iv60 Gut 2000;(Suppl IV)47:iv60–iv62 Memory in the enteric nervous system Gut: first published as 10.1136/gut.47.suppl_4.iv60 on 1 December 2000. Downloaded from J B Furness, N Clerc, WAAKunze Introduction enhanced if the bowel is inflamed but demon- Investigations in the past decade indicate that strable inflammation is not necessary for IBS to functional bowel disorders, for example, irrita- occur. Quantitative data from human studies ble bowel syndrome (IBS), involve hypersensi- show that hypersensitivity includes lowering of tivity (hyperalgesia) and hyper-reflexia of the the threshold distension for evoking pain in gut. Thus seemingly normal patients suVer IBS patients.5 Hyper-reflexia in patients with pain and discomfort during digestion, and IBS has been demonstrated by a decrease in sometimes have exaggerated enteric reflexes.1–3 threshold distension to evoke entero-enteric We have recently discovered a phenomenon reflexes and by enhanced accommodation that may be related to intestinal hypersensitiv- reflexes in the colon.6 ity and hyper-reflexia, sustained slow postsyn- There is evidence from human and animal aptic excitation (SSPE), which occurs in studies that IBS-like changes can be induced by intrinsic sensory neurones of the small intes- repeated stimulation. In healthy human volun- tine.4 SSPE can be evoked by moderate stimu- teers, conditioning jejunal distension increases lation of presynaptic inputs to intrinsic sensory the perception of discomfort invoked by a test neurones (AH neurones) in the small intestine distension at an adjacent site.7 The degree to Department of and results in substantially enhanced excitabil- which conditioning stimuli in the sigmoid colon Anatomy and Cell ity of these neurones that can outlast stimula- increase discomfort is greater in IBS than in Biology, and Howard tion by several hours. It is possible that SSPE is healthy volunteers, and accommodation to Florey Institute, involved in changed intestinal function, follow- distension of the sigmoid colon is also greater.6 University of Melbourne, Parkville, ing alterations in alimentary activity, and in the In animals, distension of suYcient amplitude Victoria 5042, genesis of functional bowel disorders. To our causes aversive behavioural responses and Australia knowledge, there is no other documented long symptoms of pain.8 A change in blood J B Furness term change in the responsiveness of enteric pressure, which is a pseudoaVective response WAAKunze neurones and thus no other candidate mech- and is regarded as an indirect index of pain, can anism for inducing hyper-reactivity within the be recorded in both anaesthetised and unan- Laboratoire de 2 Neurobiologie, CNRS, enteric nervous system. aesthetised animals. Consistent with human Marseille, France studies, pseudoaVective responses in rats are N Clerc IBS, hypersensitivity, hyper-reflexia, and enhanced by conditioning distension.9 As in cellular memory humans with IBS or inflamed intestine, the Correspondence to: There is general agreement that IBS involves eVects of distending stimuli on gut sensitivity Professor J Furness, http://gut.bmj.com/ Department of Anatomy and hypersensitivity of the bowel that is expressed in animals are enhanced if there is a back- Cell Biology, University of in several ways, including increased traYc ground of intestinal irritation.2 Even in vitro it Melbourne, Parkville, and/or changed information content carried by is possible to elicit hyper-reflexia; Holzer10 Victoria 5042, Australia. J.Furness@ spinal primary aVerent neurones and altered showed that reflex responses to distension, 12 Anatomy.Unimelb.EDU.AU intestinal motility. Hypersensitivity may be which were initially attenuated by antagonising receptors for neurotransmitters, were en- Brain stem Spinal cord Dorsal hanced by applying distending stimuli for five root ganglion second periods at two minute intervals for on September 30, 2021 by guest. Protected copyright. 10–20 minutes. In summary, IBS is associated with height- Nodose Sympathetic ganglion ened sensitivity and often with hyper-reflexia, ganglion Vagal Spinal and conditions mimicking IBS can be caused by sensory Intrinsic neurones sensory repeated distension stimuli in healthy humans neurones neurones and animals, and in the isolated intestine. LM MP Mucosal Chemical Intestinofugal Sustained slow postsynaptic excitation mechano- and stretch neurones CM (SSPE) and cell memory sensitive sensitive neurones neurones The intestine is supplied by four systems of SM sensory neurones: vagal sensory neurones (cell bodies in the nodose ganglia), spinal sensory Muc neurones (cell bodies in dorsal root ganglia), intrinsic sensory neurones (cell bodies in the gut wall), and intestinofugal neurones (cells in the intestine, terminals in sympathetic ganglia) (fig 1). Figure 1 Representation of the types of sensory neurones of the gastrointestinal tract. Extrinsic sensory neurones: some have cell bodies in the nodose ganglion, and supply the Abbreviations used in this paper: SSPE, sustained stomach through the vagus nerves, while others have cell bodies in spinal (dorsal root) slow postsynaptic excitation; EPSPs, excitatory ganglia, and supply the stomach, and small and large intestines. Intrinsic sensory neurones have cell bodies in the gut wall. These have only been demonstrated recently. Another type of postsynaptic potentials; IBS, irritable bowel syndrome; aVerent pathway, that of intestinofugal neurones, conducts sensory information from the gut LTP, long term potentiation; PKC, protein kinase C; to prevertebral sympathetic ganglia. The layers of the gut wall are represented: LM, CM CaM kinase, calcium/calmodulin dependent protein (longitudinal and circular muscle); MP,myenteric plexus; SM, submucosa; Muc, mucosa. kinase. www.gutjnl.com Memory in the enteric nervous system iv61 The intrinsic sensory neurones in which through stages of excitation. Before stimula- Gut: first published as 10.1136/gut.47.suppl_4.iv60 on 1 December 2000. Downloaded from SSPE is manifested are the least studied of tion, 500 ms depolarising pulses evoked 0–3 these. They are multipolar, with processes in action potentials (phasic response) and anode the mucosa and in the enteric ganglia, and they break action potentials were not observed. As communicate with each other through excita- excitability increased, more action potentials tory synapses and thus form networks.11 were evoked by depolarisation (the responses Morphologically, they are referred to as Dogiel became tonic), anode break action potentials type II neurones, and they are classified as AH were observed, prolonged after hyperpolarising neurones based on their electrophysiological potentials that follow multiple action potentials properties.12 When sensory stimuli, such as were diminished and, with substantial depo- distension stimuli used to demonstrate hyper- larisation of the neurones, invasion by anti- algesia in IBS, are applied to the intestine, the dromic action potentials was suppressed. intrinsic sensory neurones are activated The experiments imply that there is molecu- directly by the stimulus and indirectly through lar memory of synaptic activity, just as there is slow excitatory postsynaptic potentials memory of the eVects of distending stimuli in (EPSPs) at their synaptic connections with vivo. The molecular memory involves changes each other. in the regulation of K channels that underlie Intracellular microelectrodes record slow the gKCa, leading to an overall decrease in the EPSPs in Dogiel type II neurones when their current carried by these channels, which could presynaptic inputs are stimulated at frequen- be produced by reducing channel open life- cies from about 5 to 30 Hz in trains lasting up times, by increased close times, by reducing to about one second. The same fibre tracts, channel current, or by reducing numbers of when stimulated with low frequency main- active channels. tained stimulation, evoke the SSPE. The A likely mechanism behind SSPE is channel primary transmitter for the slow EPSP is a phosphorylation, or phosphorylation of a tachykinin (substance P) which inhibits cal- channel regulator protein. Two long term cium activated conductance (gKCa). We have changes in neurones, both involving protein recently developed methods to take patch phosphorylation, are similar to SSPE: long clamp records from the Dogiel type II term excitation of Aplysia sensory neurones22 neurones in situ and have recorded the activity and long term potentiation (LTP), particularly of potassium channels that may underlie the its postsynaptic component.23 Induction and 13 gKCa. These are calcium sensitive K channels maintenance of LTP involves several kinases, at with conductances of about 230 pS and are least PKC,24 calcium/calmodulin dependent blocked by iberiotoxin. The slow EPSP is protein kinase (CaM kinase),25 and the tyrosine almost certainly mediated by a G protein kinase Src.26 In Aplysia sensory neurones, acti- linked second messenger system.12 Firstly, slow vation of PKC also causes long term excitabil- EPSPs evoked by brief stimuli (one second or ity changes.22 Experiments on hippocampal less) have a long latency (about 100 ms) and a CA1 neurones indicate that phosphorylation long duration (one to several minutes). Sec- restricts the opening of K channels and http://gut.bmj.com/ ondly, substance P and its analogues, which dephosphorylation increases their opening.27 A mimic the slow EPSP, cause accumulation of channel phosphorylation with similar eVect cyclic 3', 5' adenosine monophosphate,