Coupling of Sharp Wave Events between Zebrafish Hippocampal and Amygdala Homologs.

The mammalian hippocampus exhibits spontaneous sharp wave events (1-30 Hz) with an often-present superimposed fast ripple oscillation (120-220 Hz) to form a sharp wave ripple (SWR) complex. During slow-wave sleep or quiet restfulness, SWRs result from the sequential spiking of hippocampal cell assemblies initially activated during learned or imagined experiences. Additional cortical/subcortical areas exhibit SWR events that are coupled to hippocampal SWRs, and studies in mammals suggest that coupling may be critical for the consolidation and recall of specific memories. In the present study, we have examined juvenile male and female zebrafish and show that SWR events are intrinsically generated and maintained within the telencephalon and that their hippocampal homolog, the anterodorsolateral lobe (ADL), exhibits SW events with ∼9% containing an embedded ripple (SWR). Single-cell calcium imaging coupled to local field potential recordings revealed that ∼10% of active cells in the dorsal telencephalon participate in any given SW event. Furthermore, fluctuations in cholinergic tone modulate SW events consistent with mammalian studies. Moreover, the basolateral amygdala (BLA) homolog exhibits SW events with ∼5% containing an embedded ripple. Computing the SW peak coincidence difference between the ADL and BLA showed bidirectional communication. Simultaneous coupling occurred more frequently within the same hemisphere, and in coupled events across hemispheres, the ADL more commonly preceded BLA. Together, these data suggest conserved mechanisms across species by which SW and SWR events are modulated, and memories may be transferred and consolidated through regional coupling.

Divergent opioid-mediated suppression of inhibition between hippocampus and neocortex across species and development.

Opioid receptors within the CNS regulate pain sensation and mood and are key targets for drugs of abuse. Within the adult rodent hippocampus (HPC), µ-opioid receptor agonists suppress inhibitory parvalbumin-expressing interneurons (PV-INs), thus disinhibiting the circuit. However, it is uncertain if this disinhibitory motif is conserved in other cortical regions, species, or across development. We observed that PV-IN mediated inhibition is robustly suppressed by opioids in HPC but not neocortex in mice and nonhuman primates, with spontaneous inhibitory tone in resected human tissue also following a consistent dichotomy. This hippocampal disinhibitory motif was established in early development when immature PV-INs and opioids already influence primordial network rhythmogenesis. Acute opioid-mediated modulation was partially occluded with morphine pretreatment, with implications for the effects of opioids on hippocampal network activity during circuit maturation as well as learning and memory. Together, these findings demonstrate that PV-INs exhibit a divergence in opioid sensitivity across brain regions that is remarkably conserved across evolution and highlights the underappreciated role of opioids acting through immature PV-INs in shaping hippocampal development.

Loss of Grin2a causes a transient delay in the electrophysiological maturation of hippocampal parvalbumin interneurons.

N-methyl-D-aspartate receptors (NMDARs) are ligand-gated ionotropic glutamate receptors that mediate a calcium-permeable component to fast excitatory neurotransmission. NMDARs are heterotetrameric assemblies of two obligate GluN1 subunits (GRIN1) and two GluN2 subunits (GRIN2A-GRIN2D). Sequencing data shows that 43% (297/679) of all currently known NMDAR disease-associated genetic variants are within the GRIN2A gene, which encodes the GluN2A subunit. Here, we show that unlike missense GRIN2A variants, individuals affected with disease-associated null GRIN2A variants demonstrate a transient period of seizure susceptibility that begins during infancy and diminishes near adolescence. We show increased circuit excitability and CA1 pyramidal cell output in juvenile mice of both Grin2a+/- and Grin2a-/- mice. These alterations in somatic spiking are not due to global upregulation of most Grin genes (including Grin2b). Deeper evaluation of the developing CA1 circuit led us to uncover age- and Grin2a gene dosing-dependent transient delays in the electrophysiological maturation programs of parvalbumin (PV) interneurons. We report that Grin2a+/+ mice reach PV cell electrophysiological maturation between the neonatal and juvenile neurodevelopmental timepoints, with Grin2a+/- mice not reaching PV cell electrophysiological maturation until preadolescence, and Grin2a-/- mice not reaching PV cell electrophysiological maturation until adulthood. Overall, these data may represent a molecular mechanism describing the transient nature of seizure susceptibility in disease-associated null GRIN2A patients.

Cochlear hair cell innervation is dependent on a modulatory function of Semaphorin-3A.

BACKGROUND: Proper connectivity between type I spiral ganglion neurons (SGNs) and inner hair cells (IHCs) in the cochlea is necessary for conveying sound information to the brain in mammals. Previous studies have shown that type I SGNs are heterogeneous in form, function and synaptic location on IHCs, but factors controlling their patterns of connectivity are not well understood. RESULTS: During development, cochlear supporting cells and SGNs express Semaphorin-3A (SEMA3A), a known axon guidance factor. Mice homozygous for a point mutation that attenuates normal SEMA3A repulsive activity (Sema3aK108N ) show cochleae with grossly normal patterns of IHC innervation. However, genetic sparse labeling and three-dimensional reconstructions of individual SGNs show that cochleae from Sema3aK108N mice lacked the normal synaptic distribution of type I SGNs. Additionally, Sema3aK108N cochleae show a disrupted distribution of GLUA2 postsynaptic patches around the IHCs. The addition of SEMA3A-Fc to postnatal cochleae led to increases in SGN branching, similar to the effects of inhibiting glutamate receptors. Ca2+ imaging studies show that SEMA3A-Fc decreases SGN activity. CONCLUSIONS: Contrary to the canonical view of SEMA3A as a guidance ligand, our results suggest SEMA3A may regulate SGN excitability in the cochlea, which may influence the morphology and synaptic arrangement of type I SGNs.

Resilient Hippocampal Gamma Rhythmogenesis and Parvalbumin-Expressing Interneuron Function Before and After Plaque Burden in 5xFAD Alzheimer's Disease Model.

Recent studies have implicated impaired Parvalbumin Fast-Spiking Interneuron (PVIN) function as a precipitating factor underlying abnormalities in network synchrony, oscillatory rhythms, and cognition associated with Alzheimer's disease (AD). However, a complete developmental investigation of potential gamma deficits, induced by commonly used carbachol or kainate in ex vivo slice preparations, within AD model mice is lacking. We examined gamma oscillations using field recordings in acute hippocampal slices from 5xFAD and control mice, through the period of developing pathology, starting at 3 months of age, when there is minimal plaque presence in the hippocampus, through to 12+ months of age, when plaque burden is high. In addition, we examined PVIN participation in gamma rhythms using targeted cell-attached recordings of genetically-reported PVINs, in both wild type and mutant mice. In parallel, a developmental immunohistochemical characterisation probing the PVIN-associated expression of PV and perineuronal nets (PNNs) was compared between control and 5xFAD mice. Remarkably, this comprehensive longitudinal evaluation failed to reveal any obvious correlations between PVIN deficits (electrical and molecular), circuit rhythmogenesis (gamma frequency and power), and Aß deposits/plaque formation. By 6-12 months, 5xFAD animals have extensive plaque formation throughout the hippocampus. However, a deficit in gamma oscillatory power was only evident in the oldest 5xFAD animals (12+ months), and only when using kainate, and not carbachol, to induce the oscillations. We found no difference in PV firing or phase preference during kainate-induced oscillations in younger or older 5xFAD mice compared to control, and a reduction of PV and PNNs only in the oldest 5xFAD mice. The lack of a clear relationship between PVIN function, network rhythmicity, and plaque formation in our study highlights an unexpected resilience in PVIN function in the face of extensive plaque pathology associated with this model, calling into question the presumptive link between PVIN pathology and Alzheimer's progression.

High-frequency head impact causes chronic synaptic adaptation and long-term cognitive impairment in mice.

Repeated head impact exposure can cause memory and behavioral impairments. Here, we report that exposure to non-damaging, but high frequency, head impacts can alter brain function in mice through synaptic adaptation. High frequency head impact mice develop chronic cognitive impairments in the absence of traditional brain trauma pathology, and transcriptomic profiling of mouse and human chronic traumatic encephalopathy brain reveal that synapses are strongly affected by head impact. Electrophysiological analysis shows that high frequency head impacts cause chronic modification of the AMPA/NMDA ratio in neurons that underlie the changes to cognition. To demonstrate that synaptic adaptation is caused by head impact-induced glutamate release, we pretreated mice with memantine prior to head impact. Memantine prevents the development of the key transcriptomic and electrophysiological signatures of high frequency head impact, and averts cognitive dysfunction. These data reveal synapses as a target of high frequency head impact in human and mouse brain, and that this physiological adaptation in response to head impact is sufficient to induce chronic cognitive impairment in mice.

Timing isn't everything: opposing roles for perisomatic inhibition.

Perisomatic inhibition from parvalbumin-containing (PV) interneurons is critical for timing, but the role of cholecystokinin-containing (CCK) interneurons remains obscure. Utilizing a novel mouse model, Dudok et al. demonstrate fundamentally distinct behavioral roles for these neuronal subpopulations during behavior.

High-Frequency Head Impact Disrupts Hippocampal Neural Ensemble Dynamics.

We have recently shown that the cognitive impairments in a mouse model of high-frequency head impact (HFHI) are caused by chronic changes to synaptic physiology. To better understand these synaptic changes occurring after repeat head impact, we used Thy1-GcCAMP6f mice to study intracellular and intercellular calcium dynamics and neuronal ensembles in HFHI mice. We performed simultaneous calcium imaging and local field potential (LFP) recordings of the CA1 field during an early-LTP paradigm in acute hippocampal slice preparations 24 h post-impact. As previously reported, HFHI causes a decrease in early-LTP in the absence of any shift in the input-output curve. Calcium analytics revealed that HFHI hippocampal slices have similar numbers of active ROIs, however, the number of calcium transients per ROI was significantly increased in HFHI slices. Ensembles consist of coordinated activity between groups of active ROIs. We exposed the CA1 ensemble to Schaffer-collateral stimulation in an abbreviated LTP paradigm and observed novel coordinated patterns of post stimulus calcium ensemble activity. HFHI ensembles displayed qualitatively similar patterns of post-stimulus ensemble activity to shams but showed significant changes in quantitative ensemble inactivation and reactivation. Previous in vivo and in vitro reports have shown that ensemble activity frequently occurs through a similar set of ROIs firing in a repeating fashion. HFHI slices showed a decrease in such coordinated firing patterns during post stimulus ensemble activity. The present study shows that HFHI alters synaptic activity and disrupts neuronal organization of the ensemble, providing further evidence of physiological synaptic adaptation occurring in the brain after a high frequency of non-pathological head impacts.

Inhibitory Parvalbumin Basket Cell Activity is Selectively Reduced during Hippocampal Sharp Wave Ripples in a Mouse Model of Familial Alzheimer's Disease.

Memory disruption in mild cognitive impairment (MCI) and Alzheimer's disease (AD) is poorly understood, particularly at early stages preceding neurodegeneration. In mouse models of AD, there are disruptions to sharp wave ripples (SWRs), hippocampal population events with a critical role in memory consolidation. However, the microcircuitry underlying these disruptions is under-explored. We tested whether a selective reduction in parvalbumin-expressing (PV) inhibitory interneuron activity underlies hyperactivity and SWR disruption. We employed the 5xFAD model of familial AD crossed with mouse lines labeling excitatory pyramidal cells (PCs) and inhibitory PV cells. We observed a 33% increase in frequency, 58% increase in amplitude, and 8% decrease in duration of SWRs in ex vivo slices from male and female three-month 5xFAD mice versus littermate controls. 5xFAD mice of the same age were impaired in a hippocampal-dependent memory task. Concurrent with SWR recordings, we performed calcium imaging, cell-attached, and whole-cell recordings of PC and PV cells within the CA1 region. PCs in 5xFAD mice participated in enlarged ensembles, with superficial PCs (sPCs) having a higher probability of spiking during SWRs. Both deep PCs (dPCs) and sPCs displayed an increased synaptic E/I ratio, suggesting a disinhibitory mechanism. In contrast, we observed a 46% spike rate reduction during SWRs in PV basket cells (PVBCs), while PV bistratified and axo-axonic cells were unimpaired. Excitatory synaptic drive to PVBCs was selectively reduced by 50%, resulting in decreased E/I ratio. Considering prior studies of intrinsic PV cell dysfunction in AD, these findings suggest alterations to the PC-PVBC microcircuit also contribute to impairment.SIGNIFICANCE STATEMENT We demonstrate that a specific subtype of inhibitory neuron, parvalbumin-expressing (PV) basket cells, have selectively reduced activity in a model of Alzheimer's disease (AD) during activity critical for the consolidation of memory. These results identify a potential cellular target for therapeutic intervention to restore aberrant network activity in early amyloid pathology. While PV cells have previously been identified as a potential therapeutic target, this study for the first time recognizes that other PV neuronal subtypes, including bistratified and axo-axonic cells, are spared. These experiments are the first to record synaptic and spiking activity during sharp wave ripple (SWR) events in early amyloid pathology and reveal that a selective decrease in excitatory synaptic drive to PV basket cells (PVBCs) likely underlies reduced function.

Increased matrix metalloproteinase levels and perineuronal net proteolysis in the HIV-infected brain; relevance to altered neuronal population dynamics.

HIV-associated neurocognitive disorders (HAND) continue to persist despite effective control of viral replication. Although the mechanisms underlying HAND are poorly understood, recent attention has focused on altered neuronal population activity as a correlate of impaired cognition. However, while alterations in neuronal population activity in the gamma frequency range are noted in the setting of HAND, the underlying mechanisms for these changes is unclear. Perineuronal nets (PNNs) are a specialized extracellular matrix that surrounds a subset of inhibitory neurons important to the expression of neuronal oscillatory activity. In the present study, we observe that levels of PNN-degrading matrix metalloproteinases (MMPs) are elevated in HIV-infected post-mortem human brain tissue. Furthermore, analysis of two PNN components, aggrecan and brevican, reveals increased proteolysis in HIV-infected brains. In addition, local field potential recordings from ex vivo mouse hippocampal slices demonstrate that the power of carbachol-induced gamma activity is increased following PNN degradation. Together, these results provide a possible mechanism whereby increased MMP proteolysis of PNNs may stimulate altered neuronal oscillatory activity and contribute to HAND symptoms.

Measuring Sharp Waves and Oscillatory Population Activity With the Genetically Encoded Calcium Indicator GCaMP6f.

GCaMP6f is among the most widely used genetically encoded calcium indicators for monitoring neuronal activity. Applications are at both the cellular and population levels. Here, we explore two important and under-explored issues. First, we have tested if GCaMP6f is sensitive enough for the detection of population activity with sparse firing, similar to the sensitivity of the local field potential (LFP). Second, we have tested if GCaMP6f is fast enough for the detection of fast network oscillations critical for the encoding and consolidation of memory. We have focused this study on the activity of the hippocampal network including sharp waves (SWs), carbachol-induced theta oscillations, and interictal-like spikes. We compare simultaneous LFP and optical GCaMP6f fluorescent recordings in Thy1-GCaMP6f mouse hippocampal slices. We observe that SWs produce a clear population GCaMP6f signal above noise with an average magnitude of 0.3% ΔF/F. This population signal is highly correlated with the LFP, albeit with a delay of 40.3 ms (SD 10.8 ms). The population GCaMP6f signal follows the LFP evoked by 20 Hz stimulation with high fidelity, while electrically evoked oscillations up to 40 Hz were detectable with reduced amplitude. GCaMP6f and LFP signals showed a large amplitude discrepancy. The amplitude of GCaMP6f fluorescence increased by a factor of 28.9 (SD 13.5) between spontaneous SWs and carbachol-induced theta bursts, while the LFP amplitude increased by a factor of 2.4 (SD 1.0). Our results suggest that GCaMP6f is a useful tool for applications commonly considered beyond the scope of genetically encoded calcium indicators. In particular, population GCaMP6f signals are sensitive enough for detecting synchronous network events with sparse firing and sub-threshold activity, as well as asynchronous events with only a nominal LFP. In addition, population GCaMP6f signals are fast enough for monitoring theta and beta oscillations (<25 Hz). Faster calcium indicators (e.g., GCaMP7) will further improve the frequency response for the detection of gamma band oscillations. The advantage of population optical over LFP recordings are that they are non-contact and free from stimulation artifacts. These features may be particularly useful for high-throughput recordings and applications sensitive to stimulus artifact, such as monitoring responses during continuous stimulation.

Venlafaxine stimulates PNN proteolysis and MMP-9-dependent enhancement of gamma power; relevance to antidepressant efficacy.

Drugs that target monoaminergic transmission represent a first-line treatment for major depression. Though a full understanding of the mechanisms that underlie antidepressant efficacy is lacking, evidence supports a role for enhanced excitatory transmission. This can occur through two non-mutually exclusive mechanisms. The first involves increased function of excitatory neurons through relatively direct mechanisms such as enhanced dendritic arborization. Another mechanism involves reduced inhibitory function, which occurs with the rapid antidepressant ketamine. Consistent with this, GABAergic interneuron-mediated cortical inhibition is linked to reduced gamma oscillatory power, a rhythm also diminished in depression. Remission of depressive symptoms correlates with restoration of gamma power. As a result of strong excitatory input, reliable GABA release, and fast firing, PV-expressing neurons (PV neurons) represent critical pacemakers for synchronous oscillations. PV neurons also represent the predominant GABAergic population enveloped by perineuronal nets (PNNs), lattice-like structures that localize glutamatergic input. Disruption of PNNs reduces PV excitability and enhances gamma activity. Studies suggest that monoamine reuptake inhibitors reduce integrity of the PNN. Mechanisms by which these inhibitors reduce PNN integrity, however, remain largely unexplored. A better understanding of these issues might encourage development of therapeutics that best up-regulate PNN-modulating proteases. We observe that the serotonin/norepinephrine reuptake inhibitor venlafaxine increases hippocampal matrix metalloproteinase (MMP)-9 levels as determined by ELISA and concomitantly reduces PNN integrity in murine hippocampus as determined by analysis of sections following their staining with a fluorescent PNN-binding lectin. Moreover, venlafaxine-treated mice (30 mg/kg/day) show an increase in carbachol-induced gamma power in ex vivo hippocampal slices as determined by local field potential recording and Matlab analyses. Studies with mice deficient in matrix metalloproteinase 9 (MMP-9), a protease linked to PNN disruption in other settings, suggest that MMP-9 contributes to venlafaxine-enhanced gamma power. In conclusion, our results support the possibility that MMP-9 activity contributes to antidepressant efficacy through effects on the PNN that may in turn enhance neuronal population dynamics involved in mood and/or memory. Cover Image for this issue: doi: 10.1111/jnc.14498.

Pacing Hippocampal Sharp-Wave Ripples With Weak Electric Stimulation.

Sharp-wave ripples (SWRs) are spontaneous neuronal population events that occur in the hippocampus during sleep and quiet restfulness, and are thought to play a critical role in the consolidation of episodic memory. SWRs occur at a rate of 30-200 events per minute. Their overall abundance may, however, be reduced with aging and neurodegenerative disease. Here we report that the abundance of SWR within murine hippocampal slices can be increased by paced administration of a weak electrical stimulus, especially when the spontaneously occurring rate is low or compromised. Resultant SWRs have large variations in amplitude and ripple patterns, which are morphologically indistinguishable from those of spontaneous SWRs, despite identical stimulus parameters which presumably activate the same CA3 neurons surrounding the electrode. The stimulus intensity for reliably pacing SWRs is weaker than that required for inducing detectable evoked field potentials in CA1. Moreover, repetitive ~1 Hz stimuli with low intensity can reliably evoke thousands of SWRs without detectable LTD or "habituation." Our results suggest that weak stimuli may facilitate the spontaneous emergence of SWRs without significantly altering their characteristics. Pacing SWRs with weak electric stimuli could potentially be useful for restoring their abundance in the damaged hippocampus.

Disruption of perineuronal nets increases the frequency of sharp wave ripple events.

Hippocampal sharp wave ripples (SWRs) represent irregularly occurring synchronous neuronal population events that are observed during phases of rest and slow wave sleep. SWR activity that follows learning involves sequential replay of training-associated neuronal assemblies and is critical for systems level memory consolidation. SWRs are initiated by CA2 or CA3 pyramidal cells (PCs) and require initial excitation of CA1 PCs as well as participation of parvalbumin (PV) expressing fast spiking (FS) inhibitory interneurons. These interneurons are relatively unique in that they represent the major neuronal cell type known to be surrounded by perineuronal nets (PNNs), lattice like structures composed of a hyaluronin backbone that surround the cell soma and proximal dendrites. Though the function of the PNN is not completely understood, previous studies suggest it may serve to localize glutamatergic input to synaptic contacts and thus influence the activity of ensheathed cells. Noting that FS PV interneurons impact the activity of PCs thought to initiate SWRs, and that their activity is critical to ripple expression, we examine the effects of PNN integrity on SWR activity in the hippocampus. Extracellular recordings from the stratum radiatum of horizontal murine hippocampal hemisections demonstrate SWRs that occur spontaneously in CA1. As compared with vehicle, pre-treatment (120 min) of paired hemislices with hyaluronidase, which cleaves the hyaluronin backbone of the PNN, decreases PNN integrity and increases SWR frequency. Pre-treatment with chondroitinase, which cleaves PNN side chains, also increases SWR frequency. Together, these data contribute to an emerging appreciation of extracellular matrix as a regulator of neuronal plasticity and suggest that one function of mature perineuronal nets could be to modulate the frequency of SWR events.