Water channel aquaporin 1 (AQP1) is present in the perineurium and perichondrium

Aquaporin 1 (AQPI) is an isoform of membrane water channels. We examined the distribution of AQP1 in peripheral nerves. Immunofluorescence microscopy with specific antibodies to AQP1 was performed in the tissue sections of the rat tongue, esophagus, trachea, and sciatic nerve. AQP1 was present in the perineurium of nerve fiber bundles in the sciatic nerve and nerve fiber bundles in the tongue, esophagus, and trachea. Laser confocal microscopy of the double-labeled specimens for AQP1 and sugar transporter GLUT1 revealed that AQP1 was localized in the outer layer(s) of the perineurial cell layers, whereas GLUT1 was present throughout the entire perineurial cell layers. AQP1 and GLUT1 were also present in blood vessels inside the nerve fiber bundle. They were rarely found in the same blood vessels, namely, AQP1 was hardly detectable in GLUT1-positive blood vessels and GLUT1 was hardly detectable in AQP1-positive vessels. AQP1 in these sites of the blood-nerve barrier may participate in the water transfer across the barrier and contribute to the maintenance of the milieu of the nerve. In addition, we showed the presence of AQP1 in the perichondrium of the tracheal cartilage suggesting a possible role in water transfer between the cartilage and surrounding connective tissues.