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

Quantitative interactions between the A-type K+ current and inositol trisphosphate receptors regulate intraneuronal Ca2+ waves and synaptic plasticity

The A-type potassium current has been implicated in the regulation of several physiological processes. Here, we explore a role for the A-type potassium current in regulating the release of calcium through inositol trisphosphate receptors (InsP3R) that reside on the endoplasmic reticulum (ER) of hippocampal pyramidal neurons. To do this, we constructed morphologically realistic, conductance-based models equipped with kinetic schemes that govern several calcium signalling modules and pathways, and constrained the distributions and properties of constitutive components by experimental measurements from these neurons. Employing these models, we establish a bell-shaped dependence of calcium release through InsP3Rs on the density of A-type potassium channels, during the propagation of an intraneuronal calcium wave initiated through established protocols. Exploring the sensitivities of calcium wave initiation and propagation to several underlying parameters, we found that ER calcium release critically depends on dendritic diameter and that wave initiation occurred at branch points as a consequence of a high surface area to volume ratio of oblique dendrites. Furthermore, analogous to the role of A-type potassium channels in regulating spike latency, we found that an increase in the density of A-type potassium channels led to increases in the latency and the temporal spread of a propagating calcium wave. Next, we incorporated kinetic models for the metabotropic glutamate receptor (mGluR) signalling components and a calcium-controlled plasticity rule into our model and demonstrate that the presence of mGluRs induced a leftward shift in a Bienenstock-Cooper-Munro-like synaptic plasticity profile. Finally, we show that the A-type potassium current could regulate the relative contribution of ER calcium to synaptic plasticity induced either through 900 pulses of various stimulus frequencies or through theta burst stimulation. Our results establish a novel form of interaction between active dendrites and the ER membrane, uncovering a powerful mechanism that could regulate biophysical/biochemical signal integration and steer the spatiotemporal spread of signalling microdomains through changes in dendritic excitability.

Differences between synaptic plasticity thresholds result in new timing rules for maximizing long-term potentiation

The fundamental observation that the temporal spacing of learning episodes plays a critical role in the efficiency of memory encoding has had little effect on either research on long-term potentiation (LTP) or efforts to develop cognitive enhancers. Here we review recent findings describing a spaced trials phenomenon for LTP that appears to be related to recent evidence that plasticity thresholds differ between synapses in the adult hippocampus. Results of tests with one memory enhancing drug suggest that the compound potently facilitates LTP via effects on 'high threshold' synapses and thus alters the temporally extended timing rules. Possible implications of these results for our understanding of LTP substrates, neurobiological contributors to the distributed practice effect, and the consequences of memory enhancement are discussed. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

The relationship between tetanus intensity and the magnitude of hippocampal long-term potentiation in vivo

In this study, we assessed the effects of varying tetanus and test-pulse intensity on the magnitude of long-term potentiation (LTP) in the perforant path–dentate gyrus projection of urethane-anaesthetized rats. We developed a novel within-subjects procedure in which test-pulse-stimulation intensity (60–1000 μA) was varied quasi-randomly under computer control throughout the recording period. After a baseline period, we applied a high-frequency tetanus, the intensity of which was varied over the same range as test-pulse intensity, but between subjects. The time-course of LTP was thus monitored continuously across a range of test-pulse intensities in each rat. Intense high-frequency tetanization at 1000 μA resulted in a paradoxical depression of the dentate field excitatory post-synaptic potential (fEPSP) slope at the lowest test intensity used (60 μA), but caused a potentiation at higher test intensities in the same animal. Moreover, intense tetanization induced less LTP than a moderate tetanus over most of the test-intensity range. Explanations for this pattern of data include a potentiation of feed-forward inhibition in conjunction with LTP of excitatory neurotransmission, or local tissue damage at the stimulation site. To address this issue, we conducted an additional experiment in which a second stimulating electrode was placed in the perforant path at a site closer to the dentate, in order to activate a common population of afferents at a location ‘downstream’ of the original stimulation site. After 1000-μA tetanization of the original (‘upstream’) site, fEPSPs were again depressed in response to test stimulation of the upstream site, but only potentiation was observed in response to stimulation of the downstream site. This is consistent with the idea that the depression induced by intense tetanization results from local changes at the stimulation site. In conclusion, while tetanus intensity must exceed the LTP induction threshold, intensities above 500 μA should be avoided; in the present study, tetanization at 250–500 μA yielded maximal levels of LTP.

Involvement of the proteasome and caspase activation in hippocampal long-term depression induced by the serine protease subtilisin

The serine protease subtilisin-A produces a long-term depression (LTD) of synaptic potentials in hippocampal slices which differs mechanistically from classical LTD. Since caspases have been implicated in hippocampal plasticity, this study examined a possible role for these enzymes in subtilisin-induced LTD. Subtilisin produced a concentration-dependent decrease in the size of field excitatory synaptic potentials (fEPSPs), which was not prevented or modified by the caspase inhibitors Z-VAD(OMe)-fmk and Z-DEVD-fmk. Similarly Z-VAD(OMe)-fmk did not modify the selective loss of protein expression produced by subtilisin. Subtilisin reduced the expression of procaspase-3 and caspase-9 but, while caspase-9 was converted to its conventionally activated form (39 kDa), caspase-3 was metabolised along a non-canonical pathway to a 29/30 kDa protein rather than the classical 17/19 kDa fragments. Both Z-VAD(OMe)-fmk and Z-DEVD-fmk were unable to prevent the reduced expression of Postsynaptic Density Protein-95, Vesicle-Associated Membrane Protein-1 and Unco-ordinated 5H3 proteins produced by subtilisin, although MG132 did produce partial recovery from subtilisin-induced depression of fEPSPs. When tested on long-term potentiation (LTP) induced by theta stimulation in the stratum radiatum, MG132 inhibited the immediate increase in fEPSP size but generated a higher plateau LTP. Twin LTP stimulation generated a further increase in LTP amplitude in control slices but not in slices exposed to MG132. The results indicate that subtilisin does produce caspase activation but that this does not contribute to its induction of LTD. However, activation of the proteasome does contribute to subtilisin-induced LTD and may also play a modulatory role in electrically induced LTP.

Protease-activated receptor-1 modulates hippocampal memory formation and synaptic plasticity

Protease-activated receptor-1 (PAR1) is an unusual G-protein coupled receptor (GPCR) that is activated through proteolytic cleavage by extracellular serine proteases. Although previous work has shown that inhibiting PAR1 activation is neuroprotective in models of ischemia, traumatic injury, and neurotoxicity, surprisingly little is known about PAR1's contribution to normal brain function. Here, we used PAR1-/- mice to investigate the contribution of PAR1 function to memory formation and synaptic function. We demonstrate that PAR1-/- mice have deficits in hippocampus-dependent memory. We also show that while PAR1-/- mice have normal baseline synaptic transmission at Schaffer collateral-CA1 synapses, they exhibit severe deficits in N-methyl-d-aspartate receptor (NMDAR)-dependent long-term potentiation (LTP). Mounting evidence indicates that activation of PAR1 leads to potentiation of NMDAR-mediated responses in CA1 pyramidal cells. Taken together, this evidence and our data suggest an important role for PAR1 function in NMDAR-dependent processes subserving memory formation and synaptic plasticity.

Synaptic plasticity in the hippocampus shows resistance to acute ethanol exposure in transgenic mice with astrocyte-targeted enhanced CCL2 expression

It has been shown that ethanol exposure can activate astrocytes and microglia resulting in the production of neuroimmune factors, including the chemokine CCL2. The role of these neuroimmune factors in the effects of ethanol on the central nervous system has yet to be elucidated. To address this question, we investigated the effects of ethanol on synaptic transmission and plasticity in the hippocampus from mice that express elevated levels of CCL2 in the brain and their non-transgenic littermate controls. The brains of the transgenic mice simulate one aspect of the alcoholic brain, chronically increased levels of CCL2. We used extracellular field potential recordings in acutely isolated hippocampal slices to identify neuroadaptive changes produced by elevated levels of CCL2 and how these neuroadaptive changes affect the actions of acute ethanol. Results showed that synaptic transmission and the effects of ethanol on synaptic transmission were similar in the CCL2-transgenic and non-transgenic hippocampus. However, long-term potentiation (LTP), a cellular mechanism thought to underlie learning and memory, in the CCL2-transgenic hippocampus was resistant to the ethanol-induced depression of LTP observed in the non-transgenic hippocampus. Consistent with these results, ethanol pretreatment significantly impaired cued and contextual fear conditioning in non-transgenic mice, but had no effect in CCL2-transgenic mice. These data show that chronically elevated levels of CCL2 in the hippocampus produce neuroadaptive changes that block the depressing effects of ethanol on hippocampal synaptic plasticity and support the hypothesis that CCL2 may provide a neuroprotective effect against the devastating actions of ethanol on hippocampal function.

Elfn1 regulates target-specific release probability at CA1-interneuron synapses

Although synaptic transmission may be unidirectional, the establishment of synaptic connections with specific properties can involve bidirectional signaling. Pyramidal neurons in the hippocampus form functionally distinct synapses onto two types of interneurons. Excitatory synapses onto oriens-lacunosum moleculare (O-LM) interneurons are facilitating and have a low release probability, whereas synapses onto parvalbumin interneurons are depressing and have a high release probability. Here, we show that the extracellular leucine-rich repeat fibronectin containing 1 (Elfn1) protein is selectively expressed by O-LM interneurons and regulates presynaptic release probability to direct the formation of highly facilitating pyramidal-O-LM synapses. Thus, postsynaptic expression of Elfn1 in O-LM interneurons regulates presynaptic release probability, which confers target-specific synaptic properties to pyramidal cell axons.

Involvement of the cholinergic system of CA1 on harmane-induced amnesia in the step-down passive avoidance test

β-carboline alkaloids such as harmane (HA) are naturally present in the human food chain. They are derived from the plant Peganum harmala and have many cognitive effects. In the present study, effects of the nicotinic system of the dorsal hippocampus (CA1) on HA-induced amnesia and exploratory behaviors were examined. One-trial step-down and hole-board paradigms were used to assess memory retention and exploratory behaviors in adult male mice. Pre-training (15 mg/kg) but not pre-testing intraperitoneal (i.p.) administration of HA decreased memory formation but did not alter exploratory behaviors. Moreover, pre-testing administration of nicotine (0.5 µg/mouse, intra-CA1) decreased memory retrieval, but induced anxiogenic-like behaviors. On the other hand, pre-test intra-CA1 injection of ineffective doses of nicotine (0.1 and 0.25 µg/mouse) fully reversed HA-induced impairment of memory after pre-training injection of HA (15 mg/kg, i.p.) which did not alter exploratory behaviors. Furthermore, pre-testing administration of mecamylamine (0.5, 1 and 2 µg/mouse, intra-CA1) did not alter memory retrieval but fully reversed HA-induced impairment of memory after pre-training injection of HA (15 mg/kg, i.p.) which had no effect on exploratory behaviors. In conclusion, the present findings suggest the involvement of the nicotinic cholinergic system in the HA-induced impairment of memory formation.

Interaction of inhibition and triplets of excitatory spikes modulates the NMDA-R-mediated synaptic plasticity in a computational model of spike timing-dependent plasticity

Spike timing-dependent plasticity (STDP) experiments have shown that a synapse is strengthened when a presynaptic spike precedes a postsynaptic one and depressed vice versa. The canonical form of STDP has been shown to have an asymmetric shape with the peak long-term potentiation at +6 ms and the peak long-term depression at -5 ms. Experiments in hippocampal cultures with more complex stimuli such as triplets (one presynaptic spike combined with two postsynaptic spikes or one postsynaptic spike with two presynaptic spikes) have shown that pre-post-pre spike triplets result in no change in synaptic strength, whereas post-pre-post spike triplets lead to significant potentiation. The sign and magnitude of STDP have also been experimentally hypothesized to be modulated by inhibition. Recently, a computational study showed that the asymmetrical form of STDP in the CA1 pyramidal cell dendrite when two spikes interact switches to a symmetrical one in the presence of inhibition under certain conditions. In the present study, I investigate computationally how inhibition modulates STDP in the CA1 pyramidal neuron dendrite when it is driven by triplets. The model uses calcium as the postsynaptic signaling agent for STDP and is shown to be consistent with the experimental triplet observations in the absence of inhibition: simulated pre-post-pre spike triplets result in no change in synaptic strength, whereas simulated post-pre-post spike triplets lead to significant potentiation. When inhibition is bounded by the onset and offset of the triplet stimulation, then the strength of the synapse is decreased as the strength of inhibition increases. When inhibition arrives either few milliseconds before or at the onset of the last spike in the pre-post-pre triplet stimulation, then the synapse is potentiated. Variability in the frequency of inhibition (50 vs. 100 Hz) produces no change in synaptic strength. Finally, a 5% variation in model's calcium parameters (calcium thresholds) proves that the model's performance is robust.

The NMDA receptor as a target for cognitive enhancement

NMDA receptors (NMDARs) play an important role in neural plasticity including long-term potentiation and long-term depression, which are likely to explain their importance for learning and memory. Cognitive decline is a major problem facing an ageing human population, so much so that its reversal has become an important goal for scientific research and pharmaceutical development. Enhancement of NMDAR function is a core strategy toward this goal. In this review we indicate some of the major ways of potentiating NMDAR function by both direct and indirect modulation. There is good evidence that both positive and negative modulation can enhance function suggesting that a subtle approach correcting imbalances in particular clinical situations will be required. Excessive activation and the resultant deleterious effects will need to be carefully avoided. Finally we describe some novel positive allosteric modulators of NMDARs, with some subunit selectivity, and show initial evidence of their ability to affect NMDAR mediated events. This article is part of a Special Issue entitled 'Cognitive Enhancers'.

Photochemical inactivation analysis of temporal dynamics of postsynaptic native AMPA receptors in hippocampal slices

Postsynaptic expression of AMPA-type glutamate receptors (AMPAR) is more mobile than previously thought. Much evidence suggests that AMPAR are delivered from intracellular reserved pools to postsynaptic sites in a constitutive, as well as activity-dependent manner by exocytosis, lateral diffusion, or diffusional trapping. These notions were supported by optical monitoring of AMPAR subunits labeled with macromolecular tags such as GFP or Immunobeads, although it remains uncertain whether the mode and rate of synaptic delivery are similar to native "unlabeled" receptors. To reveal the real-time dynamics of native AMPAR in situ, photochemical inactivation of surface receptors using 6-azido-7-nitro-1,4-dihydroquinoxaline-2,3-dione (ANQX), a photoreactive AMPAR blocker, was adopted for acute hippocampal slices of mice. Because of the irreversible block due to cross-link formation between ANQX and surface AMPAR, recovery of EPSPs after photoinactivation reflects the time course of synaptic delivery of intracellular AMPAR. Brief UV illumination with fast application of ANQX resulted in persistent suppression of EPSPs for a prolonged period of up to 3 h, suggesting minimal synaptic delivery of AMPAR by exocytosis in the resting condition. Kinetic analysis of EPSP recovery clarified that the supply of postsynaptic AMPAR from the intracellular pool is dominated in the initial, but not in the later, phase of long-term potentiation (LTP). These results suggest that constitutive synaptic delivery is minimal in the resting condition at intact hippocampal synapses in a time scale of hours, while postsynaptic AMPAR are replaced with those in intracellular pools almost exclusively in an activity-dependent manner, typically shortly after LTP induction.

Decreased afferent excitability contributes to synaptic depression during high-frequency stimulation in hippocampal area CA1

Long-term potentiation (LTP) is often induced experimentally by continuous high-frequency afferent stimulation (HFS), typically at 100 Hz for 1 s. Induction of LTP requires postsynaptic depolarization and voltage-dependent calcium influx. Induction is more effective if the same number of stimuli are given as a series of short bursts rather than as continuous HFS, in part because excitatory postsynaptic potentials (EPSPs) become strongly depressed during HFS, reducing postsynaptic depolarization. In this study, we examined mechanisms of EPSP depression during HFS in area CA1 of rat hippocampal brain slices. We tested for presynaptic terminal vesicle depletion by examining minimal stimulation-evoked excitatory postsynaptic currents (EPSCs) during 100-Hz HFS. While transmission failures increased, consistent with vesicle depletion, EPSC latencies also increased during HFS, suggesting a decrease in afferent excitability. Extracellular recordings of Schaffer collateral fiber volleys confirmed a decrease in afferent excitability, with decreased fiber volley amplitudes and increased latencies during HFS. To determine the mechanism responsible for fiber volley changes, we recorded antidromic action potentials in single CA3 pyramidal neurons evoked by stimulating Schaffer collateral axons. During HFS, individual action potentials decreased in amplitude and increased in latency, and these changes were accompanied by a large increase in the probability of action potential failure. Time derivative and phase-plane analyses indicated decreases in both axon initial segment and somato-dendritic components of CA3 neuron action potentials. Our results indicate that decreased presynaptic axon excitability contributes to depression of excitatory synaptic transmission during HFS at synapses between Schaffer collaterals and CA1 pyramidal neurons.

Frequency dependency of NMDA receptor-dependent synaptic plasticity in the hippocampal CA1 region of freely behaving mice

Hippocampal synaptic plasticity in the form of long-term potentiation (LTP) and long-term depression (LTD) is likely to enable synaptic information storage in support of memory formation. The mouse brain has been subjected to intensive scrutiny in this regard; however, a multitude of studies has examined synaptic plasticity in the hippocampal slice preparation, whereas very few have addressed synaptic plasticity in the freely behaving mouse. Almost nothing is known about the frequency or N-methyl-D-aspartate receptor (NMDAR) dependency of hippocampal synaptic plasticity in the intact mouse brain. Therefore, in this study, we investigated the forms of synaptic plasticity that are elicited at different afferent stimulation frequencies. We also addressed the NMDAR dependency of this phenomenon. Adult male C57BL/6 mice were chronically implanted with a stimulating electrode into the Schaffer collaterals and a recording electrode into the Stratum radiatum of the CA1 region. To examine synaptic plasticity, we chose protocols that were previously shown to produce either LTP or LTD in the hippocampal slice preparation. Low-frequency stimulation (LFS) at 1 Hz (900 pulses) had no effect on evoked responses. LFS at 3 Hz (ranging from 200 up to 2 × 900 pulses) elicited short-term depression (STD, <45 min). LFS at 3 Hz (1,200 pulses) elicited slow-onset potentiation, high-frequency stimulation (HFS) at 100 Hz (100 or 200 pulses) or at 50 Hz was ineffective, whereas 100 Hz (50 pulses) elicited short-term potentiation (STP). HFS at 100 Hz given as 2 × 30, 2 × 50, or 4 × 50 pulses elicited LTP (>24 h). Theta-burst stimulation was ineffective. Antagonism of the NMDAR prevented STD, STP, and LTP. This study shows for the first time that protocols that effectively elicit persistent synaptic plasticity in the slice preparation elicit distinctly different effects in the intact mouse brain. Persistent LTD could not be elicited with any of the protocols tested. Plasticity responses are NMDAR dependent, suggesting that these phenomena are relevant for hippocampus-dependent learning.

Plasticity-inducing TMS protocols to investigate somatosensory control of hand function

Hand function depends on sensory feedback to direct an appropriate motor response. There is clear evidence that somatosensory cortices modulate motor behaviour and physiology within primary motor cortex. However, this information is mainly from research in animals and the bridge to human hand control is needed. Emerging evidence in humans supports the notion that somatosensory cortices modulate motor behaviour, physiology and sensory perception. Transcranial magnetic stimulation (TMS) allows for the investigation of primary and higher-order somatosensory cortices and their role in control of hand movement in humans. This review provides a summary of several TMS protocols in the investigation of hand control via the somatosensory cortices. TMS plasticity inducing protocols reviewed include paired associative stimulation, repetitive TMS, theta-burst stimulation as well as other techniques that aim to modulate cortical excitability in sensorimotor cortices. Although the discussed techniques may modulate cortical excitability, careful consideration of experimental design is needed to isolate factors that may interfere with desired results of the plasticity-inducing protocol, specifically events that may lead to metaplasticity within the targeted cortex.

Developmental hippocampal neuroplasticity in a model of nicotine replacement therapy during pregnancy and breastfeeding

RATIONALE

The influence of developmental nicotine exposure on the brain represents an important health topic in light of the popularity of nicotine replacement therapy (NRT) as a smoking cessation method during pregnancy.

OBJECTIVES

In this study, we used a model of NRT during pregnancy and breastfeeding to explore the consequences of chronic developmental nicotine exposure on cerebral neuroplasticity in the offspring. We focused on two dynamic lifelong phenomena in the dentate gyrus (DG) of the hippocampus that are highly sensitive to the environment: granule cell neurogenesis and long-term potentiation (LTP).

METHODS

Pregnant rats were implanted with osmotic mini-pumps delivering either nicotine or saline solutions. Plasma nicotine and metabolite levels were measured in dams and offspring. Corticosterone levels, DG neurogenesis (cell proliferation, survival and differentiation) and glutamatergic electrophysiological activity were measured in pups.

RESULTS

Juvenile (P15) and adolescent (P41) offspring exposed to nicotine throughout prenatal and postnatal development displayed no significant alteration in DG neurogenesis compared to control offspring. However, NRT-like nicotine exposure significantly increased LTP in the DG of juvenile offspring as measured in vitro from hippocampal slices, suggesting that the mechanisms underlying nicotine-induced LTP enhancement previously described in adult rats are already functional in pups.

CONCLUSIONS

These results indicate that synaptic plasticity is disrupted in offspring breastfed by dams passively exposed to nicotine in an NRT-like fashion.

Analgesic effects of repetitive transcranial magnetic stimulation of the motor cortex in neuropathic pain: influence of theta burst stimulation priming

BACKGROUND

'Conventional' protocols of high-frequency repetitive transcranial magnetic stimulation (rTMS) delivered to M1 can produce analgesia. Theta burst stimulation (TBS), a novel rTMS paradigm, is thought to produce greater changes in M1 excitability than 'conventional' protocols. After a preliminary experiment showing no analgesic effect of continuous or intermittent TBS trains (cTBS or iTBS) delivered to M1 as single procedures, we used TBS to prime a subsequent session of 'conventional' 10 Hz-rTMS.

METHODS

In 14 patients with chronic refractory neuropathic pain, navigated rTMS was targeted over M1 hand region, contralateral to painful side. Analgesic effects were daily assessed on a visual analogue scale for the week after each 10 Hz-rTMS session, preceded or not by TBS priming. In an additional experiment, the effects on cortical excitability parameters provided by single- and paired-pulse TMS paradigms were studied.

RESULTS

Pain level was reduced after any type of rTMS procedure compared to baseline, but iTBS priming produced greater analgesia than the other protocols. Regarding motor cortex excitability changes, the analgesic effects were associated with an increase in intracortical inhibition, whatever the type of stimulation, primed or non-primed.

CONCLUSIONS

The present results show that the analgesic effects of 'conventional' 10 Hz-rTMS delivered to M1 can be enhanced by TBS priming, at least using iTBS. Interestingly, the application of cTBS and iTBS did not produce opposite modulations, unlike previously reported in other systems. It remains to be determined whether the interest of TBS priming is to generate a simple additive effect or a more specific process of cortical plasticity.

Target-specific output patterns are predicted by the distribution of regular-spiking and bursting pyramidal neurons in the subiculum

Pyramidal neurons in the subiculum project to a variety of cortical and subcortical areas in the brain to convey information processed in the hippocampus. Previous studies have shown that two groups of subicular pyramidal neurons--regular-spiking and bursting neurons--are distributed in an organized fashion along the proximal-distal axis, with more regular-spiking neurons close to CA1 (proximal) and more bursting neurons close to presubiculum (distal). Anatomically, neurons projecting to some targets are located more proximally along this axis, while others are located more distally. However, the relationship between the firing properties and the targets of subicular pyramidal neurons is not known. To study this relationship, we used in vivo injections of retrogradely transported fluorescent beads into each of nine different regions and conducted whole-cell current-clamp recordings from the bead-containing subicular neurons in acute brain slices. We found that subicular projections to each area were composed of a mixture of regular-spiking and bursting neurons. Neurons projecting to amygdala, lateral entorhinal cortex, nucleus accumbens, and medial/ventral orbitofrontal cortex were located primarily in the proximal subiculum and consisted mostly of regular-spiking neurons (∼80%). By contrast, neurons projecting to medial EC, presubiculum, retrosplenial cortex, and ventromedial hypothalamus were located primarily in the distal subiculum and consisted mostly of bursting neurons (∼80%). Neurons projecting to a thalamic nucleus were located in the middle portion of subiculum, and their probability of bursting was close to 50%. Thus, the fraction of bursting neurons projecting to each target region was consistent with the known distribution of regular-spiking and bursting neurons along the proximal-distal axis of the subiculum. Variation in the distribution of regular-spiking and bursting neurons suggests that different types of information are conveyed from the subiculum to its various targets.

BDNF upregulation rescues synaptic plasticity in middle-aged ovariectomized rats

Brain-derived neurotrophic factor (BDNF) has emerged as a possible broad-spectrum treatment for the plasticity losses found in rodent models of human conditions associated with memory and cognitive deficits. We have tested this strategy in the particular case of ovariectomy. The actin polymerization in spines normally found after patterned afferent stimulation was greatly reduced, along with the stabilization of long-term potentiation, in hippocampal slices prepared from middle-aged ovariectomized rats. Both effects were fully restored by a 60-minute infusion of 2 nM BDNF. Comparable rescue results were obtained after elevating endogenous BDNF protein levels in hippocampus with 4 daily injections of a short half-life ampakine (positive modulator of α-amino-3-hydroxy-5-methyl-4-isoxazolepropionate [AMPA]-type glutamate receptors). These results provide the first evidence that minimally invasive, mechanism-based drug treatments can ameliorate defects in spine plasticity caused by depressed estrogen levels.

Enhanced intrinsic excitability and EPSP-spike coupling accompany enriched environment-induced facilitation of LTP in hippocampal CA1 pyramidal neurons

Environmental enrichment (EE) is a well-established paradigm for studying naturally occurring changes in synaptic efficacy in the hippocampus that underlie experience-induced modulation of learning and memory in rodents. Earlier research on the effects of EE on hippocampal plasticity focused on long-term potentiation (LTP). Whereas many of these studies investigated changes in synaptic weight, little is known about potential contributions of neuronal excitability to EE-induced plasticity. Here, using whole-cell recordings in hippocampal slices, we address this gap by analyzing the impact of EE on both synaptic plasticity and intrinsic excitability of hippocampal CA1 pyramidal neurons. Consistent with earlier reports, EE increased contextual fear memory and dendritic spine density on CA1 cells. Furthermore, EE facilitated LTP at Schaffer collateral inputs to CA1 pyramidal neurons. Analysis of the underlying causes for enhanced LTP shows EE to increase the frequency but not amplitude of miniature excitatory postsynaptic currents. However, presynaptic release probability, assayed using paired-pulse ratios and use-dependent block of N-methyl-d-aspartate receptor currents, was not affected. Furthermore, CA1 neurons fired more action potentials (APs) in response to somatic depolarization, as well as during the induction of LTP. EE also reduced spiking threshold and after-hyperpolarization amplitude. Strikingly, this EE-induced increase in excitability caused the same-sized excitatory postsynaptic potential to fire more APs. Together, these findings suggest that EE may enhance the capacity for plasticity in CA1 neurons, not only by strengthening synapses but also by enhancing their efficacy to fire spikes—and the two combine to act as an effective substrate for amplifying LTP.

Increased size and stability of CA1 and CA3 place fields in HCN1 knockout mice

Hippocampal CA1 and CA3 pyramidal neuron place cells encode the spatial location of an animal through localized firing patterns called "place fields." To explore the mechanisms that control place cell firing and their relationship to spatial memory, we studied mice with enhanced spatial memory resulting from forebrain-specific knockout of the HCN1 hyperpolarization-activated cation channel. HCN1 is strongly expressed in CA1 neurons and in entorhinal cortex grid cells, which provide spatial information to the hippocampus. Both CA1 and CA3 place fields were larger but more stable in the knockout mice, with the effect greater in CA1 than CA3. As HCN1 is only weakly expressed in CA3 place cells, their altered activity likely reflects loss of HCN1 in grid cells. The more pronounced changes in CA1 likely reflect the intrinsic contribution of HCN1. The enhanced place field stability may underlie the effect of HCN1 deletion to facilitate spatial learning and memory.

Theta oscillations reflect a putative neural mechanism for human sensorimotor integration

Hippocampal theta oscillations (3–12 Hz) may reflect a mechanism for sensorimotor integration in rats (Bland BH. Prog Neurobiol 26: 1–54, 1986); however, it is unknown whether cortical theta activity underlies sensorimotor integration in humans. Rather, the mu rhythm (8–12 Hz) is typically found to desynchronize during movement. We measured oscillatory EEG activity for two conditions of an instructed delayed reaching paradigm. Conditions 1 and 2 were designed to differentially manipulate the contribution of the ventral visuomotor stream during the response initiation phase. We tested the hypothesis that theta activity would reflect changes in the relevant sensorimotor network: condition 2 engaged ventral stream mechanisms to a greater extent than condition 1. Theta oscillations were more prevalent during movement initiation and execution than during periods of stillness, consistent with a sensorimotor relevance for theta activity. Furthermore, theta activity was more prevalent at temporal sites in condition 2 than condition 1 during response initiation, suggesting that theta activity is present within the necessary sensorimotor network. Mu activity desynchronized more during condition 2 than condition 1, suggesting mu desynchronization is also specific to the sensorimotor network. In summary, cortical theta synchronization and mu desynchronization may represent broadly applicable rhythmic mechanisms for sensorimotor integration in the human brain.

Alterations of endocannabinoid signaling, synaptic plasticity, learning, and memory in monoacylglycerol lipase knock-out mice

Endocannabinoid (eCB) signaling is tightly regulated by eCB biosynthetic and degradative enzymes. The eCB 2-arachidonoylglycerol (2-AG) is hydrolyzed primarily by monoacylglycerol lipase (MAGL). Here, we investigated whether eCB signaling, synaptic function, and learning behavior were altered in MAGL knock-out mice. We report that MAGL⁻/⁻ mice exhibited prolonged depolarization-induced suppression of inhibition (DSI) in hippocampal CA1 pyramidal neurons, providing genetic evidence that the inactivation of 2-AG by MAGL determines the time course of the eCB-mediated retrograde synaptic depression. CB₁ receptor antagonists enhanced basal IPSCs in CA1 pyramidal neurons in MAGL⁻/⁻ mice, while the magnitude of DSI or CB₁ receptor agonist-induced depression of IPSCs was decreased in MAGL⁻/⁻ mice. These results suggest that 2-AG elevations in MAGL⁻/⁻ mice cause tonic activation and partial desensitization of CB₁ receptors. Genetic deletion of MAGL selectively enhanced theta burst stimulation (TBS)-induced long-term potentiation (LTP) in the CA1 region of hippocampal slices but had no significant effect on LTP induced by high-frequency stimulation or long-term depression induced by low-frequency stimulation. The enhancement of TBS-LTP in MAGL⁻/⁻ mice appears to be mediated by 2-AG-induced suppression of GABA(A) receptor-mediated inhibition. MAGL⁻/⁻ mice exhibited enhanced learning as shown by improved performance in novel object recognition and Morris water maze. These results indicate that genetic deletion of MAGL causes profound changes in eCB signaling, long-term synaptic plasticity, and learning behavior.

Developmental regulation of the late phase of long-term potentiation (L-LTP) and metaplasticity in hippocampal area CA1 of the rat

Long-term potentiation (LTP) is a form of synaptic plasticity thought to underlie memory; thus knowing its developmental profile is fundamental to understanding function. Like memory, LTP has multiple phases with distinct timing and mechanisms. The late phase of LTP (L-LTP), lasting longer than 3 h, is protein synthesis dependent and involves changes in the structure and content of dendritic spines, the major sites of excitatory synapses. In previous work, tetanic stimulation first produced L-LTP at postnatal day 15 (P15) in area CA1 of rat hippocampus. Here we used a more robust induction paradigm involving theta-burst stimulation (TBS) in acute slices and found the developmental onset of L-LTP to be 3 days earlier at P12. In contrast, at P8-11, TBS only reversed the synaptic depression that occurs from test-pulse stimulation in developing (P8-15) hippocampus. A second bout of TBS delivered 30-180 min later produced L-LTP at P10-11 but not at P8-9 and enhanced L-LTP at P12-15. Both the developmental onset and the enhanced L-LTP produced by repeated bouts of TBS were blocked by the N-methyl-d-aspartate receptor antagonist dl-2-amino-5-phosphonovaleric acid. Thus the developmental onset age is P12 for L-LTP induced by the more robust and perhaps more naturalistic TBS induction paradigm. Metaplasticity produced by repeated bouts of TBS is developmentally regulated, advancing the capacity for L-LTP from P12 to P10, but not to younger ages. Together these findings provide a new basis from which to investigate mechanisms that regulate the developmental onset of this important form of synaptic plasticity.

Bidirectional pattern-specific plasticity of the slow afterhyperpolarization in rats: role for high-voltage activated Ca2+ channels and I h

A burst of action potentials in hippocampal neurons is followed by a slow afterhyperpolarization (sAHP) that serves to limit subsequent firing. A reduction in the sAHP accompanies acquisition of several types of learning, whereas increases in the sAHP are correlated with cognitive impairment. The present study demonstrates in vitro that activity-dependent bidirectional plasticity of the sAHP does not require synaptic activation, and depends on the pattern of action potential firing. Whole-cell current-clamp recordings from CA1 pyramidal neurons in hippocampal slices from young rats (postnatal days 14-24) were performed in blockers of synaptic transmission. The sAHP was evoked by action potential firing at gamma-related (50 Hz, gamma-AHP) or theta frequencies (5 Hz, theta-AHP), two firing frequencies implicated in attention and memory. Interestingly, when the gamma-AHP and theta-AHP were evoked in the same cell, a gradual potentiation of the gamma-AHP (186 ± 31%) was observed that was blocked using Ca(2+) channel blockers nimodipine (10 μm) or ω-conotoxin MVIIC (1 μm). In experiments that exclusively evoked the sAHP with 50 Hz firing, the gamma-AHP was similarly potentiated (198 ± 44%). However, theta-burst firing pattern alone resulted in a decrease (65 ± 19%) of the sAHP. In these experiments, application of the h-channel blocker ZD7288 (25 μm) selectively prevented enhancement of the gamma-AHP. These data demonstrate that induction requirements for bidirectional AHP plasticity depend on the pattern of action potential firing, and result from distinct mechanisms. The identification of novel mechanisms underlying AHP plasticity in vitro provides additional insight into the dynamic processes that may regulate neuronal excitability during learning in vivo.

Protective action of mithramycin against neurodegeneration and impairment of synaptic plasticity in the hippocampal CA1 area after transient global ischemia

Mithramycin A (MTM) is an antibiotic used for the treatment of hypercalcemia and several types of cancer. We have reported previously that MTM protects against endoplasmic reticulum (ER) stress-induced neuronal death in organotypic hippocampal slice cultures. In the present study, the neuroprotective effect of MTM against ischemia/reperfusion-induced neuronal injury was evaluated in the hippocampus in mice. Neuronal damage was apparent in area CA1 of the hippocampus after transient global ischemia/reperfusion. The expression of C/EBP homologous protein (CHOP), a key transcription factor for ER stress-induced neuronal death, showed a pronounced increase in area CA1 in these mice. Treatment of the mice with MTM significantly decreased both the number of neurons stained with Fluoro-Jade B and the level of CHOP expression in the hippocampus. MTM did not affect the increase of 78-kDa glucose-regulated protein induced by ischemia/reperfusion. MTM also restored the ischemia/reperfusion-induced impairment of long-term potentiation in the hippocampus, without any change in paired pulse facilitation. These results suggest that administration of MTM protects hippocampal neurons against injury induced by transient global ischemia/reperfusion through attenuation of ER stress-associated signals, and ameliorates neuronal injury induced by ischemia/reperfusion in the hippocampus.

Theta phase modulates multiple layer-specific oscillations in the CA1 region

It was recently proposed that fast gamma oscillations (60-150 Hz) convey spatial information from the medial entorhinal cortex (EC) to the CA1 region of the hippocampus. However, here we describe 2 functionally distinct oscillations within this frequency range, both coupled to the theta rhythm during active exploration and rapid eye movement sleep: an oscillation with peak activity at ∼80 Hz and a faster oscillation centered at ∼140 Hz. The 2 oscillations are differentially modulated by the phase of theta depending on the CA1 layer; theta-80 Hz coupling is strongest at stratum lacunosum-moleculare, while theta-140 Hz coupling is strongest at stratum oriens-alveus. This laminar profile suggests that the ∼80 Hz oscillation originates from EC inputs to deeper CA1 layers, while the ∼140 Hz oscillation reflects CA1 activity in superficial layers. We further show that the ∼140 Hz oscillation differs from sharp wave-associated ripple oscillations in several key characteristics. Our results demonstrate the existence of novel theta-associated high-frequency oscillations and suggest a redefinition of fast gamma oscillations.

Local and regional network function in behaviorally relevant cortical circuits of adult mice following postnatal alcohol exposure

BACKGROUND

Ethanol consumption during pregnancy can lead to fetal alcohol spectrum disorder (FASD), which consists of the complete spectrum of developmental deficits including neurological dysfunction. FASD is associated with a variety of neurobehavioral disturbances dependent on the age and duration of exposure. Ethanol exposure in neonatal rodents can also induce widespread apoptotic neurodegeneration and long-lasting behavioral abnormalities similar to FASD. The developmental stage of neonatal rodent brains that are at the peak of synaptogenesis is equivalent to the third trimester of human gestation.

METHODS

Male and female C57BL/6By mice were injected with ethanol (20%, 2.5 g/kg, 2 s.c. injections) or an equal volume of saline (controls) on postnatal day 7 (P7). Animals were allowed to mature and at 3 months were tested on an olfactory habituation task known to be dependent on piriform cortex function, a hippocampal-dependent object place memory task, and used for electrophysiological testing of spontaneous and odor-evoked local field potential (LFP) activity in the olfactory bulb, piriform cortex, and dorsal hippocampus.

RESULTS

P7 ethanol induced widespread cell death within 1 day of exposure, with highest levels in the neocortex, intermediate levels in the dorsal hippocampus, and relatively low levels in the primary olfactory system. No impairment of odor investigation or odor habituation was detected in P7 ethanol-exposed 3-month-old mice compared to saline controls. However, hippocampal-dependent object place memory was significantly impaired in the P7 ethanol-treated adult mice. Odor-evoked LFP activity was enhanced throughout the olfacto-hippocampal pathway, primarily within the theta frequency band, although the hippocampus also showed elevated evoked delta frequency activity. In addition, functional coherence between the piriform cortex and olfactory bulb and between the piriform cortex and dorsal hippocampus was enhanced in the beta frequency range in P7 ethanol-treated adult mice compared to controls.

CONCLUSIONS

P7 ethanol induces an immediate wave of regionally selective cell death followed by long-lasting changes in local circuit and regional network function that are accompanied by changes in neurobehavioral performance. The results suggest that both the activity of local neural circuits within a brain region and the flow of information between brain regions can be modified by early alcohol exposure, which may contribute to long-lasting behavioral abnormalities known to rely on those circuits.

A distinctive subpopulation of medial septal slow-firing neurons promote hippocampal activation and theta oscillations

The medial septum-vertical limb of the diagonal band of Broca (MSvDB) is important for normal hippocampal functions and theta oscillations. Although many previous studies have focused on understanding how MSVDB neurons fire rhythmic bursts to pace hippocampal theta oscillations, a significant portion of MSVDB neurons are slow-firing and thus do not pace theta oscillations. The function of these MSVDB neurons, especially their role in modulating hippocampal activity, remains unknown. We recorded MSVDB neuronal ensembles in behaving rats, and identified a distinct physiologically homogeneous subpopulation of slow-firing neurons (overall firing <4 Hz) that shared three features: 1) much higher firing rate during rapid eye movement sleep than during slow-wave (SW) sleep; 2) temporary activation associated with transient arousals during SW sleep; 3) brief responses (latency 15∼30 ms) to auditory stimuli. Analysis of the fine temporal relationship of their spiking and theta oscillations showed that unlike the theta-pacing neurons, the firing of these "pro-arousal" neurons follows theta oscillations. However, their activity precedes short-term increases in hippocampal oscillation power in the theta and gamma range lasting for a few seconds. Together, these results suggest that these pro-arousal slow-firing MSvDB neurons may function collectively to promote hippocampal activation.

Characterizing brain cortical plasticity and network dynamics across the age-span in health and disease with TMS-EEG and TMS-fMRI

Brain plasticity can be conceptualized as nature's invention to overcome limitations of the genome and adapt to a rapidly changing environment. As such, plasticity is an intrinsic property of the brain across the lifespan. However, mechanisms of plasticity may vary with age. The combination of transcranial magnetic stimulation (TMS) with electroencephalography (EEG) or functional magnetic resonance imaging (fMRI) enables clinicians and researchers to directly study local and network cortical plasticity, in humans in vivo, and characterize their changes across the age-span. Parallel, translational studies in animals can provide mechanistic insights. Here, we argue that, for each individual, the efficiency of neuronal plasticity declines throughout the age-span and may do so more or less prominently depending on variable 'starting-points' and different 'slopes of change' defined by genetic, biological, and environmental factors. Furthermore, aberrant, excessive, insufficient, or mistimed plasticity may represent the proximal pathogenic cause of neurodevelopmental and neurodegenerative disorders such as autism spectrum disorders or Alzheimer's disease.

Coordination of size and number of excitatory and inhibitory synapses results in a balanced structural plasticity along mature hippocampal CA1 dendrites during LTP

Enlargement of dendritic spines and synapses correlates with enhanced synaptic strength during long-term potentiation (LTP), especially in immature hippocampal neurons. Less clear is the nature of this structural synaptic plasticity on mature hippocampal neurons, and nothing is known about the structural plasticity of inhibitory synapses during LTP. Here the timing and extent of structural synaptic plasticity and changes in local protein synthesis evidenced by polyribosomes were systematically evaluated at both excitatory and inhibitory synapses on CA1 dendrites from mature rats following induction of LTP with theta-burst stimulation (TBS). Recent work suggests dendritic segments can act as functional units of plasticity. To test whether structural synaptic plasticity is similarly coordinated, we reconstructed from serial section transmission electron microscopy all of the spines and synapses along representative dendritic segments receiving control stimulation or TBS-LTP. At 5 min after TBS, polyribosomes were elevated in large spines suggesting an initial burst of local protein synthesis, and by 2 h only those spines with further enlarged synapses contained polyribosomes. Rapid induction of synaptogenesis was evidenced by an elevation in asymmetric shaft synapses and stubby spines at 5 min and more nonsynaptic filopodia at 30 min. By 2 h, the smallest synaptic spines were markedly reduced in number. This synapse loss was perfectly counterbalanced by enlargement of the remaining excitatory synapses such that the summed synaptic surface area per length of dendritic segment was constant across time and conditions. Remarkably, the inhibitory synapses showed a parallel synaptic plasticity, also demonstrating a decrease in number perfectly counterbalanced by an increase in synaptic surface area. Thus, TBS-LTP triggered spinogenesis followed by loss of small excitatory and inhibitory synapses and a subsequent enlargement of the remaining synapses by 2 h. These data suggest that dendritic segments coordinate structural plasticity across multiple synapses and maintain a homeostatic balance of excitatory and inhibitory inputs through local protein-synthesis and selective capture or redistribution of dendritic resources.

Focal adhesion kinase regulates neuronal growth, synaptic plasticity and hippocampus-dependent spatial learning and memory

The focal adhesion kinase (FAK) is a non-receptor tyrosine kinase abundantly expressed in the mammalian brain and highly enriched in neuronal growth cones. Inhibitory and facilitatory activities of FAK on neuronal growth have been reported and its role in neuritic outgrowth remains controversial. Unlike other tyrosine kinases, such as the neurotrophin receptors regulating neuronal growth and plasticity, the relevance of FAK for learning and memory in vivo has not been clearly defined yet. A comprehensive study aimed at determining the role of FAK in neuronal growth, neurotransmitter release and synaptic plasticity in hippocampal neurons and in hippocampus-dependent learning and memory was therefore undertaken using the mouse model. Gain- and loss-of-function experiments indicated that FAK is a critical regulator of hippocampal cell morphology. FAK mediated neurotrophin-induced neuritic outgrowth and FAK inhibition affected both miniature excitatory postsynaptic potentials and activity-dependent hippocampal long-term potentiation prompting us to explore the possible role of FAK in spatial learning and memory in vivo. Our data indicate that FAK has a growth-promoting effect, is importantly involved in the regulation of the synaptic function and mediates in vivo hippocampus-dependent spatial learning and memory.

Transcranial magnetic brain stimulation: therapeutic promises and scientific gaps

Since its commercial advent in 1985, transcranial magnetic stimulation (TMS), a technique for stimulating neurons in the cerebral cortex through the scalp, safely and with minimal discomfort, has captured the imaginations of scientists, clinicians and lay observers. Initially a laboratory tool for neurophysiologists studying the human motor system, TMS now has a growing list of applications in clinical and basic neuroscience. Although we understand many of its effects at the system level, detailed knowledge of its actions, particularly as a modulator of neural activity, has lagged, due mainly to the lack of suitable non-human models. Nevertheless, these gaps have not blocked the therapeutic application of TMS in brain disorders. Moderate success has been achieved in treating disorders such as depression, where the U.S. Food and Drug Administration has cleared a TMS system for therapeutic use. In addition, there are small, but promising, bodies of data on the treatment of schizophrenic auditory hallucinations, tinnitus, anxiety disorders, neurodegenerative diseases, hemiparesis, and pain syndromes. Some other nascent areas of study also exist. While the fate of TMS as a therapeutic modality depends on continued innovation and experimentation, economic and other factors may be decisive.

Differential effects of strain, circadian cycle, and stimulation pattern on LTP and concurrent LTD in the dentate gyrus of freely moving rats

Because long-term potentiation (LTP) and long-term depression (LTD) are thought to be involved in learning and memory, it is important to delineate factors that modulate their induction and persistence, especially as studied in freely moving animals. Here, we investigated the effects of rat strain, circadian cycle, and high-frequency stimulation (HFS) pattern on LTP and concurrently induced LTD in the dentate gyrus (DG). Comparison of two commonly used rat strains revealed that medial perforant path field EPSP-population spike (E-S) coupling and LTP were greater in Long-Evans than Sprague-Dawley rats. Circadian cycle experiments conducted in Long-Evans rats revealed greater E-S coupling and enhanced LTP during the dark phase. Interestingly, concurrent LTD in the lateral perforant path did not significantly differ across strains or circadian cycle. Testing HFS protocols during the dark phase revealed that theta burst stimulation (100 Hz bursts at 5 Hz intervals) was ineffective in eliciting either LTP or concurrent LTD in DG, whereas 400 Hz bursts delivered at theta (5 Hz) or delta (1 Hz) frequencies produced substantial LTP and concurrent LTD. Thus, these natural and experimental factors regulate granule cell excitability, and differentially affect LTP and concurrent LTD in the DG of freely moving rats. © 2011 Wiley Periodicals, Inc.

PIKE-mediated PI3-kinase activity is required for AMPA receptor surface expression

AMPAR (α-amino-3-hydroxy-5-methyl-isoxazole-4-propionic acid receptor) is an ion channel involved in the formation of synaptic plasticity. However, the molecular mechanism that couples plasticity stimuli to the trafficking of postsynaptic AMPAR remains poorly understood. Here, we show that PIKE (phosphoinositide 3-kinase enhancer) GTPases regulate neuronal AMPAR activity by promoting GluA2/GRIP1 association. PIKE-L directly interacts with both GluA2 and GRIP1 and forms a tertiary complex upon glycine-induced NMDA receptor activation. PIKE-L is also essential for glycine-induced GluA2-associated PI3K activation. Genetic ablation of PIKE (PIKE(-/-)) in neurons suppresses GluA2-associated PI3K activation, therefore inhibiting the subsequent surface expression of GluA2 and the formation of long-term potentiation. Our findings suggest that PIKE-L is a critical factor in controlling synaptic AMPAR insertion.

Noninvasive brain stimulation in Alzheimer's disease: systematic review and perspectives for the future

BACKGROUND

A number of studies have applied transcranial magnetic stimulation (TMS) to physiologically characterize Alzheimer's disease (AD) and to monitor effects of pharmacological agents, while others have begun to therapeutically use TMS and transcranial direct current stimulation (tDCS) to improve cognitive function in AD. These applications are still very early in development, but offer the opportunity of learning from them for future development.

METHODS

We performed a systematic search of all studies using noninvasive stimulation in AD and reviewed all 29 identified articles. Twenty-four focused on measures of motor cortical reactivity and (local) plasticity and functional connectivity, with eight of these studies assessing also effects of pharmacological agents. Five studies focused on the enhancement of cognitive function in AD.

RESULTS

Short-latency afferent inhibition (SAI) and resting motor threshold are significantly reduced in AD patients as compared to healthy elders. Results on other measures of cortical reactivity, e.g. intracortical inhibition (ICI), are more divergent. Acetylcholine-esterase inhibitors and dopaminergic drugs may increase SAI and ICI in AD. Motor cortical plasticity and connectivity are impaired in AD. TMS/tDCS can induce acute and short-duration beneficial effects on cognitive function, but the therapeutic clinical significance in AD is unclear. Safety of TMS/tDCS is supported by studies to date.

CONCLUSIONS

TMS/tDCS appears safe in AD, but longer-term risks have been insufficiently considered. TMS holds promise as a physiologic biomarker in AD to identify therapeutic targets and monitor pharmacologic effects. In addition, TMS/tDCS may have therapeutic utility in AD, though the evidence is still very preliminary and cautious interpretation is warranted.

Evidence for altered hippocampal function in a mouse model of the human 22q11.2 microdeletion

22q11.2 chromosomal deletions are recurrent copy number mutations that increase the risk of schizophrenia around thirty-fold. Deletion of the orthologous chromosomal region in mice offers an opportunity to characterize changes to neuronal structure and function that may account for the development of this disease. The hippocampus has been implicated in schizophrenia pathogenesis, is reduced in volume in 22q11.2 deletion carriers and displays altered neuronal structure in a mouse model of the mutation (Df(16)A(+/-) mice). Here we investigate hippocampal CA1 physiology, hippocampal-dependent spatial memory and novelty-induced hippocampal activation in Df(16)A(+/-) mice. We found normal spatial reference memory (as assayed by the Morris water maze test) as well as modest but potentially important deficits in physiology. In particular, a reduction in the level of inhibition of CA1 pyramidal neurons was observed, implying a decrease in interneuron activity. Additionally, deficits in LTP were observed using certain induction protocols. Induction of c-Fos expression by exploration of a novel environment suggested a relative sparing of CA1 and dentate gyrus function but showed a robust decrease in the number of activated CA3 pyramidal neurons in Df(16)A(+/-) mice. Overall, experiments performed in this 22q11.2 deletion model demonstrated deficits of various degrees across different regions of the hippocampus, which together may contribute to the increased risk of developing schizophrenia.

Intracellular ATP influences synaptic plasticity in area CA1 of rat hippocampus via metabolism to adenosine and activity-dependent activation of adenosine A1 receptors

The extent to which brain slices reflect the energetic status of the in vivo brain has been a subject of debate. We addressed this issue to investigate the recovery of energetic parameters and adenine nucleotides in rat hippocampal slices and the influence this has on synaptic transmission and plasticity. We show that, although adenine nucleotide levels recover appreciably within 10 min of incubation, it takes 3 h for a full recovery of the energy charge (to ≥ 0.93) and that incubation of brain slices at 34°C results in a significantly higher ATP/AMP ratio and a threefold lower activity of AMP-activated protein kinase compared with slices incubated at room temperature. Supplementation of artificial CSF with d-ribose and adenine (Rib/Ade) increased the total adenine nucleotide pool of brain slices, which, when corrected for the influence of the dead cut edges, closely approached in vivo values. Rib/Ade did not affect basal synaptic transmission or paired-pulse facilitation but did inhibit long-term potentiation (LTP) induced by tetanic or weak theta-burst stimulation. This decrease in LTP was reversed by strong theta-burst stimulation or antagonizing the inhibitory adenosine A(1) receptor suggesting that the elevated tissue ATP levels had resulted in greater activity-dependent adenosine release during LTP induction. This was confirmed by direct measurement of adenosine release with adenosine biosensors. These observations provide new insight into the recovery of adenine nucleotides after slice preparation, the sources of loss of such compounds in brain slices, the means by which to restore them, and the functional consequences of doing so.

The theoretical model of theta burst form of repetitive transcranial magnetic stimulation

OBJECTIVE

Theta burst stimulation, a form of repetitive transcranial magnetic stimulation, can induce lasting changes in corticospinal excitability that are thought to involve long-term potentiation/depression (LTD/LTD)-like effects on cortical synapses. The pattern of delivery of TBS is crucial in determining the direction of change in synaptic efficiency. Previously we explained this by postulating (1) that a single burst of stimulation induces a mixture of excitatory and inhibitory effects and (2) those effects may cascade to produce long-lasting effects. Here we formalise those ideas into a simple mathematical model.

METHODS

The model is based on a simplified description of the glutamatergic synapse in which post-synaptic Ca(2+) entry initiates processes leading to different amount of potentiation and depression of synaptic transmission. The final effect on the synapse results from summation of the two effects.

RESULTS

The model using these assumptions can fit reported data. Metaplastic effects of voluntary contraction on the response to TBS can be incorporated by changing time constants in the model.

CONCLUSIONS

The pattern-dependent after-effects and interactions with voluntary contraction can be successfully modelled by using reasonable assumptions about known cellular mechanisms of plasticity.

SIGNIFICANCE

The model could provide insight into development of new plasticity induction protocols using TMS.

Changes in cortical plasticity across the lifespan

Deterioration of motor and cognitive performance with advancing age is well documented, but its cause remains unknown. Animal studies dating back to the late 1970s reveal that age-associated neurocognitive changes are linked to age-dependent changes in synaptic plasticity, including alterations of long-term potentiation and depression (LTP and LTD). Non-invasive brain stimulation techniques enable measurement of LTP- and LTD-like mechanisms of plasticity, in vivo, in humans, and may thus provide valuable insights. We examined the effects of a 40-s train of continuous theta-burst stimulation (cTBS) to the motor cortex (600 stimuli, three pulses at 50 Hz applied at a frequency of 5 Hz) on cortico-spinal excitability as measured by the motor evoked potentials (MEPs) induced by single-pulse transcranial magnetic stimulation before and after cTBS in the contralateral first dorsal interosseus muscle. Thirty-six healthy individuals aged 19–81 years old were studied in two sites (Boston, USA and Barcelona, Spain). The findings did not differ across study sites. We found that advancing age is negatively correlated with the duration of the effect of cTBS (r = −0.367; p = 0.028) and the overall amount of corticomotor suppression induced by cTBS (r = −0.478; p = 0.003), and positively correlated with the maximal suppression of amplitude on motor evoked responses in the target muscle (r = 0.420; p = 0.011). We performed magnetic resonance imaging (MRI)-based individual morphometric analysis in a subset of subjects to demonstrate that these findings are not explained by age-related brain atrophy or differences in scalp-to-brain distance that could have affected the TBS effects. Our findings provide empirical evidence that the mechanisms of cortical plasticity area are altered with aging and their efficiency decreases across the human lifespan. This may critically contribute to motor and possibly cognitive decline.

The mechanisms of the strong inhibitory modulation of long-term potentiation in the rat dentate gyrus

The hippocampus is essential for the formation of certain types of memory, and synaptic plasticity such as long-term potentiation (LTP) is widely accepted as a cellular basis of hippocampus-dependent memory. Although LTP in both perforant path-dentate gyrus (DG) granule cell and CA3-CA1 pyramidal cell synapses is similarly dependent on activation of postsynaptic N-methyl-D-aspartate receptors, several reports suggest that modulation of LTP by γ-aminobutyric acid (GABA) receptor-mediated inhibitory inputs is stronger in perforant path-DG granule cell synapses. However, little is known about how different the mechanism and physiological relevance of the GABAergic modulation of LTP induction are among different brain regions. We confirmed that the action of GABA(A) receptor antagonists on LTP was more prominent in the DG, and explored the mechanism introducing such difference by examining two types of GABA(A) receptor-mediated inhibition, i.e. synaptic and tonic inhibition. As synaptic inhibition, we compared inhibitory vs. excitatory monosynaptic responses and their summation during an LTP-inducing stimulus, and found that the balance of the summated postsynaptic currents was biased toward inhibition in the DG. As tonic inhibition, or sustained activation of extrasynaptic GABA(A) receptors by ambient GABA, we measured the change in holding currents of the postsynaptic cells induced by GABA(A) receptor antagonists, and found that the tonic inhibition was significantly stronger in the DG. Furthermore, we found that tonic inhibition was associated with LTP modulation. Our results suggest that both the larger tonic inhibition and the larger inhibitory/excitatory summation balance during conditioning are involved in the stronger inhibitory modulation of LTP in the DG.

The functional role of cross-frequency coupling

Recent studies suggest that cross-frequency coupling (CFC) might play a functional role in neuronal computation, communication and learning. In particular, the strength of phase-amplitude CFC differs across brain areas in a task-relevant manner, changes quickly in response to sensory, motor and cognitive events, and correlates with performance in learning tasks. Importantly, whereas high-frequency brain activity reflects local domains of cortical processing, low-frequency brain rhythms are dynamically entrained across distributed brain regions by both external sensory input and internal cognitive events. CFC might thus serve as a mechanism to transfer information from large-scale brain networks operating at behavioral timescales to the fast, local cortical processing required for effective computation and synaptic modification, thus integrating functional systems across multiple spatiotemporal scales.

Septal stimulation inhibits spinal cord dorsal horn neuronal activity

Deep brain stimulation (DBS) has been used for relieving chronic pain in patients that have been through other existing options. The septum has been one of the targets for such treatment. The purpose of this study was to determine the inhibitory effect of electrical stimulation in the medial septum diagonal band of broca (MSDB) on neuronal activity in the spinal cord of rats anesthetized with sodium pentobarbital. While unilaterally stimulating the MSDB, wide dynamic range neurons in the lumbar region of the spinal cord were recorded in response to graded mechanical stimulation of the hind paws (brush, pressure, and pinch). Stimulation was at 1, 5, 10, and 20V, at 100Hz, and 0.1ms duration. Significant bilateral reduction was observed in response to pressure (ipsilaterally: 0.90±0.05, 0.48±0.06*, 0.55±0.05*, 0.40±0.05*; and contralaterally: 0.70±0.06*, 0.59±0.08*, 0.75±0.05*, 0.49±0.07*) and pinch (ipsilaterally: 0.89±0.08, 0.46±0.05*, 0.54±0.04*, 0.50±0.05*; and contralaterally: 0.78±0.05, 0.61±0.07*, 0.64±0.04*, 0.53±0.06*). Data were expressed as a fraction of control. Significant changes were also found in responses to brush in certain groups (ipsilaterally: 1.08±0.08, 0.72±0.06*, 1.00±0.12, 0.65±0.06*; and contralaterally: 0.93±0.05, 0.77±0.07*, 0.98±0.05, 0.84±0.07). Further analysis suggested that 5V was adequate for achieving optimal inhibition. It is concluded that the MSDB can be used as alternative target for DBS in the treatment of pain.