High-affinity extrasynaptic GABA A receptors (GABA A Rs) are a prominent feature of cerebellar granule neurons and thalamic relay neurons. In both cell types, the presence of synaptic glomeruli would be expected to promote activation of these GABA A Rs, contributing to phasic spillover-mediated currents and tonic inhibition. However, the precise role of different receptor subtypes in these two phenomena is unclear. To address this question, we made recordings from neurons in acute brain slices from mice, and from tsA201 cells expressing recombinant GABA A Rs. We found that ␦ subunit-containing GABA A Rs of both cerebellar granule neurons and thalamic relay neurons of the lateral geniculate nucleus contributed to tonic conductance caused by ambient GABA but not to spillover-mediated currents. In the presence of a low "ambient" GABA concentration, recombinant "extrasynaptic" ␦ subunit-containing GABA A Rs exhibited profound desensitization, rendering them insensitive to brief synaptic-or spillover-like GABA transients. Together, our results demonstrate that phasic spillover and tonic inhibition reflect the activation of distinct receptor populations.

Most patients with N-methyl D-aspartate-receptor antibody encephalitis develop seizures but the epileptogenicity of the antibodies has not been investigated in vivo. Wireless electroencephalogram transmitters were implanted into 23 C57BL/6 mice before left lateral ventricle injection of antibody-positive (test) or healthy (control) immunoglobulin G. Mice were challenged 48 h later with a subthreshold dose (40 mg/kg) of the chemo-convulsant pentylenetetrazol and events recorded over 1 h. Seizures were assessed by video observation of each animal and the electroencephalogram by an automated seizure detection programme. No spontaneous seizures were seen with the antibody injections. However, after the pro-convulsant, the test mice (n = 9) had increased numbers of observed convulsive seizures (P = 0.004), a higher total seizure score (P = 0.003), and a higher number of epileptic 'spike' events (P = 0.023) than the control mice (n = 6). At post-mortem, surprisingly, the total number of N-methyl D-aspartate receptors did not differ between test and control mice, but in test mice the levels of immunoglobulin G bound to the left hippocampus were higher (P < 0.0001) and the level of bound immunoglobulin G correlated with the seizure scores (R(2) = 0.8, P = 0.04, n = 5). Our findings demonstrate the epileptogenicity of N-methyl D-aspartate receptor antibodies in vivo, and suggest that binding of immunoglobulin G either reduced synaptic localization of N-methyl D-aspartate receptors, or had a direct effect on receptor function, which could be responsible for seizure susceptibility in this acute short-term model.

Slow-wave sleep is thought to be important for retuning cortical synapses, but the cellular mechanisms remain unresolved. During slow-wave activity, cortical neurons display synchronized transitions between depolarized Up states and hyperpolarized Down states. Here, using recordings from LIII pyramidal neurons from acute slices of mouse medial entorhinal cortex, we find that subthreshold inputs arriving during the Up state undergo synaptic weakening. This does not reflect a process of global synaptic downscaling, as it is dependent on presynaptic spiking, with network state encoded in the synaptically evoked spine Ca2+ responses. Our data indicate that the induction of synaptic weakening is under postsynaptic control, as it can be prevented by correlated postsynaptic spiking activity, and depends on postsynaptic NMDA receptors and GSK3β activity. This provides a mechanism by which slow-wave activity might bias synapses towards weakening, while preserving the synaptic connections within active neuronal assemblies.

Self-sustained recurrent activity in cortical networks is thought to be important for multiple crucial processes, including circuit development and homeostasis. However, the precise relationship between synaptic input patterns and spiking output of individual neurons remains unresolved during spontaneous network activity. Here, using whole-network high-density microelectrode array (HD-MEA) recordings and patch clamping, we developed a novel experimental approach and analytical tools that provide a comprehensive long-term input-output characterization of individual neurons in cortical cell cultures. We found that, during in vivo-like network activity with excitation(E)-inhibition(I) balance, postsynaptic spiking coincided with the maxima of rapid, network state-dependent fluctuations in the input E/I ratio. Our approach also uncovered the underlying circuit architecture and we identified a few key inhibitory inputs — often from special hub neurons — that were instrumental in mediating these E/I ratio changes. Balanced network theory predicts dynamical regimes governed by input fluctuation and featuring a fast neuronal responsiveness. Our findings — obtained in self-organized neuronal cultures — suggest that the emergence of these favorable regimes and associated network architectures is an inherent property of all cortical networks.

Perception, thoughts, and actions are encoded by the coordinated activity of large neuronal populations spread over large areas. Using thin film electrocorticography (ECoG) arrays, this cortical activity has been used to decode speech and individual finger movements, enabling neuroprosthetics, and to localize epileptic foci. However, the connectorization of these multi-thousand channel thin-film arrays to external circuitry is challenging; current state-of-the-art methods are complex, bulky, and unscalable. We address this shortcoming by developing an electrode connector based on an ultra-conformable thin film electrode array that self-assembles onto hard silicon chip sensors, such as microelectrode arrays (MEAs) or camera sensors enabling large channel counts at high density. The interconnects are formed using microfabricated electrode pads suspended by thin support arms, termed flex2chip. Capillary-assisted assembly drives the pads to deform towards the chip surface, and van der Waals forces maintain this deformation, establishing mechanical and Ohmic contact onto individual pixels. We demonstrate a 2200-channel array with a channel density of 272 channels / mm2 connected to the MEA through the flex2chip interconnection method. Thin film electrode arrays connected through the flex2chip successfully measured extracellular action potentials ex vivo. Furthermore, in a transgenic mouse model for absence epilepsy, Scn8a+/-, we observed highly variable propagation trajectories at micrometer scales, even across the duration of a single spike-and-wave discharge (SWD).