In vitro hippocampal gamma oscillation power as an index of in vivo CA3 gamma oscillation strength and spatial reference memory

Gamma oscillation plasticity is mediated via parvalbumin interneurons

Understanding the plasticity of neuronal networks is an emerging field of (patho-) physiological research, yet the underlying cellular mechanisms remain poorly understood. Gamma oscillations (30 to 80 hertz), a biomarker of cognitive performance, require and potentiate glutamatergic transmission onto parvalbumin-positive interneurons (PVIs), suggesting an interface for cell-to-network plasticity. In ex vivo local field potential recordings, we demonstrate long-term potentiation of hippocampal gamma power. Gamma potentiation obeys established rules of PVI plasticity, requiring calcium-permeable AMPA receptors (CP-AMPARs) and metabotropic glutamate receptors (mGluRs). A microcircuit computational model of CA3 gamma oscillations predicts CP-AMPAR plasticity onto PVIs critically outperforms pyramidal cell plasticity in increasing gamma power and completely accounts for gamma potentiation. We reaffirm this ex vivo in three PVI-targeting animal models, demonstrating that gamma potentiation requires PVI-specific signaling via a Gq/PKC pathway comprising mGluR5 and a Gi-sensitive, PKA-dependent pathway. Gamma activity-dependent, metabotropically mediated CP-AMPAR plasticity on PVIs may serve as a guiding principle in understanding network plasticity in health and disease.

A therapeutic small molecule lead enhances γ-oscillations and improves cognition/memory in Alzheimer's disease model mice

Brain rhythms provide the timing and concurrence of brain activity required for linking together neuronal ensembles engaged in specific tasks. In particular, the γ-oscillations (30-120 Hz) orchestrate neuronal circuits underlying cognitive processes and working memory. These oscillations are reduced in numerous neurological and psychiatric disorders, including early cognitive decline in Alzheimer's disease (AD). Here we report on a potent brain permeable small molecule, DDL-920 that increases γ-oscillations and improves cognition/memory in a mouse model of AD, thus showing promise as a new class of therapeutics for AD. As a first in CNS pharmacotherapy, our lead candidate acts as a potent, efficacious, and selective negative allosteric modulator (NAM) of the γ-aminobutyric acid type A receptors (GABA A Rs) assembled from α1β2δ subunits. We identified these receptors through anatomical and pharmacological means to mediate the tonic inhibition of parvalbumin (PV) expressing interneurons (PV+INs) critically involved in the generation of γ-oscillations. Our approach is unique as it is meant to enhance cognitive performance and working memory in a state-dependent manner by engaging and amplifying the brain's endogenous γ-oscillations through enhancing the function of PV+INs.

The crucial role that hippocampus Cyclooxygenase-2 plays in memory

It is generally accepted that Cyclooxygenase-2 (COX-2) is activated to cause inflammation. However, COX-2 is also constitutively expressed at the postsynaptic dendrites and excitatory terminals of the cortical and spinal cord neurons. Although some evidence suggests that COX-2 release during neuronal signalling may be pivotal for regulating the function of memory, the significance of constitutively expressed COX-2 in neuron is still unclear. This research aims to discover the role of COX-2 in memory beyond neuroinflammation and to determine whether the inhibition of COX-2 can cause cognitive dysfunction by influencing dendritic plasticity and its underlying mechanism. We found COX-2 gene knockout (KO) could significantly impact the learning and memory ability, cause neuronal structure disorder and influence gamma oscillations. These might be mediated by the inhibition of prostaglandin (PG) E2/cAMP pathway and phosphorylated protein kinase A (p-PKA)-phosphorylated cAMP response element binding protein (p-CREB)-brain derived neurotrophic factor (BDNF) axis. It suggested COX-2 might play a critical role in learning, regulating neuronal structure and gamma oscillations in the hippocampus CA1 by regulating COX-2/BDNF signalling pathway.

Comparison of three common inbred mouse strains reveals substantial differences in hippocampal GABAergic interneuron populations and in vitro network oscillations

A major challenge in neuroscience is to pinpoint neurobiological correlates of specific cognitive and neuropsychiatric traits. At the mesoscopic level, promising candidates for establishing such connections are brain oscillations that can be robustly recorded as local field potentials with varying frequencies in the hippocampus in vivo and in vitro. Inbred mouse strains show natural variation in hippocampal synaptic plasticity (e.g. long-term potentiation), a cellular correlate of learning and memory. However, their diversity in expression of different types of hippocampal network oscillations has not been fully explored. Here, we investigated hippocampal network oscillations in three widely used inbred mouse strains: C57BL/6J (B6J), C57BL/6NCrl (B6N) and 129S2/SvPasCrl (129) with the aim to identify common oscillatory characteristics in inbred mouse strains that show aberrant emotional/cognitive behaviour (B6N and 129) and compare them to "control" B6J strain. First, we detected higher gamma oscillation power in the hippocampal CA3 of both B6N and 129 strains. Second, higher incidence of hippocampal sharp wave-ripple (SPW-R) transients was evident in these strains. Third, we observed prominent differences in the densities of distinct interneuron types and CA3 associative network activity, which are indispensable for sustainment of mesoscopic network oscillations. Together, these results add further evidence to profound physiological differences among inbred mouse strains commonly used in neuroscience research.

Osteocalcin ameliorates cognitive dysfunctions in a mouse model of Alzheimer's Disease by reducing amyloid β burden and upregulating glycolysis in neuroglia

Alzheimer's disease (AD) is the most common neurodegenerative disease characterized by the accumulation of amyloid β peptides (Aβ) and impaired glucose metabolism in the brain. Osteocalcin (OCN), an osteoblast-derived protein, has been shown to modulate brain functions but whether it has any effect on AD is undetermined. In this study, daily intraperitoneal injection of OCN for 4 weeks ameliorated the anxiety-like behaviors and cognitive dysfunctions in the APP/PS1 transgenic AD mice model, as shown in the increased entries into the central area in open field test, the increased time and entries into open arms in elevated plus maze test, the increased time spent in the light chamber in light-dark transition test, as well as the reduced escape latency and the increased preference for target quadrant in Morris water maze test. Aβ burden in the hippocampus and cortex of AD mice was ameliorated by OCN. Besides, OCN improved the neural network function of the brain, mainly in the enhanced power of high gamma band in the medial prefrontal cortex of AD mice. The proliferation of astrocytes in the hippocampus in AD mice was also inhibited by OCN as demonstrated by immunofluorescence. Furthermore, OCN enhanced glycolysis in astrocytes and microglia, as evidenced by elevated glucose consumption, lactate production, and increased extracellular acidification rate. Such an effect was abolished when the receptor of OCN - Gpr158 was knockdown in astrocytes. Our study revealed OCN as a novel therapeutic factor for AD potentially through reducing Aβ burden and upregulation of glycolysis in neuroglia.

Oligomeric β-Amyloid Suppresses Hippocampal γ-Oscillations through Activation of the mTOR/S6K1 Pathway

Neuronal synchronization at gamma frequency (30-100 Hz: γ) is impaired in early-stage Alzheimer's disease (AD) patients and AD models. Oligomeric Aβ_1-42 caused a concentration-dependent reduction of γ-oscillation strength and regularity while increasing its frequency. The mTOR1 inhibitor rapamycin prevented the Aβ_1-42-induced suppression of γ-oscillations, whereas the mTOR activator leucine mimicked the Aβ_1-42-induced suppression. Activation of the downstream kinase S6K1, but not inhibition of eIF4E, was required for the Aβ_1-42-induced suppression. The involvement of the mTOR/S6K1 signaling in the Aβ_1-42-induced suppression was confirmed in Aβ-overexpressing APP/PS1 mice, where inhibiting mTOR or S6K1 restored degraded γ-oscillations. To assess the network changes that may underlie the mTOR/S6K1 mediated γ-oscillation impairment in AD, we tested the effect of Aβ_1-42 on IPSCs and EPSCs recorded in pyramidal neurons. Aβ_1-42 reduced EPSC amplitude and frequency and IPSC frequency, which could be prevented by inhibiting mTOR or S6K1. These experiments indicate that in early AD, oligomer Aβ_1-42 impairs γ-oscillations by reducing inhibitory interneuron activity by activating the mTOR/S6K1 signaling pathway, which may contribute to early cognitive decline and provides new therapeutic targets.

Seizures in PPT1 Knock-In Mice Are Associated with Inflammatory Activation of Microglia

Infantile neuronal ceroid lipofuscinosis (INCL), the most severe form of neuronal ceroid lipofuscinoses, is caused by mutations in the lysosomal enzyme palmitoyl protein thioesterase 1 (PPT1). Typical symptoms of this disease include progressive psychomotor developmental retardation, visual failure, seizures, and premature death. Here, we investigated seizure activity and relevant pathological changes in PPT1 knock-in mice (PPT1 KI). The behavior studies in this study demonstrated that PPT1 KI mice had no significant seizure activity until 7 months of age, and local field potentials also displayed epileptiform activity at the same age. The expression levels of Iba-1 and CD68 demonstrated, by Western blot analysis, the inflammatory cytokine TNF-α content measured with enzyme-linked immunosorbent assay, and the number of microglia demonstrated by immunohistochemistry (IHC) were significantly increased at age of 7 months, all of which indicate microglia activation at an age of seizure onset. The increased expression of GFAP were seen at an earlier age of 4 months, and such an increase reached its peak at age of 6 months, indicating that astrocyte activation precedes microglia. The purinergic P2X7 receptor (P2X7R) is an ATP-sensitive ionic channel that is highly expressed in microglia and is fundamental to microglial activation, proliferation, cytokines release and epilepsy. We show that the ATP concentration in hippocampal tissue in PPT1 KI mice was increased using an enhanced ATP assay kit and demonstrated that the antagonist of P2X7R, A-438079, significantly reduced seizures in PPT1 KI mice. In contrast to glial cell activation and proliferation, a significant reduction in synaptic proteins GABA_AR was seen in PPT1 KI mice. These results indicate that seizure in PPT1 KI mice may be associated with microglial activation involved in ATP-sensitive P2X7R signaling and impaired inhibitory neurotransmission.

Phase coding of spatial representations in the human entorhinal cortex

The grid-like activity pattern of cells in the mammalian entorhinal cortex provides an internal reference frame for allocentric self-localization. The same neurons maintain robust phase couplings with local field oscillations. We found that neurons of the human entorhinal cortex display consistent spatial and temporal phase locking between spikes and slow gamma band local field potentials (LFPs) during virtual navigation. The phase locking maintained an environment-specific map over time. The phase tuning of spikes to the slow gamma band LFP revealed spatially periodic phase grids with environment-dependent scaling and consistent alignment with the environment. Using a Bayesian decoding model, we could predict the avatar's position with near perfect accuracy and, to a lesser extent, that of heading direction as well. These results imply that the phase of spikes relative to spatially modulated gamma oscillations encode allocentric spatial positions. We posit that a joint spatiotemporal phase code can implement the combined neural representation of space and time in the human entorhinal cortex.

Regulation of Hippocampal Gamma Oscillations by Modulation of Intrinsic Neuronal Excitability

Ion channels activated around the subthreshold membrane potential determine the likelihood of neuronal firing in response to synaptic inputs, a process described as intrinsic neuronal excitability. Long-term plasticity of chemical synaptic transmission is traditionally considered the main cellular mechanism of information storage in the brain; however, voltage- and calcium-activated channels modulating the inputs or outputs of neurons are also subjects of plastic changes and play a major role in learning and memory formation. Gamma oscillations are associated with numerous higher cognitive functions such as learning and memory, but our knowledge of their dependence on intrinsic plasticity is by far limited. Here we investigated the roles of potassium and calcium channels activated at near subthreshold membrane potentials in cholinergically induced persistent gamma oscillations measured in the CA3 area of rat hippocampal slices. Among potassium channels, which are responsible for the afterhyperpolarization in CA3 pyramidal cells, we found that blockers of SK (K_Ca2) and K_V7.2/7.3 (KCNQ2/3), but not the BK (K_Ca1.1) and IK (K_Ca3.1) channels, increased the power of gamma oscillations. On the contrary, activators of these channels had an attenuating effect without affecting the frequency. Pharmacological blockade of the low voltage-activated T-type calcium channels (Ca_V3.1–3.3) reduced gamma power and increased the oscillation peak frequency. Enhancement of these channels also inhibited the peak power without altering the frequency of the oscillations. The presented data suggest that voltage- and calcium-activated ion channels involved in intrinsic excitability strongly regulate the power of hippocampal gamma oscillations. Targeting these channels could represent a valuable pharmacological strategy against cognitive impairment.

The Impact of the Combined Administration of 1MeTIQ and MK-801 on Cell Viability, Oxidative Stress Markers, and Glutamate Release in the Rat Hippocampus

MK-801, as an N-methyl-D-aspartate (NMDA) receptor inhibitor, causes elevation in glutamate release, which may lead to an increase in excitotoxicity, oxidative stress and, consequently, cell death. 1-Methyl-1,2,3,4-tetrahydroisoquinoline (1MeTIQ) shows antioxidant activity. The aim of the present study was to evaluate the effect of combined treatment with 1MeTIQ and MK-801 on cell viability, antioxidant enzyme activity, and glutamate release in the rat hippocampus. Cytotoxicity was measured using lactate dehydrogenase leakage assay (LDH) and the methyl tetrazolium (MTT) assay; antioxidant enzyme activity (glutathione peroxidase (GPx), glutathione reductase (GR), superoxide dismutase (SOD), and catalase (CAT)) were measured by ELISA kits. The release of glutamate in the rat hippocampus was measured using in vivo microdialysis methodology. An in vitro study showed that MK-801 induced cell death in a concentration-dependent manner and that 1MeTIQ partially reduced this adverse effect of MK-801. An ex vivo study indicated that MK-801 produced an increase in antioxidant enzyme activity (GPx, GR, and SOD), whereas coadministration of MK-801 and 1MeTIQ restored the activity of these enzymes to the control level. An in vivo microdialysis study demonstrated that combined treatment with both drugs decreased the release of glutamate in the rat hippocampus. The above results revealed that 1MeTIQ shows limited neuroprotective activity under conditions of glutamate-induced neurotoxicity.

Transgenic modeling of Ndr2 gene amplification reveals disturbance of hippocampus circuitry and function

Duplications and deletions of short chromosomal fragments are increasingly recognized as the cause for rare neurodevelopmental conditions and disorders. The NDR2 gene encodes a protein kinase important for neuronal development and is part of a microduplication region on chromosome 12 that is associated with intellectual disabilities, autism, and epilepsy. We developed a conditional transgenic mouse with increased Ndr2 expression in postmigratory forebrain neurons to study the consequences of an increased gene dosage of this Hippo pathway kinase on brain circuitry and cognitive functions. Our analysis reveals reduced terminal fields and synaptic transmission of hippocampal mossy fibers, altered hippocampal network activity, and deficits in mossy fiber-dependent behaviors. Reduced doublecortin expression and protein interactome analysis indicate that transgenic Ndr2 disturbs the maturation of granule cells in the dentate gyrus. Together, our data suggest that increased expression of Ndr2 may critically contribute to the development of intellectual disabilities upon gene amplification.

Neuronal Dopamine D3 Receptors: Translational Implications for Preclinical Research and CNS Disorders

Dopamine (DA), as one of the major neurotransmitters in the central nervous system (CNS) and periphery, exerts its actions through five types of receptors which belong to two major subfamilies such as D1-like (i.e., D1 and D5 receptors) and D2-like (i.e., D2, D3 and D4) receptors. Dopamine D3 receptor (D3R) was cloned 30 years ago, and its distribution in the CNS and in the periphery, molecular structure, cellular signaling mechanisms have been largely explored. Involvement of D3Rs has been recognized in several CNS functions such as movement control, cognition, learning, reward, emotional regulation and social behavior. D3Rs have become a promising target of drug research and great efforts have been made to obtain high affinity ligands (selective agonists, partial agonists and antagonists) in order to elucidate D3R functions. There has been a strong drive behind the efforts to find drug-like compounds with high affinity and selectivity and various functionality for D3Rs in the hope that they would have potential treatment options in CNS diseases such as schizophrenia, drug abuse, Parkinson’s disease, depression, and restless leg syndrome. In this review, we provide an overview and update of the major aspects of research related to D3Rs: distribution in the CNS and periphery, signaling and molecular properties, the status of ligands available for D3R research (agonists, antagonists and partial agonists), behavioral functions of D3Rs, the role in neural networks, and we provide a summary on how the D3R-related drug research has been translated to human therapy.

Region-dependent regulation of acute ethanol on γ oscillation in the rat hippocampal slices

BACKGROUND

Ethanol use disorders are a serious medical and public health problem in the world today. Acute ethanol intoxication can lead to cognitive dysfunction such as learning and memory impairment. Gamma oscillations (γ, 30-80 Hz) are synchronized rhythmic activity generated by population of neurons within local network, and closely related to learning and memory function. The hippocampus is a critical anatomic structure that supports learning and memory. On the grounds of structure and function, hippocampus can be divided into the intermediate (IH), the dorsal (DH), and ventral hippocampus (VH). The current study is the first to investigate the effects of acute ethanol on γ oscillations in these sub-regions of rat hippocampal slices.

METHODS

The sustained γ oscillations were induced by 200 nM kainate (KA) in the CA3c of IH, DH, and VH. When KA-induced γ oscillation reached the steady state, ethanol (50 mM or 100 mM) was applied and the effects of ethanol on γ oscillation power was measured in the slices sequentially sectioned from ventral to dorsal hippocampus of adult rats.

RESULTS

In the intermediate hippocampal slices, compared with control (KA only), ethanol (50 mM) caused 36.1 ± 3.9% decrease in γ power (p < 0.05, n = 10), while ethanol (100 mM) caused 55.3 ± 5.5% decrease in γ power (p < 0.001, n = 14). In the dorsal hippocampus, only ethanol (100 mM) caused 18.1 ± 8.6% decrease in γ power (p < 0.05, n = 12). However, in the ventral hippocampus, neither 50 mM nor 100 mM ethanol affected γ oscillation.

CONCLUSIONS

Our results demonstrate that ethanol may produce the differential suppression of γ oscillations in a dose-dependent manner in different sub-regions of hippocampus, suggesting that the modulation of ethanol on hippocampal γ oscillation is region-dependent.

The novel antipsychotic cariprazine stabilizes gamma oscillations in rat hippocampal slices

Background and Purpose

Gamma oscillations are fast rhythmic fluctuations of neuronal network activity ranging from 30 to 90 Hz that establish a precise temporal background for cognitive processes such as perception, sensory processing, learning, and memory. Alterations of gamma oscillations have been observed in schizophrenia and are suggested to play crucial roles in the generation of positive, negative, and cognitive symptoms of the disease.

Experimental Approach

In this study, we investigated the effects of the novel antipsychotic cariprazine, a D₃‐preferring dopamine D₃/D₂ receptor partial agonist, on cholinergically induced gamma oscillations in rat hippocampal slices from treatment‐naïve and MK‐801‐treated rats, a model of acute first‐episode schizophrenia.

Key Results

The D₃ receptor‐preferring agonist pramipexole effectively decreased the power of gamma oscillations, while the D₃ receptor antagonist SB‐277011 had no effect. In treatment‐naïve animals, cariprazine did not modulate strong gamma oscillations but slightly improved the periodicity of non‐saturated gamma activity. Cariprazine showed a clear partial agonistic profile at D₃ receptors at the network level by potentiating the inhibitory effects when the D₃ receptor tone was low and antagonizing the effects when the tone was high. In hippocampal slices of MK‐801‐treated rats, cariprazine allowed stabilization of the aberrant increase in gamma oscillation power and potentiated resynchronization of the oscillations.

Conclusion and Implications

Data from this study indicate that cariprazine stabilizes pathological hippocampal gamma oscillations, presumably by its partial agonistic profile. The results demonstrate in vitro gamma oscillations as predictive biomarkers to study the effects of antipsychotics preclinically at the network level.

Brain gamma rhythm and potential treatment of neurodegenerative disease

In this mini-review, we illustrate the brain network oscillations in different brain areas, including the medial septal diagonal band complex (MSDB) and hippocampus, especially at gamma frequency bands (γ, 30–80 Hz) and theta frequency bands (θ, 4–12 Hz), and their induction and modulation by physical stimulation, such as light and sound, and pharmacological stimulation with agents such as agonists of the kainite subunit ionotropic glutamate receptor, metabotropic glutamate receptor, metabotropic cholinergic receptor, and nicotinic cholinergic receptor. Recent findings demonstrate that boosting gamma oscillations in specific brain areas appears to be able to restore cognitive function and reduce relative pathology in neurodegenerative diseases, such as Alzheimer’s disease. Thus, exploration of strategies to enhance or restore impaired gamma oscillations may be a new and effective method to improve the conditions in these devastating diseases.

The Modulation of Gamma Oscillations by Methamphetamine in Rat Hippocampal Slices

Gamma frequency oscillations (γ, 30–100 Hz) have been suggested to underlie various cognitive and motor functions. The psychotomimetic drug methamphetamine (MA) enhances brain γ oscillations associated with changes in psychomotor state. Little is known about the cellular mechanisms of MA modulation on γ oscillations. We explored the effects of multiple intracellular kinases on MA modulation of γ induced by kainate in area CA3 of rat ventral hippocampal slices. We found that dopamine receptor type 1 and 2 (DR1 and DR2) antagonists, the serine/threonine kinase PKB/Akt inhibitor and N-methyl-D-aspartate receptor (NMDAR) antagonists prevented the enhancing effect of MA on γ oscillations, whereas none of them affected baseline γ strength. Protein kinase A, phosphoinositide 3-kinase and extracellular signal-related kinases inhibitors had no effect on MA. We propose that the DR1/DR2-Akt-NMDAR pathway plays a critical role for the MA enhancement of γ oscillations. Our study provides an new insight into the mechanisms of acute MA on MA-induced psychosis.

Rearing-environment-dependent hippocampal local field potential differences in wild-type and inositol trisphosphate receptor type 2 knockout mice

KEY POINTS

Mice reared in an enriched environment are demonstrated to have larger hippocampal gamma oscillations than those reared in isolation, thereby confirming previous observations in rats. To test whether astrocytic Ca²⁺ surges are involved in this experience-dependent LFP pattern modulation, we used inositol trisphosphate receptor type 2 (IP₃ R2)-knockout (KO) mice, in which IP₃ /Ca²⁺ signalling in astrocytes is largely diminished. We found that this experience-dependent gamma power alteration persists in the KO mice. Interestingly, hippocampal ripple events, the synchronized events critical for memory consolidation, are reduced in magnitude and frequency by both isolated rearing and IP₃ R2 deficiency.

ABSTRACT

Rearing in an enriched environment (ENR) is known to enhance cognitive and memory abilities in rodents, whereas social isolation (ISO) induces depression-like behaviour. The hippocampus has been documented to undergo morphological and functional changes depending on these rearing environments. For example, rearing condition during juvenility alters CA1 stratum radiatum gamma oscillation power in rats. In the present study, hippocampal CA1 local field potentials (LFP) were recorded from bilateral CA1 in urethane-anaesthetized mice that were reared in either an ENR or ISO condition. Similar to previous findings in rats, gamma oscillation power during theta states was higher in the ENR group. Ripple events that occur during non-theta periods in the CA1 stratum pyramidale also had longer intervals in ISO mice. Because astrocytic Ca²⁺ elevations play a key role in synaptic plasticity, we next tested whether these changes in LFP are also expressed in inositol trisphosphate receptor type 2 (IP₃ R2)-knockout (KO) mice, in which astrocytic Ca²⁺ elevations are largely diminished. We found that the gamma power was also higher in IP₃ R2-KO-ENR mice compared to IP₃ R2-KO-ISO mice, suggesting that the rearing-environment-dependent gamma power alteration does not necessarily require the astrocytic IP₃ /Ca²⁺ pathway. By contrast, ripple events showed genotype-dependent changes, as well as rearing condition-dependent changes: ISO housing and IP₃ R2 deficiency both lead to longer inter-ripple intervals. Moreover, we found that ripple magnitude in the right CA1 tended to be smaller in IP₃ R2-KO. Because IP₃ R2-KO mice have been reported to have depression phenotypes, our results suggest that ripple events and the mood of animals may be broadly correlated.

Changes of EEG Spectra and Functional Connectivity during an Object-Location Memory Task in Alzheimer's Disease

Object-location memory is particularly fragile and specifically impaired in Alzheimer’s disease (AD) patients. Electroencephalogram (EEG) was utilized to objectively measure memory impairment for memory formation correlates of EEG oscillatory activities. We aimed to construct an object-location memory paradigm and explore EEG signs of it. Two groups of 20 probable mild AD patients and 19 healthy older adults were included in a cross-sectional analysis. All subjects took an object-location memory task. EEG recordings performed during object-location memory tasks were compared between the two groups in the two EEG parameters (spectral parameters and phase synchronization). The memory performance of AD patients was worse than that of healthy elderly adults The power of object-location memory of the AD group was significantly higher than the NC group (healthy elderly adults) in the alpha band in the encoding session, and alpha and theta bands in the retrieval session. The channels-pairs the phase lag index value of object-location memory in the AD group was clearly higher than the NC group in the delta, theta, and alpha bands in encoding sessions and delta and theta bands in retrieval sessions. The results provide support for the hypothesis that the AD patients may use compensation mechanisms to remember the items and episode.

Metabotropic glutamate receptor, mGlu5, regulates hippocampal synaptic plasticity and is required for tetanisation-triggered changes in theta and gamma oscillations

Hippocampal synaptic plasticity and learning are regulated by metabotropic glutamate receptors (mGlu) and particularly by mGlu5. In the hippocampus, synaptic plasticity is tightly linked to neuronal network oscillations in theta (5-10 Hz) and gamma (∼30-100 Hz) frequency ranges, and specific changes in theta and gamma spectral power can predict for the success of patterned afferent stimulation in inducing robust long-term potentiation (LTP). In this study, we hypothesized that activation of mGlu5 mediates tetanisation-driven changes in network oscillations and thereby determines the longevity of LTP. To explore this, we applied high-frequency stimulation (HFS) to the perforant path input to the dentate gyrus (DG), in the presence of the negative allosteric modulator, 2-methyl-6-(phenylethynyl)pyridine (MPEP), or the positive allosteric modulator (S)-(4-fluorophenyl)-[3-(3-(3-(4-fluorophenyl)-[1,2,4]oxadiazol-5-yl)-piperidin-1-yl)]methanone (ADX47273). In freely behaving rats, administration of MPEP resulted in a significant impairment, whereas treatment with ADX47273 led to a significant enhancement, of LTP (>24 h) compared to vehicle-treated controls. Allosteric potentiation of mGlu5 also resulted in a significantly greater increase of the spectral power of theta and gamma oscillations within the period of 300 s after HFS, as compared to MPEP-treated animals or controls. Our findings show that the regulation of hippocampal LTP by mGlu5 is associated with modulation of network oscillatory activity in the period shortly after LTP induction. Taken together, these data demonstrate that changes in the spectral contents of local field activity that occur in response to patterned afferent stimulation require activation of mGlu5 and may be instrumental for the successful expression of persistent LTP. This article is part of the Special Issue entitled 'Metabotropic Glutamate Receptors, 5 years on'.

Kainic acid-induced albumin leak across the blood-brain barrier facilitates epileptiform hyperexcitability in limbic regions

OBJECTIVE

Systemic administration of kainic acid (KA) is a widely used procedure utilized to develop a model of temporal lobe epilepsy (TLE). Despite its ability to induce status epilepticus (SE) in vivo, KA applied to in vitro preparations induces only interictal-like activity and/or isolated ictal discharges. The possibility that extravasation of the serum protein albumin from the vascular compartment enhances KA-induced brain excitability is investigated here.

METHODS

Epileptiform activity was induced by arterial perfusion of 6 μm KA in the in vitro isolated guinea pig brain preparation. Simultaneous field potential recordings were carried out bilaterally from limbic (CA1, dentate gyrus [DG], and entorhinal cortex) and extralimbic regions (piriform cortex and neocortex). Blood-brain barrier (BBB) breakdown associated with KA-induced epileptiform activity was assessed by parenchymal leakage of intravascular fluorescein-isothiocyanate albumin. Seizure-induced brain inflammation was evaluated by western blot analysis of interleukin (IL)-1β expression in brain tissue.

RESULTS

KA infusion caused synchronized activity at 15-30 Hz in limbic (but not extralimbic) cortical areas, associated with a brief, single seizure-like event. A second bolus of KA, 60 min after the induction of the first ictal event, did not further enhance excitability. Perfusion of serum albumin between the two administrations of KA enhanced epileptiform discharges and allowed a recurrent ictal event during the second KA infusion.

SIGNIFICANCE

Our data show that arterial KA administration selectively alters the synchronization of limbic networks. However, KA is not sufficient to generate recurrent seizures unless serum albumin is co-perfused during KA administration. These findings suggest a role of serum albumin in facilitating acute seizure generation.

Identification of Parvalbumin Interneurons as Cellular Substrate of Fear Memory Persistence

Parvalbumin-positive (PV) basket cells provide perisomatic inhibition in the cortex and hippocampus and control generation of memory-related network activity patterns, such as sharp wave ripples (SPW-R). Deterioration of this class of fast-spiking interneurons has been observed in neuropsychiatric disorders and evidence from animal models suggests their involvement in the acquisition and extinction of fear memories. Here, we used mice with neuron type-targeted expression of the presynaptic gain-of-function glycine receptor RNA variant GlyR α3L^185L to genetically enhance the network activity of PV interneurons. These mice showed reduced extinction of contextual fear memory but normal auditory cued fear memory. They furthermore displayed increase of SPW-R activity in area CA3 and CA1 and facilitated propagation of this particular network activity pattern, as determined in ventral hippocampal slice preparations. Individual freezing levels during extinction and SPW-R propagation were correlated across genotypes. The same was true for parvalbumin immunoreactivity in the ventral hippocampus, which was generally augmented in the GlyR mutant mice and correlated with individual freezing levels. Together, these results identify PV interneurons as critical cellular substrate of fear memory persistence and associated SPW-R activity in the hippocampus. Our findings may be relevant for the identification and characterization of physiological correlates for posttraumatic stress and anxiety disorders.

Long-term changes in the CA3 associative network of fear-conditioned mice

The CA3 associative network plays a critical role in the generation of network activity patterns related to emotional state and fear memory. We investigated long-term changes in the corticosterone (CORT)-sensitive function of this network following fear conditioning and fear memory reactivation. In acute slice preparations from mice trained in either condition, the ratio of orthodromic population spike (PS) to antidromic PS was reduced compared to unconditioned animals, indicating a decrease in efficacy of neuronal coupling within the associative CA3 network. However, spontaneous sharp wave-ripples (SW-R), which are thought to arise from this network, remained unaltered. Following CORT application, we observed an increase in orthodromic PS and a normalization to control levels of their ratio to antidromic PS, while SW-R increased in slices of fear conditioned and fear reactivated mice, but not in slices of unconditioned controls. Together with our previous observations of altered hippocampal gamma activity under these learning paradigms, these data suggest that fear conditioning and fear reactivation lastingly alters the CORT-sensitive configuration of different network activity patterns generated by the CA3 associational network. Observed changes in the mRNA expression of receptors for glutamate, GABA and cannabinoids in the stratum pyramidale of area CA3 may provide a molecular mechanism for these adaptive changes.

In vitro gamma oscillations following partial and complete ablation of δ subunit-containing GABAA receptors from parvalbumin interneurons

Perisynaptic and extrasynaptic δ subunit-containing GABAA receptors (δ-GABAARs) mediate tonic conductances in many neurons. On principal cells of the neocortex and hippocampus they comprise α4 subunits, whereas they usually contain α1 on various interneurons. Specific characteristics of δ-GABAARs are their pharmacology and high plasticity. In particular δ-GABAARs are sensitive to low concentrations of neurosteroids (NS) and during times of altered NS production (stress, puberty, ovarian cycle and pregnancy) δ-GABAARs expression varies in many neurons regardless of the α subunits they contain, with direct consequences for neuronal excitability and network synchrony. For example δ-GABAARs plasticity on INs underlies modifications in hippocampal γ oscillations during pregnancy or over the ovarian cycle. Most δ-GABAAR-expressing INs in CA3 stratum pyramidale (SP) are parvalbumin (PV) + INs, whose fundamental role in γ oscillations generation and control has been extensively investigated. In this study we reduced or deleted δ-subunits in PV + INs, with the use of a PV/Cre-Gabrd/floxed genetic system. We find that in vitro CA3 γ oscillations of both PV-Gabrd(+/-)and PV-Gabrd(-/-) mice are characterized by higher frequencies than WT controls. The increased frequencies could be lowered to control levels in PV-Gabrd(+/-) by the NS allopregnanolone (3α,5α-tetrahydroprogesterone, 100 nM) but not the synthetic δ-GABAAR positive allosteric modulator 4-Chloro-N-[2-(2-thienyl)imidazo[1,2-a]pyridin-3-yl] benzamide (DS-2, 10 μM). This is consistent with the idea that DS-2, in contrast to ALLO, selectively targets α4/δ-GABAARs but not the α1/δ-GABAARs found on INs. Therefore, development of drugs selective for IN-specific α1/δ-GABAARs may be useful in neurological and psychiatric conditions correlated with altered PV + IN function and aberrant γ oscillations.

Ovarian cycle-linked plasticity of δ-GABAA receptor subunits in hippocampal interneurons affects γ oscillations in vivo

GABA_A receptors containing δ subunits (δ-GABA_ARs) are GABA-gated ion channels with extra- and perisynaptic localization, strong sensitivity to neurosteroids (NS), and a high degree of plasticity. In selective brain regions they are expressed on specific principal cells and interneurons (INs), and generate a tonic conductance that controls neuronal excitability and oscillations. Plasticity of δ-GABA_ARs in principal cells has been described during states of altered NS synthesis including acute stress, puberty, ovarian cycle, pregnancy and the postpartum period, with direct consequences on neuronal excitability and network dynamics. The defining network events implicated in cognitive function, memory formation and encoding are γ oscillations (30–120 Hz), a well-timed loop of excitation and inhibition between principal cells and PV-expressing INs (PV + INs). The δ-GABA_ARs of INs can modify γ oscillations, and a lower expression of δ-GABA_ARs on INs during pregnancy alters γ frequency recorded in vitro. The ovarian cycle is another physiological event with large fluctuations in NS levels and δ-GABA_ARs. Stages of the cycle are paralleled by swings in memory performance, cognitive function, and mood in both humans and rodents. Here we show δ-GABA_ARs changes during the mouse ovarian cycle in hippocampal cell types, with enhanced expression during diestrus in principal cells and specific INs. The plasticity of δ-GABA_ARs on PV-INs decreases the magnitude of γ oscillations continuously recorded in area CA1 throughout several days in vivo during diestrus and increases it during estrus. Such recurring changes in γ magnitude were not observed in non-cycling wild-type (WT) females, cycling females lacking δ-GABA_ARs only on PV-INs (PV-Gabrd^-/-), and in male mice during a time course equivalent to the ovarian cycle. Our findings may explain the impaired memory and cognitive performance experienced by women with premenstrual syndrome (PMS) or premenstrual dysphoric disorder (PMDD).

Single and combined effects of prenatal immune activation and peripubertal stress on parvalbumin and reelin expression in the hippocampal formation

Exposure to prenatal infection and traumatizing experiences in peripubertal life are two environmental risk factors for developmental neuropsychiatric disorders. Modeling the cumulative neuronal impact of these factors in a translational animal model has led to the recent identification of pathological interactions between these environmental adversities in the development of adult brain dysfunctions. The present study explored the consequences of combined prenatal immune challenge and peripubertal stress on discrete cellular abnormalities in the γ-aminobutyric acid (GABA) system of the hippocampus. Pregnant mice were treated with the viral mimetic poly(I:C) (=polyriboinosinic-polyribocytidilic acid) or control solution, and offspring born to poly(I:C)-exposed or control mothers were then left undisturbed or subjected to unpredictable sub-chronic stress during peripubertal development. Stereological estimations of parvalbumin-expressing cells revealed a significant reduction of these GABAergic interneurons in the ventral dentate gyrus of adult offspring exposed to combined immune activation and stress. Single exposure to either environmental factor was insufficient to cause similar neuropathology. We further found that peripubertal stress exerted opposite effects on reelin-immunoreactive cells in the dorsal cornu ammonis (CA) region of the hippocampus, with stress increasing and decreasing reelin expression in control offspring and prenatally immune challenged animals, respectively. The present data suggest that the combination of two environmental risk factors, which have each been implicated in the etiology of major neuropsychiatric disease, induces significant but restricted neuropathological effects on hippocampal GABAergic cell populations known to be affected in brain disorders with neurodevelopmental components.

In sync: gamma oscillations and emotional memory

Emotional experiences leave vivid memories that can last a lifetime. The emotional facilitation of memory has been attributed to the engagement of diffusely projecting neuromodulatory systems that enhance the consolidation of synaptic plasticity in regions activated by the experience. This process requires the propagation of signals between brain regions, and for those signals to induce long-lasting synaptic plasticity. Both of these demands are met by gamma oscillations, which reflect synchronous population activity on a fast timescale (35-120 Hz). Regions known to participate in the formation of emotional memories, such as the basolateral amygdala, also promote gamma-band activation throughout cortical and subcortical circuits. Recent studies have demonstrated that gamma oscillations are enhanced during emotional situations, coherent between regions engaged by salient stimuli, and predict subsequent memory for cues associated with aversive stimuli. Furthermore, neutral stimuli that come to predict emotional events develop enhanced gamma oscillations, reflecting altered processing in the brain, which may underpin how past emotional experiences color future learning and memory.

The kainic acid model of temporal lobe epilepsy

The kainic acid model of temporal lobe epilepsy has greatly contributed to the understanding of the molecular, cellular and pharmacological mechanisms underlying epileptogenesis and ictogenesis. This model presents with neuropathological and electroencephalographic features that are seen in patients with temporal lobe epilepsy. It is also characterized by a latent period that follows the initial precipitating injury (i.e., status epilepticus) until the appearance of recurrent seizures, as observed in the human condition. Finally, the kainic acid model can be reproduced in a variety of species using either systemic, intrahippocampal or intra-amygdaloid administrations. In this review, we describe the various methodological procedures and evaluate their differences with respect to the behavioral, electroencephalographic and neuropathological correlates. In addition, we compare the kainic acid model with other animal models of temporal lobe epilepsy such as the pilocarpine and the kindling model. We conclude that the kainic acid model is a reliable tool for understanding temporal lobe epilepsy, provided that the differences existing between methodological procedures are taken into account.

Cerebral energy metabolism and the brain's functional network architecture: an integrative review

Recent functional magnetic resonance imaging (fMRI) studies have emphasized the contributions of synchronized activity in distributed brain networks to cognitive processes in both health and disease. The brain's 'functional connectivity' is typically estimated from correlations in the activity time series of anatomically remote areas, and postulated to reflect information flow between neuronal populations. Although the topological properties of functional brain networks have been studied extensively, considerably less is known regarding the neurophysiological and biochemical factors underlying the temporal coordination of large neuronal ensembles. In this review, we highlight the critical contributions of high-frequency electrical oscillations in the γ-band (30 to 100 Hz) to the emergence of functional brain networks. After describing the neurobiological substrates of γ-band dynamics, we specifically discuss the elevated energy requirements of high-frequency neural oscillations, which represent a mechanistic link between the functional connectivity of brain regions and their respective metabolic demands. Experimental evidence is presented for the high oxygen and glucose consumption, and strong mitochondrial performance required to support rhythmic cortical activity in the γ-band. Finally, the implications of mitochondrial impairments and deficits in glucose metabolism for cognition and behavior are discussed in the context of neuropsychiatric and neurodegenerative syndromes characterized by large-scale changes in the organization of functional brain networks.

Long-lasting increase of corticosterone after fear memory reactivation: anxiolytic effects and network activity modulation in the ventral hippocampus

Pathological fear and anxiety can be studied, in rodents, with fear conditioning and exposure to reminder cues. These paradigms are thought to critically involve the ventral hippocampus, which also serves as key site of glucocorticoid action in the brain. Here, we demonstrate a long-lasting reduction of kainate-induced gamma oscillations in slice preparations of the ventral hippocampal area CA3, 30 days after a single fear conditioning training. Reduction of gamma power was sensitive to corticosterone application and associated with a decrease in glucocorticoid and mineralocorticoid receptor mRNA expression across strata of the ventral hippocampal CA3. A fear reactivation session 24 h after the initial conditioning normalized receptor expression levels and attenuated the corticosterone-mediated recovery of gamma oscillations. It moreover increased both baseline and stimulus-induced corticosterone plasma levels and evoked a generalization of fear memory to the background context. Reduced ventral hippocampal gamma oscillation in both fear reactivated and non-reactivated mice were associated with a decrease of anxiety-like behavior in an elevated plus maze. Taking advantage of the circadian fluctuation in corticosterone, we demonstrated the association of high endogenous basal corticosterone plasma concentrations during morning hours with reduced anxiety-like behavior in fear reactivated mice. The anxiolytic effect of the hormone was verified with local applications to the ventral hippocampus. Our data suggest that corticosterone acting on ventral hippocampal network activity has anxiolytic-like effects following fear exposure, highlighting its potential therapeutic value for anxiety disorders.

The effect of aging-associated impaired mitochondrial status on kainate-evoked hippocampal gamma oscillations

Oscillations in hippocampal neuronal networks in the gamma frequency band have been implicated in various cognitive tasks and we showed previously that aging reduces the power of such oscillations. Here, using submerged hippocampal slices allowing simultaneous electrophysiological recordings and imaging, we studied the correlation between the kainate-evoked gamma oscillation and mitochondrial activity, as monitored by rhodamine 123. We show that the initiation of kainate-evoked gamma oscillations induces mitochondrial depolarization, indicating a metabolic response. Aging had an opposite effect on these parameters: while depressing the gamma oscillation strength, it increases mitochondrial depolarization. Also, in the aged neurons, kainate induced significantly larger Ca2+ signals. In younger slices, acute mitochondrial depolarization induced by low concentrations of mitochondrial protonophores strongly, but reversibly, inhibits gamma oscillations. These data indicating that the complex network activity required by the maintenance of gamma activity is susceptible to changes and modulations in mitochondrial status.

In Vitro and In Vivo Recording of Local Field Potential Oscillations in Mouse Hippocampus

Oscillations in hippocampal local field potentials (LFP) reflect the coordinated, rhythmic activity of constituent interneuronal and principal cell populations. Quantifying changes in oscillatory patterns and power therefore provides a powerful metric through which to infer mechanisms and functions of hippocampal network activity at the mesoscopic level, bridging single-neuron studies to behavioral assays of hippocampal function. Here, complementary protocols that enable mechanistic analyses of oscillation generation in vitro (in slices and a whole hippocampal preparation) and functional analyses of hippocampal circuits in behaving mice are described. Used together, these protocols provide a comprehensive view of hippocampal phenotypes in mouse models, highlighting oscillatory biomarkers of hippocampal function and dysfunction. Curr. Protoc. Mouse Biol. 2:273-294 © 2012 by John Wiley & Sons, Inc.

Mechanisms of physiological and epileptic HFO generation

High frequency oscillations (HFO) have a variety of characteristics: band-limited or broad-band, transient burst-like phenomenon or steady-state. HFOs may be encountered under physiological or under pathological conditions (pHFO). Here we review the underlying mechanisms of oscillations, at the level of cells and networks, investigated in a variety of experimental in vitro and in vivo models. Diverse mechanisms are described, from intrinsic membrane oscillations to network processes involving different types of synaptic interactions, gap junctions and ephaptic coupling. HFOs with similar frequency ranges can differ considerably in their physiological mechanisms. The fact that in most cases the combination of intrinsic neuronal membrane oscillations and synaptic circuits are necessary to sustain network oscillations is emphasized. Evidence for pathological HFOs, particularly fast ripples, in experimental models of epilepsy and in human epileptic patients is scrutinized. The underlying mechanisms of fast ripples are examined both in the light of animal observations, in vivo and in vitro, and in epileptic patients, with emphasis on single cell dynamics. Experimental observations and computational modeling have led to hypotheses for these mechanisms, several of which are considered here, namely the role of out-of-phase firing in neuronal clusters, the importance of strong excitatory AMPA-synaptic currents and recurrent inhibitory connectivity in combination with the fast time scales of IPSPs, ephaptic coupling and the contribution of interneuronal coupling through gap junctions. The statistical behaviour of fast ripple events can provide useful information on the underlying mechanism and can help to further improve classification of the diverse forms of HFOs.

Novel candidate genes associated with hippocampal oscillations

The hippocampus is critical for a wide range of emotional and cognitive behaviors. Here, we performed the first genome-wide search for genes influencing hippocampal oscillations. We measured local field potentials (LFPs) using 64-channel multi-electrode arrays in acute hippocampal slices of 29 BXD recombinant inbred mouse strains. Spontaneous activity and carbachol-induced fast network oscillations were analyzed with spectral and cross-correlation methods and the resulting traits were used for mapping quantitative trait loci (QTLs), i.e., regions on the genome that may influence hippocampal function. Using genome-wide hippocampal gene expression data, we narrowed the QTLs to eight candidate genes, including Plcb1, a phospholipase that is known to influence hippocampal oscillations. We also identified two genes coding for calcium channels, Cacna1b and Cacna1e, which mediate presynaptic transmitter release and have not been shown to regulate hippocampal network activity previously. Furthermore, we showed that the amplitude of the hippocampal oscillations is genetically correlated with hippocampal volume and several measures of novel environment exploration.

Fast network oscillations in vitro exhibit a slow decay of temporal auto-correlations

Ongoing neuronal oscillations in vivo exhibit non-random amplitude fluctuations as reflected in a slow decay of temporal auto-correlations that persist for tens of seconds. Interestingly, the decay of auto-correlations is altered in several brain-related disorders, including epilepsy, depression and Alzheimer's disease, suggesting that the temporal structure of oscillations depends on intact neuronal networks in the brain. Whether structured amplitude modulation occurs only in the intact brain or whether isolated neuronal networks can also give rise to amplitude modulation with a slow decay is not known. Here, we examined the temporal structure of cholinergic fast network oscillations in acute hippocampal slices. For the first time, we show that a slow decay of temporal correlations can emerge from synchronized activity in isolated hippocampal networks from mice, and is maximal at intermediate concentrations of the cholinergic agonist carbachol. Using zolpidem, a positive allosteric modulator of GABA(A) receptor function, we found that increased inhibition leads to longer oscillation bursts and more persistent temporal correlations. In addition, we asked if these findings were unique for mouse hippocampus, and we therefore analysed cholinergic fast network oscillations in rat prefrontal cortex slices. We observed significant temporal correlations, which were similar in strength to those found in mouse hippocampus and human cortex. Taken together, our data indicate that fast network oscillations with temporal correlations can be induced in isolated networks in vitro in different species and brain areas, and therefore may serve as model systems to investigate how altered temporal correlations in disease may be rescued with pharmacology.