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

Endogenous acetylcholine enhances synchronized interneuron activity in rat neocortex

Application of 4-aminopyridine (4-AP) along with EAA) receptor antagonists produces gamma-aminobutyric acid (GABAA) receptor-dependent synchronized activity in interneurons. This results in waves of activity propagating through upper cortical layers. Because interneurons in the neocortex are excited by nicotinic acetylcholine receptor (nAChR) agonists, ACh may influence synchronization of these local neocortical interneuronal networks. To study this possibility, we have used voltage-sensitive dye imaging using the fluorescent dye RH 414 (30 microM) in rat neocortical slices. Recordings were obtained in the presence of 4-AP (100 microM) and the EAA receptor antagonists D-2-amino-5-phosphonvaleric acid (20 microM) and 6-cyano-7-nitro-quinoxaline-2,3-dione (10 microM). In response to intracortical stimulation, localized or propagated activity restricted to upper cortical layers was seen. Bath application of the ACh esterase inhibitor neostigmine (10 microM) and the nAChR agonist 1,1-dimethyl-4-phenyl-piperazinium iodide (DMPP; 10 microM) increased the response amplitude, the extent of spread, and the duration of this activity. These changes were seen in 13 of 16 slices tested with neostigmine (10 microM) and 4 of 7 slices tested with DMPP (10 microM). Application of the muscarinic AChR antagonist atropine (1 microM) did not block the enhancement of activity by neostigmine (n = 7). Application of dihydro-beta-erythroidine (10 microM), known, at this concentration, to selectively antagonize alpha4beta2-like nAChRs, blocked the effect of neostigmine (n = 5). The selective alpha7-like nAChR antagonist methyllycaconitine (50 nM) was ineffective (n = 5). These results suggest that activation of alpha4beta2-like nAChRs by endogenously released ACh can enhance synchronized activity in local neocortical inhibitory networks.

Separation of the convulsions and antidepressant-like effects produced by the delta-opioid agonist SNC80 in rats

RATIONALE

Delta-opioid agonists produce a number of behavioral effects, including convulsions, antinociception, locomotor stimulation, and antidepressant-like effects. The development of these compounds as treatments for depression is limited by their convulsive effects. Therefore, determining how to separate the convulsive and antidepressant-like characteristics of these compounds is essential for their potential clinical use.

OBJECTIVE

The present study tests the hypothesis that the rate of delta-opioid agonist administration greatly contributes to the convulsive properties, but not the antidepressant-like effects, of delta-opioid agonists.

MATERIALS AND METHODS

The delta-opioid agonist SNC80 (1, 3.2, and 10 mg kg-1 or vehicle) was administered to Sprague-Dawley rats by intravenous infusion over different durations of time (20 s, 20, or 60 min). Convulsions were measured by observation prior to determining antidepressant-like effects in the forced swim test.

RESULTS

Slowing the rate of SNC80 administration minimized delta agonist-induced convulsions without altering the effects of SNC80 in the forced swim test.

CONCLUSIONS

These data suggest that delta agonist-induced antidepressant properties are independent of convulsive effects, and that it may be possible to eliminate the convulsions produced by delta agonists, further promoting their potential clinical utility.

The HMG-CoA Reductase Inhibitor Lovastatin Reverses the Learning and Attention Deficits in a Mouse Model of Neurofibromatosis Type 1

Neurofibromatosis Type 1 (NF1) is a common neurological disorder caused by mutations in the gene encoding Neurofibromin, a p21Ras GTPase Activating Protein (GAP). Importantly, NF1 causes learning disabilities and attention deficits. A previous study showed that the learning and memory deficits of a mouse model of NF1 (nf1+/-) appear to be caused by excessive p21Ras activity leading to impairments in long-term potentiation (LTP), a cellular mechanism of learning and memory. Here, we identify lovastatin as a potent inhibitor of p21Ras/Mitogen Activated Protein Kinase (MAPK) activity in the brain. Lovastatin is a specific inhibitor of three-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase, used commonly for the treatment of hypercholesterolemia. We report that lovastatin decreased the enhanced brain p21Ras-MAPK activity of the nf1+/- mice, rescued their LTP deficits, and reversed their spatial learning and attention impairments. Therefore, these results demonstrate that lovastatin may prove useful in the treatment of Neurofibromatosis Type 1.

Theta stimulation polymerizes actin in dendritic spines of hippocampus

It has been proposed that the endurance of long-term potentiation (LTP) depends on structural changes entailing reorganization of the spine actin cytoskeleton. The present study used a new technique involving intracellular and extracellular application of rhodamine-phalloidin to conventional hippocampal slices to test whether induction of LTP by naturalistic patterns of afferent activity selectively increases actin polymerization in juvenile to young adult spines. Rhodamine-phalloidin, which selectively binds to polymerized actin, was detected in perikarya and proximal dendrites of CA1 pyramidal cells that received low-frequency afferent activity but was essentially absent in spines and fine dendritic processes. Theta pattern stimulation induced LTP and caused a large (threefold), reliable increase in labeled spines and spine-like puncta in the proximal dendritic zone containing potentiated synapses. The spines frequently occurred in the absence of labeling to other structures but were also found in association with fluorescent dendritic processes. These effects were replicated (>10-fold increase in labeled spines) using extracellular applications of rhodamine-phalloidin. Increases in labeling appeared within 2 min, were completely blocked by treatments that prevent LTP induction, and occurred in slices prepared from young adult rats. These results indicate that near-threshold conditions for inducing stable potentiation cause the rapid polymerization of actin in mature spines and suggest that the effect is both sufficiently discrete to satisfy the synapse-specificity rule of LTP as well as rapid enough to participate in the initial stages of LTP consolidation.

Modulation of presynaptic plasticity and learning by the H-ras/extracellular signal-regulated kinase/synapsin I signaling pathway

Molecular and cellular studies of the mechanisms underlying mammalian learning and memory have focused almost exclusively on postsynaptic function. We now reveal an experience-dependent presynaptic mechanism that modulates learning and synaptic plasticity in mice. Consistent with a presynaptic function for endogenous H-ras/extracellular signal-regulated kinase (ERK) signaling, we observed that, under normal physiologic conditions in wild-type mice, hippocampus-dependent learning stimulated the ERK-dependent phosphorylation of synapsin I, and MEK (MAP kinase kinase)/ERK inhibition selectively decreased the frequency of miniature EPSCs. By generating transgenic mice expressing a constitutively active form of H-ras (H-rasG12V), which is abundantly localized in axon terminals, we were able to increase the ERK-dependent phosphorylation of synapsin I. This resulted in several presynaptic changes, including a higher density of docked neurotransmitter vesicles in glutamatergic terminals, an increased frequency of miniature EPSCs, and increased paired-pulse facilitation. In addition, we observed facilitated neurotransmitter release selectively during high-frequency activity with consequent increases in long-term potentiation. Moreover, these mice showed dramatic enhancements in hippocampus-dependent learning. Importantly, deletion of synapsin I, an exclusively presynaptic protein, blocked the enhancements of learning, presynaptic plasticity, and long-term potentiation. Together with previous invertebrate studies, these results demonstrate that presynaptic plasticity represents an important evolutionarily conserved mechanism for modulating learning and memory.

Impaired hippocampal long-term potentiation in melatonin MT2 receptor-deficient mice

The pineal product melatonin that acts on specific melatonin receptors has been implicated in pathobiological mechanisms of neuropsychiatric disorders including Alzheimer's disease. We used mice lacking melatonin MT(2) receptors (MT(2) knockouts) to investigate the role of these receptors in synaptic plasticity and learning-dependent behavior. In field CA1 of hippocampal slices from wild-type mice, theta burst stimulation induced robust and stable long-term potentiation that was smaller and decremental in slices from MT(2) knockouts. Tested in an elevated plus-maze on two consecutive days, wild-type mice showed shorter transfer latencies to enter a closed arm on the second day; this experience-dependent behavior did not occur in MT(2) knockouts. These results suggest that MT(2) receptors participate in hippocampal synaptic plasticity and in memory processes.

Differential translation and fragile X syndrome

Fragile X syndrome (FXS) is caused by the transcriptional silencing of the Fmr1 gene, which encodes a protein (FMRP) that can act as a translational suppressor in dendrites, and is characterized by a preponderance of abnormally long, thin and tortuous dendritic spines. According to a current theory of FXS, the loss of FMRP expression leads to an exaggeration of translation responses linked to group I metabotropic glutamate receptors. Such responses are involved in the consolidation of a form of long-term depression that is enhanced in Fmr1 knockout mice and in the elongation of dendritic spines, resembling synaptic phenotypes over-represented in fragile X brain. These observations place fragile X research at the heart of a long-standing issue in neuroscience. The consolidation of memory, and several distinct forms of synaptic plasticity considered to be substrates of memory, requires mRNA translation and is associated with changes in spine morphology. A recent convergence of research on FXS and on the involvement of translation in various forms of synaptic plasticity has been very informative on this issue and on mechanisms underlying FXS. Evidence suggests a general relationship in which the receptors that induce distinct forms of efficacy change differentially regulate translation to produce unique spine shapes involved in their consolidation. We discuss several potential mechanisms for differential translation and the notion that FXS represents an exaggeration of one 'channel' in a set of translation-dependent consolidation responses.

Dephosphorylation and internalization of cell adhesion molecule L1 induced by theta burst stimulation in rat hippocampus

The neural cell adhesion molecule L1 may participate in initiating and maintaining synaptic changes during learning in the hippocampus. One prominent form of synaptic change in the hippocampus is long-term potentiation (LTP) that occurs following specific patterns of synaptic activity. We present evidence that Y1176 of the YRSL motif within L1 cytoplasmic domain is dephosphorylated in LTP-induced hippocampus. The dephosphorylated L1 is associated with AP-2 and AP180 that are required for clathrin-mediated internalization of L1. These data suggest that clathrin-mediated recycling of L1 at presynaptic sites is enhanced by certain kinds of neural activity, and that maintenance of LTP-induced synaptic changes is regulated by L1 recycling.

Lithium attenuates stress-induced impairment of long-term potentiation induction

Stress impairs the induction of long-term potentiation in the hippocampus as well as hippocampus-dependent memory. Lithium, a classical mood stabilizer, is known to have beneficial effects on stress-induced impairment of spatial memory. In the present study, we investigated lithium effects on the impairment of long-term potentiation induction after exposure to acute immobilization stress. As previously reported, immobilization stress impaired long-term potentiation induction in the CA1 region of rat hippocampal slices. Treating the slices with 0.6 or 1 mM lithium attenuated impaired long-term potentiation induction in stressed animals. Lithium was without effect on long-term potentiation induction in unstressed animals or baseline synaptic responses in unstressed or stressed animals. These results demonstrate a protective effect of lithium against stress-induced impairment of long-term potentiation induction.

Lack of phenotype for LTP and fear conditioning learning in calpain 1 knock-out mice

We previously proposed the hypothesis that calpain activation played an important role in long-term potentiation (LTP) of synaptic transmission in hippocampus. Two forms of calpain are predominant in brain tissues, calpain 1 (mu-calpain), activated by micromolar calcium concentration and calpain 2 (m-calpain), activated by millimolar calcium concentration in vitro. In the present study, we tested the role of calpain 1 in LTP and in learning and memory using calpain 1 knock-out mice. Changes in learning and memory were assessed using both context and tone fear conditioning. No differences in freezing responses were observed between the knock-out and the wild-type animals during the acquisition phase of the training, eliminating the possibility that the knock-out animals could be differentially affected by the foot shock. Likewise, no differences in freezing responses elicited by either the context or the tone were observed during the retention phase. No differences in short-term potentiation (STP) or LTP were observed in hippocampal slices from the knock-out and matched wild-type mice. Several interpretations might explain these negative results. First, it is conceivable that calpain 2 plays a more dominant role in neurons, and that calpain 1 makes a minor contribution as opposed to its suspected predominant role in the hematopoietic system. Alternatively, it is conceivable that some as yet unknown compensatory mechanisms take effect, and that calpain 2 or another calpain isoform substitutes for the missing calpain 1.

Mouse models of impaired fear memory exhibit deficits in amygdalar LTP

Inbred mouse strains have different genetic backgrounds that can result in impairments of synaptic plasticity and memory. They are valuable models for probing the mechanisms of memory impairments. We examined fear memory in several inbred strains, along with synaptic plasticity that may underlie fear memory. Long-term potentiation (LTP) is a form of activity-dependent synaptic plasticity that is a candidate cellular mechanism for some forms of learning and memory. Strains with impaired contextual or cued fear memory may have selective LTP deficits in different hippocampal subregions, or in the amygdala. We measured fear memory and its extinction in five inbred strains: C57BL/6NCrlBR (B6), A/J, BALB/cByJ (BALB), C57BL/10J (B10), and SM/J (SM). We also measured LTP in the basolateral amygdala and in the hippocampal Schaeffer collateral-commissural (SC) and medial perforant pathways (MPP). All strains exhibited intact contextual fear memory 24 h post-training, but cued fear memory was impaired in strains A/J, BALB, and SM. At 1 h post-training, both contextual and cued fear memory deficits were more widespread: all strains except for B6 and B10 showed impairments of both types of memory. Contextual fear extinction was impaired in BALB and SM. We found that amygdalar LTP was reduced in strains A/J and BALB, but SC LTP was intact in all strains (except for a selective multi-train LTP impairment in BALB). MPPLTP was similar in all five strains. Thus, reduced amygdalar LTP is correlated with impaired cued fear memory in strains A/J and BALB. Also, hippocampal SC LTP is more strongly correlated with 24-h (long-term) than with 1-h (short-term) contextual fear memory. In this first conjoint study of amygdala-dependent memory and amygdalar LTP in inbred mice, we identified specific hippocampal and amygdalar LTP deficits that correlate with fear memory impairments. These deficits should be considered when selecting inbred strains for genetic modification.

Is the hippocampal theta rhythm related to cognition in a non-locomotor place recognition task?

The possible correlation of 4-12 Hz hippocampal field oscillations (theta rhythm) with motor and cognitive behavior was studied by recording the hippocampal electroencephalogram in non-locomoting rats as they solved a hippocampus-dependent place recognition task. The electroencephalogram (EEG) during the place recognition task was compared with the EEG during a control task, which had the same motor demands but did not require place recognition. In the place recognition task, the rat was passively transported on the periphery of a circular rotating arena, operant responses (lever pressing in Experiment 1 and licking in Experiment 2) emitted in a 60 degrees reward sector of the arena trajectory were reinforced. As expected the theta rhythm was observed during "voluntary" movements such as walking and lever pressing. During walking and lever pressing, when prominent theta was observed, the frequency increased within the first 10 min of a session. When the rats were not moving, during licking or staying motionless, both the theta amplitude and frequency were lower compared with the EEG during walking. There were no correlations between any theta characteristics and cognitive demand of the tasks.

The influence of methylphenidate on the power spectrum of ADHD children - an MEG study

BACKGROUND

The present study was dedicated to investigate the influence of Methylphenidate (MPH) on cortical processing of children who were diagnosed with different subtypes of Attention Deficit Hyperactivity Disorder (ADHD). As all of the previous studies investigating power differences in different frequency bands have been using EEG, mostly with a relatively small number of electrodes our aim was to obtain new aspects using high density magnetoencephalography (MEG).

METHODS

35 children (6 female, 29 male) participated in this study. Mean age was 11.7 years (+/- 1.92 years). 17 children were diagnosed of having an Attention-Deficit/Hyperactivity Disorder of the combined type (ADHDcom, DSM IV code 314.01); the other 18 were diagnosed for ADHD of the predominantly inattentive type (ADHDin, DSM IV code 314.0). We measured the MEG during a 5 minute resting period with a 148-channel magnetometer system (MAGNES 2500 WH, 4D Neuroimaging, San Diego, USA). Power values were averaged for 5 bands: Delta (D, 1.5-3.5 Hz), Theta (T, 3.5-7.5 Hz), Alpha (A, 7.5-12.5 Hz), Beta (B, 12.5-25 Hz) and Global (GL, 1.5-25 Hz).). Additionally, attention was measured behaviourally using the D2 test of attention with and without medication.

RESULTS

The global power of the frequency band from 1.5 to 25 Hz increased with MPH. Relative Theta was found to be higher in the left hemisphere after administration of MPH than before. A positive correlation was found between D2 test improvement and MPH-induced power changes in the Theta band over the left frontal region. A linear regression was computed and confirmed that the larger the improvement in D2 test performance, the larger the increase in Theta after MPH application.

CONCLUSION

Main effects induced by medication were found in frontal regions. Theta band activity increased over the left hemisphere after MPH application. This finding contradicts EEG results of several groups who found lower levels of Theta power after MPH application. As relative Theta correlates with D2 test improvement we conclude that MEG provide complementary and therefore important new insights to ADHD.

Involvement of GABAergic and cholinergic medial septal neurons in hippocampal theta rhythm

Hippocampal theta rhythm (HPCtheta) may be important for various phenomena, including attention and acquisition of sensory information. Two types of HPCtheta (types I and II) exist based on pharmacological, behavioral, and electrophysiological characteristics. Both types occur during locomotion, whereas only type II (atropine-sensitive) is present under urethane anesthesia. The circuit of HPCtheta synchronization includes the medial septum-diagonal band of Broca (MSDB), with cholinergic and gamma-aminobutyric acid (GABA)ergic neurons comprising the two main projections from MSDB to HPC. The primary aim of the present study was to assess the effects of GABAergic MSDB lesions on urethane- and locomotion-related HPCtheta, and compare these effects to those of cholinergic MSDB lesions. Saline, kainic acid (KA), or 192 IgG-saporin (SAP) was injected into MSDB before recording. KA preferentially destroys GABAergic MSDB neurons, whereas SAP selectively eliminates cholinergic MSDB neurons. A fixed recording electrode was placed in the dentate mid-molecular layer, and stimulating electrodes were placed in the posterior hypothalamus (PH), and medial perforant path (PP). Under urethane anesthesia, HPCtheta was induced by tail pinch, PH stimulation, and systemic physostigmine; none of the rats with KA or SAP showed HPCtheta in any of these conditions. During locomotion, HPCtheta was attenuated, but not eliminated, in rats with KA or SAP lesions. Intraseptal KA in combination with either intraseptal SAP or PP lesions reduced locomotion-related HPCtheta beyond that observed with each lesion alone, virtually eliminating HPCtheta. In contrast, intraseptal SAP combined with PP lesions did not reduce HPCtheta beyond the effect of each lesion alone. We conclude that both GABAergic and cholinergic MSDB neurons are necessary for HPCtheta under urethane, and that each of these septohippocampal projections contributes to HPCtheta during locomotion.

Long-term potentiation is impaired in middle-aged rats: regional specificity and reversal by adenosine receptor antagonists

Memory loss in humans begins early in adult life and progresses thereafter. It is not known whether these losses reflect the failure of cellular processes that encode memory or disturbances in events that retrieve it. Here, we report that impairments in hippocampal long-term potentiation (LTP), a form of synaptic plasticity associated with memory, are present by middle age in rats but only in select portions of pyramidal cell dendritic trees. Specifically, LTP induced with theta-burst stimulation in basal dendrites of hippocampal field CA1 decayed rapidly in slices prepared from 7- to 10-month-old rats but not in slices from young adults. There were no evident age-related differences in LTP in the apical dendrites. Both the adenosine A1 receptor antagonist 8-cyclopentyl-1,3-dipropylxanthine and a positive AMPA receptor modulator (ampakine) offset age-related LTP deficits. Adenosine produced greater depression of synaptic responses in middle-aged versus young adult slices and in basal versus apical dendrites. These results were not associated with variations in A1 receptor densities and may instead reflect regional and age-related differences in adenosine clearance. Pertinent to this, brief applications of A1 receptor antagonists immediately after theta stimulation fully restored LTP in middle-aged rats. We hypothesize that the build-up of extracellular adenosine during theta activity persists into the postinduction period in the basal dendrites of middle-aged slices and thereby activates the A1 receptor-dependent LTP reversal effect. Regardless of the underlying mechanism, the present results provide a candidate explanation for memory losses during normal aging and indicate that, with regard to plasticity, different segments of pyramidal neurons age at different rates.

M2 muscarinic acetylcholine receptor knock-out mice show deficits in behavioral flexibility, working memory, and hippocampal plasticity

Muscarinic acetylcholine receptors are known to play key roles in facilitating cognitive processes. However, the specific roles of the individual muscarinic receptor subtypes (M1-M5) in learning and memory are not well understood at present. In the present study, we used wild-type (M2+/+) and M2 receptor-deficient (M2-/-) mice to examine the potential role of M2 receptors in learning and memory and hippocampal synaptic plasticity. M2-/- mice showed significant deficits in behavioral flexibility and working memory in the Barnes circular maze and the T-maze delayed alternation tests, respectively. The behavioral deficits of M2-/- mice were associated with profound changes in neuronal plasticity studied at the Schaffer-CA1 synapse of hippocampal slices. Strikingly, short-term potentiation (STP) was abolished, and long-term potentiation (LTP) was drastically reduced after high-frequency stimulation of M2-/- hippocampi. Treatment of M2-/- hippocampal slices with the GABA(A) receptor antagonist, bicuculline, restored STP and significantly increased LTP. Whole-cell recordings from CA1 pyramidal cells demonstrated a much stronger disinhibition of GABAergic than glutamatergic transmission in M2-/- hippocampi, which was particularly prominent during stimulus trains. Increased strength of GABAergic inhibition is thus a likely mechanism underlying the impaired synaptic plasticity observed with M2-/- hippocampi. Moreover, the persistent enhancement of excitatory synaptic transmission in CA1 pyramidal cells induced by the transient application of a low concentration of a muscarinic agonist (referred to as LTP(m)) was totally abolished in M2-/- mice. Because impaired muscarinic cholinergic neurotransmission is associated with Alzheimer's disease and normal aging processes, these findings should be of considerable therapeutic relevance.

Dopamine D1-like receptor modulates layer- and frequency-specific short-term synaptic plasticity in rat prefrontal cortical neurons

The mesocortical dopamine (DA) input to the prefrontal cortex (PFC) is crucial for processing short-term working memory (STWM) to guide forthcoming behavior. Short-term plasticity-like post-tetanic potentiation (PTP, < 3 min) and short-term potentiation (STP, < 10 min) may underlie STWM. Using whole-cell voltage-clamp recordings, mixed glutamatergic excitatory postsynaptic currents (EPSCs) evoked by layer III or layer V stimulation (0.5 or 0.067 Hz) were recorded from layer V pyramidal neurons. With 0.5 Hz basal stimulation of layer III, brief tetani (2 x 50 Hz) induced a homosynaptic PTP (decayed: approximately 1 min). The D1-like antagonist SCH23390 (1 microm) increased the PTP amplitude and decay time without inducing changes to the tetanic response. The tetani may evoke endogenous DA release, which activates a presynaptic D1-like receptor to inhibit glutamate release to modulate PTP. With a slower (0.067 Hz) basal stimulation, the same tetani induced STP (lasting approximately 4 min, but only at 2x intensity only) that was insignificantly suppressed by SCH23390. With stimulation of layer-V-->V inputs at 0.5 Hz, layer V tetani yielded inconsisitent responses. However, at 0.067 Hz, tetani at double the intensity resulted in an STP (lasting approximately 6 min), but a long-term depression after SCH23390 application. Endogenous DA released by tetanic stimulation can interact with a D1-like receptor to induce STP in layer V-->V synapses that receive slower (0.067 Hz) frequency inputs, but suppresses PTP at layer III-->V synapses that receive higher (0.5 Hz) frequency inputs. This D1-like modulation of layer- and frequency-specific synaptic responses in the PFC may contribute to STWM processing.

Spatiotemporal visualization of long-term potentiation and depression in the hippocampal CA1 area

Long-term potentiation (LTP) in the CA1 area of the hippocampus depends critically on the statistical characteristics of its stimulus. The ability of optical imaging to record spatial distribution has made it possible to examine systematically the effect of higher-order statistical characteristics, such as the correlation between successive pairs of inter-stimulus intervals (ISIs) on the induction of LTP. Therefore, the function of frequency (first-order) and temporal pattern (second-order) was examined using this imaging technique. To investigate the dependence of LTP on frequency, periodic stimuli with the same number of pulses were applied at different frequencies (1-10 Hz, n=200) to Schaffer commissural-collateral fibers. While stimulus frequencies from 2-10 Hz induced LTP of varying magnitudes and low-frequency stimuli (1 Hz) induced long-term depression (LTD), spatial distribution remained consistent. These results suggest that induction frequency has a greater effect on the magnitude of LTP than on its spatial distribution. By employing nonperiodic stimuli at the same mean frequency (2 Hz), the effect of varying the temporal structure of a stimulus was also investigated. As the correlation of successive ISIs was increased from negative to positive, not only did the magnitude of LTP increase, there was also a statistically significant change in the spatial distribution of LTP. Interestingly, when a strong negatively correlated stimulus was applied, both LTP and LTD were simultaneously observed in the CA1 area. It was also found that the magnitude of LTP 200-300 mum distal to the cellular layer was larger than that of the LTP induced proximal (<100 microm) to that layer. These results support the hypothesis that the spatial distribution of LTP throughout the hippocampus relies principally on the temporal patterning of input stimulation. This insight into the structure of the CA1 neural network may reveal the importance of stimulus timing events in the spatial encoding of memories.

Differences in the magnitude of long-term potentiation produced by theta burst and high frequency stimulation protocols matched in stimulus number

Theta-burst stimulation (TBS: four pulses at 100 Hz repeated with 200 ms inter-burst-intervals) and another commonly used high-frequency stimulation protocol (HFS: 1 s burst of equally spaced pulses at 100 Hz) were compared for the magnitude of LTP produced in rat hippocampal slices. The total number of pulses applied during tetanus (TET) was either 40, 100, 200, or 300. In a conventional analysis of the last 10 min of the post-TET period, a two-way ANOVA revealed no difference either in LTP of the field excitatory post-synaptic potential (fEPSP) between TBS and HFS or differences across pulse number at 40, 100, or 200 pulses. At 300 pulses, there was a significant main effect by pulse number but not by protocol. A linear regression analysis showed that stimulation protocol accounted for only about 10% of the change in magnitude while pulse number contributed to 30% of the change. However, when an extended analysis of the same data was performed across the entire post-TET period with a repeated-measure ANOVA, a small but persistent increase in TBS over HFS at 200 pulses was significant. A difference between TBS and HFS at 300 pulses that occurred only during the early phase of LTP was also significant. These results suggest that, over a range of stimuli, the number of pulses in an induction protocol, rather than the pattern of stimulation, determines the magnitude of late phase LTP, while TBS produces greater potentiation than HFS in the early phase of LTP with higher TET number.

Toward establishing a therapeutic window for rTMS by theta burst stimulation

In this issue of Neuron, Huang et al. show that a version of the classic theta burst stimulation protocol used to induce LTP/LTD in brain slices can be adapted to a transcranial magnetic stimulation (TMS) protocol to rapidly produce long lasting (up to an hour), reversible effects on motor cortex physiology and behavior. These results may have important implications for the development of clinical applications of rTMS in the treatment of depression, epilepsy, Parkinson's, and other diseases.

Bidirectional activity-dependent morphological plasticity in hippocampal neurons

Dendritic spines on pyramidal neurons receive the vast majority of excitatory input and are considered electrobiochemical processing units, integrating and compartmentalizing synaptic input. Following synaptic plasticity, spines can undergo morphological plasticity, which possibly forms the structural basis for long-term changes in neuronal circuitry. Here, we demonstrate that spines on CA1 pyramidal neurons from organotypic slice cultures show bidirectional activity-dependent morphological plasticity. Using two-photon time-lapse microscopy, we observed that low-frequency stimulation induced NMDA receptor-dependent spine retractions, whereas theta burst stimulation led to the formation of new spines. Moreover, without stimulation the number of spine retractions was on the same order of magnitude as the stimulus-induced spine gain or loss. Finally, we found that the ability of neurons to eliminate spines in an activity-dependent manner decreased with developmental age. Taken together, our data show that hippocampal neurons can undergo bidirectional morphological plasticity; spines are formed and eliminated in an activity-dependent way.

Reversal of Hippocampal LTP by Spontaneous Seizure-Like Activity: Role of Group I mGluR and Cell Depolarization

Memory impairment is a common consequence of epileptic seizures. The hippocampal formation is particularly prone to seizure-induced amnesia due to its prominent role in mnemonic processes. We used the isolated CA1 slice preparation to examine effects of seizure-like activity on hippocampal plasticity, long-term potentiation (LTP), and long-term depression (LTD). Repeated spontaneous ictal events, generated in the presence of antagonists of GABAA receptor function, led to a stepwise erasure of LTP (termed spontaneous depotentiation, SDP). SDP could be initiated at various stages of LTP consolidation (tested ≤120 min after the induction of LTP). Renewed tetanic stimulation re-established LTP. SDP was remarkably specific: baseline transmission and other forms of hippocampal plasticity, i.e., Ca2+-induced LTP and two forms of LTD [(RS)-3,5-dihydroxyphenyglycine (DHPG) mediated and low-frequency stimulation mediated] were not affected by the same type of seizure activity. SDP was blocked in the presence of the group I mGluR antagonist ( S)-4-carboxyphenylglycine. The mGluR1 antagonist ( S)-(+)-α-amino-methylbenzeneacetic acid blocked ∼80%, the mGluR5-specific antagonist 2-methyl-6-(phenylethynyl)-pyridine ∼30% of SDP. Most efficient implementation of SDP was observed during seizures in the combined presence of the group I mGluR agonist DHPG and the GABAA antagonist bicuculline. However, similar ictal activity generated in the presence of DHPG alone did not lead to SDP in the vast majority of recordings. Complete disinhibition and at least partial activation of group I mGluR were necessary conditions for the induction of SDP. The depotentiating pharmacological conditions were accompanied by tonic membrane depolarization of CA1 pyramidal cells. Since hyperpolarization (by negative current injection) prevented intracellular SDP under depotentiating pharmacological conditions and depolarization (by positive current injection) led to selective intracellular SDP in the non-depotentiating seizure protocol of DHPG, it is concluded that cell depolarization was a sufficient condition for seizure-like activity to reverse hippocampal LTP.

Synaptic Enhancement Induced Through Callosal Pathways in Cat Association Cortex

The corpus callosum plays a major role in synchronizing neocortical activities in the two hemispheres. We investigated the changes in callosally elicited excitatory postsynaptic potentials (EPSPs) of neurons from cortical association areas 5 and 7 of cats under barbiturate or ketamine-xylazine anesthesia. Single pulses to callosal pathway evoked control EPSPs; pulse-trains were subsequently applied at different frequencies to homotopic sites in the contralateral cortex, as conditioning stimulation; thereafter, the single pulses were applied again to test changes in synaptic responsiveness by comparing the amplitudes of control and conditioned EPSPs. In 41 of 42 neurons recorded under barbiturate anesthesia, all frequencies of conditioning callosal stimuli induced short-term (5–30 min) enhancement of test EPSPs elicited by single stimuli. Neurons tested with successive conditioning pulse-trains at different frequencies displayed stronger enhancement with high-frequency (40–100 Hz) than with low-frequency (10–20 Hz) rhythmic pulse-trains; >100 Hz, the potentiation saturated. In a neuronal sample, microdialysis of an N-methyl-d-aspartate (NMDA) receptor blocker in barbiturate-treated cats suppressed this potentiation, and potentiation of callosally evoked EPSPs was not detected in neurons recorded under ketamine-xylazine anesthesia, thus indicating that EPSPs' potentiation implicates, at least partially, NMDA receptors. These data suggest that callosal activities occurring within low-frequency and fast-frequency oscillations play a role in cortical synaptic plasticity.

Theta oscillation in the human anterior cingulate cortex during all-night sleep: an electrocorticographic study

Ten epileptic patients each with subdural electrodes surgically attached to the anterior cingulate cortex (ACC; two cases), the orbitofrontal cortex (OFC; seven cases), or both (one case) were included in this study. We recorded each patient's ACC or OFC electrocorticogram (ECoG) during the time period that the patient was awake and naturally asleep. We performed a Fast Fourier Transformation (FFT) power spectral analysis on each ECoG to examine its frequency component. We found that the ACC showed regular and continuous theta oscillation (5-7Hz) during wakefulness and rapid eye movement (REM) sleep, but not during slow wave sleep. Theta waves observed in REM sleep were not as distinct as those found in wakefulness. We also discovered that the orbitofrontal signals represented spectral peaks in the theta band only during wakefulness. This suggests the coexistence of theta oscillation in the ACC. Considering our previous observations of gamma and beta oscillations in the human hippocampus, we hypothesize that the human limbic system manifests two oscillatory activities. The results obtained in this study suggest that electrophysiological activity in the ACC could be related to particular psychological functions in wakefulness and in REM sleep. These results are useful in elucidating the human brain mechanism.

Immunocytochemical localization of GABABR1 receptor subunits in the basolateral amygdala

Gamma-aminobutyric acid B (GABAB) receptors (GBRs) are G-protein-coupled receptors that mediate a slow, prolonged form of inhibition in the basolateral amygdala (ABL) and other brain areas. Recent studies indicate that this receptor is a heterodimer consisting of GABABR1 (GBR1) and GABABR2 subunits. In the present investigation, antibodies to the GABABR1 subunit were used to study the neuronal localization of GBRs in the rat ABL. GBR immunoreactivity was mainly found in spine-sparse interneurons and astrocytes at the light microscopic level. Very few pyramidal neurons exhibited perikaryal staining. Dual-labeling immunofluorescence analysis indicated that each of the four main subpopulations of interneurons exhibited GBR immunoreactivity. Virtually 100% of large CCK+ neurons in the basolateral and lateral nuclei were GBR+. In the basolateral nucleus 72% of somatostatin (SOM), 73% of parvalbumin (PV) and 25% of VIP positive interneurons were GBR+. In the lateral nucleus 50% of somatostatin, 30% of parvalbumin and 27% of VIP positive interneurons were GBR+. Electron microscopic (EM) analysis revealed that most of the light neuropil staining seen at the light microscopic level was due to the staining of dendritic shafts and spines, most of which probably belonged to spiny pyramidal cells. Very few axon terminals (Ats) were GBR+. In summary, this investigation demonstrates that the distal dendrites of pyramidal cells, and varying percentages of each of the four main subpopulations of interneurons in the ABL, express GBRs. Because previous studies suggest that GBR-mediated inhibition modulates NMDA-dependent EPSPs in the ABL, these receptors may play an important role in neuronal plasticity related to emotional learning.

Endocannabinoid-Mediated Metaplasticity in the Hippocampus

Repetitive activation of glutamatergic fibers that normally induces long-term potentiation (LTP) at excitatory synapses in the hippocampus also triggers long-term depression at inhibitory synapses (I-LTD) via retrograde endocannabinoid signaling. Little is known, however, about the physiological significance of I-LTD. Here, we show that synaptic-driven release of endocannabinoids is a highly localized and efficient process that strongly depresses cannabinoid-sensitive inhibitory inputs within the dendritic compartment of CA1 pyramidal cells. By removing synaptic inhibition in a restricted area of the dendritic tree, endocannabinoids selectively "primed" nearby excitatory synapses, thereby facilitating subsequent induction of LTP. This induction of local metaplasticity is a novel mechanism by which endocannabinoids can contribute to the storage of information in the brain.

Theta-contingent trial presentation accelerates learning rate and enhances hippocampal plasticity during trace eyeblink conditioning

Hippocampal theta activity has been established as a key predictor of acquisition rate in rabbit (Orcytolagus cuniculus) classical conditioning. The current study used an online brain--computer interface to administer conditioning trials only in the explicit presence or absence of spontaneous theta activity in the hippocampus-dependent task of trace conditioning. The findings indicate that animals given theta-contingent training learned significantly faster than those given nontheta-contingent training. In parallel with the behavioral results, the theta-triggered group, and not the nontheta-triggered group, exhibited profound increases in hippocampal conditioned unit responses early in training. The results not only suggest that theta-contingent training has a dramatic facilitory effect on trace conditioning but also implicate theta activity in enhancing the plasticity of hippocampal neurons.

A novel mechanism for the facilitation of theta-induced long-term potentiation by brain-derived neurotrophic factor

Brain-derived neurotrophic factor (BDNF) contributes to the induction of long-term potentiation (LTP) by theta-pattern stimulation, but the specific processes underlying this effect are not known. Experiments described here, using BDNF concentrations that have minor effects on baseline responses, show that the neurotrophin both reduces the threshold for LTP induction and elevates the ceiling on maximal potentiation. The enhanced LTP proved to be as stable and resistant to reversal as that recorded under control conditions. BDNF markedly increased the facilitation of burst responses that occurs within a theta train. This suggests that the neurotrophin acts on long-lasting events that (1) are set in motion by the first burst in a train and (2) regulate the amplitude of subsequent bursts. Whole-cell recordings established that BDNF causes a rapid reduction in the size of the long-lasting afterhyperpolarization (AHP) that follows individual theta bursts. Apamin, an antagonist of type 2 small-conductance Ca2+-activated potassium (SK2) channels, also reduced hippocampal AHPs and closely reproduced the effects of BDNF on theta-burst responses and LTP. The latter results were replicated with a newly introduced, highly selective inhibitor of SK2 channels. Immunoblot analyses indicated that BDNF increases SK2 serine phosphorylation in hippocampal slices. These findings point to the conclusion that BDNF-driven protein kinase cascades serve to depress the SK2 component, and possibly other constituents, of the AHP. It is likely that this mechanism, acting with other factors, promotes the formation and increases the magnitude of LTP.

Long-term potentiation is impaired in rat hippocampal slices that produce spontaneous sharp waves

Sharp waves (SPWs) occur in the hippocampal EEG during behaviours such as alert immobility and slow-wave sleep. Despite their widespread occurrence across brain regions and mammalian species, the functional importance of SPWs remains unknown. Experiments in the present study indicate that long-term potentiation (LTP) is significantly impaired in slices, prepared from the temporal aspect of rat hippocampus, that spontaneously generate SPW activity. This was probably not due to anatomical and/or biochemical abnormalities in temporal slices because stable LTP was uncovered in field CA1 when SPWs were eliminated by severing the projection from CA3. The same procedure did not alter LTP in slices lacking SPWs. Robust and stable LTP was obtained in the presence of SPWs in slices treated with an adenosine A1 receptor antagonist, a finding that links the present results to mechanisms related to the LTP reversal effect. In accord with this, single stimulation pulses delivered intermittently in a manner similar to the SPW pattern interfered with LTP to a similar degree as spontaneous SPWs. Taken together, these results suggest the possibility that SPWs in the hippocampus constitute a neural mechanism for forgetting.

Parallel somatic and synaptic processing in the induction of intermediate-term and long-term synaptic facilitation in Aplysia

The induction of different phases of memory depends on the amount and patterning of training, raising the question of whether specific training patterns engage different cellular mechanisms and whether these mechanisms operate in series or in parallel. We examined these questions by using a cellular model of memory formation: facilitation of the tail sensory neuron-motor neuron synapses by serotonin (5-hydroxytryptamine, 5-HT) in the CNS of Aplysia. We studied facilitation in two temporal domains: intermediate-term facilitation (1.5-3 h) and long-term facilitation (LTF, >24 h). Both forms can be induced by using several different temporal and spatial patterns of 5-HT, including (i) repeated, temporally spaced pulses of 5-HT to both the sensory neuron soma and the sensory neuron-motor neuron synapse, and (ii) temporally asymmetric exposure of 5-HT to the soma and synapse under conditions in which neither exposure alone induces LTF. We first examined the protein and RNA synthesis requirements for LTF induced by these two patterns and found that asymmetric (but not repeated) 5-HT application induced LTF that required postsynaptic protein and RNA synthesis. We next focused on the patterning and protein synthesis requirements for intermediate-term facilitation. We found that intermediate-term facilitation (i) is induced locally at the synapse, (ii) requires multiple pulses of 5-HT, and (iii) requires synaptic protein synthesis. Our findings show that different temporal and spatial patterns of 5-HT induce specific temporal phases of long-lasting facilitation in parallel by engaging different cellular and molecular mechanisms.

Modulation of AMPA receptor kinetics differentially influences synaptic plasticity in the hippocampus

Prior studies showed that positive alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor modulators facilitate long-term potentiation (LTP) and improve the formation of several types of memory in animals and humans. However, these modulators are highly diverse in their effects on receptor kinetics and synaptic transmission and thus may differ also in their efficacy to promote changes in synaptic strength. The present study examined three of these modulators for their effects on synaptic plasticity in field CA1 of hippocampal slices, two of them being the benzamide drugs 1-(quinoxalin-6-ylcarbonyl)piperidine (CX516) and 1-(1,4-benzodioxan-6-ylcarbonyl)piperidine (CX546) which prominently enhance synaptic transmission yet differ in their relative impact on amplitude versus duration of the synaptic response. The third drug was cyclothiazide which potently blocks AMPA receptor desensitization. Effects on plasticity were assessed by measuring (i) the likelihood of obtaining stable potentiation when using theta-burst stimulation with three instead of four pulses per burst, (ii) the maximum amount of potentiation under optimal stimulation conditions, and (iii) the effect on long-term depression (LTD). Both benzamides facilitated the formation of stable potentiation induced with three-pulse burst stimulation which is normally ineffective. CX546 in addition increased maximally inducible potentiation after four-pulse burst stimulation from about 50% to 100%. Burst response analysis revealed that CX546 greatly prolonged the duration of depolarization by slowing the decay of the response which thus presumably leads to a more continuous N-methyl-D-aspartate (NMDA) receptor activation. Cyclothiazide was ineffective in increasing maximal potentiation in either field or whole-cell recordings. CX546, but not CX516, also enhanced nearly two-fold the NMDA receptor-dependent long-term depression induced by heterosynaptic 2 Hz stimulation. Tests with recombinant NMDA receptors (NR1/NR2A) showed that CX516 and CX546 have no direct effects on currents mediated by these receptors. These results suggest that (1) modulation of AMPA receptors which increases either response amplitude or duration can facilitate LTP formation, (2) modulators that effectively slow response deactivation augment the maximum magnitude of LTP and LTD, and (3) receptor desensitization may have a minor impact on synaptic plasticity in the hippocampus. Taken together, our data indicate that AMPA receptor modulators differ substantially in their ability to enhance synaptic potentiation or depression, depending on their particular influence on receptor kinetics, and hence that they may also be differentially effective in influencing higher-order processes such as memory encoding.

Long-term potentiation and memory

One of the most significant challenges in neuroscience is to identify the cellular and molecular processes that underlie learning and memory formation. The past decade has seen remarkable progress in understanding changes that accompany certain forms of acquisition and recall, particularly those forms which require activation of afferent pathways in the hippocampus. This progress can be attributed to a number of factors including well-characterized animal models, well-defined probes for analysis of cell signaling events and changes in gene transcription, and technology which has allowed gene knockout and overexpression in cells and animals. Of the several animal models used in identifying the changes which accompany plasticity in synaptic connections, long-term potentiation (LTP) has received most attention, and although it is not yet clear whether the changes that underlie maintenance of LTP also underlie memory consolidation, significant advances have been made in understanding cell signaling events that contribute to this form of synaptic plasticity. In this review, emphasis is focused on analysis of changes that occur after learning, especially spatial learning, and LTP and the value of assessing these changes in parallel is discussed. The effect of different stressors on spatial learning/memory and LTP is emphasized, and the review concludes with a brief analysis of the contribution of studies, in which transgenic animals were used, to the literature on memory/learning and LTP.

Frequency-dependent changes in the paired-pulse index in the hippocampus of the freely moving adult male rat

The paired-pulse index (PPI) has been widely used as a measure of modulation of cellular excitability in the hippocampal trisynaptic circuit. This paper presents a quantification of the changes in this measure of neuronal modulation as a result of the application of pulse trains having six different train frequencies (0.1, 1, 5, 8, 15, and 30 Hz) to one of the major efferent pathways to the dentate gyrus, the medial perforant path (MPP). Our findings indicate that the modulation of the first leg of the hippocampal trisynaptic circuit is dependent on the frequency of the "burst train" applied to the perforant pathway. This methodological finding is of importance to all investigators studying hippocampal plasticity via LTP or LTD approaches. The different synaptic mechanisms implicated in being responsible for the changes in the PPI are also discussed.

Activity, modulation and role of basal forebrain cholinergic neurons innervating the cerebral cortex

The basal forebrain constitutes the ventral extra-thalamic relay from the brainstem activating system to the cerebral cortex. Cholinergic neurons form an important contingent of this relay, yet represent only a portion of the cortically projecting and other basal forebrain neurons, which include GABAergic neurons. By recording, labeling and identifying neurons by their neurotransmitter first in vitro and then in vivo, we have determined that the cholinergic neurons have different physiological and pharmacological properties than other codistributed neurons. Cholinergic neurons discharge at higher rates during cortical activation than during cortical slow wave activity, and are excited by transmitters released from brainstem afferent neurons, including glutamate from reticular formation, noradrenaline (NA) from locus coeruleus, and orexin and histamine from posterior hypothalamus. In contrast, particular GABAergic neurons discharge at higher rates during cortical slow wave activity and are inhibited by NA. When NA is administered into the basal forebrain in naturally sleeping-waking rats, it elicits an increase in fast gamma EEG activity and diminution of slow delta EEG activity while promoting waking and eliminating slow wave sleep (SWS). Cholinergic neurons also have the capacity to discharge in rhythmic bursts when activated by particular agonists, notably neurotensin (NT). When NT is administered into the basal forebrain, it stimulates theta activity in addition to gamma while promoting waking and paradoxical sleep (PS). By increasing discharge and firing in rhythmic bursts in response to transmitters of the activating systems, cholinergic neurons can thus stimulate cortical activation with gamma and theta activity along with the states of waking and PS. Colocalized GABAergic basal forebrain neurons which are inhibited by transmitters of the arousal systems can oppose these actions and promote delta activity and SWS.

Long-term synaptic plasticity in deep layer-originated associational projections to superficial layers of rat entorhinal cortex

Superficial layers of the entorhinal cortex (EC) relay the majority of cortical input projections to the hippocampus, whereas deep layers of the EC mediate a large portion of hippocampal output projections back to other cortical areas, suggesting a functional segregation between superficial and deep layers of the EC as input and output structures of the hippocampus, respectively. However, deep layers of the EC send associational projections to superficial layers, suggesting a potential interaction between neocortical input and hippocampus-processed output in superficial layers. This possibility was investigated by examining whether deep to superficial EC projections support long-term synaptic plasticity, and whether they interact with other pathways in superficial layers in rat medial EC slice preparations. Synaptic responses of the deep-to-superficial layer projections were verified based on field potential profiles, paired-pulse facilitation, physical separation between superficial and deep layers, and pharmacological manipulation. Long-term potentiation (LTP) was reliably induced in the deep-to-superficial layer projections by burst stimulations that emulated theta or sharp wave electroencephalogram (EEG),and it was blocked by an N-methyl-d-aspartate receptor antagonist (D-2-amino-5-phosphonopentanoic acid) and a calcium channel blocker (nifedipine). Prolonged low frequency stimulation induced long-term depression. A weak stimulation of deep layers, which induced a small degree of LTP by itself, generated a much larger degree of LTP when paired with a strong stimulation of superficial layers, indicating that the deep-to-superficial layer projections cooperate with other pathways in the superficial EC to enhance synaptic weights. Our results suggest that neocortical input and hippocampal output information are integrated in superficial layers of the EC.

Glutamatergic synaptic transmission participates in generating the hippocampal EEG

The participation of ionotropic glutamatergic synapses in the generation of hippocampal electroencephalography (EEG) of behaving rats has not been systematically studied. In this study, field potentials in hippocampal CA1 were recorded following injection of N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor antagonists, or vehicle control, either into the lateral ventricles or directly into the hippocampus or the medial septum. Intraventricular (i.c.v.) AMPA receptor antagonist 6,7-dinitroquinoxaline-2,3-dione (DNQX, 5-10 microg) decreased the commissural evoked potential and the amplitude of the hippocampal EEG, including the theta rhythm. Theta frequency was decreased by 10 microg, but not 5 microg DNQX i.c.v. Unilateral intrahippocampal injection of DNQX (5 microg) only decreased the amplitude, but not the frequency, of the theta rhythm near the site of injection, without affecting theta amplitude or frequency at the opposite hippocampus. Other than theta, the large irregular activity (with a delta frequency peak at 1-2 Hz) and gamma EEG (30-100 Hz) were also decreased by i.c.v. and intrahippocampal injections of DNQX. Intrahippocampal injection of NMDA receptor antagonist D-2-amino-5-phosphonovaleric acid (D-APV, 2.5 microg) decreased the amplitude of the theta rhythm and, less consistently, the gamma EEG. The frequency of the theta rhythm and the peak of the commissural evoked potential were not significantly affected by intrahippocampal D-APV injection. Medial septal injections of D-APV or D,L-APV (2.24 microg in 0.4 microl), but not DNQX (10 microg in 0.4 microl), decreased the amplitude of the hippocampal theta significantly, but theta frequency was not significantly affected. It is concluded that both NMDA and AMPA receptors in the hippocampus are involved in generating the amplitude of the hippocampal EEG of theta and gamma frequencies, while NMDA receptors in the medial septum are involved in controlling the amplitude of theta and gamma EEG in the hippocampus. Excitatory glutamatergic synaptic currents, activated by afferents from the entorhinal cortex and CA3, are suggested to participate in hippocampal EEG activities.

Theta reset produces optimal conditions for long-term potentiation

Connections among theta rhythm, long-term potentiation (LTP) and memory in hippocampus are suggested by previous research, but definitive links are yet to be established. We investigated the hypothesis that resetting of local hippocampal theta to relevant stimuli in a working memory task produces optimal conditions for induction of LTP. The timings of the peak and trough of the first wave of reset theta were determined in initial sessions and used to time stimulation (4 pulses, 200 Hz) during subsequent performance. Stimulation on the peak of stimulus-reset theta produced LTP while stimulation on the trough did not. These results suggest that a memory-relevant stimulus produces a phase shift of ongoing theta rhythm that induces optimal conditions for the stimulus to undergo potentiation.

Integrins, synaptic plasticity and epileptogenesis

A number of processes are thought to contribute to the development of epilepsy including enduring increases in excitatory synaptic transmission, changes in GABAergic inhibition, neuronal cell death and the development of aberrant innervation patterns in part arising from reactive axonal growth. Recent findings indicate that adhesion chemistries and, most particularly, activities of integrin class adhesion receptors play roles in each of these processes and thereby are likely to contribute significantly to the cell biology underlying epileptogenesis. As reviewed in this chapter, studies of long-term potentiation have shown that integrins are important for stabilizing activity-induced increases in synaptic strength and excitability. Other work has demonstrated that seizures, and in some instances subseizure neuronal activity, modulate the expression of integrins and their matrix ligands and the activities of proteases which regulate them both. These same adhesion proteins and proteases play critical roles in axonal growth and synaptogenesis including processes induced by seizure in adult brain. Together, these findings indicate that seizures activate integrin signaling and induce a turnover in adhesive contacts and that both processes contribute to lasting changes in circuit and synaptic function underlying epileptogenesis.

Priming stimulation enhances the depressant effect of low-frequency repetitive transcranial magnetic stimulation

Low-frequency (1 Hz) repetitive transcranial magnetic stimulation (rTMS) can depress the excitability of the cortex locally and has been proposed for the treatment of disorders such as schizophrenia and epilepsy. Some have speculated that the depressant effect is related to long-term depression (LTD) of cortical synapses. Because in vitro LTD can be enhanced by pretreatment of synapses with higher-frequency stimulation, we hypothesized that if rTMS depression had mechanisms in common with LTD, higher-frequency priming would increase it also. In 25 healthy volunteers in two experiments, we measured motor-evoked potentials (MEPs) from TMS of the motor cortex to define the baseline response. Subthreshold rTMS (6 Hz, fixed rate or frequency modulated) was used to prime the motor cortex, followed by suprathreshold 1 Hz stimulation for 10 min at just above the MEP threshold. Over the next 60 min, we recorded MEPs every 10 sec and found significant increases in the amount of cortical depression with both types of 6 Hz priming rTMS relative to sham. The MEP depression from 6 Hz-primed 1 Hz rTMS showed no evidence of decay after 60 min. Pretreatment with 6 Hz primes both 1 Hz rTMS depression and LTD. Although not conclusive evidence, this strengthens the case for overlapping mechanisms and suggests a potent new technique for enhancing low-frequency rTMS depression that may have experimental and clinical applications.

GABAB receptor- and metabotropic glutamate receptor-dependent cooperative long-term potentiation of rat hippocampal GABAA synaptic transmission

Repetitive stimulation of Schaffer collaterals induces activity-dependent changes in the strength of polysynaptic inhibitory postsynaptic potentials (IPSPs) in hippocampal CA1 pyramidal neurons that are dependent on stimulation parameters. In the present study, we investigated the effects of two stimulation patterns, theta-burst stimulation (TBS) and 100 Hz tetani, on pharmacologically isolated monosynaptic GABAergic responses in adult CA1 pyramidal cells. Tetanization with 100 Hz trains transiently depressed both early and late IPSPs, whereas TBS induced long-term potentiation (LTP) of early IPSPs that lasted at least 30 min. Mechanisms mediating this TBS-induced potentiation were examined using whole-cell recordings. The paired-pulse ratio of monosynaptic inhibitory postsynaptic currents (IPSCs) was not affected during LTP, suggesting that presynaptic changes in GABA release are not involved in the potentiation. Bath application of the GABAB receptor antagonist CGP55845 or the group I/II metabotropic glutamate receptor antagonist E4-CPG inhibited IPSC potentiation. Preventing postsynaptic G-protein activation or Ca2+ rise by postsynaptic injection of GDP-beta-S or BAPTA, respectively, abolished LTP, indicating a G-protein- and Ca2+-dependent induction in this LTP. Finally during paired-recordings, activation of individual interneurons by intracellular TBS elicited solely short-term increases in average unitary IPSCs in pyramidal cells. These results indicate that a stimulation paradigm mimicking the endogenous theta rhythm activates cooperative postsynaptic mechanisms dependent on GABABR, mGluR, G-proteins and intracellular Ca2+, which lead to a sustained potentiation of GABAA synaptic transmission in pyramidal cells. GABAergic synapses may therefore contribute to functional synaptic plasticity in adult hippocampus.

REM sleep enhancement induced by different procedures improves memory retention in rats

Growing evidence supports the idea that sleep following learning is critically involved in memory formation. Recent studies suggest that information acquired during waking is reactivated and possibly consolidated during subsequent sleep, especially during rapid-eye movement (REM) or paradoxical sleep (PS). Critical reviews, however, have questioned PS and memory relationships, particularly because of shortcomings of the PS deprivation paradigm applied in many studies. Therefore, in the present study we used an opposite strategy, i.e. we investigated the effects of PS enhancement on memory retention. In three experiments, we found that selective PS enhancement, induced by different procedures after discrimination training in rats, results in increased retention tested 24 h later. Moreover, calculated in all animals (n = 61), there was a highly significant correlation between post-training PS values and retention scores. Our results suggest that an experimentally induced increase of PS after learning facilitates memory consolidation.

Rap1 couples cAMP signaling to a distinct pool of p42/44MAPK regulating excitability, synaptic plasticity, learning, and memory

Learning-induced synaptic plasticity commonly involves the interaction between cAMP and p42/44MAPK. To investigate the role of Rap1 as a potential signaling molecule coupling cAMP and p42/44MAPK, we expressed an interfering Rap1 mutant (iRap1) in the mouse forebrain. This expression selectively decreased basal phosphorylation of a membrane-associated pool of p42/44MAPK, impaired cAMP-dependent LTP in the hippocampal Schaffer collateral pathway induced by either forskolin or theta frequency stimulation, decreased complex spike firing, and reduced the p42/44MAPK-mediated phosphorylation of the A-type potassium channel Kv4.2. These changes correlated with impaired spatial memory and context discrimination. These results indicate that Rap1 couples cAMP signaling to a selective membrane-associated pool of p42/44MAPK to control excitability of pyramidal cells, the early and late phases of LTP, and the storage of spatial memory.

Human theta oscillations related to sensorimotor integration and spatial learning

oscillations in the rat hippocampus have been implicated in sensorimotor integration (Bland, 1986), especially during exploratory and wayfinding behavior. We propose that human cortical activity coordinates sensory information with a motor plan to guide wayfinding behavior to known goal locations. To test this hypothesis, we analyzed invasive recordings from epileptic patients while they performed a spatially immersive, virtual taxi driver task. Consistent with this hypothesis, we found oscillations during both exploratory search and goal-seeking behavior and, in particular, during virtual movement, when sensory information and motor planning were both in flux, compared with periods of self-initiated stillness. oscillations had different topographic and spectral characteristics during searching than during goal-seeking, suggesting that different cortical networks exhibit depending on which cognitive functions are driving behavior (spatial learning during exploration vs orienting to a learned representation during goal-seeking). In contrast, oscillations in the beta band appeared to be related to simple motor planning, likely a variant of the Rolandic mu rhythm. These findings suggest that human cortical oscillations act to coordinate sensory and motor brain activity in various brain regions to facilitate exploratory learning and navigational planning.

Deficiency in short-chain fatty acid beta-oxidation affects theta oscillations during sleep

In rodents, the electroencephalogram (EEG) during paradoxical sleep and exploratory behavior is characterized by theta oscillations. Here we show that a deficiency in short-chain acyl-coenzyme A dehydrogenase (encoded by Acads) in mice causes a marked slowing in theta frequency during paradoxical sleep only. We found Acads expression in brain regions involved in theta generation, notably the hippocampus. Microarray analysis of gene expression in mice with mutations in Acads indicates overexpression of Glo1 (encoding glyoxylase 1), a gene involved in the detoxification of metabolic by-products. Administration of acetyl-L-carnitine (ALCAR) to mutant mice significantly recovers slow theta and Glo1 overexpression. Thus, an underappreciated metabolic pathway involving fatty acid beta-oxidation also regulates theta oscillations during sleep.

Developmental and regional differences in the consolidation of long-term potentiation

The alpha5beta1 integrin is present in high concentrations in the apical dendrites of pyramidal neurons in adult rats but is virtually absent in the basal dendrites. Moreover, alpha5beta1 does not appear in apical dendritic branches until the third post-natal week. Given that integrins contribute to the consolidation of synaptic plasticity, these results raise the possibility of developmental and regional differences in the stability of long-term potentiation (LTP). The present study tested this point using a LTP reversal paradigm in field CA1 of hippocampal slices. In accord with earlier reports, low-frequency afferent stimulation (5 Hz) introduced 30 s after theta burst stimulation (TBS) completely reversed LTP but was ineffective 30 min and 60 min later in slices from adult rats. The same low-frequency trains caused a partial reversal of LTP when applied 30 and 60 min post-TBS in slices from 21-day-old rats and a complete reversal at all time points in slices from 10-day-old rats. LTP in the basal dendrites of adult rats did not fully consolidate; i.e. potentiation was partially reversed by low-frequency stimulation even after delays of 30 or 60 min. Moreover, spaced (10 min) applications of 5- Hz pulses beginning at 30 min post-TBS completely erased LTP. The reversal effect in both apical and basal dendrites was blocked by N-methyl-D-aspartic acid receptor antagonists but an integrin antagonist had differential effects across the two dendritic domains. These results constitute evidence that the stability of LTP increases with age in the apical dendrites but remains incomplete even in adulthood in the basal dendrites. The possibilities that the developmental and regional variations in LTP consolidation are correlated with integrin expression and linked to different types of memory processing are discussed.

Theta oscillation in the anterior cingulate and beta-1 oscillation in the medial temporal cortices: a human case report

In previous studies we observed gamma (30-150 Hz) and beta-1 (10-20 Hz) oscillations in the medial temporal lobe (MTL) using subdural electrodes. The beta-1 was present during wake and REM sleep while gamma was present in all states. Recently we studied a patient (35 years M) with electrodes attached to the anterior cingulate cortex (ACC). This structure showed regular theta (5-6 Hz) oscillations. In the first recording, electrodes were attached to the MTL, that demonstrated the beta-1 and gamma oscillations. Two months later, electrodes were placed on orbitofrontal and ACC and an all night sleep recording was carried out. The ACC exhibited a highly regular and continuous theta oscillation during wakefulness and REM sleep, but not during NREM sleep. Since this same subject showed beta-1 oscillations in the MTL, it is probable that the theta in the ACC is independent of beta-1 in the MTL. This single case suggests the existence of two different frequency oscillators in the human limbic system. Elucidating their functional roles will be an interesting challenge for future studies.

Theta- and movement velocity-related firing of hippocampal neurons is disrupted by lesions centered on the perirhinal cortex

The hippocampus is critically involved in spatial memory and navigation. It has previously been proposed that, as part of this process, the hippocampus might have access to self-motion information. The possibility that some of this information may originate from the perirhinal cortex, a region involved in high-order multimodal processing, was tested in the present study by recording the responses of hippocampal complex-spike (place cells) and theta cells (putative interneurons) to movement velocity and to the movement-related theta rhythm EEG while rats with bilateral ibotenic acid lesions centered on the perirhinal cortex (n = 5), or control surgeries (n = 5), foraged in a rectangular environment. Perirhinal cortex lesions altered several characteristics of place and theta cell firing. First, the proportion of theta cells recorded was significantly lower in perirhinal lesion animals (8/39 units) compared to controls (22/53 units). Second, the firing of place cells recorded from lesion animals was phase-shifted so as to occur significantly earlier during the theta rhythm cycle than in place cells from controls (mean difference = 48.73 degrees). Third, the firing rates of a significantly lower proportion of place cells from lesion animals were modulated by the movement velocity of the animal compared to place cells from controls. These results indicate that the perirhinal cortex contributes to the responses of hippocampal CA1 place cells by providing information about self-movement and by controlling the timing of firing of these cells. This information may normally be utilized by the hippocampus during spatial memory and navigation processes.

Enhancement of temporal and spatial synchronization of entorhinal gamma activity by phase reset

The synchronization of cortical gamma oscillatory activity (25-80 Hz) is thought to coordinate neuronal assemblies in the processing and storage of information. The mechanism by which independently oscillating and distantly located cortical zones become synchronized is presumed to involve activity in corticocortical connections, although evidence supporting this conjecture has only been indirect. In the present study, we show that activation of synaptic inputs within and to the medial entorhinal cortex (mEC) of the in vitro isolated guinea pig brain preparation resets the phase of ongoing gamma activity induced by muscarinic receptor agonism with carbachol (frequency: 24 +/- 2 Hz at 32 degrees C). Phase reset was associated with a transient enhancement of the synchronization of gamma activity recorded at distant (>1 mm) mEC sites, across which low coherence (>0.75) was observed before stimulation. This increase in synchronization, as measured by cross-correlation analysis, was restricted to a maximal period of 200 ms after either local mEC or CA1 afferent stimulation. The results provide direct evidence that synaptic activation can enhance the rhythmic synchronization of spatially remote, independently oscillating neuronal assemblies in the mEC through a mechanism of synaptically evoked phase reset. Dynamic functional grouping of oscillatory discharges across long distances in the mEC may underlie coding processes involved in the integration and storage of incoming information and thus may be important for the role of this region in memory processes.

Cholinergic plasticity in the hippocampus

Tests were made for use-dependent plasticity in the cholinergic projections to hippocampus. Transient infusion of the cholinergic agonist carbachol into hippocampal slices induced rhythmic activity that persisted for hours after washout. Comparable effects were obtained with physostigmine, a drug that blocks acetylcholine breakdown and thereby enhances cholinergic transmission. It thus seems that activation of cholinergic synapses induces lasting changes in hippocampal physiology. Two lines of evidence indicated that cholinergic synapses are also the sites at which the plasticity is expressed. First, the induction and expression of the rhythms were not blocked by the N-methyl-D-aspartate receptor antagonist D-2-amino-5-phosphonovaleric acid, indicating that a long-term potentiation effect between pyramidal cells was not involved. Second, a muscarinic antagonist (atropine) completely abolished stable rhythmic activity after agonist washout. This result indicates that endogenous cholinergic activity is responsible for the persistence of rhythmic oscillations. These experiments suggest that short periods of intense cholinergic activity induce lasting changes in cholinergic synapses and thus extend such forms of plasticity to beyond the glutamatergic system.