GABA induces Ca 2+ transients in astrocytes

Abstract

By using the Ca 2+-sensitive indictor Fura-2/AM, the cytosolic Ca 2+ levels [Ca 2+] i were measured in type 1 astrocytes in rat cortical astroglial primary cultures, after stimulation with GABA, muscimol (GABA A agonist), or baclofen (GABA B agonist). We report the first evidence that stimulation of both GABA A and GABBA B receptors evokes Ca 2+ transients in type l astrocytes. Two types of Ca 2+ responses were seen: the single-phase curve, which was the most common, and the biphasic, which consisted of an initial rise that persisted at the maximal or submaximal level. Both types of Ca 2+ responses appeared with some latency. The responses were obtained in astrocytes grown for 12–16 days in culture and the response frequencies for all three agonists were 18% of the total number of examined cells. However, when the astrocytes were grown in a mixed astroglial/neuronal culture the response frequencies for all three agonists increased to 35% of the total number of examined cells. In some cells, the responses after GABA stimulation were blocked to baseline levels after exposure to bicuculline (GABA A antagonist). In other cells, bicuculline only slightly reduced the GABA-evoked responses, and the addition of phaclofen (GABA B antagonist) did not potentiate this partial inhibition. However, the muscimol-evoked rises in [Ca 2+] i were completely inhibited after exposure to bicuculline, while the responses after baclofen could only be partly blocked by phaclofen. GABA evoked rises in [Ca 2+] i which alternatively were inhibited (mostly) or persisted in Ca 2+-free buffer. The rises in [Ca 2+] i persisted, but were reduced, in Ca 2+-free buffer after stimulation with muscimol, but were inhibited after baclofen stimulation. The GABA uptake blockers guvacine, 4,5,6,7-tetrahydroisoxazolo(4,5-c)pyridin-3-ol and nipecotic acid were also able to reduce the GABA-evoked rises in [Ca 2+] i However, the L-type Ca 2+ chennel antagonist nifedipine failed to influence on the GABA-evoked Ca 2+ transients. The results suggest that type 1 astrocytes in primary culture express GABA receptors which can elevate [Ca 2+] i directly or indirectly via Ca 2+ channels and/or via release from internai Ca 2+ stores. The results also suggest that GABA can have intracellular Ca 2+-mobilizing sites since the GABA-evoked responses were reduced after incubation with GABA uptake blockers.