Long-lasting Long-term Potentiation Research Articles

A High-Efficiency Protein Transduction System Demonstrating the Role of PKA in Long-Lasting Long-Term Potentiation

Proteins and peptides have been demonstrated to penetrate across the plasma membrane of eukaryotic cells by protein transduction domains. We show that protein transduction by 11 arginine (11R) is an efficient method of delivering proteins into the neurons of brain slices. Here, we demonstrate that PKA inhibitory peptide, fused with 11R and nuclear localization signal, delivers the peptide exclusively into the nuclear compartment of neurons in brain slices. This inhibitory peptide blocked both cAMP responsive element-binding protein phosphorylation and long-lasting long-term potentiation (LTP) induction, but not early LTP. These results highlight transduction of proteins and peptides into specific neuronal subcellular compartments in brain slices as a powerful tool for studying neuronal plasticity.

Arg3.1/Arc mRNA Induction by Ca2+and cAMP Requires Protein Kinase A and Mitogen-Activated Protein Kinase/Extracellular Regulated Kinase Activation

Long-term potentiation (LTP) is a cellular model for persistent synaptic plasticity in the mammalian brain. Like several forms of memory, long-lasting LTP requires cAMP-mediated activation of protein kinase A (PKA) and is dependent on gene transcription. Consequently, activity-dependent genes such as c-fos that contain cAMP response elements (CREs) in their 5' regulatory region have been studied intensely. More recently, arg3.1/arc became of interest, because after synaptic stimulation, arg3.1/arc mRNA is rapidly induced and distributed to dendritic processes and may be locally translated there to facilitate synapse-specific modifications. However, to date nothing is known about the signaling mechanisms involved in the induction of this gene. Here we report that arg3.1/arc is robustly induced with LTP stimulation even at intensities that are not sufficient to activate c-fos expression. Unlike c-fos, the 5' regulatory region of arg3.1/arc does not contain a CRE consensus sequence and arg3.1/arc is unresponsive to cAMP in NIH3T3 and Neuro2a cells. However, in PC12 cells and primary cultures of hippocampal neurons, arg3.1/arc can be induced by cAMP and calcium. This induction requires the activity of PKA and mitogen-activated protein kinase, suggesting a neuron-specific pathway for the activation of arg3.1/arc expression.

Deficient long-term memory and long-lasting long-term potentiation in mice with a targeted deletion of neurotrophin-4 gene.

We examined the learning and memory of neurotrophin-4 (NT4)-/- mice by using fear conditioning. In both cue and context conditioning, we found significant deficits in the NT4 mutants at 2 and 24 h after training but not at 30 min. Hippocampal slices from the mutant mice showed normal basal synaptic transmission, short-term plasticity, and decremental long-term potentiation (LTP) at the Schaffer collateral-CA1 synapses. These findings, together with the normal short-term memory, suggest that the hippocampal development of NT4-/- mice is largely unaffected. However, consistent with the long-term memory defects, the long-lasting LTP at the same synapses was attenuated significantly in the mutant mice. Our results suggest that NT4 plays a physiological role essential for hippocampus- and amygdala-dependent long-term memory and hippocampal long-lasting LTP and that NT4 may be useful in the therapy of acquired disorders of learning and memory.

Identification and cataloging of genes induced by long-lasting long-term potentiation in awake rats.

Maintenance of long-term potentiation (LTP) requires de novo gene expression. Here we report the direct isolation, using PCR-differential display, of genes whose expression level was altered after induction of long-lasting LTP in the hippocampus of freely moving awake rats. Differential display using 480 primer combinations revealed 17 cDNA bands that showed a reproducible change in expression level. These cDNAs represented at least 10 different genes (termed RM1-10), all of which showed up-regulation at 75 min after LTP induction and a return to basal expression levels within 24 h. Three of these genes were known only from expressed sequence tags (RM1-3), two were known genes whose up-regulation by LTP has not been described (GADD153/CHOP and ler5), and five were known genes whose up-regulation by LTP has already been reported (MAPK phosphatase, NGFI-A/zif268, vesl-1S/homer-1a, Ag2, and krox-20). We characterized the expression profiles of genes in the two former categories with respect to NMDA receptor dependency, tissue specificity, and developmental regulation using northern blotting and semiquantitative RT-PCR. The up-regulation of all five of these genes was NMDA receptor-dependent and correlated with the persistence of LTP, suggesting that these genes may play functional roles in prolonged LTP maintenance.

Long-term potentiation in the reciprocal corticohippocampal and corticocortical pathways in the chronically implanted, freely moving rat.

The hippocampus and adjacent cortical structures, including the entorhinal, perirhinal, and parahippocampal cortices, appear to serve as an integrated memory system. This extended hippocampal system is believed to influence memory and consolidation through an extensive set of reciprocal connections with widespread areas of the neocortex. Long-term potentiation (LTP) has been well-examined in the intrinsic connections of the hippocampus and neocortex. However, LTP in the pathways and structures thought to convey information between the hippocampus and neocortex has received little attention. If these pathways and structures are involved in information storage, and if LTP reflects a general synaptic encoding mechanism, then these systems are also likely to support LTP. In this paper we discuss a series of experiments aimed at investigating LTP in the efferents between the hippocampus and neocortex in chronically implanted animals. In the first experiment, the efferents of the perirhinal cortex were stimulated. LTP in the dentate gyrus (DG) reached asymptote more slowly than is typically seen following perforant path stimulation, whereas the frontal area (M1) reached asymptote more quickly than reported following corticocortical stimulation. The DG and M1 LTP was long-lasting, but entorhinal cortex LTP had decayed to baseline levels after a week. In the second experiment, the hippocampal efferents were stimulated. The perirhinal, entorhinal, and frontal cortex showed a similar slow potentiation, with only the perirhinal cortex levels returning to baseline after a week. In the third experiment, the projections from M1 were tested. The perirhinal cortex and hippocampus showed a long-lasting LTP. Although LTP was found in all pathways examined, there were differences in the induction and decay rate, and these properties may correspond to differences in learning rate and longevity of information storage.

Rolipram, a type IV-specific phosphodiesterase inhibitor, facilitates the establishment of long-lasting long-term potentiation and improves memory.

In an attempt to improve behavioral memory, we devised a strategy to amplify the signal-to-noise ratio of the cAMP pathway, which plays a central role in hippocampal synaptic plasticity and behavioral memory. Multiple high-frequency trains of electrical stimulation induce long-lasting long-term potentiation, a form of synaptic strengthening in hippocampus that is greater in both magnitude and persistence than the short-lasting long-term potentiation generated by a single tetanic train. Studies using pharmacological inhibitors and genetic manipulations have shown that this difference in response depends on the activity of cAMP-dependent protein kinase A. Genetic studies have also indicated that protein kinase A and one of its target transcription factors, cAMP response element binding protein, are important in memory in vivo. These findings suggested that amplification of signals through the cAMP pathway might lower the threshold for generating long-lasting long-term potentiation and increase behavioral memory. We therefore examined the biochemical, physiological, and behavioral effects in mice of partial inhibition of a hippocampal cAMP phosphodiesterase. Concentrations of a type IV-specific phosphodiesterase inhibitor, rolipram, which had no significant effect on basal cAMP concentration, increased the cAMP response of hippocampal slices to stimulation with forskolin and induced persistent long-term potentiation in CA1 after a single tetanic train. In both young and aged mice, rolipram treatment before training increased long- but not short-term retention in freezing to context, a hippocampus-dependent memory task.

Induction of CRE-Mediated Gene Expression by Stimuli That Generate Long-Lasting LTP in Area CA1 of the Hippocampus

Gene expression regulated by the cAMP response element (CRE) has been implicated in synaptic plasticity and long-term memory. It has been proposed that CRE-mediated gene expression is stimulated by signals that induce long-term potentiation (LTP). To test this hypothesis, we made mice transgenic for a CRE-regulated reporter construct. We focused on long-lasting long-term potentiation (L-LTP), because it depends on cAMP-dependent protein kinase activity (PKA) and de novo gene expression. CRE-mediated gene expression was markedly increased after L-LTP, but not after decremental LTP (D-LTP). Furthermore, inhibitors of PKA blocked L-LTP and associated increases in CRE-mediated gene expression. These data demonstrate that the signaling required for the generation of L-LTP but not D-LTP is sufficient to stimulate CRE-mediated transcription in the hippocampus.

Metabotropic glutamate receptors in hippocampal long-term potentiation and learning and memory.

Glutamate receptors have been identified as important interfaces in learning and memory paradigms as well as in mechanisms of synaptic plasticity, such as long-term potentiation (LTP) and long-term depression (LTD), which are believed to be the underlying cellular basis of at least some forms of learning. Although investigations of G-protein-coupled receptors have a long history, those depending on ligand-binding of glutamate have only been discovered recently, and this is the reason why our knowledge about metabotropic glutamate receptors (mGluRs) is at present very limited. However, the development of relatively specific antagonists and agonists has enabled the analysis of the role of mGluRs in synaptic plasticity, mostly studied on the models of LTP and LTD. Among others, we have been able to demonstrate that activation of mGluRs is essential for induction and maintenance of long-lasting hippocampal LTP in vitro and in vivo. The work conducted by several groups, including ours, has now provided compelling evidence that mGluR activation is an important step in the cellular cascades leading to memory formation in vertebrates. This led us to assume, given that the hippocampus plays a prominent role in spatial rather than discrimination learning, that mGluRs may participate in the processing of spatial information via hippocampal mechanisms, and may thus be similarly important as N-methyl-D-aspartate receptors. This article surveys the literature dealing with mGluRs in hippocampal LTP and learning and memory. We will demonstrate that, although the understanding of cellular mechanisms of neuronal plasticity and of the pharmacology of learning and memory has advanced, the missing link to prove that LTP is a substrate for some form forms of learning still remains unsolved. Nevertheless, it appears reasonable to argue that mGluRs in LTP and learning may share some, but not all features, and it will be an interesting approach for further analysis to address the unresolved issues.

A different form of long-lasting potentiation revealed in tissue plasminogen activator mutant mice

The establishment of hippocampal long-term (LTP) requires protein and mRNA synthesis, suggesting that neuronal activity resulting in LTP initiates a cascade of gene expression. The expression of the gene for the extracellular serine protease tissue plasminogen activator (t-PA) is induced during LTP. Here we analyze long-lasting LTP (L-LTP,>4 hr)in CA1 hippocampal slices of mice homozygous for disrupted t-PA genes. Although mutant mice appear to exhibit long-term potentiation, we provide evidence that these mice are devoid of conventional homosynaptic L-LTP at the Schaffer collateral-CA1 pyramidal cell synapse. Most remarkably, t-PA-deficient mice exhibit a different form of long-lasting potentiation that is characterized by an NMDA receptor-dependent modification of GABA transmission in the CA1 region.

Postsynaptic, but not presynaptic, activity controls the early time course of long-term potentiation in the dentate gyrus

The early time course (less than 1 hr) of long-term potentiation (LTP) in the dentate gyrus of the guinea pig hippocampal slice was examined using extracellular recordings from the outer two-thirds of the dendritic layer. LTP was induced by a single brief (2-40 impulses) high-frequency (20-400 Hz) train, or by pairing a single test stimulus with a brief heterosynaptic high-frequency train. The induction of LTP was facilitated by blockade of fast GABAergic postsynaptic inhibition. It was found that, irrespective of induction conditions and the amount of LTP induced, the onset of LTP was characterized by a latency of a few seconds following the induction event, and a rapid 30 sec growth phase. After a 1-2 min period of little or no further growth, LTP decayed but in a highly variable manner, from cases in which more than 60% of the peak value remained 1 hr after the induction to cases in which LTP decayed completely within 10 min. Factors increasing presynaptic activity (frequency or number of afferent stimulations) during the induction event did not affect the relative amount of LTP decay. Repetitive presynaptic activity was found not to be a necessary condition for eliciting long-lasting LTP (greater than 1 hr), as shown by experiments in which a single presynaptic impulse was paired with a brief heterosynaptic train. Factors increasing postsynaptic activity during the induction event, such as increased stimulus intensity, temporal pairing of two weak trains, or reduced postsynaptic inhibition, all reduced the relative amount of LTP decay. Moreover, partial pharmacological blockade of NMDA receptor channels increased the relative amount of decay. In conclusion, the amount of postsynaptic activity and associated NMDA receptor activation during the induction event appeared to be the main factor governing the early stability of LTP in the dentate gyrus.

Expression of c- fos and other genes encoding transcription factors in long-term potentiation

The genes encoding transcription factors are known to be induced in many different biological phenomena of transition from one long-lasting state of cell functioning to another. It is widely believed that transcription factors control this transition by regulating the expression of other genes. Recently, several reports on gene expression after the induction of long-term potentiation (LTP) have been published. In particular, the c-fos nuclear protooncogene, encoding the Fos transcription factor, has been extensively investigated. However, the results of those studies were seemingly contradictory. The present commentary reviews available data in an attempt to resolve the apparent contradictions, showing that long-lasting LTP (i.e., lasting longer than a few hours) may involve c-fos expression, while shorter LTP may not.

Benzal glyceryl acetal

Fetal nicotine exposure from smoking during pregnancy causes long-lasting cognitive impairments in offspring, yet little is known about the mechanisms that underlie this effect. Here we demonstrate that early postnatal exposure of mouse pups to nicotine via maternal milk impairs long-term, but not short-term, hippocampus-dependent memory during adolescence. At the Schaffer collateral (SC) pathway, the most widely studied synapses for a cellular correlate of hippocampus-dependent memory, the induction of N-methyl-D-aspartate receptor-dependent transient long-term potentiation (LTP) and protein synthesis-dependent long-lasting LTP are not diminished by nicotine exposure, but rather unexpectedly the threshold for LTP induction becomes lower after nicotine treatment. Using voltage sensitive dye to visualize hippocampal activity, we found that early postnatal nicotine exposure also results in enhanced CA1 depolarization and hyperpolarization after SC stimulation. Furthermore, we show that postnatal nicotine exposure induces pervasive changes to the nicotinic modulation of CA1 activity: activation of nicotinic receptors no longer increases CA1 network depolarization, acute nicotine inhibits rather than facilitates the induction of LTP at the SC pathway by recruiting an additional nicotinic receptor subtype, and acute nicotine no longer blocks LTP induction at the temporoammonic pathway. These findings reflect the pervasive impact of nicotine exposure during hippocampal development, and demonstrate an association of hippocampal memory impairments with altered nicotinic cholinergic modulation of LTP, but not impaired LTP. The implication of our results is that nicotinic cholinergic-dependent plasticity is required for long-term memory formation and that postnatal nicotine exposure disrupts this form of plasticity.

Monographs on fragrance raw materials: Ethyl cinnamate

Fetal nicotine exposure from smoking during pregnancy causes long-lasting cognitive impairments in offspring, yet little is known about the mechanisms that underlie this effect. Here we demonstrate that early postnatal exposure of mouse pups to nicotine via maternal milk impairs long-term, but not short-term, hippocampus-dependent memory during adolescence. At the Schaffer collateral (SC) pathway, the most widely studied synapses for a cellular correlate of hippocampus-dependent memory, the induction of N-methyl-D-aspartate receptor-dependent transient long-term potentiation (LTP) and protein synthesis-dependent long-lasting LTP are not diminished by nicotine exposure, but rather unexpectedly the threshold for LTP induction becomes lower after nicotine treatment. Using voltage sensitive dye to visualize hippocampal activity, we found that early postnatal nicotine exposure also results in enhanced CA1 depolarization and hyperpolarization after SC stimulation. Furthermore, we show that postnatal nicotine exposure induces pervasive changes to the nicotinic modulation of CA1 activity: activation of nicotinic receptors no longer increases CA1 network depolarization, acute nicotine inhibits rather than facilitates the induction of LTP at the SC pathway by recruiting an additional nicotinic receptor subtype, and acute nicotine no longer blocks LTP induction at the temporoammonic pathway. These findings reflect the pervasive impact of nicotine exposure during hippocampal development, and demonstrate an association of hippocampal memory impairments with altered nicotinic cholinergic modulation of LTP, but not impaired LTP. The implication of our results is that nicotinic cholinergic-dependent plasticity is required for long-term memory formation and that postnatal nicotine exposure disrupts this form of plasticity.