The metaplastic effects of NMDA receptors blockade on reactivation of instrumental memories in rats

Metaplasticity, defined as the plasticity of synaptic plasticity, could affect learning and memory at different neural levels. It was hypothesized that metaplasticity changes on glutamate receptors may affect memory destabilization, promoting or preventing reconsolidation. We investigated the metaplastic effect of NMDA channel blocker MK-801 on sucrose instrumental memory reconsolidation in a behavioural rat model associated to the assessment of molecular markers of metaplasticity, memory retrieval, destabilization and reconsolidation. Following instrumental conditioning and forced abstinence, rats were intraperitoneally treated with MK-801 or vehicle 24\u202fh before the exposure to memory retrieval or not-retrieval. Separate groups were tested for in-vivo extinction of responding (24\u202fh and 7\u202fd after reactivation) or ex-vivo assessment of transcription factor Zif268 and ribosomal protein rpS6 phosphorylation in nucleus accumbens (NAc) and amygdala (Amy). MK-801 significantly inhibited instrumental responding at extinction test, suggesting reconsolidation blockade of instrumental memory. The decrease of Zif268 and phosphorylated-rpS6 levels in NAc and Amy in MK-801/Retrieval vs. Vehicle/Retrieval group supported the behavioural findings. An increase of GluN2B, GluA1 and mGluR5 in NAc, and GluN2B in Amy, 24\u202fh after MK-801 indicated the trigger of associated metaplastic changes. Our findings show that metaplastic changes induced by NMDA receptors blockade affected sucrose instrumental memory retrieval as shown by both behavioural and molecular changes. We hypothesize that these findings however suggested a switch to extinction rather than a reconsolidation

Involvement of endocannabinoid signaling in the neuroprotective effects of subtype 1 metabotropic glutamate receptor antagonists in models of cerebral ischemia.

Experimental evidence indicates that metabotropic glutamate (mGlu) receptors of the mGlu1 and mGlu5 subtypes play a differential role in models of cerebral ischemia and that only mGlu1 receptors are implicated in the pathways leading to postischemic neuronal injury. The localization of mGlu1 receptors in GABA-containing interneurons rather than in hippocampal CA1 pyramidal cells that are vulnerable to ischemia has prompted experimental studies that have demonstrated mGlu1 receptor antagonist agents attenuate postischemic injury by enhancing GABA-mediated neurotransmission, thus providing a new viewpoint on the neuroprotective mechanism of these pharmacological agents. In view of the recent discovery of a functional interaction between group I mGlu receptors and the cannabinoid system in the modulation of synaptic transmission, we propose a novel mechanism that predicts that the neuroprotective effects of mGlu1 receptor antagonists on CA1 pyramidal cells are mediated by a mechanism that overcomes the "synaptic circuit break" operated by endocannabinoids on GABAergic transmission

Lack of PSD-95 drives hippocampal neuronal cell death through activation of an \u3b1CaMKII transduction pathway

The PSD-95 protein family organizes the glutamatergic postsynaptic density and it is involved in the regulation of the excitatory signal at central nervous system synapses. We show here that PSD-95 deficiency by means of antisense oligonucleotides induces significant neuronal cell death within 24 h both in primary hippocampal cultures and in organotypic hippocampal slices. On the other hand, cultured cortical neurons are spared by PSD-95 antisense toxicity until they reach a NR2A detectable protein level (24 days in vitro). The neurotoxic event is characterized by increased \u3b1CaMKII association to NR2 regulatory subunits of NMDA receptor complex. As a direct consequence of \u3b1CaMKII association, we found increased GluR1 delivery to cell surface in cultured hippocampal neurons paralleled by AMPA-dependent increase in [Na+]I levels. In addition, both CaMKII specific inhibitor)KN-93 and AMPA receptor antagonists CNQX and NBQX rescued neuronal survival to control values. On the other hand, both the NMDA channel blocker MK-801 and Dantrolene, an inhibitor of calcium release from ryanodine-sensitive endoplasmic reticulum stores, failed to have any effect on neuronal survival in PSD-95 deficient neurons. Thus, our data provide clues that PSD-95 reduced expression in neurons is responsible for neuronal vulnerability mediated by direct activation of \u3b1CaMKII transduction pathway in the postsynaptic compartment

Activation of the histaminergic H3 receptor induces phosphorylation of the Akt/GSK-3 beta pathway in cultured cortical neurons and protects against neurotoxic insults

Stimulation of histamine

(+)-MCPG induces PKC\u3b5 translocation in cortical synaptosomes through a PLD-coupled mGluR

We have tested whether different agonists of metabotropic glutamate receptors could induce translocation of selective protein kinase C isozymes in nerve terminals. In rat cortical synaptosomes 1S,3R-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD; 100 \u3bcM) induced an increase in translocation to 124.6 \ub1 5.7% of basal unstimulated conditions of the Ca++-independent protein kinase C\u3b5, but not of the Ca++-dependent isozyme \u3b2. This effect was counteracted by 1-aminoindan-1,5-dicarboxylic acid (100 \u3bcM), an antagonist of metabotropic glutamate receptor 1. On the other hand, (+)-\u3b1-methyl-4-carboxyphenylglycine [(+)-MCPG], an antagonist of metabotropic glutamate receptors group I and II, did not antagonize the effect of 1S,3R-ACPD, and per se induced a translocation of protein kinase C\u3b5 of 164 \ub1 17.7% of basal unstimulated conditions. Because the (+)-MCPG induction of protein kinase C\u3b5 translocation was not antagonized by 1-aminoindan-1,5-dicarboxylic acid, it is suggested that 1S,3R-ACPD and (+)-MCPG activate this signal transduction pathway through distinct membrane receptors. Indeed (2-[2\u2033-carboxy-3'-phenylcyclopropyl]glycine)-13 (300 nM), a new compound known to antagonize metabotropic glutamate receptors coupled to phospholipase D, was able to antagonize protein kinase C\u3b5 translocation induced by (+)-MCPG. Moreover (+)-MCPG directly induced phospholipase D activity, measured as [3H]phosphoethanol production in cortical synaptosomes. These data suggest that in cortical nerve terminals (i) distinct metabotropic glutamate receptors, coupled to different signal transduction pathways, are present, (ii) (+)-MCPG is able to induce protein kinase C\u3b5 translocation, and that (iii) a metabotropic glutamate receptor associated to phospholipase D might influence translocation of protein kinase C in a calcium-independent manner

Differential mechanisms of tolerance induced by NMDA and 3,5-dihydroxyphenylglycine (DHPG) preconditioning

We investigated the molecular events triggered by NMDA and 3,5-dihydroxyphenylglycine (DHPG) preconditioning, that lead to neuroprotection against excitotoxic insults (AMPA or oxygen and glucose deprivation) in rat organotypic hippocampal slices, with particular attention on glutamate receptors and on cannabinoid system. We firstly evaluated the protein expression of NMDA and AMPA receptor subunits after preconditioning using western blot analysis performed in post-synaptic densities. We observed that following NMDA, but not DHPG preconditioning, the expression of GluA1 was significantly reduced and this reduction appeared to be associated with the internalization of AMPA receptors. Whole-cell voltage clamp recordings on CA1 pyramidal neurons of organotypic slices show that 24 hr after exposure to NMDA and DHPG preconditioning, AMPA-induced currents were significantly reduced. To clarify the mechanisms induced by DHPG preconditioning, we then investigated the involvement of the endocannabinoid system. Exposure of slices to the CB1 antagonist AM251 prevented the development of tolerance to AMPA toxicity induced by DHPG but not NMDA. Accordingly, the MAG-lipase inhibitor URB602, that increases arachidonoylglycerol (2-AG) content, but not the FAAH inhibitor URB597, that limits the degradation of anandamide, was also able to induce tolerance versus AMPA and OGD toxicity, suggesting that 2-AG is responsible for the DHPG-induced tolerance. In conclusion, preconditioning with NMDA or DHPG promotes differential neuroprotective mechanisms: NMDA by internalization of GluA1-AMPA receptors, DHPG by producing the endocannabinoid 2-AG. (Figure presented.)