Aquaporin‐4 water channels in enteric neurons

Aquaporin‐4 is a water channel predominantly found in astrocytes in the central nervous system and is believed to play a critical role in the formation and maintenance of the blood–brain barrier and in water secretion from the brain. As enteric glial cells were found to share several similarities with astrocytes, we hypothesized that enteric glia might also contain aquaporin‐4. We used immunohistochemistry to identify aquaporin‐4 in the myenteric and submucosal plexuses of the mouse and the rat colon. We found that subpopulations of neurons in both enteric plexuses were positively labeled for human aquaporin‐4. Double staining of the enteric ganglia with antibodies to the neuronal marker neurofilament–heavy chain 100 and to aquaporin‐4 showed that a minority of myenteric neurons were aquaporin‐4 positive (about 12% in the mouse and 13% in the rat). In contrast, in the submucosal plexus significant numbers of neurons were positive for aquaporin‐4 (about 79% in both the mouse and the rat). Double labeling for aquaporin‐4 and for the glial marker glial fibrillary acidic protein verified that glial cells were not immunoreactive to aquaporin‐4. We further confirmed our findings with additional aquaporin‐4 antibodies and Western blot analysis. We found that, in addition to expressing aquaporin‐4, the myenteric plexus and, to a greater extent, the submucosal plexus both expressed aquaporin‐1. We conclude that neurons rather than glial cells contain aquaporin‐4 in the colonic enteric plexuses. It is known that submucosal neurons control transport processes in the intestinal mucosa, and the high percentage of aquaporin‐4‐postive submucosal neurons suggests that aquaporin‐4 contributes to this function. © 2007 Wiley‐Liss, Inc.