• Colon
• afferent
• human
• pain
• colorectal physiology
• neurophysiology
• sensory neurones

The gastrointestinal tract receives an extensive sensory innervation, crucial both for reflex control and perception of abdominal discomfort and pain. Our understanding of sensory signalling is based almost exclusively on animal studies.1 The paper by Peiris et al published in this issue of Gut (see page 204) described recording of afferent impulse traffic from segments of the human bowel.2 Such recordings have enormous potential; first, from the perspective of understanding the stimulus transduction processes that operate in human gastrointestinal (GI) sensory endings; and second, enable the pharmacological screening of novel visceral analgesics. The need for the latter arises from the apparent lack of translation from animal models to humans with a number of novel drugs failing in the clinic despite good mechanistic data from animals (eg, Houghton et al3). This has motivated our efforts to record from human GI afferents and here we describe a methodology that shares some similarity with that described by Peiris et al but also some notable differences.

Peiris et al focused on ‘normal’ tissue but the potential is evident for investigation of the mechanisms that give rise to hypersensitivity arising from inflammation. In this respect ‘normal’ refers to tissue from hemicolectomy patients in which normal tissue is available at a distance from tumour margins. We have access to a similar patient population being treated at the Northern General Hospital in Sheffield but our efforts have focused on the colorectum in order to limit variability arising from regional differences in innervation. Informed consent was obtained from each patient (14 male/13 female, mean age 60.5±3.7, range 22–90). Full-thickness segments of colon were pinned flat with the mucosa uppermost in a tissue bath. The bath was perfused with Krebs buffer at 34°C. Nerve bundles were identified in the attached mesentery and drawn into a suction electrode made from borosilicate glass pulled on a Harvard microelectrode puller so that its tip diameter was similar to that of the dissected bundled. This ensured a good seal and high quality signal to noise ratio. Nerve activity was recorded as described previously4 and data acquired to a computer running Spike 2 software for off-line analysis.

Colonic afferent endings of several species have been characterised according to their terminal distribution and differential sensitivity to mechanical and chemical stimuli.1 Figure 1 shows preliminary data demonstrating that these various sub-types of afferent endings can be identified in the human colon based upon their differential sensitivity to mucosal stroking, circumferential stretch and blunt probing. Spontaneous activity was evident in muscular afferents with bursts of activity whose periodicity could reflect ongoing contractile activity although muscle tone was not measured (figure 1A). A mechanosensitive ‘hotspot’ could be identified by blunt probing (figure 1B) or in the case of mucosal afferents by light stroking of the mucosal epithelium overlying the receptive field (figure 1C). Compression of the receptive field with a blunt probe also activated this mucosal endings and so too did stretch (figure 1D) suggesting this ending was of a sub-class defined by Brierley et al5 as muscular/mucosal. Serosal afferents responded only to blunt probing. Sensitivity to topical application of capsaicin to the mucosal surface evoked responses in superficial endings (figure 1E) but failed to activate serosal afferents possibly because intervening tissue represented a barrier to diffusion. Responses to capsaicin also showed desenitisation. Peiris et al also used probing but applied to the serosa with tissue pinned mucosa down. This is unlikely to have a major impact of the overall sensitivity to probing but could impact on the ability to map receptive field. Moreover, from their multi-unit recordings it will be more difficult to distinguish the hot spot for a single ending among the many being simultaneously recorded. In this respect the benefits that arise from better signal to noise recordings and the ability to distinguish single units is evident. There are two further advantages to positioning the tissue with the mucosa uppermost. First, the sensitivity to probing and stroking could be differentiated thereby sub-categorising the afferent endings. Second, the mucosa will be exposed to the perfusion medium, which by maintaining pH and oxygen delivery will help preserve mucosal integrity.

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Figure 1

Panels A to E illustrate examples of three distinct populations of afferents whose characteristics are summarised in the table according to the sensitivity to mechanical stimulation delivered by light mucosal stroking, circumferential stretch between forceps and probing with a blunt von Frey filament. Numbers in brackets in the table refer to the number of afferents identified in each category. Top trace of each panel (A–E) is the neurogram showing the firing of a single afferent fibre whose frequency of firing is represented as a histogram in the lower trace (spikes s⁻¹). Smaller amplitude spikes can be observed in the background but no attempt was made to discriminate these from noise.

This data clearly demonstrate the feasibility of single unit recording of human GI afferent activity. Together with the study by Peiris et al this points to a future when drug discovery efforts can be focused on studies in humans circumventing the poor translation from animal models. However, studies on human gut are far from routine. Our success rate was just four preparations out 27. Peiris et al had greater success but with some sacrifice over recording quality. Improving the yield will be essential if this approach is to be more widely adopted. However, with the coordinated efforts of the theatre staff, surgeon and pathologist it is possible to overcome many of the logistics difficulties in order to deliver viable tissue to the electrophysiology laboratory. The rewards are enormous and include reduced compound attrition from failure of compounds in the clinic combined with mechanistic understanding of the role of key ion channels and receptors in human GI visceral afferent sensitivity.