CrossTalk proposal: 5-HT is necessary for peristalsis

Abstract

The overwhelming preponderance of the 5-HT of every known mammal is in the gut (Erspamer, 1966; Gershon & Tack, 2007; Gershon, 2013). Enteric 5-HT must be an important signalling molecule to be so conserved. Most enteric 5-HT is in enterochromaffin (EC) cells, but smaller amounts are present in myenteric neurons. Despite their small numbers, serotonergic neurons project widely throughout the enteric nervous system (ENS) and also innervate interstitial cells of Cajal (ICC) (Okamoto et al. 2014). The abundance and variety of enteric 5-HT receptors (5-HT1-7 and subtypes) also suggest that 5-HT plays a significant role in GI physiology (Smith et al. 2014). The efficacy of therapies targeting enteric 5-HT or its receptors against GI motility disorders supports this idea (Gershon & Tack, 2007; Gershon, 2013). Beyond motility and secretion, putative roles that 5-HT plays include metabolism, osteogenesis, immunity, neurogenesis and neuroprotection (Gershon, 2013). The multiplicity of enteric 5-HT targets and receptors complicates ascertaining the physiological roles of 5-HT. Controversy is thus to be expected and has appeared in recent papers, which question whether EC or neuronal 5-HT has anything to do with peristalsis (Keating & Spencer, 2010; Spencer et al. 2011; Sia et al. 2013; Spencer et al. 2013). These papers are important not because enteric 5-HT is vestigial (it is not) or that its roles in normal and abnormal GI motility can be ignored (they cannot). Instead, they highlight common misunderstandings about peristaltic reflexes and 5-HT cellular biology. The papers focus on colonic migrating motor complexes (CMMCs), which are aborally propagating propulsive contractile complexes, essentially peristaltic reflexes. The authors assert that they can evoke CMMCs after mucosal removal or depleting 5-HT with reserpine; therefore, they conclude that neither EC cells, nor neuronal 5-HT is necessary for CMMCs. To comprehend what is misunderstood, it is necessary to discuss basic information about 5-HT and GI motility. Two tryptophan hydroxylase isoforms, TPH1 and TPH2, are rate limiting in 5-HT biosynthesis, TPH1 in EC cells, and TPH2 in serotonergic neurons (Gershon, 2013). 5-HT is synthesized in the cytosol but stored in vesicles. Reuptake terminates actions of 5-HT. Because 5-HT is charged, two transporters are required for transmembrane transport, a vesicular monoamine transporter (VMAT1 in EC cells and VMAT2 in neurons) (Henry et al. 1998) and a plasmalemmal serotonin reuptake transporter (SERT) (Blakely, 2001). Reserpine inhibits VMAT (Henry et al. 1998). Intracellular 5-HT is thus reduced due to enhanced catabolism; but reserpine does not prevent 5-HT biosynthesis or constitutive release. That requires deletion or inhibition of TPH, which when isoform-selective, distinguishes mucosal from neuronal 5-HT (Li et al. 2011; Gershon, 2013). SERT deletion amplifies 5-HT effects. Because GI motility is abnormal after deletion of SERT (Chen et al. 2001), either isoform of TPH (Li et al. 2011; Gershon, 2013), or exposure to 5-HT antagonists/agonists (Monro et al. 2002; Smith et al. 2014), 5-HT clearly influences GI motility. Peristalsis is a general term applied to enteric motile behaviour that should not be conflated with the ENS-mediated peristaltic reflex that can drive propulsion (Gershon & Tack, 2007; Gershon, 2013; Furness et al. 2014). That reflex, first called the 'law of the intestine' (Bayliss & Starling, 1899), is an oral contraction and anal relaxation; it is evoked by increased intraluminal pressure and involves polarized neural pathways within the ENS (Furness et al. 2014; Smith et al. 2014). Many enteric cells, not just neurons and muscle, participate in peristaltic reflexes (Smith et al. 2014). The peristaltic reflex is only one of many activity patterns encoded within the ENS (Furness et al. 2014). Mucosal pressure/distortion or chemical stimuli release 5-HT from EC cells and evoke peristaltic reflexes (Bülbring & Lin, 1958; Bertrand et al. 2008). Luminally applied 5-HT mimics pressure (Bülbring & Crema, 1958; Bülbring & Lin, 1958). Fecal pellets apply pressure to the mucosa and thereby release 5-HT, which entrains CMMCs (Heredia et al. 2009). Mucosal removal, anaesthesia, or asphyxiation all abolish mucosally evoked peristaltic reflexes (Bülbring & Crema, 1958; Bayguinov et al. 2010; Dickson et al. 2010). Mucosally released 5-HT acts on 5-HT3 and/or 5-HT1P (or 5-HT7) receptors to stimulate intrinsic primary afferent neurons (IPANs) (Pan & Gershon, 2000; Bertrand et al. 2008; Dickson et al. 2010), which engage the ENS (Kirchgessner et al. 1992; Bayguinov et al. 2010; Okamoto et al. 2014). IPANs are found in both plexuses (Kirchgessner et al. 1992; Bayguinov et al. 2010; Okamoto et al. 2014), and appear to link mucosal and neuronal 5-HT pools together (Okamoto et al. 2014; Smith et al. 2014). 5-HT3 antagonists can block CMMCs when applied around fecal pellets, as do intraluminal 5-HT3 and 5-HT7 antagoni