Late-phase Long-term Potentiation Research Articles

Chronic Psychosocial Stress Exacerbates Impairment of Synaptic Plasticity in β-Amyloid Rat Model of Alzheimers Disease: Prevention by Nicotine

Alzheimer's disease (AD) is a degenerative disorder that leads to progressive, irreversible cognitive decline. It develops as a result of over-production and aggregation of β-amyloid (Aβ) peptides in the brain. We have recently shown that stress exacerbates, while nicotine prevents long-term memory impairment induced by β-Amyloid. In this study, we evaluated the effect of chronic psychosocial stress on synaptic plasticity (Late-phase long-term potentiation; L-LTP, and long-term depression; LTD) in the β-Amyloid rat model of AD, and the positive impact of chronic nicotine treatment. Chronic psychosocial stress was induced by an intruder method. The Rat AD model was induced by 14-day i.c.v. osmotic pump infusion of a 1:1 mixture of 300 pmol/day Aβ1-40/Aβ1-42. The rats were treated with nicotine (2 mg/kg/day) for 6 weeks. In vivo electrophysiological recordings of L-LTP, and LTD in hippocampal area CA1 showed that chronic stress by itself did not affect L-LTP. However, it markedly aggravated the impairment of this response as well as LTD in Aβ- treated rats. The effects of Aβ and the combination of stress and Aβ were totally prevented by chronic nicotine treatment. Immunoblot analysis revealed that stress and/or Aβ significantly increased the basal levels of calcineurin and prevented the expected L-LTP-induced increase in CREB phosphorylation, and CaMKIV levels. These effects were not seen in Aβ- infused rats chronically treated with nicotine. The changes in synaptic plasticity-related molecules may explain the effects of stress and/or chronic nicotine on L-LTP in Aβ animals.

Rescue of synaptic plasticity and spatial learning deficits in the hippocampus of Homer1 knockout mice by recombinant Adeno-associated viral gene delivery of Homer1c

Homer1 belongs to a family of scaffolding proteins that interact with various post-synaptic density proteins including group I metabotropic glutamate receptors (mGluR1/5). Previous research in our laboratory implicates the Homer1c isoform in spatial learning. Homer1 knockout mice (H1-KO) display cognitive impairments, but their synaptic plasticity properties have not been described. Here, we investigated the role of Homer1 in long-term potentiation (LTP) in the hippocampal CA1 region of H1-KO mice in vitro. We found that late-phase LTP elicited by high frequency stimulation (HFS) was impaired, and that the induction and maintenance of theta burst stimulation (TBS) LTP were reduced in H1-KO. To test the hypothesis that Homer1c was sufficient to rescue these LTP deficits, we delivered Homer1c to the hippocampus of H1-KO using recombinant adeno-associated virus (rAAV). We found that rAAV-Homer1c rescued HFS and TBS-LTP in H1-KO animals. Next, we tested whether the LTP rescue by Homer1c was occurring via mGluR1/5. A selective mGluR5 antagonist, but not an mGluR1 antagonist, blocked the Homer1c-induced recovery of late-LTP, suggesting that Homer1c mediates functional effects on plasticity via mGluR5. To investigate the role of Homer1c in spatial learning, we injected rAAV-Homer1c to the hippocampus of H1-KO. We found that rAAV-Homer1c significantly improved H1-KO performance in the Radial Arm Water Maze. These results point to a significant role for Homer1c in synaptic plasticity and learning.

TrkB as a Potential Synaptic and Behavioral Tag

Late-phase long-term potentiation (L-LTP), a cellular model for long-term memory (LTM), requires de novo protein synthesis. An attractive hypothesis for synapse specificity of long-term memory is "synaptic tagging": synaptic activity generates a tag, which "captures" the PRPs (plasticity-related proteins) derived outside of synapses. Here we provide evidence that TrkB, the receptor of BDNF (brain-derived neurotrophic factor), may serve as a "synaptic tag." TrkB is transiently activated by weak theta-burst stimulation (TBS) that induces only early-phase LTP (E-LTP). This TrkB activation is independent of protein synthesis, and confined to stimulated synapses. Induction of L-LTP by strong stimulation in one synaptic pathway converts weak TBS-induced E-LTP to L-LTP in a second, independent pathway. Transient inhibition of TrkB around the time of weak TBS to the second pathway diminished L-LTP in that pathway without affecting the first one. Behaviorally, weak training, which induces short-term memory (STM) but not LTM, can be consolidated into LTM by exposing animals to novel but not familiar environment 1 h before training. Inhibition of TrkB during STM training blocked such consolidation. These results suggest TrkB as a potential tag for synapse-specific expression of L-LTP and LTM.

P.2.d.009 Repeated treatment with antidepressants and risperidone changed the reactivity of the central serotonin system in mice

Anisomycin is both a well-established protein synthesis inhibitor and a potent activator of the p38/JNK MAPK pathway. It has been used to block the late phase of long-term potentiation (LTP) and long-term depression (LTD) in hippocampus. In this study, we have found that anisomycin produces a time-dependent decline in the magnitude of the field EPSP (fEPSP) in acute brain slices of mouse primary visual cortex. This anisomycin-mediated fEPSP depression occludes NMDA receptor-dependent LTD induced by low-frequency stimulation (LFS). In contrast, two other protein synthesis inhibitors, emetine and cycloheximide, have no effect either on baseline synaptic transmission or on LTD. Moreover, the decline of the fEPSP caused by anisomycin can be rescued by the application of the p38 inhibitor SB203580 but not by the JNK inhibitor SP600125. These results indicate that activation of p38 MAPK by anisomycin induces LTD and subsequently occludes electrically induced LTD. Also, the occlusion of LFS-LTD by anisomycin suggests that common mechanisms may be shared between the two forms of synaptic depression. Consistent with this view, bath application of a membrane permeant peptide derived from the carboxyl tail of GluR2 subunit of AMPA receptor, which specifically blocks regulated AMPA receptor endocytosis, thereby preventing the expression of LFS-induced LTD, significantly reduced the anisomycin-induced decline of the fEPSP. In conclusion, our results indicate that anisomycin produces long-lasting depression of AMPA receptor-mediated synaptic transmission by activating p38 MAPK-mediated endocytosis of APMA receptors in mouse primary visual cortex.

Proteomics, Ultrastructure, and Physiology of Hippocampal Synapses in a Fragile X Syndrome Mouse Model Reveal Presynaptic Phenotype

Fragile X syndrome (FXS), the most common form of hereditary mental retardation, is caused by a loss-of-function mutation of the Fmr1 gene, which encodes fragile X mental retardation protein (FMRP). FMRP affects dendritic protein synthesis, thereby causing synaptic abnormalities. Here, we used a quantitative proteomics approach in an FXS mouse model to reveal changes in levels of hippocampal synapse proteins. Sixteen independent pools of Fmr1 knock-out mice and wild type mice were analyzed using two sets of 8-plex iTRAQ experiments. Of 205 proteins quantified with at least three distinct peptides in both iTRAQ series, the abundance of 23 proteins differed between Fmr1 knock-out and wild type synapses with a false discovery rate (q-value) <5%. Significant differences were confirmed by quantitative immunoblotting. A group of proteins that are known to be involved in cell differentiation and neurite outgrowth was regulated; they included Basp1 and Gap43, known PKC substrates, and Cend1. Basp1 and Gap43 are predominantly expressed in growth cones and presynaptic terminals. In line with this, ultrastructural analysis in developing hippocampal FXS synapses revealed smaller active zones with corresponding postsynaptic densities and smaller pools of clustered vesicles, indicative of immature presynaptic maturation. A second group of proteins involved in synaptic vesicle release was up-regulated in the FXS mouse model. In accordance, paired-pulse and short-term facilitation were significantly affected in these hippocampal synapses. Together, the altered regulation of presynaptically expressed proteins, immature synaptic ultrastructure, and compromised short-term plasticity points to presynaptic changes underlying glutamatergic transmission in FXS at this stage of development.

BDNF Facilitates L-LTP Maintenance in the Absence of Protein Synthesis through PKMζ

Late-phase long term potentiation (L-LTP) is thought to be the cellular basis for long-term memory (LTM). While LTM as well as L-LTP is known to depend on transcription and translation, it is unclear why brain-derived neurotrophic factor (BDNF) could sustain L-LTP when protein synthesis is inhibited. The persistently active protein kinase ζ (PKMζ) is the only molecule implicated in perpetuating L-LTP maintenance. Here, in mouse acute brain slices, we show that inhibition of PKMζ reversed BDNF-dependent form of L-LTP. While BDNF did not alter the steady-state level of PKMζ, BDNF together with the L-LTP inducing theta-burst stimulation (TBS) increased PKMζ level even without protein synthesis. Finally, in the absence of de novo protein synthesis, BDNF maintained TBS-induced PKMζ at a sufficient level. These results suggest that BDNF sustains L-LTP through PKMζ in a protein synthesis-independent manner, revealing an unexpected link between BDNF and PKMζ.

Chronic psychosocial stress accelerates impairment of long‐term memory and late‐phase long‐term potentiation in an at‐risk model of Alzheimer's disease

Although it is generally agreed that Aβ contributes to the pathogenesis of AD, its precise role in AD and the reason for the varying intensity and time of onset of the disease have not been elucidated. In addition to genetic factors, environmental issues such as stress may also play a critical role in the etiology of AD. This study examined the effect of chronic psychosocial stress in an at-risk (treatment with a subpathogenic dose of Aβ; "subAβ") rat model of AD on long-term memory by three techniques: memory tests in the radial arm water maze, electrophysiological recordings of synaptic plasticity in anesthetized rats, and immunoblot analysis of learning- and long-term memory-related signaling molecules. Chronic psychosocial stress was induced using a rat intruder model. The subAβ rat model of AD was induced by continuous infusion of 160 pmol/day Aβ(1-42) via a 14-day i.c.v. osmotic pump. All tests showed that subAβ rats were not different from control rats. Result from behavioral tests and electrophysiological recordings showed that infusion of subAβ in chronically stressed rats (stress/subAβ group) caused significant impairment of cognitive functions and late-phase long-term potentiation (L-LTP). Molecular analysis of various signaling molecules after expression of L-LTP, revealed an increase in the levels of p-CREB in control, stress, and subAβ rats, but not in the stress/subAβ rats. These findings suggest that the chronic stress-induced molecular alteration may accelerate the impairment of cognition and synaptic plasticity in individuals "at-risk" for AD.

MRNA binding protein staufen 1-dependent regulation of pyramidal cell spine morphology via NMDA receptor-mediated synaptic plasticity

Staufens (Stau) are RNA-binding proteins involved in mRNA transport, localization, decay and translational control. The Staufen 1 (Stau1) isoform was recently identified as necessary for the protein synthesis-dependent late phase long-term potentiation (late-LTP) and for the maintenance of mature dendritic spines and synaptic activity in hippocampal CA1 pyramidal cells, strongly suggesting a role of mRNA regulation by Stau1 in these processes. However, the causal relationship between these impairments in synaptic function (spine shape and basal synaptic activity) and plasticity (late-LTP) remains unclear. Here, we determine that the effects of Stau1 knockdown on spine shape and size are mimicked by blocking NMDA receptors (or elevating extracellular Mg2+) and that Stau1 knockdown in the presence of NMDA receptor blockade (or high Mg2+) has no further effect on spine shape and size. Moreover, the effect of Stau1 knockdown on late-LTP cannot be explained by these effects, since when tested in normal medium, slice cultures that had been treated with high Mg2+ (to impair NMDA receptor function) in combination with a control siRNA still exhibited late-LTP, while siRNA to Stau1 was still effective in blocking late-LTP. Our results indicate that Stau1 involvement in spine morphogenesis is dependent on ongoing NMDA receptor-mediated plasticity, but its effects on late-LTP are independent of these changes. These findings clarify the role of Stau1-dependent mRNA regulation in physiological and morphological changes underlying long-term synaptic plasticity in pyramidal cells.

Astrocyte-Derived Adenosine and A1Receptor Activity Contribute to Sleep Loss-Induced Deficits in Hippocampal Synaptic Plasticity and Memory in Mice

Sleep deprivation (SD) can have a negative impact on cognitive function, but the mechanism(s) by which SD modulates memory remains unclear. We have previously shown that astrocyte-derived adenosine is a candidate molecule involved in the cognitive deficits following a brief period of SD (Halassa et al., 2009). In this study, we examined whether genetic disruption of soluble N-ethylmaleimide-sensitive factor attached protein (SNARE)-dependent exocytosis in astrocytes (dnSNARE mice) or pharmacological blockade of A1 receptor signaling using an adenosine A1 receptor (A1R) antagonist, 8-cyclopentyl-1,3-dimethylxanthine (CPT), could prevent the negative effects of 6 h of SD on hippocampal late-phase long-term potentiation (L-LTP) and hippocampus-dependent spatial object recognition memory. We found that SD impaired L-LTP in wild-type mice but not in dnSNARE mice. Similarly, this deficit in L-LTP resulting from SD was prevented by a chronic infusion of CPT. Consistent with these results, we found that hippocampus-dependent memory deficits produced by SD were rescued in dnSNARE mice and CPT-treated mice. These data provide the first evidence that astrocytic ATP and adenosine A1R activity contribute to the effects of SD on hippocampal synaptic plasticity and hippocampus-dependent memory, and suggest a new therapeutic target to reverse the hippocampus-related cognitive deficits induced by sleep loss.

Sleep deprivation prevents stimulation-induced increases of levels of P-CREB and BDNF: Protection by caffeine

It is well known that caffeine and sleep deprivation have opposing effects on learning and memory; therefore, this study was undertaken to determine the effects of chronic (4 wks) caffeine treatment (0.3 g/l in drinking water) on long-term memory deficit associated with 24 h sleep deprivation. Animals were sleep deprived using the modified multiple platform method. The results showed that chronic caffeine treatment prevented the impairment of long-term memory as measured by performance in the radial arm water maze task and normalized L-LTP in area CA1 of the hippocampi of sleep-deprived anesthetized rats. Sleep deprivation prevents the high frequency stimulation-induced increases in the levels of phosphorylated-cAMP response element binding protein (P-CREB) and brain-derived neurotrophic factor (BDNF) seen during the expression of late phase long-term potentiation (L-LTP). However, chronic caffeine treatment prevented the effect of sleep-deprivation on the stimulated levels of P-CREB and BDNF. The results suggest that chronic caffeine treatment may protect the sleep-deprived brain probably by preserving the levels of P-CREB and BDNF.

The Dendritic Branch Is the Preferred Integrative Unit for Protein Synthesis-Dependent LTP

The late-phase of long-term potentiation (L-LTP), the cellular correlate of long-term memory, induced at some synapses facilitates L-LTP expression at other synapses receiving stimulation too weak to induce L-LTP by itself. Using glutamate uncaging and two-photon imaging, we demonstrate that the efficacy of this facilitation decreases with increasing time between stimulations, increasing distance between stimulated spines and with the spines being on different dendritic branches. Paradoxically, stimulated spines compete for L-LTP expression if stimulated too closely together in time. Furthermore, the facilitation is temporally bidirectional but asymmetric. Additionally, L-LTP formation is itself biased toward occurring on spines within a branch. These data support the Clustered Plasticity Hypothesis, which states that such spatial and temporal limits lead to stable engram formation, preferentially at synapses clustered within dendritic branches rather than dispersed throughout the dendritic arbor. Thus, dendritic branches rather than individual synapses are the primary functional units for long-term memory storage.

Specific Involvement of Atypical PKCξ/PKMξ in Spinal Persistent Nociceptive Processing following Peripheral Inflammation in Rat

BackgroundCentral sensitization requires the activation of various intracellular signalling pathways within spinal dorsal horn neurons, leading to a lowering of activation threshold and enhanced responsiveness of these cells. Such plasticity contributes to the manifestation of chronic pain states and displays a number of features of long-term potentiation (LTP), a ubiquitous neuronal mechanism of increased synaptic strength. Here we describe the role of a novel pathway involving atypical PKCζ/PKMζ in persistent spinal nociceptive processing, previously implicated in the maintenance of late-phase LTP.ResultsUsing both behavioral tests and in vivo electrophysiology in rats, we show that inhibition of this pathway, via spinal delivery of a myristoylated protein kinase C-ζ pseudo-substrate inhibitor, reduces both pain-related behaviors and the activity of deep dorsal horn wide dynamic range neurons (WDRs) following formalin administration. In addition, Complete Freund's Adjuvant (CFA)-induced mechanical and thermal hypersensitivity was also reduced by inhibition of PKCζ/PKMζ activity. Importantly, this inhibition did not affect acute pain or locomotor behavior in normal rats and interestingly, did not inhibited mechanical allodynia and hyperalgesia in neuropathic rats. Pain-related behaviors in both inflammatory models coincided with increased phosphorylation of PKCζ/PKMζ in dorsal horn neurons, specifically PKMζ phosphorylation in formalin rats. Finally, inhibition of PKCζ/PKMζ activity decreased the expression of Fos in response to formalin and CFA in both superficial and deep laminae of the dorsal horn.ConclusionsThese results suggest that PKCζ, especially PKMζ isoform, is a significant factor involved in spinal persistent nociceptive processing, specifically, the manifestation of chronic pain states following peripheral inflammation.

SIK2 Is a Key Regulator for Neuronal Survival after Ischemia via TORC1-CREB

The cAMP responsive element-binding protein (CREB) functions in a broad array of biological and pathophysiological processes. We found that salt-inducible kinase 2 (SIK2) was abundantly expressed in neurons and suppressed CREB-mediated gene expression after oxygen-glucose deprivation (OGD). OGD induced the degradation of SIK2 protein concomitantly with the dephosphorylation of the CREB-specific coactivator transducer of regulated CREB activity 1 (TORC1), resulting in the activation of CREB and its downstream gene targets. Ca(2+)/calmodulin-dependent protein kinase I/IV are capable of phosphorylating SIK2 at Thr484, resulting in SIK2 degradation in cortical neurons. Neuronal survival after OGD was significantly increased in neurons isolated from sik2(-/-) mice, and ischemic neuronal injury was significantly reduced in the brains of sik2(-)(/-) mice subjected to transient focal ischemia. These findings suggest that SIK2 plays critical roles in neuronal survival, is modulated by CaMK I/IV, and regulates CREB via TORC1.

Calcium-dependent activator protein for secretion 2 (CAPS2) promotes BDNF secretion and is critical for the development of GABAergic interneuron network

Calcium-dependent activator protein for secretion 2 (CAPS2) is a dense-core vesicle-associated protein that is involved in the secretion of BDNF. BDNF has a pivotal role in neuronal survival and development, including the development of inhibitory neurons and their circuits. However, how CAPS2 affects BDNF secretion and its biological significance in inhibitory neurons are largely unknown. Here we reveal the role of CAPS2 in the regulated secretion of BDNF and show the effect of CAPS2 on the development of hippocampal GABAergic systems. We show that CAPS2 is colocalized with BDNF, both synaptically and extrasynaptically in axons of hippocampal neurons. Overexpression of exogenous CAPS2 in hippocampal neurons of CAPS2-KO mice enhanced depolarization-induced BDNF exocytosis events in terms of kinetics, frequency, and amplitude. We also show that in the CAPS2-KO hippocampus, BDNF secretion is reduced, and GABAergic systems are impaired, including a decreased number of GABAergic neurons and their synapses, a decreased number of synaptic vesicles in inhibitory synapses, and a reduced frequency and amplitude of miniature inhibitory postsynaptic currents. Conversely, excitatory neurons in the CAPS2-KO hippocampus were largely unaffected with respect to field excitatory postsynaptic potentials, miniature excitatory postsynaptic currents, and synapse number and morphology. Moreover, CAPS2-KO mice exhibited several GABA system-associated deficits, including reduced late-phase long-term potentiation at CA3-CA1 synapses, decreased hippocampal theta oscillation frequency, and increased anxiety-like behavior. Collectively, these results suggest that CAPS2 promotes activity-dependent BDNF secretion during the postnatal period that is critical for the development of hippocampal GABAergic networks.

‘Silent’ priming of translation-dependent LTP by β-adrenergic receptors involves phosphorylation and recruitment of AMPA receptors

The capacity for long-term changes in synaptic efficacy can be altered by prior synaptic activity, a process known as "metaplasticity." Activation of receptors for modulatory neurotransmitters can trigger downstream signaling cascades that persist beyond initial receptor activation and may thus have metaplastic effects. Because activation of β-adrenergic receptors (β-ARs) strongly enhances the induction of long-term potentiation (LTP) in the hippocampal CA1 region, we examined whether activation of these receptors also had metaplastic effects on LTP induction. Our results show that activation of β-ARs induces a protein synthesis-dependent form of metaplasticity that primes the future induction of late-phase LTP by a subthreshold stimulus. β-AR activation also induced a long-lasting increase in phosphorylation of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor (AMPAR) GluA1 subunits at a protein kinase A (PKA) site (S845) and transiently activated extracellular signal-regulated kinase (ERK). Consistent with this, inhibitors of PKA and ERK blocked the metaplastic effects of β-AR activation. β-AR activation also induced a prolonged, translation-dependent increase in cell surface levels of GluA1 subunit-containing AMPA receptors. Our results indicate that β-ARs can modulate hippocampal synaptic plasticity by priming synapses for the future induction of late-phase LTP through up-regulation of translational processes, one consequence of which is the trafficking of AMPARs to the cell surface.

Long-term memory of visually cued fear conditioning: roles of the neuronal nitric oxide synthase gene and cyclic AMP response element-binding protein

Nitric oxide (NO) produced by neuronal nitric oxide synthase (nNOS) has a role in late-phase long-term potentiation (LTP) and long-term memory (LTM) formation. Our recent studies implicated NO signaling in contextual and auditory cued fear conditioning. The present study investigated the role of NO signaling in visually cued fear conditioning. First, visually cued fear conditioning was investigated in wild-type (WT) and nNOS knockout (KO) mice. Second, the effects of pharmacological modulators of NO signaling on the acquisition of visually cued fear conditioning were investigated. Third, plasma levels of corticosterone were measured to determine a relationship between physiological and behavioral responses to fear conditioning. Fourth, levels of extracellular signal-related kinase (ERK1/2) and cyclic AMP response element binding protein (CREB) phosphorylation, downstream of NO signaling, were determined in the amygdala as potential correlates of fear learning. Mice underwent single or multiple (4) spaced trainings that consisted of a visual cue (blinking light) paired with footshock. WT mice acquired cued and contextual LTM following single and multiple trainings. nNOS KO mice acquired neither cued nor contextual LTM following a single training; however, multiple trainings improved contextual but not cued LTM. The selective nNOS inhibitor S-methyl-thiocitrulline (SMTC) impaired cued and contextual LTM in WT mice. The NO donor molsidomine recovered contextual LTM but had no effect on cued LTM in nNOS KO mice. Re-exposure to the visual cue 24 h posttraining elicited freezing response and a marked increase in plasma corticosterone levels in WT but not nNOS KO mice. The expression of CREB phosphorylation (Ser-133) was significantly higher in naive nNOS KO mice than in WT counterparts, and pharmacological modulators of NO had significant effects on levels of CREB phosphorylation and expression. These findings suggest that visual cue-dependent LTM is impaired in nNOS KO mice, and aberrant modulation of CREB in the absence of the nNOS gene may hinder cued and contextual LTM formation.

Regulation of Dendritic Development by BDNF Requires Activation of CRTC1 by Glutamate

Dendritic growth is essential for the establishment of a functional nervous system. Among extrinsic signals that control dendritic development, substantial evidence indicates that BDNF regulates dendritic morphology. However, little is known about the underlying mechanisms by which BDNF controls dendritic growth. In this study, we show that the MAPK signaling pathway and the transcription factor cAMP response element-binding protein (CREB) mediate the effects of BDNF on dendritic length and complexity. However, phosphorylation of CREB alone is not sufficient for the stimulation of dendritic growth by BDNF. Thus, using a mutant form of CREB unable to bind CREB-regulated transcription coactivator (CRTC1), we demonstrate that this effect also requires a functional interaction between CREB and CRTC1. Moreover, inhibition of CRTC1 expression by shRNA-mediated knockdown abolished BDNF-induced dendritic growth of cortical neurons. Interestingly, we found that nuclear translocation of CRTC1 results from activation of NMDA receptors by glutamate, a process that is essential for the effects of BDNF on dendritic development. Together, these data identify a previously unrecognized mechanism by which CREB and the coactivator CRTC1 mediate the effects of BDNF on dendritic growth.

Val 8-glucagon-like peptide-1 protects against Aβ1–40-induced impairment of hippocampal late-phase long-term potentiation and spatial learning in rats

Amyloid β protein (Aβ) is considered to be partly responsible for the impairment of learning and memory in Alzheimer disease (AD). In addition, it has been found recently that type 2 diabetes mellitus (T2DM) is a risk factor for developing AD. One promising treatment for AD is using analogues for the insulin-release facilitating gut hormone glucagon-like peptide-1 (GLP-1) that has been developed as a T2DM therapy. GLP-1 has been shown to have neuroprotective properties. However, if GLP-1 can protect the late phase-long term potentiation (L-LTP) and related cognitive function against Aβ-induced impairment it is still an open question. To further characterize the neuroprotective function of GLP-1 in the brain, we investigated the effects of i.c.v. injected Val 8-GLP-1(7–36) on the Aβ fragment-induced impairment of in vivo hippocampal L-LTP and spatial learning and memory in rats. The results showed that (1) Aβ1–40 (5 nmol) injection did not affect the baseline field excitatory postsynaptic potentials (fEPSPs), but significantly suppressed multiple high frequency stimulation (HFS)-induced L-LTP in hippocampal CA1 region; (2) Val 8-GLP-1(7–36) (0.05 pmol) administration alone did not affect the baseline synaptic transmission and the maintenance of L-LTP; (3) pretreatment with Val 8-GLP-1(7–36) (0.05 pmol) effectively prevented Aβ1–40-induced deficit of L-LTP; (4) i.c.v. injection of 5 nmol Aβ1–40 resulted in a significant decline learning a spatial Morris water maze (MWM) test; (5) Val 8-GLP-1(7–36) (0.05 pmol) administration alone did not affect spatial learning in this task, while pretreatment with Val 8-GLP-1(7–36) effectively reversed the impairment of spatial learning and memory induced by Aβ1–40. At the same time, the swim speeds and escape latencies of rats among all groups in the visible platform tests did not show any difference. These results suggest that increase of GLP-1 signalling in the brain may be a promising strategy to ameliorate the degenerative processes observed in AD.

SRF binding to SRE 6.9 in the Arc promoter is essential for LTD in cultured Purkinje cells

It has been suggested that gene expression and protein synthesis are required for both long-term memory consolidation and late phases of long-term potentiation and long-term depression (LTD). The necessary genes and the specific transcription factor binding sites in their promoters remain unknown. We found that inhibition of the transcription factor SRF or its cofactor MAL blocked the late phase of LTD in mouse cultured cerebellar Purkinje cells, as did deletion of the immediate early gene Arc. Using neuronal bacterial artificial chromosome (BAC) transfection, we found that, in Arc-/- cells transfected with a wild-type Arc BAC, late-phase LTD was rescued. However, mutation of one SRF-binding site in the Arc promoter (SRE 6.9) blocked this rescue. Co-transfection of wild-type Arc and SRF engineered to bind mutated SRE 6.9 restored late-phase LTD in Arc-/-, SRE 6.9 mutant BAC cells. Thus, SRF binding to SRE 6.9 in the Arc promoter is required for the late phase of cerebellar LTD.

Control of Cerebellar Long-Term Potentiation by P-Rex-Family Guanine-Nucleotide Exchange Factors and Phosphoinositide 3-Kinase

BackgroundLong-term potentiation (LTP) at the parallel fibre–Purkinje cell synapse in the cerebellum is a recently described and poorly characterized form of synaptic plasticity. The induction mechanism for LTP at this synapse is considered reciprocal to “classical” LTP at hippocampal CA1 pyramidal neurons: kinases promote increased trafficking of AMPA receptors into the postsynaptic density in the hippocampus, whereas phosphatases decrease internalization of AMPA receptors in the cerebellum. In the hippocampus, LTP occurs in overlapping phases, with the transition from early to late phases requiring the consolidation of initial induction processes by structural re-arrangements at the synapse. Many signalling pathways have been implicated in this process, including PI3 kinases and Rho GTPases.Principal FindingsWe hypothesized that analogous phases are present in cerebellar LTP, and took as the starting point for investigation our recent discovery that P-Rex – a Rac guanine nucleotide exchange factor which is activated by PtdIns(3,4,5)P3 – is highly expressed in mouse cerebellar Purkinje neurons and plays a role in motor coordination. We found that LTP evoked at parallel fibre synapses by 1 Hz stimulation or by NO donors was not sustained beyond 30 min when P-Rex was eliminated or Rac inhibited, suggesting that cerebellar LTP exhibits a late phase analogous to hippocampal LTP. In contrast, inhibition of PI3 kinase activity eliminated LTP at the induction stage.ConclusionsOur data suggest that a PI3K/P-Rex/Rac pathway is required for late phase LTP in the mouse cerebellum, and that other PI3K targets, which remain to be discovered, control LTP induction.