Abstract. Astroglial cells in vivo and in vitro respond to hormones, growth factors, and neurotransmitters by changing from an epithelial-like to stellate morphology. We have studied the temporal relationship between receptor activation, second messenger mobilization, and morphological changes using LRM55 astroglial cells. Maintenance of an altered morphology required continuous beta-adrenergic receptor activation. These changes appeared to be mediated by cAMP since they were elicited by its analogue, dihutyryl cAMP, and by forskolin, a direct activator of adenylate cyclase. Changes in cell morphology may require a relatively small increase in intracellular cAMP, since receptor-stimulated changes in cAMP levels were transient and peaked •5 min after receptor activation while changes in morphology took at least 30 min to reach a new steady state. Time-lapse videomicroscopy and high voltage electron microscopy indicated that receptor activation resulted in a sequence of morphological events. Time-lapse observations revealed the development and enlargement of openings through the cytoplasm associated with cytoplasmic withdrawal to the perinuclear region and process formation. Higher resolution high voltage electron microscopy indicated that the transition to a stellate morphology was preceded by the appearance of two distinct cytoplasmic domains. One contained an open network of filaments and organelles. The other was characterized by short broad cytoplasmic filaments. The first domain was similar to cytoplasm in control cells while the second was associated with the development and enlargement of openings through the cytoplasm and regions of obvious cytoplasmic withdrawal.D FFERENTIATION of astrocytes in vivo (9,24,36,37) and in vitro (8,13,14,16,21,25,32) is associated with dramatic changes in cell morphology. Cells change from an epithelial-like to a stellate morphology. Similar changes are observed in certain populations of astrocytes in the adult brain. Astrocytes in the supraoptic nucleus undergo dramatic morphological changes at parturition and after water deprivation by extending and withdrawing processes (24, 36, 37). When astrocytes in primary cell culture (3, 13-15, 21, 25, 31, 32) or C-6 glioma cells (22) are treated with beta-adrenergic agonists, activators of adenylate cyclase, or analogues of cAMP, cells change from an epitheliallike to a stellate morphology. These studies have demonstrated the occurrence of morphological changes, but have not thoroughly examined the relationships between beta-adrenergic receptor activation, intracellular CAMP, and morphology.Since activation of beta-adrenergic receptors stimulates the synthesis of cAMP in primary cell cultures of astrocytes (10,12,31,38) and in C-6 cells (6, 23), it has been proposed that changes in morphology result from activation of cAMPdependent protein kinase (7,20). Consistent with this hypothesis is the observation that activation of beta-adrenergic receptors results in the phosphorylation of the intermediate filament proteins, glial fibrilla...

The effects of [K+]o on taurine release from glial cells were studied with primary cultures of cerebellar astrocytes and with LRM55 cells, a continuous glial cell line. The characteristics of K(+)-stimulated taurine release were virtually identical in the 2 cell types. Both cerebellar astrocytes and LRM55 cells released taurine when stimulated with high-K+ medium prepared by isosmotically substituting KCl for NaCl, but neither cell type released taurine when stimulated with hyperosmotic high-K+ medium prepared by adding solid KCl to control medium. The membrane potential of LRM55 cells was measured by intracellular recording and was insensitive to changes in [K+]o below 20 mM. LRM55 cells released taurine when stimulated with nondepolarizing concentrations of K+ (13-22 mM) if the isosmotically prepared high-K+ medium was used, but the cells did not release taurine when treated with a depolarizing concentration of K+ (50 mM) if hyperosmotic high-K+ medium was used. The time course of K(+)-stimulated taurine release was quite slow, having a time to peak of 10-15 min. Small changes (2.5-10%) in the osmolarity of the medium strongly affected taurine release by cerebellar astrocytes and LRM55 cells. K(+)-stimulated taurine release from both cell types was inhibited when the osmolarity was increased with sucrose or NaCl and was enhanced when the osmolarity was reduced. Similarly, baseline taurine release was suppressed by small elevations in osmolarity and increased by reduced osmolarity.(ABSTRACT TRUNCATED AT 250 WORDS)

Activation of beta-adrenergic receptors on astrocytes in primary cell culture results in the release of taurine, an inhibitory transmitter. Taurine release occurs via a cyclic AMP-mediated intracellular pathway, because (a) taurine release and intracellular cyclic AMP accumulation have similar pharmacologies and time courses of activation and (b) N6,O2'-dibutyryl cyclic AMP stimulates release with a time course similar to that observed with the beta-adrenergic agonist isoproterenol. These results describe a previously unrecognized physiological function of astrocytes in the CNS-receptor-mediated release of the neuroactive amino acid taurine. This observation indicates that astrocytes may function as local regulators of neuronal activity.