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.

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.

Key pointsr 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.r 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.r 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.r By contrast, activation of granule cells worsens spontaneous seizures and can even induce acute seizures in non-epileptic animals.r 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.Abstract The dentate gyrus is a region subject to intense study in epilepsy because of its posited role as a 'gate' , acting to inhibit overexcitation in the hippocampal circuitry through its unique synaptic, cellular and network properties that result in relatively low excitability. Numerous changes predicted to produce dentate hyperexcitability are seen in epileptic patients and animal models. However, recent findings question whether changes are causative or reactive, as well as the pathophysiological relevance of the dentate in epilepsy. Critically, direct in vivo modulation of dentate 'gate' function during spontaneous seizure activity has not been explored. Therefore, using a mouse model of temporal lobe epilepsy with hippocampal sclerosis, a closed-loop system and selective optogenetic manipulation of granule cells during seizures, we directly tested the dentate 'gate' hypothesis in vivo. Consistent with the dentate gate theory, optogenetic gate restoration through granule cell hyperpolarization efficiently stopped spontaneous seizures. By contrast, optogenetic activation of granule cells exacerbated spontaneous seizures. Furthermore, activating granule cells in non-epileptic animals evoked acute seizures of increasing severity. These data indicate that the dentate gyrus is a critical node in the temporal lobe seizure network, and provide the first in vivo support for the dentate 'gate' hypothesis.

A diversity of GABAergic cell types exist within each brain area, and each cell type is thought to play a unique role in the modulation of principal cell output. Basket cells, whose axon terminals surround principal cell somata and proximal dendrites, have a privileged and influential position for regulating the firing of principal cells. This review explores the dichotomy of the two basket cell classes, cholecystokinin-(CCK) and parvalbumin (PV)-containing basket cells, beginning with differences at the level of the individual cell and subsequently focusing on two ways in which this intrinsic dichotomy is enhanced by extrinsic factors. Neuromodulatory influences, exemplified by the effects of the peptide CCK, dynamically enhance the differential functions of the two cell types. Specifications at the level of the postsynaptic principal cell, including input-specific differences in chloride handling and differences in long-range projection patterns of the principal cell targets, also enhance the distinct network function of basket cells. In this review, new findings will be highlighted concerning the roles of neuromodulatory control and postsynaptic long-range projection pattern in the definition of basket cell function.

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 interventions 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 using optogenetic intervention. This system performs with very high sensitivity and specificity, and the strategy is relevant not only to epilepsy, but can also be used to react in real time, with optogenetic or other interventions, to diverse brain states. The protocol presented here is highly modular and requires variable time to perform. We describe the basic construction of a complete system, and include our downloadable custom closed-loop detection software which can be employed for this purpose.

Feed-forward inhibition from molecular layer interneurons onto granule cells (GCs) in the dentate gyrus is thought to have major effects regulating entorhinal–hippocampal interactions, but the precise identity, properties, and functional connectivity of the GABAergic cells in the molecular layer are not well understood. We used single and paired intracellular patch clamp recordings from post-hoc-identified cells in acute rat hippocampal slices and identified a subpopulation of molecular layer interneurons that expressed immunocytochemical markers present in members of the neurogliaform cell (NGFC) class. Single NGFCs displayed small dendritic trees, and their characteristically dense axonal arborizations covered significant portions of the outer and middle one-thirds of the molecular layer, with frequent axonal projections across the fissure into the CA1 and subicular regions. Typical NGFCs exhibited a late firing pattern with a ramp in membrane potential prior to firing action potentials, and single spikes in NGFCs evoked biphasic, prolonged GABAA and GABAB postsynaptic responses in GCs. In addition to providing dendritic GABAergic inputs to GCs, NGFCs also formed chemical synapses and gap junctions with various molecular layer interneurons, including other NGFCs. NGFCs received low-frequency spontaneous synaptic events, and stimulation of perforant path fibers revealed direct, facilitating synaptic inputs from the entorhinal cortex. Taken together, these results indicate that NGFCs form an integral part of the local molecular layer microcircuitry generating feed-forward inhibition and provide a direct GABAergic pathway linking the dentate gyrus to the CA1 and subicular regions through the hippocampal fissure

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.

Effective prophylaxis for post-traumatic epilepsy currently does not exist, and clinical trials using anticonvulsant drugs have yielded no long-term antiepileptogenic effects. We report that a single, rapid post-traumatic application of the proconvulsant cannabinoid type-1 receptor antagonist SR141716A (Rimonabant--Acomplia®) abolishes the long-term increase in seizure susceptibility caused by head injury in rats. These results indicate that, paradoxically, a seizure-enhancing drug may disrupt the epileptogenic process if applied within a short therapeutic time window.