An increase in intracellular free calcium ions by nicotinic acetylcholine receptors in a single cultured rat cortical astrocyte

Neuronal nicotinic acetylcholine receptors (nAChRs) are composed of an assembly between at least seven alpha (alpha2-alpha7, alpha9) and three beta (beta2-beta4) subunits in mammals. The addition of 50 mM KCl or 1 mM nicotine immediately increased the number of cells with high fluorescence intensity in rat cortical astrocytes on fluo-3 fluorescence measurement. Nicotine was effective at increasing the fluorescence intensity in astrocytes cultured for 2 days after replating, but not in those used 1 or 5 days after replating, without markedly affecting the cellular viability irrespective of the exposure period. Nicotine markedly increased the fluorescence intensity in a concentration-dependent manner at a concentration range of 10-100 microM in cultured astrocytes when analyzed on a responsive single cell. In these responsive single cells, the increase by nicotine was significantly prevented by the heteromeric alpha4/beta2 subtype antagonist dihydro-beta-erythroidine and the homomeric alpha7 subtype antagonist methyllycaconitine, as well as by nifedipine and EGTA but not thapsigargin. Methyllycaconitine failed to inhibit further the increase by nicotine in the presence of nifedipine, however, whereas the expression of mRNA was seen for all mammalian neuronal nAChR subunits in cultured rat cortical astrocytes as well as neurons. These results suggest that nicotine may increase intracellular free Ca2+ through the influx of extracellular Ca2+ across L-type voltage-gated Ca2+ channels rather than Ca2+ release from intracellular stores, in a manner related to the alpha4/beta2 and/or alpha7 nAChR channels functionally expressed in cultured rat cortical astrocytes.