation center. Therefore, we examined whether serotonin acting on the spinal defecation center enhances colorectal motility.

Colokinetic effect of somatostatin in the spinal defecation center in rats.

Somatostatin and its receptors are expressed in the spinal cord, but the functional roles of the peptide remain unknown. In this study, we examined the colokinetic effect of somatostatin in the spinal defecation center in anesthetized rats. Intrathecal application of somatostatin into the lumbo-sacral cord caused propulsive contractions of the colorectum. However, somatostatin administered intravenously or intrathecally to the thoracic cord failed to enhance colorectal motility. Transection of the thoracic cord had no significant impact on the colokinetic action of somatostatin. The enhancement of colorectal motility by intrathecal administration of somatostatin was abolished by severing the pelvic nerves. Our results demonstrate that somatostatin acting on the spinal defecation center causes propulsive motility of the colorectum in rats. Considering that somatostatin is involved in nociceptive signal transmission in the spinal cord, our results provide a rational explanation for the concurrent appearance of chronic abdominal pain and colonic motility disorders in IBS patients.

Medullary raphe nuclei activate the lumbosacral defecation center through the descending serotonergic pathway to regulate colorectal motility in rats

Colorectal motility is regulated by two defecation centers located in the brain and spinal cord. In previous studies, we have shown that administration of serotonin (5-HT) in the lumbosacral spinal cord causes enhancement of colorectal motility. Because spinal 5-HT is derived from neurons of the medullary raphe nuclei, including the raphe magnus, raphe obscurus, and raphe pallidus, we examined whether stimulation of the medullary raphe nuclei enhances colorectal motility via the lumbosacral defecation center. Colorectal pressure was recorded with a balloon in vivo in anesthetized rats. Electrical stimulation of the medullary raphe nuclei failed to enhance colorectal motility. Because GABAergic neurons can be simultaneously activated by the raphe stimulation and released GABA masks accelerating actions of the raphe nuclei on the lumbosacral defecation center, a GABAA receptor antagonist was preinjected intrathecally to manifest excitatory responses. When spinal GABAA receptors were blocked by the antagonist, electrical stimulation of the medullary raphe nuclei increased colorectal contractions. This effect of the raphe nuclei was inhibited by intrathecal injection of 5-hydroxytryptamine type 2 (5-HT2) and type 3 (5-HT3) receptor antagonists. In addition, injection of a selective 5-HT reuptake inhibitor in the lumbosacral spinal cord augmented the raphe stimulation-induced enhancement of colorectal motility. Transection of the pelvic nerves, but not transection of the colonic nerves, prevented the effect of the raphe nuclei on colorectal motility. These results demonstrate that activation of the medullary raphe nuclei causes augmented contractions of the colorectum via 5-HT2 and 5-HT3 receptors in the lumbosacral defecation center. NEW & NOTEWORTHY We have shown that electrical stimulation of the medullary raphe nuclei causes augmented contractions of the colorectum via pelvic nerves in rats. The effect of the medullary raphe nuclei on colorectal motility is exerted through activation of 5-hydroxytryptamine type 2 and type 3 receptors in the lumbosacral defecation center. The descending serotoninergic raphespinal tract represents new potential therapeutic targets against colorectal dysmotility such as irritable bowel syndrome.

Inhibitory action of hydrogen sulfide on esophageal striated muscle motility in rats

Hydrogen sulfide (H2S) is recognized as a gaseous transmitter and has many functions including regulation of gastrointestinal motility. The aim of the present study was to clarify the effects of H2S on the motility of esophageal striated muscle in rats. An isolated segment of the rat esophagus was placed in an organ bath and mechanical responses were recorded using a force transducer. Electrical stimulation of the vagus nerve evoked contractile response in the esophageal segment. The vagally mediated contraction was inhibited by application of an H2S donor. The H2S donor did not affect the contraction induced by electrical field stimulation, which can excite the striated muscle directly, not via vagus nerves. These results show that H2S has an inhibitory effect on esophageal motility not by directly attenuating striated muscle contractility but by blocking vagal motor nerve activity and/or neuromuscular transmissions. The inhibitory actions of H2S were not affected by pretreatment with the transient receptor potential vanniloid-1 blocker, transient receptor potential ankyrin-1 blocker, nitric oxide synthase inhibitor, blockers of potassium channels, and ganglionic blocker. RT-PCR and Western blot analysis revealed the expression of H2S-producing enzymes in esophageal tissue, whereas application of inhibitors of H2S-producing enzymes did not change vagally evoked contractions in the esophageal striated muscle. These findings suggest that H2S, which might be produced in the esophageal tissue endogenously, can regulate the motor activity of esophageal striated muscle via a novel inhibitory neural pathway.

Serotonin-induced contractile responses of esophageal smooth muscle in the house musk shrew (Suncus murinus).

Serotonin (5‐hydroxytryptamine, 5‐HT) is a regulatory factor in motility of the gastrointestinal tract including the esophagus. Although we proposed that vagal cholinergic and mast cell‐derived non‐cholinergic components including serotonin coordinately shorten the esophagus, the precise mechanism of serotonin‐induced contractions in the suncus esophagus is still unclear. Therefore, the aims of this study were to determine characteristics of contractile responses induced by serotonin and to identify 5‐HT receptor subtypes responsible for regulating motility in the suncus esophagus.

Actions of probiotics on trinitrobenzenesulfonic acid-induced colitis in rats.

We investigated the actions of probiotics, Streptococcus faecalis 129 BIO 3B (SF3B), in a trinitrobenzenesulfonic acid- (TNBS-) induced colitis model in rats. After TNBS was administered into the colons of rats for induction of colitis, the rats were divided into two groups: one group was given a control diet and the other group was given a diet containing SF3B for 14 days. There were no apparent differences in body weight, diarrhea period, macroscopic colitis score, and colonic weight/length ratio between the control group and SF3B group, suggesting that induction of colitis was not prevented by SF3B. Next, we investigated whether SF3B-containing diet intake affects the restoration of enteric neurotransmissions being damaged during induction of colitis by TNBS using isolated colonic preparations. Recovery of the nitrergic component was greater in the SF3B group than in the control group. A compensatory appearance of nontachykininergic and noncholinergic excitatory components was less in the SF3B group than in the control group. In conclusion, the present study suggests that SF3B-containing diet intake can partially prevent disruptions of enteric neurotransmissions induced after onset of TNBS-induced colitis, suggesting that SF3B has therapeutic potential.