Differences in the expression of transient receptor potential channel V1, transient receptor potential channel A1 and mechanosensitive two pore-domain K+ channels between the lumbar splanchnic and pelvic nerve innervations of mouse urinary bladder and colon.

Sensory neurons innervating lower abdominal organs such as urinary bladder and distal colon are contained in two neural pathways: lumbar splanchnic (LSN) and pelvic nerves (PN) that convey sensory information to thoracolumbar and lumbosacral spinal segments, respectively (Ness and Gebhart, 1990). These two sensory pathways are not only anatomically different in their spinal level of origin, but also physiologically distinct in their relative proportions of mechanically sensitive nerve fiber types. Specifically, in the LSN innervation of the mouse bladder, 67% of mechanically sensitive afferents are of the serosal class that respond only to blunt probing of the urothelial surface, whereas in the PN, mechanosensitive muscular class afferents that respond to both probing and bladder wall stretch predominate (63%) (Xu and Gebhart, 2008). In comparison, in the LSN innervation of the mouse distal colon, ~50% of mechanically sensitive afferents are of the mesenteric class that responds to probing on the mesenteric attachments. In the PN, in contrast, mesenteric afferents are absent and four classes of mechanosensitive afferents (serosal, muscular, mucosal and muscular/mucosal) are found in relatively equal proportions (Brierley et al., 2004). The LSN and PN also differ from each other in the degree of mechanosensitivity of the same classes of afferents. For instance, the magnitude of responses to mechanical stimuli of serosal and muscular afferents in the distal colon is smaller in LSN afferents than in PN afferents of the same classes. It is poorly understood, however, what underlies the differences in proportions of mechanosensitive afferents and in the degree of their mechanosensitivity between the LSN and PN innervations, and more fundamentally, how these nerve fibers manifest heterogeneity in their responses to varying types of mechanical stimulation. One approach to begin to address these questions is to determine the expression profiles of multiple ion channels involved in mechanosensation in sensory neurons comprising the LSN or PN pathways. In this regard, it is particularly interesting that genetic deletion of transient receptor potential channel V1 (TRPV1) or TRPA1 in mice attenuates visceromotor responses to colorectal distension in vivo and action potential firing of bladder and colon PN afferents to mechanical stimulation in vitro (Brierley et al., 2009; Daly et al., 2007; Jones, III et al., 2005). In Trpv1−/− mice, low-threshold bladder PN afferents show diminished response to high intensity bladder distension, and the muscular/mucosal class of colon PN afferents is less sensitive to colon wall stretch than its wild type counterparts. In Trpa1−/− mice, the responses of colon PN afferents to blunt probing and mucosal stroking are decreased. On the contrary, mice lacking TREK-1 or/and TRAAK, mechanosensitive two pore-domain K+ (K2P) channels, exhibit an increase in response to mechanical probing of the plantar surface of their hindpaw (Alloui et al., 2006; Noel et al., 2009). This suggests that varying combinations/coexpression of these two counteracting ion channel groups (TRPs and TREKs) may contribute, at least in part, to a molecular basis that underlies differences between the LSN and PN sensory pathways and the heterogeneity in mechanosensitivity of these innervations. This idea prompted us to examine the expression of TRPV1, TRPA1 and three mechanosensitive K2P channels (TREK-1, TREK-2 and TRAAK) in thoracolumbar (representing LSN) and lumbosacral (PN) dorsal root ganglia (DRG) neurons. Because of the lack of reliable antibodies to detect several of these channels in mice immunohistochemically and technical issues associated with using multiple probes for in situ hybridization on limited numbers of DRG sections containing traced neurons, here we examined the mRNA expression of the five channels in single neurons retrogradely labeled from urinary bladder or distal colon using the single cell reverse transcription-polymerase chain reaction (RT-PCR) technique.