Receptor-coupled phospholipase C and adenylyl cyclase function with different calcium pools in astrocytes

Astrocytes in cocaine addiction and beyond

Drug addiction remains a key biomedical challenge facing current neuroscience research. In addition to neural mechanisms, the focus of the vast majority of studies to date, astrocytes have been increasingly recognized as an "accomplice." According to the tripartite synapse model, astrocytes critically regulate nearby pre- and postsynaptic neuronal substrates to craft experience-dependent synaptic plasticity, including synapse formation and elimination. Astrocytes within brain regions that are implicated in drug addiction exhibit dynamic changes in activity upon exposure to cocaine and subsequently undergo adaptive changes themselves during chronic drug exposure. Recent results have identified several key astrocytic signaling pathways that are involved in cocaine-induced synaptic and circuit adaptations. In this review, we provide a brief overview of the role of astrocytes in regulating synaptic transmission and neuronal function, and discuss how cocaine influences these astrocyte-mediated mechanisms to induce persistent synaptic and circuit alterations that promote cocaine seeking and relapse. We also consider the therapeutic potential of targeting astrocytic substrates to ameliorate drug-induced neuroplasticity for behavioral benefits. While primarily focusing on cocaine-induced astrocytic responses, we also include brief discussion of other drugs of abuse where data are available.

Effect of the branched-chain alpha-keto acids accumulating in maple syrup urine disease on S100B release from glial cells

Accumulation of the branched-chain alpha-keto acids (BCKA), alpha-ketoisocaproic acid (KIC), alpha-keto-beta-methylvaleric acid (KMV) and alpha-ketoisovaleric acid (KIV) and their respective branched-chain alpha-amino acids (BCAA) occurs in tissues and biological fluids of patients affected by the neurometabolic disorder maple syrup urine disease (MSUD). The objective of this study was to verify the effect of the BCKA on S100B release from C6 glioma cells. The cells were exposed to 1, 5 or 10 mM BCKA for different periods and the S100B release was measured afterwards. The results indicated that KIC and KIV, but not KMV, significantly enhanced S100B liberation after 6 h of exposure. Furthermore, the stimulatory effect of the BCKA on S100B release was prevented by coincubation with the energetic substrate creatine and with the N-nitro-l-arginine methyl ester (l-NAME), a nitric oxide synthase inhibitor, indicating that energy deficit and nitric oxide (NO) were probably involved in this effect. Furthermore, the increase of S100B release was prevented by preincubation with the protein kinase inhibitors KN-93 and H-89, indicating that KIC and KIV altered Ca2+/calmodulin (PKCaMII)- and cAMP (PKA)-dependent protein kinases activities, respectively. In contrast, other antioxidants such as glutathione (GSH) and trolox (soluble vitamin E) were not able to prevent KIC- and KIV-induced increase of S100B liberation, suggesting that the alteration of S100B release caused by the BCKA is not mediated by oxidation of sulfydryl or other essential groups of the enzyme as well as by lipid peroxyl radicals. Considering the importance of S100B for brain regulation, it is conceivable that enhanced liberation of this protein by increased levels of BCKA may contribute to the neurodegeneration characteristic of MSUD patients.

Regulation and organization of adenylyl cyclases and cAMP

Adenylyl cyclases are a critically important family of multiply regulated signalling molecules. Their susceptibility to many modes of regulation allows them to integrate the activities of a variety of signalling pathways. However, this property brings with it the problem of imparting specificity and discrimination. Recent studies are revealing the range of strategies utilized by the cyclases to solve this problem. Microdomains are a consequence of these solutions, in which cAMP dynamics may differ from the broad cytosol. Currently evolving methodologies are beginning to reveal cAMP fluctuations in these various compartments.

Pharmacological blockade of mGlu2/3 metabotropic glutamate receptors reduces cell proliferation in cultured human glioma cells

Glial cell proliferation in culture is under the control of metabotropic glutamate (mGlu) receptors. We have examined whether this control extends to human glioma cells. Primary cultures were prepared from surgically removed human glioblastomas. RT-PCR combined with western blot analysis showed that most of the cultures (eight out of 11) expressed group-II mGlu receptors. In two selected cultures (MZC-12 and FCN-9), the mGlu2/3 receptor antagonist, LY341495, slowed cell proliferation when applied to the growth medium from the second day after plating. This effect was reversible because linear cell growth was restored after washing out the drug. LY341495 reduced glioma cell proliferation at concentrations lower than 100 nm, which are considered as selective for mGlu2/3 receptors. In addition, its action was mimicked by the putative mGlu2/3 receptor antagonist (2S)-alpha-ethylglutamate. The anti-proliferative effect of LY341495 was confirmed by measuring [methyl-3H]-thymidine incorporation in cultures arrested in G0 phase of the cell cycle and then stimulated to proliferate by the addition of 10% fetal calf serum or 100 ng/mL of epidermal growth factor (EGF). In cultures treated with EGF, LY341495 was also able to reduce the stimulation of the mitogen-activated protein kinase (MAPK) pathway, as well as the induction of cyclin D1. Both effects, as well as decreased [methyl-3H]-thymidine incorporation, were partially reduced by co-addition of the potent mGlu2/3 receptor agonist, LY379268. We conclude that activation of group-II mGlu receptors supports the growth of human glioma cells in culture and that antagonists of these receptors should be tested for their ability to reduce tumour growth in vivo.

Group 1 mGlu receptors elevate [Ca2+]i in rat cultured cortical type 2 astrocytes: [Ca2+]i synergy with adenosine A1 receptors

Brain macroglia are known to express a diverse array of neurotransmitter receptors whose signal transduction pathways may be subject to heteroreceptor 'cross-talk'. In the current study we have examined group 1 mGlu receptor-evoked [Ca2+]i signalling, and possible heteroreceptor cross-talk, in cultured type 2 astrocytes. The selective group 1 metabotropic glutamate (mGlu) receptor agonist (S)-3,5-dihydroxyphenylglycine (DHPG) elevated [Ca2+]i (EC50 = 1.7 +/- 0.6 microM); an effect reversed by the selective mGlu receptor antagonist (S)-alpha-methyl-4-carboxyphenylglycine (IC50 = 52.7 +/- 8.7 microM). DHPG-evoked [Ca2+]i responses were abolished by (1) thapsigargin (100 nM), implicating the involvement of internal Ca2+ stores in group 1 mGlu [Ca2+]i responses and (2) the removal of extracellular Ca2+. When applied alone, the selective adenosine A1 receptor agonist, N6-cyclopentyladenosine (CPA, 100 nM) failed to influence [Ca2+]i. However, in the presence of 1 microM DHPG, CPA potently (EC50 = 12.3 +/- 1.9 nM) increased [Ca2+]i responses. In the presence of 100 nM CPA, the efficacy of DHPG was doubled without any significant change in the DHPG EC50 value. This effect was reversed by either the selective adenosine A1 receptor antagonist, 8-cyclopentyltheophylline (IC50 = 50.3 +/- 19.9 nM) or overnight incubation with Pertussis toxin (100 ng/ml). We conclude that (1) type 2 astrocytes contain group 1 mGlu receptors coupled to [Ca2+]i signalling and (2) co-activation of adenosine A1 receptors enhances group 1 mGlu-evoked [Ca2+]i responses in these cells via a Gi/o G protein-mediated mechanism.