the protective effects of vagus nerve during ischemia / reperfusion ( i / r ) have been suggested ( 1 ) . vagus nerve stimulation decreases the inflammation and the injuries resulted from i / r through affecting neutrophils while vagotomy increases these processes ( 1 ) . moreover , it has been observed that stimulation of vagus nerve can cause protective effects on gastric injury through releasing prostaglandins , nitric oxide ( no ) and calcitonin gene related peptide ( cgrp ) ( 2 ) . it is probable that vagus nerve exerts its gastric protective effects through releasing the aforementioned mediators . also , it has been reported that cholinergic activity of the efferent vagus nerve can participate in immunity modulation ( 3 ) . another study has shown that nicotine in monocytes not only decreases the production of pre - inflammatory cytokines , but also changes the response to il-10 , an anti - inflammatory cytokine ( 4 , 5 ) . cho et al have shown that alternative stimulation of cervical vagus nerve increases intra - gastric pressure and also causes bleeding ulcers in mucosal glands of stomach , and these effects can be prevented by atropine administration or sub - diaphragmatic vagotomy ( 6 ) . it has been reported that gastric vagotomy is effective in treating gastric and duodenal ulcers ( 7 ) . melatonin as a neurohormone is basically produced in the pineal gland and also in epiphysis , gastrointestinal ( gi ) mucosa and other organs . melatonin has several physiological activities such as controlling circadian rhythm , sleep induction , regulation of seasonal reproduction , and improvement of immunity . the most important effect of melatonin is an anti - oxidant effect that protects living organisms against oxidative stress ( 11 ) . in several studies , the ability of melatonin in decreasing molecular injury has been shown during i / r . melatonin prevents myocardial infarction , necrotic cell death and renal abnormality after i / r ( 8 , 12 ) , reduces brain edema in ischemia ( 12 ) , and prevents acute gastric ulcers resulted from stress . these effects can be done through direct reactive oxygen species / reactive nitrogen species ( ros / rns ) detoxification , increase in anti - oxidative enzymes , and the prevention of electron leakage in mitochondrial inner membrane ( 11 , 13 ) . melatonin has a role in expression and function of nicotinic acetylcholine receptors and increases the efficacy of beta bungarotoxin - sensitive acetylcholine receptors ( 14 ) . it has been recognized that intra - cerebral injection of melatonin is effective in prevention of acid and pepsin secretion through cholinergic activity ( 15 ) . it has been identified that 5ht3 and 5ht2 receptors are involved in stimulatory effects of melatonin in releasing pancreatic enzymes ( 11 ) . protective effects of melatonin are not only due to antioxidant activity , but it also activates capsaicin - sensitive afferent fibers ( 1 , 16 ) . melatonin may have a potential impact on the treatment of peptic ulcer via significant increase in ghrelin expression ( 17 ) . melatonin increases the release of pancreatic amylase and proteins through vagus nerve ( 18 ) . the effects of melatonin on pancreatic enzymes are reversed by vagotomy and administration of capsaicin ( 18 , 19 ) . intra - lumen administration of melatonin increases plasma cholecystokinin ( cck ) and antioxidants ( 11 , 19 , 20 ) . it has been shown that sensory nerve fibers are involved in protective effects of melatonin in the healing of acute gastric injury and peptic ulcer ( 16 ) . it has been reported that vagus nerve increases the secretion of bicarbonate from duodenum mucosa through an increase in melatonin secretion ( 21 ) . vagal pathways are the most important regulatory pathways in the gastrointestinal tract which is one of the most important sources of melatonin production . in previous studies , both protective and harmful effects of the stimulation of vagus nerve in digestive system have been reported . the role of sensory neurons in protective effects of melatonin in the healing of acute gastric injury and peptic ulcer has been reported . there is no study about the combined effects of melatonin administration and intervention of vagus nerve during gastric i / r injury . probably , vagus nerve and melatonin have interactions in their protective effects during gastric i / r events so , the present study was performed to examine this interaction during gastric i / r . this study was performed in 42 male wistar rats weighing 180 - 220 g. animals were kept at room temperature 20 - 22  c and 12 hr-12 hr light - dark cycle . animals had free access to water and food and were randomly divided into 6 groups of 7 rats . rats were fasted for 12 hr prior to the experiment but had access to water . experimental groups were : 1 ) base+i / r+vehicle ; 2 ) base+i / r+melatonin ; 3 ) vagotomy+i / r+vehicle ; 4 ) vagotomy+i / r+melatonin ; 5)vagus nerve stimulation+i / r+vehicle ; 6)vagus nerve stimulation+i / r+ melatonin . base , means condition that neither vagus nerve was stimulated nor vagotomy was performed . animals underwent tracheostomy and were cannulated ( in order to prevent probable airway occultation ) under anesthesia induced by intraperitoneal pentobarbital ( 50 mg / kg ) . after local shaving , a small incision was made in cervical area and branches of cervical vagus nerve were carefully dissected from carotid artery and cut ( 1 ) . after dissection of vagus nerve , distal end was covered with mineral oil and stimulated with bipolar electrode of a stimulator using 10 volt / msec pulses at the frequencies of 0.625 , 1.25 , 2.5 , 5 or 7.5hz for 30 sec . stimulations were performed with 2-min intervals ( 23 ) . in order to ensure the stimulation of vagus nerve , electrocardiogram gastric i / r injury was induced by dissecting celiac artery from the surrounding tissues in rats anesthetized with 50 mg / kg pentobarbital , 30 minutes after dissection of vagus nerve . then , celiac artery was closed with a microvascular clamp and returned to its place and the area was sutured using 4 - 0 silk . occlusion was continued for 30 min ( 18 ) followed by a 3-h circulation ( reperfusion ) . melatonin ( 10 mg / kg ) was injected intraperitoneally before reperfusion in groups 5 , 6 and 7 ( 18 , 25 ) . melatonin was dissolved in 1% ethanol , diluted with 0.9% saline , and injected at a final volume of 0.3 ml ( 18 ) . one part was kept in 10% formalin solution for histopathological assessment and the rest was kept at -70 c for evaluation of oxidant and antioxidant factors ( 18 , 25 ) . mda level was measured according to hiroshi ohkawa method via reaction with thiobarbituric acid and color formation . optical absorbance was determined at 532 nm and mda level was reported as nmol / mg protein ( 26 ) . glutathione peroxides activity was determined by the method described by valentine and paglia method ( 20 ) , and enzyme activity was reported as u / mg protein . superoxide dismutase activity was determined using ransod kit ( randox , uk ) and enzyme activity was reported as u / mg protein ( 19 ) . catalase activity was determined by the method described by mari and reported as u / mg protein ( 20 ) . the h & e  stained sections were examined under low power ( 40x ) to identify the areas of neutrophil aggregates within all tissue blocks . the number of neutrophils was calculated in a semiquantitative manner using the mean value of 20 non - overlapping high power fields ( hpf ; magnification of 400x ; 0.08 mm ) by using a 40x objective and a square grid mounted in a 10x microscopic eyepiece . comparisons among different groups were made by one - way and two - way anova followed by post hoc tukey s test . animals underwent tracheostomy and were cannulated ( in order to prevent probable airway occultation ) under anesthesia induced by intraperitoneal pentobarbital ( 50 mg / kg ) . after local shaving , a small incision was made in cervical area and branches of cervical vagus nerve were carefully dissected from carotid artery and cut ( 1 ) . after dissection of vagus nerve , distal end was covered with mineral oil and stimulated with bipolar electrode of a stimulator using 10 volt / msec pulses at the frequencies of 0.625 , 1.25 , 2.5 , 5 or 7.5hz for 30 sec . stimulations were performed with 2-min intervals ( 23 ) . in order to ensure the stimulation of vagus nerve , electrocardiogram gastric i / r injury was induced by dissecting celiac artery from the surrounding tissues in rats anesthetized with 50 mg / kg pentobarbital , 30 minutes after dissection of vagus nerve . then , celiac artery was closed with a microvascular clamp and returned to its place and the area was sutured using 4 - 0 silk . occlusion was continued for 30 min ( 18 ) followed by a 3-h circulation ( reperfusion ) . melatonin ( 10 mg / kg ) was injected intraperitoneally before reperfusion in groups 5 , 6 and 7 ( 18 , 25 ) . melatonin was dissolved in 1% ethanol , diluted with 0.9% saline , and injected at a final volume of 0.3 ml ( 18 ) . one part was kept in 10% formalin solution for histopathological assessment and the rest was kept at -70 c for evaluation of oxidant and antioxidant factors ( 18 , 25 ) . mda level was measured according to hiroshi ohkawa method via reaction with thiobarbituric acid and color formation . optical absorbance was determined at 532 nm and mda level was reported as nmol / mg protein ( 26 ) . glutathione peroxides activity was determined by the method described by valentine and paglia method ( 20 ) , and enzyme activity was reported as u / mg protein . superoxide dismutase activity was determined using ransod kit ( randox , uk ) and enzyme activity was reported as u / mg protein ( 19 ) . catalase activity was determined by the method described by mari and reported as u / mg protein ( 20 ) . the h & e  stained sections were examined under low power ( 40x ) to identify the areas of neutrophil aggregates within all tissue blocks . the number of neutrophils was calculated in a semiquantitative manner using the mean value of 20 non - overlapping high power fields ( hpf ; magnification of 400x ; 0.08 mm ) by using a 40x objective and a square grid mounted in a 10x microscopic eyepiece . comparisons among different groups were made by one - way and two - way anova followed by post hoc tukey s test . mean number of neutrophils in base+ i / r+ melatonin group ( 23.80.37 ) was lower than that in base+ i / r+ vehicle group ( 27.20.37 ) ( p<0.01 ) . also , this parameter in vagus stimulation + i / r+ vehicle group ( 450.32 ) was higher than that in base+ i / r+ vehicle and vagotomy+ i / r+ vehicle groups ( 27.20.36 ) ( p<0.001 ) . / r+melatonin group ( 20.6 0.4 ) in comparison to vagus stimulation+i / r+vehicle group ( p<0.001 ) . / r+ melatonin group as compared to base+i / r+ melatonin group ( p<0.01 ) . a significant increase in neutrophils count was shown in vagotomy+i / r+melatonin group ( 260.32 ) as compared to base+i / r+melatonin group ( p<0.05 ) . the number of neutrophils in gastric tissue of the study groups ( n=7 ) . # # # p<0.001 : vagus stimulation+i / r+veh group vs. vagotomy+i / r+veh group . base : condition that neither vagus was stimulated nor vagotomy was performed ; i / r : ischemia / reperfusion ; mel : melatonin ; veh : vehicle mda level in the study groups is presented in figure 2 . as it is seen , mda level in gastric tissue was lower in base+i / r+melatonin group ( 4.090.15 nmol / mg protein ) in comparison with base+i / r+vehicle group ( 5.530.11 nmol / mg protein ) ( p<0.05 ) . moreover , a significant increase in mda level was shown in vagus stimulation+i/-r+vehicle group ( 7.310.25 nmol / mg protein ) in comparison with base+i / r+vehicle and vagotomy+i/- r+vehicle ( 5.570.14 nmol / mg protein ) groups ( p<0.01 ) . / r+vehicle group ( p<0.001 ) . also , mda level was lower in vagus stimulation+i / r+melatonin group ( 2.48 0.06 nmol / mg protein ) in comparison with base+i / r+melatonin group ( p<0.01 ) . mda level was higher in vagotomy+i / r+melatonin group ( 5.75 0.63 nmol / mg protein ) in comparison with base+ i / r+melatonin group ( p<0.05 ) . * p<0.05 : base+i / r+ mel group vs. base + i / r+ veh group . # # p<0.01 : vagus stimulation+i / r+veh group vs. vagotomy+i / r+veh group . a p<0.01 : vagus stimulation+i / r+mel group vs. base + i / r+mel group . base : condition that neither vagus was stimulated nor vagotomy was performed ; i / r : ischemia / reperfusion ; mel : melatonin ; veh : vehicle catalase activity in the study groups are shown in figure 3 . catalase activity was increased in base+i / r+melatonin group ( 0.060.003 u / mg protein ) in comparison with base+i / r+vehicle group ( 0.040.003 u / mg protein ) ( p<0.001 ) . a significant increase in catalase activity was shown in vagus stimulation+ i / r+vehicle group ( 0.07 0.004 u / mg protein ) in comparison with base+i / r+vehicle and vagotomy+i / r+vehicle groups ( 0.0420.002 u / mg protein ) ( p<0.001 ) . / r+melatonin group ( 0.070.004 u / mg protein ) in comparison with base+i / r+melatonin group ( p<0.05 ) . catalase activity was lower in vagotomy+i / r+melatonin group ( 0.040.003 u / mg protein ) in comparison with base+i / r+melatonin group ( p<0.05 ) . catalase activity ( u / mg protein ) in gastric tissue of the study groups ( n=7).***p<0.001 : base+i / r+mel group vs. base+i / r+veh group . # base : condition that neither vagus was stimulated nor vagotomy was performed ; i / r : ischemia / reperfusion ; mel : melatonin ; veh : vehicle figure 4 shows glutathione peroxidase activity in the studied groups . a significant increase in gpx activity was shown in gastric tissue of basal+i / r+melatonin group ( 10.310.37 ) ( p<0.001 ) in comparison with that in basal+i / r+vehicle group ( 4.420.57 ) . / r+vehicle group ( 7.120.31 ) as compared to that observed in basal+i / r+vehicle and vagotomy+i / r+vehicle groups ( 4.430.38 ) ( p<0.001 ) . also , a significant increase in gpx activity was indicated in vagus stimulated+i this parameter was significantly lower in vagotomy+ i / r+melatonin group ( 4.610.4 ) as compared to that in basal+i / r+melatonin group ( p<0.01 ) . level of gpx activity ( u / mg protein ) in gastric tissue of the study groups ( n=7 ) * * * p<0.001 : base+i / r+mel group vs. base+i / r+veh group . # b p<0.001 : vagotomy + i / r+ mel group vs. base+i / r+mel group . base : condition that neither vagus was stimulated nor vagotomy was performed ; i / r : ischemia / reperfusion ; mel : melatonin ; veh : vehicle sod activity ( u / mg protein ) in gastric tissue of studied groups has been shown in figure 5 . as it is seen , this parameter in basal+i / r+melatonin group ( 0.310.004 ) was significantly higher ( p<0.05 ) than that in basal+i / r+vehicle group ( 0.230.016 ) . a significant decrease in sod activity / r+ vehicle group ( 0.310.004 ) in comparison with that in vagotomy+i / r+vehicle group ( 0.360.003 ) ( p<0.05 ) , but it was higher than that in basal+ i / r+vehicle group . this parameter in vagotomy+i / r+melatonin group ( 0.170.04 ) was lower than that in basal+i/-r+melatonin group ( p<0.05 ) . superoxide dismutase activity ( u / mg protein ) in gastric tissue of the study groups ( n=7 ) * p<0.05 : base+i / r+mel group vs. base+i / r+veh group . base : condition that neither vagus was stimulated nor vagotomy was performed ; i / r : ischemia / reperfusion ; mel : melatonin ; veh : vehicle mean number of neutrophils in base+ i / r+ melatonin group ( 23.80.37 ) was lower than that in base+ i / r+ vehicle group ( 27.20.37 ) ( p<0.01 ) . also , this parameter in vagus stimulation + i / r+ vehicle group ( 450.32 ) was higher than that in base+ i / r+ vehicle and vagotomy+ i / r+ vehicle groups ( 27.20.36 ) ( p<0.001 ) . / r+melatonin group ( 20.6 0.4 ) in comparison to vagus stimulation+i / r+vehicle group ( p<0.001 ) . / r+ melatonin group as compared to base+i / r+ melatonin group ( p<0.01 ) . a significant increase in neutrophils count was shown in vagotomy+i / r+melatonin group ( 260.32 ) as compared to base+i / r+melatonin group ( p<0.05 ) . the number of neutrophils in gastric tissue of the study groups ( n=7 ) . # # # p<0.001 : vagus stimulation+i / r+veh group vs. vagotomy+i / r+veh group . base : condition that neither vagus was stimulated nor vagotomy was performed ; i / r : ischemia / reperfusion ; mel : melatonin ; veh : vehicle as it is seen , mda level in gastric tissue was lower in base+i / r+melatonin group ( 4.090.15 nmol / mg protein ) in comparison with base+i / r+vehicle group ( 5.530.11 nmol / mg protein ) ( p<0.05 ) . moreover , a significant increase in mda level was shown in vagus stimulation+i/-r+vehicle group ( 7.310.25 nmol / mg protein ) in comparison with base+i / r+vehicle and vagotomy+i/- r+vehicle ( 5.570.14 nmol / mg protein ) groups ( p<0.01 ) . / r+vehicle group ( p<0.001 ) . also , mda level was lower in vagus stimulation+i / r+melatonin group ( 2.48 0.06 nmol / mg protein ) in comparison with base+i / r+melatonin group ( p<0.01 ) . mda level was higher in vagotomy+i / r+melatonin group ( 5.75 0.63 nmol / mg protein ) in comparison with base+ i / r+melatonin group ( p<0.05 ) . * p<0.05 : base+i / r+ mel group vs. base + i / r+ veh group . # a p<0.01 : vagus stimulation+i / r+mel group vs. base + i / r+mel group . base : condition that neither vagus was stimulated nor vagotomy was performed ; i / r : ischemia / reperfusion ; mel : melatonin ; veh : vehicle catalase activity was increased in base+i / r+melatonin group ( 0.060.003 u / mg protein ) in comparison with base+i / r+vehicle group ( 0.040.003 u / mg protein ) ( p<0.001 ) . a significant increase in catalase activity was shown in vagus stimulation+ i / r+vehicle group ( 0.07 0.004 u / mg protein ) in comparison with base+i / r+vehicle and vagotomy+i / r+vehicle groups ( 0.0420.002 u / mg protein ) ( p<0.001 ) . / r+melatonin group ( 0.070.004 u / mg protein ) in comparison with base+i / r+melatonin group ( p<0.05 ) . catalase activity was lower in vagotomy+i / r+melatonin group ( 0.040.003 u / mg protein ) in comparison with base+i / r+melatonin group ( p<0.05 ) . catalase activity ( u / mg protein ) in gastric tissue of the study groups ( n=7).***p<0.001 : base+i / r+mel group vs. base+i / r+veh group . # # # p<0.001 : vagus stimulation+i / r+veh group vs. vagotomy+i / r+veh group . base : condition that neither vagus was stimulated nor vagotomy was performed ; i / r : ischemia / reperfusion ; mel : melatonin ; veh : vehicle a significant increase in gpx activity was shown in gastric tissue of basal+i / r+melatonin group ( 10.310.37 ) ( p<0.001 ) in comparison with that in basal+i / r+vehicle group ( 4.420.57 ) . / r+vehicle group ( 7.120.31 ) as compared to that observed in basal+i / r+vehicle and vagotomy+i / r+vehicle groups ( 4.430.38 ) ( p<0.001 ) . also , a significant increase in gpx activity was indicated in vagus stimulated+i this parameter was significantly lower in vagotomy+ i / r+melatonin group ( 4.610.4 ) as compared to that in basal+i / r+melatonin group ( p<0.01 ) . level of gpx activity ( u / mg protein ) in gastric tissue of the study groups ( n=7 ) * * * p<0.001 : base+i / r+mel group vs. base+i / r+veh group . # # p<0.01 : vagus stimulation+i / r+veh group vs. vagotomy+i / r+veh group . b p<0.001 : vagotomy + i / r+ mel group vs. base+i / r+mel group . base : condition that neither vagus was stimulated nor vagotomy was performed ; i / r : ischemia / reperfusion ; mel : melatonin ; veh : vehicle sod activity ( u / mg protein ) in gastric tissue of studied groups has been shown in figure 5 . as it is seen , this parameter in basal+i / r+melatonin group ( 0.310.004 ) was significantly higher ( p<0.05 ) than that in basal+i / r+vehicle group ( 0.230.016 ) . a significant decrease in sod activity / r+ vehicle group ( 0.310.004 ) in comparison with that in vagotomy+i / r+vehicle group ( 0.360.003 ) ( p<0.05 ) , but it was higher than that in basal+ i / r+vehicle group . this parameter in vagotomy+i / r+melatonin group ( 0.170.04 ) was lower than that in basal+i/-r+melatonin group ( p<0.05 ) . superoxide dismutase activity ( u / mg protein ) in gastric tissue of the study groups ( n=7 ) * p<0.05 : base+i / r+mel group vs. base+i / r+veh group . # # p<0.01 : vagus stimulation+i / r+veh group vs. vagotomy+i / r+veh group . base : condition that neither vagus was stimulated nor vagotomy was performed ; i / r : ischemia / reperfusion ; mel : melatonin ; veh : vehicle the present study was performed to investigate the protective effect of interaction between melatonin and vagus nerve during gastric i / r . the results of this study showed that melatonin administration decreased neutrophils infiltration and mda level and increased sod , cat and gpx activities in gastric tissue . also , vagus stimulation increased neutrophils infiltration and mda level in gastric tissue while melatonin administration along with vagus stimulation decreased the effect of vagus stimulation in gastric tissue . finally , vagotomy prevented some of the protective effects of melatonin during gastric i / r . according to the results of this study , melatonin can reduce gastritis during i / r , probably through decreasing the activity of oxidant enzymes and increasing the activity of anti - oxidant enzymes such as sod , cat and gpx . it has also been demonstrated that melatonin decreases mad level ( 27 - 29 ) . other probable mechanisms underlying the protective effect of melatonin include an increase in gastric microcirculation ( 1 ) , a reduction in acid and pepsin secretions ( 15 ) , a reduction in production of inflammatory cytokines ( 16 ) and an increase in the expression of anti - oxidant gene ( 17 , 30 - 32 ) . moreover , in our study , a stimulation of vagus nerve increased gastritis which was associated with an increase in mda level while vagotomy after i / r was ineffective . it is probable that the stimulation of vagus nerve following gastric i / r aggravated the condition and increased the activity of oxidant enzymes . since vagotomy in basal condition did not have any effect , it seems that the control of gastritis and the activity of oxidant and anti - oxidant enzymes are not under the control of vagus nerve in basal conditions . it has been demonstrated that the stimulation of vagus nerve increases output of gastric juice and acid ( 33 ) , and decreases the activity of anti - oxidant enzymes like sod ( 33 ) . on the other hand , vagotomy showed healing effects on duodenal ulcers ( 7 ) similar to the effect of atropine ( 34 ) . in a study , diaphragmatic vagotomy did not change the activity of catalase in the intestine ( 35 ) which was similar to the result of the present study . the results of some studies are not consistent with our data . for example , it has been identified that vagal afferent fibers decrease gastrointestinal tract inflammation through nicotinic receptors leading to a decrease in inflammatory cytokines ( 16 ) . cholinergic agonists such as neostigmine decrease superoxide anion and inflammation in i / r ( 36 ) . elevation of acetylcholine transferase activity and decrease in acetylcholine esterase inhibitors increase gpx ( 35 , 37 ) . treatment with acetylcholine esterase inhibitors increases catalase half - life ( 38 ) . in the present study , melatonin administration affected the outcomes in i / r+vagus stimulation group . melatonin decreased gastritis and mda level , and increased sod and gpx activity , but not in vagotomy group . these findings show that protective effects of melatonin are probably mediated through vagus nerve . also , interaction between melatonin and vagus stimulation is suggested since these show a synergic effect in relation with anti - oxidant factors . regarding this probable interaction , it is suggested that melatonin exerts its gastric protective effect through stimulation of gi tract neurons leading to cgrp release ( 1 ) . intra - cerebral injection of melatonin inhibits acid and pepsin secretions through vagus cholinergic activity ( 15 ) . melatonin increases the release of amylase and pancreas proteins through vagus nerve ( 16 ) . effect of melatonin administration accompanied with vagotomy has not suggested in the release of amylase(18 ) . studies have shown that intra - lumen administration of melatonin or its precursor induces the release of pancreatic enzymes , while this effect is not shown in isolated pancreas . moreover , these effects of melatonin are reversed by vagotomy and capsaicin administration ( 11 , 18 , 19 ) . it has been reported that melatonin effect is mediated indirectly through the release of cck followed by vagovagal reflex ( 11 , 19 , 39 ) . it has been shown that intra - lumen administration of melatonin increases plasma cck level ( 11 , 19 ) and cck acts through stimulation of vagal afferent fibers in gi tract ( 40 , 41 ) . it has been reported that vagotomy in combination with melatonin administration inhibits the release of pancreas proteins ( 7 , 11 , 16 ) . it has been reported that melatonin increases the expression and activity of ach receptors , and synergistic effects are induced during vagus stimulation ( 42 ) . according to a study , injection of phenylephrine causes melatonin release from mucosa enterochromaffin cells in the presence of normal vagus nerve and sympathetic paths ( 39 ) . melatonin increases the release of bicarbonate from gastric mucosa ( 39 ) and decreases the release of inflammatory cytokines of gi through increasing vagus activity ( 16 ) . it is suggested that melatonin is neuroprotective in gastric i / r probably by decreasing gastritis and mda and increasing the activities of cat , sod and gpx . these effects of melatonin are probably mediated by vagus nerve . moreover , in gastric i / r , melatonin can reverse harmful effects of vagus stimulation . it is suggested that further studies are required to find the mechanisms involved in this interaction .
objectives : vagal pathways in gastrointestinal tract are the most important pathways that regulate ischemia / reperfusion ( i / r ) . 
 gastrointestinal tract is one of the important sources of melatonin production . 
 the aim of this study was to investigate probable protective effect of the interaction between vagus nerve and melatonin after i / r.materials and methods : this study was performed in male rats that were divided into six groups . 
 cervical vagus nerve was cut bilaterally after induction of i / r and the right one was stimulated by stimulator . 
 melatonin or vehicle was injected intraperitoneally . 
 the stomach was removed for histopathological and biochemical investigations.results:a significant decrease in infiltration of gastric neutrophils and malondialdehyde ( mda ) level after i / r was induced by melatonin and was disappeared after vagotomy . 
 the stimulation of vagus nerve significantly enhanced these effects of melatonin . 
 however , a stimulation of vagus nerve alone increased neutrophils infiltration and mda level . 
 melatonin significantly increased the activities of catalase , glutathione peroxidase ( gpx ) , superoxide dismutases ( sod ) . 
 unlike stimulation of vagus nerve , vagotomy decreased these effects of melatonin.conclusion:according to these results , it is probable that protective effects of melatonin after i / r may be mediated by vagus nerve . therefore , there is an interaction between melatonin and vagus nerve in their protective effects .