Physiological features of parvalbumin-expressing GABAergic interneurons contributing to high-frequency oscillations in the cerebral cortex.

Parvalbumin-expressing (PV+) inhibitory interneurons drive gamma oscillations (30-80 Hz), which underlie higher cognitive functions. In this review, we discuss two groups/aspects of fundamental properties of PV+ interneurons. In the first group (dubbed Before Axon), we list properties representing optimal synaptic integration in PV+ interneurons designed to support fast oscillations. For example: [i] Information can neither enter nor leave the neocortex without the engagement of fast PV+ -mediated inhibition; [ii] Voltage responses in PV+ interneuron dendrites integrate linearly to reduce impact of the fluctuations in the afferent drive; and [iii] Reversed somatodendritic Rm gradient accelerates the time courses of synaptic potentials arriving at the soma. In the second group (dubbed After Axon), we list morphological and biophysical properties responsible for (a) short synaptic delays, and (b) efficient postsynaptic outcomes. For example: [i] Fast-spiking ability that allows PV+ interneurons to outpace other cortical neurons (pyramidal neurons). [ii] Myelinated axon (which is only found in the PV+ subclass of interneurons) to secure fast-spiking at the initial axon segment; and [iii] Inhibitory autapses - autoinhibition, which assures brief biphasic voltage transients and supports postinhibitory rebounds. Recent advent of scientific tools, such as viral strategies to target PV cells and the ability to monitor PV cells via in vivo imaging during behavior, will aid in defining the role of PV cells in the CNS. Given the link between PV+ interneurons and cognition, in the future, it would be useful to carry out physiological recordings in the PV+ cell type selectively and characterize if and how psychiatric and neurological diseases affect initiation and propagation of electrical signals in this cortical sub-circuit. Voltage imaging may allow fast recordings of electrical signals from many PV+ interneurons simultaneously.

Overexpression of KCNN4 channels in principal neurons produces an anti-seizure effect without reducing their coding ability.

Gene therapy offers a potential alternative to the surgical treatment of epilepsy, which affects millions of people and is pharmacoresistant in ~30% of cases. Aimed at reducing the excitability of principal neurons, the engineered expression of K+ channels has been proposed as a treatment due to the outstanding ability of K+ channels to hyperpolarize neurons. However, the effects of K+ channel overexpression on cell physiology remain to be investigated. Here we report an adeno-associated virus (AAV) vector designed to reduce epileptiform activity specifically in excitatory pyramidal neurons by expressing the human Ca2+-gated K+ channel KCNN4 (KCa3.1). Electrophysiological and pharmacological experiments in acute brain slices showed that KCNN4-transduced cells exhibited a Ca2+-dependent slow afterhyperpolarization that significantly decreased the ability of KCNN4-positive neurons to generate high-frequency spike trains without affecting their lower-frequency coding ability and action potential shapes. Antiepileptic activity tests showed potent suppression of pharmacologically induced seizures in vitro at both single cell and local field potential levels with decreased spiking during ictal discharges. Taken together, our findings strongly suggest that the AAV-based expression of the KCNN4 channel in excitatory neurons is a promising therapeutic intervention as gene therapy for epilepsy.

Current Practice in Using Voltage Imaging to Record Fast Neuronal Activity: Successful Examples from Invertebrate to Mammalian Studies.

The optical imaging of neuronal activity with potentiometric probes has been credited with being able to address key questions in neuroscience via the simultaneous recording of many neurons. This technique, which was pioneered 50 years ago, has allowed researchers to study the dynamics of neural activity, from tiny subthreshold synaptic events in the axon and dendrites at the subcellular level to the fluctuation of field potentials and how they spread across large areas of the brain. Initially, synthetic voltage-sensitive dyes (VSDs) were applied directly to brain tissue via staining, but recent advances in transgenic methods now allow the expression of genetically encoded voltage indicators (GEVIs), specifically in selected neuron types. However, voltage imaging is technically difficult and limited by several methodological constraints that determine its applicability in a given type of experiment. The prevalence of this method is far from being comparable to patch clamp voltage recording or similar routine methods in neuroscience research. There are more than twice as many studies on VSDs as there are on GEVIs. As can be seen from the majority of the papers, most of them are either methodological ones or reviews. However, potentiometric imaging is able to address key questions in neuroscience by recording most or many neurons simultaneously, thus providing unique information that cannot be obtained via other methods. Different types of optical voltage indicators have their advantages and limitations, which we focus on in detail. Here, we summarize the experience of the scientific community in the application of voltage imaging and try to evaluate the contribution of this method to neuroscience research.

New Context Significantly Changes Expression of Irs2 Gene in Hippocampal Areas.

Memory formation is a complex process involving changes in the synaptic activity and gene expression encoding the insulin-like growth factors. We analyzed changes in the expression of genes encoding the insulin/insulin-like growth factors' proteins at the early period of learning in the CA1 region and dentate gyrus of the dorsal and ventral hippocampus in mice 1 hour after presentation of a new context (contextual fear conditioning) with and without negative reinforcement. It was found that in addition to changes in the expression of immediate early genes c-Fos (in all studied hippocampal fields) and Arc (in dorsal and ventral CA1, as well as in dorsal dentate gyrus), exposure to a new context significantly altered expression of the insulin receptor substrate 2 gene (Irs2) in dorsal CA1 and ventral dentate gyrus irrespectively of the negative reinforcement, which suggests participation of the insulin/IGF system in the early stages of neural activation during learning.

Nitric Oxide Regulates GluA2-Lacking AMPAR Contribution to Synaptic Transmission of CA1 Apical but Not Basal Dendrites.

The mechanisms of synaptic plasticity differ in distinct local circuits. In the CA1 region of the hippocampus, the mechanisms of long-term potentiation (LTP) at apical dendrites in stratum radiatum and basal dendrites in stratum oriens involve different molecular cascades. For instance, participation of nitric oxide in LTP induction was shown to be necessary only for apical dendrites. This phenomenon may play a key role in information processing in CA1, and one of the reasons for this difference may be differing synaptic characteristics in these regions. Here, we compared the synaptic responses to stimulation of apical and basal dendrites of CA1 pyramidal neurons and found a difference in the current-voltage characteristics of these inputs, which is presumably due to a distinct contribution of GluA2-lacking AMPA receptors to synaptic transmission. In addition, we obtained data that indicate the presence of these receptors in pyramidal dendrites in both stratum radiatum and stratum oriens. We also demonstrated that inhibition of NO synthase reduced the contribution of GluA2-lacking AMPA receptors at apical but not basal dendrites, and inhibition of soluble guanylate cyclase did not affect this phenomenon.

Impacts of OrX and cAMP-insensitive Orco to the insect olfactory heteromer activity.

Insect odorant receptors (ORs) have been suggested to function as ligand-gated cation channels, with OrX/Orco heteromers combining ionotropic and metabotropic activity. The latter is mediated by different G proteins and results in Orco self-activation by cyclic nucleotide binding. In this contribution, we co-express the odor-specific subunits DmOr49b and DmOr59b with either wild-type Orco or an Orco-PKC mutant lacking cAMP activation heterologously in mammalian cells. We show that the characteristics of heteromers strongly depend on both the OrX type and the coreceptor variant. Thus, methyl acetate-sensitive Or59b/Orco demonstrated 25-fold faster response kinetics over o-cresol-specific Or49b/Orco, while the latter required a 10-100 times lower ligand concentration to evoke a similar electrical response. Compared to wild-type Orco, Orco-PKC decreased odorant sensitivity in both heteromers, and blocked an outward current rectification intrinsic to the Or49b/Orco pair. Our observations thus provide an insight into insect OrX/Orco functioning, highlighting their natural and artificial tuning features and laying the groundwork for their application in chemogenetics, drug screening, and repellent design.

Modulation of AMPA Receptors by Nitric Oxide in Nerve Cells.

Nitric oxide (NO) is a gaseous molecule with a large number of functions in living tissue. In the brain, NO participates in numerous intracellular mechanisms, including synaptic plasticity and cell homeostasis. NO elicits synaptic changes both through various multi-chain cascades and through direct nitrosylation of targeted proteins. Along with the N-methyl-d-aspartate (NMDA) glutamate receptors, one of the key components in synaptic functioning are α-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA) receptors-the main target for long-term modifications of synaptic effectivity. AMPA receptors have been shown to participate in most of the functions important for neuronal activity, including memory formation. Interactions of NO and AMPA receptors were observed in important phenomena, such as glutamatergic excitotoxicity in retinal cells, synaptic plasticity, and neuropathologies. This review focuses on existing findings that concern pathways by which NO interacts with AMPA receptors, influences properties of different subunits of AMPA receptors, and regulates the receptors' surface expression.

Red Fluorescent Genetically Encoded Voltage Indicators with Millisecond Responsiveness.

Genetically encoded fluorescent indicators typically consist of the sensitive and reporter protein domains connected with the amino acid linkers. The final performance of a particular indicator may depend on the linker length and composition as strong as it depends on the both domains nature. Here we aimed to optimize interdomain linkers in VSD-FR189-188-a recently described red fluorescent protein-based voltage indicator. We have tested 13 shortened linker versions and monitored the dynamic range, response speed and polarity of the corresponding voltage indicator variants. While the new indicators didn't show a contrast enhancement, some of them carrying very short interdomain linkers responded 25-fold faster than the parental VSD-FR189-188. Also we found the critical linker length at which fluorescence response to voltage shift changes its polarity from negative to positive slope. Our observations thus make an important contribution to the designing principles of the fluorescent protein-derived voltage indicators.

Insertion of the voltage-sensitive domain into circularly permuted red fluorescent protein as a design for genetically encoded voltage sensor.

Visualization of electrical activity in living cells represents an important challenge in context of basic neurophysiological studies. Here we report a new voltage sensitive fluorescent indicator which response could be detected by fluorescence monitoring in a single red channel. To the best of our knowledge, this is the first fluorescent protein-based voltage sensor which uses insertion-into-circular permutant topology to provide an efficient interaction between sensitive and reporter domains. Its fluorescent core originates from red fluorescent protein (FP) FusionRed, which has optimal spectral characteristics to be used in whole body imaging techniques. Indicators using the same domain topology could become a new perspective for the FP-based voltage sensors that are traditionally based on Förster resonance energy transfer (FRET).

Cued memory reconsolidation in rats requires nitric oxide.

It is well-known that the reactivation of consolidated fear memory under boundary conditions of novelty and protein synthesis blockade results in an impairment of memory, suggesting that the reactivated memory is destabilized and requires synthesis of new proteins for reconsolidation. We tested the hypothesis of nitric oxide (NO) involvement in memory destabilization during the reconsolidation process in rats using memory reactivation under different conditions. We report that administration of NO-synthase selective blockers 3-Br-7-NI or ARL in the conditions of reactivation of memory under a protein synthesis blockade prevented destabilization of fear memory to the conditioned stimulus. Obtained results support the role of NO signaling pathway in the destabilization of existing fear memory triggered by reactivation, and demonstrate that the disruption of this pathway during memory reconsolidation may prevent changes in long-term memory.