in the gastrointestinal ( gi ) tract , 5-hydroxytryptamine ( serotonin , 5-ht ) is a major signaling molecule , which mediates gi functions ( motility and secretion ) , emesis , nausea and pain [ 1 , 2 ] .
most of colonic 5-ht is synthesized and released from mucosal enterochromaffin ( ec ) cells ; therefore , alterations in the release of 5-ht from ec cells affect both physiological and pathophysiological colonic functions .   irritable bowel syndrome ( ibs ) is a gi chronic disorder characterized by abdominal discomfort or pain associated with altered bowel habits
. abnormal levels of chromogranin a ( a marker for all endocrine cells ) in the colon of ibs patients have been demonstrated , thus indicating a role of the colonic endocrine cells ( namely , 5-ht - containg ec cells or peptide yy ( pyy)-containing l cells ) , in the regulation of colonic function in ibs patients [ 4 - 6 ] . indeed ,
a recent human study   suggests that the increased 5-ht release from colonic ec cells contributes to the development of abdominal pain in ibs patients .
therefore , an improved understanding of endogenous modulator system for 5-ht release from ec cells may provide novel insights into the etiology and pharmacotherapy of functional bowel disorders such as ibs .
our in vitro studies have demonstrated that isolated guinea - pig colon is a helpful in vitro - preparation for studying the mechanism modulating the release of 5-ht from ec cells [ 8 - 10 ] .
experimental data from the in vitro - preparations indicate that melatonin , endogenous tachykinins ( tks ; neuropeptides such as substance p and neurokinin a ) and pyy play important roles in the modulation of the release of 5-ht from ec cells via a unique endogenous tachykinergic nk2/nk3 receptor cascade system [ 8 - 11 ] .
this review focuses on the evidence establishing a potential role of the endogenous nk2/nk3 receptors cascade system controlling the release of 5-ht from ec cells , with special attention being paid to the pathophysiology of gut disorders including ibs .
a selective tachykinin nk2 receptor agonist ala - nka-(4 - 10 ) ( 0.01 - 1 m ) evoked a long - lasting 5-ht release from isolated guinea - pig colon in a concentration - dependent manner [ 8 - 11 ] .
the 5-ht - releasing action of ala - nka-(4 - 10 ) was unaffected by tetrodotoxin ( ttx ) , and was also seen in myenteric plexus - free mucosal preparations , thus suggesting that the nk2 agonist - evoked 5-ht release is mediated by activation of nk2 receptors located on non - neuronal cells in the mucosal layer .
this agrees with a previous report which showed tachykinin nk2 receptor expression on the surfaces of enterocytes of guinea - pig colon .
the l - type calcium channel blocker nicardipine or the syntaxin inhibitor botulinum toxin type c also reduced the long - lasting 5-ht release evoked by ala - nka-(4 - 10 ) , thus suggesting that the nk2 receptor - triggered 5-ht release is mediated via syntaxin - related exocytosis mechanisms .
however , an involvement of tachykinin nk1 receptors in the evoked 5-ht release is unlikely because a selective tachykinin nk1 receptor agonist , [ sar , met ( o2)]-substance p ( 100 nm ) failed to affect basal 5-ht release .  
a selective tachykinin nk3 receptor agonist senktide also evoked a transient 5-ht release from the isolated guinea - pig colon implicating tachykinin nk3 receptors , in addition to nk2 receptors in modulating 5-ht release from colonic mucosa .
1 , the enhancing effect of senktide on 5-ht release was prevented by both ttx and the nk2 receptor - selective antagonist sr48968 , thus suggesting that senktide stimulates the release of neuronal tks , which in turn enhances 5-ht release via mucosal nk2 receptors .
this agrees with previous findings in guinea - pig colon where nk3 receptor activation releases tks and 5-ht [ 13 , 14 ] .
the 5-ht - releasing action of senktide was also inhibited by hexamethonium , and was not seen in myenteric plexus - free mucosal preparations , thus suggesting that myenteric cholinergic interneurons also participate in the mechanism for nk3 receptor - triggered 5-ht release .
thus , these findings support the view that the cascade of neuronal nk3 receptors and mucosal nk2 receptors represents an endogenous modulator system for 5-ht release from ec cells .
these results are also in line with previous findings ( 1 ) in human colon where nk3 receptors present predominantly on myenteric and submucosal plexus neurons , and nk3 receptors come into a play only in pathological conditions such as rescue responses or inflammation , and ( 2 ) in healthy volunteers where the stimulation of nk2 receptors appears to induce ibs - like symptoms .
moreover , it has been shown that the 5-ht release from mucosal biopsy specimens of ibs patients is increased by 10-fold over controls , and the increased 5-ht release contributes to development of abdominal pain in ibs patients .
therefore , the view that the endogenous tachykininergic nk2/nk3 receptor cascade system can regulate the release of 5-ht from colonic ec cells , has an important implication for the potential role of endogenous tks in the generation of the symptoms associated with ibs ; it can be speculated that endogenous tks acting through the nk2/nk3 receptor cascade system facilitate the release of 5-ht which in turn affects the colonic function of ibs patients through the activation of 5-ht receptor subtypes .   a sensory nerve - derived neuropeptide , calcitonin gene - related peptide ( cgrp , 1 - 100 nm ) has been also shown to produce a ttx - sensitive but atropine - resistant 5-ht release via stimulation of myenteric plexus neurons in the isolated segments of guinea - pig colon .
the enhancing effect of cgrp on 5-ht release was also prevented by both the nk2 receptor - selective antagonist sr48968 and the nk3 receptor - selective antagonist sr142801 , but not by the nk1 receptor - selective antagonist l703606 .
thus , these findings support the view that cgrp stimulates the release of neuronal tks , which in turn enhances 5-ht release via the endogenous nk2/nk3 receptor cascade system .
thus , it appears that cgrp - sensitive tks - releasing neurons play a role in activating the endogenous nk2/nk3 receptor cascade system .
this conjecture is also supported by a previous report which showed that cgrp can stimulate intramural tachykinergic neurons in the guinea - pig colon .
furthermore , the evidence for the elevation of the levels of endogenous cgrp and tks in human diseased colonic mucosa has also been demonstrated [ 18 , 19 ] ; irritants , immunological and inflammatory mediators can release tks and cgrp within the intestinal wall where these peptides may facilitate the release of 5-ht via the endogenous nk2/nk3 receptor cascade system .
thus , these findings suggest a possible role of the endogenous nk2/nk3 receptor cascade system controlling the release of 5-ht from ec cells in pathophysiological conditions that are associated with an upregulation of cgrp or tks release .
the 36 amino - acid long peptide , peptide yy ( pyy ) is known as a major peptide hormone , which regulates colonic functions ( motility and secretion ) and appetite by acting via different neuropeptide y receptor subtypes [ 20 , 21 ] . in the colon , pyy
the blood pyy levels have been shown to increase rapidly in response to food ingestion , and the postprandial blood pyy levels remain elevated for several hours .
abnormal levels of pyy in large intestinal endocrine cells have also been reported in patients with ibs [ 6 , 22 ] , indicating a possible role of pyy in the generation of the symptoms associated with ibs .
the nk2 receptor agonist , ala - nka-(4 - 10 ) ( 10 - 100 nm ) also evoked a long - lasting and ttx - resistant pyy release from isolated guinea - pig colonic mucosa , thus suggesting that activation of nk2 receptors on the mucosal layer induces a long - lasting pyy release from l cells .
2 , pretreatment with the neuropeptide y1 receptor - selective antagonist bibo3304 attenuated the ala - nka-(4 - 10)-evoked 5-ht release .
these findings support the hypothesis that activation of nk2 receptors on pyy - containing l cells provokes the release of pyy , which in turn activates mainly y1 receptors located on ec cells to induce 5-ht release ( fig .
this hypothesis is further corroborated by findings that exogenously applied pyy ( 10 nm ) evoked a long - lasting and bibo3304-sensitive 5-ht release with a time course similar to that found for the nk2 agonist - evoked 5-ht release .
overall , the above observations indicate that endogenously released pyy play a fundamental role in the nk2 receptor - triggered 5-ht release from ec cells ; pyy - containing l cells appears to control 5-ht release from ec cells ( fig .
3 ) . given that food intake is a major and physiologically significant factor stimulating the release of pyy from mucosal l cells   and both pyy and 5-ht are well - known mediators of nausea and vomiting , it is hypothesized that the cascade of nk2 receptors on pyy - containing l cells and y1 receptors on ec cells ( fig .
3 ) might play a role in the generation of postprandial symptoms ( nausea and vomiting ) in patients with ibs ; in fact , a postprandial increase in plasma 5-ht levels have been shown to be related to the generation of postprandial symptoms in diarrhea - predominant ibs [ 24 , 25 ] .
therefore , the nk2 receptor - triggered pyy release from colonic mucosa may be expected as a pharmacological target for postprandial symptoms in functional bowel disorders including ibs .
activation of tachykinin nk3 receptors on enteric cholinergic neurons has also been shown to provoke the release of pyy from colonic mucosa .
thus , to elucidate whether the endogenous tachykinergic nk2/nk3 receptor cascade system participates in the mechanism for food intake - induced pyy release , is an interesting topic for future study .
the 5-ht - derived neurohormone , melatonin ( mt ) is mainly synthesized in the pineal gland as well as in 5-ht - rich ec cells of the intestinal mucosa .
it has been shown that exogenous mt modulates colonic motility and pain in ibs patients [ 28 , 29 ] ; therefore , mt is considered as a possible therapeutic agent for ibs [ 29 , 30 ] . in the isolated guinea - pig colonic mucosa , mt has been shown to inhibit the nk2 receptor - triggered 5-ht release by acting through mt3 melatonin receptors located on the mucosal layer , thus suggesting a possible role of mt as a negative - regulator for the nk2 receptor - triggered 5-ht release from ec cells   ( fig .
it is not yet known whether or not endogenous mt participates in the pathogenesis of ibs , but oral mt has been shown to reduce abdominal pain and the sensation of bowel urgency in ibs patients .
it is therefore to be expected that if low mt concentrations in the colonic mucosa facilitate 5-ht release from ec cells , the enhanced 5-ht release leads to abdominal pain in ibs patients .
in fact , it has been shown that ibs patients have lower mt levels compared with healthy controls .
thus , to elucidate whether the endogenous nk2/nk3 receptor cascade system participates in the modulatory effect of mt on colonic motility or pain , is an interesting topic for future study .
the present review shows that the cascade pathway of nk3 receptors on cholinergic interneurons and nk2 receptors on pyy - containing endocrine l cells represents an endogenous modulator system for 5-ht release from colonic ec cells , and that melatonin , endogenous tks and pyy play important roles in the modulation of the release of 5-ht from ec cells via the endogenous nk2/nk3 receptor cascade system ( fig .
3 ) . given that tks , 5-ht - containing ec cells , pyy - containing l cells and melatonin play important roles in the pathophysiological regulation of colonic functions in the diseased gut including ibs , the potential role of the endogenous nk2/nk3 receptor cascade system controlling the release of 5-ht from ec cells , will undoubtedly be a focus of future investigations .
 5-hydroxytryptamine ( 5-ht ) released from colonic mucosal enterochromaffin ( ec ) cells is a major signaling molecule , which participates in the pathophysiological regulation of colonic functions in gut disorder including irritable bowel syndrome ( ibs ) , but the endogenous modulator system for the 5-ht release is not yet well elucidated . 
 our in vitro studies in guinea - pig colon have indicated that the cascade pathway of neuronal tachykinergic nk3 receptors and nk2 receptors on peptide yy ( pyy)-containing endocrine l cells represents an endogenous modulator system for 5-ht release from ec cells and that melatonin , endogenous tachykinins and pyy play important roles in modulation of the release of 5-ht from ec cells via the endogenous nk2/nk3 receptor cascade system . 
 this review aims at examining the potential role of the endogenous tachykinergic nk2/nk3 receptor cascade system controlling the release of 5-ht from ec cells , with special attention being paid to the pathophysiology of gut disorders including ibs . 
INTRODUCTION
TACHYKININ RECEPTORS AND 5-HT RELEASE FROM COLONIC MUCOSA
NK
INHIBITION OF NK
CONCLUDING REMARKS
CONFLICT OF INTEREST