Mechanism of seizure-induced retrograde amnesia

Anatomical Substrates of Rapid Eye Movement Sleep Rebound in a Rodent Model of Post-sevoflurane Sleep Disruption

BACKGROUND

Previous research suggests that sevoflurane anesthesia may prevent the brain from accessing rapid eye movement (REM) sleep. If true, then patterns of neural activity observed in REM-on and REM-off neuronal populations during recovery from sevoflurane should resemble those seen after REM sleep deprivation. In this study, the authors hypothesized that, relative to controls, animals exposed to sevoflurane present with a distinct expression pattern of c-Fos, a marker of neuronal activation, in a cluster of nuclei classically associated with REM sleep, and that such expression in sevoflurane-exposed and REM sleep-deprived animals is largely similar.

METHODS

Adult rats and Targeted Recombination in Active Populations mice were implanted with electroencephalographic electrodes for sleep-wake recording and randomized to sevoflurane, REM deprivation, or control conditions. Conventional c-Fos immunohistochemistry and genetically tagged c-Fos labeling were used to quantify activated neurons in a group of REM-associated nuclei in the midbrain and basal forebrain.

RESULTS

REM sleep duration increased during recovery from sevoflurane anesthesia relative to controls (157.0 ± 24.8 min vs. 124.2 ± 27.8 min; P = 0.003) and temporally correlated with increased c-Fos expression in the sublaterodorsal nucleus, a region active during REM sleep (176.0 ± 36.6 cells vs. 58.8 ± 8.7; P = 0.014), and decreased c-Fos expression in the ventrolateral periaqueductal gray, a region that is inactive during REM sleep (34.8 ± 5.3 cells vs. 136.2 ± 19.6; P = 0.001). Fos changes similar to those seen in sevoflurane-exposed mice were observed in REM-deprived animals relative to controls (sublaterodorsal nucleus: 85.0 ± 15.5 cells vs. 23.0 ± 1.2, P = 0.004; ventrolateral periaqueductal gray: 652.8 ± 71.7 cells vs. 889.3 ± 66.8, P = 0.042).

CONCLUSIONS

In rodents recovering from sevoflurane, REM-on and REM-off neuronal activity maps closely resemble those of REM sleep-deprived animals. These findings provide new evidence in support of the idea that sevoflurane does not substitute for endogenous REM sleep.

EDITOR’S PERSPECTIVE

Amnesia after Repeated Head Impact Is Caused by Impaired Synaptic Plasticity in the Memory Engram

Subconcussive head impacts are associated with the development of acute and chronic cognitive deficits. We recently reported that high-frequency head impact (HFHI) causes chronic cognitive deficits in mice through synaptic changes. To better understand the mechanisms underlying HFHI-induced memory decline, we used TRAP2/Ai32 transgenic mice to enable visualization and manipulation of memory engrams. We labeled the fear memory engram in male and female mice exposed to an aversive experience and subjected them to sham or HFHI. Upon subsequent exposure to natural memory recall cues, sham, but not HFHI, mice successfully retrieved fearful memories. In sham mice the hippocampal engram neurons exhibited synaptic plasticity, evident in amplified AMPA:NMDA ratio, enhanced AMPA-weighted tau, and increased dendritic spine volume compared with nonengram neurons. In contrast, although HFHI mice retained a comparable number of hippocampal engram neurons, these neurons did not undergo synaptic plasticity. This lack of plasticity coincided with impaired activation of the engram network, leading to retrograde amnesia in HFHI mice. We validated that the memory deficits induced by HFHI stem from synaptic plasticity impairments by artificially activating the engram using optogenetics and found that stimulated memory recall was identical in both sham and HFHI mice. Our work shows that chronic cognitive impairment after HFHI is a result of deficiencies in synaptic plasticity instead of a loss in neuronal infrastructure, and we can reinstate a forgotten memory in the amnestic brain by stimulating the memory engram. Targeting synaptic plasticity may have therapeutic potential for treating memory impairments caused by repeated head impacts.

Closed-loop stimulation in periods with less epileptiform activity drives improved epilepsy outcomes

In patients with drug-resistant epilepsy, electrical stimulation of the brain in response to epileptiform activity can make seizures less frequent and debilitating. This therapy, known as closed-loop responsive neurostimulation (RNS), aims to directly halt seizure activity via targeted stimulation of a burgeoning seizure. Rather than immediately stopping seizures as they start, many RNS implants produce slower, long-lasting changes in brain dynamics that better predict clinical outcomes. Here we hypothesize that stimulation during brain states with less epileptiform activity drives long-term changes that restore healthy brain networks. To test this, we quantified stimulation episodes during low- and high-risk brain states-that is, stimulation during periods with a lower or higher risk of generating epileptiform activity-in a cohort of 40 patients treated with RNS. More frequent stimulation in tonic low-risk states and out of rhythmic high-risk states predicted seizure reduction. Additionally, stimulation events were more likely to be phase-locked to prolonged episodes of abnormal activity for intermediate and poor responders when compared to super-responders, consistent with the hypothesis that improved outcomes are driven by stimulation during low-risk states. These results support the hypothesis that stimulation during low-risk periods might underlie the mechanisms of RNS, suggesting a relationship between temporal patterns of neuromodulation and plasticity that facilitates long-term seizure reduction.

Reversible synaptic adaptations in a subpopulation of murine hippocampal neurons following early-life seizures

Early-life seizures (ELSs) can cause permanent cognitive deficits and network hyperexcitability, but it is unclear whether ELSs induce persistent changes in specific neuronal populations and whether these changes can be targeted to mitigate network dysfunction. We used the targeted recombination of activated populations (TRAP) approach to genetically label neurons activated by kainate-induced ELSs in immature mice. The ELS-TRAPed neurons were mainly found in hippocampal CA1, remained uniquely susceptible to reactivation by later-life seizures, and displayed sustained enhancement in α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) receptor-mediated (AMPAR-mediated) excitatory synaptic transmission and inward rectification. ELS-TRAPed neurons, but not non-TRAPed surrounding neurons, exhibited enduring decreases in Gria2 mRNA, responsible for encoding the GluA2 subunit of the AMPARs. This was paralleled by decreased synaptic GluA2 protein expression and heightened phosphorylated GluA2 at Ser880 in dendrites, indicative of GluA2 internalization. Consistent with increased GluA2-lacking AMPARs, ELS-TRAPed neurons showed premature silent synapse depletion, impaired long-term potentiation, and impaired long-term depression. In vivo postseizure treatment with IEM-1460, an inhibitor of GluA2-lacking AMPARs, markedly mitigated ELS-induced changes in TRAPed neurons. These findings show that enduring modifications of AMPARs occur in a subpopulation of ELS-activated neurons, contributing to synaptic dysplasticity and network hyperexcitability, but are reversible with early IEM-1460 intervention.

Neuronal plasticity contributes to postictal death

Repeated generalized tonic-clonic seizures (GTCSs) are the most critical risk factor for sudden unexpected death in epilepsy (SUDEP). GTCSs can cause fatal apnea. We investigated neuronal plasticity mechanisms that precipitate postictal apnea and seizure-induced death. Repeated seizures worsened behavior, precipitated apnea, and enlarged active neuronal circuits, recruiting more neurons in such brainstem nuclei as periaqueductal gray (PAG) and dorsal raphe, indicative of brainstem plasticity. Seizure-activated neurons are more excitable and have enhanced AMPA-mediated excitatory transmission after a seizure. Global deletion of the GluA1 subunit of AMPA receptors abolishes postictal apnea and seizure-induced death. Treatment with a drug that blocks Ca2+-permeable AMPA receptors also renders mice apnea-free with five-fold better survival than untreated mice. Repeated seizures traffic the GluA1 subunit-containing AMPA receptors to synapses, and blocking this mechanism decreases the probability of postictal apnea and seizure-induced death.

Chemogenetic inhibition of subicular seizure-activated neurons alleviates cognitive deficit in male mouse epilepsy model

Cognitive deficit is a common comorbidity in temporal lobe epilepsy (TLE) and is not well controlled by current therapeutics. How epileptic seizure affects cognitive performance remains largely unclear. In this study we investigated the role of subicular seizure-activated neurons in cognitive impairment in TLE. A bipolar electrode was implanted into hippocampal CA3 in male mice for kindling stimulation and EEG recording; a special promoter with enhanced synaptic activity-responsive element (E-SARE) was used to label seizure-activated neurons in the subiculum; the activity of subicular seizure-activated neurons was manipulated using chemogenetic approach; cognitive function was assessed in object location memory (OLM) and novel object recognition (NOR) tasks. We showed that chemogenetic inhibition of subicular seizure-activated neurons (mainly CaMKIIα+ glutamatergic neurons) alleviated seizure generalization and improved cognitive performance, but inhibition of seizure-activated GABAergic interneurons had no effect on seizure and cognition. For comparison, inhibition of the whole subicular CaMKIIα+ neuron impaired cognitive function in naïve mice in basal condition. Notably, chemogenetic inhibition of subicular seizure-activated neurons enhanced the recruitment of cognition-responsive c-fos+ neurons via increasing neural excitability during cognition tasks. Our results demonstrate that subicular seizure-activated neurons contribute to cognitive impairment in TLE, suggesting seizure-activated neurons as the potential therapeutic target to alleviate cognitive impairment in TLE.

Unearthing the mechanisms of responsive neurostimulation for epilepsy

Responsive neurostimulation (RNS) is an effective therapy for people with drug-resistant focal epilepsy. In clinical trials, RNS therapy results in a meaningful reduction in median seizure frequency, but the response is highly variable across individuals, with many receiving minimal or no benefit. Understanding why this variability occurs will help improve use of RNS therapy. Here we advocate for a reexamination of the assumptions made about how RNS reduces seizures. This is now possible due to large patient cohorts having used this device, some long-term. Two foundational assumptions have been that the device's intracranial leads should target the seizure focus/foci directly, and that stimulation should be triggered only in response to detected epileptiform activity. Recent studies have called into question both hypotheses. Here, we discuss these exciting new studies and suggest future approaches to patient selection, lead placement, and device programming that could improve clinical outcomes.

A New Behavioral Paradigm for Frustrative Non-reward Reveals a Global Change in Brain Networks by Frustration

Background

Irritability, defined as proneness to anger, can reach a pathological extent. It is a defining symptom of Disruptive Mood Dysregulation Disorder (DMDD) and one of the most common reasons youth presents for psychiatric evaluation and care. Aberrant responses to frustrative non-reward (FNR, the response to omission of expected reward) are central to the pathophysiology of irritability. FNR is a translational construct to study irritability across species. The development of preclinical FNR models would advance mechanistic studies of the important and relatively understudied clinical phenomenon of irritability.

Methods

We used FNR as a conceptual framework to develop a novel mouse behavioral paradigm named Alternate Poking Reward Omission (APRO). After APRO, mice were examined with a battery of behavioral tests and processed for whole brain c-Fos imaging. FNR increases locomotion and aggression in mice regardless of sex. These behavioral changes resemble the symptoms observed in youth with severe irritability. There is no change in anxiety-like, depression-like, or non-aggressive social behaviors. FNR increases c-Fos+ neurons in 13 subregions of thalamus, iso-cortex and hippocampus including the prelimbic, ACC, hippocampus, dorsal thalamus, cuneiform nucleus, pons, and pallidum areas. FNR also shifts the brain network towards a more global processing mode.

Conclusion

Our novel FNR paradigm produces a frustration effect and alters brain processing in ways resembling the symptoms and brain network reconfiguration observed in youth with severe irritability. The novel behavioral paradigm and identified brain regions lay the groundwork for further mechanistic studies of frustration and irritability in rodents.

Connectivity and Excitability Shape Seizure Circuits

Mapping neuronal circuits that generate focal to bilateral tonic-clonic seizures is essential for understanding general principles of seizure propagation and modifying the risk of death and injury due to bilateral motor seizures. We used novel techniques developed over the past decade to study these circuits. We propose the general hypothesis that at the mesoscale, seizures follow anatomical projections of the seizure focus, preferentially activating more excitable neurons.

Changes in perfusion, and structure of hippocampal subfields related to cognitive impairment after ECT: A pilot study using ultra high field MRI

BACKGROUND

Electroconvulsive therapy (ECT) in patients with major depression is associated with volume changes and markers of neuroplasticity in the hippocampus, in particular in the dentate gyrus. It is unclear if these changes are associated with cognitive side effects.

OBJECTIVES

We investigated whether changes in cognitive functioning after ECT were associated with hippocampal structural changes. It was hypothesized that 1) volume increase of hippocampal subfields and 2) changes in perfusion and diffusion of the hippocampus correlated with cognitive decline.

METHODS

Using ultra high field (7 T) MRI, intravoxel incoherent motion and volumetric data were acquired and neurocognitive functioning was assessed before and after ECT in 23 patients with major depression. Repeated measures correlation analysis was used to examine the relation between cognitive functioning and structural characteristics of the hippocampus.

RESULTS

Left hippocampal volume, left and right dentate gyrus and right CA1 volume increase correlated with decreases in verbal memory functioning. In addition, a decrease of mean diffusivity in the left hippocampus correlated with a decrease in letter fluency.

LIMITATIONS

Due to methodological restrictions direct study of neuroplasticity is not possible. MRI is used as an indirect measure.

CONCLUSION

As both volume increase in the hippocampus and MD decrease can be interpreted as indirect markers for neuroplasticity that co-occur with a decrease in cognitive functioning, our results may indicate that neuroplastic processes are affecting cognitive processes after ECT.

CRISPR: a tool with potential for genomic reprogramming in neurological disorders

The intricate neural circuitry of the brain necessitates precise and synchronized transcriptional programs. Any disturbance during embryonic or adult development, whether caused by genetic or environmental factors, may result in refractory and recurrent neurological disorders. Inadequate knowledge of the pathogenic mechanisms underlying neurological disorders is the primary obstacle to the development of effective treatments, necessitating the development of alternative therapeutic approaches to identify rational molecular targets. Recently, with the evolution of CRISPR-Cas9 technology, an engineered RNA system provides precise and highly effective correction or silencing of disease-causing mutations by modulating expression and thereby avoiding the limitations of the RNA interference strategy. This article discusses the CRISPR-Cas9 technology, its mechanisms, and the limitations of the new technology. We provide a glimpse of how the far-reaching implications of CRISPR can open new avenues for the development of tools to combat neurological disorders, as well as a review of recent attempts by neuroscientists to launch therapeutic correction.

The recycling of AMPA receptors/GABAa receptors is related to neuronal excitation/inhibition imbalance and may be regulated by KIF5A

BACKGROUND

Excitation/inhibition imbalance (E/I imbalance), which involves an increase of alpha-amino-3-hydroxy-5-methyl-4-isoxazole (AMPA) receptors (AMPARs) and decrease of gamma-aminobutyric acid type A (GABA) type A receptors (GABAaRs) on the neuron surface, has been documented in the pathogenesis of seizures. Notably, it has been established that both the glutamate receptor subunit 2 (GluR2) of AMPARs and beta 2/3 subunits of GABAaRs (Gabrb2+3) participate in the recycling mechanism mediated by the kinesin heavy chain isoform 5A (KIF5A), which determines the number of neuron surface receptors. However, it remains unclear whether receptor recycling is involved in the pathogenesis of seizures.

METHODS

Twelve adult male Sprague-Dawley rats were randomly allocated to the normal control (Ctl) group (n=6) and the pentylenetetrazol (PTZ)-induced seizure (Sez) group (n=6). The rats in the Ctl group were treated with saline. The rats in the Sez group received an intraperitoneal injection of PTZ at an initial dose of 40 mg/kg. Primary cultured neurons were obtained from newborn rats (24-hour-old). The neurons were exposed to magnesium-free (Mg2+-free) extracellular fluid for 3 hours to establish the seizure model in vitro. We detected the electrophysiology of the seizure model, the expression levels of KIF5A, GluR2, and Gabrb2+3, the recycling ratio of GluR2 and Gabrb2+3, the interaction between KIF5A and GluR2, and the interaction between KIF5A and Gabrb2+3.

RESULTS

In the Sez group, the expression of GluR2 on the cell surface was increased and the expression of Gabrb2+3 on the cell surface was decreased. The amplitude and frequency of action potentials were significantly increased in the Mg2+-free group. The amplitude and decay time of AMPAR-mediated miniature excitatory postsynaptic currents were increased in the Mg2+-free group. The amplitude and decay time of miniature inhibitory postsynaptic currents were decreased in the Mg2+-free group. The recycling ratio of GluR2 was increased and the recycling ratio of Gabrb2+3 was decreased in the Mg2+-free group. The interaction between KIF5A and GluR2 was increased, and the interaction between KIF5A and Gabrb2+3 was decreased in the seizure model in vivo and in vitro.

CONCLUSIONS

The recycling of AMPA receptors/GABAa receptors is related to E/I imbalance and may be regulated by KIF5A.

Extrahippocampal seizure and memory circuits overlap

Seizures cause retrograde amnesia. We have previously demonstrated that seizures erode recently formed memories through shared ensembles and mechanisms in the CA1 region of the hippocampus. Here, we tested whether seizure circuits overlap spatial memory circuits outside of the CA. Spatial memory is consolidated by the hippocampal-cortical coupling that are connected via multiple pathways. We tested whether a seizure invades structures involved in memory consolidation by using the activity reporter TRAP2 mice. T-maze alternation learning activated neurons in the dentate gyrus, mediodorsal thalamus, retrosplenial cortex, and medial prefrontal cortex. This spatial memory relies on the plasticity of the AMPA receptor GluA1 subunit. GluA1 knockout/TRAP2 mice did not learn to alternate, and structures interposed between the hippocampus and the cortex were not active. A seizure prevented the recall of alternation memory and activated memory-labeled structures. There was a widespread overlap between learning-activated ensembles and seizure-activated neurons, which likely contributes to retrograde amnesia.Significance StatementWe propose that seizures cause retrograde amnesia by engaging the circuits that participate in memory consolidation.

Neuronal activity mapping during exploration of a novel environment

Whole-brain mapping is an effective approach to investigate which brain areas are activated by the exploration of a novel environment. Previous studies analyzing neuronal activity promoted by novelty focused mostly on one specific area instead of the whole brain and measured activation using cfos immunohistochemistry. In this study, we utilized TRAP2 mice exposed to a novel and familiar environment to examine neuronal activity in exploratory, learning, and memory circuits. We analyzed the behavior of mice during environment exploration. Brain tissue was processed using tissue clarification and neurons active during exploration of an environment were mapped based on the cfos expression. Neuronal activity after each experience were quantified in regions of interest. We observed increased exploratory behavior in mice exposed to a novel environment in comparison to familiar (170.5 s ± 6.47 vs. 112.5 s ± 9.54, p = 0.0001). Neuronal activity was significantly increased in the dentate gyrus (115.56 ± 53.84 vs. 32.24 ± 12.32, p = 0.02) during the exploration of a novel environment. Moreover, examination of the remaining regions of interest showed some increase in the number of active neurons in the novel condition, however, those differences were not statistically significant. Brief exposure to a novel environment results in increased exploratory behavior and significant neuronal activity in the dentate gyrus.