Behavioral effects of apamin, a selective inhibitor of the SK(Ca)-channel, in mice and rats

Physiology and Therapeutic Potential of SK, H, and M Medium AfterHyperPolarization Ion Channels

SK, HCN, and M channels are medium afterhyperpolarization (mAHP)-mediating ion channels. The three channels co-express in various brain regions, and their collective action strongly influences cellular excitability. However, significant diversity exists in the expression of channel isoforms in distinct brain regions and various subcellular compartments, which contributes to an equally diverse set of specific neuronal functions. The current review emphasizes the collective behavior of the three classes of mAHP channels and discusses how these channels function together although they play specialized roles. We discuss the biophysical properties of these channels, signaling pathways that influence the activity of the three mAHP channels, various chemical modulators that alter channel activity and their therapeutic potential in treating various neurological anomalies. Additionally, we discuss the role of mAHP channels in the pathophysiology of various neurological diseases and how their modulation can alleviate some of the symptoms.

Variants in the SK2 channel gene (KCNN2) lead to dominant neurodevelopmental movement disorders

KCNN2 encodes the small conductance calcium-activated potassium channel 2 (SK2). Rodent models with spontaneous Kcnn2 mutations show abnormal gait and locomotor activity, tremor and memory deficits, but human disorders related to KCNN2 variants are largely unknown. Using exome sequencing, we identified a de novo KCNN2 frameshift deletion in a patient with learning disabilities, cerebellar ataxia and white matter abnormalities on brain MRI. This discovery prompted us to collect data from nine additional patients with de novo KCNN2 variants (one nonsense, one splice site, six missense variants and one in-frame deletion) and one family with a missense variant inherited from the affected mother. We investigated the functional impact of six selected variants on SK2 channel function using the patch-clamp technique. All variants tested but one, which was reclassified to uncertain significance, led to a loss-of-function of SK2 channels. Patients with KCNN2 variants had motor and language developmental delay, intellectual disability often associated with early-onset movement disorders comprising cerebellar ataxia and/or extrapyramidal symptoms. Altogether, our findings provide evidence that heterozygous variants, likely causing a haploinsufficiency of the KCNN2 gene, lead to novel autosomal dominant neurodevelopmental movement disorders mirroring phenotypes previously described in rodents.

Therapeutic Effects of Apamin as a Bee Venom Component for Non-Neoplastic Disease

Bee venom is a natural toxin produced by honeybees and plays an important role in defending bee colonies. Bee venom has several kinds of peptides, including melittin, apamin, adolapamine, and mast cell degranulation peptides. Apamin accounts for about 2%-3% dry weight of bee venom and is a peptide neurotoxin that contains 18 amino acid residues that are tightly crosslinked by two disulfide bonds. It is well known for its pharmacological functions, which irreversibly block Ca2+-activated K+ (SK) channels. Apamin regulates gene expression in various signal transduction pathways involved in cell development. The aim of this study was to review the current understanding of apamin in the treatment of apoptosis, fibrosis, and central nervous system diseases, which are the pathological processes of various diseases. Apamin's potential therapeutic and pharmacological applications are also discussed.

Metabotropic Glutamate Receptor 5 and 8 Modulate the Ameliorative Effect of Ultramicronized Palmitoylethanolamide on Cognitive Decline Associated with Neuropathic Pain

This study investigated whether metabotropic glutamate receptor (mGluR) 5 and 8 are involved in the effect of ultramicronizedpalmitoylethanolamide (um-PEA) on the cognitive behavior and long term potentiation (LTP) at entorhinal cortex (LEC)-dentate gyrus (DG) pathway in mice rendered neuropathic by the spare nerve injury (SNI). SNI reduced discriminative memory and LTP. Um-PEA treatment started after the development of neuropathic pain had no effects in sham mice, whereas it restored cognitive behavior and LTP in SNI mice. 2-Methyl-6-(phenylethynyl) pyridine (MPEP), a selective mGluR5 antagonist, improved cognition in SNI mice and produced a chemical long term depression of the field excitatory postsynaptic potentials (fEPSPs) in sham and SNI mice. After theta burst stimulation (TBS) MPEP restored LTP in SNI mice. In combination with PEA, MPEP antagonized the PEA effect on discriminative memory and decreased LTP in SNI mice. The (RS)-4-(1-amino-1-carboxyethyl)phthalic acid (MDCPG), a selective mGluR8 antagonist, did not affect discriminative memory, but it induced a chemical LTP and prevented the enhancement of fEPSPs after TBS in SNI mice which were treated or not treated with PEA. The effect of PEA on LTP and cognitive behavior was modulated by mGluR5 and mGluR8. In particular in the SNI conditions, the mGluR5 blockade facilitated memory and LTP, but prevented the beneficial effects of PEA on discriminative memory while the mGluR8 blockade, which was ineffective in itself, prevented the favorable action of the PEA on LTP. Thus, although their opposite roles (excitatory/inhibitory of the two receptor subtypes on the glutamatergic system), they appeared to be required for the neuroprotective effect of PEA in conditions of neuropathic pain.

Ultra-micronized palmitoylethanolamide rescues the cognitive decline-associated loss of neural plasticity in the neuropathic mouse entorhinal cortex-dentate gyrus pathway

Chronic pain is associated with cognitive deficits. Palmitoylethanolamide (PEA) has been shown to ameliorate pain and pain-related cognitive impairments by restoring glutamatergic synapses functioning in the spared nerve injury (SNI) of the sciatic nerve in mice. SNI reduced mechanical and thermal threshold, spatial memory and LTP at the lateral entorhinal cortex (LEC)-dentate gyrus (DG) pathway. It decreased also postsynaptic density, volume and dendrite arborization of DG and increased the expression of metabotropic glutamate receptor 1 and 7 (mGluR1 and mGluR7), of the GluR1, GluR1s845 and GluR1s831 subunits of AMPA receptor and the levels of glutamate in the DG. The level of the endocannabinoid 2-arachidonoylglycerol (2-AG) was instead increased in the LEC. Chronic treatment with PEA, starting from when neuropathic pain was fully developed, was able to reverse mechanical allodynia and thermal hyperalgesia, memory deficit and LTP in SNI wild type, but not in PPARα null, mice. PEA also restored the level of glutamate and the expression of phosphorylated GluR1 subunits, postsynaptic density and neurogenesis. Altogether, these results suggest that neuropathic pain negatively affects cognitive behavior and related LTP, glutamatergic synapse and synaptogenesis in the DG. In these conditions PEA treatment alleviates pain and cognitive impairment by restoring LTP and synaptic maladaptative changes in the LEC-DG pathway. These outcomes open new perspectives for the use of the N-acylethanolamines, such as PEA, for the treatment of neuropathic pain and its central behavioural sequelae.

Arthropod toxins acting on neuronal potassium channels

Arthropod venoms are a rich mixture of biologically active compounds exerting different physiological actions across diverse phyla and affecting multiple organ systems including the central nervous system. Venom compounds can inhibit or activate ion channels, receptors and transporters with high specificity and affinity providing essential insights into ion channel function. In this review, we focus on arthropod toxins (scorpions, spiders, bees and centipedes) acting on neuronal potassium channels. A brief description of the K⁺ channels classification and structure is included and a compendium of neuronal K⁺ channels and the arthropod toxins that modify them have been listed. This article is part of the Special Issue entitled 'Venom-derived Peptides as Pharmacological Tools.'

The small neurotoxin apamin blocks not only small conductance Ca2+ activated K+ channels (SK type) but also the voltage dependent Kv1.3 channel

Apamin is frequently used as a specific blocker of small-conductance Ca²⁺-activated (SK type) K⁺ channels. Here we show that the small neurotoxin is not as specific as anticipated. It is also a high-affinity inhibitor with an IC₅₀ of 13 nM of the Kv1.3 channel; it blocks the latter with potency similar to the Kv1.3 blocker PAP-1. Since SK type channels and Kv1.3 channels are frequently coexpressed in different tissues such as cells of the immune system, apamin must be used with caution as a pharmacological tool.

Chronic social isolation reduces 5-HT neuronal activity via upregulated SK3 calcium-activated potassium channels

The activity of serotonin (5-HT) neurons is critical for mood regulation. In a mouse model of chronic social isolation, a known risk factor for depressive illness, we show that 5-HT neurons in the dorsal raphe nucleus are less responsive to stimulation. Probing the responsible cellular mechanisms pinpoints a disturbance in the expression and function of small-conductance Ca²⁺-activated K⁺ (SK) channels and reveals an important role for both SK2 and SK3 channels in normal regulation of 5-HT neuronal excitability. Chronic social isolation renders 5-HT neurons insensitive to SK2 blockade, however inhibition of the upregulated SK3 channels restores normal excitability. In vivo, we demonstrate that inhibiting SK channels normalizes chronic social isolation-induced anxiety/depressive-like behaviors. Our experiments reveal a causal link for the first time between SK channel dysregulation and 5-HT neuron activity in a lifelong stress paradigm, suggesting these channels as targets for the development of novel therapies for mood disorders.

Pharmacological Alternatives for the Treatment of Neurodegenerative Disorders: Wasp and Bee Venoms and Their Components as New Neuroactive Tools

Neurodegenerative diseases are relentlessly progressive, severely impacting affected patients, families and society as a whole. Increased life expectancy has made these diseases more common worldwide. Unfortunately, available drugs have insufficient therapeutic effects on many subtypes of these intractable diseases, and adverse effects hamper continued treatment. Wasp and bee venoms and their components are potential means of managing or reducing these effects and provide new alternatives for the control of neurodegenerative diseases. These venoms and their components are well-known and irrefutable sources of neuroprotectors or neuromodulators. In this respect, the present study reviews our current understanding of the mechanisms of action and future prospects regarding the use of new drugs derived from wasp and bee venom in the treatment of major neurodegenerative disorders, including Alzheimer’s Disease, Parkinson’s Disease, Epilepsy, Multiple Sclerosis and Amyotrophic Lateral Sclerosis.

Pharmacological gating modulation of small- and intermediate-conductance Ca(2+)-activated K(+) channels (KCa2.x and KCa3.1)

This short review discusses pharmacological modulation of the opening/closing properties (gating) of small- and intermediate-conductance Ca(2+)-activated K(+) channels (KCa2 and KCa3.1) with special focus on mechanisms-of-action, selectivity, binding sites, and therapeutic potentials. Despite KCa channel gating-modulation being a relatively novel field in drug discovery, efforts in this area have already revealed a surprising plethora of pharmacological sites-of-actions and channel subtype selectivity exerted by different chemical classes. The currently published positive modulators show that such molecules are potentially useful for the treatment of various neurodegenerative disorders such as ataxia, alcohol dependence, and epilepsy as well as hypertension. The negative KCa2 modulators are very effective agents for atrial fibrillation. The prediction is that further unraveling of the molecular details of gating pharmacology will allow for the design of even more potent and subtype selective KCa modulators entering into drug development for these indications.

Mechanisms underlying vasorelaxation induced in rat aorta by galetin 3,6-dimethyl ether, a flavonoid from Piptadenia stipulacea (Benth.) Ducke

In this study, we investigated the relaxant action of galetin 3,6-dimethyl ether (FGAL) on rat aorta. The flavonoid relaxed both PMA‑ and phenylephrine (Phe)-induced contractions (pD₂ = 5.36 ± 0.11 and 4.17 ± 0.10, respectively), suggesting the involvement of PKC and Phe pathways or α₁ adrenergic receptor blockade. FGAL inhibited and rightward shifted Phe-induced cumulative concentration‑response curves, indicating a noncompetitive antagonism of α₁ adrenergic receptors. The flavonoid was more potent in relaxing 30 mM KCl- than 80 mM KCl-induced contractions (pD₂ = 5.50 ± 0.22 and 4.37 ± 0.12). The vasorelaxant potency of FGAL on Phe-induced contraction was reduced in the presence of 10 mM TEA⁺. Furthermore, in the presence of apamin, glibenclamide, BaCl₂ or 4-AP, FGAL-induced relaxation was attenuated, indicating the participation of small conductance calcium-activated K⁺ channels (SK_Ca), ATP-sensitive K⁺ channels (K_ATP), inward rectifier K⁺ channels (K_ir) and voltage-dependent K⁺ channels (K_V), respectively. FGAL inhibited and rightward shifted CaCl₂-induced cumulative concentration-response curves in both depolarizing medium (high K⁺) and in the presence of verapamil and phenylephrine, suggesting inhibition of Ca²⁺ influx through voltage-gated calcium channels (Ca_V) and receptor operated channels (ROCs), respectively. Likewise, FGAL inhibited Phe-induced contractions in Ca²⁺-free medium, indicating inhibition of Ca²⁺ release from the sarcoplasmic reticulum (SR). FGAL potentiated the relaxant effect of aminophylline and sildenafil but not milrinone, suggesting the involvement of phosphodiesterase V (PDE V). Thus, the FGAL vasorelaxant mechanism involves noncompetitive antagonism of α₁ adrenergic receptors, the non-selective opening of K⁺ channels, inhibition of Ca²⁺ influx through Ca_V or ROCs and the inhibition of intracellular Ca²⁺ release. Additionally, there is the involvement of cyclic nucleotide pathway, particularly through PDE V inhibition.

SK channel blockade reverses cognitive and motor deficits induced by nigrostriatal dopamine lesions in rats

Parkinson's disease has traditionally been viewed as a motor disorder caused by the loss of dopamine (DA) neurons. However, emotional and cognitive syndromes can precede the onset of the motor deficits and provide an opportunity for therapeutic intervention. Potassium channels have recently emerged as potential new targets in the treatment of Parkinson's disease. The selective blockade of small conductance calcium-activated K+ channels (SK channels) by apamin is known to increase burst firing in midbrain DA neurons and therefore DA release. We thus investigated the effects of systemic administration of apamin on the motor, cognitive deficits and anxiety present after bilateral nigrostriatal 6-hydroxydopamine (6-OHDA) lesions in rats. Apamin administration (0.1 or 0.3 mg/kg i.p.) counteracted the depression, anxiety-like behaviors evaluated on sucrose consumption and in the elevated plus maze, social recognition and spatial memory deficits produced by partial 6-OHDA lesions. Apamin also reduced asymmetric motor deficits on circling behavior and postural adjustments in the unilateral extensive 6-OHDA model. The partial 6-OHDA lesions (56% striatal DA depletion) produced 20% decrease of iodinated apamin binding sites in the substantia nigra pars compacta in correlation with the loss of tyrosine hydroxylase positive cells, without modifying apamin binding in brain regions receiving DAergic innervation. Striatal extracellular levels of DA, not detectable after 6-OHDA lesions, were enhanced by apamin treatment as measured by in vivo microdialysis. These results indicate that blocking SK channels may reinstate minimal DA activity in the striatum to alleviate the non-motor symptoms induced by partial striatal DA lesions.

The role of intracellular calcium stores in synaptic plasticity and memory consolidation

Memory processing requires tightly controlled signalling cascades, many of which are dependent upon intracellular calcium (Ca(2+)). Despite this, most work investigating calcium signalling in memory formation has focused on plasma membrane channels and extracellular sources of Ca(2+). The intracellular Ca(2+) release channels, ryanodine receptors (RyRs) and inositol (1,4,5)-trisphosphate receptors (IP3Rs) have a significant capacity to regulate intracellular Ca(2+) signalling. Evidence at both cellular and behavioural levels implicates both RyRs and IP3Rs in synaptic plasticity and memory formation. Pharmacobehavioural experiments using young chicks trained on a single-trial discrimination avoidance task have been particularly useful by demonstrating that RyRs and IP3Rs have distinct roles in memory formation. RyR-dependent Ca(2+) release appears to aid the consolidation of labile memory into a persistent long-term memory trace. In contrast, IP3Rs are required during long-term memory. This review discusses various functions for RyRs and IP3Rs in memory processing, including neuro- and glio-transmitter release, dendritic spine remodelling, facilitating vasodilation, and the regulation of gene transcription and dendritic excitability. Altered Ca(2+) release from intracellular stores also has significant implications for neurodegenerative conditions.

Towards therapeutic applications of arthropod venom k(+)-channel blockers in CNS neurologic diseases involving memory acquisition and storage

Potassium channels are the most heterogeneous and widely distributed group of ion channels and play important functions in all cells, in both normal and pathological mechanisms, including learning and memory processes. Being fundamental for many diverse physiological processes, K⁺-channels are recognized as potential therapeutic targets in the treatment of several Central Nervous System (CNS) diseases, such as multiple sclerosis, Parkinson's and Alzheimer's diseases, schizophrenia, HIV-1-associated dementia, and epilepsy. Blockers of these channels are therefore potential candidates for the symptomatic treatment of these neuropathies, through their neurological effects. Venomous animals have evolved a wide set of toxins for prey capture and defense. These compounds, mainly peptides, act on various pharmacological targets, making them an innumerable source of ligands for answering experimental paradigms, as well as for therapeutic application. This paper provides an overview of CNS K⁺-channels involved in memory acquisition and storage and aims at evaluating the use of highly selective K⁺-channel blockers derived from arthropod venoms as potential therapeutic agents for CNS diseases involving learning and memory mechanisms.

Roles of the Drosophila SK channel (dSK) in courtship memory

A role for SK channels in synaptic plasticity has been very well-characterized. However, in the absence of simple genetic animal models, their role in behavioral memory remains elusive. Here, we take advantage of Drosophila melanogaster with its single SK gene (dSK) and well-established courtship memory assay to investigate the contribution of this channel to memory. Using two independent dSK alleles, a null mutation and a dominant negative subunit, we show that while dSK negatively regulates the acquisition of short-term memory 30 min after a short training session, it is required for normal long-term memory 24 h after extended training. These findings highlight important functions for dSK in courtship memory and suggest that SK channels can mediate multiple forms of behavioral plasticity.

Crucial role of a shared extracellular loop in apamin sensitivity and maintenance of pore shape of small-conductance calcium-activated potassium (SK) channels

Activation of small-conductance calcium (Ca(2+))-dependent potassium (K(Ca)2) channels (herein called "SK") produces membrane hyperpolarization to regulate membrane excitability. Three subtypes (SK1-3) have been cloned and are distributed throughout the nervous system, smooth muscle, and heart. It is difficult to discern the physiological role of individual channel subtypes as most blockers or enhancers do not discriminate between subtypes. The archetypical blocker apamin displays some selectivity between SK channel subtypes, with SK2 being the most sensitive, followed by SK3 and then SK1. Sensitivity of SK1 is species specific, with the human isoform being blocked by the toxin, whereas the rat is not. Mutation studies have identified residues within the outer pore that suggest apamin blocks by an allosteric mechanism. Apamin also uses a residue within the S3-S4 extracellular loop to produce a high-sensitivity block. We have identified that a 3-amino acid motif within this loop regulates the shape of the channel pore. This motif is required for binding and block by apamin, suggesting that a change in pore shape underlies allosteric block. This motif is absent in rat SK1, explaining why it is insensitive to block by apamin. The overlapping distribution of SK channel subtype expression suggests that native heteromeric channels may be common. We show that the S3-S4 loop of one subunit overlaps the outer pore of the adjacent subunit, with apamin interacting with both regions. This arrangement provides a unique binding site for each combination of SK subunits within a coassembled channel that may be targeted to produce blockers specific for heteromeric SK channels.

The nociceptive and anti-nociceptive effects of bee venom injection and therapy: a double-edged sword

Bee venom injection as a therapy, like many other complementary and alternative medicine approaches, has been used for thousands of years to attempt to alleviate a range of diseases including arthritis. More recently, additional theraupeutic goals have been added to the list of diseases making this a critical time to evaluate the evidence for the beneficial and adverse effects of bee venom injection. Although reports of pain reduction (analgesic and antinociceptive) and anti-inflammatory effects of bee venom injection are accumulating in the literature, it is common knowledge that bee venom stings are painful and produce inflammation. In addition, a significant number of studies have been performed in the past decade highlighting that injection of bee venom and components of bee venom produce significant signs of pain or nociception, inflammation and many effects at multiple levels of immediate, acute and prolonged pain processes. This report reviews the extensive new data regarding the deleterious effects of bee venom injection in people and animals, our current understanding of the responsible underlying mechanisms and critical venom components, and provides a critical evaluation of reports of the beneficial effects of bee venom injection in people and animals and the proposed underlying mechanisms. Although further studies are required to make firm conclusions, therapeutic bee venom injection may be beneficial for some patients, but may also be harmful. This report highlights key patterns of results, critical shortcomings, and essential areas requiring further study.

Small conductance calcium-activated potassium channels: from structure to function

The cloning of K(Ca)2 channels revealed three subtypes, with each displaying distinct but partially overlapping expression distributions in the mammalian CNS and periphery. Activation of K(Ca)2 channels leads to membrane hyperpolarization and inhibition of action potential firing. Block of K(Ca)2 channels has been suggested as a novel target for cognitive enhancement, depression, myotonic muscular dystrophy and heart arrhythmias. It is clear however, that blockers selective for individual K(Ca)2 channel subtypes would be required to be therapeutically useful. K(Ca)2 channel current is blocked by apamin, with the bee venom toxin being unusual in displaying some selectivity between K(Ca)2 channel subtypes. This suboptimal selectivity is not sufficient to be therapeutically useful and the toxin has been shown in vivo to have a very narrow therapeutic window. Mutational and molecular modelling studies of the K(Ca)2 channels are beginning to determine how selective block might be achieved. Mutagenesis has indicated the importance of the outer pore region and the extracellular loop between transmembrane domains S3 and S4 for block of K(Ca)2 current by apamin. Mapping the sequence of transmembrane domains S5, pore helix and S6 onto the crystal structures of KcsA, MthK and Kv1.2 has provided an approximation of the pore structure. This approach has allowed structural modelling of the interactions between toxins and channel, demonstrating that the toxins that show little discrimination between K(Ca)2 channel subtypes interact with the outer pore and around the K(+) selectivity filter. We present the structural modelling of the interaction of apamin and K(Ca)2.2, which is superimposed onto the crystal structure of Kv1.2. This has shown that apamin interacts only with the outer pore and does not come into contact with channel's selectivity filter. It is clear that by comparing how different toxins interact with each K(Ca)2 channel subtype, a detailed picture will be generated that will aid the development of more specific K(Ca)2 channel blockers.

Ca2+-dependent K+ efflux regulates deoxycholate-induced apoptosis of BHK-21 and Caco-2 cells

BACKGROUND & AIMS

Deoxycholate (DC) has proapoptotic and tumorigenic effects in different cell types of the gastrointestinal tract. Exposure of BHK-21 (stromal) cells to DC induces Ca(2+) entry at the plasma membrane, which affects intracellular Ca(2+) signaling. We assessed whether DC-induced increases in [Ca(2+)] can impinge on plasma membrane properties (eg, ionic conductances) involved in cell apoptosis.

METHODS

Single- and double-barreled microelectrodes were used to measure membrane potential (V(m)) and extracellular [K(+)] in BHK-21 fibroblasts and Caco-2 colon carcinoma cells. Apoptosis was assessed by Hoechst labeling, propidium iodide staining, and caspase-3 and caspase-7 assays.

RESULTS

DC-induced cell membrane hyperpolarization was directly measured with intracellular microelectrodes in both cell lines. Diverse Ca(2+) mobilizing agents, such as membrane receptor agonists, an inhibitor of the sarco/endoplasmic reticulum Ca(2+) adenosine triphosphatase and a Ca(2+) ionophore, also induced increases in V(m). Removal of extracellular Ca(2+) reduced the agonist- and DC-induced membrane hyperpolarization by approximately 15% and 60%, respectively. These findings indicate a prominent role for Ca(2+) entry at the plasma membrane in the action of this bile salt. Blockade of Ca(2+)-activated K(+) conductances by charybdotoxin and apamin reduced DC-induced hyperpolarization by 75% and 64% in BHK-21 and Caco-2 cells, respectively. These inhibitors also reduced the DC-induced increase in extracellular [K(+)] by 75% and cell apoptosis by approximately 50% in both cell lines.

CONCLUSIONS

Ca(2+)-dependent K(+) conductance is an important regulator of DC-induced apoptosis in stromal and colon cancer cells.

Functions and modulation of neuronal SK channels

Small conductance (SK) channels are calcium-activated potassium channels that, when cloned in 1996, were thought solely to contribute to the afterhyperpolarisation that follows action potentials, and to control repetitive firing patterns of neurons. However, discoveries over the past few years have identified novel roles for SK channels in controlling dendritic excitability, synaptic transmission and synaptic plasticity. More recently, modulation of SK channel calcium sensitivity by casein kinase 2, and of SK channel trafficking by protein kinase A, have been demonstrated. This article will discuss recent findings regarding the function and modulation of SK channels in central neurons.

Evidence of multistability in a realistic computer simulation of hippocampus subfield CA1

The manner in which hippocampus processes neural signals is thought to be central to the memory encoding process. A theoretically oriented literature has suggested that this is carried out via "attractors" or distinctive spatio-temporal patterns of activity. However, these ideas have not been thoroughly investigated using computational models featuring both realistic single-cell physiology and detailed cell-to-cell connectivity. Here we present a 452 cell simulation based on Traub et al.'s pyramidal cell [Traub RD, Jefferys JG, Miles R, Whittington MA, Toth K. A branching dendritic model of a rodent CA3 pyramidal neurone. J Physiol (Lond) 1994;481:79-95] and interneuron [Traub RD, Miles R, Pyramidal cell-to-inhibitory cell spike transduction explicable by active dendritic conductances in inhibitory cell. J Comput Neurosci 1995;2:291-8] models, incorporating patterns of synaptic connectivity based on an extensive review of the neuroanatomic literature. When stimulated with a one second physiologically realistic input, our simulated tissue shows the ability to hold activity on-line for several seconds; furthermore, its spiking activity, as measured by frequency and interspike interval (ISI) distributions, resembles that of in vivo hippocampus. An interesting emergent property of the system is its tendency to transition from stable state to stable state, a behavior consistent with recent experimental findings [Sasaki T, Matsuki N, Ikegaya Y. Metastability of active CA3 networks. J Neurosci 2007;27:517-28]. Inspection of spike trains and simulated blockade of K(AHP) channels suggest that this is mediated by spike frequency adaptation. This finding, in conjunction with studies showing that apamin, a K(AHP) channel blocker, enhances the memory consolidation process in laboratory animals, suggests the formation of stable attractor states is central to the process by which memories are encoded. Ways that this methodology could shed light on the etiology of mental illness, such as schizophrenia, are discussed.

Selective cognitive deficits and reduced hippocampal brain-derived neurotrophic factor mRNA expression in small-conductance calcium-activated K+ channel deficient mice

Small-conductance calcium-activated K(+) channels 1-3 (SK1-3) are important for neuronal firing regulation and are considered putative CNS drug targets. For instance non-selective SK blockers improve performance in animal models of cognition. The SK subtype(s) involved herein awaits identification and the question is difficult to address pharmacologically due to the lack of subtype-selective SK-channel modulators. In this study, we used doxycycline-induced conditional SK3-deficient (T/T) mice to address the cognitive consequences of selective SK3 deficiency. In T/T mice SK3 protein is near-eliminated from the brain following doxycycline treatment. We tested T/T and wild type (WT) littermate mice in five distinct learning and memory paradigms. In Y-maze spontaneous alternations and five-trial inhibitory avoidance the performance of T/T mice was markedly inferior to WT mice. In contrast, T/T and WT mice performed equally well in passive avoidance, object recognition and the Morris water maze. Thus, some aspects of working/short-term memory are disrupted in T/T mice. Using in situ hybridization, we further found the cognitive deficits in T/T mice to be paralleled by reduced brain-derived neurotrophic factor (BDNF) mRNA expression in the dentate gyrus and CA3 of the hippocampus. BDNF mRNA levels in the frontal cortex were not affected. BDNF has been crucially implicated in many cognitive processes. Hence, the biological substrate for the cognitive impairments in T/T mice could conceivably entail reduced trophic support of the hippocampus.

Functions of SK channels in central neurons

1. SK channels are small-conductance calcium-activated potassium channels that are widely expressed in neurons. The traditional view of the functional role of SK channels is in mediating one component of the after-hyperpolarization that follows action potentials. Calcium influx via voltage-gated calcium channels active during action potentials opens SK channels and the resultant hyperpolarization lowers the firing frequency of action potentials in many neurons. 2. Recent advances have shown that, in addition to controlling action potential firing frequency, SK channels are also important in regulating dendritic excitability, synaptic transmission and synaptic plasticity. 3. In accordance with their role in modulating synaptic plasticity, SK channels are also important in regulating several learning and memory tasks and may also play a role in a number of neurological disorders. 4. The present review discusses recent findings on the role of SK channels in central neurons.

Synthesis and Radioligand Binding Studies of Bis-isoquinolinium Derivatives as Small Conductance Ca2+-Activated K+ Channel Blockers

Starting from the scaffold of N-methyllaudanosine and N-methylnoscapine, which are known small conductance Ca2+-activated K+ channel blockers, original bis-isoquinolinium derivatives were synthezised and evaluated using binding studies, electrophysiology, and molecular modeling. These quaternary compounds are powerful blockers, and the most active ones have 10 times more affinity for the channels than dequalinium. The unsubstituted compounds possess a weaker affinity than the analogues having a 6,7-dimethoxy- or a 6,7,8-trimethoxy substitution. The length of the linker has no influence in the alkane derivatives. In relation to the xylene derivatives, the affinities are higher for the ortho and meta isomers. These results are well corroborated by a molecular modeling study. Finally, the most effective compounds have been tested in electrophysiological experiments on midbrain dopaminergic neurons and demonstrate the blocking potential of the apamin-sensitive after-hyperpolarization.

Inhibitory gating modulation of small conductance Ca2+-activated K+ channels by the synthetic compound (R)-N-(benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphtylamine (NS8593) reduces afterhyperpolarizing current in hippocampal CA1 neurons

SK channels are small conductance Ca(2+)-activated K(+) channels important for the control of neuronal excitability, the fine tuning of firing patterns, and the regulation of synaptic mechanisms. The classic SK channel pharmacology has largely focused on the peptide apamin, which acts extracellularly by a pore-blocking mechanism. 1-Ethyl-2-benzimidazolinone (1-EBIO) and 6,7-dichloro-1H-indole-2,3-dione 3-oxime (NS309) have been identified as positive gating modulators that increase the apparent Ca(2+) sensitivity of SK channels. In the present study, we describe inhibitory gating modulation as a novel principle for selective inhibition of SK channels. In whole-cell patch-clamp experiments, the compound (R)-N-(benzimidazol-2-yl)-1,2,3,4-tetrahydro-1-naphtylamine (NS8593) reversibly inhibited recombinant SK3-mediated currents (human SK3 and rat SK3) with potencies around 100 nM. However, in contrast to known pore blockers, NS8593 did not inhibit (125)I-apamin binding. Using excised patches, it was demonstrated that NS8593 decreased the Ca(2+) sensitivity by shifting the activation curve for Ca(2+) to the right, only slightly affecting the maximal Ca(2+)-activated SK current. NS8593 inhibited all the SK1-3 subtypes Ca(2+)-dependently (K(d) = 0.42, 0.60, and 0.73 microM, respectively, at 0.5 microM Ca(2+)), whereas the compound did not affect the Ca(2+)-activated K(+) channels of intermediate and large conductance (hIK and hBK channels, respectively). The site of action was accessible from both sides of the membrane, and the NS8593-mediated inhibition was prevented in the presence of a high concentration of the positive modulator NS309. NS8593 was further tested on mouse CA1 neurons in hippocampal slices and shown to inhibit the apaminand tubocurarine-sensitive SK-mediated afterhyperpolarizing current, at a concentration of 3 microM.

Synthesis and Radioligand Binding Studies of Methoxylated 1,2,3,4-Tetrahydroisoquinolinium Derivatives as Ligands of the Apamin-Sensitive Ca2+-Activated K+ Channels

Several methoxylated 1,2,3,4-tetrahydroisoquinoliniums derived from N-methyl-laudanosine and N-methyl-noscapine were synthesized and evaluated for their affinity for apamin-sensitive binding sites. The quaternary ammonium derivatives have a higher affinity with regard to the tertiary amines. 6,7-Dimethoxy analogues possess a higher affinity than the 6,8- and 7,8-dimethoxy isomers. A 3,4-dimethoxybenzyl or a 2-naphthylmethyl moiety in C-1 position are more favorable than a 3,4-dimethoxyphenethyl group. Smaller groups such as propyl or isobutyl are unfavorable. In 6,7-dimethoxy analogues, increasing the size and lipophilicity with a naphthyl group in the C-1 position leads to a slight increase of affinity, while the same group in the 6,7,8-trimethoxy series is less favorable. The 6,7,8-trimethoxy derivative 3f is the first tertiary amine in the series to possess an affinity close to that of N-methyl-laudanosine and N-methyl-noscapine. Moreover, electrophysiological studies show that the most effective compound 4f blocks the apamin-sensitive afterhyperpolarization in rat dopaminergic neurons.

SK Ca2+-activated K+ channel ligands alter the firing pattern of dopamine-containing neurons in vivo

Apamin-sensitive, SK channels play an important role in generating the rhythmic firing patterns exhibited by midbrain dopamine neurons in vitro. However, their contribution to the firing properties of these cells in intact animals has yet to be determined. In the present series of experiments, extracellular single unit recording techniques were used to assess the central effects of prototypical SK channel ligands on the firing pattern of dopamine neurons in the substantia nigra of the chloral hydrate anesthetized rat. I.v. administration of the SK channel blocker apamin (0.4 mg/kg), increased bursting activity in approximately 50% of the dopamine neurons tested without altering average firing rate. The majority of these cells responded slowly to the effects of apamin, gradually transitioning from an irregular single spike to a phasic discharge composed of the same relative proportion of long (>or=three spike) and short (two spike) bursts as "natural" bursting activity recorded in drug naive animals. Local administration of apamin increased bursting activity in all cells tested. Systemic administration of the SK channel opener, 1-ethyl-2-benzimidazolinone (5-25 mg/kg) also had no effect on average firing rate but suppressed bursting activity and increased the precision of firing. The effects of 1-ethyl-2-benzimidazolinon on firing pattern were abolished when recording electrodes contained apamin (125 microM). These results suggest that SK channels actively contribute to the spontaneous firing patterns exhibited by dopamine neurons in vivo and provide additional support for the proposition that this channel could serve as a useful target for modifying their activity.

Apamin produces selective improvements of learning in rats

The effect of apamin on learning was examined using two behavioral tasks where the animals were subjected to two trials separated by a 24h interval. In the Y maze task, apamin administered before the acquisition session did not enhance performance on both the acquisition session and the restitution session. In the second behavioral task, animals were trained to press a lever to obtain a food pellet (fixed ratio 1). Then, to study the effect of apamin on extinction, animals were submitted to two sessions where a press on the lever was not reinforced. Apamin administered before the acquisition session reduced the number of lever presses during the first 3-min period of the restitution session. These results suggest that the blockade of SK channels could improve the acquisition but not when the task requires the processing of spatial information.

Synthesis and Radioligand Binding Studies of C-5- and C-8-Substituted 1-(3,4-Dimethoxybenzyl)-2,2-dimethyl-1,2,3,4-tetrahydroisoquinoliniums as SK Channel Blockers Related to N-Methyl-laudanosine and N-Methyl-noscapine

The synthesis and the (125)I-apamin binding studies of original C-5- and C-8-substituted 1-(3,4-dimethoxy-benzyl)-2,2-dimethyl-1,2,3,4-tetrahydroisoquinoliniums and 1-(3,4-dimethoxy-benzyl)-6,6-dimethyl-4,5,6,7-tetrahydrothieno[2,3-c]pyridiniums were performed in order to find a reversible and selective SK channel blocker structurally related to N-methyl-laudanosine and N-methyl-noscapine. A bulky alkyl substituent in the C-8 position of the tetrahydroisoquinoline produces a clear increase in the affinity for the apamin sensitive binding sites. The presence of an electron-withdrawing group in the C-5 and C-8 positions is not a suitable substitution for the affinity of drugs structurally related to N-methyl-laudanosine. Thiophenic analogues and 8-methoxy derivatives possess a poor affinity for the apamin sensitive binding sites. Electrophysiological studies performed with the most effective compound showed a blockade of the apamin sensitive afterhyperpolarization in rat dopaminergic neurons.

SK channels regulate excitatory synaptic transmission and plasticity in the lateral amygdala

At glutamatergic synapses, calcium influx through NMDA receptors (NMDARs) is required for long-term potentiation (LTP); this is a proposed cellular mechanism underlying memory and learning. Here we show that in lateral amygdala pyramidal neurons, SK channels are also activated by calcium influx through synaptically activated NMDARs, resulting in depression of the synaptic potential. Thus, blockade of SK channels by apamin potentiates fast glutamatergic synaptic potentials. This potentiation is blocked by the NMDAR antagonist AP5 (D(-)-2-amino-5-phosphono-valeric acid) or by buffering cytosolic calcium with BAPTA. Blockade of SK channels greatly enhances LTP of cortical inputs to lateral amygdala pyramidal neurons. These results show that NMDARs and SK channels are colocalized at glutamatergic synapses in the lateral amygdala. Calcium influx through NMDARs activates SK channels and shunts the resultant excitatory postsynaptic potential. These results demonstrate a new role for SK channels as postsynaptic regulators of synaptic efficacy.

Synthesis and biological evaluation of N-methyl-laudanosine iodide analogues as potential SK channel blockers

Neuronal action potentials are followed by an afterhyperpolarisation (AHP), which is mediated by small conductance Ca2+-activated K+ channels (SK channels or KCa2 channels). This AHP plays an important role in regulating neuronal activity and agents modulating AHP amplitude could have a potential therapeutic interest. It was previously shown that N-methyl-bicuculline iodide blocks SK channels but its GABA) activity represents a serious drawback. In view of the structural analogy between bicuculline and laudanosine 14, several N-quaternary analogues of the latter were developed. It was shown that N-methyl-laudanosine 15 (NML) and N-ethyl-laudanosine 16 induce a reversible and relatively specific blockade of the apamin sensitive AHP in dopaminergic neurones with mean IC50s of 15, and 47 microM, respectively. Laudanosine 14, N-butyl-17 and N-benzyl-18 derivatives were less potent. In order to find pharmacophore elements, modifications were performed at different positions such as C-1, C-6 and C-7. Intracellular recordings on rat midbrain dopaminergic neurones were made in order to evaluate the putative blockade of SK channels by these molecules. Simplified structures such as tetrahydroisoquinoline derivatives with H or Me at C-1 1-6 presented no significant activity at 300 microM. The presence of a 1-(3,4-dimethoxybenzyl) moiety seems an important feature. Indeed, compound 8 showed a blockade of the AHP of only 33% at 300 microM while compound 13 blocked it by 67%, respectively, at the same concentration. Binding experiments were also performed. Binding affinities for SK channels are in good agreement with electrophysiological data. These results indicate that the presence of a charged nitrogen group is an essential point for the affinity on SK channels. Finally, because of the similar activity of both enantiomers of NML 19 and 20, the interaction site may present a symmetrical configuration.

Electrophysiological characterization of the SK channel blockers methyl-laudanosine and methyl-noscapine in cell lines and rat brain slices

We have recently shown that the alkaloid methyl-laudanosine blocks SK channel-mediated afterhyperpolarizations (AHPs) in midbrain dopaminergic neurones. However, the relative potency of the compound on the SK channel subtypes and its ability to block AHPs of other neurones were unknown. Using whole-cell patch-clamp experiments in transfected cell lines, we found that the compound blocks SK1, SK2 and SK3 currents with equal potency: its mean IC(50)s were 1.2, 0.8 and 1.8 microM, respectively. IK currents were unaffected. In rat brain slices, methyl-laudanosine blocked apamin-sensitive AHPs in serotonergic neurones of the dorsal raphe and noradrenergic neurones of the locus coeruleus with IC(50)s of 21 and 19 microM, as compared to 15 microM in dopaminergic neurones. However, at 100 microM, methyl-laudanosine elicited a constant hyperpolarization of serotonergic neurones of about 9 mV, which was inconsistently (i.e. not in a reproducible manner) antagonized by atropine and hence partly due to the activation of muscarinic receptors. While exploring the pharmacology of related compounds, we found that methyl-noscapine also blocked SK channels. In cell lines, methyl-noscapine blocked SK1, SK2 and SK3 currents with mean IC(50)s of 5.9, 5.6 and 3.9 microM, respectively. It also did not block IK currents. Methyl-noscapine was slightly less potent than methyl-laudanosine in blocking AHPs in brain slices, its IC(50)s being 42, 37 and 29 microM in dopaminergic, serotonergic and noradrenergic neurones, respectively. Interestingly, no significant non-SK effects were observed with methyl-noscapine in slices. At a concentration of 300 microM, methyl-noscapine elicited the same changes in excitability in the three neuronal types than did a supramaximal concentration of apamin (300 nM). Methyl-laudanosine and methyl-noscapine produced a rapidly reversible blockade of SK channels as compared with apamin. The difference between the IC(50)s of apamin (0.45 nM) and methyl-laudanosine (1.8 microM) in SK3 cells was essentially due to a major difference in their k(-1) (0.028 s(-1) for apamin and >or=20 s(-1) for methyl-laudanosine). These experiments demonstrate that both methyl-laudanosine and methyl-noscapine are medium potency, quickly dissociating, SK channel blockers with a similar potency on the three SK subtypes. Methyl-noscapine may be superior in terms of specificity for the SK channels.

Ca2+-activated K+ channels: molecular determinants and function of the SK family

Ca(2+)-activated K(+) (K(Ca)) channels of small (SK) and intermediate (IK) conductance are present in a wide range of excitable and non-excitable cells. On activation by low concentrations of Ca(2+), they open, which results in hyperpolarization of the membrane potential and changes in cellular excitability. K(Ca)-channel activation also counteracts further increases in intracellular Ca(2+), thereby regulating the concentration of this ubiquitous intracellular messenger in space and time. K(Ca) channels have various functions, including the regulation of neuronal firing properties, blood flow and cell proliferation. The cloning of SK and IK channels has prompted investigations into their gating, pharmacology and organization into calcium-signalling domains, and has provided a framework that can be used to correlate molecularly identified K(Ca) channels with their native currents.

Matching molecules to function: neuronal Ca2+-activated K+ channels and afterhyperpolarizations

Potassium channels regulate the membrane excitability of neurons, play a major role in shaping action potentials, determining firing patterns and regulating neurotransmitter release, and thus significantly contribute to neuronal signal encoding and integration. This review focuses on the molecular and cellular basis for the specific function of small-conductance calcium-activated potassium channels (SK channels) in the nervous system. SK channels are activated by an intracellular increase of free calcium during action potentials. They mediate currents that modulate the firing frequency of neurons. Three SK channel subunits have been cloned and form channels, which are voltage-insensitive, activated by submicromolar intracellular calcium concentrations, and are blocked, with different affinities, by a number of toxins and organic compounds. Different neurons in the central and peripheral nervous system express distinct subsets of SK channel subunits. Recent progress has been made in relating cloned SK channels to their native counterparts. These findings argue in favour of regulatory mechanisms conferring to native SK channels with specific subunit compositions distinct and specific functional profiles in different neurons.

Calcium-activated potassium channels: multiple contributions to neuronal function

Calcium-activated potassium channels are a large family of potassium channels that are found throughout the central nervous system and in many other cell types. These channels are activated by rises in cytosolic calcium largely in response to calcium influx via voltage-gated calcium channels that open during action potentials. Activation of these potassium channels is involved in the control of a number of physiological processes from the firing properties of neurons to the control of transmitter release. These channels form the target for modulation for a range of neurotransmitters and have been implicated in the pathogenesis of neurological and psychiatric disorders. Here the authors summarize the varieties of calcium-activated potassium channels present in central neurons and their defining molecular and biophysical properties.

Enhancing synaptic plasticity and memory: a role for small-conductance Ca(2+)-activated K+ channels

Calcium-activated potassium (K+) channels are distributed throughout the central nervous system as well as many other peripheral tissues and comprise three distinct classes of K+ channels: small conductance (SK), intermediate conductance, and large conductance. This update focuses on SK channels. Increases in cytosolic calcium in response to depolarization activate SK channels. Activation of these channels decreases neuronal excitability. In this review, the authors discuss the role of SK channels in the induction of synaptic plasticity and their influence on learning and memory. A testable model that synthesizes the current literature is offered, suggesting that SK channels represent an important regulator of synaptic plasticity and memory.