Profile of phosphatidylinositol metabolism stimulated by carbachol and glutamate in primary cultures of rat cerebellar neurons

The quantity of calcium that appears to induce neuronal death

Excessive entry of Ca2+ through the NMDA receptor is thought to be the major cause of glutamate toxicity in brain neurons. However, actual quantitation of the calcium overload has not been achieved. Here we show that the absolute amount of 45Ca2+ taken up via the NMDA receptor correlates quantitatively with the amount of acute cell death in cultured cerebellar granule cells of the rat. Analysis of 9- and 16-day cultures reveals that the NMDA-induced Ca2+ uptake is about the same at these ages, whereas the Ca-dependent lethal process is more developed in the older neurons. The calculated lethal concentration of 45Ca taken up exceeds by approximately 10,000 times the maximal concentration of [Ca2+]i that can be measured by fluorescence imaging. It is suggested that the Ca2+ taken up induces the lethal process in a subcellular structure in which it has been segregated.

Agonist-Stimulated Inositol Polyphosphate Formation in Cerebellum

The accumulation of inositol polyphosphates in the cerebellum in response to agonists has not been demonstrated. Guinea pig cerebellar slices prelabeled with [3H]inositol showed the following increases in response to 1 mM serotonin: At 15 s, there was a peak in 3H label in the second messenger inositol 1,4,5-trisphosphate [Ins(1,4,5)P3], decreasing to a lower level in about 1 min. The level of 3H label in the putative second-messenger inositol 1,3,4,5-tetrakisphosphate [Ins(1,3,4,5)P4] increased rapidly up to 60 s and increased slowly thereafter. The accumulation of 3H label in various inositol phosphate isomers at 10 min, when steady state was obtained, showed the following increases due to serotonin: inositol 1,3,4-trisphosphate [Ins(1,3,4)P3], eight-fold; Ins(1,3,4,5)P4, 6.4-fold; Ins(1,4,5)P3, 75%; inositol 1,4-bisphosphate [Ins(1,4)P2], 0%; inositol 3,4-bisphosphate, 100%; inositol 1-phosphate/inositol 3-phosphate, 30%; and inositol 4-phosphate, 40%. [3H]Inositol 1,3-bisphosphate was not detected in controls, but it accounted for 7.2% of the total inositol bisphosphates formed in the serotonin-stimulated samples. The fact that serotonin did not increase the formation of Ins(1,4)P2 could be due to the fact that Ins(1,4)P2 is rapidly degraded or that Ins(1,4,5)P3 is metabolized primarily by Ins(1,4,5)P3-3'kinase to form Ins(1,3,4,5)P4. In the presence of pargyline (10 microM), [3H]Ins(1,3,4,5)P4 and [3H]Ins(1,3,4)P3 levels were increased, even at 1 microM serotonin. Ketanserin (7 microM) completely inhibited the serotonin effect, indicating stimulation of serotonin2 receptors. Quisqualic acid (100 microM) also increased the levels of [3H]Ins(1,4,5)P3, [3H]Ins(1,3,4,5)P4, and [3H]Ins(1,3,4)P3, but the profile of these increases was different.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ Mobilization in Cultured Rat Cerebellar Cells: Astrocytes are Activated by t-ACPD

Using primary rat cerebellar cell cultures we observed that trans-1-amino-cyclopentyl-1,3-dicarboxylic acid (t-ACPD) was able to induce an increase in intracellular [Ca2+] in different cell types. This response was not abolished by external Ca2+ withdrawal, indicating that t-ACPD triggered the release of intracellularly stored Ca2+. In neurons the t-ACPD response was monophasic and inhibited by l-2-amino-4-phosphonobutyrate (APB). In astrocytes, characterized by their immunoreactivity to antisera to glial fibrillary acidic protein and S-100 protein, the response was oscillatory and resistant to APB application. These results suggest the presence of glutamate metabotropic receptor subtypes in the mammalian brain.

Inositol 1,4,5-triphosphate-stimulated calcium release from permeabilized cerebellar granule cells

1. Muscarinic cholinoceptor stimulation of phosphoinositide hydrolysis in rat cultured cerebellar granule cells results in a rapid, transient accumulation of inositol 1,4,5-trisphosphate (Ins(1,4,5)P3), which has been implicated in the release of non-mitochondrial intracellular Ca2+ stores. In the present study, the release of Ca2+ from intracellular stores and the Ins(1,4,5)P3 receptor responsible for this process have been investigated. 2. Monolayers of saponin-permeabilized granule cells accumulate 45Ca2+ in an ATP-dependent manner and the sequestered 45Ca2+ can be concentration-dependently released by Ins(1,4,5)P3 by a stereospecific and heparin-sensitive mechanism. The EC50 for Ins(1,4,5)P3-stimulated 45Ca2+ release was 80 +/- 3 nM. 3. Radioligand binding studies performed on a crude granule cell membrane fraction indicated the presence of an apparently homogeneous population of stereo-specific Ins(1,4,5)P3 receptors (KD 54.7 +/- 2.0 nM; Bmax 1.37 +/- 0.29 pmol mg-1 protein). 4. This study provides evidence for Ins(1,4,5)P3-sensitive intracellular Ca2+ stores in primary cultures of cerebellar granule cells and suggest that these cells provide an excellent model neuronal system in which to study the relative functional roles of Ca2+ release from intracellular stores and Ca(2+)-entry in neuronal Ca2+ homeostasis.

Pretreatment of cerebellar granule cells with concanavalin A potentiates quisqualate-stimulated phosphoinositide hydrolysis

The hydrolysis of phosphoinositides (PI) elicited in cerebellar granule cell cultures by agonists of metabolotropic glutamate receptors, glutmate and quisqualate, was enhanced when the cells were pretreated with concanavalin A (Con-A). A similar effect was produced by wheat germ agglutinin, but not by several other lectins tested. Con-A produced a dose-dependent effect (EC50 = 3 microM) and increased the efficacy but not the potency of the agonists. In contrast, Con-A failed to enhance PI hydrolysis evoked by N-methyl-D-aspartate, kainate, carbachol, the calcium ionophore A23187, or 50 mM K+. The Con-A stimulatory effect was prevented by simultaneous pretreatment with the agonists of ionotropic quisqualate receptors quisqualate, kainate, and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid, but not by the antagonist 6-cyano-7-nitroquioxaline-2,3-dione (CNQX). CNQX, which did not inhibit quisqualate-stimulated PI hydrolysis in untreated cells, abolished the component of quisqualate response enhanced by Con-A pretreatment. The pretreatment with Con-A also increased the influx of 45Ca2+ in granule cells stimulated by quisqualate. This increase was inhibited by CNQX. Moreover, the potentiation of PI hydrolysis by Con-A, but not the response to quisqualate alone, was abolished in the absence of Ca2+ and Na+. Pretreatment of granule cells with pertussis toxin inhibited PI hydrolysis stimulated by the metabolotropic quisqualate receptor and the Con-A-potentiated response by the same percentage, but Ca2+ influx induced by quisqualate was not affected.(ABSTRACT TRUNCATED AT 250 WORDS)

M3 muscarinic cholinoceptors are linked to phosphoinositide metabolism in rat cerebellar granule cells

Primary cultures of rat cerebellar granule cells are shown to possess a high density (283 +/- 48 fmol/mg of protein) of muscarinic receptor sites, defined using N-[3H]methylscopolamine [( 3H]NMS), with a KD of 0.18 +/- 0.01 nM measured after culture in vitro for 7 days. Displacement of specific [3H]NMS binding demonstrated a muscarinic receptor with low affinity for pirenzepine (Ki: 240 nM); further investigation using antagonists, AF-DX 116 and 4-DAMP to discriminate between M2 and M3 receptors respectively, revealed low M2 affinity (Ki: 600 nM) and high M3 affinity (Ki: 2.4 nM), indicative of the M3 receptor subtype. The robust muscarinic receptor stimulation of [3H]inositol phosphate formation, previously observed in these cells, was confirmed. Inhibition of this response followed a similar profile to the binding data, exhibiting weak inhibitory effects for pirenzepine (Ki: 710 nM) and AF-DX 116 (Ki: 5000 nM), but a potent action for 4-DAMP (Ki: 2.4 nM). The opposite profile seen for AF-DX 116 and 4-DAMP is indicative of a M3 receptor subtype expressed on these cells and linked to phosphoinositide hydrolysis. Further studies demonstrated that M3 receptor stimulation caused a rapid, transient increase in the second messenger inositol 1,4,5-trisphosphate, suggesting that potential Ca(2+)-homeostatic and neuromodulatory effects may be mediated by this response.