Segmental differences in the non-neuronal cholinergic system in rat caecum

Abstract

Acetylcholine is not only a neurotransmitter but is also produced by several non-neuronal cell types with barrier or defence function. One of the non-neuronal tissues with expression of the key enzyme for production of acetylcholine, the choline acetyltransferase (ChAT), is the colonic surface epithelium, which releases acetylcholine after contact with the short-chain fatty acid propionate produced physiologically in the colonic lumen during the microbial fermentation of carbohydrates. Despite the fact that the caecum is the largest fermentation chamber in non-ruminant mammals, nothing is known about the expression and function of a non-neuronal cholinergic system in this part of the large intestine, which was addressed in the present study. In Ussing chamber experiments, propionate induced a concentration-dependent Cl secretion leading to an increase in short-circuit current (I), which was stronger in the aboral part (near the blind ending sac of the caecum) compared to the oral part of caecum. The propionate-induced I was blocked by atropine, but was resistant against tetrodotoxin, conotoxins (MVIIC and SVIB) or hexamethonium indicating that propionate acts via non-neuronal acetylcholine. Immunohistochemical staining revealed the expression of ChAT in the caecal surface epithelium with a significant gradient between aboral (high) and oral (low) expression. This difference combined with a higher efficiency of cholinergically induced anion secretion (as revealed by the I evoked by the cholinergic agonist carbachol) is probably responsible for the segment dependency of the response to propionate. In summary, propionate stimulates anion secretion in rat caecum via non-neuronal acetylcholine emphasizing the physiological importance of the non-neuronal cholinergic system in the communication between the gastrointestinal microbiome and the mammalian host.

The role of HCO3– in propionate-induced anion secretion across rat caecal epithelium

“…In the oral segment, these blockers did not inhibit I Prop (Table 4) indicating a segmental difference within the caecum. Interestingly, I Prop is larger in the aboral than in oral corpus caeci as reported previously [6]. This observation was confirmed in the present study: When all control series of Tables 2-5, which were performed in standard, i.e., HCO 3 − -containing buffer, were averaged, I Prop amounted to 4.08 ± 0.38 μEq•h −1 •cm −2 (n = 82) in the oral and 4.99 ± 0.32 μEq•h −1 •cm −2 (n = 82, p < 0.05 versus response in the oral corpus caeci) in the aboral part, whereas the increase in G t after propionate administration did not differ significantly between the oral (3.02 ± 0.28 mS cm −2 , n = 82) and the aboral corpus caeci (3.99 ± 0.80 mS•cm −2 , n = 82).…”

“…The functional consequence is the induction of anion secretion by paracrine stimulation of epithelial cholinergic receptors, which can be measured as an increase in short-circuit current ( I sc ) in Ussing chamber experiments. A similar response has been observed in rat caecum [ 6 ]. Acetylcholine-induced I sc has been attributed to Cl − secretion based on partial sensitivity to the Na + -K + -2Cl – -cotransport blocker, bumetanide, in rat colon [ 32 , 33 ] or strong dependence on the presence of Cl − anions in the caecum [ 6 ].…”

“…A similar response has been observed in rat caecum [ 6 ]. Acetylcholine-induced I sc has been attributed to Cl − secretion based on partial sensitivity to the Na + -K + -2Cl – -cotransport blocker, bumetanide, in rat colon [ 32 , 33 ] or strong dependence on the presence of Cl − anions in the caecum [ 6 ].…”

“…As many Cl − secreting pathways such as the dominant anion channel in the apical membrane of intestinal epithelial cells, the CFTR (cystic fibrosis transmembrane regulator) channel, are also permeable for HCO 3 − [24], it seems to be of interest to study whether HCO 3 − contributes to the electrogenic response evoked by luminal propionate in the caecum. As rat caecum exhibits large segmental differences in basal ion transport and in the epithelial expression of choline acetyltransferase (ChAT), the key enzyme for the production of acetylcholine [6], we tried to find out in the present study if HCO 3 − transport contributes to propionate-induced anion secretion in rat oral and aboral corpus caeci.…”

Unravelling the secrets of the caecum

“…Building on two previous studies published in Pflügers Arch. [1,2], in the current issue, Pouokam and Diener systematically compare differences in ion transport across two different segments of the rat caecum [8]. As in the classical model, a basolateral NKCC drives secretion via anion channels, possibly delivering HCO 3…”