Somatostatin potentiates the alpha 1-adrenergic activation of phospholipase C in striatal astrocytes through a mechanism involving arachidonic acid and glutamate.

Abstract

As previously shown with adenosine, somatostatin, which is ineffective alone, enhanced the a1-adrenergicagonist-stimulated production of inositol phosphates in cultured striatal astrocytes. This effect was suppressed in cells pretreated with pertussis toxin. It required external calcium and was selectively antagonized by both mepacrine, an inhibitor of phospholipase A2, and 5,8,11,14-eicosatetraynoic acid, a nonmetabolizable analog of arachidonic acid. In addition, a long-lasting elevation of cytosolic calcium and a release of arachidonic acid were observed only under the combined stimulation of somatostatin and a1-adrenergic receptors. Arachidonic acid could in turn inhibit glutamate uptake into astrocytes, and the resulting external accumulation of glutamate could account for the somatostatin-evoked amplifiation of the a1-adrenergic-agonist-stimulated hydrolysis of inositolphospholipids. The effect of somatostatin was indeed reproduced by glutamate or glutamate uptake inhibitors and uppressed by enzymatic removal of external glutamate. Thus, astrocytes may contribute to long-term plasticity events in glutamatergic synapses through regulation of external glutamate levels.Somatostatin (SRIF, somatotropin release-inhibiting factor, tetradecapeptide form) inhibits spontaneous neuronal firing and various secretory responses in different cell types (1-4). This could be related to its hyperpolarizing effect resulting from an increase in potassium conductances (5-10) and/or an inhibition of voltage-dependent calcium current (11)(12)(13) (19). In the present study, attempts were made to determine whether somatostatin, like the nucleoside, could enhance the a1-adrenergic-agonist-induced formation of IPs in striatal astrocytes from the mouse, since receptors for adenosine and somatostatin have generally been associated with similar transduction systems-i.e., guanine nucleotide-binding regulatory proteins (G proteins) sensitive to pertussis toxin (PTX) (20)(21)(22). As will be shown, the peptide increased the formation of IPs in the presence of methoxamine, a specific agonist of a1-adrenergic receptors.In hippocampal CAl pyramidal neurons, the hyperpolarizing effect of somatostatin seems to be mediated by a release of arachidonic acid (AA) (23). Particular attention was thus made to evaluate a possible role for AA in the potentiating effect of somatostatin seen in striatal astrocytes. In addition, as recently demonstrated in glial (Muller) cells, AA inhibits glutamate uptake (24). Furthermore, it has been shown that glutamate stimulates PLC activity in astrocytes (25). Therefore, the possible involvement of glutamate in the enhancing effect of somatostatin on the a1-adrenergic-agonist-evoked response was also investigated. (Dakopatts, Glostrup, Denmark). The remaining 5% of cells could be immature glioblasts, which are unlabeled by GFAP (26). The cultures were free of microglial cells, since no staining was observed when monoclonal antibody to mouse macrophages (anti-MAC 1; Serotec) was used. MATERIALS AND METHO...

Comparison of functional profiles at human recombinant somatostatin sst2 receptor: simultaneous determination of intracellular Ca2+ and luciferase expression in CHO‐K1 cells

“…SRIF has previously been shown to be able to increase intracellular Ca 2 þ via sst 2 receptors in recombinant cell lines including COS-7 Tomura et al, 1994), F 4 C 1 pituitary cells (Chen et al, 1997), HEK-293 (Chen & Tashjian, 1994), and to a very small extent in CCL39 Chinese hamster lung fibroblasts (Siehler & Hoyer, 1999c). The effect has also been demonstrated in native systems including cultured mouse striatal astrocytes (Marin et al, 1991), rat AR42J pancreas cells (Taylor, 1995), salamander retinal cells (Akopian et al, 2000), aortic smooth muscle (Romoser et al, 2001) and B lymphoblasts, where similar kinetics to those of this study were observed (Rosskopf et al, 2003). In the majority of studies, the increase in intracellular Ca 2 þ by SRIF parallels an increase in intracellular inositol phosphates and therefore is likely to be mediated via phospholipase C stimulation Tomura et al, 1994;Chen & Tashjian, 1994;Chen et al, 1997;Siehler & Hoyer, 1999c;Romoser et al, 2001;Rosskopf et al, 2003).…”

“…In the majority of studies, the increase in intracellular Ca 2 þ by SRIF parallels an increase in intracellular inositol phosphates and therefore is likely to be mediated via phospholipase C stimulation Tomura et al, 1994;Chen & Tashjian, 1994;Chen et al, 1997;Siehler & Hoyer, 1999c;Romoser et al, 2001;Rosskopf et al, 2003). Although, in cultured striatal astrocytes and rat AR42J pancreas cells, SRIF-induced rises in intracellular Ca 2 þ are independent of increased intracellular inositol phosphates (Marin et al, 1991;Taylor, 1995).…”

“…SRIF has previously been shown to increase intracellular Ca 2 þ levels via both recombinant and native receptors (Marin et al, 1991;Akbar et al, 1994;Tomura et al, 1994;Chen & Tashjian, 1994;Taylor, 1995;Chen et al, 1997;Siehler & Hoyer, 1999c;Akopian et al, 2000;Romoser et al, 2001;Rosskopf et al, 2003), and in this study intracellular Ca 2 þ changes were determined via fluorescence changes using a fluorometric imaging plate reader II (FLIPR II). Determination of luciferase expression was possible as a marker of functional receptor activation since the CHO-K1 cells used to express the sst 2 receptor were also stably transfected with the luciferase reporter gene under the control of the promoter sequence SRE.…”

Diazepam Attenuation of Somatostatin Binding and Effect of Somatostatin on Accumulation of Inositol 1,4,5-Trisphosphate in the Rat Frontoparietal Cortex

“…Recent studies suggest that all five human SRIF receptors ex- pressed in COS-7 cells are coupled to activation of phosphoinositide (PI)-specific PLC-␤ (Akbar et al 1994). A few studies have also reported that SRIF activates PLC followed by Ca 2ϩ mobilization in native SRIF-receptor expressing cells, such as striatal astrocytes (Marin et al 1991) and NG 108-15 cells (Okajima and Kondo 1992). Therefore, the decreased effect of SRIF on accumulation of IP 3 observed after diazepam administration at 3 or 7 days might be a consequence of the decrease in the number of Although the number of SRIF receptors returned to control values after 14 days of treatment, the effect of SRIF on IP 3 accumulation still remains decreased.…”

Metabotropic Glutamate Receptor Activation Modulates Kainate and Serotonin Calcium Response in Astrocytes

“…Rises in astrocyte calcium could also increase phospholipase A2. The resulting increase in arachidonic acid would reduce glutamate uptake, resulting in an increased extracellular glutamate level that would alter neuronal activity (Axelrod et al, 1988;Barbour et al, 1989;Marin et al, 1991). In the SCN, as in many other areas of the brain, astrocytes can surround axodendritic contacts (van den Pol et al, 1992).…”