ENaC γ-expressing astrocytes in the circumventricular organs, white matter, and ventral medullary surface: sites for Na+ regulation by glial cells

Abstract

Using a double immunofluorescence procedure, we report the discovery of a novel group of fibrous astrocytes that co-express epithelial sodium channel (ENaC) γ-subunit protein along with glial acidic fibrillary protein (GFAP). These cells are concentrated along the borders of the sensory circumventricular organs (CVOs), embedded in the white matter (e.g., optic nerve/chiasm, anterior commissure, corpus callosum, pyramidal tract) and are components of the pia mater. In the CVOs, a compact collection of ENaC γ-immunoreactive glial fibers form the lamina terminalis immediately rostral to the organum vasculosum of the lamina terminalis (OVLT). Astrocyte processes can be traced into the median preoptic nucleus - a region implicated in regulation of sodium homeostasis. In the subfornical organ (SFO), ENaC γ-GFAP astrocytes lie in its lateral border, but not in the ventromedial core. In the area postrema (AP), a dense ENaC γ-GFAP glial fibers form the interface between the AP and nucleus tractus solitarius; this area is termed the subpostremal region. Antibodies against the ENaC α- or β-subunit proteins do not immunostain these regions. In contrast, the antibodies against the ENaC γ-subunit protein react weakly with neuronal cell bodies in the CVOs. Besides affecting glial-neural functions in the CVOs, the astrocytes found in the white matter may affect saltatory nerve conduction, serving as a sodium buffer. The ENaC γ-expressing astrocytes of the ventral medulla send processes into the raphe pallidus which intermingle with the serotoninergic (5-HT) neurons found in this region as well as with the other nearby 5-HT neurons distributed along ventral medullary surface.

Keywords: AP; CC; CVO; Circumventricular organ; DMX; ENaC; Epithelial sodium channel; GFAP; Glial acidic fibrillary protein; Gr; III; IV; LT; MnPO; NA; NDB; NTS; NeuN; OVLT; OX; Pe; Psol; ROb; RPa; RVLM; SFO; Saltatory nerve conduction; SpV; Spinal trigeminal nucleus; SubP; VHC; VMS; Ventral medullary surface; Ventrolateral subnucleus of the NTS; XII; area postrema; central canal; circumventricular organ; com; commissural subnucleus of the NTS; dorsal motor nucleus of vagus nerve; epithelial sodium channel; fourth ventricle; glial fibrillary acidic protein; gracile nucleus; hypoglossal nucleus; interstitial subnucleus of the NTS; is; lamina terminalis; med; medial subnucleus of the NTS; median preoptic nucleus; neuronal nuclear marker; nucleus ambiguus; nucleus of the diagonal band; nucleus tractus solitarius; optic chiasm; optic recess; or; organum vasculosum of the lamina terminalis; parasolitary subnucleus of the NTS; periventricular hypothalamus; py; pyramidal tract; raphe obscurus nucleus; raphe pallidus nucleus; rostral ventrolateral medulla; solitary tract; subfornical organ; subpostremal region of the NTS; t; third ventricle; v; ventral hippocampal commissure; ventral medullary surface; ventral subnucleus of the NTS; vl.