Differential chemosensory function and receptor expression of splanchnic and pelvic colonic afferents in mice

Abstract

Lumbar splanchnic (LSN) and sacral pelvic (PN) nerves convey different mechanosensory information from the colon to the spinal cord. Here we determined whether these pathways also differ in their chemosensitivity and receptor expression. Using an in vitro mouse colon preparation, individual primary afferents were tested with selective P2X and transient receptor potential vanilloid receptor 1 (TRPV1) receptor ligands. Afferent cell bodies in thoracolumbar and lumbosacral dorsal root ganglia (DRG) were retrogradely labelled from the colon and analysed for P2X 3 -and TRPV1-like immunoreactivity (LI). Forty per cent of LSN afferents responded to α,β-methylene adenosine 5 -triphosphate (α,β-meATP; 1 mM), an effect that was concentration dependent and reversed by the P2X antagonist pyridoxyl5-phosphate 6-azophenyl-2 ,4 -disulphonic acid (PPADS) (100 µM). Significantly fewer PN afferents (7%) responded to α,β-meATP. Correspondingly, 36% of colonic thoracolumbar DRG neurones exhibited P2X 3 -LI compared with only 19% of colonic lumbosacral neurones. Capsaicin (3 µM) excited 61% of LSN afferents and 47% of PN afferents; 82% of thoracolumbar and 50% of lumbosacral colonic DRG neurones displayed TRPV1-LI. Mechanically insensitive afferents were recruited by α,β-meATP or capsaicin, and were almost exclusive to the LSN. Capsaicin-responsive LSN afferents displayed marked mechanical desensitization after responding to capsaicin, which did not occur in capsaicin-responsive PN afferents. Therefore, colonic LSN and PN pathways differ in their chemosensitivity to known noxious stimuli and their corresponding receptor expression. As these pathways relay information that may relate to symptoms in functional gastrointestinal disease, these results may have implications for the efficacy of therapies targeting receptor modulation.

Extrinsic primary afferent signalling in the gut

“…It is abundant in peptidergic thoracolumbar vascular afferents, 113,114 and is detectable in the majority of their perivascular, intramuscular, ganglionic and mucosal axons in the gut wall. 115 TrpV1 might be directly activated by the acidosis that accompanies frank tissue damage 116 or by local increases in temperature during inflammation.…”

TRP channels in neurogastroenterology: opportunities for therapeutic intervention

“…bradykinin, 5-HT, neuronal growth factor, tryptase), but also mild acidosis are able to sensitize TRPV1 and enhance the probability of channel gating by heat and capsaicin (Chuang et al, 2001;Ji et al, 2002;Amadesi et al, 2004;Sugiuar et al, 2004). Because TRPV1 was suggested to play a crucial role in nociception based on its expression on mostly unmyelinated visceral afferents (Ward et al, 2003;Brierley et al, 2005;Christianson et al, 2006), a vast number of studies investigating its role in visceral sensation and pain emerged (Caterina et al, 2000). (Reviews by Di Marzo et al, 2002;Geppetti and Trevisani, 2004;Holzer, 2008b;Blackshaw et al, 2010).…”

TRP Channels in Visceral Pain

“…Similar roles were suggested in bladder afferents [51]. Capsaicin potently activates splanchnic and pelvic colonic serosal afferents, which is followed by pronounced mechanical desensitization exclusively in splanchnic afferents [13]. This suggests that the coupling of TRPV1 may differ between splanchnic and pelvic pathways.…”

“…TRPV1 is more highly expressed in gut innervating afferents than skin innervating afferents [27], with up to 60% of lumbosacral and 82% of thoracolumbar colonic DRG neurones immunoreactive for TRPV1 [13,27,28], which is similar to the proportion of thoracic gastric DRG neurones [29]. TRPV1 is found correspondingly in peripheral terminals of gut afferents [30], and in 40-80% of gastric vagal afferent neurones [29,31].…”

“…These classifications are based on functional observations, and we are still working on matching anatomy and physiology. Other classifications have been made in mouse and other species, which may be confusing, but there appears to be agreement that there are fibres with low thresholds to distension, those with high thresholds, fibres that are distensioninsensitive (most likely mucosal afferents), and genuinely mechanically-insensitive fibres that respond normally only to chemical stimuli [12,13]. It is probably wrong to name fibres based on the layer of the gut wall where their receptive field appears to be, and better to wait until their structural identity is confirmed.…”