In vivo imaging of enteric neurogenesis in the deep tissue of mouse small intestine.

One of the challenges of using imaging techniques as a tool to study cellular physiology has been the inability to resolve structures that are not located near the surface of the preparation. Nonlinear optical microscopy, in particular two photon-excited fluorescence microscopy (2PM), has overcome this limitation, providing deeper optical penetration (several hundred µm) in ex vivo and in vivo preparations. We have used this approach in the gut to achieve the first in vivo imaging of enteric neurons and nerve fibers in the mucosa, submucosa, submucosal and myenteric plexuses, and circular and longitudinal muscles of the small intestine in H-line: Thy1 promoter GFP mice. Moreover, we obtained clear three-dimensional imaging of enteric neurons that were newly generated after gut transection and reanastomosis. Neurogenesis was promoted by oral application of the 5-HT(4)-receptor agonist, mosapride citrate (MOS). The number of newly generated neurons observed in mice treated with MOS for one week was 421±89 per 864,900 µm(2) (n = 5), which was significantly greater than that observed in preparations treated with MOS plus an antagonist (113±76 per 864,900 µm(2)) or in 4 week vehicle controls (100±34 per 864,900 µm(2)) (n = 4 both). Most neurons were located within 100 µm of the surface. These results confirm that activation of enteric neural 5-HT(4)-receptor by MOS promotes formation of new enteric neurons. We conclude that in vivo 2PM imaging made it possible to perform high-resolution deep imaging of the living mouse whole gut and reveal formation of new enteric neurons promoted by 5-HT(4)-receptor activation.