Patterned stimulation at the theta frequency is optimal for the induction of hippocampal long-term potentiation

Safety of theta burst transcranial magnetic stimulation: a systematic review of the literature

Theta burst stimulation (TBS) protocols have recently emerged as a method to transiently alter cortical excitability in the human brain through repetitive transcranial magnetic stimulation. TBS involves applying short trains of stimuli at high frequency repeated at intervals of 200 milliseconds. Because repetitive transcranial magnetic stimulation is known to carry a risk of seizures, safety guidelines have been established. TBS has the theoretical potential of conferring an even higher risk of seizure than other repetitive transcranial magnetic stimulation protocols because it delivers high-frequency bursts. In light of the recent report of a seizure induced by TBS, the safety of this new protocol deserves consideration. We performed an English language literature search and reviewed all studies published from May 2004 to December 2009 in which TBS was applied. The adverse events were documented, and crude risk was calculated. The majority of adverse events attributed to TBS were mild and occurred in 5% of subjects. Based on this review, TBS seems to be a safe and efficacious technique. However, given its novelty, it should be applied with caution. Additionally, this review highlights the need for rigorous documentation of adverse events associated with TBS and intensity dosing studies to assess the seizure risk associated with various stimulation parameters (e.g., frequency, intensity, and location).

Neuronal plasticity and thalamocortical sleep and waking oscillations

Throughout life, thalamocortical (TC) network alternates between activated states (wake or rapid eye movement sleep) and slow oscillatory state dominating slow-wave sleep. The patterns of neuronal firing are different during these distinct states. I propose that due to relatively regular firing, the activated states preset some steady state synaptic plasticity and that the silent periods of slow-wave sleep contribute to a release from this steady state synaptic plasticity. In this respect, I discuss how states of vigilance affect short-, mid-, and long-term synaptic plasticity, intrinsic neuronal plasticity, as well as homeostatic plasticity. Finally, I suggest that slow oscillation is intrinsic property of cortical network and brain homeostatic mechanisms are tuned to use all forms of plasticity to bring cortical network to the state of slow oscillation. However, prolonged and profound shift from this homeostatic balance could lead to development of paroxysmal hyperexcitability and seizures as in the case of brain trauma.

Enhanced hippocampal long-term potentiation following repeated MDMA treatment in Dark-Agouti rats

In rats and primates, (±)3,4-Methylenedioxymethamphetamine (MDMA, ecstasy) produces both long-lasting damage to serotonergic axons and memory impairment. Our objective was to determine effects of neurotoxic dose of MDMA on long-term potentiation (LTP) in hippocampal area CA1 in Dark-Agouti (DA) rats. One week after neurotoxic MDMA treatment in vivo (12.5mg/kg i.p., once a week, per three weeks), serotonergic deficit was evident in hippocampal slices as 56.3% reduction in 5-HT content (p=0.04) and as 68.4% reduction in the effect of endogenous 5-HT release on synaptic neurotransmission (p<0.01). In hippocampal slices from the same animals, LTP was on average 46% greater than that observed in sham-treated controls (42.9 ± 3.5%; n=12 vs. 29.2 ± 3.2%; n=12; p<0.01). Non-neurotoxic dose of MDMA (12.5 mg/kg, i.p., one time) did not change LTP one week after the treatment, suggesting correlation between serotonergic deficit and enhanced synaptic plasticity. We conclude that MDMA-induced impairment of learning and memory is not a consequence of hippocampal LTP inhibition.

A single brief burst induces GluR1-dependent associative short-term potentiation: a potential mechanism for short-term memory

Recent work showed that short-term memory (STM) is selectively reduced in GluR1 knockout mice. This raises the possibility that a form of synaptic modification dependent on GluR1 might underlie STM. Studies of synaptic plasticity have shown that stimuli too weak to induce long-term potentiation induce short-term potentiation (STP), a phenomenon that has received little attention. Here we examined several properties of STP and tested the dependence of STP on GluR1. The minimal requirement for inducing STP was examined using a test pathway and a conditioning pathway. Several closely spaced stimuli in the test pathway, forming a single brief burst, were sufficient to induce STP. Thus, STP is likely to be induced by the similar bursts that occur in vivo. STP induction is associative in nature and dependent on the NMDAR. STP decays with two components, a fast component (1.6 +/- 0.26 min) and a slower one (19 +/- 6.6 min). To test the role of GluR1 in STP, experiments were conducted on GluR1 knockout mice. We found that STP was greatly reduced. These results, taken together with the behavioral work of D. Sanderson et al. [Sanderson, D., Good, M. A., Skelton, K., Sprengel, R., Seeburg, P. H., Nicholas, J., et al. Enhanced long-term and impaired short-term spatial memory in GluA1 AMPA receptor subunit knockout mice: Evidence for a dual-process memory model. Learning and Memory, 2009], provide genetic evidence that STP is a likely mechanism of STM.

Spatiotemporal coupling between hippocampal acetylcholine release and theta oscillations in vivo

Both acetylcholine (ACh) and theta oscillations are important for learning and memory, but the dynamic interaction between these two processes remains unclear. Recent advances in amperometry techniques have revealed phasic ACh releases in vivo. However, it is unknown whether phasic ACh release co-occurs with theta oscillations. We investigated this issue in the CA1 region of urethane-anesthetized male rats using amperometric and electrophysiological recordings. We found that ACh release was highly correlated with the appearance of both spontaneous and induced theta oscillations. Moreover, the maximal ACh release was observed around or slightly above the pyramidal layer. Interestingly, such release lagged behind theta initiation by 25-60 s. The slow ACh release profile was matched by the slow firing rate increase of a subset of medial-septal low-firing-rate neurons. Together, these results establish, for the first time, the in vivo coupling between phasic ACh release and theta oscillations on spatiotemporal scales much finer than previously known. These findings also suggest that phasic ACh is not required for theta initiation and may instead operate synergistically with theta oscillations to promote neural plasticity in the service of learning and memory.

Calcium action potentials in hair cells pattern auditory neuron activity before hearing onset

We found rat central auditory neurons to fire action potentials in a precise sequence of mini-bursts before the age of hearing onset. This stereotyped pattern was initiated by hair cells in the cochlea, which trigger brief bursts of action potentials in auditory neurons each time they fire a Ca2+ spike. By generating theta-like activity, hair cells may limit the influence of synaptic depression in developing auditory circuits and promote consolidation of synapses.

A Ca-Based Computational Model for NMDA Receptor-Dependent Synaptic Plasticity at Individual Post-Synaptic Spines in the Hippocampus

Associative synaptic plasticity is synapse specific and requires coincident activity in pre-synaptic and post-synaptic neurons to activate NMDA receptors (NMDARs). The resultant Ca²⁺ influx is the critical trigger for the induction of synaptic plasticity. Given its centrality for the induction of synaptic plasticity, a model for NMDAR activation incorporating the timing of pre-synaptic glutamate release and post-synaptic depolarization by back-propagating action potentials could potentially predict the pre- and post-synaptic spike patterns required to induce synaptic plasticity. We have developed such a model by incorporating currently available data on the timecourse and amplitude of the post-synaptic membrane potential within individual spines. We couple this with data on the kinetics of synaptic NMDARs and then use the model to predict the continuous spine [Ca²⁺] in response to regular or irregular pre- and post-synaptic spike patterns. We then incorporate experimental data from synaptic plasticity induction protocols by regular activity patterns to couple the predicted local peak [Ca²⁺] to changes in synaptic strength. We find that our model accurately describes [Ca²⁺] in dendritic spines resulting from NMDAR activation during pre-synaptic and post-synaptic activity when compared to previous experimental observations. The model also replicates the experimentally determined plasticity outcome of regular and irregular spike patterns when applied to a single synapse. This model could therefore be used to predict the induction of synaptic plasticity under a variety of experimental conditions and spike patterns.

Investigating the synchronization of hippocampal neural network in response to acute nicotine exposure

Previous studies suggested that γ oscillations in the brain are associated with higher order cognitive function including selective visual attention, motor task planning, sensory perception, working memory and dreaming REM sleep. These oscillations are mainly observed in cortical regions and also occur in neocortical and subcortical areas and the hippocampus. In this paper, we investigate the influence of acute exposure to nicotine on the complexity of hippocampal γ oscillations.

Using the approximate entropy method, the influence of acute nicotine exposure on the hippocampal γ oscillations was investigated. The hippocampal γ oscillations have been generated in response to the 100 Hz stimulus and isolated using the visual inspection and spectral analysis method. Our central hypothesis is that acute exposure to nicotine significantly reduces the complexity of hippocampal γ oscillations. We used brain-slice recordings and the approximate entropy method to test this hypothesis. The approximate entropy (complexity) values of the hippocampal γ oscillations are estimated from the 14 hippocampal slices. Our results show that it takes at least 100 msec to see any hippocampal activities in response to the 100 Hz stimulus. These patterns noticeably changed after 100 msec until 300 msec after the stimulus Finally, they were less prominent after 300 msec. We have analyzed the isolated hippocampal γ oscillations (between 150 and 250 msec after the stimulus) using the approximate entropy (ApEn) method. Our results showed that the ApEn (complexity) values of hippocampal γ oscillations during nicotine exposure were reduced compared to those of hippocampal γ oscillations during control, and washout. This reduction was much more significant in response to acute nicotine exposure (p < 0.05) compared to those during control and washout conditions. These results suggest that the neural firing becomes regular and the hippocampal networks become synchronized in response to nicotine exposure.

L-type voltage-gated Ca2+ channels: a single molecular switch for long-term potentiation/long-term depression-like plasticity and activity-dependent metaplasticity in humans

The ability of synapses to undergo persistent activity-dependent potentiation or depression [long-term potentiation (LTP)/long-term depression (LTD)] may be profoundly altered by previous neuronal activity. Although natural neuronal activity can be experimentally manipulated in vivo, very little is known about the in vivo physiological mechanisms involved in regulating this metaplasticity in models of LTP/LTD. To examine whether Ca(2+) signaling may influence metaplasticity in vivo in humans, we used continuous theta burst stimulation (cTBS) (Huang et al., 2005), a noninvasive novel repetitive magnetic stimulation protocol known to induce persistent alterations of corticospinal excitability whose polarity is changed by previous voluntary motor activity. When directed to the naive motor cortex, cTBS induced long-lasting potentiation of corticospinal excitability, but depression under the influence of nimodipine (NDP), an L-type voltage-gated Ca(2+) channel (L-VGCC) antagonist. Both aftereffects were blocked by dextromethorphan, an NMDA receptor antagonist, supporting the notion that these bidirectional cTBS-induced alterations of corticospinal excitability map onto LTP and LTD as observed in animal studies. A short period of voluntary contraction and a small dose of NDP were each ineffective in blocking the cTBS-induced potentiation. However, when both interventions were combined, a depression was induced, and the magnitude of this depression increased with the dose of NDP. These findings suggest that Ca(2+) dynamics determine the polarity of LTP/LTD-like changes in vivo. L-VGCCs may act as molecular switches mediating metaplasticity induced by endogenous neuronal activation.

A microfluidic brain slice perfusion chamber for multisite recording using penetrating electrodes

To analyze the spatiotemporal dynamics of network activity in a brain tissue slice, it is useful to record simultaneously from multiple locations. When obtained from laminar structures such as the hippocampus or neocortex, multisite recordings also yield information about subcellular current distributions via current source density analysis. Multisite probes developed for in vivo recordings could serve these purposes in vitro, allowing recordings to be obtained from brain slices at sites deeper within the tissue than currently available surface recording methods permit. However, existing recording chambers do not allow for the insertion of lamina-spanning probes that enter through the edges of brain slices. Here, we present a novel brain slice recording chamber design that accomplishes this goal. The device provides a stable microfluidic perfusion environment in which tissue health is optimized by superfusing both surfaces of the slice. Multichannel electrodes can be inserted parallel to the surface of the slice, at any depth relative to the surface. Access is also provided from above for the insertion of additional recording or stimulating electrodes. We illustrate the utility of this recording configuration by measuring current sources and sinks during theta burst stimuli that lead to the induction of long-term potentiation in hippocampal slices.

Astrocytes impose postburst depression of release probability at hippocampal glutamate synapses

Many neurons typically fire action potentials in brief, high-frequency bursts with specific consequences for their synaptic output. Here we have examined short-term plasticity engaged during burst activation using electrophysiological recordings in acute rat hippocampal slices. We show that CA3-CA1 glutamate synapses exhibit a prominent depression of presynaptic release probability for approximately 1 s after such a burst. This postburst depression exhibits a distinct cooperativity threshold, is abolished by inhibiting astrocyte metabolism and astrocyte calcium signaling, and is not operational in the developing hippocampus. Our results suggest that astrocytes are actively involved in short-term synaptic depression, shaping synaptic activity during behaviorally relevant neural activity.

Effects of long-term pioglitazone treatment on peripheral and central markers of aging

BACKGROUND

Thiazolidinediones (TZDs) activate peroxisome proliferator-activated receptor gamma (PPARgamma) and are used clinically to help restore peripheral insulin sensitivity in Type 2 diabetes (T2DM). Interestingly, long-term treatment of mouse models of Alzheimer's disease (AD) with TZDs also has been shown to reduce several well-established brain biomarkers of AD including inflammation, oxidative stress and Abeta accumulation. While TZD's actions in AD models help to elucidate the mechanisms underlying their potentially beneficial effects in AD patients, little is known about the functional consequences of TZDs in animal models of normal aging. Because aging is a common risk factor for both AD and T2DM, we investigated whether the TZD, pioglitazone could alter brain aging under non-pathological conditions.

METHODS AND FINDINGS

We used the F344 rat model of aging, and monitored behavioral, electrophysiological, and molecular variables to assess the effects of pioglitazone (PIO-Actos(R) a TZD) on several peripheral (blood and liver) and central (hippocampal) biomarkers of aging. Starting at 3 months or 17 months of age, male rats were treated for 4-5 months with either a control or a PIO-containing diet (final dose approximately 2.3 mg/kg body weight/day). A significant reduction in the Ca(2+)-dependent afterhyperpolarization was seen in the aged animals, with no significant change in long-term potentiation maintenance or learning and memory performance. Blood insulin levels were unchanged with age, but significantly reduced by PIO. Finally, a combination of microarray analyses on hippocampal tissue and serum-based multiplex cytokine assays revealed that age-dependent inflammatory increases were not reversed by PIO.

CONCLUSIONS

While current research efforts continue to identify the underlying processes responsible for the progressive decline in cognitive function seen during normal aging, available medical treatments are still very limited. Because TZDs have been shown to have benefits in age-related conditions such as T2DM and AD, our study was aimed at elucidating PIO's potentially beneficial actions in normal aging. Using a clinically-relevant dose and delivery method, long-term PIO treatment was able to blunt several indices of aging but apparently affected neither age-related cognitive decline nor peripheral/central age-related increases in inflammatory signaling.

A critical role for alpha4betadelta GABAA receptors in shaping learning deficits at puberty in mice

The onset of puberty defines a developmental stage when some learning processes are diminished, but the mechanism for this deficit remains unknown. We found that, at puberty, expression of inhibitory alpha4betadelta gamma-aminobutyric acid type A (GABAA) receptors (GABAR) increases perisynaptic to excitatory synapses in CA1 hippocampus. Shunting inhibition via these receptors reduced N-methyl-D-aspartate receptor activation, impairing induction of long-term potentiation (LTP). Pubertal mice also failed to learn a hippocampal, LTP-dependent spatial task that was easily acquired by delta-/- mice. However, the stress steroid THP (3alphaOH-5alpha[beta]-pregnan-20-one), which reduces tonic inhibition at puberty, facilitated learning. Thus, the emergence of alpha4betadelta GABARs at puberty impairs learning, an effect that can be reversed by a stress steroid.

Low-frequency-induced synaptic potentiation: a paradigm shift in the field of memory-related plasticity mechanisms?

Long-term potentiation (LTP) and long-term depression (LTD) are two forms of synaptic plasticity thought to play functional roles in learning and memory processes. It is generally assumed that the direction of synaptic modifications (i.e., up- or down-regulation of synaptic strength) depends on the specific pattern of afferent inputs, with high frequency activity or stimulation effectively inducing LTP, while low-frequency patterns often elicit LTD. This dogma ("high frequency-LTP, low frequency-LTD") has recently been challenged by evidence demonstrating low frequency stimulation (LFS)-induced synaptic potentiation in the rodent hippocampus and amygdala. Extensive work in the past decades has focused on deciphering the mechanisms by which high frequency stimulation of afferent fiber systems results in LTP. With this review, we will compare and contrast the well-known synaptic and cellular mechanisms underlying classical, high-frequency-induced LTP to those mediating the more recently discovered phenomena of LFS-induced synaptic enhancement. In addition, we argue that LFS protocols provide a means to more accurately mimic some endogenous, oscillatory activity patterns present in hippocampal and extra-hippocampal (especially neocortical) circuits during periods of memory consolidation. Consequently, LFS-induced synaptic potentiation offers a novel and important avenue to investigate cellular and systems-level mechanisms mediating the encoding, consolidation, and transfer of information throughout multiple forebrain networks implicated in learning and memory processes. (c) 2009 Wiley-Liss, Inc.

Theogallin and L-theanine as active ingredients in decaffeinated green tea extract: I. electrophysiological characterization in the rat hippocampus in-vitro

The in-vitro hippocampus slice preparation was used to mimic a physiological situation where nervous tissue is exposed directly to the water soluble extract of green tea and some of its constituents. This investigation provides evidence that L-theanine- and theogallin-enriched decaffeinated green tea extract is able to change the physiological pattern of electrical hippocampus activity in a concentration dependent manner (EC50 3 mg L−1). Of the seven fractions or single components tested (fraction containing all amino acids without L-theanine, fractions containing all amino acids plus L-theanine, glutamic acid, theogallin, its metabolites quinic acid and gallic acid, and L-theanine alone), glutamic acid produced the strongest changes in terms of increased population spike amplitude after single stimuli and increased long-term potentiation, commonly taken as representative for enhancement of spatial and time dependent memory. The presence of theogallin alone shifted the activity in the same direction. Similar results as with theogallin were obtained in the presence of quinic acid. No effect was seen with gallic acid. Opposite changes (decrease of population spike amplitude and attenuated long-term potentiation) were observed in the presence of L-theanine alone. No effects were detected during the addition of the amino acid mixture unless L-theanine was added, leading to a decrease of the responses as observed for the action of L-theanine alone. The results provide evidence for the involvement of several active principles in the action of enriched green tea extract on electrical brain activity. The overall enhancement of hippocampal pyramidal cell responses as observed for the crude extract seems to be due to the combined action of glutamic acid and theogallin (or its presumable metabolite quinic acid), whereas L-theanine seems to have an opposite effect. However, this action was not strong enough to antagonize the effects of glutamic acid and theogallin. The results are in line with the observation that the tested green tea extract improves cognition at concomitant mental relaxation in man.

Neural dynamics associated with semantic and episodic memory for faces: evidence from multiple frequency bands

Prior semantic knowledge facilitates episodic recognition memory for faces. To examine the neural manifestation of the interplay between semantic and episodic memory, we investigated neuroelectric dynamics during the creation (study) and the retrieval (test) of episodic memories for famous and nonfamous faces. Episodic memory effects were evident in several EEG frequency bands: theta (4-8 Hz), alpha (9-13 Hz), and gamma (40-100 Hz). Activity in these bands was differentially modulated by preexisting semantic knowledge and by episodic memory, implicating their different functional roles in memory. More specifically, theta activity and alpha suppression were larger for old compared to new faces at test regardless of fame, but were both larger for famous faces during study. This pattern of selective semantic effects suggests that the theta and alpha responses, which are primarily associated with episodic memory, reflect utilization of semantic information only when it is beneficial for task performance. In contrast, gamma activity decreased between the first (study) and second (test) presentation of a face, but overall was larger for famous than nonfamous faces. Hence, the gamma rhythm seems to be primarily related to activation of preexisting neural representations that may contribute to the formation of new episodic traces. Taken together, these data provide new insights into the complex interaction between semantic and episodic memory for faces and the neural dynamics associated with mnemonic processes.

Differential effects of long and short train theta burst stimulation on LTP induction in rat anterior cingulate cortex slices: multi-electrode array recordings

OBJECTIVE

There is substantial evidence supporting the notion that the anterior cingulate cortex (ACC) is an important limbic structure involved in multiple brain functions such as sensory perception, motor conflict monitoring, memory, emotion and cognition. It has been shown that long term potentiation (LTP) is an important synaptic model of neural plasticity in the ACC, however, little is known about the spatiotemporal properties of ACC at network level. The present study was designed to see the LTP induction effects across different layers of the ACC by using different conditioning stimuli (CS) protocols.

METHODS

A unique multi-electrode array recording technique was used in the acutely-dissociated ACC slices of rats. Long and short train theta burst stimulation (TBS) paradigms were applied in layer V-VI as the CS and the LTP induction effects were compared across different layers of the ACC. Briefly, both long and short train TBS are composed of bursts (4 pulses at 100 Hz) with a 200 ms interval, however, the former (TBS1) was with 10 trains and the latter (TBS2) was with 5 trains. After test stimulation at layer V-VI in the ACC, network field potentials (FPs) could be simultaneously recorded across all layers of the ACC.

RESULTS

The waveforms of FPs were different across different layers. Namely, positive-going waveforms were recorded in layer I and negative-going waveforms were recorded in layers V-VI, in contrast, complex waveforms were localized mainly in layers II-III. Following application of two CS protocols, the induction rate of LTP was significantly different between TBS1 and TBS2 regardless of the spatial properties. TBS1 had more than 60% success, while TBS2 was less than 25% in induction of LTP. Moreover, both the 2 CS protocols could induce LTP in layers II-III and layers V-VI without layer-related difference. However, no LTP was inducible in layer I.

CONCLUSION

The present findings indicate that stimulation protocols may, at least in part, account for a large portion of variations among previous LTP studies, and hence highlight the importance of selecting the best LTP induction protocol when designing such experiments. Moreover, the present results demonstrate the prominent superiority of multi-electrode array recording in revealing the network properties of synaptic activities in the ACC, especially in comparing the spatiotemporal characteristics between different layers of this structure.

NMDA receptor-independent control of transcription factors and gene expression

Consolidation of synaptic plasticity seems to require transcription, but how the nucleus is informed in this context remains unknown. As NMDA receptor antagonists have been shown to interfere with action potential generation, the issue of whether or not a synaptically generated signal is required for nuclear signaling is currently unresolved. Here, we show that pharmacological maintenance of action potentials during NMDA receptor blockade allows for NMDA receptor-independent transcription factor binding and arc gene expression, both of which were previously thought to be NMDA receptor dependent. These data suggest that types of signaling in the nucleus previously attributed to NMDA-receptor-dependent synapse-to-nucleus signals can be initiated in the absence of NMDA receptor-dependent synaptic plasticity.

Functional role of gamma and theta oscillations in episodic memory

The primary aim of this review is to examine evidence for a functional role of gamma and theta oscillations in human episodic memory. It is proposed here that gamma and theta oscillations allow for the transient interaction between cortical structures and the hippocampus for the encoding and retrieval of episodic memories as described by the hippocampal memory indexing theory (Teyler and DiScenna, 1986). Gamma rhythms can act in the cortex to bind perceptual features and in the hippocampus to bind the rich perceptual and contextual information from diverse brain regions into episodic representations. Theta oscillations act to temporally order these individual episodic memory representations. Through feedback projections from the hippocampus to the cortex these gamma and theta patterns could cause the reinstatement of the entire episodic memory representation in the cortex. In addition, theta oscillations could allow for top-down control from the frontal cortex to the hippocampus modulating the encoding and retrieval of episodic memories.

Cerebellar theta oscillations are synchronized during hippocampal theta-contingent trace conditioning

The hippocampus and cerebellum are critically involved in trace eyeblink classical conditioning (EBCC). The mechanisms underlying the hippocampal-cerebellar interaction during this task are not well-understood, although hippocampal theta (3-7 Hz) oscillations are known to reflect a favorable state for EBCC. Two groups of rabbits received trace EBCC in which a brain-computer interface administered trials in either the explicit presence or absence of naturally occurring hippocampal theta. A high percentage of robust theta led to a striking enhancement of learning accompanied by rhythmic theta-band (6-7 Hz) oscillations in the interpositus nucleus (IPN) and cerebellar cortex that were time-locked both to hippocampal rhythms and sensory stimuli during training. Rhythmic oscillations were absent in the cerebellum of the non-theta group. These data strongly suggest a beneficial impact of theta-based coordination of hippocampus and cerebellum and, importantly, demonstrate that hippocampal theta oscillations can be used to index, and perhaps modulate, the functional properties of the cerebellum.

Physiology of repetitive transcranial magnetic stimulation of the human brain

During the last two decades, transcranial magnetic stimulation (TMS) has rapidly become a valuable method to investigate noninvasively the human brain. In addition, repetitive TMS (rTMS) is able to induce changes in brain activity that last after stimulation. Therefore, rTMS has therapeutic potential in patients with neurologic and psychiatric disorders. It is, however, unclear by which mechanism rTMS induces these lasting effects on the brain. The effects of rTMS are often described as LTD- or LTP-like, because the duration of these alterations seems to implicate changes in synaptic plasticity. In this review we therefore discuss, based on rTMS experiments and knowledge about synaptic plasticity, whether the physiologic basis of rTMS-effects relates to changes in synaptic plasticity. We present seven lines of evidence that strongly suggest a link between the aftereffects induced by rTMS and the induction of synaptic plasticity. It is, nevertheless, important to realize that at present it is impossible to demonstrate a direct link between rTMS on the one hand and synaptic plasticity on the other. Therefore, we provide suggestions for future, innovating research, aiming to investigate both the local effects of rTMS on the synapse and the effects of rTMS on other, more global levels of brain organization. Only in that way can the aftereffects of rTMS on the brain be completely understood.

Visual cortex plasticity evokes excitatory alterations in the hippocampus

The integration of episodic sequences in the hippocampus is believed to occur during theta rhythm episodes, when cortico-hippocampal dialog results in reconfiguration of neuronal assemblies. As the visual cortex (VC) is a major source of sensory information to the hippocampus, information processing in the cortex may affect hippocampal network oscillations, facilitating the induction of synaptic modifications. We investigated to what degree the field activity in the primary VC, elicited by sensory or electrical stimulation, correlates with hippocampal oscillatory and synaptic responsiveness, in freely behaving adult rats. We found that the spectral power of theta rhythm (4–10 Hz) in the dentate gyrus (DG), increases in parallel with high-frequency oscillations in layer 2/3 of the VC and that this correlation depends on the degree of exploratory activity. When we mimic robust thalamocortical activity by theta-burst application to dorsal lateral geniculate nucleus, a hippocampal theta increase occurs, followed by a persistent potentiation of the DG granule field population spike. Furthermore, the potentiation of DG neuronal excitability tightly correlates with the concurrently occurring VC plasticity. The concurrent enhancement of VC and DG activity is also combined with a highly negative synchronization between hippocampal and cortical low-frequency oscillations. Exploration of familiar environment decreases the degree of this synchrony. Our data propose that novel visual information can induce high-power fluctuations in intrinsic excitability for both VC and hippocampus, potent enough to induce experience-dependent modulation of cortico-hippocampal connections. This interaction may comprise one of the endogenous triggers for long-term synaptic plasticity in the hippocampus.

Specific roles of AMPA receptor subunit GluR1 (GluA1) phosphorylation sites in regulating synaptic plasticity in the CA1 region of hippocampus

Activity-dependent changes in excitatory synaptic transmission in the CNS have been shown to depend on the regulation of alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid receptors (AMPARs). In particular, several lines of evidence suggest that reversible phosphorylation of AMPAR subunit glutamate receptor 1 (GluR1, also referred to as GluA1 or GluR-A) plays a role in long-term potentiation (LTP) and long-term depression (LTD). We previously reported that regulation of serines (S) 831 and 845 on the GluR1 subunit may play a critical role in bidirectional synaptic plasticity in the Schaffer collateral inputs to CA1. Specifically, gene knockin mice lacking both S831 and S845 phosphorylation sites ("double phosphomutants"), where both serine residues were replaced by alanines (A), showed a faster decaying LTP and a deficit in LTD. To determine which of the two phosphorylation sites was responsible for the phenotype, we have now generated two lines of gene knockin mice: one that specifically lacks S831 (S831A mutants) and another that lacks only S845 (S845A mutants). We found that S831A mutants display normal LTP and LTD, whereas S845A mutants show a specific deficit in LTD. Taken together with our previous results from the "double phosphomutants," our data suggest that either S831 or S845 alone may support LTP, whereas the S845 site is critical for LTD expression.

Cytoskeletal changes underlie estrogen's acute effects on synaptic transmission and plasticity

Estrogen, in addition to its genomic effects in brain, causes rapid and reversible changes to synaptic operations. We report here that these acute actions are due to selective activation of an actin-signaling cascade normally used in the production of long-term potentiation (LTP). Estrogen, or a selective agonist of the steroid's beta-receptor, caused a modest increase in fast glutamatergic transmission and a pronounced facilitation of LTP in adult hippocampal slices; both effects were completely eliminated by latrunculin, a toxin that prevents actin filament assembly. Estrogen also increased spine concentrations of filamentous actin and strongly enhanced its polymerization in association with LTP. A search for the origins of these effects showed that estrogen activates the small GTPase RhoA and phosphorylates (inactivates) the actin severing protein cofilin, a downstream target of RhoA. Moreover, an antagonist of RhoA kinase (ROCK) blocked estrogen's synaptic effects. Estrogen thus emerges as a positive modulator of a RhoA>ROCK>LIM kinase>cofilin pathway that regulates the subsynaptic cytoskeleton. It does not, however, strongly affect a second LTP-related pathway, involving the GTPases Rac and Cdc42 and their effector p21-activated kinase, which may explain why its acute effects are reversible. Finally, ovariectomy depressed RhoA activity, spine cytoskeletal plasticity, and LTP, whereas brief infusions of estrogen rescued plasticity, suggesting that the deficits in plasticity arise from acute, as well as genomic, consequences of hormone loss.

Activity-mediated plasticity of GABA equilibrium potential in rat hippocampal CA1 neurons

The equilibrium potential (E(GABA)(-PSC)) for gamma-aminobutyric acid (GABA) A receptor mediated inhibitory postsynaptic currents (PSCs) in hippocampal CA1 pyramidal neurons shifts when theta-burst stimulation (four pulses at 100 Hz in each burst in a train consisting of five bursts with an inter-burst interval of 200 ms, the train repeated thrice at 30-s intervals) is applied to the input. E(GABA)(-PSC) is regulated by K(+)/Cl(-) co-transporter (KCC2). GABA(B) receptors are implicated in modulating KCC2 levels. In the current study, the involvement of KCC2, as well as GABA(B) receptors, in theta-burst-mediated shifts in E(GABA)(-PSC) was examined. Whole-cell patch recordings were made from hippocampal CA1 pyramidal neurons (from 9 to 12 days old rats), in a slice preparation. Glutamatergic excitatory postsynaptic currents were blocked with dl-2-amino-5-phosphonovaleric acid (50 microM) and 6,7-dinitroquinoxaline-2,3-dione (20 microM). The PSC and the E(GABA)(-PSC) were stable when stimulated at 0.05 Hz. However, both changed following a 30-min stimulation at 0.5 or 1 Hz. Furosemide (500 microM) and KCC2 anti-sense in the recording pipette but not bumetanide (20 or 100 microM) or KCC2 sense, blocked the changes, suggesting KCC2 involvement. Theta-burst stimulation induced a negative shift in E(GABA)(-PSC), which was prevented by KCC2 anti-sense; however, KCC2 sense had no effect. CGP55845 (2 microM), a GABA(B) antagonist, applied in the superfusing medium, or GDP-beta-S in the recording pipette, blocked the shift in E(GABA)(-PSC). These results indicate that activity-mediated plasticity in E(GABA)(-PSC) occurs in hippocampal CA1 pyramidal neurons and theta-burst-induced negative shift in E(GABA)(-PSC) requires KCC2, GABA(B) receptors and G-protein activation.

Nociception-induced spatial and temporal plasticity of synaptic connection and function in the hippocampal formation of rats: a multi-electrode array recording

Background

Pain is known to be processed by a complex neural network (neuromatrix) in the brain. It is hypothesized that under pathological state, persistent or chronic pain can affect various higher brain functions through ascending pathways, leading to co-morbidities or mental disability of pain. However, so far the influences of pathological pain on the higher brain functions are less clear and this may hinder the advances in pain therapy. In the current study, we studied spatiotemporal plasticity of synaptic connection and function in the hippocampal formation (HF) in response to persistent nociception.

Results

On the hippocampal slices of rats which had suffered from persistent nociception for 2 h by receiving subcutaneous bee venom (BV) or formalin injection into one hand paw, multisite recordings were performed by an 8 × 8 multi-electrode array probe. The waveform of the field excitatory postsynaptic potential (fEPSP), induced by perforant path electrical stimulation and pharmacologically identified as being activity-dependent and mediated by ionotropic glutamate receptors, was consistently positive-going in the dentate gyrus (DG), while that in the CA1 was negative-going in shape in naïve and saline control groups. For the spatial characteristics of synaptic plasticity, BV- or formalin-induced persistent pain significantly increased the number of detectable fEPSP in both DG and CA1 area, implicating enlargement of the synaptic connection size by the injury or acute inflammation. Moreover, the input-output function of synaptic efficacy was shown to be distinctly enhanced by the injury with the stimulus-response curve being moved leftward compared to the control. For the temporal plasticity, long-term potentiation produced by theta burst stimulation (TBS) conditioning was also remarkably enhanced by pain. Moreover, it is strikingly noted that the shape of fEPSP waveform was drastically deformed or split by a TBS conditioning under the condition of persistent nociception, while that in naïve or saline control state was not affected. All these changes in synaptic connection and function, confirmed by the 2-dimentional current source density imaging, were found to be highly correlated with peripheral persistent nociception since pre-blockade of nociceptive impulses could eliminate all of them. Finally, the initial pharmacological investigation showed that AMPA/KA glutamate receptors might play more important roles in mediation of pain-associated spatiotemporal plasticity than NMDA receptors.

Conclusion

Peripheral persistent nociception produces great impact upon the higher brain structures that lead to not only temporal plasticity, but also spatial plasticity of synaptic connection and function in the HF. The spatial plasticity of synaptic activities is more complex than the temporal plasticity, comprising of enlargement of synaptic connection size at network level, deformed fEPSP at local circuit level and, increased synaptic efficacy at cellular level. In addition, the multi-synaptic model established in the present investigation may open a new avenue for future studies of pain-related brain dysfunctions at the higher level of the neuromatrix.

Vesicular glutamate transporter VGLUT1 has a role in hippocampal long-term potentiation and spatial reversal learning

Vesicular glutamate transporters 1 and 2 (VGLUT1, VGLUT2) show largely complementary distribution in the mature rodent brain and tend to segregate to synapses with different physiological properties. In the hippocampus, VGLUT1 is the dominate subtype in adult animals, whereas VGLUT2 is transiently expressed during early postnatal development. We generated and characterized VGLUT1 knockout mice in order to examine the functional contribution of this transporter to hippocampal synaptic plasticity and hippocampus-dependent spatial learning. Because complete deletion of VGLUT1 resulted in postnatal lethality, we used heterozygous animals for analysis. Here, we report that deletion of VGLUT1 resulted in impaired hippocampal long-term potentiation (LTP) in the CA1 region in vitro. In contrast, heterozygous VGLUT2 mice that were investigated for comparison did not show any changes in LTP. The reduced ability of VGLUT1-deficient mice to express LTP was accompanied by a specific deficit in spatial reversal learning in the water maze. Our data suggest a functional role of VGLUT1 in forms of hippocampal synaptic plasticity that are required to adapt and modify acquired spatial maps to external stimuli and changes.

Coupling of mesoscopic brain oscillations: recent advances in analytical and theoretical perspectives

Oscillatory brain activities have been traditionally studied in the context of how oscillations at a single frequency recorded from a single area could reveal functional insights. Recent advances in methodology used in signal analysis have revealed that cross-frequency coupling, within or between functional related areas, is more informative in determining the possible roles played by brain oscillations. In this review, we begin by describing the cellular basis of oscillatory field potentials and its theorized as well as demonstrated role in brain function. The recent development of mathematical tools that allow the investigation of cross-frequency and cross-area oscillation coupling will be presented and discussed in the context of recent advances in oscillation research based on animal data. Particularly, some pitfalls and caveats of methods currently available are discussed. Data generated from the application of examined techniques are integrated back into the theoretical framework regarding the functional role of brain oscillations. We suggest that the coupling of oscillatory activities at different frequencies between brain regions is crucial for understanding the brain from a functional ensemble perspective. Effort should be directed to elucidate how cross-frequency and area coupling are modulated and controlled. To achieve this, only the correct application of analytical tools may shed light on the intricacies of information representation, generation, binding, encoding, storage and retrieval in the brain.

Hippocampal place cell firing patterns can induce long-term synaptic plasticity in vitro

In the hippocampus, synaptic strength between pyramidal cells is modifiable by NMDA receptor (NMDAR)-dependent long-term potentiation (LTP) and long-term depression (LTD), both of which require coincident presynaptic and postsynaptic activity. In vivo, many pyramidal cells exhibit location-specific activity patterns and are known as "place cells." The combination of these factors suggests that synaptic plasticity will be induced at synapses connecting place cells with overlapping firing fields, because such cells fire coincidentally when the rat is in a specific part of the environment. However, this prediction, which is important for models of how long-term synaptic plasticity can be used to encode space in the hippocampal network, has not been tested. To investigate this, action potential time series recorded simultaneously from place cells in freely moving rats were replayed concurrently into postsynaptic CA1 pyramidal cells and presynaptic inputs during perforated patch-clamp recordings from adult hippocampal slices. Place cell firing patterns induced large, pathway-specific, NMDAR-dependent LTP that was rapidly expressed within a few minutes. However, place-cell LTP was induced only if the two place cells had overlapping firing fields and if the cholinergic tone present in the hippocampus during exploration was restored by bath application of the cholinergic agonist carbachol. LTD was never observed in response to place cell firing patterns. Our findings demonstrate that spike patterns from hippocampal place cells can robustly induce NMDAR-dependent LTP, providing important evidence in support of a model in which spatial distance is encoded as the strength of synaptic connections between place cells.

ApoE isoform-dependent changes in hippocampal synaptic function

The lipoprotein receptor system in the hippocampus is intimately involved in the modulation of synaptic transmission and plasticity. The association of specific apoE isoform expression with human neurodegenerative disorders has focused attention on the role of these apoE isoforms in lipoprotein receptor-dependent synaptic modulation. In the present study, we used the apoE2, apoE3 and apoE4 targeted replacement (TR) mice along with recombinant human apoE isoforms to determine the role of apoE isoforms in hippocampus area CA1 synaptic function. While synaptic transmission is unaffected by apoE isoform, long-term potentiation (LTP) is significantly enhanced in apoE4 TR mice versus apoE2 TR mice. ApoE isoform-dependent differences in LTP induction require NMDA-receptor function, and apoE isoform expression alters activation of both ERK and JNK signal transduction. Acute application of specific apoE isoforms also alters LTP induction while decreasing NMDA-receptor mediated field potentials. Furthermore, acute apoE isoform application does not have the same effects on ERK and JNK activation. These findings demonstrate specific, isoform-dependent effects of human apoE isoforms on adult hippocampus synaptic plasticity and highlight mechanistic differences between chronic apoE isoform expression and acute apoE isoform exposure.

Effects of eicosapentaenoic acid on synaptic plasticity, fatty acid profile and phosphoinositide 3-kinase signaling in rat hippocampus and differentiated PC12 cells

Placebo-controlled clinical studies suggest that intake of n-3 polyunsaturated fatty acids improves neurological disorders such as Alzheimer's disease, Huntington's disease and schizophrenia. To evaluate the impact of eicosapentaenoic acid (EPA), we orally administered highly purified ethyl EPA (EPA-E) to rats at a dose of 1.0 mg/g per day and measured long-term potentiation of the CA1 hippocampal region, a physiological correlate of synaptic plasticity that is thought to underlie learning and memory. The mean field excitatory postsynaptic potential slope of the EPA-E group was significantly greater than that of the control group in the CA1 region. Gene expression of hippocampal p85alpha, one of the regulatory subunits of phosphatidylinositol 3-kinase (PI3-kinase), was increased with EPA-E administration. Investigation of fatty acid profiles of neuronal and glia-enriched fractions demonstrated that a single administration of EPA-E significantly increased neuronal and glial EPA content and glial docosahexaenoic acid content, clearly suggesting that EPA was indeed taken up by both neurons and glial cells. In addition, we investigated the direct effects of EPA on the PI3-kinase/Akt pathway in differentiated PC12 cells. Phosphorylated-Akt expression was significantly increased in EPA-treated cells, and nerve growth factor withdrawal-induced increases in cell death and caspase-3 activity were suppressed by EPA treatment. These findings suggest that EPA protects against neurodegeneration by modulating synaptic plasticity and activating the PI3-kinase/Akt pathway, possibly by its own functional effects in neurons and glial cells and by its capacity to increase brain docosahexaenoic acid.

Spred1 is required for synaptic plasticity and hippocampus-dependent learning

Germline mutations in SPRED1, a negative regulator of Ras, have been described in a neurofibromatosis type 1 (NF1)-like syndrome (NFLS) that included learning difficulties in some affected individuals. NFLS belongs to the group of phenotypically overlapping neuro-cardio-facial-cutaneous syndromes that are all caused by germ line mutations in genes of the Ras/mitogen-activated protein kinase extracellular signal-regulated kinase (ERK) pathway and that present with some degree of learning difficulties or mental retardation. We investigated hippocampus-dependent learning and memory as well as synaptic plasticity in Spred1(-/-) mice, an animal model of this newly discovered human syndrome. Spred1(-/-) mice show decreased learning and memory performance in the Morris water maze and visual-discrimination T-maze, but normal basic neuromotor and sensory abilities. Electrophysiological recordings on brain slices from these animals identified defects in short- and long-term synaptic hippocampal plasticity, including a disequilibrium between long-term potentiation (LTP) and long-term depression in CA1 region. Biochemical analysis, 4 h after LTP induction, demonstrated increased ERK-phosphorylation in Spred1(-/-) slices compared with those of wild-type littermates. This indicates that deficits in hippocampus-dependent learning and synaptic plasticity induced by SPRED1 deficiency are related to hyperactivation of the Ras/ERK pathway.

Mechanisms and applications of theta-burst rTMS on the human motor cortex

Theta-burst Stimulation (TBS) is a novel form of repetitive transcranial magnetic stimulation (rTMS). Applied over the primary motor cortex it has been successfully used to induce changes in cortical excitability. The advantage of this stimulation paradigm is that it is able to induce strong and long lasting effects using a lower stimulation intensity and a shorter time of stimulation compared to conventional rTMS protocols. Since its first description, TBS has been used in both basic and clinical research in the last years and more recently it has been expanded to other domains than the motor system. Its capacity to induce synaptic plasticity could lead to therapeutic implications for neuropsychiatric disorders. The neurobiological mechanisms of TBS are not fully understood at present; they may involve long-term potentiation (LTP)- and depression (LTD)-like processes, as well as inhibitory mechanisms modulated by GABAergic activity. This article highlights current hypotheses regarding the mechanisms of action of TBS and some central factors which may influence cortical responses to TBS. Furthermore, previous and ongoing research performed in the field of TBS on the motor cortex is summarized.

Transcranial magnetic stimulation, synaptic plasticity and network oscillations

Transcranial magnetic stimulation (TMS) has quickly progressed from a technical curiosity to a bona-fide tool for neurological research. The impetus has been due to the promising results obtained when using TMS to uncover neural processes in normal human subjects, as well as in the treatment of intractable neurological conditions, such as stroke, chronic depression and epilepsy. The basic principle of TMS is that most neuronal axons that fall within the volume of magnetic stimulation become electrically excited, trigger action potentials and release neurotransmitter into the postsynaptic neurons. What happens afterwards remains elusive, especially in the case of repeated stimulation. Here we discuss the likelihood that certain TMS protocols produce long-term changes in cortical synapses akin to long-term potentiation and long-term depression of synaptic transmission. Beyond the synaptic effects, TMS might have consequences on other neuronal processes, such as genetic and protein regulation, and circuit-level patterns, such as network oscillations. Furthermore, TMS might have non-neuronal effects, such as changes in blood flow, which are still poorly understood.

LTP in hippocampal area CA1 is induced by burst stimulation over a broad frequency range centered around delta

Long-term potentiation (LTP) is typically studied using either continuous high-frequency stimulation or theta burst stimulation. Previous studies emphasized the physiological relevance of theta frequency; however, synchronized hippocampal activity occurs over a broader frequency range. We therefore tested burst stimulation at intervals from 100 msec to 20 sec (10 Hz to 0.05 Hz). LTP at Schaffer collateral-CA1 synapses was obtained at intervals from 100 msec to 5 sec, with maximal LTP at 350-500 msec (2-3 Hz, delta frequency). In addition, a short-duration potentiation was present over the entire range of burst intervals. We found that N-methyl-d-aspartic acid (NMDA) receptors were more important for LTP induction by burst stimulation, but L-type calcium channels were more important for LTP induction by continuous high-frequency stimulation. NMDA receptors were even more critical for short-duration potentiation than they were for LTP. We also compared repeated burst stimulation with a single primed burst. In contrast to results from repeated burst stimulation, primed burst potentiation was greater when a 200-msec interval (theta frequency) was used, and a 500-msec interval was ineffective. Whole-cell recordings of postsynaptic membrane potential during burst stimulation revealed two factors that may determine the interval dependence of LTP. First, excitatory postsynaptic potentials facilitated across bursts at 500-msec intervals but not 200-msec or 1-sec intervals. Second, synaptic inhibition was suppressed by burst stimulation at intervals between 200 msec and 1 sec. Our data show that CA1 synapses are more broadly tuned for potentiation than previously appreciated.

Cholesterol 24-hydroxylase: an enzyme of cholesterol turnover in the brain

Cholesterol 24-hydroxylase is a highly conserved cytochrome P450 that is responsible for the majority of cholesterol turnover in the vertebrate central nervous system. The enzyme is expressed in neurons, including hippocampal and cortical neurons that are important for learning and memory formation. Disruption of the cholesterol 24-hydroxylase gene in the mouse reduces both cholesterol turnover and synthesis in the brain but does not alter steady-state levels of cholesterol in the tissue. The decline in synthesis reduces the flow of metabolites through the cholesterol biosynthetic pathway, of which one, geranylgeraniol diphosphate, is required for learning in the whole animal and for synaptic plasticity in vitro. This review focuses on how the link between cholesterol metabolism and higher-order brain function was experimentally established.

A genetically encoded fluorescent sensor of ERK activity

The activity of the ERK has complex spatial and temporal dynamics that are important for the specificity of downstream effects. However, current biochemical techniques do not allow for the measurement of ERK signaling with fine spatiotemporal resolution. We developed a genetically encoded, FRET-based sensor of ERK activity (the extracellular signal-regulated kinase activity reporter, EKAR), optimized for signal-to-noise ratio and fluorescence lifetime imaging. EKAR selectively and reversibly reported ERK activation in HEK293 cells after epidermal growth factor stimulation. EKAR signals were correlated with ERK phosphorylation, required ERK activity, and did not report the activities of JNK or p38. EKAR reported ERK activation in the dendrites and nucleus of hippocampal pyramidal neurons in brain slices after theta-burst stimuli or trains of back-propagating action potentials. EKAR therefore permits the measurement of spatiotemporal ERK signaling dynamics in living cells, including in neuronal compartments in intact tissues.

Dissecting the components of long-term potentiation

The formation of memories relies in part on plastic changes at synapses between neurons. Although the mechanisms of synaptic plasticity have been studied extensively over several decades, many aspects of this process remain controversial. The cellular locus of expression of long-term potentiation (LTP), a major form of synaptic plasticity, is one of the most important unresolved phenomena. In this article, some recent advances in this area made possible by the development of new imaging tools are summarized. These studies have demonstrated that LTP is compound in nature and consists of both presynaptic and postsynaptic components. Some features of presynaptic and postsynaptic changes during compound LTP are also reviewed.

Extracellular proteolysis by matrix metalloproteinase-9 drives dendritic spine enlargement and long-term potentiation coordinately

Persistent dendritic spine enlargement is associated with stable long-term potentiation (LTP), and the latter is thought to underlie long-lasting memories. Extracellular proteolytic remodeling of the synaptic microenvironment could be important for such plasticity, but whether or how proteolytic remodeling contributes to persistent modifications in synapse structure and function is unknown. Matrix metalloproteinase-9 (MMP-9) is an extracellular protease that is activated perisynaptically after LTP induction and required for LTP maintenance. Here, by monitoring spine size and excitatory postsynaptic potentials (EPSPs) simultaneously with combined 2-photon time-lapse imaging and whole-cell recordings from hippocampal neurons, we find that MMP-9 is both necessary and sufficient to drive spine enlargement and synaptic potentiation concomitantly. Both structural and functional MMP-driven forms of plasticity are mediated through beta1-containing integrin receptors, are associated with integrin-dependent cofilin inactivation within spines, and require actin polymerization. In contrast, postsynaptic exocytosis and protein synthesis are both required for MMP-9-induced potentiation, but not for initial MMP-9-induced spine expansion. However, spine expansion becomes unstable when postsynaptic exocytosis or protein synthesis is blocked, indicating that the 2 forms of plasticity are expressed independently but require interactions between them for persistence. When MMP activity is eliminated during theta-stimulation-induced LTP, both spine enlargement and synaptic potentiation are transient. Thus, MMP-mediated extracellular remodeling during LTP has an instructive role in establishing persistent modifications in both synapse structure and function of the kind critical for learning and memory.

AMPA Silencing Is a Prerequisite for Developmental Long-Term Potentiation in the Hippocampal CA1 Region

AMPA (α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid) unsilencing is an often proposed expression mechanism both for developmental long-term potentiation (LTP), involved in circuitry refinement during brain development, and for mature LTP, involved in learning and memory. In the hippocampal CA3–CA1 connection naïve (nonstimulated) synapses are AMPA signaling and AMPA-silent synapses are created from naïve AMPA-signaling (AMPA-labile) synapses by test-pulse synaptic activation (AMPA silencing). To investigate to what extent LTPs at different developmental stages are explained by AMPA unsilencing, the amount of LTP obtained at these different developmental stages was related to the amount of AMPA silencing that preceded the induction of LTP. When examined in the second postnatal week Hebbian induction was found to produce no more stable potentiation than that causing a return to the naïve synaptic strength existing prior to the AMPA silencing. Moreover, in the absence of a preceding AMPA silencing Hebbian induction produced no stable potentiation above the naïve synaptic strength. Thus this early, or developmental, LTP is nothing more than an unsilencing (dedepression) and stabilization of the AMPA signaling that was lost by the prior AMPA silencing. This dedepression and stabilization of AMPA signaling was mimicked by the presence of the protein kinase A activator forskolin. As the relative degree of AMPA silencing decreased with development, LTP manifested itself more and more as a “genuine” potentiation (as opposed to a dedepression) not explained by unsilencing and stabilization of AMPA-labile synapses. This “genuine,” or mature, LTP rose from close to nothing of total LTP prior to postnatal day (P)13, to about 70% of total LTP at P16, and to about 90% of total LTP at P30. Developmental LTP, by stabilization of AMPA-labile synapses, thus seems adapted to select synaptic connections to the growing synaptic network. Mature LTP, by instead strengthening existing stable connections between cells, may then create functionally tightly connected cell assemblies within this network.

Tremor entrainment by patterned low-frequency stimulation

High-frequency test stimulation for tremor suppression is a standard procedure for functional target localization during deep brain stimulation. This method does not work in cases where tremor vanishes intraoperatively, for example, due to general anaesthesia or due to an insertional effect. To overcome this difficulty, we developed a stimulation technique that effectively evokes tremor in a well-defined and quantifiable manner. For this, we used patterned low-frequency stimulation (PLFS), i.e. brief high-frequency pulse trains administered at pulse rates similar to neurons' preferred burst frequency. Unlike periodic single-pulse stimulation, PLFS enables one to convey effective and considerably greater integral charge densities without violation of safety requirements. In a computational investigation of an oscillatory neuronal network temporarily rendered inactive, we found that PLFS evokes synchronized activity, phase locked to the stimulus. While a stronger increase in the amount of synchrony in the neuronal population requires higher stimulus intensities, the portion of synchronously active neurons nevertheless becomes strongly phase locked to PLFS already at weak stimulus intensities. The phase entrainment effect of PLFS turned out to be robust against variations in the stimulation frequency, whereas enhancement of synchrony required precisely tuned stimulation frequencies. We applied PLFS to a patient with spinocerebellar ataxia type 2 (SCA2) with pronounced tremor that disappeared intraoperatively under general anaesthesia. In accordance with our computational results, PLFS evoked tremor, phase locked to the stimulus. In particular, weak PLFS caused low-amplitude, but strongly phase-locked tremor. PLFS test stimulations provided the only functional information about target localization. Optimal target point selection was confirmed by excellent post-operative tremor suppression.

Development of theta rhythmicity in entorhinal stellate cells of the juvenile rat

Mature stellate cells of the rat medial entorhinal cortex (EC), layer II, exhibit subthreshold membrane potential oscillations (MPOs) at theta frequencies (4-12 Hz) in vitro. We find that MPOs appear between postnatal days 14 (P14) and 18 (P18) but show little further change by day 28+ (P28-P32). To identify the factors responsible, we examined the electrical responses of developing stellate cells, paying attention to two currents thought necessary for mature oscillation: the h current I(h), which provides the slow rectification required for resonance; and a persistent sodium current I(NaP), which provides amplification of resonance. Responses to injected current revealed that P14 cells were often nonresonant with a relatively high resistance. Densities of I(h) and I(NaP) both rose by about 50% from P14 to P18. However, I(h) levels fell to intermediate values by P28+. Given the nonrobust trend in I(h) expression and a previously demonstrated potency of even low levels of I(h) to sustain oscillation, we propose that resonance and MPOs are limited at P14 more by low levels of I(NaP) than of I(h). The relative importance of I(NaP) for the development of MPOs is supported by simulations of a conductance-based model, which also suggest that general shunt conductance may influence the precise age when MPOs appear. In addition to our physiological study, we analyzed spine densities at P14, P18, and P28+ and found a vigorous synaptogenesis across the whole period. Our data predict that functions that rely on theta rhythmicity in the hippocampal network are limited until at least P18.

Spontaneous plasticity of multineuronal activity patterns in activated hippocampal networks

Using functional multineuron imaging with single-cell resolution, we examined how hippocampal networks by themselves change the spatiotemporal patterns of spontaneous activity during the course of emitting spontaneous activity. When extracellular ionic concentrations were changed to those that mimicked in vivo conditions, spontaneous activity was increased in active cell number and activity frequency. When ionic compositions were restored to the control conditions, the activity level returned to baseline, but the weighted spatial dispersion of active cells, as assessed by entropy-based metrics, did not. Thus, the networks can modify themselves by altering the internal structure of their correlated activity, even though they as a whole maintained the same level of activity in space and time.

Potential adaptive function for altered long-term potentiation mechanisms in aging hippocampus

Age-dependent alterations in the induction of long-term potentiation (LTP) are well documented, providing a likely neural basis for memory decline associated with aging. Studies of neural plasticity are also important to understand the neural basis of individual differences in aging, ranging from significant cognitive impairment to preservation of function on a par with younger adults. To examine the cellular mechanisms that distinguish such outcomes, we studied the induction of LTP in male outbred young and aged rats behaviorally characterized in hippocampal-dependent spatial learning. We evaluated, in vitro, the magnitude of NMDA receptor (NMDAR)-dependent and -independent forms of LTP induced in the Schaffer collateral to CA1 synapses. We found that age substantially reduces NMDAR-dependent LTP across the spectrum of cognitive outcomes, whereas increased NMDAR-independent LTP occurs distinctively in high-performing aged rats. Moreover, in young rats, behavioral performance correlates strongly with the magnitude of NMDAR-LTP, whereas NMDAR-independent LTP correlates with behavioral performance only in aged rats. Together with similar previous findings on the mechanisms for LTD in this model, these results support the notion that a shift from NMDAR-dependent to NMDAR-independent mechanisms for neural plasticity during aging is associated with better cognitive outcomes.