In vitro Functional Characterization of Mouse Colorectal Afferent Endings

Summary

This video demonstrates a protocol for conducting single-fiber electrophysiological recordings on an in vitro mouse colorectum-nerve preparation.

Abstract

This video demonstrates in detail an in vitro single-fiber electrophysiological recording protocol using a mouse colorectum-nerve preparation. The approach allows unbiased identification and functional characterization of individual colorectal afferents. Extracellular recordings of propagated action potentials (APs) that originate from one or a few afferent (i.e., single-fiber) receptive fields (RFs) in the colorectum are made from teased nerve fiber fascicles. The colorectum is removed with either the pelvic (PN) or lumbar splanchnic (LSN) nerve attached and opened longitudinally. The tissue is placed in a recording chamber, pinned flat and perfused with oxygenated Krebs solution. Focal electrical stimulation is used to locate the colorectal afferent endings, which are further tested by three distinct mechanical stimuli (blunt probing, mucosal stroking and circumferential stretch) to functionally categorize the afferents into five mechanosensitive classes. Endings responding to none of these mechanical stimuli are categorized as mechanically-insensitive afferents (MIAs). Both mechanosensitive and MIAs can be assessed for sensitization (i.e., enhanced response, reduced threshold, and/or acquisition of mechanosensitivity) by localized exposure of RFs to chemicals (e.g., inflammatory soup (IS), capsaicin, adenosine triphosphate (ATP)). We describe the equipment and colorectum–nerve recording preparation, harvest of colorectum with attached PN or LSN, identification of RFs in the colorectum, single-fiber recording from nerve fascicles, and localized application of chemicals to the RF. In addition, challenges of the preparation and application of standardized mechanical stimulation are also discussed.

Introduction

Pain and hypersensitivity are the predominant complaints of patients suffering from functional gastrointestinal disorders, including irritable bowel syndrome (IBS), which exist in the absence of apparent pathobiological cause or tissue damage. For example, IBS patients exhibit hypersensitivity, including heightened responses to rectal balloon distension and increased sensitivity during normal bowel function, as well as hypersensitivity of somatic referral (i.e., tenderness to palpation of the abdominal area)¹. Because targeting colorectal afferents has proven to be effective in alleviating pain and hypersensitivity in IBS patients (e.g., i....

Protocol

NOTE: This protocol was reviewed and approved by the University of Pittsburgh Institutional Animal Care and Use Committee.

1. Preparation of Modified Krebs Solution and Test Drug Aliquots

1. Make 6 L of modified Krebs solution that contains (in mM): 117.9 NaCl, 4.7 KCl, 25 NaHCO₃, 1.3 NaH₂PO₄, 1.2 MgSO₄, 2.5 CaCl₂, 11.1 D-glucose, 2 sodium butyrate, 20 sodium acetate, 0.004 nifedipine (to block spontaneous muscle contractions), and 0.003 indomethacin (to block synthesis of endogenous prostaglandins). Use ice-cold and warm Krebs solutions for tissue dissection and single-fiber r....

Results

The setup is illustrated in Figure 1. It includes a custom-made tissue chamber that houses the colorectum in a silicone-lined bath compartment and the attached nerve in a contiguous mineral oil-filled compartment. The two-compartment chamber was machined from a solid block of acrylic plastic by a CNC machine; the bottom of both compartments was subsequently lined with firm silicone to allow easy pin down of the colorectal tissue. Extracellular APs from teased nerve fascicles are recorded using a low-nois.......

Discussion

The in vitro colorectum-nerve preparation described here has proven to be a powerful approach to study neural encoding functions of individual colorectal afferents, which nicely complements other non-functional approaches (e.g., cellular, molecular, and histological studies) on visceral sensory neurons (see review ²⁷ for details). Neuronal mechanisms contributing to nociception and long-term colorectal hypersensitivity have been revealed and pharmacological manipulations have been performed t.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Leica MZ16 stereo microscope	Leica Microsystems Inc.
Leica IC D camera	Leica Microsystems Inc.
Amplifier	World Precision instruments, Inc.	SYS-DAM80	Low-noise differential amplifier
Two-compartment tissue chamber	Custom made
Power1401	Cambridge Electronic Design Limited	Power1401	Data acquisition, analog signal input/out
Spike2 v5.02	Cambridge Electronic Design Limited		Software package that works with the Power1401
Audio monitor	Natus	Am 8
Square pulse stimulator	Natus	S48	To deliver electrical stimuli
Photoelectric isolation unit	Natus	PSIU6	Stimulus isolation to reduce noise
Concentric bipolar microelectrode	FHC Inc.	CBFFG75	To deliver electrical stimuli
Dual-mode lever system	Aurora Scientific Inc.	Series 300C	To deliver mechanical stimuli
Forceps	Fine Science Tools	11252-00	Forceps with fine tips