International Union of Pharmacology. LI. Nomenclature and structure-function relationships of cyclic nucleotide-regulated channels

Natural Compounds Purified from the Leaves of Aristotelia chilensis: Makomakinol, a New Alkaloid and the Effect of Aristoteline and Hobartine on NaV Channels

Aristotelia chilensis or "maqui" is a tree native to Chile used in the folk medicine of the Mapuche people as an anti-inflammatory agent for the treatment of digestive ailments, fever, and skin lesions. Maqui fruits are black berries which are considered a "superfruit" with notable potential health benefits, promoted to be an antioxidant, cardioprotective, and anti-inflammatory. Maqui leaves contain non-iridoid monoterpene indole alkaloids which have previously been shown to act on nicotinic acetylcholine receptors, potassium channels, and calcium channels. Here, we isolated a new alkaloid from maqui leaves, now called makomakinol, together with the known alkaloids aristoteline, hobartine, and 3-formylindole. Moreover, the polyphenols quercetine, ethyl caffeate, and the terpenes, dihydro-β-ionone and terpin hydrate, were also obtained. In light of the reported analgesic and anti-nociceptive properties of A. chilensis, in particular a crude mixture of alkaloids containing aristoteline and hobartinol (PMID 21585384), we therefore evaluated the activity of aristoteline and hobartine on NaV1.8, a key NaV isoform involved in nociception, using automated whole-cell patch-clamp electrophysiology. Aristoteline and hobartine both inhibited Nav1.8 with an IC50 of 68 ± 3 µM and 54 ± 1 µM, respectively. Hobartine caused a hyperpolarizing shift of the voltage-dependence of the activation, whereas aristoteline did not change the voltage-dependence of the activation or inactivation. The inhibitory activity of these alkaloids on NaV channels may contribute to the reported analgesic properties of Aristotelia chilensis used by the Mapuche people.

Flexible, Transparent, and Wafer-Scale Artificial Synapse Array Based on TiOx /Ti3 C2 Tx Film for Neuromorphic Computing

A high-density neuromorphic computing memristor array based on 2D materials paves the way for next-generation information-processing components and in-memory computing systems. However, the traditional 2D-materials-based memristor devices suffer from poor flexibility and opacity, which hinders the application of memristors in flexible electronics. Here, a flexible artificial synapse array based on TiOx /Ti3 C2 Tx film is fabricated by a convenient and energy-efficient solution-processing technique, which realizes high transmittance (≈90%) and oxidation resistance (>30 days). The TiOx /Ti3 C2 Tx memristor shows low device-to-device variability, long memory retention and endurance, a high ON/OFF ratio, and fundamental synaptic behavior. Furthermore, satisfactory flexibility (R = 1.0 mm) and mechanical endurance (104 bending cycles) of the TiOx /Ti3 C2 Tx memristor are achieved, which is superior to other film memristors prepared by chemical vapor deposition. In addition, high-precision (>96.44%) MNIST handwritten digits recognition classification simulation indicates that the TiOx /Ti3 C2 Tx artificial synapse array holds promise for future neuromorphic computing applications, and provides excellent high-density neuron circuits for new flexible intelligent electronic equipment.

Regulation of Neurotransmitter Release by K+ Channels

K+ channels play potent roles in the process of neurotransmitter release by influencing the action potential waveform and modulating neuronal excitability and release probability. These diverse effects of K+ channel activation are ensured by the wide variety of K+ channel genes and their differential expression in different cell types. Accordingly, a variety of K+ channels have been implicated in regulating neurotransmitter release, including the Ca2+- and voltage-gated K+ channel Slo1 (also known as BK channel), voltage-gated K+ channels of the Kv3 (Shaw-type), Kv1 (Shaker-type), and Kv7 (KCNQ) families, G-protein-gated inwardly rectifying K+ (GIRK) channels, and SLO-2 (a Ca2+-. Cl-, and voltage-gated K+ channel in C. elegans). These channels vary in their expression patterns, subcellular localization, and biophysical properties. Their roles in neurotransmitter release may also vary depending on the synapse and physiological or experimental conditions. This chapter summarizes key findings about the roles of K+ channels in regulating neurotransmitter release.

Peripheral Voltage-Gated Cation Channels in Neuropathic Pain and Their Potential as Therapeutic Targets

The persistence of increased excitability and spontaneous activity in injured peripheral neurons is imperative for the development and persistence of many forms of neuropathic pain. This aberrant activity involves increased activity and/or expression of voltage-gated Na+ and Ca2+ channels and hyperpolarization activated cyclic nucleotide gated (HCN) channels as well as decreased function of K+ channels. Because they display limited central side effects, peripherally restricted Na+ and Ca2+ channel blockers and K+ channel activators offer potential therapeutic approaches to pain management. This review outlines the current status and future therapeutic promise of peripherally acting channel modulators. Selective blockers of Nav1.3, Nav1.7, Nav1.8, Cav3.2, and HCN2 and activators of Kv7.2 abrogate signs of neuropathic pain in animal models. Unfortunately, their performance in the clinic has been disappointing; some substances fail to meet therapeutic end points whereas others produce dose-limiting side effects. Despite this, peripheral voltage-gated cation channels retain their promise as therapeutic targets. The way forward may include (i) further structural refinement of K+ channel activators such as retigabine and ASP0819 to improve selectivity and limit toxicity; use or modification of Na+ channel blockers such as vixotrigine, PF-05089771, A803467, PF-01247324, VX-150 or arachnid toxins such as Tap1a; the use of Ca2+ channel blockers such as TTA-P2, TTA-A2, Z 944, ACT709478, and CNCB-2; (ii) improving methods for assessing "pain" as opposed to nociception in rodent models; (iii) recognizing sex differences in pain etiology; (iv) tailoring of therapeutic approaches to meet the symptoms and etiology of pain in individual patients via quantitative sensory testing and other personalized medicine approaches; (v) targeting genetic and biochemical mechanisms controlling channel expression using anti-NGF antibodies such as tanezumab or re-purposed drugs such as vorinostat, a histone methyltransferase inhibitor used in the management of T-cell lymphoma, or cercosporamide a MNK 1/2 inhibitor used in treatment of rheumatoid arthritis; (vi) combination therapy using drugs that are selective for different channel types or regulatory processes; (vii) directing preclinical validation work toward the use of human or human-derived tissue samples; and (viii) application of molecular biological approaches such as clustered regularly interspaced short palindromic repeats (CRISPR) technology.

Left Ventricular Noncompaction: New Insights into a Poorly Understood Disease

Left ventricular noncompaction (LVNC) is a congenital pathology that directly affects the lining walls of myocardial tissue, causing trabeculations with blood filling in the inner wall of the heart, concomitantly with the development of a mesocardial thinning. Although LVNC was described for the first time as long ago as 1984, our understanding of the disease with regard to its genetic pattern, diagnosis, clinical presentation and treatment is still scanty. LVNC can present as an isolated condition or associated with congenital heart disease, genetic syndromes or neuromuscular disease. This suggests that LVNC is not a distinct form of cardiomyopathy, but rather a morphological expression of different diseases. Recognition of the disease is of fundamental importance because its clinical manifestations are variable, ranging from the absence of any symptom to congestive heart failure, lethal arrhythmias and thromboembolic events. The study of this disease has emphasized its genetic aspects, as it may be of sporadic origin or hereditary, in which case it most commonly has an autosomal dominant inheritance or one linked to the X chromosome. Echocardiography is the gold standard for diagnosis, and magnetic resonance imaging may refine the identification of the disease, especially in those patients with non-conclusive echocardiography. This article sets out to review the main characteristics of LVNC and present updates, especially in the genetic pattern, diagnosis and treatment of the disease.

Defined extracellular ionic solutions to study and manipulate the cellular resting membrane potential

All cells possess an electric potential across their plasma membranes and can generate and receive bioelectric signals. The cellular resting membrane potential (RMP) can regulate cell proliferation, differentiation and apoptosis. Current approaches to measure the RMP rely on patch clamping, which is technically challenging, low-throughput and not widely available. It is therefore critical to develop simple strategies to measure, manipulate and characterize the RMP. Here, we present a simple methodology to study the RMP of non-excitable cells and characterize the contribution of individual ions to the RMP using a voltage-sensitive dye. We define protocols using extracellular solutions in which permeable ions (Na+, Cl- and K+) are substituted with non-permeable ions [N-Methyl-D-glucamine (NMDG), gluconate, choline, SO42-]. The resulting RMP modifications were assessed with both patch clamp and a voltage sensitive dye. Using an epithelial and cancer cell line, we demonstrate that the proposed ionic solutions can selectively modify the RMP and help determine the relative contribution of ionic species in setting the RMP. The proposed method is simple and reproducible and will make the study of bioelectricity more readily available to the cell biology community.This article has an associated First Person interview with the first author of the paper.

New Structures and Gating of Voltage-Dependent Potassium (Kv) Channels and Their Relatives: A Multi-Domain and Dynamic Question

Voltage-dependent potassium channels (Kv channels) are crucial regulators of cell excitability that participate in a range of physiological and pathophysiological processes. These channels are molecular machines that display a mechanism (known as gating) for opening and closing a gate located in a pore domain (PD). In Kv channels, this mechanism is triggered and controlled by changes in the magnitude of the transmembrane voltage sensed by a voltage-sensing domain (VSD). In this review, we consider several aspects of the VSD–PD coupling in Kv channels, and in some relatives, that share a common general structure characterized by a single square-shaped ion conduction pore in the center, surrounded by four VSDs located at the periphery. We compile some recent advances in the knowledge of their architecture, based in cryo-electron microscopy (cryo-EM) data for high-resolution determination of their structure, plus some new functional data obtained with channel variants in which the covalent continuity between the VSD and PD modules has been interrupted. These advances and new data bring about some reconsiderations about the use of exclusively a classical electromechanical lever model of VSD–PD coupling by some Kv channels, and open a view of the Kv-type channels as allosteric machines in which gating may be dynamically influenced by some long-range interactional/allosteric mechanisms.

Retinal Cyclic Nucleotide-Gated Channels: From Pathophysiology to Therapy

The first step in vision is the absorption of photons by the photopigments in cone and rod photoreceptors. After initial amplification within the phototransduction cascade the signal is translated into an electrical signal by the action of cyclic nucleotide-gated (CNG) channels. CNG channels are ligand-gated ion channels that are activated by the binding of cyclic guanosine monophosphate (cGMP) or cyclic adenosine monophosphate (cAMP). Retinal CNG channels transduce changes in intracellular concentrations of cGMP into changes of the membrane potential and the Ca²⁺ concentration. Structurally, the CNG channels belong to the superfamily of pore-loop cation channels and share a common gross structure with hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and voltage-gated potassium channels (KCN). In this review, we provide an overview on the molecular properties of CNG channels and describe their physiological role in the phototransduction pathways. We also discuss insights into the pathophysiological role of CNG channel proteins that have emerged from the analysis of CNG channel-deficient animal models and human CNG channelopathies. Finally, we summarize recent gene therapy activities and provide an outlook for future clinical application.

Cell signal transduction: hormones, neurotransmitters and therapeutic drugs relate to purine nucleotide structure

Purine nucleotides transduce cell membrane receptor responses and modulate ion channel activity. This is accomplished through conformational change in the structure of nucleotides and cell membrane associated proteins. The aim of this study is to enhance our understanding of nucleotide dependence in regard to signal transduction events, drug action and pharmacological promiscuity. Nucleotides and ligand structures regulating Gα protein subunits, voltage- and ligand-gated ion channels are investigated for molecular similarity using a computational program. Results differentiate agonist and antagonist structures, identify molecular similarity within nucleotide and ligand structures and demonstrate the potential of ligands to regulate nucleotide conformational change. Relative molecular similarity within nucleotides and the ligands of the major receptor classes provides insight into mechanisms of receptor and ion channel regulation. The nucleotide template model has some merit as an initial screening tool in the study and comparison of drug and hormone structures.

Identification of methylated genes in salivary gland adenoid cystic carcinoma xenografts using global demethylation and methylation microarray screening

Salivary gland adenoid cystic carcinoma (ACC) is a rare head and neck malignancy without molecular biomarkers that can be used to predict the chemotherapeutic response or prognosis of ACC. The regulation of gene expression of oncogenes and tumor suppressor genes (TSGs) through DNA promoter methylation may play a role in the carcinogenesis of ACC. To identify differentially methylated genes in ACC, a global demethylating agent, 5-aza-2′-deoxycytidine (5-AZA) was utilized to unmask putative TSG silencing in ACC xenograft models in mice. Fresh xenografts were passaged, implanted in triplicate in mice that were treated with 5-AZA daily for 28 days. These xenografts were then evaluated for genome-wide DNA methylation patterns using the Illumina Infinium HumanMethylation27 BeadChip array. Validation of the 32 candidate genes was performed by bisulfite sequencing (BS-seq) in a separate cohort of 6 ACC primary tumors and 6 normal control salivary gland tissues. Hypermethylation was identified in the HCN2 gene promoter in all 6 control tissues, but hypomethylation was found in all 6 ACC tumor tissues. Quantitative validation of HCN2 promoter methylation level in the region detected by BS-seq was performed in a larger cohort of primary tumors (n=32) confirming significant HCN2 hypomethylation in ACCs compared with normal samples (n=10; P=0.04). HCN2 immunohistochemical staining was performed on an ACC tissue microarray. HCN2 staining intensity and H-score, but not percentage of the positively stained cells, were significantly stronger in normal tissues than those of ACC tissues. With our novel screening and sequencing methods, we identified several gene candidates that were methylated. The most significant of these genes, HCN2, was actually hypomethylated in tumors. However, promoter methylation status does not appear to be a major determinant of HCN2 expression in normal and ACC tissues. HCN2 hypomethylation is a biomarker of ACC and may play an important role in the carcinogenesis of ACC.

Fetal heart extract facilitates the differentiation of human umbilical cord blood-derived mesenchymal stem cells into heart muscle precursor cells

Human umbilical cord blood-derived mesenchymal stem cells (UCB-MSCs) are a promising stem cell source with the potential to modulate the immune system as well as the capacity to differentiate into osteoblasts, chondrocytes, and adipocytes. In previous publications, UCB-MSCs have been successfully differentiated into cardiomyocytes. This study aimed to improve the efficacy of differentiation of UCB-MSCs into cardiomyocytes by combining 5-azacytidine (Aza) with mouse fetal heart extract (HE) in the induction medium. UCB-MSCs were isolated from umbilical cord blood according to a published protocol. Murine fetal hearts were used to produce fetal HE using a rapid freeze-thaw procedure. MSCs at the 3rd to 5th passage were differentiated into cardiomyocytes in two kinds of induction medium: complete culture medium plus Aza (Aza group) and complete culture medium plus Aza and fetal HE (Aza + HE group). The results showed that the cells in both kinds of induction medium exhibited the phenotype of cardiomyocytes. At the transcriptional level, the cells expressed a number of cardiac muscle-specific genes such as Nkx2.5, Gata 4, Mef2c, HCN2, hBNP, α-Ca, cTnT, Desmin, and β-MHC on day 27 in the Aza group and on day 18 in the Aza + HE group. At the translational level, sarcomic α-actin was expressed on day 27 in the Aza group and day 18 in the Aza + HE group. Although they expressed specific genes and proteins of cardiac muscle cells, the induced cells in both groups did not contract and beat spontaneously. These properties are similar to properties of heart muscle precursor cells in vivo. These results demonstrated that the fetal HE facilitates the differentiation process of human UCB-MSCs into heart muscle precursor cells.

cAMP-PKA inhibition of SK3 channel reduced both Ca2+ entry and cancer cell migration by regulation of SK3-Orai1 complex

SK3 channel mediates the migration of various cancer cells. When expressed in breast cancer cells, SK3 channel forms a complex with Orai1, a voltage-independent Ca(2+) channel. This SK3-Orai1 complex associates within lipid rafts where it controls a constitutive Ca(2+) entry leading to cancer cell migration and bone metastases development. Since cAMP was found to modulate breast cancer cell migration, we hypothesized that this could be explained by a modulation of SK3 channel activity. Herein, we study the regulation of SK3 channel by the cAMP-PKA pathway and the consequences for SK3-dependent Ca(2+) entry and cancer cell migration. We established that the beta-adrenergic receptor agonist, isoprenaline, or the direct adenylyl cyclase activator forskolin alone or in combination with the PDE4 inhibitor, CI-1044, decreased SK3 channel activity without modifying the expression of SK3 protein at the plasma membrane. Forskolin and CI-1044 reduced the SK3-dependent constitutive Ca(2+) entry and the SK3-dependent migration of MDA-MB-435s cells. PKA inhibition with KT 5720 reduced: (1) the effect of forskolin and CI-1044 by 50 % on Ca(2+) entry and (2) SK3 activity by inhibiting the serine phosphorylation of SK3. These cAMP-elevating agents displaced Orai1 protein outside lipid rafts in contrast to SK3, which remained in the lipid rafts fractions. All together, these results show that activation of the cAMP-PKA pathway decreases SK3 channel and SK3-Orai1 complex activities, leading to a decrease in both Ca(2+) entry and cancer cell migration. This work supports the potential use of cAMP-elevating agents to reduce cancer cell migration and may provide novel opportunities to address/prevent bone metastasis.

The Concise Guide to PHARMACOLOGY 2013/14: ion channels

The Concise Guide to PHARMACOLOGY 2013/14 provides concise overviews of the key properties of over 2000 human drug targets with their pharmacology, plus links to an open access knowledgebase of drug targets and their ligands (www.guidetopharmacology.org), which provides more detailed views of target and ligand properties. The full contents can be found at http://onlinelibrary.wiley.com/doi/10.1111/bph.12444/full. Ion channels are one of the seven major pharmacological targets into which the Guide is divided, with the others being G protein-coupled receptors, ligand-gated ion channels, catalytic receptors, nuclear hormone receptors, transporters and enzymes. These are presented with nomenclature guidance and summary information on the best available pharmacological tools, alongside key references and suggestions for further reading. A new landscape format has easy to use tables comparing related targets. It is a condensed version of material contemporary to late 2013, which is presented in greater detail and constantly updated on the website www.guidetopharmacology.org, superseding data presented in previous Guides to Receptors and Channels. It is produced in conjunction with NC-IUPHAR and provides the official IUPHAR classification and nomenclature for human drug targets, where appropriate. It consolidates information previously curated and displayed separately in IUPHAR-DB and the Guide to Receptors and Channels, providing a permanent, citable, point-in-time record that will survive database updates.

Channel-noise-induced stochastic facilitation in an auditory brainstem neuron model

Neuronal membrane potentials fluctuate stochastically due to conductance changes caused by random transitions between the open and closed states of ion channels. Although it has previously been shown that channel noise can nontrivially affect neuronal dynamics, it is unknown whether ion-channel noise is strong enough to act as a noise source for hypothesized noise-enhanced information processing in real neuronal systems, i.e., "stochastic facilitation". Here we demonstrate that biophysical models of channel noise can give rise to two kinds of recently discovered stochastic facilitation effects in a Hodgkin-Huxley-like model of auditory brainstem neurons. The first, known as slope-based stochastic resonance (SBSR), enables phasic neurons to emit action potentials that can encode the slope of inputs that vary slowly relative to key time constants in the model. The second, known as inverse stochastic resonance (ISR), occurs in tonically firing neurons when small levels of noise inhibit tonic firing and replace it with burstlike dynamics. Consistent with previous work, we conclude that channel noise can provide significant variability in firing dynamics, even for large numbers of channels. Moreover, our results show that possible associated computational benefits may occur due to channel noise in neurons of the auditory brainstem. This holds whether the firing dynamics in the model are phasic (SBSR can occur due to channel noise) or tonic (ISR can occur due to channel noise).

New perspectives in cyclic nucleotide-mediated functions in the CNS: the emerging role of cyclic nucleotide-gated (CNG) channels

Cyclic nucleotides play fundamental roles in the central nervous system (CNS) under both physiological and pathological conditions. The impact of cAMP and cGMP signaling on neuronal and glial cell functions has been thoroughly characterized. Most of their effects have been related to cyclic nucleotide-dependent protein kinase activity. However, cyclic nucleotide-gated (CNG) channels, first described as key mediators of sensory transduction in retinal and olfactory receptors, have been receiving increasing attention as possible targets of cyclic nucleotides in the CNS. In the last 15 years, consistent evidence has emerged for their expression in neurons and astrocytes of the rodent brain. Far less is known, however, about the functional role of CNG channels in these cells, although several of their features, such as Ca(2+) permeability and prolonged activation in the presence of cyclic nucleotides, make them ideal candidates for mediators of physiological functions in the CNS. Here, we review literature suggesting the involvement of CNG channels in a number of CNS cellular functions (e.g., regulation of membrane potential, neuronal excitability, and neurotransmitter release) as well as in more complex phenomena, like brain plasticity, adult neurogenesis, and pain sensitivity. The emerging picture is that functional and dysfunctional cyclic nucleotide signaling in the CNS has to be reconsidered including CNG channels among possible targets. However, concerted efforts and multidisciplinary approaches are still needed to get more in-depth knowledge in this field.

Role of Ih in the firing pattern of mammalian cold thermoreceptor endings

Mammalian peripheral cold thermoreceptors respond to cooling of their sensory endings with an increase in firing rate and modification of their discharge pattern. We recently showed that cultured trigeminal cold-sensitive (CS) neurons express a prominent hyperpolarization-activated current ( Ih), mainly carried by HCN1 channels, supporting subthreshold resonance in the soma without participating in the response to acute cooling. However, peripheral pharmacological blockade of Ih, or characterization of HCN1−/− mice, reveals a deficit in acute cold detection. Here we investigated the role of Ih in CS nerve endings, where cold sensory transduction actually takes place. Corneal CS nerve endings in mice show a rhythmic spiking activity at neutral skin temperature that switches to bursting mode when the temperature is lowered. Ih blockers ZD7288 and ivabradine alter firing patterns of CS nerve endings, lengthening interspike intervals and inducing bursts at neutral skin temperature. We characterized the CS nerve endings from HCN1−/− mouse corneas and found that they behave similar to wild type, although with a lower slope in the firing frequency vs. temperature relationship, thus explaining the deficit in cold perception of HCN1−/− mice. The firing pattern of nerve endings from HCN1−/− mice was also affected by ZD7288, which we attribute to the presence of HCN2 channels in the place of HCN1. Mathematical modeling shows that the firing phenotype of CS nerve endings from HCN1−/− mice can be reproduced by replacing HCN1 channels with the slower HCN2 channels rather than by abolishing Ih. We propose that Ih carried by HCN1 channels helps tune the frequency of the oscillation and the length of bursts underlying regular spiking in cold thermoreceptors, having important implications for neural coding of cold sensation.

Molecular identity, ontogeny, and cAMP modulation of the hyperpolarization-activated current in vestibular ganglion neurons

Properties, developmental regulation, and cAMP modulation of the hyperpolarization-activated current (I(h)) were investigated by the whole cell patch-clamp technique in vestibular ganglion neurons of the rat at two postnatal stages (P7-10 and P25-28). In addition, by RT-PCR and immunohistochemistry the identity and distribution of hyperpolarization-activated and cyclic nucleotide-gated channel (HCN) isoforms that generate I(h) were investigated. I(h) current density was larger in P25-28 than P7-10 rats, increasing 410% for small cells (<30 pF) and 200% for larger cells (>30 pF). The half-maximum activation voltage (V(1/2)) of I(h) was -102 mV in P7-10 rats and in P25-28 rats shifted 7 mV toward positive voltages. At both ages, intracellular cAMP increased I(h) current density, decreased its activation time constant (τ), and resulted in a rightward shift of V(1/2) by 9 mV. Perfusion of 8-BrcAMP increased I(h) amplitude and speed up its activation kinetics. I(h) was blocked by Cs(+), zatebradine, and ZD7288. As expected, these drugs also reduced the voltage sag caused with hyperpolarizing pulses and prevented the postpulse action potential generation without changes in the resting potential. RT-PCR analysis showed that HCN1 and HCN2 subunits were predominantly amplified in vestibular ganglia and end organs and HCN3 and HCN4 to a lesser extent. Immunohistochemistry showed that the four HCN subunits were differentially expressed (HCN1 > HCN2 > HCN3 ≥ HCN4) in ganglion slices and in cultured neurons at both P7-10 and P25-28 stages. Developmental changes shifted V(1/2) of I(h) closer to the resting membrane potential, increasing its functional role. Modulation of I(h) by cAMP-mediated signaling pathway constitutes a potentially relevant control mechanism for the modulation of afferent neuron discharge.

Quantitative single-cell ion-channel gene expression profiling through an improved qRT-PCR technique combined with whole cell patch clamp

Cellular excitability originates from a concerted action of different ion channels. The genomic diversity of ion channels (over 100 different genes) underlies the functional diversity of neurons in the central nervous system (CNS) and even within a specific type of neurons large differences in channel expression have been observed. Patch-clamp is a powerful technique to study the electrophysiology of excitability at the single cell level, allowing exploration of cell-to-cell variability. Only a few attempts have been made to link electrophysiological profiling to mRNA transcript levels and most suffered from experimental noise precluding conclusive quantitative correlations. Here we describe a refinement to the technique that combines patch-clamp analysis with quantitative real-time (qRT) PCR at the single cell level. Hereto the expression of a housekeeping gene was used to normalize for cell-to-cell variability in mRNA isolation and the subsequent processing steps for performing qRT-PCR. However, the mRNA yield from a single cell was insufficient for performing a valid qRT-PCR assay; this was resolved by including a RNA amplification step. The technique was validated on a stable Ltk(-) cell line expressing the Kv2.1 channel and on embryonic dorsal root ganglion (DRG) cells probing for the expression of Kv2.1. Current density and transcript quantity displayed a clear correlation when the qRT-PCR assay was done in twofold and the data normalized to the transcript level of the housekeeping gene GAPD. Without this normalization no significant correlation was obtained. This improved technique should prove very valuable for studying the molecular background of diversity in cellular excitability.

ATDB 2.0: A database integrated toxin-ion channel interaction data

We have developed an updated version of the Animal Toxin Database (ATDB 2.0) that provides a new bioinformatics resource for analyzing toxin-channel (T-C) interactions. Data on more than 54,000 T-C interactions, including 9193 high-confidence interactions, has been extracted, formatted and mapped to toxin and ion channel databases. The interaction data can be accessed easily through a new network browser on the website at http://protchem.hunnu.edu.cn/toxin. This resource may be useful for sequence pattern recognition and prediction of the function of new toxins.

Properties of low-threshold motor axons in the human median nerve

This study investigated the excitability and accommodative properties of low-threshold human motor axons to test whether these motor axons have greater expression of the persistent Na(+) conductance, I(NaP). Computer-controlled threshold tracking was used to study 22 single motor units and the data were compared with compound motor potentials of various amplitudes recorded in the same experimental session. Detailed comparisons were made between the single units and compound potentials that were 40% or 5% of maximal amplitude, the former because this is the compound potential size used in most threshold tracking studies of axonal excitability, the latter because this is the compound potential most likely to be composed entirely of motor axons with low thresholds to electrical recruitment. Measurements were made of the strength-duration relationship, threshold electrotonus, current-voltage relationship, recovery cycle and latent addition. The findings did not support a difference in I(NaP). Instead they pointed to greater activity of the hyperpolarization-activated inwardly rectifying current (I(h)) as the basis for low threshold to electrical recruitment in human motor axons. Computer modelling confirmed this finding, with a doubling of the hyperpolarization-activated conductance proving the best single parameter adjustment to fit the experimental data. We suggest that the hyperpolarization-activated cyclic nucleotide-gated (HCN) channel(s) expressed on human motor axons may be active at rest and contribute to resting membrane potential.

Functional impact of the hyperpolarization-activated current on the excitability of myelinated A-type vagal afferent neurons in the rat

1. The hyperpolarization-induced, cation-selective current I(h) is widely observed in peripheral sensory neurons of the vagal and dorsal root ganglia, but the peak magnitude and voltage- and time-dependent properties of this current vary widely across afferent fibre type. 2. Using patch clamp investigations of rat isolated vagal ganglion neurons (VGN) identified as myelinated A-type afferents, we established a compendium of functional correlates between changes in membrane potential and the dynamic discharge properties of these sensory neurons as a result of the controlled recruitment of I(h) using the current clamp technique. 3. Two robust responses were observed in response to hyperpolarizing step currents: (i) upon initiation of the negative step current, there was a rapid hyperpolarization of membrane potential followed by a depolarizing voltage sag (DVS) towards a plateau in membrane potential as a result of steady state recruitment of I(h); and (ii) upon termination of the negative step current, there was a rapid return to the pretest resting membrane potential that often led to spontaneous action potential discharge. These data were strongly correlated (r(2) > 0.9) with a broad compendium of dynamic discharge characteristics in these A-type VGN. 4. In response to depolarizing step currents of increasing magnitude, the discharge frequency of the A-type VGN responded with increases in the rate of sustained repetitive discharge. Upon termination of the depolarizing step current, there was a post-excitatory membrane hyperpolarization of a magnitude that was strongly correlated with action potential discharge rate (r(2) > 0.9). 5. Application of the selective hyperpolarization-activated cyclic nucleotide gated (HCN) channel blockers ZD7288 (10 micromol/L) or CsCl (1.0 mmol/L) abolished I(h) and all of the aforementioned functional correlates. In addition to reducing the excitability of the A-type VGN to step depolarizing currents. 6. Because there is increasing evidence that the HCN channel current may represent a valid target for pharmacological intervention, the quantitative relationships described in the present study could potentially help guide the molecular and/or chemical modification of HCN channel gating properties to effect a particular outcome in VGN discharge properties, ideally well beyond merely selective blockade of a particular HCN channel subtype.

Phototransduction motifs and variations

Seeing begins in the photoreceptors, where light is absorbed and signaled to the nervous system. Throughout the animal kingdom, photoreceptors are diverse in design and purpose. Nonetheless, phototransduction-the mechanism by which absorbed photons are converted into an electrical response-is highly conserved and based almost exclusively on a single class of photoproteins, the opsins. In this Review, we survey the G protein-coupled signaling cascades downstream from opsins in photoreceptors across vertebrate and invertebrate species, noting their similarities as well as differences.

Presence of cyclic nucleotide-gated channels in the rat urethra and their involvement in nerve-mediated nitrergic relaxation

We have addressed the distribution of cGMP-gated channels (CNG) in the rat urethra for the first time, as well as their putative role in mediating of the relaxation elicited by electrical field stimulation of nitrergic nerves. Functional studies have shown that specifically blocking CNG with L-cis-diltiazem leads to the rapid inhibition of urethral relaxation induced either by nitric oxide (NO) released by the nerves or by soluble guanylate cyclase activated with YC-1. By contrast, nerve-mediated noradrenergic contractions were only slowly and mildly reduced by L-cis-diltiazem. This effect was mimicked by lower concentrations of the D-diltiazem isomer, probably due to the nonspecific inhibition of voltage-dependent calcium channels. However, D-diltiazem did not affect relaxation responses. The expression of heteromeric retinal-like CNGA1 channels was demonstrated by conventional PCR on mRNA from the rat urethra. These channels were located in a subpopulation of intramuscular interstitial cells of Cajal (ICC) as well as in smooth muscle cells, although they were less abundant in the latter. CNG channels could not be visualized in any nervous structure within the urethral wall, in agreement with the emerging view that a subset of ICC serves as a target for NO. These channels could provide a suitable ionic mechanism to associate the changes in cytosolic calcium with the activation of the nitric NO-cGMP pathway and relaxation although the precise mechanisms involved remain to be elucidated.

Characteristics and physiological role of hyperpolarization activated currents in mouse cold thermoreceptors

Hyperpolarization-activated currents (I(h)) are mediated by the expression of combinations of hyperpolarization-activated, cyclic nucleotide-gated (HCN) channel subunits (HCN1-4). These cation currents are key regulators of cellular excitability in the heart and many neurons in the nervous system. Subunit composition determines the gating properties and cAMP sensitivity of native I(h) currents. We investigated the functional properties of I(h) in adult mouse cold thermoreceptor neurons from the trigeminal ganglion, identified by their high sensitivity to moderate cooling and responsiveness to menthol. All cultured cold-sensitive (CS) neurons expressed a fast activating I(h), which was fully blocked by extracellular Cs(+) or ZD7288 and had biophysical properties consistent with those of heteromeric HCN1-HCN2 channels. In CS neurons from HCN1(-/-) animals, I(h) was greatly reduced but not abolished. We find that I(h) activity is not essential for the transduction of cold stimuli in CS neurons. Nevertheless, I(h) has the potential to shape the excitability of CS neurons. First, I(h) blockade caused a membrane hyperpolarization in CS neurons of about 5 mV. Furthermore, impedance power analysis showed that all CS neurons had a prominent subthreshold membrane resonance in the 5-7 Hz range, completely abolished upon blockade of I(h) and absent in HCN1 null mice. This frequency range matches the spontaneous firing frequency of cold thermoreceptor terminals in vivo. Behavioural responses to cooling were reduced in HCN1 null mice and after peripheral pharmacological blockade of I(h) with ZD7288, suggesting that I(h) plays an important role in peripheral sensitivity to cold.

Movements of native C505 during channel gating in CNGA1 channels

We investigated conformational changes occurring in the C-linker and cyclic nucleotide-binding (CNB) domain of CNGA1 channels by analyzing the inhibition induced by thiol-specific reagents in mutant channels Q409C and A414C in the open and closed state. Cd(2+) (200 microM) inhibited irreversibly mutant channels Q409C and A414C in the closed but not in the open state. Cd(2+) inhibition was abolished in the mutant A414C(cys-free), in the double mutant A414C + C505T and in the tandem construct A414C + C505T/CNGA1, but it was present in the construct A414C + C505(cys-free). The cross-linker reagent M-2-M inhibited mutant channel Q409C in the open state. M-2-M inhibition in the open state was abolished in the double mutant Q409C + C505T and in the tandem construct Q409C + C505T/CNGA1. These results show that C(alpha) of C505 in the closed state is located at a distance between 4 and 10.5 A from the C(alpha) of A414 of the same subunit, but in the open state C505 moves towards Q409 of the same subunit at a distance that ranges from 10.5 to 12.3 A from C(alpha) of this residue. These results are not consistent with a 3-D structure of the CNGA1 channel homologous to the structure of HCN2 channels either in the open or in the closed state.

cGMP signalling in the mammalian brain: role in synaptic plasticity and behaviour

The second messenger cyclic guanosine 3',5'-monophosphate (cGMP) plays a crucial role in the control of cardiovascular and gastrointestinal homeostastis, but its effects on neuronal functions are less established. This review summarizes recent biochemical and functional data on the role of the cGMP signalling pathway in the mammalian brain, with a focus on the regulation of synaptic plasticity, learning, and other complex behaviours. Expression profiling, along with pharmacological and genetic manipulations, indicates important functions of nitric oxide (NO)-sensitive soluble guanylyl cyclases (sGCs), cGMP-dependent protein kinases (cGKs), and cGMP-regulated phosphodiesterases (PDEs) as generators, effectors, and modulators of cGMP signals in the brain, respectively. In addition, neuronal cGMP signalling can be transmitted through cyclic nucleotide-gated (CNG) or hyperpolarization-activated cyclic nucleotide-gated (HCN) ion channels. The canonical NO/sGC/cGMP/cGK pathway modulates long-term changes of synaptic activity in the hippocampus, amygdala, cerebellum, and other brain regions, and contributes to distinct forms of learning and memory, such as fear conditioning, motor adaptation, and object recognition. Behavioural studies indicate that cGMP signalling is also involved in anxiety, addiction, and the pathogenesis of depression and schizophrenia. At the molecular level, different cGK isoforms appear to mediate effects of cGMP on presynaptic transmitter release and postsynaptic functions. The cGKs have been suggested to modulate cytoskeletal organization, vesicle and AMPA receptor trafficking, and gene expression via phosphorylation of various substrates including VASP, RhoA, RGS2, hSERT, GluR1, G-substrate, and DARPP-32. These and other components of the cGMP signalling cascade may be attractive new targets for the treatment of cognitive impairment, drug abuse, and psychiatric disorders.

A nomenclature for ligand-gated ion channels

The ligand-gated ion channels that participate in fast synaptic transmission comprise the nicotinic acetylcholine, 5-hydroxytryptamine3 (5-HT3), gamma-aminobutyric acidA (GABA(A)), glycine, ionotropic glutamate and P2X receptor families. A consistent and systematic nomenclature for the individual subunits that comprise these receptors and the receptors that result from their co-assembly is highly desirable. There is also a need to develop criteria that aid in deciding which of the vast number of heteromeric combinations of subunits that can be assembled in heterologous expression systems in vitro, are known, or likely, to exist as functional receptors in vivo. The aim of this short article is to summarize the progress being made by the nomenclature committee of IUPHAR (NC-IUPHAR) in formulating recommendations that attempt to address these issues.

Cyclic nucleotide-gated channels

Cyclic nucleotide-gated (CNG) channels are ion channels which are activated by the binding of cGMP or cAMP. The channels are important cellular switches which transduce changes in intracellular concentrations of cyclic nucleotides into changes of the membrane potential and the Ca2+ concentration. CNG channels play a central role in the signal transduction pathways of vision and olfaction. Structurally, the channels belong to the superfamily of pore-loop cation channels. They share a common domain structure with hyperpolarization-activated cyclic nucleotide-gated (HCN) channels and Eag-like K+ channels. In this chapter, we give an overview on the molecular properties of CNG channels and describe the signal transduction pathways these channels are involved in. We will also summarize recent insights into the physiological and pathophysiological role of CNG channel proteins that have emerged from the analysis of CNG channel-deficient mouse models and human channelopathies.

Cyclic nucleotide-regulated cation channels

Cyclic nucleotide-regulated cation channels are ion channels whose activation is regulated by the direct binding of cAMP or cGMP to the channel protein. Two structurally related families of channels regulated by cyclic nucleotides have been identified, the cyclic nucleotide-gated channels and the hyperpolarization-activated cyclic nucleotide-gated channels. Cyclic nucleotide-gated channels play a key role in visual and olfactory transduction. Hyperpolarization-activated cyclic nucleotide-gated channels are present in the conduction system of the heart and are involved in the control of cardiac automaticity. Moreover, these channels are widely expressed in central and peripheral neurons, where they control a variety of fundamental processes.

Evolutionary genomics reveals lineage-specific gene loss and rapid evolution of a sperm-specific ion channel complex: CatSpers and CatSperbeta

The mammalian CatSper ion channel family consists of four sperm-specific voltage-gated Ca2+ channels that are crucial for sperm hyperactivation and male fertility. All four CatSper subunits are believed to assemble into a heteromultimeric channel complex, together with an auxiliary subunit, CatSperbeta. Here, we report a comprehensive comparative genomics study and evolutionary analysis of CatSpers and CatSperbeta, with important correlation to physiological significance of molecular evolution of the CatSper channel complex. The development of the CatSper channel complex with four CatSpers and CatSperbeta originated as early as primitive metazoans such as the Cnidarian Nematostella vectensis. Comparative genomics revealed extensive lineage-specific gene loss of all four CatSpers and CatSperbeta through metazoan evolution, especially in vertebrates. The CatSper channel complex underwent rapid evolution and functional divergence, while distinct evolutionary constraints appear to have acted on different domains and specific sites of the four CatSper genes. These results reveal unique evolutionary characteristics of sperm-specific Ca2+ channels and their adaptation to sperm biology through metazoan evolution.

Cyclic nucleotide-activated currents in cultured olfactory receptor neurons of the hawkmoth Manduca sexta

Moth pheromones cause rises in intracellular Ca(2+) concentrations that activate Ca(2+)-dependent cation channels in antennal olfactory receptor neurons. In addition, mechanisms of adaptation and sensitization depend on changes in cyclic nucleotide concentrations. Here, cyclic nucleotide-activated currents in cultured olfactory receptor neurons of the moth Manduca sexta are described, which share properties with currents through vertebrate cyclic nucleotide-gated channels. The cyclic nucleotide-activated currents of M. sexta carried Ca(2+) and monovalent cations. They were directly activated by cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), modulated by Ca(2+)/calmodulin, and inhibited by lanthanum. M. sexta cyclic nucleotide-activated currents developed in an all-or-none manner, which suggests that the underlying channels are coupled and act coordinately. At least one cAMP- and two cGMP-activated nonselective cation currents could be distinguished. Compared with the cAMP-activated current, both cGMP-activated currents appeared to conduct more Ca(2+) and showed a stronger down-regulation by Ca(2+)/calmodulin-dependent negative feedback. Furthermore, both cGMP-activated currents differed in their Ca(2+)-dependent inhibition. Thus M. sexta olfactory receptor neurons, like vertebrate sensory neurons, appear to express nonselective cyclic nucleotide-activated cation channels with different subunit compositions. Besides the nonselective cyclic nucleotide-activated cation currents, olfactory receptor neurons express a cAMP-dependent current. This current resembled a protein kinase-modulated low-voltage-activated Ca(2+) current.

How vision begins: an odyssey

Retinal rods and cones, which are the front-end light detectors in the eye, achieve wonders together by being able to signal single-photon absorption and yet also able to adjust their function to brightness changes spanning 10(9)-fold. How these cells detect light is now quite well understood. Not surprising for almost any biological process, the intial step of seeing reveals a rich complexity as the probing goes deeper. The odyssey continues, but the knowledge gained so far is already nothing short of remarkable in qualitative and quantitative detail. It has also indirectly opened up the mystery of odorant sensing. Basic science aside, clinical ophthalmology has benefited tremendously from this endeavor as well. This article begins by recapitulating the key developments in this understanding from the mid-1960s to the late 1980s, during which period the advances were particularly rapid and fit for an intricate detective story. It then highlights some details discovered more recently, followed by a comparison between rods and cones.

Gonadotropin-releasing hormone-1 neuronal activity is independent of hyperpolarization-activated cyclic nucleotide-modulated channels but is sensitive to protein kinase a-dependent phosphorylation

Pulsatile release of GnRH-1 stimulates the anterior pituitary and induces secretion of gonadotropin hormones. GnRH-1 release is modulated by many neurotransmitters that act via G protein-coupled membrane receptors. cAMP is the most ubiquitous effector for these receptors. GnRH-1 neurons express hyperpolarization-activated cyclic nucleotide-modulated (HCN) channel protein in vivo. HCN channels are involved in neuronal pacemaking and can integrate cAMP signals. cAMP-dependent protein kinase (PKA) is also activated by cAMP signals, and PKA-dependent phosphorylation modulates voltage-activated channels. In this report, these two pathways were examined in GnRH-1 neurons as integrators of forskolin (FSK)-induced stimulation. The HCN3 isoform was detected in GnRH-1 neurons obtained from mouse nasal explants. ZD7288, a HCN channel blocker, significantly reduced the efficiency of FSK to stimulate GnRH-1 neurons, whereas blockade of PKA with Rp-adenosine-3',5'-cyclic monophosphorothioate triethylammonium did not attenuate the FSK-induced stimulation. To ensure that disruption of HCN channels on GnRH-1 neurons was responsible for reduction of FSK stimulation, experiments were performed removing gamma-aminobutyric acid (GABA), the major excitatory input to GnRH-1 neurons in nasal explants. Under these conditions, Rp-adenosine-3',5'-cyclic monophosphorothioate triethylammonium, but not ZD7288, altered the FSK-induced response of GnRH-1 neurons. These studies indicate that PKA-dependent phosphorylation is involved in the FSK-induced stimulation of GnRH-1 neurons rather than HCN channels, and HCN channels integrate the FSK-induced stimulation on GABAergic neurons. In addition, blockade of HCN channels did not modify basal GnRH-1 neuronal activity when GABAergic input was intact or removed, negating a role for these channels in basal GABAergic or GnRH-1 neuronal activity.

Function and dysfunction of CNG channels: insights from channelopathies and mouse models

Channels directly gated by cyclic nucleotides (CNG channels) are important cellular switches that mediate influx of Na+ and Ca2+ in response to increases in the intracellular concentration of cAMP and cGMP. In photoreceptors and olfactory receptor neurons, these channels serve as final targets for cGMP and cAMP signaling pathways that are initiated by the absorption of photons and the binding of odorants, respectively. CNG channels have been also found in other types of neurons and in non-excitable cells. However, in most of these cells, the physiological role of CNG channels has yet to be determined. CNG channels have a complex heteromeric structure. The properties of individual subunits that assemble in specific stoichiometries to the native channels have been extensively investigated in heterologous expression systems. Recently, mutations in human CNG channel genes leading to inherited diseases (so-called channelopathies) have been functionally characterized. Moreover, mouse knockout models were generated to define the role of CNG channel proteins in vivo. In this review, we will summarize recent insights into the physiological and pathophysiological role of CNG channel proteins that have emerged from genetic studies in mice and humans.

NO/cGMP-dependent modulation of synaptic transmission

Nitric oxide (NO) is a multifunctional messenger in the CNS that can signal both in antero- and retrograde directions across synapses. Many effects of NO are mediated through its canonical receptor, the soluble guanylyl cyclase, and the second messenger cyclic guanosine-3',5'-monophosphate (cGMP). An increase of cGMP can also arise independently of NO via activation of membrane-bound particulate guanylyl cyclases by natriuretic peptides. The classical targets of cGMP are cGMP-dependent protein kinases (cGKs), cyclic nucleotide hydrolysing phosphodiesterases, and cyclic nucleotide-gated (CNG) cation channels. The NO/cGMP/cGK signalling cascade has been linked to the modulation of transmitter release and synaptic plasticity by numerous pharmacological and genetic studies. This review focuses on the role of NO as a retrograde messenger in long-term potentiation of transmitter release in the hippocampus. Presynaptic mechanisms of NO/cGMP/cGK signalling will be discussed with recently identified potential downstream components such as CaMKII, the vasodilator-stimulated phosphoprotein, and regulators of G protein signalling. NO has further been suggested to increase transmitter release through presynaptic clustering of a-synuclein. Alternative modes of NO/cGMP signalling resulting in inhibition of transmitter release and long-term depression of synaptic activity will also be addressed, as well as anterograde NO signalling in the cerebellum. Finally, emerging evidence for cGMP signalling through CNG channels and hyperpolarization-activated cyclic nucleotide-gated (HCN) channels will be discussed.

Differential patterning of cGMP in vascular smooth muscle cells revealed by single GFP-linked biosensors

Here, we report the design of unprecedented, non-FRET based cGMP-biosensors, named FlincGs, to assess the dynamics of nitric oxide (NO) and atrial natriuretic peptide (ANP) induced synthesis of intracellular cGMP, [cGMP](i). Regulatory fragments of PKG I alpha, PKG I beta, and an N-terminal deletion mutant of PKG I alpha were fused to circular permutated EGFP to generate alpha-, beta-, and delta-FlincG, with high dynamic ranges and apparent K(D,cGMP) values of 35 nM, 1.1 microM, and 170 nM, respectively. All indicators displayed significant selectivity for cGMP over cAMP, and 1.5- to 2.1-fold increases in fluorescence intensity at 510 nm when excited at 480 nm. Surprisingly, FlincGs displayed an additional excitation peak at 410 nm. delta-FlincG permitted ratiometric (480/410 nm) measurements, with a cGMP-specific 3.5-fold ratio change. In addition, delta-FlincG presented cGMP association and dissociation kinetics sufficiently fast to monitor rapid changes of [cGMP](i) in intact cells. In unpassaged, adenoviral transfected vascular smooth muscle (VSM) cells, delta-FlincG had an EC(50,cGMP) of 150 nM, and revealed transient global cGMP elevations to sustained physiological NO (EC(50,DEA/NO) = 4 nM), and the decay phase depended on the activity of PDE-5. In contrast, ANP elicited sustained submembrane elevations in [cGMP](i), which were converted to global cGMP elevations by inhibition of PDE-5 by sildenafil. These results indicate that FlincG is an innovative tool to elucidate the dynamics of a central biological signal, cGMP, and that NO and natriuretic peptides induce distinct cGMP patterning under the regulation of PDE-5, and therefore likely differentially engage cGMP targets.

Mapping ligand interactions with the hyperpolarization activated cyclic nucleotide modulated (HCN) ion channel binding domain using a soluble construct

Hyperpolarization activated cyclic nucleotide modulated (HCN) ion channel currents are activated by hyperpolarization and modulated in response to changes in cytosolic adenosine 3',5'-cyclic monophosphate (cAMP) concentrations. A cDNA chimera combining the rat HCN2 cyclic nucleotide binding domain and the DNA binding domain of the cAMP receptor protein (CRP) from E. coli and the histidine tag (HCN2/CRP) was expressed and purified. The construct is capable of forming only non-ligand dependent dimers because the C-linker region of the channel is not present in this construct. The construct binds 8-[[2-[(fluoresceinylthioureido) amino] ethyl] thio] adenosine-3',5'-cyclic monophosphate (8-fluo cAMP) with a Kd of 0.299 microM as determined with a monomer binding model. The Ki values of 20 ligands related to cAMP were measured in order to determine the properties necessary for a ligand to bind to the HCN2 binding domain. This is the first report of cAMP and gunaosine 3',5'-cyclic monophosphate (cGMP) affinities to the HCN2 binding domain being equivalent, even though they modulate the channel with a 10-fold difference in K0.5. Furthermore, the array of ligands measured allows the preference rank order for each purine ring position to be determined: position 1, H > NH2 > O; position 2, NH2 > Cl > H > O; position 6, NH2 > Cl > H > O; and position 8, NH2 > Cl > H > O. Finally, the ability of HCN2/CRP to bind cyclic nucleotide pyrimidine rings at concentrations approximately 1.33 times greater than cAMP suggests that ribofuranose is key for binding.

The vicissitudes of the pacemaker current I Kdd of cardiac purkinje fibers

The mechanisms underlying the pacemaker current in cardiac tissues is not agreed upon. The pacemaker potential in Purkinje fibers has been attributed to the decay of the potassium current I (Kdd). An alternative proposal is that the hyperpolarization-activated current I (f) underlies the pacemaker potential in all cardiac pacemakers. The aim of this review is to retrace the experimental development related to the pacemaker mechanism in Purkinje fibers with reference to findings about the pacemaker mechanism in the SAN as warranted. Experimental data and their interpretation are critically reviewed. Major findings were attributed to K(+) depletion in narrow extracellular spaces which would result in a time dependent decay of the inward rectifier current I (K1). In turn, this decay would be responsible for a "fake" reversal of the pacemaker current. In order to avoid such a postulated depletion, Ba(2+) was used to block the decay of I (K1). In the presence of Ba(2+) the time-dependent current no longer reversed and instead increased with time and more so at potentials as negative as -120 mV. In this regard, the distinct possibility needs to be considered that Ba(2+) had blocked I (Kdd) (and not only I (K1)). That indeed this was the case was demonstrated by studying single Purkinje cells in the absence and in the presence of Ba(2+). In the absence of Ba(2+), I (Kdd) was present in the pacemaker potential range and reversed at E (K). In the presence of Ba(2+), I (Kdd) was blocked and I (f) appeared at potentials negative to the pacemaker range. The pacemaker potential behaves in a manner consistent with the underlying I (Kdd) but not with I (f). The fact that I (f) is activated on hyperpolarization at potential negative to the pacemaker range makes it suitable as a safety factor to prevent the inhibitory action of more negative potentials on pacemaker discharge. It is concluded that the large body of evidence reviewed proves the pacemaker role of I (Kdd) (but not of I (f)) in Purkinje fibers.

Direct Inhibition of Cardiac Hyperpolarization-Activated Cyclic Nucleotide–Gated Pacemaker Channels by Clonidine

BACKGROUND

Inhibition of cardiac sympathetic tone represents an important strategy for treatment of cardiovascular disease, including arrhythmia, coronary heart disease, and chronic heart failure. Activation of presynaptic alpha2-adrenoceptors is the most widely accepted mechanism of action of the antisympathetic drug clonidine; however, other target proteins have been postulated to contribute to the in vivo actions of clonidine.

METHODS AND RESULTS

To test whether clonidine elicits pharmacological effects independent of alpha2-adrenoceptors, we have generated mice with a targeted deletion of all 3 alpha2-adrenoceptor subtypes (alpha2ABC-/-). Alpha2ABC-/- mice were completely unresponsive to the analgesic and hypnotic effects of clonidine; however, clonidine significantly lowered heart rate in alpha2ABC-/- mice by up to 150 bpm. Clonidine-induced bradycardia in conscious alpha2ABC-/- mice was 32.3% (10 microg/kg) and 26.6% (100 microg/kg) of the effect in wild-type mice. A similar bradycardic effect of clonidine was observed in isolated spontaneously beating right atria from alpha2ABC-knockout and wild-type mice. Clonidine inhibited the native pacemaker current (I(f)) in isolated sinoatrial node pacemaker cells and the I(f)-generating hyperpolarization-activated cyclic nucleotide-gated (HCN) 2 and HCN4 channels in transfected HEK293 cells. As a consequence of blocking I(f), clonidine reduced the slope of the diastolic depolarization and the frequency of pacemaker potentials in sinoatrial node cells from wild-type and alpha2ABC-knockout mice.

CONCLUSIONS

Direct inhibition of cardiac HCN pacemaker channels contributes to the bradycardic effects of clonidine gene-targeted mice in vivo, and thus, clonidine-like drugs represent novel structures for future HCN channel inhibitors.

Sp-tetraKCNG: A novel cyclic nucleotide gated K(+) channel

The sequence of a novel cGMP-regulated, tetrameric, K(+) selective channel (Sp-tetraKCNG) was discovered in the sea urchin Strongylocentrotus purpuratus. The Sp-tetraKCNG is a single polypeptide made of four KCNG domains similar to voltage-dependent Na(+) and Ca(2+) channels. Each KCNG domain has six transmembrane segments (S1-S6), the ion pore having the K(+) selectivity signature GYGD and a cyclic nucleotide-binding domain (CNBD). This novel channel is evolutionary located between K(+)-selective and voltage-dependent EAG channels and voltage-independent cationic CNG channels. Bilayer reconstitutions demonstrate such a cGMP-regulated K(+) selective channel in sea urchin spermatozoa.

Cyclic nucleotide-gated ion channels in sensory transduction

Cyclic nucleotide-gated (CNG) channels, directly activated by the binding of cyclic nucleotides, were first discovered in retinal rods, cones and olfactory sensory neurons. In the visual and olfactory systems, CNG channels mediate sensory transduction by conducting cationic currents carried primarily by sodium and calcium ions. In olfactory transduction, calcium in combination with calmodulin exerts a negative feedback on CNG channels that is the main molecular mechanism responsible for fast adaptation in olfactory sensory neurons. Six mammalian CNG channel genes are known and some human visual disorders are caused by mutations in retinal rod or cone CNG genes.