LTP In Dentate Gyrus Research Articles

Analysis of the presynaptic signaling mechanisms underlying the inhibition of LTP in rat dentate gyrus by the tyrosine kinase inhibitor, genistein.

A great deal of recent evidence points to a role for tyrosine kinase in expression of LTP. Data have been presented that are consistent with the idea that tyrosine phosphorylation of proteins occurs in both the presynaptic and postsynaptic areas. In this study, we set out to investigate the role that tyrosine kinase might play presynaptically to modulate release of glutamate in an effort to understand the mechanism underlying the persistent increase in release that accompanies LTP in perforant path-granule cell synapses. We report that LTP was associated with increased calcium influx and glutamate release. LTP was also associated with an increase in phosphorylation of the alpha-subunit of calcium channels and ERK in synaptosomes prepared from dentate gyrus, and these effects were inhibited when LTP was blocked by the tyrosine kinase inhibitor, genistein. LTP was accompanied by increased protein synthesis and increased phosphorylation of CREB in entorhinal cortex, effects that were also blocked by genistein. We conclude that tetanic stimulation leads to enhanced tyrosine phosphorylation of certain presynaptically located proteins that modulate glutamate release and contribute to expression of LTP.

Regulated transcription of the immediate-early gene Zif268: mechanisms and gene dosage-dependent function in synaptic plasticity and memory formation.

The immediate-early gene Zif268 is a member of the Egr family of inducible transcription factors. Data from gene expression studies have suggested that this gene may play a critical role in initial triggering of the genetic machinery that has long been considered a necessary mechanism for maintenance of the later phases of LTP and also for the consolidation or stabilization of long-lasting memories. Until recently, however, the data supporting this assumption have been based primarily on circumstantial evidence, with no direct evidence to suggest that Zif268 is required for long-lasting synaptic plasticity and memory. In this report, we review our own data using Zif268 mutant mice; we show that although the early phase of dentate gyrus LTP is normal in these mice, the later phases are not present, and the ability of the mice to maintain learned information over a 24-h period is deficient. In addition, we present new information showing a task-dependent gene dosage effect in Zif268 heterozygous mice. We show that spatial learning is particularly sensitive to reduced levels of Zif268, as one-half of the complement of Zif268 in heterozygous mice is insufficient to maintain spatial long-term memories.

Neuropeptide Y inhibits glutamate release and long-term potentiation in rat dentate gyrus

The effect of intracerebroventricular injection of neuropeptide Y (NPY) was assessed on LTP in dentate gyrus. We report that NPY attenuated LTP and inhibited KCl-induced glutamate release in synaptosomes prepared from dentate gyrus. Activity of the stress-activated kinase, c-Jun NH2-terminal kinase (JNK) in synaptosomes was increased by incubation with NPY or following intracerebroventricular injection. Activation of JNK might underlie the inhibitory effect of NPY on LTP.

Enhanced Hippocampal CA1 LTP but Normal Spatial Learning in Inositol 1,4,5-trisphosphate 3-kinase(A)-Deficient Mice

To define the physiological role of IP(3)3-kinase(A) in vivo, we have generated a mouse strain with a null mutation of the IP(3)3-kinase(A) locus by gene targeting. Homozygous mutant mice were fully viable, fertile, apparently normal, and did not show any morphological anomaly in brain sections. In the mutant brain, the IP4 level was significantly decreased whereas the IP3 level did not change, demonstrating a major role of IP(3)3-kinase(A) in the generation of IP4. Nevertheless, no significant difference was detected in the hippocampal neuronal cells of the wild-type and the mutant mice in the kinetics of Ca2+ regulation after glutamate stimulation. Electrophysiological analyses carried out in hippocampal slices showed that the mutation significantly enhanced the LTP in the hippocampal CA1 region, but had no effect on the LTP in dentate gyrus (DG). No difference was noted, however, between the mutant and the wild-type mice in the Morris water maze task. Our results indicate that IP(3)3-kinase(A) may play an important role in the regulation of LTP in hippocampal CA1 region through the generation of IP4, but the enhanced LTP in the hippocampal CA1 does not affect spatial learning and memory.

Deficits in Memory Tasks of Mice with CREB Mutations Depend on Gene Dosage

Studies in Aplysia, Drosophila, and mice have shown that the transcription factor CREB is involved in formation and retention of long-term memory. To analyze the impact of differential CREB levels on learning and memory, we varied the gene dosage of CREB in two strains of mutant mice: (1) CREBalphadelta mice, in which the alpha and delta isoforms are disrupted, but a third isoform beta is strongly up-regulated; (2) CREBcomp, a compound strain with one alphadelta allele and one CREBnull allele in which all CREB isoforms are disrupted. To minimize genetic background effects, CREB mutations were backcrossed into a C57BL/6 and a FVB/N strain, respectively, and studies were performed in F1 hybrids from these lines. CREBcomp but not CREBalphadelta F1 hybrids were impaired in water maze learning and fear conditioning, demonstrating a CREB gene dosage effect. However, analysis of the platform searching strategies in the water maze task suggested that CREBcomp mutants are impaired in behavioral flexibility rather than in spatial memory. In contrast to previous experiments using CREBalphadelta mice with different genetic background, the F1 hybrid CREBalphadelta and CREBcomp mice did not show deficits in a social transmission of food preference task nor in dentate gyrus and CA1 LTP as recorded from slice preparations. These data indicate that the hybrid vigor typical for F1 hybrids may compensate for a reduction in CREB levels in some tests. On the other hand, the persistence of clear behavioral deficits as shown by the F1 hybrid CREBcomp mice in water maze and fear conditioning indicates a robust and repeatable phenomenon that will permit further functional analysis of CREB.

Analysis of the interaction between arachidonic acid and metabotropic glutamate receptor activation reveals that phospholipase C acts as a coincidence detector in the expression of long‐term potentiation in the rat dentate gyrus

We have reported that arachidonic acid and the metabotropic glutamate receptor agonist, trans-1-amino-cyclopentyl-1,3-dicarboxylate (ACPD), act in synergy to increase release of glutamate from synaptosomes prepared from rat dentate gyrus. The observation that prior induction of LTP in perforant path-granule cell synapses occluded this synergism suggested that the interaction between arachidonic acid and ACPD might trigger the increase in glutamate release that accompanies LTP in dentate gyrus. Our objective was to identify the mechanism underlying the synergism between arachidonic acid and ACPD in LTP. The data indicate that both agents activate phospholipase C(PLC); the arachidonic acid-induced increase in phospholipase C activation was inhibited by the tyrosine kinase inhibitor, genistein, suggesting that PLCγ, which is stimulated by tyrosine phosphorylation may be activated by arachidonic acid. The ACPD-induced increase was inhibited by neomycin, indicating the involvement of a G-protein and suggesting that PLCβ may be activated by ACPD. We report that arachidonic acid stimulated phosphorylation of the specific tyrosine kinase substrate, poly(Glu80,Tyr20) and direct analysis indicated that arachidonic acid increased phosphorylation of PLCγ. PLCγ phosphorylation was assessed in control dentate gyrus and dentate gyrus in which LTP was induced in vivo.We report that the tyrosine kinase inhibitor, genistein, blocked expression of LTP and also blocked the associated increase in phosphorylation of PLCγ. The data presented here indicate that tyrosine phosphorylation of PLCγ was significantly enhanced following induction of LTP, but in separate experiments, in which LTP was inhibited by intraventricular injection of genistein, phosphorylation of PLCγ was inhibited. The evidence presented is consistent with the hypothesis that PLC acts as a coincidence detector in LTP. The data indicate that PLCβ is activated by ACPD, PLCγ is activated by arachidonic acid, and coincident activation of both isoforms is necessary to stimulate an increase in glutamate release. Hippocampus 1998;8:48–56. © 1998 Wiley-Liss, Inc.

Analysis of the interaction between arachidonic acid and metabotropic glutamate receptor activation reveals that phospholipase C acts as a coincidence detector in the expression of long-term potentiation in the rat dentate gyrus.

We have reported that arachidonic acid and the metabotropic glutamate receptor agonist, trans-1-amino-cyclopentyl-1,3-dicarboxylate (ACPD), act in synergy to increase release of glutamate from synaptosomes prepared from rat dentate gyrus. The observation that prior induction of LTP in perforant path-granule cell synapses occluded this synergism suggested that the interaction between arachidonic acid and ACPD might trigger the increase in glutamate release that accompanies LTP in dentate gyrus. Our objective was to identify the mechanism underlying the synergism between arachidonic acid and ACPD in LTP. The data indicate that both agents activate phospholipase C(PLC); the arachidonic acid-induced increase in phospholipase C activation was inhibited by the tyrosine kinase inhibitor, genistein, suggesting that PLCgamma, which is stimulated by tyrosine phosphorylation may be activated by arachidonic acid. The ACPD-induced increase was inhibited by neomycin, indicating the involvement of a G-protein and suggesting that PLCbeta may be activated by ACPD. We report that arachidonic acid stimulated phosphorylation of the specific tyrosine kinase substrate, poly(Glu80,Tyr20) and direct analysis indicated that arachidonic acid increased phosphorylation of PLCgamma. PLCgamma phosphorylation was assessed in control dentate gyrus and dentate gyrus in which LTP was induced in vivo. We report that the tyrosine kinase inhibitor, genistein, blocked expression of LTP and also blocked the associated increase in phosphorylation of PLCgamma. The data presented here indicate that tyrosine phosphorylation of PLCgamma was significantly enhanced following induction of LTP, but in separate experiments, in which LTP was inhibited by intraventricular injection of genistein, phosphorylation of PLCgamma was inhibited. The evidence presented is consistent with the hypothesis that PLC acts as a coincidence detector in LTP. The data indicate that PLCbeta is activated by ACPD, PLCgamma is activated by arachidonic acid, and coincident activation of both isoforms is necessary to stimulate an increase in glutamate release.

Effects of the novel NMDA antagonist, NPC 12626, on long-term potentiation, learning and memory

NPC 12626 (2-amino-4,5-(1,2-cyclohexyl)-7-phosphonoheptanoic acid), a newly developed drug which crosses the blood-brain barrier, is a competitive antagonist ofN-methyl- d-aspartate receptors. In Experiment I, the effects of NPC 12626 on perforant path - dentate gyrus LTP were tested. NPC 12626 (100 mg/kg, i.p.), injected 150 min prior to tetanization, prevented potentiation of the EPSP slope and population spike amplitude. EPSP-spike potentiation was also prevented. Post-tetanus administration was ineffective. In Experiment II, mice were injected with NPC 12626 (35 mg/kg, i.p.) or saline 35 min prior to spontaneous alternation testing. NPC 12626 significantly decreased alternation rates, but did not affect turn bias or the mean delay between arm entries. This pattern of results may reflect impaired learning or memory. In Experiment III, mice were tested on an inhibitory avoidance task. NPC 12626 (35 mg/kg, i.p.), administered before but not after training, significantly impaired performance. When the drug was administered before training as well as before testing, performance was similarly impaired, indicating that the observed deficits were not attributable to state-dependent learning. Pre-test injections were ineffective. Overall, these results support the hypothesis that some forms of learning require the participation of NMDA receptors and that this participation is largely limited to acquisition processes. In addition, these results point to the utility of peripherally administered NPC 12626 as a tool with which to examine the involvement of NMDA receptors in LTP and learning.