Spermine (SPM) regulates neuronal receptors and channels and is primarily accumulated in glia, not in neurons (Glia 19(1997)171; Glia 31(2000)84). Glia lack the biosynthetic enzymes ornithine decarboxylase and spermidine synthase required for synthesis of SPM precursors (Prog. Neurobiol. 57(1999)485; SFN 2004, No. 754.15), but the mechanism of accumulation and release in glia remains unclear. Because polyamines pass through unapposed Cx38 hemichannels in a voltage-dependent manner (J. Physiol. 553(2003)95), we suggest that endogenous SPM acts as a signaling molecule between glial and neuronal cells via release and uptake through glial Cx43 hemichannels. Here we show that: (1) when glial K+ channels are blocked and extracelluar Ca2+ and Mg2+ are 2.5 and 2.0 mM, respectively, no currents were recorded in cultured rat cortical astrocytes. (2) A large non-selective ion current (1-5 nA) was activated by switching to Ca2+-Mg2+-free solution and (3) this current was blocked by many hemichannel blockers, but (4) n...