Intraintestinal capsaicin transiently reduces CGRP-like immunoreactivity in rat submucosal plexus

Intestinal epithelial stem/progenitor cells are controlled by mucosal afferent nerves

Background

The maintenance of the intestinal epithelium is of great importance for the survival of the organism. A possible nervous control of epithelial cell renewal was studied in rats and mice.

Methods

Mucosal afferent nerves were stimulated by exposing the intestinal mucosa to capsaicin (1.6 mM), which stimulates intestinal external axons. Epithelial cell renewal was investigated in the jejunum by measuring intestinal thymidine kinase (TK) activity, intestinal ³H-thymidine incorporation into DNA, and the number of crypt cells labeled with BrdU. The influence of the external gut innervation was minimized by severing the periarterial nerves.

Principal Findings

Luminal capsaicin increased all the studied variables, an effect nervously mediated to judge from inhibitory effects on TK activity or ³H-thymidine incorporation into DNA by exposing the mucosa to lidocaine (a local anesthetic) or by giving four different neurotransmitter receptor antagonists i.v. (muscarinic, nicotinic, neurokinin1 (NK1) or calcitonin gene related peptide (CGRP) receptors). After degeneration of the intestinal external nerves capsaicin did not increase TK activity, suggesting the involvement of an axon reflex. Intra-arterial infusion of Substance P (SP) or CGRP increased intestinal TK activity, a response abolished by muscarinic receptor blockade. Immunohistochemistry suggested presence of M3 and M5 muscarinic receptors on the intestinal stem/progenitor cells. We propose that the stem/progenitor cells are controlled by cholinergic nerves, which, in turn, are influenced by mucosal afferent neuron(s) releasing acetylcholine and/or SP and/or CGRP. In mice lacking the capsaicin receptor, thymidine incorporation into DNA and number of crypt cells labeled with BrdU was lower than in wild type animals suggesting that nerves are important also in the absence of luminal capsaicin, a conclusion also supported by the observation that atropine lowered thymidine incorporation into DNA by 60% in control rat segments.

Conclusion

Enteric nerves are of importance in maintaining the intestinal epithelial barrier.

Pilot studies to demonstrate that intestinal mucosal afferent nerves are functionally linked to visceral adipose tissue

Dietary capsaicin reduces rodent visceral fat weight. We tested the hypothesis that intact intestinal mucosal afferent nerve function is necessary for fat deposition in visceral adipose tissue sites. Rats were treated daily for 2 weeks with intragastric (chronic treatment) vehicle or capsaicin. Superior mesenteric artery blood flow and mesenteric and inguinal fat blood flow were measured before and after capsaicin was administered into the duodenum (acute treatment). Fat from all sites was dissected and weighed. Chronic capsaicin significantly attenuated acute capsaicin-induced mesenteric hyperemia but did not abolish the reflex wiping of the eye exposed to capsaicin, indicating that functional ablation was limited to the intestinal mucosal afferent nerves. The associated vasoconstriction in adipose tissue was inhibited at the visceral (mesenteric) site and maintained but attenuated at the subcutaneous (inguinal) site. The onset of vasoconstriction was instantaneous, indicating a reflex mechanism. There was a redistribution of fat from visceral to subcutaneous sites, reflected by a decrease and an increase in the percentage of body fat in the visceral and subcutaneous sites, respectively. These pilot studies reveal for the first time that normal intestinal mucosal afferent nerve function is necessary for the physiologic accumulation of fat in visceral adipose tissue sites.

Gastrointestinal mechanisms of satiation for food

Satiation for food comprises the physiological processes that result in the termination of eating. Satiation is evoked by physical and chemical qualities of ingested food, which trigger afferent signals to the brain from multiple sites in the GI tract, including the stomach, the proximal small intestine, the distal small intestine and the colon. The physiological nature of each signal's contribution to satiation and overall control of food intake is likely to vary, depending on the level of the GI tract from which the signal arises. This article is a critical, though non-exhaustive, review of our current understanding of the mechanisms and adaptive value of satiation signals from the stomach and intestine.

The capsaicin VR1 receptor mediates substance P release in toxin A-induced enteritis in rats

The mechanism by which Clostridium difficile toxin A causes substance P (SP) release and subsequent inflammation in the rat ileum is unknown. Pretreatment with the vanilloid receptor subtype 1 (VR1) antagonist, capsazepine, before toxin A administration significantly inhibited toxin A-induced SP release and intestinal inflammation. Intraluminal administration of the VR1 agonist capsaicin caused intestinal inflammation similar to the effects of toxin A. Pretreatment with capsazepine before capsaicin administration also significantly inhibited capsaicin-induced intestinal inflammation. These results suggest that intraluminal toxin A causes SP release from primary sensory neurons via stimulation of VR1 receptors resulting in intestinal inflammation.