The hippocampal intrinsic network oscillator

Excitatory synchronization of rat hippocampal interneurons during network activation in vitro

Introduction

Hippocampal interneurons (INs) are known to synchronize their electrical activity via mechanisms, which are poorly defined due to immense complexity of neural tissue but seem to depend on local cell interactions and intensity of network activity.

Methods

Here, synchronization of INs was studied using paired patch-clamp recordings in a simplified culture model with intact glutamate transmission. The level of network activity was moderately elevated by field electric stimulation, which is probably an analogue of afferent processing in situ.

Results

Even in baseline conditions, ∼45% of spontaneous inhibitory postsynaptic currents (sIPSCs) resulting from firing of individual presynaptic INs coincided between cells within ±1 ms due to simple divergence of inhibitory axons. Brief network activation induced an appearance of ‘hypersynchronous’ (∼80%) population sIPSCs occurring in response to coherent discharges of several INs with jitter ±4 ms. Notably, population sIPSCs were preceded by transient inward currents (TICs). Those were excitatory events capable to synchronize firing of INs, in this respect being reminiscent of so-called fast prepotentials observed in studies on pyramidal neurons. TICs also had network properties consisting of heterogeneous components: glutamate currents, local axonal and dendritic spikelets, and coupling electrotonic currents likely via gap junctions; putative excitatory action of synaptic gamma-aminobutyric acid (GABA) was not involved. The appearance of population excitatory-inhibitory sequences could be initiated and reproduced by firing of a single excitatory cell reciprocally connected with one IN.

Discussion

Our data demonstrate that synchronization of INs is initiated and dominated by glutamatergic mechanisms, which recruit, in a whole-sale manner, into supporting action other excitatory means existing in a given neural system.

The mechanisms shaping CA2 pyramidal neuron action potential bursting induced by muscarinic acetylcholine receptor activation

Recent studies have revealed that hippocampal area CA2 plays an important role in hippocampal network function. Disruption of this region has been implicated in neuropsychiatric disorders. It is well appreciated that cholinergic input to the hippocampus plays an important role in learning and memory. While the effect of elevated cholinergic tone has been well studied in areas CA1 and CA3, it remains unclear how changes in cholinergic tone impact synaptic transmission and the intrinsic properties of neurons in area CA2. In this study, we applied the cholinergic agonist carbachol and performed on-cell, whole-cell, and extracellular recordings in area CA2. We observed that under conditions of high cholinergic tone, CA2 pyramidal neurons depolarized and rhythmically fired bursts of action potentials. This depolarization depended on the activation of M1 and M3 cholinergic receptors. Furthermore, we examined how the intrinsic properties and action-potential firing were altered in CA2 pyramidal neurons treated with 10 µM carbachol. While this intrinsic burst firing persisted in the absence of synaptic transmission, bursts were shaped by synaptic inputs in the intact network. We found that both excitatory and inhibitory synaptic transmission were reduced upon carbachol treatment. Finally, we examined the contribution of different channels to the cholinergic-induced changes in neuronal properties. We found that a conductance from K_v7 channels partially contributed to carbachol-induced changes in resting membrane potential and membrane resistance. We also found that D-type potassium currents contributed to controlling several properties of the bursts, including firing rate and burst kinetics. Furthermore, we determined that T-type calcium channels and small conductance calcium-activated potassium channels play a role in regulating bursting activity.

Parvalbumin expression and gamma oscillation occurrence increase over time in a neurodevelopmental model of NMDA receptor dysfunction

Dysfunction of the N-methyl-d-aspartate receptor (NMDAR) is thought to play a role in the pathophysiology of neurodevelopmental diseases like schizophrenia. To study the effects of NMDAR dysfunction on synaptic transmission and network oscillations, we used hippocampal tissue of NMDAR subunit GluN2A knockout (KO) mice. Field excitatory postsynaptic potentials were recorded in acute hippocampal slices of adult animals. Synaptic transmission was impaired in GluN2A KO slices compared to wild-type (WT) slices. Further, to investigate whether NMDAR dysfunction would alter neurodevelopment in vitro, we used organotypic hippocampal slice cultures of WT and GluN2A KO mice. Immunostaining performed with cultures kept two, seven, 14, 25 days in vitro (DIV) revealed an increasing expression of parvalbumin (PV) over time. As a functional readout, oscillatory activity induced by the cholinergic agonist carbachol was recorded in cultures kept seven, 13, and 26 DIV using microelectrode arrays. Initial analysis focused on the occurrence of delta, theta, beta and gamma oscillations over genotype, DIV and hippocampal area (CA1, CA3, dentate gyrus (DG)). In a follow-up analysis, we studied the peak frequency and the peak power of each of the four oscillation bands per condition. The occurrence of gamma oscillations displayed an increase by DIV similar to the PV immunostaining. Unlike gamma occurrence, delta, theta, and beta occurrence did not change over time in culture. The peak frequency and peak power in the different bands of the oscillations were not different in slices of WT and GluN2A KO mice. However, the level of PV expression was lower in GluN2A KO compared to WT mice. Given the role of PV-containing fast-spiking basket cells in generation of oscillations and the decreased PV expression in subjects with schizophrenia, the study of gamma oscillations in organotypic hippocampal slices represents a potentially valuable tool for the characterization of novel therapeutic drugs.

Loss of Nogo-A, encoded by the schizophrenia risk gene Rtn4, reduces mGlu3 expression and causes hyperexcitability in hippocampal CA3 circuits

Recent investigations of Nogo-A, a well characterized protein inhibitor of neurite outgrowth in the brain, have revealed additional functions including a role in neuropsychiatric disorders such as schizophrenia. Here we examined Nogo-A functions in mouse CA3 hippocampal circuitry. Patch clamp recordings showed that the absence of Nogo-A results in a hyperactive network. In addition, mGlu3 metabotropic glutamate receptors, which exhibit mutations in certain forms of schizophrenia, were downregulated specifically in the CA3 area. Furthermore, Nogo-A^-/- mice showed disordered theta oscillations with decreased incidence and frequency, similar to those observed in mGlu3^-/- mice. As disruptions in theta rhythmicity are associated with impaired spatial navigation, we tested mice using modified Morris water maze tasks. Mice lacking Nogo-A exhibited altered search strategies, displaying greater dependence on global as opposed to local reference frames. This link between Nogo-A and mGlu3 receptors may provide new insights into mechanisms underlying schizophrenia.

Cellular and System Biology of Memory: Timing, Molecules, and Beyond

The storage of information in the mammalian nervous systems is dependent on a delicate balance between change and stability of neuronal networks. The induction and maintenance of processes that lead to changes in synaptic strength to a multistep process which can lead to long-lasting changes, which starts and ends with a highly choreographed and perfectly timed dance of molecules in different cell types of the central nervous system. This is accompanied by synchronization of specific networks, resulting in the generation of characteristic "macroscopic" rhythmic electrical fields, whose characteristic frequencies correspond to certain activity and information-processing states of the brain. Molecular events and macroscopic fields influence each other reciprocally. We review here cellular processes of synaptic plasticity, particularly functional and structural changes, and focus on timing events that are important for the initial memory acquisition, as well as mechanisms of short- and long-term memory storage. Then, we cover the importance of epigenetic events on the long-time range. Furthermore, we consider how brain rhythms at the network level participate in processes of information storage and by what means they participating in it. Finally, we examine memory consolidation at the system level during processes of sleep.

Focal Scn1a knockdown induces cognitive impairment without seizures

Cognitive impairment is a common comorbidity in pediatric epilepsy that can severely affect quality of life. In many cases, antiepileptic treatments fail to improve cognition. Therefore, a fundamental question is whether underlying brain abnormalities may contribute to cognitive impairment through mechanisms independent of seizures. Here, we examined the possible effects on cognition of Nav1.1 down-regulation, a sodium channel principally involved in Dravet syndrome but also implicated in other cognitive disorders, including autism and Alzheimer's disease. Using an siRNA approach to knockdown Nav1.1 selectively in the basal forebrain region, we were able to target a learning and memory network while avoiding the generation of spontaneous seizures. We show that reduction of Nav1.1 expression in the medial septum and diagonal band of Broca leads to a dysregulation of hippocampal oscillations in association with a spatial memory deficit. We propose that the underlying etiology responsible for Dravet syndrome may directly contribute to cognitive impairment in a manner that is independent from seizures.

Synaptic integration by different dendritic compartments of hippocampal CA1 and CA2 pyramidal neurons

Pyramidal neurons have a complex dendritic arbor containing tens of thousands of synapses. In order for the somatic/axonal membrane potential to reach action potential threshold, concurrent activation of multiple excitatory synapses is required. Frequently, instead of a simple algebraic summation of synaptic potentials in the soma, different dendritic compartments contribute to the integration of multiple inputs, thus endowing the neuron with a powerful computational ability. Most pyramidal neurons share common functional properties. However, different and sometimes contrasting dendritic integration rules are also observed. In this review, we focus on the dendritic integration of two neighboring pyramidal neurons in the hippocampus: the well-characterized CA1 and the much less understood CA2. The available data reveal that the dendritic integration of these neurons is markedly different even though they are targeted by common inputs at similar locations along their dendrites. This contrasting dendritic integration results in different routing of information flow and generates different corticohippocampal loops.

Slow waves during sleep in crayfish. Origin and spread

Previous results show that when unrestrained crayfish sleep, the electrical activity of the brain changes from multiple spikes (frequencies above 300 Hz) on a flat baseline to continuous slow waves at a frequency of 15–20 Hz. To study the temporal organization of such activity, we developed a tethered crayfish preparation that allows us to place electrodes on visually identified regions of the brain. Recording the electrical activity of different brain areas shows that when the animal is active (awake), slow waves are present only in the central complex. However, simultaneously with the animal becoming limp (sleeping), slow waves spread first to deuto- and then to protocerebrum, suggesting that the central complex of the crayfish brain acts as the sleep generator.

Effects of XE991, retigabine, losigamone and ZD7288 on kainate-induced theta-like and gamma network oscillations in the rat hippocampus in vitro

Ion currents such as M-currents (I(M)), persistent sodium currents (I(NaP)) and H-currents (I(h)) have been observed in a variety of brain regions, including the hippocampal formation, where storage and retrieval of information are facilitated by oscillatory network activities. They have been suggested to play an important role in neuronal excitability, synaptic transmission, membrane oscillatory activity, and in shaping resonance. Resonance and membrane potential oscillations have been implied in the generation of theta but not gamma oscillations. Here, we performed extracellular field potential recordings in hippocampal slices from adult rats and applied either the I(M) blocker XE991, the I(M) activator retigabine, the I(NaP) blocker losigamone or the I(h) inhibitor ZD7288 to test if these currents contribute to the generation of network oscillations. Kainate application induced network theta-like frequency oscillations in coronal slices as well as network gamma frequency oscillations in horizontal slices, and these remained stable for up to 3h. Power spectrum analysis revealed that all agents dose-dependently reduced the network oscillations in both frequency bands in areas CA3 and CA1. In contrast, the peak oscillation frequency was affected differentially. These results confirm that theta-like frequency oscillations are induced in longitudinal slices while gamma frequency oscillations dominate in horizontal slices. They also suggest that modifying neuronal excitability and transmitter release alters hippocampal network oscillations which are thought to be crucial for memory processing.

Comparison between spontaneous and kainate-induced gamma oscillations in the mouse hippocampus in vitro

Neuronal synchronization at gamma frequency, implicated in cognition, can be evoked in hippocampal slices by pharmacological activation. We characterized spontaneous small-amplitude gamma oscillations (SgammaO) recorded in area CA3 of mouse hippocampal slices and compared it with kainate-induced gamma oscillations (KgammaO). SgammaO had a lower peak frequency, a more sinusoidal waveform and was spatially less coherent than KgammaO, irrespective of oscillation amplitude. CA3a had the smallest oscillation power, phase-led CA3c by approximately 4 ms and had the highest SgammaO frequency in isolated subslices. During SgammaO CA3c neurons fired at the rebound of inhibitory postsynaptic potentials (IPSPs) that were associated with a current source in stratum lucidum, whereas CA3a neurons often fired from spikelets, 3-4 ms earlier in the cycle, and had smaller IPSPs. Kainate induced faster/larger IPSPs that were associated with an earlier current source in stratum pyramidale. SgammaO and KgammaO power were dependent on alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptors, gap junctions and gamma-aminobutyric acid (GABA)(A) receptors. SgammaO was suppressed by elevating extracellular KCl, blocking N-methyl-d-aspartate (NMDA) receptors or muscarinic receptors, or activating GluR5-containing kainate receptors. SgammaO was not affected by blocking metabotropic glutamate receptors or hyperpolarization-activated currents. The adenosine A(1) receptor antagonist 8-cyclopentyl-1,3-dimethoxyxanthine (8-CPT) and the CB1 cannabinoid receptor antagonist N-(piperidin-1-yl)-5-(4-iodophenyl)-1-(2,4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide (AM251) increased SgammaO power, indicating that endogenous adenosine and/or endocannabinoids suppress or prevent SgammaO in vitro. SgammaO emerges from a similar basic network as KgammaO, but differs in involvement of somatically projecting interneurons and pharmacological modulation profile. These observations advocate the use of SgammaO as a natural model for hippocampal gamma oscillations, particularly during less activated behavioural states.

Satellite NG2 progenitor cells share common glutamatergic inputs with associated interneurons in the mouse dentate gyrus

Several studies have provided evidence that NG2-expressing (NG2(+)) progenitor cells are anatomically associated to neurons in gray matter areas. By analyzing the spatial distribution of NG2(+) cells in the hilus of the mouse dentate gyrus, we demonstrate that NG2(+) cells are indeed closely associated to interneurons. To define whether this anatomical proximity reflected a specific physiological interaction, we performed patch-clamp recordings on hilar NG2(+) cells and interneurons between 3 and 21 postnatal days. We first observed that hilar NG2(+) cells exhibit spontaneous glutamatergic EPSCs (sEPSCs) whose frequency and amplitude increase during the first 3 postnatal weeks. At the same time, the rise time and decay time of sEPSCs significantly decreased, suggesting that glutamatergic synapses in NG2(+) cells undergo a maturation process that is reminiscent of what has been reported in neurons during the same time period. We also observed that hilar interneurons and associated NG2(+) cells are similarly integrated into the local network, receiving excitatory inputs from both granule cells and CA3 pyramidal neurons. By performing pair recordings, we found that bursts of activity induced by GABAergic antagonists were strongly synchronized between both cell types and that the amplitude of these bursts was positively correlated. Finally, by applying carbachol to increase EPSC activity, we observed that closely apposed cells were more likely to exhibit synchronized EPSCs than cells separated by >200 microm. The finding that NG2(+) cells are sensing patterns of activity arising in closely associated neurons suggests that NG2(+) cell function is finely regulated by the local network.

Synapses on NG2-expressing progenitors in the brain: multiple functions?

Progenitor cells expressing the proteoglycan NG2 represent approximately 5% of the total cells in the adult brain, and are found both in grey and white matter regions where they give rise to oligodendrocytes. The finding that these cells receive synaptic contacts from excitatory and inhibitory neurons has not only raised major interest in the possible roles of these synapses, but also stimulated further research on the developmental and cellular functions of NG2-expressing (NG2(+)) progenitors themselves in the context of neural circuit physiology. Here we review recent findings on the functional properties of the synapses on NG2(+) cells in grey and white matter regions of the brain. In this review article we make an attempt to integrate current knowledge on the cellular and developmental properties of NG2(+) progenitors with the functional attributes of their synapses, in order to understand the physiological relevance of neuron-NG2(+) progenitor signal transmission. We propose that, although NG2(+) progenitors receive synaptic contact in all brain regions where they are found, their synapses might have different developmental and functional roles, probably reflecting the distinct functions of NG2(+) progenitors in the brain.

Activity deprivation leads to seizures in hippocampal slice cultures: is epilepsy the consequence of homeostatic plasticity?

SUMMARY

Neural networks operate robustly despite destabilizing factors, ranging from gene product turnover to circuit refinement, throughout life. Maintaining functional robustness of neuronal networks critically depends upon forms of homeostatic plasticity including synaptic scaling. Synaptic strength and intrinsic excitability have been shown to "scale" (up or down) in response to altered ambient activity levels, and this has led to the general idea that homeostatic plasticity operates along a continuum. After 48 hours of activity deprivation, cultured hippocampal networks exhibited a homeostatic-type reconfiguration that was discrete: a switch from spontaneous spiking to oscillatory bursting. Blockade of fast glutamatergic and GABAergic transmission abolished spontaneous network bursting, but the majority of neurons exhibited intrinsic bursting in response to current injection, which was not the case in control tissue. This de novo intrinsic bursting could be blocked by cadmium chloride, suggesting that this bursting involves calcium mechanisms. Immunohistochemistry confirmed that activity-deprived slice cultures exhibited a widespread upregulation of voltage-dependent calcium channels compared with controls. Calcium imaging studies from activity-deprived slices demonstrated that spontaneous bursting was not a local behavior, but rather a global, synchronous phenomenon, reminiscent of seizure activity. These data suggest that the input/output transformation of individual neurons undergoing homeostatic remodeling is more complex than simple scaling. Network consequences of this transformation include network destabilization of epileptic proportions. Spontaneous activity plays a critical role in actively maintaining homeostatic balance in networks, which is lost after activity deprivation.

Mechanism of spontaneous and self-sustained oscillations in networks connected through axo-axonal gap junctions

Electrical coupling of clusters of neurons via axo-axonal gap junctions is a candidate mechanism underlying the ultra-fast (> 100 Hz) oscillations recorded in various in vitro and in vivo normal and pathological conditions [Traub et al. (1999)Neuroscience, 92, 407-426]. The poor characterization of axo-axonal gap junctions, however, limits experimental verification of this mechanism. We simulated networks of prototype multi-compartmental neurons in order to identify the parameters constraining the production and frequency of ultra-fast oscillations. Weak axo-axonal coupling was found to synchronize networks preferentially at the gamma-range frequency (30-100 Hz). Networks with strong axo-axonal coupling were able to produce 200 Hz oscillations, a finding we extended with several new observations. Ultra-fast oscillations arose spontaneously during dendritic excitation, i.e. in the absence of extrinsic axonal background spikes, as the spike trigger zone concomitantly shifted from soma to axon. The all-or-none oscillations could be transitory or self-sustained, and lasted longer in larger networks. They occurred more likely during low to modest soma firing rates, as strong afterhyperpolarizing currents tended to impair them. The rate of the rhythm was independent of network size and of the level of excitation, but inversely proportional to the distance of the junctions from the soma. As a matter of fact, the resulting axonal firing rate was the highest one at which antidromic spikes would not collide with spikes reflected from the soma. Taken together, the observed model dynamics lends further credibility to axo-axonal coupling as a mechanism of ultra-fast oscillations.

Beta/gamma oscillatory activity in the CA3 hippocampal area is depressed by aberrant GABAergic transmission from the dentate gyrus after seizures

Oscillatory activity in the CA3 region is thought to be involved in the encoding and retrieval of information. These oscillations originate from the recurrent excitation between pyramidal cells that are entrained by the synchronous rhythmic inhibition of local interneurons. We show here that, after seizures, the dentate gyrus (DG) tonically inhibits beta/gamma (20-24 Hz) field oscillations in the CA3 area through GABA-mediated signaling. These oscillations originate in the interneuron network because they are maintained in the presence of ionotropic glutamate receptor antagonists, and they can be blocked by GABA(A) receptor antagonists or by perfusion of a calcium-free extracellular medium. Inhibition of this oscillatory activity requires intact DG-to-CA3 connections, and it is suppressed by the activation of metabotropic glutamate receptors (mGluR). The influence of mGluR activation was reflected in the spontaneous subthreshold membrane oscillations of CA3 interneurons after one seizure but could also be observed in pyramidal cells after several seizures. Coincident stimulation of the DG at and beta/gamma frequencies produced a frequency-dependent excitation of interneurons and the inhibition of pyramidal cells. Indeed, these effects were maximal at the frequency that matched the mGluR-sensitive spontaneous field oscillations, suggesting a resonance phenomenon. Our results shed light on the mechanisms that may underlie the deficits in memory and cognition observed after epileptic seizures.

Null mutation of the alpha4 nicotinic receptor subunit increases the propensity of muscarinic-mediated neuronal bursting in mouse hippocampal slices

Alpha4 subunit nicotinic cholinergic receptor (nAChR) knock out mice (KO) have a greater susceptibility to proconvulsant-induced seizures than do wild type (WT). The underlying mechanisms remain obscure. We tested whether such seizure-like activity was reflected in bursting activity of hippocampal neurons by recording with intracellular microelectrodes from CA1 pyramidal neurons in slices from WT and KO mice. Intriguingly, while carbachol-induced bursting activity occurred in only 21% of WT slices, qualitatively identical patterns of bursting occurred in 72% of KO slices. Extracellular recordings from CA1 and CA3 regions suggest that carbachol-mediated population activity was regionalized in our preparations. The relative weighting of excitatory to inhibitory synaptic potentials was similar between WT and alpha4 KO mice. However, burst-firing cells had a smaller input time constant than non-bursters. Low-concentration DHbetaE (selective alpha4beta2 nAChR antagonist) did not increase the propensity of WT slices to burst-fire, indicating that absence of alpha4 subunits per se, cannot explain the differences in activity between slices from WT and KO mice. These observations suggest that alpha4 nAChRs are unlikely to be involved in modulating the pattern of bursting neural activity, but their absence could induce subtle developmental changes in the sensitivity of hippocampal circuits to develop this behaviour.

Regulation of IPSP Theta Rhythm by Muscarinic Receptors and Endocannabinoids in Hippocampus

Theta rhythms are behaviorally relevant electrical oscillations in the mammalian brain, particularly the hippocampus. In many cases, theta oscillations are shaped by inhibitory postsynaptic potentials (IPSPs) that are driven by glutamatergic and/or cholinergic inputs. Here we show that hippocampal theta rhythm IPSPs induced in the CA1 region by muscarinic acetylcholine receptors independent of all glutamate receptors can be briefly interrupted by action potential–induced, retrograde endocannabinoid release. Theta IPSPs can be recorded in CA1 pyramidal cell somata surgically isolated from CA3, subiculum, and even from their own apical dendrites. These results suggest that perisomatic-targeting interneurons whose output is subject to inhibition by endocannabinoids are the likely source of theta IPSPs. Interneurons having these properties include the cholecystokinin-containing cells. Simultaneous recordings from pyramidal cell pairs reveal synchronous theta-frequency IPSPs in neighboring pyramidal cells, suggesting that these IPSPs may help entrain or modulate small groups of pyramidal cells.

Perisomatic Feedback Inhibition Underlies Cholinergically Induced Fast Network Oscillations in the Rat Hippocampus In Vitro

Gamma frequency network oscillations are assumed to be important in cognitive processes, including hippocampal memory operations, but the precise functions of these oscillations remain unknown. Here, we examine the cellular and network mechanisms underlying carbachol-induced fast network oscillations in the hippocampus in vitro, which closely resemble hippocampal gamma oscillations in the behaving rat. Using a combination of planar multielectrode array recordings, imaging with voltage-sensitive dyes, and recordings from single hippocampal neurons within the CA3 gamma generator, active current sinks and sources were localized to the stratum pyramidale. These proximal currents were driven by phase-locked rhythmic inhibitory inputs to pyramidal cells from identified perisomatic-targeting interneurons. AMPA receptor-mediated recurrent excitation was necessary for the synchronization of interneuronal discharge, which strongly supports a synaptic feedback model for the generation of hippocampal gamma oscillations.

Hippocampal gamma-frequency oscillations: from interneurones to pyramidal cells, and back

GABAergic interneurones are necessary for the emergence of hippocampal gamma-frequency network oscillations, during which they play a key role in the synchronization of pyramidal cell firing. However, it remains to be resolved how distinct interneurone subtypes contribute to gamma-frequency oscillations, in what way the spatiotemporal pattern of interneuronal input affects principal cell activity, and by which mechanisms the interneurones themselves are synchronized. Here we summarize recent evidence from cholinergically induced gamma-frequency network oscillations in vitro, showing that perisomatic-targeting GABAergic interneurones provide prominent rhythmic inhibition in pyramidal cells, and that these interneurones are synchronized by recurrent excitation. We conclude by presenting a minimal integrate-and-fire network model which demonstrates that this excitatory-inhibitory feedback loop is sufficient to explain the generation of intrahippocampal gamma-frequency oscillations.

Orexins/hypocretins cause sharp wave- and theta-related synaptic plasticity in the hippocampus via glutamatergic, gabaergic, noradrenergic, and cholinergic signaling

Orexins (OX), also called hypocretins, are bioactive peptides secreted from glucose-sensitive neurons in the lateral hypothalamus linking appetite, arousal and neuroendocrine-autonomic control. Here, OX-A was found to cause a slow-onset long-term potentiation of synaptic transmission (LTPOX) in the hippocampus of young adult mice. LTPOX was induced at Schaffer collateral-CA1 but not mossy fiber-CA3 synapses, and required transient sharp wave-concurrent population field-burst activity generated by the autoassociative CA3 network. Exogenous long theta-frequency stimulation of Schaffer collateral axons erased LTPOX in intact hippocampal slices but not mini slices devoid of the CA3 region. Pharmacological analysis revealed that LTPOX requires co-activation of ionotropic and metabotropic glutamatergic, GABAergic, as well as noradrenergic and cholinergic receptors. Together these data indicate that OX-A induces a state-dependent metaplasticity in the CA1 region associated with sharp-wave and theta rhythm activity as well as glutamatergic, GABAergic, aminergic, and cholinergic transmission. Thus, orexins not only regulate arousal threshold and body weight but also threshold and weight of synaptic connectivity, providing a molecular prerequisite for homeostatic and behavioral state-dependent control of neuronal plasticity and presumably memory functions.

Mesial temporal lobe epilepsy: a proton magnetic resonance spectroscopy study and a histopathological analysis

Object. Proton magnetic resonance (MR) spectroscopy imaging of the ratio of N-acetylaspartate (NAA) to creatine (Cr) has proved efficacious as a localizing tool in demonstrating the metabolic changes associated with temporal lobe epilepsy. To analyze the significance of these MR spectroscopy findings further, the authors explored the relationship between regional alterations in the NAA/Cr ratio in hippocampi measured preoperatively and histopathological findings in hippocampi resected in patients with intractable mesial temporal lobe epilepsy (MTLE).

Methods. Twelve patients in whom the diagnosis of MTLE had been made and 12 healthy volunteers with no known history of neurological disease underwent high-resolution 1H MR spectroscopy imaging of NAA and Cr (0.64 cm3 nominal voxel resolution) in five voxels spanning the anteroposterior length of the hippocampus. The authors correlated the NAA/Cr ratio with neuropathological findings in resected hippocampi, specifically glial fibrillary acidic protein (GFAP) immunoreactivity and pyramidal neuronal loss. A linear regression analysis of the ipsilateral NAA/Cr ratio revealed a statistically significant relation to the extent of hippocampal neuronal loss in only the CA2 sector (correlation coefficient [r] = −0.66, p < 0.03). The ipsilateral NAA/Cr ratio displayed significant regressions with GFAP immunoreactivity from all the CA sectors (r values ranged from −0.69 and p < 0.01 for the CA4 sector to −0.88 and p < 0.001 for the CA2 sector) except for the CA1. The extent of neuronal cell loss in every hippocampal subfield (r = 0.71−0.74, p < 0.007), except the CA2 (p = 0.08), correlated to the extent of neuronal cell loss in the dentate gyrus. There was no significant relationship between the duration or frequency of seizures and the mean ipsilateral NAA/Cr ratio; however, the mean density of GFAP-immunopositive cells correlated with seizure frequency (p < 0.03).

Conclusions. The NAA/Cr ratio may not measure the full extent of hippocampal neuronal cell loss. The significant association of the NAA/Cr ratio with the GFAP immunoreactivity of most CA sectors indicates that the NAA/Cr ratio may provide a more accurate measurement of recent neuronal injury caused by epileptic activity. The coupling between neuronal impairment and astroglial GFAP expression may indicate the close association between neuronal and glial dysfunction in patients with epilepsy.