sirolimus and alpha-methyl-4-carboxyphenylglycine

Hippocampal long-term depression (LTD) is a long-lasting decrease in synaptic strength that is most commonly studied at glutamatergic inputs to pyramidal cells in hippocampal area CA1. Activation of G-protein-coupled group I (including types 1 and 5) metabotropic glutamate receptors (mGluRs) by the pharmacological agonist (RS)-3,5-dihydroxyphenylglycine (DHPG) elicits LTD in area CA1 of the hippocampus. Recent reports have shown that de novo protein synthesis is necessary for DHPG-induced LTD. However, relatively little is known about the signaling pathways that couple mGluRs to translation initiation. In this study, we investigated whether the activation of the phosphoinositide 3-kinase (PI3K)-Akt-mammalian target of rapamycin (mTOR) pathway, which has been shown to regulate translation initiation, is necessary for mGluR-LTD induced by DHPG. We found that brief incubations of mouse hippocampal slices with DHPG resulted in increased phosphorylation of Akt and mTOR in hippocampal area CA1. Two structurally unrelated PI3K inhibitors, LY294002 and wortmannin, blocked the DHPG-induced increases in phosphorylation of Akt and mTOR. Biochemical fractionation studies showed that the DHPG-induced increase in the phosphorylation of Akt and mTOR could be detected in synaptoneurosome preparations, and immunohistochemical analysis revealed that similar increases could be detected in both stratum pyramidale and stratum radiatum in area CA1. Finally, we observed that both PI3K inhibitors and rapamycin, an mTOR inhibitor, prevented mGluR-LTD induced by DHPG. Together, our findings indicate that activation of the PI3K-Akt-mTOR signaling cascade is required for mGluR-LTD and suggest that this pathway may couple group I mGluRs to translation initiation in hippocampal area CA1.

Topics: 3-Phosphoinositide-Dependent Protein Kinases; Androstadienes; Animals; Benzoates; Chromones; Dendrites; Enzyme Inhibitors; Excitatory Amino Acid Agonists; Glycine; Hippocampus; Long-Term Synaptic Depression; Male; Mice; Mice, Inbred C57BL; Morpholines; Phosphatidylinositol 3-Kinases; Phosphoinositide-3 Kinase Inhibitors; Phosphorylation; Protein Kinases; Protein Processing, Post-Translational; Protein Serine-Threonine Kinases; Proto-Oncogene Proteins; Proto-Oncogene Proteins c-akt; Pyramidal Cells; Pyridines; Receptor, Metabotropic Glutamate 5; Receptors, Metabotropic Glutamate; Resorcinols; Signal Transduction; Sirolimus; Synaptosomes; Tacrolimus; TOR Serine-Threonine Kinases; Wortmannin

Inducing long-term depression (LTD), one type of synaptic plasticity, typically requires electrical stimulation. Using rat hippocampal slices, we show that increasing the Ca(2+) of the extracellular solution to 4 mM and decreasing the Mg(2+) to 0.1 mM induced LTD in the CA1 region in the absence of electrical stimulation. The ionically-induced-LTD occluded electrical stimulation-induced LTD and was not blocked by application of 50 microM D-2-amino-5-phosphonopentanoic acid (an N-methyl-D-aspartate receptor antagonist) or by 1 mM (RS)-alpha-methyl-4-carboxyphenylglycine (a metabotropic glutamate receptor antagonist) alone. However, LTD was not ionically induced in the presence of both inhibitors. Our findings suggest that altering tissue excitability by changing the ionic environment mimics electrical stimulation in the induction of LTD.

Topics: 2-Amino-5-phosphonovalerate; Animals; Benzoates; Calcium; Calcium Channel Blockers; Electric Stimulation; Excitatory Amino Acid Antagonists; Glycine; Hippocampus; In Vitro Techniques; Magnesium; Neuronal Plasticity; Pyramidal Cells; Rats; Rats, Sprague-Dawley; Sirolimus; Solutions; Tacrolimus; Verapamil