High potassium enhances secretion of neurotrophic factors from cultured astrocytes

Elevation of extracellular potassium concentration ([K+]o) in the central nervous system (CNS), which is observed such after physiological stimuli and during ischemia, is known to be regulated by astrocytes. We suspected that in response to increased [K+]o, astrocytes might secrete some neurotrophic factor(s) to promote the survival of active and/or ischemically damaged neurons. In the present study, we examined neurotrophic activity contained in HK-ACM, i.e., astrocyte-conditioned medium (ACM) obtained after culturing astrocytes in 40 mM potassium-containing medium (HK medium). Addition of HK-ACM to basal forebrain cultures from postnatal 2-week-old (P2w) rats increased both the choline acetyltransferase (ChAT) activity (4.40-fold) and the number of ChAT-positive neurons (2.01-fold) as compared with non-conditioned HK medium. On the other hand, the neurotrophic effects of LK-ACM, i.e., ACM collected after culturing astrocytes in 4 mM potassium-containing medium (LK medium), were much weaker (2.85- and 1.41-fold for ChAT activity and number of ChAT-positive neurons, respectively) than those of HK-ACM. The neurotrophic effects of ACMs increased in a manner dependent on potassium concentration and on astrocyte culture time. Addition of an antibody against nerve growth factor (NGF) neutralized the neurotrophic effects of HK- and LK-ACMs. Direct quantification of NGF protein in ACMs by the two-site ELISA method demonstrated that a high concentration of potassium enhanced NGF secretion from cultured astrocytes. These results suggested that astrocytes secrete NGF in response to [K+]o elevation in the CNS.