Downregulation of KCC2 following LTP contributes to EPSP-spike potentiation in rat hippocampus

GABAergic synaptic inhibition plays a critical role in regulating long-term potentiation (LTP) of glutamatergic synaptic transmission and circuit output. The K(+)-Cl(-) cotransporter 2 (KCC2) is an important factor in determining inhibitory GABAergic synaptic strength besides the contribution of GABA(A) receptor. Although much knowledge has been gained regarding activity-dependent downregulation of KCC2 in many pathological conditions, the potential change and contribution of KCC2 in LTP expression is still unknown. In this study, we found that downregulation of KCC2 was accompanied with the occurrence of LTP but not that of long-term depression in hippocampal CA1 region. Meanwhile, KCC2 level in CA3/DG and adjacent cortex was stable in the process of LTP expression in Schaffer collateral synapses. Blockade of NMDA receptor with APV not only prevented LTP induction also abolished the reduction of KCC2. Furthermore, the inhibition of KCC2 function with furosemide directly induced EPSP-spike (E-S) potentiation, an important component of LTP in hippocampus. The present data suggest a novel mechanism that LTP formation is accompanied by the downregulation of KCC2, which is underlying GABAergic strength and most likely contributes to the E-S potentiation following LTP.     

Preservation of plastic properties of synaptic transmission in long-lasting hippocampal slices under the effects of a peptide analog of piracetam, L-pGlu-D-Ala-NH2

The tetanic stimulation of the Schaffer collaterals (SC) in rat hippocamp slices after 6 hrs in vitro conditions did not produce long-term potentiation (LTP) of the field response amplitude in the CA1 pyramidal cell layer. In contrast, LTP after the late tetanization was well preserved in the slices that were perfused for 20 minutes with 0.5 mkM L-pGlu-D-Ala-NH2 (PGAA) after 4-4.5 hrs in vitro. There were no significant reactivity changes during the perfusion of the slices with this drug concentration. Two other drugs with nootropic activity, piracetam (100 mkM) and gamma-hydroxybutyrate (100 mkM, Na-salt) did not prevent the disappearance of LTP in the late period in vitro, while enhanced the reactivity during perfusion period. The maintenance of the plastic properties of the SC-CA1 synaptic transmission under the influence of PGAA is thought to be the result of some specific interaction of the drug with LTP induction mechanisms. LTP damaged in the late period in vitro might be a new model of memory disturbances and this model can turn out to be useful for the comparative estimation of the effectiveness of the drugs with proposed nootropic activity and for the analysis of the possible mechanisms of their action.     

Inhibition of long-term potentiation by CuZn superoxide dismutase injection in rat dentate gyrus: involvement of muscarinic M1 receptor

Long-term potentiation (LTP) and long-term depression represent important processes that modulate synaptic transmission that carries out a key role in neural mechanisms of memory. Many studies give strong evidences on a role of the reactive oxygen species in the induction of LTP in CA1 region of hippocampal slices that was inhibited by adding the scavenger enzyme superoxide dismutase (SOD1). Previous data showed that SOD1 is secreted by many cellular lines, including neuroblastoma SK-N-BE cells through microvesicles by an ATP-dependent mechanism; moreover, it has been shown that SOD1 interacts with human neuroblastoma cell membranes increasing intracellular calcium levels via a phospholipase C-protein kinase C pathway activation. The aim of this study was to investigate the effect of intracerebral injection of SOD1 or the inactive form of enzyme (ApoSOD) on the modulation of synaptic transmission in dentate gyrus of the hippocampus in urethane anesthetized rats. The results of the present research showed that intracerebral injection of SOD1 and ApoSOD in the dentate gyrus of the rat hippocampal formation inhibits LTP induced by high-frequency stimulation of the perforant path. This result cannot be only explained by the dismutation of oxygen radical induced by SOD1 since also ApoSOD, that lacks the enzymatic activity, carries out the same inhibitory effect on LTP induction.     

Eugenol depresses synaptic transmission but does not prevent the induction of long-term potentiation in the CA1 region of rat hippocampal slices

Using field potential recording in the CA1 region of the rat hippocampal slices, the effects of eugenol on synaptic transmission and long-term potentiation (LTP) were investigated. Population spikes (PS) were recorded in the stratum pyramidal following stimulation of stratum fibers. To induce LTP, eight episodes of theta pattern primed-bursts (PBs) were delivered. Eugenol decreased the amplitude of PS in a concentration-dependent manner. The effect was fast and completely reversible. Eugenol had no effect on PBs-induced LTP of PS. It is concluded that while eugenol depresses synaptic transmission it does not affect the ability of CA1 synapses for tetanus-induced LTP and plasticity.     

CaM kinase II and protein kinase C activations mediate enhancement of long-term potentiation by nefiracetam in the rat hippocampal CA1 region

Nefiracetam is a pyrrolidine-related nootropic drug exhibiting various pharmacological actions such as cognitive-enhancing effect. We previously showed that nefiracetam potentiates NMDA-induced currents in cultured rat cortical neurons. To address questions whether nefiracetam affects NMDA receptor-dependent synaptic plasticity in the hippocampus, we assessed effects of nefiracetam on NMDA receptor-dependent long-term potentiation (LTP) by electrophysiology and LTP-induced phosphorylation of synaptic proteins by immunoblotting analysis. Nefiracetam treatment at 1-1000 nM increased the slope of fEPSPs in a dose-dependent manner. The enhancement was associated with increased phosphorylation of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor through activation of calcium/calmodulin-dependent protein kinase II (CaMKII) without affecting synapsin I phosphorylation. In addition, nefiracetam treatment increased PKCalpha activity in a bell-shaped dose-response curve which peaked at 10 nM, thereby increasing phosphorylation of myristoylated alanine-rich protein kinase C substrate and NMDA receptor. Nefiracetam treatment did not affect protein kinase A activity. Consistent with the bell-shaped PKCalpha activation, nefiracetam treatment enhanced LTP in the rat hippocampal CA1 region with the same bell-shaped dose-response curve. Furthermore, nefiracetam-induced LTP enhancement was closely associated with CaMKII and PKCalpha activation with concomitant increases in phosphorylation of their endogenous substrates except for synapsin I. These results suggest that nefiracetam potentiates AMPA receptor-mediated fEPSPs through CaMKII activation and enhances NMDA receptor-dependent LTP through potentiation of the post-synaptic CaMKII and protein kinase C activities. Together with potentiation of nicotinic acetylcholine receptor function, nefiracetam-enhanced AMPA and NMDA receptor functions likely contribute to improvement of cognitive function.     

Synaptic Homeostasis and Restructuring across the Sleep-Wake Cycle

Sleep is critical for hippocampus-dependent memory consolidation. However, the underlying mechanisms of synaptic plasticity are poorly understood. The central controversy is on whether long-term potentiation (LTP) takes a role during sleep and which would be its specific effect on memory. To address this question, we used immunohistochemistry to measure phosphorylation of Ca²⁺/calmodulin-dependent protein kinase II (pCaMKIIα) in the rat hippocampus immediately after specific sleep-wake states were interrupted. Control animals not exposed to novel objects during waking (WK) showed stable pCaMKIIα levels across the sleep-wake cycle, but animals exposed to novel objects showed a decrease during subsequent slow-wave sleep (SWS) followed by a rebound during rapid-eye-movement sleep (REM). The levels of pCaMKIIα during REM were proportional to cortical spindles near SWS/REM transitions. Based on these results, we modeled sleep-dependent LTP on a network of fully connected excitatory neurons fed with spikes recorded from the rat hippocampus across WK, SWS and REM. Sleep without LTP orderly rescaled synaptic weights to a narrow range of intermediate values. In contrast, LTP triggered near the SWS/REM transition led to marked swaps in synaptic weight ranking. To better understand the interaction between rescaling and restructuring during sleep, we implemented synaptic homeostasis and embossing in a detailed hippocampal-cortical model with both excitatory and inhibitory neurons. Synaptic homeostasis was implemented by weakening potentiation and strengthening depression, while synaptic embossing was simulated by evoking LTP on selected synapses. We observed that synaptic homeostasis facilitates controlled synaptic restructuring. The results imply a mechanism for a cognitive synergy between SWS and REM, and suggest that LTP at the SWS/REM transition critically influences the effect of sleep: Its lack determines synaptic homeostasis, its presence causes synaptic restructuring.     

Cellular and molecular mechanisms of memory: the LTP connection.

Studies of the cellular and molecular mechanisms of memory formation have focused on the role of long-lasting forms of synaptic plasticity such as long-term potentiation (LTP). A combination of genetic, electrophysiological and behavioral techniques have been used to examine the possibility that LTP is a cellular mechanism of memory storage in the mammalian brain. Although a definitive answer remains elusive, it is clear that in many cases manipulations that alter LTP alter memory, and training regimens that produce memory can produce LTP-like potentiation of synaptic transmission.     

Synaptic Plasticity in the Hippocampus of a APP/PS1 Mouse Model of Alzheimer's Disease Is Impaired in Old but Not Young Mice

Background

Alzheimer disease (AD) is a neurodegenerative disorder for which there is no cure. We have investigated synaptic plasticity in area CA1 in a novel AD mouse model (APPPS1-21) which expresses the Swedish mutation of APP and the L166P mutation of human PS-1. This model shows initial plaque formation at 2 months in the neocortex and 4 months in the hippocampus and displays β−amyloid-associated pathologies and learning impairments.

Methodology/Principal Findings

We tested long-term potentiation (LTP) and short term potentiation (paired-pulse facilitation, PPF) of synaptic transmission in vivo in area CA1 of the hippocampus. There was no difference in LTP or PPF at 4–5 months of age in APPPS1-21 mice compared to littermate controls. At 6 months of age there was also no difference in LTP but APPPS1-21 mice showed slightly increased PPF (p<0.03). In 8 months old mice, LTP was greatly impaired in APPPS-21 animals (p<0.0001) while PPF was not changed. At 15 months of age, APPPS1-21 mice showed again impaired LTP compared to littermate controls (p<0.005), and PPF was also significantly reduced at 80 ms (p<0.005) and 160 ms (p<0.01) interstimulus interval. Immunohistological analysis showed only modest amyloid deposition in the hippocampus at 4 and 6 months with a robust increase up to 15 months of age.

Conclusions

Our results suggest that increased formation and aggregation of beta amyloid with aging is responsible for the impaired LTP with aging in this mouse model, while the transient increase of PPF at 6 months of age is caused by some other mechanism.     

Clonidine Suppresses the Induction of Long-Term Potentiation by Inhibiting HCN Channels at the Schaffer Collateral–CA1 Synapse in Anesthetized Adult Rats

Activation of alpha2-adrenoceptors inhibits long-term potentiation and long-term depression in many brain regions. However, effectiveness and mechanism of alpha2-adrenoceptors for synaptic plasticity at the Schaffer collateral–CA1 synapses in rat in vivo is unclear. In the present study, we investigated the effects of alpha2-adrenoceptors agonist clonidine on high-frequency stimulation (HFS)-induced long-term potentiation (LTP) and paired-pulse facilitation (PPF) at the Schaffer collateral–CA1 synapse of rat hippocampus in vivo. Clonidine (0.05, 0.1 mg/kg, ip) inhibited synaptic plasticity in a dose-dependent manner, accompanying with the decreasing of aortic pressure and heart rate (HR) in anesthetized rats. Clonidine (1.25, 2.5 μg/kg, icv, 10 min before HFS) also dose-dependently inhibited synaptic plasticity, which had no remarkable effect on HR and aortic pressure. But, 20 min after HFS, administration of clonidine (2.5 μg/kg) had no effect on LTP. The inhibitory effect of clonidine (2.5 μg/kg) on LTP was completely reversed by yohimbine (18 μg/kg, icv) and ZD7288 (5 μg/kg, icv). Moreover, the inhibition was accompanied by a significant increase of the normalized PPF ratio. Furthermore, clonidine at 1 and 10 μM significantly decreased glutamate (Glu) content in the culture supernatants of hippocampal neurons, and yohimbine at 1 and 10 μM had no effect on Glu release, while it could reverse the inhibition of clonidine (1 and 10 μM) on Glu release. In conclusion, clonidine can suppress the induction of LTP at the Schaffer collateral–CA1 synapse, and the possible mechanism is that activation of presynaptic alpha2-adrenoceptors reduces the Glu release by inhibiting HCN channels.     

An in vitro study of long-term potentiation in the carp (Cyprinus carpio L.) olfactory bulb

Long-term potentiation (LTP) of synaptic transmission is considered a cellular mechanism for neural plasticity and memory formation. Previously, we showed that in the carp olfactory bulb, LTP occurs at the dendrodendritic mitral-to-granule cell synapse following tetanic electrical stimulation applied to the olfactory tract, and suggested that it is involved in the process of olfactory memory formation. As a first step towards understanding mechanisms underlying plasticity at this synapse, we examined the effects of various drugs (glutamate and GABA receptor agonists and antagonists, noradrenaline, and drugs affecting cAMP signaling) on dendrodendritic mitral-to-granule cell synaptic transmission in an in vitro preparation. Two forms of LTP are involved: a postsynaptic form (tetanus-evoked LTP) and a presynaptic form. The postsynaptic form is evoked at the granule cell dendrite following tetanic olfactory tract stimulation and is suppressed by the NMDA receptor antagonist, D-AP5, enhanced by noradrenaline, and occluded by the metabotropic glutamate receptor agonist, trans-ACPD. The presynaptic form occurs at the mitral cell dendrite following blockade of the GABA_A receptor by picrotoxin and bicuculline, or via activation of cAMP signaling by forskolin and 8-Br-cAMP.     

Neurabin contributes to hippocampal long-term potentiation and contextual fear memory

Neurabin is a scaffolding protein that interacts with actin and protein phosphatase-1. Highly enriched in the dendritic spine, neurabin is important for spine morphogenesis and synaptic formation. However, less is known about the role of neurabin in hippocampal plasticity and its possible effect on behavioral functions. Using neurabin knockout (KO) mice, here we studied the function of neurabin in hippocampal synaptic transmission, plasticity and behavioral memory. We demonstrated that neurabin KO mice showed a deficit in contextual fear memory but not auditory fear memory. Whole-cell patch clamp recordings in the hippocampal CA1 neurons showed that long-term potentiation (LTP) was significantly reduced, whereas long-term depression (LTD) was unaltered in neurabin KO mice. Moreover, increased AMPA receptor but not NMDA receptor-mediated synaptic transmission was found in neurabin KO mice, and is accompanied by decreased phosphorylation of GluR1 at the PKA site (Ser845) but no change at the CaMKII/PKC site (Ser831). Pre-conditioning with LTD induction rescued the following LTP in neurabin KO mice, suggesting the loss of LTP may be due to the saturated synaptic transmission. Our results indicate that neurabin regulates contextual fear memory and LTP in hippocampal CA1 pyramidal neurons.     

Tyrosine phosphatase STEP is a tonic brake on induction of long-term potentiation

The functional roles of protein tyrosine phosphatases (PTPs) in the developed CNS have been enigmatic. Here we show that striatal enriched tyrosine phosphatase (STEP) is a component of the N-methyl-D-aspartate receptor (NMDAR) complex. Functionally, exogenous STEP depressed NMDAR single-channel activity in excised membrane patches. STEP also depressed NMDAR-mediated synaptic currents whereas inhibiting endogenous STEP enhanced these currents. In hippocampal slices, administering STEP into CA1 neurons did not affect basal glutamatergic transmission evoked by Schaffer collateral stimulation but prevented tetanus-induced long-term potentiation (LTP). Conversely, inhibiting STEP in CA1 neurons enhanced transmission and occluded LTP induction through an NMDAR-, Src-, and Ca(2+)-dependent mechanism. Thus, STEP acts as a tonic brake on synaptic transmission by opposing Src-dependent upregulation of NMDARs.     

Ionotropic glutamate receptors

In the brain, most fast excitatory synaptic transmission is mediated through L-glutamate acting on postsynaptic ionotropic glutamate receptors. These receptors are of two kinds—the α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA)/kainate (non-NMDA) and theN-methyl-D-aspartate (NMDA) receptors, which are thought to be colocalized onto the same postsynaptic elements. This excitatory transmission can be modulated both upward and downward, long-term potentiation (LTP) and long-term depression (LTD), respectively. Whether the expression of LTP/LTD is pre-or postsynaptically located (or both) remains an enigma. This article will focus on what postsynaptic modifications of the ionotropic glutamate receptors may possibly underly long-term potentiation/depression. It will discuss the character of LTP/LTD with respect to the temporal characteristics and to the type of changes that appears in the non-NMDA and NMDA receptor-mediated synaptic currents, and what constraints these findings put on the possible expression mechanism(s) for LTP/LTD. It will be submitted that if a modification of the glutamate receptors does underly LTP/LTD, an increase/decrease in the number of functional receptors is the most plausible alternative. This change in receptor number will have to include a coordinated change of both the non-NMDA and the NMDA receptors.     

Long-lasting potentiation of excitatory synaptic signaling to the crayfish lateral giant neuron

The neural circuit that underlies the lateral giant fiber (LG)-mediated reflex escape in crayfish has provided findings relating synaptic change to nonassociative learning such as sensitization and habituation. The LGs receive sensory inputs from the primary sensory afferents and a group of mechanosensory interneurons (MSIs). An increase of excitability by suprathreshold repetitive excitation of this circuit, which is similar to Hebbian long-term potentiation (LTP), has been reported [Miller et al. (1987) J Neurosci 7:1081]. This potentiation was previously thought to result from the enhancement of transmission at cholinergic synapses between primary afferents and MSIs but not the electrical synapses onto LG. In this study, we found that potentiation of synaptic signaling at the electrical synapse onto LG can also be induced when the synapse was activated with subthreshold repetitive pulses or with a few strong suprathreshold shocks. LG LTP was induced in the preparation which had received pulses at limited frequency range. Although whether this LTP is involved in the learning process of escape behavior in crayfish is not clear, the intensity and amount of sensory stimulation used here mimicked those that could easily be produced by a predator trying to catch a crayfish and could be of adaptive significance in life.     

Long-term depression of kainate receptor-mediated synaptic transmission

Kainate receptors (KARs) have been shown to be involved in hippocampal mossy fiber long-term potentiation (LTP); however, it is not known if KARs are involved in the induction or expression of long-term depression (LTD), the other major form of long-term synaptic plasticity. Here we describe LTD of KAR-mediated synaptic transmission (EPSC(KA) LTD) in perirhinal cortex layer II/III neurons that is distinct from LTD of AMPAR-mediated transmission, which also coexists at the same synapses. Induction of EPSC(KA) LTD requires a rise in postsynaptic Ca(2+) but is independent of NMDARs or T-type voltage-gated Ca(2+) channels; however, it requires synaptic activation of inwardly rectifying KARs and release of Ca(2+) from stores. The synaptic KARs are regulated by tonically activated mGluR5, and expression of EPSC(KA) LTD occurs via a mechanism involving mGluR5, PKC, and PICK1 PDZ domain interactions. Thus, we describe the induction and expression mechanism of a form of synaptic plasticity, EPSC(KA) LTD.     

Ca2+ entry via postsynaptic voltage-sensitive Ca2+ channels can transiently potentiate excitatory synaptic transmission in the hippocampus.

We have studied the role of Ca2+ entry via voltage-sensitive Ca2+ channels in long-term potentiation (LTP) in the CA1 region of the hippocampus. Repeated depolarizing pulses, in the presence of the NMDA receptor antagonist D-APV and without synaptic stimulation, resulted in a potentiation of excitatory postsynaptic potentials (EPSPs) or currents (EPSCs). This depolarization-induced potentiation was augmented in raised extracellular Ca2+ and was blocked by intracellular BAPTA, a Ca2+ chelator, or by nifedipine, a Ca2+ channel antagonist, indicating that the effect was mediated by Ca2+ entry via voltage-sensitive Ca2+ channels. Although the peak potentiation could be as large as 3-fold, the EPSP(C)s decayed back to baseline values within approximately 30 min. However, synaptic activation paired with depolarizing pulses in the presence of D-APV converted the transient potentiation into a sustained form. These results indicate that a rise in postsynaptic Ca2+ via voltage-sensitive Ca2+ channels can transiently potentiate synaptic transmission, but that another factor associated with synaptic transmission may be required for LTP.     

Glutamate uptake determines pathway specificity of long-term potentiation in the neural circuitry of fear conditioning

Long-term synaptic modifications in afferent inputs to the amygdala underlie fear conditioning in animals. Fear conditioning to a single sensory modality does not generalize to other cues, implying that synaptic modifications in fear conditioning pathways are input specific. The mechanisms of pathway specificity of long-term potentiation (LTP) are poorly understood. Here we show that inhibition of glutamate transporters leads to the loss of input specificity of LTP in the amygdala slices, as assessed by monitoring synaptic responses at two independent inputs converging on a single postsynaptic neuron. Diffusion of glutamate ("spillover") from stimulated synapses, paired with postsynaptic depolarization, is sufficient to induce LTP in the heterosynaptic pathway, whereas an enzymatic glutamate scavenger abolishes this effect. These results establish active glutamate uptake as a crucial mechanism maintaining the pathway specificity of LTP in the neural circuitry of fear conditioning.     

Activation of Rho GTPases by synaptic transmission in the hippocampus

Ras-related GTPases of the Rho family, such as RhoA and RhoB, are well-characterised mediators of morphological change in peripheral tissues via their effects on the actin cytoskeleton. We tested the hypothesis that Rho family GTPases are involved in synaptic transmission in the CA1 region of the hippocampus. We show that GTPases are activated by synaptic transmission. RhoA and RhoB were activated by low frequency stimulation, while the induction of long-term potentiation (LTP) by high frequency stimulation was associated with specific activation of RhoB via NMDA receptor stimulation. This illustrates that these GTPases are potential mediators of synaptic transmission in the hippocampus, and raises the possibility that RhoB may play a role in plasticity at hippocampal synapses during LTP.     

Modulation of learning and hippocampal, neuronal plasticity by repetitive transcranial magnetic stimulation (rTMS)

The influence of high-frequency repetitive transcranial magnetic stimulation (rTMS) on learning process in mice and on neuronal excitability of the hippocampal tissue obtained from stimulated animals were investigated. While the stimulation with rTMS at higher frequency (15 Hz) improved animals' performance in novel object recognition test (NOR), lower frequency (1 and 8 Hz) impaired the memory. The effect was observed when evaluated immediately after rTMS exposure and declined with time. In parallel to the results of behavioral test, there was a significant enhancement of the synaptic efficiency expressed as of the long-term potentiation (LTP) recorded from hippocampal slices prepared from the animals exposed to 15 Hz rTMS. The stimulation with 1 and 8 Hz had no influence on the magnitude of LTP. Our results demonstrate that rTMS modifies mechanisms involved in memory formation. The effects of rTMS in vivo are preserved and expressed in the hippocampus tested in vitro.     

MGluRs regulate the expression of neuronal calcium sensor proteins NCS-1 and VILIP-1 and the immediate early gene arg3.1/arc in the hippocampus in vivo

The metabotropic glutamate receptor (mGluR) agonist (R,S)-3,5-dihydroxyphenylglycine (DHPG) is involved in several forms of hippocampal synaptic plasticity. DHPG application can induce slow-onset potentiation, a form of long-term potentiation (LTP), in the dentate gyrus and in the CA1 region in vivo. The induction of LTP correlates with increased expression levels of neuronal calcium sensor (NCS), considered as key elements for plasticity. In this study we investigated mGluR- and time-dependent changes in the expression of two different NCS proteins. Following DHPG application in vivo NCS-1 and VILIP-1 expression increased, with significant levels reached after 8 and 24h. The effect was attenuated by treatment with the group I mGluR specific antagonist S-4-carboxyphenylglycine. The immediate early gene (IEG) arg3.1/arc showed highest expression levels 2h after DHPG-treatment. Therefore, mGluRs at concentrations which induce synaptic plasticity regulate the expression of IEGs and NCS proteins in different time frames and thus contribute to late phases of synaptic plasticity.