Linopirdine (DuP 996) improves performance in several tests of learning and memory by modulation of cholinergic neurotransmission

Structural Basis for the Modulation of Human KCNQ4 by Small-Molecule Drugs

Among the five KCNQ channels, also known as the K_v7 voltage-gated potassium (K_v) channels, KCNQ2-KCNQ5 control neuronal excitability. Dysfunctions of KCNQ2-KCNQ5 are associated with neurological disorders such as epilepsy, deafness, and neuropathic pain. Here, we report the cryoelectron microscopy (cryo-EM) structures of human KCNQ4 and its complexes with the opener retigabine or the blocker linopirdine at overall resolutions of 2.5, 3.1, and 3.3 Å, respectively. In all structures, a phosphatidylinositol 4,5-bisphosphate (PIP₂) molecule inserts its head group into a cavity within each voltage-sensing domain (VSD), revealing an unobserved binding mode for PIP₂. Retigabine nestles in each fenestration, inducing local shifts. Instead of staying within the central pore, linopirdine resides in a cytosolic cavity underneath the inner gate. Electrophysiological analyses of various mutants corroborated the structural observations. Our studies reveal the molecular basis for the modulatory mechanism of neuronal KCNQ channels and provide a framework for structure-facilitated drug discovery targeting these important channels.

The Role of Kv7 Channels in Neural Plasticity and Behavior

Activity-dependent persistent changes in neuronal intrinsic excitability and synaptic strength are widely thought to underlie learning and memory. Voltage-gated KCNQ/K_v7 potassium channels have been of great interest as the potential targets for memory disorders due to the beneficial effects of their antagonists in cognition. Importantly, de novo dominant mutations in their neuronal subunits KCNQ2/K_v7.2 and KCNQ3/K_v7.3 are associated with epilepsy and neurodevelopmental disorder characterized by developmental delay and intellectual disability. The role of K_v7 channels in neuronal excitability and epilepsy has been extensively studied. However, their functional significance in neural plasticity, learning, and memory remains largely unknown. Here, we review recent studies that support the emerging roles of K_v7 channels in intrinsic and synaptic plasticity, and their contributions to cognition and behavior.

In Vivo Attenuation of M-Current Suppression Impairs Consolidation of Object Recognition Memory

The M-current is a low voltage-activated potassium current generated by neuronal Kv7 channels. A prominent role of the M-current is to a create transient increase of neuronal excitability in response to neurotransmitters through the suppression of this current. Accordingly, M-current suppression is assumed to be involved in higher brain functions including learning and memory. However, there is little evidence supporting such a role to date. To address this gap, we examined behavioral tasks to assess learning and memory in homozygous Kv7.2 knock-in mice, Kv7.2(S559A), which show reduced M-current suppression while maintaining a normal basal M-current activity in neurons. We found that Kv7.2(S559A) mice had normal object location memory and contextual fear memory, but impaired long-term object recognition memory. Furthermore, short-term memory for object recognition was intact in Kv7.2(S559A) mice. The deficit in long-term object recognition memory was restored by the administration of a selective Kv7 channel inhibitor, XE991, when delivered during the memory consolidation phase. Lastly, c-Fos induction 2 h after training in Kv7.2(S559A) mice was normal in the hippocampus, which corresponds to intact object location memory, but was reduced in the perirhinal cortex, which corresponds to impaired long-term object recognition memory. Together, these results support the overall conclusion that M-current suppression is important for memory consolidation of specific types of memories.SIGNIFICANCE STATEMENT Dynamic regulation of neuronal excitation is a fundamental mechanism for information processing in the brain, which is mediated by changes in synaptic transmissions or by changes in ion channel activity. Some neurotransmitters can facilitate action potential firing by suppression of a low voltage-activated potassium current, M-current. We demonstrate that M-current suppression is critical for establishment of long-term object recognition memory, but is not required for establishment of hippocampus-dependent location memory or contextual memory. This study suggests that M-current suppression is important for stable encoding of specific types of memories.

One physical exercise session promotes recognition learning in rats with cognitive deficits related to amyloid beta neurotoxicity

Alzheimer's disease is a progressive neurodegenerative pathological process that causes memory loss and cognitive impairment. One of the pathological characteristics of Alzheimer's disease is the amyloid-β protein aggregation on the brain. The regular practice of physical exercise is a consolidated strategy on the prevention of cognitive deficits; however, little is known about the effects of acute exercise on memory. We hypothesize that one physical exercise session could act as a modulator of learning. Here we investigated the effects of one single session of running (aerobic) or strength (anaerobic) exercise on memory deficits related to neurotoxicity induced by amyloid-β. Male Wistar rats were submitted to stereotaxic surgery to intrahippocampal infusion of amyloid-β protein or saline (control). Ten days after the surgery the rats were submitted to the object recognition (OR) memory task. Immediately after the OR learning session, some rats were submitted to one treadmill running or strength exercise session. Then, the animals were submitted to memory tests 24 h, 7, and 14 days after the OR learning. We demonstrated that one physical exercise session, both aerobic as anaerobic, performed after learning improves learning and memory, promoting memory persistence in control rats and memory consolidation in rats submitted to amyloid-β neurotoxicity model. Notably, the effects of the aerobic exercise session seem to be more prominent, since they also reflect in an improvement of object discrimination index for 7 days in control animals. We verified that the mechanisms involved in the effects of aerobic exercise include the dopaminergic system activation. The mechanisms involved in the anaerobic exercise effects seem to be others since no alterations on hippocampal dopamine or noradrenaline levels were detected.

Recent Status of Nanomaterial Fabrication and Their Potential Applications in Neurological Disease Management

Nanomaterials (NMs) are receiving remarkable attention due to their unique properties and structure. They vary from atoms and molecules along with those of bulk materials. They can be engineered to act as drug delivery vehicles to cross blood-brain barriers (BBBs) and utilized with better efficacy and safety to deliver specific molecules into targeted cells as compared to conventional system for neurological disorders. Depending on their properties, various metal chelators, gold nanoparticles (NPs), micelles, quantum dots, polymeric NPs, liposomes, solid lipid NPs, microparticles, carbon nanotubes, and fullerenes have been utilized for various purposes including the improvement of drug delivery system, treatment response assessment, diagnosis at early stage, and management of neurological disorder by using neuro-engineering. BBB regulates micro- and macromolecule penetration/movement, thus protecting it from many kinds of illness. This phenomenon also prevents drug delivery for the neurological disorders such as Alzheimer's disease (AD), Parkinson's disease (PD), multiple sclerosis, amyotrophic lateral sclerosis, and primary brain tumors. For some neurological disorders (AD and PD), the environmental pollution was considered as a major cause, as observed that metal and/or metal oxide from different sources are inhaled and get deposited in the lungs/brain. Old age, obesity, diabetes, and cardiovascular disease are other factors for rapid deterioration of human health and onset of AD. In addition, gene mutations have also been examined to cause the early onset familial forms of AD. AD leads to cognitive impairment and plaque deposits in the brain leading to neuronal cell death. Based on these facts and considerations, this review elucidates the importance of frequently used metal chelators, NMs and/or NPs. The present review also discusses the current status and future challenges in terms of their application in drug delivery for neurological disease management.

XE991 and Linopirdine Are State-Dependent Inhibitors for Kv7/KCNQ Channels that Favor Activated Single Subunits

M-channel inhibitors, especially XE991, are being used increasingly in animal experiments; however, insufficient characterization of XE991 at times confounds the interpretation of results when using this compound. Here, we demonstrate that XE991 and linopirdine are state-dependent inhibitors that favor the activated-subunit of neuronal Kv7/KCNQ channels. We performed patch-clamp experiments on homomeric Kv7.2 or heteromeric Kv7.2/3 channels expressed in Chinese hamster ovary cells to characterize XE991 and linopirdine. Neither inhibitor was efficacious around the resting membrane potential of cells in physiologic conditions. Inhibition of Kv7.2 and Kv7.2/3 channels by XE991 was closely related with channel activation. When the voltage dependence of activation was left-shifted by retigabine or right-shifted by the mutation, Kv7.2(R214D), the shift in half-activation voltage proportionally coincided with the shift in the half-effective potential for XE991 inhibition. Inhibition kinetics during XE991 wash-in was facilitated at depolarized potentials. Ten-minute washout of XE991 resulted in ∼30% current recovery, most of which was attributed to surface transport of Kv7.2 channels. Linopirdine also exhibited similar inhibition characteristics, with the exception of near- complete current recovery after washout at depolarized potentials. Inhibition kinetics of both XE991 and linopirdine was not as sensitive to changes in voltage as would be predicted by open- channel inhibition. Instead, they were well explained by binding to a single activated subunit. The characteristics of XE991 and linopirdine should be taken into account when these M-channel inhibitors are used in experiments.

Potassium channel gene associations with joint processing speed and white matter impairments in schizophrenia

Patients with schizophrenia show decreased processing speed on neuropsychological testing and decreased white matter integrity as measured by diffusion tensor imaging, two traits shown to be both heritable and genetically associated indicating that there may be genes that influence both traits as well as schizophrenia disease risk. The potassium channel gene family is a reasonable candidate to harbor such a gene given the prominent role potassium channels play in the central nervous system in signal transduction, particularly in myelinated axons. We genotyped members of the large potassium channel gene family focusing on putatively functional single nucleotide polymorphisms (SNPs) in a population of 363 controls, 194 patients with schizophrenia spectrum disorder (SSD) and 28 patients with affective disorders with psychotic features who completed imaging and neuropsychological testing. We then performed three association analyses using three phenotypes - processing speed, whole-brain white matter fractional anisotropy (FA) and schizophrenia spectrum diagnosis. We extracted SNPs showing an association at a nominal P value of <0.05 with all three phenotypes in the expected direction: decreased processing speed, decreased FA and increased risk of SSD. A single SNP, rs8234, in the 3' untranslated region of voltage-gated potassium channel subfamily Q member 1 (KCNQ1) was identified. Rs8234 has been shown to affect KCNQ1 expression levels, and KCNQ1 levels have been shown to affect neuronal action potentials. This exploratory analysis provides preliminary data suggesting that KCNQ1 may contribute to the shared risk for diminished processing speed, diminished white mater integrity and increased risk of schizophrenia.

K+ channels as potential targets for the treatment of gastrointestinal motor disorders

K(+) channels play important functional roles in excitable cells, as neurons and muscle cells. The activation or inhibition of K(+) channels hyperpolarizes or depolarizes the cell membrane, respectively. These effects determine in the smooth muscle decrease or increase in Ca(2+) influx through voltage-gated Ca(2+) (CaV1.2) channels and relaxation or contraction, respectively. Recent studies highlight the importance of voltage-dependent type 7 K(+) (KV7 or KCNQ) channels in regulating muscle tone and contractility in stomach and colon. KV7 channels, that include 5 subtypes (KV7.1-7.5), are activated at relatively negative potential values, close to those of the resting membrane potential for the smooth muscle cells of some segments of the gastrointestinal tract. Thus, they contribute to set the resting membrane potential and their blockade induces increase in smooth muscle contractility in stomach and colon. In addition, KV7 channel activation produces profound relaxations of gastric and colonic smooth muscle. Therefore, KV7 channel activators could be used to relax the smooth muscle and relieve symptoms in diseases such as functional dyspepsia and irritable bowel syndrome with prevalent diarrhea. The discovery of activators selective for the channel subtypes present in the smooth muscle, mainly KV7.4 and 7.5, would allow avoiding adverse cardiac and nervous system effects. A further step forward would be characterizing putative differences among the KV7 channel subtypes expressed in the various smooth muscles and synthesizing molecules specific for the gastrointestinal smooth muscle.

Kv7 channels as targets for anti-epileptic and psychiatric drug-development

The Kv7 channels, a family of voltage-dependent K(+) channels (Kv7.1-Kv7.5), have gained much attention in drug discovery especially because four members are genetically linked to diseases. For disorders of the CNS focus was originally on epilepsy and pain, but it is becoming increasingly evident that Kv7 channels can also be valid targets for psychiatric disorders, such as anxiety and mania. The common denominator is probably neuronal hyperexcitability in different brain areas, which can be successfully attenuated by pharmacological increment of Kv7 channel activity. This perspective attempts to review the current status and challenges for CNS drug discovery based on Kv7 channels as targets for neurological and psychiatric indications with special focus on selectivity and mode-of-actions.

Major channels involved in neuropsychiatric disorders and therapeutic perspectives

Voltage-gated ion channels are important mediators of physiological functions in the central nervous system. The cyclic activation of these channels influences neurotransmitter release, neuron excitability, gene transcription, and plasticity, providing distinct brain areas with unique physiological and pharmacological response. A growing body of data has implicated ion channels in the susceptibility or pathogenesis of psychiatric diseases. Indeed, population studies support the association of polymorphisms in calcium and potassium channels with the genetic risk for bipolar disorders (BPDs) or schizophrenia. Moreover, point mutations in calcium, sodium, and potassium channel genes have been identified in some childhood developmental disorders. Finally, antibodies against potassium channel complexes occur in a series of autoimmune psychiatric diseases. Here we report recent studies assessing the role of calcium, sodium, and potassium channels in BPD, schizophrenia, and autism spectrum disorders, and briefly summarize promising pharmacological strategies targeted on ion channels for the therapy of mental illness and related genetic tests.

The Voltage-Sensing Domain of K(v)7.2 Channels as a Molecular Target for Epilepsy-Causing Mutations and Anticonvulsants

Understanding the molecular mechanisms underlying voltage-dependent gating in voltage-gated ion channels (VGICs) has been a major effort over the last decades. In recent years, changes in the gating process have emerged as common denominators for several genetically determined channelopathies affecting heart rhythm (arrhythmias), neuronal excitability (epilepsy, pain), or skeletal muscle contraction (periodic paralysis). Moreover, gating changes appear as the main molecular mechanism by which several natural toxins from a variety of species affect ion channel function. In this work, we describe the pathophysiological and pharmacological relevance of the gating process in voltage-gated K⁺ channels encoded by the K_v7 gene family. After reviewing the current knowledge on the molecular mechanisms and on the structural models of voltage-dependent gating in VGICs, we describe the physiological relevance of these channels, with particular emphasis on those formed by K_v7.2–K_v7.5 subunits having a well-established role in controlling neuronal excitability in humans. In fact, genetically determined alterations in K_v7.2 and K_v7.3 genes are responsible for benign familial neonatal convulsions, a rare seizure disorder affecting newborns, and the pharmacological activation of K_v7.2/3 channels can exert antiepileptic activity in humans. Both mutation-triggered channel dysfunction and drug-induced channel activation can occur by impeding or facilitating, respectively, channel sensitivity to membrane voltage and can affect overlapping molecular sites within the voltage-sensing domain of these channels. Thus, understanding the molecular steps involved in voltage-sensing in K_v7 channels will allow to better define the pathogenesis of rare human epilepsy, and to design innovative pharmacological strategies for the treatment of epilepsies and, possibly, other human diseases characterized by neuronal hyperexcitability.

Neurotherapeutic applications of nanoparticles in Alzheimer's disease

A rapid increase in incidence of neurodegenerative disorders has been observed with the aging of the population. Alzheimer's disease (AD) is the most common neurodegenerative disorder among the elderly. It is characterized by memory dysfunction, loss of lexical access, spatial and temporal disorientation and impairment of judgement clinically. Unfortunately, clinical development of drugs for the symptomatic and disease-modifying treatment of AD has resulted in both promise and disappointment. Indeed, a large number of drugs with differing targets and mechanisms of action were investigated with only a few of them being clinically available. The targeted drug delivery to the central nervous system (CNS), for the diagnosis and treatment of neurodegenerative disorders such as AD, is restricted due to the limitations posed by the blood-brain barrier (BBB) as well as due to opsonization by plasma proteins in the systemic circulation and peripheral side-effects. Over the last decade, nanoparticle-mediated drug delivery represents one promising strategy to successfully increase the CNS penetration of several therapeutic moieties. Different nanocarriers are being investigated to treat and diagnose AD by delivering at a constant rate a host of therapeutics over times extending up to days, weeks or even months. This review provides a concise incursion on the current pharmacotherapies for AD besides reviewing and discussing the literature on the different drug molecules that have been successfully encapsulated in nanoparticles (NPs). Some of them have been shown to cross the BBB and have been tested either for diagnosis or treatment of AD. Finally, the route of NPs administration and the future prospects will be discussed.

Pharmacological modulation of voltage-gated potassium channels as a therapeutic strategy

IMPORTANCE OF THE FIELD

The human genome encodes at least 40 distinct voltage-gated potassium channel subtypes, which vary in regional expression, pharmacological and biophysical properties. Voltage-dependent potassium (Kv) channels help orchestrate many of the physiological and pathophysiological processes that promote and sometimes hinder the healthy functioning of our bodies.

AREAS COVERED IN THIS REVIEW

This review summarizes patent and scientific literature reports from the past decade highlighting the opportunities that Kv channels offer for the development of new therapeutic interventions for a wide variety of disorders.

WHAT THE READER WILL GAIN

The reader will gain an insight from an analysis of the associations of different Kv family members with disease processes, summary and evaluation of the development of therapeutically relevant pharmacological modulators of these channels, particularly focusing on proprietary agents being developed.

TAKE HOME MESSAGE

Development of new drugs that target Kv channels continue to be of great interest but is proving to be challenging. Nevertheless, opportunities for Kv channel modulators to have an impact on a wide range of disorders in the future remain an exciting prospect.

Voltage-gated potassium channels as therapeutic targets

The human genome encodes 40 voltage-gated K(+) channels (K(V)), which are involved in diverse physiological processes ranging from repolarization of neuronal and cardiac action potentials, to regulating Ca(2+) signalling and cell volume, to driving cellular proliferation and migration. K(V) channels offer tremendous opportunities for the development of new drugs to treat cancer, autoimmune diseases and metabolic, neurological and cardiovascular disorders. This Review discusses pharmacological strategies for targeting K(V) channels with venom peptides, antibodies and small molecules, and highlights recent progress in the preclinical and clinical development of drugs targeting the K(V)1 subfamily, the K(V)7 subfamily (also known as KCNQ), K(V)10.1 (also known as EAG1 and KCNH1) and K(V)11.1 (also known as HERG and KCNH2) channels.

Disruption of the ether-a-go-go K+ channel gene BEC1/KCNH3 enhances cognitive function

The K+ channel, one of the determinants for neuronal excitability, is genetically heterogeneous, and various K+ channel genes are expressed in the CNS. The therapeutic potential of K+ channel blockers for cognitive enhancement has been discussed, but the contribution each K+ channel gene makes to cognitive function remains obscure. BEC1 (KCNH3) is a member of the K+ channel superfamily that shows forebrain-preferential distribution. Here, we show the critical involvement of BEC1 in cognitive function. BEC1 knock-out mice performed behavioral tasks related to working memory, reference memory, and attention better than their wild-type littermates. Enhanced performance was also observed in heterozygous mutants. The knock-out mice had neither the seizures nor the motor dysfunction that are often observed in K+ channel-deficient mice. In contrast to when it is disrupted, overexpression of BEC1 in the forebrain caused the impaired performance of those tasks. It was also found that altering BEC1 expression could change hippocampal neuronal excitability and synaptic plasticity. The results indicate that BEC1 may represent the first K+ channel that contributes preferentially and bidirectionally to cognitive function.

Receptor-mediated suppression of potassium currents requires colocalization within lipid rafts

Expression of KCNQ2/3 (Kv7.2 and -7.3) heteromers underlies the neuronal M current, a current that is suppressed by activation of a variety of receptors that couple to the hydrolysis of phosphatidylinositol 4,5-bisphosphate. Expression of Kv7.2/7.3 channels in human embryonic kidney (HEK) 293 cells produced a noninactivating potassium current characteristic of M current. Muscarinic receptors endogenous to HEK293 cells were identified as being M3 by pharmacology and Western blotting, producing a rise of intracellular calcium ([Ca2+](i)) upon activation. Activation of these endogenous muscarinic receptors however, failed to suppress expressed Kv7.2/7.3 current. Current suppression was reconstituted by coexpression of HA-tagged muscarinic m1 or m3 receptors. Examination of membrane fractions showed that both expressed receptors and Kv7.2 and -7.3 channel subunits resided within lipid rafts. Disruption of lipid rafts by pretreatment of cells expressing either m1 or m3 muscarinic receptors with methyl-beta-cyclodextrin produced a loss of localization of proteins within lipid raft membrane fractions. This pretreatment also abolished both the increase of [Ca2+](i) and suppression of expressed Kv7.2/7.3 current evoked by activation of expressed m1 or m3 muscarinic receptors. A similar loss of muscarinic receptor-mediated suppression of M current native to rat dorsal root ganglion neurons was observed after incubating dissociated cells with methyl-beta-cyclodextrin. These data suggested that lipid rafts colocalized both muscarinic receptors and channel subunits to enable receptor-mediated suppression of channel activity, a spatial colocalization that enables specificity of coupling between receptor and ion channel.

Decreased acetylcholine release is correlated to memory impairment in the Tg2576 transgenic mouse model of Alzheimer's disease

Acetylcholine (ACh) release is one of the key factors in memory mechanisms. To clarify whether beta-amyloid (Abeta) induces a disturbance of the cholinergic system leading to memory impairment, we examined memory impairment and measured hippocampal ACh release in Tg2576 (Tg) mice that over-express the Swedish mutant amyloid precursor protein (APPsw). Furthermore, we examined Abeta burden with aging. Tg mice aged 9-11 months, but not aged 4-6 months, showed memory impairment in the 8-arm radial maze behavior test. Spontaneous ACh release was not altered in Tg mice compared with age-matched control mice at 4-6 or 9-11 months of age. On the other hand, high-K(+)-evoked ACh release was decreased in Tg mice aged 9-11 months, but not in Tg mice aged 4-6 months. Hippocampal Abeta increased in an age-dependent manner, but evident amyloid plaques were not found in the hippocampus of Tg mice aged 11 months. These results suggest that memory impairment in Tg mice could be attributed to cholinergic synapse dysfunction that could not be caused predominantly by amyloid plaques. Measuring ACh release in this model might be a useful index for the screening of new drugs to treat the early-phase of Alzheimer's disease.

In vivo profile of ICA-27243 [N-(6-chloro-pyridin-3-yl)-3,4-difluoro-benzamide], a potent and selective KCNQ2/Q3 (Kv7.2/Kv7.3) activator in rodent anticonvulsant models

Openers or activators of neuronal KCNQ2/Q3 potassium channels decrease neuronal excitability and may provide benefit in the treatment of disorders of neuronal excitability such as epilepsy. In the present study, we evaluate the effects of ICA-27243 [N-(6-chloro-pyridin-3-yl)-3,4-difluoro-benzamide], an orally bioavailable, potent, and selective KCNQ2/Q3 opener, in a broad range of rodent seizure models. ICA-27243 was effective against maximal electroshock (MES) and pentylenetetrazole (PTZ)-induced seizures in both rats (MES, ED(50) = 1.5 mg/kg p.o.; PTZ, ED(50) = 2.2 mg/kg p.o.) and mice (MES, ED(50) = 8.6 mg/kg p.o.; PTZ, ED(50) = 3.9 mg/kg p.o.) in the rat amygdala kindling model of partial seizures (full protection from seizure at 9 mg/kg p.o.) and in the 6-Hz model of psychomotor seizures in mice (active at 10 mg/kg i.p.). Antiseizure efficacy in all models was observed at doses significantly less than those shown to effect open-field locomotor activity (rat ED(50) = 40 mg/kg p.o.) or ability to remain on a Rotorod (no effect in rat at doses up to 100 mg/kg p.o.). There was no evidence of cognition impairment as measured in the Morris water maze in the rat (10 and 30 mg/kg p.o.), nor was there evidence of the development of tolerance after multiple doses of ICA-27243. Our findings suggest that selective KCNQ2/Q3 opening activity in the absence of effects on KCNQ3/Q5 or GABA-activated channels may be sufficient for broad-spectrum antiepileptic activity in rodents.

Regulation of the voltage-gated K+ channels KCNQ2/3 and KCNQ3/5 by serum- and glucocorticoid-regulated kinase-1

The voltage-gated KCNQ2/3 and KCNQ3/5 K+ channels regulate neuronal excitability. We recently showed that KCNQ2/3 and KCNQ3/5 channels are regulated by the ubiquitin ligase Nedd4-2. Serum- and glucocorticoid-regulated kinase-1 (SGK-1) plays an important role in regulation of epithelial ion transport. SGK-1 phosphorylation of Nedd4-2 decreases the ability of Nedd4-2 to ubiquitinate the epithelial Na+ channel, which increases the abundance of channel protein in the cell membrane. In this study, we investigated the mechanism(s) of SGK-1 regulation of M-type KCNQ channels expressed in Xenopus oocytes. SGK-1 significantly upregulated the K+ current amplitudes of KCNQ2/3 and KCNQ3/5 channels ∼1.4- and ∼1.7-fold, respectively, whereas the kinase-inactive SGK-1 mutant had no effect. The cell surface levels of KCNQ2-hemagglutinin/3 were also increased by SGK-1. Deletion of the KCNQ3 channel COOH terminus in the presence of SGK-1 did not affect the K+ current amplitude of KCNQ2/3/5-mediated currents. Coexpression of Nedd4-2 and SGK-1 with KCNQ2/3 or KCNQ3/5 channels did not significantly alter K+ current amplitudes. Only the Nedd4-2 mutant S448ANedd4-2 exhibited a significant downregulation of the KCNQ2/3/5 K+ current amplitudes. Taken together, these results demonstrate a potential mechanism for regulation of KCNQ2/3 and KCNQ3/5 channels by SGK-1 regulation of the activity of the ubiquitin ligase Nedd4-2.

Polarized axonal surface expression of neuronal KCNQ channels is mediated by multiple signals in the KCNQ2 and KCNQ3 C-terminal domains

The M channels, important regulators of neuronal excitability, are voltage-gated potassium channels composed of KCNQ2-5 subunits. Mutations in KCNQ2 and KCNQ3 cause benign familial neonatal convulsions (BFNC), dominantly inherited epilepsy and myokymia. Crucial for their functions in controlling neuronal excitability, the M channels must be placed at specific regions of the neuronal membrane. However, the precise distribution of surface KCNQ channels is not known. Here, we show that KCNQ2/KCNQ3 channels are preferentially localized to the surface of axons both at the axonal initial segment and more distally. Whereas axonal initial segment targeting of surface KCNQ channels is mediated by ankyrin-G binding motifs of KCNQ2 and KCNQ3, sequences mediating targeting to more distal portion of the axon reside in the membrane proximal and A domains of the KCNQ2 C-terminal tail. We further show that several BFNC mutations of KCNQ2 and KCNQ3 disrupt surface expression or polarized surface distribution of KCNQ channels, thereby revealing impaired targeting of KCNQ channels to axonal surfaces as a BFNC etiology.

Axo-somatic and apical dendritic Kv7/M channels differentially regulate the intrinsic excitability of adult rat CA1 pyramidal cells

Kv7/KCNQ/M channel subunits are widely expressed in peripheral and central neurons, where they give rise to a muscarinic-sensitive, subthreshold, and noninactivating K+ current (M current). Immunohistochemical data suggest that Kv7/M channels are expressed in both axons, somata and dendrites, but their distinctive roles in these compartments are not known. Here we used intracellular microelectrode recordings to monitor the effects of selective Kv7/M channel modulators focally applied to the axo-somatic region and to the apical dendrites of adult rat CA1 pyramidal cells. We show that both compartments express functional Kv7/M channels that synergistically control intrinsic neuronal excitability, albeit in different ways. Axo-somatic Kv7/M channels activate during the spike afterdepolarization (ADP) and counteract the depolarizing drive furnished by conjointly activated persistent Na+ channels. Thereby they limit the size and duration of the spike ADP and prevent its escalation into a somatic spike burst. Apical dendritic Kv7/M channels do not ordinarily regulate the somatic spike ADP and spike output. In hyperexcitable conditions that promote Ca2+ electrogenesis in these dendrites, they elevate the threshold for initiating Ca2+ spikes and associated downstream spike bursts. Thus the concerted activity of Kv7/M channels in both compartments serves to reduce the propensity to generate self-sustained burst responses and fosters a regular, stimulus-graded spike output of the neuron. Given that the activity of Kv7/M channels is regulated by multiple neurotransmitters, they may provide a substrate for neuromodulation of neuronal input/output relations at both the axo-somatic and apical dendritic regions.

KCNQ/Kv7 channel regulation of hippocampal gamma-frequency firing in the absence of synaptic transmission

Synchronous neuronal firing can be induced in hippocampal slices in the absence of synaptic transmission by lowering extracellular Ca2+ and raising extracellular K+. However, the ionic mechanisms underlying this nonsynaptic synchronous firing are not well understood. In this study we have investigated the role of KCNQ/Kv7 channels in regulating this form of nonsynaptic bursting activity. Incubation of rat hippocampal slices in reduced (<0.2 mM) [Ca2+]o and increased (6.3 mM) [K+]o, blocked synaptic transmission, increased neuronal firing, and led to the development of spontaneous periodic nonsynaptic epileptiform activity. This activity was recorded extracellularly as large (4.7 +/- 1.9 mV) depolarizing envelopes with superimposed high-frequency synchronous population spikes. These intraburst population spikes initially occurred at a high frequency (about 120 Hz), which decayed throughout the burst stabilizing in the gamma-frequency band (30-80 Hz). Further increasing [K+]o resulted in an increase in the interburst frequency without altering the intraburst population spike frequency. Application of retigabine (10 microM), a Kv7 channel modulator, completely abolished the bursts, in an XE-991-sensitive manner. Furthermore, application of the Kv7 channel blockers, linopirdine (10 microM) or XE-991 (10 microM) alone, abolished the gamma frequency, but not the higher-frequency population spike firing observed during low Ca2+/high K+ bursts. These data suggest that Kv7 channels are likely to play a role in the regulation of synchronous population firing activity.

Sound sequence discrimination learning is dependent on cholinergic inputs to the rat auditory cortex

In rat auditory cortex (AC) slices, synaptic potentiation following heterosynaptic stimulation is affected by the stimulus sequence used for induction. It was hypothesized that this sequence-dependent plasticity might be partly involved in the cellular mechanisms underlying sound sequence discrimination. Sequence dependence is abolished by muscarinic receptor antagonists. Therefore, dependence of sound sequence discrimination learning on cholinergic inputs to the rat AC was investigated. Rats were trained to discriminate the sequences of two sound components and a licking behavior in response to one of two possible sequences was rewarded with water. Atropine, a muscarinic receptor antagonist, attenuated sound sequence discrimination learning. The acquired sound sequence discrimination was not affected by atropine. Injections of the cholinergic immunotoxin 192IgG-saporin into the AC suppressed sound sequence discrimination learning, while discrimination between the two sound components was not affected. An inhibitor of M-current, linopirdine, restores the sequence dependence of synaptic potentiation in the AC slices suppressed by atropine. In this study, sound sequence discrimination learning attenuated by 192IgG-saporin was also restored by linopirdine. These similarities between sequence dependent plasticity in the AC slices and sound sequence discrimination learning support the hypothesis that the former is involved in the cellular mechanisms underlying the latter.

KCNQ/M channels control spike afterdepolarization and burst generation in hippocampal neurons

KCNQ channel subunits are widely expressed in peripheral and central neurons, where they give rise to a muscarinic-sensitive, subthreshold, and noninactivating K+ current (M-current). It is generally agreed that activation of KCNQ/M channels contributes to spike frequency adaptation during sustained depolarizations but is too slow to influence the repolarization of solitary spikes. This concept, however, is based mainly on experiments with muscarinic agonists, the multiple effects on membrane conductances of which may overshadow the distinctive effects of KCNQ/M channel block. Here, we have used selective modulators of KCNQ/M channels to investigate their role in spike electrogenesis in CA1 pyramidal cells. Solitary spikes were evoked by brief depolarizing current pulses injected into the neurons. The KCNQ/M channel blockers linopirdine and XE991 markedly enhanced the spike afterdepolarization (ADP) and, in most neurons, converted solitary ("simple") spikes to high-frequency bursts of three to seven spikes ("complex" spikes). Conversely, the KCNQ/M channel opener retigabine reduced the spike ADP and induced regular firing in bursting neurons. Selective block of BK or SK channels had no effect on the spike ADP or firing mode in these neurons. We conclude that KCNQ/M channels activate during the spike ADP and limit its duration, thereby precluding its escalation to a burst. Consequently, down-modulation of KCNQ/M channels converts the neuronal firing pattern from simple to complex spiking, whereas up-modulation of these channels exerts the opposite effect.

Effects of histamine and cholinergic systems on memory retention of passive avoidance learning in rats

In the present study, the effects of the histamine and cholinergic systems on memory retention in adult male rats were investigated. Post-training intracerebroventricular injections were carried out in all the experiments. Cholinoceptor agonist, acetylcholine (1-10 microg/rat) or nicotine (1-10 microg/rat), increased, while a cholinoceptor antagonist, scopolamine (5-20 microg/rat), decreased memory retention. The response to acetylcholine was attenuated by scopolamine. Administration of histamine (5-20 microg/rat) reduced, but the histamine H(1) receptor antagonist, pyrilamine (10-50 microg/rat), and the histamine H(2) receptor antagonist, cimetidine (1-50 microg/rat), increased memory retention in rats. The histamine receptor antagonists attenuated the response to histamine. Histamine reduced the acetylcholine- or nicotine-induced enhancement. The histamine receptor antagonists enhanced the nicotine- or acetylcholine-induced response. Histamine potentiated the inhibitory effect induced by scopolamine. It is concluded that histaminergic and cholinergic systems have opposing effects on memory retention. Also, the histaminergic system elicits an interaction with the cholinergic system in memory retention.

Sequence dependence of post-tetanic potentiation after sequential heterosynaptic stimulation in the rat auditory cortex

1. To investigate the mechanisms for the coding stimulus sequence in the auditory cortex (AC), post-tetanic potentiation (PTP) was recorded after sequentially combined heterosynaptic stimulation was applied in rat AC slices. 2. Brief tetanic stimulation (TS) was applied at two sites on AC slices at intervals of 0.5-10 s. PTP of field potentials was induced by the earlier TS, rather than the later TS. PTP was followed by sequence-dependent long-term potentiation (LTP). 3. Using Ca(2+) imaging in the slices loaded with rhod-2, a Ca(2+) indicator, a sequence-dependent distribution of PTP was found in AC slices. 4. The sequence-dependent PTP in excitatory postsynaptic potentials (EPSPs) was observed in supragranular pyramidal neurons. 5. The sequence dependence of PTP was not significantly affected by 1 microM bicuculline, an antagonist of GABA(A) receptors, or 100 microM 2-hydroxysaclofen, an antagonist of GABA(B) receptors. 6. Depolarization and firing recorded in pyramidal neurons during the later TS were less vigorous than when the slices were incubated in the control medium. However, this suppression of the responses during the later TS was not observed in the presence of 50 microM atropine, an antagonist of muscarinic receptors. 7. PTP was induced by the earlier and later TS in the presence of 50 microM atropine, so that the sequence dependence of PTP was abolished. Pirenzepine (50 microM), an antagonist of muscarinic M1 receptors, but not methoctramine (30 microM), an antagonist of M2 receptors, eliminated the sequence dependence of PTP. 8. These findings suggest that the sequence dependence of PTP in AC might have a role in the temporal processing of auditory information on the scale of seconds.

The acetylcholine release enhancer linopirdine induces Fos in neocortex of aged rats

Centrally acting cholinergic agents induce the immediate early gene c-fos in the rat brain resulting in transient increases of Fos protein, most notably in the cerebral cortex. In this study we have monitored by Fos immunohistochemistry the effect of the acetylcholine release enhancer linopirdine (DUP996) on the immediate early gene c-fos in brains of 3 months and 30 months old rats. In young rats linopirdine had only a marginal effect on Fos expression. In contrast, in aged rats linopirdine caused widespread expression of Fos throughout neocortex. In somatosensory cortex, the induction of the c-fos gene by linopirdine was nearly completely blocked by atropine and scopolamine and strongly attenuated by the NMDA receptor blockers CPP and MK-801. The results suggest that the age-related decline in acetylcholine release in rodents can be partially compensated for by administration of linopirdine.

Influence of cholinergic system on motor learning during aging in mice

Three, 12- and 20-month-old C57Bl6 mice, reared in standard conditions or in an enriched environment, were administered subcutaneously either scopolamine hydrobromide (SIGMA), 0.6 and 1.2 mg kg(-1), or physiological saline 15 min before testing their motor skills (muscular strength, dynamic equilibrium and motor coordination) and motor learning abilities (number of trials needed to reach a learning criterion on a rotorod rotating at 27 revolutions per min). The results demonstrated a lack of correlation between motor skill scores and between motor skill and motor learning scores, suggesting that the rotorod training procedure measures motor learning and not motor skills or is insensitive to changes in motor skills. They also demonstrated that motor skills decreased with age but were insensitive to environmental rearing and to scopolamine. In contrast, the learning scores, which also decreased with age, were very sensitive to scopolamine, particularly in the oldest mice. These results are discussed according to the role of cholinergic system in motor learning during aging.

Cognitive impairment in old rats: a comparison of object displacement, object recognition and water maze

The behavioral performance of young and aged rats was studied in a repeated-trials test. Young animals reacted to both spatial displacement and novelty, whereas most aged rats lost the ability to react to novelty although maintaining spatial memory. The cluster analysis procedure performed on all the tested subjects enabled the recognition of a consistent group of the aged sample (35%) with a mild degree of spatial and non-spatial memory impairment. Spatial memory impairment of some of the aged animals was also evaluated in the Morris water maze test. On the fifth day of the task, we observed a very low percentage of impaired aged animals, which partially corresponded to the impaired group identified by the object recognition test. In contrast, the subgroup of mildly impaired rats performed similarly to the young animals. We advance that the Morris water maze might represent a stressful experimental condition for aged rats, enhancing the motivational level of animals subjected to this procedure. This condition may alter the cognitive responses. As a consequence, the "mildly impaired" rats, which may be considered an interesting group for investigating memory-enhancing drugs, will infrequently be recognized with the Morris water maze test. Cognitive impairment in aged rats should be studied utilizing a sensitive test in which motivation does not substantially influence the results of the test.

Linopirdine (DUP 996): cholinergic treatment of older adults using successive and non-successive tests

The purpose of this study was to examine whether cholinergic treatment of age-associated memory impairment with Linopirdine (DUP 996), a derivate of phenylindoline, affects explicit memory, implicit memory, and primary memory. We also assessed cognitive decision making in a reaction time test. Explicit memory was assessed by face recognition, word recall and a word recognition test, being part of a successive test paradigm. Implicit memory was assessed by primed word fragment completion in the same successive test paradigm. Primary memory was studied by means of digit recall. Thirty-eight elderly subjects fulfilled the criteria for memory impairment. Four groups of subjects were given 10, 20 or 30 mg of DUP 996 or placebo during 4 weeks. A double-blind procedure was applied. No significant treatment effects for recognition memory and priming were obtained in the successive test paradigm. Analysis of dependence/independence between tests did not show any clear pattern of treatment effects. The other explicit memory tests and the reaction time test showed no effect with DUP 996. Because of the range of the different tests used here, the result and the general evidence in other investigations of the cholinergic depletion among aged people, the conclusion is that DUP 996 does not improve memory performance either in explicit, implicit or primary tests.

Influence of acetylcholine on binding of 4-[125I]iododexetimide to muscarinic brain receptors

The distribution of nicotinic and muscarinic cholinergic receptors in the human brain in vivo has been successfully characterized using radiolabeled tracers and emission tomography. The effect of acetylcholine release into the synaptic cleft on receptor binding of these tracers has not yet been investigated. The present study examined the influence of acetylcholine on binding of 4-[125I]iododexetimide to muscarinic cholinergic receptors of porcine brain synaptosomes in vitro. 4-Iododexetimide is a subtype-unspecific muscarinic receptor antagonist with high affinity. Acetylcholine competed with 4-[125I]iododexetimide in a dose-dependent manner. A concentration of 500 microM acetylcholine inhibited 50% of total specific 4-[125I]iododexetimide binding to synaptosomes when both substances were given simultaneously. An 800 microM acetylcholine solution reduced total specific 4-[125I]iododexetimide binding by about 35%, when acetylcholine was given 60 min after incubation of synaptosomes with 4-[125I]iododexetimide. Variations in the synaptic acetylcholine concentration might influence muscarinic cholinergic receptor imaging in vivo using 4-[123I]iododexetimide. Conversely, 4-[123I]iododexetimide might be an appropriate molecule to investigate alterations of acetylcholine release into the synaptic cleft in vivo using single photon emission computed tomography.

Linopirdine reduces stimulus intensity threshold for induction of long-term potentiation in the Schaffer collateral/CA1 pathway in rat hippocampal slices

Linopirdine, a putative cognition enhancing agent, increases neurotransmitter release and blocks M-current in rat brain. Its effects on long-term potentiation (LTP) in the Schaffer collateral/CA1 pathway were investigated using standard, extracellular recording techniques in rat hippocampal slice preparation. When using a half maximal stimulus intensity for tetanic stimulation, a 30 min exposure to 3 or 10 microM linopirdine exerted no significant effect on excitatory postsynaptic potential (EPSP) slope, post-tetanic potentiation or LTP. In contrast, when a weak stimulus was employed, linopirdine enhanced the incidence and amplitude of LTP in a dose-dependent manner. These results indicate that linopirdine reduced stimulus intensity threshold for induction of LTP, an effect which may be mediated by its ability to enhance presynaptic glutamate release and cause CA1 membrane depolarization.

Species differences in [3H]linopirdine (DuP 996) binding to brain membranes

Binding of [3H]linopirdine was evaluated in membranes prepared from rat, mouse, calf, pig, and human brain tissue. Saturation and homologous competition experiments with freshly prepared and subsequently frozen brain membranes of young adult rats yielded biphasic binding curves. Analysing binding data with two-site models confirmed the existence of specific, high-affinity binding sites for [3H]linopirdine with a Kd value of 7.8 +/- 3.5 nM and revealed that another site with micromolar affinity for the radioligand may exist. Almost identical data were obtained with mouse brain membranes. However, high-affinity binding of [3H]linopirdine could not be detected in cerebral cortical membranes from calf, pig or an aged human subject, respectively. In these tissues [3H]linopirdine bound only with moderate affinity (Kd about 200 nM). In subsequent experiments using brain membranes either freshly prepared from aged (25-month-old) rats or prepared from young adult (3-month-old) rats after a post-mortem delay of up to 15 h, it could be excluded that the factors age or post-mortem delay were responsible for the lack of high-affinity [3H]linopirdine binding sites in calf, pig or human brain. It is concluded that [3H]linopirdine binding data obtained from rodent studies, and consequently physiological drug effects mediated by this drug target, cannot be readily extrapolated to other species including man.

Stereoselective effects of (R)- and (S)-zacopride on cognitive performance in a spatial navigation task in rats

In the present studies we investigated the actions of (R)- and (S)-zacopride, potent 5-HT3 receptor antagonists with 5-HT4 receptor agonists properties, on performance in a spatial learning and memory task in rats, the Morris water maze. A significant cognitive/performance deficit, as indicated by the increased escape latency across several trials, was produced by systemic administration of the muscarinic receptor antagonist atropine (30 mg/kg, IP). (R)-zacopride (0.001-1 microgram/kg, but not 10 or 100 micrograms/kg) significantly reduced escape latency in atropine-treated animals. (S)-Zacopride was inactive over the entire dose range examined (0.001-100 micrograms/kg, i.p.). Moreover, pretreatment with (S)-zacopride (1 or 100 micrograms/kg) did not alter the procognitive effects of (R)-zacopride (1 microgram/kg). These data demonstrate that the cognition enhancing properties of zacopride in this model of cholinergic hypofunction are exclusive to its (R)-enantiomer and imply that this action is unrelated to 5-HT, receptor antagonism or 5-HT4 receptor agonism. The possibility that the procognitive effects of (R)-zacopride may be related to actions at the novel "(R)-zacopride site" is discussed.

Ondansetron improves cognitive performance in the Morris water maze spatial navigation task

In the present studies we investigated the actions of ondansetron, a prototypic 5-hydroxytryptamine3 (5-HT3) receptor antagonist, on performance in a complex spatial navigation/memory task in rats. Specifically, we compared the activity of ondansetron to that of the cholinesterase inhibitor physostigmine in attenuating two distinct cognitive deficits in the Morris water maze. In the first model, rats treated with the muscarinic receptor antagonist atropine (30 mg/kg) had significantly longer latencies to find the submerged platform across two days of testing. Physostigmine (0.03, 0.1 and 0.3 mg/kg) and ondansetron (0.03-1 mg/kg) significantly reduced the latencies to find the submerged platform in atropine-treated animals, suggesting an increase in cognitive performance. There was little evidence of a dose-response relationship for either compound, and a loss of efficacy for ondansetron was seen at 3 mg/kg. In the second model, pre-screened, aged (23 months), cognition-impaired and nonimpaired rats were tested. Ondansetron (0.1 mg/kg), but not physostigmine (0.1 mg/kg), decreased the latencies to find the submerged platform in the aged-impaired rats, while neither compound improved performance of aged-nonimpaired rats. These data suggest that ondansetron may have cognition enhancing properties in animal models of aging and cholinergic hypofunction.