Function of the hyperpolarization-activated inward rectification in nonmyelinated peripheral rat and human axons

Mutual Suppression of Proximal and Distal Axonal Spike Initiation Determines the Output Patterns of a Motor Neuron

Axonal spike initiation at sites far from somatodendritic integration occurs in a range of systems, but its contribution to neuronal output activity is not well understood. We studied the interactions of distal and proximal spike initiation in an unmyelinated motor axon of the stomatogastric nervous system in the lobster, Homarus americanus. The peripheral axons of the pyloric dilator (PD) neurons generate tonic spiking in response to dopamine application. Centrally generated bursting activity and peripheral spike initiation had mutually suppressive effects. The two PD neurons and the electrically coupled oscillatory anterior burster (AB) neuron form the pacemaker ensemble of the pyloric central pattern generator, and antidromic invasion of central compartments by peripherally generated spikes caused spikelets in AB. Antidromic spikes suppressed burst generation in an activity-dependent manner: slower rhythms were diminished or completely disrupted, while fast rhythmic activity remained robust. Suppression of bursting was based on interference with the underlying slow wave oscillations in AB and PD, rather than a direct effect on spike initiation. A simplified multi-compartment circuit model of the pacemaker ensemble replicated this behavior. Antidromic activity disrupted slow wave oscillations by resetting the inward and outward current trajectories in each spike interval. Centrally generated bursting activity in turn suppressed peripheral spike initiation in an activity-dependent manner. Fast bursting eliminated peripheral spike initiation, while slower bursting allowed peripheral spike initiation to continue during the intervals between bursts. The suppression of peripheral spike initiation was associated with a small after-hyperpolarization in the sub-millivolt range. A realistic model of the PD axon replicated this behavior and showed that a sub-millivolt cumulative after-hyperpolarization across bursts was sufficient to eliminate peripheral spike initiation. This effect was based on the dynamic interaction between slow activity-dependent hyperpolarization caused by the Na⁺/K⁺-pump and inward rectification through the hyperpolarization-activated inward current, I _h . These results demonstrate that interactions between different spike initiation sites based on spike propagation can shift the relative contributions of different types of activity in an activity-dependent manner. Therefore, distal axonal spike initiation can play an important role in shaping neural output, conditional on the relative level of centrally generated activity.

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.

Acupoint Sensitization is Associated with Increased Excitability and Hyperpolarization-Activated Current (Ih) in C- But Not Aδ-Type Neurons

Under pathological conditions, acupoint sensitization is the phenomenon of acupoints transforming from the stable state to the dynamic state. Evidences suggest that hyperpolarization-activated current (I_h), conducted by the hyperpolarization-activated/cyclic nucleotide-gated (HCN) channel, greatly contributes to the peripheral and central sensitization. However, the role of the I_h current in acupoint sensitization has not been explained. In the present study, changes in excitability, I_h density and the HCN channel of dorsal root ganglion (DRG) nociceptive neurons were examined in the later phase of knee osteoarthritis (KOA) rats. To investigate the neuronal specificity of acupoint sensitization, retrograde dyes were injected into the acupoints ST35 and GB37. The results showed that acupoint sensitization occurred in bilateral ST35 but not GB37 acupoints. The excitability and I_h density of C- but not A_δ-type neurons innervating ST35 acupoint increased in bilateral L5 DRG of acupoint sensitized rats than that of sham rats. No obvious changes were found in the excitability or I_h density of C- and A_δ-type neurons innervating the GB37 acupoint in the bilateral L5 DRG. HCN channel subtype 2 (HCN2) expression levels significantly increased after acupoint sensitization. Furthermore, ZD7288, an HCN current (I_h) blocker, attenuated the acupoint sensitization of the ST35 acupoint. Taken together, our findings suggest that the increased excitability of C- but not A_δ-type neurons and the upregulation of I_h/HCN2 channels contribute to the formation of acupoint sensitization.

State-Dependent Modification of Sensory Sensitivity via Modulation of Backpropagating Action Potentials

Neuromodulators play a critical role in sensorimotor processing via various actions, including pre- and postsynaptic signal modulation and direct modulation of signal encoding in peripheral dendrites. Here, we present a new mechanism that allows state-dependent modulation of signal encoding in sensory dendrites by neuromodulatory projection neurons. We studied the impact of antidromic action potentials (APs) on stimulus encoding using the anterior gastric receptor (AGR) neuron in the heavily modulated crustacean stomatogastric ganglion (STG). We found that ectopic AP initiation in AGR's axon trunk is under direct neuromodulatory control by the inferior ventricular (IV) neurons, a pair of descending projection neurons. IV neuron activation elicited a long-lasting decrease in AGR ectopic activity. This modulation was specific to the site of AP initiation and could be mimicked by focal application of the IV neuron co-transmitter histamine. IV neuron actions were diminished after blocking H2 receptors in AGR's axon trunk, suggesting a direct axonal modulation. This local modulation did not affect the propagation dynamics of en passant APs. However, decreases in ectopic AP frequency prolonged sensory bursts elicited distantly near AGR's dendrites. This frequency-dependent effect was mediated via the reduction of antidromic APs, and the diminishment of backpropagation into the sensory dendrites. Computational models suggest that invading antidromic APs interact with local ionic conductances, the rate constants of which determine the sign and strength of the frequency-dependent change in sensory sensitivity. Antidromic APs therefore provide descending projection neurons with a means to influence sensory encoding without affecting AP propagation or stimulus transduction.

Ionic mechanisms underlying history-dependence of conduction delay in an unmyelinated axon

Axonal conduction velocity can change substantially during ongoing activity, thus modifying spike interval structures and, potentially, temporal coding. We used a biophysical model to unmask mechanisms underlying the history-dependence of conduction. The model replicates activity in the unmyelinated axon of the crustacean stomatogastric pyloric dilator neuron. At the timescale of a single burst, conduction delay has a non-monotonic relationship with instantaneous frequency, which depends on the gating rates of the fast voltage-gated Na⁺ current. At the slower timescale of minutes, the mean value and variability of conduction delay increase. These effects are because of hyperpolarization of the baseline membrane potential by the Na⁺/K⁺ pump, balanced by an h-current, both of which affect the gating of the Na⁺ current. We explore the mechanisms of history-dependence of conduction delay in axons and develop an empirical equation that accurately predicts this history-dependence, both in the model and in experimental measurements.

Dopamine modulation of Ih improves temporal fidelity of spike propagation in an unmyelinated axon

We studied how conduction delays of action potentials in an unmyelinated axon depended on the history of activity and how this dependence was changed by the neuromodulator dopamine (DA). The pyloric dilator axons of the stomatogastric nervous system in the lobster, Homarus americanus, exhibited substantial activity-dependent hyperpolarization and changes in spike shape during repetitive activation. The conduction delays varied by several milliseconds per centimeter, and, during activation with realistic burst patterns or Poisson-like patterns, changes in delay occurred over multiple timescales. The mean delay increased, whereas the resting membrane potential hyperpolarized with a time constant of several minutes. Concomitantly with the mean delay, the variability of delay also increased. The variability of delay was not a linear or monotonic function of instantaneous spike frequency or spike shape parameters, and the relationship between these parameters changed with the increase in mean delay. Hyperpolarization was counteracted by a hyperpolarization-activated inward current (I(h)), and the magnitude of I(h) critically determined the temporal fidelity of spike propagation. Pharmacological block of I(h) increased the change in delay and the variability of delay, and increasing I(h) by application of DA diminished both. Consequently, the temporal fidelity of pattern propagation was substantially improved in DA. Standard measurements of changes in excitability or delay with paired stimuli or tonic stimulation failed to capture the dynamics of spike conduction. These results indicate that spike conduction can be extremely sensitive to the history of axonal activity and to the presence of neuromodulators, with potentially important consequences for temporal coding.

Beyond faithful conduction: short-term dynamics, neuromodulation, and long-term regulation of spike propagation in the axon

Most spiking neurons are divided into functional compartments: a dendritic input region, a soma, a site of action potential initiation, an axon trunk and its collaterals for propagation of action potentials, and distal arborizations and terminals carrying the output synapses. The axon trunk and lower order branches are probably the most neglected and are often assumed to do nothing more than faithfully conducting action potentials. Nevertheless, there are numerous reports of complex membrane properties in non-synaptic axonal regions, owing to the presence of a multitude of different ion channels. Many different types of sodium and potassium channels have been described in axons, as well as calcium transients and hyperpolarization-activated inward currents. The complex time- and voltage-dependence resulting from the properties of ion channels can lead to activity-dependent changes in spike shape and resting potential, affecting the temporal fidelity of spike conduction. Neural coding can be altered by activity-dependent changes in conduction velocity, spike failures, and ectopic spike initiation. This is true under normal physiological conditions, and relevant for a number of neuropathies that lead to abnormal excitability. In addition, a growing number of studies show that the axon trunk can express receptors to glutamate, GABA, acetylcholine or biogenic amines, changing the relative contribution of some channels to axonal excitability and therefore rendering the contribution of this compartment to neural coding conditional on the presence of neuromodulators. Long-term regulatory processes, both during development and in the context of activity-dependent plasticity may also affect axonal properties to an underappreciated extent.

Axon Physiology

Axons are generally considered as reliable transmission cables in which stable propagation occurs once an action potential is generated. Axon dysfunction occupies a central position in many inherited and acquired neurological disorders that affect both peripheral and central neurons. Recent findings suggest that the functional and computational repertoire of the axon is much richer than traditionally thought. Beyond classical axonal propagation, intrinsic voltage-gated ionic currents together with the geometrical properties of the axon determine several complex operations that not only control signal processing in brain circuits but also neuronal timing and synaptic efficacy. Recent evidence for the implication of these forms of axonal computation in the short-term dynamics of neuronal communication is discussed. Finally, we review how neuronal activity regulates both axon morphology and axonal function on a long-term time scale during development and adulthood.

Dopamine modulates Ih in a motor axon

We studied the axons of the pyloric dilator neurons in the stomatogastric nervous system of the lobster. The several-centimeters-long portions of these axons in the motor nerves depolarize in response to low concentrations of dopamine (DA) and exhibit peripheral spike initiation in the absence of centrally generated activity. This effect is inhibited by blockers of hyperpolarization-activated inward current (I(h)). We show here that peripheral spike initiation was also elicited by D(1)-type receptor agonists and drugs that increase cAMP. This suggests that DA acts via a D(1)-type receptor mechanism to modulate hyperpolarization-activated cyclic nucleotide-gated channels. We used two-electrode voltage clamp of the axon to directly study the effect of DA on I(h). Surprisingly, DA decreased the maximal conductance. However, because of a shift of the activation curve to more depolarized potentials, and a change in the slope, conductance was increased at biologically relevant membrane potentials. These changes were solely caused by modulation of I(h), as DA had no discernible effect when I(h) was blocked. In addition, they were not induced by repeated activation and could be mimicked by application of drugs that increase cAMP concentration. DA modulation of I(h) persisted in the presence of a protein kinase A inhibitor and is therefore potentially mediated by a phosphorylation-independent direct effect of cAMP on the ion channel. A computer model of the axon showed that the changes in maximal conductance and voltage dependence were not qualitatively affected by space-clamp problems.

Axonal ion channels from bench to bedside: a translational neuroscience perspective

Over recent decades, the development of specialised techniques such as patch clamping and site-directed mutagenesis have established the contribution of neuronal ion channel dysfunction to the pathophysiology of common neurological conditions including epilepsy, multiple sclerosis, spinal cord injury, peripheral neuropathy, episodic ataxia, amyotrophic lateral sclerosis and neuropathic pain. Recently, these insights from in vitro studies have been translated into the clinical realm. In keeping with this progress, novel clinical axonal excitability techniques have been developed to provide information related to the activity of a variety of ion channels, energy-dependent pumps and ion exchange processes activated during impulse conduction in peripheral axons. These non-invasive techniques have been extensively applied to the study of the biophysical properties of human peripheral nerves in vivo and have provided important insights into axonal ion channel function in health and disease. This review will provide a translational perspective, focusing on an overview of the investigational method, the clinical utility in assessing the biophysical basis of ectopic symptom generation in peripheral nerve disease and a review of the major findings of excitability studies in acquired and inherited neurological disease states.

Complex intrinsic membrane properties and dopamine shape spiking activity in a motor axon

We studied the peripheral motor axons of the two pyloric dilator (PD) neurons of the stomatogastric ganglion in the lobster, Homarus americanus. Intracellular recordings from the motor nerve showed both fast and slow voltage- and activity-dependent dynamics. During rhythmic bursts, the PD axons displayed changes in spike amplitude and duration. Pharmacological experiments and the voltage dependence of these phenomena suggest that inactivation of sodium and A-type potassium channels are responsible. In addition, the "resting" membrane potential was dependent on ongoing spike or burst activity, with more hyperpolarized values when activity was strong. Nerve stimulations, pharmacological block and current clamp experiments suggest that this is due to a functional antagonism between a slow after-hyperpolarization (sAHP) and inward rectification through hyperpolarization-activated current (IH). Dopamine application resulted in modest depolarization and "ectopic" peripheral spike initiation in the absence of centrally generated activity. This effect was blocked by CsCl and ZD7288, consistent with a role of IH. High frequency nerve stimulation inhibited peripheral spike initiation for several seconds, presumably due to the sAHP. Both during normal bursting activity and antidromic nerve stimulation, the conduction delay over the length of the peripheral nerve changed in a complex manner. This suggests that axonal membrane dynamics can have a substantial effect on the temporal fidelity of spike patterns propagated from a spike initiation site to a synaptic target, and that neuromodulators can influence the extent to which spike patterns are modified.

The H-current secures action potential transmission at high frequencies in rat cerebellar parallel fibers

Most axons in the mammalian brain are unmyelinated and thin with pre-synaptic specializations (boutons) along their entire paths. The parallel fibers in the cerebellum are examples of such axons. Unlike most thin axons they have only one branch point. The granule cell soma, where they originate, can fire bursts of action potentials with spike intervals of about 2 ms. An important question is whether the axons are able to propagate spikes with similarly short intervals. By using extracellular single-unit and population-recording methods we showed that parallel fibers faithfully conduct spikes at high frequencies over long distances. However, when adding 20 microm ZD7288 or 1 mm Cs(+), or reducing the temperature from 35 to 24 degrees C, the action potentials often failed even when successfully initiated. Ba(2+)(1 mm), which blocks Kir channels, did not reproduce these effects. The conduction velocity was reduced by ZD7288 but not by Ba(2+). This suggests that the parallel fibers have an H-current that is active at rest and that is important for their frequency-following properties. Interestingly, failures occurred only when the action potential had to traverse the axonal branch point, suggesting that the branch point is the weakest point in these axons.

Characteristics of HCN channels and their participation in neuropathic pain

Neuropathic pain is induced by the injury to nervous systems and characterized by hyperalgesia, allodynia and spontaneous pain. The underlying mechanisms include peripheral and central sensitization resulted from neuronal hyperexcitability. A number of ion channels are considered to contribute to the neuronal hyperexcitability. Here, we particularly concentrate on an interesting ion channel, hyperpolarization-activated cyclic nucleotide gated (HCN) channels. We overview its biophysical properties, physiological functions, followed by focusing on the current progress in the study of its role in the development of neuropathic pain. We attempt to provide a comprehensive review of the potential valuable target, HCN channels, in the treatment of neuropathic pain.

Hyperpolarization-activated current (I(h)) contributes to excitability of primary sensory neurons in rats

In various excitable tissues, the hyperpolarization-activated, cyclic nucleotide-gated current (I(h)) contributes to burst firing by depolarizing the membrane after a period of hyperpolarization. Alternatively, conductance through open channels I(h) channels of the resting membrane may impede excitability. Since primary sensory neurons of the dorsal root ganglion show both loss of I(h) and elevated excitability after peripheral axonal injury, we examined the contribution of I(h) to excitability of these neurons. We used a sharp electrode intracellular technique to record from neurons in nondissociated ganglia to avoid potential artefacts due to tissue dissociation and cytosolic dialysis. Neurons were categorized by conduction velocity. I(h) induced by hyperpolarizing voltage steps was completely blocked by ZD7288 (approximately 10 microM), which concurrently eliminated the depolarizing sag of transmembrane potential during hyperpolarizing current injection. I(h) was most prominent in rapidly conducting Aalpha/beta neurons, in which ZD7288 produced resting membrane hyperpolarization, slowed conduction velocity, prolonged action potential (AP) duration, and elevated input resistance. The rheobase current necessary to trigger an AP was elevated and repetitive firing was inhibited by ZD7288, indicating an excitatory influence of I(h). Less I(h) was evident in more slowly conducting Adelta neurons, resulting in diminished effects of ZD7288 on AP parameters. Repetitive firing in these neurons was also inhibited by ZD7288, and the peak frequency of AP transmission during tetanic bursts was diminished by ZD7288. Slowly conducting C-type neurons showed minimal I(h), and no effect of ZD7288 on excitability was seen. After spinal nerve ligation, axotomized neurons had less I(h) compared to control neurons and showed minimal effects of ZD7288 application. We conclude that I(h) supports sensory neuron excitability, and loss of I(h) is not a factor contributing to increased neuronal excitability after peripheral axonal injury.

Axonal accumulation of hyperpolarization-activated cyclic nucleotide-gated cation channels contributes to mechanical allodynia after peripheral nerve injury in rat

Peripheral nerve injury causes neuropathic pain including mechanical allodynia and thermal hyperalgesia due to central and peripheral sensitization. Spontaneous ectopic discharges derived from dorsal root ganglion (DRG) neurons and from the sites of injury are a key factor in the initiation of this sensitization. Numerous studies have focused primarily on DRG neurons; however, the injured axons themselves likely play an equally important role. Previous studies of neuropathic pain rats with spinal nerve ligation (SNL) showed that the hyperpolarization-activated cyclic nucleotide-gated cation (HCN) channel in DRG neuronal bodies is important for the development of neuropathic pain. Here, we investigate the role of the axonal HCN channel in neuropathic pain rats. Using the chronic constriction injury (CCI) model, we found abundant axonal accumulation of HCN channel protein at the injured sites accompanied by a slight decrease in DRG neuronal bodies. The function of these accumulated channels was verified by local application of ZD7288, a specific HCN blocker, which significantly suppressed the ectopic discharges from injured nerve fibers with no effect on impulse conduction. Moreover, mechanical allodynia, but not thermal hyperalgesia, was relieved significantly by ZD7288. These results suggest that axonal HCN channel accumulation plays an important role in ectopic discharges from injured spinal nerves and contributes to the development of mechanical allodynia in neuropathic pain rats.

Post-spike excitability indicates changes in membrane potential of isolated C-fibers

Recording of action potentials from single unmyelinated nerve fibers by microneurography is an important tool to investigate peripheral neural functions in human neuropathies. However, the interpretation of microneurography recordings can be difficult because axonal membrane potential is not revealed by this method. We tested the hypothesis that the recovery cycle of excitability after a single action potential is correlated with changes in the axonal membrane potential. To this end, we used the threshold tracking technique to study how different chemical mediators, with known effects on the membrane potential, influence the post-spike superexcitability of C-fiber compound action potentials in isolated rat sural and vagus nerves. We found that: (1) some chemical mediators (e.g., adenosine 5'-triphosphate) produce a reduction or loss of superexcitability together with increased axonal excitability, indicating membrane depolarization; (2) blockade of axonal hyperpolarization-activated (Ih) currents produces an enhancement of superexcitability together with a decreased excitability, indicating membrane hyperpolarization; and (3) application of calcium produces an increase in membrane threshold without an alteration in superexcitability, indicating a non-specific increase in surface charge and a change in the voltage-dependent activation of sodium channels. In addition, we demonstrated that membrane depolarization and hyperpolarization induce opposite post-spike latency shifts (changes in supernormality) in rat and human nerve segments. Thus, recordings of post-spike excitability and shifts in latency are sensitive techniques for detection of various types of neuromodulation, which are correlated with changes in membrane potential of unmyelinated peripheral axons and may help to understand observations obtained by microneurography in peripheral human neuropathies.

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

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

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.

Multiple measures of axonal excitability in peripheral sensory nerves: An in vivo rat model

Excitability measurements on human motor and sensory nerves have provided new insights into axonal membrane changes in peripheral nerve disorders. The aim of this study was to establish an in vivo rat preparation suitable for threshold tracking of sensory nerve action potentials (SNAPs) to model clinical sensory nerve excitability studies. In Sprague-Dawley rats anesthetized with ketamine and xylazine, current stimuli were applied to the base of the tail and SNAPs recorded from distal needle electrodes. Multiple excitability data were obtained as previously described for human nerves and compared to recordings from the motor tail axons and to sensory recordings from human median and ulnar nerves. The pattern of excitability changes in rats was broadly similar to that in humans, although some parameters differed significantly. Individual recordings were stable for at least 3 h. These data show that the rat tail enables excitability properties of sensory as well as motor axons to be studied experimentally, e.g., in models of nerve disease and during pharmacological interventions.

Activity-dependent slowing of conduction velocity in uninjured L4 C fibers increases after an L5 spinal nerve injury in the rat

Growing evidence suggests that uninjured afferents may play an important role in neuropathic pain following nerve injury. The excitability of nociceptive neurons in the L4 spinal nerve appears to be enhanced following an injury to the adjacent L5 spinal nerve. In this study, we investigated whether the action-potential conduction properties of unlesioned, unmyelinated fibers are also altered. A teased-fiber technique was used to record from single C fibers from the L4 spinal nerve of the rat in vitro. Repeated electrical stimulation of the tibial nerve was used to investigate activity-dependent slowing of conduction velocity. Twin pulse stimulation at a 50 ms interpulse interval allowed investigation of supranormal conduction velocity. Blinded experiments were performed 8-10 days after sham surgery and after an L5 spinal nerve ligation (L5 SNL). Activity-dependent slowing revealed two populations of C fibers, a "nociceptor" population with a large degree of activity-dependent slowing and a "non-nociceptor" population with a smaller degree of activity-dependent slowing. Both populations showed enhanced activity-dependent slowing of conduction velocity and enhanced supranormal conduction velocities in lesioned animals compared to sham animals. Activity-dependent slowing was also enhanced after an L5 SNL in the mouse. These alterations in conduction velocity may reflect changes in expression of ion channels responsible for the membrane excitability. These data provide additional evidence that a nerve injury leads to persistent alterations in the properties of adjacent uninjured, unmyelinated fibers.

Hyperpolarization-activated and cyclic nucleotide-gated cation channel subunit 2 ion channels modulate synaptic transmission from nociceptive primary afferents containing substance P to secondary sensory neurons in laminae I-IIo of the rodent spinal dorsal horn

We have previously demonstrated that hyperpolarization-activated and cyclic nucleotide-gated cation channel subunit 2 (HCN2) is expressed by terminals of peptidergic nociceptive primary afferents in laminae I-IIo of the rat spinal dorsal horn. In this study, we investigated the possible neurotransmitters and postsynaptic targets of these HCN2-expressing primary afferent terminals in the superficial spinal dorsal horn by using immunocytochemical methods. We demonstrated that HCN2 widely colocalizes with substance P (SP), and that HCN2-positive terminals that are also immunoreactive for SP form serial close appositions with dendrites and perikarya of neurokinin 1 receptor-immunoreactive neurons. It was also found that HCN2-immunoreactive terminals are frequently apposed to neurons that are immunoreactive for calbindin, micro-opioid receptor and the alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate receptor subunit GluR2, markers for excitatory interneurons. Investigating HCN2 immunoreactivity in glutamic acid decarboxylase 65-green fluorescent protein transgenic mice, we found that HCN2-positive terminals occasionally also contact cells that contain an isoform of glutamic acid decarboxylase (glutamic acid decarboxylase 65), a marker for GABAergic inhibitory neurons. Application of ZD7288, an antagonist of HCN channels, onto neurons that were recorded in spinal cord slices with whole-cell patch-clamp electrodes reduced the number of monosynaptic excitatory postsynaptic potentials evoked by electrical stimulation of primary afferents at nociceptive intensities. The results suggest that HCN2 may contribute to the modulation of membrane excitability of SP-containing nociceptive primary afferent terminals, may increase the reliability of synaptic transmission from primary afferents to secondary sensory neurons and thus may play a role in the fine-tuning of pain transmission from nociceptive primary afferents to neurons in the spinal dorsal horn.

Chemical mediators enhance the excitability of unmyelinated sensory axons in normal and injured peripheral nerve of the rat

Ectopic excitation of nociceptive axons by chemical mediators may contribute to symptoms in neuropathic pain. In this study, we have measured the excitability of unmyelinated rat C-fiber axons in isolated segments of sural nerves under different experimental conditions. (1) We demonstrate in normal rats that several mediators including ATP, serotonin (5-HT), 1-(3-chlorophenyl)biguanide (5-HT3 receptor agonist), norepinephrine, acetylcholine and capsaicin alter electrophysiological parameters of C-fibers which indicate an increase of axonal excitability. Other mediators such as histamine, glutamate, prostaglandin E(2) and the cytokines tumor necrosis factor alpha, interleukin-1beta and interleukin-6 did not produce such effects. (2) The effects of several mediators were tested after peripheral nerve injury (partial ligation or spared nerve injury). Sural nerves from such animals did not show significant changes when compared with controls. (3) We tested whether the effects of chemical mediators on axonal excitability are due to actions on the sensory C-fiber afferents or the postganglionic sympathetic efferents. In order to distinguish these effects, we performed surgical sympathectomy of the lumbar sympathetic chain, including the L3, L4 and L5 ganglia. Sympathectomy did not markedly influence the effects of mediators on axonal excitability (except that the norepinephrine effect was significantly diminished). In conclusion, our data suggest a constitutive rather than inducible expression of axonal receptors for some chemical mediators on the axonal membrane of unmyelinated fibers. Most of the changes in axonal excitability take place in sensory C-fiber afferents rather than in postganglionic sympathetic efferents. Thus, it is possible that certain immune and glial cell mediators released in or around the nerve following injury or inflammation influence the excitability of intact nociceptive fibers. This mechanism could contribute to ectopic excitation of axons in neuropathic pain.

Partial reversal of conduction slowing during repetitive stimulation of single sympathetic efferents in human skin

AIMS

To describe and identify the function of a class of human C fibre with an unusual response to repetitive electrical stimulation. Other C fibres slow progressively at 2 Hz (type 1), reach a latency plateau (type 2) or hardly slow at all (type 3).

METHODS

C fibres innervating hairy skin were recorded by microneurography in the superficial peroneal nerves of 19 healthy volunteers. Baseline electrical stimulation of the skin was at 0.25 Hz, and activity-dependent slowing recorded during stimulation at 2 Hz for 3 min and after a 3-min pause in stimulation.

RESULTS

In 41 units, there was a partial recovery of latency during repetitive stimulation. These were classified as 'type-4' units, and identified as sympathetic efferents, since they exhibited spontaneously activity, which was enhanced by manoeuvres that increase sympathetic outflow (15 of 16 cases) and/or suppressed by a proximal anaesthetic block (eight of eight cases). The peak slowing during 2 Hz trains averaged 6.47 +/- 2.06% (mean +/- SD, n=41), but after 3 min the slowing had reduced to 4.90 +/- 2.20%, which was less than in all type 1 (nociceptor) fibres but similar to that in type 2 (cold) fibres. Compared with cold fibres, type-4 sympathetic fibres slowed more after the first 10 impulses at 2 Hz (2.57 +/- 0.45%) and also after a pause in stimulation (1.66 +/- 0.51%).

CONCLUSIONS

The distinctive activity-dependent slowing profiles of these type-4 sympathetic C units may help identification in vitro, and suggest that hyperpolarization-activated channels have a particularly prominent role in the axonal membrane.

Unmyelinated axons in the rat hippocampus hyperpolarize and activate an H current when spike frequency exceeds 1 Hz

The mammalian cortex is densely populated by extensively branching, thin, unmyelinated axons that form en passant synapses. Some thin axons in the peripheral nervous system hyperpolarize if action potential frequency exceeds 1-5 Hz. To test the hypothesis that cortical axons also show activity-induced hyperpolarization, we recorded extracellularly from individual CA3 pyramidal neurons while activating their axon with trains consisting of 30 electrical stimuli. Synaptic excitation was blocked by kynurenic acid. We observed a positive correlation between stimulation strength and the number of consecutive axonal stimuli that resulted in soma spikes, suggesting that the threshold increased as a function of the number of spikes. During trains without response failures there was always a cumulative increase in the soma response latency. Intermittent failures, however, decreased the latency of the subsequent response. At frequencies of > 1 Hz, the threshold and latency increases were enhanced by blocking the hyperpolarization-activated H current (Ih)by applying the specific Ih blocker ZD7288 (25 microM) or 2 mM Cs+. Under these conditions, response failures occurred after 15-25 stimuli, independent of the stimulation strength. Adding GABA receptor blockers (saclofen and bicuculline) and a blocker of metabotropic glutamate receptors did not change the activity-induced latency increase in recordings of the compound action potential. We interpret these results as an activity-induced hyperpolarization that is partly counteracted by Ih. Such a hyperpolarization may influence transmitter release and the conduction reliability of these axons.

Expression of hyperpolarization-activated and cyclic nucleotide-gated cation channel subunit 2 in axon terminals of peptidergic nociceptive primary sensory neurons in the superficial spinal dorsal horn of rats

Hyperpolarization-activated cyclic nucleotide-gated cation channel proteins (HCN1-4), which are potentially able to modulate membrane excitability, are abundantly expressed by neurons in spinal dorsal root ganglia (DRG). In the present experiment, we investigated whether HCN2 protein is confined exclusively to the perikarya of DRG neurons or is transported from the somata to the central axons of DRG neurons that terminate in the spinal dorsal horn. Using immunohistochemical methods, we have demonstrated that laminae I-IIo of the superficial spinal dorsal horn of the adult rat spinal cord show a strong punctate immunoreactivity for HCN2. Dorsal rhizotomy resulted in a complete loss of immunostaining in the dorsal horn, suggesting that HCN2 is confined to axon terminals of primary afferents. In double labelling immunohistochemical studies, we have also shown that HCN2 widely co-localizes with calcitonin gene-related peptide, but is almost completely segregated from isolectin-B4 binding, indicating that HCN2 is primarily expressed in peptidergic nociceptive primary afferents. The expression of HCN2 in central terminals of peptidergic primary afferents was also verified with electron microscopy. Utilizing the pre-embedding nanogold method, we found that HCN2 is largely confined to axon terminals with dense-core vesicles. Within these terminals, some of the silver grains marking the accurate location of HCN2 molecules were associated with the cell membrane, and others were scattered in the axoplasm. Within the cell membrane, HCN2 was found almost exclusively in extrasynaptic locations. The results suggest that HCN2 may contribute to the modulation of membrane excitability of nociceptive primary afferent terminals in the spinal dorsal horn.

Characterization of neuronal nicotinic acetylcholine receptors in the membrane of unmyelinated human C-fiber axons by in vitro studies

Application of acetylcholine to peripheral nerve terminals in the skin is a widely used test in studies of human small-fiber functions. However, a detailed pharmacological profile and the subunit composition of nicotinic acetylcholine receptors in human C-fiber axons are not known. In the present study, we recorded acetylcholine-induced changes of the excitability and of the intracellular Ca2+ concentration in C-fiber axons of isolated human nerve segments. In addition, using immunohistochemistry, an antibody of a subtype of nicotinic acetylcholine receptor was tested. Acetylcholine and agonists reduced the current necessary for the generation of action potentials in C fibers by <or=30%. This increase in axonal excitability was accompanied by a rise in the free intracellular Ca2+ concentration. The following rank order of potency for agonists was found: epibatidine >> 5-Iodo-A-85380 > 1,1-dimethyl-4-phenylpiperazinium iodide > nicotine > cytisine > acetylcholine; choline had no effect. The epibatidine-induced increase in axonal excitability was blocked by mecamylamine and, less efficiently, by methyllycacontine and dihydro-beta-erythroidine. Many C-fiber axons were labeled by an antibody that recognizes the alpha5 subunit of nicotinic acetylcholine receptors. In summary, electrophysiological and immunohistochemical data indicate the functional expression of nicotinic acetylcholine receptors composed of alpha3, alpha5, and beta4 but not of alpha4/beta2 or of alpha7 subunits in the axonal membrane of unmyelinated human C fibers. In addition, the observations suggest that the axonal membrane of C fibers in isolated segments of human sural nerve can be used as a model for presumed cholinergic chemosensitivity of axonal terminals.

Impulse propagation over tactile and kinaesthetic sensory axons to central target neurones of the cuneate nucleus in cat

Paired, simultaneous recordings were made in anaesthetized cats from the peripheral and central axons of individual tactile and kinaesthetic sensory fibres. The aim was to determine whether failure of spike propagation occurred at any of the three major axonal branch points in the path to their cuneate target neurones, and whether propagation failure may contribute, along with synaptic transmission failures, to limitations in transmission security observed for the cuneate synaptic relay. No evidence for propagation failure was found at the two major axonal branch points prior to the cuneate nucleus, namely, the T-junction at the dorsal root ganglion, and the major branch point near the cord entry point, even for the highest impulse rates (approximately 400 impulses s(-1)) at which these fibres could be driven. However, at the highest impulse rates there was evidence at the central, intra-cuneate recording site of switching between two states in the terminal axonal spike configuration. This appears to reflect a sporadic propagation failure into one of the terminal branches of the sensory axon. In conclusion, it appears that central impulse propagation over group II sensory axons occurs with complete security through branch points within the dorsal root ganglion and at the spinal cord entry zone. However, at high rates of afferent drive, terminal axonal propagation failure may contribute to the observed decline in transmission security within the cuneate synaptic relay.

Activation of dopamine D2-like receptors attenuates pulmonary C-fiber hypersensitivity in rats

This study was performed to determine whether activation of dopamine D2-like receptors inhibits the hyperresponsiveness of pulmonary C fibers induced by inflammatory mediators such as prostaglandin E2 (PGE2). In anesthetized, open-chest rats, constant infusion of PGE2 (1.5-4.5 microg/kg per minute, 2 minutes) significantly enhanced the C-fiber response to capsaicin injection. At 20 minutes after pretreatment with quinpirole (3 mg/kg, intravenous), a D2-like receptor agonist, the hyperresponsiveness to capsaicin of the same C fibers induced by PGE2 infusion was markedly attenuated, and this inhibitory effect lasted for more than 90 minutes. The effect of quinpirole was dose dependent and was antagonized by pretreatment with domperidone (5 mg/kg, intravenous), a D2-like receptor antagonist, administrated 10 minutes before the quinpirole injection. In a separate series of experiments, C-fiber responses to injections of phenyl biguanide and lactic acid and to constant-pressure lung inflation were augmented by PGE2; these potentiating responses were also significantly reduced by quinpirole. Furthermore, the effect of quinpirole was equally effective in inhibiting the increase in excitability of pulmonary C fibers induced by alveolar hypercapnia or constant infusion of adenosine. In conclusion, these results clearly show that activation of the dopamine D2-like receptors attenuates the hyperresponsiveness of pulmonary C fibers to both chemical stimuli and lung inflation.

Action potential conduction in the terminal arborisation of nociceptive C-fibre afferents

Recordings of single human peroneal C-fibres and rat saphenous C-fibres confirm two different patterns of conduction at branching points. In general, an action potential (AP) arising from one terminal branch may be propagated not only centrally, but also antidromically into the other branches of the terminal arborisation. If a stimulus activates several converging branches of one unit, at each branching point only the AP arriving first from the simultaneously activated daughter branches will be propagated centrally, resetting the slower branches. However, occasionally a single electrical stimulus may evoke a double response in the parent axon. In this case, these two responses apparently originate from different terminal branches and require unidirectional conduction block to prevent the faster AP from invading and resetting the slower-conducting terminal. This conclusion is supported by the notion that when such a double response occurs, both responses immediately show additional activity-dependent slowing of the conduction velocity due to frequency increase in the parent axon (two spikes per stimulus, one from each of the two excited branches). A comparable discharge pattern in the stem axon can be induced by repetitive paired stimulation of one terminal branch. Then the slowing is induced by the doubled frequency along the whole nerve fibre including the terminal branch. Since in this case not only the stem axon, but also the terminal branches carry two spikes per pulse, activity-dependent slowing is predictably more pronounced. Unidirectional block thus provides insight into the differential amount of activity-dependent slowing (and hence postexcitatory hyperpolarisation) in the stem axon and terminal branches of cutaneous C-fibres. This comparison reveals that more than two-thirds of the slowing can be attributed to the terminal branches, since it is two- to fourfold that observed during double stimulation as compared with the unidirectional block condition. This indicates that the terminal branches are equipped with membrane proteins that are different from those of the parent axon.

ATP stimulates peripheral axons in human, rat and mouse--differential involvement of A(2B) adenosine and P2X purinergic receptors

Receptors for ATP have been reported on peripheral nerve terminals. It is a widespread assumption that the axonal membrane does not possess this kind of chemosensitivity, although P2X purinoceptors have been found in isolated rat vagus nerve. Therefore, in the present study, effects of ATP and analogues were tested on the excitability of unmyelinated axons in isolated rat sural nerve, mouse dorsal roots, and human sural nerve. Bath application of ATP to all three types of axonal preparations increased axonal excitability, but the underlying receptors appear to differ in the various preparations. In rat sural nerve, alpha,beta-adenosine-5'-methylene triphosphate produced the strongest excitation. This effect was blocked by pyridoxal-phosphate-6-azophenyl-2',5'-disulphonic acid and indicates the presence of P2X receptors. In mouse dorsal roots, differences were found between fast and slow C-fibres. The latter responded to both P2X receptor and adenosine receptor agonists. In contrast, effects of ATP on faster-conducting C-fibres seem to be caused exclusively by effects of ATP on adenosine receptors. Application of ATP also excited C-fibres in fascicles of isolated human nerve. The pharmacological profile indicates activation of A(2B) adenosine receptors. However, we could not detect P2X receptors in this preparation with our techniques. These data show that the ATP sensitivity of sensory neurones is not restricted to their terminals. Activation of axonal purinergic receptors may contribute to the transduction of sensory, including nociceptive, stimuli.

Effect of halothane on neuronal excitation in the superficial dorsal horn of rat spinal cord slices: evidence for a presynaptic action

The action of the volatile anaesthetic halothane on optically recorded neuronal excitation in juvenile rat spinal cord slices was investigated. Prolonged neuronal excitation lasting approximately 100 ms was evoked in the superficial dorsal horn after single-pulse dorsal root stimulation that activated both A- and C-fibres. Halothane depressed the neuronal excitation in a concentration-dependent manner (IC(50) 0.21 mm, I(max) 28%). In Ca(2+)-free solution, dorsal root stimulation induced excitation with a short duration of several tens of milliseconds, in which the excitation of the postsynaptic component was largely eliminated. Under these conditions, halothane also depressed the excitation concentration-dependently (IC(50) 0.46 mm, I(max) 60%). Most of the suppression occurred within 5 min of halothane application, and the effect of halothane was fully reversible upon washout of the anaesthetic. Application of bicuculline and strychnine or picrotoxin, or reduction of extracellular Cl(-) concentration ([Cl(-)](o)), had no effect on halothane inhibition. Applications of K(+) channel blockers tetraethyl ammonium, 4-aminopyridine, Cs(+) or Ba(2+) either had no effect or augmented the inhibitory effect of halothane. On the other hand, the degree of inhibition by halothane was found to be dependent on [K(+)](o); the higher [K(+)](o), the larger the depression. In addition, decreases in [Na+]o and [Mg(2+)](o) reduced the excitation similar to that of halothane treatment, and the degree of halothane inhibition became larger with lower [Mg(2+)](o). These results lead to a hypothesis that halothane suppresses the excitation of presynaptic elements by inhibiting presynaptic Na(+) channels by shifting the steady-state inactivation curve in the hyperpolarizing direction.

Specificity of cold thermotransduction is determined by differential ionic channel expression

Sensations of cold are mediated by specific thermoreceptor nerve endings excited by low temperature and menthol. Here we identify a population of cold-sensitive cultured mouse trigeminal ganglion neurons with a unique set of biophysical properties. Their impulse activity during cooling and menthol application was similar to that of cold thermoreceptor fibers in vivo. We show that cooling closes a background K+ channel, causing depolarization and firing that is limited by the slower reduction of a cationic inward current (Ih). In cold-insensitive neurons, firing is prevented by a slow, transient, 4-AP-sensitive K+ current (IKD) that acts as an excitability brake. In addition, pharmacological blockade of IKD induced thermosensitivity in cold-insensitive neurons, a finding that may explain cold allodynia in neuropathic pain. These results suggest that cold sensitivity is not associated to a specific transduction molecule but instead results from a favorable blend of ionic channels expressed in a small subset of sensory neurons.

ATP affects both axons and Schwann cells of unmyelinated C fibres

Recent studies indicate that effects of ATP on unmyelinated afferent nerve fibres contribute to the transduction of nociceptive and non-nociceptive stimuli. In the present study, effects of ATP were studied on axons and Schwann cells of C fibres in isolated rat vagus nerves. A combination of a computerised threshold tracking technique with photometric and confocal measurements of the free intracellular Ca2+ concentration revealed differences in the effect of ATP and related compounds. Pyridoxal-phosphate-6-azophenyl-2',5'-disulphonic acid (iso-PPADS, an antagonist of ionotropic P2X receptors) completely blocked the excitatory effect of alpha,beta-meATP on unmyelinated axons, whereas the effects of ATP and 2-Cl-ATP were only slightly changed. Moreover, the threshold lowering effects of ATP and 2-Cl-ATP, but not of alpha,beta-meATP, were accompanied by intracellular Ca2+ transients. In confocal imaging experiments, the lectin IB4 was used to identify unmyelinated nerve fibres and their ensheathing Schwann cells. The Schwann cells were identified as the cellular elements underlying ATP-induced Ca2+ transients. In addition, an increase in axonal excitability of C fibres was seen during a rise in [Ca2+]i induced by inhibition of the endoplasmic Ca2 ATPase with cyclopiazonic acid. These data show that an increase of the extracellular ATP concentration in an intact peripheral nerve trunk activates both axons and Schwann cells. It appears that P2 nucleotide receptors on Schwann cells may contribute to the excitatory effect of ATP observed on unmyelinated, including nociceptive, axons.

Modulation of the hyperpolarisation-activated current, Ih, in rat facial motoneurones in vitro by ZD-7288

ZD-7288 [4-(N-ethyl-N-phenylamino)-1,2-dimethyl-6-(methylamino) pyrimidinium chloride] and Cs(+) have been used to distinguish the currents contributing to inward rectification in neonatal rat facial motoneurones (FMs). ZD-7288 (0.1-10 microM) inhibited a current that reversed at -43.7+/-3.7 mV in artificial cerebrospinal fluid (ACSF) containing 3 mM K(+) (n=9), and displayed the time and voltage dependence of the hyperpolarisation-activated current, I(h). Depolarisation-activated transient (I(K(A))) and sustained outward currents were unaffected by ZD-7288. The IC(50) for block of I(h) by ZD-7288 was around 0.2 microM. Onset of inhibition was slow and no recovery was seen after washing in ZD-7288-free ACSF for up to 4 h. In the presence of ZD-7288, Ba(2+) and Rb(+) blocked an inwardly rectifying potassium (K(+)) current, confirming both the presence of I(K(IR)) and its insensitivity to ZD-7288. Cs(+) rapidly and reversibly blocked both I(h) and I(K(IR)). Inhibition of I(h) by ZD-7288 showed no use dependence, internally applied ZD-7288 also blocked I(h), and tail current analysis indicated inhibition to be voltage-independent. In the presence of internal guanosine 5'-O-(3-thiotriphosphate) (GTP-gamma-S) and after previous exposure to ZD-7288, 5-hydroxytryptamine (5-HT), but not noradrenaline, promoted a recovery of I(h) that was not observed if ZD-7288 was present throughout the recording period. This interaction between ZD-7288 and irreversible 5-HT-receptor activation may be related to the mechanism underlying ZD-7288-mediated block of these channels.

Diadenosine pentaphosphate is more potent than ATP at P2X receptors in isolated rat vagus nerve

The effects of diadenosine pentaphosphate (Ap5A), diadenosine tetraphosphate (Ap4A), alpha,beta-methyleneATP (alpha,beta-meATP), and ATP were studied on the excitability of unmyelinated axons in isolated rat vagus nerve by means of a computerized threshold tracking technique. All purinergic compounds produced an increase in excitability, however, only the effects of alpha,beta-meATP and of Ap5A were strongly reduced by 2'- (or 3') -O-(2,4,6-trinitrophenyl)-ATP (TNP-ATP), a selective blocker for P2X1, P2X3, and heteromeric P2X2/3 receptors. The rank order of potency for TNP-ATP-sensitive excitation was determined as follows (30 microM each): alpha,beta-meATP >Ap5A >> Ap4A = ATP. These data suggest that Ap5A might be an important naturally occurring agonist for P2X receptors at the axonal membrane of unmyelinated, including nociceptive, nerve fibres.

Cellular expression and functional characterization of four hyperpolarization-activated pacemaker channels in cardiac and neuronal tissues

Hyperpolarization-activated cation currents (I(h)) have been identified in cardiac pacemaker cells and a variety of central and peripheral neurons. Four members of a gene family encoding hyperpolarization-activated, cyclic nucleotide-gated cation channels (HCN1--4) have been cloned recently. Native I(h) currents recorded from different cell types exhibit distinct activation kinetics. To determine if this diversity of I(h) currents may be caused by differential expression of HCN channel isoforms, we investigated the cellular distribution of the transcripts of HCN1--4 in the murine sinoatrial node, retina and dorsal root ganglion (DRG) by in situ hybridization. In the sinoatrial node, the most prominently expressed HCN channel is HCN4, whereas HCN2 and HCN1 are detected there at moderate and low levels, respectively. Retinal photoreceptors express high levels of HCN1, whereas HCN2, 3 and 4 were not found in these cells. In DRG neurons, the dominant HCN transcript is HCN1, followed by HCN2. We next determined the functional properties of recombinant HCN1--4 channels expressed in HEK293 cells. All four channel types gave rise to I(h) currents but displayed marked differences in their activation kinetics. Our results suggest that the heterogeneity of native I(h) currents is generated, at least in part, by the tissue-specific expression of HCN channel genes.

Time course of post-excitatory effects separates afferent human C fibre classes

1. To study post-excitatory changes of conduction velocity, action potentials were recorded from 132 unmyelinated nerve fibres (C fibres) in cutaneous fascicles of the peroneal nerve using microneurography in healthy human subjects. The 'marking' technique was used to assess responsiveness to mechanical and heat stimuli or sympathetic reflex provocation. 2. C fibres were classified into three major classes: mechano-responsive afferent (n = 76), mechano-insensitive afferent (n = 48) and sympathetic efferent C fibres (n = 8). 3. During regular stimulation at 0.25 Hz, conditioning pulses were intermittently interposed. Changes of conduction velocity were assessed for different numbers of conditioning impulses and varying interstimulus intervals (ISIs). For all three fibre classes the latency shift following conditioning pulses at an ISI of 1000 ms increased linearly with their number (n = 1, 2 and 4). However, the absolute degree of conduction velocity slowing was much higher in the 32 mechano-insensitive fibres as compared with 56 mechano-responsive or 8 sympathetic fibres. 4. Single additional pulses were interposed at different ISIs from 20 to 2000 ms. For 20 mechano-responsive fibres conduction velocity slowing increased with decreasing ISI (subnormal phase). In contrast, for 16 mechano-insensitive C fibres the conduction velocity slowing decreased with shorter ISIs, and at values lower than 417 +/- 49 ms (mean +/- s.e.m.) the conduction velocity of the conditioned action potential was faster than before (conduction velocity speeding). This supernormal phase had its maximum at 69 +/- 10 ms. 5. In this study we provide, for the first time, direct evidence of relative supernormal conduction in human mechano-insensitive C fibres. The implications for temporal coding in different afferent C fibre classes are discussed.

Hyperpolarization-activated currents in presynaptic terminals of mouse cerebellar basket cells

Using patch-clamp techniques, a hyperpolarization-activated current (I(h)) was recorded from synaptic terminals of mouse cerebellar basket cells. Ih was blocked quickly and reversibly by 2 mM Cs(+), and subtraction revealed a rapidly activating and deactivating I(h) current. Similar gating and block of presynaptic I(h) were also seen with the more selective inhibitor ZD 7288 (10 microM). The time constant of activation (tau (a))of presynaptic I(h) current became faster with membrane hyperpolarization, being approximately 74 ms at -130 mV, changing e-fold for a 33 mV change in membrane potential. Whole-cell recordings from basket cell somata also revealed an I(h) current, which was similarly sensitive to block by ZD 7288. Inhibition of I(h) by 10 microM ZD 7288 reduced the frequency ( approximately 34 %) and amplitude ( approximately 26 %) of spontaneous IPSCs (sIPSCs) recorded in Purkinje cells, one of the principal synaptic targets of basket neurones. This is the first report of an I(h) current in mammalian inhibitory presynaptic terminals, which may be an important target for neuromodulation in the cerebellum. Comparing the biophysical properties and distribution of cloned hyperpolarization-activated cation channels, we also suggest a molecular candidate underlying I(h) at these synapses.

Mechanisms of paresthesias arising from healthy axons

Paresthesias are common manifestations of central and peripheral pathological processes and are due to ectopic impulse activity in cutaneous afferents or their central projections. Cutaneous afferents are more excitable than motor axons, due to differences in their biophysical properties. These differences probably include more persistent Na(+) conductance and inward rectification on cutaneous afferents, properties which probably confer greater protection from impulse-dependent conduction failure but create a greater tendency to ectopic activity. Ectopic discharges can be induced in normal afferents by four maneuvers: hyperventilation, ischemia, release of ischemia, and prolonged tetanization. The alkaline shift produced by hyperventilation selectively increases the persistent Na(+) conductance, while the membrane depolarization produced by ischemia affects both transient and persistent Na(+) channels. Postischemic and posttetanic paresthesias occur when hyperpolarization by the Na(+)/K(+) pump is transiently prevented by raised extracellular K(+). The electrochemical gradient for K(+) is reversed, and inward K(+) currents trigger regenerative depolarization. These mechanisms of paresthesia generation can account for paresthesias in normal subjects and may be relevant in some peripheral nerve disorders.

Enhancement of synaptic transmission by cyclic AMP modulation of presynaptic Ih channels

Presynaptic activation of adenylyl cyclase and subsequent generation of cAMP represent an important mechanism in the modulation of synaptic transmission. In many cases, short- to medium-term modulation of synaptic strength by cAMP is due to activation of protein kinase A and subsequent covalent modification of presynaptic ion channels or synaptic proteins. Here we show that presynaptic cAMP generation via serotonin receptor activation directly modulated hyperpolarization-activated cation channels (Ih channels) in axons. This modulation of Ih produced an increase in synaptic strength that could not be explained solely by depolarization of the presynaptic membrane. These studies identify a mechanism by which cAMP and Ih regulate synaptic plasticity.