Abstract
An important function of astrocytes is the clearance of excess extracellular glutamate via specific carriers whose expression has become an astrocytic marker. In the present study, we found that a large population of astrocytes in the supraoptic nucleus (SON) of the rat hypothalamus lacks glutamate uptake currents and receptor responses but expresses GABA~A~ receptors. Patch clamp recordings in acute hypothalamic slices that included the SON showed typical astrocytic membrane currents and demonstrated that GABA, via GABA~A~ receptor activation, triggered a conductance increase with the reversal potential close to the Cl^β^ equilibrium potential and a decrease in resting K^+^ conductance. Intracellular labeling with Lucifer Yellow revealed that these cells had a radial gliaβlike morphology, with cell bodies lined up along the base of the brain and long processes traversing the nucleus; they were not dyeβcoupled. Parallel immunocytochemical labelings showed that they expressed strong GABA~A~ receptor and glial fibrillary acidic protein (GFAP) immunoreactivities. In addition, our electrophysiological and morphological analyses revealed another population of astrocytes in this nucleus, located next to the subarachnoid space. They were less numerous than the radial type, had a round morphology and few processes, and were dyeβcoupled. Unlike the radial astrocytes, they showed little immunoreactivity for GABA~A~ receptor or GFAP. Moreover, they did not respond to GABA but to glutamate, a response that was partially mimicked by aspartate, indicating glutamate transporter expression. Taken together, our observations add to growing evidence illustrating heterogeneity of astrocytes in the adult brain, a heterogeneity that reflects striking differences in form and function of astrocytic populations in regions as discrete as the SON of the hypothalamus. Β© 2003 WileyβLiss, Inc.