Functional heteromerization of HCN1 and HCN2 pacemaker channels

Intrinsic electrophysiological properties of entorhinal cortex stellate cells and their contribution to grid cell firing fields

The medial entorhinal cortex (MEC) is an increasingly important focus for investigation of mechanisms for spatial representation. Grid cells found in layer II of the MEC are likely to be stellate cells, which form a major projection to the dentate gyrus. Entorhinal stellate cells are distinguished by distinct intrinsic electrophysiological properties, but how these properties contribute to representation of space is not yet clear. Here, we review the ionic conductances, synaptic, and excitable properties of stellate cells, and examine their implications for models of grid firing fields. We discuss why existing data are inconsistent with models of grid fields that require stellate cells to generate periodic oscillations. An alternative possibility is that the intrinsic electrophysiological properties of stellate cells are tuned specifically to control integration of synaptic input. We highlight recent evidence that the dorsal-ventral organization of synaptic integration by stellate cells, through differences in currents mediated by HCN and leak potassium channels, influences the corresponding organization of grid fields. Because accurate cellular data will be important for distinguishing mechanisms for generation of grid fields, we introduce new data comparing properties measured with whole-cell and perforated patch-clamp recordings. We find that clustered patterns of action potential firing and the action potential after-hyperpolarization (AHP) are particularly sensitive to recording condition. Nevertheless, with both methods, these properties, resting membrane properties and resonance follow a dorsal-ventral organization. Further investigation of the molecular basis for synaptic integration by stellate cells will be important for understanding mechanisms for generation of grid fields.

An essential role for modulation of hyperpolarization-activated current in the development of binaural temporal precision

In sensory circuits of the brain, developmental changes in the expression and modulation of voltage-gated ion channels are a common occurrence, but such changes are often difficult to assign to clear functional roles. We have explored this issue in the binaural neurons of the medial superior olive (MSO), whose temporal precision in detecting the coincidence of binaural inputs dictates the resolution of azimuthal sound localization. We show that in MSO principal neurons of gerbils during the first week of hearing, a hyperpolarization-activated current (I(h)) progressively undergoes a 13-fold increase in maximal conductance, a >10-fold acceleration of kinetics, and, most surprisingly, a 30 mV depolarizing shift in the voltage dependence of activation. This period is associated with an upregulation of the hyperpolarization-activated and cyclic nucleotide-gated (HCN) channel subunits HCN1, HCN2, and HCN4 in the MSO, but only HCN1 and HCN4 were expressed strongly in principal neurons. I(h) recorded in nucleated patches from electrophysiologically mature MSO neurons (>P18) exhibited kinetics and an activation range nearly identical to the I(h) found in whole-cell recordings before hearing onset. These results indicate that the developmental changes in I(h) in MSO neurons can be explained predominantly by modulation from diffusible intracellular factors, and not changes in channel subunit composition. The exceptionally large modulatory changes in I(h), together with refinements in synaptic properties transform the coding strategy from one of summation and integration to the submillisecond coincidence detection known to be required for transmission of sound localization cues.

Exploring HCN channels as novel drug targets

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels have a key role in the control of heart rate and neuronal excitability. Ivabradine is the first compound acting on HCN channels to be clinically approved for the treatment of angina pectoris. HCN channels may offer excellent opportunities for the development of novel anticonvulsant, anaesthetic and analgesic drugs. In support of this idea, some well-established drugs that act on the central nervous system - including lamotrigine, gabapentin and propofol - have been found to modulate HCN channel function. This Review gives an up-to-date summary of compounds acting on HCN channels, and discusses strategies to further explore the potential of these channels for therapeutic intervention.

Colocalization of hyperpolarization-activated, cyclic nucleotide-gated channel subunits in rat retinal ganglion cells

The current-passing pore of mammalian hyperpolarization-activated, cyclic nucleotide-gated (HCN) channels is formed by subunit isoforms denoted HCN1-4. In various brain areas, antibodies directed against multiple isoforms bind to single neurons, and the current (I(h)) passed during hyperpolarizations differs from that of heterologously expressed homomeric channels. By contrast, retinal rod, cone, and bipolar cells appear to use homomeric HCN channels. Here, we assess the generality of this pattern by examining HCN1 and HCN4 immunoreactivity in rat retinal ganglion cells, measuring I(h) in dissociated cells, and testing whether HCN1 and HCN4 proteins coimmunoprecipitate. Nearly half of the ganglion cells in whole-mounted retinae bound antibodies against both isoforms. Consistent with colocalization and physical association, 8-bromo-cAMP shifted the voltage sensitivity of I(h) less than that of HCN4 channels and more than that of HCN1 channels, and HCN1 coimmunoprecipitated with HCN4 from membrane fraction proteins. Finally, the immunopositive somata ranged in diameter from the smallest to the largest in rat retina, the dendrites of immunopositive cells arborized at various levels of the inner plexiform layer and over fields of different diameters, and I(h) activated with similar kinetics and proportions of fast and slow components in small, medium, and large somata. These results show that different HCN subunits colocalize in single retinal ganglion cells, identify a subunit that can reconcile native I(h) properties with the previously reported presence of HCN4 in these cells, and indicate that I(h) is biophysically similar in morphologically diverse retinal ganglion cells and differs from I(h) in rods, cones, and bipolar cells.

The magnitudes of hyperpolarization-activated and low-voltage-activated potassium currents co-vary in neurons of the ventral cochlear nucleus

In the ventral cochlear nucleus (VCN), neurons have hyperpolarization-activated conductances, which in some cells are enormous, that contribute to the ability of neurons to convey acoustic information in the timing of their firing by decreasing the input resistance and speeding-up voltage changes. Comparisons of the electrophysiological properties of neurons in the VCN of mutant mice that lack the hyperpolarization-activated cyclic nucleotide-gated channel α subunit 1 (HCN1(-/-)) (Nolan et al. 2003) with wild-type controls (HCN1(+/+)) and with outbred ICR mice reveal that octopus, T stellate, and bushy cells maintain their electrophysiological distinctions in all strains. Hyperpolarization-activated (I(h)) currents were smaller and slower, input resistances were higher, and membrane time constants were longer in HCN1(-/-) than in HCN1(+/+) in octopus, bushy, and T stellate cells. There were significant differences in the average magnitudes of I(h), input resistances, and time constants between HCN1(+/+) and ICR mice, but the resting potentials did not differ between strains. I(h) is opposed by a low-voltage-activated potassium (I(KL)) current in bushy and octopus cells, whose magnitudes varied widely between neuronal types and between strains. The magnitudes of I(h) and I(KL) were correlated across neuronal types and across mouse strains. Furthermore, these currents balanced one another at the resting potential in individual cells. The magnitude of I(h) and I(KL) is linked in bushy and octopus cells and varies not only between HCN1(-/-) and HCN1(+/+) but also between "wild-type" strains of mice, raising the question to what extent the wild-type strains reflect normal mice.

Projection-specific neuromodulation of medial prefrontal cortex neurons

Mnemonic persistent activity in the prefrontal cortex (PFC) constitutes the neural basis of working memory. To understand how neuromodulators contribute to the generation of persistent activity, it is necessary to identify the intrinsic properties of the layer V pyramidal neurons that transfer this information to downstream networks. Here we show that the somatic dynamic and integrative properties of layer V pyramidal neurons in the rat medial PFC depend on whether they project subcortically to the pons [corticopontine (CPn)] or to the contralateral cortex [commissural (COM)]. CPn neurons display low temporal summation and accelerate in firing frequency when depolarized, whereas COM neurons have high temporal summation and display spike frequency accommodation. In response to dynamic stimuli, COM neurons act as low-pass filters, whereas CPn neurons act as bandpass filters, resonating in the theta frequency range (3-6 Hz). The disparate subthreshold properties of COM and CPn neurons can be accounted for by differences in the hyperpolarization-activated cyclic nucleotide gated cation h-current. Interestingly, neuromodulators hypothesized to enhance mnemonic persistent activity affect COM and CPn neurons distinctly. Adrenergic modulation shifts the dynamic properties of CPn but not COM neurons and increases the excitability of CPn neurons significantly more than COM neurons. In response to cholinergic modulation, CPn neurons were much more likely to display activity-dependent intrinsic persistent firing than COM neurons. Together, these data suggest that the two categories of projection neurons may subserve separate functions in PFC and may be engaged differently during working memory processes.

H-channel dysfunction in generalized epilepsy: it takes two

Impaired Regulation of Thalamic Pacemaker Channels Through an Imbalance of Subunit Expression in Absence Epilepsy

Budde T, Caputi L, Kanyshkova T, Staak R, Abrahamczik C, Munsch T, Pape HC

J Neurosci 2005;25(43):9871–9882

The role of hyperpolarization-activated, cyclic nucleotide-modulated (HCN) channel isoforms and hyperpolarization-activated cation current ( Ih) for seizure-related burst firing in thalamocortical (TC) neurons was investigated in a rat genetic model of absence epilepsy [Wistar Albino Glaxo rats, bred in Rijswijk (WAG/Rij)]. Burst discharges in TC neurons locked to seizure activity in vivo were prolonged during blockade of Ih by Cs+ and ZD7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride). In vitro analyses revealed a hyperpolarizing shift of half-maximal Ih activation ( Vh) in WAG/Rij ( Vh = −93.2 mV) compared with nonepileptic controls [August × Copenhagen-Irish (ACI) ( Vh = −88.0 mV)]. This effect is explained by a shift of the responsiveness of Ih to cAMP toward higher concentrations in TC neurons from WAG/Rij, as revealed by application of 8-bromo-cAMP and the phosphodiesterase inhibitor IBMX. During blockade of adenylyl cyclase activity, Ih activation was similar in the two strains, whereas the difference in cAMP responsiveness persisted, thereby voting against different ambient cAMP levels between strains. Increasing the intracellular cAMP level and shifting Ih activation led to a change from burst to tonic firing mode in WAG/Rij but not in ACI rats. Furthermore, HCN1 expression was significantly increased on mRNA and protein levels, with no changes in HCN2–4 expression. In conclusion, there is an increase in HCN1 expression in the epileptic thalamus, associated with a decrease in cAMP responsiveness of Ih in TC neurons and resulting impairment to control the shift from burst to tonic firing, which, in turn, will prolong burst activity after recruitment of Ih during absence seizures.

Local control of postinhibitory rebound spiking in CA1 pyramidal neuron dendrites

Postinhibitory rebound spiking is characteristic of several neuron types and brain regions, where it sustains spontaneous activity and central pattern generation. However, rebound spikes are rarely observed in the principal cells of the hippocampus under physiological conditions. We report that CA1 pyramidal neurons support rebound spikes mediated by hyperpolarization-activated inward current (I(h)), and normally masked by A-type potassium channels (K(A)). In both experiments and computational models, K(A) blockage or reduction consistently resulted in a somatic action potential upon release from hyperpolarizing injections in the soma or main apical dendrite. Rebound spiking was systematically abolished by the additional blockage or reduction of I(h). Since the density of both K(A) and I(h) increases in these cells with the distance from the soma, such "latent" mechanism may be most effective in the distal dendrites, which are targeted by a variety of GABAergic interneurons. Detailed computer simulations, validated against the experimental data, demonstrate that rebound spiking can result from activation of distal inhibitory synapses. In particular, partial K(A) reduction confined to one or few branches of the apical tuft may be sufficient to elicit a local spike following a train of synaptic inhibition. Moreover, the spatial extent and amount of K(A) reduction determines whether the dendritic spike propagates to the soma. These data suggest that the plastic regulation of K(A) can provide a dynamic switch to unmask postinhibitory spiking in CA1 pyramidal neurons. This newly discovered local modulation of postinhibitory spiking further increases the signal processing power of the CA1 synaptic microcircuitry.

Dendritic HCN channels shape excitatory postsynaptic potentials at the inner hair cell afferent synapse in the mammalian cochlea

Synaptic transmission at the inner hair cell (IHC) afferent synapse, the first synapse in the auditory pathway, is specialized for rapid and reliable signaling. Here we investigated the properties of a hyperpolarization-activated current (I(h)), expressed in the afferent dendrite of auditory nerve fibers, and its role in shaping postsynaptic activity. We used whole cell patch-clamp recordings from afferent dendrites directly where they contact the IHC in excised postnatal rat cochlear turns. Excitatory postsynaptic potentials (EPSPs) of variable amplitude (1-35 mV) were found with 10-90% rise times of about 1 ms and time constants of decay of about 5 ms at room temperature. Current-voltage relations recorded in afferent dendrites revealed I(h). The pharmacological profile and reversal potential (-45 mV) indicated that I(h) is mediated by hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channels. The HCN channel subunits HCN1, HCN2, and HCN4 were found to be expressed in afferent dendrites using immunolabeling. Raising intracellular cAMP levels sped up the activation kinetics, increased the magnitude of I(h) and shifted the half activation voltage (V(half)) to more positive values (-104 +/- 3 to -91 +/- 2 mV). Blocking I(h) with 50 microM ZD7288 resulted in hyperpolarization of the resting membrane potential (approximately 4 mV) and slowing the decay of the EPSP by 47%, suggesting that I(h) is active at rest and shortens EPSPs, thereby potentially improving rapid and reliable signaling at this first synapse in the auditory pathway.

Trafficking and surface expression of hyperpolarization-activated cyclic nucleotide-gated channels in hippocampal neurons

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels mediate the hyperpolarization-activated current I(h) and thus play important roles in the regulation of brain excitability. The subcellular distribution pattern of the HCN channels influences the effects that they exert on the properties and activity of neurons. However, little is known about the mechanisms that control HCN channel trafficking to subcellular compartments or that regulate their surface expression. Here we studied the dynamics of HCN channel trafficking in hippocampal neurons using dissociated cultures coupled with time lapse imaging of fluorophore-fused HCN channels. HCN1-green fluorescence protein (HCN1-GFP) channels resided in vesicle-like organelles that moved in distinct patterns along neuronal dendrites, and these properties were isoform-specific. HCN1 trafficking required intact actin and tubulin and was rapidly inhibited by activation of either NMDA or AMPA-type ionotropic glutamate receptors in a calcium-dependent manner. Glutamate-induced inhibition of the movement of HCN1-GFP-expressing puncta was associated with increased surface expression of both native and transfected HCN1 channels, and this surface expression was accompanied by augmented I(h). Taken together, the results reveal the highly dynamic nature of HCN1 channel trafficking in hippocampal neurons and provide a novel potential mechanism for rapid regulation of I(h), and hence of neuronal properties, via alterations of HCN1 trafficking and surface expression.

Hyperpolarization-activated and cyclic nucleotide-gated channels are differentially expressed in juxtaglomerular cells in the olfactory bulb of mice

In the olfactory bulb, input from olfactory receptor neurons is processed by neuronal networks before it is relayed to higher brain regions. In many neurons, hyperpolarization-activated and cyclic nucleotide-gated (HCN) channels generate and control oscillations of the membrane potential. Oscillations also appear crucial for information processing in the olfactory bulb. Four channel isoforms exist (HCN1-HCN4) that can form homo- or heteromers. Here, we describe the expression pattern of HCN isoforms in the olfactory bulb of mice by using a novel and comprehensive set of antibodies against all four isoforms. HCN isoforms are abundantly expressed in the olfactory bulb. HCN channels can be detected in most cell populations identified by commonly used marker antibodies. The combination of staining with marker and HCN antibodies has revealed at least 17 different staining patterns in juxtaglomerular cells. Furthermore, HCN isoforms give rise to an unexpected wealth of co-expression patterns but are rarely expressed in the same combination and at the same level in two given cell populations. Therefore, heteromeric HCN channels may exist in several cell populations in vivo. Our results suggest that HCN channels play an important role in olfactory information processing. The staining patterns are consistent with the possibility that both homomeric and heteromeric HCN channels are involved in oscillations of the membrane potential of juxtaglomerular cells.

Probing the bradycardic drug binding receptor of HCN-encoded pacemaker channels

I_f (or I_h), encoded by the hyperpolarization-activated, cyclic nucleotide-gated (HCN1–4) channel gene family, contributes significantly to cardiac pacing. Bradycardic agents such as ZD7288 that target HCN channels have been developed, but the molecular configuration of their receptor is poorly defined. Here, we probed the drug receptor by systematically introducing alanine scanning substitutions into the selectivity filter (C347A, I348A, G349A, Y350A, G351A in the P-loop), outer (P355A, V356A, S357A, M358A in the P-S6 linker), and inner (M377A, F378A, V379A in S6) pore vestibules of HCN1 channels. When heterologously expressed in human embryonic kidney 293 cells for patch-clamp recordings, I348A, G349A, Y350A, G351A, P355A, and V356A did not produce measurable currents. The half-blocking concentration (IC₅₀) of wild type (WT) for ZD7288 was 25.8 ± 9.7 μM. While the IC₅₀ of M358A was identical to WT, those of C347A, S357A, F378A, and V379A markedly increased to 137.6 ± 56.4, 113.3 ± 34.1, 587.1 ± 167.5, and 1726.3 ± 673.4 μM, respectively (p < 0.05). Despite the proximity of the S6 residues studied, M377A was hypersensitive (IC₅₀ = 5.1 ± 0.7 μM; p < 0.05) implicating site specificity. To explore the energetic interactions among the S6 residues, double and triple substitutions (M377A/F378A, M377A/V379A, F378A/V379A, and M377A/F378A/V379A) were generated for thermodynamic cycle analysis. Specific interactions with coupling energies (ΔΔG) >1 kT for M377–F378 and F378–V379 but not M377–V379 were identified. Based on these new data and others, we proposed a refined drug-blocking model that may lead to improved antiarrhythmics and bioartificial pacemaker designs.

The modulation of orexin A on HCN currents of pyramidal neurons in mouse prelimbic cortex

The hyperpolarization-activated/cyclic nucleotide (HCN)-gated channels make important contributions to neural excitability. In prefrontal cortex, HCN channels are localized on the distal dendrites of layer V pyramidal neurons and decrease neural excitability when they are open. In the present study, using whole-cell voltage clamp recordings, the effect of an arousal peptide, orexin A, on HCN currents in layer V pyramidal neurons from mouse prelimbic cortex (PL), the homolog of the prefrontal cortex was investigated. The results demonstrated that orexin A suppressed HCN currents and shifted their activation curve to a more negative direction. This action of orexin A was blocked by SB334867, an orexin receptor 1 (OXR1) blocker and bisindolylmaleimide, a protein kinase C (PKC) inhibitor, indicating the involvement of OXR1 and PKC. The excitatory effect of orexin A on PL pyramidal neurons was enhanced when HCN currents were diminished, while attenuated when HCN currents were enlarged. In summary, orexin A inhibits HCN currents and enhances excitability of pyramidal neurons in PL, which may contribute to arousal and cognition.

Anatomical and electrophysiological comparison of CA1 pyramidal neurons of the rat and mouse

The study of learning and memory at the single-neuron level has relied on the use of many animal models, most notably rodents. Although many physiological and anatomical studies have been carried out in rats, the advent of genetically engineered mice has necessitated the comparison of new results in mice to established results from rats. Here we compare fundamental physiological and morphological properties and create three-dimensional compartmental models of identified hippocampal CA1 pyramidal neurons of one strain of rat, Sprague-Dawley, and two strains of mice, C57BL/6 and 129/SvEv. We report several differences in neuronal physiology and anatomy among the three animal groups, the most notable being that neurons of the 129/SvEv mice, but not the C57BL/6 mice, have higher input resistance, lower dendritic surface area, and smaller spines than those of rats. A surprising species-specific difference in membrane resonance indicates that both mouse strains have lower levels of the hyperpolarization-activated nonspecific cation current I(h). Simulations suggest that differences in I(h) kinetics rather than maximal conductance account for the lower resonance. Our findings indicate that comparisons of data obtained across strains or species will need to account for these and potentially other physiological and anatomical differences.

Hyperpolarization-activated cation channels: from genes to function

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels comprise a small subfamily of proteins within the superfamily of pore-loop cation channels. In mammals, the HCN channel family comprises four members (HCN1-4) that are expressed in heart and nervous system. The current produced by HCN channels has been known as I(h) (or I(f) or I(q)). I(h) has also been designated as pacemaker current, because it plays a key role in controlling rhythmic activity of cardiac pacemaker cells and spontaneously firing neurons. Extensive studies over the last decade have provided convincing evidence that I(h) is also involved in a number of basic physiological processes that are not directly associated with rhythmicity. Examples for these non-pacemaking functions of I(h) are the determination of the resting membrane potential, dendritic integration, synaptic transmission, and learning. In this review we summarize recent insights into the structure, function, and cellular regulation of HCN channels. We also discuss in detail the different aspects of HCN channel physiology in the heart and nervous system. To this end, evidence on the role of individual HCN channel types arising from the analysis of HCN knockout mouse models is discussed. Finally, we provide an overview of the impact of HCN channels on the pathogenesis of several diseases and discuss recent attempts to establish HCN channels as drug targets.

The cardiac pacemaker current

In mammals cardiac rate is determined by the duration of the diastolic depolarization of sinoatrial node (SAN) cells which is mainly determined by the pacemaker I(f) current. f-channels are encoded by four members of the hyperpolarization-activated cyclic nucleotide-gated gene (HCN1-4) family. HCN4 is the most abundant isoform in the SAN, and its relevance to pacemaking has been further supported by the discovery of four loss-of-function mutations in patients with mild or severe forms of cardiac rate disturbances. Due to its selective contribution to pacemaking, the I(f) current is also the pharmacological target of a selective heart rate-reducing agent (ivabradine) currently used in the clinical practice. Albeit to a minor extent, the I(f) current is also present in other spontaneously active myocytes of the cardiac conduction system (atrioventricular node and Purkinje fibres). In working atrial and ventricular myocytes f-channels are expressed at a very low level and do not play any physiological role; however in certain pathological conditions over-expression of HCN proteins may represent an arrhythmogenic mechanism. In this review some of the most recent findings on f/HCN channels contribution to pacemaking are described.

Proteolytic processing of HCN2 and co-assembly with HCN4 in the generation of cardiac pacemaker channels

In sino-atrial and atrio-ventricular nodal cells, hyperpolarization-activated cyclic nucleotide-gated (HCN) inward current carrying cationic channels, I(f), are expressed that contribute importantly to the diastolic depolarization critical for cardiac pacemaker activity. Although previous studies have demonstrated myocardial expression of both the HCN2 and HCN4 subunits, the specific roles of these subunits in the generation of functional myocardial I(f) channels remain unclear. To explore the molecular compositions of functional cardiac I(f) channels, antibodies targeted against specific C- and N-terminal sequences in HCN2 and HCN4 were exploited to examine HCN2 and HCN4 subunit expression in adult (mouse) heart and to immunoprecipitate endogenous HCN-encoded cardiac I(f) channel complexes. Western blot experiments revealed that although the full-length HCN2 (105 kDa) and HCN4 (160 kDa) proteins are readily detected in transiently transfected HEK-293 cells and in adult (mouse) brain, the molecular mass of the HCN2 protein in the myocardium is approximately 60 kDa. In addition, the myocardial 60-kDa HCN2 protein lacks the C terminus, which contains the cAMP binding domain. In heterologous cells, the C-terminal-truncated HCN2 protein co-assembles with HCN4 to form functional heteromeric HCN channels, which activate faster than homomeric HCN2 or homomeric HCN4 channels, and display properties similar to endogenous myocardial I(f) channels Taken together, these results suggest that functional myocardial I(f) channels reflect the heteromeric assembly of HCN2 and HCN4 subunits and further that the HCN4 subunit underlies the cAMP-mediated regulation of cardiac I(f) channels.

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.

Associated changes in HCN2 and HCN4 transcripts and I(f) pacemaker current in myocytes

The time- and voltage-dependent inward current generated by the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels contributes to the tissue-specific rhythmic activities in the brain and heart. Four isoforms (HCN1-HCN4) have been identified. Previous studies showed that different HCN isoforms may form functional heteromeric channels. We report here that when HCN2 and HCN4 mRNA were injected into Xenopus oocytes with various ratios of HCN2 over HCN4 at 1:1, 10:1, and 1:10, respectively, the resultant channels showed a depolarized current activation and significantly faster activation kinetics near the midpoint of activation compared with HCN4 homomeric channels. In adult rat myocytes overexpressing HCN4, there was an associated increase in HCN2 mRNA. In neonatal rat myocytes in which HCN2 was knocked down, there was also a simultaneous decrease in HCN4 mRNA. Coimmunoprecipitation experiments showed that HCN2 and HCN4 channel proteins can associate with each other in adult rat ventricles. Finally, in adult myocytes overexpressing HCN4, the hyperpolarization-activated inward current activation, I(f), was shifted to physiological voltages from non-physiological voltages, associated with faster activation kinetics. These data suggested that different ratios of HCN2 and HCN4 transcripts overlapping in different tissues also contribute to the tissue-specific properties of I(f).

Role of the hyperpolarization-activated current Ih in somatosensory neurons

The hyperpolarization-activated current (I(h)) is an inward current activated by hyperpolarization from the resting potential and is an important modulator of action potential firing frequency in many excitable cells. Four hyperpolarization-activated, cyclic nucleotide-modulated subunits, HCN1-4, can form I(h) ion channels. In the present study we investigated the function of I(h) in primary somatosensory neurons. Neuronal firing in response to current injection was promoted by elevating intracellular cAMP levels and inhibited by blockers of I(h), suggesting that I(h) plays a critical role in modulating firing frequency. The properties of I(h) in three size classes of sensory neurons were next investigated. In large neurons I(h) was fast activating and insensitive to elevations in cAMP, consistent with expression of HCN1. I(h) was ablated in most large neurons in HCN1(-/-) mice. In small neurons a slower activating, cAMP-sensitive I(h) was observed, as expected for expression of HCN2 and/or HCN4. Consistent with this, I(h) in small neurons was unchanged in HCN1(-/-) mice. In a neuropathic pain model HCN1(-/-) mice exhibited substantially less cold allodynia than wild-type littermates, suggesting an important role for HCN1 in neuropathic pain. This work shows that I(h) is an important modulator of action potential generation in somatosensory neurons.

Hyperpolarization activated cyclic-nucleotide gated (HCN) channels in developing neuronal networks

Developing neuronal networks evolve continuously, requiring that neurons modulate both their intrinsic properties and their responses to incoming synaptic signals. Emerging evidence supports roles for the hyperpolarization-activated cyclic nucleotide-gated (HCN) channels in this neuronal plasticity. HCN channels seem particularly suited for fine-tuning neuronal properties and responses because of their remarkably large and variable repertoire of functions, enabling integration of a wide range of cellular signals. Here, we discuss the involvement of HCN channels in cortical and hippocampal network maturation, and consider potential roles of developmental HCN channel dysregulation in brain disorders such as epilepsy.

Decreases in HCN mRNA expression in the hippocampus after kindling and status epilepticus in adult rats

PURPOSE

Studies in animal models and patients have implicated changes in hyperpolarization-activated cyclic nucleotide-gated cation channel (HCN) expression in the pathogenesis of temporal lobe epilepsy (TLE). However, the nature of HCN changes during the epileptogenic process and their commonality across different TLE models is unknown. Here HCN1 and HCN2 mRNA expression was quantitatively measured at different time points during epileptogenesis in two distinct animal models of TLE; the kainic acid (KA)-induced status epilepticus (SE) and amygdala kindling models.

METHODS

Hippocampal subregions (CA1, CA3, and dentate gyrus [DG]) and entorhinal cortex were dissected at different time-points. For KA-induced SE animals this was 24 h, 7 days (preepileptic), and 6 weeks (epileptic) post status. For amygdala kindling animals this was 2 weeks after reaching either "partially kindled" (one class II/III seizure) or "fully kindled" (five class V seizures) states. Quantification of regional hippocampal neuronal loss in the KA-treated animals was done using NeuN immunofluorescence and confocal microscopy.

RESULTS

HCN mRNA levels decreased in an isoform and region specific manner at all time points after KA-induced SE. The decrease in neuronal number could not account for all reductions in HCN mRNA levels post-KA insult, implicating transcriptional changes. A reduction in HCN2 mRNA levels was also observed in fully kindled animals in the CA3 region.

CONCLUSIONS

A reduction in HCN mRNA levels is present in two different models of TLE. This supports the case that a reduction in HCN channel expression is an accompaniment of epileptogenesis in different adult models of TLE.

Biological pacemakers as a therapy for cardiac arrhythmias

PURPOSE OF REVIEW

Cardiac rhythm disorders are caused by malfunctions of impulse generation or conduction. Malfunctions of impulse generation, that is, defects in pacemaking, are often life-threatening. Present therapies span a wide array of approaches, but remain largely palliative. Recent progress in understanding of the underlying biology of pacemaking opens up new prospects for better alternatives to the present routine. Specifically, development and use of biological pacemakers could prove to be advantageous to the conventional approaches.

RECENT FINDINGS

We review the current state of the art in gene and cell-based approaches to correct cardiac rhythm disturbances. These include genetic suppression of an ionic current, embryonic as well as adult stem cell therapies, novel synthetic pacemaker channels, and adult somatic cell-fusion approach.

SUMMARY

Biological pacemaking can be achieved by modulating ionic currents by gene transfer or by delivering engineered pacemaker cells into normally quiescent myocardium. The present state of development is proof-of-concept; we are now working on reducing to practice a stable, reliable biological product as an alternative to electronic pacemakers.

Activity-dependent heteromerization of the hyperpolarization-activated, cyclic-nucleotide gated (HCN) channels: role of N-linked glycosylation

Formation of heteromeric complexes of ion channels via co-assembly of different subunit isoforms provides an important mechanism for enhanced channel diversity. We have previously demonstrated co-association of the hyperpolarization activated cyclic-nucleotide gated (HCN1/HCN2) channel isoforms that was regulated by network (seizure) activity in developing hippocampus. However, the mechanisms that underlie this augmented expression of heteromeric complexes have remained unknown. Glycosylation of the HCN channels has been implicated in the stabilization and membrane expression of heteromeric HCN1/HCN2 constructs in heterologous systems. Therefore, we used in vivo and in vitro systems to test the hypothesis that activity modifies HCN1/HCN2 heteromerization in neurons by modulating the glycosylation state of the channel molecules. Seizure-like activity (SA) increased HCN1/HCN2 heteromerization in hippocampus in vivo as well as in hippocampal organotypic slice cultures. This activity increased the abundance of glycosylated HCN1 but not HCN2-channel molecules. In addition, glycosylated HCN1 channels were preferentially co-immunoprecipitated with the HCN2 isoforms. Provoking SA in vitro in the presence of the N-linked glycosylation blocker tunicamycin abrogated the activity-dependent increase of HCN1/HCN2 heteromerization. Thus, hippocampal HCN1 molecules have a significantly higher probability of being glycosylated after SA, and this might promote a stable heteromerization with HCN2.

Hyperpolarization-activated cyclic nucleotide-gated channel mRNA and protein expression in large versus small diameter dorsal root ganglion neurons: correlation with hyperpolarization-activated current gating

Hyperpolarization-activated cyclic nucleotide-gated channels (HCN) are responsible for the functional hyperpolarization-activated current (I(h)) in dorsal root ganglion (DRG) neurons. We studied HCN1-4 channel mRNA and protein expression and correlated these findings with I(h) functional properties in rat DRG neurons of different size. Quantitative RT-PCR (TaqMan) analysis demonstrated that HCN2 and HCN1 mRNAs were more abundantly expressed in large diameter (55-80 microm) neurons, while HCN3 mRNA was preferentially expressed in small diameter (20-30 microm) neurons. HCN4 mRNA expression was very low in neurons of all sizes. At the protein level, subunit-selective polyclonal antibodies and immunofluorescence indicated that HCN1 and HCN3 are present in large diameter neurons and small diameter neurons. Staining in small diameter neurons was in IB4-positive (non-peptidergic) and IB4-negative (peptidergic) cells. HCN2 immunofluorescent staining was heterogeneous and predominantly in large diameter neurons and in small diameter IB4-negative neurons. HCN4 was poorly expressed in all neurons. Functionally, I(h) amplitude and density were significantly larger, and activation kinetics faster, in large diameter neurons when compared with small neurons. I(h) activation rates in small and large diameter DRG neurons were consistent with the relative abundance of HCN subunits in the respective cell type, considering the reported HCN channel activation rates in heterologous systems (HCN1>HCN2 approximately HCN3>HCN4), suggesting exclusivity of roles of different HCN subunits contributing to the excitability of DRG neurons of different size. Additionally, a functional role of I(h) in small DRG neuron excitability was evaluated using a computational model.

Dendritic Ih ensures high-fidelity dendritic spike responses of motion-sensitive neurons in rat superior colliculus

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels that generate I(h) currents are widely distributed in the brain and have been shown to contribute to various neuronal functions. In the present study, we investigated the functions of I(h) in the motion-sensitive projection neurons [wide field vertical (WFV) cells] of the superior colliculus, a pivotal visual center for detection of and orientating to salient objects. Combination of whole cell recordings and immunohistochemical investigations suggested that HCN1 channels dominantly contribute to the I(h) in WFV cells among HCN isoforms expressed in the superficial superior colliculus and mainly located on their expansive dendritic trees. We found that blocking I(h) suppressed the initiation of short- and fixed-latency dendritic spike responses and led instead to long- and fluctuating-latency somatic spike responses to optic fiber stimulations. These results suggest that the dendritic I(h) facilitates the dendritic initiation and/or propagation of action potentials and ensures that WFV cells generate spike responses to distal synaptic inputs in a sensitive and robustly time-locked manner, probably by acting as continuous depolarizing drive and fixing dendritic membrane potentials close to the spike threshold. These functions are different from known functions of dendritic I(h) revealed in hippocampal and neocortical pyramidal cells, where they spatiotemporally limit the propagations of synaptic inputs along the apical dendrites by reducing dendritic membrane resistance. Thus we have revealed new functional aspects of I(h), and these dendritic properties are likely critical for visual motion processing in these neurons.

A hyperpolarization-activated, cyclic nucleotide-gated, (Ih-like) cationic current and HCN gene expression in renal inner medullary collecting duct cells

The cation conductancein primary cultures of rat renal inner medullary collecting duct was studied using perforated-patch and conventional whole cell clamp techniques. Hyperpolarizations beyond -60 mV induced a time-dependent inward nonselective cationic current (I(vti)) that resembles the well-known hyperpolarization-activated, cyclic nucleotide-gated I(h) and I(f) currents. I(vti) showed a half-maximal activation around -102 mV with a slope factor of 25 mV. It had a higher conductance (but, at its reversal potential, not a higher permeability) for K(+) than for Na(+) (gK(+)/gNa(+) = 1.5), was modulated by cAMP and blocked by external Cd(2+) (but not Cs(+) or ZD-7288), and potentiated by a high extracellular K(+) concentration. We explored the expression of the I(h) channel genes (HCN1 to -4) by RT-PCR. The presence of transcripts corresponding to the HCN1, -2, and -4 genes was observed in both the cultured cells and kidney inner medulla. Western blot analysis with HCN2 antibody showed labeling of approximately 90- and approximately 120-kDa proteins in samples from inner medulla and cultured cells. Immunocytochemical analysis of cell cultures and inner medulla showed the presence of HCN immunoreactivity partially colocalized with the Na(+)-K(+)-ATPase at the basolateral membrane of collecting duct cells. This is the first evidence of an I(h)-like cationic current and HCN immunoreactivity in either kidney or any other nonexcitable mammalian cells.

HCN1 channels control resting and active integrative properties of stellate cells from layer II of the entorhinal cortex

Whereas recent studies have elucidated principles for representation of information within the entorhinal cortex, less is known about the molecular basis for information processing by entorhinal neurons. The HCN1 gene encodes ion channels that mediate hyperpolarization-activated currents (I(h)) that control synaptic integration and influence several forms of learning and memory. We asked whether hyperpolarization-activated, cation nonselective 1 (HCN1) channels control processing of information by stellate cells found within layer II of the entorhinal cortex. Axonal projections from these neurons form a major component of the synaptic input to the dentate gyrus of the hippocampus. To determine whether HCN1 channels control either the resting or the active properties of stellate neurons, we performed whole-cell recordings in horizontal brain slices prepared from adult wild-type and HCN1 knock-out mice. We found that HCN1 channels are required for rapid and full activation of hyperpolarization-activated currents in stellate neurons. HCN1 channels dominate the membrane conductance at rest, are not required for theta frequency (4-12 Hz) membrane potential fluctuations, but suppress low-frequency (<4 Hz) components of spontaneous and evoked membrane potential activity. During sustained activation of stellate cells sufficient for firing of repeated action potentials, HCN1 channels control the pattern of spike output by promoting recovery of the spike afterhyperpolarization. These data suggest that HCN1 channels expressed by stellate neurons in layer II of the entorhinal cortex are key molecular components in the processing of inputs to the hippocampal dentate gyrus, with distinct integrative roles during resting and active states.

Novel pharmacological activity of loperamide and CP-339,818 on human HCN channels characterized with an automated electrophysiology assay

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels underlie the pacemaker currents in neurons (I(h)) and cardiac (I(f)) cells. As such, the identification and characterization of novel blockers of HCN channels is important to enable the dissection of their function in vivo. Using a new IonWorks HT electrophysiology assay with human HCN1 and HCN4 expressed stably in cell lines, four HCN channel blockers are characterized. Two blockers known for their activity at opioid/Ca(2+) channels and K(+) channels, loperamide and CP-339,818 (respectively), are described to block HCN1 more potently than HCN4. The known HCN blocker ZD7288 was also found to be more selective for HCN1 over HCN4, while the HCN blocker DK-AH269 was equipotent on HCN4 and HCN1. Partial replacement of the intracellular Cl(-) with gluconate reduced the potency on both channels, but to varying degrees. For both HCN1 and HCN4, ZD7288 was most sensitive in lower Cl(-) solutions, while the potency of loperamide was not affected by the differing solutions. The block of HCN1 for all compounds was voltage-dependent, being relieved at more negative potentials. The voltage-dependent, Cl(-) dependent, HCN1 preferring compounds described here elaborate on the current known pharmacology of HCN channels and may help provide novel tools and chemical starting points for the investigation of HCN channel function in natively expressing systems.

Structural and functional determinants in the S5-P region of HCN-encoded pacemaker channels revealed by cysteine-scanning substitutions

Hyperpolarization-activated cyclic nucleotide-modulated (HCN) channels are responsible for the membrane pacemaker current that underlies the spontaneous generation of bioelectrical rhythms. However, their structure-function relationship is poorly understood. Previously, we identified several pore residues that influence HCN gating properties and proposed a pore-to-gate mechanism. Here, we systematically introduced cysteine-scanning substitutions into the descending portion of the P loop (residues 339-345) of HCN1-R (where R is resistance to sulfhydryl-reactive agents) channels, in which all endogenous cysteines except C303 have been removed or replaced. F339C, K340C, A341C, M342C, S343C, and M345C did not produce functional currents. Interestingly, the loss of function phenotype of F339C could be rescued by the reducing agent dithiothreitol (DTT). H344C but not HCN1-R and DTT-treated F339C channels were sensitive to blockade by divalent Cd(2+) (current with 100 microM Cd(2+)/control current at -140 mV = 67.6 +/- 2.9%, 109.3 +/- 3.1%, and 103.8 +/- 1.7%, respectively). Externally applied methanethiosulfate ethylammonium, a covalent sulfhydryl-reactive compound, irreversibly modified H344C by reducing the current at -140 mV (to 43.7 +/- 6.5%), causing a hyperpolarizing steady-state activation shift (change in half-activation voltage: approximately 6 mV) and decelerated gating kinetics (by up to 3-fold). Based on these results, we conclude that pore residues 339-345 are important determinants of the structure-function properties of HCN channels and that the side chain of H344 is externally accessible.

Local and global effects of I(h) distribution in dendrites of mammalian neurons

The hyperpolarization-activated cation current I(h) exhibits a steep gradient of channel density in dendrites of pyramidal neurons, which is associated with location independence of temporal summation of EPSPs at the soma. In striking contrast, here we show by using dendritic patch-clamp recordings that in cerebellar Purkinje cells, the principal neurons of the cerebellar cortex, I(h) exhibits a uniform dendritic density, while location independence of EPSP summation is observed. Using compartmental modeling in realistic and simplified dendritic geometries, we demonstrate that the dendritic distribution of I(h) only weakly affects the degree of temporal summation at the soma, while having an impact at the dendritic input location. We further analyze the effect of I(h) on temporal summation using cable theory and derive bounds for temporal summation for any spatial distribution of I(h). We show that the total number of I(h) channels, not their distribution, governs the degree of temporal summation of EPSPs. Our findings explain the effect of I(h) on EPSP shape and temporal summation, and suggest that neurons are provided with two independent degrees of freedom for different functions: the total amount of I(h) (controlling the degree of temporal summation of dendritic inputs at the soma) and the dendritic spatial distribution of I(h) (regulating local dendritic processing).

Dendritic HCN2 channels constrain glutamate-driven excitability in reticular thalamic neurons

Hyperpolarization activated cyclic nucleotide (HCN) gated channels conduct a current, I(h); how I(h) influences excitability and spike firing depends primarily on channel distribution in subcellular compartments. For example, dendritic expression of HCN1 normalizes somatic voltage responses and spike output in hippocampal and cortical neurons. We reported previously that HCN2 is predominantly expressed in dendritic spines in reticular thalamic nucleus (RTN) neurons, but the functional impact of such nonsomatic HCN2 expression remains unknown. We examined the role of HCN2 expression in regulating RTN excitability and GABAergic output from RTN to thalamocortical relay neurons using wild-type and HCN2 knock-out mice. Pharmacological blockade of I(h) significantly increased spike firing in RTN neurons and large spontaneous IPSC frequency in relay neurons; conversely, pharmacological enhancement of HCN channel function decreased spontaneous IPSC frequency. HCN2 deletion abolished I(h) in RTN neurons and significantly decreased sensitivity to 8-bromo-cAMP and lamotrigine. Recapitulating the effects of I(h) block, HCN2 deletion increased both temporal summation of EPSPs in RTN neurons as well as GABAergic output to postsynaptic relay neurons. The enhanced excitability of RTN neurons after I(h) block required activation of ionotropic glutamate receptors; consistent with this was the colocalization of HCN2 and glutamate receptor 4 subunit immunoreactivities in dendritic spines of RTN neurons. The results indicate that, in mouse RTN neurons, HCN2 is the primary functional isoform underlying I(h) and expression of HCN2 constrains excitatory synaptic integration.

HCN-encoded pacemaker channels: from physiology and biophysics to bioengineering

The depolarizing membrane ionic current I(h) (also known as I(f), "f" for funny), encoded by the hyperpolarization-activated cyclic-nucleotide-modulated (HCN1-4) channel gene family, was first discovered in the heart over 25 years ago. Later, I(h) was also found in neurons, retina, and taste buds. HCN channels structurally resemble voltage-gated K(+) (Kv) channels but the molecular features underlying their opposite gating behaviors (activation by hyperpolarization rather than depolarization) and non-selective permeation profiles (> or =25 times less selective for K(+) than Kv channels) remain largely unknown. Although I(h) has been functionally linked to biological processes from the autonomous beating of the heart to pain transmission, the underlying mechanistic actions remain largely inferential and, indeed, somewhat controversial due to the slow kinetics and negative operating voltage range relative to those of the bioelectrical events involved (e.g., cardiac pacing). This article reviews the current state of our knowledge in the structure-function properties of HCN channels in the context of their physiological functions and potential HCN-based therapies via bioengineering.

HCN2 and HCN4 isoforms self-assemble and co-assemble with equal preference to form functional pacemaker channels

Hyperpolarization-activated cyclic nucleotide-modulated (HCN) "pacemaker" channel subunits are integral membrane proteins that assemble as tetramers to form channels in cardiac conduction tissue and nerve cells. Previous studies have suggested that the HCN2 and HCN4 channel isoforms physically interact when overexpressed in mammalian cells, but whether they are able to co-assemble and form functional channels remains unclear. The extent to which co-assembly occurs over self-assembly and whether HCN2-HCN4 heteromeric channels are formed in native tissue are not known. In this study, we show co-assembly of HCN2 and HCN4 in live Chinese hamster ovary cells using bioluminescence resonance energy transfer (BRET(2)), a novel approach for studying tetramerization of ion channel subunits. Together with results from electrophysiological and imaging approaches, the BRET(2) data show that HCN2 and HCN4 subunits self-assemble and co-assemble with equal preference. We also demonstrate colocalization of HCN2 and HCN4 and a positive correlation of their intensities in the embryonic mouse heart using immunohistochemistry, as well as physical interactions between these isoforms in the rat thalamus by coimmunoprecipitation. Together, these data support the formation of HCN2-HCN4 heteromeric channels in native tissue.

Alpha2A-adrenoceptors strengthen working memory networks by inhibiting cAMP-HCN channel signaling in prefrontal cortex

Spatial working memory (WM; i.e., "scratchpad" memory) is constantly updated to guide behavior based on representational knowledge of spatial position. It is maintained by spatially tuned, recurrent excitation within networks of prefrontal cortical (PFC) neurons, evident during delay periods in WM tasks. Stimulation of postsynaptic alpha2A adrenoceptors (alpha2A-ARs) is critical for WM. We report that alpha2A-AR stimulation strengthens WM through inhibition of cAMP, closing Hyperpolarization-activated Cyclic Nucleotide-gated (HCN) channels and strengthening the functional connectivity of PFC networks. Ultrastructurally, HCN channels and alpha2A-ARs were colocalized in dendritic spines in PFC. In electrophysiological studies, either alpha2A-AR stimulation, cAMP inhibition or HCN channel blockade enhanced spatially tuned delay-related firing of PFC neurons. Conversely, delay-related network firing collapsed under conditions of excessive cAMP. In behavioral studies, either blockade or knockdown of HCN1 channels in PFC improved WM performance. These data reveal a powerful mechanism for rapidly altering the strength of WM networks in PFC.

Panulirus interruptus Ih-channel gene PIIH: modification of channel properties by alternative splicing and role in rhythmic activity

We cloned 10 full-length variants of PIIH, the gene for I(h) from the spiny lobster, Panulirus interruptus, using reverse transcription-PCR (RT-PCR) and rapid amplification of cDNA ends (RACE). This gene shows a significant amount of alternative splicing in the S3-S4 and S4-S5 linkers, in the P-loop and the entire S6 transmembrane domain, in the cyclic nucleotide binding domain (CNBD), and near the 3' end of the gene. Functional expression of seven splice variants in Xenopus oocytes generated slowly activating hyperpolarization-activated inward currents, which were blocked by the I(h) channel blockers CsCl and ZD7288. The different splice variants had markedly varying activation kinetics and voltage dependence of activation. Bath application of 8-Br-cAMP shifted the V(1/2) to more positive potentials and accelerated the activation kinetics in an isoform-specific manner. Two variants containing a segment with an ER-retention motif in the S4-S5 loop did not produce currents in oocytes. Overexpression of one splice variant, PIIH AB(S)-I, in pyloric dilator (PD) neurons in the lobster stomatogastric ganglion produced an average threefold increase in I(h) without evoking a compensatory increase in I(A). The voltage for half-maximal activation of I(h) in PIIH AB(S)-I-expressing PDs was shifted in the depolarizing direction by 9 mV, whereas the slope factor decreased by 3.8 mV. Moreover, its activation kinetics were significantly faster than in control PDs. PIIH AB(S)-I overexpression enhanced PD neuron rhythmic firing in an amplitude-dependent manner above a minimal threshold two- to threefold increase in amplitude.

Functional presynaptic HCN channels in the rat globus pallidus

Hyperpolarization-activated, cyclic nucleotide-gated cation (HCN) channels are expressed postsynaptically in the rodent globus pallidus (GP), where they play several important roles in controlling GP neuronal activity. To further elucidate the role of HCN channels in the GP, immunocytochemical and electrophysiological approaches were used to test the hypothesis that HCN channels are also expressed presynaptically on the local axon collaterals of GP neurons. At the electron microscopic level, immunoperoxidase labelling for HCN1 and HCN2 was localized in GP somata and dendritic processes, myelinated and unmyelinated axons, and axon terminals. One population of labelled terminals formed symmetric synapses with somata and proximal dendrites and were immunoreactive for parvalbumin, consistent with the axon collaterals of GABAergic GP projection neurons. In addition, labelling for HCN2 and, to a lesser degree, HCN1 was observed in axon terminals that formed asymmetric synapses and were immunoreactive for the vesicular glutamate transporter 2. Immunogold labelling demonstrated that HCN1 and HCN2 were located predominantly at extrasynaptic sites along the plasma membrane of both types of terminal. To determine the function of presynaptic HCN channels in the GP, we performed whole-cell recordings from GP neurons in vitro. Bath application of the HCN channel blocker ZD7288 resulted in an increase in the frequency of mIPSCs but had no effect on their amplitude, implying that HCN channels tonically regulate the release of GABA. Their presence, and predicted role in modulating transmitter release, represents a hitherto unidentified mechanism whereby HCN channels influence the activity of GP neurons.

Reversed somatodendritic I(h) gradient in a class of rat hippocampal neurons with pyramidal morphology

In CA1 and neocortical pyramidal neurons, I(h) is present primarily in the dendrites. We asked if all neurons of a pyramidal morphology have a similar density of I(h). We characterized a novel class of hippocampal neurons with pyramidal morphology found in the stratum radiatum, which we termed the 'pyramidal-like principal' (PLP) neuron. Morphological similarities to pyramidal neurons were verified by filling the neurons with biocytin. PLPs did not stain for markers associated with interneurons, and projected to both the septum and olfactory bulb. By using cell-attached patch-clamp recordings, we found that these neurons expressed a high density of I(h) in the soma that declined to a lower density in the dendrites, a pattern that is reversed compared to pyramidal neurons. The voltage-dependent activation and activation time constants of I(h) in the PLPs were similar to pyramidal neurons. Whole-cell patch-clamp recordings from the soma and dendrites of PLP neurons showed no significant differences in input resistance and local temporal summation between the two locations. Blockade of I(h) by ZD7288 increased the input resistance and temporal summation of simulated EPSPs, as in pyramidal neurons. When NMDA receptors were blocked, temporal summation at the soma of distal synaptic potentials was similar to that seen with current injections at the soma, suggesting a 'normalization' of temporal summation similar to that observed in pyramidal neurons. Thus, we have characterized a principal neuronal subtype in the hippocampus with a similar morphology but reversed I(h) somatodendritic gradient to that previously observed in CA1 hippocampal and neocortical pyramidal neurons.

Creation of a biological pacemaker by gene- or cell-based approaches

Cardiac rhythm-associated disorders are caused by mal-functions of impulse generation and conduction. Present therapies for the impulse generation span a wide array of approaches but remain largely palliative. The progress in the understanding of the biology of the diseases with related biological tools beckons for new approaches to provide better alternatives to the present routine. Here, we review the current state of the art in gene- and cell-based approaches to correct cardiac rhythm disturbances. These include genetic suppression of an ionic current, stem cell therapies, adult somatic cell-fusion approach, novel synthetic pacemaker channel, and creating a self-contained pacemaker activity in non-excitable cells. We then conclude by discussing advantages and disadvantages of the new possibilities.

The pacemaker current: from basics to the clinics

Activation of the pacemaker ("funny," I(f)) current during diastole is the main process underlying generation of the diastolic depolarization and spontaneous activity of cardiac pacemaker cells. I(f) modulation by autonomic transmitters is responsible for the chronotropic regulation of heart rate. Given its role in pacemaking, I(f) has been a major target of investigation aimed to exploit its rate-controlling function in a clinical perspective. In this short review, we describe some of the most recent clinically relevant applications of the concept of I(f)-based pacemaking.

Use-dependent inhibition of hHCN4 by ivabradine and relationship with reduction in pacemaker activity

BACKGROUND AND PURPOSE

Ivabradine, a specific and use-dependent I(f) inhibitor, exerts anti-ischaemic activity purely by reducing heart rate. The aim of this work was to characterize its effect on the predominant HCN channel isoform expressed in human sino-atrial nodes (hSAN), to determine its kinetics in HCN channels from multicellular preparations and rate-dependency of its action.

EXPERIMENTAL APPROACH

RT-PCR analysis of the four HCN channel isoforms was carried out on RNAs from hSAN. Patch-clamp and intracellular recordings were obtained from CHO cells stably expressing hHCN4 and isolated SAN, respectively. Beating rate of rat isolated atria was followed using a transducer.

KEY RESULTS

hHCN4 mRNAs were predominant in hSAN. Ivabradine induced a time-dependent inhibition of hHCN4 with an IC(50) of 0.5 microM. In rabbit SAN, ivabradine progressively reduced the frequency of action potentials: by 10% after 3 h at 0.1 microM, by 14% after 2 h at 0.3 microM and by 17% after 1.5 h at 1 microM. After 3h, ivabradine reduced the beating rate of rat right atria with an IC(30) of 0.2 microM. The onset of action of ivabradine was use-dependent rather than time-dependent with slower effects than caesium, an extracellular I (f) blocker. Ivabradine 3 microM decreased the frequency of action potentials in SAN from guinea-pig, rabbit and pig by 33%, 21% and 15% at 40 min, respectively.

CONCLUSIONS AND IMPLICATIONS

The use-dependent inhibition of hHCN4 current by ivabradine probably contributes to its slow developing effect in isolated SAN and right atria and to its increased effectiveness in species with rapid SAN activity.

Gene transfer of a synthetic pacemaker channel into the heart: a novel strategy for biological pacing

BACKGROUND

One key element of natural pacemakers is the pacemaker current encoded by the hyperpolarization-activated nucleotide-gated channel (HCN) gene family. Although HCN gene transfer has been used to engineer biological pacemakers, this strategy may be confounded by unpredictable consequences of heteromultimerization with endogenous HCN family members and limited flexibility with regard to frequency tuning of the engineered pacemaker.

METHODS AND RESULTS

To circumvent these limitations, we converted a depolarization-activated potassium-selective channel, Kv1.4, into a hyperpolarization-activated nonselective channel by site-directed mutagenesis (R447N, L448A, and R453I in S4 and G528S in the pore). Gene transfer into ventricular myocardium demonstrated the ability of this construct to induce pacemaker activity with spontaneous action potential oscillations in adult ventricular myocytes and idioventricular rhythms by in vivo electrocardiography.

CONCLUSIONS

Given the sparse expression of Kv1 family channels in the human ventricle, gene transfer of a synthetic pacemaker channel based on the Kv1 family has novel therapeutic potential as a biological alternative to electronic pacemakers.