Novel candidate genes associated with hippocampal oscillations

Adiabatic dynamic causal modelling

This technical note introduces adiabatic dynamic causal modelling, a method for inferring slow changes in biophysical parameters that control fluctuations of fast neuronal states. The application domain we have in mind is inferring slow changes in variables (e.g., extracellular ion concentrations or synaptic efficacy) that underlie phase transitions in brain activity (e.g., paroxysmal seizure activity). The scheme is efficient and yet retains a biophysical interpretation, in virtue of being based on established neural mass models that are equipped with a slow dynamic on the parameters (such as synaptic rate constants or effective connectivity). In brief, we use an adiabatic approximation to summarise fast fluctuations in hidden neuronal states (and their expression in sensors) in terms of their second order statistics; namely, their complex cross spectra. This allows one to specify and compare models of slowly changing parameters (using Bayesian model reduction) that generate a sequence of empirical cross spectra of electrophysiological recordings. Crucially, we use the slow fluctuations in the spectral power of neuronal activity as empirical priors on changes in synaptic parameters. This introduces a circular causality, in which synaptic parameters underwrite fast neuronal activity that, in turn, induces activity-dependent plasticity in synaptic parameters. In this foundational paper, we describe the underlying model, establish its face validity using simulations and provide an illustrative application to a chemoconvulsant animal model of seizure activity.

Genetic Variation in CNS Myelination and Functional Brain Connectivity in Recombinant Inbred Mice

Myelination greatly increases the speed of action potential propagation of neurons, thereby enhancing the efficacy of inter-neuronal communication and hence, potentially, optimizing the brain’s signal processing capability. The impact of genetic variation on the extent of axonal myelination and its consequences for brain functioning remain to be determined. Here we investigated this question using a genetic reference panel (GRP) of mouse BXD recombinant inbred (RI) strains, which partly model genetic diversity as observed in human populations, and which show substantial genetic differences in a variety of behaviors, including learning, memory and anxiety. We found coherent differences in the expression of myelin genes in brain tissue of RI strains of the BXD panel, with the largest differences in the hippocampus. The parental C57BL/6J (C57) and DBA/2J (DBA) strains were on opposite ends of the expression spectrum, with C57 showing higher myelin transcript expression compared with DBA. Our experiments showed accompanying differences between C57 and DBA in myelin protein composition, total myelin content, and white matter conduction velocity. Finally, the hippocampal myelin gene expression of the BXD strains correlated significantly with behavioral traits involving anxiety and/or activity. Taken together, our data indicate that genetic variation in myelin gene expression translates to differences observed in myelination, axonal conduction speed, and possibly in anxiety/activity related behaviors.

Regulation of intrinsic excitability: Roles for learning and memory, aging and Alzheimer's disease, and genetic diversity

Plasticity of intrinsic neuronal excitability facilitates learning and memory across multiple species, with aberrant modulation of this process being linked to the development of neurological symptoms in models of cognitive aging and Alzheimer's disease. Learning-related increases in intrinsic excitability of neurons occurs in a variety of brain regions, and is generally thought to promote information processing and storage through enhancement of synaptic throughput and induction of synaptic plasticity. Experience-dependent changes in intrinsic neuronal excitability rely on activity-dependent gene expression patterns, which can be influenced by genetic and environmental factors, aging, and disease. Reductions in baseline intrinsic excitability, as well as aberrant plasticity of intrinsic neuronal excitability and in some cases pathological hyperexcitability, have been associated with cognitive deficits in animal models of both normal cognitive aging and Alzheimer's disease. Genetic factors that modulate plasticity of intrinsic excitability likely underlie individual differences in cognitive function and susceptibility to cognitive decline. Thus, targeting molecular mediators that either control baseline intrinsic neuronal excitability, subserve learning-related intrinsic neuronal plasticity, and/or promote resilience may be a promising therapeutic strategy for maintaining cognitive function in aging and disease. In this review, we discuss the complementary relationship between intrinsic excitability and learning, with a particular focus on how this relationship varies as a function of age, disease state, and genetic make-up, and how targeting these factors may help to further elucidate our understanding of the role of intrinsic excitability in cognitive function and cognitive decline.

Differential Expression of Munc13-2 Produces Unique Synaptic Phenotypes in the Basolateral Amygdala of C57BL/6J and DBA/2J Mice

C57BL/6J (B6) and DBA/2J (D2) mice are well known to differentially express a number of behavioral phenotypes, including anxiety-like behavior, fear conditioning, and drug self-administration. However, the cellular mechanisms contributing to these differences remain unclear. Given the basolateral amygdala (BLA) contributes to these behaviors, we characterized strain-dependent differences in presynaptic and postsynaptic function in BLA neurons by integrating electrophysiological, biochemical, and genetic approaches to identify specific molecular mechanisms. We found that D2 glutamatergic synapses expressed enhanced release probability and lower sensitivity to both the inhibitory effects of low extracellular calcium and facilitation by phorbol esters. Furthermore, repetitive stimulation of BLA afferents at low (2 Hz) or high (40 Hz) frequencies revealed that B6 terminals, relative to D2 terminals, were more sensitive to synaptic fatigue principally because of reduced vesicle recycling rates. Additionally, B6 synapses exhibited more robust augmentation of spontaneous release after repetitive stimulation relative to the D2 strain. In silico analysis of the inheritance of synaptic physiology from an array of BXD recombinant inbred strains (Jansen et al., 2011) identified a segment on chromosome 4 containing the gene encoding Munc13-2, which has calcium-/phorbol ester-binding domains and controls presynaptic function. We subsequently found that B6 mice express substantially higher levels of Munc13-2 compared with the D2 strain whereas expression of several release-related proteins, including Munc13-1, was equivalent. We then knocked down the expression of Munc13-2 in B6 mice using a short hairpin RNA and found this recapitulated the presynaptic phenotype of D2 BLA synapses.

SIGNIFICANCE STATEMENT

DBA/2J and C57BL/6J mice have been used to understand the genetic mechanisms controlling behaviors related to a number of psychiatric illnesses. However, the fundamental neurobiological mechanisms producing these behavioral characteristics remain unresolved. Here we identify a critical family of presynaptic proteins differentially expressed by these strains that control strain-dependent synaptic physiology. This family of proteins regulates excitation/secretion coupling, vesicle recycling, and short-term plasticity throughout the CNS. Thus, differential inheritance of proteins like Munc13-2 has broad implications for genetic control over a wide variety of pathological behaviors. Importantly, these proteins also contain a large number of modulatory sites, making them attractive potential targets for the development of novel neuropharmaceutical treatments.

Multiple Single-Unit Long-Term Tracking on Organotypic Hippocampal Slices Using High-Density Microelectrode Arrays

A novel system to cultivate and record from organotypic brain slices directly on high-density microelectrode arrays (HD-MEA) was developed. This system allows for continuous recording of electrical activity of specific individual neurons at high spatial resolution while monitoring at the same time, neuronal network activity. For the first time, the electrical activity patterns of single neurons and the corresponding neuronal network in an organotypic hippocampal slice culture were studied during several consecutive weeks at daily intervals. An unsupervised iterative spike-sorting algorithm, based on PCA and k-means clustering, was developed to assign the activities to the single units. Spike-triggered average extracellular waveforms of an action potential recorded across neighboring electrodes, termed “footprints” of single-units were generated and tracked over weeks. The developed system offers the potential to study chronic impacts of drugs or genetic modifications on individual neurons in slice preparations over extended times.

Altered theta oscillations and aberrant cortical excitatory activity in the 5XFAD model of Alzheimer's disease

Alzheimer's disease (AD) is an age-related neurodegenerative disorder characterized by impairment of memory function. The 5XFAD mouse model was analyzed and compared with wild-type (WT) controls for aberrant cortical excitability and hippocampal theta oscillations by using simultaneous video-electroencephalogram (EEG) monitoring. Seizure staging revealed that 5XFAD mice exhibited cortical hyperexcitability whereas controls did not. In addition, 5XFAD mice displayed a significant increase in hippocampal theta activity from the light to dark phase during nonmotor activity. We also observed a reduction in mean theta frequency in 5XFAD mice compared to controls that was again most prominent during nonmotor activity. Transcriptome analysis of hippocampal probes and subsequent qPCR validation revealed an upregulation of Plcd4 that might be indicative of enhanced muscarinic signalling. Our results suggest that 5XFAD mice exhibit altered cortical excitability, hippocampal dysrhythmicity, and potential changes in muscarinic signaling.

Sporadic and familial subgroups of schizophrenia do not differ on dense array spontaneous gamma oscillatory activity

Genetic variations and developmental insults independently have been proposed to underlie aberrant gamma activity in schizophrenia. We investigated differences in spectral power in gamma (30-100Hz) frequency in patients with familial and sporadic schizophrenia. Subjects underwent resting-awake EEG recording on 192 channels. The two patient subgroups did not significantly differ in any of the gamma bands and regions. We conclude that complex gene-environment interactions are responsible for the limited power of familial-sporadic distinction in schizophrenia.

Evaluation of spontaneous dense array gamma oscillatory activity and minor physical anomalies as a composite neurodevelopmental endophenotype in schizophrenia

OBJECTIVE

Minor physical anomalies (MPAs) and gamma oscillatory activity have been proposed as associated endophenotypes in schizophrenia. Combining these endophenotypes to create a composite endophenotype may help identify those at risk for schizophrenia better. The present study aims to investigate MPAs and gamma oscillatory activity in schizophrenia patients, their unaffected first degree relatives and healthy controls and appreciate whether they can be used together as a composite endophenotype.

METHODS

This was a cross sectional family study conducted at a tertiary care mental health setup. Ninety participants including schizophrenia patients, their first degree relatives and controls (thirty each) were assessed for MPAs on the Extended Waldrop Scale. All participants underwent an awake, resting 192-channel EEG recording. Spectral power and coherence in 30-100Hz gamma bands were estimated using Welch's averaged periodogram method. One-way ANOVA, chi square test were used for comparing socio-demographic-clinical variables. MANOVA supplemented by one-way ANOVAs (post hoc Tukey HSD) were done for comparison of spectral measures. Pearson's correlation, step-by-step linear discriminant functional and intra-familial correlation analysis were subsequently performed.

RESULTS

An endophenotype pattern of finding was found for MPAs in the craniofacial region, the total number of MPAs, spectral power in right temporal region on all bands and in the right parietal region in 50-70Hz and 70-100Hz gamma bands. The three groups were most accurately classified when MPA total score, right temporal 30-50Hz gamma power and right occipital 'intra hemispheric' 50-70Hz gamma coherence were considered together than when considered independently. Significant intra familial correlation was seen for MPA total score and right temporal gamma 30-50Hz power.

CONCLUSION

Composite evaluation of two developmentally linked markers i.e. MPAs and gamma spectral measures may prove useful in categorizing schizophrenia and identifying at-risk individuals.

Inflammation subverts hippocampal synaptic plasticity in experimental multiple sclerosis

Abnormal use-dependent synaptic plasticity is universally accepted as the main physiological correlate of memory deficits in neurodegenerative disorders. It is unclear whether synaptic plasticity deficits take place during neuroinflammatory diseases, such as multiple sclerosis (MS) and its mouse model, experimental autoimmune encephalomyelitis (EAE). In EAE mice, we found significant alterations of synaptic plasticity rules in the hippocampus. When compared to control mice, in fact, hippocampal long-term potentiation (LTP) induction was favored over long-term depression (LTD) in EAE, as shown by a significant rightward shift in the frequency–synaptic response function. Notably, LTP induction was also enhanced in hippocampal slices from control mice following interleukin-1β (IL-1β) perfusion, and both EAE and IL-1β inhibited GABAergic spontaneous inhibitory postsynaptic currents (sIPSC) without affecting glutamatergic transmission and AMPA/NMDA ratio. EAE was also associated with selective loss of GABAergic interneurons and with reduced gamma-frequency oscillations in the CA1 region of the hippocampus. Finally, we provided evidence that microglial activation in the EAE hippocampus was associated with IL-1β expression, and hippocampal slices from control mice incubated with activated microglia displayed alterations of GABAergic transmission similar to those seen in EAE brains, through a mechanism dependent on enhanced IL-1β signaling. These data may yield novel insights into the basis of cognitive deficits in EAE and possibly of MS.

α2-containing GABAA receptors expressed in hippocampal region CA3 control fast network oscillations

GABA(A) receptors are critically involved in hippocampal oscillations. GABA(A) receptor α1 and α2 subunits are differentially expressed throughout the hippocampal circuitry and thereby may have distinct contributions to oscillations. It is unknown which GABA(A) receptor α subunit controls hippocampal oscillations and where these receptors are expressed. To address these questions we used transgenic mice expressing GABA(A) receptor α1 and/or α2 subunits with point mutations (H101R) that render these receptors insensitive to allosteric modulation at the benzodiazepine binding site, and tested how increased or decreased function of α subunits affects hippocampal oscillations. Positive allosteric modulation by zolpidem prolonged decay kinetics of hippocampal GABAergic synaptic transmission and reduced the frequency of cholinergically induced oscillations. Allosteric modulation of GABAergic receptors in CA3 altered oscillation frequency in CA1, while modulation of GABA receptors in CA1 did not affect oscillations. In mice having a point mutation (H101R) at the GABA(A) receptor α2 subunit, zolpidem effects on cholinergically induced oscillations were strongly reduced compared to wild-type animals, while zolpidem modulation was still present in mice with the H101R mutation at the α1 subunit. Furthermore, genetic knockout of α2 subunits strongly reduced oscillations, whereas knockout of α1 subunits had no effect. Allosteric modulation of GABAergic receptors was strongly reduced in unitary connections between fast spiking interneurons and pyramidal neurons in CA3 of α2H101R mice, but not of α1H101R mice, suggesting that fast spiking interneuron to pyramidal neuron synapses in CA3 contain α2 subunits. These findings suggest that α2-containing GABA(A) receptors expressed in the CA3 region provide the inhibition that controls hippocampal rhythm during cholinergically induced oscillations.