The Role Of 5-hydroxytryptamine Signaling Pathway In The Esophageal Mucosal Injury Induced By Reflux

Gastroesophageal reflux (GER) is defined as the gastric contents reflux into the esophagus or oropharyngeal area. It is usually known as a physiological phenomenon. However, when the frequency and duration of reflux exceed a certain extent, leading to the pathological condition and (or) discomfort in individuals, it was termed gastroesophageal reflux disease (GERD). Typical GERD symptoms are often considered to be associated with the reflux of gastric acid, therefore decreasing the gastric acid secretion is clinically used as a routine treatment, including proton pump inhibitor (PPI) as the preferred drug. However,15-20% of patients had acid reflux or non-acid reflux during the treatments even after the dose was increased. If the refluent acid and non-acid materials were not effectively removed from the esophagus, it would induce the esophageal mucosal damage and even reflux esophagitis (RE).GERD is a motility disorder disease. Its pathogenesis is still unclear. The main etiological factors related to GERD are lower esophageal sphincter (LES) dysfunction, decreased esophageal clearance capacity, esophageal mucosal barrier dysfunction and esophageal visceral hypersensitivity. Scavenging capacity of esophageal mucosa depends on esophageal motor function, which may play an important role in the reflux-induced esophageal mucosal injury. Normal esophageal peristalsis depends on the coordination of esophageal smooth muscle contraction and relaxation, and its physiological basis is the normal reaction of esophageal smooth muscle to different neurotransmitters. Some studies reported that RE induced by acid reflux or mixed reflux was associated with impaired esophageal smooth muscle reactivity, while the impaired esophageal smooth muscle reactivity is a key factor in esophageal motility disorders. Therefore, much attention should be paid to the role of esophageal smooth muscle reactivity to different neurotransmitters in the pathogenesis of RE.It is found that 5-hydroxytryptamine (5-HT; serotonin) signaling pathway dysfunction might lead to gastrointestinal motility and secretion abnormalities and visceral hypersensitivity. Therefore 5-HT was an important neurotransmitter to regulate gastrointestinal motility, secretion and sensory function. About 90% of 5-HT is stored in the gastrointestinal mucosa, its synthesis, release, reuptake, and receptor status abnormalities would lead to a series of pathophysiological changes.5-HT may bind to 5-HT 4 receptor (5-HT4R), activating adenylyl cyclase (AC) to increase cyclic adenosine monophosphate (cAMP), leading to activation of cAMP-dependent protein kinase A (PKA), which causes the hyperpolarization of smooth muscle cells and Ca2+ influx reduction, resulting in smooth muscle relaxation. The esophageal smooth muscle relaxation plays an important role in the maintenance of normal esophageal motility. 5-HT4R partial agonist tegaserod can reduce the postprandial esophageal acid reflux, lower esophageal mechanical pain threshold and reflux, and improve esophageal peristalsis.5-HT transporter inhibitor citalopram can reduce the esophageal sensitivity to chemical and mechanical stimuli. It indicates that the 5-HT signaling pathway damage may be the key factor in the pathogenesis of GERD, but the underlying mechanism need to be investigated in the future.Objective:To establish an animal model of GER to evaluate the reflux degree by esophageal pH monitor, to assess the extent of esophageal mucosal injury by pathological slides, and to detect 5-HT concentration, SERT and 5-HT4R mRNA transcription and protein expression in the esophageal tissue, so as to explore the role of 5-HT signaling pathway in the esophageal mucosal injury induced by reflux for providing a new strategy in the treatment of GERD.Methods:Fifty 8-week male SD rats were randomly divided into the GER model group (30 cases) and the sham operation control group (20 cases). GER model was established through gastric cardia muscle longitudinal cut, transverse suture and gastric fundus ligament ligation. Animals were executed after esophageal pH value monitor in the age of 12 weeks. The anatomical forms and histopathological changes of esophageal mucosa were observed. High performance liquid chromatography- electrochemical determination (HPLC-ECD) was used to detect 5-HT/5-HIAA concentrations of the esophageal tissue. SERT and 5-HT4R mRNA expression were investigated by real-time RT-PCR. The level of 5-HT4R protein expression was also measured by western blot method. The SERT inhibitor citalopram hydrobromide was used to treat GER and RE induced by reflux.Results:Four weeks after operation,27 rats in GER model group had survivors, and other three cases were dead. The increased average body weight in GER model group were significantly lower than the control group, and the values of esophageal pH were also significantly lower than the control ones. Anatomically, different degrees of esophageal mucosal congestion and erosion were observed in the most of GER model group, while esophageal mucosa was smooth in the control group. Histopathologically, twenty rats in GER model group had esophageal mucosal inflammation (reflux esophagitis, RE group), other seven cases had no obvious inflammation (no esophagitis, NE group). No inflammatory pathological changes were observed in the esophageal mucosa of control group. The 5-HT levels of esophageal tissue in the RE group were significantly higher than those in the control and NE groups. The differences of 5-HT levels in esophageal tissue between the NE group and control group had not reached significance. The 5-HIAA concentrations of esophageal mucosa both in RE group and in NE group were significantly higher than that in control group. The SERT mRNA expressions of esophageal mucosa both in RE group and in NE group were increased significantly than that in the control group, and the SERT mRNA levels in RE group was significantly higher than that in NE group. There were no significant differences in the 5-HT4R mRNA levels among RE group, NE group and control group. The 5-HT4R protein level of esophageal mucosa in RE group was significantly lower than that in the control group, but no significant difference of 5-HT4R protein level was observed between the NE and control group. The positive rate of esophageal mucosal inflammation after citalopram hydrobromide treatment in the GER model was decreased significantly both 20mg/kg group and 10mg/kg group compared with the model group without citalopram hydrobromide administration. However,20mg/kg citalopram hydrobromide-treated model group did not show better therapeutic effect than the 10mg/kg-treated model group.Conclusions:It was concluded that the 5-HT signaling pathway disorder might be a major factor in the pathogenesis of gastroesophageal reflux and reflux esophagitis.5-HT transporter inhibitors could be used for the treatment of reflux esophagitis.