Primary afferents: In vitro characterization of the mechano- and chemo-sensitivity of pelvic nerve afferent fibers innervating the mouse colon

Abstract

Primary afferent fibers in the pelvic nerves transmit noxious mechanical and chemical information from the distal colon/rectum, thereby placing this sensory pathway in a prominent position for targeting novel visceral analgesic drugs. However, the precise nature of pelvic afferent receptive endings has remained elusive because in vivo electrophysiologic techniques hinder access to receptive fields lying within the colon itself. We have adapted a previously described in vitro organ-nerve electrophysiologic technique to the mouse colon-pelvic nerve to better characterize the this crucial sensory pathway. Fine mucosal stroking (10mg) and circular stretch applied to afferent receptive fields discriminated four distinct classes of pelvic afferent: muscular afferents that responded only to stretch, mucosal afferents that responded only to stroking, muscular/mucosal afferents that responded to both stroking and stretch, and serosal afferents that responded to neither stimulus but could be activated by direct stimulation with von Frey probes. These afferents occur in relatively equal proportions (21%, 23%, 23%, 33%, respectively), are distributed throughout the distal 3 cm of the colon/rectum, and have small punctate receptive fields. We have also tested afferent chemosensitivity to focal application of an inflammatory soup (in μM: KCl 1000, histamine 0.5, serotonin 0.5, prostaglandin E2 0.5, bradykinin 0.5; 1 min). We have demonstrated that not only are mechanosensitive afferents activated by exposure to inflammatory soup, reflected by a sustained increase in spontaneous activity from a silent baseline to an average maximum firing rate (+ sem) of 23 + 7 Hz, but also are sensitized to subsequent mechanical stimulation following chemical exposure, with significant leftward shifts in the mechanical stimulus-response functions. These in vitro results bolster previous in vivo descriptions of pelvic nerve properties and provide the basis for studies on pelvic afferent contributions to changes occurring in animal models of visceral hyperalgesia.