Differences between mammalian ventral and dorsal spinal roots in response to blockade of potassium channels during maturation

Real-time CARS imaging reveals a calpain-dependent pathway for paranodal myelin retraction during high-frequency stimulation

High-frequency electrical stimulation is becoming a promising therapy for neurological disorders, however the response of the central nervous system to stimulation remains poorly understood. The current work investigates the response of myelin to electrical stimulation by laser-scanning coherent anti-Stokes Raman scattering (CARS) imaging of myelin in live spinal tissues in real time. Paranodal myelin retraction at the nodes of Ranvier was observed during 200 Hz electrical stimulation. Retraction was seen to begin minutes after the onset of stimulation and continue for up to 10 min after stimulation was ceased, but was found to reverse after a 2 h recovery period. The myelin retraction resulted in exposure of Kv 1.2 potassium channels visualized by immunofluorescence. Accordingly, treating the stimulated tissue with a potassium channel blocker, 4-aminopyridine, led to the appearance of a shoulder peak in the compound action potential curve. Label-free CARS imaging of myelin coupled with multiphoton fluorescence imaging of immuno-labeled proteins at the nodes of Ranvier revealed that high-frequency stimulation induced paranodal myelin retraction via pathologic calcium influx into axons, calpain activation, and cytoskeleton degradation through spectrin break-down.

Functional specializations of primary auditory afferents on the Mauthner cells: interactions between membrane and synaptic properties

Primary auditory afferents are usually perceived as passive, timing-preserving, lines of communication. Contrasting this view, a special class of auditory afferents to teleost Mauthner cells, a command neuron that organizes tail-flip escape responses, undergoes potentiation of their mixed (electrical and chemical) synapses in response to high frequency cellular activity. This property is likely to represent a mechanism of input sensitization as these neurons provide the Mauthner cell with essential information for the initiation of an escape response. We review here the anatomical and physiological specializations of these identifiable auditory afferents. In particular, we discuss how their membrane and synaptic properties act in concert to more efficaciously activate the Mauthner cells. The striking functional specializations of these neurons suggest that primary auditory afferents might be capable of more sophisticated contributions to auditory processing than has been generally recognized.

The effects of oxaliplatin, an anticancer drug, on potassium channels of the peripheral myelinated nerve fibres of the adult rat

Oxaliplatin is a novel chemotherapeutic agent which is effective against advanced colorectal cancer, but at the same time causes severe neuropathy in the peripheral nerve fibres, affecting mainly the voltage-gated sodium (Na(+)) channels (VGNaCs), according to literature. In this study the effects of oxaliplatin on the peripheral myelinated nerve fibres (PMNFs) were investigated in vitro using the isolated sciatic nerve of the adult rat. The advantage of this nerve-preparation was that stable in amplitude evoked compound action potentials (CAP) were recorded for over 1000min. Incubation of the sciatic nerve fibres in 25, 100 and 500microM oxaliplatin, for 300-700min caused dramatic distortion of the waveform of the CAP, namely broadening the repolarization phase, repetitive firing and afterhyperpolarization (AHP), related to the malfunction of voltage-gated potassium (K(+)) channels (VGKCs). At a concentration of 5microM, oxaliplatin caused broadening of the repolarization phase of the CAP only, while the no observed effect concentration was estimated to be 1microM. These findings are indicative of severe effects of oxaliplatin on the VGKCs. In contrast, the amplitude and the rise-time of the depolarization of the CAP did not change significantly, a clear indication that the VGNaCs of the particular nerve preparation were not affected by oxaliplatin. The effects of oxaliplatin on the PMNFs were similar to those of 4-aminopyridine (4-AP), a classical antagonist of VGKCs. These similarities in the pattern of action between oxaliplatin and 4-AP combined with the fact that the effects of oxaliplatin were more pronounced and developed at lower concentrations suggest that oxaliplatin acts as a potent VGKCs antagonist.

Manually-stimulated recovery of motor function after facial nerve injury requires intact sensory input

We have recently shown in rat that daily manual stimulation (MS) of vibrissal muscles promotes recovery of whisking and reduces polyinnervation of muscle fibers following repair of the facial nerve (facial-facial anastomosis, FFA). Here, we examined whether these positive effects were: (1) correlated with alterations of the afferent connections of regenerated facial motoneurons, and (2) whether they were achieved by enhanced sensory input through the intact trigeminal nerve. First, we quantified the extent of total synaptic input to motoneurons in the facial nucleus using synaptophysin immunocytochemistry following FFA with and without subsequent MS. We found that, without MS, this input was reduced compared to intact animals. The number of synaptophysin-positive terminals returned to normal values following MS. Thus, MS appears to counteract the deafferentation of regenerated facial motoneurons. Second, we performed FFA and, in addition, eliminated the trigeminal sensory input to facial motoneurons by extirpation of the ipsilateral infraorbital nerve (IONex). In this paradigm, without MS, vibrissal motor performance and pattern of end-plate reinnervation were as aberrant as after FFA without MS. MS did not influence the reinnervation pattern after IONex and functional recovery was even worse than after IONex without MS. Thus, when the sensory system is intact, MS restores normal vibrissal function and reduces the degree of polyinnervation. When afferent inputs are abolished, these effects are eliminated or even reversed. We conclude that rehabilitation strategies must be carefully designed to take into account the extent of motor and/or sensory damage.

Subthreshold sodium current underlies essential functional specializations at primary auditory afferents

Primary auditory afferents are generally perceived as passive, timing-preserving lines of communication. Contrasting this view, identifiable auditory afferents to the goldfish Mauthner cell undergo potentiation of their mixed--electrical and chemical--synapses in response to high-frequency bursts of activity. This property likely represents a mechanism of input sensitization because they provide the Mauthner cell with essential information for the initiation of an escape response. Consistent with this synaptic specialization, we show here that these afferents exhibit an intrinsic ability to respond with bursts of 200-600 Hz and this property critically relies on the activation of a persistent sodium current, which is counterbalanced by the delayed activation of an A-type potassium current. Furthermore, the interaction between these conductances with the membrane passive properties supports the presence of electrical resonance, whose frequency preference is consistent with both the effective range of hearing in goldfish and the firing frequencies required for synaptic facilitation, an obligatory requisite for the induction of activity-dependent changes. Thus our data show that the presence of a persistent sodium current is functionally essential and allows these afferents to translate behaviorally relevant auditory signals into patterns of activity that match the requirements of their fast and highly modifiable synapses. The functional specializations of these neurons suggest that auditory afferents might be capable of more sophisticated contributions to auditory processing than has been generally recognized.

Multiple interacting sites of ectopic spike electrogenesis in primary sensory neurons

Ectopic discharge generated in injured afferent axons and cell somata in vivo contributes significantly to chronic neuropathic dysesthesia and pain after nerve trauma. Progress has been made toward understanding the processes responsible for this discharge using a preparation consisting of whole excised dorsal root ganglia (DRGs) with the cut nerve attached. In the in vitro preparation, however, spike activity originates in the DRG cell soma but rarely in the axon. We have now overcome this impediment to understanding the overall electrogenic processes in soma and axon, including the resulting discharge patterns, by modifying the bath medium in which recordings are made. At both sites, bursts can be triggered by subthreshold oscillations, a phasic stimulus, or spikes arising elsewhere in the neuron. In the soma, once triggered, bursts are maintained by depolarizing afterpotentials, whereas in the axon, an additional process also plays a role, delayed depolarizing potentials. This alternative process appears to be involved in "clock-like" bursting, a discharge pattern much more common in axons than somata. Ectopic spikes arise alternatively in the soma, the injured axon end (neuroma), and the region of the axonal T-junction. Discharge sequences, and even individual multiplet bursts, may be a mosaic of action potentials that originate at these alternative electrogenic sites within the neuron. Correspondingly, discharge generated at these alternative sites may interact, explaining the sometimes-complex firing patterns observed in vivo.

Early Excision of a Full-Thickness Burn Prevents Peripheral Nerve Conduction Deficits in Mice

BACKGROUND

A full-thickness 20 percent body surface area burn in mice produces a significant decrease in tibial motor nerve conduction velocity within 6 hours of the burn and in sensory conduction velocity within 7 days. This suggests that cutaneous burn injury produces a systemic response that affects peripheral motor and sensory nerve function at a distance from the burn site. The authors tested the hypothesis that burn wound excision either 30 minutes or 3 hours after burn would prevent neuropathy.

METHODS

A 20 percent body surface area third-degree burn was applied to the backs of anesthetized mice using procedures that followed National Institutes of Health guidelines. Motor nerve conduction velocity and sensory conduction velocity were determined in intact, anesthetized mice by percutaneous nerve stimulation. Burn wounds were excised and closed at 30 minutes or 3 hours after burn. Motor nerve conduction velocity and sensory conduction velocity were measured before burn and 1, 3, 7, 14, and 21 days after a burn or sham procedure. The number of circulating neutrophils and serum concentrations of tumor necrosis factor-alpha, nitrite, and electrolytes were also determined in each group.

RESULTS

Motor nerve conduction velocity and sensory conduction velocity in the 30-minute excision (n = 10) and sham group (n = 5) were not significantly different. Motor nerve conduction velocity and sensory conduction velocity in the nonexcised group (n = 10) and 3-hour excision group (n = 10) were significantly decreased. Serum tumor necrosis factor-alpha concentration was elevated 6 hours after burn in nonexcised animals (n = 9) and in 3-hour excision mice (n = 8) but was not significantly different in the sham (n = 8) and the 30-minute excision group (n = 7).

CONCLUSION

The authors conclude that burn wound excision at 30 minutes but not at 3 hours prevents the nerve conduction deficits measured in mice with 20 percent body surface area burns. The cellular basis of burn-induced neuropathy is unknown, but nitric oxide and tumor necrosis factor alpha-alpha appear to play a role.

Fibroblast growth factor homologous factor 2B: association with Nav1.6 and selective colocalization at nodes of Ranvier of dorsal root axons

Voltage-gated sodium channels interact with cytosolic proteins that regulate channel trafficking and/or modulate the biophysical properties of the channels. Na(v)1.6 is heavily expressed at the nodes of Ranvier along adult CNS and PNS axons and along unmyelinated fibers in the PNS. In an initial yeast two-hybrid screen using the C terminus of Na(v)1.6 as a bait, we identified FHF2B, a member of the FGF homologous factor (FHF) subfamily, as an interacting partner of Na(v)1.6. Members of the FHF subfamily share approximately 70% sequence identity, and individual members demonstrate a cell- and tissue-specific expression pattern. FHF2 is abundantly expressed in the hippocampus and DRG neurons and colocalizes with Na(v)1.6 at mature nodes of Ranvier in myelinated sensory fibers in the dorsal root of the sciatic nerve. However, retinal ganglion cells and spinal ventral horn motor neurons show very low levels of FHF2 expression, and their axons exhibit no nodal FHF2 staining within the optic nerve and ventral root, respectively. Thus, FHF2 is selectively localized at nodes of dorsal root sensory but not ventral root motor axons. The coexpression of FHF2B and Na(v)1.6 in the DRG-derived cell line ND7/23 significantly increases the peak current amplitude and causes a 4 mV depolarizing shift of voltage-dependent inactivation of the channel. The preferential expression of FHF2B in sensory neurons may provide a basis for physiological differences in sodium currents that have been reported at the nodes of Ranvier in sensory versus motor axons.

Axonal oscillations in developing mammalian nerve axons

We study neuronal spike propagation in a developing myelinated axon in various stages of its development through detailed computational modeling. Recently, a form of bursting (axonal bursting), has been reported in axons in developing nerves in the absence of potassium channels. We present a computational study using a detailed model for a myelinated nerve in development to explore under what circumstances such an effect can be expected. It is shown that axonal oscillation may be caused by backfiring between the nodes of Ranvier or through backfiring from internodal sodium channels or by reducing the thickness of the myelin wrapping the axon between the nodes of Ranvier.

Voltage-dependent enhancement of electrical coupling by a subthreshold sodium current

Voltage-dependent changes in electrical coupling are often attributed to a direct effect on the properties of gap junction channels. Identifiable auditory afferents terminate as mixed (electrical and chemical) synapses on the distal portion of the lateral dendrite of the goldfish Mauthner cells, a pair of large reticulospinal neurons involved in the organization of sensory-evoked escape responses. At these afferents, the amplitude of the coupling potential produced by the retrograde spread of signals from the postsynaptic Mauthner cell is dramatically enhanced by depolarization of the presynaptic terminal. We demonstrate here that this voltage-dependent enhancement of electrical coupling does not represent a property of the junctions themselves but the activation of a subthreshold sodium current present at presynaptic terminals that acts to amplify the synaptic response. We also provide evidence that this amplification operates under physiological conditions, enhancing synaptic communication from the Mauthner cells to the auditory afferents where electrical and geometrical properties of the coupled cells are unfavorable for retrograde transmission. Retrograde electrical communication at these afferents may play an important functional role by promoting cooperativity between afferents and enhancing transmitter release. Thus, the efficacy of an electrical synapse can be dynamically modulated in a voltage-dependent manner by properties of the nonjunctional membrane. Finally, asymmetric amplification of electrical coupling by intrinsic membrane properties, as at the synapses between auditory afferents and the Mauthner cell, may ensure efficient communication between neuronal processes of dissimilar size and shape, promoting neuronal synchronization.

Effects of K+ channel blockers on developing rat myelinated CNS axons: identification of four types of K+ channels

Four blockers of voltage-gated potassium channels (Kv channels) were tested on the compound action potentials (CAPs) of rat optic nerves in an attempt to determine the regulation of Kv channel expression during the process of myelination. Before myelination occurred, 4-aminopyridine (4-AP) increased the amplitude, duration, and refractory period of the CAPs. On the basis of their pharmacological sensitivity, 4-AP-sensitive channels were divided in two groups, the one sensitive to kaliotoxin (KTX), dendrotoxin-I (DTX-I), and 4-AP, and the other sensitive only to 4-AP. In addition, tetraethylammonium chloride (TEA) applied alone broadened the CAPs. At the onset of myelination, DTX-I induced a more pronounced effect than KTX; this indicates that a fourth group of channels sensitive to 4-AP and DTX-I but insensitive to KTX had developed. The effects of KTX and DTX-I gradually disappeared during the period of myelination. Electron microscope findings showed that the disappearance of these effects was correlated with the ongoing process of myelination. This was confirmed by the fact that DTX-I and KTX enlarged the CAPs of demyelinated adult optic nerves. These results show that KTX- and DTX-sensitive channels are sequestrated in paranodal regions. During the process of myelination, KTX had less pronounced effects than DTX-I on demyelinated nerves, which suggests that the density of the KTX-sensitive channels decreased during this process. By contrast, 4-AP increased the amplitude, duration, and refractory period of the CAPs at all the ages tested and to a greater extent than KTX and DTX-I. The effects of TEA alone also gradually disappeared during this period. However, effects of TEA on CAPs were observed when this substance was applied after 4-AP to the adult optic nerve; this shows that TEA-sensitive channels are not masked by the myelin sheath. In conclusion, the process of myelination seems to play an important part in the regulation and setting of Kv channels in optic nerve axons.

A distal upstream enhancer from the myelin basic protein gene regulates expression in myelin-forming schwann cells

In peripheral nerves, large caliber axons are ensheathed by myelin-elaborating Schwann cells. Multiple lines of evidence demonstrate that expression of the genes encoding myelin structural proteins occurs in Schwann cells in response to axonal instructions. To gain further insight into the mechanisms controlling myelin gene expression, we used reporter constructs in transgenic mice to search for the DNA elements that regulate the myelin basic protein (MBP) gene. Through this in vivo investigation, we provide evidence for the participation of multiple, widely distributed, positive and negative elements in the overall control of MBP expression. Notably, all constructs bearing a 0.6 kb far-upstream sequence, designated Schwann cell enhancer 1 (SCE1), expressed at high levels in myelin-forming Schwann cells. In addition, robust targeting activity conferred by SCE1 was shown to be independent of other MBP 5' flanking sequence. These observations suggest that SCE1 will make available a powerful tool to drive transgene expression in myelinating Schwann cells and that a focused analysis of the SCE1 sequence will lead to the identification of transcription factor binding sites that positively regulate MBP expression.

Effect of maturation on nerve excitability in an experimental model of threshold electrotonus

Threshold electrotonus (TE) is a new tool for investigating axonal function noninvasively in vivo. To increase its potential clinical value, we developed a rat model of TE, and examined the effects of maturation and pharmacological intervention. We recorded TE in 92 male rats (body weight 90-650 g) by stimulating the motor nerve in the tail, and applying 100-ms conditioning currents. Motor conduction velocities increased up to a body weight of 330 g, and remained constant thereafter. TE in mature rats was similar to that in humans, and two parameters were analyzed: TEd(10-20) or the mean threshold reduction 10-20 ms after the onset of the depolarizing conditioning current at 40% of threshold intensity; and TEh(10-20) or the corresponding threshold decrease on hyperpolarization. Like latency, the absolute value of TEh(10-20) decreased up to 330 g, and then stabilized thereafter, probably reflecting the progressive increase in the axonal diameter and relative reduction in internodal impedance. In contrast, TEd(10-20) gradually decreased up to 330 g, and then jumped to a higher level, which was maintained for animals of >400 g. 4-Aminopyridine, a blocker of fast potassium channels, selectively increased TEd(10-20) only in the immature or young (<330 g) rats. This suggests that, in the mature animals, fast potassium channels become sequestrated from the nodal membrane and not activated in response to nodal depolarization. These findings indicate that mature rats (>400 g) may provide a useful experimental model for interpreting abnormal TE responses in humans, and provide evidence for nonlinear maturation of potassium channel function in myelinated axons.

Dynamic potassium channel distributions during axonal development prevent aberrant firing patterns

The distribution and function of Shaker-related K+ channels were studied with immunofluorescence and electrophysiology in sciatic nerves of developing rats. At nodes of Ranvier, Na+ channel clustering occurred very early (postnatal days 1-3). Although K+ channels were not yet segregated at most of these sites, they were directly involved in action potential generation, reducing duration, and the refractory period. At approximately 1 week, K+ channel clusters were first seen but were within the nodal gap and in paranodes, and only later (weeks 2-4) were they shifted to juxtaparanodal regions. K+ channel function was most dramatic during this transition period, with block producing repetitive firing in response to single stimuli. As K+ channels were increasingly sequestered in juxtaparanodes, conduction became progressively insensitive to K+ channel block. Over the first 3 weeks, K+ channel clustering was often asymmetric, with channels exclusively in the distal paranode in approximately 40% of cases. A computational model suggested a mechanism for the firing patterns observed, and the results provide a role for K+ channels in the prevention of aberrant excitation as myelination proceeds during development.

■ REVIEW : Ionic Channels with Weak Voltage-dependence

In light of the recent identification of a class of voltage-dependent Na+ channels with a comparatively weak peak conductance/voltage relationship, this article reviews the structural elements by which proteins are rendered voltage-dependent and the manner in which these proteins are studied. The physicochemical principles used to analyze voltage-dependent ionic channels are reviewed, together with examples drawn from Na+ currents recorded from the somatic membranes of dorsal root ganglion neurons. The structure of the voltage dependent Na + channel and known structural/functional correlates are described. A functional definition of ionic channels with weak voltage-dependence and a discussion of inherent pitfalls of classical analytic methods when applied to the study of such channels are offered. Ultimately, determining how these channels function as they do, i.e., elucidating their gating properties, will be accomplished by cloning and reexpressing the channels in otherwise non-excitable cell systems, and recording their gating currents. Na+ and K+ channels with weak voltage-dependence may serve an important functional role by modulating threshold in excitable cells. NEUROSCIENTIST 3:102-111, 1997

Action potentials and membrane currents in the human node of Ranvier

Action potentials and membrane currents were recorded in single human myelinated nerve fibres under current- and voltage-clamp conditions at room temperature. Nerve material was obtained from patients undergoing nerve graft operations. Successful recordings were made in 11 nerve fibres. In Ringer's solution, large transient Na currents were recorded, which could be blocked completely with tetrodotoxin. Partial block of these currents with 3 nM tetrodotoxin was used to reduce the voltage-clamp error due to series resistance. Outward K currents were very small in intact nerve fibres, but had a large amplitude in fibres showing signs of paranodal demyelination. In isotonic KCl, the K current could be separated into three components: two fast components (Kf1 and Kf2) and one slow component (Ks). Time constants and steady-state activation and inactivation of Na permeability and of fast and slow K conductance were measured within the potential range of -145 mV to +115 mV. From these parameters, the corresponding rate constants were calculated and a mathematical model based on the Frankenhaeuser-Huxley equations was derived. Calculated action potentials closely matched those recorded. Single calculated action potentials were little affected by removing the fast or slow K conductance, but the slow K conductance was required to limit the repetitive response of the model to prolonged stimulating currents.

Delayed depolarization and slow sodium currents in cutaneous afferents

1. Intraaxonal recordings were obtained in vitro from the sural nerve (SN), the muscle branch of the anterior tibial nerve (ATN), or the deafferented ATN (dATN) in 5- to 7-wk-old rats. Whole-nerve sucrose gap recordings were obtained from the SN and the ATN. This allowed study of cutaneous (SN), mixed motor and muscle afferent (ATN), and isolated muscle afferent (dATN) axons. 2. Application of the potassium channel blocking agent 4-aminopyridine (4-AP) to ATN or dATN resulted in a slight prolongation of the action potential. In contrast, a distinct delayed depolarization followed the axonal action potential in cutaneous afferents (SN) exposed to 4-AP. The delayed depolarization could be induced by a single whole-nerve stimulus or by injection of constant-current depolarizing pulses into individual axons. The delayed depolarization often gave rise to bursts of action potentials and was followed by a prominent afterhyperpolarization (AHP). 3. In paired-pulse experiments on single SN axons, the recovery time (half-amplitude of the action potential) was 3.06 +/- 1.82 (SE) ms (n = 12). After exposure to 4-AP the recovery time of the delayed depolarization was considerably longer (half-recovery time: 99.0 +/- 28.3 ms; n = 15) than that of the action potential (18.8 +/- 9.1 ms; n = 16). 4. Application of tetraethylammonium (TEA) to cutaneous or muscle afferents alone had little effect on single action potential waveform. However, TEA reduced the amplitude of the AHP elicited by a single stimulus in cutaneous afferent axons after exposure to 4-AP and resulted in repetitive spike discharge. 5. The delayed depolarization and spike burst activity induced by 4-AP in SN was present in Ca(2+)-free solutions containing 1 mM ethylene glycol-bis (beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid and was not blocked by Cd2+ (1.0 mM). 6. We obtained whole-cell patch-clamp recordings to study Na+ currents from either randomly selected dorsal root ganglion neurons or cutaneous afferent neurons identified by retrograde labeling with Fluoro-Gold. The majority of the randomly selected neurons had a singular kinetically fast Na+ current. In contrast, no identified cutaneous afferent neurons had a singular fast Na+ current. Rather, they had a combination of kinetically separable fast and slow currents or a singular relatively slow Na+ current.(ABSTRACT TRUNCATED AT 400 WORDS)

Long lasting excitability changes in human peripheral nerve

When pairs of equal but submaximal electrical stimuli are delivered to a peripheral nerve, the second stimulus does not always excite the same number of fibers as the first. The number of fibers responding to the second stimulus depends on the interstimulus interval; the refractory period, a well-defined period of hypoexcitability, is followed by longer lasting and less well-characterized periods of hyper- and hypoexcitability. These cycles last at least 200 ms after the initial stimulus. We have carefully studied these cycles of excitability in human peripheral nerve in 12 normal subjects. The magnitude of excitability changes were found to be much greater in motor fibers than in mixed nerve; under some conditions, the motor response was reduced by more than 80% at interstimulus intervals of 40 ms, while the mixed nerve response never varied by more than 20%. In addition, the amplitude of the excitability changes varied as a function of the stimulus strength, so that stimuli that were near threshold or evoked near maximal responses were associated with smaller excitability changes than stimuli evoking midrange responses. Given that the excitability fluctuations are of large magnitude and occur at interresponse intervals easily achieved during physiological firing, it is suggested that they may be important modifiers of firing rate under experimental or physiological conditions.

Microneurography, impulse conduction, and paresthesias

It is possible to learn more about peripheral nerve function in human subjects than is obtainable with routine nerve conduction studies, and thereby to study the basis of "positive" symptoms, such as paresthesias. Using microneurography, ectopic impulse activity in cutaneous afferents has been recorded in patients suffering from neurologic disorders and in normal subjects in whom paresthesias were provoked by hyperventilation, prolonged tetanization of cutaneous nerves and ischemia. Using relatively simple modifications of standard nerve conduction techniques, the increases in axonal excitability responsible for this ectopic activity have been documented in human volunteers. Hyperventilation increases axonal excitability but does not change supernormality, probably because Na+ channels are activated by the decrease in [Ca2+] on the axonal membrane. Prolonged tetanic stimulation and ischemia probably share similar mechanisms. At least in motor axons, postischemic ectopic activity occurs when the hyperpolarization that results from activation of the Na+/K+ pump lowers the membrane potential below the equilibrium potential for K+. A high extracellular [K+] can then result in an inward current producing depolarization and possibly triggering regenerative processes.

Hyperglycaemic hypoxia alters after-potential and fast K+ conductance of rat axons by cytoplasmic acidification

1. The effects of hyperglycaemic hypoxia (a condition possibly involved in the pathogenesis of diabetic neuropathy) on the depolarizing after-potential and the potassium conductance of myelinated rat spinal root axons were investigated using electrophysiological recordings from intact spinal roots and from excised, inside-out axonal membrane patches. 2. Isolated spinal roots were exposed to hypoxia in solutions containing normal or high glucose concentrations. The depolarizing after-potential of compound action potentials was only enhanced in spinal roots exposed to hyperglycaemic (25 mM D-glucose) hypoxia. A maximal effect was seen in bathing solutions with low buffering power. 3. The depolarizing after-potential was also enhanced by cytoplasmic acidification after replacement of 10-30 mM chloride in the bathing solution by propionate. 4. Multi-channel current recordings from excised, inside-out axonal membrane patches were used to study the effects of cytoplasmic acidification on voltage-dependent K+ conductances with fast (F channels) and intermediate (I channels) kinetics of deactivation. 5. F channels were blocked by small changes in cytoplasmic pH (50% inhibition at pH 6.9). I channels were much less sensitive to intra-axonal acidification. 6. In conclusion, our data show that hyperglycaemic hypoxia enhances the depolarizing after-potential in peripheral rat axons. The underlying mechanism seems to be an inhibition of a fast, voltage-dependent axonal K+ conductance by cytoplasmic acidification. This alteration in membrane conductance may contribute to positive symptoms in diabetic neuropathy.

Differences in sensitivity to hyperglycemic hypoxia of isolated rat sensory and motor nerve fibers

We explore whether the prevalence of sensory deficits in diabetic neuropathy can be explained by diffuse endoneurial hypoxia. Isolated ventral and dorsal rat spinal roots incubated in 2.5 or 25 mM extracellular glucose were transiently exposed to hypoxia (30 min) in a solution of low buffering power. Compound nerve action potentials and extracellular direct current potentials were continuously recorded before, during, and after hypoxia. In both ventral and dorsal roots incubated in 2.5 mM glucose, sensitivity to hypoxia and posthypoxic recovery were similar. In contrast, hypoxia in 25 mM glucose preferentially induced electrophysiological damage in dorsal roots as indicated by a lack of posthypoxic recovery. This observation was not made in the presence of 25 mM bicarbonate, which suggests involvement of nerve acidosis. In conclusion, the different sensitivity of sensory and motor fibers to hyperglycemic hypoxia supports the hypothesis that hypoxia has an important role in the pathogenesis of diabetic neuropathy.

Ectopic activity in demyelinated spinal root axons of the rat

1. We have provoked ectopic discharges from demyelinated rat spinal roots by applying 1 mM-4-aminopyridine (4-AP), and recorded membrane currents and action potentials extracellularly by spike-triggered averaging. The demyelination was caused by intrathecal injection of diphtheria toxin, 6-9 days previously. 2. Mapping the distribution of membrane currents in the vicinity of an ectopic site showed that in most cases (eight out of twelve recorded) the impulses arose from one end of a continuously conducting internode, and conducted in both directions. In the remaining cases the impulses also arose from a site of demyelination. 3. The 4-AP-induced activity resembled the activity occurring spontaneously in some preparations, and was often highly regular (5-20 Hz). Recordings of membrane potential revealed a pacemaker potential, which was localized to the site of impulse initiation. One ectopic site was tested with applied currents and found to have a linear current-frequency relation for steady currents. 4. The time course of the pacemaker potential resembled that of the small after-hyperpolarization seen in normal fibres, due to a slow K+ conductance (GKs). Tetraethylammonium and barium ions, which block GKs, made spontaneously active fibres fire much more rapidly, or to fire bursts of action potentials. 5. Possible mechanisms for these ectopic discharges are discussed. GKs appears to contribute to the pacing of the activity, but not its generation. The increased excitability of the active fibres could not be attributed directly to the loss of myelin, nor to extracellular K+ accumulation. We suggest that they may have been depolarized by stretch-activated or ligand-gated channels in the demyelinated axon membrane.

Physiological evidence for a slow K+ conductance in human cutaneous afferents

1. The depression in axonal excitability that follows short trains of impulses (H1) may lead to spike frequency adaptation to a sustained stimulus, and has been attributed to a slow K+ conductance. The present experiments sought indirect evidence for slow K+ channels at the node of Ranvier of human cutaneous afferents based on the demonstration of post-tetanic changes in excitability typical of H1. 2. The excitability changes in low-threshold cutaneous afferents in the digital nerves of the index finger were explored using a submaximal test pulse conditioned by trains of supramaximal stimuli, containing up to 100 impulses. Changes in the amplitude of the compound sensory action potential set up by a constant test stimulus were used as a measure of the changes in excitability. These changes in amplitude were paralleled by inverse changes in latency. 3. When the conditioning stimulus was a single supramaximal pulse, excitability was enhanced at conditioning-test intervals of 4-40 ms, with a peak at 6-8 ms. When the conditioning stimulus consisted of a train of ten pulses delivered at 200 Hz, the recovery cycle was dominated by subnormality that was maximal at 20 ms and subsided gradually over 50 ms. 4. The post-train depression in excitability increased as the number of pulses in the conditioning train increased to ten but changed little with further increases in train duration. The degree of depression increased with the pulse frequency within the train. Cooling the hand from a skin temperature of 35 to 25 degrees C slowed the recovery processes but did not alter the magnitude of the post-train depression. 5. These characteristics are typical of the H1 phase of post-tetanic depression in axonal excitability. The extent of the depression in excitability suggests, first, that there may be a significant K+ conductance at the nodes of human cutaneous afferents and, secondly, that H1 may play a significant role in limiting repetitive discharge in normal and pathological afferents.

TEA-sensitive potassium channels and inward rectification in regenerated rat sciatic nerve

Sucrose gap and intra-axonal recording techniques were used to identify the types of ion channels and inward rectification that are present in regenerated axons of adult (greater than 8 weeks) rat sciatic nerve after crush injury. In sucrose gap recordings, 4-aminopyridine (4-AP) led to slight broadening of the compound action potential (CAP) in normal nerve, and a greater broadening in regenerated nerves. By 12 days after sciatic nerve crush, regenerated nerves manifested an afterhyperpolarization (AHP) lasting up to 250 ms that was sensitive to tetraethylammonium (TEA). A similar TEA-sensitive AHP could be elicited with repetitive stimulation. Hyperpolarizing constant current steps (0.1 to 0.5 mA; 600-900 ms duration) applied across the sucrose gap through regenerated axons evoked membrane hyperpolarizations with a depolarizing, Cs(+)-sensitive relaxation in the response to hyperpolarization, which is characteristic of inward rectification, occurring after about 70 ms. The relaxation was present as early as 21 days after nerve crush. Intra-axonal recordings showed burst firing in 4-AP that was terminated by an AHP that temporally correlated with the TEA-sensitive AHP, and a relaxation in the response to hyperpolarizing current, similar to that of whole nerve recordings. The results demonstrate that in addition to voltage-sensitive sodium channels and 4-AP-sensitive potassium channels, there are TEA-sensitive and inwardly rectifying channels on mammalian regenerated peripheral nerve axons.

Depolarization changes the mechanism of accommodation in rat and human motor axons

1. We have previously studied accommodation in rat and human motor axons by testing excitability with combinations of long and short current pulses. We found that normally polarized axons accommodate slowly and partially (over about 50 ms) to subthreshold depolarizing currents, and that the principal mechanism is the activation of slow potassium channels (Bostock & Baker, 1988). To understand the response of human nerves to ischaemia, we have now extended these observations to axons already depolarized before the testing currents were applied. 2. Rat ventral root axons were depolarized by passing continuous currents or by raising the extracellular potassium concentration. Human forearm nerves were depolarized by ischaemia, induced by inflating a sphygmomanometer cuff on the upper arm. Depolarized rat and human motor axons accommodated much more rapidly and completely than normally polarized axons (e.g. accommodation in rat axons was 50% complete within 2 ms at about 15 mV depolarized to rest). 3. The fast component of accommodation in depolarized rat fibres was not blocked by tetraethylammonium ions or 4-aminopyridine, was not accompanied by a conductance or potential change, and had a time constant of 1.7 ms at 30 degrees C. It was attributed to inactivation of closed sodium channels. 4. In depolarized rat fibres exhibiting fast accommodation, a brief rise in excitability was seen at the break of an anodal current. Our prediction that human motor axons would show anode-break excitation during ischaemia was readily confirmed. 5. The results are discussed in relation to Hill's (1936) mathematical description of accommodation in nerve, and it is concluded that his description is only applicable to depolarized axons.

Physiological effects of 4-aminopyridine on demyelinated mammalian motor and sensory fibers

The selective response of demyelinated sensory fibers to 4-aminopyridine (4-AP) has been proposed as a mechanism underlying the reported paresthesias that complicate the use of this potassium-channel blocking agent in clinical trials for the treatment of multiple sclerosis and neuromuscular disorders. To identify differences in the electrophysiological response of specific fiber types to the application of 4-AP, rat ventral and dorsal spinal roots, demyelinated by intrathecal injections of lysophosphatidylcholine, were examined in vitro before and during potassium-channel blockade. The compound action potentials of demyelinated ventral roots showed a prominent postspike negativity associated with a broadening of single action potentials following application of 4-AP. Under similar conditions, whole root responses of demyelinated dorsal root axons also developed a late negativity, but individual fibers were observed to fire repetitively in response to a single stimulus. The data support the hypothesis that the prominent sensory dysfunctions reported in clinical trials of 4-AP are due to the selective response characteristics of sensory fibers.

Differential sensitivity of amphibian nodal and paranodal K+ channels to 4-aminopyridine and TEA

Voltage-dependent K+ channels are blocked by several drugs, including 4-aminopyridine (4-AP) and tetraethylammonium (TEA). 4-AP is most widely used to localize K+ channels in mammalian and non-mammalian nerve fibers, but 4-AP and TEA alter various K+ channels and/or preparations in specific ways. The reason is not known, in part because dissociation constants for 4-AP and TEA have not been measured for nodal and internodal K+ channels in the same fibers. Smith and Schauf showed that the density of nodal versus paranodal K+ channels in frog nerves depends on fiber diameter. The size dependence was used to determine the relative sensitivity of nodal and internodal K+ channels to 4-AP and TEA, and to compare voltage- and time-dependent activation. The results show nodal and internodal K+ channels activate similarly. However, internodal channels are selectivity blocked by 4-AP while TEA is more effective on nodal channels. A high sensitivity of internodal K+ channels may explain why 4-AP improves symptoms in diseases such as multiple sclerosis.

Function and distribution of three types of rectifying channel in rat spinal root myelinated axons

1. The nature, distribution and function of rectifying channels in rat spinal root myelinated axons has been assessed with selective blocking agents and a variety of intracellular and extracellular recording techniques. 2. The electrotonic responses of roots poisoned with tetrodotoxin (TTX) to constant current pulses had fast (rise time much less than 1 ms) and slow components, which were interpreted in terms of Barrett & Barrett's (1982) revised cable model for myelinated nerve. Depolarization evoked a rapid outward rectification (time constant, tau approximately 0.5 ms), selectively blocked by 4-aminopyridine (4AP, 1 mM), and a slow outward rectification (tau approximately 15 ms), selectively blocked by tetraethylammonium (TEA, 1 mM) or Ba2+ (0.5 mM). Hyperpolarization evoked an even slower inward rectification, selectively blocked by Cs+ (3 mM) but not by Ba2+. 3. From the different effects of the blocking agents on the fast and slow components of electrotonus, it was deduced (a) that the inward rectification is a property of the internodal axon, (b) that the slow outward rectifier is present at the nodes, and probably the internodes as well, and (c) that the 4AP-sensitive channels have a minor nodal and a major internodal representation. 4. TEA and Ba2+ reduced the accommodation of roots and fibres not poisoned with TTX to long current pulses, whereas 4AP facilitated short bursts of impulses in response to a single brief stimulus. 5. TEA and Ba2+ also abolished a late hyperpolarizing after-potential (peaking at 20-80 ms), while 4AP enhanced the depolarizing after-potential in normal fibres, and abolished an early hyperpolarizing after-potential (peaking at 1-3 ms) in depolarized fibres. Corresponding to the later after-potentials were post-spike changes in excitability and conduction velocity, which were affected similarly by the blocking agents. Cs+ increased the post-tetanic depression attributable to electrogenic hyperpolarization. 6. The physiological roles of the three different rectifying conductances are discussed. It is also argued that the prominent ohmic 'leak conductance', usually ascribed to the nodal axon, must arise in an extracellular pathway in series with the rectifying internodal axon.