Dendritic mechanisms controlling the threshold and timing requirement of synaptic plasticity

Novel Psychopharmacological Herbs Relieve Behavioral Abnormalities and Hippocampal Dysfunctions in an Animal Model of Post-Traumatic Stress Disorder

Post-traumatic stress disorder (PTSD) is an anxiety disorder caused by traumatic or frightening events, with intensified anxiety, fear memories, and cognitive impairment caused by a dysfunctional hippocampus. Owing to its complex phenotype, currently prescribed treatments for PTSD are limited. This study investigated the psychopharmacological effects of novel COMBINATION herbal medicines on the hippocampus of a PTSD murine model induced by combining single prolonged stress (SPS) and foot shock (FS). We designed a novel herbal formula extract (HFE) from Chaenomeles sinensis, Glycyrrhiza uralensis, and Atractylodes macrocephala. SPS+FS mice were administered HFE (500 and 1000 mg/kg) once daily for 14 days. The effects of HFE of HFE on the hippocampus were analyzed using behavioral tests, immunostaining, Golgi staining, and Western blotting. HFE alleviated anxiety-like behavior and fear response, improved short-term memory, and restored hippocampal dysfunction, including hippocampal neurogenesis alteration and aberrant migration and hyperactivation of dentate granule cells in SPS+FS mice. HFE increased phosphorylation of the Kv4.2 potassium channel, extracellular signal-regulated kinase, and cAMP response element-binding protein, which were reduced in the hippocampus of SPS+FS mice. Therefore, our study suggests HFE as a potential therapeutic drug for PTSD by improving behavioral impairment and hippocampal dysfunction and regulating Kv4.2 potassium channel-related pathways in the hippocampus.

Epileptiform activity influences theta-burst induced LTP in the adult hippocampus: a role for synaptic lipid raft disruption in early metaplasticity?

Non-epileptic seizures are identified as a common epileptogenic trigger. Early metaplasticity following seizures may contribute to epileptogenesis by abnormally altering synaptic strength and homeostatic plasticity. We now studied how in vitro epileptiform activity (EA) triggers early changes in CA1 long-term potentiation (LTP) induced by theta-burst stimulation (TBS) in rat hippocampal slices and the involvement of lipid rafts in these early metaplasticity events. Two forms of EA were induced: (1) interictal-like EA evoked by Mg²⁺ withdrawal and K⁺ elevation to 6 mM in the superfusion medium or (2) ictal-like EA induced by bicuculline (10 μM). Both EA patterns induced and LTP-like effect on CA1 synaptic transmission prior to LTP induction. LTP induced 30 min post EA was impaired, an effect more pronounced after ictal-like EA. LTP recovered to control levels 60 min post interictal-like EA but was still impaired 60 min after ictal-like EA. The synaptic molecular events underlying this altered LTP were investigated 30 min post EA in synaptosomes isolated from these slices. EA enhanced AMPA GluA1 Ser831 phosphorylation but decreased Ser845 phosphorylation and the GluA1/GluA2 ratio. Flotillin-1 and caveolin-1 were markedly decreased concomitantly with a marked increase in gephyrin levels and a less prominent increase in PSD-95. Altogether, EA differentially influences hippocampal CA1 LTP thorough regulation of GluA1/GluA2 levels and AMPA GluA1 phosphorylation suggesting that altered LTP post-seizures is a relevant target for antiepileptogenic therapies. In addition, this metaplasticity is also associated with marked alterations in classic and synaptic lipid raft markers, suggesting these may also constitute promising targets in epileptogenesis prevention.

Hippocampal CA1 theta burst-induced LTP from weaning to adulthood: Cellular and molecular mechanisms in young male rats revisited

Long-term potentiation (LTP) is a highly studied cellular process, yet determining the transduction and gamma aminobutyric acid (GABAergic) pathways that are the essential versus modulatory for LTP elicited by theta burst stimulation (TBS) in the hippocampal Cornu Ammonis 1 (CA1) area is still elusive, due to the use of different TBS intensities, patterns or different rodent/cellular models. We now characterised the developmental maturation and the transduction and GABAergic pathways required for mild TBS-induced LTP in hippocampal CA1 area in male rats. LTP induced by TBS (5x4) (five bursts of four pulses delivered at 100 Hz) lasted for up to 3 h and was increasingly larger from weaning to adulthood. Stronger TBS patterns - TBS (15x4) or three TBS (15x4) separated by 6 min induced nearly maximal LTP not being the best choice to study the value of LTP-enhancing drugs. LTP induced by TBS (5x4) in young adults was fully dependent on N-methyl D-aspartate (NMDA) receptor and calmodulin-dependent protein kinase II (CaMKII) activity but independent of protein kinase A (PKA) or protein kinase C (PKC) activity. Furthermore, it was partially dependent on GABA_B receptor activation and was potentiated by GABA_A receptor blockade and less by GAT-1 transporter blockade. AMPA GluA1 phosphorylation on Ser₈₃₁ (CaMKII target) but not GluA1 Ser₈₄₅ (PKA target) was essential for LTP expression. The phosphorylation of the Kv4.2 channel was observed at Ser₄₃₈ (CaMKII target) but not at Thr₆₀₂ or Thr₆₀₇ (ERK/MAPK pathway target). This suggests that cellular kinases like PKA, PKC, or kinases of the ERK/MAPK family although important modulators of TBS (5x4)-induced LTP may not be essential for its expression in the CA1 area of the hippocampus.

Determination of Diffusion Kinetics of Ketamine in Brain Tissue: Implications for in vitro Mechanistic Studies of Drug Actions

Ketamine has been in use for over 50 years as a general anesthetic, acting primarily through blockade of N-methyl-D-aspartate receptors in the brain. Recent studies have demonstrated that ketamine also acts as a potent and rapid-acting antidepressant when administered at sub-anesthetic doses. However, the precise mechanism behind this effect remains unclear. We examined the diffusion properties of ketamine in brain tissue to determine their effects in in vitro studies related to the antidepressant action of ketamine. Brain slices from adult mice were exposed to artificial cerebrospinal fluid (aCSF) containing ∼17 μM ketamine HCl for varying amounts of time. The amount of ketamine within each slice was then measured by tandem high-performance liquid chromatography - mass spectrometry to characterize the diffusion of ketamine into brain tissue over time. We successfully modeled the diffusion of ketamine into brain tissue using a mono-exponential function with a time constant of τ = 6.59 min. This curve was then compared to a one-dimensional model of diffusion yielding a diffusion coefficient of approximately 0.12 cm²⋅s^-1 for ketamine diffusing into brain tissue. The brain:aCSF partition coefficient for ketamine was determined to be approximately 2.76. Our results suggest that the diffusion properties of ketamine have a significant effect on drug concentrations achieved within brain tissue during in vitro experiments. This information is vital to determine the ketamine concentration necessary for in vitro slice preparation to accurately reflect in vivo doses responsible for its antidepressant actions.

Leveraging deep learning to control neural oscillators

Modulation of the firing times of neural oscillators has long been an important control objective, with applications including Parkinson's disease, Tourette's syndrome, epilepsy, and learning. One common goal for such modulation is desynchronization, wherein two or more oscillators are stimulated to transition from firing in phase with each other to firing out of phase. The optimization of such stimuli has been well studied, but this typically relies on either a reduction of the dimensionality of the system or complete knowledge of the parameters and state of the system. This limits the applicability of results to real problems in neural control. Here, we present a trained artificial neural network capable of accurately estimating the effects of square-wave stimuli on neurons using minimal output information from the neuron. We then apply the results of this network to solve several related control problems in desynchronization, including desynchronizing pairs of neurons and achieving clustered subpopulations of neurons in the presence of coupling and noise.

16p11.2 deletion is associated with hyperactivation of human iPSC-derived dopaminergic neuron networks and is rescued by RHOA inhibition in vitro

Reciprocal copy number variations (CNVs) of 16p11.2 are associated with a wide spectrum of neuropsychiatric and neurodevelopmental disorders. Here, we use human induced pluripotent stem cells (iPSCs)-derived dopaminergic (DA) neurons carrying CNVs of 16p11.2 duplication (16pdup) and 16p11.2 deletion (16pdel), engineered using CRISPR-Cas9. We show that 16pdel iPSC-derived DA neurons have increased soma size and synaptic marker expression compared to isogenic control lines, while 16pdup iPSC-derived DA neurons show deficits in neuronal differentiation and reduced synaptic marker expression. The 16pdel iPSC-derived DA neurons have impaired neurophysiological properties. The 16pdel iPSC-derived DA neuronal networks are hyperactive and have increased bursting in culture compared to controls. We also show that the expression of RHOA is increased in the 16pdel iPSC-derived DA neurons and that treatment with a specific RHOA-inhibitor, Rhosin, rescues the network activity of the 16pdel iPSC-derived DA neurons. Our data suggest that 16p11.2 deletion-associated iPSC-derived DA neuron hyperactivation can be rescued by RHOA inhibition.

Activity-dependent isomerization of Kv4.2 by Pin1 regulates cognitive flexibility

Voltage-gated K+ channels function in macromolecular complexes with accessory subunits to regulate brain function. Here, we describe a peptidyl-prolyl cis-trans isomerase NIMA-interacting 1 (Pin1)-dependent mechanism that regulates the association of the A-type K+ channel subunit Kv4.2 with its auxiliary subunit dipeptidyl peptidase 6 (DPP6), and thereby modulates neuronal excitability and cognitive flexibility. We show that activity-induced Kv4.2 phosphorylation triggers Pin1 binding to, and isomerization of, Kv4.2 at the pThr607-Pro motif, leading to the dissociation of the Kv4.2-DPP6 complex. We generated a novel mouse line harboring a knock-in Thr607 to Ala (Kv4.2TA) mutation that abolished dynamic Pin1 binding to Kv4.2. CA1 pyramidal neurons of the hippocampus from these mice exhibited altered Kv4.2-DPP6 interaction, increased A-type K+ current, and reduced neuronal excitability. Behaviorally, Kv4.2TA mice displayed normal initial learning but improved reversal learning in both Morris water maze and lever press paradigms. These findings reveal a Pin1-mediated mechanism regulating reversal learning and provide potential targets for the treatment of neuropsychiatric disorders characterized by cognitive inflexibility.

Phase model-based neuron stabilization into arbitrary clusters

Deep brain stimulation (DBS) is a common method of combating pathological conditions associated with Parkinson's disease, Tourette syndrome, essential tremor, and other disorders, but whose mechanisms are not fully understood. One hypothesis, supported experimentally, is that some symptoms of these disorders are associated with pathological synchronization of neurons in the basal ganglia and thalamus. For this reason, there has been interest in recent years in finding efficient ways to desynchronize neurons that are both fast-acting and low-power. Recent results on coordinated reset and periodically forced oscillators suggest that forming distinct clusters of neurons may prove to be more effective than achieving complete desynchronization, in particular by promoting plasticity effects that might persist after stimulation is turned off. Current proposed methods for achieving clustering frequently require either multiple input sources or precomputing the control signal. We propose here a control strategy for clustering, based on an analysis of the reduced phase model for a set of identical neurons, that allows for real-time, single-input control of a population of neurons with low-amplitude, low total energy signals. After demonstrating its effectiveness on phase models, we apply it to full state models to demonstrate its validity. We also discuss the effects of coupling on the efficacy of the strategy proposed and demonstrate that the clustering can still be accomplished in the presence of weak to moderate electrotonic coupling.

Down-Regulation of KV4 Channel in Drosophila Mushroom Body Neurons Contributes to Aβ42-Induced Courtship Memory Deficits

Accumulation of amyloid-β (Aβ) is widely believed to be an early event in the pathogenesis of Alzheimer's disease (AD). K_v4 is an A-type K⁺ channel, and our previous report shows the degradation of K_v4, induced by the Aβ42 accumulation, may be a critical contributor to the hyperexcitability of neurons in a Drosophila AD model. Here, we used well-established courtship memory assay to investigate the contribution of the K_v4 channel to short-term memory (STM) deficits in the Aβ42-expressing AD model. We found that Aβ42 over-expression in Drosophila leads to age-dependent courtship STM loss, which can be also induced by driving acute Aβ42 expression post-developmentally. Interestingly, mutants with eliminated K_v4-mediated A-type K⁺ currents (I_A) by transgenically expressing dominant-negative subunit (DNK_v4) phenocopied Aβ42 flies in defective courtship STM. K_v4 channels in mushroom body (MB) and projection neurons (PNs) were found to be required for courtship STM. Furthermore, the STM phenotypes can be rescued, at least partially, by restoration of K_v4 expression in Aβ42 flies, indicating the STM deficits could be partially caused by K_v4 degradation. In addition, I_A is significantly decreased in MB neurons (MBNs) but not in PNs, suggesting K_v4 degradation in MBNs, in particular, plays a critical role in courtship STM loss in Aβ42 flies. These data highlight causal relationship between region-specific K_v4 degradation and age-dependent learning decline in the AD model, and provide a mechanism for the disturbed cognitive function in AD.

Morvan's syndrome and the sustained absence of all sleep rhythms for months or years: An hypothesis

Despite the predation costs, sleep is ubiquitous in the animal realm. Humans spend a third of their life sleeping, and the quality of sleep has been related to co-morbidity, Alzheimer disease, etc. Excessive wakefulness induces rapid changes in cognitive performances, and it is claimed that one could die of sleep deprivation as quickly as by absence of water. In this context, the fact that a few people are able to go without sleep for months, even years, without displaying any cognitive troubles requires explanations. Theories ascribing sleep to memory consolidation are unable to explain such observations. It is not the case of the theory of sleep as the hebbian reinforcement of the inhibitory synapses (ToS-HRIS). Hebbian learning (Long Term Depression - LTD) guarantees that an efficient inhibitory synapse will lose its efficiency just because it is efficient at avoiding the activation of the post-synaptic neuron. This erosion of the inhibition is replenished by hebbian learning (Long Term Potentiation - LTP) when pre and post-synaptic neurons are active together - which is exactly what happens with the travelling depolarization waves of the slow-wave sleep (SWS). The best documented cases of months-long insomnia are reports of patients with Morvan's syndrome. This syndrome has an autoimmune cause that impedes - among many things - the potassium channels of the post-synaptic neurons, increasing LTP and decreasing LTD. We hypothesize that the absence of inhibitory efficiency erosion during wakefulness (thanks to a decrease of inhibitory LTD) is the cause for an absence of slow-wave sleep (SWS), which results also in the absence of REM sleep.

Deletion of novel protein TMEM35 alters stress-related functions and impairs long-term memory in mice

The mounting of appropriate emotional and neuroendocrine responses to environmental stressors critically depends on the hypothalamic-pituitary-adrenal (HPA) axis and associated limbic circuitry. Although its function is currently unknown, the highly evolutionarily conserved transmembrane protein 35 (TMEM35) is prominently expressed in HPA circuitry and limbic areas, including the hippocampus and amygdala. To investigate the possible involvement of this protein in neuroendocrine function, we generated tmem35 knockout (KO) mice to characterize the endocrine, behavioral, electrophysiological, and proteomic alterations caused by deletion of the tmem35 gene. While capable of mounting a normal corticosterone response to restraint stress, KO mice showed elevated basal corticosterone accompanied by increased anxiety-like behavior. The KO mice also displayed impairment of hippocampus-dependent fear and spatial memories. Given the intact memory acquisition but a deficit in memory retention in the KO mice, TMEM35 is likely required for long-term memory consolidation. This conclusion is further supported by a loss of long-term potentiation in the Schaffer collateral-CA1 pathway in the KO mice. To identify putative molecular pathways underlying alterations in plasticity, proteomic analysis of synaptosomal proteins revealed lower levels of postsynaptic molecules important for synaptic plasticity in the KO hippocampus, including PSD95 and N-methyl-d-aspartate receptors. Pathway analysis (Ingenuity Pathway Analysis) of differentially expressed synaptic proteins in tmem35 KO hippocampus implicated molecular networks associated with specific cellular and behavioral functions, including decreased long-term potentiation, and increased startle reactivity and locomotion. Collectively, these data suggest that TMEM35 is a novel factor required for normal activity of the HPA axis and limbic circuitry.

Update on the implication of potassium channels in autism: K(+) channelautism spectrum disorder

Autism spectrum disorders (ASDs) are characterized by impaired ability to properly implement environmental stimuli that are essential to achieve a state of social and cultural exchange. Indeed, the main features of ASD are impairments of interpersonal relationships, verbal and non-verbal communication and restricted and repetitive behaviors. These aspects are often accompanied by several comorbidities such as motor delay, praxis impairment, gait abnormalities, insomnia, and above all epilepsy. Genetic analyses of autistic individuals uncovered deleterious mutations in several K⁺ channel types strengthening the notion that their intrinsic dysfunction may play a central etiologic role in ASD. However, indirect implication of K⁺ channels in ASD has been also reported. For instance, loss of fragile X mental retardation protein (FMRP) results in K⁺ channels deregulation, network dysfunction and ASD-like cognitive and behavioral symptoms. This review provides an update on direct and indirect implications of K⁺ channels in ASDs. Owing to a mounting body of evidence associating a channelopathy pathogenesis to autism and showing that nearly 500 ion channel proteins are encoded by the human genome, we propose to classify ASDs - whose susceptibility is significantly enhanced by ion channels defects, either in a monogenic or multigenic condition - in a new category named “channelAutismSpectrumDisorder” (channelASD; cASD) and introduce a new taxonomy (e.g., K_vx.y-channelASD and likewise Na_vx.y-channelASD, Ca_vx.y-channelASD; etc.). This review also highlights some degree of clinical and genetic overlap between K⁺ channelASDs and K⁺ channelepsies, whereby such correlation suggests that a subcategory characterized by a channelASD-channelepsy phenotype may be distinguished. Ultimately, this overview aims to further understand the different clinical subgroups and help parse out the distinct biological basis of autism that are essential to establish patient-tailored treatments.

Distribution and functional expression of Kv4 family α subunits and associated KChIP β subunits in the bed nucleus of the stria terminalis

Regulation of BNSTALG neuronal firing activity is tightly regulated by the opposing actions of the fast outward potassium current, IA , mediated by α subunits of the Kv4 family of ion channels, and the transient inward calcium current, IT . Together, these channels play a critical role in regulating the latency to action potential onset, duration, and frequency, as well as dendritic back-propagation and synaptic plasticity. Previously we have shown that Type I-III BNSTALG neurons express mRNA transcripts for each of the Kv4 α subunits. However, the biophysical properties of native IA channels are critically dependent on the formation of macromolecular complexes of Kv4 channels with a family of chaperone proteins, the potassium channel-interacting proteins (KChIP1-4). Here we used a multidisciplinary approach to investigate the expression and function of Kv4 channels and KChIPs in neurons of the rat BNSTALG . Using immunofluorescence we demonstrated the pattern of localization of Kv4.2, Kv4.3, and KChIP1-4 proteins in the BNSTALG . Moreover, our single-cell reverse-transcription polymerase chain reaction (scRT-PCR) studies revealed that mRNA transcripts for Kv4.2, Kv4.3, and all four KChIPs were differentially expressed in Type I-III BNSTALG neurons. Furthermore, immunoelectron microscopy revealed that Kv4.2 and Kv4.3 channels were primarily localized to the dendrites and spines of BNSTALG neurons, and are thus ideally situated to modulate synaptic transmission. Consistent with this observation, in vitro patch clamp recordings showed that reducing postsynaptic IA in these neurons lowered the threshold for long-term potentiation (LTP) induction. These results are discussed in relation to potential modulation of IA channels by chronic stress.

Local plasticity of dendritic excitability can be autonomous of synaptic plasticity and regulated by activity-based phosphorylation of Kv4.2

While plasticity is typically associated with persistent modifications of synaptic strengths, recent studies indicated that modulations of dendritic excitability may form the other part of the engram and dynamically affect computational processing and output of neuronal circuits. However it remains unknown whether modulation of dendritic excitability is controlled by synaptic changes or whether it can be distinct from them. Here we report the first observation of the induction of a persistent plastic decrease in dendritic excitability decoupled from synaptic stimulation, which is localized and purely activity-based. In rats this local plasticity decrease is conferred by CamKII mediated phosphorylation of A-type potassium channels upon interaction of a back propagating action potential (bAP) with dendritic depolarization.

The Kvβ2 subunit of voltage-gated potassium channels is interacting with ProSAP2/Shank3 in the PSD

The postsynaptic density is an electron dense meshwork composed of a variety of molecules facilitating neuronal signal transmission. ProSAP2/Shank3 represents a crucial player at postsynaptic sites, assembling large multimeric platforms and anchoring numerous other molecules, thereby linking the functional synapse with the cytoskeleton. ProSAP2/Shank3 is also implicated in the pathogenesis of numerous diseases, including autism spectrum disorders. KvBeta2 (Kvβ2) on the other hand serves as a regulatory subunit of voltage-gated potassium channels. Kvβ2 is located at various sites in the neuron including the axon (binding to Kv1.2), the dendrites (binding to Kv4.2) and the synapse. Binding of Kvβ2 to either Kv1.2 or Kv4 modulates not only the channel conformation but directs targeting of the channel protein complex to distinct loci within the cell. Thus an interaction between ProSAP2 and Kvβ2 could have important roles at diverse cellular compartments and moreover during maturation stages. We report here on the direct protein-protein interaction of the postsynaptic density anchoring molecule ProSAP2 and the potassium channel subunit Kvβ2, initially identified in a yeast-two-hybrid-screen. Furthermore, we characterize this interaction at synapses using primary hippocampal neurons in vitro.

Iron deficiency with or without anemia impairs prepulse inhibition of the startle reflex

Iron deficiency (ID) during early life causes long-lasting detrimental cognitive sequelae, many of which are linked to alterations in hippocampus function, dopamine synthesis, and the modulation of dopaminergic circuitry by the hippocampus. These same features have been implicated in the origins of schizophrenia, a neuropsychiatric disorder with significant cognitive impairments. Deficits in sensorimotor gating represent a reliable endophenotype of schizophrenia that can be measured by prepulse inhibition (PPI) of the acoustic startle reflex. Using two rodent model systems, we investigated the influence of early-life ID on PPI in adulthood. To isolate the role of hippocampal iron in PPI, our mouse model utilized a timed (embryonic day 18.5), hippocampus-specific knockout of Slc11a2, a gene coding an important regulator of cellular iron uptake, the divalent metal transport type 1 protein (DMT-1). Our second model used a classic rat dietary-based global ID during gestation, a condition that closely mimics human gestational ID anemia (IDA). Both models exhibited impaired PPI in adulthood. Furthermore, our DMT-1 knockout model displayed reduced long-term potentiation (LTP) and elevated paired-pulse facilitation (PPF), electrophysiological results consistent with previous findings in the IDA rat model. These results, in combination with previous findings demonstrating impaired hippocampus functioning and altered dopaminergic and glutamatergic neurotransmission, suggest that iron availability within the hippocampus is critical for the neurodevelopmental processes underlying sensorimotor gating. Ultimately, evidence of reduced PPI in both of our models may offer insights into the roles of fetal ID and the hippocampus in the pathophysiology of schizophrenia.

Dynamic regulation of synaptic maturation state by voltage-gated A-type K+ channels in CA1 hippocampal pyramidal neurons

Neuronal activity is critical for the formation and modification of neural circuits during brain development. In hippocampal CA1 pyramidal dendrites, A-type voltage-gated K(+) currents, formed primarily by Kv4.2 subunits, control excitability. Here we used Kv4.2 knock-out (Kv4.2-KO) mice along with acute in vivo expression of Kv4.2 or its dominant-negative pore mutant to examine the role of Kv4.2 in the development of CA1 synapses. We found that Kv4.2 expression induces synaptic maturation in juvenile WT mice and rescues developmentally delayed synapses in adult Kv4.2-KO mice. In addition, we show that NMDAR subunit composition can be reverted back to the juvenile form in WT adult synapses by functionally downregulating Kv4.2 levels. These results suggest that Kv4.2 regulation of excitability determines synaptic maturation state, which can be bidirectionally adjusted into adulthood.

Quantitative interactions between the A-type K+ current and inositol trisphosphate receptors regulate intraneuronal Ca2+ waves and synaptic plasticity

The A-type potassium current has been implicated in the regulation of several physiological processes. Here, we explore a role for the A-type potassium current in regulating the release of calcium through inositol trisphosphate receptors (InsP3R) that reside on the endoplasmic reticulum (ER) of hippocampal pyramidal neurons. To do this, we constructed morphologically realistic, conductance-based models equipped with kinetic schemes that govern several calcium signalling modules and pathways, and constrained the distributions and properties of constitutive components by experimental measurements from these neurons. Employing these models, we establish a bell-shaped dependence of calcium release through InsP3Rs on the density of A-type potassium channels, during the propagation of an intraneuronal calcium wave initiated through established protocols. Exploring the sensitivities of calcium wave initiation and propagation to several underlying parameters, we found that ER calcium release critically depends on dendritic diameter and that wave initiation occurred at branch points as a consequence of a high surface area to volume ratio of oblique dendrites. Furthermore, analogous to the role of A-type potassium channels in regulating spike latency, we found that an increase in the density of A-type potassium channels led to increases in the latency and the temporal spread of a propagating calcium wave. Next, we incorporated kinetic models for the metabotropic glutamate receptor (mGluR) signalling components and a calcium-controlled plasticity rule into our model and demonstrate that the presence of mGluRs induced a leftward shift in a Bienenstock-Cooper-Munro-like synaptic plasticity profile. Finally, we show that the A-type potassium current could regulate the relative contribution of ER calcium to synaptic plasticity induced either through 900 pulses of various stimulus frequencies or through theta burst stimulation. Our results establish a novel form of interaction between active dendrites and the ER membrane, uncovering a powerful mechanism that could regulate biophysical/biochemical signal integration and steer the spatiotemporal spread of signalling microdomains through changes in dendritic excitability.

Kv4.2 potassium channels segregate to extrasynaptic domains and influence intrasynaptic NMDA receptor NR2B subunit expression

Neurons of the intercalated cell clusters (ITCs) represent an important relay site for information flow within amygdala nuclei. These neurons receive mainly glutamatergic inputs from the basolateral amygdala at their dendritic domains and provide feed-forward inhibition to the central nucleus. Voltage-gated potassium channels type-4.2 (Kv4.2) are main players in dendritic signal processing and integration providing a key component of the A currents. In this study, the subcellular localization and distribution of the Kv4.2 was studied in ITC neurons by means of light- and electron microscopy, and compared to other types of central principal neurons. Several ultrastructural immunolocalization techniques were applied including pre-embedding techniques and, most importantly, SDS-digested freeze-fracture replica labeling. We found Kv4.2 densely expressed in somato-dendritic domains of ITC neurons where they show a differential distribution pattern as revealed by nearest neighbor analysis. Comparing ITC neurons with hippocampal pyramidal and cerebellar granule cells, a cell type- and domain-dependent organization in Kv4.2 distribution was observed. Kv4.2 subunits were localized to extrasynaptic sites where they were found to influence intrasynaptic NMDA receptor subunit expression. In samples of Kv4.2 knockout mice, the frequency of NR1-positive synapses containing the NR2B subunit was significantly increased. This indicates a strong, yet indirect effect of Kv4.2 on the synaptic content of NMDA receptor subtypes, and a likely role in synaptic plasticity at ITC neurons.

Kv4 potassium channels modulate hippocampal EPSP-spike potentiation and spatial memory in rats

Kv4 channels regulate the backpropagation of action potentials (b-AP) and have been implicated in the modulation of long-term potentiation (LTP). Here we showed that blockade of Kv4 channels by the scorpion toxin AmmTX3 impaired reference memory in a radial maze task. In vivo, AmmTX3 intracerebroventricular (i.c.v.) infusion increased and stabilized the EPSP-spike (E-S) component of LTP in the dentate gyrus (DG), with no effect on basal transmission or short-term plasticity. This increase in E-S potentiation duration could result from the combination of an increase in excitability of DG granular cells with a reduction of GABAergic inhibition, leading to a strong reduction of input specificity. Radioactive in situ hybridization (ISH) was used to evaluate the amounts of Kv4.2 and Kv4.3 mRNA in brain structures at different stages of a spatial learning task in naive, pseudoconditioned, and conditioned rats. Significant differences in Kv4.2 and Kv4.3 mRNA levels were observed between conditioned and pseudoconditioned rats. Kv4.2 and Kv4.3 mRNA levels were transiently up-regulated in the striatum, nucleus accumbens, retrosplenial, and cingulate cortices during early stages of learning, suggesting an involvement in the switch from egocentric to allocentric strategies. Spatial learning performance was positively correlated with the levels of Kv4.2 and Kv4.3 mRNAs in several of these brain structures. Altogether our findings suggest that Kv4 channels could increase the signal-to-noise ratio during information acquisition, thereby allowing a better encoding of the memory trace.

Effects of genetic deletion of the Kv4.2 voltage-gated potassium channel on murine anxiety-, fear- and stress-related behaviors

Background

Potassium channels have been proposed to play a role in mechanisms of neural plasticity, and the Kv4.2 subunit has been implicated in the regulation of action-potential back-propagation to the dendrites. Alterations in mechanisms of plasticity have been further proposed to underlie various psychiatric disorders, but the role of Kv4.2 in anxiety or depression is not well understood.

Methods

In this paper, we analyzed the phenotype Kv4.2 knockout mice based on their neurological function, on a battery of behaviors including those related to anxiety and depression, and on plasticity-related learning tasks.

Results

We found a novelty-induced hyperactive phenotype in knockout mice, and these mice also displayed increased reactivity to novel stimulus such as an auditory tone. No clear anxiety- or depression-related phenotype was observed, nor any alterations in learning/plasticity-based paradigms.

Conclusions

We did not find clear evidence for an involvement of Kv4.2 in neuropsychiatric or plasticity-related phenotypes, but there was support for a role in Kv4.2 in dampening excitatory responses to novel stimuli.

DPP6 establishes the A-type K(+) current gradient critical for the regulation of dendritic excitability in CA1 hippocampal neurons

Subthreshold-activating A-type K(+) currents are essential for the proper functioning of the brain, where they act to delay excitation and regulate firing frequency. In CA1 hippocampal pyramidal neuron dendrites, the density of A-type K(+) current increases with distance from the soma, playing an important role in synaptic integration and plasticity. The mechanism underlying this gradient has, however, remained elusive. Here, dendritic recordings from mice lacking the Kv4 transmembrane auxiliary subunit DPP6 revealed that this protein is critical for generating the A-current gradient. Loss of DPP6 led to a decrease in A-type current, specifically in distal dendrites. Decreased current density was accompanied by a depolarizing shift in the voltage dependence of channel activation. Together these changes resulted in hyperexcitable dendrites with enhanced dendritic AP back-propagation, calcium electrogenesis, and induction of synaptic long-term potentiation. Despite enhanced dendritic excitability, firing behavior evoked by somatic current injection was mainly unaffected in DPP6-KO recordings, indicating compartmentalized regulation of neuronal excitability.