Mammalian protoplasmic astrocytes are extensively coupled through gap junction channels but the biophysical properties of these channels under physiological and ischemic conditions in situ are not well defined. Using confocal morphometric analysis of biocytin-filled astrocytic syncytia in rat hippocampal CA1 stratum radiatum we found that each astrocyte directly couples, on average, to 11 other astrocytes with a mean interastrocytic distance of 45 pro. Voltage-independent and bidirectional transjunctional currents were always measured between directly coupled astrocyte pairs in dual voltage-clamp recordings, but never from astrocyte-NG2 glia or astrocyte-interneuron pairs. The electrical coupling ratio varied considerably among astrocytes in developing postnatal day 14 rats (P14, 0.5-12.4%, mean = 3.6%), but became more constant in young adult P21 rats (0.18-3.9%, mean = 1.6%), and the coupling ratio declined exponentially with increasing pair distance. Electrical coupling was not affected by short-term oxygen-glucose deprivation (OGD) treatment, but showed delayed inhibition in an acidic extracellular pH of 6.4. Combination of acidic pH (6.4) and OGD, a condition that better represents cerebral ischemia in vivo, accelerated the inhibition of electrical coupling. Our results show that, under physiological conditions, 20.7-24.2% of K+ induced currents can travel from any astrocytic soma in CA1 stratum radiatum to the gap junctions of the nearest neighbor astrocytes, but this should be severely inhibited as a consequence of the OGD and acidosis seen in the ischemic brain. (C) 2009 Wiley-Liss, Inc.