Targeting tachykinin receptors for the treatment of functional gastrointestinal disorders with a focus on irritable bowel syndrome

BACKGROUND

Tachykinins (TKs) are a family of endogenous peptides widely expressed in the central and in the peripheral nervous systems as well as in the gastrointestinal (GI) tract. They act as full agonists at three different membrane receptors neurokinin (NK) 1, NK2, and NK3, which are G protein-coupled receptors and in the GI tract are expressed both on neurons and effector cells.

PURPOSE

This article reviews the literature concerning the role of TKs in the GI tract function in physiological and pathological conditions and their potential relevance in the treatment of functional GI disorders with particular reference to irritable bowel syndrome (IBS). The efficacy of NK1 antagonists in chemotherapy-induced and postoperative nausea and vomiting is well established. While pharmacodynamic studies have reported conflicting and negative results concerning the effects of NK1 and of NK3 antagonists, respectively, on the GI tract function in humans, clinical studies applying the NK3 antagonist talnetant in IBS-D were negative. Pharmacodynamic studies applying NK2 antagonists have suggested a role for antagonism of NK2 receptors in modulation of GI chemical-induced altered motility and of stress-induced altered bowel habits. Clinical studies and in particular a recently completed Phase 2 study have reported that the NK2 antagonist ibodutant is effective and safe in treating symptoms of D-IBS, especially in females.

Sciatic nerve transection triggers release and intercellular transfer of a genetically expressed macromolecular tracer in dorsal root ganglia

We recently developed a genetic transneuronal tracing approach that allows for the study of circuits that are altered by nerve injury. We generated transgenic (ZW-X) mice in which expression of a transneuronal tracer, wheat germ agglutinin (WGA), is induced in primary sensory neurons, but only after transection of their peripheral axon. By following the transneuronal transport of the tracer into the central nervous system (CNS) we can label the circuits that are engaged by the WGA-expressing damaged neurons. Here we used the ZW-X mouse line to analyze dorsal root ganglia (DRG) for intraganglionic connections between injured sensory neurons and their neighboring "intact" neurons. Because neuropeptide Y (NPY) expression is strongly induced in DRG neurons after peripheral axotomy, we crossed the ZW-X mouse line with a mouse that expresses Cre recombinase under the influence of the NPY promoter. As expected, sciatic nerve transection triggered WGA expression in NPY-positive DRG neurons, most of which are of large diameter. As expected, double labeling for ATF-3, a marker of cell bodies with damaged axons, showed that the tracer predominated in injured (i.e., axotomized) neurons. However, we also found the WGA tracer in DRG cell bodies of uninjured sensory neurons. Importantly, in the absence of nerve injury there was no intraganglionic transfer of WGA. Our results demonstrate that intraganglionic, cell-to-cell communication, via transfer of large molecules, occurs between the cell bodies of injured and neighboring noninjured primary afferent neurons.

Distinct differences in tachykinin gene expression in ulcerative colitis, Crohn's disease and diverticular disease: a role for hemokinin-1?

BACKGROUND

In the intestine, the tachykinins substance P (SP) and neurokinin A (NKA) are found in neurons and have key roles in motility, secretion, and immune functions. A new tachykinin, hemokinin (HK-1), has been identified in non-neuronal cells in recent years and its role in intestinal inflammation is unclear. We aimed to examine the expression of genes encoding tachykinin peptides and receptors in colon from patients with ulcerative colitis (UC), Crohn's disease (CD), and acute diverticular disease (DD).

METHODS

Human colon segments were dissected into mucosa and muscle, and evaluated for tachykinin and tachykinin receptor gene expression by real-time PCR.

KEY RESULTS

In UC mucosa, the TAC4 gene (encoding HK-1) was 10-fold more abundant than in control mucosa (P < 0.01). Similarly, TAC1 (encoding SP and NKA) and TACR1 (encoding NK1 receptor) displayed 6-fold and 12-fold upregulation, respectively, in UC mucosa, but no change occurred in UC muscle. In contrast to UC, no difference was observed for any tachykinin genes in CD mucosa. In CD muscle, expression of TAC1 (P < 0.01), TAC4 and TACR1 (both P < 0.05) were moderately upregulated. In DD, there was a decrease in TACR1 (P < 0.05), and TACR2 (encoding NK2 receptor, P < 0.0001) in muscle compared with control. Histological staining showed increased collagen fibers between muscle bundles in DD smooth muscle.

CONCLUSIONS & INFERENCES

We provide evidence for the first time that HK-1, like SP, may be involved in the pathophysiology of inflammatory bowel disease. Distinctly different expression patterns of tachykinin-related genes occur in UC, CD and DD.

Effect of otilonium bromide and ibodutant on the internalization of the NK2 receptor in human colon

BACKGROUND

The present aim was to study the modulation of NK2 receptor internalization by two compounds, the spasmolytic otilonium bromide (OB) endowed with NK2 receptor antagonistic properties and the selective NK2 receptor antagonist ibodutant.

METHODS

Full-thickness human colonic segments were incubated in the presence of OB (0.1-10 μmol L(-1)) or ibodutant (0.001-0.1 μmol L(-1)), with or without the NK2 receptor selective agonist [ßAla8]NKA(4-10) and then fixed in 4% paraformaldehyde. Cryosections were processed for NK2 receptor immunohistochemical revelation. Quantitative analysis evaluated the number of the smooth muscle cells that had internalized the NK2 receptor.

KEY RESULTS

Immunohistochemistry revealed that in basal condition, the NK2 receptor was internalized in about 23% of total smooth muscle cells. The exposure to the selective NK2 receptor agonist induced internalization of the receptor in more than 77% of the cells. Previous exposure to both OB or ibodutant, either alone or in the presence of the agonist, concentration-dependently reduced the number of the cells with the internalized receptor.

CONCLUSIONS & INFERENCES

Both OB and ibodutant antagonize the internalization of the NK2 receptor in the human colon. As NK2 receptors are the predominant receptor mediating spasmogenic activity of tachykinins on enteric smooth muscle, we hypothesize that the antagonistic activity found for both OB and ibodutant should play a specific therapeutic role in gut diseases characterized by hypermotility.