Loss of neurotrophin-3 from smooth muscle disrupts vagal gastrointestinal afferent signaling and satiation

Abstract

A large proportion of vagal afferents are dependent on neurotrophin-3 (NT-3) for survival. NT-3 is expressed in developing gastrointestinal (GI) smooth muscle, a tissue densely innervated by vagal mechanoreceptors, and thus could regulate their survival. We genetically ablated NT-3 from developing GI smooth muscle and examined the pattern of loss of NT-3 expression in the GI tract and whether this loss altered vagal afferent signaling or feeding behavior. Meal-induced c-Fos activation was reduced in the solitary tract nucleus and area postrema in mice with a smooth muscle-specific NT-3 knockout (SM-NT-3KO) compared with controls, suggesting a decrease in vagal afferent signaling. Daily food intake and body weight of SM-NT-3KO mice and controls were similar. Meal pattern analysis revealed that mutants, however, had increases in average and total daily meal duration compared with controls. Mutants maintained normal meal size by decreasing eating rate compared with controls. Although microstructural analysis did not reveal a decrease in the rate of decay of eating in SM-NT-3KO mice, they ate continuously during the 30-min meal, whereas controls terminated feeding after 22 min. This led to a 74% increase in first daily meal size of SM-NT-3KO mice compared with controls. The increases in meal duration and first meal size of SM-NT-3KO mice are consistent with reduced satiation signaling by vagal afferents. This is the first demonstration of a role for GI NT-3 in short-term controls of feeding, most likely involving effects on development of vagal GI afferents that regulate satiation.

Reduced Intestinal Brain-Derived Neurotrophic Factor Increases Vagal Sensory Innervation of the Intestine and Enhances Satiation

“…In addition, our laboratory and others have used Rosa26 reporter mice to map the spatiotemporal expression of SM22␣ cre -mediated recombination at several developmental ages. These studies found that SM22␣ cre mice produced sufficient Cre expression to drive recombination in SM of the GI wall and associated blood vessels at ages when vagal GI afferents enter the gut (Pan et al, 2011;Fox et al, 2013a). This was determined to begin at approximately E13 and continued until at least E17, based on the pattern of ␤-galactosidase expression in the stomach and intestines.…”

“…Amplification of 1 g of cDNA from the first-strand reaction was performed in triplicate using previously established protocols (Unger et al, 2007;Cordeira et al, 2010;Fox et al, 2013a). Based on these protocols, primer sequences used included the following: BDNF forward, 5Ј-GAA AGT CCC GGT ATC CAA AG-3Ј; BDNF reverse, 5Ј-CCA GCC AAT TCT CTT TTT-3Ј; ␤-actin forward, 5Ј-GGC TGT ATT CCCC TCC ATC G-3Ј; and ␤-actin reverse, 5Ј-CCA GTT GGT AAC AAT GCC ATG T-3Ј.…”

“…To visualize vagal afferent axons and terminals in the GI tract of control, INT-BDNF ϩ/Ϫ , and INT-BDNF Ϫ/Ϫ mice, the nerve tracer horseradish peroxidase conjugated to wheat-germ agglutinin (WGA-HRP) was injected into the left nodose ganglion (controls, n ϭ 15; INT-BDNF ϩ/Ϫ , n ϭ 11; and INT-BDNF Ϫ/Ϫ , n ϭ 12). This method was used because, to date, it has been the most successful for labeling consistent, large numbers of vagal sensory elements supplying the GI wall and, therefore, has been the most useful for quantification of vagal sensory axons and their terminal endings, as well as for making quantitative comparisons of these elements between groups of mutant and control animals (Fox et al, , 2001a(Fox et al, ,b, 2002Wang and Powley, 2000). The left nodose was chosen for injections because it was known to provide much greater innervation to the proximal intestine compared with the right nodose ganglion in mice and rats (Fox et al, , 2001bWang and Powley, 2000).…”

“…Several developing and mature peripheral tissues also express BDNF, including those of the gastrointestinal (GI) tract, some at even higher levels than in brain (Lommatzsch et al, 1999;Fox, 2006;Fox and Murphy, 2008;Fox et al, 2013a). However, the anorexigenic potential of GI BDNF has not been explored.…”

“…One of these pathways, vagal afferents, expresses trkB during development and maturity (Ernfors et al, 1992;Wetmore and Olson, 1995), and BDNF knock-out (KO) results in substantial loss of vagal afferents and disrupts survival or formation of vagal mechanoreceptors that supply the neonatal stomach wall (Jones et al, 1994;Erickson et al, 1996;Fox and Murphy, 2008;Murphy and Fox, 2010). These findings suggest that some vagal afferents may depend on GI BDNF for survival or other aspects of development.…”