Electrophysiological characterization of mouse pelvic nerve afferent activity in response to mechanical and chemical stimulation of the colon

Functional gastrointestinal disorders are characterized in part by hypersensitivity to distention of the distal colon and rectum, causing normally non-noxious stimuli to be perceived as painful. This feature suggests abnormalities in primary afferent neurons, specifically those of the pelvic nerve (PN). The goal of this work was to become proficient in single-fiber electrophysiological characterization of mouse PN primary afferents using an in vitro colon preparation coupled with a novel electrical search strategy, which allowed for unbiased characterization of all excitable receptive endings (REs) including those of mechanically insensitive afferents (MIAs). Using previously established criteria, mechanosensitive fibers were classified as mucosal, muscular, muscular/mucosal, and serosal based on their responses to three distinct mechanical stimuli: probing (0.4, 1.0, and 1.4 g), circumferential stretch (>2 mm), and mucosal stroking (10 mg). Located MIAs were tested for mechanical sensitization by exposing their REs to inflammatory soup and capsaicin. The proportions of afferent subtypes among the 39 PN fibers investigated were as follows: serosal (33%), muscular/mucosal (23%), muscular (26%), mucosal (10%), and MIA (8%). These proportions do not entirely match previous data. Also differing from previous investigations was the topographic distribution of afferents. Despite these discrepancies, which are most likely associated with limited sample size, the present study largely confirms previous findings, thus demonstrating the reliability of the experimenter with this particular technique. In addition to confirming the remarkable detail and fidelity in which mechanical events in the colon are encoded and transmitted, this project establishes the foundation for future work investigating the role of specific gene products in sensory function and the changes which may occur in peripheral innervation in pathophysiological states such as inflammatory/irritable bowel syndrome.