Delayed effects of combined stress and Aβ infusion on L-LTP of the dentate gyrus: Prevention by nicotine

Alzheimer's Disease (AD) is a progressive dementia hallmarked by the presence in the brain of extracellular beta-amyloid (Aβ) plaques and intraneuronal fibrillary tangles. Chronic stress is associated with heightened Aβ buildup and acceleration of development of AD, however, stress alone has no significant effect on synaptic plasticity in the dentate gyrus (DG) area. Previously, we have reported that the combination of stress and AD causes more severe inhibition of synaptic plasticity of hippocampal area CA1 than chronic stress or AD alone, and that chronic nicotine treatment prevents this impairment. To investigate the effect of stress and nicotine on synaptic plasticity in the relatively injury-resistant DG area, the present experiments analyzed the effect of chronic stress and the neuroprotective effect of nicotine on LTP in the DG area of a rat model of AD. Wistar rats were chronically stressed and treated with nicotine (1 mg/kg/twice daily; s.c.) for six weeks. Then, at weeks 5-6, AD model was generated by 14-day i.c.v osmotic pump infusion of Aβ peptides (300 pmol/day) into the brains of these rats. Field potential recordings from the DG area of anesthetized rats, revealed that while chronic stress did not accentuate Aβ-induced impairments of E-LTP, it markedly augmented Aβ effect on L-LTP that was only seen 100 min after multiple high frequency stimulation. This delayed action is likely to be due to impairment of process of de novo protein synthesis required for maintenance phase of L-LTP. Chronic nicotine treatment prevented stress-enhanced suppression of synaptic plasticity.

HFS-Triggered AMPK Activation Phosphorylates GSK3β and Induces E-LTP in Rat Hippocampus In Vivo

BACKGROUND

The AMP-activated protein kinase (AMPK) is a sensor of cellular energy and nutrient status, with substantial amount of cross talk with other signaling pathways, including its phosphorylation by Akt, PKA, and GSK3β.

AIMS

Various signaling pathways and energy-consuming transport of glutamate receptors subunits are required in synaptic plasticity. However, it is unknown which energy sensors integrate the signaling pathways in these processes. In this article, we elucidated the role of AMPK activation and GSK3β phosphorylation after HFS during the inducement of early-phase long-term potentiation (E-LTP).

METHODS

Synaptic LTP in vivo was induced by high-frequency stimulation (HFS at 200 Hz at a 5-s interval). In addition, phosphorylation of AMPK and glycogen synthase kinase 3β (GSK3β) were measured using Western blotting. The amount of hippocampal AMP, ADP and ATP was measured by HPLC.

RESULTS

We showed that the phosphorylation of AMPK and GSK3β was significantly increased by HFS in vivo. HFS-induced AMPK activation occurred via increased (AMP + ADP)/ATP ratio and activation of Ca(2+) /calmodulin-dependent kinase kinase beta (CaMKKβ). Pharmacological inhibition of AMPK by compound C (CC) prevented HFS-induced inhibitory phosphorylation of GSK3β and the induction of LTP in dentate gyrus (DG) area in vivo.

CONCLUSIONS

Our findings reveal that HFS-triggered AMPK activation phosphorylates GSK3β and induces E-LTP in vivo.

BDNF mechanisms in late LTP formation: A synthesis and breakdown

Unraveling the molecular mechanisms governing long-term synaptic plasticity is a key to understanding how the brain stores information in neural circuits and adapts to a changing environment. Brain-derived neurotrophic factor (BDNF) has emerged as a regulator of stable, late phase long-term potentiation (L-LTP) at excitatory glutamatergic synapses in the adult brain. However, the mechanisms by which BDNF triggers L-LTP are controversial. Here, we distill and discuss the latest advances along three main lines: 1) TrkB receptor-coupled translational control underlying dendritic protein synthesis and L-LTP, 2) Mechanisms for BDNF-induced rescue of L-LTP when protein synthesis is blocked, and 3) BDNF-TrkB regulation of actin cytoskeletal dynamics in dendritic spines. Finally, we explore the inter-relationships between BDNF-regulated mechanisms, how these mechanisms contribute to different forms of L-LTP in the hippocampus and dentate gyrus, and outline outstanding issues for future research. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.

BDNF-induced local protein synthesis and synaptic plasticity

Brain-derived neurotrophic factor (BDNF) is an important regulator of synaptic transmission and long-term potentiation (LTP) in the hippocampus and in other brain regions, playing a role in the formation of certain forms of memory. The effects of BDNF in LTP are mediated by TrkB (tropomyosin-related kinase B) receptors, which are known to be coupled to the activation of the Ras/ERK, phosphatidylinositol 3-kinase/Akt and phospholipase C-γ (PLC-γ) pathways. The role of BDNF in LTP is best studied in the hippocampus, where the neurotrophin acts at pre- and post-synaptic levels. Recent studies have shown that BDNF regulates the transport of mRNAs along dendrites and their translation at the synapse, by modulating the initiation and elongation phases of protein synthesis, and by acting on specific miRNAs. Furthermore, the effect of BDNF on transcription regulation may further contribute to long-term changes in the synaptic proteome. In this review we discuss the recent progress in understanding the mechanisms contributing to the short- and long-term regulation of the synaptic proteome by BDNF, and the role in synaptic plasticity, which is likely to influence learning and memory formation. This article is part of the Special Issue entitled 'BDNF Regulation of Synaptic Structure, Function, and Plasticity'.