Reduction of NT-3 or TrkC results in fewer putative vagal mechanoreceptors in the mouse esophagus

Intraganglionic laminar endings (IGLEs) represent major vagal afferent structures throughout the gastrointestinal tract. Both morphological and functional data suggested a mechanosensory role. Elucidation of their functional significance in a particular organ would be facilitated by the availability of animal models with significantly altered numbers of IGLEs. The present study was aimed at searching for mouse strains fulfilling this criterion in the esophagus. Anterograde wheat germ agglutinin-horseradish peroxidase tracing (WGA-HRP) from nodose ganglion was used in order to label esophageal IGLEs in mice deficient for neurotrophin-3 (NT-3) or tyrosine kinase C-receptor (TrkC) and in control littermates. This approach was feasible only in heterozygous mutants which are viable. IGLEs were counted in tetramethylbenzidine (TMB) processed wholemounts using a standardised protocol. Quantification of myenteric neurons was done in cuprolinic blue-stained specimens. Nodose neuron counts were performed in cryostat sections stained with cresyl violet. Numbers of IGLEs in the esophagus were significantly reduced in both heterozygous NT-3 (NT-3+/-) and heterozygous TrkC (TrkC+/-) mutants (65% and 40% reduction, respectively). Numbers of nodose neurons were also significantly reduced in NT-3+/- mice (48% reduction), while their reduction in TrkC+/- mutants was insignificant (11% reduction). There was no reduction of myenteric neurons in the esophagus of either mutant strain. The numeric deficiency of IGLEs was unlikely to be secondary to reduction of myenteric neurons. Although only heterozygous mutants could be studied, these results suggest that esophageal IGLEs share neurotrophin dependence on NT-3/TrkC with spinal proprioceptors and some cutaneous mechanosensors. This concurs with their proposed function as vagal mechanosensors crucial for reflex peristalsis.