Detailed localization of aquaporin-4 messenger RNA in the CNS: Preferential expression in periventricular organs

We have performed a detailed in situ hybridization study of the distributio n of aquaporin-4 messenger RNA in the CNS. Contrary to expectation, we demo nstrate that aquaporin-4 is ubiquitously expressed in the CNS. Strong hybri dization labeling was detected in multiple olfactory areas, cortical cells, medial habenular nucleus, bed nucleus of the stria terminalis, tenia tecta , pial surface, pontine nucleus, hippocampal formation and multiple thalami c and hypothalamic areas. A low but significant hybridization signal was fo und, among others, in the choroid plexus of the lateral ventricles, ependym al cells, dorsal raphe and cerebellum. Overall, a preferential distribution of aquaporin-4, messenger RNA-expressing cells was evident in numerous per iventricular organs. From the distribution study, the presence of aquaporin -4 messenger RNA-expressing cells in neuronal layers was evident in neurona l layers including the CA1-CA3 hippocampal pyramidal cells, granular dentat e cells and cortical cells. Further evidence of neuronal expression comes f rom the semicircular arrangement of aquaporin-4 messenger RNA-expressing ce lls in the bed nucleus of the stria terminalis and medial habenular nucleus exhibiting Nissl-stained morphological features typical of neurons. Combin ed glial fibrillary acidic protein immunohistochemistry and aquaporin-4 mes senger RNA in situ hybridization demonstrated that aquaporin-4 messenger RN A is expressed by glial fibrillary acidic protein-lacking cells. We conclude that aquaporin-4 messenger RNA is present in a collection of st ructures typically involved in the regulation of water and sodium intake an d that aquaporin-4 water channels could be the osmosensor mechanism respons ible for detecting changes in cell volume by these cells. (C) 1999 IBRO. Pu blished by Elsevier Science Ltd.