HCN pacemaker channels and pain: a drug discovery perspective

Cryo-EM structure of human HCN3 channel and its regulation by cAMP

HCN channels are important for regulating heart rhythm and nerve activity and have been studied as potential drug targets for treating depression, arrhythmia, nerve pain, and epilepsy. Despite possessing unique pharmacological properties, HCN channels share common characteristics in that they are activated by hyperpolarization and modulated by cAMP and other membrane lipids. However, the mechanisms of how these ligands bind and modulate HCN channels are unclear. In this study, we solved structures of full-length human HCN3 using cryo-EM and captured two different states, including a state without any ligand bound and a state with cAMP bound. Our structures reveal the novel binding sites for cholesteryl hemisuccinate in apo state and show how cholesteryl hemisuccinate and cAMP binding cause conformational changes in different states. These findings explain how these small modulators are sensed in mammals at the molecular level. The results of our study could help to design more potent and specific compounds to influence HCN channel activity and offer new therapeutic possibilities for diseases that lack effective treatment.

Ih current stabilizes excitability in rodent DRG neurons and reverses hyperexcitability in a nociceptive neuron model of inherited neuropathic pain

We show here that hyperpolarization-activated current (Ih ) unexpectedly acts to inhibit the activity of dorsal root ganglion (DRG) neurons expressing WT Nav1.7, the largest inward current and primary driver of DRG neuronal firing, and hyperexcitable DRG neurons expressing a gain-of-function Nav1.7 mutation that causes inherited erythromelalgia (IEM), a human genetic model of neuropathic pain. In this study we created a kinetic model of Ih and used it, in combination with dynamic-clamp, to study Ih function in DRG neurons. We show, for the first time, that Ih increases rheobase and reduces the firing probability in small DRG neurons, and demonstrate that the amplitude of subthreshold oscillations is reduced by Ih . Our results show that Ih , due to slow gating, is not deactivated during action potentials (APs) and has a striking damping action, which reverses from depolarizing to hyperpolarizing, close to the threshold for AP generation. Moreover, we show that Ih reverses the hyperexcitability of DRG neurons expressing a gain-of-function Nav1.7 mutation that causes IEM. In the aggregate, our results show that Ih unexpectedly has strikingly different effects in DRG neurons as compared to previously- and well-studied cardiac cells. Within DRG neurons where Nav1.7 is present, Ih reduces depolarizing sodium current inflow due to enhancement of Nav1.7 channel fast inactivation and creates additional damping action by reversal of Ih direction from depolarizing to hyperpolarizing close to the threshold for AP generation. These actions of Ih limit the firing of DRG neurons expressing WT Nav1.7 and reverse the hyperexcitability of DRG neurons expressing a gain-of-function Nav1.7 mutation that causes IEM. KEY POINTS: Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels, the molecular determinants of hyperpolarization-activated current (Ih ) have been characterized as a 'pain pacemaker', and thus considered to be a potential molecular target for pain therapeutics. Dorsal root ganglion (DRG) neurons express Nav1.7, a channel that is not present in central neurons or cardiac tissue. Gain-of-function mutations (GOF) of Nav1.7 identified in inherited erythromelalgia (IEM), a human genetic model of neuropathic pain, produce DRG neuron hyperexcitability, which in turn produces severe pain. We found that Ih increases rheobase and reduces firing probability in small DRG neurons expressing WT Nav1.7, and demonstrate that the amplitude of subthreshold oscillations is reduced by Ih . We also demonstrate that Ih reverses the hyperexcitability of DRG neurons expressing a GOF Nav1.7 mutation (L858H) that causes IEM. Our results show that, in contrast to cardiac cells and CNS neurons, Ih acts to stabilize DRG neuron excitability and prevents excessive firing.

Peripheral CCL2 induces inflammatory pain via regulation of Ih currents in small diameter DRG neurons

The C-C motif chemokine ligand 2 (CCL2) has been implicated in chronic pain, but its exact mechanism of peripheral sensitization is unknown. In this study, we aimed to clarify the mechanism of CCL2 regulation of ion channels. Our behavioral experiments revealed that ZD7288, a blocker of Ih current, can inhibit CFA and CCL2-mediated mechanical and thermal nociceptive sensitization. Furthermore, patch clamp studies demonstrated that CFA-induced peripheral sensitization primarily affects the excitability of small-diameter DRG neurons. Further studies revealed that inflammatory pain caused by CFA or incubation of DRG with CCL2 mainly affected Ih currents in small-diameter DRG neurons, which were blocked by co-incubation CCR2 antagonist INCB3344 or adenylate cyclase inhibitor SQ22536. Immunohistochemical staining showed that both intraplantar injection of CFA as well as DRG injection of CCL2 resulted in significant upregulation of CCR2+/HCN2+ expression. In conclusion, we suggest in the inflammatory pain state, CCL2 can act on small-diameter DRG neurons, leading to upregulation of HCN2 expression and consequently Ih, which in turn leads to neuronal hyperexcitability.

Optogenetics and Optical Tools in Automated Patch Clamping

Automated patch clamping (APC) has been used for almost two decades to increase the throughput of electrophysiological measurements, especially in preclinical safety screening of drug compounds. Typically, cells are suctioned onto holes in planar surfaces and a stronger subsequent suction allows access to a whole cell configuration for electrical measurement of ion channel activity. The development of optogenetic tools over a wide range of wavelengths (UV to IR) provides powerful tools for improving spatiotemporal control of in vivo and in vitro experiments and is emerging as a powerful means of investigating cell networks (neuronal), single cell transduction, and subcellular pathways.Combining APC and optogenetic tools paves the way for improved investigation and control of cell kinetics and provides the opportunity for collecting robust data for new and exciting applications and therapeutic areas. Here, we present an APC optogenetics capability on the Qube Opto 384 system including experiments on light activated ion channels and photoactivated ligands.

HCN channel-mediated neuromodulation can control action potential velocity and fidelity in central axons

Hyperpolarization-activated cyclic-nucleotide-gated (HCN) channels control electrical rhythmicity and excitability in the heart and brain, but the function of HCN channels at the subcellular level in axons remains poorly understood. Here, we show that the action potential conduction velocity in both myelinated and unmyelinated central axons can be bidirectionally modulated by a HCN channel blocker, cyclic adenosine monophosphate (cAMP), and neuromodulators. Recordings from mouse cerebellar mossy fiber boutons show that HCN channels ensure reliable high-frequency firing and are strongly modulated by cAMP (EC₅₀ 40 µM; estimated endogenous cAMP concentration 13 µM). In addition, immunogold-electron microscopy revealed HCN2 as the dominating subunit in cerebellar mossy fibers. Computational modeling indicated that HCN2 channels control conduction velocity primarily by altering the resting membrane potential and are associated with significant metabolic costs. These results suggest that the cAMP-HCN pathway provides neuromodulators with an opportunity to finely tune energy consumption and temporal delays across axons in the brain.

Prostanoid EP4 Receptor-Mediated Augmentation of I h Currents in Aβ Dorsal Root Ganglion Neurons Underlies Neuropathic Pain

An injury of the somatosensory system causes neuropathic pain, which is usually refractory to conventional analgesics, thus warranting the development of novel drugs against this kind of pain. The mechanism of neuropathic pain in rats that had undergone left L5 spinal nerve transection was analyzed. Ten days after surgery, these rats acquired neuropathic pain. The patch-clamp technique was used on the isolated bilateral L5 dorsal root ganglion neurons. The current-clamped neurons on the ipsilateral side exhibited significantly higher excitability than those on the contralateral side. However, only neurons with diameters of 40-50 μm on the ipsilateral side exhibited significantly larger voltage sags in response to hyperpolarizing current pulses than those on the contralateral side. Under the voltage clamp, only these neurons on the ipsilateral side showed a significantly larger density of an inward current at < -80 mV [hyperpolarization-activated nonselective cation (I _h) current] with a rightward-shifted activation curve than that on the contralateral side. Ivabradine-an I _h current inhibitor-inhibited I _h currents in these neurons on both sides in a similar concentration-dependent manner, with an IC₅₀ value of ∼3 μM. Moreover, the oral administration of ivabradine significantly alleviated the neuropathic pain on the ipsilateral side. An inhibitor of adenylyl cyclase or an antagonist of prostanoid EP4 receptors (CJ-023423) inhibited ipsilateral, but not contralateral I _h, currents in these neurons. Furthermore, the intrathecal administration of CJ-023423 significantly attenuated neuropathic pain on the ipsilateral side. Thus, ivabradine and/or CJ-023423 may be a lead compound for the development of novel therapeutics against neuropathic pain.

The change of HCN1/HCN2 mRNA expression in peripheral nerve after chronic constriction injury induced neuropathy followed by pulsed electromagnetic field therapy

Neuropathic pain is usually defined as a chronic pain state caused by peripheral or central nerve injury as a result of acute damage or systemic diseases. It remains a difficult disease to treat. Recent studies showed that the frequency of action potentials in nociceptive afferents is affected by the activity of hyperpolarization-activated cyclic nucleotide-gated cation channels (HCN) family. In the current study, we used a neuropathy rat model induced by chronic constriction injury (CCI) of sciatic nerve to evaluate the change of expression of HCN1/HCN2 mRNA in peripheral nerve and spinal cord. Rats were subjected to CCI with or without pulsed electromagnetic field (PEMF) therapy. It was found that CCI induced neural cell degeneration while PEMF promoted nerve regeneration as documented by Nissl staining. CCI shortened the hind paw withdrawal latency (PWL) and hind paw withdrawal threshold (PWT) and PEMF prolonged the PWL and PWT. In addition, CCI lowers the expression of HCN1 and HCN2 mRNA and PEMF cannot restore the expression of HCN1 and HCN2 mRNA. Our results indicated that PEMF can promote nerve regeneration and could be used for the treatment of neuropathic pain.

HCN2 ion channels: basic science opens up possibilities for therapeutic intervention in neuropathic pain

Nociception — the ability to detect painful stimuli — is an invaluable sense that warns against present or imminent damage. In patients with chronic pain, however, this warning signal persists in the absence of any genuine threat and affects all aspects of everyday life. Neuropathic pain, a form of chronic pain caused by damage to sensory nerves themselves, is dishearteningly refractory to drugs that may work in other types of pain and is a major unmet medical need begging for novel analgesics. Hyperpolarisation-activated cyclic nucleotide (HCN)-modulated ion channels are best known for their fundamental pacemaker role in the heart; here, we review data demonstrating that the HCN2 isoform acts in an analogous way as a ‘pacemaker for pain’, in that its activity in nociceptive neurons is critical for the maintenance of electrical activity and for the sensation of chronic pain in pathological pain states. Pharmacological block or genetic deletion of HCN2 in sensory neurons provides robust pain relief in a variety of animal models of inflammatory and neuropathic pain, without any effect on normal sensation of acute pain. We discuss the implications of these findings for our understanding of neuropathic pain pathogenesis, and we outline possible future opportunities for the development of efficacious and safe pharmacotherapies in a range of chronic pain syndromes.

ZD7288, a selective hyperpolarization-activated cyclic nucleotide-gated channel blocker, inhibits hippocampal synaptic plasticity

The selective hyperpolarization-activated cyclic nucleotide-gated (HCN) channel blocker 4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride (ZD7288) blocks the induction of long-term potentiation in the perforant path–CA3 region in rat hippocampus in vivo. To explore the mechanisms underlying the action of ZD7288, we recorded excitatory postsynaptic potentials in perforant path–CA3 synapses in male Sprague-Dawley rats. We measured glutamate content in the hippocampus and in cultured hippocampal neurons using high performance liquid chromatography, and determined intracellular Ca²⁺ concentration [Ca²⁺]_i) using Fura-2. ZD7288 inhibited the induction and maintenance of long-term potentiation, and these effects were mirrored by the nonspecific HCN channel blocker cesium. ZD7288 also decreased glutamate release in hippocampal tissue and in cultured hippocampal neurons. Furthermore, ZD7288 attenuated glutamate-induced rises in [Ca²⁺]_i in a concentration-dependent manner and reversed 8-Br-cAMP-mediated facilitation of these glutamate-induced [Ca²⁺]_i rises. Our results suggest that ZD7288 inhibits hippocampal synaptic plasticity both glutamate release and resultant [Ca²⁺]_i increases in rat hippocampal neurons.

Hyperpolarization-activated, cyclic nucleotide-gated cation channels in Aplysia: Contribution to classical conditioning

Hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels are critical regulators of neuronal excitability, but less is known about their possible roles in synaptic plasticity and memory circuits. Here, we characterized the HCN gene organization, channel properties, distribution, and involvement in associative and nonassociative forms of learning in Aplysia californica. Aplysia has only one HCN gene, which codes for a channel that has many similarities to the mammalian HCN channel. The cloned acHCN gene was expressed in Xenopus oocytes, which displayed a hyperpolarization-induced inward current that was enhanced by cGMP as well as cAMP. Similarly to its homologs in other animals, acHCN is permeable to K(+) and Na(+) ions, and is selectively blocked by Cs(+) and ZD7288. We found that acHCN is predominantly expressed in inter- and motor neurons, including LFS siphon motor neurons, and therefore tested whether HCN channels are involved in simple forms of learning of the siphon-withdrawal reflex in a semiintact preparation. ZD7288 (100 μM) significantly reduced an associative form of learning (classical conditioning) but had no effect on two nonassociative forms of learning (intermediate-term sensitization and unpaired training) or baseline responses. The HCN current is enhanced by nitric oxide (NO), which may explain the postsynaptic role of NO during conditioning. HCN current in turn enhances the NMDA-like current in the motor neurons, suggesting that HCN channels contribute to conditioning through this pathway.

Pulsed Radiofrequency Treatment Enhances Dorsal Root Ganglion Expression of Hyperpolarization-Activated Cyclic Nucleotide-Gated Channels in a Rat Model of Neuropathic Pain

Pulsed radiofrequency (PRF) treatment is a minimally invasive technique with multiple therapeutic applications. Hyperpolarization-activated cyclic nucleotide-gated (HCN) channel mediating Ih may regulate neuropathic pain signaling. This study aimed to determine whether PRF suppresses neuropathic pain by altering HCN channel expression in dorsal root ganglion (DRG) neurons. Male Sprague Dawley rats with sciatic nerve chronic constriction injury (CCI) were randomly assigned to PRF (n = 60) and sham control (n = 60) groups, respectively. On postoperative day 7 (D07), PRF or sham treatment was delivered to the proximal sciatic nerve for 8 min. Behavioral tests were performed before surgery (D0), on D07, and on D1, D7, and D14 after PRF or sham treatment. HCN1 and HCN2 expression levels in the DRG were examined by immunohistochemistry and Western blot. The results showed that thermal hyperalgesia, mechano-allodynia, and mechano-hyperalgesia were lower, and DRG expression levels of HCN1 and HCN2 higher, in the PRF group compared with sham control animals (all P < 0.05 at D14). In conclusion, PRF can upregulate HCN channel expression in the DRG of rats with sciatic nerve CCI. How this regulation of Ih in nociceptive afferents contributes to the suppression of neuropathic pain by PRF remains to be determined.

Dysfunctional HCN ion channels in neurological diseases

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels are expressed as four different isoforms (HCN1-4) in the heart and in the central and peripheral nervous systems. HCN channels are activated by membrane hyperpolarization at voltages close to resting membrane potentials and carry the hyperpolarization-activated current, dubbed I_f (funny current) in heart and I_h in neurons. HCN channels contribute in several ways to neuronal activity and are responsible for many important cellular functions, including cellular excitability, generation, and modulation of rhythmic activity, dendritic integration, transmission of synaptic potentials, and plasticity phenomena. Because of their role, defective HCN channels are natural candidates in the search for potential causes of neurological disorders in humans. Several data, including growing evidence that some forms of epilepsy are associated with HCN mutations, support the notion of an involvement of dysfunctional HCN channels in different experimental models of the disease. Additionally, some anti-epileptic drugs are known to modify the activity of the I_h current. HCN channels are widely expressed in the peripheral nervous system and recent evidence has highlighted the importance of the HCN2 isoform in the transmission of pain. HCN channels are also present in the midbrain system, where they finely regulate the activity of dopaminergic neurons, and a potential role of these channels in the pathogenesis of Parkinson’s disease has recently emerged. The function of HCN channels is regulated by specific accessory proteins, which control the correct expression and modulation of the neuronal I_h current. Alteration of these proteins can severely interfere with the physiological channel function, potentially predisposing to pathological conditions. In this review we address the present knowledge of the association between HCN dysfunctions and neurological diseases, including clinical, genetic, and physiopathological aspects.

Characterizing human stem cell-derived sensory neurons at the single-cell level reveals their ion channel expression and utility in pain research

The generation of human sensory neurons by directed differentiation of pluripotent stem cells opens new opportunities for investigating the biology of pain. The inability to generate this cell type has meant that up until now their study has been reliant on the use of rodent models. Here, we use a combination of population and single-cell techniques to perform a detailed molecular, electrophysiological, and pharmacological phenotyping of sensory neurons derived from human embryonic stem cells. We describe the evolution of cell populations over 6 weeks of directed differentiation; a process that results in the generation of a largely homogeneous population of neurons that are both molecularly and functionally comparable to human sensory neurons derived from mature dorsal root ganglia. This work opens the prospect of using pluripotent stem-cell–derived sensory neurons to study human neuronal physiology and as in vitro models for drug discovery in pain and sensory disorders.

HCN2 channels account for mechanical (but not heat) hyperalgesia during long-standing inflammation

There is emerging evidence that hyperpolarization-activated cation (HCN) channels are involved in the development of pathological pain, including allodynia and hyperalgesia. Mice lacking the HCN isoform 2 display reduced heat but unchanged mechanical pain behavior, as recently shown in preclinical models of acute inflammatory pain. However, the impact of HCN2 to chronic pain conditions is less clear and has not been examined so far. In this report, we study the role of HCN2 in the complete Freund's adjuvant inflammation model reflecting chronic pain conditions. We used sensory neuron-specific as well as inducible global HCN2 mutants analyzing pain behavior in persistent inflammation and complemented this by region-specific administration of an HCN channel blocker. Our results demonstrate that the absence of HCN2 in primary sensory neurons reduces tactile hypersensitivity in chronic inflammatory conditions but leaves heat hypersensitivity unaffected. This result is in remarkable contrast to the recently described role of HCN2 in acute inflammatory conditions. We show that chronic inflammation results in an increased expression of HCN2 and causes sensitization in peripheral and spinal terminals of the pain transduction pathway. The contribution of HCN2 to peripheral sensitization mechanisms was further supported by single-fiber recordings from isolated skin-nerve preparations and by conduction velocity measurements of saphenous nerve preparations. Global HCN2 mutants revealed that heat hypersensitivity-unaffected in peripheral HCN2 mutants-was diminished by the additional disruption of central HCN2 channels, suggesting that thermal hyperalgesia under chronic inflammatory conditions is mediated by HCN2 channels beyond primary sensory afferents.

Capsazepine concentration dependently inhibits currents in HEK 293 cells mediated by human hyperpolarization-activated cyclic nucleotide-gated 2 and 4 channels

Recent studies indicate that blockade of currents (Ih) mediated by hyperpolarization-activated cyclic nucleotide-gated (HCN) channels (particularly HCN1) may partly account for the antinociceptive effects of capsazepine (CPZ). Unfortunately, determining whether capsazepine is a selective HCN channel blocker and determining its adverse effects when it is used for the treatment of neuropathic pain, have been thus far understudied. In this study, we aimed to elucidate the effects of capsazepine on human HCN2 (hHCN2) and HCN4 (hHCN4) channels in HEK293 cells. The vectors that expressed hHCN2 and hHCN4 cDNA were constructed and transfected into HEK293 cells. Enhanced green fluorescent protein (EGFP) fluorescence and the reverse transcription polymerase chain reaction (RT-PCR) were used to confirm the successful transfection of the vectors. After G418 (neomycin) screening, cell lines that expressed hHCN2 and hHCN4 were obtained. The whole-cell voltage-clamp technique was used to determine the currents from hHCN2 and hHCN4 channels, which were perfused with five concentrations (0.1 µM, 1 µM, 5 µM, 10 µM and 50 µM) of capsazepine. The results showed that capsazepine at the range from 0.1 to 50 µM markedly inhibited hHCN2 and hHCN4 currents in a concentration-dependent manner, with most inhibition achieved at a concentration of 10 µM of capsazepine. When compared with the control group, a V0.5 for the hHCN2 and hHCN4 channel showed that 10 µM capsazepine significantly shifted the membrane potential towards hyperpolarization. The present results indicate that capsazepine is not a selective HCN1 channel blocker and that it may have adverse effects when used to treat neuropathic pain.

Characterization of hyperpolarization-activated currents in deep dorsal horn neurons of neonate mouse spinal cord in vitro

Emerging evidence suggests that blockade of hyperpolarization-activated current (Ih) produces analgesia acting at peripheral sites. However, little is known about the role of this current in central pain-processing structures. The aim of the present work was to characterize the Ih in deep dorsal horn neurons and to assess the role of the current in the transmission of somatosensory signals across spinal circuits. To these purpose in vitro preparations of the spinal cord from mice pups were used in combination with whole cell recordings to characterize the current in native neurons. Extracellular recordings from sensory and motor pathways were performed to assess the role of the current in spinal somatosensory processing. Cesium chloride and ZD7288 were used as current blockers. Most deep dorsal horn neurons showed a functional Ih that was blocked by ZD7288 and cesium. Ih blockade caused hyperpolarization, increased input resistance and potentiation of synaptic responses. Excitatory effects of Ih blockade on synaptic transmission were confirmed in projecting anterolateral axons and ventral roots. Ih modulation by cAMP produced a rightward shift in the voltage dependency curve and blocked excitatory effects of ZD7288 on sensory pathways. Results indicate that Ih currents play a stabilizing role in the spinal cord controlling transmission across sensory and motor spinal pathways via cellular effects on input resistance and excitability. In addition, results suggest that current modulation may alter significantly the role of the current in somatosensory processing.

Hyperpolarization activated cation current (I(f)) in cardiac myocytes from pulmonary vein sleeves in the canine with atrial fibrillation

Objective

To investigate the characteristics of ectopic automaticity and cation current (I_f) of cardiac myocytes from pulmonary vein sleeves (PVs) in canines with atrial fibrillation.

Methods

The canines (8–10 years old) were subjected to long-term, rapid atrial pacing (RAP) for 10 weeks, which induced the atrial fibrillation model. Disassociation of PVs of canines yielded single cardiac myocytes from a Landengorff column. Action potential, I_f and hyperpolarisation activated cyclic nucleotide-gated (HCN) currents were measured with the patch-clamp technique.

Results

Compared with the control group, cardiac myocytes from the RAP canine PVs had spontaneous diastolic depolarization, shorter action potential duration, and larger I_f densities. In the group of RAP cells, the half maximal activation potential (V_1/2) was found to be less negative (−105.5 ± 5.2 mV) compared to control cells (−87.3 ± 4.9 mV). Current densities of I_f were increased significantly by β-adrenergic receptor stimulation with isoproterenol and caused an acceleration of current activation. In contrast, I_f currents in the RAP were reduced by carvedilol, a selective beta-adrenergic receptor. Another important finding is that HCN4-based channels may make a significant contribution to I_f in PVs cells, but not HCN2. Meanwhile, HCN4 current significantly increases in canine PVs cardiac myocytes with RAP.

Conclusions

The spontaneous action potential and larger I_f current were observed in the PVs cardiac myocytes using RAP, which may contribute to more ectopic activity events to trigger and maintain atrial fibrillation.

Novel blockers of hyperpolarization-activated current with isoform selectivity in recombinant cells and native tissue

BACKGROUND AND PURPOSE Selective hyperpolarization activated, cyclic nucleotide-gated channel (HCN) blockers represent an important therapeutic goal due to the wide distribution and multiple functions of these proteins, representing the molecular correlate of f- and h-current (I(f) or I(h) ). Recently, new compounds able to block differentially the homomeric HCN isoforms expressed in HEK293 have been synthesized. In the present work, the electrophysiological and pharmacological properties of these new HCN blockers were characterized and their activities evaluated on native channels. EXPERIMENTAL APPROACH HEK293 cells expressing mHCN1, mHCN2 and hHCN4 isoforms were used to verify channel blockade. Selected compounds were tested on native guinea pig sinoatrial node cells and neurons from mouse dorsal root ganglion (DRG) by patch-clamp recordings and on dog Purkinje fibres by intracellular recordings. KEY RESULTS In HEK293 cells, EC18 was found to be significantly selective for HCN4 and MEL57A for HCN1 at physiological membrane potential. When tested on guinea pig sinoatrial node cells, EC18 (10 µM) maintained its activity, reducing I(f) by 67% at -120 mV, while MEL57A (3 µM) reduced I(f) by 18%. In contrast, in mouse DRG neurons, only MEL57A (30 and 100 µM) significantly reduced I(h) by 60% at -80 mV. In dog cardiac Purkinje fibres, EC18, but not MEL57A, reduced the amplitude and slowed the slope of the spontaneous diastolic depolarization. CONCLUSIONS Our results have identified novel and highly selective HCN isoform blockers, EC18 and MEL57A; the selectivity found in recombinant system was maintained in various tissues expressing different HCN isoforms.

A Novel Carbamoyloxy Arylalkanoyl Arylpiperazine Compound (SKL-NP) Inhibits Hyperpolarization-Activated Cyclic Nucleotide-Gated (HCN) Channel Currents in Rat Dorsal Root Ganglion Neurons

In this study, we determined mode of action of a novel carbamoyloxy arylalkanoyl arylpiperazine compound (SKL-NP) on hyperpolarization-activated cyclic nucleotide-gated (HCN) channel currents (I(h)) that plays important roles in neuropathic pain. In small or medium-sized dorsal root ganglion (DRG) neurons (<40 µm in diameter) exhibiting tonic firing and prominent I(h), SKL-NP inhibited I(h) and spike firings in a concentration dependent manner (IC(50)=7.85 µM). SKL-NP-induced inhibition of I(h) was blocked by pretreatment of pertussis toxin (PTX) and N-ethylmaleimide (NEM) as well as 8-Br-cAMP, a membrane permeable cAMP analogue. These results suggest that SKL-NP modulates I(h) in indirect manner by the activation of a Gi-protein coupled receptor that decreases intracellular cAMP concentration. Taken together, SKL-NP has the inhibitory effect on HCN channel currents (I(h)) in DRG neurons of rats.

HCN2 ion channels: an emerging role as the pacemakers of pain

Acute nociceptive pain is caused by the direct action of a noxious stimulus on pain-sensitive nerve endings, whereas inflammatory pain (both acute and chronic) arises from the actions of a wide range of inflammatory mediators released following tissue injury. Neuropathic pain, which is triggered by nerve damage, is often considered to be very different in its origins, and is particularly difficult to treat effectively. Here we review recent evidence showing that members of the hyperpolarization-activated cyclic nucleotide-modulated (HCN) ion channel family - better known for their role in the pacemaker potential of the heart - play important roles in both inflammatory and neuropathic pain. Deletion of the HCN2 isoform from nociceptive neurons abolishes heat-evoked inflammatory pain and all aspects of neuropathic pain, but acute pain sensation is unaffected. This work shows that inflammatory and neuropathic pain have much in common, and suggests that selective blockers of HCN2 may have value as analgesics in the treatment of pain.

The voltage dependence of I(h) in human myelinated axons

HCN channels are responsible for I(h), a voltage-gated inwardly rectifying current activated by hyperpolarization. This current appears to be more active in human sensory axons than motor and may play a role in the determination of threshold. Differences in I(h) are likely to be responsible for the high variability in accommodation to hyperpolarization seen in different subjects. The aim of this study was to characterise this current in human axons, both motor and sensory. Recordings of multiple axonal excitability properties were performed in 10 subjects, with a focus on the changes in threshold evoked by longer and stronger hyperpolarizing currents than normally studied. The findings confirm that accommodation to hyperpolarization is greater in sensory than motor axons in all subjects, but the variability between subjects was greater than the modality difference. An existing model of motor axons was modified to take into account the behaviour seen with longer and stronger hyperpolarization, and a mathematical model of human sensory axons was developed based on the data collected. The differences in behaviour of sensory and motor axons and the differences between different subjects are best explained by modulation of the voltage dependence, along with a modest increase of expression of the underlying conductance of I(h). Accommodation to hyperpolarization for the mean sensory data is fitted well with a value of -94.2 mV for the mid-point of activation (V(0.5)) of I(h) as compared to -107.3 mV for the mean motor data. The variation in response to hyperpolarization between subjects is accounted for by varying this parameter for each modality (sensory: -89.2 to -104.2 mV; motor -87.3 to -127.3 mV). These voltage differences are within the range that has been described for physiological modulation of I(h) function. The presence of slowly activated I(h) isoforms on both motor and sensory axons was suggested by modelling a large internodal leak current and a masking of the Na(+)/K(+)-ATPase pump activity by a tonic depolarization. In addition to an increased activation of I(h), the modelling suggests that in sensory axons the nodal slow K(+) conductance is reduced, with consequent depolarization of resting membrane potential, and action potential of shorter duration.

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.

Organisation of potassium channels on the neuronal surface

Potassium channels are a family of ion channels that govern the intrinsic electrical properties of neurons in the brain. Molecular cloning has revealed over 100 genes encoding the pore-forming alpha subunits of potassium channels in mammals, making them the most diverse subset of ion channels. Multiplicity in this ion channel family is further generated through alternative splicing. The precise location of potassium channels along the dendro-somato-axonic surface of the neurons is an important factor in determining its functional impact. Today, it is widely accepted that potassium channels can be located at any subcellular compartment on the neuronal surface, at synaptic and extrasynaptic sites, from somata to dendritic shafts, dendritic spines, axons or axon terminals. However, they are not evenly distributed on the neuronal surface and depending on the potassium channel subtype, are instead concentrated at different compartments. This selective localization of ion channels to specific neuronal compartments has many different functional implications. One factor necessary to understand the role of potassium channels in neuronal function is to unravel their specialized distribution and subcellular localization within a cell, and this can only be achieved by electron microscopy. In this review, I summarize anatomical findings, describing their distribution in the central nervous system. The distinct regional, cellular and subcellular distribution of potassium channels in the brain will be discussed in view of their possible functional implications.