Inhibition of hyperpolarization-activated cyclic nucleotide-gated channels with natural flavonoid quercetin

Quercetin is a natural flavonoid which has been reported to be analgesic in different animal models of pain. However, the mechanism underlying the pain-relieving effects is still unclear. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels play critical roles in controlling pacemaker activity in cardiac and nervous systems, making the channel a new target for therapeutic exploration. In this study, we explored a series of flavonoids for their modulation on HCN channels. Among all tested flavonoids, quercetin was the most potent inhibitor for HCN channels with an IC₅₀ value of 27.32 ± 1.19 μM for HCN2. Furthermore, quercetin prominently left shifted the voltage-dependent activation curves of HCN channels and decelerated deactivation process. The results presented herein firstly characterize quercetin as a novel and potent inhibitor for HCN channels, which represents a novel structure for future drug design of HCN channel inhibitors.

HCN channels in the mammalian cochlea: Expression pattern, subcellular location, and age-dependent changes

Neuronal diversity in the cochlea is largely determined by ion channels. Among voltage‐gated channels, hyperpolarization‐activated cyclic nucleotide‐gated (HCN) channels open with hyperpolarization and depolarize the cell until the resting membrane potential. The functions for hearing are not well elucidated and knowledge about localization is controversial. We created a detailed map of subcellular location and co‐expression of all four HCN subunits across different mammalian species including CBA/J, C57Bl/6N, Ly5.1 mice, guinea pigs, cats, and human subjects. We correlated age‐related hearing deterioration in CBA/J and C57Bl/6N with expression levels of HCN1, −2, and −4 in individual auditory neurons from the same cohort. Spatiotemporal expression during murine postnatal development exposed HCN2 and HCN4 involvement in a critical phase of hair cell innervation. The huge diversity of subunit composition, but lack of relevant heteromeric pairing along the perisomatic membrane and axon initial segments, highlighted an active role for auditory neurons. Neuron clusters were found to be the hot spots of HCN1, −2, and −4 immunostaining. HCN channels were also located in afferent and efferent fibers of the sensory epithelium. Age‐related changes on HCN subtype expression were not uniform among mice and could not be directly correlated with audiometric data. The oldest mice groups revealed HCN channel up‐ or downregulation, depending on the mouse strain. The unexpected involvement of HCN channels in outer hair cell function where HCN3 overlaps prestin location emphasized the importance for auditory function. A better understanding may open up new possibilities to tune neuronal responses evoked through electrical stimulation by cochlear implants.

The hyperpolarization-activated cyclic nucleotide-gated channel currents contribute to oxaliplatin-induced hyperexcitability of DRG neurons

Humans are likely to experience mechanical allodynia and cold hyperalgesia after oxaliplatin intravenous injection. The mechanism by which oxaliplatin leads to these side effects is unknown. Since the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are involved in the automatic depolarization of action potentials, we speculated that HCN channels are involved in oxaliplatin-induced hyperalgesia through action potentials. Our results showed that the density of HCN channel currents and the excitability of dorsal root ganglion neurons both increased after oxaliplatin perfusion at the cellular level. The neuronal hyperexcitability could be alleviated by ivabradine. Ivabradine inhibited oxaliplatin-induced mechanical allodynia and cold hyperalgesia at the individual rat level. Oxaliplatin enhanced the function of HCN channels, which in turn promoted the automatic depolarization of action potentials. The acceleration of automatic depolarization excited the neurons and caused more rapid firing of action potentials. Therefore, the HCN channel is a potential therapeutic target for the hyperalgesia induced by oxaliplatin.

Understanding Docking Complexes of Macromolecules Using HADDOCK: The Synergy between Experimental Data and Computations

This protocol illustrates the modelling of a protein-peptide complex using the synergic combination of in silico analysis and experimental results. To this end, we use the integrative modelling software HADDOCK, which possesses the powerful ability to incorporate experimental data, such as NMR Chemical Shift Perturbations and biochemical protein-peptide interaction data, as restraints to guide the docking process. Based on the modelling results, a rational mutagenesis approach is used to validate the generated models. The experimental results allow to select a final structural model best representing the bona fide protein-peptide complex. The described protocol can also be applied to model protein-protein complexes. There is no size limit for the macromolecular complexes that can be characterized by HADDOCK as long as the 3D structures of the individual components are available.

Constitutively active HCN channels constrain detrusor excitability and modulate evoked contractions of human bladder

OBJECTIVE

Expression of Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels is reported in bladder, but the functional role remains unsettled. Here, we immunolocalized the HCN1 and HCN4 subtype in human bladder and investigated their functional significance.

METHODS

Bladder procured from ten organ donors was dissected into mucosa (containing urothelium and submucosa) and detrusor for double immunofluorescence of HCN1 and 4 subtypes with gap junction and neural proteins together with isometric tension recordings. Mucosa intact and denuded detrusor strips were stretched to a basal tension of 10 mN for eliciting either tetrodotoxin (TTX) resistant spontaneous, carbachol evoked contractions and TTX sensitive electrical field stimulated (EFS), pre and post-addition of HCN blocker, ZD7288 or the activator, Lamotrigine or the cholinesterase inhibitor, Neostigmine.

RESULTS

Double immunofluorescence revealed immunolocalization of HCN1 and HCN4 subtype with calcitonin gene related peptide (CGRP), choline acetyl transferase and gap junction proteins in mucosa and detrusor. Removal of mucosa significantly raised the resting tension and the force of spontaneous contractions upon cumulative addition of ZD7288 in micromolar range relative to Lamotrigine treated strips (P<0.05). ZD7288 [10 nM] did not affect the contractile response evoked by EFS or carbachol, but the addition of ZD7288 [10 nM] in presence of Neostigmine [1 µM] significantly enhanced the atropine and TTX sensitive EFS evoked contractions of mucosa denuded strips.

CONCLUSIONS

Overall, HCN channels immunolocalized in mucosa, smooth muscle, gap junctions and nerve fibers exert a tonic constraint on detrusor excitability, enable spatio-temporal integration of evoked contractions and constrain the release of neurotransmitters, respectively. In contrast to the pacemaker role in other organs, findings argue for a non-pacemaking role of HCN channels in human bladder.

Neuropathy in sporadic inclusion body myositis: A multi-modality neurophysiological study

OBJECTIVE

Sporadic inclusion body myositis (sIBM) has been associated with neuropathy. This study employs nerve excitability studies to re-examine this association and attempt to understand underlying pathophysiological mechanisms.

METHODS

Twenty patients with sIBM underwent median nerve motor and sensory excitability studies, clinical assessments, conventional nerve conduction testing (NCS) and quantitative thermal threshold studies. These results were compared to established normal controls, or results from a normal cohort of older control individuals.

RESULTS

Seven sIBM patients (35%) demonstrated abnormalities in conventional NCS, with ten patients (50%) demonstrating abnormalities in thermal thresholds. Median nerve motor and sensory excitability differed significantly in sIBM patients when compared to normal controls. None of these neurophysiological markers correlated significantly with clinical markers of sIBM severity.

CONCLUSION

A concurrent neuropathy exists in a significant proportion of sIBM patients, with nerve excitability studies revealing changes possibly consistent with axolemmal depolarization or concurrent neuronal adaptation to myopathy. Neuropathy in sIBM does not correlate with muscle disease severity and may reflect a differing tissue response to a common pathogenic factor.

SIGNIFICANCE

This study affirms the presence of a concurrent neuropathy in a large proportion of sIBM patients that appears independent of the severity of myopathy.

Effects of ghrelin on the electrical activities of substantia nigra dopaminergic neurons treated with MPP

Ghrelin, a 28 amino acid brain-gut peptide, has attracted increasing attention for its neuroprotective effect in Parkinson's disease (PD). In view of the pivotal role of excitability of dopaminergic neurons in substantia nigra pars compacta (SNc) in the function of nigrostriatal system, it is of great significance to elucidate the regulation of electrical activity of dopaminergic neurons by ghrelin, especially in PD pathogenesis. In this study, we tackled this issue by probing the effects of ghrelin on the electrophysiological properties of dopaminergic neurons in acute application of Methyl-4-phenylpyridinium (MPP⁺), a potent parkinsonizing agent in primates and rodents, with whole cell patch clamp technique. We first observed that MPP⁺ (10, 20 and 50 μM) inhibited the spontaneous firing activity of dopaminergic neurons with dose-dependent and time-dependent properties. MPP⁺ also hyperpolarized the membrane potential, inhibited the evoked firing activity and reduced the amplitude of the inward rectification characteristic (Sag) in dopaminergic neurons. Importantly, ghrelin (100 nM) could improve the above effects of MPP⁺ on the electrical activities of dopaminergic neurons. The potential mechanism of this phenomenon may be that ghrelin upregulated hyperpolarization-activated cyclic nucleotide-gated channel current (I_h) to antagonize the inhibition of MPP⁺ on I_h, thereby improving the electrical activities of dopaminergic neurons.

Effect of Ginkgo biloba extract on pacemaker channels encoded by HCN gene

BACKGROUND

In the present study, the electropharmacological activity of traditional Chinese medicine, Ginkgo biloba extract (GBE), on human hyperpolarization-activated nucleotide-gated (HCN) channels and the underlying "funny" currents was investigated.

METHODS

Standard two-electrode voltage-clamp recordings were employed to examine the properties of cloned HCN subunit currents expressed in Xenopus oocytes under controlled conditions and GBE administration.

RESULTS

We found that GBE irreversibly inhibited the HCN2 and HCN4 channel currents in a concentration-dependent fashion and that the HCN4 current was more sensitive to GBE compared with HCN2. In addition, GBE inhibition of the current amplitudes of HCN2 and HCN4 currents was accompanied by a decrease in the activation and deactivation kinetics.

CONCLUSION

The results of this study contribute toward illustrating the antiarrhythmic mechanism of GBE, which might be useful for the treatment of arrhythmia.

Top-down machine learning approach for high-throughput single-molecule analysis

Single-molecule approaches provide enormous insight into the dynamics of biomolecules, but adequately sampling distributions of states and events often requires extensive sampling. Although emerging experimental techniques can generate such large datasets, existing analysis tools are not suitable to process the large volume of data obtained in high-throughput paradigms. Here, we present a new analysis platform (DISC) that accelerates unsupervised analysis of single-molecule trajectories. By merging model-free statistical learning with the Viterbi algorithm, DISC idealizes single-molecule trajectories up to three orders of magnitude faster with improved accuracy compared to other commonly used algorithms. Further, we demonstrate the utility of DISC algorithm to probe cooperativity between multiple binding events in the cyclic nucleotide binding domains of HCN pacemaker channel. Given the flexible and efficient nature of DISC, we anticipate it will be a powerful tool for unsupervised processing of high-throughput data across a range of single-molecule experiments.

A Brief History of Pacemaking

Cardiac pacemaking is a most fundamental cardiac function, thoroughly investigated for decades with a multiscale approach at organ, tissue, cell and molecular levels, to clarify the basic mechanisms underlying generation and control of cardiac rhythm. Understanding the processes involved in pacemaker activity is of paramount importance for a basic physiological knowledge, but also as a way to reveal details of pathological dysfunctions useful in the perspective of a therapeutic approach. Among the mechanisms involved in pacemaking, the "funny" (I_f) current has properties most specifically fitting the requirements for generation and control of repetitive activity, and has consequently received the most attention in studies of the pacemaker function. Present knowledge of the basic mechanisms of pacemaking and the properties of funny channels has led to important developments of clinical relevance. These include: (1) the successful development of heart rate-reducing agents, such as ivabradine, able to control cardiac rhythm and useful in the treatment of diseases such as coronary artery disease, heart failure and tachyarrhythmias; (2) the understanding of the genetic basis of disorders of cardiac rhythm caused by HCN channelopathies; (3) the design of strategies to implement biological pacemakers based on transfer of HCN channels or of stem cell-derived pacemaker cells expressing I_f, with the ultimate goal to replace electronic devices. In this review, I will give a brief historical account of the discovery of the funny current and the development of the concept of I_f-based pacemaking, in the context of a wider, more complex model of cardiac rhythmic function.

Potassium channels in the neuronal homeostasis and neurodegenerative pathways underlying Alzheimer's disease: An update

With more than 80 subunits, potassium (K⁺) channels represent a group of ion channels showing high degree of diversity and ubiquity. They play important role in the control of membrane depolarization and cell excitability in several tissues, including the brain. Controlling the intracellular and extracellular K⁺ flow in cells, they also modulate the hormone and neurotransmitter release, apoptosis and cell proliferation. It is therefore not surprising that an improper functioning of K⁺ channels in neurons has been associated with pathophysiology of a wide range of neurological disorders, especially Alzheimer's disease (AD). This review aims to give a comprehensive overview of the basic properties and pathophysiological functions of the main classes of K⁺ channels in the context of disease processes, also discussing the progress, challenges and opportunities to develop drugs targeting these channels as potential pharmacological approach for AD treatment.

Intra- and Extracellular Pillars of a Unifying Framework for Homeostatic Plasticity: A Crosstalk Between Metabotropic Receptors and Extracellular Matrix

In the face of chronic changes in incoming sensory inputs, neuronal networks are capable of maintaining stable conditions of electrical activity over prolonged periods of time by adjusting synaptic strength, to amplify or dampen incoming inputs [homeostatic synaptic plasticity (HSP)], or by altering the intrinsic excitability of individual neurons [homeostatic intrinsic plasticity (HIP)]. Emerging evidence suggests a synergistic interplay between extracellular matrix (ECM) and metabotropic receptors in both forms of homeostatic plasticity. Activation of dopaminergic, serotonergic, or glutamate metabotropic receptors stimulates intracellular signaling through calmodulin-dependent protein kinase II, protein kinase A, protein kinase C, and inositol trisphosphate receptors, and induces changes in expression of ECM molecules and proteolysis of both ECM molecules (lecticans) and ECM receptors (NPR, CD44). The resulting remodeling of perisynaptic and synaptic ECM provides permissive conditions for HSP and plays an instructive role by recruiting additional signaling cascades, such as those through metabotropic glutamate receptors and integrins. The superimposition of all these signaling events determines intracellular and diffusional trafficking of ionotropic glutamate receptors, resulting in HSP and modulation of conditions for inducing Hebbian synaptic plasticity (i.e., metaplasticity). It also controls cell-surface delivery and activity of voltage- and Ca2+-gated ion channels, resulting in HIP. These mechanisms may modify epileptogenesis and become a target for therapeutic interventions.

Membrane and calcium clock mechanisms contribute variably as a function of temperature to setting cardiac pacemaker rate in zebrafish Danio rerio

Here, we show that heart rate in zebrafish Danio rerio is dependent upon two pacemaking mechanisms and it possesses a limited ability to reset the cardiac pacemaker with temperature acclimation. Electrocardiogram recordings, taken from individual, anaesthetised zebrafish that had been acclimated to 18, 23 or 28°C were used to follow the response of maximum heart rate (f_Hmax ) to acute warming from 18°C until signs of cardiac failure appeared (up to c. 40°C). Because f_Hmax was similar across the acclimation groups at almost all equivalent test temperatures, warm acclimation was limited to one significant effect, the 23°C acclimated zebrafish had a significantly higher (21%) peak f_Hmax and reached a higher (3°C) test temperature than the 18°C acclimated zebrafish. Using zatebradine to block the membrane hyperpolarisation-activated cyclic nucleotide-gated channels (HCN) and examine the contribution of the membrane clock mechanisms to cardiac pacemaking, f _Hmax was significantly reduced (by at least 40%) at all acute test temperatures and significantly more so at most test temperatures for zebrafish acclimated to 28°C vs. 23°C. Thus, HCN channels and the membrane clock were not only important, but could be modified by thermal acclimation. Using a combination of ryanodine (to block sarcoplasmic calcium release) and thapsigargin (to block sarcoplasmic calcium reuptake) to examine the contribution of sarcoplasmic reticular handling of calcium and the calcium clock, f _Hmax was again consistently reduced independent of the test temperature and acclimation temperature, but to a significantly lesser degree than zatebradine for zebrafish acclimated to both 28 and 18°C. Thus, the calcium clock mechanism plays an additional role in setting pacemaker activity that was independent of temperature. In conclusion, the zebrafish cardiac pacemaker has a limited temperature acclimation ability compared with known effects for other fishes and involves two pacemaking mechanisms, one of which was independent of temperature.

N 6-modified cAMP derivatives that activate protein kinase A also act as full agonists of murine HCN2 channels

cAMP acts as a second messenger in many cellular processes. Three protein types mainly mediate cAMP-induced effects: PKA, exchange protein directly activated by cAMP (Epac), and cyclic nucleotide-modulated channels (cyclic nucleotide-gated or hyperpolarization-activated and cyclic nucleotide-modulated (HCN) channels). Discrimination among these cAMP signaling pathways requires specific targeting of only one protein. Previously, cAMP modifications at position N ⁶ of the adenine ring (PKA) and position 2'-OH of the ribose (Epac) have been used to produce target-selective compounds. However, cyclic nucleotide-modulated ion channels were usually outside of the scope of these previous studies. These channels are widely distributed, so possible channel cross-activation by PKA- or Epac-selective agonists warrants serious consideration. Here we demonstrate the agonistic effects of three PKA-selective cAMP derivatives, N ⁶-phenyladenosine-3',5'-cyclic monophosphate (N ⁶-Phe-cAMP), N ⁶-benzyladenosine-3',5'-cyclic monophosphate (N ⁶-Bn-cAMP), and N ⁶-benzoyl-adenosine-3',5'-cyclic monophosphate (N ⁶-Bnz-cAMP), on murine HCN2 pacemaker channels. Electrophysiological characterization in Xenopus oocytes revealed that these derivatives differ in apparent affinities depending on the modification type but that their efficacy and effects on HCN2 activation kinetics are similar to those of cAMP. Docking experiments suggested a pivotal role of Arg-635 at the entrance of the binding pocket in HCN2, either causing stabilizing cation-π interactions with the aromatic ring in N ⁶-Phe-cAMP or N ⁶-Bn-cAMP or a steric clash with the aromatic ring in N ⁶-Bnz-cAMP. A reduced apparent affinity of N ⁶-Phe-cAMP toward the variants R635A and R635E strengthened that notion. We conclude that some PKA activators also effectively activate HCN2 channels. Hence, when studying PKA-mediated cAMP signaling with cAMP derivatives in a native environment, activation of HCN channels should be considered.

Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels: An Emerging Role in Neurodegenerative Diseases

Neurodegenerative diseases such as Parkinson’s disease (PD), Alzheimer’s disease (AD), amyotrophic lateral sclerosis (ALS), and spinal muscular atrophy (SMA) are chronic, progressive, and age-associated neurological disorders characterized by neuronal deterioration in specific brain regions. Although the specific pathological mechanisms underlying these disorders have remained elusive, ion channel dysfunction has become increasingly accepted as a potential mechanism for neurodegenerative diseases. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are encoded by the HCN1-4 gene family and conduct the hyperpolarization-activated current (I_h). These channels play important roles in modulating cellular excitability, rhythmic activity, dendritic integration, and synaptic transmission. In the present review, we first provide a comprehensive picture of the role of HCN channels in PD by summarizing their role in the regulation of neuronal activity in PD-related brain regions. Dysfunction of I_h may participate in 1-methyl-4-phenylpyridinium (MPP⁺)-induced toxicity and represent a pathogenic mechanism in PD. Given current reports of the critical role of HCN channels in neuroinflammation and depression, we also discussed the putative contribution of HCN channels in inflammatory processes and non-motor symptoms in PD. In the second section, we summarize how HCN channels regulate the formation of β-amyloid peptide in AD and the role of these channels in learning and memory. Finally, we briefly discuss the effects of HCN channels in ALS and SMA based on existing discoveries.

Response to coincident inputs in electrically coupled primary afferents is heterogeneous and is enhanced by H-current (IH) modulation

Electrical synapses represent a widespread modality of interneuronal communication in the mammalian brain. These contacts, by lowering the effectiveness of random or temporally uncorrelated inputs, endow circuits of coupled neurons with the ability to selectively respond to simultaneous depolarizations. This mechanism may support coincidence detection, a property involved in sensory perception, organization of motor outputs, and improvement signal-to-noise ratio. While the role of electrical coupling is well established, little is known about the contribution of the cellular excitability and its modulations to the susceptibility of groups of neurons to coincident inputs. Here, we obtained dual whole cell patch-clamp recordings of pairs of mesencephalic trigeminal (MesV) neurons in brainstem slices from rats to evaluate coincidence detection and its determinants. MesV neurons are primary afferents involved in the organization of orofacial behaviors whose cell bodies are electrically coupled mainly in pairs through soma-somatic gap junctions. We found that coincidence detection is highly heterogeneous across the population of coupled neurons. Furthermore, combined electrophysiological and modeling approaches reveal that this heterogeneity arises from the diversity of MesV neuron intrinsic excitability. Consistently, increasing these cells' excitability by upregulating the hyperpolarization-activated cationic current (I_H) triggered by cGMP results in a dramatic enhancement of the susceptibility of coupled neurons to coincident inputs. In conclusion, the ability of coupled neurons to detect coincident inputs is critically shaped by their intrinsic electrophysiological properties, emphasizing the relevance of neuronal excitability for the many functional operations supported by electrical transmission in mammals. NEW & NOTEWORTHY We show that the susceptibility of pairs of coupled mesencephalic trigeminal (MesV) neurons to coincident inputs is highly heterogenous and depends on the interaction between electrical coupling and neuronal excitability. Additionally, upregulating the hyperpolarization-activated cationic current (I_H) by cGMP results in a dramatic increase of this susceptibility. The I_H and electrical synapses have been shown to coexist in many neuronal populations, suggesting that modulation of this conductance could represent a common strategy to regulate circuit operation supported by electrical coupling.

Analysis of excitatory and inhibitory neuron types in the inferior colliculus based on properties

The inferior colliculus (IC) is a large midbrain nucleus that integrates inputs from many auditory brainstem and cortical structures. Despite its prominent role in auditory processing, the various cell types and their connections within the IC are not well characterized. To further separate GABAergic and non-GABAergic neuron types according to their physiological properties, we used a mouse model that expresses channelrhodopsin and enhanced yellow fluorescent protein in all GABAergic neurons and allows identification of GABAergic cells by light stimulation. Neuron types were classified upon electrophysiological measurements of the hyperpolarizing-activated current (I_h) in acute brain slices of young adult mice. All GABAergic neurons from our sample displayed slow-activating I_h with moderate amplitudes, whereas a subset of excitatory neurons showed fast-activating I_h with large amplitudes. This is in agreement with our finding that immunoreactivity against the fast-gating hyperpolarization-activated and cyclic-nucleotide-gated 1 (HCN1) channel was present around excitatory neurons, whereas the slow-gating HCN4 channel was found perisomatically around most inhibitory neurons. I_h properties and neurotransmitter types were correlated with firing patterns to depolarizing current pulses. All GABAergic neurons displayed adapting firing patterns very similar to the majority of glutamatergic neurons. About 15% of the glutamatergic neurons showed an onset spiking pattern, always in combination with large and fast I_h. We conclude that HCN channel subtypes are differentially distributed in IC neuron types and correlate with neurotransmitter type and firing pattern. In contrast to many other brain regions, membrane properties and firing patterns were similar in GABAergic neurons and about one-third of the excitatory neurons. NEW & NOTEWORTHY Neuron types in the central nucleus of the auditory midbrain are not well characterized regarding their transmitter type, ion channel composition, and firing pattern. The present study shows that GABAergic neurons have slowly activating hyperpolarizing-activated current (I_h) and an adaptive firing pattern whereas at least four types of glutamatergic neurons exist regarding their I_h properties and firing patterns. Many of the glutamatergic neurons were almost indistinguishable from the GABAergic neurons regarding I_h properties and firing pattern.

Upper and lower limb motor axons demonstrate differential excitability and accommodation to strong hyperpolarizing currents during induced hyperthermia

Length-dependent peripheral neuropathy typically involves the insidious onset of sensory loss in the lower limbs before later progressing proximally. Recent evidence proposes hyperpolarization-activated cyclic nucleotide-gated (HCN) channels as dysfunctional in rodent models of peripheral neuropathy, and therefore differential expression of HCN channels in the lower limbs was hypothesized as a pathophysiological mechanism accounting for the pattern of symptomatology within this study. We studied six healthy participants, using motor axon excitability including strong and long [-70% and -100% hyperpolarizing threshold electrotonus (TEh)] hyperpolarizing currents to preferably study HCN channel function from the median and tibial nerves from high (40%) and low (20%) threshold. This was recorded at normothermia (~32°C) and then repeated during hyperthermia (~40°C) as an artificial hyperpolarizing axon stress. Significant differences between recovery cycle, superexcitability, accommodation to small depolarizing currents, and alterations in late stages of the inward-rectifying currents of strongest (-70% and -100% TEh) currents were observed in the lower limbs during hyperthermia. We demonstrate differences in late I_H current flow, which implies higher expression of HCN channel isoforms. The findings also indicate their potential inference in the symptomatology of length-dependent peripheral neuropathies and may be a unique target for minimizing symptomatology and pathogenesis in acquired disease. NEW & NOTEWORTHY This study demonstrates nerve excitability differences between the upper and lower limbs during hyperthermia, an experimentally induced axonal stress. The findings indicate that there is differential expression of slow hyperpolarization-activated cyclic nucleotide-gated (HCN) channel isoforms between the upper and lower limbs, which was demonstrated through strong, long hyperpolarizing currents during hyperthermia. Such mechanisms may underlie postural control but render the lower limbs susceptible to dysfunction in disease states.

Protein and surface expression of HCN2 and HCN4 subunits in mesocorticolimbic areas after cocaine sensitization

The I_h is a mixed depolarizing current present in neurons which, upon activation by hyperpolarization, modulates neuronal excitability in the mesocorticolimbic (MCL) system, an area which regulates emotions such as pleasure, reward, and motivation. Its biophysical properties are determined by HCN protein expression profiles, specifically HCN subunits 1-4. Previously, we reported that cocaine-induced behavioral sensitization increases HCN2 protein expression in all MCL areas with the Ventral Tegmental Area (VTA) showing the most significant increase. Recent evidence suggests that HCN4 also has an important expression in the MCL system. Although there is a significant expression of HCN channels in the MCL system their role in addictive processes is largely unknown. Thus, in this study we aim to compare HCN2 and HCN4 expression profiles and their cellular compartmental distribution in the MCL system, before and after cocaine sensitization. Surface/intracellular (S/I) ratio analysis indicates that VTA HCN2 subunits are mostly expressed in the cell surface in contrast to other areas tested. Our findings demonstrate that after cocaine sensitization, the HCN2 S/I ratio in the VTA was decreased whereas in the Prefrontal Cortex it was increased. In addition, HCN4 total expression in the VTA was decreased after cocaine sensitization, although the S/I ratio was not altered. Together, these results demonstrate differential cocaine effects on HCN2 and HCN4 protein expression profiles and therefore suggest a diverse I_h modulation of cellular activity during cocaine addictive processes.

HCN channels: New targets for the design of an antidepressant with rapid effects

BACKGROUND

Major depressive disorder (MDD) is a prevalent neuropsychiatric disease that carries a staggering global burden. Although numerous antidepressants are available on the market, unfortunately, many patients die by committing suicide as a result of the therapeutic lag between treatment initiation and the improvement of depressive symptoms. This therapeutic lag highlights the need for new antidepressants that provide rapid relief of depressive symptoms.

METHOD

In this review, we discuss the seminal researches on hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in animal models of depression and highlight the substantial evidence supporting the development of rapid-acting antidepressants targeting HCN channels.

RESULTS

HCN channels are associated with the risk of depression and targeting HCN channels or its auxiliary subunit tetratricopeptide repeat-containing Rab8b-interacting protein (TRIP8b) function may exert a rapid antidepressant-like effect.

CONCLUSIONS

Compounds acting on HCN subunits or the TRIP8b-HCN interaction site may be excellent candidates for development into effective drugs with rapid antidepressant action.

Alkylphenol inverse agonists of HCN1 gating: H-bond propensity, ring saturation and adduct geometry differentially determine efficacy and potency

BACKGROUND AND PURPOSE

In models of neuropathic pain, inhibition of HCN1 is anti-hyperalgesic. 2,6-di-iso-propyl phenol (propofol) and its non-anesthetic congener, 2,6-di-tert-butyl phenol, inhibit HCN1 channels by stabilizing closed state(s).

EXPERIMENTAL APPROACH

Using in vitro electrophysiology and kinetic modeling, we systematically explore the contribution of ligand architecture to alkylphenol-channel coupling.

KEY RESULTS

When corrected for changes in hydrophobicity (and propensity for intra-membrane partitioning), the decrease in potency upon 1-position substitution (NCO∼OH >> SH >>> F) mirrors the ligands' H-bond acceptor (NCO > OH > SH >>> F) but not donor profile (OH > SH >>> NCO∼F). H-bond elimination (OH to F) corresponds to a ΔΔG of ∼4.5 kCal mol^-1 loss of potency with little or no disruption of efficacy. Substitution of compact alkyl groups (iso-propyl, tert-butyl) with shorter (ethyl, methyl) or more extended (sec-butyl) adducts disrupts both potency and efficacy. Ring saturation (with the obligate loss of both planarity and π electrons) primarily disrupts efficacy.

CONCLUSIONS AND IMPLICATIONS

A hydrophobicity-independent decrement in potency at higher volumes suggests the alkylbenzene site has a volume of ≥800 ų. Within this, a relatively static (with respect to ligand) H-bond donor contributes to initial binding with little involvement in generation of coupling energy. The influence of π electrons/ring planarity and alkyl adducts on efficacy reveals these aspects of the ligand present towards a face of the channel that undergoes structural changes during opening. The site's characteristics suggest it is "druggable"; introduction of other adducts on the ring may generate higher potency inverse agonists.

HCN4 knockdown in dorsal hippocampus promotes anxiety-like behavior in mice

Hyperpolarization‐activated and cyclic nucleotide‐gated (HCN) channels mediate the I_h current in the murine hippocampus. Disruption of the I_h current by knockout of HCN1, HCN2 or tetratricopeptide repeat‐containing Rab8b‐interacting protein has been shown to affect physiological processes such as synaptic integration and maintenance of resting membrane potentials as well as several behaviors in mice, including depressive‐like and anxiety‐like behaviors. However, the potential involvement of the HCN4 isoform in these processes is unknown. Here, we assessed the contribution of the HCN4 isoform to neuronal processing and hippocampus‐based behaviors in mice. We show that HCN4 is expressed in various regions of the hippocampus, with distinct expression patterns that partially overlapped with other HCN isoforms. For behavioral analysis, we specifically modulated HCN4 expression by injecting recombinant adeno‐associated viral (rAAV) vectors mediating expression of short hairpin RNA against hcn4 (shHcn4) into the dorsal hippocampus of mice. HCN4 knockdown produced no effect on contextual fear conditioning or spatial memory. However, a pronounced anxiogenic effect was evident in mice treated with shHcn4 compared to control littermates. Our findings suggest that HCN4 specifically contributes to anxiety‐like behaviors in mice.

Cardiomyocyte Progenitor Cells as a Functional Gene Delivery Vehicle for Long-Term Biological Pacing

Sustained pacemaker function is a challenge in biological pacemaker engineering. Human cardiomyocyte progenitor cells (CMPCs) have exhibited extended survival in the heart after transplantation. We studied whether lentivirally transduced CMPCs that express the pacemaker current I_f (encoded by HCN4) can be used as functional gene delivery vehicle in biological pacing. Human CMPCs were isolated from fetal hearts using magnetic beads coated with Sca-1 antibody, cultured in nondifferentiating conditions, and transduced with a green fluorescent protein (GFP)- or HCN4-GFP-expressing lentivirus. A patch-clamp analysis showed a large hyperpolarization-activated, time-dependent inward current (−20 pA/pF at −140 mV, n = 14) with properties typical of I_f in HCN4-GFP-expressing CMPCs. Gap-junctional coupling between CMPCs and neonatal rat ventricular myocytes (NRVMs) was demonstrated by efficient dye transfer and changes in spontaneous beating activity. In organ explant cultures, the number of preparations showing spontaneous beating activity increased from 6.3% in CMPC/GFP-injected preparations to 68.2% in CMPC/HCN4-GFP-injected preparations (P < 0.05). Furthermore, in CMPC/HCN4-GFP-injected preparations, isoproterenol induced a significant reduction in cycle lengths from 648 ± 169 to 392 ± 71 ms (P < 0.05). In sum, CMPCs expressing HCN4-GFP functionally couple to NRVMs and induce physiologically controlled pacemaker activity and may therefore provide an attractive delivery platform for sustained pacemaker function.

Control of Xenopus Tadpole Locomotion via Selective Expression of Ih in Excitatory Interneurons

Locomotion relies on the coordinated activity of rhythmic neurons in the hindbrain and spinal cord and depends critically on the intrinsic properties of excitatory interneurons. Therefore, understanding how ion channels sculpt the properties of these interneurons, and the consequences for circuit function and behavior, is an important task. The hyperpolarization-activated cation current, Ih, is known to play important roles in shaping neuronal properties and for rhythm generation in many neuronal networks. We show in stage 42 Xenopus laevis frog tadpoles that Ih is strongly expressed only in excitatory descending interneurons (dINs), an important ipsilaterally projecting population that drives swimming activity. The voltage-dependent HCN channel blocker ZD7288 completely abolished a prominent depolarizing sag potential in response to hyperpolarization, the hallmark of Ih, and hyperpolarized dINs. ZD7288 also affected dIN post-inhibitory rebound firing, upon which locomotor rhythm generation relies, and disrupted locomotor output. Block of Ih also unmasked an activity-dependent ultraslow afterhyperpolarization (usAHP) in dINs following swimming, mediated by a dynamic Na/K pump current. This usAHP, unmasked in dINs by ZD7288, resulted in suprathreshold stimuli failing to evoke swimming at short inter-swim intervals, indicating an important role for Ih in maintaining swim generation capacity and in setting the post-swim refractory period of the network. Collectively, our data suggest that the selective expression of Ih in dINs determines specific dIN properties that are important for rhythm generation and counteracts an activity-dependent usAHP to ensure that dINs can maintain coordinated swimming over a wide range of inter-swim intervals.

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Ih is strongly expressed in Xenopus locomotor-rhythm-generating dIN interneurons

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Ih is active at rest in dINs, contributing to their distinct electrical properties

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dINs normally lack a Na pump-dependent ultra-slow afterhyperpolarization (usAHP)

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Ih counterbalances dIN usAHPs to preserve tadpole rhythm generating capacity