Two types of Na ⁺‐currents in cultured rat optic nerve astrocytes : Changes with time in culture and with age of culture derivation

Na⁺ channel expression was studied in cultures of rat optic nerve astrocytes using whole‐cell voltage‐clamp recordings. Astrocytes from postnatal day 7 rat optic nerve (RON) expressed two distinct types of Na⁺ currents, which had significantly different h_∞ curves. Stellate, GFAP ⁺/A2B5 ⁺ astrocytes showed currents with h_∞ curve midpoints close to−65 mV, similar to Na ⁺ currents in most neurons. In contrast, flat fibroblast‐like GFAP ⁺/A2B5⁻ astrocytes showed Na⁺ currents with h_∞ midpoints around −85 mV, almost 20 mV more hyperpolarized than in neurons or A2B5 ⁺ astrocytes. Interestingly, Na ⁺ current expression was maintained in A2B5 Na⁺ astrocytes but began to decrease in A2B5⁻ astrocytes after 6 days in vitro (DIV) and fell to or below the level of detection (i.e., 1 pA/pF) at 12 DIV. Astrocytes cultured from neonatal rats (P0) are almost exclusively GFAP ⁺/A2B5‐. These cells did not display measurable Na ⁺ currents when studied at 2 DIV; however, Na⁺ current was observed after 5 DIV in A2B5⁻ astrocytes from these neonatal (P0) cultures. These findings were substantiated by immunocytochemical experiments using 7493, an antibody raised against purified rat brain Na ⁺ channels; in P0‐derived astrocyte cultures 7493 antibody staining was initially lacking (up to 3 DIV), but it was prominent in cultures after 5 DIV, suggesting that Na⁺ current expression in RON astrocytes occurs postnatally. Copyright © 1991 Wiley‐Liss, Inc.