AGONIST-EVOKED CA2+ TRANSIENTS IN PRIMARY ASTROGLIAL CULTURES - MODULATORY EFFECTS OF VALPROIC ACID

Cytosolic Ca2+ ([Ca2+]i) activity was measured in individual type 1 astroglial cells in primary culture after exposure to glutamate (Glu), quisqualate (QA), gamma-aminobutyric acid (GABA), 5-hydroxytryptamine (5HT), and noradrenaline (NA) by using the Ca2+ indicator dye fura-2/AM in a computerized microspectrofluorimetric system. Various patterns of Ca2+ transients were observed, but the most common was biphasic, having an initial sharp peak, rising immediately after stimulation, and then declining to a lower but sustained Ca2+ level. The only substance that diverged from this pattern was GABA, which induced a Ca2+ response with longer latency and a single-phase curve. The effects of the anticonvulsive drug Na+-valproate (VPA) were also investigated. After both acute and chronic (5-7 days) exposure to 10(-4) M VPA, the GABA-evoked rises in [Ca2+]i were completely inhibited. VPA also had acute effects on the 5HT- and Glu-evoked Ca2+ spikes. The Ca2+ responses after 5HT stimulation were greatly reduced after exposure to 10(-4) M VPA. The responses after glutamate stimulation were, on the contrary, increased after a similar exposure. No VPA effects were seen on the curve patterns of QA and NA stimulations. The most frequent agonist-evoked responses were seen after stimulation with 5HT and NA, where over 80% of the tested cells responded. For QA and Glu, the response frequencies were about 40% each, while for GABA it was 20%. The responses after 5HT and NA stimulation were blocked to baseline levels after exposure to ketanserin (5HT2 receptor antagonist) and a combination of prazosin, yohimbine, and propranolol (alpha-1, alpha-2, and beta-adrenoceptor antagonists, respectively). The rises in [Ca2+]i after stimulation with QA, Glu, and GABA were partly antagonized by 6-nitro-7-cyano-quinoxaline-2,3-dione (CNQX; a QA receptor antagonist, also tested on the glutamate responses) and bicuculline (GABA receptor antagonists). The biphasic curve forms, after stimulation with Glu, QA, 5HT, and NA, were reduced to single-phase curves with unaffected or slightly reduced initial peaks when the stimulations were performed in Ca2+-free medium. Interestingly, in some cells Glu induced a curve form that, different from the other Glu-evoked responses, consisted of an immediate rise in [Ca2+]i, which persisted at the level of maximal response. Furthermore, this response was completely inhibited in Ca2+-free buffer. The GABA response was also totally inhibited in Ca2+-free buffer. The results suggest that astrocytes in primary culture respond differently to various receptor agonists, with regard to both the intracellular Ca2+ spiking behavior and the response frequency. Furthermore, the data also show that certain agonist-evoked rises in [Ca2+]i can be significantly modulated by the neuroactive agent VPA.