Esther Krook-Magnuson

On-demand optogenetic control of spontaneous seizures in temporal lobe epilepsy

Temporal lobe epilepsy is the most common type of epilepsy in adults, is often medically refractory, and due to broad actions and long-time scales, current systemic treatments have major negative side-effects. However, temporal lobe seizures tend to arise from discrete regions before overt clinical behaviour, making temporally and spatially specific treatment theoretically possible. Here we report the arrest of spontaneous seizures using a real-time, closed-loop, response system and in vivo optogenetics in a mouse model of temporal lobe epilepsy. Either optogenetic inhibition of excitatory principal cells, or activation of a subpopulation of GABAergic cells representing <5% of hippocampal neurons, stops seizures rapidly upon light application. These results demonstrate that spontaneous temporal lobe seizures can be detected and terminated by modulating specific cell populations in a spatially restricted manner. A clinical approach built on these principles may overcome many of the side-effects of currently available treatment options.

Cerebellar directed optogenetic intervention inhibits spontaneous hippocampal seizures in a mouse model of temporal lobe epilepsy

Epilepsy is a condition of spontaneous recurrent seizures. Current treatment options for epilepsy can have major negative side effects and for many patients fail to control seizures. We detected seizures on-line and tested a new selective intervention using a mouse model of temporal lobe epilepsy. Abstract Cover Figure Krook-Magnuson et al. report a bidirectional functional connectivity between the hippocampus and the cerebellum in a mouse model of temporal lobe epilepsy, and demonstrate that cerebellar directed on-demand optogenetic intervention can stop seizures recorded from the hippocampus. Temporal lobe epilepsy is often medically refractory and new targets for intervention are needed. We used a mouse model of temporal lobe epilepsy, on-line seizure detection, and responsive optogenetic intervention to investigate the potential for cerebellar control of spontaneous temporal lobe seizures. Cerebellar targeted intervention inhibited spontaneous temporal lobe seizures during the chronic phase of the disorder. We further report that the direction of modulation as well as the location of intervention within the cerebellum can affect the outcome of intervention. Specifically, on-demand optogenetic excitation or inhibition of parvalbumin-expressing neurons, including Purkinje cells, in the lateral or midline cerebellum results in a decrease in seizure duration. In contrast, a consistent reduction in spontaneous seizure frequency occurs uniquely with on-demand optogenetic excitation of the midline cerebellum, and was not seen with intervention directly targeting the hippocampal formation. These findings demonstrate that the cerebellum is a powerful modulator of temporal lobe epilepsy, and that intervention targeting the cerebellum as a potential therapy for epilepsy should be revisited.

In vivo evaluation of the dentate gate theory in epilepsy

A key mechanistic concept in epilepsy is the dentate gate hypothesis, which argues that the dentate gyrus protects hippocampal circuits from overexcitation and that a breakdown of this gate leads to epilepsy. Direct in vivo evidence for the dentate gate hypothesis is lacking and it is therefore unclear whether interventions selectively targeting the dentate gyrus would inhibit seizures. We demonstrate that on‐demand optogenetic restoration of the dentate gate through selective inhibition of granule cells is sufficient to inhibit spontaneous seizures in a mouse model of temporal lobe epilepsy. By contrast, activation of granule cells worsens spontaneous seizures and can even induce acute seizures in non‐epileptic animals. These data provide direct evidence for the dentate gate hypothesis, indicate that the dentate gyrus is indeed a critical node in temporal lobe seizure circuitry, and illustrate that the dentate gyrus can be an effective target for seizure inhibition.

Closed-loop optogenetic intervention in mice

Optogenetic interventions offer novel ways of probing, in a temporally specific manner, the roles of specific cell types in neuronal network functions of awake, behaving animals. Despite the unique potential for temporally specific optogenetic intervention in disease states, a major hurdle in its broad application to unpredictable brain states in a laboratory setting is constructing a real-time responsive system. We recently created a closed-loop system for stopping spontaneous seizures in chronically epileptic mice by using optogenetic intervention. This system performs with a very high sensitivity and specificity, and the strategy is not only relevant to epilepsy but also can also be used to react to diverse brain states in real time, with optogenetic or other interventions. The protocol presented here is highly modular and requires variable amounts of time to perform. We describe the basic construction of a complete system, and we include our downloadable custom closed-loop detection software, which can be used for this purpose.

New dimensions of interneuronal specialization unmasked by principal cell heterogeneity

Although the diversity of neocortical and hippocampal GABAergic interneurons is recognized in terms of their anatomical, molecular and functional properties, principal cells are usually assumed to constitute homogenous populations. However, even within a single layer, subpopulations of principal cells can often be differentiated by their distinct long-range projection targets. Such subpopulations of principal cells can have different local connection properties and excitatory inputs, forming subnetworks that may serve as separate information-processing channels. Interestingly, as reviewed here, recent evidence has revealed specific instances where interneuron cell types selectively innervated distinct subpopulations of principal cells, targeting only those with particular long-distance projection targets. This organization represents a novel form of interneuron specialization, providing interneurons with the potential to selectively regulate specific information-processing streams.

Neurogliaform and Ivy Cells: A Major Family of nNOS Expressing GABAergic Neurons

Neurogliaform and Ivy cells are members of an abundant family of neuronal nitric oxide synthase (nNOS) expressing GABAergic interneurons found in diverse brain regions. These cells have a defining dense local axonal plexus, and display unique synaptic properties including a biphasic postsynaptic response with both a slow GABAA component and a GABAB component following even a single action potential. The type of transmission displayed by these cells has been termed “volume transmission,” distinct from both tonic and classical synaptic transmission. Electrical connections are also notable in that, unlike other GABAergic cell types, neurogliaform family cells will form gap junctions not only with other neurogliaform cells, but also with non-neurogliaform family GABAergic cells. In this review, we focus on neurogliaform and Ivy cells throughout the hippocampal formation, where recent studies highlight their role in feedforward inhibition, uncover their ability to display a phenomenon called persistent firing, and reveal their modulation by opioids. The unique properties of this family of cells, their abundance, rich connectivity, and modulation by clinically relevant drugs make them an attractive target for future studies in vivo during different behavioral and pharmacological conditions.

Tonically active inhibition selectively controls feedforward circuits in mouse barrel cortex

Tonic inhibition mediated by extrasynaptic gamma-aminobutyric acid type A (GABA A) receptors is a powerful conductance that controls cell excitability. Throughout the CNS, tonic inhibition is expressed at varying degrees across different cell types. Despite a rich history of cortical interneuron diversity, little is known about tonic inhibition in the different classes of cells in the cerebral cortex. We therefore examined the cell-type specificity and functional significance of tonic inhibition in layer 4 of the mouse somatosensory barrel cortex. In situ hybridization and immunocytochemistry showed moderate delta-subunit expression across the barrel structures. Whole cell patch-clamp recordings additionally indicated that significant levels of tonic inhibition can be found across cell types, with differences in the magnitude of inhibition between cell types. To activate tonic currents, we used 4,5,6,7-tetrahydroisoxazolo[5,4-c]pyridin-3-ol (THIP, a superagonist at delta-subunit-containing GABA A receptors) at a concentration that did not affect synaptic decay kinetics. THIP produced greater shifts in baseline holding current in inhibitory cells (low-threshold spiking [LTS], 109 +/- 17 pA; fast spiking [FS], 111 +/- 15 pA) than in excitatory cells (39 +/- 10 pA; P < 0.001). In addition to these differences across cell types, there was also variability within inhibitory cells. FS cells with faster action potentials had larger baseline shifts. Because FS cells are known mediators of feedforward inhibition, we tested whether THIP-induced tonic conductance selectively controls feedforward circuits. THIP application resulted in the abolishment of the inhibitory postsynaptic potential in thalamic-evoked disynaptic responses in a subset of excitatory neurons. These data suggest multiple feedforward circuits can be differentiated by the inhibitory control of the presynaptic inhibitory neuron.

Beyond the hammer and the scalpel: selective circuit control for the epilepsies

Current treatment options for epilepsy are inadequate, as too many patients suffer from uncontrolled seizures and from negative side effects of treatment. In addition to these clinical challenges, our scientific understanding of epilepsy is incomplete. Optogenetic and designer receptor technologies provide unprecedented and much needed specificity, allowing for spatial, temporal and cell type-selective modulation of neuronal circuits. Using such tools, it is now possible to begin to address some of the fundamental unanswered questions in epilepsy, to dissect epileptic neuronal circuits and to develop new intervention strategies. Such specificity of intervention also has the potential for direct therapeutic benefits, allowing healthy tissue and network functions to continue unaffected. In this Perspective, we discuss promising uses of these technologies for the study of seizures and epilepsy, as well as potential use of these strategies for clinical therapies.

Neuroelectronics and Biooptics: Closed-Loop Technologies in Neurological Disorders

Brain-implanted devices are no longer a futuristic idea. Traditionally, therapies for most neurological disorders are adjusted based on changes in clinical symptoms and diagnostic measures observed over time. These therapies are commonly pharmacological or surgical, requiring continuous or irreversible treatment regimens that cannot respond rapidly to fluctuations of symptoms or isolated episodes of dysfunction. In contrast, closed-loop systems provide intervention only when needed by detecting abnormal neurological signals and modulating them with instantaneous feedback. Closed-loop systems have been applied to several neurological conditions (most notably epilepsy and movement disorders), but widespread use is limited by conceptual and technical challenges. Herein, we discuss how advances in experimental closed-loop systems hold promise for improved clinical benefit in patients with neurological disorders.

Ivy and neurogliaform interneurons are a major target of μ-Opioid receptor modulation

μ-Opioid receptors (μORs) are selectively expressed on interneurons in area CA1 of the hippocampus. Fast-spiking, parvalbumin-expressing, basket cells express μORs, but circumstantial evidence suggests that another major, unidentified, GABAergic cell class must also be modulated by μORs. Here we report that the abundant, dendritically targeting, neurogliaform family of cells (Ivy and neurogliaform cells) is a previously unrecognized target of direct modulation by μORs. Ivy and neurogliaform cells are not only numerous but also have unique properties, including promiscuous gap junctions formed with various interneuronal subtypes, volume transmission, and the ability to produce a postsynaptic GABAB response after a single presynaptic spike. Using a mouse line expressing green fluorescent protein under the neuropeptide Y promoter, we find that, across all layers of CA1, activation of μORs hyperpolarizes Ivy and neurogliaform cells. Furthermore, paired recordings between synaptically coupled Ivy and pyramidal cells show that Ivy cell terminals are dramatically inhibited by μOR activation. Effects in Ivy and neurogliaform cells are seen at similar concentrations of agonist as those producing inhibition in fast-spiking parvalbumin basket cells. We also report that Ivy cells display the recently described phenomenon of persistent firing, a state of continued firing in the absence of continued input, and that induction of persistent firing is inhibited by μOR activation. Together, these findings identify a major, previously unrecognized, target of μOR modulation. Given the prominence of this cell type in and beyond CA1, as well as its unique role in microcircuitry, opioid modulation of neurogliaform cells has wide implications.