Journal of neurochemistry
-
Journal of neurochemistry · Apr 2008
Enhanced behavioral sensitivity to the competitive GABA agonist, gaboxadol, in transgenic mice over-expressing hippocampal extrasynaptic alpha6beta GABA(A) receptors.
The behavioral and functional significance of the extrasynaptic inhibitory GABA(A) receptors in the brain is still poorly known. We used a transgenic mouse line expressing the GABA(A) receptor alpha6 subunit gene in the forebrain under the Thy-1.2 promoter (Thy1alpha6) mice ectopically expressing alpha6 subunits especially in the hippocampus to study how extrasynaptically enriched alphabeta(gamma2)-type receptors alter animal behavior and receptor responses. In these mice extrasynaptic alpha6beta receptors make up about 10% of the hippocampal GABA(A) receptors resulting in imbalance between synaptic and extrasynaptic inhibition. ⋯ Gaboxadol inhibition of the binding could be partially prevented by GABA. Electrophysiology of recombinant GABA(A) receptors revealed that GABA was a partial agonist at alpha6beta3 and alpha6beta3delta receptors, but a full agonist at alpha6beta3gamma2 receptors when compared with gaboxadol. The results suggest strong behavioral effects via selective pharmacological activation of enriched extrasynaptic alphabeta GABA(A) receptors, and the mouse model represents an example of the functional consequences of altered balance between extrasynaptic and synaptic inhibition.
-
Journal of neurochemistry · Apr 2008
Activity-dependent heteromerization of the hyperpolarization-activated, cyclic-nucleotide gated (HCN) channels: role of N-linked glycosylation.
Formation of heteromeric complexes of ion channels via co-assembly of different subunit isoforms provides an important mechanism for enhanced channel diversity. We have previously demonstrated co-association of the hyperpolarization activated cyclic-nucleotide gated (HCN1/HCN2) channel isoforms that was regulated by network (seizure) activity in developing hippocampus. However, the mechanisms that underlie this augmented expression of heteromeric complexes have remained unknown. ⋯ In addition, glycosylated HCN1 channels were preferentially co-immunoprecipitated with the HCN2 isoforms. Provoking SA in vitro in the presence of the N-linked glycosylation blocker tunicamycin abrogated the activity-dependent increase of HCN1/HCN2 heteromerization. Thus, hippocampal HCN1 molecules have a significantly higher probability of being glycosylated after SA, and this might promote a stable heteromerization with HCN2.
-
Journal of neurochemistry · Feb 2008
Comparative StudyPhysiological changes in GRK2 regulate CCL2-induced signaling to ERK1/2 and Akt but not to MEK1/2 and calcium.
G protein-coupled receptor (GPCR) kinase 2 (GRK2) regulates G protein-coupled receptor signaling via agonist-induced receptor phosphorylation and desensitization. GRK2 can also modulate cellular activation by interacting with downstream signaling molecules. The intracellular GRK2 level changes during inflammatory conditions. ⋯ These data suggest that altered GRK2 expression modulates chemokine signaling downstream of the receptor. We found that GRK2 kinase activity was not required to decrease chemokine-induced ERK1/2 phosphorylation, whereas regulation of CCL2-induced Akt phosphorylation did require an active GRK2 kinase domain. Collectively, these data suggest that changes in endogenous GRK2 expression in primary astrocytes regulate chemokine receptor signaling to ERK1/2 and to PDK-1-Akt downstream of receptor coupling via kinase-dependent and kinase-independent mechanisms, respectively.
-
Journal of neurochemistry · Jan 2008
Glial cell-derived glutamate mediates autocrine cell volume regulation in the retina: activation by VEGF.
Astroglial cells are a source for gliotransmitters such as glutamate and ATP. We demonstrate here that gliotransmitters have autocrine functions in the regulation of cellular volume. Hypoosmotic stress in the presence of inflammatory mediators or oxidative stress, and during blockade or down-regulation of potassium channels, induces swelling of retinal glial cells. ⋯ This cascade involved the release of ATP and adenosine, and the activation of purinergic P2Y(1) and adenosine A1 receptors, resulting in the opening of potassium and chloride channels and inhibition of cellular swelling. The glutamatergic-purinergic regulation of the glial cell volume may be functionally important in the homeostasis of the extracellular space volume during intense neuronal activation which is associated with a swelling of neuronal cell structures in the retina. However, glial cell-derived glutamate may also contribute to the swelling of activated neurons since metabolic poisoning of glial cells by iodoacetate inhibits the neuronal cell swelling mediated by activation of ionotropic glutamate receptors.
-
Journal of neurochemistry · Jan 2008
Effect of glutamate intake during gestation on adenosine A(1) receptor/adenylyl cyclase pathway in both maternal and fetal rat brain.
Pregnant Wistar rats were orally treated with 1 g/L l-glutamate during the entire gestational period and the status of adenosine A(1) receptor (A(1)R)/adenylyl cyclase transduction pathway from maternal and fetal brain was analyzed. Glutamate consumption, estimated from the loss of water from the drinking bottles, was 110 +/- 4.6 mg/kg/day. In mother brains glutamate intake did not significantly alter the B(max) value, although the K(d) value was significantly decreased. ⋯ On the other hand, basal, forskolin, and forskolin plus GTPgammaS-stimulated adenylyl cyclase activity was significantly decreased in both maternal and fetal brain, and this was more prominent in fetal than in maternal brain. Finally, A(1)R functionality was significantly decreased in mother brain whereas no significant differences were detected in fetus brain. These results suggest that glutamate administered to pregnant rats modulates A(1)R signaling pathways in both tissues, showing an A(1)R down-regulation in fetal brain, and desensitization in maternal brain.