Differences between the channels, currents and mechanisms of conduction slowing/block and accommodative processes in simulated cases of focal demyelinating neuropathies

To clarify the differences between the mechanisms of conduction slowing/block and accommodative processes in focal demyelinating neuropathies, this computational study presents the kinetics of the ionic, transaxonal and transmyelin currents defining the intracellular and electrotonic potentials in different segments of human motor nerve fibres. The computations use our previous double cable model of the fibres. The simulated fibres have focal demyelination of internodes, paranodes or both together. The intracellular potentials are defined mainly by the Na(+) current, as the contribution of the K(+) fast and K(+) slow currents to the total nodal ionic current is negligible. The paranodal demyelinations cause an increase in the transaxonal current and a decrease in the transmyelin current at the paranodal segments. However, there is an inverse relationship between the transaxonal and transmyelin currents at the same segments in the cases of internodal demyelination. The internodal ionic channels beneath the myelin sheath do not contribute to the intracellular potentials, but they show a high sensitivity to long-lasting pulses. The slow components of the electrotonic potentials depend on the activation of the channel types in the nodal or internodal axolemma, whereas the fast components of the potentials are determined mainly by the passive cable responses. However, the current kinetics changes (defining the investigated electrotonic changes) are relatively weak. The study summarizes the results from these modelling investigations on the mechanisms underlying the conduction slowing/block and accommodative processes in focal demyelinating neuropathies such as Guillain-Barré syndrome and multifocal motor neuropathy.     

Is sodium current present in human sinoatrial node cells?

Pacemaker activity of the sinoatrial node has been studied extensively in various animal species, but is virtually unexplored in man. As such, it is unknown whether the fast sodium current (I_Na) plays a role in the pacemaker activity of the human sinoatrial node. Recently, we had the unique opportunity to perform patch-clamp experiments on single pacemaker cells isolated from a human sinoatrial node. In 2 out of the 3 cells measured, we observed large inward currents with characteristics of I_Na. Although we were unable to analyze the current in detail, our findings provide strong evidence that I_Na is present in human sinoatrial node pacemaker cells, and that this I_Na is functionally available at potentials negative to -60 mV.     

Bifurcation analysis and effects of changing ionic conductances on pacemaker rhythm in a sinoatrial node cell model

The electrical excitation (action potential generation) of sinoatrial node (cardiac pacemaker) cells is directly related to various ion channels (pore-forming proteins) in cell membranes. In order to analyze the relation between action potential generation and ion channels, we use the Yanagihara-Noma-Irisawa (YNI) model of sinoatrial node cells, which is described by the Hodgkin-Huxley-type equations with seven variables. In this paper, we analyze the global bifurcation structure of the YNI model by varying various conductances of ion channels, and examine the effects of these conductance changes on pacemaker rhythm (frequency of action potential generation). The coupling effect on pacemaker rhythm is also examined approximately by applying external current to the YNI model.     

Role of pacemaking current in cardiac nodes: insights from a comparative study of sinoatrial node and atrioventricular node

Cardiac pacemaking in the sinoatrial (SA) node and atrioventricular (AV) node is generated by an interplay of many ionic currents, one of which is the funny pacemaker current (I_f). To understand the functional role of I_f in two different pacemakers, comparative studies of spontaneous activity and expression of the HCN channel in mouse SA node and AV node were performed. The intrinsic cycle length (CL) is 179±2.7 ms (n=5) in SA node and 258±18.7 ms (n=5) in AV node. Blocking of I_f current by 1 μmol/L ZD7288 increased the CL to 258±18.7 ms (n=5) and 447±92.4 ms (n=5) in SA node and AV node, respectively. However, the major HCN channel, HCN4 expressed at low level in the AV node compared to the SA node. To clarify the discrepancy between the functional importance of I_f and expression level of HCN4 channel, a SA node cell model was used. Increasing the I_f conductance resulted in decreasing in the CL in the model, which explains the high pacemaking rate and high expression of HCN channel in the SA node. Resistance to the blocking of I_f in the SA node might result from compensating effects from other currents (especially voltage sensitive currents) involved in pacemaking. The computer simulation shows that the difference in the intrinsic CL could explain the difference in response to I_f blocking in these two cardiac nodes.     

Selected contribution: axial stretch increases spontaneous pacemaker activity in rabbit isolated sinoatrial node cells.

Isolated, spontaneously beating rabbit sinoatrial node cells were subjected to longitudinal stretch, using carbon fibers attached to both ends of the cell. Their electrical behavior was studied simultaneously in current-clamp or voltage-clamp mode using the perforated patch configuration. Moderate stretch ( approximately 7%) caused an increase in spontaneous beating rate (by approximately 5%) and a reduction in maximum diastolic and systolic potentials (by approximately 2.5%), as seen in multicellular preparations. Mathematical modeling of the stretch intervention showed the experimental results to be compatible with stretch activation of cation nonselective ion channels, similar to those found in other cardiac cell populations. Voltage-clamp experiments validated the presence of a stretch-induced current component with a reversal potential near -11 mV. These data confirm, for the first time, that the positive chronotropic response of the heart to stretch is, at least in part, encoded on the level of individual sinoatrial node pacemaker cells; all reported data are in agreement with a major contribution of stretch-activated cation nonselective channels to this response.     

Changes in activation sequence of embryonic chick atria correlate with developing myocardial architecture

To characterize developmental changes in impulse propagation within atrial musculature, we performed high-speed optical mapping of activation sequence of the developing chick atria using voltage-sensitive dye. The activation maps were correlated with detailed morphological studies using scanning electron microscopy, histology, and whole mount confocal imaging with three-dimensional reconstruction. A preferential pathway appeared during development within the roof of the atria, transmitting the impulse rapidly from the right-sided sinoatrial node to the left atrium. The morphological substrate of this pathway, the bundle of Bachman, apparent from stage 29 onward, was a prominent ridge of pectinate muscles continuous with the terminal crest. Further acceleration of impulse propagation was noted along the ridges formed by the developing pectinate muscles, ramifying from the terminal crest toward the atrioventricular groove. In contrast, when the impulse reached the interatrial septum, slowing was often observed, suggesting that the septum acts as a barrier or sink for electrical current. We conclude that these inhomogeneities in atrial impulse propagation are consistent with existence of a specialized network of fast-conducting tissues. The purpose of these preferential pathways appears to be to assure synchronous atrial activation and contraction rather than rapid impulse conduction between the sinoatrial and atrioventricular nodes.     

Effects of a slow potassium permeability on repetitive activity of the frog node of ranvier

Adding a potassium permeability with slow kinetics to the Frankenhaeuser-Huxley equations describing action potential generation at a frog node of Ranvier has a twofold effect on the maintained repetitive firing the model can show. If the contribution of the slow to the total potassium permeability is increased, the maintained discharge frequency for a given stimulating current experiences a decrease. On the other hand, addition of the slow channel narrows the range of currents for which the model can generate repetitive activity. If as little as 6.2% of the total potassium permeability are provided by the slow channels, the Frankenhaeuser-Huxley equations completely lose the ability to show maintained firing. The introduction of the slow potassium current abolishes especially repetitive activity at low values of stimulating current. This effect is so marked that the minimal discharge frequency the model can maintain increases with increasing contribution of the slow channel. Therefore, an important purpose of the slow potassium channel present at the frog nodal membrane could consist of preventing the node of Ranvier from generating consistent firing on its own.     

Mouse models for studying pacemaker channel function and sinus node arrhythmia

Pacemaker activity of the heart is generated by a small group of cells forming the sinoatrial node (SAN). Cells of the SAN are spontaneously active and generate action potentials with remarkable regularity and stability under all physiological conditions. The exact molecular mechanisms underlying pacemaker potentials in the SAN have not yet been fully elucidated. Several voltage-dependent ion channels as well as intracellular calcium cycling processes are thought to contribute to the pacemaker activity. Hyperpolarization-activated cation channels, which generate the I_f current, have biophysical properties which seem ideally suited for the initiation of spontaneous electrical activity. This review describes recent work on several transgenic mice lacking different cardiac HCN channel subtypes. The role of I_f for normal pacemaking and sinus node arrhythmia as revealed by these genetic models will be discussed. In addition, a new mouse line is described which enables gene targeting in a temporally-controlled manner selectively in SAN cells. Elucidating the function of HCN and other ion channels in well-controlled mouse models should ultimately lead to a better understanding of the mechanisms underlying human sinoatrial arrhythmias.     

Interactions within the intrinsic cardiac nervous system contribute to chronotropic regulation

The objective of this study was to determine how neurons within the right atrial ganglionated plexus (RAGP) and posterior atrial ganglionated plexus (PAGP) interact to modulate right atrial chronotropic, dromotropic, and inotropic function, particularly with respect to their extracardiac vagal and sympathetic efferent neuronal inputs. Surgical ablation of the PAGP (PAGPx) attenuated vagally mediated bradycardia by 26%; it reduced heart rate slowing evoked by vagal stimulation superimposed on sympathetically mediated tachycardia by 36%. RAGP ablation (RAGPx) eliminated vagally mediated bradycardia, while retaining the vagally induced suppression of sympathetic-mediated tachycardia (-83%). After combined RAGPx and PAGPx, vagal stimulation still reduced sympathetic-mediated tachycardia (-47%). After RAGPx alone and after PAGPx alone, stimulation of the vagi still produced negative dromotropic effects, although these changes were attenuated compared with the intact state. Negative dromotropic responses to vagal stimulation were further attenuated after combined ablation, but parasympathetic inhibition of atrioventricular nodal conduction was still demonstrable in most animals. Finally, neither RAGPx nor PAGPx altered autonomic regulation of right atrial inotropic function. These data indicate that multiple aggregates of neurons within the intrinsic cardiac nervous system are involved in sinoatrial nodal regulation. Whereas parasympathetic efferent neurons regulating the right atrium, including the sinoatrial node, are primarily located within the RAGP, prejunctional parasympathetic-sympathetic interactions regulating right atrial function also involve neurons within the PAGP.     

Developmental switch in neuropeptide Y and melanocortin effects in the paraventricular nucleus of the hypothalamus

Homeostatic regulation of energy balance in rodents changes dramatically during the first 3 postnatal weeks. Neuropeptide Y (NPY) and melanocortin neurons in the arcuate nucleus, a primary energy homeostatic center in adults, do not fully innervate the paraventricular nucleus (PVN) until the third postnatal week. We have identified two classes of PVN neurons responsive to these neuropeptides, tonically firing neurosecretory (NS) and burst-firing preautonomic (PA) cells. In neonates, NPY could inhibit GABAergic inputs to nearly all NS and PA neurons, while melanocortin regulation was minimal. However, there was a dramatic, age-dependent decrease in NPY responses specifically in the PA neurons, and a 3-fold increase in melanocortin responses in NS cells. These age-dependent changes were accompanied by changes in spontaneous GABAergic currents onto these neurons. This primarily NPYergic regulation in the neonates likely promotes the positive energy balance necessary for growth, while the developmental switch correlates with maturation of homeostatic regulation of energy balance.     

Developmental changes of BKCa channels depend on differentiation status in cultured podocytes

The podocyte is a remarkable cell type, which encases the capillaries of the kidney glomerulus. Podocytes are of keen interests because of their key roles in kidney development and disease. Large-conductance Ca²⁺-activated K⁺ channels (BK_Ca channels) are important ion channels located in podocytes and play the essential role in regulating calcium homeostasis cell signaling. In this research, we studied the undergoing developmental changes of BK_Ca channels and their contribution to functional maturation of podocytes. Our results showed that the distribution of BK_Ca channels changed with the maturity of differentiation in a conditionally immortalized mouse podocyte cell line. Additionally, the increase of BK_Ca channel protein expression was detected by immunofluorescence staining with confocal microscopy in podocytes, which was consistent with the increase in the current density of BK_Ca channels examined by whole-cell patch-clamp technique. Our results suggested that the developmental changes of BK_Ca channels may help podocytes adapt to changes in pressure gradients occurring in physiological conditions. Those findings may have implications for understanding the physiology and development of kidney and will also serve as a baseline for future studies designed to investigate developmental changes of ion channel expression in podocytes.     

Eye opening rapidly induces synaptic potentiation and refinement

NMDA receptor (NMDAR)-mediated increases in AMPA receptor (AMPAR) currents are associated with long-term synaptic potentiation (LTP). Here, we provide evidence that similar changes occur in response to normal increases in sensory stimulation during development. Experiments discriminated between eye opening-induced and age-dependent changes in synaptic currents. At 6 hr after eye opening (AEO), a transient population of currents mediated by NR2B-rich NMDARs increase significantly, and silent synapses peak. Sustained increases in evoked and miniature AMPAR currents occur at 12 hr AEO. Significant changes in AMPAR:NMDAR evoked current ratios, contacts per axon, and inputs per cell are present at 24 hr AEO. The AMPAR current changes are those seen in vitro during NMDAR-dependent LTP. Here, they are a consequence of eye opening and are associated with a new wave of synaptic refinement. These data also suggest that new NR2B-rich NMDAR currents precede and may initiate this developmental synaptic potentiation and functional tuning.     

Pacemaker activity of the rabbit sinoatrial node. A comparison of mathematical models.

In the past decade, three mathematical models describing the pacemaker activity of the rabbit sinoatrial node have been developed: the Bristow-Clark model, the Irisawa-Noma model, and the Noble-Noble model. In a comparative study it is demonstrated that these models, as well as subsequent modifications, all have several drawbacks. A more accurate model, describing the pacemaker activity of a single pacemaker cell isolated from the rabbit sinoatrial node, was constructed. Model equations, including equations for the T-type calcium current, are based on experimental data from voltage clamp experiments on single cells that were published during the last few years. In contrast to the other models, only a small amount of background current contributes to the overall electrical charge flow. The action potential parameters of the model cell, its responses to voltage clamp steps and its current-voltage relationships have been computed. The model is used to discuss the relative contribution of membrane current components to the slow diastolic depolarization phase of the action potential.     

Immunocytochemical demonstration of the equilibrative nucleoside transporter rENT1 in rat sinoatrial node.

Adenosine exerts multiple receptor-mediated effects in the heart, including a negative chronotropic effect on the sinoatrial node. The aim of this study was to investigate the distribution of the equilibrative nucleoside transporter rENT1 in rat sinoatrial node and atrial muscle. Immunocytochemistry and/or immunoblotting revealed abundant expression of this protein in plasma membranes of sinoatrial node and in atrial and ventricular cells. Because rENT1-mediated transport is likely to regulate the local concentrations of adenosine in the sinoatrial node and other parts of the heart, it represents a potential pharmacological target that might be exploited to ameliorate ischemic damage during heart surgery.     

Critical Roles of Voltage-Dependent Sodium Channels in the Process of Synaptogenesis During the Postnatal Cortical Development of Rats

The developmental changes of the sodium channel and construction of synapse connection were studied in cerebral cortical pyramidal neurons of rats at different age groups. We used whole-cell patch-clamp recordings to characterize electrophysiological properties of cortical neurons at different age stages, including the sodium currents, APs evoked by depolarizing current and short-term plasticity of the eEPSCs. The result shows that the sodium currents undergo a hyperpolarizing shift in activation process and acceleration of activation and inactivation with age. The maximal sodium current also increased with maturation, and the evident difference appeared from P7–P11 (with the day of birth as P0) to P12–P15 group. The tendency of the sodium current density changes which exhibited the same properties as that of sodium current, showed the significant increases from P19–P21 to P ≥ 22 group. The APs’ parameters exhibited the age-dependent changes except the threshold, including the increase of the peak amplitude from P ≤ 6 to P16–P18 groups, and the curtailment of duration and the time-to-peak with age. The amplitude of 1st eEPSC increased with maturation, and STP displays depression at all observed groups. In addition, although STP also exhibited depression in response to last three stimulations in P ≥ 22 group, the 2nd response showed the tendency of facilitation compared with the younger age groups. Our results indicated that the cerebral cortical pyramidal neurons of rats are undergoing marked changes in the characteristics of their sodium channels with maturation, which play a critical role in synaptogenesis and construction of the neuronal network.     

Age-dependent axonal expression of potassium channel proteins during development in mouse hippocampus

The development of the hippocampal network requires neuronal activity, which is shaped by the differential expression and sorting of a variety of potassium channels. Parallel to their maturation, hippocampal neurons undergo a distinct development of their ion channel profile. The age-dependent dimension of ion channel occurrence is of utmost importance as it is interdependently linked to network formation. However, data regarding the exact temporal expression of potassium channels during postnatal hippocampal development are scarce. We therefore studied the expression of several voltage-gated potassium channel proteins during hippocampal development in vivo and in primary cultures, focusing on channels that were sorted to the axonal compartment. The Kv1.1, Kv1.2, Kv1.4, and Kv3.4 proteins showed a considerable temporal variation of axonal localization among neuronal subpopulations. It is possible, therefore, that hippocampal neurons possess cell type-specific mechanisms for channel compartmentalization. Thus, age-dependent axonal sorting of the potassium channel proteins offers a new approach to functionally distinguish classes of hippocampal neurons and may extend our understanding of hippocampal circuitry and memory processing.