Serotonin and Its Implication for the Management of Irritable Bowel Syndrome Michael D. Gershon, MD

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1 IRRITABLE BOWEL SYNDROME Serotonin and Its Implication for the Management of Irritable Bowel Syndrome Michael D. Gershon, MD Department of Anatomy and Cell Biology, College of Physicians and Surgeons, Columbia University, New York, NY Our understanding of the enteric nervous system (ENS) has evolved from the classical" view, in which the brain controls all enteric behavior, to the current view, which holds that enteric innervation is one of local control within the bowel, modified by a bidirectional dialogue" with the brain. The ENS independently controls enteric reflexes through intrinsic primary afferent s, which monitor intraluminal conditions. This monitoring is accomplished through the use of enteroendocrine cells in the mucosa, the best known of which are the serotonin-containing enterochromaffin cells. This article describes the roles that serotonin, specific serotonin-receptor subtypes, and the serotonin reuptake transporter play in the ENS and in the communication between the ENS and central nervous system. The way in which these findings have implicated serotonin in irritable bowel syndrome is discussed. [Rev Gastroenterol Disord. 2003;3(suppl 2):S25 S34] 2003 MedReviews, LLC Key words: Serotonin Enteric nervous system Irritable bowel syndrome Serotonin reuptake transporter Peristaltic reflex The impact of 19th-century English research on the control of gastrointestinal motility has been both major and lasting. Three critical papers from this period were published by Bayliss and Starling, 1-3 who demonstrated that intestinal motility could be regulated by the enteric nervous system (ENS) without input from the brain or spinal cord. Essentially, Bayliss and Starling found that when pressure was raised inside a loop of dog s intestine, the gut responded with a propulsive wave of oral contraction and anal relaxation. They called this behavior the law of the intestine and went on to show that it could be elicited even VOL. 3 SUPPL REVIEWS IN GASTROENTEROLOGICAL DISORDERS S25

2 Serotonin in the Management of IBS continued Classical View Brain ENS Gut Wall Muscle Muscle Muscle A) Relay Ganglia IPAN Muscle Figure 1. Evolution of the concept of the enteric nervous system (ENS). (A) The classical" view considers the ENS to be a collection of relay s in the path of the vagal parasympathetic" innervation of the bowel. The brain is assigned the full task of regulating enteric behavior. (B) The classical model of the control of enteric behavior by the brain implies that vagotomy would, by cutting the efferent vagal nerves, cause paralysis of the intestine. Vagotomy does not have this effect; the classical model is wrong. (C) As was demonstrated at the end of the 19th century, the ENS is capable of regulating enteric behavior in the absence of any input from the central nervous system (CNS). The independence of the ENS depends on the presence in the bowel of intrinsic primary afferent s (IPANs). (D) The ability of the ENS to monitor intraluminal conditions is a transepithelial phenomenon that involves the intermediation of epithelial transducers, such as the 5-HT secreting enterochromaffin (EC) cells. (E) Despite the ability of the ENS to function independently, it does not always do so. The brain can powerfully influence the function of the ENS, necessitating that CNS input via the vagus (and, in the colon, sacral nerves) be coordinated with the local al activity of the ENS. (F) The bowel can influence the CNS through extrinsic primary afferent (sensory) s, the cell bodies of which reside in cranial nerve and dorsal root ganglia. There are far more extrinsic primary afferent nerves carrying information from the gut to the CNS than there are extrinsic efferent nerves carrying information from the CNS to the gut. The extrinsic enteric innervation, therefore, is two-way, implying that the CNS and ENS influence one another. The current concept of the enteric innervation is thus one of local control within the bowel, modified by a bidirectional dialogue" with the brain. after they had cut all of the extrinsic nerves to the stimulated segment of bowel. Bayliss and Starling thus attributed the law of the intestine to the intrinsic neural mechanism of the bowel. They were aware of the much IPAN B) No Denervation Paralysis C) ENS Independence Vagus Brain Vagotomy Brain IPAN Denervated Muscle Muscle D) Sensory Transduction E) CNS Affects ENS F) CNS-ENS Interaction ENS EC Cell EC Cell EC Cell Vagus Brain IPAN Inter Inter Inter Inter Current View earlier discoveries of Auerbach 4,5 and Meissner 6 that the gut contains extremely large neural plexuses, and they surmised correctly, it turned out that this large nervous system, later to be called the ENS, is capable of the independent control of intestinal behavior. The conclusions of Bayliss and Starling were dramatically and emphatically confirmed in 1917 when Trendelenburg 7 reported that he was able to evoke, in an in vitro loop of guinea pig bowel, a behavior identical to that described by Bayliss and Starling. Of course, when a loop of gut is suspended in vitro, it lacks a connection to the central nervous system (CNS). Since Trendelenburg s publication, the law of the intestine has been known more mundanely as the peristaltic reflex. The independence of the ENS was incorporated by Langley into his seminal description of the autonomic nervous system. 8 Langley s autonomic classification, which was based on the anatomy of the pathways connecting preganglionic s to the CNS, included three divisions: the sympathetic (thoracolumbar), the parasympathetic (craniosacral), and the enteric (none). Langley thought that the number of efferent fibers in the vagus nerves was trivial in comparison to the number of s in the intestines; in humans, these numbers are, respectively, approximately 10 3 and approximately ,10 The total number of s in the bowel probably exceeds that in the spinal cord and the aggregate of all of the other (nonenteric) s of the peripheral nervous system (PNS). The discrepancy in numbers between vagal axons and enteric s suggested to Langley that most enteric s are probably not directly innervated by the CNS, a supposition that has subsequently received both experimental support 11 and criticism. 12 Even if the number of enteric s innervated directly by the CNS should some day be shown to be higher than Langley originally believed, it seems clear from the five orders of magnitude of difference S26 VOL. 3 SUPPL REVIEWS IN GASTROENTEROLOGICAL DISORDERS

3 Serotonin in the Management of IBS between the numbers of vagal axons and enteric s that the vagal innervation cannot be an instrument of precise control over the ENS. After Langley's description of the ENS and its elegant ancient history, research on the ENS entered a dark age from which it has emerged only recently. The dark age began at no precise moment in time, but it was manifested by a widely accepted and stifling simplification of the organization of the autonomic nervous system that virtually deleted the ENS from the body. This wrong-minded classical" view of the enteric innervation considered the s in the wall of the bowel to be parasympathetic relays inserted in the neural pathway between the CNS and its enteric effectors, smooth muscle, blood vessels, and glands (Figure 1A). 13 This view used to be taught in most, if not all, medical schools and is probably still the most common version presented today. It is a testament to the power of empirical observation that surgeons for so long happily undertook vagotomies when the concept of the enteric innervation they accepted predicted that the operation would paralyze the bowel (Figure 1B). It is also a testament to the power of acceptance that the patent failure of cutting the vagus nerves to paralyze the bowel, which proved that the classical" view of the enteric innervation was incorrect, did not cause that idea to be abandoned or even revised. The fact that the ENS is able, by itself, to control intestinal motility (Figure 1C, D) does not imply that it always does so. In practice, the CNS can, and does, exert powerful effects on intestinal motility (Figure 1E). The relationship between anxiety and gastrointestinal motility, for example, is painfully and sometimes embarrassingly apparent to almost everyone. It is also obvious that the enteric innervation carries information from the gut to the brain (Figure 1F), at Interestingly, more than 90% of the fibers of the vagus nerves are sensory and, when vagal afferents are cut, they have a remarkable ability to regenerate. least when sensations from the bowel, such as pain, bloating, nausea, and satiety, reach consciousness. What these sensations from the gut have in common is unpleasantness. The information from the bowel that reaches consciousness, therefore, makes the gut an organ from which no one wishes to receive frequent progress reports. Interestingly, more than 90% of the fibers of the vagus nerves are sensory and, when vagal afferents are cut, they have a remarkable ability to regenerate. 14 Not all of the sensory information that is sent from the bowel to the brain by way of the vagus nerves reaches consciousness. Some may influence the brain in ways that are beneficial and not readily apparent. For example, vagal stimulation has been used to treat epilepsy and depression. 16,18,19 Conceivably, therefore, information from the gut delivered via the vagus nerves to the brain could influence mood or other brain functions. The bilateral nature of the relationship between the ENS and the CNS (Figure 1F) has not received a great deal of attention from investigators, who have often taken note of the frequency with which various psychoneuroses are correlated with functional and other diseases of the bowel. As a result of the strength of these correlations, peptic ulcer, Crohn s disease, and ulcerative colitis were all once considered to be psychosomatic in origin Correlations, however, do not establish causation, and we now understand that infection with Helicobacter pylori underlies many, if not all, cases of peptic ulcer disease, 20,25,26 whereas Crohn s disease and ulcerative colitis are autoimmune conditions. 20,27 The ability of the ENS to affect the CNS (especially mood), moreover, means that it is just as possible that abnormal gastrointestinal function is a primary cause of anxiety, depression, or even personality disorders as it is for these mental traits to be a cause of abnormal gastrointestinal function. Role of the ENS in the Pathogenesis of Irritable Bowel Syndrome The two-way nature of the relationship between the ENS and the CNS may be particularly significant in The two-way nature of the relationship between the ENS and the CNS may be particularly significant in relation to the pathogenesis of irritable bowel syndrome. relation to the pathogenesis of irritable bowel syndrome (IBS). The absence of an identifiable anatomical pathology in IBS means that the condition has to be diagnosed as a symptom complex, using an accepted definition, such as that of Manning or its replacement, the ROME criteria (I and II) Unfortunately, the variety of abnormal behaviors that the bowel is capable of manifesting is VOL. 3 SUPPL REVIEWS IN GASTROENTEROLOGICAL DISORDERS S27

4 Serotonin in the Management of IBS continued limited. As a result, many different underlying diseases could give rise to similar symptom complexes, bringing each within the definition of IBS. If so, then IBS might, in fact, reflect a variety of disorders. In some patients, a history of abuse or neurosis might lead to IBS, while in others a primary disorder of the gut could be the cause of the patients neurosis. In fact, an identifiable neurosis is not one of the criteria that define IBS, which can thus exist without one. Whether the involvement of the ENS in IBS is primary or secondary, however, it would be impossible for IBS to occur without the intense participation of the ENS. It follows that an understanding of the ENS will help in ultimately comprehending the pathogenesis of IBS and learning to treat it effectively. Independent Functioning of the ENS via Intrinsic Primary Afferent Neurons In order for the ENS to control peristaltic, secretory, and vascular reflexes independently, it is necessary for it to monitor conditions prevailing in the intestinal lumen. The ENS is able to do so because it contains, as ganglia in other organs do not, intrinsic primary afferent s (IPANs). 35,36 These s are functionally related to the sensory s of dorsal root and cranial nerve ganglia and even share molecular markers with them. 37 Enteric IPANs, however, differ from their dorsal root and cranial nerve counterparts in that they are themselves innervated 38 and thus may be dual-function cells that also act as inters. 36,39,40 IPANs are found in both the submucosal 35,41,42 and myenteric plexuses. 36,38,43-45 In both cases, the targets to which IPANs project are inters that are critical for the regulation of peristaltic and secretory reflexes. IPANs, inters, and final common motor s (to muscle and secretory cells) constitute Neither 5-HT 3 nor 5-HT 4 antagonists are able, by themselves, to abolish peristaltic and secretory reflexes, although both types of antagonist can alter intestinal motility. the microcircuits of the ENS. These microcircuits contain many different chemically defined types of s, and their organization may be quite complex. Every class of transmitter that has been identified in the CNS has now also been found in the ENS Roles of Serotonin and Specific Serotonin-Receptor Subtypes in Enteric Transepithelial Sensory Transduction Not surprisingly, given the nature of the contents of the intestinal lumen and the digestive process that takes place within it, there are no intraluminal enteric nerves. IPANs must therefore monitor intraluminal conditions across the mucosal epithelium. They evidently do so by using the system of enteroendocrine cells in the mucosa (Figure 1D). 49,50 Among the best-known cells of this system are the serotonin (5-HT)-containing enterochromaffin (EC) cells. 51 These cells contain so much 5-HT that the gastrointestinal tract accounts for 95% of the body's 5-HT. EC cells release 5-HT in response to increases in intraluminal pressure or chemical stimuli, and the 5-HT they secrete plays a role in the initiation of peristaltic and secretory reflexes (Figure 2). 41,42,53,57-63 To initiate peristaltic and secretory reflexes, the 5-HT that is secreted by EC cells must stimulate IPANs that project to the mucosa (Figure 2), which IPANs of both the myenteric 36,64 and submucosal plexus 42 do. In order for 5-HT to stimulate them, IPANs must express 5-HT receptors; moreover, it follows that antagonism of the 5-HT receptors that activate the relevant IPANs will block peristaltic and secretory reflexes stimulated either by the mucosal application of 5- HT releasing stimuli or by exogenous 5-HT. Enteric s have been found to express 5-HT 1A, HT 1P, HT 2A, 76 5-HT 2B, 77 5-HT 3, 78,79 and 5-HT 4 receptors; 53,67,80-83 however, of these, only 5-HT 1P, 5-HT 3, and 5- HT 4 exert excitatory actions on enteric s. Neither 5-HT 3 nor 5-HT 4 antagonists are able, by themselves, to abolish peristaltic 84,85 and secretory 61,62,86 reflexes, although both types of antagonist can alter intestinal motility These observations suggest that neither peristaltic nor secretory reflexes are initiated by 5- HT 3 or 5-HT 4 receptors. Instead, these 5-HT receptor subtypes probably modulate mucosa-initiated reflexes by affecting neurotransmission within the ENS and, indeed, it is possible to abolish peristaltic reflexes by inhibiting both 5-HT 3 and 5-HT 4 receptors simultaneously. 84,85,91 5-HT 3 receptors are present on the mucosal terminals of myenteric IPANs, and these cells are stimulated by mucosal 5-HT. 43,44 The inability of 5-HT 3 antagonists to prevent initiation of peristaltic and secretory reflexes, therefore, suggests that myenteric IPANs are not the ones responsible for peristaltic and secretory reflexes. Submucosal IPANs respond to the mucosal application of 5-HT; however, 5-HT 3 antagonists do not affect this response, and 5-HT 3 agonists are unable to activate submucosal IPANs. 42 In contrast, 5-HT 1P antagonists block the activation of mucosal reflexes and submucosal IPANs, 42 and 5-HT 4 receptors enhance the release S28 VOL. 3 SUPPL REVIEWS IN GASTROENTEROLOGICAL DISORDERS

5 Serotonin in the Management of IBS Alosetron Enterocytes 5-HT 3 EC Dorsal root or cranial nerve ganglion cell 5-HT ACh + CGRP To CNS Figure 2. Release of serotonin (5-HT) by enterochromaffin (EC) cells to initiate peristaltic and secretory reflexes. Increase in intraluminal pressure causes EC cells to secrete 5-HT into the wall of the bowel. The released 5-HT stimulates the mucosal processes of submucosal intrinsic primary afferent s (IPANs) via 5-HT 1P receptors and extrinsic primary afferent s via 5-HT 3 receptors. Submucosal IPANs are cholinergic s that also contain calcitonin gene related peptide (CGRP). Acetylcholine (ACh) is the transmitter responsible for fast excitatory postsynaptic potentials (EPSPs), whereas CGRP is responsible for slow EPSPs in second-order follower s. Submucosal IPANs activate the enteric microcircuits responsible for peristaltic and secretory reflexes. Stimulation of 5-HT 4 receptors enhances the secretion of both ACh and CGRP, promoting reflex activity. A 5-HT 4 agonist, such as tegaserod, thus promotes peristaltic activity by enhancing the release of transmitters in the reflex pathway. The ability of the brain to receive 5-HT mediated noxious information from the gut is inhibited by 5-HT 3 antagonists, such as ondansetron, granisetron, alosetron, and cilansetron, which thus are useful in the treatment of nausea associated with cancer chemotherapy and the pain (and other untoward enteric sensations) associated with diarrhea-predominant IBS. SERT, serotonin reuptake transporter; ENS, enteric nervous system; CNS, central nervous system. of the submucosal IPAN neurotransmitters acetylcholine (ACh) and calcitonin gene related peptide (CGRP) (Figure 2). 42,53,57,92 Submucosal IPANs are stimulated by 5-HT 1P agonists but not by 5-HT 4 agonists; moreover, Cell 5-HT 4 5-HT 1P Pressure to wall of gut SERT-mediated uptake Submucosal IPAN Tegaserod Submucosal or myenteric second order To ENS Microcircuit the only 5-HT 4 antagonist that inhibits the stimulation of submucosal IPANs by 5-HT is tropisetron, which works only at a high concentration, at which it is not very specific. 42 These observations suggest that the elicitation of peristaltic and secretory reflexes by the release of 5-HT from mucosal EC cells depends on the stimulation of 5-HT 1P and 5-HT 4 receptors on submucosal (as opposed to myenteric) IPANs. The data summarized above are most compatible with the view (illustrated in Figure 2) that transmission at the EC cell-to-submucosal IPAN synapse is 5-HT 1P mediated, whereas 5-HT 4 receptors act at the terminals of these cells to promote both fast (ACh) and slow (CGRP) neurotransmission. This idea accounts for the ability of 5-HT 4 agonists to enhance peristaltic reflexes, 57 even though these reflexes persist when 5-HT 4 receptors are blocked. The theory that 5-HT 4 receptors act presynaptically to increase transmitter release is supported by recent immunocytochemical observations (unpublished) that 5-HT 4 receptors are located on nerve processes in enteric ganglia and smooth muscle but are not present on mucosal axons, which are the nerve processes stimulated by mucosal 5-HT. Roles of 5-HT 3 Receptors within the ENS If 5-HT 3 receptors are not involved in the initiation of peristaltic and secretory reflexes, what do they do? First, 5-HT 3 receptors activate myenteric IPANs 43 and thus may play a role in the initiation of other types of intestinal reflex. In addition, postsynaptic 5-HT 3 receptors are present on many enteric s in both plexuses, 79,93,94 including the motor s that innervate smooth muscle. 5-HT 3 receptors, therefore, are well located to mediate enteric neurotransmission and, in fact, they do so. The 5-HT 3 receptor is a ligandgated ion channel and not, like all other 5-HT receptor subtypes, a G protein coupled receptor The fast, inward current evoked by 5-HT 3 receptor stimulation is responsible VOL. 3 SUPPL REVIEWS IN GASTROENTEROLOGICAL DISORDERS S29

6 Serotonin in the Management of IBS continued for mediating a subset of fast excitatory postsynaptic potentials in the ENS. 96,97 Because this subset is small, 5-HT 3 antagonists do not normally block propulsive reflexes, although alosetron, which is a potent 5-HT 3 antagonist effective in the treatment of female patients with diarrhea-predominant IBS, has been associated with dose-dependent constipation in a minority of treated patients. 103 It is possible that alosetron-associated constipation is due to the drug s interference with the small proportion of ENS synapses at which fast transmission is mediated by 5-HT and not, as in the large majority, by ACh. Activation of Extrinsic Sensory Neurons by 5-HT 3 Receptors The most striking function of 5-HT 3 receptors is not the stimulation of IPANs or ENS neurotransmission, but the activation of extrinsic sensory nerves of the gut (Figure 2). The release of 5-HT from EC cells can activate these fibers and cause the profound nausea that accompanies cancer chemotherapy 109,110 and perhaps also the pain and bloating 111 associated with IBS. The presence of 5-HT 3 receptors on the sensory nerves that carry unwanted and unpleasant information from the bowel to the brain is undoubtedly responsible for the beneficial effects of 5-HT 3 antagonists in combating nausea 109 and possibly in blunting the visceral hypersensitivity that accounts for many IBS symptoms. 98, , The separation of powers that distinguishes the roles among 5-HT receptor subtypes in the bowel provides an opportunity for therapeutic intervention (Figure 2). Because 5-HT 3 receptors are primarily involved with the transmission of signals from the bowel to the brain, it is possible to The presence of 5-HT 3 receptors on the sensory nerves that carry unwanted and unpleasant information from the bowel to the brain is undoubtedly responsible for the beneficial effects of 5-HT 3 antagonists in combating nausea and possibly in blunting the visceral hypersensitivity that accounts for many IBS symptoms. protect the brain from these signals by antagonizing 5-HT 3 receptors with ondansetron, granisetron, or alosetron. Obviously, this type of intervention could not be used if 5-HT 3 receptors were responsible for initiating peristalsis. Substituting paralytic ileus for nausea would not be received well by most patients. Peristaltic and secretory reflexes, however, are not 5-HT 3 dependent, but instead are activated by 5-HT 1P and 5-HT 4 receptors, as discussed above. 5-HT 3 antagonists have thus been successfully used to alleviate both the nausea associated with cancer chemotherapy 109 and the pain, urgency, and other symptoms associated with the visceral hypersensitivity of diarrhea-predominant IBS in women. 32,101,103,104,115,116 Similarly, it is possible to promote motility, when it is deficient in patients with constipation-predominant IBS, by stimulating 5-HT 4 receptors with an agonist such as tegaserod (Figure 2) Clearly, that intervention could not be used if 5-HT 4 receptors were present on extrinsic sensory nerves. Because 5-HT 4 receptors are not located on these axons, 5-HT 4 agonists can amplify peristaltic reflexes without sending volleys of nausea- or painproducing signals to the CNS. The probable presynaptic location of 5-HT 4 receptors, moreover, allows 5- HT 4 agonists to enhance motility in response to endogenous mucosal stimulation rather than to initiate constitutive propulsive activity, which, if it were to occur, would probably be manifest as a painful and debilitating diarrhea. Although a 5-HT 4 agonist, tegaserod, has thus far been approved only for the treatment of IBS with constipation, it seems likely that a multitude of other uses will evolve in time. In fact, it will probably fill all the therapeutic niches previously occupied by cisapride. Serotonin Reuptake Transporter: Its Expression in the Bowel, Role in 5-HT Inactivation, and Possible Involvement in IBS The large amount of 5-HT that is released by EC cells implies that the body must be very efficient in inactivating extracellular 5-HT. Circulating 5-HT is potentially Tegaserod will probably fill all the therapeutic niches previously occupied by cisapride. lethal 121,122 and, in any case, 5-HT cannot be allowed to linger in contact with its receptors, which become desensitized in its continued presence. Evolution, moreover, has had to overcome a serious problem in developing a means of removing 5-HT from the extracellular space. At a physiological ph, 5-HT is highly charged. As a result, it will not cross the lipid bilayer of a plasma membrane to enter cells unless the cell membranes contain a transporter that S30 VOL. 3 SUPPL REVIEWS IN GASTROENTEROLOGICAL DISORDERS

7 Serotonin in the Management of IBS will enable 5-HT to get across. All of the enzymes that catabolize 5-HT are intracellular ; therefore, reuptake with the assistance of a 5-HT transporter is essential for the inactivation of 5-HT. In both the brain and the ENS, serotonergic neurites take up 5-HT. This reuptake, which is the primary mechanism of terminating the action of 5-HT at synapses, is mediated by a plasma membrane transporter called serotonin reuptake transporter (SERT) or 5-HT transporter (5-HTT) SERT is the target of tricyclic antidepressants, cocaine, and the selective serotonin reuptake inhibitors (SSRIs). The mucosa contains no serotonergic neurites; however, enterocytes express SERT, take up 5-HT, and participate critically in the inactivation of the 5-HT that EC cells release (Figure 2). 59,60,132 The uptake and consequent termination of the action of 5-HT on its receptors is as important in the initiation of mucosal reflexes as the 5-HT receptors themselves. The receptors and EC-cell 5-HT constitute the on" switch for these reflexes. SERT constitutes the off" switch. Acute inhibition of SERT in the gastrointestinal mucosa thus leads first to the recruitment of more IPANs than would normally respond after the application of a stimulus. 42,60 This recruitment occurs because 5-HT released from EC cells diffuses further when 5-HT inactivation is inhibited. The potentiation of 5-HT amplifies peristaltic reflexes. 59,60 Eventually, if SERT inhibition persists, 5-HT receptors become desensitized and peristaltic reflexes are lost. This phenomenon occurs cyclically in transgenic mice that lack SERT. 132 The potentiation of 5-HT in these mice causes colorectal motility to be more rapid than normal, which leads to decreased water absorption in the colon and thus to increased water in stools. Periodically, 5-HT receptors become desensitized, colorectal motility slows, and the mice become constipated. Diarrhea and constipation thus alternate, as they do in a subset of patients with IBS. In fact, the SERT knockout mice might be useful animal models for the study of phenomena relevant to IBS. The importance of SERT in the inactivation of 5-HT raises the question of how mice that lack SERT manage to survive, and indeed how humans taking or abusing tricyclic antidepressants, SSRIs, or cocaine do as well. The answer for the mice, and probably also for the humans, is that there are backup transporters that can mediate the uptake and inactivation of 5-HT when SERT is knocked out or inhibited. 132 These backup transporters include the dopamine transporter (DAT), which is expressed in the bowel as well as in the brain, 132,133 and organic cation transporters (OCTs) 1 and , The affinity of DAT and the OCTs for 5-HT is three orders of magnitude less than that of SERT; thus, they are imperfect in their function and only partially compensate for the knockout or inhibition of SERT. As a result, animals are enabled to survive the loss of SERT activity, but there are consequences, which make themselves apparent in the gut. SSRIs are associated with gastrointestinal side effects in many patients, and the SERT knockout mice, as noted above, exhibit a syndrome that resembles IBS. Since SERT is expressed and functions in the bowel, the gastrointestinal side effects" of SSRIs are really direct effects. The most common of these is nausea, which probably is due to the potentiation of the effect of 5-HT on the 5-HT 3 receptors of extrinsic sensory nerves. Among 5-HT receptor subtypes, the 5-HT 3 receptor becomes desensitized the most rapidly; therefore, patients treated with an SSRI usually lose the nausea associated with the drugs if they persist with the therapy. The next most common untoward gastrointestinal effect of SSRIs is diarrhea, which might be related to the potentiation of the effects of 5-HT at 5-HT 1P and 5-HT 4 receptors. If these receptors become desensitized, then constipation results The 5-HT receptors and EC-cell 5-HT constitute the on" switch for mucosal reflexes. SERT constitutes the off" switch. and SSRI treatment probably needs to be stopped, at least temporarily to give the receptors a chance to recover. The alternating diarrhea and constipation in SERT knockout mice is likely to be related to environmental variables that alter the rate of 5-HT release. When something occurs that causes a major increase in 5-HT release, perhaps inflammation, the backup transporters are overwhelmed and 5-HT receptors become desensitized. The resulting constipation abates after the environmental conditions that were responsible for the excessive secretion of 5-HT disappear. The relationship among neurotransmitters, receptors, and transporters, which are collectively responsible for neurotransmission, is so closely interknit that a perturbation in one affects others. As a result, the sensitivity of 5-HT 3 receptors and their propensity to become desensitized both change in SERT knockout mice. The concentration-effect curve for activation of 5-HT 3 receptors shifts to the right, indicating that the 5-HT sensitivity of these receptors decreases. The receptors also become desensitized more readily. These changes are secondary to a downregulation in the VOL. 3 SUPPL REVIEWS IN GASTROENTEROLOGICAL DISORDERS S31

8 Serotonin in the Management of IBS continued expression of the B subunit of the 5-HT 3 receptor. (The receptor is a dimer of A and B subunits.) Both the decrease in sensitivity and the increased tendency of the receptors to become desensitized can be characterized as adaptations to an internal milieu of increased 5-HT availability. Both changes tend to prevent the effects of receptor stimulation from becoming excessive. Whether or not similar changes in 5-HT 3 receptors take place after SERT is inhibited during use of an SSRI, tricyclic antidepressant, or cocaine remains to be determined. References 1. Bayliss WM, Starling EH. The movements and innervation of the small intestine. J Physiol (Lond). 1899;24: Bayliss WM, Starling EH. The movements and innervation of the small intestine. J Physiol (Lond). 1900;26: Bayliss WM, Starling EH. The movements and innervation of the large intestine. J Physiol (Lond). 1900;26: Auerbach L. 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